JP2012172243A - High-tensile steel sheet having excellent toughness and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-tensile steel sheet having excellent strength and toughness of a base material, and excellent toughness of a weld heat affected zone, and a favorable method for manufacturing the same.SOLUTION: Steel material containing, by mass, C:0.005-0.2%, Si:0.05-0.3%, Mn:0.5-5%, Cr:≤3%, Ni:≤5%, Ti:0.005-0.02%, and B:0.0003-0.003%, and satisfying expression of Mn+Ni+Cr-12.5×C≥2.6% is heated to the temperature between Actransformation point to 1,200°C, subjected to the hot working with the cumulative draft of ≥50%, quenched as it is from the temperature of ≥Artransformation point until the temperature of a thickness center part becomes ≤350°C, or naturally cooled, and reheated to the temperature of Actransformation point to 1,050°C, and quenched so that the temperature of the thickness center part becomes ≤350°C. Then, the steel material is tempered at the temperature of 450-650°C, and the average area of the island-like martensite formed at a weld heat-affected zone is set to ≤3 μm.

Description

  The present invention relates to a high-strength steel plate used for steel structures such as ships, offshore structures, pressure vessels, and penstocks, and a method for producing the same. -It relates to a high-tensile steel sheet not only excellent in toughness but also excellent in the toughness of the weld heat-affected zone and its manufacturing method. In the present invention, the yield strength refers to the yield point (YP) or 0.2% yield strength (YS).

  Various steel structures such as ships, offshore structures and pressure vessels are usually finished by welding steel materials such as thick steel plates and joining them to a desired shape and structure. For this reason, the steel materials used for these are required not only to be excellent in toughness of the base material but also to be excellent in toughness of the weld heat affected zone from the viewpoint of ensuring safety.

  However, for example, a thick steel plate having a thickness of 30 mm or more is generally constructed by multi-layer welding with an amount of heat input of 80 kJ / cm or less, but this welding heat affected zone receives a complicated heat history that repeats heating and cooling. The embrittlement zone is likely to occur locally, and in particular, in the heat affected zone, remarkable embrittlement occurs in the vicinity of the bond zone (also called fusion line (FL)) heated to the two-phase zone of ferrite and austenite. It has been known. The cause of this is the island martensite where transformation occurs many times when subjected to multiple heating / cooling in the region as described above, the structure becomes finer, hardenability is reduced, and toughness is reduced. It is said that this is because the upper bainite structure is formed.

  As a countermeasure against the above-mentioned problem, a technique has been put into practical use in which TiN is uniformly finely dispersed in steel to suppress coarsening of austenite and also used as a ferrite transformation nucleus. Patent Document 1 discloses that the toughness of the weld heat affected zone is obtained by finely dispersing Ti oxide or the like in the steel to prevent coarsening of the austenite grains in the weld heat affected zone and miniaturizing the ferrite grains. A technique for improving the above is disclosed. However, since these technologies are intended for relatively low-strength steel materials with ferrite as the matrix, high-strength steel sheets whose weld heat-affected zone has a structure that does not contain ferrite are produced by this refinement of ferrite grains. It cannot be expected to improve the toughness of the heat affected zone.

  The reason why the two-phase zone heating zone, that is, the zone heated to the vicinity of the melting point during the first welding, becomes the most brittle in the zone where the two-phase zone of ferrite and austenite becomes due to the subsequent reheating of welding. This is because C is concentrated in the austenite region due to reheating, and this portion forms an upper bainite structure accompanied by generation of island martensite during cooling. Further, in this region, since coarse island martensite is easily formed, it is likely to become a starting point of brittle fracture.

  For the toughness reduction of the weld heat affected zone due to this C enrichment, for example, in Patent Document 2, the steel is made low C and low Si, and after suppressing the formation of island martensite, Cu is further added. A technique for ensuring the strength of the base material by adding the metal is disclosed. Patent Document 3 discloses a technique for increasing strength by adding Cu after lowering C to improve the toughness of the weld heat affected zone. However, these techniques increase the strength by utilizing the precipitation strengthening of Cu by aging treatment. However, since a large amount of Cu is required, brittleness is likely to occur during hot working, and the productivity and quality are reduced. There are many problems.

