JP7444343B1 - Thick steel plate and its manufacturing method - Google Patents

Abstract

To provide a thick steel plate that has high strength, low yield ratio, and high toughness, and has excellent toughness at a joint bond part even in high heat input welding where the welding heat input exceeds 40 kJ/mm. The area fraction of the average concentration region of Mn, which is defined as a region having a predetermined component composition and microstructure and having a Mn concentration of 0.9 to 1.1 times the average Mn content (mass%), is 90. %, the area fraction of the Mn enriched region is defined as a region having a Mn concentration of 1.15 times or more than the average Mn content (mass %) is 1.0% or more, and the Mn enriched region A thick steel plate having an Mn concentration distribution in which the average equivalent circle diameter is 7.0 μm or less, and a Charpy absorbed energy vE0 at 0° C. of 70 J or more.

Description

The present invention relates to a thick steel plate, and in particular, it combines high strength, low yield ratio, and high toughness, and has excellent toughness at the bond part of a welded joint even in high heat input welding where the welding heat input exceeds 40 kJ/mm. Regarding thick steel plates. In addition to having the above-mentioned excellent properties, the thick steel plate of the present invention is also suitable for industrial production and can be extremely suitably used as a steel material for construction. The present invention also relates to a method for manufacturing the thick steel plate.

In recent years, as building structures become taller and span longer, the steel materials used are becoming thicker. The thick steel plates used as such steel materials are required to have excellent mechanical properties, specifically, to have high tensile strength, yield stress, and toughness.

Furthermore, from the viewpoint of the safety of steel structures, the thick steel plates used are also required to have a low yield ratio (ratio of yield stress to tensile strength). The lower the yield ratio, the better the plastic deformability, which contributes to improving the seismic safety of the structure.

As a manufacturing process for thick steel plates with a reduced yield ratio, a process using multistage heat treatment has been put into practical use. In the multistage heat treatment, the thick steel plate after hot rolling is reheated to a ferrite+austenite two-phase region, quenched, and then tempered. However, in the multi-stage heat treatment, the yield point increases due to the recovery of the structure due to tempering, so it has been difficult to achieve both high strength and low yield ratio.

Furthermore, when building steel structures, it is common to join steel materials together by welding. Therefore, the thick steel plate used is required to have excellent toughness not only of the thick steel plate itself (base metal toughness) but also of the weld heat-affected zone. In particular, from the viewpoint of improving the earthquake resistance of building structures, it is required that welded joints have excellent toughness, especially the bond portions of welded joints.

Additionally, as the steel materials used are becoming thicker, the scope of application of high heat input welding (welding with a large amount of heat input during welding) is expanding to improve construction efficiency and reduce construction costs. In particular, in recent years, the application of high heat input welding such as submerged arc welding and electroslag welding where the welding heat input exceeds 40 kJ/mm has become common. Therefore, thick steel plates are required to have excellent toughness in the weld heat affected zone even when subjected to high heat input welding where the welding heat input exceeds 40 kJ/mm.

Generally, when high heat input welding is applied to steel materials, the biggest problem is the deterioration of the toughness of the bond part of the welded joint. Among the welding heat affected zones, the bond zone is exposed to high temperatures just below the melting point during high heat input welding, so coarsening of austenite crystal grains is most noticeable. Then, the coarsened austenite crystal grains transform into a brittle upper bainite structure due to the temperature drop after welding, and furthermore, a coarse island-like martensite (Martensite-Austenite constituent, MA), which is a brittle structure, is generated. The toughness decreases. Therefore, if the toughness of the bond portion of a welded joint in high heat input welding can be improved, the safety of steel structures can be significantly improved.

Thus, thick steel plates are required not only to have excellent mechanical properties such as strength, yield ratio, and toughness, but also to have excellent toughness in the weld heat affected zone. Various techniques have been proposed to meet these demands.

For example, Patent Document 1 proposes a technology for manufacturing high-strength steel by quenching a hot-rolled steel plate, then heating and quenching it again to a two-phase region of ferrite + austenite, and then performing a tempering treatment. has been done.

Patent Document 2 proposes a low yield ratio high tensile strength steel plate having a predetermined composition and an amount of retained austenite of 1.0% or more.

Patent Document 3 discloses a low yield ratio, high tensile strength steel plate that has a predetermined component composition, has a microstructure containing bainite and island martensite, and has a controlled circular equivalent diameter and average aspect ratio of prior austenite grains. is proposed.

Japanese Patent Application Publication No. 06-248337 Japanese Patent Application Publication No. 2001-226740 Japanese Patent Application Publication No. 2018-090872

However, the technology proposed in Patent Document 1 aims to prevent cracking during low heat input welding, and does not pay attention to the toughness of the weld heat affected zone during high heat input welding. There wasn't.

On the other hand, the technique proposed in Patent Document 2 takes into consideration the toughness of the welded portion. However, evaluations have only been conducted at relatively low heat inputs of 5 kJ/mm or 15 kJ/mm, and the toughness of the weld heat affected zone in high heat input welding where the welding heat input exceeds 40 kJ/mm has not been considered. . Furthermore, in the manufacturing process disclosed in Patent Document 2, the volume fraction of ferrite and martensite tends to change depending on the manufacturing conditions and the position within the steel sheet. Therefore, in order to obtain the desired product, it is necessary to strictly adjust the manufacturing conditions, and it is not suitable for industrial production due to the high operational load.

Further, the technology proposed in Patent Document 3 achieves high bond part toughness in high heat input welding where the welding heat input exceeds 40 kJ/mm in addition to low yield ratio and high strength. However, in Patent Document 3, in order to realize the above characteristics, it is essential that the area fraction of island-shaped martensite be 5% or more. In the manufacturing process in Patent Document 3, in order to make the area fraction of island-like martensite 5% or more, after reheating the hot rolled steel sheet, a first water cooling step and an air cooling step are performed under controlled temperature conditions. , and a second water cooling step to control the formation of island martensite. During cooling, temperature variations tend to occur in the longitudinal and width directions of the steel plate, so controlling the structure during such a cooling process requires extremely strict adjustment of manufacturing conditions, resulting in a high operational load.

The present invention has been made in view of the above circumstances. The present invention combines high strength, low yield ratio, and high toughness, and also has excellent toughness of the bond part of the welded joint even in large heat input welding where the welding heat input exceeds 40 kJ/mm, and is suitable for industrial production. The purpose is to provide thick steel plates suitable for.

The present inventors conducted extensive research to achieve the above object and obtained the following knowledge.

(1) In a thick steel plate that contains bainite and island martensite and has an area fraction of bainite of 80.0% or more, by forming a predetermined Mn concentration distribution, the area fraction of island martensite can be reduced to 5%. Despite being relatively low at less than .0%, high strength, low yield ratio, and high toughness can be achieved.

(2) The above Mn concentration distribution can be obtained by controlling the component composition, especially the content of C and Mn, within a specific range, and by appropriately controlling the temperature raising conditions in the reheating step after hot rolling. can.

The present invention was completed based on the above findings. The gist of the present invention is as follows.

1. In mass%,
C: 0.010-0.14%,
Si: 0.01 to 0.50%,
Mn: 0.9-3.0%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.002-0.080%,
Contains Ti: 0.003 to 0.030%, and N: 0.0015 to 0.0080%,
the remainder consists of Fe and unavoidable impurities, and
4.83C+Mn expressed as C content (mass%) and Mn content (mass%) is 1.4 to 3.3 mass%,
The ratio Ti/N of Ti content (mass%) to N content (mass%) is 2.0 to 4.3,
It has a component composition in which _PCM represented by the following formula (1) is 0.30% by mass or less,
Contains bainite and island martensite, and
The area fraction of bainite is 80.0% or more,
having a microstructure in which the area fraction of island martensite is less than 5.0%,
The area fraction of the average concentration region of Mn, defined as a region having a Mn concentration of 0.9 to 1.1 times the average Mn content (mass%), is less than 90%,
The area fraction of the Mn enriched region, defined as a region having a Mn concentration of 1.15 times or more than the average Mn content (mass%), is 1.0% or more, and the average circle of the Mn enriched region has a Mn concentration distribution with an equivalent diameter of 7.0 μm or less,
A thick steel plate whose Charpy absorbed energy: _vE0 at 0°C is 70J or more.
P _CM = [C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5 [B]…(1)
However, the parentheses in the above formula represent the content (mass%) of the element within the parentheses, and when the element is not contained, it is set to 0.

