Thick steel plate and method for producing same

Abstract

Provided is a thick steel plate which combines high strength, low yield ratio and high toughness, and which has excellent toughness at the joint even when subjected to high heat input welding in which the welding heat input exceeds 40 kJ / mm. A thick steel plate having a prescribed component composition and microstructure, and having an Mn concentration distribution in which the area fraction of an average Mn concentration region defined as a region having an Mn concentration of 0.9-1.1 times the average Mn content (mass%) is less than 90%; the area fraction of an Mn-rich region defined as a region having an Mn concentration of 1.15 times or more of the average Mn content (mass%) is 1.0% or more, the average circle-equivalent diameter of the Mn-rich region is 7.0 [mu] m or less, and the Charpy absorption energy vE0 at 0 DEG C is 70 J or more.

Description

Technical Field

[0001] The present invention relates to a thick steel plate, and more particularly to a thick steel plate having high strength, low yield ratio and high toughness, and having excellent toughness of the joint portion of the welded joint even in high heat input welding with a welding heat input exceeding 40 kJ / mm. In addition to having the above-mentioned excellent characteristics, the thick steel plate of the present invention is also suitable for industrial production and can be very suitable for use as a steel material for construction. In addition, the present invention relates to a method for manufacturing the above-mentioned thick steel plate. Background Art

[0002] In recent years, as building structures have become taller and longer span, the thickness of steel used has been increasing. In addition, thick steel plates used as such steel are required to have excellent mechanical properties, specifically, high tensile strength, yield stress, and toughness.

[0003] In addition, from the perspective 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 higher the plastic deformation capacity, which helps to improve the earthquake safety of the structure.

[0004] As a manufacturing process for thick steel plates with a reduced yield ratio, a process using a multi-stage heat treatment has been put into practical use. In the multi-stage heat treatment, the hot-rolled thick steel plate is reheated to the ferrite + austenite two-phase region, quenched, and then tempered. However, in the multi-stage heat treatment, the yield point rises due to the structural recovery caused by tempering, so it is difficult to achieve both high strength and low yield ratio.

[0005] Furthermore, when constructing steel structures, steel materials are generally joined together by welding. Therefore, the thick steel plates used are required to have not only the toughness of the thick steel plates themselves (the toughness of the parent material), but also the toughness of the heat-affected zone of the weld. In particular, from the perspective of improving the earthquake resistance of building structures, the toughness of the welded joints, especially the toughness of the joints of the welded joints, is required to be excellent.

[0006] In addition, as the steel used is becoming thicker, the application range of high heat input welding (welding with a large heat input during welding) is expanding in order 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 with a welding heat input exceeding 40kJ / mm has become common. Therefore, it is required that the toughness of the heat affected zone of the weld is excellent even when high heat input welding such as welding heat input exceeding 40kJ / mm is performed on thick steel plates.

[0007] Generally speaking, when high-heat input welding is applied to steel, the most problematic issue is the deterioration of the toughness of the joint of the welded joint. In the heat-affected zone of the weld, the joint is exposed to a high temperature just below the melting point during high-heat input welding, so the coarsening of the austenite grains is most obvious. Moreover, the coarsened austenite grains are transformed into a fragile upper bainite structure due to the temperature drop after welding, and then coarse island martensite (Martensite-Austenite constituent, MA) as a brittle structure is generated, and the toughness is reduced. Therefore, if the toughness of the joint of the welded joint in high-heat input welding can be improved, the safety of the steel structure can be significantly improved.

[0008] As described above, thick steel plates are required to have excellent mechanical properties such as strength, yield ratio, and toughness, as well as excellent toughness of the weld heat affected zone. Various technologies have been proposed to meet such requirements.

[0009] For example, Patent Document 1 proposes a technique of manufacturing high-tensile steel by quenching a hot-rolled steel sheet, heating it again to a two-phase region of ferrite and austenite and quenching it, and then tempering it.

[0010] Patent Document 2 proposes a low yield ratio high tensile strength steel sheet having a predetermined chemical composition and a retained austenite content of 1.0% or more.

[0011] Patent Document 3 proposes a low yield ratio, high tensile strength thick steel plate having a predetermined chemical composition, a microstructure including bainite and island martensite, and a controlled equivalent circle diameter and average aspect ratio of prior austenite grains.

[0012] Prior art literature

[0013] Patent Literature

[0014] Patent Document 1: Japanese Patent Application Laid-Open No. 06-248337

[0015] Patent Document 2: Japanese Patent Application Publication No. 2001-226740

[0016] Patent Document 3: Japanese Patent Application Publication No. 2018-090872 Summary of the invention

[0017] However, the technology proposed in Patent Document 1 aims to prevent cracking during low heat input welding, but does not focus on the toughness of the heat affected zone during high heat input welding.

[0018] On the other hand, the technology proposed in Patent Document 2 takes the toughness of the weld into consideration. However, the evaluation was only performed at a relatively low heat input of 5 kJ / mm or 15 kJ / mm, and the toughness of the heat-affected zone of the weld in high heat input welding exceeding 40 kJ / mm was not considered. In addition, in the manufacturing process disclosed in Patent Document 2, the volume fractions of ferrite and martensite are easily changed depending on the manufacturing conditions and the position in the steel plate. Therefore, in order to obtain the desired product, the manufacturing conditions need to be strictly adjusted, and it is not suitable for industrial production due to the high operating load.

[0019] In addition, in the technology proposed in Patent Document 3, in addition to low yield ratio and high strength, high joint toughness in high heat input welding with a welding heat input exceeding 40 kJ / mm is also achieved. However, in Patent Document 3, in order to achieve the above characteristics, the area fraction of island martensite must be 5% or more. Moreover, in the manufacturing process of Patent Document 3, in order to make the area fraction of island martensite more than 5%, after reheating the hot-rolled steel plate, it is necessary to perform a first water cooling process, an air cooling process, and a hot second water cooling process under controlled temperature conditions to control the formation of island martensite. During cooling, temperature deviations are likely to occur in the length and width directions of the steel plate. Therefore, in order to control the structure during this cooling process, it is necessary to strictly adjust the manufacturing conditions, and the operating load is high.

[0020] The present invention is made in view of the above-mentioned actual situation. The object of the present invention is to provide a thick steel plate which has high strength, low yield ratio and high toughness, and has excellent toughness of the joint part of the welded joint even in high heat input welding exceeding 40 kJ / mm, and is suitable for industrial production.

[0021] The present inventors have conducted intensive studies to achieve the above-mentioned objects and have obtained the following findings.

[0022] (1) By forming a predetermined Mn concentration distribution in a thick steel plate containing bainite and island martensite with an area fraction of bainite of 80.0% or more, high strength, low yield ratio and high toughness can be achieved despite the relatively low area fraction of island martensite of less than 5.0%.

[0023] (2) The above-mentioned Mn concentration distribution can be obtained by controlling the component composition, particularly the contents of C and Mn, within specific ranges and appropriately controlling the temperature increase conditions in the reheating step after hot rolling.

[0024] The present invention has been accomplished based on the above findings. The gist of the present invention is as follows.

[0025] 1. A thick steel plate having the following composition:

[0026] Contains, in mass%, 0.010 to 0.14% C, 0.01 to 0.50% Si, 0.9 to 3.0% Mn, 0.015% or less P, 0.0050% or less S, 0.002 to 0.080% Al, 0.003 to 0.030% Ti, and 0.0015 to 0.0080% N, with the remainder being Fe and unavoidable impurities, and,

[0027] 4.83C+Mn represented by the C content (mass %) and the Mn content (mass %) is 1.4 to 3.3 mass %,

[0028] The ratio of Ti content (mass %) to N content (mass %), Ti / N, is 2.0 to 4.3,

[0029] P represented by the following formula (1) CM 0.30 mass % or less;

[0030] The above thick steel plate has the following microstructure:

[0031] Contains bainite and island martensite, and,

[0032] The area fraction of bainite is 80.0% or more.

