JP2009235548A - Low yield ratio high tensile strength thick steel plate having excellent toughness in super-large heat input weld affected zone, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low yield ratio high tensile strength thick steel plate having excellent toughness in a super-large heat input weld affected zone, which has a tensile strength of ≥590 MPa and a yield ratio of ≤80%, can obtain a high toughness HAZ by super-large heat input welding, and used in the fields of civil engineering, building, bridges or the like, and to provide a method for producing the same. <P>SOLUTION: A steel stock having a composition comprising prescribed amounts of C, Si, Mn, P, S, Al, Cr, Nb, Mo, V, B, Ti, Ca, N and O, and further comprising one or more kinds selected from Cu and Ni, and in which Ceq satisfies 0.44 to 0.50, Pcm satisfies ≤0.21 and ACR satisfies 0.2 to 0.8, and the balance Fe with inevitable impurities is subjected to hot rolling, so as to be a thick steel plate. The thick steel plate is then reheated at 900 to 1,000°C, is held for ≥20 min, is thereafter subjected to reheat quenching treatment, is further heated to the two phase region temperature of (Ac1+20°C) to (Ac1+80°C), is held for ≥30 min, is subsequently subjected to two phase region quenching treatment, and is further subjected to tempering treatment at 400 to 600°C. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a low-yield-ratio high-tensile steel plate suitable for welded structures constructed in the fields of civil engineering, architecture, bridges, and the like, and in particular, high heat input welding (hereinafter referred to as super-large size) exceeding 400 kJ / cm. Excellent heat-affected zone toughness by repeated welding (hereinafter referred to as small heat input multi-pass welding) with a low heat input of 20 to 50 kJ / cm, yield strength of 440 MPa or more, and tensile strength of 590 MPa As described above, the present invention relates to a low yield ratio high tensile steel plate having a yield ratio of 80% or less and a method for manufacturing the same.

  In recent years, the size of welded structures has been increased, and along with this, there has been an increasing demand for higher strength and thicker steel sheets. Further, from the viewpoint of improvement in construction efficiency and construction cost in construction for constructing a welded structure, improvement in welding efficiency is demanded, and high heat input welding is widely adopted. In particular, in the joining of box columns used in high-rise buildings, super-high heat input welding (for example, submerged arc welding, electroslag welding, etc.) is performed such that the welding heat input exceeds 400 kJ / cm.

  On the other hand, welded structures in the fields of civil engineering, architecture, bridges, etc. are required to have earthquake resistance, and a low yield ratio (for example, a yield ratio of 80% or less) in order to ensure plastic deformation of the welded structure and prevent collapse ) Steel plate is used. Furthermore, in order to further improve the earthquake resistance of the welded structure, high toughness has been required for the welded joints of steel plates. For example, in high-rise buildings, high toughness such as Charpy absorbed energy at 0 ° C. of 70 J or more is also required for welded joints between box columns and welded joints between box columns and beams.

  When super-high heat input welding is performed, it is the weld heat affected zone (hereinafter referred to as HAZ) that significantly reduces toughness, and is most reduced in a region called a bond portion in the vicinity of the melt line. In the bond part, the austenite grains become coarse by being exposed to a high temperature close to the melting point during super-high heat input welding, and subsequently cooled to form a structure with low toughness such as upper bainite and martensite in the austenite grains. . As a result, the toughness of HAZ decreases.

In addition, when small heat input multi-pass welding is performed, a region (hereinafter referred to as ICCGHAZ) is reheated to a two-phase region by a subsequent pass in the bond portion. In this ICCGHAZ, island-like martensite is generated and the toughness is lowered.
Particularly in high-strength steel with a tensile strength exceeding 590 MPa, a large amount of alloying elements is added, so that the yield ratio of the steel plate increases, and HAZ (for example, bond portion of super-high heat input welding, ICCGHAZ of small heat input multi-pass welding, etc.) ) Tendencies to decrease. Therefore, various techniques for producing high-strength thick steel plates having excellent HAZ toughness and low yield ratio have been studied.

  For example, Patent Document 1 discloses that a rare earth element (hereinafter referred to as REM) oxysulfide (that is, oxysulfide) and TiN are combined and dispersed in a steel sheet to suppress coarsening of austenite grains, and high heat input welding. A technique for improving the toughness of HAZ in is disclosed. However, in ultra-high heat input welding, TiN is exposed to a high temperature close to the melting point for a long time and is dissolved, so that there is a problem that the effect of suppressing the growth of austenite grains cannot be obtained. In addition, when a large amount of alloying elements such as REM and Ti are added to increase the strength of the steel sheet, the hardenability of the steel sheet increases, so a structure with low toughness such as upper bainite and martensite is generated in the austenite grains. There is a problem that it becomes easy.

  Patent Document 2 discloses a technique for improving the toughness of HAZ by dispersing Ti oxide in a steel sheet and utilizing it as a ferrite nucleus. However, it is difficult to uniformly and finely disperse the Ti oxide, and it is difficult to stably maintain the quality of the steel plate produced in large quantities. Further, when various alloy elements are added in a large amount in order to increase the strength of the steel sheet, the hardenability of the steel sheet is increased, so that upper bainite and martensite are easily generated and ferrite is difficult to generate.

