JP6048627B2 - Manufacturing method of steel plates for high heat input welding - Google Patents

Description

  The present invention relates to a non-tempered high strength steel having a yield stress of 460 MPa or more and a plate thickness of 25 mm or more and 50 mm or less, which is used for various steel structures in the fields of ships, construction, civil engineering, etc., and in particular, the heat input is 200 kJ. The present invention relates to a method for producing a steel plate for high heat input welding having excellent joint characteristics even when high heat input welding exceeding / cm is performed.

Steel materials used in various steel structures in the fields of ships, construction, civil engineering, etc. are becoming stronger and thicker. As steel materials are strengthened and thickened, high heat input welding with excellent production efficiency such as submerged arc welding, electrogas welding, and electroslag welding is applied when the steel materials are welded. Opportunities are increasing.
Various steel structures in the fields of ships, construction, civil engineering, etc. are required to have excellent joint properties such as strength and toughness of the welded part in addition to the properties of the base material. However, the weld heat affected zone (hereinafter also referred to as “HAZ”) after high heat input welding has a toughness because the characteristics of the base material created in the manufacturing process by the structure control or the like are invalidated by the heat effect. Is known to decrease. On the other hand, various high heat input welding steels have been proposed in order to suppress HAZ toughness deterioration.

  As a technique for improving the toughness of HAZ, for example, TiN is finely dispersed in steel to suppress coarsening of HAZ austenite grains, or dispersed TiN is used as a ferrite transformation nucleus in HAZ. Have been put to practical use. However, the technique of finely dispersing TiN in steel cannot obtain an effect for suppressing a decrease in toughness when HAZ is equal to or higher than the temperature at which TiN dissolves. Furthermore, the technique of finely dispersing TiN in steel has a problem that the solid structure becomes brittle due to the solid solution Ti and solid solution N generated by dissolution of TiN, and the toughness is remarkably lowered.

To solve the problem of dissolution of TiN in HAZ, Patent Document 1 discloses a technique for finely dispersing TiO _x (where x: 0.65 to 1.3) having a particle size of 5 μm or less in steel. In Patent Document 1, a decrease in the toughness of HAZ is suppressed by finely dispersing a Ti oxide that does not dissolve even in a high temperature region of HAZ and using the Ti oxide as a production nucleus of acicular ferrite. In addition, in the technique using Ti oxide as in Patent Document 1, it is difficult to uniformly disperse the oxide uniformly, and therefore, studies have been made to improve the dispersibility by combining oxides. ing.

  Further, as a technique for improving the toughness of HAZ, for example, Patent Document 2 discloses that BN for refining the structure of HAZ is positively precipitated, so that B, N and sol. A technique for adjusting the amount of Al is disclosed. Furthermore, Patent Document 3 discloses a technique of adjusting the amount of Ti—B—N so that the toughness of HAZ becomes a high toughness region, and further adding Ca or Ce to control the form of inclusions. Yes. Furthermore, Patent Document 4 discloses a technique of adding REM with a steel composition having a low N-low Ti system in order to form a stable sulfur / oxide in a weld bond.

  However, in the techniques described in Patent Documents 1 to 4, it is difficult to sufficiently suppress the grain growth of HAZ austenite in large heat input welding in which the heat input exceeds 200 kJ / cm, and the HAZ toughness is prevented from being lowered. It was difficult. On the other hand, as a technique for improving the toughness of HAZ even in high heat input welding, Patent Document 5 discloses a Ca-based nonmetallic inclusion by appropriately controlling the amounts of Ca, O, and S in the steel composition. A technique for finely dispersing in steel is disclosed. According to Patent Document 5, since Ca-based metal inclusions become transformation nuclei and promote ferrite transformation in HAZ, the toughness of HAZ can be improved even in high heat input welding exceeding 400 kJ / cm.

