KR20070003918A - Steel plate excellent in machinability, toughness and weldability, and method for production thereof - Google Patents

Abstract

A steel plate which has a thickness of 4 mm or more and less than 10 mm, wherein interior portions thereof having distances from its surface of 1/4 and 3/4 of its thickness have a ferrite proportion of 30 to 90 %, and an interior portion having a distance from the surface thereof of 1/2 of its thickness has a ferrite proportion of 20 to 90 %; and a steel plate which has a thickness of 10 to 100 mm, wherein interior portions thereof having distances from the surface and the back surface thereof of 2 mm have a ferrite proportion of 30 to 90 %, and interior portions thereof having distances from its surface of 1/4, 1/2 and 3/4 of its thickness have a ferrite proportion of 20 to 90 %. The above steel plate is produced by a method comprising using a steel having the contents of C, Si, Mn, P, S, Al and N being limited to prescribed ranges and optionally further containing Mo, Cr, Nb, Ti, V, Cu, Ni, B, REM, Ca, Zr and/or Mg, and strictly specifying the balance between components of the steel, and comprising strictly controlling the conditions in rolling, washing with water, and the like, and is excellent in machinability, toughness and weldability. ® KIPO & WIPO 2007

Description

Steel plate excellent in machinability, toughness and weldability and its manufacturing method {STEEL PLATE EXCELLENT IN MACHINABILITY, TOUGHNESS AND WELDABILITY, AND METHOD FOR PRODUCTION THEREOF}

The present invention relates to a steel sheet excellent in machinability, toughness and weldability, in particular a steel sheet having a sheet thickness of about 4 to 100 mm and a tensile strength of about 570 to 720 MPa and a method of manufacturing the same. The steel sheet manufactured by this manufacturing method can be used for general welding structures such as shipbuilding, bridges, construction, offshore structures, pressure vessels, line pipes, etc. It is effective for use in the field.

In addition to high strength, steel sheets used in welded structures often require suppression of weld cracking and high weld heat affected zone toughness as weldability. In the case of steel materials having a tensile strength of 570 MPa or more, the addition amount of alloying elements is extremely suppressed, and the main structure constituting the steel is bainite or martensite, thereby attempting to achieve both high strength and weldability. However, when manufacturing structures such as architecture, bridges, and ships, there are cutting processes such as hole machining and surface cutting. When bainite and martensite are the main structures, the frequency of replacement and regrinding associated with tool life increases, and cutting resistance. The productivity of the work is lowered due to the decrease in cutting speed through increase, and as a result, the manufacturing cost of the structure is increased. For example, Japanese Unexamined Patent Application Publication No. 9-310117 discloses both high strength and weldability by making the structure mainly of bainite with a relatively low alloy. However, since the structure of the steel is a hard bainite main body, the machinability is poor and the cutting work costs are high.

Steel sheets used in major parts of the welded structure, for example, webs, flanges, etc., are welded and relatively large stresses are applied at the time of use, and therefore, excellent weldability and toughness are required. On the other hand, there are some parts where welding is not performed and some parts where high toughness is not required. For example, a bonding plate or the like used for bolting the main structure of a bridge preferably has toughness that satisfies the specification, but does not require the same level as the main structure. The use of a steel plate composed mainly of bainite and martensite at such a site causes poor machinability, which requires time for cutting, and greatly increases the production cost of the structure.

It is known that the addition of S is effective for machinability, in particular for prolonging tool life and reducing cutting resistance. However, when S is added in a large amount, the base metal toughness is lowered and the weldability is lowered. On the other hand, the method of making both machinability improvement and weldability ensure by S addition is disclosed by Unexamined-Japanese-Patent No. 6-184695. However, the weldability secured here is only the omission of preheating and the suppression of the weld crack, and the weld part toughness and the base metal toughness are low and cannot be used as weld structural steel. Moreover, the method of making machinability and base material toughness compatible is disclosed by Unexamined-Japanese-Patent No. 2000-87179. The anisotropy of the base metal toughness is improved by controlling the shape of the sulfide by the addition of Ca and Mg. However, since the absolute value is low and the weldability is also poor, it cannot be used as a welding structural steel.

Machinability also depends on the microstructure configuration, and it is known that ferrite pearlite and ferrite bainite structure are superior to the structure mainly composed of bainite and martensite. For example, Unexamined-Japanese-Patent No. 7-54100, Unexamined-Japanese-Patent No. 7-109518, and Unexamined-Japanese-Patent No. 7-166235 disclose the steel whose structure is a ferrite bainite structure. In addition, Japanese Unexamined Patent Publication No. 2000-63988, Japanese Unexamined Patent Publication No. 2000-63989, Japanese Unexamined Patent Publication No. 2000-282172, and Japanese Unexamined Patent Publication No. 2001-214241 disclose steels defining the fraction of ferrite. The machinability of steel sheets with microstructures of ferrite bainite and steel sheets with a constant ferrite fraction is qualitatively superior to that of bainite or martensite, but the absolute improvement is in the manufacturing process of welded structures. It is not enough to improve productivity in hole making and surface cutting. In addition, since the said technique has many addition amounts of alloying elements, and since toughness and weldability are low, it cannot be used as steel for welded structures. From the above, it is impossible to manufacture a steel sheet having a tensile strength of 570 MPa or more and high toughness, weldability, and machinability with the current technology.

The present invention is excellent in machinability, toughness and weldability, or extremely excellent in machinability, and has a toughness of the degree applicable to the weld structure, and has a thickness of about 4 to 100 mm and a level of tensile strength of about 570 to 720 MPa. By providing the steel plate and its manufacturing method, the summary is as follows.

 (1) the steel, in mass%,

C: 0.005 to 0.2%,

Si: 0.01 to 1%,

Mn: 0.01-2%,

P: 0.02% or less,

S: 0.035% or less

Al: 0.OO1 to 0.1%

N: 0.1% or less, the remainder being a steel composition composed of Fe and unavoidable impurities, X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + X1 represented by V / 10 + 5 × B is 0.24 or less,

If the sheet thickness is 4 mm or more and less than 10 mm, the ferrite fraction of the portion that enters from the steel plate label surface by 1/4 and 3/4 of the sheet thickness is 30% or more and 90% or less, and the steel sheet is half the plate thickness. A steel sheet excellent in machinability, toughness and weldability, characterized in that the ferrite fraction of the portion that enters from the surface to the inside is 20% or more and 90% or less.

 (2) The steel is, in mass%, in the steel composition described in (1),

Mn: 0.1 to 1.4%,

S: 0.01% or less,

X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr X2 represented by / 2) / Mn is 0.15 or more, 10.0 or less, or the structure constituting the steel has a ferrite fraction of 30% or more and 90% or less, and the remainder consists of hard tissue mainly composed of bainite and martensite. The steel sheet excellent in machinability, toughness, and weldability, whose structure is 20% or more of micro Vickers hardness of 190HV or less, and Vickers hardness of steel is 165HV or more and 300HV or less.

 (3) The steel is mass%, and in the steel composition as described in (1),

Mn: 0.01-1.4%,

S: more than 0.01% to 0.035% or less,

X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr X2 represented by / 2) / Mn is 0.15 or more and 10.0 or less,

In addition, the steel constituting structure is a structure in which the ferrite fraction is 30% or more and 90% or less, and the remainder consists of hard tissue mainly composed of bainite and martensite, or 20% or more, in which the micro-Vickers hardness is 190HV or less. And the Vickers hardness of the steel is 165 HV or more and 300 HV or less, the steel sheet excellent in machinability, toughness and weldability.

 (4) the steel, in mass%,

Mo: 0.01 to 1%,

Cr: 0.1 to 1%,

Nb: 0.001-0.1%,

Ti: 0.001-0.1%,

V: 0.001 to 0.1%,

Cu: 0.005 to 1%,

Ni: 0.01-2%,

B: 0.0002 to 0.005%,

REM: 0.0005 to 0.1%,

Ca: 0.0005 to 0.02%,

Zr: 0.0005 to 0.02%,

Mg: The steel plate excellent in the machinability, toughness, and weldability in any one of (1)-(3) characterized by containing 1 type (s) or 2 or more types of 0.0005 to 0.02%.

 (5) has the steel composition described in (1),

+1070, X2=(Si/5+Mo+Cr/2)/Mn으로 나타내는 T1(℃) 이하 720℃ 이상, 전체 압하율을 30% 이상 95% 이하로 하는 마무리 압연을 실시하고, 압연 종료 후에, 수량 밀도가 0.2 m³/m²·mm. 이상 5.O m³/m²·mm. 이하인 수냉을 개시하고, 600℃ 이하에서 수냉을 종료하는 것을 특징으로 하는 피삭성과 인성 및 용접성이 우수한 강판의 제조 방법. 다만, t는 판 두께이다. The total reduction ratio is 30 after heating the steel sheet or cast steel having X1 of 0.24 or less represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B. Rough rolling of not less than 95% and not more than 95% is performed, and then the first pass bite temperature is set to T1 = 351 n (X2 / 2) -25.

T1 (° C) or less represented by +1070, X2 = (Si / 5 + Mo + Cr / 2) / Mn, 720 ° C or more, and finish rolling with a total reduction ratio of 30% or more and 95% or less. , The quantity density is 0.2 m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling at or below and ending water cooling at 600 ° C or lower. Where t is the plate thickness.

 (6) In the water cooling which starts after completion | finish of rolling, the average cooling rate of water cooling start temperature or more and more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and the average cooling rate more than 650 degreeC or less water cooling stop temperature is 10 degreeC. It is / s or more and 100 degrees C / s or less, The manufacturing method of the steel plate excellent in the machinability, toughness, and weldability as described in (5).

 (7) has the steel composition described in (1),

The total reduction ratio is 30 after heating the steel sheet or cast steel having X1 of 0.24 or less represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B. Rough rolling with% or more and 95% or less, finish rolling with a total rolling reduction of 30% or more and 95% or less, followed by air cooling, and the steel sheet surface temperature of T2 = 910-310 × C-80 × Mn-20 × Cu -15 × Cr-55 × Ni- 80 × Mo + 0.0006t 2 -0.56t-8.6 in more than 650 ℃ below T2 (℃) represented by the water density is ^{0.2 m 3 / m 2 · mm} . 5.O m ³ / m ² · mm or more. A water production process excellent in the machinability, toughness, and weldability which starts water cooling below and completes water cooling below 50 degreeC. Where t is the plate thickness.

(8) X1 which has the steel composition as described in (1), and is represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5xB is 0.24 or less After heating the steel or cast steel, rough rolling with a total reduction ratio of 30% or more and 95% or less is performed, and finish rolling with a total reduction ratio of 30% or more and 95% or less is cooled to 500 ° C or less, and the steel sheet is T3 = 0.0017t. ^A method for producing a steel sheet excellent in machinability, toughness and weldability, wherein water cooling is terminated after reheating to T 3 (° C.) or higher and 850 ° C. or lower represented by ² +0.17 t + 730. Where t is the plate thickness.

+1100로 나타내는 T4(℃) 이하 Ar₃ 점 이상, 전체 압하율을 30% 이상 95% 이하로 하는 마무리 압연을 실시하고, 그 후 신속하게 수량밀도가 0.2 m³/m²·mm. 이상 5.O m³/m²·mm. 이하인 수냉을 개시하고, 600℃ 이하에서 수냉을 종료하는 것을 특징으로 하는 피삭성과 인성 및 용접성이 우수한 강판의 제조 방법. 다만, t는 판 두께이다. (9) X1 having a steel composition as described in (2) and represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less And heating the slab or cast steel having X2 represented by X2 = (Si / 5 + Mo + Cr / 2) / Mn of 0.15 or more and 10.0 or less, followed by rough rolling with a total reduction ratio of 30% or more and 95% or less, Then the first pass bite temperature is T4 = 351n (X2 / 2) -25

T4 (° C.) or less represented by +1100, at least ₃ points of Ar and a finish rolling with a total reduction ratio of 30% or more and 95% or less were performed, and thereafter, the yield density was quickly 0.2 m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling at or below and ending water cooling at 600 ° C or lower. Where t is the plate thickness.

 (10) In the water cooling started after the end of rolling, the average cooling rate of the water cooling start temperature or less and more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and the average cooling rate more than 650 degreeC or less water cooling stop temperature is 10 degreeC. It is / s or more and 100 degrees C / s or less, The manufacturing method of the steel plate excellent in the machinability, toughness, and weldability as described in (9).

 (11) X1 having a steel composition as described in (2) and represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less X2 = (Si / 5 + Mo + Cr / 2) / Mn, after heating and rolling a steel sheet or cast steel having 0.15 or more and 10.0 or less, cooling to 500 ° C or less, and further heating the steel sheet to 900 ° C or more and 1050 ° C or less. Reheating the furnace, water cooling at an average cooling rate of 1 ° C./s or more and 100 ° C./s, and terminating water cooling at 500 ° C. or less, characterized in that the steel sheet is excellent in machinability, toughness and weldability.

 (12) has the steel composition described in (2),

X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) / Xn represented by Mn, after heating the steel or cast steel of 0.15 or more, 10.0 or less, rough rolling with a total reduction of 30% or more and 95% or less, and finish rolling with a total reduction of 30% or more and 95% or less after cooling to the air-cooling after the Ar ₃ point or more and 150 ℃ temperature lower than the Ar ₃ point,

Quantity density is 0.2 m ³ / m ² · mm. 5. The manufacturing method of the steel plate excellent in machinability, toughness, and weldability which starts water cooling of more than 5.Om <^3> / m <^2> mm or less and completes water cooling at 500 degrees C or less.

 (13) has the steel composition described in (2),

X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) After heating the steel or cast steel with X2 of 0.15 or more and 10.0 or less represented by / Mn, rough rolling of 30% or more and 95% or less of total rolling reduction, and finish rolling of 30% or more and 95% or less of total rolling reduction are performed. Of the steel sheet excellent in machinability, toughness and weldability, wherein the steel sheet is cooled to 500 ° C. or lower, and then reheated the steel sheet to 730 ° C. or higher and less than 900 ° C., and then cooled to end water cooling at 500 ° C. or lower. Manufacturing method.

+1100으로 나타내는 T4(℃) 이하 Ar₃점 이상, 전체 압하율을 30% 이상 95% 이하로 하는 마무리 압연을 실시하고, 압연 종료 후에, 수량 밀도가 0.2 m³/m²·mm. 이상 5.O m³/m²·mm. 이하인 수냉을 개시하고, 600℃ 이하에서 수냉을 종료하는 것을 특징으로 하는 피삭성과 인성 및 용접성이 우수한 강판의 제조 방법. 다만, t는 판 두께이다. (14) X1 having a steel composition as described in (3) and represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less And heating the slab or cast steel having X2 represented by X2 = (Si / 5 + Mo + Cr / 2) / Mn of 0.15 or more and 10.0 or less, and then rough rolling having a total reduction ratio of 30% or more and 95% or less, and The first pass bit temperature after T4 = 351n (X2 / 2) -25

Represented by the +1100 T4 (℃) less than Ar ₃ point or higher, subjected to finish rolling to a total reduction ratio in a range from 30% to 95% and, after rolling end, the water density ^{0.2 m 3 / m 2 · mm} . 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling at or below and ending water cooling at 600 ° C or lower. Where t is the plate thickness.

 (15) In the water cooling started after the end of rolling, the average cooling rate of the water cooling start temperature or less and more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and the average cooling rate of 650 degrees C or less and water cooling stop temperature or more is 10. It is C / s or more and 100 degrees C / s or less, The manufacturing method of the steel plate excellent in the machinability, toughness, and weldability as described in (14).

 (16) has the steel composition described in (3),

X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr X2 represented by / 2) / Mn is heated to 0.15 or more and 10.0 or less, and after heating and rolling the steel slab or cast, it is cooled to 500 degrees C or less, and the steel plate is reheated at 900 degrees C or more and 1050 degrees C or less, and the average cooling rate is 1 degrees C / s. It is water-cooled to 100 degrees C / s or less and completes water cooling at 500 degrees C or less, The manufacturing method of the steel plate excellent in the machinability, toughness, and weldability.

 (17) has the steel composition described in (3),

X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + M / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) After heating the steel or cast steel with X2 represented by Mn of 0.15 or more and 10.0 or less, rough rolling of 30% or more and 95% or less of total rolling reduction, and finish rolling of 30% or more and 95% or less of total rolling reduction are performed. was cooled to, and then air-cooled to Ar ₃ point or more and 150 ℃ temperature lower than the Ar ₃ point and then,

Quantity density is 0.2 m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling below and ending water cooling at 500 ° C or lower.

