JP2013139610A - HIGH-TENSILE STRENGTH THICK STEEL SHEET HAVING TENSILE STRENGTH OF 780 MPa OR MORE, AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-tensile strength thick steel sheet which has small deterioration in the material quality after warm working and has a tensile strength of 780 MPa or more, and to provide a method for manufacturing the same.SOLUTION: The steel sheet has a composition including, by mass, 0.06-0.12% of C, 0.05-0.40% of Si, 0.80-1.20% of Mn, 0.015% or less of P, 0.003% or less of S, 0.005-0.060% of Al, 0.0040% or less of N, 0.20-0.50% of Mo, 0.020-0.080% of V, and one or more of 0.005-0.030% of Nb, 0.10-0.50% of Cu, 0.1-1.0% of Ni, 0.10-0.80% of Cr, and 0.0003-0.0030% of B, wherein 0.45≤(Mo+4.9V+5.8Nb)≤0.85, 4.0≤Mo/V≤16.0, and 0.20≤C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B≤0.27 are satisfied. The steel sheet has a tempered martensite structure in which a microstructure has an area ratio of 80% or more and the old austenite grain diameter is 12-30 μm.

Description

  The present invention relates to a high-strength thick steel plate having a tensile strength of 780 MPa or more, which is suitable as a material for a circular steel pipe used for offshore structures or building structural members, and a method for producing the same, and particularly, a warm press bend or a warm roll. The present invention relates to a material that is suitable as a material for a thick-walled large-diameter steel pipe formed by a bend and has excellent warm workability with little material deterioration after warm working.

  In recent years, with the increase in the height of building structures and the increase in the span between columns, there has been a strong demand for higher strength and thicker steel materials. For example, in the case of circular steel pipes used mainly as pillar materials for building structures, conventionally, steel pipes having an outer diameter of 600 to 800 mm and a wall thickness of 20 to 40 mm have been mainly used. : 800 mm or more and wall thickness: A large diameter and thick steel pipe having a size of more than 40 mm is required.

  Thick-walled and large-diameter steel pipes are usually manufactured by molding by press bend or roll bend. The press bend or roll bend is usually carried out cold for the convenience of molding. However, with the increase in thickness and strength of the material (steel material) to be used, there is a problem that cold forming increases the load applied to the molding apparatus to be used and makes the molding itself impossible.

  In addition, the cold forming also has a problem that it is accompanied by significant material deterioration of the molded product, such as a decrease in plastic deformability and a decrease in toughness due to work hardening of the steel material that occurs during the forming. Therefore, in order to reduce the deformation resistance of the raw material (steel material) at the time of molding or to reduce work hardening, the raw material (steel material) may be formed in a hot region or a warm region.

For example, Patent Document 1 describes a method of manufacturing a thick-walled steel pipe round column. The technique described in Patent Document 1 is weight%, C: 0.06 to 0.17%, Si: 0.06 to 0.5%, Mn: 0.5 to 1.6%, Mo: 0 0.1 to 0.25%, Ti: 0.01 to 0.02%, B: 0.0005 to 0.002%, Al: 0.07% or less, N: 0.004% or less, A steel containing one or two selected from Nb: 0.005 to 0.05% and V: 0.01 to 0.1% is rolled, the steel plate is heated to 900 to 1000 ° C., and Ar This is a method for producing a thick-walled steel tube round column in which bending is finished in a temperature range of _three or more transformation points. According to the technique described in Patent Document 1, it is described that a thick steel pipe round column having a uniform material and a high strength can be manufactured without requiring a large manufacturing facility.

  However, in the case of pipe forming by press bend, in which the material (steel material) is a steel pipe in a hot zone or a warm zone, as shown in FIG. The steel plate temperature is inevitably lowered gradually because the steel mold is formed by pressing the press mold 2 toward the part. In the case of thick-walled large-diameter steel pipes that require a long time from the start of processing to completion, the amount of temperature decrease during pipe forming is maximum, for example, the processing starts at 550 ° C and the completion temperature reaches about 400 ° C. It reaches about 150 ° C.

Therefore, it has been extremely difficult to manage the material of the product (steel pipe) after hot or warm forming within a certain range. In order to ensure stable product characteristics in mass production using press bends, it is necessary to solve the decrease in ductility due to work hardening and increase in yield ratio. The temperature drop at the end and the variation in the forming temperature of each steel pipe are inevitable, and the material variation of the resulting product (steel pipe) has been a major problem. Patent Document 2 describes a steel pipe for thick construction with a low yield ratio. With respect to the method for producing the pillar, by weight, C: 0.05 to 0.25%, Si: 0.10 to 0.50%, Mn: 0.5 to 2.0%, sol. Al: 0.005 to 0.10% Mo: a steel sheet containing 0.05 to 0.25%, was heated to a temperature range of Ac ₁ or Ac ₃ the following two-phase regions, processing the Ar ₁ or more The steel pipe manufacturing method starts from the end of the plate in the temperature range, ends at the center of the plate at a temperature range higher than the transformation end temperature, and is air-cooled. It is described that it is possible to manufacture a thick steel tubular round column for construction having high strength and a low yield ratio in each part of the plate thickness without damaging.