JP 57-51243 A JP-A-62-170459 JP 60-204863 A Japanese Patent Publication No. 4-14180 Japanese Patent No. 3546308

By the way, in recent years, the opportunity to apply high heat input welding to high strength steels whose yield strength exceeds 460 MPa class has been increasing. Among these high-strength steel plates, especially for medium-thickness and high-strength steels with a thickness of 25 mm or more and 50 mm or less, the steel material weight can be reduced by thinning due to the increase in strength, and therefore, it is applied to highly efficient transport vessels. Demand is increasing.
However, the technique described in the cited document 5 is intended for steel materials having a yield strength of 390 MPa class, and is applied to steel materials having a carbon equivalent lower than that of high strength steel having a yield strength exceeding 460 MPa class. For this reason, when the technique of the cited document 5 is applied to high strength steel whose yield strength exceeds 460 MPa class, since the carbon equivalent is high, the HAZ crystal grains have a mixed structure of ferrite and bainite. It was difficult to improve joint characteristics such as toughness. Furthermore, in the techniques described in the cited documents 1 to 4, joint characteristics such as HAZ toughness were not improved in the large heat input welding in which the heat input amount exceeds 200 kJ / cm as described above.
Therefore, the present invention has been made paying attention to the above-mentioned problems, and has excellent joint characteristics even under large heat input welding where the welding heat input is 200 kJ / cm or more, and the yield strength is 460 MPa or more. An object of the present invention is to provide a method for producing a steel plate for high heat input welding having a thickness of 25 mm or more and 50 mm or less.

In order to achieve the above object, the method for manufacturing a steel plate for high heat input welding according to one embodiment of the present invention is, in mass%, C: 0.03% or more and 0.10% or less, Si: 0.01% or more. 0.10% or less, Mn: 0.8% or more and 2.0% or less, P: 0.020% or less, S: 0.0005% or more and 0.0050% or less, Al: 0.005% or more and 0.100 %: Nb: 0.003% to 0.030%, Ti: 0.005% to 0.050%, Cu: 0.10% to 0.50%, Ni: 0.30% to 2 0.000% or less, N: 0.0030% or more and 0.0100% or less, B: 0.0003% or more and 0.0025% or less, Ca: 0.0005% or more and 0.0030% or less, O: 0.0040% And ACR defined by the following formula (1) is more than 0 and less than 1, and the following formula (2) Being defined _{C eq} is contained in each component satisfies 0.38 or more 0.43 or less, the steel material and the balance being Fe and unavoidable impurities, was heated to 1050 ° C. or higher 1200 ° C. or less, heated steel material , Hot rolled at a temperature range of 850 ° C. or less and Ar ₃ transformation point or more at the end of rolling, so that the sheet thickness t after rolling is 25 mm or more and 50 mm or less and the cumulative reduction ratio is 40% or more. The rolled steel material is water-cooled at a cooling rate of 5 ° C./second or more until the surface temperature becomes (−t × 1.5) + 400 ° C. or higher and (−t × 1.5) + 620 ° C. or lower. A steel sheet for high heat input welding with a yield strength of 460 MPa or more is manufactured by air-cooling the steel material.
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S)
... (1)
C _eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
... (2)
In addition, t in the surface temperature conditional expression indicates the thickness of the steel material after hot rolling, and in the expressions (1) and (2), each element symbol indicates the content (mass%) of each element in the steel material. Indicates.

The steel material is, in mass%, one or more selected from V: more than 0% to 0.20% or less, Cr: more than 0% to 0.40% or less, and Mo: more than 0% to 0.40% or less You may contain.
The steel material is, in mass%, selected from Mg: 0.0005% to 0.0050%, Zr: 0.0010% to 0.0200%, REM: 0.0010% to 0.0200%. One or more types may be further contained.

  According to the present invention, even under high heat input welding with a heat input of 200 kJ / cm or more, high heat input welding having excellent joint characteristics, yield strength of 460 MPa or more, and plate thickness of 25 mm or more and 50 mm or less. A method for manufacturing a steel sheet is provided.

  The steel plate for high heat input welding of the present invention is a non-tempered high strength steel plate for high heat input welding having a plate thickness of 25 mm or more and 50 mm or less, a yield strength of 460 MPa or more, and a welding heat input of 200 kJ / cm or more. is there. In order to ensure the tensile strength of the joint, such high heat input welding steel is specifically designed in consideration of the reduction in the plastic restraint of the base metal due to the reduction of the plate thickness for the thick steel plate having a plate thickness of more than 50 mm. There is a need. On the other hand, when the component design for ensuring the joint strength is performed in the high heat input welding steel, it becomes difficult to ensure the toughness in the coarse grain region of the HAZ. Furthermore, when the conventional method for producing thick steel sheets is applied to the steel for high heat input welding to which the above-described component design for ensuring joint strength is applied, the base metal strength becomes excessive. descend.

