JP2020204072A - High strength steel sheet for high heat input welding - Google Patents

Abstract

To provide a high strength steel sheet for high heat input welding based on new guidelines on component design.SOLUTION: A high strength steel sheet for high heat input welding contains, in mass%, C: 0.12% or more and 0.18% or less, Mn: 0.5% or more and 1.5% or less, Ni: 1.0% or more and 3.0% or less, Al: 0.05% or more and 0.20% or less, B: 0.0003% or more and 0.0030% or less, with content limits of Si: 0.30% or less, Ti: 0.004% or less, O: 0.0040% or less, and N: 0.0100% or less, Mn/Ni: 0.80 or less, and carbon equivalent CeqWES: 0.43% or more and 0.53% or less. CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14, where C, Mn, Si, Ni, Cr, Mo, V each denote a content (mass%) of each element. One or more of Cu, Cr, Mo, W, Co, Nb, V, Ca, Mg, REM, Zr can be contained.SELECTED DRAWING: None

Description

The present invention relates to a high-strength steel sheet to which high heat input welding is applied.

In recent years, the demand for steel frames of welded structures represented by high-rise buildings has been increasing from the viewpoints of increasing the size of buildings, improving the efficiency of construction, and improving safety (seismic resistance) against destruction during earthquakes. .. The thick steel plate used for the steel frame of the welded structure is required to secure the toughness of the large heat-affected zone HAZ in addition to increasing the strength and thickness. The "high heat input welding HAZ" means a welding heat affected zone (HAZ) formed by the large heat input welding. Hereinafter, the large heat-affected zone HAZ may be simply referred to as a large heat-affected zone HAZ. Large heat input welding is high heat input welding, and examples thereof include highly efficient electroslag welding and submerged arc welding.

Conventionally, when the above-mentioned large heat input welding is applied to a high-strength thick steel sheet, it has been considered difficult to secure good toughness in HAZ. For example, the HAZ toughness of an electroslag welded portion of a 780 MPa class thick steel sheet with a tensile strength is shown in Non-Patent Document 1 and Non-Patent Document 2. According to FIG. 6 of Non-Patent Document 1, the average value of Charpy absorption energy at notch positions of fusion line (Fusion Line, FL), FL to 1 mm (HAZ1), FL to 3 mm (HAZ3), and FL to 5 mm (HAZ5). Is 40J or less. Further, according to FIGS. 3 and 5 of Non-Patent Document 2, the average value of Charpy absorption energy at the notch position of FL is 50 J or less.

To solve this problem, thick steel sheets are subjected to a two-phase region quenching treatment to reduce the yield ratio, and alloying elements such as Mn, Cu, and Ni are distributed at the boundary between ferrite and austenite. A 780 MPa class thick steel plate having an improved toughness has been proposed (see, for example, Patent Document 1). In this technique, the shades of alloying elements that enhance hardenability are generated, and in HAZ, intragranular bainite is nucleated in a region where the alloy concentration is low to make the structure finer and enhance toughness.

Further, using V carbonitride (V (C, N)), intragranular ferrite is generated in HAZ, and the toughness is enhanced by refining the structure. Yield strength is 325 to 500 MPa class thickness. Steel sheets have been proposed (see, for example, Patent Document 2). This technique utilizes MnS as a precipitation nucleus for reducing the N content, increasing the C content, and precipitating V (C, N) in the large heat-affected zone HAZ.

Japanese Unexamined Patent Publication No. 2010-280977 Japanese Unexamined Patent Publication No. 2007-327099

Kazushige Tokuno, 7 others, "Large heat input welding type preheating reduction 780N / mm class 2 high-strength steel plate for construction", Nippon Steel Technical Report, 1997, No. 365, p. 37-43 Minoru Hirota and 5 others, "Low yield ratio for building structures by online manufacturing process, 780 N / mm class 2 steel materials, Part 3 Characteristics of joints with large heat input welds", Architectural Institute of Japan Conference Academic Lecture Abstracts, 2012, No. 1017

In order to increase the strength of the steel sheet, it is effective to increase the carbon equivalent CeqWES, which is an index of the hardenability of the steel. However, when the content of alloying elements such as Mn and Ni is increased, the large heat input HAZ becomes a hardened structure mainly composed of bainite, and the embrittled phase of the martensite-austenite mixed phase (Martensite-Austenite component, MA). Generation is promoted. The formation of MA is due to the microsegregation portion formed by locally thickening alloying elements such as Mn and Ni contained in the steel sheet. The microsegregated zone is heated by the heat of welding, cooled, and then becomes MA by phase transformation. MA is a hard phase that serves as a starting point for fracture and reduces HAZ toughness.

Further, since the large heat-affected zone HAZ is heated to a high temperature, the grain growth of austenite is promoted and the crystal grains of steel are coarsened. Further, increasing the content of the alloying element cures the HAZ. These also cause a decrease in HAZ toughness. As described above, when the carbon equivalent CeqWES is increased in order to increase the strength of the steel sheet, a coarse bainite-based structure produced by MA is formed in the large heat-affected zone HAZ, and the toughness tends to decrease.

As described above, in the case of a thick steel sheet containing Mn and Ni, which are alloying elements that increase the strength, the toughness of the large heat-affected zone HAZ is determined by the formation of MA due to microsegregation, the coarsening of old austenite, and the hardening of bainite. Significantly reduced. Therefore, it has been difficult to achieve both high strength of the steel sheet (base material) and ensuring toughness of the large heat-affected zone HAZ based on the conventional guideline for component design of the thick steel sheet.

The present invention has been made in view of such circumstances, and it is an object of the present invention to propose a new guideline for component design and to provide a high-strength steel sheet for high heat input welding based on the guideline. ..

The present inventors consider that the causes of brittleness of the heat-affected zone HAZ are (1) formation of MA, (2) micron-sized deposits / inclusions, and (3) HAZ hardness. From each viewpoint, studies were conducted to achieve both high strength of the steel plate (base material) and ensuring the toughness of the large heat-affected zone HAZ.