Further, Patent Document 4, in order to suppress the decrease in toughness due to island martensite two-phase region heating unit defines a base material and components of the weld metal, after further welding, (A ₁ transformation point + 100 ° C.) ~ A technique for reheating to 1000 ° C. is disclosed. However, in the technique of Patent Document 4, in order to refine the island-shaped martensite generated in the two-phase zone heating unit, it is necessary to heat it to a high temperature once again, which significantly impedes productivity. Moreover, this technique is a technique applied to a steel pipe using a steel plate having a relatively thin plate thickness, and is difficult to apply to a large steel structure such as a marine structure.

JP 2001-323336 A JP 05-186823 A Japanese Patent Laid-Open No. 2001-335484 JP 2000-256777 A

Furthermore, the following problems to be solved remain in the conventional technology.
For example, a technique using Ti oxide or the like has a problem that it is difficult to uniformly disperse the oxide or the like in steel. Furthermore, in recent years, with the increase in size of structures, it has been required to increase the strength and thickness of steel materials used. In order to meet these requirements, alloying elements have been added over conventional technologies. It is necessary to do. However, excessive addition of alloy elements also has the problem that it is not preferable because it reduces the toughness of the weld heat affected zone.

  The present invention has been made in view of the above-mentioned problems of the prior art, and its objectives are a plate thickness of 30 mm or more, a yield strength of 630 MPa or more, excellent base material strength and toughness, and a weld heat affected zone. The present invention proposes a high-tensile steel sheet having excellent toughness and an advantageous manufacturing method thereof.

  The inventors have made extensive studies on a method for improving not only the strength and toughness of the base material of a steel sheet that is required to be thick and having high strength, but also the toughness of the weld heat affected zone. As a result, the toughness reduction in the vicinity of the bond portion of the weld heat affected zone is due to the formation of an embrittled structure including island martensite formed in the portion heated to the two-phase region during multilayer welding. While confirming, it was found that the countermeasures against the island-shaped martensite are still insufficient with the prior art.

  Thus, as a result of further study on the method for improving the above-mentioned embrittlement structure, the inventors simply do not reduce C as in the prior art, and further, the size of island martensite to be generated ( Area) and the hardness of the island martensite must be reduced to reduce the hardness difference from the matrix structure, and as means for achieving this, Mn, Ni and Cr in the base material component It was found that it is effective to add a proper amount of C and reduce C, and the present invention was completed.

That is, the present invention is C: 0.005-0.2 mass%, Si: 0.05-0.3 mass%, Mn: 0.5-5 mass%, P: 0.015 mass% or less, S: 0.005 mass %: Cr: 3 mass% or less, Ni: 5 mass% or less, Ti: 0.005-0.02 mass%, Al: 0.004 mass% or less, N: 0.007 mass% or less, B: 0.0003-0. 003 mass%, and Mn, Ni, Cr and C are represented by the following formula (1):
Mn + Ni + Cr-12.5 × C ≧ 2.6 (mass%) (1)
And the balance is a high-tensile steel sheet having a component composition composed of Fe and inevitable impurities, and having an average area of island martensite formed in the weld heat affected zone of 3 μm ² or less.
Here, each element symbol in the above formula indicates the content (mass%) of the element.

  The high-tensile steel sheet of the present invention is further selected from Cu: 0.5 mass% or less, Mo: 1 mass% or less, V: 0.2 mass% or less, and Nb: 0.1 mass% or less in addition to the above component composition. 1 type or 2 types or more are contained, It is characterized by the above-mentioned.

  In addition to the above component composition, the high-tensile steel plate of the present invention further includes one or two selected from Ca: 0.0005 to 0.003 mass% and REM: 0.0003 to 0.003 mass%. It is characterized by containing.

In addition, the present invention is to heat a steel material having any of the above-described component compositions to a temperature of Ac ₃ transformation point to 1200 ° C., then subject to hot working with a cumulative rolling reduction of 50% or more, and then continue with Ar as it is. The temperature at the center of the plate thickness is either rapidly cooled from the temperature of the _third transformation point or higher until the temperature at the center of the plate thickness becomes 350 ° C. or lower, or after re-cooling to the temperature of Ac ₃ transformation point to 1050 ° C. We propose a method for producing a high-tensile steel sheet that is rapidly cooled to 350 ° C. or lower and then tempered at a temperature of 450 to 650 ° C.