Patent Document 3 relates to a method for producing a high-strength steel excellent in material properties after warm working, in terms of weight percent, C: 0.03 to 0.20%, Si: 0.6% or less, Mn: 0.00. 5 to 2.0%, sol. Al: 0.005 to 0.08%, and further accumulated at 900 ° C. or less in steel containing one or more selected from Nb, V, Ti, Cu, Cr, Ni, Mo, B After hot rolling with a reduction ratio of at least 30% or after accelerated cooling after hot rolling, the steel is further heated to 750 to 400 ° C., preferably Ac _{1 to} 400 ° C. and immediately or allowed to cool. In addition, a manufacturing method of high-strength steel is described in which hot working is performed at a processing temperature of 750 to 250 ° C., preferably Ac _{1 to} 400 ° C.

JP-A-9-279244 JP-A-8-283850 JP 62-54018 A

  In the case of thick and large diameter steel pipes manufactured by warm or hot working, conventionally, a relatively low strength circular steel pipe having a tensile strength of 490 MPa to 570 MPa or less has been required. In a building structure where designability is important, demand for high-tensile steel pipes with excellent weldability and tensile strength of 780 MPa or more is increasing.

  The steel pipes according to the techniques described in Patent Documents 1 to 3 all have a tensile strength of less than 780 MPa, and there is no description regarding weldability. In order to achieve both high strength and excellent weldability, TMCP technology is applied at the time of manufacturing a steel plate as a raw material. However, in Patent Document 1, the steel plate is reheated to the γ region and bent to form a steel pipe round column. Since the effect of the TMCP technology on the steel sheet is lost, the toughness and weldability of the steel pipe deteriorates because the high-component system that can ensure high strength with air cooling after bending is deteriorated.

  Therefore, the present invention has an object to provide a high-tensile steel plate for high-tensile steel pipes with a material thickness of 40 mm or more and a tensile strength of 780 MPa or more, and a method for producing the same, in which material deterioration after warm working is small. Specifically, by the warm working (warm forming) at 400 to 550 ° C., the yield strength: 630 MPa or more and 900 MPa or less, the tensile strength: 780 MPa or more, the yield ratio 95% or less, the fracture surface transition temperature of the Charpy impact test vTrs: An object of the present invention is a high-tensile thick steel plate that can industrially easily and stably mass-produce a circular steel pipe having an excellent weldability of −40 ° C. or less and a method for producing the same.

In order to achieve the above-mentioned object, the present inventors diligently studied the influence of various factors on the material deterioration after warm working (warm forming), and obtained the following knowledge. Note that the “material degradation” refers to a decrease and excessive increase in yield strength, a decrease in tensile strength, an increase in yield ratio, and an increase in the fracture surface transition temperature of the Charpy test.
1. The steel sheet was composed of Mo, V, and Nb, and the steel sheet structure was mainly composed of a tempered martensite phase, and precipitates such as Mo, V, and Nb were controlled to an optimum state. By using a microstructure, it is possible to suppress material deterioration after warm working (warm forming).
2. Inclusion of Mo, V, and Nb can be expected to increase the strength due to precipitation strengthening, thereby compensating for a decrease in strength associated with heating to a warm working (warm forming) temperature. Excessive precipitation strengthening is accompanied by embrittlement, but if the contents of Mo, V, and Nb satisfying the expression (1) are used, embrittlement accompanying precipitation strengthening can be suppressed.
0.45 ≦ (Mo + 4.9V + 5.8Nb) ≦ 0.85 (1)
(Here, Mo, V, Nb: content of each element (mass%))
3. Mo precipitate (carbide) greatly contributes to securing the strength after warm working (warm forming). However, since Mo is dissolved in the Mo precipitate (carbide), and the stability of the Mo precipitate (carbide) is greatly changed, it is possible to stably secure a desired high strength after warm working. It can be difficult. In order to suppress an excessive increase and decrease in strength after warm processing and to ensure the desired strength and desired toughness stably, in addition to the formula (1), the contents of Mo and V are further increased ( 2) It is necessary to adjust so as to satisfy the equation.
4.0 ≦ Mo / V ≦ 16.0 (2)
(Where Mo, V: content of each element (mass%))
4). In order to ensure the desired strength and weldability, it is possible to adjust the weld cracking susceptibility composition (Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B) to satisfy the expression (3). is necessary.
0.20 ≦ C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.27 (3)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content (mass%) of each element, and 0 if not contained.)
The present invention has been completed based on such findings and further completed, that is, the present invention
1. % By mass
C: 0.06 to 0.12%,
Si: 0.05-0.40%,
Mn: 0.80 to 1.20%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.005 to 0.060%,
N: 0.0040% or less,
Mo: 0.20 to 0.50%,
V: 0.020-0.080% is contained,
Further, Nb: 0.005 to 0.030%, Cu: 0.10 to 0.50%, Ni: 0.1 to 1.0%, Cr: 0.10 to 0.80%, B: 0.0003 1 or 2 or more types selected from -0.0030% satisfy the following formula (1), the following (2) and the following (3) formula, and have a composition comprising the balance Fe and inevitable impurities, The microstructure is composed of a tempered martensite phase with an area ratio of 80% or more, the nominal austenite grain size of the tempered martensite phase is 12 μm or more and 30 μm or less, and is characterized by excellent properties after warm working. A high-tensile thick steel plate with a tensile strength of 780 MPa or more.