 (18) has the steel composition described in (3),

X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) / Xn represented by Mn, after heating the steel or cast steel of 0.15 or more, 10.0 or less, rough rolling with a total reduction rate of 30% or more and 95% or less, and finish rolling with a total reduction rate of 30% or more and 95% or less After that, the steel sheet is cooled to 500 ° C. or lower by air cooling, and after the steel sheet is reheated to 730 ° C. or higher and less than 900 ° C., water is cooled, and the water cooling is terminated at 500 ° C. or higher, thereby producing a steel sheet excellent in machinability, toughness and weldability. Way.

 (19) The said slab or cast steel is in mass%,

Mo: 0.1 to 1%,

Cr: 0.1 to 1%,

Nb: 0.OO1 to 0.1%,

Ti: 0.OO1 to 0.1%,

V: 0.001 to 0.1%,

Cu: 0.005 to 1%,

Ni: 0.01-2%,

B: 0.0002 to 0.005%,

REM: 0.0005 to 0.1%,

Ca: 0.0005 to 0.02%,

Zr: 0.0005 to 0.02%,

Mg: 0.0005 to 0.02% of 1 type or 2 or more types characterized by the manufacturing method of the steel plate excellent in the machinability, toughness, and weldability in any one of (5)-(18).

According to the present invention, the structure constituting the steel is a composite structure of soft ferrite and hard bainite and martensite, and (1) the ferrite fraction of the front and back surfaces of the steel sheet, which has a great influence on the wear and tear of the tool, Strictly stipulating, or (2) adjusting the steel component balance that can greatly reduce the cutting resistance during cutting at a high temperature, or (3) increasing the amount of S added in a range that does not cause toughness deterioration. It is possible to provide a steel sheet having a high level of machinability which has not been achieved in the conventional welded structural steel sheet, and a tensile strength of about 570 to 720 MPa, which is also excellent in strength, toughness and weldability, and a manufacturing method thereof. It is a high invention of industrial value.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the site | part which measures the micro-Vickers hardness prescribed | regulated by this invention.

The present invention will be described in detail.

The inventors earnestly examine the method for producing the steel sheet according to (1) of the present invention having a sheet thickness of about 4 to 100 mm and a strength of the base material of about 570 to 720 MPa and excellent in base material toughness, weldability, and machinability. It was. As a result, the soft tissue mainly composed of ferrite and the hard tissue mainly composed of bainite and martensite are used as the main structure of the steel, especially the start and end of the cutting operation. Strictly defining the ferrite fraction of the front and back surfaces of the steel sheet, which has a great influence, and strict regulations of the manufacturing method requiring water cooling, have been found to significantly improve machinability while securing strength, base material toughness and weldability. .

In addition, in this invention, weldability refers to both a welding crack and a weld heat affected zone toughness, and it is assumed that the harder a weld crack is generated and the higher the weld heat affected zone toughness, the better the weldability. On the other hand, machinability refers to tool life, cutting resistance and cutting debris treatment, and the longer the tool life, the lower the cutting resistance, and the easier the cutting debris treatment, the better the machinability.

The most important thing in the steel plate as described in (1) of this invention is to produce a large amount of ferrite in the vicinity of the steel plate surface. In the cutting process, the surface of the steel sheet corresponds to the start point and the end point, and since a large load is placed on the tool, cutting in this area has a great influence on the cutting resistance and cutting debris treatment in the tool life or subsequent cutting. Gives. That is, by making the structure of the front and back surfaces of the steel sheet a composite structure of the soft tissue and the hard tissue, the soft portion is easily deformed at the time when the tool starts to cut the workpiece and at the time of the end, while the soft part and the hard part are in the vicinity of the interface of the soft part and the hard part. Stress concentration occurs, and as a result, cutting can start and end with very little plastic deformation. As a result, the tool life is long, the cutting resistance is lowered, and the cutting chips are easily processed. The inventor evaluates the machinability of various steel sheets having different ferrite distribution in the plate thickness direction, and represents a value representing the ferrite distribution in the plate thickness direction. 3 parts of the part (hereinafter called t / 4 part, t / 2 part, 3t / 4 part respectively) which entered the steel plate inside from the steel plate surface by 1/2, 3/4 are 10 mm or more of sheet thickness, and 100 mm or less In steel sheet of 5, five places were added in addition to the above three places, and the part (2 mm below the surface and 2 mm below the back surface) which entered the inside of the steel sheet by 2 mm from the steel plate surface and the back surface was defined. It was found to be desirable.

In the case of steel sheets having a sheet thickness of 4 mm or more and less than 10 mm, the machinability was found to be good when the ferrite fraction of t / 4 part and 3t / 4 part was 30% or more and the ferrite fraction of t / 2 part was 20% or more. On the other hand, when any one of the ferrite fractions of t / 4 part, t / 2 part, and 3t / 4 part exceeds 90%, intensity | strength falls largely. From this, it is prescribed | regulated that the ferrite fraction of t / 4 part and 3t / 4 part is 30% or more and 90% or less, and the ferrite fraction of t / 2 part is 20% or more and 90% or less with respect to the steel plate of 4 mm or more and less than 10 mm. It was. In addition, when the ferrite fraction of t / 4 part, t / 2 part, and 3t / 4 part is 50% or more, the machinability is significantly improved. Therefore, the ferrite fraction is preferably t / 4 part, t / 2 part and 3t / 4 part. It is prescribed that this is 50% or more and 90% or less.

In the case of a steel plate with a sheet thickness of 10 mm or more and 100 mm or less, the ferrite fraction of 30 mm or more in the 2 mm bottom surface and the 2 mm bottom surface is 20% or more, and the ferrite fraction in t / 4 part, t / 2 part, and 3 t / 4 part is 20%. It turned out that machinability becomes favorable when it becomes abnormal. On the other hand, in the case of more than 90% of one of the ferrite fractions of 2 mm under the surface, 2 mm under the back, t / 4, t / 2, and 3t / 4, the strength is greatly reduced. From this, the ferrite fraction of 2 mm below the surface and 2 mm below the surface of the steel sheet having a sheet thickness of 10 mm or more and 10 mm or less is 30% or more and 90% or less, t / 4 parts, t / 2 parts and 3t / 4 Negative ferrite fraction was defined as 20% or more and 90% or less. Further, when the ferrite fraction of 2 mm below the surface, 2 mm below the back surface, t / 4 portion, t / 2 portion, and 3t / 4 portion is 50% or more, the machinability is significantly improved. The ferrite fractions of 2 parts, t / 4 parts, t / 2 parts, and 3t / 4 parts under the back and back surface are 50% or more and 90% or less.

At this time, the measuring method of a ferrite fraction is prescribed | regulated. The measurement is performed along a plane parallel to both the rolling direction and the plate thickness direction (hereinafter referred to as L plane). Avoiding between the edge part of the width direction of the steel plate and the site | part which entered inside the width direction by the length corresponding to the board | board thickness from the edge part, and extracting the test piece of the test piece thickness whole thickness extremely from the site | part near the center part of the steel plate width direction, Polish and nital etch. Then, L surface is observed with an optical microscope. The magnification is 500 times. Observation is carried out using an eyepiece containing a reticle, and the number of lattice points corresponding to ferrite is counted, and the fraction of the lattice points corresponding to the ferrite occupying all the lattice points (percentage indication) is used as the ferrite fraction. The measurement is performed at least 10 o'clock for each site, and the displacement amount moving from one field of view to the next field of view is kept constant. At this time, when it is difficult to judge whether it is a ferrite or another phase, it counts as 0.5. In addition, as a criterion of the ferrite at the time of measuring, the ferrite in this invention points out ferrite generally called a bulk ferrite, polygonal ferrite, an equiaxed ferrite, etc., and the needle-like ferrite produced at a low temperature is included. Do not do it. However, even in the case of the bulk ferrite, anisotropy may be generated in the growth direction depending on the control of the austenite before transformation, and a long form of the bulk ferrite may be produced in the rolling direction, but this is included in the ferrite in the present invention.

In addition, in order to make it excellent in weldability and toughness, it is necessary to adjust the addition amount of an alloying element. When X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, the welding crack may be greatly reduced. Moreover, since it is excellent also in toughness and toughness of a weld heat affected zone, X1 is prescribed | regulated as 0.24 or less. In addition, when X1 is 0.21 or less, this effect is more remarkable. Therefore, X1 is preferably 0.21 or less. In addition, when calculating X1, C, Mn, Cu, Cr, Si, Ni, Mo, V, and B are all addition amounts expressed in mass%.

In addition, the present inventors have a sheet thickness of about 4 to 100 mm, the tensile strength of the base material is about 570 to 720 MPa, and the method for producing the steel sheet according to (2) of the present invention excellent in both machinability, base material toughness and weldability. A polite review was made. As a result, the composite structure of the soft structure mainly composed of ferrite and the hard structure mainly composed of bainite and martensite is used as the main structure of the steel, the strict regulation of the balance of addition amount of Si, Cr, Mo and Mn in the steel components, It has been found that the machinability is greatly improved while ensuring strength, base material toughness and weldability, etc., by strict regulations of a manufacturing method mainly based on temperature control in a manufacturing method requiring water cooling.

Further, the inventors of the present invention also have a plate thickness of about 4 to 100 mm, a base material of about 570 to 720 MPa, extremely excellent machinability, and toughness that can be applied to a welded structure (3). The method of manufacturing the steel plate of the above-mentioned) was earnestly examined. As a result, the composite structure of the soft structure mainly composed of ferrite and the hard structure mainly composed of bainite and martensite is used as the main structure of the steel, the strict regulation of the balance of addition amount of Si, Cr, Mo and Mn in the steel components, Increasing the amount of S added in the range that does not significantly reduce the toughness, strict regulations of the manufacturing method mainly in the temperature control in the manufacturing method that requires water cooling, etc., while maintaining the strength and base material toughness It has been found to improve.

In addition, weldability in this invention refers to both a weld crack and a weld heat affected zone toughness, and it is assumed that weldability is excellent, so that weld crack generate | occur | produces and the weld weld heat affected zone toughness is high. On the other hand, machinability refers to tool life, cutting resistance, and cutting chip processing property. The longer the tool life, the lower the cutting resistance, and the easier the cutting chip processing, the better the machinability.

The most important thing as a structural requirement which expresses the excellent machinability by this invention is the following two points in the steel plate of (2) and (3) of this invention, but in the steel plate of (3) of this invention, it mentions later Point (third) is added. First, the structure of the steel sheet is a complex structure of soft ferrite, hard bainite, and martensite. In particular, since the steel sheet having a sheet thickness of about 4 to 100 mm is targeted, it is important to have a soft and hard composite structure in a wide portion in the sheet thickness direction. By controlling the structure in this way, the soft part is easily deformed at the time of cutting, while the soft fracture is promoted by stress concentration in the vicinity of the interface between the soft part and the hard part, and as a result, the cutting proceeds with very little plastic deformation. As a result, the tool life is long, the cutting resistance is lowered, and the cutting chips are easily processed. Even in the case of a composite structure of soft ferrite, hard bainite, and martensite, even if the soft ferrite fraction is less than 30%, the machinability is greatly reduced, and if it is more than 90%, the ferrite fraction is 50%. In the above-mentioned 90% or less, remainder prescribes that it mainly consists of bainite and martensite. In the case where the ferrite fraction is 45% or more, the machinability is more excellent. Therefore, the ferrite fraction is preferably 55% or more and 90% or less, and the balance is mainly composed of bainite and martensite. When the ferrite fraction is 60% or more, the machinability is remarkably excellent, and more preferably, the ferrite fraction is 50% or more and 90% or less, and the balance is mainly composed of bainite and martensite. In addition, the hard structure is mainly composed of bainite and martensite, but even when pearlite, acicular ferrite, and other inclusions are mixed in part, the machinability is excellent without being deteriorated within the scope of the present invention. .

The ferrite fraction defined above is measured by optical microscope tissue observation. A measurement surface is made into the surface (henceforth L surface) which a rolling direction and plate | board thickness direction make | form. When the plate thickness is 8 mm or less, the measured portion in the plate thickness direction is the portion that enters the inside of the steel sheet from the surface of the steel sheet by the length corresponding to 1/4, 1/2, 3/4 of the plate thickness of the steel sheet (hereinafter, t / 4 parts, t / 2 parts, and 3t / 4 parts), and when the plate thickness exceeds 8 mm, only t / 4 parts, t / 2 parts, and 3t / 4 parts in the plate thickness direction In addition, the site | part which entered inside by 2 mm from the steel plate surface, and the site | part which entered inside by 2 mm from the back surface of steel plate (henceforth 2 mm part under a surface, and 2 mm part under back surface) are also added into 5 parts. The line segments connecting each measuring point should be parallel to the plate thickness direction. The measurement site | part in the width direction avoids measuring between the edge part of the width direction of a steel plate, and the site | part which entered inside by the length corresponded to plate | board thickness from the end part, and is measured in the site | part extremely close to the center part of the width direction. The measurement is preferably carried out at a magnification of about 100 to 500 times, and the measurement is performed by the point count method using an eyepiece with a grating. It has an average value of the ferrite fraction in all the measurement parts, and sets it as the ferrite fraction in this invention. In addition, about the criterion of the ferrite at the time of making a measurement, the ferrite in this invention points out ferrite generally called a bulk ferrite, polygonal ferrite, an equiaxed ferrite, etc., and the needle-like ferrite produced at lower temperature is included. Do not do it. However, even in the case of the bulk ferrite, anisotropy in the growth direction may occur depending on the control of the austenite before transformation, and the bulk ferrite having a long shape in the rolling direction may be produced, but this is included in the ferrite in the present invention.

Next, the second requirement of the constituent requirement for expressing excellent machinability is as follows. The composite structure of the soft structure mainly composed of ferrite and the hard structure of the bainite and martensite main body is excellent in machinability as described above, but by itself, the machinability required for hole processing or surface cutting in the production of a welded structure alone. It is not necessarily enough. It is necessary to optimize the ratio of the addition amount of specific alloying elements on the premise of the complex structure of the soft structure and the hard structure. Specifically, the addition ratio of Mn amount, Si amount, Cr amount and Mo amount is strictly defined. Cutting such as hole processing or surface cutting is a phenomenon of destruction of the workpiece by the tool under high temperature and high strain rate, and it is important to know how much energy required for this is reduced. Therefore, the difference in strength between the soft part and the hard part at a high temperature is greatly increased. Needs to be. When the amount of Mn added is large, the amount of Mn added is good because the solid solution strengthening amount of the soft ferrite is increased to reduce the difference in strength between the hard part and the soft part. Increasing the addition amount of Si, Cr and Mo not only contributes to the increase of the room temperature strength of the hard part mainly composed of bainite and martensite, but also increases the resistance to the decrease in the strength of the hard part at high temperatures. It is effective to enlarge. The inventor produced a composite tissue steel of soft and hard tissues using a component ingot in which the amounts of Mn, Si, Cr, and Mo were changed in various ways, and examined the machinability and component balance. When X2 represented by / 5 + Mo + Cr / 2) / Mn was less than 0.15, it was found that the absolute level of machinability was insufficient, and conversely, when X2 exceeded 10.0, weldability greatly decreased. Therefore, in this invention, X2 represented by X2 = (Si / 5 + Mo + Cr / 2) / Mn is prescribed | regulated that it is 0.15 or more and 10.0 or less. Moreover, since machinability improves more when the value of X2 is 0.3 or more, Preferably, X2 is 0.3 or more and 10.0 or less. In addition, since machinability improves remarkably when the value of X2 is 0.4 or more, More preferably, X2 is made into 0.4 or more and 10.0 or less. In addition, Si, Mo, Cr, and Mn at the time of calculating X2 are all addition amounts expressed in mass%. Cr and Mo are important elements in the present invention, but are added as necessary after considering the alloy cost. When Cr and Mo are not added, the value of X2 is calculated from Si and Mn amounts.

As described above, in the steel sheet described in (3) of the present invention, in addition to the above two constituent requirements which are most important for expressing excellent machinability, S is weldable as a third requirement of constituent requirement for expressing superior machinability. It is important to add a large amount in the range which does not significantly reduce toughness. S has a function of reducing cutting resistance and extending tool life by the effect of MnS as a stress concentration source. When the addition amount exceeds 0.01%, the machinability is remarkably improved. On the other hand, when the addition amount exceeds 0.035%, the toughness and weldability decrease together, so that the amount of S is set to be more than 0.01% and 0.035% or less.