  According to the present invention, a high-strength steel sheet having a thickness of 30 mm or more, a yield strength of 630 MPa or more, excellent base material toughness, and excellent heat-affected zone toughness in multi-layer welding can be stabilized. And can be manufactured.

As described above, the inventors have repeatedly studied a method for improving the strength and toughness of a steel plate (base material) that is required to be thick and having high strength, and also improving the toughness of the weld heat affected zone. It was confirmed that the decrease in toughness in the vicinity of the bond portion of the weld heat affected zone was caused by the formation of an embrittled structure including island martensite formed in the two-phase zone heating zone during multi-layer welding. And, in order to improve the toughness reduction in the vicinity of the bond portion of the weld heat affected zone due to the island-shaped martensite, it is not sufficient to simply reduce C as in the prior art. The average area must be reduced to 3 μm ² or less, and the hardness of the island martensite must be reduced to reduce the hardness difference from the matrix structure. In addition, the C, Mn, Ni and Cr in the base material component are represented by the following formula (1):
Mn + Ni + Cr-12.5 × C ≧ 2.6 (mass%) (1)
(Each element symbol in the above formula indicates the content (mass%) of the element.)
It was found that it is necessary to satisfy and contain.

Here, the meaning of the above formula (1) is as follows.
Since Mn and Ni are austenite stabilizing elements, increasing the content of these elements can suppress an increase in the concentration of C that dissolves in austenite. Furthermore, the concentration of C dissolved in austenite can be further reduced by adding Cr, which is a carbide stabilizing element, and suppressing re-dissolution of the precipitated carbide. Due to the effect of reducing the solute C concentration, each of the island martensites generated in the weld heat affected zone can be refined to an average area of 3 μm ² or less. The hardness can be reduced, and the hardness difference from the matrix structure can be reduced. As a result, island martensite is less likely to be a starting point of fracture, and the toughness of the welded portion can be significantly improved.

  In addition, the average area of the island martensite formed in the welding heat affected zone in the present invention means the average value of the area of individual island martensite observed in the cross section of the weld heat affected zone. In addition, as a measuring method, for example, the cross section of the weld heat affected zone is etched in two steps to reveal island martensite, and then the vicinity of the bond portion heated in the two-phase region is scanned with a scanning electron microscope ( It can be obtained by taking 10 fields of view using a SEM) at a magnification of 3000 times, analyzing the image, measuring the area of each island-like martensite, and averaging them.

Next, the component composition that the steel sheet of the present invention should have will be specifically described.
C: 0.005-0.2 mass%
C is an indispensable element for ensuring the strength (yield strength ≧ 630 MPa) required for the steel plate of the present invention as structural steel. If C is less than 0.005 mass%, the required strength cannot be ensured, or a large amount of other alloy elements must be added, leading to an increase in raw material costs. On the other hand, if it exceeds 0.2 mass%, the amount of island martensite generated in the weld heat affected zone increases, and the individual island martensite is likely to be coarsened. Further, in the island martensite Since C density | concentration also becomes high and hardness rises, the toughness of a welding heat affected zone will fall large. Therefore, C is in the range of 0.005 to 0.2 mass%. Preferably it is the range of 0.01-0.15 mass%.

Si: 0.05-0.3 mass%
Si is an essential element added as a deoxidizer for steel, and in the present invention, it is necessary to add 0.05 mass% or more. However, if added over 0.3 mass%, not only the base material toughness but also the formation of island martensite is promoted and the toughness of the weld heat affected zone is also lowered. Therefore, in this invention, Si shall be 0.3 mass% or less.

Mn: 0.5-5 mass%
Mn needs to be added in an amount of 0.5 mass% or more as a deoxidizing material and from the viewpoint of securing the strength of the base material. On the other hand, if added over 5 mass%, the hardenability is excessively increased and the toughness of the weld heat affected zone is lowered. Therefore, Mn is set to a range of 0.5 to 5 mass%. Preferably, it is the range of 0.5-2 mass%.

