Steel plate and its manufacturing method

Abstract

To provide a steel sheet that has high strength, can reduce the use of expensive Mo, does not require reheating after hot rolling, and has excellent toughness in the heat-affected zone. [Solution] A steel sheet having a composition containing C, Si, Mn, P, S, Al, Ti, N, Ca, and O, with the remainder being Fe and unavoidable impurities, and a manganese-molybdenum factor (MMF) defined by the formula MMF = Mn + 2Mo being 1.70% or more and 2.50% or less; a microstructure in which the total area ratio of martensite and island martensite is 3% or more and 30% or less, with the remainder being bainite; and mechanical strength having a tensile strength of 590 MPa or more and 740 MPa or less, and a yield ratio of 80% or less.

Description

Technical Field

[0001] The present invention relates to a steel plate having high strength and excellent toughness in the heat affected zone after welding even when subjected to high heat input welding with a heat input of 400 kJ / cm or more using submerged arc welding or electro-slag welding, and a method for manufacturing the same.

Background Art

[0002] In recent years, with the enlargement of welded steel structures, the strength and thickness of steel plates have been increased. At the same time, from the viewpoint of improving the construction efficiency of welded steel structures and reducing the construction cost, the application of a welding method called high heat input welding with a large heat input during welding has been expanding. For example, in the manufacture of box columns used in high-rise buildings, steel plates are welded using high heat input welding with a heat input exceeding 400 kJ / cm, such as submerged arc welding or electro-slag welding.

[0003] In addition, in building structures, improvement of seismic resistance is required, and steel materials used in building structures are required to have a low YR characteristic with a yield ratio YR of 80% or less in order to ensure plastic deformation ability. Furthermore, when a large load such as an earthquake is applied, brittle fracture may occur from the welded part before plastic deformation occurs in the welded steel structure. Therefore, excellent toughness is also required in the welded joint part.

[0004] In high-strength steel plates with a tensile strength exceeding 590 MPa, it is necessary to add a large amount of alloying elements to ensure strength. When high heat input welding is performed on a high-strength steel plate with a large amount of alloying elements added, deterioration of toughness in the heat affected zone during welding (hereinafter sometimes referred to as "HAZ") becomes a problem. This deterioration of toughness is considered to be caused by the fact that when a welding method with slow cooling such as high heat input welding is adopted, the residence time in the high temperature range becomes long, and hard and brittle phases such as island martensite are likely to be generated in the HAZ. The greater the amount of alloying elements added, the more likely the influence of the brittle phase is to occur. Therefore, there is a demand for steel plates having both high strength and excellent toughness of the HAZ, and various proposals have been made.

[0005] Patent Document 1 describes an invention for a method of manufacturing high-tensile steel with excellent toughness and ductility, characterized by reheating a hot-rolled steel sheet to a two-phase region of ferrite + austenite, and then performing accelerated cooling.

[0006] Patent Document 2 describes an invention for a method of manufacturing a high-strength thick steel sheet with excellent toughness of the heat-affected zone (HAZ), characterized by hot-rolling a steel material in which the atomic concentration ratio ACR and carbon equivalent Ceq calculated from the component composition are adjusted to a predetermined range, accelerating the cooling of the hot-rolled steel sheet, reheating it to the ferrite + austenite two-phase region, and then tempering it. In this manufacturing method, the toughness of the HAZ is improved by using TiN to suppress the coarsening of austenite grains in the HAZ, and by using a composite sulfide of CaS and MnS to promote nucleation of intragranular ferrite, thereby refining the structure of the HAZ.

[0007] Patent Document 3 describes an invention of a high-tensile steel sheet with excellent HAZ toughness, characterized by containing a predetermined amount of Mo and having the atomic concentration ratio ACR and weld cracking susceptibility index Pcm, calculated from the component composition, adjusted to a predetermined range. In this high-tensile steel sheet, the formation of island martensite is suppressed by reducing Si and P, thereby improving toughness. Furthermore, in this high-tensile steel sheet, reheating to the ferrite + austenite two-phase region after hot rolling is unnecessary. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2001-288512 [Patent Document 2] Japanese Patent Publication No. 2005-68519 [Patent Document 3] Japanese Patent Publication No. 2013-147742 [Non-patent literature]

[0009] [Non-Patent Document 1] FSLePera, Journal of metals, 32(1980), p.38-39. [Overview of the project] [Problems that the invention aims to solve]

[0010] However, the manufacturing methods described in Patent Documents 1 and 2 require reheating of the steel sheet after hot rolling, which increases manufacturing costs due to the increased number of steps. Furthermore, the manufacturing method described in Patent Document 2 adds Cu and Ni as elements useful for increasing strength. These elements promote cracking during continuous casting of slabs, requiring repair of the deteriorated surface properties of the slabs, which creates challenges in productivity and manufacturing costs. On the other hand, while the high-tensile steel sheet described in Patent Document 3 does not require reheating after hot rolling, it requires the addition of expensive Mo, which increases raw material costs.

[0011] This invention has been made in view of the above-mentioned problems, and aims to provide a steel sheet and a method for manufacturing the same that have high strength, can reduce the use of expensive Mo, do not require reheating after hot rolling, and have excellent toughness in the heat-affected zone. [Means for solving the problem]

[0012] To achieve the above objectives, the inventors diligently conducted research based on the following design concept.

[0013] First, to prevent cracking of the cast slab, the content of Cu and Ni, which promote slab cracking, was adjusted to an appropriate range. Furthermore, to reduce the amount of island martensite formation that degrades the toughness of the heat-induced zone (HAZ) during high-heat-input welding, the content of C, Si, Mo, Cr, Nb, and V, which promote the formation of island martensite, was adjusted to an appropriate range. In addition, to appropriately utilize CaS and improve the toughness of the HAZ during high-heat-input welding, the atomic concentration ratio (ACR) was adjusted to between 0.20 and 0.80.

