JP2001303169A - High tensile strength steel plate excellent in weldability and toughness of surface layer part and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thick high tensile strength steel plate having tensile strength of >=780 MPa and excellent in weldability and the toughness of the surface layer part. SOLUTION: This high tensile strength steel plate excellent in weldability and the toughness of the surface layer part has a composition containing, by mass, 0.05 to 0.15% C, 0.05 to 0.30% Si, 0.5 to 2.0% Mn, 0.5 to 3.5% Ni, 0.1 to 1.0% Cr and 0.1 to 1.0% Mo, in which the content of B is limited to <=0.0004%, and also, the value of the componential parameter H obtained from the following formula is >=4, and the balance substantially iron with inevitable impurities: H=(%C)× 1+0.5(%Si)}× 1+3(%Mn)}×1+0.5(%Ni)}× 1+2(% Cr)}× 1+3(%Mo)}×1+0.3(%Cu)}×1+1.5(%V)}.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet having excellent weldability and surface layer toughness used for welding structures such as marine structures, tanks, bridges, and penstocks, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, with the increase in the size of a welded structure, a steel plate to be used is required to be thicker and have higher strength. In particular, for welded structures such as marine structures, tanks, bridges, and penstocks, thick high-strength steels having a tensile strength of 780 MPa class or 950 MPa class are used. On the other hand, if the strength of the thick steel plate is improved, the required plate thickness can be reduced, which leads to a reduction in the number of welding steps. As a result, the construction period can be shortened. Therefore, in order to smoothly shorten the construction period by reducing the number of welding steps, the steel sheet used for the welded structure is required to have not only high strength but also further improved weldability.

[0003] For improving the weldability, it is effective to reduce the carbon content. In recent years, the carbon content of steel sheets for welded structures has been suppressed to 0.10% by mass or less. Also,
Reducing the carbon content alone causes a significant decrease in hardenability, so that a large amount of elements such as Ni, Cr, and Mo that improve the hardenability are added. As a result, it becomes easy to be baked, and a martensite structure or a lower bainite structure can be stably formed even in the center portion of the plate thickness, and has a tensile strength such as 780 MPa class or 950 MPa class. However, Ni,
Since elements such as Cr and Mo are expensive, the addition of a large amount of these elements causes an increase in cost.

Therefore, in order to design a low cost type alloy, expensive elements such as Ni, Cr and Mo are reduced, and the strength is increased by adding a precipitation strengthening element such as Cu, V and Nb. The hardenability is complemented by securing or adding a B element having a remarkable effect of improving the hardenability.

When a high-strength steel sheet having a thickness of 50 mm or more is manufactured, the cooling rate at the center of the thickness during quenching decreases. For this reason, it is necessary to design a component that ensures the hardenability of the central part of the sheet thickness so as to have the required strength and toughness. However, when the component design is intended to ensure the hardenability of the central part of the sheet thickness, the surface layer of the steel sheet in the vicinity of the 1 / 4t thickness from the surface thereof is excessively hardened, so that the toughness is deteriorated. In other words, for thick steel plates,
Material variations occur in the thickness direction. Since the fracture properties of the entire steel sheet are determined by the most embrittled part, the thick steel sheet whose surface layer has deteriorated toughness as described above is
Even if the central part of the sheet thickness has high toughness, the degree of toughness of the surface layer part determines the fracture characteristics of the steel sheet as a whole.

Conventionally, a high-strength high-strength steel sheet having a tensile strength of 780 MPa or higher has a chemical composition that can secure hardenability at the center of the sheet thickness, and is manufactured by various working heat treatments so as not to deteriorate the toughness of the surface layer. Has been proposed and implemented.

[0007] Specifically, Japanese Patent Application Laid-Open No. 9-041077 discloses that the reheating and quenching are performed by controlling the reheating and quenching in order to secure a required quenched structure in the surface layer, specializing in improving the toughness of the surface layer. Have been. Further, Japanese Patent Application Laid-Open No. 10-9
Japanese Patent No. 6042 discloses a composition in which the temperature at which high-density dislocations defined by the composition stably remain is equal to or higher than the austenitizing temperature determined by the composition.
After heating the temperature range from the point to the Ac3 point at a specific critical heating rate or more, cooling, and then performing tempering at the Ac1 point or less, it is possible to obtain a thick steel plate having good toughness even in the surface layer. Has been disclosed.

