JP2001200334A - 60 kilo class high tensile strength steel excellent in weldability and toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce thick 60 kilo class steel excellent in low temperature toughness and weldability and to provide a method for producing the same. SOLUTION: This 60 kilo class high tensile strength steel excellent in weldability an toughness satisfies, by mass, P cm<=0.2% and Ceq (WES)<=0.42%, preferably has components containing, by mass, 0.04 to 0.09% C, 0.1 to 0.5% Si, 1.2 to 1.8% Mn, 0.1 to 0.5% Cr, 0.01 to 0.05% Nb, 0.002 to 0.07% sol.Al and 0.001 to 0.005% N, also satisfying 0.50<=Si+3Cr<=1.25%, and the balance iron with inevitable impurities as a steel composition and has a structure essentially consisting of bainite with a bainitic lath width of 2 μm or less, and in which filmy ferrite with a thickness of 0.1 to 5 μm is precipitated on the old austenitic grain boundaries in an occupancy ratio of 20% or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 60-kg class welded structural steel used for construction, bridges, penstocks, low-temperature storage tanks, pressure vessels, line pipes, ships and marine structures, and particularly to low-temperature toughness. The present invention relates to a method for producing an excellent steel material.

[0002]

2. Description of the Related Art When large-scale structures such as buildings, bridges, penstocks, low-temperature storage tanks, pressure vessels, line pipes, ships, and marine structures undergo brittle fracture, the impact on the economy and the environment is great. A high degree of safety is required.
For this reason, low-temperature toughness is often required for steel materials used in these structures, and the required level is becoming more stringent year after year due to development in extremely cold regions, enlargement of structures, and increase in reliability requirements. ing. In addition, there is an increasing demand for thick-walled materials in which it is relatively difficult to ensure low-temperature toughness.

As means for improving the low temperature toughness of steel,
Conventionally, a method of increasing the Ni content has been known,
9% for liquefied natural gas (LNG) storage tanks
Ni steel is used on a commercial scale. However, since an increase in the amount of Ni greatly increases costs, it is difficult to add a large amount of Ni to uses other than the LNG storage tank.

[0004] On the other hand, from the viewpoint of welding workability, it is required to lower the preheating temperature for preventing low-temperature cracking, and in recent years, demand for preheating-free steel materials has been rapidly increasing. To lower the preheating temperature, Ceq (WES), P
cm needs to be reduced, and reducing C is most effective.

[0005] In particular, when C is set to 0.09% or less, preheating free is realized. However, since the decrease in C lowers the strength, it is necessary to add an alloying element. However, when an element effective for improving toughness such as Ni is not added due to cost constraints, the upper bainite structure, that is, the bainite lath And a structure in which cementite is precipitated, and it is extremely difficult to obtain excellent low-temperature toughness.

As a method for solving such a problem, Japanese Patent Application Laid-Open No. H10-158778 discloses a 570 MPa class thick high-tensile steel sheet which has been toughened by thermomechanical treatment and a method for manufacturing the same.

In the present technology, the ferrite volume ratio of a steel sheet is set to 1
By setting the content to 0 to 40%, it is intended to divide the structure and improve the toughness. However, when the ferrite volume ratio is increased, it is difficult to secure the yield strength, and it is difficult for the thick steel to satisfy the yield strength of 440 MPa or more required for a tensile strength steel of 60 kg.

[0008] In addition, the manufacturing method is to apply a strong pressure in the non-recrystallization temperature range of austenite, which causes anisotropy in the steel sheet, and may cause defective inspection in an ultrasonic inspection test of a welded portion.

[0009]

As described above, a technique for imparting excellent low-temperature toughness to a 60 kg-class thick steel material excellent in welding workability has not yet been completed. An object of the present invention is to provide a 60-kg class high-strength steel having excellent welding workability and excellent low-temperature toughness, and a method for producing the same. Specifically, an object of the present invention is to satisfy the following performance. Yield strength: 440MPa or more, tensile strength: 570-610M
Pa class, vTs at the 1/4 position in the thickness direction: -70 ° C or less, Preheating during welding: unnecessary.

[0010]

Means for Solving the Problems The inventors of the present invention have made intensive studies to achieve the above object, and have achieved both excellent welding workability and low-temperature toughness without adding a large amount of expensive alloying elements. The steel and its manufacturing method have been completed. In other words, with the addition of a small amount of alloying elements, the basic policy is to optimize the metallographic structure in order to improve the low-temperature toughness without lowering the strength, and to study at the optical microscope level, and further, at the electron microscope level. Was obtained.