Although the above is the most important structural requirement for expressing excellent machinability in the steel sheets described in (2) and (3) of the present invention, the first requirement, that is, in the case where the tissue structure is complicated or very fine grained tissue, In some cases, it may be difficult to define the complex tissue of the soft tissue and the hard tissue. In the present invention, a method of determining that the composite tissue is a composite tissue by micro-Vickers hardness is also generally defined. Since the micro-Vickers hardness has a smaller measurement area than the Vickers hardness, in the case of a composite tissue, the measured value greatly varies depending on the structure of the tissue. In particular, it is possible to define that the hardness of the region mainly composed of ferrite is low, and that the composite tissue is composed of soft tissue and hard tissue by the ratio of the measurement score having low hardness. The inventor performed the micro-Vickers hardness test with respect to various structures, and made clear the range of the micro-Vickers hardness excellent in machinability. As a result, since the machinability is excellent when the ratio whose micro-Vickers hardness is 190 HV or less is 20% or more, the ratio whose micro-Vickers hardness is 190 HV or less is made into 20% or more. In addition, since the machinability is more excellent when the ratio of the micro Vickers hardness is 180 HV or less is 20% or more, the ratio of the micro Vickers hardness of 180 HV or less is preferably 20% or more. In addition, since the machinability is excellent when the ratio of the micro Vickers hardness is 170 HV or less is 20% or more, the ratio of the Vickers hardness is 170 HV or less is more preferably 20% or more. Further, when the ratio of the micro Vickers hardness is 170HV or less is 40% or more, the machinability is more excellent, and even more preferably, the ratio of the micro Vickers hardness of 170 HV or less is 40% or more.

In the present invention, the micro-Vickers hardness is a value measured based on the method specified in JIS Z 2244, and measurement methods other than those defined by the standard are described here in detail. Test force shall be 0.09807N. The measurement surface shall be L surface. When the plate thickness is 8 mm or less, the measurement site in the plate thickness direction is composed of three parts of t / 4 part, t / 2 part, and 3t / 4 part. The lower 2 mm part is also to be added to 5 parts. The line segments connecting each measuring point should be parallel to the plate thickness direction. The measurement site | part in the width direction avoids measuring between the edge part of the width direction of a steel plate, and the site | part which entered inside by the length corresponded to plate thickness, and is measured in the site | part extremely close to the center part of the width direction. As shown in FIG. 1, it measures at intervals of 100 micrometers, and makes a measurement score 121 points. Of these 121 points, the ratio of the scores of the micro Vickers hardness of 190 HV or less was measured, the average value of three parts was calculated when the plate thickness was 8 mm or less, and the average value of the five points was calculated when the plate thickness was more than 8 mm, Let this be the ratio whose micro-Vickers hardness is 190 HV or less. The same method is also used for the ratio of the micro Vickers hardness of 180 HV or less and the ratio of 170 HV or less.

In order to improve machinability, the above provisions are important, but in order to secure strength, toughness and weldability, the following provisions are required.

First, in order to secure 570 MPa or more in tensile strength, it is necessary to define Vickers hardness. If the Vickers hardness is less than 165 HV, it is difficult to secure the tensile strength of 570 MPa or more, and if the Vickers hardness exceeds 300 HV, the weldability is greatly reduced. Therefore, the Vickers hardness is defined to be 165 HV or more and 300 HV or less.

In this invention, a Vickers hardness is a value measured based on the method prescribed | regulated to JISZ2244, and measurement methods other than that prescribed | regulated by the specification are demonstrated in detail here. Test force is to be 98.07 N. The measurement surface shall be L surface. When the plate thickness is 8 mm or less, the measuring part in the plate thickness direction is composed of three parts of t / 4 part, t / 2 part, and 3t / 4 part. The lower 2 mm part is also to be added to 5 parts. The line segments connecting each measuring point should be parallel to the plate thickness direction. The measurement site | part in the width direction is measured between the edge part of the width direction of a steel plate, and the site | part which entered inside by the length equivalent to plate thickness from the end part, and is measured in the site | part extremely close to the center part of the width direction. A measurement is performed 5 or more points in each site | part, and calculates the average value of each site | part. When the plate thickness is 8 mm or less, the average value of three parts is calculated, and when the plate thickness is 8 mm or more, the average value of 5 points is calculated, and this is referred to as Vickers hardness.

In addition, in order to make it excellent in weldability and toughness, it is necessary to adjust the addition amount of an alloying element. When X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, the welding crack may be greatly reduced. Moreover, since it is excellent also in toughness and toughness of a weld heat affected zone, X1 is prescribed | regulated as 0.24 or less. In addition, when X1 is 0.21 or less, this effect is more remarkable. Therefore, X1 is preferably 0.21 or less. In addition, when calculating X1, C, Mn, Cu, Cr, Si, Ni, Mo, V, and B are all addition amounts expressed in mass%.

Below, the range of the alloying element of the steel plate of this invention is prescribed | regulated.

The following provisions are common to the steel sheets described in (1), (2) and (3) of the present invention unless otherwise specified.

Since C is an element essential for securing strength, the addition amount thereof is made 0.005% or more. On the other hand, an increase in the amount of C causes a decrease in base metal toughness and weldability due to the formation of coarse precipitates, and the upper limit thereof is made 0.2%. In addition, when the amount of C is at least 0.07%, it is easy to secure the tensile strength of 570 MPa or more, and at 0.14% or less, the toughness and the weldability are better, so that the amount of C is preferably 0.07% or more and 0.14% or less.

Si is a very important element for the present invention. In the manufacturing method which improves machinability while increasing strength, and presupposes water cooling after rolling, ferrite is produced | generated in the wide plate | board thickness range, and the remainder is mainly transformed to bainite and martensite, and soft ferrite It is an effective element to obtain a complex structure mainly composed of and hard bainite and martensite. In order to exert the effect, addition of 0.01% or more is required, and addition of more than 1% lowers the weldability, so the amount of addition is made 0.01% or more and 1% or less. Further, in order to exert the above effects more remarkably, addition of 0.2% or more is effective, while 0.55% or less is very good in weldability, so it is preferably 0.2% or more and 0.55% or less.

Mn is an effective element for increasing the strength, and in order to achieve a tensile strength of 570 MPa or more of the present invention, at least 0.01% or more is required. On the contrary, when it is added in excess of 2%, weldability is lowered. Therefore, it is prescribed | regulated to 0.01% or more and 2% or less. In addition, when Mn is added beyond 1.4%, since machinability falls, it is good to set it as 1.4% or less from a machinability viewpoint. Therefore, about the steel plate as described in (2) and (3) of this invention, it prescribed | regulated to 0.01% or more and 1.4% or less.

P is an impurity element and it is good that the addition amount is low. Since the addition exceeding 0.02% reduces the ductility, toughness, or weldability of a base material, it is prescribed | regulated to 0.02% or less.

S is an important element in the present invention and is an element that is frequently actively added to improve machinability.

By adding S, MnS is produced, and acting as a local stress concentration source additionally improves machinability. This effect is larger the larger the amount of S added, but the addition exceeding 0.035% extremely lowers the base material toughness, so the upper limit is defined as 0.035%.

Moreover, when the addition amount of S is made small, the effect of the machinability improvement by S becomes small, but base material toughness and weldability improve. Therefore, the addition amount of S is large in the case where importance is placed on machinability, and on the contrary, in the case where emphasis is placed on toughness of base metal and weldability, it is better to use a small amount of addition.

That is, in the steel plate as described in (2) of this invention, since there is no relationship with S addition in the machinability improvement mechanism, it is preferable that addition amount is low. Since the addition exceeding 0.01% reduces base material toughness by production | generation of MnS, it is prescribed | regulated to 0.01% or less. In addition, when the amount of S is 0.006% or less, since the toughness of the base material is further improved, the amount of S is preferably defined to be 0.006% or less.

In addition, in the steel sheet according to (3) of the present invention, the addition of S has a function of reducing the cutting resistance and increasing the tool life by the effect as the stress concentration source of MnS. When the addition amount exceeds 0.01%, the machinability is remarkably improved. On the other hand, when the addition amount exceeds 0.035%, both the toughness and the weldability deteriorate, so the amount of S is specified to be more than 0.01% and 0.035% or less.

Al is an effective element as a deoxidation material, and the addition amount shall be 0.001% or more. However, on the other hand, since the increase of Al amount causes the base metal toughness to fall, the upper limit is made into 0.1%.

N is an impurity element, and since the addition exceeding 0.01% reduces base material toughness and weldability, it is prescribed | regulated to 0.01% or less.

Mo is a very important element in the present invention, and can be added if necessary in view of cost. In the manufacturing method which improves machinability while increasing strength, and presupposes water-cooling after rolling, ferrite is produced in a wide plate thickness range, and the remainder is mainly transformed into bainite and martensite, and soft ferrite and hard It is an effective element in order to obtain the complex structure of bainite and martensite of, and in order to exert its effect, 0.01% or more is required, and when it is added in excess of 1%, the weldability is lowered. It should be less than%. In addition, since 0.1% or more of addition is effective in order to exhibit the said effect more remarkably, Preferably it is 0.1% or more and 1% or less.

Cr is a very important element in the present invention and can be added if necessary in view of cost. In the manufacturing method which improves machinability while increasing strength, and presupposes water cooling after rolling, after producing a ferrite in a wide plate | board thickness range, the remainder is mainly transformed to bainite and martensite, and soft ferrite and hard It is an effective element in order to obtain the composite structure of bainite and martensite, and in order to exert its effect, it is required to add 0.01% or more, and the addition of more than 1% decreases the weldability. Let it be as follows. In addition, since 0.1% or more of addition is effective in order to exhibit the said effect more remarkably, Preferably it is 0.1% or more and 1% or less.

In the present invention, Nb, Ti, and V are also important elements. Nb, Ti, and V are elements effective for increasing the strength due to precipitation strengthening and the like and for improving the toughness by miniaturization of the structure, and are added as necessary from the viewpoint of securing strength and toughness. The inventor evaluated the tool life at the time of drill drilling about what strengthened the steel plate which consists of soft and hard composite structures with these elements. As a result, for example, even in the case of a soft and hard composite structure, when the precipitation strengthening amount is large, it was found that the hardness difference between the soft part and the hard part decreases and the drill life decreases. When the addition amount of Nb, Ti, and V exceeds 0.1%, the machinability is remarkably decreased. On the other hand, the addition of Nb, Ti, and V is not effective in increasing the strength. It was made into% or less. In addition, when the addition amounts of Nb, Ti, and V are respectively 0.05%, 0.04%, and 0.05% or less, the machinability accompanying the increase in strength is particularly small, so that the addition amounts of Nb, Ti, and V are preferably 0.05%, It is made into 0.04% and 0.05% or less.

Cu, Ni, and B are added as needed from a viewpoint of securing strength. Cu is an element effective for securing strength. When the addition is less than 0.005%, the effect is small, and since the addition exceeds 1%, the weldability is lowered, so the range is made 0.005 to 1%. Ni is added as needed to secure strength. When the addition is less than 0.01%, the effect is small, and since the addition exceeds 2%, the weldability is lowered, so the range is made 0.01 to 2%. B is an element effective for increasing hardenability, and the added amount thereof is 0.0002% or more. However, on the other hand, since the increase in the amount of B causes the lowering of the base metal toughness due to the formation of coarse precipitates, the upper limit thereof is made 0.005%.

Base metal toughness and weld heat affected zone toughness by controlling the base material inclusions, miniaturization of the heating austenite of the weld heat affected zone, or transformation nuclei from the particles by adding one or two or more of REM, Ca, Zr, and Mg. Since it can raise, it is added as needed. In order to achieve this effect, it is necessary to add 0.0005% or more of REM, Ca, Zr, and Mg. On the other hand, since excessive addition of sulfides and oxides leads to coarsening and lowering of base metal toughness and ductility, or the upper limit is made 0.1% for REM and 0.02% for Ca, Zr and Mg.

In addition, in the steel of the present invention, O, Zn, Sn, Sb, Te, Ta, W, which can be mixed as an unavoidable impurity eluted from a furnace material or the like in the used raw material or solvent including an additive alloy; When Pb, Bi, etc. are mixed at 0.005% or less, the effect of this invention is not impaired at all.

Next, the method of manufacturing the steel plate as described in (1) of this invention is demonstrated. There are three manufacturing methods largely divided, and the 1st manufacturing method (manufacturing method 1) performs rolling at comparatively low temperature, and the method of quick water cooling after that, and the 2nd manufacturing method (manufacturing method 2) after rolling Air cooling is carried out until ferrite is produced, and water cooling is continued, and the third manufacturing method (Manufacturing Method 3) is a method of performing water cooling after heating again after the temperature of the steel sheet decreases after rolling. In order to produce ferrite in the wide range of thickness direction, and to ensure a high ferrite fraction in the vicinity of the steel plate surface, it is necessary to perform strict temperature control according to plate thickness.

In addition, when manufacturing the steel plate as described in (1), the steel composition as described in (1) of this invention, ie, C: 0.005 to 0.2%, Si: 0.01 to 1%, Mn: 0.01 to 2%, P: 0.02 % Or less, S: 0.035% or less, Al: 0.001 to 0.1%, N: 0.01% or less, the balance has a composition of steel composed of Fe and unavoidable impurities, and X1 = C + (Mn + Cu + Cr) / 20 A slab or cast steel having X1 of 0.24 or less, represented by + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B, is used.

First, the manufacturing method of the 1st manufacturing method (manufacturing method 1), ie, rolling at comparatively low temperature, performing water cooling quickly after that, and (5) of this invention are demonstrated. In this manufacturing method, strict regulations in rough rolling, finish rolling and water cooling become important.

Rough rolling is important because the austenite is refined by recrystallization to produce ferrite stably and consequently to improve machinability. If the total rolling reduction of the rough rolling is less than 30%, ferrite is not produced stably, whereas if the total rolling reduction is more than 95%, the productivity greatly decreases, so the total rolling reduction of the rough rolling is defined to be 30% or more and 95% or less. In addition, since the machinability is further improved when the total reduction ratio of the rough rolling is 50% or more, the total reduction rate of the rough rolling is preferably 50% or more and 95% or less. In addition, since machinability improves more when the total rolling reduction of rough rolling is 80% or more, more preferably, the total rolling reduction of rough rolling is made into 80% or more and 95% or less. The rough rolling temperature can be arbitrarily set as long as the conditions of the finish rolling temperature are satisfied. In addition, the total rolling reduction rate of rough rolling is taken as a percentage display of the value which divided | stacked the value which subtracted the board thickness after rough rolling from the board thickness before rough rolling, divided by the plate thickness before rough rolling.