2. The component composition is in mass%,
Cu: 3.0% or less,
Ni: 3.0% or less,
Cr: 3.0% or less,
Mo: 1.5% or less,
W: 3.0% or less,
Nb: 0.10% or less,
V: 0.10% or less,
B: 0.0050% or less,
Ca: 0.005% or less,
REM: 0.020% or less,
The thick steel plate according to 1 above, further containing at least one selected from the group consisting of Mg: 0.005% or less and Zr: 0.020% or less.

3. In the microstructure,
The thick steel plate according to 1 or 2 above, wherein the area fraction of the island-like martensite is 1.0% or more and less than 5.0%, and the average equivalent circle diameter of the island-like martensite is 5.0 μm or less. .

4. A hot rolling process of hot rolling a steel material having the composition described in 1 or 2 above into a thick steel plate;
a first cooling step of cooling the thick steel plate after the hot rolling step;
The thick steel plate after the first cooling step is heated at the 1/4 position of the plate thickness, average heating rate in the temperature range from Ac1 point to Ac3 point: 2.0 ° C / s or less, Ac3 point -100 ° C to Ac3 point Residence time in the temperature range up to: Raise the temperature to a reheating temperature of 3 Ac or more and 3 Ac + 60°C or less under conditions of 60 seconds or more, and then hold at the reheating temperature for a holding time of 10 minutes or more. a reheating step;
The thick steel plate after the reheating process,
Average cooling rate at 1/4 plate thickness position: 1.0 to 200.0°C/s, accelerated cooling to an accelerated cooling stop temperature of 100°C to 600°C, and then air cooling to a temperature of 100°C or less A method for manufacturing a thick steel plate, including a cooling step.

5. Furthermore, a heat treatment step is included after the first cooling step and before the reheating step,
In the heat treatment step,
The thick steel plate after the first cooling step is heated to a heat treatment temperature of 3 Ac or more and 1050 ° C or less,
Holding at the heat treatment temperature for a holding time of 5 minutes or more,
4. The method for manufacturing a thick steel plate according to 4 above, wherein the steel plate is then cooled to a cooling stop temperature of 500° C. or less.

According to the present invention, a thick steel plate that has high strength, low yield ratio, and high toughness, and has excellent toughness at the bond part of welded joints even in high heat input welding where the welding heat input exceeds 40 kJ/mm is produced. Obtainable. The thick steel plate of the present invention can be extremely suitably used as a steel material for construction, and contributes to increasing the size of steel structures and improving earthquake resistance. Moreover, the thick steel plate of the present invention can be manufactured by a process with low operational load, and is suitable for industrial production.

FIG. 3 is a schematic diagram showing a groove shape in electroslag welding performed for evaluation of bond part toughness. FIG. 2 is a schematic diagram showing the sampling positions of Charpy impact test pieces from electroslag welds.

Embodiments of the present invention will be described below. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description. Further, in this specification, the toughness of the unwelded thick steel plate itself is sometimes referred to as "base metal toughness" to distinguish it from the bond part toughness after welding.

[Component composition]
The thick steel plate of the present invention and the steel material used for manufacturing the thick steel plate need to have the above-mentioned composition. Each component included in the component composition will be explained below. In addition, unless otherwise specified, "%" representing the content of each component means "mass %".

C: 0.010-0.14%
C is an element that has the effect of increasing the strength of thick steel plates. If the C content is less than 0.010%, the desired tensile strength cannot be obtained. Therefore, the C content is set to 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, when the C content exceeds 0.14%, the formation of coarse island-like martensite and cementite is promoted, and the toughness of the base material decreases, and the toughness of the bond portion also deteriorates significantly. Therefore, the C content is set to 0.14% or less, preferably 0.10% or less, and more preferably 0.08% or less.

Si: 0.01~0.50%
Si is an element that functions as a deoxidizing agent and has the effect of increasing the strength of thick steel plates. In order to obtain the above effect, the Si content is set to 0.01% or more. On the other hand, when the Si content exceeds 0.50%, the formation of coarse island-like martensite is promoted, and a decrease in the toughness of the base material and the toughness of the bond portion becomes obvious. Therefore, the Si content is set to 0.50% or less, preferably 0.35% or less.

Mn: 0.9-3.0%
Mn is an element that has the effect of increasing the strength of thick steel plates. Further, by controlling the Mn concentration distribution as described later, high strength, low yield ratio, and high toughness can be achieved. If the Mn content is less than 0.9%, the above effects cannot be obtained. Therefore, the Mn content is set to 0.9% or more, preferably 1.2% or more. On the other hand, when the Mn content exceeds 3.0%, the area fraction of the Mn-enriched portion increases and coarse MA is generated, resulting in a decrease in base material toughness. In addition, the weld heat affected zone hardens, and the toughness of the bonded part decreases significantly. Therefore, the Mn content is 3.0% or less, preferably 2.6% or less.

In addition, the component composition explained here is the average composition of a thick steel plate. Therefore, the value of the above Mn content is used as the "average Mn content" in the definition of the Mn concentration distribution described later.

P: 0.015% or less P is an element that deteriorates the toughness of the base material and the toughness of the bond part, and it is desirable to reduce it as much as possible. When the P content exceeds 0.015%, the base material toughness and the bonding part toughness decrease significantly. This is considered to be because when the P content is high, P segregates in the Mn-enriched area and hardens the structure. Therefore, the P content is set to 0.015% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction leads to an increase in cost. Therefore, from the viewpoint of manufacturing cost, it is preferable that the P content is 0.001% or more.

S: 0.0050% or less S is an element that deteriorates the toughness of the base material, and it is desirable to reduce it as much as possible. If the S content is higher than 0.0050%, desired base material toughness and bond portion toughness cannot be obtained. Therefore, the S content is set to 0.0050% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction leads to an increase in cost. Therefore, from the viewpoint of manufacturing cost, it is preferable that the S content is 0.0003% or more.

Al: 0.002-0.080%
Al is an element that acts as a deoxidizing agent. Furthermore, Al fixes N in the steel as AlN, contributing to improving the toughness of the base material. In order to obtain the above effect, the Al content is set to 0.002% or more, preferably 0.010% or more. On the other hand, when the Al content exceeds 0.080%, the toughness of the base material decreases. Therefore, the Al content is 0.080% or less, preferably 0.060% or less.

Ti: 0.003~0.030%
Ti is an element that functions as a deoxidizing agent and also contributes to improving the strength of thick steel plates. Further, Ti combines with N and precipitates as TiN, which is a nitride that is stable even at high temperatures. Therefore, the pinning effect of TiN prevents austenite grains from becoming coarser when heated, and improves base material toughness and bonding part toughness. In order to obtain the above effect, the Ti content is set to 0.003% or more, preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.030%, the base material toughness and the bond toughness will deteriorate, so the Ti content should be 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less. % or less.

N: 0.0015-0.0080%
N combines with Al and Ti to precipitate nitrides. The nitride suppresses coarsening of austenite grains and improves the toughness of the base material and bond portion. In order to obtain the above effect, the N content is set to 0.0015% or more, preferably 0.0025% or more. On the other hand, if the N content exceeds 0.0080%, the toughness of the base material and the bond portion will deteriorate due to the increase in the amount of solid solution N. Therefore, the N content is set to 0.0080% or less, preferably 0.0065% or less, and more preferably 0.0060% or less.

The component composition in one embodiment of the present invention may have a component composition consisting of the above-mentioned elements, the remainder Fe, and unavoidable impurities. However, other trace elements may be included as long as they do not impair the effects of the present invention. Note that the inevitable impurity includes, for example, oxygen (O). The content of oxygen contained as an unavoidable impurity is preferably 0.0030% or less.

