JP2013147741A - High-strength steel sheet for heavy heat input welding excellent in material uniformity within the steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet that is excellent in low-temperature toughness in a weld heat affected zone of heavy heat input welding and is excellent in material uniformity within the steel sheet, and a method for producing the same.SOLUTION: A steel sheet has a composition comprising, by mass%, 0.030-0.150% C, ≤0.50% Si, 1.00-2.00% Mn, P, S, Al, Ti, N, Ca, O, Ca, O and S in amounts satisfying a prescribed formula, one or more chosen from B, V, Nb, Ni, Cu, Cr, Mo, W, Mg, Zr and REM if required and the balance being Fe and unavoidable impurities, has a metallic structure substantially comprising ferrite and a bainite structure, has a variation of hardness in the sheet thickness direction of ≤ΔHV50 and has a variation in hardness in the sheet width direction of ≤ΔHV50. The steel having the steel composition is heated to a prescribed temperature, subjected to hot-rolling with the steel sheet surface temperature being 700-900°C, subsequently subjected to a first cooling starting from a steel sheet surface temperature of ≥700°C to reach a steel sheet surface temperature of 400-600°C with a cooling rate at steel sheet surface of 20-100°C/s and subsequently subjected to a second cooling with a steel sheet average cooling rate of ≥4°C/s to reach ≤400°C.

Description

  The present invention is suitable for steel materials used in various welded structures such as offshore structures, architecture, civil engineering, construction industry machines, line pipes, etc. The present invention relates to a high-strength steel sheet having excellent low-temperature toughness and excellent material uniformity within the steel sheet, and a method for producing the same.

  From the viewpoint of increasing the size of steel structures and reducing costs, there is an increasing demand for steel sheets having higher strength and higher toughness. Conventionally, for the purpose of improving the properties of steel sheets, reducing alloy elements, and omitting heat treatment, high-strength steel sheets have been manufactured by applying so-called TMCP technology, which combines controlled rolling and controlled cooling.

  In order to increase the strength of steel using the TMCP technology, it is effective to increase the cooling rate during controlled cooling and to lower the temperature at which cooling is stopped. However, when controlled cooling is performed at a high cooling rate, the surface layer portion of the steel sheet is rapidly cooled, so that the hardness of the surface layer portion is higher than that inside the steel plate, and the hardness distribution in the thickness direction varies.

  In addition, the variation in hardness distribution in the thickness direction tends to increase as the cooling stop temperature is lowered and the thickness increases, which is a problem in terms of ensuring material uniformity in the steel plate. .

  Patent Document 1 discloses a method of suppressing an increase in surface hardness with respect to the center portion of the plate thickness by controlling the cooling rate to a relatively low cooling rate of 3 to 12 ° C./s during controlled cooling.

  In Patent Document 2, by waiting in a cooling process in the temperature range where ferrite precipitates, the steel sheet has a two-phase structure of ferrite and bainite, and the difference in hardness between the surface layer and the thickness center is reduced. A method of manufacturing a steel sheet having a small material difference in the thickness direction is disclosed.

  In Patent Documents 3 and 4, the manufacture of steel sheets having a small material difference in the sheet thickness direction after performing rolling and controlled cooling at a high cooling rate for reheating the surface before the surface layer portion completes the bainite transformation. A method is disclosed.

  In addition, if there is unevenness in the scale properties on the surface of the steel sheet, the cooling rate of the lower steel sheet varies depending on the thickness of the scale during cooling, that is, the cooling stop temperature varies in the part of the steel sheet, Corresponding to the properties, the steel plate material varies in the plate width direction.

  Patent Documents 5 and 6 disclose a method for reducing uneven cooling due to scale properties and improving the steel plate shape by performing descaling immediately before cooling.

  On the other hand, steel materials used in the fields of shipbuilding, construction, civil engineering, etc. are generally finished in a desired shape by welding, so of course the base material toughness of the steel materials used is of course from the viewpoint of safety. That is, it is required that the toughness of the welded portion is excellent.

  These structures and ships are becoming larger in size, and with the increase in strength and thickness of the steel materials used, the welding work is highly efficient, such as submerged arc welding, electrogas welding, and electroslag welding. Thermal welding is applied. For this reason, when welding is performed by high heat input welding, a steel material excellent in toughness of the welded portion is required.