P: 0.015 mass% or less P is a harmful element that lowers toughness. In particular, when it contains exceeding 0.015 mass%, the toughness of a base material and a welding heat affected zone will be reduced. Therefore, in the present invention, P is limited to 0.015 mass% or less.

S: 0.005 mass% or less S is also a harmful element that reduces toughness. In particular, when it contains exceeding 0.005 mass%, the toughness of a base material and a welding heat affected zone will be reduced. Therefore, in the present invention, S is limited to 0.005 mass% or less.

Cr: 3 mass% or less Cr is an element effective for increasing the strength of steel (base material). However, if excessively contained, the toughness is lowered, and therefore, the upper limit is set to 3 mass% in the present invention. Preferably it is 0.1-2.7 mass%, More preferably, it is the range of 0.4-2.5 mass%.

Ni: 5 mass% or less Ni is an element effective for improving the strength of steel (base material) and the toughness of the weld heat affected zone. However, since Ni is an expensive element, the upper limit is set to 5 mass%. Preferably it is 0.5-5 mass%, More preferably, it is the range of 0.7-3 mass%.

Ti: 0.005-0.02 mass%
Ti forms a nitride in the steel, reduces the amount of dissolved nitrogen, and suppresses the precipitation of BN, and is therefore an effective element for securing B necessary for quenching. Furthermore, Ti nitride is a stable precipitate even in the austenite temperature range, and effectively suppresses austenite coarsening in the weld heat affected zone. Such an effect can be obtained by adding 0.005 mass% or more. However, if added over 0.02 mass%, the deposited nitride is coarsened and the toughness of the base metal and the weld heat affected zone is lowered, so the upper limit needs to be 0.02 mass%.

Al: 0.004 mass% or less In the present invention, Si and / or Mn deoxidized steel is used as a steel material for producing a high-strength steel sheet, not Al deoxidized steel. However, as a preliminary treatment, prior to Si or Mn deoxidation, Al may be added to perform preliminary deoxidation. However, when Al is added as a deoxidizing material, the Al remaining during dissolution needs to be 0.004 mass% or less. This is because when Al exceeds 0.004 mass%, the formation of island martensite in the weld heat-affected zone is promoted to reduce toughness.

N: 0.007 mass% or less N, when excessively dissolved in steel (base material), lowers the toughness of the base material. Further, excessive N content also forms coarse nitrides and carbonitrides in the weld heat affected zone, thereby reducing toughness. Therefore, in the present invention, N is limited to 0.007 mass% or less.

B: 0.0003 to 0.003 mass%
B segregates at the austenite grain boundaries and suppresses the ferrite transformation from the grain boundaries, and has the effect of promoting the bainite transformation and martensite transformation. Therefore, B is an effective element for increasing the strength and toughness. In order to acquire this effect, 0.0003 mass% or more needs to be contained. However, when added over 0.003 mass%, it becomes carbonitride and precipitates, resulting in a decrease in hardenability and a decrease in toughness. Therefore, in the present invention, B is in the range of 0.0003 to 0.003 mass%. Preferably it is the range of 0.0005-0.0020 mass%.

In addition to the above essential components, the high-tensile steel sheet of the present invention further contains one or more selected from Cu, Mo, V and Nb in the following ranges for the purpose of increasing strength and toughness. Can do.
Cu: 0.5 mass% or less Cu is an element that can increase the strength of steel without impairing low-temperature toughness. However, if the content exceeds 0.5 mass%, the steel sheet surface is cracked during hot working. Therefore, when Cu is added, the upper limit is preferably 0.5 mass%.

Mo: 1 mass% or less Mo is an element effective for increasing the strength of the base material. However, if the content exceeds 1 mass%, the hardness increases due to precipitation of carbides, and the toughness decreases. Therefore, when it contains Mo, it is preferable to set it as 1 mass% or less.