4.83C+Mn: 1.4-3.3% by mass
The thick steel plate of the present invention achieves excellent mechanical properties by forming a predetermined Mn concentration distribution as described later. In order to realize the Mn concentration distribution, the value of "4.83C+Mn" determined from the C content and Mn content in the component composition needs to be 1.4% by mass or more. The reason for this will be explained below.

In order to form the Mn concentration distribution in the finally obtained thick steel plate, it is necessary to cause microscopic variations in the Mn concentration inside the thick steel plate during the manufacturing process of the thick steel plate. For this purpose, it is necessary to make 4.83C+Mn 1.4% by mass or more, and to control the temperature raising conditions in the reheating step as described later. By reheating a steel sheet containing 4.83C+Mn of 1.4% by mass or more under predetermined conditions, the distribution of Mn into the reversely transformed austenite is promoted, and the microscopic variations in the Mn concentration are reduced. can be formed. If the value of 4.83C+Mn is less than 1.4% by mass, the distribution of Mn into the reversely transformed austenite will be insufficient, making it impossible to obtain the desired Mn concentration distribution. As a result, the strength of the thick steel plate decreases and the yield ratio increases. Therefore, the content of 4.83C+Mn is 1.4% by mass or more, preferably 1.7% by mass or more. On the other hand, when 4.83C+Mn exceeds 3.3% by mass, the effect is saturated. Therefore, 4.83C+Mn is set to 3.3% by mass or less.

Ti/N: 2.0-4.3
TiN has the effect of suppressing the growth of austenite grains in the weld heat affected zone due to the pinning effect, and improving the toughness of the bond part. If Ti/N is less than 2.0, the amount of TiN required to obtain the above effect cannot be secured, and the toughness of the bond portion deteriorates. Therefore, Ti/N is set to 2.0 or more, preferably 2.4 or more. On the other hand, when Ti/N exceeds 4.3, the base material toughness and the bond toughness deteriorate due to the generation of TiC particles and coarsening of TiN. Therefore, Ti/N is set to 4.3 or less, preferably 4.0 or less.

_PCM : 0.30% by mass or less If _PCM defined by the following formula (1) is higher than 0.30% by mass, good bond toughness cannot be obtained. Therefore, the _PCM content is 0.30% by mass or less, preferably 0.28% by mass or less, more preferably 0.26% by mass or less.
P _CM = [C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5 [B]…(1)
However, the parentheses in the above formula represent the content (mass%) of the element within the parentheses, and when the element is not contained, it is set to 0.

On the other hand, the lower limit of P _CM is not particularly limited, but if P _CM is too low, the strength of the thick steel plate will decrease. Therefore, _PCM is preferably 0.15% by mass or more, more preferably 0.17% by mass or more, and even more preferably 0.19% by mass or more.

In another embodiment of the present invention, the component composition is optionally selected from the group consisting of Cu, Ni, Cr, Mo, W, Nb, V, B, Ca, REM, Mg, and Zr. It may further contain at least one of the following.

Cu: 3.0% or less Cu is an element that further improves the strength of the thick steel plate while maintaining its high toughness, and can be contained arbitrarily depending on the required strength. However, if the Cu content exceeds 3.0%, hot embrittlement occurs and the surface quality of the steel sheet deteriorates. Therefore, when containing Cu, the Cu content is 3.0% or less. The Cu content is preferably 2.0% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but in order to sufficiently obtain the above effects, the Cu content is preferably 0.01% or more, more preferably 0.05% or more.

Ni: 3.0% or less Ni, like Cu, is an element that further improves the strength of the thick steel plate while maintaining its high toughness, and can be contained arbitrarily depending on the required strength. However, if the Ni content exceeds 3.0%, the effect of addition becomes saturated, which is economically disadvantageous. Therefore, when Ni is contained, the Ni content is set to 3.0% or less. The Ni content is preferably 2.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but in order to sufficiently obtain the above effects, the Ni content is preferably 0.01% or more, more preferably 0.10% or more.

Cr: 3.0% or less Cr is an element that further improves the strength of the thick steel plate, and can be contained arbitrarily depending on the required strength. However, if the Cr content exceeds 3.0%, the toughness of the base material and the bond portion will deteriorate, so when Cr is contained, the Cr content is set to 3.0% or less. The Cr content is preferably 2.0% or less. On the other hand, the lower limit of the Cr content is not particularly limited, but from the viewpoint of sufficiently obtaining the strength-improving effect of Cr, the Cr content is preferably 0.01% or more, and preferably 0.10% or more. is more preferable.

Mo: 1.5% or less Like Cr, Mo is an element that further improves the strength of a thick steel plate, and can be contained arbitrarily depending on the required strength. However, when the Mo content exceeds 1.5%, the toughness of the base material and the bond portion deteriorates. In addition, quench cracking is likely to occur during the manufacturing process of thick steel plates, resulting in reduced productivity. Therefore, when Mo is contained, the Mo content is set to 1.5% or less, preferably 1.0% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of sufficiently obtaining the strength-improving effect of Mo, the Mo content is preferably 0.01% or more, and preferably 0.10% or more. is more preferable.

W: 3.0% or less W, like Cr and Mo, is an element that further improves the strength of a thick steel plate, and can be contained arbitrarily depending on the required strength. However, when the W content exceeds 3.0%, the toughness of the base material and the bond portion deteriorates. Therefore, when containing W, the W content is set to 3.0% or less, preferably 2.0% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of sufficiently obtaining the strength-improving effect of W, the W content is preferably 0.01% or more, and preferably 0.10% or more. is more preferable.

Nb: 0.10% or less Nb, like Cr, Mo, and W, is an element that further improves the strength of a thick steel plate, and can be contained arbitrarily depending on the required strength. However, if the Nb content exceeds 0.10%, the toughness of the base material and bond part will decrease, so when Nb is included, the Nb content should be 0.10% or less, preferably 0.05% or less. . On the other hand, the lower limit of the Nb content is not particularly limited, but from the viewpoint of sufficiently obtaining the strength-improving effect of Nb, it is preferable that the Nb content is 0.005% or more.

V: 0.10% or less V, like Cr, Mo, W, and Nb, is an element that further improves the strength of a thick steel plate, and can be contained arbitrarily depending on the required strength. However, if the V content exceeds 0.10%, the toughness of the base material and bond part will decrease, so when V is included, the V content should be 0.10% or less, preferably 0.05% or less. . On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of sufficiently obtaining the strength-improving effect of V, it is preferable that the V content is 0.005% or more.

B: 0.0050% or less B is an element that has the effect of further improving the strength of thick steel plates by improving hardenability. In addition, B has the effect of further improving the toughness of the bond portion by fixing solid solution nitrogen as nitride in the weld heat affected zone during high heat input welding. However, when the B content exceeds 0.0050%, the hardenability becomes excessively high, and the toughness of the base material and the bond part deteriorates. Therefore, when B is contained, the B content is set to 0.0050% or less, preferably 0.0020% or less. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of B addition, the B content is preferably 0.0003% or more.

Ca: 0.005% or less Ca is an element that has the effect of further improving base material toughness by refining crystal grains, and can be contained arbitrarily depending on the required base material toughness. However, when the Ca content exceeds 0.005%, the effect of addition is saturated, so when Ca is contained, the Ca content is set to 0.005% or less. On the other hand, the lower limit of the Ca content is not particularly limited, but from the viewpoint of sufficiently obtaining the toughness improving effect of Ca, the Ca content is preferably 0.001% or more.

REM: 0.020% or less REM (rare earth metal), like Ca, is an element that has the effect of further improving base material toughness by refining crystal grains, and can be used as desired depending on the required base material toughness. It can be contained in However, when the REM content exceeds 0.020%, the effect of addition is saturated, so when REM is contained, the REM content is set to 0.020% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of improving toughness due to REM, it is preferable that the REM content is 0.002% or more.