  However, it is generally known that as the welding heat input increases, the structure of the weld heat affected zone (HAZ) becomes coarse, and the toughness of the weld heat affected zone decreases. As measures for improving the reduction in toughness due to such high heat input welding, the use of austenite as a ferrite transformation nucleus has been put into practical use so far by suppressing the coarsening of austenite by fine dispersion of TiN.

  Further, in Patent Document 7 and Patent Document 8 aiming to improve toughness at a weld bond portion with a heat input of 230 kJ / cm, a rare earth element (REM) is added in combination with Ti to disperse fine particles in steel. Thus, a method of suppressing the austenite grain growth and improving the toughness of the welded portion is shown.

  Furthermore, a technique for combining a Ti oxide dispersion technique and a BN ferrite nucleation ability has been developed. In addition, it is known that by adding Ca or REM, the form of sulfide can be controlled and higher toughness can be obtained.

Japanese Patent Publication No.7-116504 Japanese Patent No. 3911834 Japanese Patent No. 3951428 Japanese Patent No. 3951429 JP-A-9-57327 Japanese Patent No. 3796133 Japanese Patent Publication No. 03-53367 JP 60-184663 A

  However, since the technology of Patent Document 1 has a relatively high cooling stop temperature of 400 ° C. or higher, the effect of controlled cooling such as high strength by high cooling rate, reduction of alloy elements, simplification of controlled rolling, etc. is fully utilized. Can not do it.

  In the manufacturing method of Patent Document 2, since ferrite is precipitated during cooling standby below the Ar3 transformation point, the strength is lowered, and the cooling standby time is required, so that manufacturing efficiency is deteriorated.

  In the production methods of Patent Document 3 and Patent Document 4, if the transformation behavior differs depending on the components of the steel sheet, sufficient material homogenization effect due to recuperation may not be obtained. Moreover, since highly accurate cooling control is required, the application range is limited and the manufacturing efficiency is deteriorated.

  Moreover, although the method of patent document 5 and the patent document 6 has improved the steel plate shape by reducing the dispersion | variation nonuniformity of the cooling stop temperature of a steel plate by descaling, the consideration with respect to the hardness distribution of a plate | board thickness direction is not made | formed.

  The variation in the cooling stop temperature due to the unevenness of the scale is large in the case of cooling in a range where the cooling rate is relatively low. Moreover, in the case of a low cooling rate, there exists a possibility that the average cooling rate of a steel plate will fall by descaling and intensity | strength may fall. In that case, in order to prevent a decrease in strength, it is necessary to attach a scale having a certain scale thickness to the steel plate surface.

  On the other hand, when cooling at a high cooling rate to increase the strength, the variation in cooling stop temperature due to unevenness in scale can be reduced, but the hardness of the surface layer is sensitive to the influence of the scale thickness. Suppression of hardness rise becomes a problem.

  The technique of improving the toughness of the heat-affected zone with high heat input welding by adding rare earth elements (REM) described in Patent Document 7 and Patent Document 8 together with Ti is difficult to produce a steel material with stable toughness. In addition, there is a problem that sufficient toughness cannot be obtained by high heat input welding exceeding 300 kJ / cm.

  In the technique mainly using TiN, there is no effect in the welded portion heated to a temperature range where TiN dissolves, and the toughness is significantly lowered due to the embrittlement of the ground structure by solid solution Ti and N. It was.

  Further, the technique using Ti oxide has a problem that the fine dispersion of the oxide cannot be made sufficiently uniform. Further, even in the technique of adding Ca or REM, it has been difficult to ensure high toughness of the heat affected zone by high heat input welding exceeding 300 kJ / cm.

  Therefore, the object of the present invention is to solve such problems of the prior art, have excellent material uniformity in the steel sheet, and toughness of the heat affected zone when high heat input welding exceeding 300 kJ / cm is performed. It is to provide a high-strength steel sheet excellent in resistance and a method for producing the same.

