JP5699798B2 - Low yield ratio high strength steel with excellent toughness of heat affected zone of high heat input welding and its manufacturing method - Google Patents

Description

  The present invention relates to steel materials used in structures such as buildings, bridges and shipbuilding fields, and in particular, steel materials that require a low yield ratio and are subjected to high heat input welding; tensile strength; TS is 590 MPa or more. The yield ratio; YR (= YS / TS, YS; yield strength) is about 80% or less of high-tensile steel material and its manufacturing method.

  In submerged arc welding, electroslag welding, and the like applied to assembly welding of box columns of building structures, large heat input welding exceeding 500 kJ / cm may be performed in order to improve construction efficiency. In general, when the heat input of welding increases, the structure of the weld heat affected zone becomes coarse and the toughness decreases. Various methods for improving the toughness of the weld heat affected zone have been proposed so far.

  In addition, high-strength steel materials used in buildings and structures are required to have a low yield ratio in order to guarantee energy absorption due to plastic deformation during earthquakes. A manufacturing method for realizing such a low yield ratio has also been proposed.

  Patent Document 1 discloses a low yield ratio steel material having a TS of 590 MPa or more and a yield ratio of 80% or less, and a method for manufacturing the same, which are excellent in toughness of the weld heat affected zone. Here, rare earth elements (REM) and Zr are added, and inclusions (oxides, sulfides, carbides, nitrides or oxysulfides and composites thereof) containing them are finely dispersed in the steel. The toughness of the weld heat affected zone is improved.

  Since these inclusions are affected by heat during welding and do not lose their solid solution even at a high temperature of 1400 ° C. or higher, if these inclusions are contained in the steel material, the austenite grains are coarse in the weld heat affected zone. It is said that the structure of the weld heat-affected zone can be made finer and the toughness of the weld heat-affected zone can be further improved.

Further, in Patent Document 1, in order to realize a low yield ratio of steel, after hot rolling, quenching is performed from a temperature range of Ar ₃ point or higher, quenching is performed from a temperature range of Ac ₁ point to Ac ₃ point, Ac ₁ point. A manufacturing method by three-stage heat treatment of quenching, two-phase zone quenching, and tempering, in which each step of tempering is sequentially performed in a temperature range below is disclosed.

  Patent Document 2 discloses a low yield ratio steel material having a TS of 520 MPa or more and a yield ratio of 80% or less, which is excellent in the toughness of the weld heat affected zone, and a method for producing the same. In addition to the proper amount of Cu and Ni, in addition to that, the balance of Ti, B, N and Al is optimized, and the toughness of the weld heat affected zone is improved by optimizing the extremely low S and Ca content.

Patent Document 2 states that in order to obtain a low yield ratio characteristic, it is preferable that the steel material has a composite structure composed of a soft phase and a hard phase. Further, as a manufacturing method for making the steel structure into the composite structure as described above, after hot rolling of the steel material, the cooling start surface temperature is Ar ₃ transformation point ± 20 ° C., the cooling stop temperature is 400 ° C. ± 20 ° C. Accelerated cooling is performed at a rate of 10 ° C./second or higher, and subsequently, as a heat treatment, heating is performed from the average temperature of Ac ₁ point and Ac ₃ point to a temperature range of + 60 ° C. and −20 ° C. Furthermore, a method of performing a tempering treatment in a temperature range of 450 to 650 ° C. is disclosed.

  Patent Document 3 discloses a high-tensile steel material having excellent weldability (cold crack resistance and weld heat-affected zone toughness), a reduced yield ratio (particularly YR ≦ 82%), and a TS of 780 MPa or more, and a method for producing the same. It is disclosed.

  Here, the content of C in the steel is reduced as much as possible, and Mn, Cr and Mo, which are hardenability improving elements, are positively added, and the amount of Mn, Cr and Mo is appropriately controlled, and further B is added. By adding, the components of the steel material are controlled so that low-temperature bainite is generated at both high cooling rate and low cooling rate. In such a component system, the hardness of the weld heat-affected zone is suppressed even at a high cooling rate by low heat input welding, so that low temperature cracking resistance is improved, and toughness is reduced even at a low cooling rate by high heat input welding. The generation of coarse island-shaped martensite to be suppressed is suppressed, and the weld heat-affected zone toughness can be secured.