In a structure mainly composed of upper bainite, which is indispensable for securing a strength of 60 kg class, crack generation or propagation which governs toughness occurs preferentially at the former austenite grain boundary. In order to prevent this, it is effective to precipitate ferrite at the grain boundary and set a point where cracks are generated or propagated as a bainite packet boundary.

However, when the volume fraction of ferrite is too large, the strength is reduced, and the yield strength and tensile strength of a 60-kg class steel cannot be satisfied. At least, a microstructure that improves low-temperature toughness is desirable, and a film-like ferrite is precipitated on the prior austenite grain boundaries, and the bainite phase is refined to a bainite lath interval of a certain value or less, and the strengthened structure has the lowest strength. And is effective in improving low-temperature toughness.

The present invention has been made by further study based on these findings.

1. In mass%, Pcm ≦ 0.2%, Ce
q (WES) ≦ 0.42%, bainite having a bainite lath width of 2 μm or less as a main component, and a film-like ferrite having a thickness of 0.1 to 5 μm on an old austenite grain boundary.
A high-strength steel of 60 kg class having excellent weldability and toughness, characterized by having a structure precipitated at an occupancy of at least%.

However, Pcm = C + Mn / 20 + Si / 3
0 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5B, Ceq (WES) = C + Mn / 6 +
Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 1
4 2. As steel composition, in mass%, C: 0.04 to 0.0
9%, Si: 0.1 to 0.5%, Mn: 1.2 to 1.8
%, Cr: 0.1 to 0.5%, Nb: 0.01 to 0.0
5%, Sol. Al: 0.002 to 0.07%, N:
0.001 to 0.005%, and 0.50 ≦ S
60 kg high strength steel excellent in weldability and toughness according to 1 above, wherein the balance satisfying i + 3Cr ≦ 1.25% is substantially composed of iron and inevitable impurities.

3. As a steel composition, M
3. A 60-kilometer high-strength steel excellent in weldability and toughness according to 2, which contains one or two kinds of o: 0.02 to 0.3% and Cu: 0.1 to 0.6%.

4. As a steel composition, in mass%, N
i: 60 kg class high strength steel excellent in weldability and toughness according to 2 or 3 containing 0.1 to 0.5%.

5. As a steel composition, V:
The high strength steel of 60 kg class having excellent weldability and toughness according to any one of 2 to 4, containing 0.01 to 0.08%.

6. As steel composition, T
i: 0.005 to 0.02%, Ca: 0.001 to 0.
6. A 60 kg high strength steel excellent in weldability and toughness according to any one of 2 to 5 containing one or two kinds of 004%.

[7] FIG. As a steel composition, B: 0.
The high strength steel of 60 kg class excellent in weldability and toughness according to any of items 2 to 6, wherein the steel content is not more than 0002%.

8. As a steel composition, P: 0.
The high strength steel of 60 kg class excellent in weldability and toughness according to any one of 2 to 7, characterized in that the content is 0.01% or less and S: 0.002% or less.

9. A method for producing a 60 kg high strength steel excellent in weldability and toughness, comprising the following steps.

(1) a step of heating a slab having the steel composition according to any of 2 to 8 below 1150 ° C .;
(2) Id / hm ≧ 1 in a temperature range of 900 to 1000 ° C.
0: one or more passes, where ld: projected contact arc length ld = (R · (hi−ho)) / 2 hm: average plate thickness hm = (hi + 2ho) / 3 R: roll radius, hi : Sheet thickness before rolling, ho: Sheet thickness after rolling (3) Cumulative rolling reduction 1 in the temperature range of Ar3 or more and less than 900 ° C
A step of rolling at 0 to 50%, and (4) a steel material surface temperature of A
Step of starting cooling from r3 or more, performing cooling such that the cooling rate of the steel material surface becomes 40 ° C / sec or more in a temperature range where the steel material surface temperature is 200 ° C or more, and stopping cooling when the steel material average temperature is 500 ° C or less. .

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the reasons for limiting the microstructure, component composition, and manufacturing conditions in the present invention will be described.

1. Microstructure The microstructure of the steel of the present invention is a bainite-based structure in order to secure a strength of 60 kg class, and further defines the bainite lath width and the precipitation state of ferrite on the former austenite grain boundary from the viewpoint of strength and toughness. . In the present invention, the structure mainly composed of bainite may contain a part of ferrite, pearlite, etc. within a range that does not lose its function and effect, and bainite means bainite mainly composed of upper bainite. I do.