In contrast, the present inventors have made various studies and obtained the following findings (a) to (c).
(A) In order to improve the toughness of the heat-affected zone with high heat input welding, it is possible to suppress the austenite grain coarsening in the high-temperature region, and to generate intragranular ferrite in the subsequent cooling process. , Also referred to as “MA”.) It is important to reduce the amount. Furthermore, in order to reduce the amount of MA, it is important to reduce the C, Si and P contents in the steel composition.
(B) By adjusting the components so that the carbon equivalent (C _eq ), which is an index of hardenability, falls within an appropriate range, both the tensile strength and toughness of the joint can be achieved.
(C) Self-tempering by recuperation is effective for suppressing the base metal strength, and the base metal strength can be controlled within an appropriate range by controlling the cooling stop temperature according to the plate thickness in the cooling after rolling. Furthermore, by controlling the cooling stop temperature according to the plate thickness, it is possible to achieve other characteristics other than the base material strength such as ductility and toughness.

<Ingredient composition>
[Basic component composition]
Next, embodiments of the present invention will be described in detail. First, the basic component composition that the steel material of the present invention should have will be described. In the description, all percentages relating to chemical components mean mass%.

C: 0.03% or more and 0.10% or less C is an element that increases the strength of the steel material, and in order to ensure the strength necessary for structural steel, it is necessary to contain 0.03% or more. On the other hand, if the C content exceeds 0.10%, MA tends to be generated in the HAZ near the bond portion, so the upper limit is made 0.10% or less. Preferably, the C content is 0.05% or more and 0.08% or less. Here, the vicinity of the bond portion means a region where the coarsening is most remarkable in the HAZ immediately adjacent to the melting line.

Si: 0.01% or more and 0.10% or less Si is an element added as a deoxidizer when melting steel, and it is necessary to add 0.01% or more. However, if the Si content exceeds 0.10%, the toughness of the base material decreases. Furthermore, if the Si content exceeds 0.10%, MA is generated in the HAZ in the vicinity of the bond portion after the high heat input welding, so that the toughness is easily lowered. Therefore, the Si content is in the range of 0.01% to 0.10%. Preferably, the Si content is 0.08% or less.

Mn: 0.8% or more and 2.0% or less Mn is added in an amount of 0.8% or more in order to ensure the strength of the base material. On the other hand, if the Mn content exceeds 2.0%, the toughness of the HAZ is remarkably deteriorated, so the Mn content is set to 0.8% or more and 2.0% or less. Preferably, the Mn content is 1.2% or more and 2.0% or less.
P: 0.020% or less P has a content of 0.020% or less in order to promote MA formation in HAZ near the bond portion and greatly reduce toughness. Preferably, the P content is 0.010% or less.

S: 0.0005% or more and 0.0050% or less S is an element necessary for forming MnS or CaS acting as a nucleation site of ferrite. For this reason, content of S shall be 0.0005% or more. However, since an excessive content causes a decrease in the toughness of the base material, the upper limit of the S content is set to 0.0050%.

Al: 0.005% or more and 0.100% or less Al is an element added for deoxidation of steel, and it is necessary to contain 0.005% or more. However, if the Al content exceeds 0.100%, not only the toughness of the base metal but also the toughness of the weld metal is lowered. Therefore, the Al content is set to be 0.005% or more and 0.100% or less. Preferably, the Al content is 0.010% or more and 0.100% or less.

Nb: 0.003% to 0.030% Nb is an element necessary for ensuring the strength of the base material and the joint. However, when the Nb content is less than 0.003%, the effect of improving the strength is small. On the other hand, when the Nb content exceeds 0.030%, MA is generated in the HAZ in the vicinity of the bond portion, so that the toughness is lowered. Therefore, the Nb content is in the range of 0.003% to 0.030%. Preferably, the Nb content is 0.008% or more and 0.0020% or less.

Ti: 0.005 or more and 0.050% or less Ti is an element that contributes to the improvement of the base metal toughness by becoming TiN during solidification of the molten steel and precipitating in the base metal and suppressing the coarsening of the austenite grains. Addition is essential. At the same time, Ti reduces N that can be combined with B and ensures solid solution B in the steel, so that it works effectively in securing the strength of the base material. In addition, TiN becomes a ferrite transformation nucleus in HAZ and contributes to high toughness of HAZ. In order to obtain such an effect, the Ti content needs to be 0.005% or more, and preferably 0.015% or more. On the other hand, when the Ti content exceeds 0.050%, the precipitated TiN becomes coarse and the above effect cannot be obtained. Therefore, the Ti content is in the range of 0.005% to 0.050%. Preferably, the Ti content is 0.010% or more and 0.0035% or less.