Patent Document 3 includes a quenching process (hereinafter referred to as a two-phase region quenching process) in which Ca oxysulfide having an optimized form is dispersed in a steel sheet to promote the formation of ferrite and rapidly cooled from a two-phase region temperature. A technique for obtaining a steel sheet with a low yield ratio by applying the steel sheet to a two-phase structure composed of a hard phase and a soft phase is disclosed. Furthermore, it is disclosed that the toughness of HAZ in high heat input welding can be improved by regulating the contents of Ca, O, and S within an appropriate range. If this technique is applied, a thick steel plate having excellent HAZ toughness, low yield ratio and high strength can be produced. However, when a large amount of C or various alloy elements is added to increase the strength of the steel sheet, the hardenability of the steel sheet is increased, so that a structure with low toughness such as upper bainite or martensite is easily generated in the HAZ. In order to prevent the HAZ toughness from being lowered, the addition of C and alloy elements must be suppressed. That is, it is difficult to stably maintain the characteristics of HAZ excellent in toughness, low yield ratio and high strength.
JP-A-60-152626 JP 57-51243 A JP 2005-68519 JP

  An object of this invention is to provide the manufacturing method of the low yield ratio high tension thick steel plate used in field | areas, such as a civil engineering, a building, and a bridge. Specifically, it has a yield strength of 440 MPa or more, a tensile strength of 590 MPa or more, and a yield ratio of 80% or less, and a super-high heat input heat-affected zone toughness that provides high toughness HAZ by super-high heat input welding. An object of the present invention is to provide a high yield strength high strength steel plate and a method for producing the same.

  Here, super high heat input welding refers to welding performed with a heat input exceeding 400 kJ / cm, and the excellent toughness of the weld heat affected zone (ie HAZ) is the bond obtained by super high heat input welding exceeding 400 kJ / cm. The Charpy absorbed energy (0 ° C.) of the part is 70 J or more. Furthermore, small heat input multi-pass welding that repeats welding with a small heat input of 20 to 50 kJ / cm is also classified here as super large heat input welding, and the excellent toughness of the weld heat affected zone is the small heat input multi-pass. It means that the Charpy absorbed energy (0 ° C.) of ICCGHAZ obtained by welding is 70 J or more.

In order to improve the toughness of HAZ and ICCGHAZ obtained by performing ultra-high heat input welding, as conventionally known,
(1) Finely disperse TiN.
(2) It is effective to maintain the ACR value within the range of 0.2 to 0.8 by adjusting the addition amount of Ca, S, and O.

  However, according to the research by the inventors, in the case of a thick steel plate having a thickness of 80 to 100 mm, the Ceq value must be set high and maintained within the range of 0.44 to 0.50 in order to ensure the strength. As a result, the hardenability of the thick steel plate is increased, so that a structure with low toughness such as upper bainite and martensite is likely to be formed in HAZ and ICCGHAZ, and sufficient effects can be obtained only by the above (1) and (2). Not found out.

Therefore, the inventors have intensively studied a low yield ratio high tensile steel plate that can improve the toughness of HAZ and ICCGHAZ in super high heat input welding while maintaining the strength of the steel plate while maintaining the Ceq value within the above range. did. As a result, the following findings (A) to (C) were obtained. The unit of the content of each element is mass%.
(A) In order to improve the toughness of ICCGHAZ in small heat input multi-pass welding, it is necessary to suppress an excessive increase in the C content of the thick steel plate. By making the C content 0.070% or less (preferably 0.064% or less), the amount of island-like martensite produced is reduced, and excellent toughness with Charpy absorbed energy _V E _{0 at} 0 ° C. of 70 J or more can be secured. .

  (B) In order to reduce the yield ratio of the thick steel plate, a two-phase quenching treatment is effective. A specific alloy element (for example, Mn, Cu, Ni, etc.) is unevenly distributed at the interface between ferrite and austenite while maintaining the temperature at the two-phase region temperature, and thus the content of the alloy element in the thick steel plate is shaded. Since the density of the alloy element does not disappear by heating in a short time such that the HAZ is exposed, ferrite is easily generated when heated by welding in a region where the content of the alloy element is low. Therefore, even when the Ceq value is high, Ca oxysulfide can be used as a ferrite nucleation because of the presence of a region having a low content of alloy elements. In order to obtain such an effect, it is necessary to form a microstructure in which Mn, Cu, and Ni are thinned to about 90% or less of the average concentration, and the fraction of the lightened region must be 10% or more. There is.

  (C) Alloy elements such as Mn, Cu, and Ni are substitutional alloy elements, and the distance traveled by diffusion is small while being maintained at the two-phase temperature. Therefore, in order to efficiently produce the light and shade in the content of the alloy element while maintaining the two-phase region temperature, it is necessary to make the structure of the thick steel plate before heating fine. Therefore, quenching is performed before heating to the two-phase region temperature. This quenching process is a process in which a thick steel plate obtained by hot rolling is reheated to 900 to 1000 ° C. and quenched (hereinafter referred to as reheating quenching process), and is different from the two-phase region quenching process. Austenite is formed at the high-density lath interface of martensite and bainite generated by reheating and quenching while maintaining the temperature in the two-phase region, and as a result, a fine mixed structure of austenite and ferrite is formed on the thick steel plate. The In addition, high-density dislocations existing in martensite and bainite promote the diffusion of Mn, Cu, Ni, and the like. Therefore, even if the diffusion distance of Mn, Cu, Ni or the like is short, it is possible to obtain the effect of causing the density of the content.

The present invention has been made based on the above findings.
That is, the present invention is, in mass%, C: 0.040 to 0.070%, Si: 0.05 to 0.45%, Mn: 1.0 to 1.6%, P: 0.020% or less, S: 0.0007 to 0.0040%, Al: 0.005 to 0.05%, Cr: 0.03-1.0%, Nb: 0.003% or less, Mo: 0.04% or less, V: 0.030% or less, B: 0.0005-0.0030%, Ti: 0.005-0.030%, Ca: 0.0005-0.0035%, N: 0.0030- 0.0070%, O: 0.0010 to 0.0040%, and Cu: 0.05 to 1.0% and Ni: 0.05 to 2.0%, the total of Cu content and Ni content is 1.0 to 2.0% And Ceq defined by the formula (1) satisfies 0.44 to 0.50, and Pcm defined by the formula (2) satisfies 0.21 or less, using the content of each element, (3) The ACR defined by the formula satisfies 0.2 to 0.8, the balance has a composition composed of Fe and inevitable impurities, and the microstructure has a concentration region in which the content of Mn, Cu, and Ni is higher than the steel average Including low lightening area, (Mn / 6 + Cu / 15 Ni / 15 value of) the low yield ratio high-strength thick steel plate fraction and excellent ultra-large heat input welded heat affected zone toughness is 10% or more of Awaka area is less than 90% of the steel average.