The object of the present invention can be achieved by the following means.
1. Steel composition is mass%,
C: 0.030 to 0.150%,
Si: 0.50% or less,
Mn: 1.00 to 2.00%,
P: 0.030% or less,
S: 0.0005 to 0.0040%,
Al: 0.005 to 0.100%,
Ti: 0.004 to 0.030%,
N: 0.0035 to 0.0075%,
Ca: 0.0005 to 0.0030%,
O: 0.0040% or less,
Each content of Ca, O, and S satisfies the following formula (1), consists of the balance Fe and unavoidable impurities, the metal structure is substantially a ferrite and bainite structure, and there is a variation in hardness in the thickness direction. A high-strength steel sheet for high heat input welding with excellent material uniformity in a steel sheet, characterized by having a Vickers hardness of ΔHV50 or less and a variation in hardness in the plate width direction being a Vickers hardness of ΔHV50 or less.
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.8 (1)
However, Ca, O, and S are the contents of each component (mass%).
2. 2. The steel composition further comprises one or two selected from B: 0.0003 to 0.0030 and V: 0.20% or less in mass%. High strength steel plate for high heat input welding with excellent material uniformity in the steel plate.
3. Further, the steel composition is mass%, Nb: 0.030% or less, Ni: 1.00% or less, Cu: 1.00% or less, Cr: 0.70% or less, Mo: 0.70% or less, W: For high heat input welding with excellent material uniformity in the steel sheet according to 1 or 2, characterized by containing one or more selected from 0.05 to 0.40% High strength steel plate.
4). The steel composition is further selected by mass%, Mg: 0.0005-0.0050%, Zr: 0.001-0.020%, REM: 0.001-0.020% The high-strength steel plate for high heat input welding excellent in material uniformity in the steel plate according to any one of 1 to 3 containing two or more kinds.
The steel having the steel composition according to any one of 5.1 to 4 is heated to a temperature of 900 ° C. or higher and 1300 ° C. or lower, and the rolling end temperature is 700 ° C. or higher and 900 ° C. or lower at the steel sheet surface temperature. Then, the steel sheet surface temperature is 700 ° C. or higher, the cooling rate on the steel plate surface is 20 ° C./s or higher and 100 ° C./s or lower, and the steel sheet surface temperature is 400 ° C. or higher and 600 ° C. or lower. Large heat input with excellent material uniformity in the steel sheet, characterized by cooling the second stage and then cooling the second stage until the average cooling rate of the steel sheet is 4 ° C / s or higher and 400 ° C or lower. A method for producing high-strength steel sheets for welding.
3 ≦ (700−T) / V (2)
T: Finishing temperature of the steel sheet surface cooling in the first stage cooling (° C.), V: Cooling rate on the steel sheet surface in the first stage cooling (° C./s)
6). After hot rolling, before performing the first stage cooling, the impact pressure of the jet flow on the steel sheet surface is descaled at 1 MPa or more, and then the first stage cooling is performed within 5 seconds. The manufacturing method of the high strength steel plate for high heat input welding excellent in the material uniformity in the steel plate of 5.

  According to the present invention, it greatly contributes to improving the quality of a large structure constructed by high heat input welding such as submerged arc welding, electrogas welding, and electroslag welding.

In the present invention, variations in hardness in the plate thickness direction and the plate width direction are defined as indicators of component composition, metal structure, and material uniformity.
[Component Composition] In the following description, “%” is mass%.
C: 0.030 to 0.150%
The lower limit of C content is 0.030% in order to obtain the strength required for structural steel, and the upper limit is 0.150% in order to suppress the amount of island martensite produced.

Si: 0.50% or less Si is added because it has the effect of improving the strength of the base material. Since this effect is exhibited by containing 0.05% or more, it is preferable to contain 0.05% or more, and more preferably to contain more than 0.10%, but 0.50%. If contained in excess, the toughness of the high heat input welding heat-affected zone is deteriorated, so the upper limit is made 0.50%.

Mn: 1.00 to 2.00%
Mn needs to be 1.00% or more in order to ensure the strength of the base material, and if it exceeds 2.00%, the toughness of the welded portion is deteriorated. Preferably, it is in the range of 1.00% to 1.60%.

P: 0.030% or less P is an impurity that is inevitably mixed. If P exceeds 0.030%, the toughness of the base metal and the welded portion is lowered, so the upper limit is made 0.030%.