As a result, first, as a countermeasure for the cause (1), controlling Mn / Ni of the steel component to 0.80 or less and increasing the C content to 0.12% or more is effective in reducing MA. I got the finding that there is.

Next, as a countermeasure for cause (2), in order to suppress the formation of coarse TiN and aluminum-based oxides, the Ti content of the steel component is 0.004% or less, and the O (oxygen) content is 0. It was found that it was necessary to control it to .004% or less.

Further, as a countermeasure for the cause (3), the carbon equivalent CeqWES is suppressed to 0.53% or less, the Al content is increased to suppress the formation of BN, and the amount of solid solution B that contributes to quenching is secured. As a result, we have obtained a new finding that it is possible to secure both the strength of the base metal and the toughness of the large heat-affected zone HAZ.

The present invention has been made based on such findings, and the gist thereof is as follows.

[1] By mass%
C: 0.12% or more, 0.18% or less,
Mn: 0.5% or more, 1.5% or less,
Ni: 1.0% or more, 3.0% or less,
Al: 0.05% or more, 0.20% or less,
B: 0.0003% or more, 0.0030% or less,
Cu: 0% or more, 2.0% or less,
Cr: 0% or more, 1.0% or less,
Mo: 0% or more, 1.0% or less,
W: 0% or more, 1.0% or less,
Co: 0% or more, 1.0% or less,
Nb: 0% or more, 0.10% or less,
V: 0% or more, 0.10% or less,
Ca: 0% or more, 0.005% or less,
Mg: 0% or more, 0.005% or less,
REM: 0% or more, 0.005% or less,
Zr: Contains 0% or more and 0.005% or less,
Si: 0.30% or less,
P: 0.015% or less,
S: 0.005% or less,
Ti: 0.004% or less,
O: 0.0040% or less,
N: Limited to 0.0100% or less,
The rest consists of Fe and impurities
The Mn / Ni content ratio Mn / Ni is 0.80 or less,
A high-strength steel plate for large heat input welding in which the carbon equivalent CeqWES calculated by the following equation (1) is 0.43% or more and 0.53% or less.
CeqWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)
Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the content [mass%] of each element, and 0 is substituted for the term of the element not contained.
[2] Furthermore, by mass%,
Cu: 0.1% or more, 2.0% or less,
Cr: 0.1% or more, 1.0% or less,
Mo: 0.1% or more, 1.0% or less,
W: 0.1% or more, 1.0% or less,
Co: 0.1% or more, 1.0% or less,
Nb: 0.005% or more, 0.10% or less,
V: The high-strength steel sheet for high heat input welding according to [1], which contains one or more of 0.005% or more and 0.10% or less.
[3] Furthermore, by mass%,
Ca: 0.0001% or more, 0.005% or less,
Mg: 0.0001% or more, 0.005% or less,
REM: 0.0001% or more, 0.005% or less,
Zr: The high-strength steel sheet for large heat input welding according to [1] or [2], which contains one or more of 0.0001% or more and 0.005% or less.

According to the present invention, it is possible to provide a high-strength steel sheet for high heat input welding based on a new guideline for component design.

It is a figure which shows the collecting procedure of the Charpy test piece in the electroslag welding T-shaped joint.

Hereinafter, a high-strength steel sheet for large heat input welding according to an embodiment of the present invention will be described. First, the examination results of the present inventors who have completed the present invention and the new findings obtained will be described in detail.

The high-strength steel sheet for high heat input welding (hereinafter, also simply referred to as “steel sheet”) according to the present embodiment contains alloy elements C, Mn, and Ni that enhance hardenability. The steel sheet according to the present embodiment is manufactured by hot rolling a steel piece obtained by melting and casting steel. The steel sheet produced in this manner has a microsegregated portion formed at the interface of the solidified structure by solidification during casting. The concentration of alloying elements such as Mn and Ni in the microsegregated zone is difficult to eliminate by heating for a short time such as the heat effect of welding. Therefore, when high heat input welding is applied to a steel sheet containing C, Mn, and Ni, the microsegregated zone of HAZ becomes retained austenite in which C is concentrated by heating, and becomes hard MA after cooling. Since such MA serves as a starting point of fracture and lowers HAZ toughness, it is desirable to suppress the retention of stable austenite, in other words, the formation of retained austenite. Then, as a result of the study, the present inventors have obtained a new finding that Mn delays the decomposition of retained austenite during cooling of the large heat-affected zone HAZ as compared with Ni.

As described above, in the large heat-affected zone HAZ, when the large heat-affected zone HAZ is cooled to room temperature without decomposing the retained austenite in the microsegregation zone, the retained austenite becomes MA and deteriorates the toughness of the HAZ. Compared with Ni, Mn delays the decomposition of retained austenite, and is therefore likely to cause an increase in MA. In other words, Ni is considered to have a smaller adverse effect on the large heat input HAZ toughness than Mn. Therefore, the present inventors focused on the balance between the Mn content and the Ni content in the steel, and by optimizing the ratio between the two, the hardenability of the steel was improved and the amount of MA produced was suppressed. I thought I could do it. Specifically, the present inventors, when Mn / Ni, which is the ratio of the Mn content in the steel divided by the Ni content, becomes 0.80 or less, produces MA in the HAZ of the large heat-affected zone. We found a phenomenon that the amount decreases. It is presumed that this phenomenon is due to the partitioning behavior of Mn and Ni atoms on the partitioning behavior of C atoms at the heterophase interface when the retained austenite is decomposed, that is, when the retained austenite transforms into ferrite and cementite.

Furthermore, the present inventors, as the content of C concentrated in the microsegregated zone increases when heated by the large heat-affected zone, the decomposition of retained austenite during cooling is promoted, and the large heat-affected zone HAZ It was found that the production of MA was suppressed. As described above, it is presumed that the phenomenon that the MA of HAZ decreases as the content of C in the steel increases is that the driving force for producing cementite from retained austenite increases due to C. As a result of further studies, the present inventors have limited Mn / Ni to 0.80 or less and increased the C content to 0.12% or more in the steel component. It was found that the decomposition of retained austenite during cooling was further promoted.