Cu: 0.10% to 0.50% Cu is an element that contributes to securing the strength of the base material and the joint. In particular, in the HAZ in the vicinity of the bond portion, addition is essential because it contributes to the improvement of joint strength without accompanying significant MA generation. In order to obtain such an effect, the Cu content is 0.10% or more. On the other hand, when the Cu content exceeds 0.50%, the effect of securing the strength of the base material and the joint is saturated. For this reason, the upper limit of the Cu content is 0.50%. Preferably, the Cu content is 0.020% or more and 0.040% or less.

Ni: 0.30% or more and 2.00% or less Ni is an element that improves the toughness of the base material and increases the strength of the base material. Ni also has the effect of suppressing the occurrence of cracks during continuous casting due to the addition of Cu. In order to obtain such an effect, the Ni content is set to 0.30% or more. On the other hand, when the Ni content exceeds 2.0%, the effect of improving the strength of the base material is saturated. For this reason, content of Ni shall be 0.30% or more and 2.00% or less. Preferably, the Ni content is 0.50% or more and 1.50% or less.

N: 0.0030% or more and 0.0100% or less N is an element that contributes to the improvement of the base material toughness by forming TiN during the solidification of the molten steel and precipitating in the base material and suppressing the coarsening of the austenite grains. is there. In order to obtain such an effect, the N content is set to 0.0030% or more. On the other hand, when the N content exceeds 0.0100%, the toughness deteriorates due to the increase in the solid solution N in the region where the TiN is dissolved by the welding heat cycle. For this reason, content of N shall be 0.0030% or more and 0.0100% or less. Preferably, the N content is 0.0040% or more and 0.0080% or less.

B: 0.0003% or more and 0.0025% or less B is an element that reduces solute N by becoming BN in HAZ, and is an effective ferrite transformation nucleus in combination with ACR (Atomic Concentration Ratio) control. Thus, ferrite is generated to improve the toughness of the HAZ. In order to obtain these effects, the B content is set to 0.0003% or more. However, when the B content exceeds 0.0025%, the toughness of the steel plate and the HAZ as the base material is reduced. For this reason, the B content is in the range of 0.0003% to 0.0025%. Preferably, the B content is 0.008% or more and 0.0020% or less.

Ca: 0.0005% or more and 0.0030% or less Ca is an element that improves toughness by fixing S as CaS used as a ferrite nucleus, and is an essential element for ACR control. . In order to obtain such an effect, the Ca content is set to 0.0005% or more. On the other hand, when the content of Ca exceeds 0.0030%, the effect of improving toughness is saturated. For this reason, content of Ca shall be 0.0005% or more and 0.0030% or less of range.
O: Less than 0.0040% O is an element that indirectly affects the formation of a composite granulated product in which MnS is precipitated on CaS. Therefore, the O content is less than 0.0040%. Preferably, the O content is less than 0.0030%.

In the steel sheet for high heat input welding according to the present invention, the composition component of the steel material satisfies the above composition range, and further, ranges of ACR and carbon equivalent C _eq defined in the following formulas (1) and (2) Meet.
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (1)
C _eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (2)
In addition, in (1) Formula and (2) Formula, each element symbol shows content (mass%) of each element in steel materials.

ACR: more than 0 and less than 1 ACR is a parameter formula indicating the toughness of HAZ when steel of each component range is subjected to high heat input welding, and is set to more than 0 and less than 1. By defining the contents of Ca, O and S so as to satisfy the above ACR range, a composite granulated product in which MnS is precipitated on CaS is generated, which becomes a ferrite generation nucleus. By finely dispersing the composite granulated material, the transformation structure is refined and the toughness of the heat affected zone is improved. When ACR is 0 or less, CaS does not crystallize but S precipitates as MnS simple substance. Precipitated MnS extends in the rolling direction during the production of the steel sheet, thereby lowering the base material toughness. Moreover, since MnS melts in HAZ, excellent toughness cannot be obtained. On the other hand, when ACR is 1 or more, most of S is fixed by Ca, and composite inclusions that become ferrite nuclei cannot be obtained. For this reason, the HAZ structure is not refined and the effect of improving toughness cannot be obtained. Furthermore, by setting the ACR to more than 0 and less than 1, BN precipitates on the finely dispersed composite sulfide of MnS and CaS, and can be used as a ferrite nuclei with higher ability. Improvement can be achieved.