Bainite lath width: 2 μm or less When ferrite is precipitated at grain boundaries in order to improve low-temperature toughness, strength decreases. Despite the fact that the amount of alloy addition was reduced to eliminate the need for preheating during welding work, due to cost constraints, if an expensive alloy element was not added, the strength of a 60-kilometer thick steel material could not be satisfied. Occurs.

Therefore, in the present invention, the bainite lath width, which affects the strength of the bainite phase, is adjusted to optimize the bainite lower structure. As the bainite lath width increases, the strength of the bainite decreases. If the lath width exceeds 2 μm, the thickness of a thick-walled material is insufficient for a 60-kilometer steel, and is set to 2 μm or less. It was confirmed that the size of the lath did not affect the toughness.

Grain-boundary ferrite: Precipitate in the form of a film having a thickness of 0.1 to 5 μm on the former austenite grain boundary at an occupation ratio of 20% or more. Of ferrite. Ferrite precipitated in prior austenite grains is also effective in increasing the toughness of bainite, but since the effect is smaller than on the grain boundaries, the ferrite precipitation sites are located on the grain boundaries.

Further, since precipitation of polygonal ferrite causes a large decrease in strength, the precipitation form of ferrite is in the form of a film. When the thickness of the ferrite is more than 5 μm, the decrease in strength is large. Since the effect of toughening is not clear, the thickness is set to 0.1 to 5 μm. Furthermore, if the occupation ratio of ferrite on the former austenite grain boundary is less than 20%, the effect of toughening is not clear, so that it is set to 20% or more.

The grain boundary occupancy is determined by observing a cross section parallel to the sheet thickness direction with an optical microscope or a scanning electron microscope, on the former austenite grain boundary relative to the total length of the former austenite grain boundary included in a certain area. Is the ratio of the sum of the lengths of the film-like ferrites present in the film.

2. Component Composition In the present invention, the strength as a 60-kilometer steel is ensured by the microstructure described above. However, in order to impart necessary properties such as weldability to structural steel, the following limitations are imposed on the component composition.

Pcm: 0.2% or less, Ceq (WE
S): 0.42% or less Pcm is weld cold cracking property, Ceq (WES) is an index of toughness of the weld heat affected zone, and when Pcm exceeds 0.2%,
Welding without preheating may cause low temperature cracking.
When eq (WES) exceeds 0.42%, the heat-affected zone toughness of large heat input welding is significantly deteriorated.
2% or less, 0.42% or less.

Further, the preferred specific composition of the steel of the present invention is as follows.

C: 0.04% or more and 0.09% or less C is added to secure a predetermined strength. If it is less than 0.04%, it is difficult to secure a tensile strength of 60 kg class in the case of a thick material, and if it exceeds 0.09%, welding workability is impaired. % Or less.

Si: 0.1% or more and 0.5% or less Si is a strong ferrite-forming element, and is added to form a film-like ferrite at a prior austenite grain boundary. If less than 0.1%, the effect is not enough, and 0.5%
If the content exceeds 0.1%, the effect is saturated, and the toughness of the weld heat affected zone is significantly deteriorated.

Mn: 1.2% or more and 1.8% or less Mn is added to secure a predetermined strength. 1.2%
If the thickness is less than 60%, it is difficult to secure a tensile strength of 60 kg in the case of a thick material, and if it exceeds 1.8%, the toughness of the heat affected zone is significantly deteriorated. % Or less.

P: 0.010% or less, S: 0.002%
Hereafter, P and S are impurities, and P is 0.010% in order to reduce central segregation and improve toughness and weldability at the center of the sheet thickness.
Hereinafter, S is set to 0.002% or less.

Cr: 0.1% or more and 0.5% or less Cr is a strong ferrite-forming element and is added to form a film-like ferrite at a prior austenite grain boundary.
If it is less than 0.1%, the effect is insufficient, and if it exceeds 0.5%, hardenability is remarkably increased, and it becomes difficult to form film-like ferrite. I do.

Nb: 0.01% or more and 0.05% or less Nb suppresses austenite recrystallization during rolling, activates austenite grain boundaries, and facilitates formation of film ferrite. Further, Nb is added as it precipitates as Nb carbonitride and is effective in increasing the strength. If it is less than 0.01%, these effects are insufficient, and if it exceeds 0.05%, the toughness deteriorates due to remarkable precipitation strengthening of Nb carbonitride.
% Or more and 0.05% or less.