Further, in a high-strength steel plate for high heat-affected welding, hardening of HAZ and coarsening of crystal grains cause deterioration of toughness of high heat-affected HAZ. In order to suppress the hardening of HAZ and secure sufficient strength of the base material, the hardenability is remarkably reduced even in a small amount in order to supplement the hardenability while reducing the content of the alloying element that enhances the hardenability. It is effective to use B which enhances. Since the effect of enhancing the hardenability of B is impaired by the formation of BN, it is desirable to contain an alloying element that forms a nitride to fix N and suppress the formation of BN. However, in high-strength steel sheets for high heat input welding, micron-sized deposits and inclusions also cause deterioration of HAZ toughness. The present inventors investigated the fracture surface of the Charpy impact piece showing a low HAZ toughness value, and clarified that micron-sized TiN and Al-based oxides are the fracture starting points. Therefore, in order to secure the toughness of the large heat-affected zone HAZ, it is necessary to suppress the formation of micron-sized TiN and Al-based oxides.

Therefore, in the high-strength steel sheet for high heat input welding according to the present embodiment, Ti is reduced to 0.004% or less to avoid the formation of coarse TiN of micron size. On the other hand, if Ti is reduced, it becomes difficult to fix N by forming TiN. Therefore, in order to suppress the formation of BN, secure the amount of B that dissolves in the steel, and improve the hardenability, Al is used in this embodiment, and the formation of BN is suppressed by the formation of AlN. In order to exert this effect, the Al content in the steel needs to be 0.05% or more. Then, the content of O (oxygen), which is an impurity, must be strictly limited to 0.0040% or less so that a coarse aluminum oxide is not generated as the Al content increases.

Furthermore, in the high-strength steel plate for large heat-affected welding according to the present embodiment, the upper limit of the carbon equivalent CeqWES is limited in order to suppress the hardening of the large heat-affected zone HAZ, which causes deterioration of the toughness of the large heat-affected zone. The contents of C, Mn, Si, Ni, Cr, Mo and V are controlled. As a result of the study by the present inventors, it was found that the toughness of the large heat-affected zone HAZ can be ensured by limiting the carbon equivalent CeqWES to 0.53% or less. Although there is a concern that the strength may be insufficient by limiting the upper limit of the carbon equivalent CeqWES, as described above, even if the carbon equivalent CeqWES is limited by utilizing the hardenability improving effect of the solid solution B, the steel sheet The strength of the (base material) can be ensured. The carbon equivalent CeqWES can be calculated by the following formula (1) according to the content of the alloying element.

CeqWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)

Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the content [mass%] of each element, and 0 is substituted for the term of the element not contained.

Hereinafter, the high-strength steel sheet for large heat input welding according to this embodiment will be described.

First, the chemical composition (steel composition) of the high-strength steel sheet for high heat input welding according to this embodiment will be described. In the following description of the chemical composition, mass% is simply expressed as%.

(C: 0.12% or more, 0.18% or less)
C is an element that enhances the hardenability of steel, contributes to high strength, and affects the formation of MA. In this embodiment, the C content is 0.12% or more. As a result, decomposition of retained austenite, that is, transformation to ferrite and precipitation of cementite, is promoted in the large heat-affected zone HAZ, MA is reduced, and deterioration of toughness is suppressed. The content of C is preferably 0.13% or more, and more preferably 0.14% or more. On the other hand, from the viewpoint of preventing excessive formation of cementite and ensuring toughness, the content of C is 0.18% or less in this embodiment. The content of C is preferably 0.17% or less, and more preferably 0.16% or less.

(Mn: 0.5% or more, 1.5% or less)
Mn is an element that enhances the hardenability of steel and contributes to high strength, and in the present embodiment, the content of Mn is 0.5% or more. The Mn content is preferably 0.8% or more. On the other hand, in the present embodiment, the Mn content is 1.5% or less from the viewpoint of suppressing the formation of MA in the large heat-affected zone and ensuring toughness. The Mn content is preferably 1.4% or less, more preferably 1.3% or less, still more preferably 1.2% or less.

(Ni: 1.0% or more, 3.0% or less)
Ni is an element that enhances the hardenability of steel and contributes to high strength, and at the same time, it is an element that enhances the toughness of the large heat-affected zone HAZ. From the viewpoint of ensuring strength and toughness, the content of Ni in this embodiment is 1.0% or more. The Ni content is preferably 1.2% or more, more preferably 1.4% or more, still more preferably 1.5% or more. On the other hand, Ni is an expensive element, and the content of Ni is 3.0% or less in this embodiment from the viewpoint of suppressing an increase in manufacturing cost. The Ni content is preferably 2.5% or less, more preferably 2.2% or less, still more preferably 2.0% or less.

(Mn / Ni: 0.80 or less)
Both Mn and Ni are elements that contribute to increasing the strength of steel, but in high heat-affected zone HAZ, Mn is more likely to promote the formation of MA than Ni, so the content of Mn is higher than the content of Ni. Is also preferable. From the viewpoint of ensuring toughness while increasing the strength of the large heat-affected zone HAZ, in the steel sheet of the present embodiment, Mn / Ni, which is the ratio of the Mn content in the steel divided by the Ni content, is 0. It is 80 or less. Mn / Ni is preferably 0.70 or less, more preferably 0.60 or less. The lower limit of Mn / Ni may be the ratio obtained by dividing the lower limit of the Mn content by the upper limit of the Ni content, that is, 0.17 or more. Mn / Ni may be 0.20 or more.

(Ti: 0.004% or less)
Ti is an element that forms TiN, and has been conventionally used for refining the HAZ structure and suppressing the precipitation of BN. However, as a result of the investigation by the present inventors, it was found that when a micron-sized coarse TiN is present in the microsegregated portion of the large heat input HAZ, it acts as a fracture starting point and exhibits extremely low toughness. Therefore, in this embodiment, the Ti content is 0.004% or less. It is preferable that Ti is not intentionally contained, and the content of Ti may be 0%. Since Ti may be mixed as an impurity, the Ti content may be more than 0% or 0.001% or more.