Record
0.45 ≦ (Mo + 4.9V + 5.8Nb) ≦ 0.85 (1)
4.0 ≦ Mo / V ≦ 16.0 (2)
0.20 ≦ C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 +
Mo / 15 + V / 10 + 5B ≦ 0.27 (3)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, B: Content (mass%) of each element, but 0 is not included.
2. The high-strength thick steel plate having a tensile strength of 780 MPa or more according to 1, wherein the composition further contains Ti: 0.005 to 0.020% by mass.
3. In addition to the composition,
Ca: 0.0005 to 0.0050%,
REM: 0.001 to 0.0050% of 1 type or 2 types of high tension thick steel plates with a tensile strength of 780 MPa or more according to 1 or 2.
4). After the steel material is heated, it is hot rolled into a thick steel plate by hot rolling, an accelerated cooling step in which accelerated cooling is performed on the thick steel plate after the hot rolling step, and after the accelerated cooling step is completed. In the method of producing a high-tensile thick steel plate having a tensile strength of 780 MPa or more, which is subjected to a tempering step in which reheating and tempering is performed, the steel material has the composition according to any one of 1 to 3, and the hot rolling After the process is heated to a heating temperature of 1050 to 1200 ° C., the cumulative reduction amount at 950 ° C. or less is 30 to 60%, and the rolling finish temperature is 900 ° C. or less and the hot rolling to the Ar3 transformation point or more. After completion of hot rolling, the cooling step is accelerated cooling of 2 ° C./s or more at an average cooling rate of 700 to 500 ° C. from a temperature of Ar 3 transformation point to 400 ° C. or less, and the tempering step is a tempering temperature. : Reheated to 450-650 ° C Method of manufacturing a tensile strength of 780MPa or more high-tensile steel plate according to claim Rukoto.
5. The steel material is heated, hot-rolled, cooled to a temperature of 400 ° C. or lower, and then subjected to a reheating and quenching step and a reheating and tempering step after the reheating and quenching step. High tensile strength with a tensile strength of 780 MPa or more In the method for producing a thick steel plate,
The steel material has the composition according to any one of 1 to 3, and after the reheating and quenching step is reheated to 880 to 980 ° C, the temperature of the steel material is 700 to 500 ° C. It is a step of cooling at an average cooling rate of 2 ° C./s or more, and the reheating and tempering step is a step of reheating to a tempering temperature: 450 to 650 ° C. A high tensile strength of 780 MPa or more A method for producing a tension thick steel plate.

  According to the present invention, it is possible to stably produce a steel plate having a plate thickness of 40 mm or more and a tensile strength of 780 MPa or more, capable of suppressing deterioration of the material after warm forming, and warm-molding the obtained thick steel plate. Yield strength: 630 MPa to 900 MPa, tensile strength: 780 MPa or more, yield ratio 95% or less, fracture surface transition temperature vTrs of Charpy impact test: −40 ° C. or less, excellent toughness and excellent weldability Steel pipes can be industrially easily manufactured with little material variation, and can be stably mass-produced, and have remarkable industrial effects such as increasing the size of steel structures, improving safety, and improving construction efficiency.

Explanatory drawing which shows typically an example of the manufacturing method of the circular steel pipe by press bend (press bending).

In the present invention, the component composition and the microstructure are defined. In the description,% is mass%.
[Ingredient composition]
C: 0.06 to 0.12%
C is an element that solidifies to increase the strength of the steel and combines with carbide-forming elements such as Mo, V, and Nb to form a carbide, and contributes to an increase in the strength of the steel by precipitation strengthening. In order to secure a desired high strength as a structural steel material, in the present invention, C needs to be contained by 0.06% or more. On the other hand, if the content exceeds 0.12%, the base metal toughness and the weld heat affected zone (HAZ) toughness are remarkably reduced, and weld cracks are induced to deteriorate the weld crack resistance. For this reason, C was limited to the range of 0.06 to 0.12%. In addition, Preferably it is 0.06-0.11%.

Si: 0.05-0.40%
Si is an element that acts as a deoxidizer, and has an effect of increasing strength by solid solution strengthening. In order to ensure these effects, a content of at least 0.05% is required. On the other hand, when it contains exceeding 0.40%, base material toughness and HAZ toughness will be reduced. For this reason, Si was limited to the range of 0.05 to 0.40%. In addition, Preferably, it is 0.05 to 0.35%.