  Further, Patent Document 3 shows that island martensite is actively used to achieve a reduction in yield ratio. In order to finely disperse island martensite, steel material is accelerated and cooled after hot rolling. A manufacturing method for tempering as required is disclosed.

JP 2010-121200 A JP 2008-240033 A Japanese Patent No. 3602396

  In the steel material disclosed in Patent Document 1, a steel in which inclusions containing REM and Zr are finely dispersed is obtained by adding and removing REM and Zr by controlling the amount of dissolved oxygen in the molten steel in the steelmaking stage. Yes.

  However, the reaction rate of oxygen, REM, and Zr in molten steel is extremely high, and the generated REM and Zr oxides are likely to be aggregated and coarsened. Therefore, in order to disperse inclusions containing REM and Zr in a desired form, it is necessary to strictly control not only the amount of dissolved oxygen but also the molten steel temperature and the time from the addition of REM and Zr to the casting, It is not easy to obtain a steel material in which inclusions that effectively act to improve the weld heat affected zone toughness are dispersed.

  In addition, in the method for manufacturing a steel material disclosed in Patent Document 1, after the hot rolling, a three-stage heat treatment of quenching, two-phase quenching, and tempering is performed to reduce the yield ratio, but three heat treatment steps are required. The heat treatment cost and the processing time are increased, and the manufacturing cost is increased and the manufacturing period is lengthened.

  In the production method disclosed in Patent Document 2, accelerated cooling is performed after rolling, and then two-phase region quenching and tempering are performed. In accelerated cooling, it is difficult to accurately control the cooling start temperature, cooling rate, and cooling stop temperature, and variations in these cooling conditions tend to lead to variations in the characteristics of steel materials. In accelerated cooling, the steel surface is water-cooled, resulting in a difference in the characteristics between the steel surface and the interior, and the cooling of the steel material by water cooling is affected by the amount of cooling water and the surface condition of the steel material. Easy to come out. In addition, since heat treatment of the steel material requires hot rolling and accelerated cooling, followed by two-phase quenching and tempering, heat treatment costs and processing time are required.

  The technique disclosed in Patent Document 3 does not consider the weld heat-affected zone toughness in welding with a heat input of 500 kJ / cm or more, which is the subject of the present invention, but considers only the weld heat-affected zone toughness up to a heat input of 150 kJ / cm. Not done. In addition, in the manufacturing method disclosed in Patent Document 3, the steel material is accelerated and cooled after hot rolling and tempered as necessary in order to reduce the yield ratio.

  The present invention has been made in view of such a situation, and the purpose thereof is excellent in weld heat-affected zone toughness even when large heat input with a heat input of 500 kJ / cm or more is performed, It is to provide industrially stable low yield ratio steel material having a TS of 590 MPa or more that can realize the low yield ratio required from the viewpoint of earthquake resistance, without requiring special accelerated cooling or heat treatment.

The low-yield-ratio high-strength steel excellent in the toughness of the heat-affected zone with high heat input welding according to the present invention and the manufacturing method thereof are as follows.
(1) By mass%, C: 0.025 to 0.050%, Si: 0.01 to 0.40%, Mn: 1.20 to 2.50%, P: 0.003 to 0.050% , S: 0.0004 to 0.0100%, Cr: 1.50 to 3.50%, Mo: 0.10 to 0.50%, Al: 0.010 to 0.050%, Ti: 0.005 -0.050%, N: 0.0015-0.0060% each, B is limited to 0.0003% or less, Mn + 0.4Cr is 2.50-3.00%, the balance is iron And low yield ratio high strength steel with excellent toughness of heat-affected zone with high heat input, which is an inevitable impurity. However, Mn and Cr in the formula indicate the content (mass%) of each element.
(2) The steel material is further mass%, Ni: 0.05 to 2.00%, Cu: 0.02 to 2.00%, Nb: 0.002 to 0.050%, V: 0.010. The low yield ratio high-tensile steel material excellent in toughness of the high heat input welding heat-affected zone according to (1), which contains at least one of ˜0.150% and Ca: 0.0005 to 0.0100%.
(3) The steel slab is heated to 1000 ° C. or more and 1250 ° C. or less, hot-rolled, and cooled by air cooling, which is excellent in toughness of the high heat input welding heat-affected zone according to (1) or (2) A method for producing high yield steel with low yield ratio.