[0008]

However, in Japanese Unexamined Patent Publication No. 9-041077, the cooling process after rolling is interrupted, and a reheating process is performed by an external heat source or heat inside the steel sheet. There is a possibility that the production efficiency may be significantly reduced. Further, in Japanese Patent Application Laid-Open No. Hei 10-96042, a reheating treatment is required, which leads to an increase in energy cost, and addition of B element is indispensable for securing base material strength. However, the toughness may be significantly deteriorated.

An object of the present invention is to provide a thick high-strength steel sheet having a tensile strength of 780 MPa class or higher, which is excellent in weldability and excellent in surface layer toughness.

Another object of the present invention is to provide a manufacturing method for manufacturing the above-described high-strength steel sheet at high manufacturing efficiency and at low cost.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on low-carbon high-strength steel sheets mainly containing martensite and containing bainite, and as a result, have low anisotropy by so-called ausforming and have high strength toughness. Excellent 7
It has been found that high-strength steel sheets of 80 MPa class or higher can be manufactured. In addition, the present inventors have further studied the toughening effect by ausforming, and found that by combining a specific chemical component and specific manufacturing conditions, markedly high toughness is achieved in the surface layer portion of the steel sheet. Obtained knowledge.

The present invention has been completed on the basis of the above-mentioned knowledge on a low-carbon high-tensile steel sheet.

According to the present invention, in mass%, C: 0.05
0.15%, Si: 0.05 to 0.30%, Mn:
0.5 to 2.0%, Ni: 0.5 to 3.5%, Cr:
0.1-1.0%, Mo: 0.1-1.0%,
B: The content is limited to 0.0004% by mass or less, and the value of the component parameter H obtained from the following formula is 4 or more, and the balance is substantially excellent in weldability and surface layer toughness consisting of iron and unavoidable impurities. A high strength steel sheet is provided. H = (% C) × {1 + 0.5 (% Si)} × {1 + 3
(% Mn)｝ × {1 + 0.5 (% Ni)} × ｛1 + 2
(% Cr)｝ × {1 + 3 (% Mo)} × ｛1 + 0.3
(% Cu)｝ × {1 + 1.5 (% V)} The high-tensile steel sheet further contains Cu: 0.1 to
1.0%, V: 0.01 to 0.1%, Nb: 0.005
You may contain at least 1 sort chosen from -0.050%.

Further, the high-tensile steel sheet is characterized by having a martensite-based structure having a lower bainite fraction of 15% by volume or less.

The method for producing a high-strength steel sheet having excellent weldability and surface layer toughness according to the present invention is as follows.
0.15%, Si: 0.05 to 0.30%, Mn:
0.5 to 2.0%, Ni: 0.5 to 3.5%, Cr:
0.1-1.0%, Mo: 0.1-1.0%,
B: a step of preparing a steel material which is limited to 0.0004 or less, the value of a component parameter H obtained from the following formula is 4 or more, and the balance is substantially composed of iron and unavoidable impurities; A step of heating to a temperature range of 1,050 to 1,250 ° C., and a hot rolling step of rolling the steel material at a rolling reduction of 30% or less in a temperature range of 800 ° C. to a strain recovery temperature,
After the hot rolling step, a cooling rate of 2 ° C./min or more
And a cooling step of rapidly cooling to a temperature range of 0 ° C. or lower. H = (% C) × {1 + 0.5 (% Si)} × {1 + 3
(% Mn)｝ × {1 + 0.5 (% Ni)} × ｛1 + 2
(% Cr)｝ × {1 + 3 (% Mo)} × ｛1 + 0.3
(% Cu)｝ × {1 + 1.5 (% V)} Further, the steel material is represented by mass% and Cu: 0.1 to 1.0.
%, V: 0.01-0.1%, Nb: 0.005-0.
At least one selected from 050% may be contained.

In the hot rolling step, it is preferable to use a rolling roll having an uneven surface.