Mn: 0.80 to 1.20%
Mn is an inexpensive element having an effect of increasing the strength of steel through solid solution or through an increase in hardenability. In the present invention, Mn is required to be 0.80% or more in order to secure the desired strength (tensile strength: 780 MPa or more) by minimizing the content of other more expensive elements. On the other hand, if the content exceeds 1.20%, the concentration in the central segregation part during solidification becomes remarkable, and there is a problem of increasing slab defects. Further, a large amount of Mn exceeding 1.20% further causes a significant decrease in the base material toughness and the HAZ toughness. For this reason, Mn was limited to the range of 0.80 to 1.20%.

P: 0.015% or less P is an element that segregates at the prior γ grain boundaries and lowers the toughness of the steel, and has a particularly large adverse effect on the toughness of a steel material having a martensite phase or a bainite phase. For this reason, although it is desirable to reduce P as much as possible, if it reduces to 0.015% or less, the above-mentioned bad influence will become an acceptable range. For this reason, P was limited to 0.015% or less.

S: 0.003% or less S combines with Mn to form MnS. When the S content increases, coarse MnS elongated by hot rolling increases. When coarse MnS increases, in particular, the Charpy test absorbed energy in the plate thickness direction (Z direction) decreases, and the toughness in the plate thickness direction decreases. For this reason, it is desirable to reduce S as much as possible, but if it is reduced to 0.003% or less, such an adverse effect is to an acceptable level. For this reason, S was limited to 0.003% or less.

Al: 0.005-0.060%
Al is an element that acts as a deoxidizer, and is the most widely used element in the molten steel deoxidation process of high-tensile steel. Al also has the effect of fixing N in steel as AlN and preventing toughness reduction and cracking due to N. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.060%, the toughness of the base metal is lowered, and the toughness is lowered by mixing with the weld metal during welding. For this reason, Al was limited to 0.005 to 0.060% of range. In addition, Preferably, it is 0.010 to 0.045%.

N: 0.0040% or less N is an element that dissolves in steel and lowers the base metal toughness and the HAZ toughness. In the present invention, N is desirably reduced as much as possible. When the content exceeds 0.0040%, the above-described decrease in toughness becomes significant. For this reason, N was limited to 0.0040% or less.

Mo: 0.20 to 0.50%
Mo has an action of suppressing softening due to an increase in molding temperature during warm forming by precipitation strengthening of Mo carbide formed by combining with C in steel, and further, by generating hardenability, ferrite is generated. It is an essential element for suppressing and forming a structure mainly composed of a martensite phase. In order to obtain these effects, a content of 0.20% or more is required. On the other hand, the content exceeding 0.50% lowers the HAZ toughness and weld crack resistance. For this reason, Mo was limited to 0.20 to 0.50% of range.

V: 0.020-0.080%
V, like Nb, is an element that forms carbides and has the effect of suppressing softening due to an increase in the forming temperature during warm forming by precipitation strengthening, and is one of the important elements in the present invention. Moreover, V has the effect | action which dissolves in Mo carbide | carbonized_material and raises the stability of Mo carbide | carbonized_material and suppresses the coarsening of Mo carbide | carbonized_material during warm forming. In order to obtain such an effect, a content of 0.020% or more is required. On the other hand, if the content exceeds 0.080%, the base material toughness and the HAZ toughness are significantly reduced. For this reason, V was limited to 0.020 to 0.080% of range. In addition, Preferably, it is 0.040 to 0.060%.

  Nb: 0.005 to 0.030%, Cu: 0.10 to 0.50%, Ni: 0.1 to 1.0%, Cr: 0.10 to 0.80%, B: 0.0003 to One or two of 0.0030%. In order to obtain desired strength and toughness, one or more of Nb, Cu, Ni, Cr, and B are contained.

Nb: 0.005 to 0.030%
Nb is an element that forms fine carbides and suppresses softening due to an increase in the molding temperature during warm molding by precipitation strengthening, and is one of the important elements in the present invention. Moreover, Nb has the effect | action which suppresses recrystallization of austenite, and has the effect | action which promotes formation of a fine crystal grain by controlled rolling. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if it exceeds 0.030%, the HAZ toughness is significantly lowered. For this reason, when adding Nb, it limited to 0.005 to 0.030% of range. In addition, Preferably, it is 0.008 to 0.025%.

Cu: 0.10 to 0.50%
Cu is an element that increases the strength of steel through solid solution strengthening and hardenability improvement. In order to obtain such an effect, it is necessary to contain 0.10% or more. However, if it exceeds 0.50%, the material (alloy) cost increases and the surface properties deteriorate due to hot brittleness. Invite. For this reason, when it contains, it is preferable to limit Cu to the range of 0.10 to 0.50%.