(Al: 0.05% or more, 0.20% or less)
Al is an important element that forms AlN and fixes N. In this embodiment, the Al content is 0.05% or more in order to suppress the precipitation of BN and secure the solid solution B effective for hardenability. The Al content is preferably 0.06% or more, more preferably 0.07% or more. On the other hand, in the present embodiment, the Al content is 0.20% or less from the viewpoint of suppressing the formation of a coarse aluminum-based oxide that serves as a fracture starting point and lowers toughness. The Al content is preferably 0.18% or less, more preferably 0.16% or less, still more preferably 0.15% or less.

(B: 0.0003% or more, 0.0030% or less)
B is an important element for ensuring the hardenability of steel while limiting the carbon equivalent CeqWES. B is an element that can remarkably improve hardenability even if the content in steel is very small, and in the present embodiment, the content of B is 0.0003% or more. The content of B is preferably 0.0005% or more, more preferably 0.0007% or more. On the other hand, in the present embodiment, the content of B is 0.0030% or less from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat-affected zone HAZ. The content of B is preferably 0.0020% or less, more preferably 0.0015% or less.

(Si: 0.30% or less)
Si is an element contained in steel for deoxidation and high strength. On the other hand, Si is also an element that promotes the formation of MA, and the present inventors have obtained the finding that Si has an extremely large effect on the formation of MA in the microsegregation zone of the large heat-affected zone HAZ. Therefore, in order to secure the toughness of the large heat-affected zone HAZ, it is necessary to limit the Si content, and in the present embodiment, the Si content is 0.30% or less. The Si content is preferably 0.25% or less, more preferably 0.20% or less, still more preferably 0.15% or less. The lower limit of the Si content is not limited, but the Si content may be 0.01% or more from the viewpoint of manufacturing cost.

(P: 0.015% or less)
P is an impurity harmful to toughness. The content of P needs to be limited in order to stably secure the toughness of the large heat-affected zone HAZ, and is 0.015% or less in this embodiment. The content of P is preferably 0.010% or less, more preferably 0.008% or less. The lower limit of the P content is not limited, but the P content may be 0.001% or more from the viewpoint of manufacturing cost.

(S: 0.005% or less)
S is an impurity, and if it is contained in a large amount in steel, it may form coarse inclusions and reduce toughness. Therefore, the content of S needs to be limited in order to stably secure the toughness of the large heat-affected zone HAZ, and in the present embodiment, S is 0.005% or less. The content of S is preferably 0.004% or less, more preferably 0.003% or less. The lower limit of the S content is not limited, but the S content may be 0.0001% or more from the viewpoint of manufacturing cost. The content of S may be 0.001% or more.

(O: 0.0040% or less)
O is an impurity, and the coarse oxide acts as a fracture starting point and reduces toughness. In particular, when a coarse aluminum oxide is present in the microsegregated zone of the large heat-affected zone, it exhibits extremely low toughness. Therefore, from the viewpoint of ensuring toughness, the content of O in this embodiment is 0.0040% or less. The content of O is preferably 0.0030% or less, and more preferably 0.0025% or less. It is desirable that the O content is low, but from the viewpoint of manufacturing cost, the O content may be 0.0001% or more, or 0.0010% or more.

(N: 0.0100% or less)
N is an element that forms a nitride. From the viewpoint of preventing the formation of coarse nitrides and ensuring toughness, and from the viewpoint of suppressing the formation of BN and ensuring hardenability, the content of N is 0.0100% or less in this embodiment. Is. The content of N is preferably 0.0080% or less, more preferably 0.0060% or less. In addition, an excessive increase in the N content may significantly reduce the solid solution B that produces BN and contributes to the improvement of hardenability. Therefore, it is desirable that the content of N is small, but from the viewpoint of manufacturing cost, the content of N may be 0.0001% or more, or 0.0020% or more.

(Carbon equivalent CeqWES: 0.43% or more, 0.53% or less)
The carbon equivalent CeqWES is an index of hardenability that affects the strength of the steel sheet (base material) and the hardness of HAZ. In order to secure the strength of the base metal, the carbon equivalent CeqWES is 0.43% or more in this embodiment. The carbon equivalent CeqWES is preferably 0.44% or more, more preferably 0.45% or more. On the other hand, in the present embodiment, the carbon equivalent CeqWES is 0.53% or less from the viewpoint of suppressing the curing of the large heat-affected zone HAZ and ensuring toughness. The carbon equivalent CeqWES is preferably 0.52% or less, more preferably 0.51% or less. The carbon equivalent CeqWES is calculated by the following equation (1) according to the content of the alloying element.

CeqWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)

Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the content [mass%] of each element, and 0 is substituted for the term of the element not contained.

The rest of the chemical composition of the steel sheet according to this embodiment is iron (Fe) and impurities. Impurities are components that are mixed in by raw materials such as ores and scraps and other factors when steel materials are industrially manufactured, and are allowed as long as they do not adversely affect the steel sheet according to the present embodiment. means. However, among impurities, the upper limit of the content of P, S and O is limited as described above. Further, it is necessary to limit the upper limit of Ti when it is mixed as an impurity.

In order to improve the strength and toughness of the steel sheet (base material), the steel sheet according to the present embodiment may be one of the selective elements Cu, Cr, Mo, W, Co, Nb, V shown below or, if necessary. Two or more kinds may be contained.

(Cu: 0% or more, 2.0% or less)
Cu is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the Cu content is not limited and may be 0%. Further, Cu is also an element that has little adverse effect on weldability and toughness of HAZ and improves the strength and toughness of the base metal. Therefore, in this embodiment, the Cu content may be 0.1% or more. However, from the viewpoint of suppressing the generation of Cu cracks during hot rolling of the steel sheet, the Cu content is 2.0% or less in this embodiment. The Cu content is preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less.