[0033] The area fraction of island martensite is less than 5.0%;

[0034] The above thick steel plate has the following Mn concentration distribution:

[0035] The area fraction of the average Mn concentration region defined as a region having a Mn concentration of 0.9 to 1.1 times the average Mn content (mass %) is less than 90%,

[0036] The area fraction of the Mn-rich region, defined as a region having a Mn concentration of 1.15 times or more the average Mn content (mass %), is 1.0% or more, and

[0037] The average equivalent circle diameter of the Mn-enriched region is less than 7.0 μm;

[0038] The thick steel plate has a Charpy absorbed energy vE0 at 0°C of 70 J or more.

[0039] P CM =[C]+[Si] / 30+[Mn] / 20+[Cu] / 20+[Ni] / 60+[Cr] / 20+[Mo] / 15+

[0040] [V] / 10+5[B]…(1)

[0041] The brackets in the above formula represent the content (mass %) of the element within the brackets, and are 0 when the element is not contained.

[0042] 2. The thick steel plate according to item 1 above, wherein the above-mentioned component composition further contains, in terms of mass %, at least one selected from the group consisting of: 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, Mg: 0.005% or less, and Zr: 0.020% or less.

[0043] 3. The thick steel plate according to 1 or 2 above, wherein in the microstructure,

[0044] The area fraction of the island martensite is 1.0% or more and less than 5.0%, and

[0045] The average equivalent circle diameter of the island martensite is 5.0 μm or less.

[0046] 4. A method for manufacturing a thick steel plate, comprising the following steps:

[0047] A hot rolling step of hot rolling the steel billet having the composition described in 1 or 2 above into a thick steel plate;

[0048] A first cooling step of cooling the thick steel plate after the hot rolling step;

[0049] a reheating step of heating the thick steel plate after the first cooling step to a reheating temperature of Ac3 to Ac3+60°C under the conditions of an average heating rate of 2.0°C / s or less in the temperature range from Ac1 to Ac3 at a position of 1 / 4 of the plate thickness and a retention time of 60 seconds or more in the temperature range from Ac3-100°C to Ac3, and then maintaining the reheating temperature for a retention time of 10 minutes or more; and

[0050] In the second cooling step, the thick steel plate after the above reheating step is accelerated cooled 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 a position of 1 / 4 of the plate thickness, and then air-cooled to a temperature below 100°C.

[0051] 5. The method for manufacturing a thick steel plate according to 4 above, further comprising a heat treatment step after the first cooling step and before the reheating step,

[0052] In the above heat treatment process,

[0053] After the first cooling step, the thick steel plate is heated to a heat treatment temperature of Ac3 point to 1050°C.

[0054] Keep the heat treatment temperature for more than 5 minutes.

[0055] Next, the steel is cooled to a cooling stop temperature of 500° C. or lower.

[0056] According to the present invention, a thick steel plate having high strength, low yield ratio and high toughness can be obtained, and the toughness of the joint of the welded joint is excellent even in high-heat input welding with a welding heat input exceeding 40 kJ / mm. The thick steel plate of the present invention can be very suitable for use as a steel material for construction, which contributes to the enlargement of steel structures and the improvement of earthquake resistance. In addition, the thick steel plate of the present invention can be manufactured by a process with low operating load, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a schematic diagram showing the groove shape in electroslag welding performed to evaluate the toughness of the joint.

[0058] Figure 2 This is a schematic diagram showing the position where a Charpy impact test specimen is taken from an electroslag weld. DETAILED DESCRIPTION

[0059] The following is a description of an embodiment of the present invention. It should be noted that the following description represents a preferred embodiment of the present invention, and the present invention is not limited by the following description. In addition, in this specification, in order to distinguish it from the toughness of the joint after welding, the toughness of the unwelded thick steel plate itself is sometimes referred to as "parent material toughness".

[0060] [Ingredients]

[0061] The thick steel plate of the present invention and the steel billet for manufacturing the thick steel plate need to have the above-mentioned composition. The components contained in the above-mentioned composition are described below. It should be noted that, unless otherwise specified, the "%" indicating the content of each component means "mass %".

[0062] C: 0.010~0.14%

[0063] 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 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, if the C content exceeds 0.14%, the formation of coarse island martensite and cementite is promoted, the toughness of the base material is reduced, and the toughness of the joint is also significantly deteriorated. Therefore, the C content is 0.14% or less, preferably 0.10% or less, and more preferably 0.08% or less.

[0064] Si: 0.01~0.50%

[0065] Si is an element that functions as a deoxidizer and has the effect of increasing the strength of thick steel plates. In order to obtain the above-mentioned effect, the Si content is set to 0.01% or more. On the other hand, if the Si content exceeds 0.50%, the formation of coarse island martensite is promoted, and the decrease in the toughness of the base material and the toughness of the joint becomes obvious. Therefore, the Si content is set to 0.50% or less, preferably 0.35% or less.

[0066] Mn: 0.9~3.0%

[0067] Mn is an element that has the effect of increasing the strength of thick steel plates. In addition, as described later, by controlling the Mn concentration distribution, 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 be 0.9% or more, preferably 1.2% or more. On the other hand, if the Mn content exceeds 3.0%, the area fraction of the Mn-enriched portion increases, and coarse MA is generated, so the toughness of the base material decreases. In addition, the heat-affected zone of the weld solidifies, and the toughness of the joint portion is significantly reduced. Therefore, the Mn content is less than 3.0%, preferably less than 2.6%.

[0068] It should be noted that the chemical composition described here is an average composition of the thick steel plate. Therefore, the above-mentioned Mn content value is used as the "average Mn content" in the definition of the Mn concentration distribution described later.

[0069] P: 0.015% or less

[0070] P is an element that deteriorates the toughness of the base material and the toughness of the joint, and it is desirable to reduce it as much as possible. If the P content exceeds 0.015%, the reduction in the toughness of the base material and the toughness of the joint becomes significant. It is believed that this is because if the P content is high, P segregates in the Mn-rich part and the structure solidifies. Therefore, the P content is less than 0.015%. On the other hand, the lower limit of the P content is not particularly limited and can be 0%. However, excessive reduction will lead to increased costs. Therefore, from the perspective of manufacturing cost, it is preferred to make the P content more than 0.001%.

[0071] S: 0.0050% or less

[0072] 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%, the desired base material toughness and joint toughness cannot be obtained. Therefore, the S content is 0.0050% or less. On the other hand, the lower limit of the S content is not particularly limited and can be 0%. However, excessive reduction will lead to increased costs. Therefore, from the perspective of manufacturing cost, it is preferred that the S content be 0.0003% or more.

[0073] Al: 0.002~0.080%

[0074] Al is an element that acts as a deoxidizer. In addition, Al fixes N in the steel in the form of AlN, which helps to improve the toughness of the base material. In order to obtain the above-mentioned effect, the Al content is set to 0.002% or more, preferably 0.010% or more. On the other hand, if 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.

[0075] Ti: 0.003~0.030%

[0076] Ti is an element that functions as a deoxidizer and helps to improve the strength of thick steel plates. In addition, Ti combines with N and precipitates in the form of TiN, which is a nitride that is stable even at high temperatures. Therefore, the pinning effect of TiN prevents the coarsening of austenite grains during heating, and the toughness of the base material and the toughness of the joint are improved. In order to obtain the above-mentioned 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 toughness of the base material and the toughness of the joint deteriorate, so the Ti content is 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less.

[0077] N: 0.0015~0.0080%

[0078] N combines with Al and Ti to precipitate nitrides. The above-mentioned nitrides inhibit the coarsening of austenite grains and improve the toughness of the parent material and the joint. In order to obtain the above-mentioned effect, the N content is 0.0015% or more, preferably 0.0025% or more. On the other hand, if the N content exceeds 0.0080%, the toughness of the parent material and the joint is reduced due to the increase in the amount of solid-solution N. Therefore, the N content is 0.0080% or less, preferably 0.0065% or less, and more preferably 0.0060% or less.