Further, after the cooling step, a step of performing tempering in a temperature range equal to or lower than the Ac1 transformation point may be provided.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

The high-strength steel sheet according to the present invention contains C, Si, Mn, Ni, Cr and Mo as basic components of the steel. Further, in addition to the above components, one or more of Cu, V and Nb may be contained. These component elements are contained so as to satisfy that the value of H obtained from the following formula is 4 or more. H = (% C) × {1 + 0.5 (% Si)} × {1 + 3
(% Mn)｝ × {1 + 0.5 (% Ni)} × ｛1 + 2
(% Cr)｝ × {1 + 3 (% Mo)} × ｛1 + 0.3
(% Cu)｝ × {1 + 1.5 (% V)} The function of each of the above components and the reason for limiting the component range will be described below. In addition, “%” in the following component ranges means “% by mass”.

(1) C: 0.05 to 0.15% C has a function to secure necessary strength as a steel sheet, and its content is required to be 0.05% or more. If it exceeds, the weldability may be reduced and the weld cracking sensitivity may be increased.

(2) Si: 0.05 to 0.30% Si has a deoxidizing action and a solid solution strengthening action. If the Si content is less than 0.05%, these functions may not be sufficiently exerted. If the Si content is more than 0.30%, the toughness of the heat affected zone may be adversely affected, and the workability may be deteriorated.

(3) Mn: 0.5 to 2.0% Mn has a function of ensuring the hardenability of the base material, and its content is required to be 0.5% or more. If the content exceeds the range, the weld hardening property may be significantly increased.

(4) Ni: 0.5 to 3.5% Ni has a function of improving the strength and toughness of the steel sheet without impairing the weldability, and its content is required to be 0.5% or more. However, if it exceeds 3.5%, there is a possibility that the susceptibility to weld cracking may be increased, and a cost may be increased.

(5) Cr: 0.1 to 1.0% Cr has the function of improving corrosion resistance, improving hardenability and improving base metal strength by precipitation hardening, and its content is 0.1%. % Or more, but if the content exceeds 1.0%, the toughness of the weld heat affected zone may be deteriorated and hardened.

(6) Mo: 0.1 to 1.0% Mo has a function of improving hardenability and a function of improving base material strength by precipitation hardening.
1% or more is necessary, but if the content exceeds 1.0%, the toughness of the weld heat affected zone may be deteriorated.

(7) Cu: 0.1 to 1.0% Cu has a function of increasing the strength of the base material by solid solution strengthening and precipitation strengthening. When Cu is contained, if its content is less than 0.1%, its function may not be sufficiently performed,
If the content exceeds 1.0%, there is a possibility that the susceptibility to weld cracking may be increased.

(8) V: 0.01 to 0.10% V has the function of increasing the strength of the base material by solid solution strengthening and precipitation strengthening. When V is contained, its function can be effectively achieved when it is contained at 0.01% or more.
If the content exceeds 10%, the weld heat affected zone may be embrittled.

(9) Nb: 0.005 to 0.050% Nb precipitates as Nb (C, N) during rolling, and works to prevent coarsening of recrystallized grains due to the pinning effect of this precipitate. It has the function of increasing the strength of the base material by reinforcement. When Nb is contained, if it is contained at 0.005% or more, these functions can be effectively performed. 0.
If the content is more than 050%, the hardenability of the heat affected zone is increased, and the susceptibility to weld cracking may be increased.

(10) Other elements B: 0.0004% or less The high-strength steel sheet according to the present invention is allowed to contain the element B in addition to the components (1) to (9) described above. The content is limited to 0.0004% by mass or less. this is,
If the content exceeds 0.0004% by mass, the hardness of the weld heat-affected zone in the coarse-grained region is significantly increased, and the susceptibility to weld cracking is increased.

(11) Component parameter H ≧ 4 The above component parameter H is a parameter expressing the structure of the steel sheet after cooling. By defining the value of H to be 4 or more, it becomes possible to obtain a martensite-based structure having a lower bainite fraction of 15% by volume or less. A steel sheet having such a structure has improved toughness not only in the center part of the sheet thickness but also in the surface layer part.

As described above, by adjusting the content of each component so as to satisfy the above-mentioned component range and the component parameter H, a tensile strength of 780 MPa class having excellent weldability and surface layer toughness is obtained. It is possible to provide a high-strength steel sheet having the above thickness.