+1070, X2= (Si/5+Mo+Cr/2)/Mn로 나타내는 T1(℃) 이하의 온도로 하였을 경우에, 폭 넓은 판 두께 방향으로 페라이트가 생성되고, 피삭성, 용접성, 인성의 어느 것에도 우수한 것을 확인하였다. 따라서, 마무리 압연의 제1 패스 물림 온도를, T1=351n (X2/2)-25

+1070, X2=(Si/5+Mo+Cr/2)/Mn로 나타내는 T1(℃) 이하로 규정한다. 또한, Si, Mo, Cr, Mn는 질량%로 나타나는 첨가량이며, t는 강판의 판 두께( mm)이다. 마무리 압연의 제1 패스 물림 온도를 720℃ 미만으로 하면, 페라이트의 가공에 의하여 모재 인성과 피삭성이 큰 폭으로 저하되므로, 압연의 제1 패스 물림 온도의 하한을 720℃로 한다. 또한, 마무리 압연의 제1 패스 물림 온도를 T1보다 40℃ 낮게 하면, 한층 현저하게 피삭성이 향상되기 때문에, 좋기로는 마무리 압연 의 제1 패스 물림 온도를 T1보다 40℃ 낮은 온도 이하로 한다. 또한 마무리 압연의 제1 패스 물림 온도를 T1보다 80℃ 낮게 하면, 더 현저하게 피삭성이 향상되기 때문에, 더 좋기로는, 마무리 압연의 제1 패스 물림 온도를 T1보다 80℃ 낮은 온도 이하로 한다. 마무리 압연의 최종 패스 물림 온도는 700℃ 미만에서는 페라이트의 가공에 의하여 모재 인성과 피삭성이 큰 폭으로 저하되므로, 또한 T1(℃) 초과에서는 페라이트가 판 두께 방향으로 폭 넓게 생성되지 않으므로, 마무리 압연의 최종 패스 물림 온도의 하한은 700℃, 상한은 T1(℃)인 것이 좋다. 마무리 압연에 대하여서는 전체 압하율도 중요하고, 이것이 30% 미만이면, 판 두께의 폭 넓은 범위에서 페라이트가 생성되지 않고, 반대로 95%를 넘으면 생산성이 대폭으로 저하되므로, 마무리 압연의 전체 압하율을 30% 이상 95% 이하로 규정한다. 또한, 마무리 압연의 전체 압하율이60% 이상인 경우에는 한층 페라이트가 안정적으로 생성되어 피삭성이 향상되므로, 좋기로는 마무리 압연의 전체 압하율을 60% 이상 95% 이하로 한다. Finish rolling is mainly important for promoting or minimizing ferrite generation in various forms by using dislocations introduced at temperatures not recrystallized, and as a result, improving machinability, toughness and weldability. The inventor manufactured the steel plate of various alloy components and plate | board thickness by this manufacturing method, and evaluated the machinability, weldability, and base material toughness. As a result, the first pass bite temperature of finish rolling is set to T1 = 351n (X2 / 2) -25

When the temperature is equal to or less than T1 (° C) represented by +1070, X2 = (Si / 5 + Mo + Cr / 2) / Mn, ferrite is formed in a wide sheet thickness direction, and the machinability, weldability and toughness are increased. It confirmed that it was excellent in all. Therefore, the first pass bite temperature of finish rolling is T1 = 351n (X2 / 2) -25

It is prescribed | regulated below T1 (degreeC) shown by +1070 and X2 = (Si / 5 + Mo + Cr / 2) / Mn. In addition, Si, Mo, Cr, and Mn are addition amounts represented by the mass%, t is the plate | board thickness (mm) of a steel plate. If the first pass bite temperature of finish rolling is lower than 720 ° C, the base metal toughness and machinability are greatly reduced by the processing of ferrite, so the lower limit of the first pass bite temperature of rolling is set to 720 ° C. Further, when the first pass bite temperature of the finish rolling is 40 ° C lower than T1, the machinability is significantly improved. Therefore, the first pass bite temperature of the finish rolling is preferably 40 ° C or lower lower than T1. Moreover, since machinability improves more remarkably when the 1st pass bite temperature of finish rolling is made 80 degreeC lower than T1, More preferably, the 1st pass bite temperature of finish rolling shall be 80 degrees C or less lower than T1. . The final pass bite temperature of the finish rolling is significantly lowered by the machining of ferrite at a temperature below 700 ° C., and the ferrite is not widely produced in the sheet thickness direction at a temperature above T1 (° C.). It is preferable that the lower limit of the final pass stitch temperature is 700 ° C, and the upper limit is T1 (° C). The overall rolling reduction is also important for finish rolling. If it is less than 30%, ferrite is not produced in a wide range of sheet thickness, whereas if it exceeds 95%, the productivity greatly decreases. It is prescribed in more than% and less than 95%. Moreover, when the total rolling reduction of finish rolling is 60% or more, since ferrite is produced more stably and machinability improves, the total rolling reduction of finishing rolling is preferably 60% or more and 95% or less.

Moreover, in this invention, the rolling performed with a rough rolling mill is made into rough rolling, and the rolling performed with a finish rolling mill is used as finish rolling. If rough rolling and finish rolling are performed with the same rolling mill, if there is a clear set temperature for dividing the first half and the second half of the rolling, the first half is rough rolling and the second half is finish rolling. When there is no presence or when two or more set temperatures exist, all of the subsequent rolling passes including the rolling pass in which the steel sheet surface temperature before the rolling pass starts to become 950 ° C. or less are regarded as finish rolling. The 1st pass bite temperature of finish rolling refers to the temperature measured on the steel plate surface before the initial rolling reduction of finish rolling. The final pass bite temperature of finish rolling refers to the temperature measured on the steel sheet surface before the last rolling reduction of finish rolling. In addition, a steel plate surface temperature can be measured by using a radiation thermometer, for example.

Next, water cooling is demonstrated. Water cooling is effective to increase the weldability through securing a tensile strength of about 570 to 720 MPa, and to secure the strength with a low alloy, and to increase the base material toughness by further microstructure. The water density at the time of water cooling is 0.2 m ³ / m ² · mm. If it falls below, the strength decreases, while if it exceeds 5.Om ³ / m ² · mm., Ferrite is not produced stably in a wide range in the sheet thickness direction, and machinability decreases. ^{0.2 m 3 / m 2 · mm} . 5.O m ³ / m ² · mm or more. It is prescribed as follows. When the end temperature of water cooling exceeds 60 degreeC, since the remainder austenite after ferrite production does not transform into low temperature and intensity | strength falls, the end temperature of water cooling shall be 600 degrees C or less. At this time, the end temperature of water cooling refers to the maximum value of the steel plate surface temperature measured after waiting for reheating after water cooling. It is air cooled after water cooling.

In the water cooling carried out after the end of rolling, ferrite can be produced more stably by changing the cooling rate of the first half and the second half, so that this method can be taken as necessary. The cooling rate of the first half prescribed | regulated by water cooling start temperature or less more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and the second half cooling rate prescribed | regulated by water cooling end temperature or more is 10 degreeC / s or more and 100 degreeC / s. By making it below, the steel plate which is more excellent in machinability and whose strength is equal or more can be manufactured. The lowering of the cooling rate in the first half of the cooling is to increase the amount of ferrite production and to lower the transformation temperature of bainite or the like formed by the latter cooling through the concentration of C in the unmodified austenite, and to increase the cooling rate in the second half. The purpose is to make the transformation temperature of untransformed austenite extremely low. In addition, the temperature and cooling rate in the cooling of this two stage are made into the average cooling rate computed based on the temperature measured in t / 4 part of steel plate, and the value, and the preliminary sample which put the thermocouple in the steel plate. The measurement can be carried out by using a liquid crystal and simulating the actual liquid cooling.

Below, other preferable conditions are described in a 1st manufacturing method (manufacturing method 1). Prior to rough rolling and finish rolling, the steel or cast steel is heated. When the heating temperature is less than 900 ° C., a part of the tissue before heating remains unaffected, the material becomes uneven, while when the heating temperature exceeds 1350 ° C., the austenite coarsens and the final structure coarsens. Not only the toughness is greatly reduced, but also the production of ferrite is suppressed and the machinability is lowered, so that the heating temperature is preferably at least 90 ° C and at most 1350 ° C. Water cooling starts extremely quickly after the end of finish rolling. For example, it is good to start within 200 s from the end of finish rolling. This is because if the time until the start of water cooling exceeds 200 s, the potential introduced by rolling decreases due to recovery, and ferrite is not produced stably in a wide range in the plate thickness direction and the machinability is lowered. At this time, the end of finish rolling refers to the time point at which the foremost part of the steel sheet was pressed down in the final pass of the finish rolling, and the start of water cooling refers to the time point at which the foremost part of the steel plate reached the water cooling facility and water cooling was started. In addition, it is possible to give heat processing to the steel plate cooled by water after air cooling as needed. For example, tempering can be performed from a viewpoint of improving base material toughness.

Next, the manufacturing method of 2nd manufacturing method (manufacturing method 2), ie, air-cooling until it produces | generates after rolling, continuing water cooling, and the manufacturing method as described in (7) of this invention are prescribed | regulated. About heating, it is the same as that of the said 1st manufacturing method. Although the temperature of the rough rolling can be arbitrarily set, the toughness is greatly reduced when the total rolling reduction of the rough rolling is less than 30%, and the productivity greatly decreases when it exceeds 95%. Therefore, the overall rolling reduction of the rough rolling is more than 30%. It is regulated to 95% or less. The temperature of finish rolling does not have the same definition as that of the first manufacturing method, and can be carried out under arbitrary conditions. If the total rolling reduction of the finish rolling falls below 30%, the toughness greatly decreases, and if it exceeds 95%, the productivity decreases greatly. Therefore, the total reduction of the finish rolling is defined to be 30% or more and 95% or less. After heating, rough rolling, and finish rolling are completed, air cooling is performed. After ferrite is formed during air cooling, water cooling is performed. The inventors varied and examined the steel plate surface temperature at the time of transition from air cooling after finishing rolling to water cooling for various components, and the steel plate surface temperature at the time of transition to water cooling was T2 = 910-310 × C. -80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.0006t ² At or below T2 (° C.) represented by -0.56t-8.6, ferrite is produced in a wide range of plate thickness directions. It was found that the machinability was improved and the strength greatly decreased when the steel plate surface temperature was below 650 ° C. Therefore, the steel plate surface temperature at the time of transition to water cooling is represented by T2 = 910-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.0006t ² -0.56t-8.6 It is prescribed | regulated to T2 (degreeC) or less and 650 degreeC or more. At this time, the steel plate surface temperature at the time of transition to water cooling refers to the steel plate surface temperature measured before water cooling. C, Mn, Cu, Cr, Ni, and Mo are the addition amounts (mass%) of each element, and t is plate | board thickness (mm). When the water density during water cooling falls below 0.2 m ³ / m ² · mm, the strength decreases. On the other hand, when the water density exceeds 5.O m ³ / m ² · mm, ferrite does not stably form in a wide range in the plate thickness direction. Since machinability is reduced, the water density at the time of water cooling is 0.2 m ³ / m ² · mm ³ / m ² · mm. It is prescribed as follows. When the end temperature of water cooling exceeds 500 degreeC, since the remainder austenite after ferrite production does not transform at low temperature and intensity | strength falls, the end temperature of water cooling shall be 500 degrees C or less. At this time, the end temperature of water cooling refers to the maximum value of the steel plate surface temperature measured after waiting for reheating after water cooling. It is air cooled after water cooling. In addition, it is possible to heat-treat the steel plate cooled by water after air cooling as needed. For example, tempering can be performed from a viewpoint of improving base material toughness.

Next, the 3rd manufacturing method (manufacturing method 3), ie, the method of heating again after the temperature of a steel plate after rolling falls, is prescribed | regulated. About heating before rolling, it is the same as that of the said 1st manufacturing method. Although the temperature of rough rolling can be arbitrarily set, toughness falls significantly when the total rolling reduction rate of rough rolling falls below 30%, and when it exceeds 95%, productivity falls significantly, and therefore the overall rolling reduction rate of rough rolling is 30 It is prescribed in more than% and less than 95%. The temperature of finish rolling does not have the same definition as that of the first manufacturing method, and can be carried out under arbitrary conditions. If the total rolling reduction of the finish rolling falls below 30%, the toughness decreases significantly, and if it exceeds 95%, the productivity decreases greatly. Therefore, the total rolling reduction of the finish rolling is made 30% or more and 95% or less. Heating, rough rolling, and finish rolling are complete | finished, and after cooling a steel plate to arbitrary methods to 500 degrees C or less, it heats again. The inventor conducted irradiation with various changes in the reheating temperature, and when the reheating temperature was lower than T3 (° C) represented by T3 = 0.0017t ² +0.17 t + 730 or exceeded 850 ° C, sufficient strength could not be obtained. The reheating temperature is defined to be T3 (° C) or higher and 850 ° C or lower, represented by T3 = 0.0017t ² + 0.17t + 730. After reheating, perform water cooling. The water density at the time of water cooling decreases the strength below 0.2 m ³ / m ² · mm. On the other hand, when the water density exceeds 5.O m ³ / m ² · mm., Ferrite is stably formed in a wide range of plate thickness directions. As the machinability is lowered, the water density at the time of water cooling is 0.2 m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. It is good to set it as follows. When the end temperature of water cooling exceeds 50 degreeC, since the remainder austenite after ferrite production does not change to low temperature and intensity | strength falls, the end temperature of water cooling is prescribed | regulated to 500 degrees C or less. At this time, the end temperature of water cooling refers to the maximum value of the steel plate surface temperature measured after waiting for reheating after water cooling. It is air cooled after water cooling.

Next, the method of manufacturing the steel plate as described in (2) and (3) of this invention is demonstrated. There are four manufacturing methods largely divided, and each steel plate is described as manufacturing methods 4-7, 4'-7 '. In addition, in the manufacturing method of the steel plate as described in (2) and (3) of this invention, since the conditions other than the steel composition of a steel piece or a cast steel are common in four manufacturing methods, about the following description, (2) And the steel plate of (3) is demonstrated together. In the first manufacturing method (manufacturing methods 4, 4 '), the method of rapidly cooling water after rolling, and the second manufacturing method (manufacturing methods 5, 5') are heated again after the temperature of the steel sheet decreases after rolling. After that, the method of performing water cooling and the third manufacturing method (manufacturing methods 6 and 6 ') perform air cooling until ferrite is produced after rolling, and then the method of performing water cooling and fourth manufacturing method (manufacturing Method 7, 7 ') is a method of heating up to two phases again after the temperature of a steel plate falls after rolling, and continuing water cooling.

Moreover, when manufacturing the steel plate as described in (2) of this invention, in the steel composition as described in (2) of this invention, ie, the steel composition as described in (1), Mn is 0.1%-1.4%, S: 0.01% X1 having a steel composition of less than or equal to X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / A steel or cast steel having X2 represented by 5 + Mo + Cr / 2) / Mn of 0.15 or more and 10.0 or less is used,

Moreover, when manufacturing the steel plate as described in (3) of this invention, in the composition of the steel as described in (3) of this invention, ie, the steel composition as described in (1) of this invention, Mn is 0.1%-1.4%, S: X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B having a composition of steel that is greater than 0.01% and 0.035% or less and is 0.24 or less A slab or cast steel having X2 of 0.15 or more and 10.0 or less, represented by X2 = (Si / 5 + Mo + Cr / 2) / Mn, is used.

Among the methods for producing the steel sheet described in (2) and (3) for the first time, a first manufacturing method (manufacturing methods 4 and 4 '), that is, a method of starting water cooling immediately after rolling, (9) and (14) of the present invention. The manufacturing method described in) will be described. Rough rolling, finishing rolling, and water cooling become important in this manufacturing method.

First, rough rolling will be described. Rough rolling is important in terms of promoting ferrite production through refining of austenite. If the total rolling reduction of rough rolling is less than 30%, ferrite is not produced stably. On the other hand, if the rolling reduction exceeds 95%, productivity decreases significantly. Therefore, the total rolling reduction of rough rolling is defined to be 30% or more and 95% or less. . Further, if the total reduction rate of the rough rolling is 50% or more, the ferrite is more stably generated and the machinability is further improved. Therefore, the reduction rate of the rough rolling is preferably 50% or more and 95% or less. Further, if the total reduction rate of rough rolling is 80% or more, the ferrite is more stably produced and the machinability is further improved. More preferably, the rolling reduction rate of rough rolling is 80% or more and 95% or less. The bite temperature of rough rolling and the steel plate surface temperature before final bus can be set arbitrarily as long as the conditions of finish rolling temperature are satisfy | filled. In addition, the total rolling reduction rate of rough rolling is taken as a percentage display of the value which divided | stacked the value which subtracted the board thickness after rough rolling from the board thickness before rough rolling, divided by the plate thickness before rough rolling.