  The steel of the present invention does not require special heat treatment or accelerated cooling to lower the yield ratio, and a high yield steel with a low yield ratio can be obtained simply by air-cooling by ordinary hot rolling. In addition, the steel material yield can be improved and the steel material manufacturing period can be shortened, resulting in excellent productivity.

  Moreover, even when high heat input welding exceeding 500 kJ / cm is performed by using the steel material of the present invention, excellent weld heat-affected zone toughness can be secured, and highly safe building structures and the like can be manufactured with high efficiency. can do.

  The present inventor has intensively studied to obtain a high-tensile steel material having excellent toughness of the weld heat-affected zone when high heat input welding is performed. As a result, in order to stably secure the high toughness of the heat affected zone, the content of C is reduced as much as possible to suppress the formation of island martensite in the weld heat affected zone, and Mn, Cr, Ni, Si, It turned out that it is necessary to add the element which improves hardenability, such as Mo, and to lower a transformation temperature and to make the whole welding heat affected zone into a uniform bainite structure.

  On top of that, when an austenite-generating element (Mn, Ni, etc.) and a ferrite-forming element (Cr, Si, Mo, etc.) are added at the same time and the ferrite-forming element amount is made appropriate relative to the austenite-forming element amount, It was found that the formation of island martensite between the bainite laths was reduced and the weld heat affected zone toughness became excellent.

  Mn is a strong austenite stabilizing element and lowers the transformation temperature and promotes bainite transformation. However, even if Mn alone is added in a large amount, Mn excessively stabilizes austenite, so that the formation of island martensite is increased and the weld heat affected zone toughness is not improved. That is, when the amount of Mn is excessive, when bainite transformation occurs in the weld heat affected zone, the portion where C between bainite laths is concentrated is transformed into ferrite because austenite is excessively stabilized by Mn. This portion remains, and this portion is further transformed into martensite by martensite transformation at a lower temperature.

  Ni is a strong austenite stabilizing element next to Mn, and lowers the transformation temperature and promotes bainite transformation, but its effect is smaller than that of Mn. For this reason, even if Ni is added alone, the effect is not sufficient unless a large amount is added. When a large amount of Ni is added alone, the weld heat-affected zone has a uniform bainite structure, and unlike the case of Mn addition, almost no martensite is harmful to toughness between the bainite laths of the weld heat-affected zone. Therefore, excellent weld heat affected zone toughness can be obtained. However, since Ni is expensive and requires a large amount of addition, the alloy cost of the steel material becomes extremely high. Therefore, it is practical that Mn is mainly used as an austenite generating element and Ni is used as an auxiliary.

  Addition of ferrite-stabilizing elements in addition to these austenite-generating elements alleviates excessive austenite stabilization due to the addition of Mn, reduces island martensite between bainite laths, and improves weld heat-affected zone toughness. .

  In particular, Cr is a strong ferrite-forming element and is effective in suppressing excessive austenite stabilization due to Mn.

  Si is also a strong ferrite-forming element. However, since Si strongly suppresses the formation of cementite, when it is added excessively, the generation of island martensite increases.

  Mo is also a strong ferrite stabilizing element and is an element effective for preventing temper embrittlement. However, it is expensive, and a large amount of addition leads to an increase in cost.

  Therefore, it is effective to mainly use Cr as a ferrite-forming element and use a proper amount of Si and Mo in an auxiliary manner.

  As described above, in order to increase the weld heat-affected zone toughness and provide other properties such as the strength and toughness of the steel (base material), the content of the components needs to be within the following range. In addition, all the component% is the mass%.