Next, a method of manufacturing a high-tensile steel according to the present invention will be described.

In the method of manufacturing a high-tensile steel sheet according to the present invention, first, the respective components of C, Si, Mn, Ni, Cr and Mo are converted into the component ranges shown in the above (1) to (6) and the component parameter H
A steel material is prepared so as to satisfy ≧ 4 and the balance substantially consists of iron and unavoidable impurities.

In the steel preparation process, the above (1) to
In addition to the components shown in (6), the C listed in (7) to (9)
One or more of u, V, and Nb may be further contained. In this case, each component is as described in (1) to (9) above.
And the above component parameter H ≧ 4.

In the steel material preparation step, the amount of B added is limited to 0.0004% by mass or less as shown in the above (10).

Next, the prepared steel material was mixed with 1050 to 1250
Heat to the temperature range of ° C. By heating to 1050 ° C. or more, the structure can be completely austenitized. In addition, the efficiency of a rolling step described later can be increased. On the other hand, when heated to a temperature range exceeding 1250 ° C., austenite grains are coarsened, and there is a possibility that the fineness by rolling described later is not sufficiently achieved. Further, in the case of a steel material prepared by adding an Nb element, if this is heated to a temperature range exceeding 1250 ° C., almost all of Nb will form a solid solution,
It becomes difficult to precipitate as (C, N). For this reason, it is difficult to exhibit the pinning effect of the Nb precipitate.

Next, the steel material heated to the temperature range of 1050 to 1250 ° C. is subjected to hot rolling at a rolling reduction of 30% or less in a temperature range of 800 ° C. to a strain recovery temperature.

The above-mentioned strain recovery temperature varies depending on various conditions such as the composition of the steel. However, in the steel having the above-mentioned specified composition, it is usually in the range of 890 to 940 ° C.

If the temperature at the time of rolling is lower than 800 ° C., the hardenability may decrease in the cooling step described later. When the temperature is higher than the strain recovery temperature, the strain applied by rolling becomes easy to recover, and it becomes difficult to achieve toughness.

FIG. 1 shows the results of examining the effect of the temperature during rolling on the toughening effect of the steel sheet by taking the tensile strength TS (MPa) on the horizontal axis and the transition temperature vTrs (° C.) on the vertical axis. FIG. The composition of the steel sheet used here is shown in Table 1 below.

[0041]

[Table 1]

In FIG. 1, open circles represent the results of rolling at a temperature of over 900 ° C. and at most 950 ° C., which is a temperature range exceeding the strain recovery temperature of a steel having the composition shown in Table 1 as a rolling temperature at a rolling reduction of 30%. And black circles represent the result of rolling in a temperature range of 850 ° C. or more and 900 ° C. or less, which is the strain recovery temperature or lower and the temperature at the time of rolling at a rolling reduction of 30%. The structures after rolling are all martensite.

As is apparent from FIG. 1, white circles are distributed in groups in the upper left area in the figure, while black circles are distributed in groups in the lower right area in the figure. You can see that. This indicates that a steel material to which a reduction of 30% is applied at a temperature equal to or lower than the strain recovery temperature is significantly toughened compared to a steel material to which no reduction is applied at a temperature equal to or lower than the strain recovery temperature.

On the other hand, in the above-mentioned rolling at a specific temperature range, when the rolling reduction is more than 30%, the material inside the steel sheet becomes anisotropic or hardenability is reduced, and the lower bainite fraction described later is reduced. In some cases, a martensite-based structure of 15% by volume or less cannot be formed.

In the above-mentioned rolling step, it is preferable to use a rolling roll having an uneven surface. By using such a roll, the toughness of the surface layer portion of the steel sheet can be further increased.