S: 0.0005 to 0.0040%
S is required to be 0.0005% or more in order to produce required CaS or MnS, and if it exceeds 0.0040%, the toughness of the base material is deteriorated.

Al: 0.005 to 0.100%
Al needs to be 0.005% or more, preferably 0.010% or more in terms of deoxidation of steel, and if contained over 0.100%, the toughness of the base metal is lowered and the toughness of the weld metal is deteriorated at the same time. .

Ti: 0.004 to 0.030%
Ti precipitates as TiN during solidification and contributes to high toughness by suppressing austenite coarsening in the weld heat affected zone and becoming a ferrite transformation nucleus. If less than 0.004%, the effect is small, and if it exceeds 0.030%, the expected effect cannot be obtained due to the coarsening of TiN particles.

N: 0.0035 to 0.0075%
N is an element necessary for securing the necessary amount of TiN, and if it is less than 0.0035%, a sufficient amount of TiN cannot be obtained, and if it exceeds 0.0075%, TiN is dissolved in a region where it is melted by the welding heat cycle. The toughness is remarkably lowered by increasing the amount of dissolved N.

Ca: 0.0005 to 0.0030%
Ca is an element having an effect of improving toughness by fixing S. In order to exhibit such an effect, it is preferable to contain at least 0.0005% or more, but even if it exceeds 0.0030%, the effect is saturated. For this reason, in this invention, it limits to 0.0005% to 0.0030% of range.

O: 0.0040% or less O precipitates as an oxide during solidification. When it contains exceeding 0.0040%, the toughness of a base material and a welding heat affected zone will fall.

0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.8 (1) where Ca, O, and S are the contents of each component (mass%). Represent.
Ca, O, and S must be contained so as to satisfy the relationship of 0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.8. In this case, it becomes the form of the composite sulfide in which MnS is deposited on CaS. When the value of Ca− (0.18 + 130 × Ca) × 1.25) / S exceeds 0.8, S is almost fixed by Ca, and MnS that functions as a ferrite nucleation does not precipitate on CaS. The toughness of the heat affected zone cannot be secured. When Ca− (0.18 + 130 × Ca) × O) /1.25/S is 0 or less, since CaS does not crystallize, S precipitates in the form of MnS alone, and is fine in the weld heat affected zone. Dispersion is not achieved.

  The above is the balance Fe and unavoidable impurities in the basic steel composition of the present invention, but when further improving the strength toughness of the steel sheet and the weld heat affected zone toughness, B, V, Nb, Ni, Cu, Cr, Mo, W , Mg, Zr, or REM may be contained.

B: 0.0003 to 0.0030%
B is an element that generates BN in the weld heat affected zone to reduce solid solution N and act as a ferrite generation nucleus. Such an effect is exhibited when the content is 0.0003% or more, but if added over 0.0.0030%, the hardenability is excessively increased and the toughness is deteriorated. Therefore, when it contains B, it is preferable to set it as 0.0003 to 0.0030%.

V: 0.20% or less V increases the strength and toughness of the base metal and acts as a ferrite nucleation core as VN. However, if it exceeds 0.20%, it causes a decrease in toughness. Therefore, when V is contained, the content is preferably 0.20% or less.

Nb: 0.030% or less Nb is an element effective for securing the strength and toughness of the base metal and the strength of the joint, but if it exceeds 0.030%, island martensite is contained in the weld heat affected zone. The toughness deteriorates due to the formation. Therefore, when it contains Nb, it is preferable to set it as 0.030% or less.

Ni: 1.00% or less Ni increases the strength while maintaining the high toughness of the base material, but the effect is saturated even if it exceeds 1.00%. Therefore, when it contains Ni, it is preferable to set it as 1.00% or less.

Cu: 1.00% or less Cu has a function similar to that of Ni, but when it exceeds 1.00%, hot brittleness is caused and the surface properties of the steel sheet are deteriorated. Therefore, when it contains Cu, it is preferable to set it as 1.00% or less.

Cr: 0.70% or less Cr is an element effective for increasing the strength of the base material, but if added in a large amount, it adversely affects toughness. Therefore, when Cr is contained, the content is preferably 0.70% or less.