  Moreover, this invention is the mass%, C: 0.040-0.070%, Si: 0.05-0.45%, Mn: 1.0-1.6%, P: 0.020% or less, S: 0.0007-0.0040%, Al: 0.005-0.05%, Cr: 0.03-1.0%, Nb: 0.003% or less, Mo: 0.04% or less, V: 0.030% or less, B: 0.0005-0.0030%, Ti: 0.005-0.030%, Ca: 0.0005-0.0035%, N: 0.0030- 0.0070%, O: 0.0010 to 0.0040%, and Cu: 0.05 to 1.0% and Ni: 0.05 to 2.0%, the total of Cu content and Ni content is 1.0 to 2.0% And Ceq defined by the formula (1) satisfies 0.44 to 0.50, and Pcm defined by the formula (2) satisfies 0.21 or less, using the content of each element, (3) A steel material having a composition in which the ACR defined by the formula satisfies 0.2 to 0.8 and the balance is composed of Fe and inevitable impurities is made into a thick steel plate by hot rolling, and then the thick steel plate is reheated to a temperature of 900 to 1000 ° C. 800 minutes after holding for more than 20 minutes Reheat quenching treatment is performed in which the temperature range of 500 ° C is cooled at an average cooling rate of 1 ° C / second or higher, and then heated to a two-phase temperature range of (Ac1 + 20 ° C) to (Ac1 + 80 ° C) for 30 minutes or longer After holding, a two-phase quenching process is performed in which the temperature range of (Ac1−20 ° C) to 500 ° C is cooled at an average cooling rate of 1 ° C / second or more, and then heated to a temperature of 400 to 600 ° C. This is a method for producing a low yield ratio high-tensile thick steel plate excellent in toughness of a heat-affected zone with a super-high heat input weld subjected to tempering treatment.

Ceq = [% C] + ([% Mn] / 6) + {([% Ni] + [% Cu]) / 15}
+ {([% Cr] + [% Mo] + [% V]) / 5} (1)
Pcm = [% C] + ([% Si] / 30) + ([% Mn] / 20) + ([% Cu] / 20)
+ ([% Ni] / 60) + ([% Cr] / 20) + ([% Mo] / 15)
+ ([% V] / 10) + (5 × [% B]) (2)
ACR = {[% Ca] − (0.18 + 130 × [% Ca]) × [% O]} / (1.25 × [% S])
... (3)
Where [% C] is the C content, [% Mn] is the Mn content, [% Ni] is the Ni content, [% Cu] is the Cu content, and [% Cr] is the Cr content. [% Mo] is the Mo content, [% V] is the V content, [% Si] is the Si content, [% B] is the B content, and [% Ca] is the Ca content. The amount, [% O] indicates the O content, and [% S] indicates the S content. The unit of content is mass%.

In the manufacturing method of the low yield ratio high-tensile steel plate excellent in the super high heat input welding heat-affected zone toughness of the present invention, in addition to the above-described composition, the steel material is REM: 0.0010 to 0.020% and Mg: It is preferable to contain one or more of 0.0010 to 0.0050%.
Moreover, it is preferable that the plate | board thickness of the thick steel plate obtained by hot-rolling a steel raw material is 100 mm or less. The thickness of the thick steel plate is more preferably in the range of 80 to 100 mm.

  According to the present invention, it has a yield strength of 440 MPa or more, a tensile strength of 590 MPa or more and a yield ratio of 80% or less, which is suitable in the fields of civil engineering, architecture, bridges, etc., and has high toughness HAZ by super-high heat input welding. In addition, it is possible to manufacture a high yield thick steel plate having a low yield ratio and excellent in super heat input welding heat affected zone toughness from which ICCGHAZ can be obtained.

First, the reason for limiting the components of the steel material that is hot-rolled in order to apply the present invention to produce a thick steel plate will be described. The unit of the content of each element is mass%. Hereinafter, HAZ and ICCGHAZ by super-high heat input welding are collectively referred to as HAZ.
C: 0.040-0.070%
C has an effect of increasing the strength of the thick steel plate, and is an important element in securing the strength required as a structural steel material. If the C content is less than 0.040%, a tensile strength of 590 MPa or more cannot be obtained. On the other hand, if it exceeds 0.070%, island-shaped martensite is generated in a region heated to Ac3 or higher (so-called fine-grained region HAZ) in the HAZ of super high heat input welding, and the toughness of the HAZ is reduced. Therefore, C needs to satisfy the range of 0.040 to 0.070%. Preferably it is 0.050 to 0.064%.

Si: 0.05-0.45%
Si acts as a deoxidizer in the molten metal for super high heat input welding. When the Si content is less than 0.05%, the effect as a deoxidizer cannot be obtained. On the other hand, if it exceeds 0.45%, the toughness of the thick steel plate deteriorates and the toughness of the HAZ significantly deteriorates. Therefore, Si needs to satisfy the range of 0.05 to 0.45%. Preferably it is 0.05 to 0.35%.

Mn: 1.0-1.6%
Mn has an effect of increasing the strength of the thick steel plate, and is an important element for ensuring the strength required as a structural steel material. If the Mn content is less than 1.0%, a tensile strength of 590 MPa or more cannot be obtained. On the other hand, if it exceeds 1.6%, the toughness of the HAZ as well as the thick steel plate deteriorates significantly within the range of the Ceq value (that is, 0.44 to 0.50) described later. Therefore, Mn needs to satisfy the range of 1.0 to 1.6%.

P: 0.020% or less P is an element that increases the strength of a thick steel plate while degrading toughness. Therefore, it is necessary to reduce P as much as possible from the viewpoint of preventing the deterioration of the toughness of the HAZ due to super large heat input welding. When the P content exceeds 0.020%, the toughness of the HAZ is significantly deteriorated. Therefore, P is set to 0.020% or less. In addition, although the lower limit of P content is not specifically limited, if P is reduced excessively, the refining cost will increase at the stage of melting molten steel. Therefore, P is preferably 0.005 to 0.020%. More preferably, it is 0.005 to 0.015%.