V: 0.2 mass% or less V is an element effective for improving the strength and toughness of the base material, and is also an element effective for precipitation as VN and for reducing solid solution N. However, if the content exceeds 0.2 mass%, the toughness decreases due to precipitation of hard VC. Therefore, when V is contained, the upper limit is preferably 0.2 mass%. More preferably, it is 0.1 mass% or less.

Nb: 0.1 mass% or less Nb is an element effective for increasing the strength of steel. However, if the content exceeds 0.1 mass%, the toughness of the weld heat-affected zone is lowered. Therefore, when Nb is contained, the upper limit is preferably set to 0.1 mass%. More preferably, it is 0.010 to 0.020%.

In addition to the above components, the high-tensile steel sheet of the present invention further includes one or two selected from Ca and REM for the purpose of improving mechanical properties, Ca: 0.0005 to 0.003 mass%, REM: It can be contained in the range of 0.0005 to 0.003 mass%.
Since Ca and REM have the effect of fixing harmful O and N as oxides and sulfides and improving the mechanical properties of steel, they can be contained in amounts of 0.0005 mass% or more, respectively. However, even if both are contained exceeding 0.003 mass%, the effect is saturated, so the upper limit is preferably set to 0.003 mass%. The REM (rare earth metal) refers to rare earth elements including La and Ce.

  In the high-tensile steel sheet of the present invention, the balance other than the above components is composed of Fe and inevitable impurities. However, other components may be contained as long as the effects of the present invention are not impaired.

Next, the manufacturing method of the high-tensile steel plate of this invention is demonstrated.
Steel material The steel material (slab, billet, etc.) that is the material of the high-strength steel sheet of the present invention is obtained by melting steel having the above-described composition in a normal smelting process such as a converter, electric furnace, vacuum melting furnace, etc. After the production, it can be produced by a generally known method such as a continuous casting method or an ingot-bundling rolling method, and there is no particular limitation.

Hot working (hot rolling and / or hot forging)
Next, in the present invention, the steel material is heated to a temperature of Ac ₃ transformation point to 1200 ° C. and then hot-worked with a cumulative rolling reduction of 50% or more to obtain a high-tensile steel plate. Here, the above hot working means any one of hot rolling, hot forging, or a combination of hot forging and hot rolling.
The heating temperature of the steel material, the reason for the Ac ₃ transformation point or higher at temperatures below Ac ₃ transformation point, and since they are not in the austenite single-phase region, the uniformity of the steel structure is poor, manufacturing stability is remarkably lowered Because. Moreover, the reason for setting it as 1200 degrees C or less is because even if it heats exceeding 1200 degreeC, it will only be a loss of thermal energy. Here, the said heating temperature shall point out the temperature of the thickness center part of a steel raw material. The temperature at the center of the thickness can be obtained by simulation calculation or the like from the thickness of the steel material, the surface temperature, the cooling conditions, and the like. For example, the temperature at the center of the thickness can be obtained by calculating the temperature distribution in the thickness direction using the difference method. The reason why the cumulative reduction ratio in hot working is 50% or more is to refine the steel structure by applying sufficient working to the center of the plate thickness of the steel material.
Here, the Ac ₃ transformation point can be obtained by actual measurement, but may be calculated from the following equation (2).
Notation Ac ₃ transformation point (° C.) = 937.2−476.5 × C + 56 × Si−19.7 × Mn−16.3 × Cu−26.6 × Ni−4.9 × Cr + 38.1 × Mo + 1244.8 * V + 136.3 * Ti-19.1 * Nb + 198.4 * Al + 3315 * B
... (2)
(Each element symbol in the above formula indicates the content (mass%) of the element.)
Incidentally, the end temperature of the hot working, in order to ensure the uniformity of the steel sheet structure, preferably the Ar ₃ transformation point or more.