<C: 0.025 to 0.050%>
C is an element necessary for ensuring the strength of the base metal and suppressing the coarsening of the austenite grains to ensure the toughness of the weld heat affected zone. There is a need. On the other hand, when the amount of C is excessive, island-shaped martensite increases, so that the weld heat affected zone toughness deteriorates. Therefore, the C amount is 0.050% or less.

<Si: 0.01-0.40%>
Si is an element necessary for deoxidation at the time of steelmaking, but if the purpose of deoxidation is achieved, the addition amount is at least good. Addition of 0.01% or more is necessary for deoxidation. Si is a strong ferrite-forming element and has the effect of suppressing the excessive stabilization of austenite by Mn to suppress the formation of island martensite. However, since Si strongly suppresses the formation of cementite, On the other hand, island-shaped martensite increases and the weld heat-affected zone toughness deteriorates, so the content is made 0.40% or less. A more preferable Si addition amount is 0.03 to 0.20%.

<Mn: 1.20 to 2.50%>
Mn is a strong austenite stabilizing element and is an element useful for lowering the transformation point and ensuring the strength of the base material. It is an element that promotes bainite transformation. In order to ensure the strength of the base material, 1.20% or more is contained. However, when Mn is excessive, austenite is excessively stabilized, island-like martensite is precipitated between bainite laths, and the weld heat affected zone toughness deteriorates. Therefore, the Mn content is 2.50% or less. A more preferable amount of Mn is 1.65 to 2.10%.

<P: 0.003 to 0.050%>
P is preferably as small as possible because it impairs the toughness of the steel material. For this reason, it is 0.050% or less. However, considering the dephosphorization cost in the steelmaking process, it is made 0.003% or more.

<S: 0.0004 to 0.0100%>
The smaller the S, the more preferable it is because it impairs the toughness of the steel material. For this reason, it is made into 0.0100% or less. However, considering the desulfurization cost in the steelmaking process, the content is made 0.0004% or more.

<Cr: 1.50 to 3.50%>
Cr is an element that is useful for improving the hardenability and securing the strength and toughness of the base metal and is a ferrite stabilizing element, and prevents excessive stabilization of austenite due to the addition of Mn. It is an element useful for suppressing the generation. In order to exhibit these effects, it is contained 1.50% or more. However, if Cr is present excessively, the hardness of the weld heat affected zone increases and the weld heat affected zone toughness deteriorates. Therefore, Cr is 3.50% or less. A more preferable addition amount of Cr is 1.70 to 2.40%.

<Mo: 0.10 to 0.50%>
Mo is an element that is useful for improving the hardenability and ensuring the strength and toughness of the base metal and is a ferrite stabilizing element. It prevents the excessive stabilization of austenite due to the addition of Mn and the generation of island martensite. It is an element useful to suppress In order to exert these effects, the content is 0.10% or more. However, if Mo is present excessively, the hardness of the weld heat affected zone increases and the weld heat affected zone toughness deteriorates. Therefore, Mo is 0.50% or less.

<Al: 0.010 to 0.050%>
Al is an element necessary for deoxidation at the time of steelmaking, but the addition amount is at least good if the purpose of deoxidation is achieved. Add 0.010% or more for deoxidation. However, when Al is excessive, coarse inclusions such as alumina increase and the base material toughness deteriorates. In addition, island martensite increases and the weld heat affected zone toughness deteriorates, so the content is made 0.050% or less.

<Ti: 0.005 to 0.050%>
Ti is an element that combines with N to form TiN, and this TiN effectively suppresses the growth of austenite grains in the weld heat affected zone and effectively contributes to the improvement of the weld heat affected zone toughness. In order to exert this effect, 0.005% or more of Ti is added. On the other hand, when Ti becomes excessive, TiN becomes coarse and both the base metal toughness and the weld heat affected zone toughness deteriorate, so the content is made 0.050% or less. A more preferable addition amount of Ti is 0.010 to 0.025%.