S: 0.0007-0.0040%
S combines with Ca in the solidification stage of molten steel to crystallize CaS particles. CaS particles act as ferrite nuclei during cooling after hot rolling and contribute to lowering the yield ratio of thick steel plates. Furthermore, when performing ultra-high heat input welding of the thick steel plate, MnS precipitates on the CaS particles and acts as ferrite nuclei, thereby improving the toughness of the weld metal. When the S content is less than 0.0007%, this effect cannot be obtained. On the other hand, if it exceeds 0.0040%, a large amount of MnS is segregated in the center of the slab by continuous casting of molten steel, not only causing defects inside the slab, but also the toughness of the thick steel plate produced from the slab. to degrade. Therefore, S needs to satisfy the range of 0.0007 to 0.0040%.

Al: 0.005-0.05%
Al is used as a deoxidizer at the stage of melting molten steel. Moreover, N in molten steel is fixed as AlN, and it also has the effect of maintaining the effect of improving the hardenability by B described later. If the Al content is less than 0.005%, these effects cannot be obtained. On the other hand, if it exceeds 0.05%, the toughness of the thick steel plate deteriorates, and at the time of performing super-high heat input welding of the thick steel plate, it is mixed into the weld metal to deteriorate the toughness of the weld metal. Therefore, Al needs to satisfy the range of 0.005-0.05%. Preferably it is 0.010 to 0.045%.

Cr: 0.03-1.0%
Cr has the effect of increasing the strength of the thick steel sheet by improving the hardenability of the thick steel sheet. Moreover, it is a useful element because it has little adverse effect on HAZ. If the Cr content is less than 0.03%, the effect of increasing the strength of the thick steel plate cannot be obtained. On the other hand, if it exceeds 1.0%, the toughness of the HAZ as well as the thick steel plate is degraded. Therefore, Cr needs to satisfy the range of 0.03-1.0%. Preferably it is 0.1 to 0.5%.

Nb: 0.003% or less
Nb is an element that promotes the formation of upper bainite and martensite in the HAZ and degrades the toughness of the HAZ. Therefore, it is necessary to reduce Nb as much as possible. When the Nb content exceeds 0.003%, the toughness of the HAZ is significantly deteriorated. Therefore, Nb is 0.003% or less.
Mo: 0.04% or less
Mo is an element that promotes the formation of upper bainite and martensite in the HAZ and degrades the toughness of the HAZ. Therefore, it is necessary to reduce Mo as much as possible. When the Mo content exceeds 0.04%, the toughness of the HAZ is significantly deteriorated. Therefore, Mo is set to 0.04% or less.

V: 0.030% or less V, like Nb and Mo, is an element that degrades the toughness of HAZ, but its degree of embrittlement is low compared to Nb and Mo, so we want to increase the strength of the base metal and HAZ. Sometimes it can be added. However, if it exceeds 0.030%, precipitation hardening becomes remarkable, and the toughness of the base material and HAZ is remarkably reduced, so the content is made 0.030% or less. Preferably V is 0.020% or less.

B: 0.0005-0.0030%
B is an element that increases the strength of the thick steel plate by improving the hardenability of the thick steel plate by adding a small amount. Further, B combines with N at the stage of melting the molten steel to produce BN, and the BN acts as a ferrite nucleation during cooling after hot rolling, contributing to a decrease in the yield ratio of the thick steel plate. Furthermore, B generates BN in HAZ by super-high heat input welding in which TiN is dissolved. This BN not only acts as a ferrite nuclei but also has an effect of reducing the solid solution N, contributing to the improvement of HAZ toughness. If the B content is less than 0.0005%, these effects cannot be obtained. On the other hand, if it exceeds 0.0030%, not only the thick steel plate but also the toughness of the HAZ is deteriorated, and the yield strength of the thick steel plate is remarkably increased, so that it is difficult to control the yield ratio. Therefore, B needs to satisfy the range of 0.0005 to 0.0030%. Preferably it is 0.0007 to 0.0020%.

Ti: 0.005-0.030%
Ti has a strong affinity for N and precipitates as TiN in the continuous casting of molten steel, and TiN acts as a ferrite nucleation during cooling after hot rolling, contributing to a decrease in the yield ratio of the thick steel plate. Furthermore, since TiN suppresses the growth of austenite in the HAZ of super high heat input welding and acts as a ferrite formation nucleus, it contributes to improving the toughness of the HAZ. If the Ti content is less than 0.005%, these effects cannot be obtained. On the other hand, if it exceeds 0.030%, TiN particles are coarsened, so these effects cannot be obtained. Therefore, Ti needs to satisfy the range of 0.005-0.030%. Preferably it is 0.010 to 0.020%.

Ca: 0.0005-0.0035%
Ca has the effect of improving the toughness of the thick steel plate by combining with S to produce CaS. Ca is added at the melting stage of molten steel, but after adjusting the dissolved oxygen amount to 0.0050% or less, Ca is added. In this way, the production of CaO is suppressed and the production of CaS is promoted. CaS crystallizes in molten steel at a lower temperature than oxides, and is uniformly and finely dispersed. These fine CaS particles are combined with MnS during welding of thick steel plates to act as ferrite nuclei and contribute to the improvement of HAZ toughness. If the Ca content is less than 0.0005%, these effects cannot be obtained. On the other hand, if it exceeds 0.0035%, excess Ca forms an oxide and deteriorates the toughness of the thick steel plate. Therefore, Ca needs to satisfy the range of 0.0005 to 0.0035%.

N: 0.0030-0.0070%
N precipitates as TiN in the continuous casting of molten steel, and TiN acts as a ferrite nucleation during cooling after hot rolling, contributing to a decrease in the yield ratio of the thick steel plate. Furthermore, since TiN suppresses the growth of austenite in the HAZ of super high heat input welding and acts as a ferrite formation nucleus, it contributes to improving the toughness of the HAZ. If the N content is less than 0.0030%, these effects cannot be obtained. On the other hand, if it exceeds 0.0070%, the TiN of HAZ is melted by super-high heat input welding, and as a result, the amount of solid solution N increases and the toughness of HAZ is significantly deteriorated. Therefore, N needs to satisfy the range of 0.0030 to 0.0070%.