Mo: 0.70% or less Mo is an element effective for increasing the strength of the base material, but if added in a large amount, it adversely affects toughness. Therefore, when Mo is contained, the content is preferably 0.70% or less.

W: 0.05-0.40%
W is an element effective for increasing the strength of the base material, and this effect is exhibited when it is contained in an amount of 0.05% or more, but if added excessively, the toughness may be adversely affected. Therefore, when it contains W, it is preferable to set it as 0.05 to 0.4%.

Mg: 0.0005 to 0.0050%
Mg is an element having an effect of improving toughness due to dispersion of oxides. In order to exhibit such an effect, it is preferable to contain 0.0005 or more, but even if it exceeds 0.0050%, the effect is saturated. Therefore, when it contains Mg, it is preferable to set it as 0.0005 to 0.0050%.

Zr: 0.001 to 0.020%
Zr is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is preferable to contain 0.001% or more, but even if it exceeds 0.020%, the effect is saturated. Therefore, when it contains Zr, it is preferable to set it as 0.001-0.020%.

REM: 0.001-0.020%
REM is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is preferable to contain 0.001% or more, but even if it exceeds 0.020%, the effect is saturated. Therefore, when it contains REM, it is preferable to set it as 0.001-0.020%.

[Microstructure]
In order to increase the tensile strength of 490 MPa or more, the metal structure is substantially a mixed structure of ferrite and bainite. Substantially the mixed structure of ferrite and bainite means that the total volume fraction of ferrite and bainite is 95% or more, and the balance is martensite, pearlite, island martensite, residual austenite, etc. It is a structure containing 2 or more types by volume fraction of 5% or less.

  In addition to ferrite and bainite, the metal structure contains one or more of martensite, pearlite, island martensite, and retained austenite in a volume fraction exceeding 5%, resulting in a decrease in strength and toughness deterioration. Since the surface hardness increases, the volume fraction of these tissues is set to 5% or less. However, when a hard phase such as martensite or island-like martensite (MA) is generated in the surface layer portion, the surface layer hardness increases and the hardness variation in the steel sheet described later increases. It is desirable to use a mixed structure consisting of only a bainite structure.

[Hardness variation]
The variation in hardness in the plate thickness direction is set to ΔHV50 or less in terms of Vickers hardness (load 10 kg, hereinafter the same), and the variation in hardness in the plate width direction is set to ΔHV50 or less in terms of Vickers hardness.

  From the viewpoint of strength, elongation, formability, and the like of the steel sheet, it is required to suppress the variation in hardness within the steel sheet. If the variation in hardness in the thickness direction exceeds ΔHV50 in terms of Vickers hardness, or if the variation in hardness in the width direction of the plate exceeds ΔHV50 in terms of Vickers hardness, the above characteristics are adversely affected.

  For example, when the hardness of the steel sheet surface layer portion exceeds ΔHV50 as compared with the inside of the steel sheet, springback is likely to occur after forming. Also, if the hardness distribution in the width direction of the steel sheet exceeds ΔHV50, when the shape is not as expected due to the difference in deformation method between the hard part and the soft part at the time of forming, or when cut into small plates The strength and elongation of the small plates are different. The method for measuring the hardness will be described in detail in the examples. The variation is a difference between the maximum value and the minimum value in the measurement result.

Next, the manufacturing method of this invention is demonstrated.
The high-strength, high-toughness steel sheet according to the present invention is a first-stage cooling (also referred to as primary cooling), in which the steel material is melted and then hot-rolled, and then the cooling process performed subsequently is divided into two stages. In order to increase the strength of the steel sheet as a whole, a microstructure that suppresses hardening in the surface layer of the steel sheet is built, and the steel sheet is strengthened and toughened exclusively in the second stage cooling (also called secondary cooling). It can be manufactured.
In the present invention, 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 a steel material such as a slab by a conventional method such as a continuous casting method or an ingot-bundling method. It is preferable that The melting method and the casting method are not limited to the methods described above.
Next, the obtained steel material such as slab is heated and hot rolled to obtain a steel plate.