Ni: 0.1 to 1.0%
Ni is an element that increases the strength of steel with almost no deterioration in toughness. Moreover, Ni has little adverse effect on the HAZ toughness. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, if the content is larger than 1.0%, Ni is expensive, which causes a rise in material (alloy) cost. For this reason, when it contains, it is preferable to limit Ni to 0.1 to 1.0% of range.

Cr: 0.10 to 0.80%
Cr is an element that increases the strength of steel through the improvement of hardenability. In order to obtain such an effect, it is necessary to contain 0.10% or more. However, if it contains more than 0.80%, the material (alloy) cost increases. For this reason, when contained, Cr is preferably limited to a range of 0.10 to 0.80%.

B: 0.0003 to 0.0030%
B is an element having an effect of improving the hardenability by containing a small amount and increasing the strength of the steel through the improvement of the hardenability. In addition, B forms BN in the vicinity of the weld bond portion of HAZ that is exposed to a high temperature at which TiN is dissolved, and acts as a ferrite transformation nucleus, while reducing solid solution N and improving HAZ toughness. Let In order to acquire such an effect, 0.0003% or more needs to be contained.

  On the other hand, when the content exceeds 0.0030%, the base material toughness and the HAZ toughness are lowered, and the yield strength of the base material is remarkably increased to make it difficult to ensure a desired low yield ratio. For this reason, when it contains, it is preferable to limit B to 0.0003 to 0.0030% of range. In addition, More preferably, it is 0.0007 to 0.0020%.

0.45 ≦ (Mo + 4.9V + 5.8Nb) ≦ 0.85 (1)
(Here, Mo, V, Nb: content of each element (mass%))
In the present invention, in order to compensate for the strength reduction accompanying heating to the warm forming temperature and to suppress embrittlement accompanying precipitation strengthening, the contents of Mo, V, and Nb are set within the content range of each element (1 ) Adjust to satisfy the equation.

  Mo, V, and Nb all form precipitates (carbides) and have a great influence on the strength and toughness of the steel (steel pipe) after warm forming through precipitation strengthening. By forming precipitates (carbides), an increase in strength due to precipitation strengthening can be expected, and a decrease in strength associated with heating to the warm forming temperature can be compensated. Becomes brittle.

  The precipitation strengthening ability of Mo, V, and Nb is greatest for Nb, then V, and Mo is the smallest. The total precipitation strengthening ability of each element (Mo + 4.9V + 5.8Nb) is less than 0.45, but the amount of precipitates is not sufficient, the precipitation strengthening is insufficient, and the strength decreases with an increase in warm forming temperature. Too big.

  On the other hand, when (Mo + 4.9V + 5.8Nb) exceeds 0.85, the amount of precipitates becomes excessive, precipitation strengthening becomes too large and becomes brittle, and the base material toughness decreases and the yield ratio increases remarkably. Therefore, (Mo + 4.9V + 5.8Nb) was limited to the range of 0.45-0.85. In addition, Preferably, it is 0.55-0.77.

4.0 ≦ Mo / V ≦ 16.0 (2)
(Where Mo, V: content of each element (mass%))
Further, in the present invention, Mo and V are adjusted so as to satisfy the expression (2). By dissolving an appropriate amount of V in the Mo carbide, the stability of the Mo carbide can be improved, and the coarsening of the Mo carbide during warm forming can be suppressed, which is accompanied by heating to the warm forming temperature. It is possible to stably compensate for the strength reduction and to suppress the embrittlement of the steel material accompanying a great amount of precipitation strengthening.

  The V concentration in the Mo carbide depends on the Mo / V ratio, Mo / V. When Mo / V exceeds 16.0, the V concentration in the Mo carbide is too small and the above-described effect cannot be expected. On the other hand, if the Mo / V is less than 4.0, the V concentration in the Mo carbide becomes excessive, and embrittlement accompanying excessive precipitation strengthening increases. For this reason, Mo / V was limited to the range of 4.0-16.0. In addition, Preferably, it is the range of 5.0-12.0.

0.20 ≦ C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ≦ 0.27 (3)
(Here, each element is a content (mass%), and the one not contained is 0.)
If the weld cracking susceptibility composition (Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B) is less than 0.20, the hardenability is insufficient and a martensite-based structure cannot be obtained, resulting in insufficient strength. Or toughness decreases.

On the other hand, if it exceeds 0.27, the weldability is reduced, preheating at a temperature exceeding 50 ° C. is essential, and the welding efficiency is remarkably reduced. For this reason, it limited to the range of 0.20 or more and 0.27 or less.
The above is the balance Fe and inevitable impurities in the basic component composition of the present invention. When further improving the characteristics, the selected element is selected from Ti: 0.005 to 0.020%, Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050% It can contain seeds or two or more.

Ti: 0.005-0.020%
Ti is an element that contributes to improving the toughness of HAZ, and can be contained as necessary. Ti has a strong affinity with N and precipitates as TiN during solidification. The finely precipitated TiN particularly suppresses the coarsening of austenite grains in the HAZ and contributes to increasing the toughness of the HAZ as a ferrite transformation nucleus. In order to obtain such an effect, 0.005% or more of Ti is required. On the other hand, the content exceeding 0.020% leads to coarsening of TiN particles and impairs the precipitation strengthening ability of Nb by dissolving Nb in TiN. For this reason, when it contains, it is preferable to limit Ti to 0.005 to 0.020% of range. In addition, More preferably, it is 0.008 to 0.015%.