O: 0.0010 to 0.0040%
O is an impurity that is inevitably mixed at the stage of melting molten steel, and forms an oxide by combining with other elements. Therefore, it is preferable to reduce the O content. However, in order to make it less than 0.0010%, the refining cost is increased at the stage of melting the molten steel. On the other hand, if it exceeds 0.0040%, the oxide becomes coarse and the toughness of the thick steel plate is deteriorated. Therefore, O needs to satisfy the range of 0.0010 to 0.0040%.

One or more of Cu: 0.05-1.0% and Ni: 0.05-2.0%
Cu has the effect of increasing the strength without reducing the toughness of the thick steel plate. Moreover, it is a useful element because it has little adverse effect on HAZ. If the Cu content is less than 0.05%, the effect of increasing the strength of the thick steel plate cannot be obtained. On the other hand, if it exceeds 1.0%, hot brittleness occurs and the surface properties of the thick steel plate deteriorate. Therefore, Cu needs to satisfy the range of 0.05 to 1.0%. Preferably it is 0.1 to 0.5%. Preferably it is 0.1 to 0.5%.

  Ni has the effect of increasing the strength without reducing the toughness of the thick steel plate. Moreover, it is a useful element because it has little adverse effect on HAZ. If the Ni content is less than 0.05%, the effect of increasing the strength of the thick steel plate cannot be obtained. On the other hand, if it exceeds 2.0%, hot brittleness occurs and the surface properties of the thick steel plate deteriorate. Therefore, Ni needs to satisfy the range of 0.05 to 2.0%. Preferably it is 0.2 to 1.5%.

Total Cu content and Ni content: 1.0-2.0%
As already described, in the present invention, Mn, Cu, Ni, etc. are unevenly distributed at the interface between ferrite and austenite while maintaining the temperature in the two-phase region prior to the two-phase region quenching treatment, thereby causing the Mn, Concentration is produced in the content of Cu, Ni, etc., and used as ferrite nuclei. In order to obtain such an effect, it is necessary to set the content of Mn, Cu, Ni, etc. high. However, when Mn is added excessively, the toughness of the thick steel plate and the HAZ deteriorates. Therefore, the contents of Cu and Ni are set large, and the total content is defined. If the total of Cu content and Ni content is less than 1.0%, ferrite nuclei cannot be obtained sufficiently. On the other hand, if it exceeds 2.0%, the ferrite nuclei are saturated, an effect commensurate with the increase in content cannot be obtained, and the manufacturing cost of the thick steel plate increases. Therefore, the total of Cu content and Ni content is in the range of 1.0 to 2.0%.

Ceq: 0.44-0.50
Ceq is a value defined by the following equation (1). When Ceq is less than 0.44, the hardenability in the reheating quenching process and the two-phase quenching process is insufficient, and thus ferrite is generated. As a result, the desired tensile strength (ie, 590 MPa or more) cannot be obtained with a thick steel plate having a thickness of 80 mm or more. Further, even if the reheating quenching process is performed, a fine structure of martensite or bainite cannot be obtained in the thick steel plate, so that it becomes difficult to cause the density of the alloy elements by the two-phase region quenching process. On the other hand, if it exceeds 0.50, the toughness of the HAZ is significantly deteriorated. Therefore, Ceq needs to satisfy the range of 0.44 to 0.50. Preferably it is 0.45-0.48.

Ceq = [% C] + ([% Mn] / 6) + {([% Ni] + [% Cu]) / 15}
+ {([% Cr] + [% Mo] + [% V]) / 5} (1)
In the above formula (1), [% C] is C content, [% Mn] is Mn content, [% Ni] is Ni content, [% Cu] is Cu content, [% Cr] is Cr content The amount, [% Mo] indicates the Mo content, and [% V] indicates the V content. The unit of content is mass%.

Pcm: 0.21 or less Pcm is a value defined by the following equation (2). When Pcm exceeds 0.21, the low temperature cracking sensitivity is increased, and cracking is likely to occur in the weld metal. Therefore, Pcm is 0.21 or less.
Pcm = [% C] + ([% Si] / 30) + ([% Mn] / 20) + ([% Cu] / 20)
+ ([% Ni] / 60) + ([% Cr] / 20) + ([% Mo] / 15)
+ ([% V] / 10) + (5 × [% B]) (2)
Where [% C] is the C content, [% Si] is the Si content, [% Mn] Mn content, [% Cu] is the Cu content, [% Ni] is the Ni content, [% Cr ] Indicates the Cr content, [% Mo] indicates the Mo content, [% V] indicates the V content, and [% B] indicates the B content. The unit of content is mass%.

ACR: 0.2-0.8
ACR is a value defined by the following equation (3). If the ACR is less than 0.2, CaS is not generated and MnS is precipitated. This MnS is not uniformly and finely dispersed in the thick steel plate, but is elongated by hot rolling. As a result, not only the thick steel plate but also the HAZ toughness deteriorates. On the other hand, when it exceeds 0.8, a large amount of CaS is generated and MnS is insufficient. As a result, when welding thick steel plates, MnS does not precipitate on the CaS particles, and a sufficient amount of ferrite forming nuclei cannot be obtained. Therefore, the ACR needs to satisfy the range of 0.2 to 0.8. By maintaining ACR within this range, a composite sulfide in which MnS is precipitated on CaS particles is obtained. The composite sulfide acts as a ferrite formation nucleus, the HAZ structure is refined, and the toughness of the HAZ is improved.