[0014] Next, in order to satisfy the excellent toughness of the HAZ while simultaneously achieving high strength and low YR of the base material, the carbon equivalent (Ceq) was adjusted to a range of 0.40% to 0.46%. Furthermore, the microstructure was a multiphase structure in which the total area ratio of martensite and island martensite was 3% to 30%, with the remainder being bainite.

[0015] Furthermore, in order to reduce raw material costs, we decided to limit the use of expensive Mo and instead add an appropriate amount of Mn. As a result of our investigation, we introduced a new control index called the "manganese-molybdenum factor" (MMF), defined by the formula MMF = Mn + 2Mo. We found that by adjusting the MMF to between 1.70% and 2.50%, we could achieve steel sheets that possessed both high strength and excellent HAZ toughness.

[0016] This invention builds upon the above-mentioned findings and summarizes them as follows.

[0017] [1] In mass%, C: 0.040% or more and 0.100% or less, Si: 0.01% or more and 0.25% or less, Mn: 1.50% or more and 2.50% or less, P: 0.020% or less, S: 0.0005% or more and 0.0030% or less, Al: 0.002% or more and 0.080% or less, Ti: 0.003% or more and 0.030% or less, N: 0.0060% or less, Ca: 0.0005% or more and 0.0040% or less, O: 0.0010% or more and 0.0040% or less It contains, with the remainder consisting of Fe and unavoidable impurities. The carbon equivalent Ceq, as defined by the following formula (1), is 0.40% or more and 0.46% or less. The atomic concentration ratio ACR, defined by equation (2) below, is 0.20 or more and 0.80 or less, A steel sheet having a component composition in which the manganese-molybdenum factor MMF defined by the following formula (3) is 1.70% or more and 2.50% or less, and a microstructure in which the total area ratio of martensite and island martensite is 3% or more and 30% or less at the 1 / 4 thickness position, and the balance is composed of bainite, a tensile strength of 590 MPa or more and 740 MPa or less, and a yield ratio of 80% or less for mechanical strength. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ···(1) ACR = {Ca - (0.18 + 130×Ca)×O} / (1.25×S) ···(2) MMF = Mn + 2Mo ···(3) Here, C, Si, Mn, Ni, Cr, Mo, V, Ca, O, and S in formulas (1) to (3) are the contents of the respective elements expressed in mass%, and the content when not contained is 0 (zero).

[0018] [2] The component composition is, in mass%, Mo: 0.20% or less, B: 0.0020% or less, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.50% or less, Nb: 0.10% or less, V: 0.10% or less, Mg: 0.0050% or less, REM: 0.020% or less, and Zr: 0.020% or less and further contains one or more selected from the group consisting of the steel sheet according to [1] above.

[0019] [3] A step of heating a steel piece having the component composition according to [1] or [2] above to a temperature of 1000°C or more and 1200°C or less, and then hot rolling in a temperature range of Ar3 point or more to obtain a steel sheet. ​Subsequently, the process involves performing accelerated cooling in a temperature range from an accelerated cooling start temperature of Ar3 or higher to an accelerated cooling end temperature of 400°C to 600°C, such that the cooling rate of the steel plate at the 1 / 4 thickness position is 3°C / s or more and 40°C / s or less. Subsequently, the steel plate is air-cooled, A method for manufacturing steel plates, comprising:

[0020] [4] A step of obtaining a steel sheet by heating a steel billet having the component composition described in [1] or [2] above to a temperature of 1000°C or more and 1200°C or less, and then hot rolling it in a temperature range of Ar3 or higher, Subsequently, the process involves performing accelerated cooling in a temperature range from an accelerated cooling start temperature of Ar3 or higher to an accelerated cooling end temperature of 50°C to 600°C, such that the cooling rate of the steel plate at the 1 / 4 thickness position is 3°C / s or more and 40°C / s or less. Subsequently, the steel plate is air-cooled, Subsequently, the steel plate is subjected to a tempering treatment at a temperature of 550°C or lower. A method for manufacturing steel plates, comprising: [Effects of the Invention]

[0021] According to the present invention, it is possible to obtain steel plates with high strength and excellent toughness of the heat-induced zone (HAZ) even in high-heat-input welding where the welding heat input exceeds 400 kJ / cm. Furthermore, according to the present invention, steel plates can be stably manufactured with fewer heat treatment processes. Therefore, the present invention greatly contributes to the scaling up of steel structures, improvement of seismic resistance, and improvement of construction efficiency, resulting in remarkable industrial benefits. [Brief explanation of the drawing]

[0022] [Figure 1] This is a schematic diagram showing the groove shape in electroslag welding performed to evaluate the toughness of HAZ. [Figure 2] This is a schematic diagram showing the sampling locations for Charpy impact test specimens from electroslag welds. [Modes for carrying out the invention]

[0023] Embodiments of the present invention are described below. The following description illustrates a preferred embodiment of the present invention, and the modes for carrying out the present invention are not limited in any way by the following description.

[0024] [Basic ingredients] First, the basic components of the composition of the steel sheet according to the present invention will be described. In this specification, unless otherwise specified, the symbol "%" means mass percentage.

[0025] C: 0.040% or more and 0.100% or less Carbon (C) has a relatively large effect in increasing the strength of steel and is a useful element for ensuring the strength required for structural steel. However, if the C content is too low, it becomes impossible to minimize the amount of other alloying elements added in order to ensure sufficient strength. Therefore, the C content should be 0.040% or more, preferably 0.050% or more. On the other hand, if the C content is too high, the amount of cementite and island martensite, which are the starting points for brittle fracture, increases, reducing toughness. Therefore, the C content should be 0.100% or less, preferably 0.090% or less.

[0026] Si: 0.01% or more and 0.25% or less Si is an effective element for increasing the strength of the base material. However, if the Si content is too low, this effect cannot be obtained. Therefore, the Si content should be 0.01% or more, preferably 0.05% or more. On the other hand, if the Si content is too high, a large amount of island-like martensite will be formed in the HAZ structure, and suitable toughness of the HAZ cannot be obtained. Therefore, the Si content should be 0.25% or less, preferably 0.20% or less.