(Cr: 0% or more, 1.0% or less)
Cr is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the Cr content is not limited and may be 0%. Cr is also an element that improves the strength of the base metal. Therefore, in the present embodiment, the Cr content may be 0.1% or more. The Cr content is preferably 0.2% or more, more preferably 0.3% or more. However, in this embodiment, the Cr content is 1.0% or less from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat-affected zone HAZ. The Cr content is preferably 0.8% or less, more preferably 0.5% or less.

(Mo: 0% or more, 1.0% or less)
Mo is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the Mo content is not limited and may be 0%. Mo is also an element that improves the strength and toughness of the base metal. Therefore, in the present embodiment, the Mo content may be 0.1% or more. The Mo content is preferably 0.2% or more, more preferably 0.3% or more. However, in the present embodiment, the Mo content is 1.0% or less from the viewpoint of suppressing deterioration of toughness and weldability of the large heat-affected HAZ and suppressing an increase in alloy cost. The Mo content is preferably 0.5% or less.

(W: 0% or more, 1.0% or less)
W is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the W content is not limited and may be 0%. W is also an element that improves the strength and toughness of the base metal. Therefore, in the present embodiment, the W content may be 0.1% or more. The W content is preferably 0.2% or more, more preferably 0.3% or more. However, in the present embodiment, the W content is 1.0% or less from the viewpoint of suppressing deterioration of toughness and weldability of the large heat-affected zone HAZ and suppressing an increase in alloy cost. The W content is preferably 0.5% or less.

(Co: 0% or more, 1.0% or less)
Co is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the Co content is not limited and may be 0%. In addition, Co has little adverse effect on weldability and HAZ toughness, and is also an element that improves the strength and toughness of the base metal. Therefore, in the present embodiment, the Co content may be 0.1% or more. However, from the viewpoint of suppressing an increase in alloy cost, the Co content is 1.0% or less in this embodiment. The Co content is preferably 0.5% or less.

(Nb: 0% or more, 0.10% or less)
Nb is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the Nb content is not limited and may be 0%. Nb is also an element that improves the strength and toughness of the base metal. Therefore, in the present embodiment, the Nb content may be 0.005% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat input HAZ, the content of Nb in this embodiment is 0.10% or less. The content of Nb is preferably 0.05% or less, more preferably 0.03% or less.

(V: 0% or more, 0.10% or less)
V is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the V content is not limited and may be 0%. V is also an element that improves the strength of the base metal. Therefore, in this embodiment, the V content may be 0.005% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat input HAZ, the V content is 0.10% or less in this embodiment. The V content is preferably 0.08% or less, more preferably 0.06% or less.

Further, the steel sheet according to the present embodiment may contain one or more of the selective elements Ca, Mg, REM, and Zr shown below, if necessary, in order to control the form of inclusions.

(Ca: 0% or more, 0.005% or less)
Ca is an element that forms oxides, sulfides, and acid sulfides to suppress the formation of coarse inclusions and enhance the toughness of the base metal and HAZ. Therefore, in the present embodiment, the Ca content may be 0.0001% or more. However, in the present embodiment, the Ca content is 0.005% or less from the viewpoint of suppressing an increase in Ca-based inclusions that may act as a starting point for brittle fracture. The Ca content is preferably 0.004% or less. The Ca content may be 0%.

(Mg: 0% or more, 0.005% or less)
Like Ca, Mg is an element that forms oxides, sulfides, and acid sulfides to suppress the formation of coarse inclusions and enhance the toughness of the base metal and HAZ. Therefore, the Mg content may be 0.0001% or more. However, in the present embodiment, the Mg content is 0.005% or less from the viewpoint of suppressing an increase in Mg-based inclusions that may act as a starting point for brittle fracture. The Mg content is preferably 0.003% or less. The Mg content may be 0%.

(REM: 0% or more, 0.005% or less)
REM (rare earth element) is a general term for two elements, Sc and Y, and 15 lanthanoid elements such as La, Ce and Nd. The REM referred to in the present embodiment is composed of one or more kinds selected from these rare earth elements, and the REM content described below is the total amount of the rare earth elements.

Like Ca, REM is an element that forms oxides, sulfides, and acid sulfides to suppress the formation of coarse inclusions and enhance the toughness of the base metal and HAZ. Therefore, in the present embodiment, the content of REM may be 0.0001% or more. However, from the viewpoint of suppressing the increase of REM-based inclusions that may act as the starting point of brittle fracture, the content of REM is 0.005% or less in the present embodiment. The content of REM is preferably 0.003% or less. The content of REM may be 0%.

(Zr: 0% or more, 0.005% or less)
Like Ca and REM, Zr is an element that forms oxides, sulfides, and acid sulfides to suppress the formation of coarse inclusions and enhance the toughness of the base metal and HAZ. Therefore, in the present embodiment, the Zr content may be 0.0001% or more. However, in the present embodiment, the Zr content is 0.005% or less from the viewpoint of suppressing an increase in Zr-based inclusions that may act as a starting point for brittle fracture. The Zr content is preferably 0.003% or less. The Zr content may be 0%.

The high-strength steel sheet for large heat input welding according to this embodiment is suitable for applications that require a high-strength and thick thick steel sheet. The high-strength steel plate for high heat input welding according to the present embodiment is particularly suitable for applications in which high heat input welding with high welding efficiency is performed and the required level for toughness of HAZ is high. Specifically, the high-strength steel plate for large heat input welding according to the present embodiment is subjected to diaphragm welding (electroslag welding) such as a four-sided box column for a building steel frame, and has a high-strength thickness that requires HAZ toughness. Suitable for steel plates.