ACR = {[% Ca] − (0.18 + 130 × [% Ca]) × [% O]} / (1.25 × [% S])
... (3)
Here, [% Ca] indicates the Ca content, [% O] indicates the O content, and [% S] indicates the S content. The unit of content is mass%.
In this invention, in addition to said component, you may contain 1 or more types of rare earth elements (namely, REM): 0.0010-0.020% and Mg: 0.0010-0.0050%.

One or more of REM: 0.0010-0.020% and Mg: 0.0010-0.0050%
REM has the effect | action which improves the toughness of a thick steel plate and HAZ. If the REM content is less than 0.0010%, this effect cannot be obtained. On the other hand, if it exceeds 0.020%, the effect of improving toughness is saturated, an effect commensurate with the increase in content cannot be obtained, and the production cost of the thick steel plate increases. Therefore, REM is preferably in the range of 0.0010 to 0.020%.

Mg has the effect | action which improves the toughness of a thick steel plate and HAZ. If the Mg content is less than 0.0010%, this effect cannot be obtained. On the other hand, if it exceeds 0.0050%, the effect of improving toughness is saturated, an effect commensurate with the increase in content cannot be obtained, and the production cost of the thick steel plate increases. Therefore, Mg is preferably in the range of 0.0010 to 0.0050%.
The balance other than the above components is Fe and inevitable impurities.

  In the present invention, the toughness of HAZ is improved by forming Mn, Cu, Ni enriched regions and lightened regions at the micro level and promoting intragranular ferrite nucleation from the lightened regions. In order to promote ferrite nucleation, the local (Mn / 6 + Cu / 15 + Ni / 15) value measured by EPMA or the like needs to be 90% or less of the steel average value. In addition, when the fraction of such a lightened region is less than 10%, a sufficient amount of intragranular ferrite cannot be generated. Therefore, the fraction of the lightened region where the value of (Mn / 6 + Cu / 15 + Ni / 15) is 90% or less of the average steel material value is limited to 10% or more.

Next, the manufacturing process of a thick steel plate will be described.
The technique employed in the process of producing the steel material having the above-described components by melting and casting the molten steel is not particularly limited, and a conventionally known technique is used. However, considering the production of a large number of thick steel plates, the molten steel is melted in a converter, electric furnace, vacuum melting furnace, etc., degassed to adjust the gas components, and then inclusions using CaSi wire It is preferable to carry out the control and further perform continuous casting to produce a steel material (that is, a slab).

In addition, in order to promote the production | generation of CaS in the melting stage of molten steel, it is preferable to add Ca after adjusting O with strong binding force with Ca to 0.0050% or less. Ca is added by a method such as charging a CaSi wire.
This steel material is heated and further hot-rolled to obtain a thick steel plate. Hot rolling uses a conventionally known technique. When the thickness of the thick steel plate exceeds 100 mm, it is difficult to improve the toughness of the HAZ by super-high heat input welding throughout the thickness direction even if the present invention is applied. If the plate thickness of the thick steel plate is 100 mm or less, the toughness of the HAZ can be improved throughout the plate thickness direction, so the lower limit value of the plate thickness is not particularly limited. However, when the present invention is applied to a thick steel plate having a thickness of 80 mm or more, which is generally said to be difficult to improve the toughness of HAZ by super-high heat input welding, a great effect is obtained. Accordingly, the thickness of the thick steel plate is preferably 100 mm or less, and more preferably in the range of 80 to 100 mm.

  The obtained thick steel plate is reheated. When the reheating temperature is less than 900 ° C., the hardenability in the subsequent quenching (that is, the reheating quenching process) is lowered, and a suitable structure (that is, a fine structure of martensite or bainite) cannot be obtained. As a result, the strength of the thick steel plate decreases and the toughness of the HAZ deteriorates. On the other hand, when it exceeds 1000 ° C., austenite coarsens and the toughness of the thick steel plate deteriorates. Therefore, the reheating temperature needs to satisfy the range of 900 to 1000 ° C.

In addition, if the time for maintaining in the above temperature range by reheating is less than 20 minutes, sufficient time for segregating B at the grain boundaries is not secured, so that the effect of improving hardenability by B cannot be obtained. Therefore, the reheating holding time is set to 20 minutes or longer. Preferably it is 30 minutes or more.
After the reheating is completed, the thick steel plate is subjected to a reheating quenching process. In the reheating quenching process, the temperature range of 800 to 500 ° C. is cooled at an average cooling rate of 1 ° C./second or more. When the average cooling rate is less than 1 ° C./second, a suitable structure (that is, a fine structure of martensite or bainite) cannot be obtained.

  After reheating and quenching, the thick steel plate is heated to a two-phase region temperature. If the temperature at which the two-phase region is heated is less than Ac1 + 20 ° C., the fraction of austenite being held at the heating temperature is too low, so that the fraction of the hard phase of the base material is insufficient and the strength of the thick steel plate is lowered. On the other hand, if it exceeds Ac1 + 80 ° C., the fraction of austenite increases excessively, so that C concentrated in the austenite decreases. As a result, the hardenability in the subsequent quenching (that is, the two-phase region quenching process) is lowered, and the strength of the thick steel plate is lowered. Therefore, the temperature for heating in the two-phase region needs to satisfy the range of (Ac1 + 20 ° C.) to (Ac1 + 80 ° C.).

In addition, if the time for maintaining in the above temperature range by the two-phase region heating is less than 30 minutes, sufficient time is not ensured for producing the density of Mn, Cu and Ni, and the effect of improving the toughness of HAZ is obtained. I can't. Therefore, the holding time for the two-phase region heating is 30 minutes or more. Preferably it is 40 minutes or more.
After the two-phase region heating is completed, the thick steel plate is subjected to a two-phase region quenching process. In the two-phase quenching treatment, the temperature range of (Ac1-20 ° C) to 500 ° C is cooled at an average cooling rate of 1 ° C / second or more. When the average cooling rate is less than 1 ° C./second, the hardness of the hard phase is insufficient and a suitable structure cannot be obtained.