FIG. 3 is a graph showing the results of examining the effect of the rolling roll on the toughening effect of the steel sheet, with the horizontal axis representing the tensile strength TS (MPa) and the vertical axis representing the transition temperature vTrs (° C.). It is. The steel plate here (plate thickness 50 mm)
A steel material having a composition similar to that described with reference to FIG.
The following rolling was performed. The white circles in the figure are the results when a normal roll is used, and the black circles are the results when a roll having an uneven surface is used. From this figure, the strength and toughness at the center of the sheet thickness are almost the same regardless of whether a normal roll or an uneven roll is used. On the other hand, the strength and toughness in the surface layer portion of the steel sheet are improved respectively when the roll having the uneven surface is used as compared with the case where the normal roll is used, and it has been found that the toughness is particularly greatly improved. .

It is considered that the reason why the toughness of the surface layer of the steel sheet is remarkably improved by the rolling roll having such an uneven surface is as follows. That is, by the rolling using the rolling roll having the uneven surface, the surface layer portion of the steel sheet is in a state in which portions having large processing strain and portions having small processing strain are alternately present. In other words, since a large rolling strain due to the convex portion of the roll and a small rolling strain due to the concave portion of the roll are applied alternately, the bending of the martensite lath in the surface layer portion of the steel sheet is promoted, and the surface layer portion toughness is reduced. It is thought that it greatly improves.

In the case of a rolling roll having an uneven surface,
The pitch of the convex or concave to concave is 5 mm or less, and the height of the convex portion is 1 m
It is preferably a continuous wave shape of m or less. In addition, even if the uneven surface of the rolling roll is in the above-described form, if the number of rolling passes and the rolling direction are variously changed, the effect of increasing the toughness of the surface layer portion of the steel sheet may increase or decrease accordingly. It should be noted that after the hot rolling step, the rolled steel sheet is immediately quenched immediately at a cooling rate of 2 ° C./sec or more to a temperature range of 500 ° C. or less. Forced cooling after rolling is necessary to complete the martensitic transformation. By the cooling operation as described above, a martensite-based structure having a lower bainite fraction of 15% by volume or less can be obtained as a final steel sheet structure.

FIG. 2 shows the cooling rate after rolling (° C. /
FIG. 5 is a graph showing the results of examining the effect of the structure fraction of lower austenite in the martensite structure on the toughening effect, taking the component parameter H on the vertical axis. In FIG. 3, a region on the lower left side from the line A1 is a mixed structure of martensite and lower bainite, and a region in which the lower bainite fraction exceeds 15% by volume.
The band-like region between the two lines is a mixed structure of martensite and lower bainite, and the region where the lower bainite fraction is 15% or less by volume, and the region above and above the A2 line is a region of a substantially martensite single phase structure. It is. In the figure, open circles indicate steel sheets that have been successfully toughened, and black circles indicate steel sheets that have not been successfully toughened. As is clear from FIG. 3, all white circles are distributed in the upper right area from the A1 line. From this, it was found that a steel sheet having a martensite-based structure or a martensite single-phase structure having a lower bainite fraction of 15% by volume or less has a remarkable toughening effect.

After the cooling operation step, an aging treatment for tempering at a temperature lower than the Ac1 transformation point may be performed. This tempering is performed mainly when it is necessary to adjust the balance between the strength and the toughness of the base material. At this time, care must be taken because excessive heating and holding promotes agglomeration and coarsening of precipitates, which leads to deterioration of toughness.

As described above, according to the production method of the present invention in which a specific component composition and a specific production condition are combined, the weldability and the production cost can be reduced without lowering the production efficiency and increasing the production cost. A thick high-strength steel sheet having excellent surface layer toughness can be manufactured.

[0052]

Examples of the present invention will be described below.

First, No. 2 shown in Table 2 below was used. Steel materials having compositions of 1 to 19 were prepared respectively. All the component values in Table 2 are values obtained by ladle analysis.

[0054]

[Table 2]

These Nos. As shown in Table 3 below, thick steel plates (thickness of 50 to 100 mm) were manufactured by using steel materials having compositions of 1 to 19 and changing manufacturing conditions variously. In addition,
The composition of each steel plate in Table 3 corresponds to the steel material composition with the same reference in Table 2 above. In Table 3, N
o. Each of the steel plates 1-1 and 1-2 is N in Table 2
o. No. 1 in the same manner. 6-1
The steel sheets No. 6-2 were both No. 2 in Table 2. The composition shown in FIG. Each of the steel sheets 8-1 and 8-2 was No. 2 in Table 2. The composition shown in FIG.