With the increase in size of buildings, the efficiency of construction, and the improvement of safety, the demand for thick steel sheets for welded structures is increasing. Therefore, in the high-strength steel sheet for high heat input welding according to the present embodiment, the plate thickness is preferably 50 mm or more and 100 mm or less, and the yield strength is preferably 630 MPa or more from the viewpoint of strength. The upper limit of the yield strength is not limited, and for example, the yield strength may be 750 MPa or less. Further, from the viewpoint of seismic resistance, the yield ratio of the high-strength steel sheet for large heat input welding according to this embodiment is preferably 85% or less. The lower limit of the yield ratio is not limited, and for example, the yield ratio may be 70% or more. Further, from the viewpoint of high construction efficiency and seismic resistance, it is preferable that the average value of Charpy absorption energy (test temperature 0 ° C.) in HAZ of the large heat-affected zone is 70 J or more. The large heat input welding includes, for example, electroslag welding and submerged arc welding.

Next, a method for manufacturing a high-strength steel sheet for large heat input welding according to this embodiment will be described.

The high-strength steel plate for high-intensity heat welding according to the present embodiment is manufactured by melting and casting steel to produce steel pieces, and hot-rolling the obtained steel pieces. The method for producing the steel piece is not limited, and the steel piece may be produced by a known method. For example, steel pieces are melted by a normal refining process such as a converter or an electric furnace, and then manufactured by a method such as a continuous casting method or a ingot-breaking method. The steel pieces may be hot-rolled and then subjected to controlled cooling such as water cooling as they are, or may be air-cooled and then heat-treated. Further, the steel piece may be hot-rolled as it is after being produced by melting and casting steel. However, as will be described later, the steel pieces are preferably cooled after casting, reheated to a temperature of Ac ₃ or higher, and hot-rolled.

Hereinafter, preferable manufacturing conditions for the high-strength steel sheet for high heat input welding according to the present embodiment will be described.

A steel piece having a thickness of 200 mm or more, which is composed of the above-mentioned chemical components and is produced by a continuous casting method, is once cooled to 400 ° C. or lower. After that, the steel pieces are heated to a temperature range of 900 ° C. or higher and 1250 ° C. or lower and hot-rolled to produce steel sheets having a plate thickness of 50 mm or more and 100 mm or less. The steel sheet is subjected to various heat treatments as needed.

When the steel pieces after continuous casting are charged into a heating furnace by hot charging without being cooled to 400 ° C. or lower, the coarse γ structure generated during casting remains in the steel pieces after heating, and the structure of the steel sheet. However, the low temperature toughness may deteriorate due to insufficient fineness. Therefore, it is preferable that the steel pieces after continuous casting are once cooled to 400 ° C. or lower.

The heating temperature of the steel piece is preferably 900 ° C. or higher in order to dissolve the BN precipitated on the steel piece after casting. N in the heated steel piece is fixed as AlN during hot rolling, and the formation of BN is suppressed. As a result, in the steel sheet, a solid solution B that improves the hardenability of the steel is sufficiently secured. On the other hand, the heating temperature of the steel piece is 1250 ° C. or lower from the viewpoint of suppressing the coarsening of γ grains, refining the metal structure after hot rolling, and suppressing the deterioration of low temperature toughness. preferable. The heating temperature is more preferably 1200 ° C. or lower.

In the case of direct quenching after hot rolling, the end temperature (finishing temperature) of hot rolling is preferably the austenite (γ) single phase region, that is, the Ar ₃ transformation point or higher at which the ferrite transformation starts. At this time, even if the temperature of the surface layer of the steel sheet is in the two-phase region of austenite (γ) / ferrite (α) at the end of hot rolling, there is no problem if the temperature of the central portion in the plate thickness direction is in the γ single-phase region. Absent. The end temperature of hot rolling may be 750 ° C. or higher. The end temperature of hot rolling is preferably 900 ° C. or lower from the viewpoint of miniaturization of the metal structure. The Ar ₃ transformation point (° C.) can be obtained by the following equation (2).

Ar ₃ transformation point = 868-396 × C + 24.6 × Si-68.1 × Mn-36.1 × Ni-20.7 × Cu-24.8 × Cr + 29.1 × Mo… (2)

Here, C, Si, Mn, Ni, Cu, Cr, and Mo in the above equation (2) are the contents of each element expressed in mass% in the steel sheet, and 0 is substituted for the term of the element not contained. To do.

Further, in the case of direct quenching after hot rolling, hot rolling is completed in the γ single-phase region, and water cooling is continuously performed in order to adjust the material of the steel sheet. On the other hand, when the steel sheet is air-cooled after hot rolling, the steel sheet is subjected to reheating to the γ single-phase region and subsequent quenching (γ reheating quenching). Further, the steel sheet that has been directly hardened or γ-reheated and hardened after hot rolling may be subjected to various heat treatments in order to adjust the material.

These hardened steel sheets (direct quenching or γ reheating quenching) are reheated to a two-phase region where austenite (γ) and ferrite (α) coexist in order to reduce the yield ratio. Subsequent quenching (γ / α reheating quenching) may be performed. Here, the two-phase region is equal to or more than the Ac ₁ transformation point and less than the Ac ₃ transformation point, and the Ac ₁ transformation point and the Ac ₃ transformation point can be obtained by the following equations (3) and (4), respectively.

Ac ₁ transformation point = 750.8-26.6C + 17.6Si-11.6Mn-22.9Cu-23.0Ni + 24.1Cr + 22.5Mo-39.7V-5.7Ti + 232.4Nb-169.4Al-894.7B723 + 29.1Si -10.7Mn-16.9Ni + 6.38W + 16.9Cr ... (3)
Ac ₃ transformation point = 910-203√C + 44.7Si-30Mn-400Al-15.2Ni + 104V + 31.5Mo + 13.1W + 11Cr + 20Cu-700P-400Ti ... (4)

Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Al, B, W, and P in the above equations (3) and (4) are the elements represented by mass%. Substitute 0 for the element content that is not contained in the steel sheet.

Further, the steel sheet may be tempered in order to finally adjust the strength, yield ratio and toughness of the steel sheet. When tempering is carried out, the tempering temperature is preferably 350 ° C. or higher and 600 ° C. or lower.