<N: 0.0015 to 0.0060%>
N is an element that combines with Ti to form TiN. This TiN suppresses the growth of austenite grains in the weld heat affected zone and effectively contributes to the improvement of the weld heat affected zone toughness. In order to exhibit this effect, N is contained 0.0015% or more. On the other hand, when Ti becomes excessive, TiN becomes coarse and both the base material toughness and the weld heat affected zone toughness deteriorate, so the content is made 0.0060% or less. A more preferable N content is 0.0025% to 0.0050%.

<B: 0.0003% or less>
Even if B is a very small amount, it segregates at the austenite grain boundary and strongly suppresses the transformation from austenite to ferrite. More specifically, ferrite nucleation at the austenite grain boundary is suppressed. For this reason, it is usually handled as an element that promotes the transformation of bainite. However, as in the present invention, it contains a large amount of Mn and Cr, and in the steel type that is extremely harder than usual, austenite is excessively stabilized. In order to deposit island martensite between the laths, both the base metal toughness and the weld heat affected zone toughness are deteriorated even in a very small amount. In consideration of this point, addition is not performed, but even when impurities are mixed, the content is made 0.0003% or less.

<Mn + 0.4Cr: 2.50 to 3.00%>
Although the addition amount of Cr should be determined according to the addition amount of Mn, Cr addition of Mn + 0.4Cr ≧ 2.50% (wherein Mn and Cr indicate the content (mass%) of each element) This prevents excessive stabilization of austenite by Mn, suppresses the generation of island martensite, and makes the weld heat-affected zone a fine bainite structure with appropriate hardness so that the weld heat-affected zone toughness is excellent. become. However, if Cr becomes excessive and Mn + 0.4Cr> 3.00%, the hardness of the weld heat affected zone becomes too hard and the weld heat affected zone toughness decreases, so Mn + 0.4Cr ≦ 3.00%.

The above is the basic component of the present invention, and the balance is Fe and inevitable impurities.
Furthermore, in the present invention, in addition to the above basic components, the following elements can be selectively added as necessary.

<Ni: 0.05-2.00%>
Ni is a strong austenite stabilizing element and is an element useful for lowering the transformation point and ensuring the strength of the base material. It is an element that promotes bainite transformation. Furthermore, the toughness of the base material and the weld heat affected zone is increased by increasing the toughness of the matrix. For this reason, when adding, it is preferable to add 0.05% or more. However, since it is an expensive element, it is preferably made 2.00% or less from the viewpoint of alloy cost.

<Cu: 0.02-2.00%>
Cu is an element that strengthens the steel by solid solution. For this purpose, 0.02% or more is preferably added. However, if the content exceeds 2.00%, the toughness of the base material is lowered, so Cu is preferably made 2.00% or less.

<Nb: 0.002 to 0.050% or less>
Nb is an element that contributes to refinement of the structure by suppressing recrystallization and improves the toughness of the base material and the weld heat affected zone. For this purpose, it is preferable to add 0.002% or more. However, if it exceeds 0.050%, the structure becomes too fine and the yield ratio becomes high, so Nb is preferably 0.050% or less.

<V: 0.010 to 0.150%>
V is an element that generates carbides and increases strength. For this purpose, 0.010% or more is preferably added. However, if it exceeds 0.150%, the weldability deteriorates and the toughness of the base material deteriorates, so V is preferably 0.150% or less.

<Ca: 0.0005 to 0.0100%>
Ca controls the form of inclusions (specifically, spheroidizes MnS) to improve toughness. For this purpose, 0.0005% or more is preferably added. However, if added excessively, a coarse oxide is formed, and the toughness deteriorates instead. Therefore, Ca is preferably 0.0100% or less.

  Next, the manufacturing method of the steel material of this invention is described. The steel having the above-described composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and is made into a steel material such as a slab by a conventional method using a continuous casting method or ingot-bundling method. Is preferred. The melting method and casting method are not limited to the above methods. A steel material having a TS of 590 MPa or more and a yield ratio of 80% or less is obtained by heating the slab having the above-described composition components to 1000 to 1250 ° C., performing normal hot rolling, and air cooling.