Ca: 0.0005-0.0050%, REM: One or two kinds selected from 0.0010-0.0050% Ca and REM are both of the base material through the form control of sulfide. It is an element that contributes to the improvement of toughness and ductility. Also, when fine sulfide particles are dispersed in steel, it acts as a ferrite transformation nucleus and contributes to the improvement of HAZ toughness. It can be selected according to the content. In order to exert these effects, it is necessary to contain at least 0.0005% for Ca and at least 0.0010% for REM, but if both contain more than 0.0050%, inclusions are generated. And toughness may be reduced. For this reason, when it contains, it is preferable to limit Ca to 0.0005 to 0.0050% and REM to 0.0010 to 0.0050%.

[Microstructure]
In the present invention, the microstructure is a tempered martensite phase with an area ratio of 80% or more and a nominal grain size of prior austenite grains of 12 μm or more and 30 μm or less.

  In order to ensure a tensile strength of 780 MPa or more, the tempered martensite phase has a structure with an area ratio of 80% or more. Since the tempered martensite phase is composed of fine substructures such as laths and blocks, excellent toughness can be obtained, and precipitation and growth of various carbides and cementite have progressed to some extent by tempering, so warm forming Small change in structure with temperature.

  A tempered martensite phase may be a 100% single phase (tempered martensite single phase). When it is not a single phase, examples of the second phase other than the main phase include ferrite, pearlite, and bainite. The total content of the second phase is 20% or less in terms of area ratio, in order to ensure predetermined toughness, ductility, and strength.

  During warm forming, recrystallized grains are easily generated from the prior austenite grain boundaries, and precipitates on the prior austenite grain boundaries are likely to be coarsened. For this reason, the more the prior austenite grain boundaries exist, that is, the smaller the prior austenite grain size, the easier the material changes.

  On the other hand, if the prior austenite particle size is too large, the toughness is lowered. For this reason, in this invention, the nominal particle diameter of the prior austenite grain boundary was limited to the range of 12-30 micrometers. The method for observing the microstructure will be described in Examples.

[Production conditions]
The slab heating temperature, hot rolling conditions, accelerated cooling conditions, and tempering temperature in the preferred method for producing a thick steel plate according to the present invention are as follows. After hot rolling, reheating quenching and tempering may be performed. Unless otherwise specified, the temperature and cooling rate are average values in the thickness direction.

Slab heating temperature: 1050 to 1200 ° C
The slab (sometimes referred to as a steel material) is reheated to a heating temperature of 1050 to 1200 ° C. and then subjected to a hot rolling process. When the heating temperature is less than 1050 ° C., precipitates (carbides) forming elements such as V and Nb are not sufficiently dissolved, and the effects of these elements may not be sufficiently exhibited, and deformation resistance increases and rolling is performed. The load on the machine increases.

  On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grains become coarse during heating, and the microstructure after rolling becomes coarse, so that the base material toughness decreases. For this reason, the heating temperature of the steel material is preferably in the range of 1050 to 1200 ° C. The method for producing the steel material is not particularly limited, but a slab having the above-described composition is melted by a conventional melting method such as a converter, and a conventional casting method such as a continuous casting method is performed.

Hot rolling When performing accelerated cooling after hot rolling, the steel material heated to 1050 to 1200 ° C has a cumulative reduction amount of 30 to 60% at 950 ° C or lower, and the rolling end temperature: 900 ° C or lower Ar ₃ It is preferable to perform hot rolling at a transformation point or higher.

  In the present invention, controlled rolling is performed at 950 ° C. or lower in order to appropriately refine the microstructure. If the cumulative reduction at 950 ° C. or less is less than 30%, the effect of controlled rolling is not sufficient, the structure becomes coarse and the toughness decreases, or the hardenability increases more than necessary, and the surface layer is excessively hardened. is there.

  On the other hand, if the cumulative reduction amount at 950 ° C. or less exceeds 60%, the martensite packet block is remarkably miniaturized, and there are excessive large-angle boundaries that promote material changes due to warm working. For this reason, it is preferable to limit the cumulative reduction amount at 950 ° C. or less to a range of 30 to 60%.

When the rolling end temperature exceeds 900 ° C. and becomes high, the structure becomes coarse and the toughness is lowered, or the hardenability increases more than necessary, and the surface layer may be hardened excessively. On the other hand, if the rolling end temperature is less than the Ar ₃ transformation point, ferrite may be generated during rolling or immediately after rolling, resulting in coarsening and reduced toughness. For this reason, it is preferable to limit the rolling end temperature to 900 ° C. or lower and the Ar ₃ transformation point or higher.