Mg: 0.005% or less Mg, like Ca and REM, is an element that has the effect of further improving base material toughness by refining crystal grains, and may be optionally included depending on the required base material toughness. can. However, when the Mg content exceeds 0.005%, the effect of addition is saturated, so when Mg is contained, the Mg content is set to 0.005% or less. On the other hand, the lower limit of the Mg content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of improving toughness due to Mg, it is preferable that the Mg content is 0.001% or more.

Zr: 0.020% or less Like Ca, REM, and Mg, Zr is an element that has the effect of further improving base material toughness by refining crystal grains, and can be added arbitrarily depending on the required base material toughness. It can be contained in However, if the Zr content exceeds 0.020%, the effect of addition is saturated, so when containing Zr, the Zr content is set to 0.020% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of improving toughness due to Zr, it is preferable that the Zr content is 0.002% or more.

[Microstructure]
The thick steel plate of the present invention has a microstructure that contains bainite and island martensite and has an area fraction of bainite of 80.0% or more. The reason why the microstructure is limited to the above range will be explained below. Note that "area fraction" in the following description refers to the area fraction with respect to the entire microstructure, unless otherwise specified. Further, the above-mentioned microstructure refers to the microstructure at a position of 1/4 of the thickness of the steel plate.

Microstructure containing bainite and island martensite Bainite is a structure necessary to improve the strength and toughness of thick steel plates, as described below. On the other hand, island martensite (MA) has a structure with higher hardness than bainite because it is enriched with C. Therefore, by forming MA, tensile strength can be improved. In addition, since a large amount of mobile dislocations are introduced around the MA, an increase in yield stress is suppressed. Therefore, in order to achieve both high strength and low yield ratio, it is necessary to have a microstructure containing bainite and island martensite.

Area fraction of bainite: 80.0% or more If the area fraction of bainite is less than 80.0%, sufficient strength and base material toughness cannot be obtained. Therefore, the area fraction of bainite is set to 80.0% or more, preferably 85.0% or more, and more preferably 90.0% or more. On the other hand, although the upper limit of the area fraction of bainite is not particularly limited, if it is too high, the area fraction of island-like martensite becomes relatively low, making it difficult to sufficiently reduce the yield ratio. Therefore, the bainite area fraction is preferably 99.0% or less. Note that the area fraction of bainite can be measured by the method described in Examples.

Area Fraction of Island Martensite As a result of studies by the present inventors, it was found that even if the base material is MA, if the area fraction is 5.0% or more, the toughness of the bond portion decreases. This is considered to be due to the following reasons. That is, during welding, the bond portion is heated to a high temperature close to the melting point, so MA contained in the thick steel plate is once decomposed by the heating. However, during the cooling process after welding, MA is regenerated at the bond portion. At this time, the amount of regenerated MA increases as the amount of MA contained in the thick steel plate before welding increases. When the amount of MA regenerated in the bond portion is large, the toughness of the bond portion decreases. Therefore, in the present invention, in order to improve the bond toughness, the area fraction of island martensite in the microstructure of a thick steel plate is less than 5.0%, preferably 4.9% or less, and more preferably 4.7%. The content is more preferably 4.5% or less.

On the other hand, the lower limit of the area fraction of MA is not particularly limited, but as described above, MA has the effect of improving tensile strength and suppressing the increase in yield stress. From the viewpoint of fully exhibiting these effects, the area fraction of MA in the microstructure is preferably 1.0% or more, more preferably 2.0% or more.

Note that in Patent Document 3, desired mechanical properties are achieved by increasing the area fraction of MA, so it is essential that the area fraction of MA be 5% or more. In contrast, in the present invention, desired mechanical properties are achieved by controlling the Mn concentration distribution, as will be described later. Therefore, excellent mechanical properties can be obtained even though the area fraction of MA is less than 5.0%. Furthermore, since the area fraction of MA in the base material is low at less than 5.0%, the bond portion toughness is superior to that in Patent Document 3. From this, the difference between the thick steel plate of the present invention and the thick steel plate disclosed in Patent Document 3 is clear. Furthermore, unlike Patent Document 3, there is no need to perform complex cooling control consisting of a first water cooling process, an air cooling process, and a second water cooling process to control the generation of MA.

The size of the average circular equivalent diameter MA of island-shaped martensite is not particularly limited, and may be any size. However, if the MA is excessively coarse, the toughness of the thick steel plate decreases. Therefore, from the viewpoint of further improving the toughness, the average circular equivalent diameter of MA is preferably 5.0 μm or less, more preferably 4.0 μm or less. On the other hand, the lower limit of the average equivalent circle diameter of MA is not particularly limited, but usually may be 0.8 μm or more, and may be 1.0 μm or more.

The area fraction and average circular equivalent diameter of MA were determined using a scanning electron microscope (SEM) at a magnification of 1000 after subjecting a steel plate as a sample to repeller corrosion (Journal of Metals, March, 1980, p.38-39). It can be determined by observing at magnification and analyzing the photographed image using an image analysis device.

Other Structures The microstructure in one embodiment of the present invention may consist of bainite and island martensite. Moreover, the microstructure in other embodiments of the present invention may contain other structures in addition to bainite and island martensite. The other tissue may be any tissue without particular limitation. For example, the other structure may be at least one selected from the group consisting of ferrite, pearlite, martensite, and retained austenite. When the microstructure includes other structures, the total area fraction of the other structures is preferably 19% or less, more preferably 15% or less, and even more preferably 10% or less. .

The thick steel plate in one embodiment of the present invention has an area fraction of
80-99.0% bainite,
1.0 to 15.0% island martensite,
and remnant tissue,
The residual structure may be at least one selected from the group consisting of ferrite, pearlite, martensite, and retained austenite.

[Mn concentration distribution]
The present inventors intentionally created microscopic variations in Mn concentration inside a thick steel plate by controlling the C and Mn contents and the temperature raising conditions in the reheating process after hot rolling. I found out what I can do. It was also found that by appropriately controlling the microscopic variation in the Mn concentration, that is, the Mn concentration distribution, a thick steel plate having high strength, low yield ratio, and high toughness can be obtained. This is considered to be because the distribution and size of MA are affected by the Mn concentration distribution. For example, MA tends to be formed in regions where Mn is concentrated. As mentioned earlier, MA has a harder structure than bainite, so it has the effect of improving strength. In addition, MA has the effect of lowering the yield ratio through the introduction of mobile dislocations. Therefore, by controlling the Mn concentration distribution, the MA distribution is controlled, and as a result, both high strength and low yield ratio can be achieved.

The present invention is based on the above technical idea, and specifically, the thick steel plate of the present invention has a Mn concentration distribution that satisfies the following conditions (1) to (3). Note that the Mn concentration distribution in the present invention refers to the Mn concentration distribution at a position of 1/4 of the thickness of the steel plate.
(1) The area fraction of the average Mn concentration region is less than 90%. (2) The area fraction of the Mn concentration region is 1.0% or more. (3) The average equivalent circle diameter of the Mn concentration region is 7. 0μm or less

(1) Area fraction of the average concentration region of Mn: less than 90% If the area fraction of the average concentration region of Mn is 90% or more, the hard structure containing MA becomes insufficient, making it difficult to obtain the desired strength. I can't. Further, Mn concentration in the Mn-concentrated region becomes insufficient, and the effect of reducing yield stress due to the formation of MA becomes insufficient. As a result, the yield ratio also tends to increase. Therefore, the area fraction of the average Mn concentration region is less than 90%, preferably less than 85%, more preferably less than 80%. On the other hand, the lower limit of the area fraction of the average concentration region of Mn is not particularly limited. However, if the area fraction of the average concentration region of Mn is too low, the size of the Mn enriched region becomes large, and as a result, the MA formed in the enriched region becomes coarse. Therefore, the area fraction of the average Mn concentration region is preferably 50% or more, more preferably 60% or more. Note that the "average Mn concentration region" here is defined as a region having a Mn concentration of 0.9 to 1.1 times the average Mn content (mass %).