Sol. Al: 0.002% or more, 0.0
7% or less Al is added for deoxidation. sol. Al content is 0.0
If the content is less than 02%, the effect is not sufficient, and if the content exceeds 0.07%, the surface flaw of the steel material is apt to occur, so the content is added 0.002% or more and 0.07% or less.

N: not less than 0.001% and not more than 0.005% N is combined with Al or Ti at the time of rolling and heating, and AlN,
Generates TiN and refines austenite. 0.
If it is less than 001%, the effect is not sufficient, and 0.005%
If the content exceeds 0.001%, the toughness deteriorates.
At least 0.005%.

B: 0.0002% or less B is treated as an impurity element in the present invention. At the start of cooling, if it is present as solid solution B, the formation of film-like ferrite at the prior austenite grain boundaries is suppressed.

Si + 3Cr: 0.50% or more, 1.25
% Or less This parameter is for forming a film-like ferrite at the former austenite grain boundary of the steel composed of the above component range. When Si + 3Cr is less than 0.50%, the effect is not sufficient, and %, The formation of film-like ferrite is suppressed due to excessive hardenability.
0% or more and 1.25% or less.

Although the basic component composition is as described above, the object of the present invention can be sufficiently achieved. However, when the characteristics are further improved, Mo, Cu, Ni, V, Ti, and Ca can be added.

Mo: 0.02% or more, 0.3% or less, C
u: One or more of 0.1% or more and 0.6% or less Mo is added because it improves the strength and is particularly effective for a thick material. If the content is less than 0.02%, the effect is not sufficient.
If it exceeds 3%, the weldability and the toughness of the heat affected zone are significantly deteriorated.

Cu is added to improve the strength.
If the content is less than 0.1%, the effect is not sufficient, and if the content is more than 0.6%, the possibility of Cu cracking increases.
At least 0.6%.

Ni: 0.1% or more and 0.5% or less Ni is added to improve toughness. If it is less than 0.1%, the effect is not sufficient, and if it exceeds 0.5%, the cost of steel material increases significantly, so it is made 0.1% or more and 0.5% or less.

V: 0.01% or more and 0.08% or less V precipitates as carbide and is added to improve the strength. If it is less than 0.01%, the effect is not sufficient, and
If it exceeds 8%, the toughness is deteriorated due to remarkable precipitation strengthening of V carbide, so that the content is made 0.01% or more and 0.08% or less.

One or two types of Ti: 0.005% or more and 0.02% or less, Ca: 0.001% or more and 0.004% or less Ti, Ca is the base material toughness and the toughness of the weld heat affected zone. Is added to improve the content. Ti generates TiN at the time of rolling heating or welding and refines austenite grain size. However, if the content is less than 0.005%, the effect is not sufficient. On the other hand, if added over 0.02%, TiNb is generated at the time of rolling. The precipitation of the complex carbide of Nb and the precipitation amount of Nb carbonitride becomes insufficient and the strength is reduced.
0.02% or less.

Ca is present in the steel as Ca sulfate oxide,
Austenite grain size is reduced during rolling or welding. If less than 0.001%, the effect is not enough,
If added in excess of 0.004%, the cleanliness will be significantly degraded by a large amount of Ca sulphate, so the content is made 0.001% or more and 0.004% or less.

3. Manufacturing Conditions The microstructure of the present invention is achieved, for example, by a manufacturing method including the following steps. Each step will be described.

Step of heating the slab to 1150 ° C. or less If the slab heating temperature exceeds 1150 ° C., the austenite crystal grains are rapidly coarsened, and it becomes difficult to reduce the grain size by subsequent rolling, and the low-temperature toughness deteriorates. Therefore, the temperature is set to 1150 ° C. or less.

Ld / hm in the temperature range of 900 to 1000 ° C.
Step of performing rolling of ≧ 1.0 for one or more passes, where ld: projected contact arc length ld = (R · (hi−h)
o)) / 2 hm: average sheet thickness hm = (hi + 2ho) / 3 R: roll radius, hi: sheet thickness before rolling, ho: sheet thickness after rolling In order to refine the austenite grain size and improve toughness, Nb-containing steel has a low recrystallization temperature range of 90.
At 0 to 1000 ° C., rolling satisfying ld / hm ≧ 1.0 is performed for one or more passes, and a working strain is effectively introduced to a central portion of the sheet thickness in a temperature range in which recovery is quick.