[0027] Mn: 1.50% or more and 2.50% or less Mn is an inexpensive element that dissolves in steel and increases the strength of the steel sheet. By including Mn, the amount of expensive alloying elements can be minimized. However, if the Mn content is too low, this effect cannot be obtained. Therefore, the Mn content should be 1.50% or more, preferably 1.70% or more. On the other hand, if the Mn content is too high, it reduces the toughness of the base material. Therefore, the Mn content should be 2.50% or less, preferably 2.40% or less, and more preferably 2.30% or less.

[0028] P:0.020% or less P is an impurity element and negatively affects toughness, so it is desirable to reduce it as much as possible. If the P content is too high, the decrease in toughness becomes significant. For this reason, the P content should be 0.020% or less, preferably 0.015% or less. Since a lower P content is preferable, there is no particular lower limit, but from the viewpoint of manufacturing cost, it is preferable that the P content be 0.001% or more.

[0029] S: 0.0005% or more and 0.0030% or less S is one of the elements that make up the formula for calculating the atomic concentration ratio ACR, which will be described later, and forms CaS, which is the nucleus for the formation of MnS. MnS formed with CaS as the nucleus has an effective effect on the formation of intragranular ferrite in high heat input welds and the refinement of the crystal structure. However, if the S content is too low, these effects cannot be obtained. For this reason, the S content should be 0.0005% or more, preferably 0.0010% or more. On the other hand, if the S content is too high, material degradation such as a decrease in ductility in the thickness direction due to the formation of MnS becomes significant. For this reason, the S content should be 0.0030% or less, preferably 0.0025% or less.

[0030] Al: 0.002% or more and 0.080% or less Al is an element that acts as a deoxidizing agent and is commonly used in the molten steel deoxidation process. Al also fixes nitrogen in the steel as AlN, contributing to improved toughness of the base material. However, if the Al content is too low, this effect cannot be obtained. Therefore, the Al content should be 0.002% or higher, preferably 0.003% or higher. On the other hand, if the Al content is too high, coarse AlN and alumina clusters are formed, reducing the toughness of the base material and the HAZ (heat-affected zone). Therefore, the Al content should be 0.080% or lower, preferably 0.060% or lower.

[0031] Ti: 0.003% or more and 0.030% or less Ti has a strong affinity for N, and precipitates as TiN during solidification, suppressing the coarsening of austenite grains in the HAZ. Furthermore, TiN also functions as a nucleus in the phase transformation from austenite to ferrite, contributing to improved toughness of the HAZ. However, if the Ti content is too low, these effects cannot be obtained. Therefore, the Ti content should be 0.003% or more, preferably 0.008% or more. On the other hand, if the Ti content is too high, the TiN particles will coarse, and the above effects cannot be expected. Therefore, the Ti content should be 0.030% or less, preferably 0.020% or less.

[0032] N: 0.0060% or less N is an element that dissolves in steel and induces strain aging after cold working, thereby reducing toughness. If the N content is excessive, the deterioration of toughness becomes significant. Therefore, the N content should be 0.0060% or less, preferably 0.0050% or less. Since a lower N content is preferable, there is no particular lower limit, but from the viewpoint of manufacturing cost, it is preferable that the N content be 0.0025% or more.

[0033] Ca: 0.0005% or more and 0.0040% or less Ca is one of the elements that make up the formula for calculating the atomic concentration ratio ACR, which will be described later, and forms CaS, which is the nucleus for the formation of MnS. MnS formed with CaS as the nucleus has an effective effect on the formation of intragranular ferrite in high heat input welds and on the refinement of the crystal structure. However, if the Ca content is too low, these effects cannot be obtained. For this reason, the Ca content should be 0.0005% or more, preferably 0.0010% or more. On the other hand, if the Ca content is too high, the amount of Ca-based oxides increases, and the cleanliness of the steel decreases. For this reason, the Ca content should be 0.0040% or less, preferably 0.0030% or less.

[0034] O: 0.0010% or more and 0.0040% or less Oxygen (O) is an unavoidable impurity element, and a low content is desirable. If the O content is too low, reducing the oxygen will lead to increased manufacturing costs in the melting process. Therefore, the O content should be 0.0010% or more, preferably 0.0015% or more. On the other hand, if the O content is too high, the amount of oxide inclusions increases, degrading the cleanliness of the steel. Therefore, the O content should be 0.0040% or less, preferably 0.0025% or less.

[0035] The remaining components of the steel sheet according to the present invention, other than those mentioned above, consist of Fe and unavoidable impurities.

[0036] [Management indicators] Next, we will explain three types of control indicators calculated from the content of the elements that make up the basic components mentioned above and the optional components described later.

[0037] Carbon equivalent (Ceq): 0.40% to 0.46% "Equivalent carbon content" (Ceq) refers to the influence of carbon and other elements on strength and weldability, converted into carbon content. In this invention, the carbon equivalent defined by the following formula (1) as specified in Japanese Industrial Standard JIS G 3475:2014 is used as one of the control indicators. Ceq=C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, in equation (1), C, Si, Mn, Ni, Cr, Mo, and V represent the content of each element as mass percent, and the content is considered to be 0 (zero) if no element is present at all.

[0038] Steel plates with a carbon equivalent controlled within an appropriate range exhibit excellent strength and weldability. However, if the carbon equivalent is too low, the strength of the base material and the heat-affected zone (HAZ) cannot be ensured in thick materials up to a maximum thickness of 100 mm. Therefore, the carbon equivalent should be 0.40% or higher, preferably 0.41% or higher. On the other hand, if the carbon equivalent is too high, weldability decreases, and the toughness of the HAZ also decreases. Therefore, the carbon equivalent should be 0.46% or lower, preferably 0.45% or lower.