(2) Area fraction of Mn enriched region: 1.0% or more If the area fraction of Mn enriched region is less than 1.0%, MA formation will be insufficient, resulting in high strength and low yield. It is not possible to balance the ratio. Therefore, the area fraction of the Mn enriched region is set to 1.0% or more, preferably 1.5% or more, more preferably 2.0% or more, and still more preferably 6.2% or more. On the other hand, the upper limit of the area fraction of the Mn-enriched region is not particularly limited, but if the area fraction of the Mn-enriched region is too high, the size of the enriched region becomes large, and it is formed in the enriched region. MA tends to become coarse. Therefore, the area fraction of the Mn enriched region is preferably less than 50%, more preferably less than 40%, and even more preferably 20% or less. Note that since the Mn enriched region disappears by heating during welding, the presence of the Mn enriched region does not adversely affect the bond toughness after welding. Therefore, according to the present invention, it is possible to achieve both excellent mechanical properties of a thick steel plate and high bond portion toughness.

Average circular equivalent diameter of the Mn enriched region: 7.0 μm or less If the average equivalent circular diameter of the Mn enriched region is larger than 7.0 μm, the MA formed in the Mn enriched region also becomes coarse. Toughness of thick steel plate decreases. Therefore, the average equivalent circular diameter of the Mn-enriched region is 7.0 μm or less, preferably 4.0 μm or less. On the other hand, the lower limit of the average equivalent circle diameter of the Mn enriched region is not particularly limited, but generally may be 1.0 μm or more, and may be 1.5 μm or more.

The Mn concentration distribution can be measured using an electron beam microanalyzer (EPMA). Specifically, a test piece is taken from a thick steel plate so that the observation position is 1/4 the thickness of the plate. The Mn concentration distribution in the test piece is measured by EPMA, and the area fraction of the average Mn concentration region, the area fraction of the Mn enriched region, and the average equivalent circular diameter of the Mn enriched region are calculated. The measurement of the Mn concentration distribution by EPMA is performed in two or more fields selected at random, each field having a size of 50 μm×50 μm, and each field having 250×250 measurement points.

[Mechanical properties]
- Toughness The thick steel plate of the present invention has a Charpy absorbed energy: vE ₀ of 70 J or more at 0°C. Charpy absorbed energy is one of the indicators of toughness, and the thick steel plate of the present invention with vE ₀ of 70 J or more exhibits excellent seismic safety even when used in high-rise buildings. vE ₀ is preferably 80J or more, more preferably 100J or more. On the other hand, from the viewpoint of earthquake resistance, the higher vE ₀ is, the better, so the upper limit of vE ₀ is not particularly limited. However, in general, it may be 250J or less, 220J or less, or 210J or less.

The Charpy absorbed energy at 0°C of the above-mentioned thick steel plate was determined according to the regulations of JIS Z 2242 using a V-notch test piece taken from the 1/4th position of the thick steel plate in accordance with the regulations of JIS Z 2202. can be measured in accordance with

・Yield stress The yield stress (YS) of the thick steel plate of the present invention is not particularly limited, but from the viewpoint of increasing the strength as building structures become taller, it is preferably 600 MPa or more, and preferably 620 MPa or more. preferable. The upper limit of the yield stress is also not particularly limited, but may be, for example, 900 MPa or less, 880 MPa or less, or 850 MPa or less.

・Tensile strength The tensile strength (TS) of the thick steel plate of the present invention is not particularly limited, but from the viewpoint of increasing strength as building structures become taller, it is preferably 780 MPa or more, and 800 MPa or more. It is more preferable. The upper limit of the tensile strength is also not particularly limited, but may be, for example, 1100 MPa or less, or 1000 MPa or less.

・Yield ratio The yield ratio (YR) of the thick steel plate of the present invention is not particularly limited, but from the viewpoint of improving the deformation performance of the building structure considering the allowable margin for destruction during earthquakes, it is preferably 85% or less. preferable. On the other hand, the lower limit of the yield ratio is also not limited, but may be, for example, 70% or more, or 75% or more. Note that the yield ratio here is a value expressed as a percentage of the ratio of yield stress (YS) to tensile strength (TS), that is, YS/TS×100 (%).

The yield stress and tensile strength can be measured by a tensile test according to JIS Z 2241 using a JIS No. 4 tensile test piece taken from a quarter of the thickness of a thick steel plate. Further, the yield ratio can be calculated from the yield stress and tensile strength measured by the above method.

- Bond Part Toughness The bond part toughness of the thick steel plate of the present invention is not particularly limited, but it is preferable that the Charpy absorbed energy (vE ₀ ) of the bond part at 0° C. is 47 J or more. The upper limit of vE ₀ in the bond portion is also not particularly limited, but may generally be 150 J or less.

Note that vE ₀ at the bond portion is a JIS No. 4 Charpy impact sample obtained from a welded joint fabricated by electroslag welding with a welding heat input of 40 kJ/mm or more so that the notch position is at the bond portion. This is the value measured using a test piece. More specifically, it can be measured by the method described in Examples.

- Plate Thickness The thickness of the above-mentioned thick steel plate is not particularly limited, and can be any thickness. The thickness of the thick steel plate is preferably 6 mm or more, more preferably 9 mm or more, and even more preferably 12 mm or more. From the viewpoint of coping with the increase in the height of building structures, the thickness is preferably 40 mm or more, and more preferably 60 mm or more. On the other hand, the upper limit of the thickness of the thick steel plate is not particularly limited either, but it is preferably 100 mm or less.

[Production method]
Next, a method for manufacturing a thick steel plate according to an embodiment of the present invention will be described. The thick steel plate can be manufactured by sequentially performing the steps (a) to (d) on a steel material having the above-mentioned composition.
(a) Hot rolling process (b) First cooling process (c) Reheating process (d) Second cooling process

The conditions in each step will be specifically explained below. In the following description, unless otherwise specified, temperature means the temperature at the center of the plate thickness (1/2 position of the plate thickness). The temperature at the center of the plate thickness can be determined by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer. In addition, the temperature under the heating conditions and cooling conditions after the hot rolling process was the temperature at the 1/4 position of the plate thickness, and the heating rate and cooling rate were also calculated based on the temperature at the 1/4 position of the plate thickness. Means average heating rate or average cooling rate.

(Steel material)
Any form of material can be used as the steel material. The steel material may be, for example, a steel slab. Although the method for manufacturing the steel material is not particularly limited, for example, it can be manufactured by melting steel having the above-mentioned composition and casting. The melting can be performed by any method such as a converter, an electric furnace, an induction furnace, or the like. Further, from the viewpoint of productivity, the casting is preferably carried out by a continuous casting method, but it can also be carried out by an ingot-forming-decomposition rolling method.

(hot rolling process)
In the hot rolling process, the steel material is hot rolled into a thick steel plate. The hot rolling conditions are not particularly limited and can be carried out under any conditions. Typically, a steel material is heated to a predetermined heating temperature and then rolled. The heating may be performed after once cooling the steel material obtained by a method such as casting, or the obtained steel material may be directly subjected to the heating without cooling.

In addition, in the present invention, the microstructure and properties of the thick steel plate are controlled in the reheating step and second cooling step after hot rolling. Therefore, the heating temperature in the hot rolling process is not particularly limited and can be set to any temperature. However, if the heating temperature is less than 1000° C., the deformation resistance of the steel material is high, so the load on the rolling mill during hot rolling increases, and hot rolling may become difficult. Therefore, the heating temperature is preferably 1000°C or higher. On the other hand, if the heating temperature is higher than 1250° C., oxidation of the steel becomes significant, and as a result, loss due to oxidation increases, resulting in a decrease in yield. Therefore, the heating temperature is preferably 1250°C or less.

The rolling end temperature is not particularly limited, but is preferably 1000°C or less. Moreover, it is preferable that the said rolling completion temperature is 750 degreeC or more.