C _eq : 0.38 or more and 0.43 or less C _eq is a parameter formula that serves as an index for achieving both joint tensile strength and joint toughness when steel of each component range is subjected to high heat input welding. More than 0.43. By defining the content of the element in the formula (2) so as to satisfy the above range of C _eq , good toughness can be ensured while achieving joint strength exceeding 570 MPa in HAZ. When C _eq is 0.38 or less, the hardenability of the steel sheet is insufficient and the hardness of the HAZ softened region is remarkably lowered, so that the desired joint strength cannot be obtained. On the other hand, when C _eq is 0.43 or more, the hardenability of the steel sheet becomes excessive, and ferrite formation is suppressed in the vicinity of the bond portion and MA formation is promoted, so that excellent toughness cannot be obtained. In addition, in the formula (2), Cr, Mo and V are included, but in the basic component composition of the present invention, Cr, Mo and V are not included unless they are inevitably mixed. May be.
The above is the basic component composition of the steel plate for high heat input welding of the present invention. The balance other than the above components is composed of Fe and inevitable impurities.

[Modification of component]
Next, the modification of the component composition of the steel plate for high heat input welding of this invention is demonstrated. In addition to the above basic component composition, the steel plate for high heat input welding of the present invention contains at least one element selected from V, Cr and Mo within the following range and the range satisfying the above formula (2). Furthermore, it can contain. By including at least one selected from V, Cr and Mo as a selective element, effects such as strength improvement can be obtained.

V: more than 0% and 0.20% or less V is an element that precipitates as VN and contributes to the improvement of strength and toughness of the base material, and also acts as a ferrite forming nucleus. In order to obtain such an effect, the V content is preferably 0.005% or more. However, when the V content is excessive, the toughness is lowered and the alloy cost is further increased. Therefore, the upper limit of the V content is preferably 0.20%.

Cr: more than 0% to 0.40% or less Cr is an element effective for increasing the strength of the base material. In order to obtain such an effect, the Cr content is preferably 0.02% or more. However, when the Cr content is excessive, Cr adversely affects toughness and further increases the alloy cost. For this reason, the upper limit of the Cr content is preferably 0.40%.

Mo: more than 0% and 0.40% or less Mo, like Cr, is an element effective for increasing the strength of the base material. In order to obtain such an effect, the Mo content is preferably 0.02% or more. However, when the Mo content is excessive, Mo adversely affects toughness and further increases the alloy cost. For this reason, the upper limit of the Mo content is preferably 0.40%.

Further, the component composition of the steel plate for high heat input welding of the present invention is in addition to the above component composition containing one or more elements selected from V, Cr and Mo in the basic component composition or the basic component composition, One or more selected from Mg, Zr and REM can be contained as selective elements in the following range.
Mg: 0.0005% or more and 0.0050% or less Mg is an element having an effect of improving toughness due to oxide dispersion. In order to exhibit such an effect, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0050%, the toughness improving effect is saturated. For this reason, the content of Mg is preferably in the range of 0.0005% to 0.0050%.

Zr: 0.0010% or more and 0.0200% or less Zr is an element having an effect of improving toughness due to oxide dispersion, like Mg. In order to exhibit such an effect, the Zr content is preferably 0.0005% or more. On the other hand, when the content of Zr exceeds 0.0200%, the toughness improving effect is saturated. For this reason, it is preferable to make content of Zr into the range of 0.0005% or more and 0.0200% or less.

REM: 0.0010% or more and 0.0200% or less REM is an element having an effect of improving toughness due to oxide dispersion, similarly to Mg and Zr. In order to exhibit such an effect, the REM content is preferably 0.0010% or more. On the other hand, when the content of REM exceeds 0.0200%, the toughness improving effect is saturated. For this reason, it is preferable to make content of REM into the range of 0.0010% or more and 0.0200% or less.

<Manufacturing method of steel plate for large heat input welding>
Next, the manufacturing method of the steel plate for high heat input welding which concerns on this invention is demonstrated. In the method for producing a steel plate for large heat input according to the present invention, first, the molten steel having the above composition is melted and melted by a normal refining method using refining equipment such as a converter, an electric furnace, a vacuum melting furnace and the like. The molten steel is cast by a casting method such as a continuous casting method or an ingot casting method to produce a steel material such as a slab. In the following description of the manufacturing method, the descriptions of the steel plate temperature all indicate the temperature of the steel plate surface.