[Slab heating temperature]
Slab heating temperature shall be 900 degreeC or more and 1300 degrees C or less. When the heating temperature is less than 900 ° C., the homogenization of the microstructure is insufficient and the required strength and toughness cannot be obtained. When the heating temperature exceeds 1300 ° C., the toughness deteriorates. Here, the temperature is the furnace temperature of the heating furnace, and the slab is sufficiently heated to this temperature to the center.

[Rolling end temperature]
The rolling end temperature is 700 ° C. or higher and 900 ° C. or lower as the surface temperature of the steel sheet. If the temperature is lower than 700 ° C., the cooling start temperature is lowered, and sufficient strength cannot be obtained. If the temperature exceeds 900 ° C., the microstructure becomes coarse and the toughness deteriorates. In addition, the surface temperature of a steel plate can be measured with a radiation thermometer or the like.

[First stage cooling condition]
As described above, it is a feature of the present invention that the cooling process after hot rolling is divided into two stages.
The cooling start temperature of the first stage cooling is 700 ° C. or more at the surface temperature of the steel sheet. If the temperature is lower than 700 ° C, sufficient strength cannot be obtained.

  The cooling rate is 20 ° C./s to 100 ° C./s on the steel sheet surface. It is important to control the cooling rate in order to reduce the hardness variation in the steel sheet and improve the material uniformity while increasing the strength. When the cooling rate of the steel sheet surface is less than 20 ° C./s, sufficient strength cannot be obtained in the whole steel sheet, and when it exceeds 100 ° C./s, a hard phase such as martensite or island martensite (MA) is formed in the steel sheet surface layer. Is generated and the surface hardness is remarkably increased, so that the cooling rate on the surface of the steel sheet is 20 ° C./s or more and 100 ° C./s or less.

  The cooling stop temperature is 400 ° C. or more and 600 ° C. or less at the surface temperature of the steel sheet. By cooling at a rate of 20 ° C./s or more and 100 ° C./s or less from a temperature range of 700 ° C. or more to a temperature range of 400 ° C. or more and 600 ° C. or less, ferrite and a bainite phase are generated in the steel sheet surface layer portion.

  If the cooling stop temperature is less than 400 ° C., the start of the second-stage cooling is delayed and the cooling effect becomes insufficient, and high strength and toughness cannot be achieved.

  On the other hand, when the temperature exceeds 600 ° C., the generation of ferrite and bainite is not sufficient, and when cooling in the second stage is started in that state, martensite and island martensite (MA) are generated in the surface layer portion. The eye cooling stop temperature is 400 ° C. or more and 600 ° C. or less as the surface temperature of the steel sheet.

Further, the first stage cooling is performed so as to satisfy the following expression (2).
3 ≦ (700−T) / V (2) where T is the cooling end temperature (° C.) at the first stage cooling steel plate surface, and V is the cooling rate at the first stage cooling steel plate surface ( ° C / s)
Equation (2) indicates that the cooling time for the first stage cooling requires 3 seconds or more. This is because it takes 3 seconds or more for the ferrite and bainite phases to be sufficiently formed so that the structure of the surface layer does not become hard.

  When the formula (2) is not satisfied, martensite and island-like martensite (MA) are generated in the steel sheet surface layer portion, the hardness of the surface layer portion is significantly increased, and the variation in hardness is increased.

[Second stage cooling conditions]
The cooling rate is 4 ° C./s or more on the average steel plate. If it is less than 4 ° C./s, the effect of increasing the strength cannot be obtained sufficiently, so that it is 4 ° C./s or more. The average temperature and cooling rate of the steel sheet cannot be directly measured physically, but can be obtained in real time by simulation calculation based on the temperature change of the surface. In the case of a thick steel plate having a plate thickness of 60 mm or more and an average steel plate of 4 ° C./s or more, the cooling rate on the steel plate surface exceeds 100 ° C./s in the temperature range of 200 ° C. or more. Cooling is necessary.

  Cooling stop temperature shall be 400 degrees C or less with the average temperature of a steel plate. In steels with a streamlined steel composition with reduced alloying elements, if cooling is stopped at a temperature exceeding 400 ° C., sufficient strength cannot be obtained, so the cooling stop temperature is 400 ° C. or lower. With the above manufacturing method, it is possible to manufacture a high-strength steel sheet with excellent material uniformity within the steel sheet. However, when further improving the material uniformity, the high impact pressure should be descaled just before the first stage cooling. It is desirable to do.