[0079] The composition of the components in one embodiment of the present invention may have a composition consisting of the above-mentioned elements and the remaining Fe and inevitable impurities. However, other trace elements may be contained as long as the effect of the present invention is not impaired. It should be noted that as the above-mentioned inevitable impurities, for example, oxygen (O) can be cited. The content of oxygen contained as an inevitable impurity is preferably 0.0030% or less.

[0080] 4.83C+Mn: 1.4-3.3 mass%

[0081] In the thick steel plate of the present invention, as described later, excellent mechanical properties are achieved by forming a predetermined Mn concentration distribution. In addition, in order to achieve the above-mentioned Mn concentration distribution, the value of "4.83C+Mn" calculated from the C content and the Mn content in the above-mentioned component composition needs to be 1.4 mass % or more. The reason is explained below.

[0082] In order to form the above-mentioned Mn concentration distribution in the thick steel plate finally obtained, it is necessary to generate a microscopic deviation of the Mn concentration inside the thick steel plate during the manufacturing process of the thick steel plate. For this reason, it is necessary to make 4.83C+Mn more than 1.4 mass%, and control the heating conditions in the reheating process as described later. By reheating the steel plate with 4.83C+Mn more than 1.4 mass% under specified conditions, the distribution of Mn to the reverse transformation austenite can be promoted to form a microscopic deviation of the Mn concentration. If the value of 4.83C+Mn is less than 1.4 mass%, the distribution of Mn to the reverse transformation austenite is insufficient, so the desired Mn concentration distribution cannot be obtained. As a result, the strength of the thick steel plate decreases and the yield ratio increases. Therefore, 4.83C+Mn is more than 1.4 mass%, preferably more than 1.7 mass%. On the other hand, if 4.83C+Mn exceeds 3.3 mass%, the effect is saturated. Therefore, 4.83C+Mn is less than 3.3 mass%.

[0083] Ti / N: 2.0~4.3

[0084] TiN has the effect of suppressing the growth of austenite grains in the weld heat affected zone by the pinning effect and improving the toughness of the joint. If Ti / N is less than 2.0, it is impossible to ensure the amount of TiN required for the above effect, and the toughness of the joint deteriorates. Therefore, Ti / N is made to be more than 2.0, preferably more than 2.4. On the other hand, if Ti / N exceeds 4.3, the base material toughness and the toughness of the joint deteriorate due to the generation of TiC particles and the coarsening of TiN. Therefore, Ti / N is made to be less than 4.3, preferably less than 4.0.

[0085] P CM : 0.30 mass % or less

[0086] If P defined by the following formula (1) CM If the content of P is higher than 0.30 mass %, good joint toughness cannot be obtained. CM It is 0.30 mass % or less, preferably 0.28 mass % or less, and more preferably 0.26 mass % or less.

[0087] P CM =[C]+[Si] / 30+[Mn] / 20+[Cu] / 20+[Ni] / 60+[Cr] / 20+[Mo] / 15+

[0088] [V] / 10+5[B]…(1)

[0089] The brackets in the above formula represent the content (mass %) of the element within the brackets, and are 0 when the element is not contained.

[0090] On the other hand, P CM There is no particular lower limit, but if P CM If the value is too low, the strength of the thick steel plate will decrease. CM It is preferably 0.15 mass % or more, more preferably 0.17 mass % or more, and further preferably 0.19 mass % or more.

[0091] In addition, in other embodiments of the present invention, the above-mentioned component composition may arbitrarily further contain at least one selected from Cu, Ni, Cr, Mo, W, Nb, V, B, Ca, REM, Mg and Zr.

[0092] Cu: 3.0% or less

[0093] Cu is an element that maintains the high toughness of thick steel plates and further improves the strength, and can be contained arbitrarily according to the required strength. However, if the Cu content exceeds 3.0%, hot brittleness occurs and the surface properties of the steel plate deteriorate. Therefore, when Cu is contained, 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 fully obtain the above-mentioned effect, it is preferred that the Cu content is 0.01% or more, and more preferably 0.05% or more.

[0094] Ni: 3.0% or less

[0095] Ni, like Cu, is an element that maintains the high toughness of thick steel plates and further improves strength, and can be contained arbitrarily according to the required strength. However, if the Ni content exceeds 3.0%, the addition effect is saturated, so it becomes 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 fully obtain the above-mentioned effect, it is preferred that the Ni content is 0.01% or more, and more preferably 0.10% or more.

[0096] Cr: 3.0% or less

[0097] Cr is an element that further improves the strength of thick steel plates, and can be contained arbitrarily according to the required strength. However, if the Cr content exceeds 3.0%, the toughness of the base material and the joint deteriorates, 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 fully obtaining the strength improvement effect based on Cr, the Cr content is preferably set to 0.01% or more, and more preferably 0.10% or more.

[0098] Mo: 1.5% or less

[0099] Mo, like Cr, is an element that further improves the strength of thick steel plates and can be contained arbitrarily according to the required strength. However, if the Mo content exceeds 1.5%, the toughness of the base material and the joint deteriorates. In addition, quenching cracks are easily generated during the manufacturing process of the thick steel plate, and the manufacturability is reduced. 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 fully obtaining the strength improvement effect based on Mo, it is preferred that the Mo content is 0.01% or more, and more preferably 0.10% or more.

[0100] W: 3.0% or less

[0101] W, like Cr and Mo, is an element that further improves the strength of thick steel plates and can be contained arbitrarily according to the required strength. However, if the W content exceeds 3.0%, the toughness of the base material and the joint deteriorates. Therefore, when W is contained, 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 fully obtaining the strength improvement effect based on W, the W content is preferably set to 0.01% or more, and more preferably 0.10% or more.

[0102] Nb: 0.10% or less

[0103] Nb, like Cr, Mo, and W, is an element that further improves the strength of thick steel plates, and can be arbitrarily contained according to the required strength. However, if the Nb content exceeds 0.10%, the toughness of the base material and the joint portion decreases, so when Nb is contained, the Nb content is set to 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 fully obtaining the strength improvement effect based on Nb, the Nb content is preferably set to 0.005% or more.

[0104] V: 0.10% or less

[0105] V, like Cr, Mo, W, and Nb, is an element that further improves the strength of thick steel plates and can be arbitrarily contained according to the required strength. However, if the V content exceeds 0.10%, the toughness of the base material and the joint portion decreases, so when V is contained, the V content is set to 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 fully obtaining the strength improvement effect based on V, it is preferred that the V content be 0.005% or more.

[0106] B: 0.0050% or less

[0107] 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 joint by fixing the solid-solution nitrogen in the form of nitrides in the heat-affected zone of the weld during high-input energy welding. However, if the B content exceeds 0.0050%, the hardenability becomes too high, and the toughness of the base material and the joint is reduced. 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 fully obtaining the effect of adding B, it is preferred that the B content is set to 0.0003% or more.

[0108] Ca: 0.005% or less

[0109] Ca is an element that has the effect of further improving the toughness of the base material by making the crystal grains finer, and can be arbitrarily contained according to the required toughness of the base material. However, if the Ca content exceeds 0.005%, the addition effect 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 fully obtaining the toughness improvement effect based on Ca, it is preferred that the Ca content be 0.001% or more.

[0110] REM: 0.020% or less

[0111] REM (rare earth metal) is an element that has the effect of further improving the toughness of the base material by making the grains finer, as with Ca, and can be arbitrarily contained according to the required toughness of the base material. However, if the REM content exceeds 0.020%, the addition effect 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 fully obtaining the toughness improvement effect based on REM, it is preferred that the REM content be 0.002% or more.

[0112] Mg: 0.005% or less

[0113] Mg, like Ca and REM, is an element that has the effect of further improving the toughness of the base material by making the crystal grains finer, and can be arbitrarily contained according to the required toughness of the base material. However, if the Mg content exceeds 0.005%, the addition effect 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 fully obtaining the toughness improvement effect based on Mg, it is preferred that the Mg content be 0.001% or more.