+1100으로 나타내는 T4(℃) 이하인 경우에 페라이트가 안정적으로 생성되는 것을 밝혀내었다. 따라서, 마무리 압연의 제1 패스 물림 온도를 T4(℃) 이하라고 규정한다. 이 때, X2는 이미 나타낸 바와 같이 X2= (Si/5+Mo+Cr/2)/Mn으로 계산되는 값이며, t는 판 두께(mm)이다. T4의 식에 판 두께의 항이 포함되는 것은 최종 판 두께가 큰 만큼 압연에 있어서의 압하율이 일반적으로 저하되기 때문에, 재결정 입자 지름의 조대화나 잔존 전위 밀도의 저하에 의하여 페라이트의 생성이 억제되고, 이것에 의하여 저온 압연이 필요하기 때문이다. 또한, 마무리 압연의 제1 패스 물림 온도를 T4보다 40℃ 낮게 하면, 한층 현저하게 피삭성이 향상되기 때문에, 좋기로는 마무리 압연의 제1 패스 물림 온도를 T4보다 40℃ 낮은 온도 이하로 한다. 또는 마무리 압연의 제1 패스 물림 온도를 T4보다 80℃ 낮게 하면, 더욱 현저하게 피삭성이 향상되기 때문에, 더 좋기로는, 마무리 압 연의 제1 패스 물림 온도를 T4보다 80℃ 낮은 온도 이하로 한다. 또한, 마무리 압연의 제1 패스 물림 온도가 Ar₃점을 밑돌면, 페라이트의 가공에 수반하는 경도 증대에 의하여 피삭성이 저하되기 때문에, 마무리 압연의 제1 패스 물림 온도의 하한을 Ar₃점으로 규정한다. 마무리 압연의 최종 패스 물림 온도는 페라이트의 가공에 수반하는 피삭성 저하를 극도로 억제하는 관점에서, 그 하한을 Ar₃점보다 100℃ 낮은 온도 이상으로 하는 것이 좋고, 상한은 T4+50(℃)으로 하는 것이 좋다. Finish rolling is important for producing ferrite stably in the manufacturing method which performs water cooling. Rolling at a low temperature results in a higher introduction dislocation density per unit reduction rate, and further restrains the recovery of the potential in the rolling phase between the rolling passes and from the rolling mill to the water-cooling equipment, thereby facilitating the formation of ferrite. Since the formation behavior of ferrite is strongly influenced by the alloy component, the finish rolling temperature needs to be defined in relation to the component. In general, in a thick steel sheet having a tensile strength of 570 MPa or more, it is difficult to produce ferrite by a method of rapidly cooling water after rolling, and in order to achieve this, a very low finish rolling temperature is required and productivity is lowered. However, in the range of the component ratio regulation of Si, Mn, Mo, and Cr prescribed in order to improve the machinability in the present invention, it has been newly found that it can be manufactured without lowering the productivity. The inventor examines the 1st pass bite temperature of the finish finishing rolling optimal with respect to the steel of various components, and the 1st pass bite temperature of the finish rolling is T4 = 351n (X2 / 2) -25

It was found that ferrite was produced stably when below T4 (° C.) indicated by +1100. Therefore, the 1st pass bite temperature of finish rolling is prescribed | regulated as T4 (degreeC) or less. At this time, X2 is a value calculated as X2 = (Si / 5 + Mo + Cr / 2) / Mn as already shown, and t is the plate thickness (mm). The fact that the term T4 is included in the formula of T4 means that the reduction ratio in rolling generally decreases because the final sheet thickness is large, so that the production of ferrite is suppressed due to coarsening of the recrystallized grain diameter and lowering of the residual dislocation density. This is because low temperature rolling is necessary. Moreover, since machinability improves remarkably when the 1st pass bite temperature of finish rolling is 40 degreeC lower than T4, Preferably, the 1st pass bite temperature of finish rolling is 40 degrees C or less lower than T4. Alternatively, if the first pass bite temperature of finish rolling is lowered by 80 ° C. than T4, the machinability is more remarkably improved. More preferably, the first pass bite temperature of finish rolling is lower than or equal to 80 ° C. lower than T4. . When the first pass bite temperature of finish rolling falls below Ar ₃ , the machinability decreases due to the increase in hardness associated with the processing of ferrite, so the lower limit of the first pass bite temperature of finish rolling is defined as Ar ₃ point. do. The final pass bite temperature of the finish rolling is preferably at least 100 ° C lower than the Ar ₃ point from the viewpoint of extremely suppressing the machinability deterioration associated with the processing of ferrite, and the upper limit is T4 + 50 (° C). It is good to do.

Moreover, in this invention, the rolling performed with a rough rolling mill is made into rough rolling, and the rolling performed with a finish rolling mill is used as finish rolling. If the rough rolling and finish rolling are carried out by the same rolling mill, when there is a clear set temperature dividing the first half and the second half of the rolling, the first half is rough rolling and the second half is finish rolling. In the case where there is no presence or when two or more set temperatures exist, all of the subsequent rolling passes including the rolling buses at which the steel sheet surface temperature before the start of the rolling pass becomes 950 ° C. or less are regarded as finish rolling. The 1st pass bite temperature of finish rolling refers to the temperature measured on the steel plate surface before the initial rolling reduction of finish rolling. The final pass bite temperature of finish rolling refers to the temperature measured on the steel plate surface before the last rolling reduction of finish rolling. Although Ar ₃ point cannot be measured directly, it can be estimated by performing the processing heat processing which simulated actual manufacturing conditions, measuring an expansion curve. In addition, a steel plate surface temperature can be measured by using a radiation thermometer, for example.

The overall rolling reduction of finish rolling is also important in view of the stable production of ferrite. If the total rolling reduction of the finish rolling is 30% or more, the machinability is improved for stable generation of ferrite. On the other hand, when the total rolling reduction of finish rolling exceeds 95%, productivity will fall largely. Therefore, the total rolling reduction of finish rolling is prescribed | regulated as 30% or more and 95% or less. Moreover, since machinability improves further by making the total rolling reduction rate of finish rolling into 60% or more, Preferably, the total rolling reduction rate of finish rolling is made into 60% or more and 95% or less. In addition, the total rolling reduction rate of finish rolling is taken as the percentage display of the value which divided | segmented the value which subtracted the final board thickness from the plate thickness before finishing rolling, by the plate thickness before finishing rolling.

Next, the conditions of water cooling are demonstrated. Water cooling improves machinability through stable formation of ferrite and formation of bainite and martensite main structure by low-temperature transformation of the remainder, improvement of base material toughness by miniaturization of particle diameter, and improvement of weldability through securing strength in low alloy It is important to achieve this at the same time. If the water density at the time of water cooling is less than 0.2 m ³ / m ² · mm., The strength decreases. On the other hand, if the water density exceeds 5.O m ³ / m ² · mm. , The water density at the time of water cooling 0.2 m ³ / m ² · mm. 5.Om ³ / m ² · mm or less When the end temperature of water cooling exceeds 600 degreeC, since the remainder austenite after ferrite production does not transform into low temperature and intensity | strength falls, the end temperature of water cooling shall be 600 degrees C or less. At this time, the end temperature of water cooling refers to the maximum value of the steel plate surface temperature measured after waiting for reheating after water cooling. It is air cooled after water cooling.

In addition, it is good to start water cooling promptly after completion | finish rolling of finishing. This is because, when the time from the completion of finish rolling to the start of water cooling becomes long, the potential introduced by rolling decreases due to recovery, and ferrite is not produced stably, and machinability deteriorates. In addition, specifically, it is good to start water cooling within 200s after completion | finish of finish rolling. At this time, the end of finish rolling refers to the time point at which the foremost part of the steel sheet was pressed down in the final pass of the finish rolling, and the start of water cooling refers to the time point at which the foremost part of the steel plate reached the water cooling facility and water cooling was started.

In water cooling, since the ferrite can be produced more stably by changing the cooling rate of the first half and the latter half, this method can be taken as needed. The cooling rate of the first half prescribed | regulated by water cooling start temperature or more and more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and the second half cooling rate prescribed | regulated by water cooling end temperature or more is 10 degrees C / s or more and 100 degrees C /. By setting it as s or less, it is further excellent in machinability and can manufacture the steel plate more than equal in strength. Lowering the cooling rate in the first half of the cooling increases the amount of ferrite production and lowers the transformation temperature of bainite or the like, which is formed by the latter cooling through the concentration of C in the unmodified austenite. The purpose is to make the transformation temperature of untransformed austenite extremely low. In addition, the temperature and cooling rate in the cooling of this two stage are made into the average cooling rate calculated based on the temperature measured in t / 4 part of steel plate, and the value, using the preliminary sample which put the thermocouple in the steel plate, Measurement can be performed by performing water cooling simulating actual water cooling.

Below, other preferable conditions are described in a manufacturing method. Prior to rolling, the slabs or slabs are heated. If the heating temperature is less than 900 ° C, a part of the tissue before heating remains unaffected, and the material becomes uneven. On the other hand, if the heating temperature exceeds 1350 ° C, austenite is coarsened and the final structure is also coarsened. Not only the toughness is greatly reduced, but also the production of ferrite is suppressed and the machinability is lowered, so that the heating temperature is 900 ° C or more and 1350 ° C or less. Moreover, it is possible to give heat processing to the steel plate cooled by water after air cooling as needed. For example, tempering can be performed from a viewpoint of improving base material toughness.

Next, in the method of manufacturing the steel plate as described in (2) and (3) of this invention, 2nd manufacturing method (manufacturing methods 5 and 5 '), ie, reheating is carried out after the temperature of the steel plate after rolling falls, After that, the method of performing water cooling and the manufacturing method as described in (11) and (16) of this invention are prescribed | regulated. About heating, rough rolling, and finish rolling, it can implement on arbitrary conditions. Heating, rough rolling, and finish rolling are complete | finished, and after cooling a steel plate by arbitrary methods to 500 degrees C or less, it heats again at 900 degreeC or more and 1050 degrees C or less again. After heating, water cooling is performed at a cooling rate of 1 ° C / s or more and 100 ° C / s or less. The end temperature of water cooling is 500 degrees C or less, and air-cooled after water cooling. By reheating after rolling, fine austenite can be obtained, and ferrite can be stably produced. When the reheating temperature is lower than 900 ° C, a high concentration of austenite is produced, and this becomes martensite after transformation, so that the base metal toughness is greatly reduced, and when the reheating temperature exceeds 1050 ° C, ferrite is stably formed. Since machinability falls, the reheating temperature is prescribed | regulated to 900 degreeC or more and 1050 degrees C or less. The cooling rate after reheating is lower than 1 ° C / s, so that the remainder of austenite after ferrite generation does not undergo low temperature transformation, and the strength is lowered. It is prescribed | regulated to 1 degrees C / s or more and 100 degrees C / s or less. When the end temperature of water cooling exceeds 500 degreeC, since the remainder austenite after ferrite transformation does not transform at low temperature and intensity falls, the stop temperature of water cooling is prescribed | regulated to 500 degrees C or less. Moreover, it is possible to give heat processing to the steel plate cooled by water after air cooling as needed. For example, tempering can be performed from a viewpoint of improving base material toughness.

At this time, the end temperature of water cooling is made into the maximum value of the temperature in the steel plate surface measured rapidly after waiting for reheating after water cooling. The cooling rate at the time of water cooling is made into the average cooling rate calculated based on the temperature measured in t / 4 part of steel plates, and the estimate is made by performing the water cooling which simulates actual water cooling using the preliminary sample which put the thermocouple in the steel plate. It is possible to estimate.

Next, in the method of manufacturing the steel plate as described in (2) and (3) of this invention, air cooling is performed until the 3rd manufacturing method (manufacturing methods 6 and 6 '), ie, production of ferrite after rolling starts, Then, the method of performing water cooling and the manufacturing method as described in (12) and (17) of this invention are prescribed | regulated. About heating, it is made the same as the said 1st manufacturing method (manufacturing method 4). In rough rolling, when the total rolling reduction is less than 30%, the toughness is lowered, and when it exceeds 95%, the productivity is greatly reduced. Therefore, the overall rolling reduction of rough rolling is defined to be 30% or more and 95% or less. The finish rolling is not the same as the first method described above with respect to temperature, and can be carried out under any conditions. If the total rolling reduction of the finish rolling falls below 30%, the toughness decreases, and if it exceeds 95%, the productivity greatly decreases. Therefore, the overall rolling reduction of the finish rolling is defined to be 30% or more and 95% or less. After heating, rough rolling, and finish rolling are completed, air cooling is carried out, and after the production of ferrite is started during air cooling, water cooling is performed. When the temperature for starting the water cooling exceeds the Ar ₃ point, and the ferrite is the machinability decreases not stably generated. On the other hand, because the 150 ℃ temperature lower than the Ar ₃ point falls below the strength is lowered, the starting temperature of the water-cooled Ar ₃ It is defined as a point or less, more than 150 ℃ temperature lower than the Ar ₃ point. At this time, the start temperature of water cooling refers to the steel plate surface temperature measured before water cooling. The Ar ₃ point can be estimated by performing a processing heat treatment simulating actual manufacture while measuring the expansion curve. When the water density at the time of water cooling is less than 0.2 m ³ / m ² · mm., The strength decreases, while when it exceeds 5.O m ³ / m ² · mm., The productivity decreases, so the water density at the time of water cooling is 0.2 m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. It is prescribed as follows. When the end temperature of water cooling exceeds 50 degreeC, since the remainder austenite after ferrite production does not transform into low temperature and intensity | strength falls, the end temperature of water cooling shall be 500 degrees C or less. At this time, the end temperature of water cooling refers to the maximum value of the steel plate surface temperature measured promptly after waiting for reheating after water cooling. It is air cooled after water cooling. In addition, it is possible to give heat processing to the steel plate cooled by water after air cooling as needed. For example, tempering can be performed from a viewpoint of improving base material toughness.

Next, in the method of manufacturing the steel plate as described in (2) and (3) of this invention, the 4th manufacturing method (manufacturing method 7, 7 '), ie, after the temperature of a steel plate after rolling falls, it will go back to two phases again. The manufacturing method as described in (13) and (18) of this invention and the method of heating are prescribed | regulated. Heating, rough rolling, and finish rolling are the same as in the third production method (manufacturing methods 6 and 6 '). Heating, rough rolling, and finish rolling are complete | finished, and after cooling a steel plate to arbitrary methods to 500 degrees C or less, it heats again. The reheating temperature is lower than 730 ° C, the machinability is lowered, and the strength is lowered at 900 ° C or higher, so the reheating temperature is defined to be 730 ° C or higher and lower than 900 ° C. After reheating, it is possible to cool the water by any method. When the end temperature of water cooling exceeds 500 degreeC, since the remainder austenite after ferrite production does not transform into low temperature and intensity | strength falls, the end temperature of water cooling shall be 500 degrees C or less. It is air cooled after water cooling. It is possible to apply heat processing to the cooled steel plate as needed. For example, tempering can be performed from a viewpoint of improving base material toughness.

Example 1

The Example of the steel plate as described in (1) of this invention is demonstrated.

The test steels of various chemical components were used to evaluate the properties of steel sheets having a thickness of 6, 18, 40, and 100 mm manufactured under various manufacturing conditions. Among the evaluation items, the tensile stress, the tensile strength, the Charpy impact absorption energy as the toughness, the Charpy impact absorption energy as the weld heat affected zone toughness among the weldability, and the drill hole processing characteristics as the machinability were shown. The ferrite fractions measured at the chemical composition of the steel sheet, the plate thickness, X1, and various sites are shown in Tables 1 to 6, the manufacturing conditions (manufacturing methods 1 to 3) are shown in Tables 7 to 9, and the evaluation results of the characteristics are shown in Table 10 To Table 12.

Yield stress and tensile strength were measured by the metallic material tensile test method described in JIS Z 2241. The test piece is a metallic material tensile test piece described in JIS Z 2201, and a No. 5 test piece is used from a steel plate with a sheet thickness of 6 mm and 18 mm and a No. 10 test piece taken from t / 4 part with a steel plate with a plate thickness of 40 mm and 100 mm. It was. The test piece was sampled so that a longitudinal direction might become perpendicular | vertical to a rolling direction. The yield stress was 0.2% yield strength calculated by the lower yield stress or the offset method. Two tests were performed at normal temperature, and the average value was employ | adopted.

Base metal toughness was measured by the metal material impact test method described in JIS Z 2242. The test piece is a metal material impact test piece described in JIS Z 2202, and the steel sheet having a sheet thickness of 6 mm has a subsize test piece having a width of 5 mm from the sheet thickness center, and the steel sheet having a sheet thickness of 18 mm has a width of 10 mm from the sheet thickness center. As for the test piece, the steel plate of 40 mm of thickness and 100 mm took the test piece of width 10mm from t / 4 part. The shapes were all taken as a V notch test piece, and it was extract | collected so that the line which a notch bottom may make parallel with a plate thickness direction, and the longitudinal direction of a test piece may be perpendicular to a rolling direction. The test temperature was -5 degreeC and the average value which performed three tests was employ | adopted.

HAZ toughness was measured for the absorbed energy in a Charpy test piece sampled, and -5 ℃ from the weld joint created by the CO ₂ gas shielded arc welding and submerged arc welding. Weld heat input is 2 to 3 kJ / mm for CO ₂ gas shield arc welding, 3 kJ / mm for plate thickness 6 mm for submerged arc welding, 5 kJ / mm for plate thickness 18 mm, and 40 mm for plate thickness. It was 7 kJ / mm by 100 mm material. The test piece was sampled so that 0.5 mm from the bond part of a weld corresponded to the notch position of a Charpy test piece. The average value of three shock absorption energy was employ | adopted.