  In the steel sheet after rolling, if the scale thickness is large, the scale thickness may become uneven due to descaling, etc. before rolling or during rolling, or part of the scale may peel off, resulting in partial cooling of the steel sheet surface. The uniformity of the material is impaired.

  However, if the descaling is performed immediately before the first stage cooling so that the scale thickness of the steel sheet after the secondary cooling is 15 μm or less, even if unevenness occurs in the thickness of the scale, there is no great difference in the cooling rate. Thus, the variation in hardness in the plate width direction can be stabilized to ΔHV30 or less. Although it is practically difficult to measure the scale thickness of the steel sheet immediately before the control cooling, that is, immediately before the first stage cooling in the present invention, the scale thickness before the control cooling is the value after the control cooling (the second stage in the present invention). It was elucidated that the desired effect can be obtained by performing descaling immediately before cooling so that the scale thickness of the steel sheet after controlled cooling is 15 μm or less. .

  When descaling is performed, the scale thickness after the second stage cooling is set to 15 μm or less, so that the collision pressure of the jet flow on the steel plate surface is set to 1 MPa or more, and then the first stage cooling is performed within 5 seconds. It is desirable. If the collision pressure of the jet flow on the steel plate surface is less than 1 MPa, the scaling may be uneven due to insufficient descaling, so the collision pressure of the jet flow is 1 MPa or more. Descaling is performed using high-pressure water, but other jet streams may be used as long as the collision pressure of the jet stream on the steel sheet surface is 1 MPa or more.

  After the descaling, if the first stage cooling is performed after 5 seconds, the scale grows and the scale thickness after the second stage cooling exceeds 15 μm. It is preferable to start the first stage cooling within seconds.

  Steel having the composition shown in Table 1 was melted and rolled and cooled under the conditions shown in Table 2 to produce a thick steel plate having a thickness of 60 mm. The metal structure and scale properties of the obtained steel sheet were observed with an optical microscope and a scanning electron microscope.

  In the metal structure observation, a cross-sectional structure photograph of 10 fields of view was obtained from ¼ part of the plate thickness t, and the phase fraction (volume fraction) was measured using an image analyzer. Moreover, scale thickness was measured from the cross-sectional structure | tissue photograph containing the steel plate surface, and it evaluated by the average value of 10 visual fields. Since the scale does not grow after the second stage cooling is completed, the scale thickness observed above may be regarded as the scale thickness immediately after the second stage cooling.

  Mechanical properties are obtained by collecting a round bar tensile test piece with a parallel part of 14φ x 85 mm and a distance between gauge points of 70 mm from the direction perpendicular to the rolling of 1/4 part of the sheet thickness t, and conducting a tensile test to evaluate the strength of the base material. did. In addition, a 2 mm V notch Charpy test piece was taken with the 1/4 direction rolling direction of the plate thickness t as the longitudinal direction, and the absorbed energy vE (−20 ° C.) (average value of 3 pieces) at a test temperature of −20 ° C. was measured. The toughness of the base material was evaluated.

  Further, the hardness in the plate thickness direction and the hardness in the plate width direction were measured with a Vickers hardness tester. The hardness in the plate width direction was measured at 1 mm below the surface, 1/4 position of the plate thickness t, and 1/2 position of the plate thickness t (plate thickness center portion). Since the variation in hardness showed the maximum at the position, the variation in hardness in the plate width direction was evaluated at a position 1 mm below the surface.

  Furthermore, after producing a joint by electrogas welding (EGW) of high heat input welding (about 400 kJ / cm), the surface 1 mm position in the plate thickness direction (1 mm position from the surface to the inside of the steel plate) and the back surface 1 mm position (from the back surface to the steel plate) Using a Charpy test piece with a notch in the bond part of the weld heat-affected zone, the absorption energy (vE-20 ° C.) at a test temperature of −20 ° C. (average value of three) was evaluated. The evaluation results are shown in Table 3.