[0114] Zr: 0.020% or less

[0115] Zr, like Ca, REM, and Mg, is an element that has the effect of further improving the toughness of the base material by making the crystal grains finer, and can be arbitrarily contained according to the required toughness of the base material. However, if the Zr content exceeds 0.020%, the addition effect is saturated, so when Zr is contained, 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 fully obtaining the toughness improvement effect based on Zr, it is preferred that the Zr content be 0.002% or more.

[0116] [Microstructure]

[0117] The thick steel plate of the present invention has a microstructure containing bainite and island martensite, and the area fraction of bainite is 80.0% or more. The reasons for limiting the microstructure to the above range are explained below. It should be noted that, unless otherwise specified, the "area fraction" in the following description refers to the area fraction relative to the entire microstructure. In addition, the above microstructure refers to the microstructure at the 1 / 4 position of the plate thickness of the steel plate.

[0118] Microstructure containing bainite and island martensite

[0119] As described later, bainite is a necessary structure for improving the strength and toughness of thick steel plates. On the other hand, island martensite (MA) is a structure with higher hardness than bainite due to the concentration of C. Therefore, by forming MA, the tensile strength can be improved. In addition, since a large number of movable dislocations are introduced around MA, the increase in yield stress is suppressed. Therefore, in order to achieve both high strength and low yield ratio, a microstructure containing bainite and island martensite is required.

[0120] Area fraction of bainite: 80.0% or more

[0121] 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, the upper limit of the area fraction of bainite is not particularly limited, but if it is too high, the area fraction of island martensite becomes relatively low, so it is difficult to fully reduce the yield ratio. Therefore, the area fraction of bainite is preferably 99.0% or less. It should be noted that the area fraction of bainite can be measured by the method described in the examples.

[0122] Area fraction of island martensite

[0123] The research results of the present inventors show that even if the MA of the base material is 5.0% or more in area fraction, the toughness of the joint is reduced. This is believed to be due to the following reasons. That is, during welding, the joint is heated to a high temperature close to the melting point, so the MA contained in the thick steel plate is temporarily decomposed due to the above heating. However, during the cooling process after welding, MA is regenerated in the joint. At this time, the amount of regenerated MA increases as the amount of MA contained in the thick steel plate before welding increases. Moreover, if the amount of regenerated MA in the joint is large, the toughness of the joint is reduced. Therefore, in the present invention, in order to improve the toughness of the joint, the area fraction of island martensite in the microstructure of the thick steel plate is made less than 5.0%, preferably less than 4.9%, more preferably less than 4.7%, and further preferably less than 4.5%.

[0124] On the other hand, the lower limit of the area fraction of MA is not particularly limited. As mentioned above, MA has the effect of improving tensile strength and the effect of suppressing the increase of yield stress. From the viewpoint of fully exerting these effects, it is preferred that the area fraction of MA in the above-mentioned microstructure is 1.0% or more, and more preferably 2.0% or more.

[0125] It should be noted that in Patent Document 3, since the desired mechanical properties are achieved by increasing the area fraction of MA, the area fraction of MA must be 5% or more. In contrast, in the present invention, as described later, the desired mechanical properties are achieved by controlling the Mn concentration distribution. Therefore, even if the area fraction of MA is less than 5.0%, excellent mechanical properties can be obtained. In addition, since the area fraction of MA in the base material is as low as less than 5.0%, the toughness of the joint is excellent compared to Patent Document 3. This also makes it clear that the difference between the thick steel plate of the present invention and the thick steel plate disclosed in Patent Document 3. Furthermore, there is no need to perform a complex cooling control consisting of a first water cooling process, an air cooling process, and a hot second water cooling process to control the generation of MA as in Patent Document 3.

[0126] Average equivalent circle diameter of island martensite

[0127] The size of MA is not particularly limited and can be any size. However, if MA is too coarse, the toughness of the thick steel plate decreases. Therefore, from the viewpoint of further improving toughness, it is preferred that the average equivalent circle diameter of MA is 5.0 μm or less, 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 and can generally be 0.8 μm or more, or 1.0 μm or more.

[0128] The area fraction and average equivalent circle diameter of MA can be determined by subjecting a steel plate as a sample to LePera corrosion (Journal of Metals, March, 1980, p. 38-39), observing the steel plate at a magnification of 1000 times using a scanning electron microscope (SEM), and analyzing the image obtained using an image analyzer.

[0129] Other organizations

[0130] The microstructure in one embodiment of the present invention may be composed of bainite and island martensite. In addition, the microstructure in other embodiments of the present invention may contain other tissues in addition to bainite and island martensite. The above-mentioned other tissues are not particularly limited and may be any tissue. For example, the above-mentioned other tissues may be at least one selected from ferrite, pearlite, martensite and residual austenite. When other tissues are included in the microstructure, the total area fraction of the above-mentioned other tissues is preferably 19% or less, more preferably 15% or less, and further preferably 10% or less.

[0131] The thick steel plate in one embodiment of the present invention is composed of 80 to 99.0% of bainite, 1.0 to 15.0% of island martensite, and the remainder in terms of area fraction:

[0132] The remaining structure may be at least one selected from ferrite, pearlite, martensite and retained austenite.

[0133] [Mn concentration distribution]

[0134] The inventors of the present invention have found that by controlling the contents of C and Mn and the heating conditions in the reheating process after hot rolling, microscopic deviations of Mn concentration can be intentionally formed inside the thick steel plate. It is also known that by appropriately controlling the microscopic deviations of the above-mentioned Mn concentration, i.e., the Mn concentration distribution, a thick steel plate having high strength, low yield ratio and high toughness can be obtained. It is believed that this is because the distribution and size of MA are affected by the above-mentioned Mn concentration distribution. For example, there is a tendency to form MA in Mn-rich areas. As mentioned above, MA is a harder structure than bainite, and therefore has the effect of increasing strength. In addition, MA has the effect of reducing the yield ratio by introducing movable dislocations. Therefore, by controlling the distribution of Mn concentration to control the distribution of MA, the result is that both high strength and low yield ratio can be taken into account.

[0135] The present invention is based on the above technical concept. Specifically, the thick steel plate of the present invention has a Mn concentration distribution that satisfies the following conditions (1) to (3). It should be noted that the Mn concentration distribution in the present invention refers to the Mn concentration distribution at the 1 / 4 position of the steel plate thickness.

[0136] (1) The area fraction of the average Mn concentration region is less than 90%

[0137] (2) The area fraction of the Mn-rich region is 1.0% or more

[0138] (3) The average equivalent circle diameter of the Mn-rich region is 7.0 μm or less

[0139] (1) Area fraction of the average Mn concentration region: less than 90%

[0140] If the area fraction of the average concentration region of Mn is more than 90%, the hard structure containing MA becomes insufficient and the desired strength cannot be obtained. In addition, the enrichment of Mn in the Mn-enriched region becomes insufficient, and the yield stress reduction effect caused by the formation of MA becomes insufficient. As a result, the yield ratio is also prone to become high. Therefore, the area fraction of the average concentration region of Mn is less than 90%, preferably less than 85%, and 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 larger, and as a result, the MA formed in the enriched region becomes coarse. Therefore, the area fraction of the average concentration region of Mn is preferably more than 50%, and more preferably more than 60%. It should be noted that, herein, the "average concentration region of Mn" is defined as a region with a Mn concentration of 0.9 to 1.1 times the average Mn content (mass %).