  Usually, steel materials with TS of 590 MPa or more require quenching and tempering or accelerated cooling. However, the steel materials of the present invention have a large amount of Mn and Cr and high hardenability. A steel material of 590 MPa or more is obtained. In addition, since the amount of C is extremely small, after hot rolling and air cooling, YS is lower than TS and low YR is realized.

  In the present invention, the heating temperature, rolling finishing temperature, etc. were determined by calculation from the surface temperature of the slab or steel (steel plate, shape steel, etc., hereinafter referred to as a steel plate) in consideration of the plate thickness, thermal conductivity, etc. Average temperature of slab or steel plate.

  The reasons for limiting the slab heating temperature and the cooling conditions after hot rolling will be described below.

<Slab heating temperature: 1000 to 1250 ° C.>
If the heating temperature is less than 1000 ° C., the deformation resistance is large, and subsequent hot rolling becomes difficult. If the heating temperature exceeds 1250 ° C., the crystal grains become coarse and the toughness of the base material decreases. For this reason, the slab heating temperature before hot rolling shall be 1000-1250 degreeC.

<Cooling after hot rolling: Air cooling>
When the steel sheet is accelerated and cooled by means of water cooling or the like, the strength of the steel sheet increases, but the yield strength increases more than the increase in tensile strength, and the yield ratio exceeds 80%. For this reason, cooling after hot rolling is air cooling. A preferable cooling rate in this case is 2 ° C./s or less as an average cooling rate from 800 ° C. to 500 ° C.

  Hereinafter, the present invention will be described more specifically with reference to examples. After melting the steel of the component composition shown in Table 1 into a slab, it was heated to 1200 ° C. and hot-rolled to a finishing temperature of 950 ° C. and a plate thickness of 15 mm. Cooling after hot rolling was air cooling. Using the steel plate thus obtained, the base material strength and toughness were measured and the weld heat affected zone toughness was evaluated as follows.

[Measurement of base material strength and toughness]
A round bar test piece (parallel portion: diameter 6.0 mm × length 30 mm, distance between gauge points 25 mm) is sampled from the rolling direction at a half thickness position of each steel plate, and a tensile test is performed in accordance with JISZ 2241. Yield strength; YS (MPa), tensile strength; TS (MPa), elongation; EL (%) were measured. And those having a tensile strength of 590 MPa or more and a yield ratio; YR (= YS / TS) of 80% or less were evaluated as having a low yield ratio and high tension. In addition, as an evaluation of the toughness of the base metal, three Charpy V-notch specimens were taken perpendicularly to the rolling direction and subjected to a Charpy impact test in accordance with JISZ 2242, and the absorbed energy at a test temperature of 0 ° C .; vE0 (J) and The brittle fracture surface ratio (%) was measured. It was evaluated that the average of the three absorbed energy was 70 J or more and the minimum value of 3 was 50 J or more was excellent in toughness (the brittle fracture surface ratio is a reference value with no particular standard). These results are shown in Table 2.

[Evaluation of weld heat-affected zone toughness]
Simulating the heat history corresponding to the heat affected zone near the bond of the skin plate when electroslag welding with a heat input of 550 kJ / cm is performed by combining a skin plate material (50 mm thickness) and a diaphragm material (50 mm thickness). A 12 mm thick × 12 mm wide square bar specimen taken from the rolling direction is heated and held at 1420 ° C. for 1 second and subjected to heat treatment with a high-frequency induction heating device in a cycle of 800 to 500 ° C. and a cooling time of 510 seconds. did.

Then, three V-notch test pieces of JIS Z 2202 were sampled from the test pieces subjected to heat treatment simulating the welding heat history, and subjected to the Charpy impact test according to the procedure of JISZ 2242, and the absorbed energy at the test temperature of 0 ° C .; vE ₀ (J) and the brittle fracture surface ratio (%) were measured. It was evaluated that the average of three absorbed energies was 70 J or more and the minimum value of three was 50 J or more as being excellent in toughness of the weld heat affected zone (the brittle fracture surface ratio is a reference value with no particular standard) ). Further, the average of the three absorbed energy was 100 J or more, and the toughness of the weld heat-affected zone was particularly excellent when the minimum value of three was 70 J or more. These results are shown in Table 3.