[0039] Atomic concentration ratio ACR: 0.20 or more and 0.80 or less The "atomic concentration ratio" (ACR) refers to the ratio of the effective atomic concentration of calcium, which contributes to the formation of Ca-based sulfides, to the atomic concentration of sulfur. In this invention, the atomic concentration ratio defined by the following equation (2) is used as one of the control indicators. ACR={Ca-(0.18+130×Ca)×O} / (1.25×S) ···(2) Here, in equation (2), Ca, O, and S represent the mass percentage of each element, and the content is considered to be 0 (zero) if no element is present at all.

[0040] The atomic concentration ratio is a good indicator of the amount of Ca-based sulfides produced in steel plates. Furthermore, there is a strong correlation between the atomic concentration ratio and the toughness of the heat-induced zone (HAZ) in high-heat-input welding. If the atomic concentration ratio is too low, the amount of Ca-based sulfides necessary for ferrite formation decreases, and the effect of improving the toughness of the HAZ in high-heat-input welding is not obtained. Therefore, the atomic concentration ratio should be 0.20 or higher, preferably 0.25 or higher. On the other hand, if the atomic concentration ratio is too high, Ca-based sulfides are produced, but MnS is not formed using them as nuclei, so the effect of HAZ refinement due to ferrite formation cannot be obtained. Therefore, the atomic concentration ratio should be 0.80 or lower, preferably 0.75 or lower.

[0041] Manganese-molybdenum factor (MMF): 1.70% to 2.50% The manganese-molybdenum factor (MMF) is a control index used in preparing the component composition when substituting some of the expensive Mo with inexpensive Mn, and is defined by the following formula (3). The manganese-molybdenum factor is a third control index that is applied together with carbon equivalent and atomic concentration ratio, and is introduced for the first time in this invention. MMF = Mn + 2Mo ... (3) Here, Mn and Mo in equation (3) represent the content of each element as expressed in mass percent. As described above, in this invention, Mo is an element that is optionally added as needed. If the steel sheet does not contain Mo, the manganese-molybdenum factor is calculated by setting the value of Mo in equation (3) to zero.

[0042] Mo is an effective element for ensuring the strength of the base material, but as mentioned above, the inclusion of Mo increases the raw material cost. By substituting a portion of Mo with Mn, the decrease in strength due to the reduced Mo content is compensated for by solid solution strengthening due to the addition of Mn and dislocation strengthening due to the lowering of the bainite transformation point, making it possible to secure a tensile strength of 590 MPa or more. This makes it possible to achieve both high strength and low cost. However, if the manganese-molybdenum factor is too low, this effect cannot be achieved. Therefore, the manganese-molybdenum factor should be 1.70% or more, preferably 1.90% or more. On the other hand, if the manganese-molybdenum factor is too high, the hardness of the center of the plate thickness increases significantly, and the toughness of the base material deteriorates significantly. Therefore, the manganese-molybdenum factor should be 2.50% or less, preferably 2.40% or less.

[0043] In this invention, in addition to individually adjusting the Mn content and Mo content, the inventors believe that a steel sheet possessing both high strength and excellent HAZ toughness can be obtained by adjusting the newly introduced manganese-molybdenum factor to an appropriate range, although the exact reason is unclear. As shown in equation (3), in the manganese-molybdenum factor, the coefficient of Mn is 1, while the coefficient of Mo content is twice that, at 2. As a result, the Mo content changes the manganese-molybdenum factor with twice the weight compared to the Mn content.

[0044] As mentioned above, if the Mn content is excessive, central segregation occurs during casting, increasing the hardness of the center of the plate thickness and degrading the toughness of the base material. When Mo is added in combination with Mn, when Mo segregates together with Mn, the effect of Mo in increasing hardenability is added, resulting in a significant increase in the hardness of the center of the plate thickness and a significant degradation of the toughness of the base material. By introducing a manganese-molybdenum factor with a coefficient of 2 applied to the Mo content, the effect of Mo content is more strongly reflected. It is thought that by adjusting the component composition using such a manganese-molybdenum factor, the balance of Mn and Mo content can be adjusted more appropriately.

[0045] [Microorganisms] The steel sheet according to the present invention has a microstructure in which, at the 1 / 4 thickness position, the total area ratio of martensite and island martensite is 3% or more and 30% or less, with the remainder being bainite.

[0046] Martensite and island martensite are significantly harder phases compared to bainite. Therefore, steel sheets containing martensite and island martensite in their microstructure exhibit improved tensile strength, while the large number of introduced mobile dislocations suppress an increase in the yield ratio. This allows for both high strength and a low yield ratio. It is preferable that the martensite and island martensite are dispersed within the bainite.

[0047] However, if the area ratio of martensite and island martensite is too small, the effects of high strength and low yield ratio cannot be obtained. Therefore, the total area ratio of martensite and island martensite should be 3% or more, preferably 5% or more. On the other hand, if the area ratio of martensite and island martensite is too large, the ductility and toughness of the base material will deteriorate. Therefore, the total area ratio of martensite and island martensite should be 30% or less, preferably 20% or less.

[0048] The size of the martensite and island martensite is not particularly limited, but if the size is too small, the effects of high strength and low yield ratio cannot be obtained. For this reason, the size of the martensite and island martensite is preferably 1 μm or more in terms of average circle equivalent diameter, and more preferably 3 μm or more. On the other hand, if the size is too large, the ductility and toughness of the base material deteriorate. For this reason, the size of the martensite and island martensite is preferably 10 μm or less in terms of average circle equivalent diameter, and more preferably 8 μm or less.

[0049] The area ratio of martensite and island martensite can be determined by polishing the surface of the steel plate sample at a position 1 / 4 of the plate thickness, then etching it with an etching solution, taking a photograph with an optical microscope, and quantitatively evaluating it using an image analysis device. It is preferable to use an etching solution called "Repera reagent" as described in Non-Patent Literature 1.