[0141] (2) Area fraction of Mn-rich region: 1.0% or more

[0142] If the area fraction of the Mn-rich region is less than 1.0%, the formation of MA becomes insufficient, and high strength and low yield ratio cannot be achieved at the same time. Therefore, the area fraction of the Mn-rich region is 1.0% or more, preferably 1.5% or more, more preferably 2.0% or more, and further preferably 6.2% or more. On the other hand, the upper limit of the area fraction of the Mn-rich region is not particularly limited, but if the area fraction of the Mn-rich region is too high, the size of the enriched region becomes larger, and the MA formed in the enriched region tends to become coarse. Therefore, the area fraction of the Mn-rich region is preferably less than 50%, more preferably less than 40%, and further preferably less than 20%. It should be noted that the above-mentioned Mn-rich region disappears due to heating during welding, so the presence of the enriched region will not have an adverse effect on the toughness of the joint after welding. Therefore, according to the present invention, it is possible to achieve both excellent mechanical properties of thick steel plates and high joint toughness.

[0143] Average equivalent circle diameter of Mn-rich region: 7.0 μm or less

[0144] If the average equivalent circle diameter of the Mn-rich region is greater than 7.0 μm, the MA formed in the Mn-rich region also coarsens, so the toughness of the thick steel plate decreases. Therefore, the average equivalent circle diameter of the Mn-rich 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-rich region is not particularly limited, and can generally be 1.0 μm or more, or 1.5 μm or more.

[0145] The above-mentioned Mn concentration distribution can be measured using an electron microscope (EPMA). Specifically, a test piece is taken from a thick steel plate in such a way that the position of 1 / 4 of the plate thickness becomes the observation position. The Mn concentration distribution in the above-mentioned test piece is measured using EPMA, and the area fraction of the average Mn concentration area, the area fraction of the Mn enriched area, and the average equivalent circle diameter of the Mn enriched area are calculated. The above-mentioned measurement of the Mn concentration distribution using EPMA is performed in more than two randomly selected fields of view, the size of one field of view is 50μm×50μm, and the measurement points in each field of view are 250×250 points.

[0146] [Mechanical properties]

[0147] ·toughness

[0148] The Charpy absorbed energy at 0°C of the thick steel plate of the present invention: vE0 is 70J or more. Charpy absorbed energy is one of the indicators of toughness, and the thick steel plate of the present invention with vE0 of 70J or more exhibits excellent earthquake safety even when used in high-rise buildings. vE0 is preferably 80J or more, and more preferably 100J or more. On the other hand, from the perspective of earthquake resistance, the higher the vE0, the better, and the upper limit of vE0 is not particularly limited. However, it can generally be 250J or less, 220J or less, or 210J or less.

[0149] The Charpy absorbed energy at 0°C of the thick steel plate can be measured in accordance with JIS Z 2242 using a V-notch test piece taken from a position 1 / 4 of the plate thickness of the thick steel plate in accordance with JIS Z 2202.

[0150] Yield stress

[0151] The yield stress (YS) of the thick steel plate of the present invention is not particularly limited, but from the viewpoint of high strength accompanying the high-rise of building structures, it is preferably 600 MPa or more, preferably 620 MPa or more. The upper limit of the yield stress is also not particularly limited, for example, it can be 900 MPa or less, 880 MPa or less, or 850 MPa or less.

[0152] Tensile strength

[0153] The tensile strength (TS) of the thick steel plate of the present invention is not particularly limited, but from the viewpoint of high strength accompanying the high-rise of building structures, it is preferably 780 MPa or more, more preferably 800 MPa or more. The upper limit of the tensile strength is also not particularly limited, for example, it can be 1100 MPa or less, and it can also be 1000 MPa or less.

[0154] Yield ratio

[0155] The yield ratio (YR) of the thick steel plate of the present invention is not particularly limited, but considering the allowable margin for damage during an earthquake, it is preferably 85% or less from the viewpoint of improving the deformation performance of the building structure. On the other hand, the lower limit of the yield ratio is also not limited, for example, it can be 70% or more, or 75% or more. It should be noted that the yield ratio here refers to the value of the ratio of the yield stress (YS) to the tensile strength (TS) expressed as a percentage, that is, YS / TS×100(%).

[0156] The yield stress and tensile strength can be measured by using a JIS No. 4 tensile test piece taken from a 1 / 4 thickness position of a thick steel plate through a tensile test in accordance with JIS Z 2241. The yield ratio can be calculated from the yield stress and tensile strength measured by the above method.

[0157] ·Joint toughness

[0158] The toughness of the joint of the thick steel plate of the present invention is not particularly limited, and the Charpy absorbed energy (vE0) of the joint at 0°C is preferably 47 J or more. The upper limit of vE0 in the joint is also not particularly limited, and may generally be 150 J or less.

[0159] It should be noted that vE0 in the above-mentioned joint is a value measured by using a JIS No. 4 Charpy impact test piece taken from a welded joint by electroslag welding with a welding line energy of 40 kJ / mm or more, with the notch position forming a joint. More specifically, it can be measured by the method described in the Examples.

[0160] ·Plate thickness

[0161] The thickness of the 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 further preferably 12 mm or more. From the perspective of coping with the high-rise of building structures, it 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 also not particularly limited, and is preferably 100 mm or less.

[0162] [Manufacturing method]

[0163] Next, a method for producing a thick steel plate according to an embodiment of the present invention will be described. The thick steel plate can be produced by sequentially subjecting a steel billet having the above-mentioned chemical composition to the steps (a) to (d).

[0164] (a) Hot rolling process

[0165] (b) First Cooling Step

[0166] (c) Reheating process

[0167] (d) Second Cooling Step

[0168] The conditions in each process are described in detail below. It should be noted that in the following description, unless otherwise specified, the temperature refers to 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 calculated by heat transfer based on the surface temperature of the steel plate measured by a radiation thermometer. In addition, the temperature in the heating condition and cooling condition after the hot rolling process is the temperature at the 1 / 4 position of the plate thickness, and the heating rate and cooling rate also refer to the average heating rate or average cooling rate calculated based on the temperature at the 1 / 4 position of the plate thickness.

[0169] (Steel Billet)

[0170] As the steel billet material, any form of billet material can be used. The steel billet material can be, for example, a steel billet. The method for manufacturing the steel billet material is not particularly limited, and for example, the steel billet material can be manufactured by melting steel having the above-mentioned component composition and casting. The melting can be performed by any method such as a converter, an electric furnace, an induction furnace, etc. In addition, from the viewpoint of productivity, the casting is preferably performed by a continuous casting method, and can also be performed by an ingot casting-decomposition rolling method.

[0171] (Hot rolling process)

[0172] In the hot rolling process, the steel billet is hot rolled to form a thick steel plate. The conditions for the hot rolling are not particularly limited and can be performed under any conditions. Typically, the steel billet is heated to a predetermined heating temperature and then rolled. The heating can be performed after temporarily cooling the steel billet obtained by casting or the like, or the steel billet obtained can be directly heated without being cooled.

[0173] It should be noted that in the present invention, the microstructure and properties of the thick steel plate are controlled in the reheating process and the second cooling process after hot rolling. Therefore, the heating temperature in the hot rolling process is not particularly limited and can be any temperature. However, if the heating temperature is less than 1000°C, the deformation resistance of the steel billet is high, so the load on the rolling mill during hot rolling increases, and hot rolling is sometimes difficult. Therefore, the heating temperature is preferably above 1000°C. On the other hand, if the heating temperature is higher than 1250°C, the oxidation of the steel becomes significant, the loss caused by oxidation increases, and the yield is reduced as a result. Therefore, the heating temperature is preferably below 1250°C.

[0174] The rolling end temperature is not particularly limited, but is preferably 1000° C. or lower. The rolling end temperature is preferably 750° C. or higher.