  Next, the manufactured steel material is heated to a temperature of 1050 ° C. or higher and 1200 ° C. or lower in a heating furnace. In the present invention, in order to completely dissolve Nb carbonitride in the steel material, the heating temperature of the steel material is set to 1050 ° C. or higher. On the other hand, when heating temperature exceeds 1200 degreeC, TiN will become coarse and toughness will deteriorate.

Further, the heated steel material is heated at a temperature range of 850 ° C. or lower and an Ar ₃ transformation point or higher at the end of rolling so that the sheet thickness t after rolling is 25 mm or more and 50 mm or less and the cumulative rolling reduction is 40% or more. It is rolled into a steel plate. Ar ₃ transformation point (° C.) is a temperature calculated by the following equation (3) according to the composition of the steel material.
Ar ₃ transformation point = 900-332C + 6Si-77Mn-20Cu-50Ni-18Cr-68Mo (3)
In the formula (3), C, Si, Mn, Cu, Ni, Cr, and Mo indicate the content (% by mass) of each element.

In hot rolling, rolling with a cumulative reduction of 40% or more is performed in a temperature range of 850 ° C. or lower in order to refine the microstructure of the steel sheet. When the cumulative rolling reduction is less than 40%, the structure becomes coarse and the toughness of the steel sheet is lowered. In addition to the above rolling conditions, rolling is performed in a temperature range where the temperature at the end of rolling is equal to or higher than the Ar ₃ transformation point. When the temperature at the end of rolling is lower than the Ar ₃ transformation point, ferrite is generated during or immediately after rolling, the surface structure becomes processed ferrite, and the ductility is significantly reduced. In the hot rolling, it is only necessary to include rolling with a cumulative reduction of 40% or more in a temperature range of 850 ° C. or lower, and other rolling is not excluded.

After hot rolling, the steel sheet is water-cooled at a cooling rate of 5 ° C./second or more until the surface temperature becomes (−t × 1.5) + 400 ° C. or more and (−t × 1.5) + 620 ° C. or less. The Here, t indicates the thickness of the steel sheet. When the cooling stop temperature is less than (−t × 1.5) + 400 ° C., the self-temper effect due to recuperation cannot be sufficiently obtained, the base material strength becomes excessive, and the ductility and toughness are lowered. On the other hand, when the cooling stop temperature is higher than (−t × 1.5) + 620 ° C., the base material becomes a mixed structure of ferrite or ferrite + bainite, and the base material strength is insufficient. In the steel sheet of the present invention, the yield strength of 460 MPa or more is obtained, so that the metal structure of the steel sheet is a bainite-based structure. When the cooling rate at the time of accelerated cooling is less than 5 ° C./sec, sufficient quenching does not occur and a ferrite-based microstructure is exhibited, making it difficult to ensure a yield strength of 460 MPa or more.
After water-cooling the steel sheet, the steel sheet for high heat input welding is manufactured by air-cooling the steel sheet.

<Summary>
(1) The manufacturing method of the steel plate for high heat input welding which concerns on this invention is the mass%, C: 0.03% or more and 0.10% or less, Si: 0.01% or more and 0.10% or less, Mn: 0.8% or more and 2.0% or less, P: 0.020% or less, S: 0.0005% or more and 0.0050% or less, Al: 0.005% or more and 0.100% or less, Nb: 0.003 %: 0.030% or less, Ti: 0.005% or more and 0.050% or less, Cu: 0.10% or more and 0.50% or less, Ni: 0.30% or more and 2.00% or less, N: 0 .0030% or more and 0.0100% or less, B: 0.0003% or more and 0.0025% or less, Ca: 0.0005% or more and 0.0030% or less, O: less than 0.0040%, and The ACR defined by the formula (1) is more than 0 and less than 1, and the C _eq defined by the following formula (2) is 0.38. The steel material containing 0.43 or less and containing the above components, the balance being Fe and inevitable impurities is heated to 1050 ° C. or more and 1200 ° C. or more, and the heated steel material has a sheet thickness t after rolling of A steel material that has been hot-rolled and hot-rolled at a temperature range of 850 ° C. or lower and an Ar ₃ transformation point or higher at the end of rolling so that the cumulative reduction ratio is 25% or more and 50 mm or less and 40% or more is obtained. The steel material is cooled with water at a cooling rate of 5 ° C./sec or more until it becomes (−t × 1.5) + 400 ° C. or more and (−t × 1.5) + 620 ° C. or less, and the steel material is air-cooled.
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (1)
C _eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (2)
In addition, t in the surface temperature conditional expression indicates the thickness of the steel material after hot rolling, and in the expressions (1) and (2), each element symbol indicates the content (mass%) of each element in the steel material. Indicates.