From Table 3, Example No. of the present invention. Both the 1-8 yield stress 355N / mm ² or more, a tensile strength of 490 N / mm ² or more, vE-40 ° C. or higher 100 J, the scale thickness is 30μm or less, the microstructure of the steel sheet is substantially a ferrite In the bainite structure, the hardness variation in the sheet thickness direction is ΔHV50 or less and the hardness variation in the sheet width direction is ΔHV50 or less. It was.

  Further, the steel of the present invention has an absorption energy vE-20 ° C. of 50 J or more in the weld heat affected zone (notch position: bond portion) and is excellent in weld heat affected zone toughness. In particular, Invention Example No. 5: Descaling before cooling the first stage The impact pressure of the jet flow was 2 MPa, and after the descaling, the first stage cooling was performed in 4 seconds, so the scale thickness was 3 μm, the thickness direction, the width The variation in direction hardness is small, and the material uniformity is particularly excellent.

  On the other hand, at least one of the manufacturing conditions or the steel composition is a comparative example that is outside the scope of the present invention. Nos. 9 to 25 are inferior to the examples of the present invention in any one or more of the above characteristics. No. In Nos. 9 to 15, at least one of the production conditions is outside the scope of the present invention. 16-25 is a comparative example whose steel composition is outside the scope of the present invention.

Claims (6)

Steel composition is mass%,
C: 0.030 to 0.150%,
Si: 0.50% or less,
Mn: 1.00 to 2.00%,
P: 0.030% or less,
S: 0.0005 to 0.0040%,
Al: 0.005 to 0.100%,
Ti: 0.004 to 0.030%,
N: 0.0035 to 0.0075%,
Ca: 0.0005 to 0.0030%,
O: 0.0040% or less,
Each content of Ca, O, and S satisfies the following formula (1), consists of the balance Fe and unavoidable impurities, the metal structure is substantially a ferrite and bainite structure, and there is a variation in hardness in the thickness direction. A high-strength steel sheet for high heat input welding with excellent material uniformity in a steel sheet, characterized by having a Vickers hardness of ΔHV50 or less and a variation in hardness in the plate width direction being a Vickers hardness of ΔHV50 or less.
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.8 (1)
However, Ca, O, and S are the contents of each component (mass%).   The steel composition further comprises one or two selected from B: 0.0003 to 0.0030 and V: 0.20% or less in mass%. A high-strength steel plate for high heat input welding with excellent material uniformity in the steel plate described.   Further, the steel composition is mass%, Nb: 0.030% or less, Ni: 1.00% or less, Cu: 1.00% or less, Cr: 0.70% or less, Mo: 0.70% or less, W: One or two or more selected from 0.05 to 0.40% are contained, and the material excellent in material uniformity in the steel sheet according to claim 1 or 2, High strength steel plate for heat input welding.   The steel composition is further selected by mass%, Mg: 0.0005-0.0050%, Zr: 0.001-0.020%, REM: 0.001-0.020% The high-strength steel sheet for high heat input welding excellent in material uniformity in the steel sheet according to any one of claims 1 to 3, comprising two or more kinds. The steel having the steel composition according to any one of claims 1 to 4 is heated to a temperature of 900 ° C or higher and 1300 ° C or lower, and the rolling finish temperature is 700 ° C or higher and 900 ° C or lower at a steel sheet surface temperature. Then, the steel sheet surface temperature is 700 ° C. or higher, the cooling rate on the steel plate surface is 20 ° C./s or higher and 100 ° C./s or lower, and the steel sheet surface temperature is 400 ° C. or higher and 600 ° C. or lower. Large heat input with excellent material uniformity in the steel sheet, characterized by cooling the second stage and then cooling the second stage until the average cooling rate of the steel sheet is 4 ° C / s or higher and 400 ° C or lower. A method for producing high-strength steel sheets for welding.
3 ≦ (700−T) / V (2)
T: Finishing temperature of the steel sheet surface cooling in the first stage cooling (° C.), V: Cooling rate on the steel sheet surface in the first stage cooling (° C./s)   After hot rolling, before performing the first stage cooling, the impact pressure of the jet flow on the steel sheet surface is descaled at 1 MPa or more, and then the first stage cooling is performed within 5 seconds. The manufacturing method of the high strength steel plate for high heat input welding excellent in the material uniformity in the steel plate of Claim 5.