[0175] (First Cooling Step)

[0176] 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 characteristics of the thick steel plate in the subsequent reheating process and the second cooling process, the cooling in the first cooling process is not particularly limited and can be performed under any conditions. However, in the subsequent reheating process, it is necessary to control the average heating rate in the temperature range from point Ac1 to point Ac3, so the cooling stop temperature in the first cooling process only needs to be below point Ac1. The above-mentioned cooling stop temperature is preferably below 500°C. If the above-mentioned cooling stop temperature is below 500°C, the coarsening of the precipitates can be suppressed, and the Mn-rich area can be generated 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. The above-mentioned cooling stop temperature is more preferably below 250°C. On the other hand, the lower limit of the above-mentioned cooling stop temperature is also not limited, and it can be cooled to any temperature. However, excessive cooling will reduce productivity, so the above-mentioned cooling stop temperature is preferably above 0°C, more preferably above 10°C, and more preferably above 20°C. Typically, it is preferred that the cooling stop temperature be room temperature or higher.

[0177] The cooling in the first cooling step is not particularly limited and may be performed by any method. For example, the cooling may 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 more preferably performed by at least one selected from spray cooling, mist cooling and laminar cooling.

[0178] (Reheating process)

[0179] Next, the steel plate after the first cooling step is heated to a reheating temperature in a predetermined heating pattern and maintained at the reheating temperature.

[0180] Average heating rate in the temperature range from point Ac1 to point Ac3: less than 2.0℃ / s

[0181] During the heating process, when passing through the temperature range of Ac1 point to Ac3 point (two-phase zone), Mn is distributed to austenite generated from bainite and martensite through reverse phase transformation, resulting in a microscopic deviation of Mn concentration. However, if the average heating rate in the temperature range from Ac1 point to Ac3 point is higher than 2.0°C / s, the distribution of Mn will not be fully carried out, and as a result, the desired Mn concentration distribution cannot be obtained. Therefore, in the above-mentioned reheating process, the thick steel plate after the above-mentioned first cooling process is heated at an average heating rate of less than 2.0°C / s in the temperature range from Ac1 point to Ac3 point at 1 / 4 of the plate thickness. On the other hand, the lower limit of the above-mentioned average heating rate is not particularly limited. However, if the heating rate is too slow, the effect of controlling the heating rate is saturated, and the time required for heating becomes longer, and the productivity decreases. Therefore, the above-mentioned average heating rate is preferably above 0.01°C / s.

[0182] The residence time in the temperature range from Ac3 point - 100°C to Ac3 point: more than 60 seconds

[0183] Similarly, if the residence time in the temperature range from Ac3 point -100°C to Ac3 point is less than 60 seconds, the distribution of Mn will not be fully carried out, and as a result, the desired Mn concentration distribution cannot be obtained. Therefore, in the temperature rise process of the above-mentioned 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 above-mentioned residence time is too long, the productivity decreases. Therefore, the above-mentioned residence time is preferably 60 minutes or less.

[0184] It should be noted that the temperature increase pattern in the above-mentioned temperature increase process is not particularly limited as long as it satisfies the above-mentioned conditions. For example, the temperature may be continuously increased to the reheating temperature, or may be intentionally maintained in the two-phase region.

[0185] It should be noted that the Ac1 point and the Ac3 point are obtained by the following equations (2) and (3).

[0186] Ac1(℃)=751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B···(2)

[0187] Ac3(℃)=937-476.5C+56Si-19.7Mn-16.3Cu-4.9Cr-26.6Ni+38.1Mo+124.8V+136.3Ti+198Al+3315B···(3)

[0188] The element symbols in the above formula (2) and formula (3) represent the content (mass %) of each element, and are 0 when the element is not contained.

[0189] Reheating temperature: Ac3 point ~ Ac3 point + 60°C

[0190] In the reheating step, the steel plate is heated to a reheating temperature of Ac3 point to Ac3 point + 60° C. Thus, bainite, martensite and other structures contained in the steel plate at the end of the first cooling step are reversely transformed into austenite, and become a microstructure composed of austenite single phase.

[0191] If the above-mentioned 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. In addition, coarse MA is generated in the Mn-rich area, and the toughness of the base material is reduced. Therefore, the above-mentioned reheating temperature is above the Ac3 point. On the other hand, if the above-mentioned reheating temperature is higher than the Ac3 point + 60°C, the austenite grains coarsen and the homogenization of the composition progresses, resulting in the failure to obtain the desired Mn concentration distribution. Therefore, the above-mentioned reheating temperature is below the Ac3 point + 60°C, more preferably below the Ac3 point + 55°C, and further preferably below the Ac3 point + 50°C.

[0192] Reheating temperature holding time: more than 10 minutes

[0193] In the above-mentioned reheating process, after the thick steel plate is heated to the above-mentioned reheating temperature, it is maintained at the reheating temperature for a prescribed holding time. If the above-mentioned holding time is less than 10 minutes, the average equivalent circle diameter of the Mn-enriched area in the finally obtained thick steel plate cannot be less than 7.0 μm. It is believed that this is because the holding time is short, so the deviation of the grain size of the reverse phase transformation austenite becomes larger, and as a result, the size of the Mn-enriched area becomes uneven. Therefore, the above-mentioned holding time is set to more than 10 minutes. On the other hand, the upper limit of the above-mentioned holding time is not particularly limited, but if it is maintained for too long, the productivity decreases, so it is preferably less than 100 minutes.

[0194] Any heating method may be used in the reheating step. An example of the heating method is furnace heating. The furnace heating is not particularly limited, and a general heat treatment furnace may be used.

[0195] (Second Cooling Step)

[0196] Next, the thick steel plate after the reheating process is cooled. Specifically, the thick steel plate after the reheating process is accelerated cooled 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 a position of 1 / 4 of the plate thickness, and then air-cooled to a temperature below 100°C.

[0197] Average cooling rate: 1.0~200.0℃ / s

[0198] By carrying out accelerated cooling under the above conditions, the reverse transformation austenite can be transformed into bainite, and a microstructure with bainite as the main body can be obtained. If the above average cooling rate is less than 1.0°C / s, ferrite is generated, so the area fraction of bainite cannot be made greater than 80.0%. Therefore, the above average cooling rate is greater than 1.0°C / s, preferably greater than 5.0°C / s. On the other hand, if the above average cooling rate is higher than 200.0°C / s, the temperature control of each position in the steel plate becomes difficult, and material deviations are likely to occur in the plate width direction and rolling direction, resulting in material deviations such as strength characteristics. Therefore, the above average cooling rate is less than 200.0°C / s, preferably less than 150.0°C / s, and more preferably less than 100.0°C / s.

[0199] The method for accelerating cooling is not particularly limited, and 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 (e.g., spray cooling, mist cooling, laminar cooling, etc.) can be used. As the water cooling method, mist cooling is preferably used.

[0200] Accelerated cooling stop temperature: 100~600℃

[0201] In the above-mentioned second cooling process, accelerated cooling is performed to an accelerated cooling stop temperature of 100°C to 600°C, followed by air cooling. If the above-mentioned accelerated cooling stop temperature is less than 100°C, all austenite is transformed into bainite, and therefore a microstructure containing island martensite cannot be obtained. In addition, since the tempering effect cannot be obtained, the toughness of the thick steel plate is reduced. Therefore, the above-mentioned accelerated cooling stop temperature is above 100°C, preferably above 200°C. On the other hand, if the above-mentioned accelerated cooling stop temperature is higher than 600°C, ferrite is easily generated, and the area fraction of bainite cannot be made to be above 80.0%. Therefore, the above-mentioned accelerated cooling stop temperature is below 600°C, preferably below 500°C.

[0202] Air cooling to a temperature below 100°C

[0203] After stopping the accelerated cooling, the thick steel plate is further air-cooled to a temperature below 100°C. The air cooling is not forced cooling, but can be cooling, i.e., natural cooling. In the air cooling, the thick steel plate can be cooled to a temperature below 100°C, but it is not necessary to stop the air cooling at a specific temperature, and it is usually sufficient to air-cool to room temperature (ambient temperature).