As the Ar ₃ transformation point, a value calculated by the following equation is used.
Ar ₃ (° C.) = 900-332C + 6Si-77Mn-20Cu-50Ni-18Cr-68Mo
(Here, C, Si, Mn, Cu, Ni, Cr, Mo: content (mass%) of each element, and elements not contained are 0.)
Accelerated cooling Accelerated cooling is an average cooling rate of 700 to 500 ° C. (hereinafter referred to as average cooling rate) from 2 ° C./s or higher from the temperature above the Ar ₃ transformation point to a temperature of 400 ° C. or lower after the end of hot rolling. It is preferable to cool. When the cooling rate of accelerated cooling is less than 2 ° C./s at an average cooling rate of 700 to 500 ° C., a large amount of ferrite precipitates, so that a microstructure with martensite as the main phase (80% or more in area ratio) is obtained. It becomes difficult. The upper limit of the cooling rate for accelerated cooling need not be specified.

  When the cooling stop temperature of accelerated cooling exceeds 400 ° C., ferrite, pearlite, and bainite are generated, and it becomes difficult to secure a microstructure having martensite as a main phase. For this reason, it is preferable to limit the cooling stop temperature of accelerated cooling to 400 ° C. or lower. It is not necessary to specify the lower limit of the cooling stop temperature for accelerated cooling.

Tempering After accelerated cooling, the balance between strength and toughness is further adjusted, and tempering is performed at 450 to 650 ° C. in order to promote precipitation and growth of various carbides and cementite.

  When the tempering temperature is less than 450 ° C., it is impossible to secure a desired tempering effect that promotes precipitation and growth of various carbides and cementite before warm working and suppresses a change in structure during warm working. On the other hand, if the temperature exceeds 650 ° C., the precipitates become coarse and the strength decreases, so that the effect of increasing the strength due to Mo, V, and Nb cannot be ensured. For this reason, a tempering temperature shall be 450-650 degreeC.

  In the present invention, instead of accelerated cooling after hot rolling, hot rolling, cooling to a temperature of 400 ° C. or lower, reheating to 880 to 980 ° C., and then quenching, reheating quenching is performed. May be. In this case, the cooling conditions after rolling are not particularly limited, and may be air cooling or accelerated cooling. When the quenching temperature is less than 880 ° C., the austenite grain size is fine and the hardenability is insufficient, and the prior austenite grain size in the microstructure is less than 12 μm. On the other hand, when the quenching temperature exceeds 980 ° C., the prior austenite grain size exceeds 30 μm, and the toughness decreases, so the quenching temperature is preferably 880 to 980 ° C. After reheating and quenching, tempering is performed under the same conditions as in accelerated cooling. In hot rolling in the case of performing reheating and quenching, a desired plate thickness may be obtained, and controlled rolling in the case of accelerated cooling is not necessary. However, slab heating temperature shall be 1050-1200 degreeC. Hereinafter, based on an Example, this invention is demonstrated further.

  A steel material (slab) having the composition shown in Table 1 is subjected to hot rolling and accelerated cooling under the conditions shown in Table 2, and some steel plates are tempered to produce a steel plate having a thickness of 60 mm. And used as test steel. Some steel plates were subjected to reheating quenching and tempering after hot rolling.

Test pieces were sampled from the obtained thick steel plates and subjected to structure observation, tensile test and Charpy impact test by the methods described below. In addition, the mechanical properties after warm forming were investigated.
(1) Microstructure observation A specimen for microstructural observation is collected from the test steel, the cross section in the rolling direction (L cross section) is polished, corroded with a nital liquid, an optical microscope (magnification: 400 times), and a scanning electron microscope. The tissue was observed and imaged at (magnification: 2000 times), and the type and fraction of the tissue were measured using an image analyzer. Further, prior austenite grain boundaries were revealed by corrosion using a saturated aqueous picric acid solution, and the nominal particle diameter (square root of the average area of the region) was determined by image analysis.

(2) Tensile test Thickness of the test steel: JIS No. 4 tensile test piece (round bar test piece) was collected from the 1 / 4t position so that the length direction coincided with the rolling direction, and specified in JIS Z 2241. The tensile properties (yield strength YS, tensile strength TS, elongation El, yield ratio YR) were determined by conducting a tensile test.
(3) Charpy impact test V-notch specimens were taken from the 1/4 position of the steel thickness (t) of the test steel with the length direction as the rolling direction, and the Charpy impact test was conducted in accordance with the provisions of JISZ2242. The fracture surface transition temperature vTrs (° C.) was determined.
(4) Tensile test and impact test after warm working A test material for bending (size: rolling direction 1000 × width direction 1500 mm) was sampled from the test steel. The obtained test material was heated to heating temperatures of 400 ° C., 500 ° C., and 550 ° C., and then subjected to warm press bending so that the bending direction was perpendicular to the rolling direction.