In the present invention, especially by setting the alloy content of the steel material to the configuration of (1) above, coarsening of austenite grains in the high temperature region is suppressed in the steel sheet structure subjected to welding. Then, in the subsequent cooling process, the structure is refined by intragranular ferrite using finely dispersed composite inclusions as production nuclei, and the amount of MA in bainite is further reduced, so that the toughness of HAZ is improved. In addition, by setting the range of C _eq to the configuration of (1) above, it is possible to simultaneously improve the tensile strength and toughness of the joint.

  Furthermore, in this invention, the temperature which heats a steel raw material is made into the structure of said (1), and deterioration of toughness accompanying the coarsening of TiN can be prevented. Moreover, the fall of toughness by coarsening of a base material structure | tissue can be prevented by making the cumulative reduction rate by hot rolling and the temperature at the time of hot rolling into the structure of said (1). Furthermore, by setting the temperature after hot rolling to the configuration of (1) above, it is possible to prevent a decrease in ductility due to the surface structure becoming processed ferrite. Furthermore, in the present invention, by setting the surface temperature after stopping cooling to the configuration of (1) above, the strength of the base material can be controlled within an appropriate range, and other properties such as ductility and toughness can be improved. Both can be achieved.

  Therefore, according to the configuration of (1) above, the fracture surface has excellent joint characteristics even under high heat input welding with a heat input of 200 kJ / cm or more, yield strength is 460 MPa or more, and exhibits bond HAZ toughness. A steel plate for high heat input welding having a transition temperature vTrs of −40 ° C. or less and a plate thickness of 25 mm or more and 50 mm or less can be stably produced.

(2) In the configuration of (1) above, the steel material is, in mass%, V: more than 0% to 0.20% or less, Cr: more than 0% to 0.40% or less, and Mo: more than 0% to 0.40%. It further contains one or more selected from the following.
With configuration (2) above, it is possible to improve the strength of the base material while suppressing a decrease in the toughness of the base material.
(3) In the configuration of (1) or (2), the steel material is in mass%, Mg: 0.0005% to 0.0050%, Zr: 0.0010% to 0.0200%, REM : It further contains 1 or more types chosen from 0.0010% or more and 0.0200% or less.
According to the configuration of (3) above, an effect of improving toughness due to oxide dispersion can be obtained.

Next, examples performed by the present inventors will be described.
First, using a 150 kg high frequency melting furnace, No. 1 having the component composition shown in Table 1. Steel ingots (steel materials) were manufactured by melting and casting molten steels 1 to 23, respectively. Subsequently, the steel ingots of various thickness were manufactured by hot-rolling each steel ingot. Furthermore, the obtained steel slab was rolled and accelerated cooled under various rolling and accelerated cooling conditions to produce a steel plate having a thickness of 25 mm or more and less than 50 mm. Then, the 1A No. 1A test piece described in JISZ2201 was collected from each steel plate so that the longitudinal direction coincided with the plate width direction, and yield stress YS (MPa), tensile strength TS (MPa), and total elongation El (%) were determined. Measured. In Table 1, steel No. 1 to 8 are examples of the present invention. 9-23 are comparative examples in which the component composition is outside the scope of the present invention.

  In addition, a V-notch Charpy impact test piece described in JISZ2202 was taken from a position at which the thickness of each steel piece was ¼, and the Charpy test piece was collected with appropriate Charpy within a test temperature range of −100 ° C. to 40 ° C. An impact test was performed. From the result of the Charpy test, the fracture surface transition temperature vTrs (° C.) at which the ductile fracture surface ratio was 50% was determined, and the base material toughness was evaluated.

  Furthermore, in order to evaluate the HAZ toughness in the vicinity of the bond part, a test piece of width 80 mm × length 80 mm × thickness 15 mm was taken from each thick steel plate, and the collected test piece was heated to 1450 ° C. and then 800 ° C. The reproduction | regeneration thermal cycle which cools between -500 degreeC in 250 seconds was given. A 2 mmV notch Charpy test piece was collected from these heat-treated test pieces, and the collected Charpy test piece was appropriately subjected to a Charpy impact test within a test temperature range of −100 ° C. to 40 ° C. From the result of the Charpy test, the fracture surface transition temperature vTrs (° C.) at which the ductile fracture surface ratio was 50% was determined, and the toughness in the vicinity of the bond portion was evaluated. The conditions of the reproducible heat cycle correspond to the heat cycle in the vicinity of the bond part in the case of electrogas welding with a heat input of 300 kJ / cm, and simulated one-pass welding with a plate thickness of 40 mm corresponding to the assumed maximum heat input.