[0204] The other conditions in the above-mentioned air cooling have no substantial effect on the structure of the thick steel plate, etc., and therefore are not particularly limited. For example, the cooling rate in the above-mentioned air cooling is not particularly limited, and cooling can be performed at any speed. Typically, the cooling rate in the above-mentioned air cooling can be less than 1.0°C / s, or less than 0.5°C / s. In addition, the lower limit of the cooling rate in the above-mentioned air cooling is not particularly limited, and can be greater than 0.001°C / s, greater than 0.01°C / s, or greater than 0.07°C / s.

[0205] Similarly, the time required for the above-mentioned air cooling (air cooling time) is not particularly limited. In the present embodiment, since no special treatment is required after the above-mentioned air cooling, there is no problem even if time is spent on air cooling. Typically, the above-mentioned air cooling time may exceed 300s, may be more than 310s, or may be more than 320s. On the other hand, the upper limit of the air cooling time is not particularly limited, for example, it may be less than 24 hours, or may be less than 12 hours. It should be noted that, here, in the second cooling step, the time taken from the start of air cooling to reaching 100°C is defined as the air cooling time.

[0206] As described above, the thick steel plate of the present invention can be manufactured by controlling the component composition, especially the contents of C and Mn, within a specific range and appropriately controlling the temperature increase conditions in the reheating process after hot rolling. Therefore, the thick steel plate of the present invention is easier to produce and is suitable for industrial production than the thick steel plate of Patent Document 3 which requires controlling the conditions in the cooling process after reheating to control the generation of MA.

[0207] In addition, in the present invention, as described above, microscopic deviations in Mn concentration are intentionally generated by controlling the temperature increase conditions in the reheating process. And, as a result, the above-mentioned Mn concentration distribution is formed in the thick steel plate finally obtained. In contrast, in Patent Document 3, there is no control of the temperature increase conditions for generating microscopic deviations in Mn concentration as in the present invention. Therefore, in the manufacturing process described in Patent Document 3, a thick steel plate having a Mn concentration distribution that satisfies the conditions of the present invention cannot be obtained. MA exists in the thick steel plate of Patent Document 3, but it is believed that the Mn concentration around the MA is the same average concentration as the Mn concentration in the matrix, and there is no enriched area that satisfies the conditions of the present invention.

[0208] In other embodiments of the present invention, in order to further increase the strength and reduce the yield ratio of the thick steel plate, a heat treatment may be further performed after the first cooling step and before the reheating step. That is, in the above embodiment, the thick steel plate may be manufactured by sequentially performing the steps (a) to (e) on the steel billet having the above composition.

[0209] (a) Hot rolling process

[0210] (b) First Cooling Step

[0211] (c) Heat treatment process

[0212] (d) Reheating process

[0213] (e) Second Cooling Step

[0214] (Heat treatment process)

[0215] In the above-mentioned heat treatment process, the thick steel plate after the above-mentioned first cooling process is heated to a heat treatment temperature of Ac3 point ~ 1050°C, maintained at the above-mentioned heat treatment temperature for more than 5 minutes, and then cooled to a cooling stop temperature below 500°C. By performing the above-mentioned heat treatment, high strength and low yield ratio can be taken into account at a higher level. The reasons are believed to be as follows. By implementing a reheating process after the above-mentioned heat treatment process, the existence position of the Mn-enriched part is homogenized, and the frequency of reverse phase transformation nucleation is increased. As a result, the formation of microscopic deviations in Mn concentration is promoted, and the area fraction of the Mn-enriched area is finally increased. Specifically, the area fraction of the enriched area can be made to be more than 6.2%. As mentioned above, since MA is formed in the Mn-enriched area, high strength and low yield ratio are taken into account.

[0216] Heat treatment temperature: Ac3 point ~ 1050℃

[0217] By setting the heat treatment temperature to Ac3 point ~ 1050°C, hardenability can be ensured and the desired bainite and martensite structures can be obtained. If the heat treatment temperature is less than the Ac3 point, the desired base material toughness cannot be obtained. It is believed that this is because coarse ferrite is generated during the 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. It is believed that this is because coarse bainite and coarse martensite are generated during the heat treatment, resulting in the formation of a coarse bainite structure in the final microstructure.

[0218] Holding time: more than 5 minutes

[0219] In order to reduce the deviation of austenite grain size, the holding time at the above-mentioned heat treatment temperature is more than 5 minutes, preferably more than 10 minutes. On the other hand, the upper limit of the above-mentioned holding time is not particularly limited, but even if it is too long, the effect is also saturated, so considering productivity, it is preferably below 100 minutes, more preferably below 60 minutes.

[0220] For the heating in the above-mentioned 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. As an example of a heating method that can be used, furnace heating can be cited. The above-mentioned furnace heating is not particularly limited, and a general heat treatment furnace can be used.

[0221] Cooling stop temperature: below 500℃

[0222] After being maintained at the above-mentioned heat treatment temperature, the thick steel plate is cooled to a cooling stop temperature of less than 500°C. Through the above-mentioned cooling, the austenite generated in the above-mentioned heat treatment process is made into a low-temperature phase transformation phase of bainite and martensite, and in the subsequent above-mentioned reheating process, higher strength and lower yield ratio can be obtained. If the above-mentioned cooling stop temperature is higher than 500°C, the desired strength and toughness cannot be ensured. Therefore, the above-mentioned cooling temperature is set to less than 500°C, preferably less than 400°C, and more preferably less than 200°C. On the other hand, the lower limit of the above-mentioned cooling stop temperature is not particularly limited. However, excessive cooling will reduce productivity, so the above-mentioned cooling stop temperature is preferably above 0°C, more preferably above 10°C, and further preferably above 20°C. Typically, it is preferred that the above-mentioned cooling stop temperature is above room temperature.

[0223] The cooling method is not particularly limited, and the cooling may be performed by any method such as air cooling, water cooling, etc. As the water cooling, any cooling method using water, such as spray cooling, mist cooling, laminar cooling, etc., may be used.

[0224] When the heat treatment step is performed, the steel plate after cooling may be subjected to the next reheating step. When the heat treatment step is not performed, the steel plate after the first cooling step may be subjected to the next reheating step without being subjected to heat treatment.

[0225] In other embodiments of the present invention, a tempering process may be further performed after the reheating process for the purpose of correcting the shape of the thick steel plate. In addition, the lower the cooling stop temperature of the cooling process after reheating, the more it can be expected that the toughness of the base material will be further improved by the effect of tempering. The heating temperature during tempering is preferably 200°C to 500°C. In the cooling of the steel plate after the tempering process, the material does not change depending on the cooling rate, so it can be performed by any method.

[0226] Example

[0227] Thick steel plates were produced according to the following procedure, and their properties were evaluated.

[0228] First, molten steel having the composition shown in Table 1 was smelted in a converter, and a billet (thickness: 260 mm) was produced as a steel billet material by continuous casting. CM (mass %), the Ac1 point (° C.) obtained by the formula (2), and the Ac3 point (° C.) obtained by the formula (3) are shown in Table 1.

[0229] The steel slabs were heated to 1150° C. and then hot rolled to produce thick steel plates having the thicknesses shown in Tables 2 and 3 (hot rolling process). Tables 2 and 3 show the rolling end temperatures in the hot rolling.

[0230] The obtained thick steel plate was cooled to the cooling stop temperature shown in Tables 2 and 3 by the cooling method shown in Tables 2 and 3 (first cooling step).

[0231] Next, the thick steel plate after the first cooling step is 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 is heat treated under the conditions shown in Tables 2 and 3 (heat treatment step), and then reheated. The reheating is performed using a heat treatment furnace.

[0232] Finally, after the above reheating, accelerated cooling was performed under the conditions shown in Tables 2 and 3, and after stopping the above accelerated cooling, air cooling (letting cool) was performed to room temperature. The above accelerated cooling was performed by water cooling. In addition, the air cooling time (the time required to reach 100° C.) in the above air cooling was 1 hour or more.

[0233] The microstructure, Mn concentration distribution, mechanical properties, and joint toughness after welding of each thick steel plate obtained as described above were evaluated. The above evaluations were performed by the following method.