  In Table 1, steel types A1 to A3, A5, A8, A14, A18 to A21, and A26 are comparative examples whose Cr or (and) Mn + 0.4Cr value is outside the scope of the present invention. Steel plates A4, A6, A7, A10 to A13, A16, A17, A22 to A25, A28 to A32 are examples of the present invention. Steel type A27 is a comparative example outside the scope of the present invention in which 0.0017% of B, which is limited to 0.0003% or less in the present invention, is added.

  It can be seen from Table 2 that all the steel plates of the present invention satisfy TS ≧ 590 MPa and are excellent in the toughness of the base material. Steel plates B14 and B20 are comparative examples whose component compositions are outside the scope of the present invention, but YR exceeds 80%. Steel plate B27 is an example in which 0.0017% of B, which is limited to 0.0003% or less in the present invention, is added, but the toughness of the base material is reduced.

  As is clear from Table 3, all of the examples of the present invention have an average of three absorbed energy of 70 J or more, and the minimum value of three is 50 J or more, which is excellent in the toughness of the heat affected zone. However, in all of the comparative examples, the toughness of the weld heat affected zone is not sufficient.

  A slab having the same composition as steel types A4 and A12 in Table 1, which is the component range of the present invention, was heated to 1200 ° C and hot-rolled to a sheet thickness of 15 mm at a finishing temperature of 950 ° C. The cooling after hot rolling was air-cooled and water-cooled (mist cooling by mist shower and high-density water cooling by high-density high-pressure water flow), and the difference in characteristics due to the difference in cooling rate was investigated. Water cooling was performed after the hot rolling was completed until the temperature of the steel sheet reached 940 ° C. until it reached 100 ° C. or less. Using the steel plate thus obtained, the base material strength and toughness were measured and the weld heat affected zone toughness was evaluated in the same manner as in Example 1. Table 4 shows the base metal strength and toughness results, and Table 5 shows the weld heat affected zone toughness results.

  The steel plates C1 and C4 are examples of the present invention in which after the hot rolling is completed, cooling is not performed and air cooling is performed. These inventive examples satisfy YR ≦ 80% and TS ≧ 590 MPa. Table 4 shows that the toughness of the base material is also excellent. In contrast, in the comparative examples (steel plates C2, C3, C5, C6) in which water cooling was performed after completion of hot rolling, the YR exceeded 80%.

  However, as apparent from Table 5, the presence or absence of water cooling after the completion of hot rolling had little effect on the weld heat affected zone toughness, and all showed excellent weld heat affected zone toughness.

Claims (3)

% By mass
C: 0.025 to 0.050%,
Si: 0.01-0.40%,
Mn: 1.20 to 2.50%,
P: 0.003 to 0.050%
S: 0.0004 to 0.0100%,
Cr: 1.50 to 3.50%
Mo: 0.10 to 0.50%,
Al: 0.010 to 0.050%,
Ti: 0.005 to 0.050%,
N: each containing 0.0015 to 0.0060%,
B is limited to 0.0003% or less,
Mn + 0.4Cr is 2.50 to 3.00%,
A low-yield ratio high-strength steel material with excellent toughness in the heat-affected zone of high heat input welding, with the balance being iron and inevitable impurities. However, Mn and Cr in the formula indicate the content (mass%) of each element. Furthermore, in mass%,
Ni: 0.05-2.00%,
Cu: 0.02 to 2.00%,
Nb: 0.002 to 0.050%,
V: 0.010 to 0.150% and Ca: 0.0005 to 0.0100%
The low-yield-ratio high-tensile steel material excellent in toughness of the high heat input welding heat-affected zone according to claim 1, comprising at least one of the above. The steel slab is heated to 1000 ° C. or higher and 1250 ° C. or lower, hot-rolled and air-cooled. 3. The low yield ratio high-tensile steel material having excellent toughness of the high heat input welding heat-affected zone according to claim 1 or 2. Production method.