Cooling after hot working, or reheating after hot working and subsequent cooling The steel sheet that has been hot worked (hot rolled and / or hot forged) as described above then has an Ar ₃ transformation point or higher as it is. From this temperature, the sheet thickness center is rapidly cooled to 350 ° C. or lower, or after standing to cool to a temperature of Ac ₃ transformation point to 1050 ° C., until the thickness center reaches 350 ° C. or lower. Quench by either method of rapid cooling.
Here, when quenching as it is after hot working, the reason for setting the rapid cooling start temperature to the Ar ₃ transformation point or more is to make the structure before the cooling start an austenite single phase structure. The rapid cooling start temperature is the steel sheet surface temperature.
On the other hand, when the hot-worked steel sheet is rapidly cooled after being reheated, the reason for setting the reheating temperature to 1050 ° C. or less is that the austenite grains become coarse at temperatures exceeding this, and the toughness of the base material is reduced. . The reason why the reheating temperature is set to the Ac ₃ transformation point or higher is to make the steel sheet structure and material after quenching and tempering uniform by making the steel sheet before quenching an austenite single phase structure. is there. Thereby, desired strength and toughness can be achieved. Here, the said reheating temperature shall point out the temperature of the plate | board thickness center part of a steel plate. The temperature at the center of the plate thickness can be obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the temperature at the center of the plate thickness can be obtained by calculating the temperature distribution in the plate thickness direction using the difference method.
The Ar ₃ transformation point may be actually measured, but may be obtained from the following equation (3).
Ar ₃ transformation point (° C.) = 910-273 × C-74 × Mn-16 × Cr-9 × Mo-5 × Cu (3)
(Each element symbol in the above formula indicates the content (mass%) of the element.)

The reason why the cooling stop temperature when quenching from the temperature above the Ar ₃ transformation point is 350 ° C. or less at the center of the plate thickness is to quench the entire steel plate. Thereby, since the bainite or martensite transformation is surely started over the entire plate thickness, the tempered bainite structure and / or the martensite structure can be formed over the entire plate thickness when the tempering process described later is completed.

Therefore, the cooling rate from the temperature above the Ar ₃ transformation point does not pass through the Fs point (ferrite transformation start temperature) of the continuous cooling transformation diagram (CCT curve), but the Bs point (bainite transformation start temperature) or Ms point ( There is no particular limitation as long as it can pass through the martensite transformation start temperature), but it is preferably about 1 ° C./second or more. Moreover, although the water cooling can be employ | adopted for the method of the said rapid cooling, if it is a method which can ensure the said cooling rate, it will not be limited to water cooling, Gas cooling may be sufficient.

The steel sheet after quenching has a bainite structure and / or a martensite structure.
Industrially, quenching may be repeated for the purpose of toughening steel, and in the present invention, quenching may be repeated. However, at the time of final quenching, it is necessary to satisfy the above cooling conditions.

Tempering treatment After hot working, the steel sheet quenched and quenched by any of the above methods must be reheated and tempered at a temperature of 450 to 650 ° C. When the tempering temperature is less than 450 ° C, the effect of removing the residual stress accompanying quenching is not sufficient. On the other hand, when the temperature exceeds 650 ° C, various carbonitrides are precipitated in the steel and the microstructure obtained by transformation is obtained. This disappears, and the strength and toughness are greatly reduced. By the tempering treatment, the steel sheet structure becomes a tempered bainite structure and / or a tempered martensite structure. In addition, a ferrite structure, a pearlite structure, a retained austenite structure, and the like may be present. However, if the total of these is 5% or less in area ratio, there is no influence on the function and effect of the present invention. As described above, it is important that the high-tensile steel sheet of the present invention is tempered after quenching, and by performing these heat treatments, it is possible to produce a steel sheet that is excellent in the strength and toughness of the base material. .