As an evaluation of machinability, the boring test was done using the boring machine and a high speed drill. For the steel sheet with a sheet thickness of 6 mm, the perforation distance is 7 sheets stacked 42 mm, two sheets 36 mm thick for a steel plate 18 mm thick and 40 mm thick for a sheet 40 mm thick. In the case of steel sheet, the test was performed as 100 mm per sheet. The drill was perforated using a high-speed drill SKH51 with a diameter of 6 mm. The rotation speed was 1610 rpm, the transmission speed was 190 mm / min., And the cutting oil used the water-soluble cutting oil. Under the above conditions, the hole processing was performed until it became impossible to drill, and the number of hole processing to a limit was measured.

Inventive Examples 1-1 to 1-11 produce a steel sheet by a first manufacturing method (manufacturing method 1), that is, a method of rapidly cooling water after rolling (the manufacturing method according to (5) of the present invention), and Comparative Examples 1-1 to 1-11 are also shown.

Inventive Example 1-1 is a steel sheet having a tensile strength of about 570 MPa, and a steel sheet having a sheet thickness of 6 mm is manufactured by a method of controlling the balance, rolling and water cooling conditions of alloying elements in order to achieve high toughness, weldability and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-1 is a component and a manufacturing method similar to Inventive Example 1-1, but because the amount of C and X1 are outside the scope of the present invention, the base metal toughness and the weld heat affected zone toughness are inferior.

Inventive Example 1-2 is a steel sheet having a sheet thickness of 18 mm by a manufacturing method that controls the balance, rolling and water-cooling conditions of alloying components, etc. in order to achieve high toughness, weldability and machinability in a steel sheet having a tensile strength of about 570 MPa It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-2 is a component and a manufacturing method similar to Inventive Example 1-2, but since the Si is out of the range of the present invention, the base metal toughness and the weld heat affected zone toughness are poor.

Inventive Example 1-3 is a steel sheet having a tensile strength of about 570 MPa, and a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance, rolling and water cooling conditions of alloying elements in order to achieve high toughness, weldability and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the surface of the steel sheet, the tensile strength is about 57O MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-3 is a component and a manufacturing method similar to Inventive Example 1-3, but the machinability is inferior because the overall reduction ratio and the ferrite fraction of the rough rolling are outside the scope of the present invention.

Inventive Example 1-4 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 10 mm is manufactured by a manufacturing method that controls the balance of the alloying components, rolling and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, particularly near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-4 is similar in composition and production method to Inventive Example 1-4, but the machinability is inferior because the first pass bite temperature and the ferrite fraction of finish rolling are outside the scope of the present invention.

Inventive Example 1-5 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability, and machinability, a steel sheet having a sheet thickness of 6 mm is manufactured by a method of controlling the balance of alloy components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-5 is a component and a manufacturing method similar to Inventive Example 1-5, but since P amount is out of the range of the present invention, the base metal toughness and the weld heat affected zone toughness are inferior.

Inventive Example 1-6 is a steel sheet having a tensile strength of about 570 MPa, and a steel sheet having a thickness of 18 mm is manufactured by a manufacturing method that controls balance, rolling, and water cooling conditions of alloying components in order to achieve high toughness, weldability and machinability. It is. Since ferrite is produced stably in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base material toughness, the weld heat affected zone toughness, and the machinability are excellent. On the other hand, Comparative Example 1-6 is a component and a manufacturing method similar to Inventive Example 1-6, but since the amount of Mo and the first pass bite temperature of finish rolling are outside the scope of the present invention, the toughness of the base metal and the weld heat affected zone , Machinability is poor.

Inventive Example 1-7 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance of the alloying components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-7 is similar in composition and production method to Inventive Example 1-7, but the machinability is inferior because the total reduction ratio and the ferrite fraction of finish rolling are outside the scope of the present invention.

Inventive Example 1-8 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 10 mm is manufactured by a method of controlling the balance of the alloying components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-8 is a component similar to the invention example 1-8 and a manufacturing method, but since the Mn amount and X1 are out of the scope of the present invention, the weld heat affected zone toughness is inferior.

Inventive Example 1-9 is a steel sheet with a tensile strength of about 570 MPa, and a steel sheet having a sheet thickness of 6 mm is manufactured by a method of controlling the balance of the alloying components, rolling, and water cooling conditions in order to achieve high toughness, weldability and machinability. It is. Since ferrite is produced stably in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base material toughness, the weld heat affected zone toughness, and the machinability are excellent. On the other hand, Comparative Example 1-9 is similar in composition and production method to Inventive Example 1-9, but since the N content and the water-cooled end temperature are outside the scope of the present invention, the strength, toughness and toughness of the weld heat affected zone are inferior.

Inventive Example 1-10 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 18 mm is manufactured by a manufacturing method that controls the balance of the alloying components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-10 is a component and a manufacturing method similar to Inventive Example 1-10, but because the S amount and the water content density are outside the scope of the present invention, the strength, toughness, and toughness of the weld heat affected zone are poor.

Inventive Example 1-11 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm is manufactured by a manufacturing method which controls the balance of alloying components, rolling and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-11 is a component and manufacturing method similar to Inventive Example 1-11, but since the Al amount, Cr amount, ferrite fraction, and water content density are outside the scope of the present invention, the toughness, toughness of the weld heat affected zone, Machinability is inferior.

Inventive Examples 1-12 to 1-17 are air-cooled until a second production method (manufacturing method 2), that is, production of ferrite after rolling starts, and then water cooling thereafter ((7) The steel plate is manufactured by the manufacturing method of ()), and also Comparative Examples 1-12 to 1-17 are shown.

Inventive Example 1-12 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability, and machinability, a steel sheet having a sheet thickness of 6 mm is manufactured by a method of controlling the balance of alloy components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-12 is a component and a manufacturing method similar to Inventive Example 1-12, but since the amount of V and the water-cooling start temperature are out of the range of the present invention, the strength, toughness, toughness of the weld heat affected zone, and machinability are Falls.

Inventive Example 1-13 is a steel sheet having a tensile strength of about 570 MPa, and a steel sheet having a thickness of 18 mm is manufactured by a manufacturing method that controls balance, rolling, and water cooling conditions of alloy components in order to achieve high toughness, weldability, and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-13 is a component similar to invention example 1-13, and a manufacturing method, but since water cooling end temperature and S amount are out of the range of this invention, strength, toughness, and toughness of a weld heat influence part are inferior.

Inventive Example 1-14 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability, and machinability, a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance of the alloying components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-14 is a component and a manufacturing method similar to Inventive Example 1-14, but the machinability is inferior because the water cooling start temperature and the ferrite fraction are outside the scope of the present invention.

Inventive Example 1-15 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability, and machinability, a steel sheet having a thickness of 100 mm is manufactured by a manufacturing method that controls the balance of alloy components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-15 is similar in composition and production method to Inventive Example 1-15, but since the amount of Nb and the total reduction ratio of finish rolling are outside the scope of the present invention, the toughness, toughness of the weld heat affected zone, and machinability Falls.

Inventive Example 1-16 is a steel sheet having a tensile strength of about 570 MPa, in order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 6 mm is manufactured by a method of controlling the balance of alloy components, rolling, and water cooling conditions. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-16 is a component and a manufacturing method similar to Inventive Example 1-16, but Mg amount and quantity density are beyond the scope of the present invention, and the strength and toughness are inferior.

Inventive Example 1-17 is a steel sheet having a tensile strength of about 570 MPa, and a steel sheet having a thickness of 18 mm is manufactured by a method of controlling the balance, rolling and water cooling conditions of alloying elements in order to achieve high toughness, weldability and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Examples 1-17 are similar components and production methods to Inventive Examples 1-17, but the toughness, toughness of the weld heat affected zone, and machinability, since the amount of Ti and the total rolling reduction of rough rolling are outside the scope of the present invention. Falls.

Inventive examples 1-18 to 1-21 are a third manufacturing method (manufacturing method 3), that is, a method of heating up to a two-phase station again after the temperature of the steel sheet is reduced after rolling (described in (8) of the present invention). The steel sheet is manufactured by the manufacturing method), and also Comparative Examples 1-18 to 1-21 are shown.

Inventive Example 1-18 is a steel sheet having a tensile strength of about 570 MPa to produce a steel sheet having a thickness of 18 mm by a manufacturing method that controls the balance of the alloying components, rolling and water cooling conditions in order to achieve high toughness, weldability and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-18 is similar in composition and production method to Inventive Example 1-18, but because the amount of Cu, X1, and reheating temperature are out of the range of the present invention, the toughness and toughness of the weld heat affected zone are inferior.

Inventive Example 1-19 is a steel sheet with a tensile strength of about 570 MPa, and a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance, rolling, and water cooling conditions of alloying components in order to achieve high toughness, weldability and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-19 is a component and a manufacturing method similar to Inventive Example 1-19, but since toughness and reheating temperature are out of the range of this invention, toughness is inferior.

Inventive Example 1-20 is a steel sheet having a tensile strength of about 570 MPa to produce a steel sheet having a sheet thickness of 6 mm by a manufacturing method that controls the balance, rolling and water cooling conditions of the alloying components in order to achieve high toughness, weldability and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-20 is similar in composition and production method to Inventive Example 1-20, but since the total reduction ratio of X1 and finish rolling is outside the scope of the present invention, the toughness and the weld heat affected zone toughness are inferior.

Inventive Example 1-21 is a steel sheet having a tensile strength of about 570 MPa, which achieves high toughness, weldability, and machinability. It is. Since ferrite is stably produced in the entire sheet thickness range, especially near the steel plate surface, the tensile strength is about 570 MPa, and the base metal toughness, the weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 1-21 is a component and a manufacturing method similar to Inventive Example 1-21, but since the amount of Ni, the total reduction ratio of rough rolling, and the water cooling end temperature are out of the range of the present invention, the strength, toughness, and heat of welding Influence toughness is inferior.

From the above examples, the steel sheets of Inventive Examples 1-1 to 1-21, which are steels produced by the present invention, have a tensile strength of about 570 to 720 MPa, have high base material toughness, high weld heat affected zone toughness, and high machinability. It is obvious that it is an excellent steel.

Example 2

The Example of the steel plate as described in (2) of this invention is demonstrated.

Absorption of Charpy impact of the base material as the yield stress, tensile strength, and toughness of the base material as the strength of the steel sheets of 6, 20, 40, and 100 mm thickness manufactured by various manufacturing conditions using various steels of various chemical components. Among the energy and weldability, the Charpy impact absorption energy as the weld heat affected zone toughness and the drill hole machining characteristics as machinability were evaluated. The chemical composition, sheet thickness, X1, X2, ferrite fraction, the ratio of the micro-Vickers hardness in the specific range, the Vickers hardness are shown in Tables 13 to 22, and the production conditions (manufacturing methods 4 to 7) are shown in Tables 23 to 27. In Table 28-32, the evaluation result of a characteristic is shown.

Yield stress and tensile strength were measured by the metallic material tensile test method described in JIS Z 2241. The test piece is a metal material tensile test piece described in JIS Z 2201, and a No. 5 test piece for a 6 mm and 20 mm steel plate, and a No. 10 test piece taken from t / 4 part for a steel plate having a plate thickness of 40 mm and 100 mm is used. It was. The test piece was sampled so that a longitudinal direction might become perpendicular | vertical to a rolling direction. The yield stress was 0.2% yield strength calculated by the lower yield stress or the offset method. Two tests were performed at normal temperature, and the average value was employ | adopted.

Base metal toughness was measured by the metal material impact test method described in JIS Z 2242. The test piece is a metal material impact test piece according to JIS Z 2202, and the steel sheet having a sheet thickness of 6 mm is a sub-size test piece having a width of 5 mm from the sheet thickness center, and the steel sheet having a sheet thickness of 18 mm is a test piece having width 10 mm from the sheet thickness center. The steel plate with a plate thickness of 40 mm and 100 mm collected the test piece of width 10mm from t / 4 part. The shapes were all taken as a V-notch test piece, and it extract | collected so that the line which a notch bottom may make parallel to a plate thickness direction, and the longitudinal direction of a test piece is perpendicular to a rolling direction. The test temperature was -5 degreeC and the average value which performed three tests was employ | adopted.

The weld heat affected zone toughness was taken from a weld joint created by CO ₂ gas shield arc welding and submerged arc welding to measure a Charpy test piece, and measured the absorbed energy at -5 ° C. Weld heat input is 2 to 3 kJ / mm for CO ₂ gas shield arc welding, 3 kJ / mm for 6 mm sheet thickness for submerged arc welding, 5 kJ / mm for 20 mm sheet thickness, 40 mm for sheet thickness 100 It was 7 kJ / mm in mm material. The test piece was sampled so that the place 0.5 mm from the bond part of a welding corresponded to the notch position of a Charpy test piece. The average value of three shock absorption energy was employ | adopted.

As an evaluation of machinability, the boring test was done using the boring machine and a high speed drill. Perforation length is 7 sheets for 42 mm of steel plate with plate thickness of 6 mm, 2 sheets for 40 mm sheet for steel plate of 20 mm plate thickness, 40 mm for one sheet of steel plate with plate thickness of 40 mm and 40 mm sheet thickness of 100 mm In the case of a steel plate, it tested as 100 mm.

The drill was perforated using a high-speed drill SKH51 with a diameter of 6 mm. The rotation speed was 1610 rpm, the transmission speed was 190 mm / min., And the cutting oil used the water-soluble cutting oil. Under the above conditions, the hole processing was performed until it became impossible to drill, and the number of hole processing to a limit was measured.

Inventive example 2-1 to 2-19, the 1st manufacturing method (manufacturing method 4) of the steel plate as described in (2) of this invention, ie, the method of performing water cooling rapidly after rolling, and (9) of this invention The steel plate is manufactured by the manufacturing method, and also Comparative Examples 2-1 to 2-19 are shown.

Inventive Example 2-1 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 6 mm is manufactured by controlling the balance of alloy components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-1 is similar in composition and production method to Inventive Example 2-1, but inferior in machinability since X2 is outside the scope of the present invention.

Inventive Example 2-2 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 6 mm is controlled by a manufacturing method that controls the balance of the alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-2 is similar in composition and production method to Inventive Example 2_2, but since the amount of Si is out of the range of the present invention, the toughness and the weld heat affected zone toughness are inferior.

Inventive Example 2-3 is a steel sheet having a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 6 mm is manufactured by a method of controlling the balance of the alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-3 is a component and a manufacturing method similar to Inventive Example 2-3, but because the amount of Mn is out of the range of the present invention, the weld heat affected zone toughness is inferior.

Inventive Example 2-4 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 6 mm is controlled by a manufacturing method that controls the balance of alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-4 is similar in composition and production method to Inventive Example 2-4, but the machinability is inferior because the first pass bite temperature and the ferrite fraction of finish rolling are outside the scope of the present invention.

Inventive Example 2-5 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by a method of controlling the balance of alloy components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-5 is a component and a manufacturing method similar to Inventive Example 2-5, but the toughness is inferior because Al amount and X2 are out of the range of the present invention.

Inventive Example 2-6 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by a method of controlling the balance of the alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-6 is a component and a manufacturing method similar to Inventive Example 2-6, but because the amount of S is out of the range of the present invention, the toughness and the weld heat affected zone toughness are inferior.

Inventive Example 2-7 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by a method of controlling the balance of alloy components, rolling conditions, water cooling conditions, and the like. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-7 is a component and a manufacturing method similar to Inventive Example 2-7, but because the amount of P is out of the range of the present invention, the toughness and the weld heat affected zone toughness are inferior.

Inventive Example 2-8 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by a method of controlling the balance of alloy components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-8 is similar in composition and production method to Inventive Example 2-8, but the machinability is inferior because the first pass bite temperature and micro-Vickers hardness of finish rolling are outside the scope of the present invention. Inventive Example 2-9 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance of alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-9 is similar in composition and production method to Inventive Example 2-9, but since the Mo amount and X1 are outside the scope of the present invention, the weld heat affected zone toughness is inferior.

Inventive Example 2-10 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance of the alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, toughness of weld heat-affected zone, and machinability. On the other hand, Comparative Example 2-10 is similar in composition and production method to Inventive Example 2-10, but because the amount of Cr is out of the range of the present invention, the weld heat affected zone toughness is inferior.

Inventive Example 2-11 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance of alloying components, rolling conditions, water cooling conditions, and the like. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-11 is similar in composition and production method to Inventive Example 2-11, but the strength is poor because Vickers hardness and water density are outside the scope of the present invention.