  Table 2 also shows the test results of rolling conditions, accelerated cooling conditions, tensile properties (YS, TS, El) of the base material evaluated by the above procedure, and HAZ toughness in the vicinity of the bond part. In Table 2, the steel plate No. 1 to 16 are examples of the present invention. 17-22 are steel No. This is a comparative example in which the steel plates taken from 1 to 8 were rolled and cooled under conditions that fall outside the scope of the present invention. Nos. 23 to 44 are steel Nos. It is the comparative example which rolled and cooled the steel plate extract | collected from 9-30 on the conditions which become outside the scope of the present invention.

  Steel plate No. which is an example. Nos. 1 to 16 are excellent base materials having a yield stress YS of 460 MPa or more, a tensile strength TS of 570 MPa or more, a total elongation El of 16% or more, and a fracture surface transition temperature vTrs for evaluating base material toughness of −50 ° C. or less. It was confirmed that it had characteristics. Steel plate No. In Nos. 1 to 16, it was confirmed that the fracture surface transition temperature vTrs for evaluating the HAZ toughness in the vicinity of the bond part was −40 ° C. or lower, and excellent toughness was obtained in the high heat input welded part.

  On the other hand, in steel plate Nos. 17 to 22, which are comparative examples, although the component composition of steel is included in the present invention, since the manufacturing conditions are out of the range, one or more of the tensile properties, toughness or elongation of the base material is present. It was confirmed that it was lower than that of the example. Moreover, in steel plate No. 23-44 which is a comparative example, although manufacturing conditions correspond to this invention, since the component composition of steel has remove | deviated, especially either bond part vicinity HAZ toughness or the tensile strength of a joint It was also confirmed that both values were lower than those of the examples.

Claims (3)

% By mass
C: 0.03% to 0.10%,
Si: 0.01% or more and 0.10% or less,
Mn: 0.8% or more and 2.0% or less,
P: 0.020% or less,
S: 0.0005% or more and 0.0050% or less,
Al: 0.005% or more and 0.100% or less,
Nb: 0.003% to 0.030%,
Ti: 0.005% or more and 0.050% or less,
Cu: 0.10% or more and 0.50% or less,
Ni: 0.30% or more and 2.00% or less,
N: 0.0030% or more and 0.0100% or less,
B: 0.0003% or more and 0.0025% or less,
Ca: 0.0005% or more and 0.0030% or less,
O: less than 0.0040%, and
ACR defined by the following formula (1) is more than 0 and less than 1, C _eq defined by the following formula (2) satisfies 0.38 or more and 0.43 or less, each component is contained, and the balance is Fe and inevitable A steel material composed of mechanical impurities is heated to 1050 ° C. or higher and 1200 ° C. or lower,
Hot rolling the heated steel material at a temperature range of 850 ° C. or less and Ar ₃ transformation point or more at the end of rolling so that the thickness after rolling is 25 mm or more and 50 mm or less and the cumulative rolling reduction is 40% or more. And
The hot-rolled steel material is water-cooled at a cooling rate of 5 ° C./sec or higher until the surface temperature becomes (−t × 1.5) + 400 ° C. or higher and (−t × 1.5) + 620 ° C. or lower. ,
Method for producing a high heat input welding steel plates, characterized in that the steel material was cooled, the yield strength by cooling to produce a high heat input welding steel plates above 460 MPa.
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S)
... (1)
C _eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
... (2)
Note that t in the surface temperature conditional expression indicates the thickness of the steel material after hot rolling, and in the expressions (1) and (2), each element symbol indicates the content (mass of each element in the steel material). %).   The steel material contains at least one selected from the group consisting of V: more than 0% and 0.20% or less, Cr: more than 0% and 0.40% and Mo: more than 0% and 0.40% or less. Furthermore, it contains, The manufacturing method of the steel plate for high heat input welding of Claim 1 characterized by the above-mentioned.   The steel material is in mass%, Mg: 0.0005% or more and 0.0050% or less, Zr: 0.0010% or more and 0.0200% or less, REM: 0.0010% or more and 0.0200% or less. The method for producing a steel sheet for high heat input welding according to claim 1 or 2, further comprising one or more selected.