(First cooling process)
Next, the thick steel plate obtained in the hot rolling process is cooled (first cooling process). In the present invention, in order to control the microstructure and properties of the thick steel plate in the subsequent reheating step and second cooling step, cooling in the first cooling step can be performed under any conditions without particular limitations. . However, in the next reheating step, it is necessary to control the average temperature increase rate in the temperature range from Ac1 point to Ac3 point, so the cooling stop temperature in the first cooling step only needs to be below Ac1 point. It is preferable that the cooling stop temperature is 500°C or less. If the cooling stop temperature is 500° C. or lower, coarsening of precipitates can be suppressed and Mn-enriched regions can be formed more uniformly. As a result, the strength and toughness of the thick steel plate can be further improved, and the yield ratio can be further reduced. More preferably, the cooling stop temperature is 250°C or less. On the other hand, the lower limit of the cooling stop temperature is not limited either, and cooling can be performed to any temperature. However, since excessive cooling reduces productivity, the cooling stop temperature is preferably 0°C or higher, more preferably 10°C or higher, and even more preferably 20°C or higher. Typically, it is preferable that the cooling stop temperature is set to room temperature or higher.

Cooling in the first cooling step can be performed by any method without particular limitation. For example, the cooling can be performed by one or both of air cooling and water cooling. From the viewpoint of further improving the strength and toughness of the thick steel plate, the cooling is preferably performed by water cooling, and the water cooling is at least one selected from the group consisting of spray cooling, mist cooling, and laminar cooling. It is more preferable to do so.

(Reheating process)
Next, the thick steel plate after the first cooling step is heated to a reheating temperature in a predetermined heating pattern, and held at the reheating temperature.

Average temperature increase rate in the temperature range from Ac1 point to Ac3 point: 2.0℃/s or less During the temperature increase process, bainite and martensite are generated when passing through the temperature range from Ac1 point to Ac3 point (two-phase region). Mn is distributed into austenite produced by reverse transformation, resulting in microscopic variations in Mn concentration. However, if the average temperature increase rate in the temperature range from Ac1 point to Ac3 point is higher than 2.0°C/s, the distribution of Mn will not proceed sufficiently, and as a result, the desired Mn concentration distribution cannot be obtained. . Therefore, in the reheating process, the average heating rate of the thick steel plate after the first cooling process in the temperature range from Ac1 point to Ac3 point at the 1/4 position of the plate thickness: 2.0°C/s or less Raise the temperature with On the other hand, the lower limit of the average temperature increase rate is not particularly limited. However, if the temperature increase rate is too slow, the effect of controlling the temperature increase rate becomes saturated, and the time required for heating increases, resulting in a decrease in productivity. Therefore, it is preferable that the average temperature increase rate is 0.01° C./s or more.

Residence time in the temperature range from Ac3 point -100°C to Ac3 point: 60 seconds or more Similarly, if the residence time in the temperature range from Ac3 point -100°C to Ac3 point is less than 60 seconds, Mn distribution is sufficient. As a result, the desired Mn concentration distribution cannot be obtained. Therefore, in the temperature raising process of the reheating step, the residence time in the temperature range from Ac3 point -100°C to Ac3 point is set to 60 seconds or more. On the other hand, if the residence time is too long, productivity will decrease. Therefore, it is preferable that the residence time is 60 minutes or less.

Note that the temperature increase pattern in the above temperature increase process only needs to satisfy the above conditions, and other conditions are not particularly limited. For example, the temperature may be raised continuously to the reheating temperature, or may be intentionally maintained in a two-phase region.

Note that the Ac1 point and Ac3 point are determined by the following equations (2) and (3).
Ac1 (℃)=751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B...(2)
Ac3(℃)=937-476.5C+56Si-19.7Mn-16.3Cu-4.9Cr-26.6Ni+38.1Mo+124.8V+136.3Ti+198Al+3315B...(3)
However, the element symbols in the above formulas (2) and (3) represent the content (mass%) of each element, and are set to 0 if the element is not contained.

Reheating temperature: 3 points Ac or higher, 3 points Ac + 60°C or lower In the reheating step, the thick steel plate is heated to a reheating temperature of 3 points Ac or higher and 3 points Ac + 60°C or lower. As a result, structures such as bainite and martensite contained in the thick steel plate at the time when the first cooling step is completed are reversely transformed into austenite, resulting in a microstructure consisting of a single phase of austenite.

If the reheating temperature is less than the Ac3 point, a soft recovery structure is generated in the microstructure, and the strength of the base material is reduced. Furthermore, coarse MA is generated in the Mn-enriched region, reducing the toughness of the base material. Therefore, the reheating temperature is set to Ac3 point or higher. On the other hand, if the reheating temperature is higher than the Ac3 point +60° C., the austenite crystal grains become coarser and the composition becomes more uniform, making it impossible to obtain the desired Mn concentration distribution. Therefore, the reheating temperature is set to below Ac3 point + 60°C, more preferably below Ac3 point + 55°C, even more preferably below Ac3 point + 50°C.

Holding time at reheating temperature: 10 minutes or more In the reheating step, the thick steel plate is heated to the reheating temperature and then held at the reheating temperature for a predetermined holding time. If the holding time is less than 10 minutes, the average equivalent circular diameter of the Mn enriched region in the finally obtained thick steel plate cannot be 7.0 μm or less. This is considered to be because the short holding time increased the variation in grain size of the reversely transformed austenite, and as a result, the size of the Mn-enriched region became non-uniform. Therefore, the holding time is set to 10 minutes or more. On the other hand, the upper limit of the holding time is not particularly limited, but it is preferably 100 minutes or less since productivity decreases if holding is carried out for an excessively long time.

In the reheating step, any heating method can be used. An example of the heating method is furnace heating. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

(Second cooling process)
Next, the thick steel plate after the reheating step is cooled. Specifically, the thick steel plate after the reheating step is accelerated to an accelerated cooling stop temperature of 100°C to 600°C at an average cooling rate of 1.0 to 200.0°C/s at the 1/4 position of the plate thickness. Cool and then air cool to a temperature below 100°C.

Average cooling rate: 1.0 to 200.0°C/s
By performing accelerated cooling under the above conditions, reversely transformed austenite can be transformed into bainite, and a microstructure consisting mainly of bainite can be obtained. If the average cooling rate is less than 1.0° C./s, ferrite will be generated, making it impossible to make the area fraction of bainite 80.0% or more. Therefore, the average cooling rate is 1.0°C/s or more, preferably 5.0°C/s or more. On the other hand, if the average cooling rate is higher than 200.0°C/s, it becomes difficult to control the temperature at each position within the steel plate, and variations in material quality tend to occur in the width direction and rolling direction, resulting in changes in strength properties etc. There will be variations in the material. Therefore, the average cooling rate is 200.0°C/s or less, preferably 150.0°C/s or less, and more preferably 100.0°C/s or less.

The accelerated cooling method is not particularly limited, but cooling can be performed by any method. Typically, the accelerated cooling is preferably performed by one or both of air cooling and water cooling, and more preferably by water cooling. As the water cooling, any method using water (eg, spray cooling, mist cooling, laminar cooling, etc.) can be used. As the water cooling method, it is preferable to use mist cooling.

Accelerated cooling stop temperature: 100-600℃
In the second cooling step, accelerated cooling is performed to an accelerated cooling stop temperature of 100° C. to 600° C., and then air cooling is performed. If the accelerated cooling stop temperature is less than 100° C., all austenite will be transformed into bainite, making it impossible to obtain a microstructure containing island martensite. In addition, since the tempering effect cannot be obtained, the toughness of the thick steel plate decreases. Therefore, the accelerated cooling stop temperature is set to 100°C or higher, preferably 200°C or higher. On the other hand, if the accelerated cooling stop temperature is higher than 600° C., ferrite is likely to be generated, and the area fraction of bainite cannot be increased to 80.0% or more. Therefore, the accelerated cooling stop temperature is set to 600°C or lower, preferably 500°C or lower.

Air Cooling to a Temperature of 100°C or Less After stopping the accelerated cooling described above, the thick steel plate is further air cooled to a temperature of 100°C or less. The air cooling may be not forced cooling but natural cooling. In the air cooling, the thick steel plate may be cooled to a temperature of 100° C. or less, but there is no need to stop the air cooling at a specific temperature, and it is usually sufficient to air cool the steel plate to ambient temperature.