After the two-phase quenching treatment, the thick steel plate is heated and tempered. When the heating temperature in the tempering treatment is less than 400 ° C., the toughness of the thick steel plate embrittled by the two-phase region quenching treatment cannot be improved. On the other hand, when it exceeds 600 degreeC, the intensity | strength of a thick steel plate will fall. Therefore, the heating temperature in the tempering process needs to satisfy the range of 400 to 600 ° C.
In addition, the time for maintaining the temperature range in the tempering process is not particularly limited, and is appropriately set according to the use of the thick steel plate and the required characteristics. However, if it exceeds 60 minutes, the strength of the thick steel plate is remarkably lowered, so that the holding time of the tempering treatment is preferably 60 minutes or less.

  The steel plate produced by the above procedure has both a low yield ratio and high strength, and HAZ by super-high heat input welding exhibits excellent toughness.

  Ladle refining was performed on molten steel melted using a converter, and a steel material (ie, slab) was produced by continuous casting. The thickness of the steel material is 310 mm, and its components are as shown in Table 1. The invention examples (namely, steel types A to I) shown in Table 1 are examples in which the components satisfy the scope of the present invention. Among the comparative examples, steel types J and K are examples in which the C content is outside the range of the present invention, steel type L is an example in which the sum of the Cu content and the Ni content is outside the range of the present invention, and steel type M has an N content. Examples outside the scope of the present invention, steel grades N and O are examples where ACR is outside the scope of the present invention, steel grade P is an example where Ceq is outside the scope of the present invention, steel grade Q is an example where Ceq and Pcm are outside the scope of the present invention It is.

  These steel materials were hot-rolled to obtain a steel plate having a thickness of 100 mm, and then subjected to reheating quenching treatment, two-phase region quenching treatment, and tempering treatment under the conditions shown in Table 2. Thick steel plate symbols A, B, and D to Q shown in Table 2 are thick steel plates manufactured from steel materials of steel types A, B, and D to Q (see Table 1), respectively, reheating and quenching, and two-phase quenching. The conditions for the treatment and the tempering treatment both satisfy the scope of the present invention. Thick steel plate symbols C-1 to C-8 are thick steel plates manufactured from a steel material of steel type C (see Table 1), and the conditions of the reheating quenching process, the two-phase region quenching process, and the tempering process are all the present invention. Satisfies the range.

  Thick steel plate symbols C-9 and C-10 are thick steel plates manufactured from a steel material of steel type C (see Table 1), but the heating temperature of the reheating quenching process is out of the scope of the present invention. Thick steel plate symbol C-11 is a thick steel plate manufactured from a steel material of steel type C, but the holding time of the reheating quenching treatment is out of the scope of the present invention. Thick steel plate symbol C-12 is a thick steel plate manufactured from a steel material of steel type C, but the cooling rates of the reheating quenching process and the two-phase quenching process are out of the scope of the present invention. Thick steel plate symbols C-13 and C-14 are thick steel plates manufactured from a steel material of steel type C, but the heating temperature of the two-phase quenching treatment is out of the scope of the present invention. Thick steel plate symbol C-15 is a thick steel plate manufactured from a steel material of steel type C, but the holding time of the two-phase region quenching treatment is out of the scope of the present invention. Thick steel plate symbols C-16 and C-17 are thick steel plates manufactured from a steel material of steel type C, but the heating temperature of the tempering treatment is out of the scope of the present invention.

JIS No. 4 tensile test specimens are taken from the position of the depth of 1/4 in the plate thickness direction of these thick steel plates and subjected to a tensile test in accordance with the provisions of JIS standard Z2241, yield strength (YS) and tensile strength. (TS) was investigated. Furthermore, the yield ratio was calculated from the obtained yield strength and tensile strength. The results are shown in Table 3.
In addition, a V-notch test piece was taken from a position at a depth of 1/4 in the plate thickness direction of the thick steel plate in accordance with the JIS standard Z2202, and a Charpy impact test was conducted in accordance with the JIS standard Z2242. We investigated the absorption energy _(V E ₀₎ in ℃. The results are shown in Table 3.

  Further, test pieces for measuring the fraction of the thinned region are taken from the position of the depth in the plate thickness direction 1/4 of these thick steel plates, and the cross section parallel to the rolling direction (that is, the L cross section) is subjected to surface analysis by EPMA. Then, the concentration distribution of Mn, Cu, and Ni was measured. The measurement area of the concentration distribution was 50 μm × 50 μm, and each field of view was measured, and the fraction of the lightened region (that is, the region where the value of (Mn / 6 + Cu / 15 + Ni / 15) was 90% or less of the average steel material value) was calculated. . The average value of these four visual fields was used as the fraction of the lightened area. The results are shown in Table 3.

Next, two joint test plates (400 mm × 600 mm) were sampled from each thick steel plate to form a groove as shown in FIG. That is, the joint test plate 1 was combined at a right angle, and a gap 3 having a width of 25 mm was provided, and a groove 2 was formed by disposing the stopper 2 on both sides of the gap 3.
This groove was electroslag welded (hereinafter referred to as ESW) to produce a welded joint. The welding heat input of ESW was 1000 kJ / cm, the welding wire was JIS standard 3353YES62 equivalent, and the welding flux was JIS standard 3353FS-FG3 equivalent.

From the obtained welded joint, as shown in FIG. 2, a V-notch test piece 5 is taken with the notch position as a bond part, and subjected to a Charpy impact test in accordance with the JIS standard Z2242, and the absorbed energy at 0 ° C. ( _V E ₀ ) was investigated. The results are shown in Table 3.
Furthermore, from a test piece corresponding to ICCGHAZ of small heat input multi-pass welding to which a double heat cycle with an average cooling rate of 50 ° C./second was given by induction heating at peak temperatures of 1400 ° C. and 800 ° C., a V-notch test piece The Charpy impact test was conducted in accordance with the provisions of JIS standard Z2242, and the absorbed energy ( _V E ₀ ) at 0 ° C. was investigated. The results are shown in Table 3.