Inventive Example 2-12 is a steel sheet having a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance of alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-12 is a component and a manufacturing method similar to Inventive Example 2-12, but because the N amount is outside the scope of the present invention, the toughness and toughness of the weld heat affected zone are inferior.

Inventive Example 2-13 is a steel sheet having a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-13 is similar in composition and production method to Inventive Example 2-13, but the machinability is inferior because the overall rolling reduction and the micro Vickers hardness of the finish rolling are outside the scope of the present invention.

Inventive Example 2-14 is a steel sheet with a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-14 is a component and a production method similar to Inventive Example 2-14, but because the amount of B, X1 is outside the scope of the present invention, the toughness is inferior.

Inventive Example 2-15 is a steel sheet with a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, toughness of weld heat-affected zone, and machinability. On the other hand, Comparative Example 2-15 is similar in composition and production method to Inventive Example 2-15, but inferior in machinability since the ferrite fraction and the total rolling reduction of rough rolling are outside the scope of the present invention.

Inventive Example 2-16 is a steel sheet having a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-16 is similar in composition and production method to Inventive Example 2-16, but because X2 is outside the scope of the present invention, the weld heat affected zone toughness is poor.

Inventive Example 2-17 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by controlling the balance of alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-17 is a component and a production method similar to Inventive Example 2-17, but the strength is lowered because the water cooling end temperature and the Vickers hardness are outside the scope of the present invention.

Inventive Example 2-18 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by controlling a balance of alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-18 is similar in composition and production method to Inventive Example 2-18, but the machinability is inferior because the yield density and the ferrite fraction are outside the scope of the present invention.

Inventive Example 2-19 is a steel sheet having a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-19 is similar in composition and production method to Inventive Example 2-19, but since the amount of C and X1 are outside the scope of the present invention, the toughness and the weld heat affected zone toughness are inferior.

Inventive examples 2-20 to 2-23 are heated again after the temperature of the steel plate after rolling is reduced (the manufacturing method 5) of the 2nd manufacturing method as described in (2) of this invention, ie, water cooling after that The steel plate is manufactured by the method of implementing and the manufacturing method of (11) of this invention, and also Comparative Examples 2-20-2-23 are also shown.

Inventive Example 2-20 is a steel sheet having a tensile strength of about 570 MPa, and a steel sheet having a sheet thickness of 6 mm was manufactured by a method of controlling the balance of alloy components, water cooling conditions, etc. in order to achieve high toughness, weldability and machinability. The tensile strength is about 570 MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 2-20 is similar in composition and production method to Inventive Example 2-20, but the machinability is inferior because the reheating temperature and the ferrite fraction are outside the scope of the present invention.

Inventive Example 2-21 is a steel sheet having a tensile strength of about 570 MPa, and a steel sheet having a thickness of 20 mm was manufactured by a manufacturing method that controlled the balance of alloy components, water cooling conditions, etc. in order to achieve high toughness, weldability and machinability. The tensile strength is about 570 MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent. On the other hand, Comparative Example 2-21 is a component and a production method similar to Inventive Example 2-21, but since the water-cooling end temperature and Vickers hardness after reheating are outside the scope of the present invention, the strength is inferior.

Inventive Example 2-22 is a steel sheet having a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm was manufactured by a manufacturing method that controlled the balance of alloy components, water cooling conditions, and the like. The tensile strength is about 570 MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent.

On the other hand, Comparative Example 2-22 is similar in composition and production method to Inventive Example 2-22, but since the amount of Nb and the reheating temperature are outside the scope of the present invention, the toughness, the toughness of the weld heat affected zone, and the machinability are poor.

Inventive Example 2-23 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 10 mm is manufactured by a method of controlling the balance of alloy components, water cooling conditions, and the like. The tensile strength is about 570 MPa, and the base metal toughness, weld heat affected zone toughness and machinability are excellent.

On the other hand, Comparative Example 2-23 is a component and a manufacturing method similar to Inventive Example 2-23, but the strength, toughness, weld heat affected zone toughness, since the Ti amount, the cooling rate after reheating, and the Vickers hardness are outside the scope of the present invention. , Machinability is poor.

Inventive Examples 2-24 to 2-29 were subjected to air cooling until the third production method (manufacturing method 6) of the steel sheet according to (2) of the present invention, that is, production of ferrite after rolling was started, followed by water cooling. The steel sheet is manufactured by the method of carrying out and the manufacturing method of (12) of this invention, and also Comparative Examples 2-24-2-29 are shown.

Inventive Example 2-24 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability, and machinability, a steel sheet having a sheet thickness of 6 mm is manufactured by controlling the balance of alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-24 is a component and a manufacturing method similar to Inventive Example 2-24, but the machinability is inferior because the water cooling start temperature and the micro-Vickers hardness are out of the range of the present invention.

Inventive Example 2-25 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by a method of controlling the balance of alloy components, rolling conditions, water cooling conditions, and the like. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-25 is a component and a manufacturing method similar to Inventive Example 2-25, but the strength and toughness of the Zr amount, the water cooling end temperature, and the Vickers hardness are outside the scope of the present invention.

Inventive Example 2-26 is a steel sheet with a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-26 is a component and manufacturing method similar to Inventive Example 2-26, but since the V amount, the water cooling start temperature, and the Vickers hardness are outside the scope of the present invention, the strength, toughness, toughness of the weld heat affected zone, Machinability is inferior.

Inventive Example 2-27 is a steel sheet having a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-27 is a component and a manufacturing method similar to Inventive Example 2-27, but since the amount of Ni, X1, and the total reduction ratio of finish rolling are outside the scope of the present invention, the toughness and the weld heat affected zone toughness Falls.

Inventive Example 2-28 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by a method of controlling the balance of the alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-28 is a component and a manufacturing method similar to Inventive Example 2-28, but the strength is inferior because the yield density and the Vickers hardness are out of the range of the present invention.

Inventive Example 2-29 is a steel sheet with a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-29 is a component and a manufacturing method similar to Inventive Example 2-29, but the toughness and the toughness of the weld heat affected zone are inferior because the amount of Cu and the total reduction ratio of rough rolling are outside the scope of the present invention.

Inventive Examples 2-30 to 2-34 perform the heating to the two-phase region again after the fourth manufacturing method (manufacturing method 7) of the steel sheet according to (2) of the present invention, that is, the temperature of the steel sheet after rolling decreases. The steel plate is manufactured by the manufacturing method as described in (13) of this invention, and Comparative Examples 2-30 and 2-34 are also shown.

Inventive Example 2-30 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a sheet thickness of 6 mm is manufactured by controlling the balance of alloy components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-30 is similar in composition and production method to Inventive Example 2-30, but the toughness and machinability are inferior because the amount of REM and the reheating temperature are outside the scope of the present invention.

Inventive Example 2-31 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 20 mm is manufactured by a method of controlling the balance of alloying components, rolling conditions, water cooling conditions, and the like. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-31 is a component and a manufacturing method similar to Inventive Example 2-31, but the strength and toughness are inferior because the amount of Ca, the water cooling end temperature after reheating, and the Vickers hardness are out of the range of the present invention.

Inventive Example 2-32 is a steel sheet with a tensile strength of about 570 MPa. In order to achieve high toughness, weldability and machinability, a steel sheet having a thickness of 40 mm is manufactured by a method of controlling the balance of alloying components, rolling conditions, and water cooling conditions. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-32 is similar in composition and production method to Inventive Example 2-32, but since the Mg amount, reheating temperature, and Vickers hardness are outside the scope of the present invention, the strength and toughness are inferior.

Inventive Example 2-33 is a steel sheet having a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-33 is similar in composition and production method to Inventive Example 2-33, but since the V amount and the total rolling reduction of finish rolling are outside the scope of the present invention, toughness, toughness of weld heat affected zone, and machinability Falls.

Inventive Example 2-34 is a steel sheet having a tensile strength of about 570 MPa. It is manufactured, and the tensile strength is about 570 MPa, and excellent in base material toughness, weld heat affected zone toughness, and machinability. On the other hand, Comparative Example 2-34 is a component and manufacturing method similar to Inventive Example 2-34, but the toughness, toughness and machinability of the weld heat-affected zone, because the Nb amount and the total rolling reduction of rough rolling are outside the scope of the present invention. Falls.

From the above examples, it is clear that the steel sheets of Inventive Examples 2-1 to 2-34, which are steels produced by the present invention, are steels having excellent tensile strength, weldability, and machinability, all of which are about 570 to 720 MPa.

Example 3

The Example described in (3) of this invention is demonstrated.

For steel sheets of 6, 18, 40, and 100 mm thickness manufactured using various chemically-formed steels and manufactured under various manufacturing conditions, the Charpy impact absorption of the base material as the yield stress, tensile strength, and toughness of the base material as strength. Drill hole processing characteristics were evaluated as energy and machinability. The production conditions (manufacturing methods 4 'to 7') are shown in Tables 33 to 40 for the ratios of the chemical composition of the steel sheet, the plate thickness, X1, X2, the ferrite fraction, the Vickers hardness, and the micro Vickers hardness in the specific ranges. In Tables 44-48, evaluation results of the properties are shown in Tables 45-48.

The yield stress and the tensile strength were measured by the metal sheet and tensile test method described in JIS Z 2241. The test piece was a metal material tensile test piece described in JIS Z 220 1, and a No. 5 test piece was used from a steel plate of 6 mm and a plate thickness of 18 mm and a No. 10 test piece taken from t / 4 part of a steel plate of 40 mm and 100 nm thickness. It was. The test piece was sampled so that a longitudinal direction might become perpendicular | vertical to a rolling direction. The yield stress was 0.2% yield strength calculated by the lower yield stress or the offset method. Two tests were performed at room temperature and an average value was employed.

Base metal toughness was measured by the metal material impact test method described in JIS Z 2242. The test piece is a metal material impact test piece described in JIS Z 2202, and the steel sheet having a sheet thickness of 6 mm has a sub-size test piece having a width of 5 mm from the sheet thickness center, and the steel sheet having a sheet thickness of 18 mm has a width of 10 mm from the sheet thickness center. The steel plate with a plate thickness of 40 mm and 100 mm collected the test piece of width 10mm from t / 4 part. The shapes were all taken as V notch test pieces, and the samples were taken so that the lines formed by the notched bottom were parallel to the plate thickness direction, and the longitudinal direction of the test pieces was perpendicular to the rolling direction. The test temperature was -5 degreeC and the average value which performed three tests was employ | adopted.

As an evaluation of machinability, the boring test was done using the boring machine and a high speed drill. Steel plate of 6mm in thickness stacks 7 sheets 42mm, steel sheet of 18mm in thickness stacks 2 sheets 36mm, 40mm in sheet thickness 40mm in one piece, and 100mm in sheet thickness 100mm in one piece Was carried out. The drill was perforated using a high-speed drill SKH51 having a diameter of 6 mm. The rotation speed was 1610 rpm, the transmission speed was 190 mm / min., And the cutting oil used the water-soluble cutting oil. Under the above conditions, the hole processing was performed until it became impossible to drill, and the total number of hole processing to a limit was measured.

Inventive examples 3-1 to 3-14 refer to the first manufacturing method (manufacturing method 4 ') of the steel sheet according to (3) of the present invention, that is, a method of rapidly cooling water after rolling, (14) of the present invention. Steel sheets are manufactured by the manufacturing method described, and the comparative examples 3-1 to 3-14 are also shown.

Inventive example 3-1 manufactures the steel plate of plate thickness 18mm by a 1st manufacturing method (manufacturing method 4 '). Since the requirements specified in the present invention are fully satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-1 is a component and a manufacturing method similar to Inventive Example 3-1, but the toughness is extremely low because the amount of C and X1 are outside the scope of the present invention.

Inventive example 3-2 manufactures the steel plate of 18 mm of plate | board thickness by the 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-2 is similar in composition and production method to Inventive Example 3-2, but the machinability is extremely low since the first pass bite temperature and the ferrite fraction of finish rolling are outside the scope of the present invention.

Inventive example 3-3 manufactures the steel plate of plate thickness 18mm by a 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-3 is similar in composition and production method to Inventive Example 3-3, but the machinability is extremely low because X2 is outside the scope of the present invention.

Inventive example 3-4 manufactures the steel plate of 6 mm of plate | board thickness by the 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-4 is a component and a production method similar to Inventive Example 3-4, but the toughness is extremely low because the amount of Si is outside the scope of the present invention.

Invention Example 3-5 manufactures the steel plate of 6 mm of plate | board thickness by the 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-5 is similar in composition and production method to Inventive Example 3-5, but the toughness and machinability are extremely low since the first pass bite temperature and micro-Vickers hardness of finish rolling are outside the scope of the present invention. .

Inventive example 3-6 manufactures the steel plate of 6 mm of plate | board thickness by the 1st manufacturing method (manufacturing method 4 '). Since all the requirements prescribed | regulated by this invention are satisfy | filled, it exhibits the outstanding machinability and favorable toughness simultaneously with the tensile strength of 570 Mpa or more. On the other hand, Comparative Example 3-6 is similar in composition and production method to Inventive Example 3-6, but the strength is extremely low since the end temperature of the water cooling is outside the range of the present invention.

Inventive example 3-7 manufactures the steel plate of plate thickness 40mm by a 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-7 is similar in composition and production method to Inventive Example 3-7, but the machinability is extremely low because Mn amount and X2 are outside the scope of the present invention.

Inventive example 3-8 manufactures the steel plate of plate thickness 40mm by a 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-8 is a component and a manufacturing method similar to Inventive Example 3-8, but the toughness is extremely low because Mo amount is out of the range of the present invention.

Inventive example 3-9 manufactures the steel plate of plate thickness 40mm by a 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-9 is similar in composition and production method to Inventive Example 3-9, but the toughness is extremely low since P amount is out of the range of the present invention.

Inventive example 3-10 manufactures the steel plate of plate thickness 100mm by a 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-10 is similar in composition and production method to Inventive Example 3-10, but the machinability is extremely low since the total reduction ratio and the ferrite fraction of finish rolling are outside the scope of the present invention.

Inventive example 3-11 manufactures the steel plate of plate thickness 100mm by a 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-11 is a component and a manufacturing method similar to Inventive Example 3-11, but the toughness is extremely low because S amount is out of the range of the present invention.

Inventive example 3-12 manufactures the steel plate of plate thickness 100mm by a 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-12 is a component and manufacturing method similar to Inventive Example 3-12, but the machinability and toughness are extremely low because the Al amount, the overall reduction ratio of the rough rolling, and the ferrite fraction are outside the scope of the present invention.

Inventive example 3-13 manufactures the steel plate of 6 mm of plate | board thickness by the 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-13 is similar in composition and production method to Inventive Example 3-13, but the strength is extremely low because the yield density and Vickers hardness are outside the scope of the present invention.

Inventive example 3-14 manufactures the steel plate of 6 mm of plate | board thickness by the 1st manufacturing method (manufacturing method 4 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-14 is similar in composition and production method to Inventive Example 3-14, but the machinability is extremely low because the yield density and the ferrite fraction are outside the scope of the present invention.

Inventive Examples 3-15 to 3-19 are heated again after the second manufacturing method (manufacturing method 5 ') of the steel plate as described in (3) of this invention, ie, the temperature of a steel plate after rolling falls, and after that Steel plate is manufactured by the method of performing water cooling, and the manufacturing method of (16) of this invention, and also Comparative Examples 3-15-3-19 are shown.

Inventive example 3-15 manufactures the steel plate of 6 mm of plate | board thickness by the 2nd manufacturing method (manufacturing method 5 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-15 is similar in composition and production method to Inventive Example 3-15, but the machinability and toughness are extremely low since N amount, cooling rate of water cooling after reheating, and micro-Vickers hardness are outside the scope of the present invention.

Inventive example 3-16 manufactures the steel plate of 18 mm of plate | board thickness by the 2nd manufacturing method (manufacturing method 5 '). In order to satisfy all of the requirements specified in the present invention, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-16 is similar in composition and production method to Inventive Example 3-16, but the strength is extremely low because the amount of C, the cooling rate of water cooling after reheating, and the Vickers hardness are outside the scope of the present invention.

Inventive example 3-17 manufactures the steel plate with a plate thickness of 40 mm by the 2nd manufacturing method (manufacturing method 5 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-17 is a component and a production method similar to Inventive Example 3-17, but the strength and toughness are extremely low because the amount of Cr, the end temperature of water cooling after reheating, and the Vickers hardness are out of the range of the present invention.