[0050] [Tensile strength and yield ratio] The steel plate according to the present invention has a tensile strength of 590 MPa or more and 740 MPa or less, and a yield ratio of 80% or less. The tensile strength is set to 590 MPa or more, preferably 610 MPa or more, from the viewpoint of increasing strength in line with the increasing height of building structures. On the other hand, the upper limit of the tensile strength is not particularly limited, but may be 740 MPa or less, or 720 MPa or less. The yield ratio is set to 80% or less from the viewpoint of improving the deformation performance of building structures, taking into account the allowable margin against failure during earthquakes. The yield stress is not particularly limited, but is preferably 440 MPa or more. The tensile strength and yield stress can be measured, for example, by the method described in the examples, and the yield ratio (%) can be calculated as (yield stress / tensile strength) × 100. In the present invention, the tensile strength and yield ratio of the base material are sometimes collectively referred to as "mechanical strength".

[0051] [Charpy absorption energy] The steel sheet according to the present invention preferably has a Charpy impact energy (vE0) of 70 J or more at 0°C. The upper limit of the Charpy impact energy of the base material is not particularly limited. The Charpy impact energy can be measured, for example, by conducting a Charpy impact test in accordance with the provisions of Japanese Industrial Standard JIS Z 2242.

[0052] Furthermore, the steel plate according to the present invention preferably has a Charpy absorption energy (vE0) of 47 J or more at 0°C in the HAZ when a welded joint is formed. The upper limit of the Charpy absorption energy of the HAZ at 0°C is not particularly limited, but is generally 150 J or less. Test specimens used to measure the Charpy absorption energy of the HAZ can be prepared, for example, by the method described in the examples.

[0053] [plate thickness] The thickness of the steel plate according to the present invention is not particularly limited and can be any thickness. The thickness of the steel plate is preferably 6 mm or more, more preferably 12 mm or more, and even more preferably 19 mm or more. From the viewpoint of responding to the increasing height 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 steel plate is not particularly limited, but it is preferably 100 mm or less.

[0054] [Optional ingredients] In a preferred embodiment, the steel sheet according to the present invention may further contain, in addition to the above basic components, one or more selected from the group consisting of Mo, B, Cu, Ni, Cr, Nb, V, Mg, REM, and Zr.

[0055] Mo: 0.20% or less Mo is an element that contributes to increasing the strength of steel by forming precipitates such as Mo carbides. For this reason, it may be included as needed. However, if the Mo content is excessive, it will lead to an increase in raw material costs. Also, if the amount of Mo added increases, irregularities will occur on the cut surface when the steel plate is gas cut. For this reason, when Mo is included, the Mo content should be 0.20% or less, and more preferably 0.15% or less. There is no particular lower limit to the Mo content, but from the viewpoint of obtaining a sufficient effect of increasing strength, it is preferable that the Mo content be 0.01% or more.

[0056] B: 0.0020% or less B is an effective element that improves hardenability and thereby increases the strength of steel plates, even in trace amounts. For this reason, it may be included as needed. However, if the B content is excessive, weldability decreases. Therefore, when B is included, the B content should be 0.0020% or less, and more preferably 0.0017% or less. There is no particular lower limit to the B content, but from the viewpoint of improving hardenability, when B is included, it is preferable that the B content be 0.0003% or more.

[0057] Cu:1.00% or less Cu is an element that dissolves in steel to increase its strength. For this reason, it may be included as needed. However, if the Cu content is too high, it will reduce weldability. Therefore, when Cu is included, the Cu content should be 1.00% or less, and more preferably 0.80% or less. There is no particular lower limit to the Cu content, but in order to fully obtain the above effect, it is preferable that the Cu content be 0.01% or more.

[0058] Ni: 1.00% or less Ni is an element that contributes to increasing the strength of steel, improving low-temperature toughness, and effectively improving hot brittleness that occurs when Cu is included. For this reason, it may be included as needed. However, if the Ni content is too high, it will reduce weldability. Therefore, when Ni is included, the Ni content should be 1.00% or less, and more preferably 0.80% or less. There is no particular lower limit to the Ni content, but in order to fully obtain the above effects, it is preferable that the Ni content be 0.01% or more.

[0059] Cr:0.50% or less Cr is an element that contributes to increasing the strength of steel. For this reason, it may be included as needed. However, if the Cr content is excessive, weldability and toughness will decrease. For this reason, when Cr is included, the Cr content should be 0.50% or less, and more preferably 0.40% or less. There is no particular lower limit to the Cr content, but from the viewpoint of obtaining sufficient strength-improving effect from Cr, it is preferable that the Cr content be 0.01% or more.

[0060] Nb: 0.10% or less Nb is an element that increases the strength of steel through matrix strengthening and precipitation strengthening. For this reason, it may be included as needed. However, if the Nb content is excessive, the toughness will decrease. For this reason, when Nb is included, the Nb content should be 0.10% or less, and more preferably 0.08% or less. There is no particular lower limit to the Nb content, but from the viewpoint of obtaining sufficient strength-enhancing effect from Nb, it is preferable that the Nb content be 0.005% or more.

[0061] V: 0.10% or less V, like Nb, is an element that increases the strength of steel through matrix strengthening and precipitation strengthening. For this reason, it may be included as needed. However, if the V content is excessive, weldability and toughness will decrease. Therefore, when V is included, the V content should be 0.10% or less, and more preferably 0.08% or less. There is no particular lower limit to the V content, but from the viewpoint of obtaining sufficient strength-enhancing effect from V, it is preferable that the V content be 0.005% or more.

[0062] Mg: 0.0050% or less Mg, like Ca and REM described later, is an element that further improves the toughness of the matrix material by refining the crystal grains. For this reason, it may be included depending on the required toughness of the matrix material. However, if the Mg content is excessive, the additive effect will saturate, so if Mg is included, the Mg content should be 0.0050% or less, and more preferably 0.0040% or less. There is no particular lower limit to the Mg content, but from the viewpoint of obtaining sufficient toughness improvement effect from Mg, it is preferable that the Mg content be 0.0010% or more.

[0063] REM: 0.020% or less Rare earth metals (REMs), like calcium (Ca), are elements that further improve the toughness of the matrix material by refining the crystal grains. For this reason, they may be included depending on the desired toughness of the matrix material. However, if the REM content is excessive, the additive effect will saturate. Therefore, when REM is included, the REM content should be 0.020% or less, and more preferably 0.015% or less. There is no particular lower limit to the REM content, but from the viewpoint of obtaining sufficient toughness improvement effect from REM, it is preferable that the REM content be 0.002% or more.