The structure of each of the obtained steel sheets has the composition parameter No. H of less than 4, Except for 19, each was a martensite-based structure having a lower bainite fraction of 15% by volume or less.

The center of the thickness of each of the manufactured steel sheets (thickness 1 /
A test piece was sampled from each of the 2t portion) and the surface layer portion (including the surface layer portion), and the following test was performed. The tensile strength TS (MPa) and the transition temperature vTrs (° C.) are obtained by performing a tensile test and an impact test on the central part of the sheet thickness, and the transition temperature vTrs is obtained by performing an impact test on the surface layer part.
(° C.). In addition, in order to evaluate the weldability of each steel sheet, an oblique y-shaped weld crack test was performed, and the y crack prevention preheating temperature ystop (° C.) was examined. Each of these tests was performed in accordance with JIS. In addition, an acoustic anisotropy test was also performed on the surface layer, and the sound velocity ratio (L / C) was examined.

The target performance of the manufactured steel sheet is as follows: tensile strength TS ≧ 780 MPa, transition temperature vTrs ≦ −60 ° C.
All of the setting conditions of the y-break prevention preheating temperature ≦ 75 ° C. and the sound speed ratio ≦ 1.02 were satisfied.

The results of the above examination are shown in Table 3 below. In addition,
Table 3 shows the yield point YS (MP
The results of a) are also shown.

[0060]

[Table 3]

In Table 3 above, the rolling reduction rate of 50% and the No. 3 which deviated from the above-mentioned reduction rate were specified. Although the steel sheet of 1-2 has the tensile strength and transition temperature at the center of the thickness satisfying the above-mentioned set conditions, the transition temperature at the surface layer thereof is −56 ° C., which does not satisfy the above-mentioned set conditions, and is a high value. And the sound speed ratio does not satisfy the above set conditions, and
This is a high value of 3. The heating temperature before rolling was 1280 ° C., which deviated from the above-mentioned specification. The steel sheet of No. 8-2 has a transition temperature of −58 ° C. and −45 ° C. at the surface layer portion and the center portion of the thickness, respectively.
And the above conditions are not satisfied, and the value is high, so that the toughness of the entire steel sheet is low.

In addition, No. 2 which deviates from the regulation of the C content. 1
In the steel sheet No. 5, although the tensile strength and the transition temperature both satisfy the above set conditions in the center part of the thickness, the transition temperature is as high as −56 ° C. in the surface layer part, and the y-break prevention preheating temperature is also 100.
Since the temperature is as high as ° C., the toughness and weldability of the surface layer are low. No. which deviates from the regulation of the Mn content. 1
The steel sheet No. 6 has a low weldability because the transition temperature at the center of the thickness and the transition temperature at the surface layer satisfy the above-mentioned set conditions, but the preheating temperature for preventing the y-crack is as high as 100 ° C. N that deviates from the regulation of Ni content
o. The steel sheet No. 17 has a low toughness in the entire steel sheet because the transition temperature in both the center part and the surface part of the sheet thickness does not satisfy the above-mentioned set conditions, and both are high values. No. B deviating from the regulation of the B content. The steel sheet No. 18 has a low weldability since the y-break prevention preheating temperature is 100 ° C., which does not satisfy the above set conditions and is a high value. No. deviating from the definition of the component parameter H In the steel sheet No. 19, the transition temperature does not satisfy the above-mentioned set conditions in both the center part and the surface part of the sheet thickness, and both are high values, so that the toughness of the whole steel sheet is reduced.
Therefore, it was found that none of the steel sheets of the comparative examples described above could achieve the target performance.

On the other hand, no. 1-1, 2-6-1,
Each of the steel plates 6-2 to 8-1 and 9 to 14 (Example) not only satisfies the above-mentioned set conditions in both the tensile strength and the transition temperature at the center of the plate thickness, but also the transition temperature and the sound velocity in the surface layer. The ratio satisfies the above set conditions and y
The crack prevention preheating temperature also satisfies the above set conditions. As described above, the steel sheets of each of the above-described examples that satisfy the above-described chemical composition and the manufacturing conditions can achieve the above-described target performance, and have a high thickness and high toughness at the center of the sheet thickness. Not only that, it was found that the steel had both excellent surface layer toughness and excellent weldability. Further, in the examples in Table 3, No. 1 using a roll having an uneven surface was used.
No. 6-2 and No. 6 using a normal roll. Paying attention to the value of the transition temperature of the surface layer for the steel sheet 6-1 and
o. No. 6-1 is No. 6 steel plate. The transition temperature was significantly lower than that of the steel sheet No. 6-2, and it was found that the toughness of the surface layer was significantly improved.