Other conditions in the air cooling are not particularly limited because they do not substantially affect the structure of the thick steel plate. For example, the cooling rate in the air cooling is not particularly limited, and cooling can be performed at any rate. Typically, the cooling rate in the air cooling may be less than 1.0°C/s, and may be 0.5°C/s or less. Further, the lower limit of the cooling rate in the air cooling is not particularly limited, but may be 0.001°C/s or more, 0.01°C/s or more, or 0.07°C/s or more. It's okay.

Similarly, the time required for the air cooling (air cooling time) is not particularly limited. In this embodiment, since there is no need to perform any special processing after the air cooling, there is no problem even if the air cooling takes a long time. Typically, the air cooling time may be more than 300 seconds, may be 310 seconds or more, or may be 320 seconds or more. On the other hand, the upper limit of the air cooling time is not particularly limited either, but may be, for example, 24 hours or less, or 12 hours or less. Note that here, in the second cooling step, the time taken from the start of air cooling until the temperature reaches 100° C. is defined as the air cooling time.

As described above, the thick steel plate of the present invention can be manufactured by controlling the chemical composition, especially the content of C and Mn, within a specific range, and by appropriately controlling the temperature increase conditions in the reheating process after hot rolling. can do. Therefore, the thick steel plate of the present invention is easier to produce than the steel plate of Patent Document 3, which requires controlling the conditions in the cooling process after reheating to control the generation of MA, and is suitable for industrial production. ing.

Furthermore, in the present invention, as described above, microscopic variations in the Mn concentration are intentionally caused by controlling the temperature raising conditions in the reheating step. As a result, the above-mentioned Mn concentration distribution is formed in the finally obtained thick steel plate. On the other hand, in Patent Document 3, unlike the present invention, the heating conditions are not controlled to cause microscopic variations in the Mn concentration. Therefore, with the manufacturing process described in Patent Document 3, it is not possible to obtain a thick steel plate having a Mn concentration distribution that satisfies the conditions of the present invention. MA exists in the thick steel plate of Patent Document 3, but the Mn concentration around this MA is the same average concentration as the Mn concentration in the matrix, and there is no enriched region that satisfies the conditions of the present invention. It is considered not to do so.

In another embodiment of the present invention, heat treatment can be further performed after the first cooling step and before the reheating step in order to further increase the strength and lower the yield ratio of the thick steel plate. That is, in the embodiment, a thick steel plate can be manufactured by sequentially performing the steps (a) to (e) on a steel material having the above-mentioned composition.
(a) Hot rolling process (b) First cooling process (c) Heat treatment process (d) Reheating process (e) Second cooling process

(Heat treatment process)
In the heat treatment step, the steel plate after the first cooling step is heated to a heat treatment temperature of 3 Ac or more and 1050°C or less, held at the heat treatment temperature for a holding time of 5 minutes or more, and then heated to 500°C or less. Cool down to the cooling stop temperature. By performing the heat treatment, it is possible to achieve both high strength and low yield ratio at a higher level. The reason is thought to be as follows. By performing the reheating step after the heat treatment step, the location of the Mn-enriched portion becomes homogenized, and the frequency of reverse transformation nucleation increases. As a result, the formation of microscopic variations in the Mn concentration is promoted, and the final area fraction of the Mn-enriched region increases. Specifically, the area fraction of the concentrated region can be 6.2% or more. As described above, since MA is formed in the Mn-enriched region, both high strength and low yield ratio can be achieved.

Heat treatment temperature: Ac 3 points or more and 1050° C. or less By setting the heat treatment temperature to Ac 3 points or more and 1050° C. or less, hardenability can be ensured and desired bainite and martensitic structures can be obtained. If the heat treatment temperature is less than the Ac3 point, desired base material toughness cannot be obtained. This is considered to be because coarse ferrite is produced during heat treatment, resulting in the formation of an upper bainite structure containing coarse carbides in the final microstructure. On the other hand, when the heat treatment temperature is higher than 1050°C, the desired base material toughness cannot be obtained. This is considered to be because coarse bainite and coarse martensite are generated during heat treatment, resulting in the formation of a coarse bainite structure in the final microstructure.

Holding time: 5 minutes or more The holding time at the heat treatment temperature is 5 minutes or more, preferably 10 minutes or more, in order to reduce variations in austenite grain size. On the other hand, the upper limit of the holding time is not particularly limited, but if it is too long, the effect will be saturated, so in consideration of productivity, it is preferably 100 minutes or less, and more preferably 60 minutes or less.

For heating in the heat treatment step, any heating method can be used as long as the heat treatment temperature and holding time can be controlled as described above. An example of a heating method that can be used is furnace heating. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

Cooling stop temperature: 500°C or less After being maintained at the above heat treatment temperature, the thick steel plate is cooled to a cooling stop temperature of 500°C or less. By the cooling, the austenite produced in the heat treatment step is transformed into a low-temperature transformed phase of bainite and martensite, and in the subsequent reheating step, higher strength and lower yield ratio can be obtained. If the cooling stop temperature is higher than 500°C, desired strength and toughness cannot be ensured. Therefore, the cooling temperature is set to 500°C or lower, preferably 400°C or lower, and more preferably 200°C or lower. On the other hand, the lower limit of the cooling stop temperature is not particularly limited. However, since excessive cooling reduces productivity, the cooling stop temperature is preferably 0°C or higher, more preferably 10°C or higher, and even more preferably 20°C or higher. Typically, it is preferable that the cooling stop temperature is set to room temperature or higher.

The method for performing the cooling is not particularly limited, and any method such as air cooling or water cooling can be used. As the water cooling, any cooling method using water, such as spray cooling, mist cooling, laminar cooling, etc., can be used.

When the above heat treatment step is performed, the thick steel plate after cooling may be subjected to the next reheating step. In addition, when the above-mentioned heat treatment process is not performed, the thick steel plate after the first cooling process may be subjected to the next reheating process without performing heat treatment.

In other embodiments of the present invention, a tempering step may optionally be further performed after the reheating step for the purpose of correcting the shape of the thick steel plate. Further, when the cooling stop temperature in the cooling step after reheating is lower, it can be expected that the toughness of the base material is further improved due to the effect of tempering. The heating temperature for tempering is preferably 200°C to 500°C. The steel plate after the above-mentioned tempering process can be cooled by any method since the material quality does not change depending on the cooling rate.

A thick steel plate was manufactured using the procedure described below, and its properties were evaluated.

First, molten steel having the composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 260 mm) as a steel material was made by continuous casting. In addition, the P _CM (mass%) determined by the above-mentioned formula (1), the Ac1 point (℃) determined by the formula (2), and the Ac3 point (℃) determined by the formula (3) are also listed in Table 1. .

After heating the steel slab to 1150° C., it was hot rolled into a thick steel plate having the thickness shown in Tables 2 and 3 (hot rolling process). The rolling end temperatures in the hot rolling are shown in Tables 2 and 3.

The obtained thick steel plate was cooled to the cooling stop temperature shown in Tables 2 and 3 using the cooling method shown in Tables 2 and 3 (first cooling step).

Next, the thick steel plate after the first cooling step was reheated under the conditions shown in Tables 2 and 3 (reheating step). However, in some examples, the thick steel plate after the first cooling step was heat treated under the conditions shown in Tables 2 and 3 (heat treatment step), and then reheated. The reheating was performed using a heat treatment furnace.

Finally, after the reheating, accelerated cooling was performed under the conditions shown in Tables 2 and 3, and after the accelerated cooling was stopped, it was air cooled (left to cool) to room temperature. The accelerated cooling was performed by water cooling. Further, the air cooling time (time taken to reach 100° C.) in the air cooling was 1 hour or more.

For each of the thick steel plates obtained as described above, the microstructure, Mn concentration distribution, mechanical properties, and bond toughness after welding were evaluated. The evaluation was performed by the method described below.