No. shown in Table 1. After steel 1-29 was melted and made into a steel material (slab), it was heated under the conditions shown in Table 2 and subjected to hot rolling with a cumulative rolling reduction of 50-90%, resulting in a plate thickness of 50- The steel plate was made into a 150 mm steel plate, and the thick steel plate was quenched and quenched as it was, or allowed to cool and then reheated and then quenched and quenched, and then subjected to a tempering treatment. 1-33 product steel plates were produced. The steel plate thus obtained was subjected to the following test.
<Tensile test>
A JIS No. 4 tensile test piece with the rolling direction as the tensile direction was taken from the position of the thickness ¼ of each steel plate, and the tensile test was performed to measure the yield strength and the tensile strength (TS).
<Impact test>
Three V-notch Charpy impact test pieces with the rolling direction as the longitudinal direction were sampled from the position of the thickness ¼ of each steel plate, and Charpy impact test was performed on each test piece at −60 ° C., and the absorbed energy (vE -60) was measured and the average value thereof was determined.
<Impact test after welding>
Two welding test pieces collected from each steel plate were subjected to X groove processing (groove angle 45 °), and then subjected to submerged arc welding with a heat input of 50 kJ / cm to produce a multilayer welded joint. Next, three Charpy impact test pieces each having a bond portion as a V-notch position were sampled from the position of the plate thickness 1/4 of the welded portion of this joint, and subjected to a Charpy impact test at −60 ° C., and the absorbed energy ( vE-60) was measured and the average value thereof was determined.
<Area measurement of island martensite>
The cross section of the weld heat affected zone is etched in two steps to reveal island martensite, and then the vicinity of the bond portion heated in the two-phase region is 10 times with a magnification of 3000 times using a scanning electron microscope (SEM). A field of view was taken and image analysis was performed to measure the average area of the island martensite.

The test results are shown in Table 3. From these results, the steel sheets (steel plates Nos. 1 to 25) of the invention examples in which the component composition of the steel is suitable for the present invention have a base material yield strength of 630 MPa or more, a tensile strength of 720 MPa or more, and a base material vE. It can be seen that −60 is 120 J or more, and the strength and toughness of the base material are excellent. Furthermore, it can be seen that the toughness vE-60 of the welded portion has a toughness of 70 J or more and is excellent in the toughness of the welded portion.
On the other hand, the steel plates of comparative examples (steel plates Nos. 21 to 29) that deviate from the component composition of the present invention have a base material yield strength of less than 630 MPa, a tensile strength of less than 720 MPa, and a toughness vE-60 of less than 120 J or The toughness vE-60 of the welded portion is any one or more of less than 70 J, and any one or more characteristics of the strength / toughness of the base material and the toughness of the welded portion are inferior. Moreover, even if each component composition is an appropriate range, the steel plate (No. 28) which does not satisfy Formula (1) has large island martensite.
In addition, steel plate No. As shown in 30 to 33, even if the steel component composition conforms to the present invention, if the quenching condition or tempering condition does not conform to the present invention, one of the strength and toughness of the base material is selected. It is recognized that the above characteristics are inferior.

Claims (4)

C: 0.005-0.2 mass%, Si: 0.05-0.3 mass%, Mn: 0.5-5 mass%, P: 0.015 mass% or less, S: 0.005 mass% or less, Cr: 3 mass %: Ni: 5 mass% or less, Ti: 0.005 to 0.02 mass%, Al: 0.004 mass% or less, N: 0.007 mass% or less, B: 0.0003 to 0.003 mass%, And Mn, Ni, Cr, and C satisfy the following formula (1), the balance has a component composition consisting of Fe and inevitable impurities, and the average of island martensite formed in the weld heat affected zone A high-tensile steel sheet having an area of 3 μm ² or less.
Mn + Ni + Cr-12.5 × C ≧ 2.6 (mass%) (1)
Here, each element symbol in the above formula indicates the content (mass%) of the element. In addition to the above component composition, Cu: 0.5 mass% or less, Mo: 1 mass% or less, V: 0.2 mass% or less, and Nb: 0.1 mass% or less are included. The high-tensile steel plate according to claim 1. 2. In addition to the said component composition, it further contains 1 type or 2 types chosen from Ca: 0.0005-0.003mass% and REM: 0.0003-0.003mass%, It is characterized by the above-mentioned. Or the high-tensile steel sheet according to 2; A steel material having the composition according to any one of claims 1 to 3 is heated to a temperature of Ac ₃ transformation point to 1200 ° C, and then subjected to hot working with a cumulative reduction of 50% or more, and then as it is. Rapid cooling from the temperature above the Ar ₃ transformation point until the temperature at the center of the plate thickness becomes 350 ° C. or below, or after cooling and reheating to a temperature at the Ac ₃ transformation point to 1050 ° C. A method for producing a high-tensile steel sheet, which is rapidly cooled until the temperature becomes 350 ° C. or lower, and then tempered at a temperature of 450 to 650 ° C.