As is clear from Table 3, the investigation results of the mechanical properties of the thick steel plates of the inventive examples were yield strength (YS) of 446 to 489 MPa, and all were 440 MPa or more. Moreover, tensile strength (TS) was 593-647 MPa, and all were 590 MPa or more. Furthermore, the yield ratio was 74 to 79%, and all were 80% or less.
Invention Example steel plate and its weld joint toughness findings of the steel plates absorbed energy _(V E ₀₎ is 112~264J the absorbed energy _(V E ₀₎ of the bond portion of the ESW joint 79~156J, ICCGHAZ The absorbed energy ( _V E ₀ ) of considerably reproduced HAZ was 94 to 188 J, and all of them had excellent toughness with _V E ₀ of 70 J or more.

  On the other hand, in the comparative example, the steel plate symbols C-9, C-17, and P were insufficient in yield strength and tensile strength of the thick steel plate. Thick steel plate symbol C-10 was insufficient in yield strength and toughness of the thick steel plate. Thick steel plate symbols C-11, C-15, L to O, and Q had insufficient toughness at the ESW bond part. Thick steel plate symbols C-12 to C-14, J were insufficient in yield strength and tensile strength of the thick steel plate and toughness of the ESW bond part. Thick steel plate symbol C-16 was insufficient in toughness of the thick steel plate. Thick steel plate symbol K had insufficient ICCGHAZ toughness.

It is sectional drawing which shows typically the example of the groove | channel which welds by applying this invention. It is sectional drawing which shows typically the collection position of a Charpy test piece.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Test plate for joints 2 Lid 3 Clearance 4 Weld metal 5 V notch test piece

Claims (3)

In mass%, C: 0.040-0.070%, Si: 0.05-0.45%, Mn: 1.0-1.6%, P: 0.020% or less, S: 0.0007-0.0040%, Al: 0.005-0.05%, Cr: 0.03-1.0 %, Nb: 0.003% or less, Mo: 0.04% or less, V: 0.030% or less, B: 0.0005 to 0.0030%, Ti: 0.005 to 0.030%, Ca: 0.0005 to 0.0035%, N: 0.0030 to 0.0070%, O: Containing 0.0010 to 0.0040%, further containing one or more of Cu: 0.05 to 1.0% and Ni: 0.05 to 2.0% so that the total of Cu content and Ni content is 1.0 to 2.0%, And using the content of each element, Ceq defined by equation (1) satisfies 0.44 to 0.50, Pcm defined by equation (2) satisfies 0.21 or less, and is defined by equation (3). ACR satisfies 0.2 to 0.8, the balance is composed of Fe and inevitable impurities, and the microstructure includes a thickening region and a low lightening region in which the contents of Mn, Cu, and Ni are higher than the steel average. , (Mn / 6 + Cu / 15 + Ni / 15) value is steel Low yield ratio high-strength thick steel plate fraction of Awaka region of 90% or less and excellent ultra-large heat input welded heat affected zone toughness, characterized in that at least 10% of the average.
Ceq = [% C] + ([% Mn] / 6) + {([% Ni] + [% Cu]) / 15}
+ {([% Cr] + [% Mo] + [% V]) / 5} (1)
Pcm = [% C] + ([% Si] / 30) + ([% Mn] / 20) + ([% Cu] / 20)
+ ([% Ni] / 60) + ([% Cr] / 20) + ([% Mo] / 15)
+ ([% V] / 10) + (5 × [% B]) (2)
ACR = {[% Ca] − (0.18 + 130 × [% Ca]) × [% O]} / (1.25 × [% S])
... (3) In mass%, C: 0.040-0.070%, Si: 0.05-0.45%, Mn: 1.0-1.6%, P: 0.020% or less, S: 0.0007-0.0040%, Al: 0.005-0.05%, Cr: 0.03-1.0 %, Nb: 0.003% or less, Mo: 0.04% or less, V: 0.030% or less, B: 0.0005 to 0.0030%, Ti: 0.005 to 0.030%, Ca: 0.0005 to 0.0035%, N: 0.0030 to 0.0070%, O: Containing 0.0010 to 0.0040%, further containing one or more of Cu: 0.05 to 1.0% and Ni: 0.05 to 2.0% so that the total of Cu content and Ni content is 1.0 to 2.0%, And using the content of each element, Ceq defined by equation (1) satisfies 0.44 to 0.50, Pcm defined by equation (2) satisfies 0.21 or less, and is defined by equation (3). A steel material having a composition with ACR of 0.2 to 0.8 and the balance of Fe and inevitable impurities is made into a thick steel plate by hot rolling, and then the thick steel plate is reheated to a temperature of 900 to 1000 ° C. for 20 minutes. After holding above, the temperature of 800 ~ 500 ℃ After reheating and quenching treatment in which the range is cooled at an average cooling rate of 1 ° C./second or more and quenched, and further heated to a two-phase temperature range of (Ac1 + 20 ° C.) to (Ac1 + 80 ° C.) and held for 30 minutes or more, (Ac1-20 ° C) to 500 ° C is cooled at an average cooling rate of 1 ° C / second or more and subjected to a two-phase quenching process, and further tempered by heating to 400 to 600 ° C. A method for producing a high yield thick steel sheet having a low yield ratio and excellent heat-affected zone toughness.
Ceq = [% C] + ([% Mn] / 6) + {([% Ni] + [% Cu]) / 15}
+ {([% Cr] + [% Mo] + [% V]) / 5} (1)
Pcm = [% C] + ([% Si] / 30) + ([% Mn] / 20) + ([% Cu] / 20)
+ ([% Ni] / 60) + ([% Cr] / 20) + ([% Mo] / 15)
+ ([% V] / 10) + (5 × [% B]) (2)
ACR = {[% Ca] − (0.18 + 130 × [% Ca]) × [% O]} / (1.25 × [% S])
... (3) The super high heat input according to claim 2, wherein the steel material contains one or more of REM: 0.0010 to 0.020% and Mg: 0.0010 to 0.0050% by mass% in addition to the composition. A method for producing a low-yield ratio, high-tensile steel plate with excellent weld heat-affected zone toughness.

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