If the rolling temperature is less than 900 ° C., recrystallization is not sufficient, and if it exceeds 1000 ° C., the recrystallized grains become large. ld / hm is 1.
When the value is less than 0, a processing strain sufficient to excite recrystallization is not applied to the central portion of the sheet thickness, so that the value is set to 1.0 or more.

In order to improve the process toughness of rolling at a cumulative rolling reduction of 10 to 50% in a temperature range of Ar3 or more and less than 900 ° C., work strain is accumulated by cumulative rolling in a non-recrystallization temperature range of Nb-containing steel, and ferrite transformation is performed. To promote. If the rolling temperature is less than Ar3, the hardenability decreases due to the progress of ferrite transformation at the start of cooling, and the predetermined strength cannot be obtained.
If the temperature is higher than 00 ° C., the processing strain is not accumulated due to recrystallization, and the ferrite transformation becomes insufficient.

If the cumulative rolling reduction is less than 10%, the ferrite transformation is not promoted, and if it exceeds 50%, the effect is saturated and the anisotropy of the steel material sharply increases.
And

Cooling is started when the surface temperature of the steel material is Ar3 or higher, and cooling is performed so that the cooling rate of the surface of the steel material becomes 40 ° C / second or more in a temperature range where the steel material surface temperature is 200 ° C or more. A step of stopping cooling in the following. For cooling after rolling, the steel material surface temperature is set to Ar3 or higher and the structure at the start of cooling is a single austenitic phase in order to secure hardenability to obtain predetermined strength and toughness. Ar3 is, for example, Ar3 = 910-310C-80Mn-20Cu-
It is determined as 15Cr-55Ni-80Mo.

The cooling rate is set to 40 ° C./sec or more in order to obtain a bainite lath width at which a predetermined strength can be obtained. The cooling rate is obtained by dividing the difference between the steel surface temperature at the start of cooling and the surface temperature at the time of cooling stop by the cooling time as an industrial measurement method. Since the temperature difference becomes small and the apparent cooling rate sharply decreases, the steel material surface temperature is set to a temperature range of 200 ° C. from the start of cooling.

When the cooling stop is higher than 200 ° C., the temperature range is from the start of cooling to the temperature range of the cooling stop. After the cooling is stopped, the cooling is stopped at an average steel material temperature of 500 ° C. or less in order to transform into a bainite-based structure having a predetermined strength. In this case, the average temperature of the steel material can be approximated by the reheat temperature of the surface of the steel material since the temperature of the steel material after cooling is stopped is substantially equal to the temperature after the surface of the steel material is regained by the latent heat inside the steel material.

[0060]

EXAMPLES Table 1 shows the components of the test steels used in the examples (the remainder not shown consists essentially of Fe and unavoidable impurities). After heating the slabs having these chemical components, they were rolled into steel plates having a thickness of 30 to 100 mm and cooled. Table 2 shows the manufacturing conditions, and Table 3 shows the properties of the steel sheet.

The microstructure was observed with an optical microscope, a scanning electron microscope, and a transmission electron microscope, and the bainite lath width and the state of precipitation of ferrite were measured. Strength and toughness were determined as mechanical properties. The tensile test was a test using a JIS G14 (14Φ) test piece sampled from a 1/4 sheet thickness, and the impact test was a 2 mm V notch sampled so that the longitudinal direction was perpendicular to the rolling direction from a 1/4 sheet thickness. The test was performed using a Charpy impact test piece (JIS No. 4 standard test piece).

Hereinafter, embodiments will be described in detail. Steel types A to H in Table 1 satisfy the component composition described in any of claims 1 to 8, and steel types I and J are Ceq (WES), P
cm is out of the range of the present invention, and steel types K and L are Si + 3C
r is a comparative example outside the scope of the invention. Steel numbers 1 to 13 in Tables 2 and 3 are production examples using steel types A to H, and are examples of the invention according to any one of claims 1 to 11. Good low temperature toughness is obtained at a vTs of -70 ° C or less.

Steel No. 14 has a high heating temperature, and Steel No. 15 has a heating temperature of 9
Since the rolling at 00 to 1000 ° C. was omitted, vTs exceeded −70 ° C. and the toughness was low. Steel No. 16 has a low cooling rate, a large bainite lath width, and a low strength. Steel number 17
Since the rolling of austenite in the non-recrystallization temperature region was omitted, the amount of ferrite produced was small and the toughness was poor.
Steel No. 18 has a slow cooling rate and a large bainite lath width,
The ferrite is also too thick, and is inferior in both strength and toughness.