[0064]

As described above, according to the present invention, it is possible to provide a thick high-strength steel sheet having excellent weldability and surface layer toughness, which is useful as a member for large-sized welded structures.

According to the manufacturing method of the present invention, such a high-tensile steel sheet can be manufactured with high manufacturing efficiency and low manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of an investigation on the effect of the temperature during rolling on the toughening effect of a steel sheet.

FIG. 2 is a view showing a result of an examination on an influence of a roll used on a toughening effect of a surface layer portion of a steel sheet.

FIG. 3 is a graph showing the results of an investigation on the effect of the lower austenite fraction on the toughening effect of a steel sheet.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA02 AA04 AA05 AA11 AA14 AA16 AA19 AA22 AA23 AA24 AA31 AA36 BA01 CA02 CA03 CD02 CD06 CF02

Claims (7)

[Claims] 1. A mass% of C: 0.05 to 0.15%,
Si: 0.05 to 0.30%, Mn: 0.5 to 2.0
%, Ni: 0.5 to 3.5%, Cr: 0.1 to 1.0
%, Mo: 0.1 to 1.0%, B: 0.000%
Excellent in weldability and surface layer toughness characterized in that the content is limited to 4% or less and the value of the component parameter H obtained from the following formula is 4 or more, and the balance substantially consists of iron and unavoidable impurities. High strength steel sheet. H = (% C) × {1 + 0.5 (% Si)} × {1 + 3
(% Mn)｝ × {1 + 0.5 (% Ni)} × ｛1 + 2
(% Cr)｝ × {1 + 3 (% Mo)} × ｛1 + 0.3
(% Cu)｝ × {1 + 1.5 (% V)} 2. Further, in mass%, Cu: 0.1-1.
0%, V: 0.01-0.1%, Nb: 0.005-
The high-tensile steel sheet according to claim 1, comprising at least one kind selected from 0.050%. 3. The high-tensile steel sheet according to claim 1, further comprising a martensite-based structure having a lower bainite fraction of 15% by volume or less. C: 0.05 to 0.15% by mass%,
Si: 0.05 to 0.30%, Mn: 0.5 to 2.0
%, Ni: 0.5 to 3.5%, Cr: 0.1 to 1.0
%, Mo: 0.1 to 1.0%, B: 0.000%
A step of preparing a steel material in which the value of the component parameter H obtained from the following formula is 4 or more and the balance is substantially composed of iron and unavoidable impurities; and C .; a step of heating to a temperature range of 800 ° C .; a hot rolling step of performing rolling at a rolling reduction of 30% or less in a temperature range of 800 ° C. to a strain recovery temperature; 50 at cooling rate
A method for producing a high-strength steel sheet having excellent weldability and surface layer portion toughness, comprising: a cooling step of rapidly cooling to a temperature range of 0 ° C. or lower. H = (% C) × {1 + 0.5 (% Si)} × {1 + 3
(% Mn)｝ × {1 + 0.5 (% Ni)} × ｛1 + 2
(% Cr)｝ × {1 + 3 (% Mo)} × ｛1 + 0.3
(% Cu)｝ × {1 + 1.5 (% V)} 5. The steel material according to claim 1, wherein said steel material is Cu:
0.1-1.0%, V: 0.01-0.1%, Nb:
The method for producing a high-tensile steel sheet according to claim 5, comprising at least one selected from 0.005 to 0.050%. 6. A rolling roll having an uneven surface in the hot rolling step.
The method for producing a high-tensile steel sheet according to item 1. 7. The high-tensile steel sheet according to claim 5, further comprising, after the cooling step, a step of performing tempering in a temperature range equal to or lower than an Ac1 transformation point. Manufacturing method.