(microstructure)
A test piece for microstructure observation was taken from the thick steel plate so that the observation position was 1/4 of the plate thickness. The test piece was buried in resin so that the cross section perpendicular to the rolling direction served as the observation surface, and mirror-polished. Next, after performing repeller corrosion, it was observed with a scanning electron microscope at a magnification of 1000 times, an image of the structure was taken, and an island-like martensite structure was identified. The captured images of five fields of view were analyzed by an image analysis device, and the area fraction and average circular equivalent diameter of the island-like martensite structure were determined.

In addition, a test piece for microstructure observation was taken from the thick steel plate so that the observation position was 1/4 of the thickness of the plate. The test piece was buried in resin so that the cross section perpendicular to the rolling direction served as the observation surface, and mirror-polished. Next, after performing nital corrosion, it was observed with a scanning electron microscope at a magnification of 200 times, an image of the structure was taken, and a bainite structure was identified. The captured images of five fields of view were analyzed by an image analysis device, and the area fraction of the bainite structure was determined.

(Mn concentration distribution)
A test piece was taken from the thick steel plate so that the observation position was 1/4 the thickness of the plate. The Mn concentration distribution in the test piece was measured by EPMA, and the area fraction of the average Mn concentration region, the area fraction of the Mn enriched region, and the average equivalent circular diameter of the Mn enriched region were calculated. The measurement of the Mn concentration distribution by EPMA was carried out in two or more fields selected at random, the size of one field was 50 μm x 50 μm, and the number of measurement points per field was 250 x 250.

(mechanical properties)
A JIS No. 4 tensile test piece was taken from the 1/4th thickness position of the thick steel plate. Using the tensile test piece, a tensile test was conducted in accordance with the provisions of JIS Z 2241, and the yield stress, tensile strength, and yield ratio of the thick steel plate were measured. Further, a V-notch test piece was taken from a position of 1/4 of the thickness of the thick steel plate in accordance with the regulations of JIS Z 2202. Using the V-notch test piece, Charpy absorbed energy (vE ₀ ) was determined by a Charpy impact test at 0° C. in accordance with JIS Z 2242, and base material toughness was evaluated.

(Bond toughness)
In order to evaluate the toughness of the welded heat-affected zone of the thick steel plate, a welded joint was created according to the following procedure, and the Charpy absorbed energy at the bond portion was measured.

First, a set of joint test plates having the same thickness as the thick steel plate were taken from the thick steel plate. Using one of the joint test plates as the skin plate 1 and the other as the diaphragm 2, a groove 3 having the shape shown in FIG. 1 was prepared. Next, electroslag welding was performed with a welding heat input of 40 kJ/mm or more to produce a welded joint 5.

Next, as shown in FIG. 2, a JIS No. 4 Charpy impact test piece 8 was taken from the welded joint 5 so that the notch 9 was located at the bond portion. The notch 9 was located at the intersection of the welding line and a straight line passing 6 mm from the surface of the skin plate 1. Further, Charpy impact test piece 8 was taken such that the longitudinal direction of the test piece was perpendicular to the weld line.

Using the Charpy test piece, the absorbed energy (vE ₀ ) at the bond portion of the welded joint was measured in a Charpy impact test at 0°C. Note that the bond toughness was not evaluated for some thick steel plates whose base material properties did not meet the target.

The results obtained are shown in Tables 4 and 5. In addition, the yield stress is 600 MPa or more, the tensile strength is 780 MPa or more, the yield ratio is 85% or less, the absorbed energy at 0°C (vE ₀ ) is 70 J or more, and the absorbed energy (vE ₀ ) at 0°C of the bond part of the welded joint is A score of 47J or higher was considered a pass.

All thick steel plates that meet the conditions of the present invention have a yield stress of 600 MPa or more, a tensile strength of 780 MPa or more, a yield ratio (YR) of 85% or less, and an absorbed energy vE ₀ at 0 ° C. of 70 J or more, It had high strength and low yield ratio, as well as excellent base material toughness. In addition, even when high heat input welding was performed with a welding heat input exceeding 40 kJ/mm, vE ₀ at the welded joint bond was 47 J or more, and the welded joint had excellent bond toughness. . On the other hand, thick steel plates that did not meet the conditions of the present invention were poor in at least one of the following properties: base metal strength, yield ratio, base metal toughness, and welded joint bond toughness.

1 Skin plate 2 Diaphragm 3 Bevel 4 Weld metal 5 Weld joint 6 Weld metal 7 Heat affected zone (HAZ)
8 Charpy impact test piece 9 Notch

Claims (5)

In mass%,
C: 0.010-0.14%,
Si: 0.01 to 0.50%,
Mn: 0.9-3.0%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.002-0.080%,
Contains Ti: 0.003 to 0.030%, and N: 0.0015 to 0.0080%,
the remainder consists of Fe and unavoidable impurities, and
4.83C+Mn expressed as C content (mass%) and Mn content (mass%) is 1.4 to 3.3 mass%,
The ratio Ti/N of Ti content (mass%) to N content (mass%) is 2.0 to 4.3,
It has a component composition in which _PCM represented by the following formula (1) is 0.30% by mass or less,
Contains bainite and island martensite, and
The area fraction of bainite is 80.0% or more,
having a microstructure in which the area fraction of island martensite is less than 5.0%,
The area fraction of the average concentration region of Mn, defined as a region having a Mn concentration of 0.9 to 1.1 times the average Mn content (mass%), is less than 90%,
The area fraction of the Mn enriched region, defined as a region having a Mn concentration of 1.15 times or more than the average Mn content (mass%), is 1.0% or more, and the average circle of the Mn enriched region has a Mn concentration distribution with an equivalent diameter of 7.0 μm or less,
A thick steel plate whose Charpy absorbed energy: _vE0 at 0°C is 70J or more.
P _CM = [C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5 [B]…(1)
However, the parentheses in the above formula represent the content (mass%) of the element within the parentheses, and when the element is not contained, it is set to 0. The component composition is in mass%,
Cu: 3.0% or less,
Ni: 3.0% or less,
Cr: 3.0% or less,
Mo: 1.5% or less,
W: 3.0% or less,
Nb: 0.10% or less,
V: 0.10% or less,
B: 0.0050% or less,
Ca: 0.005% or less,
REM: 0.020% or less,
The thick steel plate according to claim 1, further comprising at least one selected from the group consisting of Mg: 0.005% or less, and Zr: 0.020% or less. In the microstructure,
The thickness according to claim 1 or 2, wherein the area fraction of the island-like martensite is 1.0% or more and less than 5.0%, and the average equivalent circular diameter of the island-like martensite is 5.0 μm or less. steel plate. The method for manufacturing a thick steel plate according to claim 1 or 2,
a hot rolling process of hot rolling a steel material having the above-mentioned composition into a thick steel plate;
a first cooling step of cooling the thick steel plate after the hot rolling step;
The thick steel plate after the first cooling step is heated at the 1/4 position of the plate thickness, average heating rate in the temperature range from Ac1 point to Ac3 point: 2.0 ° C / s or less, Ac3 point -100 ° C to Ac3 point Residence time in the temperature range up to: Raise the temperature to a reheating temperature of 3 Ac or more and 3 Ac + 60°C or less under conditions of 60 seconds or more, and then hold at the reheating temperature for a holding time of 10 minutes or more. a reheating step;
The thick steel plate after the reheating process,
Average cooling rate at 1/4 plate thickness position: 1.0 to 200.0°C/s, accelerated cooling to an accelerated cooling stop temperature of 100°C to 600°C, and then air cooling to a temperature of 100°C or less A method for manufacturing a thick steel plate, including a cooling step. Furthermore, a heat treatment step is included after the first cooling step and before the reheating step,
In the heat treatment step,
The thick steel plate after the first cooling step is heated to a heat treatment temperature of 3 Ac or more and 1050 ° C or less,
Holding at the heat treatment temperature for a holding time of 5 minutes or more,
The method for manufacturing a thick steel plate according to claim 4, wherein the steel plate is then cooled to a cooling stop temperature of 500°C or less.

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