In steel No. 19, the cumulative reduction in the austenite non-recrystallized region is too large, and the anisotropy of the steel plate is large.
Steel No. 20 had a too high cooling stop temperature, and Steel No. 21 had a cooling start temperature lower than Ar3.
Low strength. Ceq and Pcm of steel numbers 22 and 23 are out of the range of the present invention, and the weldability is deteriorated. Steel numbers 24 and 25 are S
The value of i + 3Cr is not within the range of the invention, and the precipitation of ferrite is not sufficient and the toughness is poor.

[0065]

[Table 1]

[0066]

[Table 2]

[0067]

[Table 3]

[0068]

[Table 4]

[0069]

According to the present invention, it is possible to provide a thick 60-kg class steel excellent in low-temperature toughness and weldability without using expensive alloy elements and a method for producing the same. Is extremely large.

Continued on the front page F term (reference) 4K032 AA01 AA02 AA04 AA08 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CC03 CC04 CD03 CF01 CF02

Claims (9)

[Claims] 1. Pcm ≦ 0.2% by mass%, Ceq
(WES) ≦ 0.42%, bainite having a bainite lath width of 2 μm or less, and 20% of 0.1 to 5 μm thick film-like ferrite on the prior austenite grain boundary
A 60 kg high strength steel excellent in weldability and toughness characterized by having a structure precipitated at the above occupancy. However,
Pcm = C + Mn / 20 + Si / 30 + Cu / 20 + N
i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B,
Ceq (WES) = C + Mn / 6 + Si / 24 + Ni /
40 + Cr / 5 + Mo / 4 + V / 14 2. As a steel composition, C: 0.04 by mass%.
0.09%, Si: 0.1 to 0.5%, Mn: 1.2
To 1.8%, Cr: 0.1 to 0.5%, Nb: 0.01
~ 0.05%, Sol. Al: 0.002 to 0.07
%, N: 0.001 to 0.005%, and 0.1%.
The 60 kg class high tensile strength steel excellent in weldability and toughness according to claim 1, wherein the balance satisfying 50 ≦ Si + 3Cr ≦ 1.25% substantially consists of iron and inevitable impurities. 3. The steel composition further comprises Mo:
The high strength steel of 60 kg class having excellent weldability and toughness according to claim 2, containing one or two kinds of 0.02 to 0.3% and Cu: 0.1 to 0.6%. 4. The steel composition further includes Ni:
The 60 kg class high strength steel excellent in weldability and toughness according to claim 2 or 3 containing 0.1 to 0.5%. 5. The steel composition, in which V: 0.
The high strength steel of 60 kg class having excellent weldability and toughness according to any one of claims 2 to 4, containing 0.01 to 0.08%. 6. The steel composition further includes Ti:
0.005 to 0.02%, Ca: 0.001 to 0.00
The 60-kilometer high-strength steel excellent in weldability and toughness according to any one of claims 2 to 5, containing 1% or 2% of 4%. 7. The steel composition, in mass%, B: 0.00
The high-strength steel of 60 kg class excellent in weldability and toughness according to any one of claims 2 to 6, characterized in that the content is not more than 02%. 8. The steel composition, in mass%, P: 0.01
% Or less, S: 0.002% or less, 60 kg class high strength steel excellent in weldability and toughness according to any one of claims 2 to 7, characterized in that: 9. A method for producing a 60 kg high strength steel having excellent weldability and toughness, comprising the following steps. (1) a step of heating the slab having the steel composition according to claim 2 to 1150 ° C. or less;
A step of performing one or more passes of ld / hm ≧ 1.0 in a temperature range of 0 to 1000 ° C., where ld: projected contact arc length ld = (R · (hi−ho)) / 2 hm: average plate Thickness hm = (hi + 2ho) / 3 R: roll radius, hi: sheet thickness before rolling, ho: sheet thickness after rolling (3) Cumulative rolling reduction 1 in a temperature range of Ar3 or more and less than 900 ° C.
A step of rolling at 0 to 50%, and (4) a steel material surface temperature of A
Step of starting cooling from r3 or more, performing cooling such that the cooling rate of the steel material surface becomes 40 ° C / sec or more in a temperature range where the steel material surface temperature is 200 ° C or more, and stopping cooling when the steel material average temperature is 500 ° C or less. .