[0064] Zr: 0.020% or less Zr, like Ca, REM, and Mg, is an element that further improves the toughness of the matrix material by refining the crystal grains. For this reason, it may be included depending on the required toughness of the matrix material. However, if the Zr content is excessive, the additive effect will saturate, so if Zr is included, the Zr content should be 0.020% or less, and more preferably 0.015% or less. On the other hand, there is no particular lower limit to the Zr content, but from the viewpoint of obtaining sufficient toughness improvement effect from Zr, it is preferable that the Zr content be 0.002% or more.

[0065] [Method of manufacturing steel plates] In another embodiment, the present invention relates to a method for manufacturing a steel sheet, comprising the steps of: heating a steel billet having the above-mentioned component composition to a temperature of 1000°C to 1200°C, then hot-rolling it in a temperature range of Ar3 or higher to obtain a steel sheet; then performing accelerated cooling in a temperature range from an accelerated cooling start temperature of Ar3 or higher to an accelerated cooling end temperature of 400°C to 600°C, such that the cooling rate of the steel sheet at the 1 / 4 thickness position is 3°C / s to 40°C / s; and then air-cooling the steel sheet. In this invention relating to a method for manufacturing a steel sheet, "temperature" means the temperature (°C) at the 1 / 4 thickness position unless otherwise specified.

[0066] In the present invention, steel having the above-mentioned component composition can be melted using a melting means such as a converter or electric furnace, and then formed into steel billets by conventional methods such as continuous casting or ingot formation / splitting. However, in the present invention, the method of melting steel and the method of manufacturing steel billets are not limited to the above-mentioned manufacturing methods. The shape of the steel billet is preferably a large, flat slab.

[0067] Heating temperature: 1000 or more and 1200℃ or less If the heating temperature of the steel billet is too low, the resistance to hot deformation will be high, making rolling difficult. Therefore, the heating temperature should be 1000°C or higher, preferably 1050°C or higher. On the other hand, if the heating temperature is too high, the initial microstructure during heating will coarseen, causing the base material microstructure to coarseen and degrading its toughness. Therefore, the heating temperature should be 1200°C or lower, preferably 1150°C or lower.

[0068] Hot rolling temperature range: Ar3 or higher By setting the hot rolling temperature range to above the Ar3 point, microstructure coarsening can be prevented, and the toughness and high strength of the base material can be ensured. Here, "setting the hot rolling temperature range to above the Ar3 point" means that the temperature at the end of hot rolling is above the Ar3 point. The temperature of the Ar3 point (°C) can be calculated from the following equation (4). Ar3=868-396C+25Si-68Mn-21Cu-36Ni-25Cr-30Mo...(4) However, in formula (4), C, Si, Mn, Cu, Ni, Cr, and Mo represent the content of each element, and if an element is not present, it is set to 0 (zero).

[0069] If the hot rolling temperature range is too low, ferrite will form during rolling, reducing the strength of the base material. Therefore, the hot rolling temperature range should be above the Ar3 point. There is no particular upper limit to the hot rolling temperature range, but if the rolling temperature is too high, the base material structure may coarseen, degrading its toughness. Therefore, it is preferable that the hot rolling temperature range be 950°C or lower.

[0070] Accelerated cooling start temperature: Temperature above the Ar3 point After hot rolling, accelerated cooling is initiated at an accelerated cooling start temperature of Ar3 or higher. Since the temperature at the end of hot rolling is above the Ar3 point, accelerated cooling is initiated before the temperature of the steel sheet after hot rolling drops below the Ar3 point. There is no particular limit to the cooling rate of the steel sheet from the end of hot rolling until the start of accelerated cooling.

[0071] Cooling rate: 3℃ / s or more and 40℃ / s or less In the accelerated cooling process, accelerated cooling is performed in a temperature range from the start temperature of accelerated cooling at the Ar3 point or higher to the end temperature of accelerated cooling between 400°C and 600°C, such that the cooling rate of the steel plate at the 1 / 4 thickness position is between 3°C / s and 40°C / s. If the cooling rate is too low, the microstructure after accelerated cooling will be mainly composed of ferrite, and the target tensile strength of 590 MPa or more cannot be satisfied. Therefore, the cooling rate should be 3°C / s or higher, preferably 5°C / s or higher. On the other hand, if the cooling rate is too high, the material becomes unstable within the steel plate, resulting in material variation. Therefore, the cooling rate should be 40°C / s or lower, preferably 35°C / s or lower.

[0072] To determine the cooling rate at the 1 / 4 thickness position during accelerated cooling, for example, a thermocouple can be placed at the 1 / 4 thickness position, the temperature at the 1 / 4 thickness position from the accelerated cooling start temperature to the accelerated cooling end temperature can be measured, and the average cooling rate can be calculated.

[0073] Accelerated cooling termination temperature: 400°C to 600°C If the accelerated cooling termination temperature is too low, it becomes difficult to maintain the normal shape of the steel plate due to cooling distortion, etc. Therefore, the accelerated cooling termination temperature should be 400°C or higher, preferably 420°C or higher. On the other hand, if the accelerated cooling start temperature is too high, it becomes difficult to ensure sufficient strength. Therefore, the accelerated cooling start temperature should be 600°C or lower, preferably 580°C or lower.

[0074] After accelerated cooling is complete, air cooling is performed. There are no particular limitations on the cooling rate during air cooling.

[0075] By subjecting a steel billet having the above-described component composition to hot rolling, accelerated cooling, and air cooling using the above-described manufacturing method, the microstructure and mechanical strength of the steel sheet according to the present invention can be achieved.