  The bending radius R was R = 500 mm. After bending, JIS No. 4 round bar tensile test specimen and V-notch test specimen were collected from the 1/4 position of the thickness (t) on the outer surface side of the processed part so that the test specimen length direction coincided with the rolling direction. Then, a tensile test was conducted in accordance with the provisions of JIS Z 2241, and a Charpy impact test was conducted in accordance with the provisions of JIS Z 2242, and tensile properties (yield strength YS, tensile strength TS, elongation El) , Yield ratio YR) and fracture surface transition temperature vTrs (° C.) were determined, and warm formability was evaluated.

  Table 3 shows the test results. After warm working at 400 to 550 ° C., yield strength: 630 MPa or more and 900 MPa or less, tensile strength: 780 MPa or more, yield ratio 95% or less, fracture surface transition temperature vTrs of Charpy impact test: −40 ° C. or less Within the scope of the invention. All the steel plates (No. 1-19, No. 30-32) satisfying the provisions of the present invention have a yield strength of 630 MPa to 900 MPa after bending at 400 ° C., 500 ° C., and 550 ° C., Tensile strength: 780 MPa or more, yield ratio 95% or less, fracture surface transition temperature vTrs of Charpy impact test: −40 ° C. or less were obtained.

  On the other hand, among the comparative examples, No. In Nos. 20 to 24, the component composition is within the scope of the present invention, but the microstructure is outside the scope of the present invention. Nos. 26 to 29 were inferior in any of the properties after bending at 400 ° C., 500 ° C., and 550 ° C. because the component composition was outside the scope of the present invention.

1 Steel plate 2 Press die

Claims (5)

% By mass
C: 0.06 to 0.12%,
Si: 0.05-0.40%,
Mn: 0.80 to 1.20%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.005 to 0.060%,
N: 0.0040% or less,
Mo: 0.20 to 0.50%,
V: 0.020-0.080% is contained,
Further, Nb: 0.005 to 0.030%, Cu: 0.10 to 0.50%, Ni: 0.1 to 1.0%, Cr: 0.10 to 0.80%, B: 0.0003 1 or 2 or more types selected from -0.0030% satisfy the following formula (1), the following (2) and the following (3) formula, and have a composition comprising the balance Fe and inevitable impurities, The microstructure is composed of a tempered martensite phase with an area ratio of 80% or more, the nominal austenite grain size of the tempered martensite phase is 12 μm or more and 30 μm or less, and is characterized by excellent properties after warm working. A high-tensile thick steel plate with a tensile strength of 780 MPa or more.
Record
0.45 ≦ (Mo + 4.9V + 5.8Nb) ≦ 0.85 (1)
4.0 ≦ Mo / V ≦ 16.0 (2)
0.20 ≦ C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 +
Mo / 15 + V / 10 + 5B ≦ 0.27 (3)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, B: Content (mass%) of each element, but 0 is not included.   The high-strength thick steel plate having a tensile strength of 780 MPa or more according to claim 1, wherein the composition further contains Ti: 0.005 to 0.020% by mass. In addition to the composition,
Ca: 0.0005 to 0.0050%,
REM: 0.0010-0.0050% of 1 type or 2 types is contained, The high strength thick steel plate of tensile strength 780 Mpa or more of Claim 1 or 2 characterized by the above-mentioned.   After the steel material is heated, it is hot rolled into a thick steel plate by hot rolling, an accelerated cooling step in which accelerated cooling is performed on the thick steel plate after the hot rolling step, and after the accelerated cooling step is completed. In the method for producing a high-tensile thick steel plate having a tensile strength of 780 MPa or more, which is subjected to a tempering step in which reheating and tempering is performed, the steel material has the composition according to any one of claims 1 to 3, and the heat After the hot rolling process is heated to a heating temperature of 1050 to 1200 ° C., the cumulative rolling amount at 950 ° C. or lower is 30 to 60%, and the rolling end temperature is 900 ° C. or lower and Ar 3 transformation point or higher. The accelerated cooling step is an accelerated cooling of 2 ° C./s or more at an average cooling rate of 700 to 500 ° C. from a temperature of Ar 3 transformation point or higher to a temperature of 400 ° C. or lower after completion of hot rolling, and the tempering step is Tempering temperature: 450-650 ° C Method of manufacturing a tensile strength of 780MPa or more high-tensile steel plates, characterized in that the heating. The steel material is heated, hot-rolled, cooled to a temperature of 400 ° C. or lower, and then subjected to a reheating and quenching step and a reheating and tempering step after the reheating and quenching step. High tensile strength with a tensile strength of 780 MPa or more In the method for producing a thick steel plate,
The steel material has the composition according to any one of claims 1 to 3, and after the reheating and quenching step is reheated to 880 to 980 ° C, the temperature of the steel material is 700 to 500 ° C. A tensile strength of 780 MPa or more, characterized in that it is a step of cooling at 2 ° C./s or more at an average cooling rate of ° C., and the reheating and tempering step is a step of reheating to a tempering temperature: 450 to 650 ° C. Manufacturing method for high-tensile thick steel plates.

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