Inventive example 3-18 manufactures the steel plate of plate thickness 100mm by the 2nd manufacturing method (manufacturing method 5 '). Since all the requirements specified in the present invention are satisfied, the tensile strength is excellent at the same time as the tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-18 is similar in composition and production method to Inventive Example 3-18, but the machinability is extremely low because the reheating temperature and the ferrite fraction are outside the scope of the present invention.

Inventive example 3-19 manufactures the steel plate of 18 mm of plate | board thickness by the 2nd manufacturing method (manufacturing method 5 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-19 is a component and a manufacturing method similar to Inventive Example 3-19, but the toughness is extremely low because the amount of S and the reheating temperature are outside the scope of the present invention.

Inventive Examples 3-20 to 3-24 carry out air cooling until the 3rd manufacturing method (manufacturing method 6 ') of the steel plate as described in (3) of this invention, ie, the production of ferrite after rolling starts, and after that Steel sheet is manufactured by the method of performing water cooling, and the manufacturing method as described in (17) of this invention, and also Comparative Examples 3-20-3-24 are also shown.

Inventive example 3-20 manufactures the steel plate of 6 mm of plate | board thickness by the 3rd manufacturing method (manufacturing method 6 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-20 is similar in composition and production method to Inventive Example 3-20, but the strength is extremely low since the water cooling start temperature and the Vickers hardness are outside the scope of the present invention.

Inventive example 3-21 manufactures the steel plate of plate thickness 18mm by the 3rd manufacturing method (manufacturing method 6 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-21 is a component and a manufacturing method similar to Inventive Example 3-21, but the strength is extremely low because the yield density and Vickers hardness are outside the scope of the present invention.

Inventive example 3-22 manufactures the steel plate of plate thickness 40mm by the 3rd manufacturing method (manufacturing method 6 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-22 is similar in composition and production method to Inventive Example 3-22, but since V amount and total rolling reduction in finish rolling are outside the scope of the present invention, toughness and machinability are extremely low.

Inventive example 3-23 manufactures the 3rd manufacturing method (manufacturing method 6 ') of the steel plate of plate thickness 100mm. Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-23 is a component and a manufacturing method similar to Inventive Example 3-23, but since the total reduction ratio, the water cooling stop temperature and the Vickers hardness in the rough rolling are outside the scope of the present invention, the strength and toughness Extremely low

Inventive example 3-24 manufactures the steel plate of 18 mm of plate | board thickness by the 3rd manufacturing method (manufacturing method 6 '). Since all the requirements specified in the present invention are satisfied, tensile strength of 570 MPa or more and excellent machinability and good toughness are exhibited. On the other hand, Comparative Example 3-24 is similar in composition and production method to Inventive Example 3-24, but the machinability is extremely low since the water cooling start temperature and the ferrite fraction are outside the scope of the present invention.

Inventive Examples 3-25 to 3-29 perform the fourth production method (manufacturing method 7 ') of the steel sheet according to (3) of the present invention, ie, heating to the two-phase region again after the temperature of the steel sheet decreases after rolling. The steel plate is manufactured by the manufacturing method as described in (18) of this invention, and Comparative Examples 3-25 to 3-29 are also shown.

Inventive example 3-25 manufactures the steel plate of 6 mm of plate | board thickness by the 4th manufacturing method (manufacturing method 7 '). Since all the requirements specified in the present invention are satisfied, tensile strength of 570 MPa or more and excellent machinability and good toughness are exhibited. On the other hand, Comparative Example 3-25 is similar in composition and production method to Inventive Example 3-25, but the machinability is extremely low since X2, reheating temperature and ferrite fraction are outside the scope of the present invention.

Inventive example 3-26 manufactures the steel plate of 18 mm of plate | board thickness by the 4th manufacturing method (manufacturing method 7 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more.

On the other hand, Comparative Example 3-26 is similar in composition and production method to Inventive Example 3-26, but the strength is extremely low because Vickers hardness and reheating temperature are outside the scope of the present invention.

Inventive example 3-27 manufactures the steel plate of plate thickness 40mm by the 4th manufacturing method (manufacturing method 7 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-27 is a component and a manufacturing method similar to Inventive Example 3-27, but the strength is extremely low because the water cooling end temperature and the Vickers hardness are outside the scope of the present invention.

Inventive example 3-28 manufactures the steel plate of plate thickness 100mm by the 4th manufacturing method (manufacturing method 7 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-28 is similar in composition and production method to Inventive Example 3-28, but since the V amount and the total rolling reduction of finish rolling are outside the scope of the present invention, toughness and machinability are extremely low.

Inventive example 3-29 manufactures the steel plate of plate thickness 100mm by the 4th manufacturing method (manufacturing method 7 '). Since all of the requirements specified in the present invention are satisfied, it exhibits excellent machinability and good toughness at the same time as a tensile strength of 570 MPa or more. On the other hand, Comparative Example 3-29 is similar in composition and production method to Inventive Example 3-29, but the toughness is extremely low because the amount of B and the total rolling reduction of rough rolling are outside the scope of the present invention.

From the above examples, it is apparent that the steel sheets of Inventive Examples 3-1 to 3-29, which are steels produced by the present invention, have a tensile strength of about 570 to 720 MPa, are high in base metal toughness, and are excellent in machinability.

According to the present invention, the structure constituting the steel is a composite structure of soft ferrite and hard bainite and martensite, and (1) the ferrite fraction of the front and back surfaces of the steel sheet, which has a great influence on the wear and tear of the tool, Strictly stipulating, or (2) adjusting the steel component balance that can greatly reduce the cutting resistance during cutting at a high temperature, or (3) increasing the amount of S added in a range that does not cause toughness deterioration. It is possible to provide a steel sheet having a high level of machinability which has not been achieved in the conventional welded structural steel sheet, and a tensile strength of about 570 to 720 MPa, which is also excellent in strength, toughness and weldability, and a manufacturing method thereof. It is a high invention of industrial value.

Claims (19)

In terms of mass, C: 0.005 to 0.2%, Si: 0.01 to 1%, Mn: 0.01-2%, P: 0.02% or less, S: 0.035% or less Al: 0.OO1 to 0.1% N: 0.1% or less, the balance being a steel composition composed of Fe and unavoidable impurities, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, If the sheet thickness is 4 mm or more and less than 10 mm, the ferrite fraction of the portion that enters from the steel plate label surface by 1/4 and 3/4 of the sheet thickness is 30% or more and 90% or less, and the steel sheet is half the plate thickness. A steel sheet excellent in machinability, toughness and weldability, characterized in that the ferrite fraction of the portion that enters from the surface to the inside is 20% or more and 90% or less. In the steel composition according to claim 1, the steel is in mass%, Mn: 0.01-1.4%, S: 0.01% or less, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr X2 represented by / 2) / Mn is 0.15 or more and 10.0 or less, and the structure constituting the steel has a ferrite fraction of 30% or more and 90% or less, and the remainder consists of hard tissue mainly composed of bainite and martensite. The steel sheet excellent in machinability, toughness, and weldability, whose structure is 20% or more of micro Vickers hardness of 190HV or less, and Vickers hardness of steel is 165HV or more and 300HV or less. In the steel composition according to claim 1, the steel is in mass%, Mn: 0.01-1.4%, S: more than 0.01% to 0.035% or less, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr X2 represented by / 2) / Mn is 0.15 or more and 10.0 or less, In addition, the steel constituting structure is a structure in which the ferrite fraction is 30% or more and 90% or less, and the remainder consists of hard tissue mainly composed of bainite and martensite, or 20% or more, in which the micro-Vickers hardness is 190HV or less. And the Vickers hardness of the steel is 165 HV or more and 300 HV or less, the steel sheet excellent in machinability, toughness and weldability. The steel according to any one of claims 1 to 3, wherein, in terms of mass%, Mo: 0.01 to 1%, Cr: 0.1 to 1%, Nb: 0.001-0.1%, Ti: 0.001-0.1%, V: 0.001 to 0.1%, Cu: 0.005 to 1%, Ni: 0.01-2%, B: 0.0002 to 0.005%, REM: 0.0005 to 0.1%, Ca: 0.0005 to 0.02%, Zr: 0.0005 to 0.02%, Mg: Steel plate excellent in machinability, toughness, and weldability containing 1 type (s) or 2 or more types of 0.0005 to 0.02%. With the steel composition of claim 1, The total reduction ratio is 30 after heating the steel sheet or cast steel having X1 of 0.24 or less represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B. Rough rolling of not less than 95% and not more than 95% is performed, and then the first pass bite temperature is set to T1 = 351 n (X2 / 2) -25.

T1 (° C) or less represented by +1070, X2 = (Si / 5 + Mo + Cr / 2) / Mn, 720 ° C or more, and finish rolling with a total reduction ratio of 30% or more and 95% or less. , The quantity density is 0.2 m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling at or below and ending water cooling at 600 ° C or lower. Where t is the plate thickness. In the water cooling started after completion | finish of rolling, the average cooling rate of water cooling start temperature or more and more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and the average cooling rate more than 650 degreeC or less water cooling stop temperature. Is 10 ° C / s or more and 100 ° C / s or less, wherein the steel sheet has excellent machinability, toughness and weldability. With the steel composition of claim 1, The total reduction ratio is 30 after heating the steel sheet or cast steel having X1 of 0.24 or less represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B. Rough rolling with% or more and 95% or less, finish rolling with a total rolling reduction of 30% or more and 95% or less, followed by air cooling, and the steel sheet surface temperature of T2 = 910-310 × C-80 × Mn-20 × Cu -15 × Cr-55 × Ni- 80 × Mo + 0.0006t 2 and -0.56t-8.6 T2 (℃) or less than 650 ℃ shown, the water density is 0.2m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. The method of manufacturing a steel sheet excellent in machinability, toughness and weldability, wherein water cooling below is started and water cooling is finished at 5O <0> C or lower. Where t is the plate thickness. With the steel composition of claim 1, The total reduction ratio is 30 after heating a steel sheet or cast steel having X1 of 0.24 or less represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B Rough rolling of not less than 95% and not more than 95%, total rolling reduction of not less than 30% and not more than 95% of the final rolling, cooling to 500 ° C or less, and T3 (° C), in which the steel sheet is represented by T3 = 0.0017t ² + 0.17t +730 The method of manufacturing a steel sheet excellent in machinability, toughness and weldability, wherein water cooling is terminated after reheating to 850 ° C. or lower. Where t is the plate thickness. With the steel composition of claim 2, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, After heating the slab or cast steel whose X2 represented by X2 = (Si / 5 + Mo + Cr / 2) / Mn is 0.15 or more and 10.0 or less, rough rolling of 30% or more and 95% or less of total rolling reduction is performed after that. First pass bite temperature T4 = 351n (X2 / 2) -25

+1100 represented by T4 (℃) less than Ar ₃ point or higher, subjected to finish rolling to a total reduction ratio in a range from 30% to 95%, after which the fast water density ^{0.2 m 3 / m 2 · mm} . 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling at or below and ending water cooling at 600 ° C or lower. Where t is the plate thickness. In the water cooling started after completion | finish of rolling, the average cooling rate of water cooling start temperature or more and more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and the average cooling rate more than 650 degreeC or less water cooling stop temperature Is 10 ° C / s or more and 100 ° C / s or less, wherein the steel sheet has excellent machinability, toughness and weldability.  With the steel composition of claim 2, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, X2 = (Si / 5 + Mo + Cr / 2) / Mn, after heating and rolling the slab or cast steel whose 0.12 or more and 10.0 or less are heated and rolled, it cools to 500 degrees C or less, and also reheats a steel plate to 900 degrees C or more and 1050 degrees C or less. And water cooling at an average cooling rate of 1 ° C./s or more and 100 ° C./s and ending water cooling at 500 ° C. or less, wherein the steel sheet has excellent machinability, toughness, and weldability. With the steel composition of claim 2, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, After heating the slab or cast steel whose X2 represented by X2 = (Si / 5 + Mo + Cr / 2) / Mn is 0.15 or more and 10.0 or less, the rough rolling rate of 30% or more and 95% or less of total rolling reduction is 30 conducting% or more and 95% or less, and the finish rolling, after cooling to the air-cooling after the Ar ₃ point or more and 150 ℃ temperature lower than the Ar ₃ point, Quantity density is 0.2 m ³ / m ² · mm. 5. The manufacturing method of the steel plate excellent in machinability, toughness, and weldability which starts water cooling of more than 5.Om <^3> / m <^2> mm or less and completes water cooling at 500 degrees C or less. With the steel composition of claim 2, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) After heating the steel or cast steel with X2 of 0.15 or more and 10.0 or less represented by / Mn, rough rolling of 30% or more and 95% or less of total rolling reduction, and finish rolling of 30% or more and 95% or less of total rolling reduction are performed. Of the steel sheet excellent in machinability, toughness and weldability, wherein the steel sheet is cooled to 500 ° C. or lower, and then reheated the steel sheet to 730 ° C. or higher and less than 900 ° C., and then cooled to end water cooling at 500 ° C. or lower. Manufacturing method. With the steel composition of claim 3, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / l0 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) heating the steel or cast steel with X2 represented by / Mn of 0.15 or more and 10.0 or less, then performing rough rolling with a total reduction ratio of 30% or more and 95% or less, after which the first pass bite temperature is T4 = 351n (X2). / 2) -25

Represented by the +1100 T4 (℃) less than Ar ₃ point or higher, subjected to finish rolling to a total reduction ratio in a range from 30% to 95% and, after rolling end, the water density ^{0.2 m 3 / m 2 · mm} . 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling at or below and ending water cooling at 600 ° C or lower. Where t is the plate thickness. The average cooling rate of the water cooling started after completion | finish of rolling WHEREIN: The average cooling rate which is below water cooling start temperature more than 650 degreeC is 1 degreeC / s or more and 5 degrees C / s or less, and is 650 degreeC or less and average cooling more than water cooling stop temperature. The speed | rate is 10 degreeC / s or more and 100 degrees C / s or less, The manufacturing method of the steel plate excellent in the machinability, toughness, and weldability. With the steel composition of claim 3, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr X2 represented by / 2) / Mn is heated to 0.15 or more and 10.0 or less, and after heating and rolling the steel slab or cast, it is cooled to 500 degrees C or less, and the steel plate is reheated at 900 degrees C or more and 1050 degrees C or less, and the average cooling rate is 1 degrees C / s. It is water-cooled to 100 degrees C / s or less and completes water cooling at 500 degrees C or less, The manufacturing method of the steel plate excellent in the machinability, toughness, and weldability. With the steel composition of claim 3, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + M / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) After heating the steel or cast steel with X2 represented by Mn of 0.15 or more and 10.0 or less, rough rolling of 30% or more and 95% or less of total rolling reduction, and finish rolling of 30% or more and 95% or less of total rolling reduction are performed. was cooled to, and then air-cooled to Ar ₃ point or more and 150 ℃ temperature lower than the Ar ₃ point and then, Quantity density is 0.2 m ³ / m ² · mm. 5.O m ³ / m ² · mm or more. A method for producing a steel sheet excellent in machinability, toughness and weldability, characterized by starting water cooling below and ending water cooling at 500 ° C or lower. With the steel composition of claim 3, X1 represented by X1 = C + (Mn + Cu + Cr) / 20 + Si / 30 + Ni / 60 + Mo / 15 + V / 10 + 5 × B is 0.24 or less, and X2 = (Si / 5 + Mo + Cr / 2) / Xn represented by Mn, after heating the steel or cast steel of 0.15 or more, 10.0 or less, rough rolling with a total reduction rate of 30% or more and 95% or less, and finish rolling with a total reduction rate of 30% or more and 95% or less After that, the steel sheet is cooled to 500 ° C. or lower by air cooling, and after the steel sheet is reheated to 730 ° C. or higher and less than 900 ° C., water is cooled, and the water cooling is terminated at 500 ° C. or higher, thereby producing a steel sheet excellent in machinability, toughness and weldability. Way. The steel sheet or cast steel according to any one of claims 5 to 18, in mass%, Mo: 0.1 to 1%, Cr: 0.1 to 1%, Nb: 0.OOl to 0.1%, Ti: 0.OO1 to 0.1%, V: 0.001 to 0.1%, Cu: 0.005 to 1%, Ni: 0.01-2%, B: 0.0002 to 0.005%, REM: 0.0005 to 0.1%, Ca: 0.0005 to 0.02%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02% of l-type or 2 or more types, characterized in that the steel sheet is excellent in machinability, toughness and weldability.

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