[0076] [Tempering process] In yet another embodiment, the present invention relates to a method for manufacturing a steel sheet, comprising the steps of: heating a steel billet having the above-mentioned component composition to a temperature of 1000°C to 1200°C, then hot-rolling it in a temperature range of Ar3 or higher to obtain a steel sheet; then performing accelerated cooling in a temperature range from an accelerated cooling start temperature of Ar3 or higher to an accelerated cooling start temperature of 50°C to 600°C, such that the cooling rate of the steel sheet at the 1 / 4 thickness position is 3°C / s to 40°C / s; then air-cooling the steel sheet; and then tempering the steel sheet at 550°C or lower. The differences from the steel sheet manufacturing method described above are, firstly, that the lower limit of the accelerated cooling end temperature is 50°C instead of 400°C, and secondly, that tempering is performed after air-cooling.

[0077] Accelerated cooling termination temperature: 50°C to 600°C In this embodiment, by setting the accelerated cooling termination temperature to less than 400°C, even if the shape of the steel plate deforms due to cooling strain, the shape of the steel plate can be corrected by the subsequent tempering treatment. Therefore, the accelerated cooling termination temperature can be extended to a lower temperature, specifically to 50°C or higher, preferably 100°C or higher. On the other hand, as with the case where tempering treatment is not performed, if the accelerated cooling termination temperature is excessively high, it becomes difficult to ensure strength. Therefore, the accelerated cooling termination temperature is set to 600°C or lower, preferably 580°C or lower.

[0078] Tempering process: Below 550℃ After accelerated cooling, the material is air-cooled and then tempered at a temperature of 550°C or lower. If the tempering temperature is too high, the strength of the base material will decrease. Therefore, the tempering temperature should be 550°C or lower, preferably 530°C or lower. There is no particular lower limit to the tempering temperature, but if the tempering temperature is too low, the effect of correcting the shape of the steel sheet by tempering will not be obtained. Therefore, it is preferable that the tempering temperature be 400°C or higher. [Examples]

[0079] Next, embodiments of the present invention will be described. However, the present invention is not limited to these embodiments.

[0080] [Example 1] Molten steel refined using a converter was poured into a ladle, and 20 types of slabs were produced by continuous casting, which did not contain B and whose component composition was adjusted to the values ​​shown in Table 1. The thickness of the slabs was 250 mm. Slabs of steel grades A1 to F1 shown in Table 1 are conforming materials in which the basic components and the three control indicators all conform to the numerical ranges specified in the present invention. Slabs of steel grades G1 to T1 are comparative materials in which either the basic components or one of the three control indicators falls outside the numerical ranges specified in the present invention. Table 1 also shows the Ar3 point temperature calculated using formula (4) from the content of each alloying element.

[0081] [Table 1]

[0082] Next, the obtained slabs were hot-rolled according to the heating temperature, thickness, and end temperature shown in Table 2 to obtain steel sheets. Then, the steel sheets were accelerated-cooled according to the accelerated-cooling start temperature, cooling rate, and accelerated-cooling end temperature shown in Table 2, and subsequently air-cooled to obtain steel sheet samples. Furthermore, some of the samples were tempered at the tempering temperatures shown in Table 2. Steel sheets No. 1 to 12 shown in Table 2 are examples of the invention in which the manufacturing conditions all satisfy the conditions specified in the present invention. Steel sheets No. 13 to 26 are comparative examples in which the component composition falls outside the numerical range specified in the present invention. Steel sheets No. 27 to 31 are comparative examples in which one or more of the manufacturing conditions falls outside the numerical range specified in the present invention.

[0083] [Table 2]

[0084] Next, the microstructure of the obtained steel sheet was photographed at a position 1 / 4 of the sheet thickness using an optical microscope, and the area ratios of bainite, martensite, island martensite, and ferrite were evaluated using an image analysis device.

[0085] Next, tensile tests were conducted on the obtained steel plates in accordance with the provisions of the Japanese Industrial Standard JIS Z 2201 to determine the yield strength, tensile strength, and yield ratio. For steel plates with a thickness of 19 mm, JIS No. 5 tensile test specimens were taken from the entire thickness. For steel plates with thicknesses of 40 mm and 60 mm, JIS No. 4 tensile test specimens were taken from the 1 / 4 position of the plate thickness.

[0086] Furthermore, the obtained steel plates were subjected to Charpy impact tests in accordance with the provisions of the Japanese Industrial Standard JIS Z 2242, and the Charpy impact energy absorbed at a test temperature of 0°C was measured. During this process, V-notch Charpy impact test specimens were taken from the 1 / 2 thickness position of the plate. In the Charpy impact test, measurements were taken from three test specimens taken from the same type of steel plate, and the average value was calculated as the measured value.

[0087] Next, using the obtained steel plate, a groove shape 3 consisting of a skin plate 1, a diaphragm 2, and a backing plate 4 as shown in Figure 1 was constructed, and electroslag welding with a heat input of 400 kJ / cm was performed to produce a welded joint 5 as shown in Figure 2. Next, a Charpy impact test was performed on the obtained welded joint 5 in accordance with the provisions of Japanese Industrial Standard JIS Z 2242, and the Charpy absorbed energy at a test temperature of 0°C was measured. At that time, a V-notch Charpy impact test piece 8 was taken so that the intersection point of the surface of the skin plate 1, which is 6 mm deep from the surface, and the interface between the weld metal 6 and the heat-affected zone (HAZ) 7 was at the position of the notch 9. In the Charpy impact test of the welded joint, measurements were also taken on three test pieces taken from the same type of welded joint, and the average value was taken as the measured value. Note that the toughness of the welded joint was not evaluated for comparative examples of steel plates No. 27 to 31, whose manufacturing conditions did not meet any of the conditions specified in the present invention. The above evaluation results are summarized in Table 3.

[0088] [Table 3]

[0089] As shown in Table 3, the inventive examples of steel plates No. 1 to 12 satisfied the microstructure defined in the present invention. Furthermore, the tensile strength of the base material was between 590 MPa and 740 MPa, and the yield ratio was 80% or less, thus satisfying the mechanical strength defined in the present invention. On the other hand, in the comparative examples of steel plates No. 13 to 31, in which the basic components, control indicators, or manufacturing conditions fell outside the numerical range defined in the present invention, the tensile strength of the base material either did not satisfy the numerical range defined in the present invention, or the Charpy absorption energy of the HAZ was below the preferred upper limit of 47 J.