[0234] (Microstructure)

[0235] A test piece for organization observation is taken from the thick steel plate in such a way that the position of 1 / 4 of the plate thickness becomes the observation position. The test piece is embedded in resin in such a way that the cross section perpendicular to the rolling direction becomes the observation surface, and mirror polishing is performed. Next, after LePera corrosion is performed, the image of the organization is taken with a scanning electron microscope at a magnification of 1000 times, and the island martensite organization is identified. The images of the five fields of view taken are analyzed using an image analysis device to obtain the area fraction and average equivalent circle diameter of the island martensite organization.

[0236] In addition, a test piece for structural observation was taken from the thick steel plate in such a way that the position of 1 / 4 of the plate thickness became the observation position. The test piece was embedded in resin in such a way that the cross section perpendicular to the rolling direction became the observation surface, and mirror polishing was performed. Then, after nitric acid etching, the sample was observed with a scanning electron microscope at a magnification of 200 times, and an image of the structure was taken to identify the bainite structure. The images of the five fields of view taken were analyzed using an image analysis device to obtain the area fraction of the bainite structure.

[0237] (Mn concentration distribution)

[0238] A test piece was taken from the thick steel plate in such a way that the position of 1 / 4 of the plate thickness became the observation position. The Mn concentration distribution in the test piece was measured by EPMA, and the area fraction of the average Mn concentration area, the area fraction of the Mn enriched area, and the average equivalent circle diameter of the Mn enriched area were calculated. The measurement of the Mn concentration distribution by EPMA was performed in more than two randomly selected fields of view, the size of one field of view was 50 μm × 50 μm, and the measurement points in each field of view were 250 × 250 points.

[0239] (Mechanical properties)

[0240] A JIS No. 4 tensile test piece was taken from the 1 / 4 position of the plate thickness of the thick steel plate. Using the tensile test piece, a tensile test was carried out in accordance with the provisions of JIS Z2241 to measure the yield stress, tensile strength and yield ratio of the thick steel plate. In addition, in accordance with the provisions of JIS Z 2202, a V-notch test piece was taken from the 1 / 4 position of the plate thickness of the thick steel plate. Using the V-notch test piece, the Charpy absorbed energy (vE0) based on the Charpy impact test at 0°C was calculated in accordance with the provisions of JIS Z 2242 to evaluate the toughness of the base material.

[0241] (Joint toughness)

[0242] In order to evaluate the toughness of the weld heat affected zone of the thick steel plate, a welded joint was prepared according to the following procedure, and the Charpy absorbed energy in the joint was measured.

[0243] First, a set of joint test plates having the same plate thickness as the thick steel plate are taken from the thick steel plate. One of the joint test plates is used as the surface plate 1 and the other is used as the spacer plate 2. Figure 1 The groove 3 is of the shape shown. Next, electroslag welding is performed at a welding line energy of 40 kJ / mm or more to produce a welded joint 5.

[0244] Next, if Figure 2As shown in FIG. 1 , a JIS No. 4 Charpy impact test piece 8 is taken from the welded joint 5 in such a manner that the position of the notch 9 becomes the joint portion. It should be noted that the position of the notch 9 is set as the intersection of a straight line passing through a position 6 mm from the surface of the surface plate 1 and the weld line. In addition, the Charpy impact test piece 8 is taken in such a manner that the length direction of the test piece is perpendicular to the weld line.

[0245] Using the Charpy test piece, the absorbed energy (vE0) in the joint of the welded joint in the Charpy impact test at 0° C. was measured. For some thick steel plates whose base material properties did not meet the target, the evaluation of the joint toughness was not performed.

[0246] The obtained results are shown in Tables 4 and 5. It should be noted that the yield stress was 600 MPa or more, the tensile strength was 780 MPa or more, the yield ratio was 85% or less, the absorbed energy at 0°C (vE0) was 70 J or more, and the absorbed energy at 0°C (vE0) of the joint of the welded joint was 47 J or more.

[0247] Thick steel plates that meet the conditions of the present invention all 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, an absorbed energy vE0 of 70 J or more at 0°C, high strength and low yield ratio, and excellent parent material toughness. In addition, even when high-energy-input welding with a welding energy input exceeding 40 kJ / mm is performed, the vE0 in the joint of the welded joint is 47 J or more, and has excellent joint toughness of the welded joint. On the other hand, thick steel plates that do not meet the conditions of the present invention have poor parent material strength, yield ratio, parent material toughness, and joint toughness of the welded joint.

[0248]

[0249]

[0250]

[0251]

[0252]

[0253] Explanation of symbols

[0254] 1 Surface panel

[0255] 2 partitions

[0256] 3 slope

[0257] 4 Liner

[0258] 5. Welding joints

[0259] 6. Weld Metal

[0260] 7Heat Affected Zone (HAZ)

[0261] 8Charpy impact test piece

[0262] 9Gap

Claims

1. A thick steel plate having the following composition: Contains, by mass%, C: 0.010-0.14%, Si: 0.01-0.50%, Mn: 0.9-3.0%, P: 0.015% or less, S: 0.0050% or less, Al: 0.002-0.080%, Ti: 0.003-0.030% and N: 0.0015-0.0080%, with the remainder being Fe and unavoidable impurities, and, 4.83C+Mn represented by the C content and the Mn content is 1.4 to 3.3 mass%, The ratio of Ti content to N content, Ti / N, is 2.0 to 4.3, and the unit of content is mass %. P represented by the following formula (1) CM 0.30 mass % or less; The thick steel plate has the following microstructure: comprising bainite and island martensite, wherein the area fraction of bainite is greater than 80.0% and the area fraction of island martensite is less than 5.0%; The thick steel plate has the following Mn concentration distribution: The area fraction of the average Mn concentration region defined as a region having a Mn concentration of 0.9 to 1.1 times the average Mn content is less than 90%, and the unit of the content is mass %. The area fraction of the Mn-rich region defined as a region having a Mn concentration of 1.15 times or more of the average Mn content is 1.0% or more, the unit of the content is mass %, and The average equivalent circle diameter of the Mn-enriched region is less than 7.0 μm; The Charpy absorbed energy vE0 of the thick steel plate at 0°C is greater than 70 J; Q CM =[C]+[Si] / 30+[Mn] / 20+[Cu] / 20+[Ni] / 60+[Cr] / 20+[Mo] / 15+ [V] / 10+5[B]…(1) in, The brackets in the above formula represent the content of the element within the brackets, and when the element is not contained, it is 0, and the unit of the content is mass %.

2. The thick steel plate according to claim 1, wherein: The component composition further contains, in mass %, at least one selected from the group consisting of: 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, Mg: 0.005% or less, and Zr: 0.020% or less.

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

4. A method for manufacturing a thick steel plate, comprising the following steps: A hot rolling step of hot rolling the steel billet having the composition of claim 1 or 2 to produce a thick steel plate; The first cooling step is to cool the thick steel plate after the hot rolling step. a reheating step of heating the thick steel plate after the first cooling step to a reheating temperature of Ac3 point to Ac3 point + 60°C under the conditions that the average heating rate in the temperature range from point Ac1 to point Ac3 at a position of 1 / 4 of the plate thickness is 2.0°C / s or less and the retention time in the temperature range from point Ac3 point - 100°C to point Ac3 point is 60 seconds or more, and then maintaining the temperature for a retention time of 10 minutes or more; and In the second cooling step, the thick steel plate after the reheating step is accelerated cooled 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 a position of 1 / 4 of the plate thickness, and then air-cooled to a temperature below 100°C.

5. The method for manufacturing a thick steel plate according to claim 4, wherein: A heat treatment step is further included after the first cooling step and before the reheating step, In the heat treatment process, After the first cooling step, the thick steel plate is heated to a heat treatment temperature of Ac3 point to 1050°C. Maintaining the heat treatment temperature for a holding time of 5 minutes or more, Next, the steel is cooled to a cooling stop temperature of 500° C. or lower.

Images