Here, the above-mentioned hot rolling finishing temperature, γ reheating quenching temperature, γ / α reheating quenching temperature, and tempering temperature all refer to the temperature at the center in the plate thickness direction. The temperature of the central portion in the plate thickness direction can be obtained by heat transfer calculation from the temperature of the steel plate surface measured by a radiation thermometer.

By the above manufacturing method, the high-strength steel sheet for large heat input welding according to the present embodiment can be manufactured.

The high-strength steel plate for large heat input welding according to the present embodiment has good HAZ toughness even if large heat input welding such as electroslag welding or submerged arc welding is performed so that the welding heat input exceeds 50 kJ / mm. Secured.

Further, the large heat-affected high-strength steel plate according to the present embodiment preferably has a yield strength of 630 MPa or more and a Charpy absorption energy (test temperature 0 ° C.) in HAZ of a large heat-affected zone (for example, an electroslag weld). The average value is 70J or more. Therefore, the high-strength steel plate for large heat input welding according to the present embodiment is suitable for a building steel frame, and the high-strength steel plate for high heat input welding according to the present embodiment promotes high-rise buildings and large spans. It can be promoted, and construction efficiency and seismic safety can be further improved.

Examples of the present invention are shown below. However, the examples shown below are examples of the present invention, and the present invention is not limited to the examples described below.

The thickness of the steel pieces produced by melting and continuous casting of steel by a converter is 300 mm. After continuous casting, the steel pieces were cooled to room temperature, reheated within a temperature range of 1000 ° C. or higher and 1200 ° C. or lower, and hot-rolled. The finishing temperature of hot rolling is 750 ° C. or higher and 900 ° C. or lower. When the steel sheet after hot rolling is directly hardened, the finishing temperature of hot rolling is in the γ single-phase region (Ar ₃ transformation point or higher).

Next, the steel sheet after hot rolling was heat-treated under the conditions shown in Tables 3 and 4. In Tables 3 and 4, the “γ reheating quenching temperature” is the heating temperature when the steel sheet air-cooled after hot rolling is subjected to γ reheating quenching. On the other hand, the "γ / α reheating quenching temperature" is the heating temperature when direct quenching or γ reheating quenching is performed after hot rolling, and further, γ / α reheating quenching is performed.

Samples were taken from the thick steel plates produced in this way and chemically analyzed. The chemical composition of each thick steel sheet is shown in Tables 1 and 2, and the sheet thickness is shown in Tables 5 and 6. The carbon equivalent CeqWES shown in Tables 1 and 2 was calculated by the following formula (5).

CeqWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (5)

Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (5) are the content [mass%] of each element, and 0 is substituted for the term of the element not contained.

<Mechanical properties of base material>
The test pieces used for the evaluation of the mechanical properties of the base material, that is, the tensile test and the Charpy impact test, were taken from the position of 1/4 of the plate thickness of the thick steel plate.
The tensile test was performed at room temperature using two test pieces in accordance with JIS Z 2241: 2011. YS (0.2% yield strength) and TS (tensile strength) are the average values of the two test pieces, respectively. YR (yield ratio) is the ratio of YS to TS and is expressed as a percentage, that is, 100 × (YS / TS). The unit of YR (yield ratio) is%.
The Charpy impact test was performed in accordance with JIS Z 2242: 2018 using three V-notch test pieces, and the absorbed energy was measured. The test temperature is 0 ° C. The absorbed energy of the base material (KV ₂ (0 ° C.)) is the average value (arithmetic mean) of the absorbed energies of the three test pieces measured in this way.

<HAZ toughness of welded joints>
The HAZ toughness of the welded joint was evaluated using the welded joint of each thick steel plate manufactured by electroslag welding (ESW).
The electroslag welding method (ESW) produced the T-joint illustrated in FIG. Welding was performed in one pass, and large heat input welding with a welding heat input amount of 70 kJ / mm or more and 150 kJ / mm or less was applied. The heat input amount is an average value of the heat input amount in the entire welding length of the T-shaped joint shown in FIG.

The T-shaped joint of FIG. 1 is manufactured by ESW as follows. First, the diaphragm 2 made of thick steel plate is arranged in a T shape with a gap with respect to the skin plate 1 made of thick steel plate. Next, the back pads 3 and 4 are arranged along the diaphragm 2 so as to sandwich the gap from the longitudinal direction of the skin plate 1. The backing metal 3 and 4 surround the gap so that the molten slag and the molten metal during welding do not flow out from the welded portion. Then, inside this gap, the welding wire is supplied into the molten slag bath. The weld wire is mainly melted by the resistance heat of the molten slag, and the weld metal portion 5 is formed to form a T-shaped joint.

At the welded portion of this T-shaped joint, a test piece 7 for a Charpy impact test was taken along the plate thickness center line of the diaphragm 2. Specifically, as shown in FIG. 1, from the weld metal portion 5, the weld metal portion 5 passes through the weld heat affected zone (HAZ) 6 on the skin plate 1 side beyond the melt fusion wire (FL) to the inside side of the skin plate 1. Test pieces 7 were collected from all parts. FIG. 1 shows the sampling position of the Charpy test piece whose notch position is 1 mm from FL. In addition, although not shown, a Charpy test piece having a notch position of FL was also collected. The skin plate 1 and the diaphragm 2 are of the same steel type, and both have the same plate thickness.

The test piece 7 produced in this manner is a V-notch test piece having a notch in the HAZ portion 1 mm away from the melt fusion wire (FL), and a V-notch test piece having a notch on the FL. The test results using these are shown in Tables 5 and 6, respectively, as "FL + 1 mm" and "FL". Charpy impact tests were performed using each V-notch test piece at 0 ° C. and −20 ° C. in accordance with JIS Z 2242: 2018. A Charpy impact test was performed using three V-notch test pieces for each notch position and one test temperature, and the average value of absorbed energy (arithmetic mean) under each condition was adopted as the evaluation result. Tables 5 and 6 show the thickness of the thick steel plate, the mechanical properties of the base metal, the amount of heat input in electroslag welding, and the HAZ toughness of the electroslag welded joint. KV ₂ (0 ° C.) and KV ₂ (-20 ° C.) are the absorbed energy at 0 ° C. and the absorbed energy at −20 ° C., respectively.