[0090] [Example 2] Twenty-one types of slabs were manufactured using the same method as in Example 1, except that the component composition included B, with the component composition adjusted to the values ​​shown in Table 4. The steel materials from steel grades A2 to F2 shown in Table 4 are conforming materials in which the basic components and the three control indicators all conform to the numerical ranges specified in the present invention. The steel materials from steel grades G2 to U2 are comparative materials in which either the basic components or one of the three control indicators falls outside the numerical ranges specified in the present invention. Table 4 also shows the Ar3 point temperature calculated using formula (4) from the content of each alloying element.

[0091] [Table 4]

[0092] Next, the obtained slabs were hot-rolled according to the heating temperature, thickness, and end temperature shown in Table 5 to obtain steel sheets. Then, accelerated cooling was performed according to the accelerated cooling start temperature, cooling rate, and accelerated cooling end temperature shown in Table 5, followed by air cooling to obtain steel sheet samples. Furthermore, some of the samples were subjected to tempering treatment at the tempering temperatures shown in Table 5. Steel sheets No. 32 to 46 shown in Table 5 are examples of the invention in which the manufacturing conditions all satisfy the conditions specified in the present invention. Steel sheets No. 47 to 61 are comparative examples in which the component composition falls outside the numerical range specified in the present invention. Steel sheets No. 62 to 66 are comparative examples in which one or more of the manufacturing conditions falls outside the numerical range specified in the present invention.

[0093] [Table 5]

[0094] Next, the microstructure and mechanical strength of the obtained steel plates were evaluated using the same method as in Example 1. For steel plates with a thickness of 100 mm, JIS No. 4 tensile test specimens were taken from the entire thickness. These evaluation results are summarized in Table 6.

[0095] [Table 6]

[0096] As shown in Table 6, the inventive examples of steel plates No. 32 to 46 satisfied the microstructure defined in the present invention. Furthermore, the tensile strength of the base material was between 590 MPa and 740 MPa, and the yield ratio was 80% or less, thus satisfying the mechanical strength defined in the present invention. On the other hand, in the comparative examples of steel plates No. 47 to 66, in which the basic components, control indicators, or manufacturing conditions fell outside the numerical range defined in the present invention, the tensile strength of the base material either did not satisfy the numerical range defined in the present invention, or the Charpy absorption energy of the HAZ was below the preferred upper limit of 47 J. [Explanation of Symbols]

[0097] 1 Skin Plate 2 diaphragms 3 Bevel shape 4. Receipt 5. Welded joints 6. Weld metal 7 Heat affected zone (HAZ) 8 Charpy impact test specimens 9 Notches

Claims

1. In mass percent, C: 0.040% or more and 0.100% or less, Si: 0.01% or more and 0.25% or less, Mn: 1.50% or more and 2.50% or less, P: 0.020% or less, S: 0.0005% or more and 0.0030% or less, Al: 0.002% or more and 0.080% or less, Ti: 0.003% or more and 0.030% or less, N: 0.0060% or less, Ca: 0.0005% or more and 0.0040% or less, O: 0.0010% or more and 0.0040% or less It contains, with the remainder consisting of Fe and unavoidable impurities. The carbon equivalent Ceq, as defined by the following formula (1), is 0.40% or more and 0.46% or less. The atomic concentration ratio ACR, as defined by equation (2) below, is 0.20 or more and 0.80 or less, The composition of the manganese-molybdenum factor (MMF), as defined by equation (3) below, is 1.70% or more and 2.50% or less. At the 1 / 4 thickness position, the total area ratio of martensite and island martensite is 3% or more and 30% or less, with the remainder being bainite, and the microstructure is as follows. The tensile strength is 590 MPa or more and 740 MPa or less, and Mechanical strength with a yield ratio of 80% or less, A steel plate having [a certain characteristic]. Ceq=C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 14...(1) ACR={Ca-(0.18+130×Ca)×O} / (1.25×S)...(2) MMF=Mn+2Mo...(3) Here, in equations (1) to (3), C, Si, Mn, Ni, Cr, Mo, V, Ca, O, and S represent the content of each element in mass percent, and the content of an element that is not present is considered to be 0 (zero).

2. The above component composition is, in mass%, Mo: 0.20% or less B: 0.0020% or less, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.50% or less, Nb: 0.10% or less, V: 0.10% or less, Mg: 0.0050% or less, REM: 0.020% or less, Zr: 0.020% or less It further contains one or more selected from the group consisting of, The steel plate according to claim 1.

3. A steel billet having the component composition described in claim 1 or 2 is heated to a temperature of 1000°C to 1200°C, and then hot-rolled in a temperature range of Ar3 or higher to obtain a steel sheet. Subsequently, the process involves performing accelerated cooling in a temperature range from an accelerated cooling start temperature of Ar3 or higher to an accelerated cooling end temperature of 400°C to 600°C, such that the cooling rate of the steel plate at the 1 / 4 thickness position is 3°C / s or more and 40°C / s or less. Subsequently, the steel plate is air-cooled, A method for manufacturing steel plates, comprising:

4. A steel billet having the component composition described in claim 1 or 2 is heated to a temperature of 1000°C to 1200°C, and then hot-rolled in a temperature range of Ar3 or higher to obtain a steel sheet. Subsequently, the process involves performing accelerated cooling in a temperature range from an accelerated cooling start temperature of Ar3 or higher to an accelerated cooling end temperature of 50°C to 600°C, such that the cooling rate of the steel plate at the 1 / 4 thickness position is 3°C / s or more and 40°C / s or less. Subsequently, the steel plate is air-cooled, Subsequently, the steel plate is subjected to a tempering treatment at a temperature of 550°C or lower. A method for manufacturing steel plates, comprising:

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