As shown in Table 5, the steel sheet of the present invention has a yield strength (YS) of 630 MPa or more and a yield ratio (YR) of 85% or less when the sheet thickness is 50 mm or more and 100 mm or less. Further, the ESW joint produced by using the steel plate of the present invention has an excellent HAZ toughness of 70 J or more at 0 ° C. Further, the ESW joint manufactured by using the steel plate of the present invention has a very excellent HAZ toughness of 27 J or more even when the test temperature is −20 ° C.

On the other hand, as shown in Table 6, since the chemical composition of the conventional steel (comparative steel) is out of the scope of the present invention, the mechanical properties of the base material and the HAZ toughness of the ESW joint are inferior.

The symbol B1 has a poor yield strength because the amount of C is too low, and the code B2 has a poor HAZ toughness because the amount of C is too high. Reference numeral B3 is inferior in yield strength because the amount of Mn is too low, and symbol B4 is inferior in HAZ toughness because the amount of Mn is too high. The sign B5 has poor HAZ toughness because the amount of Ni is too low.

The code B6 has a poor yield strength because the Al content is too low, and the code B7 has a poor HAZ toughness because the Al content is too high. The sign B8 has a poor yield strength because the amount of B is too low, and the sign B9 has a poor HAZ toughness because the amount of B is too high. The sign B10 has too high a Si amount, the sign B11 has a too high P amount, the sign B12 has a too high S amount, the sign B13 has a too high Ti amount, and the sign B14 has an O amount. Since it is too high, the sign B15 has a poor yield strength and HAZ toughness because the amount of N is too high.

The code 16 has a poor yield strength because the CeqWES is too low, and the code 17 has a poor HAZ toughness because the CeqWES is too high. The code B18 is inferior in HAZ toughness because Mn / Ni is too high.

1 ... skin plate,
2 ... Diaphragm,
3, 4 ... Back allowance,
5 ... Welded metal part,
6 ... Welding heat affected zone (HAZ),
7 ... Test piece

The present invention applies to thick steel sheets manufactured in the steel industry. The present invention can also be applied to steel products other than thick steel plates, such as shaped steel. The high-strength and thick steel plate to which the present invention is applied is mainly used as a steel frame for high-rise buildings, and is particularly suitable as a steel frame for a four-sided box column which is roughly composed of four skin plates and a diaphragm arranged inside. is there. In the joining of each member of the four-sided box column, so-called large heat input welding, which has a large welding heat input, is performed. For example, high-efficiency large heat input welding such as electroslag welding and submerged arc welding is applied to diaphragm welding for attaching a diaphragm to a skin plate and square welding for assembling a skin plate, respectively. Further, the high-strength steel plate for high heat input welding according to the present invention can also be used for welded structures such as bridges, shipbuilding, tanks, marine structures, and line pipes.

The high-strength steel sheet for high-intensity welding according to the present invention is suitable when high-strength and thick thick steel sheet is subjected to high-intensity welding with high welding efficiency and the required level of HAZ toughness is high. Specifically, the high-strength steel plate according to the present invention has a yield strength of 630 MPa or more, a plate thickness of 50 mm or more and 100 mm or less, and an average value of Charpy absorption energy (test temperature 0 ° C.) in HAZ of an electroslag weld is 70 J. The above is the thick steel plate for large heat input welding. Therefore, the high-strength steel plate for high heat input welding according to the present invention has a high toughness required for high heat input HAZ, such as a building steel frame four-sided box pillar diaphragm to which large heat input such as electroslag welding is applied. Suitable for strong thick steel plates.

Claims (3)

By mass%
C: 0.12% or more, 0.18% or less,
Mn: 0.5% or more, 1.5% or less,
Ni: 1.0% or more, 3.0% or less,
Al: 0.05% or more, 0.20% or less,
B: 0.0003% or more, 0.0030% or less,
Cu: 0% or more, 2.0% or less,
Cr: 0% or more, 1.0% or less,
Mo: 0% or more, 1.0% or less,
W: 0% or more, 1.0% or less,
Co: 0% or more, 1.0% or less,
Nb: 0% or more, 0.10% or less,
V: 0% or more, 0.10% or less,
Ca: 0% or more, 0.005% or less,
Mg: 0% or more, 0.005% or less,
REM: 0% or more, 0.005% or less,
Zr: Contains 0% or more and 0.005% or less,
Si: 0.30% or less,
P: 0.015% or less,
S: 0.005% or less,
Ti: 0.004% or less,
O: 0.0040% or less,
N: Limited to 0.0100% or less,
The rest consists of Fe and impurities
The Mn / Ni content ratio Mn / Ni is 0.80 or less,
A high-strength steel plate for large heat input welding in which the carbon equivalent CeqWES calculated by the following equation (1) is 0.43% or more and 0.53% or less.
CeqWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)
Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the content [mass%] of each element, and 0 is substituted for the term of the element not contained. Furthermore, in% by mass,
Cu: 0.1% or more, 2.0% or less,
Cr: 0.1% or more, 1.0% or less,
Mo: 0.1% or more, 1.0% or less,
W: 0.1% or more, 1.0% or less,
Co: 0.1% or more, 1.0% or less,
Nb: 0.005% or more, 0.10% or less,
V: The high-strength steel sheet for high heat input welding according to claim 1, which contains one or more of 0.005% or more and 0.10% or less. Furthermore, in% by mass,
Ca: 0.0001% or more, 0.005% or less,
Mg: 0.0001% or more, 0.005% or less,
REM: 0.0001% or more, 0.005% or less,
The high-strength steel sheet for large heat input welding according to claim 1 or 2, which contains one or more of Zr: 0.0001% or more and 0.005% or less.

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