KR20240059623A - Steel plate and its manufacturing method - Google Patents

Abstract

The purpose is to propose a steel plate with excellent joint toughness, excellent base material toughness, and high tensile strength even at low temperatures of -60°C or lower in a high-heat input welding heat-affected zone where the welding heat input is 50kJ/cm or more, and a manufacturing method thereof. do. A steel sheet that contains specific components to satisfy the following formulas (1) and (2), has Nb: 0.003% or less, and has a specific structure and specific characteristics.
0＜(Ca－(0.18＋130×Ca)×O)/1.25/S＜1···(1)
Ceq.≤0.36···(2)
However, Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and in equations (1) and (2), each element symbol represents the content (% by mass) of each element.

Description

Steel plate and its manufacturing method

The present invention relates to steel materials used in various steel structures in the fields of ships, architecture, civil engineering, etc., and has excellent weld zone toughness even when high-heat input welding is performed, especially when the welding heat input exceeds 200 kJ/cm. It relates to high-strength steel and its manufacturing method.

With the increase in strength and thickness of steel materials, there is an increasing demand for the application of high heat input welding with excellent production efficiency, such as submerged arc welding, electro gas welding, and electro slag welding, in welding construction. Since the toughness of the heat-affected zone (Heat-Affected Zone, also referred to as HAZ) welded with high heat input decreases, various steels for high heat input welding have been proposed. For example, a technology is being put into practical use by finely dispersing TiN in steel to suppress coarsening of austenite grains in the weld heat-affected zone, or to use it as a ferrite transformation nucleus in the weld heat-affected zone.

Suppression of tissue coarsening using TiN is economically useful and is widely used. However, these effects cannot be obtained in a high temperature range where TiN is dissolved in the weld heat-affected zone, and in addition, the base metal structure is formed of dissolved Ti and There was a problem that toughness was significantly reduced due to embrittlement due to dissolved N.

Therefore, in Patent Document 1, among Ti oxides that are difficult to dissolve even in the high temperature range of the weld heat-affected zone, TiOx with a particle size of 5 μm or less (however, x: 0.65 to 1.3) is finely dispersed in steel, and is used in the weld heat-affected zone. A technology using it as a production nucleus for needle-shaped ferrite has been proposed. Patent Document 2 proposes a technique for improving the toughness of the weld heat-affected zone by adjusting the amounts of B, N, and sol.Al in the steel composition to actively precipitate BN, which refines the weld heat-affected zone. In addition, in Patent Document 3, the amount of Ti-B-N in the steel composition is adjusted so that the HAZ toughness is in the high toughness region, and Ca or Ce is added to provide a toughness improvement effect by controlling the shape of inclusions. In addition, in Patent Document 4, a technology for improving the toughness of a high heat input weld zone is also proposed by setting the steel composition to a low N-low Ti system and adding REM, which forms stable sulfur oxides even in the bond portion of the weld. In addition, in Patent Document 5, Ca-based inclusions that become transformation nuclei and promote ferrite transformation in the weld heat-affected zone are finely dispersed in steel by appropriately controlling the Ca, O, and S contents, resulting in a large heat input exceeding 400 kJ/cm. A technology to improve the toughness of the heat-affected zone of a weld has been proposed.

Japanese Patent Publication No. 57-51243 Japanese Patent Publication No. 62-170459 Japanese Patent Publication No. 60-204863 Japanese Patent Publication No. 4-14180 Japanese Patent No. 3546308 Publication

The technology described in Patent Documents 1 and 2 has the problem that stable manufacturing is difficult in industrial production. In addition, in the techniques described in Patent Documents 3 to 5, it is not possible to stably secure high shock absorption performance such as exceeding 100 J in a high heat input joint toughness test at low temperatures such as -60°C or lower. It is a difficult task.

Therefore, the present invention provides a steel sheet with excellent joint toughness, base material toughness, and high tensile strength even at low temperatures of -60°C or lower in a high-heat input welding heat-affected zone where the welding heat input is 50 kJ/cm or more, and a method for manufacturing the same. The purpose is to suggest

Excellent joint toughness means that a test piece measuring 80 mm wide The 2 mmV notched Charpy test pieces taken from these test pieces were subjected to a Charpy impact test at a test temperature of -60°C to evaluate the toughness. As a result, the average shock absorption value of the three test results exceeded 100 J, and the lowest value was This refers to exceeding 50J. In addition, excellent base material toughness is indicated by collecting a V-notch Charpy impact test piece as described in JISZ2202 from a position of 1/4 of the plate thickness, and performing a Charpy impact test appropriately at a test temperature in the range of -120 to 40°C to determine the ductile fracture surface. The fracture surface transition temperature vTrs(℃) at which the rate is 50% is determined and the base metal toughness is evaluated, indicating that vTrs(℃) is -60℃ or lower. In addition, high tensile strength means that a No. 1A test piece described in JISZ2201 is taken from a steel plate so that the length direction of the test piece coincides with the sheet width direction, the tensile strength: TS (MPa) is measured, and TS is 450 MPa or more.

In order to solve the above problems, the inventors conducted various studies and obtained the following knowledge.

In other words, in order to suppress the coarsening of the structure in the weld heat-affected zone using TiN precipitates with excellent industrial productivity, research was conducted to suppress the formation of the coarse structure in addition to maximizing the use of TiN precipitates in the base steel sheet. is needed. To achieve this, by adding more than a certain amount of B in addition to Ti and N, keeping Ceq. below a certain level, and adding as little Nb as possible, even in high-heat input welded joints with a welding heat input of 50 kJ/cm or more, welding joints can be kept at temperatures below -60°C. It was discovered that joint toughness could be reliably secured. On the other hand, it was found that in the composition system with low alloy and B addition, partial coarse structure is generated in the base material structure due to suppression of nucleation by solid solution B, making securing base material toughness an issue. Therefore, the inventors conducted additional studies and derived manufacturing conditions that achieve both excellent base material toughness and high tensile strength while stably securing joint toughness below -60°C.

The present invention was completed through further examination based on the knowledge obtained above, that is, the present invention is:

[1] Steel composition, in mass%,

C: 0.01 to 0.07%,

Si: 0.01 to 0.20%,

Mn: 0.80 to 1.80%,

P: 0.020% or less,

S: 0.0005 to 0.0050%,

Al: 0.030 to 0.100%,

Ti: 0.005 to 0.030%,

N: 0.0045 to 0.0090%,

B: 0.0010 to 0.0030%,

Ca: 0.0005 to 0.0030%,

O: 0.0040% or less,

Nb: 0.003% or less, and contained so as to satisfy the following formulas (1) and (2),

The balance is Fe and inevitable impurities,

A steel plate that has a microstructure in which grains with an average local orientation difference exceeding 1° are 50% or less as an area fraction of all grains, and an average grain size is 50 μm or less, and a tensile strength of 450 MPa or more.

0＜(Ca－(0.18＋130×Ca)×O)/1.25/S＜1···(1)

Ceq.≤0.36···(2)

However, Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and in equations (1) and (2), each element symbol represents the content (% by mass) of each element. Elements not contained are 0.

[2] The steel composition is further expressed in mass%,

Cu: 0.50% or less,

Ni: 1.00% or less,

Cr: 0.50% or less,

Mo: 0.30% or less,

V: The steel sheet according to [1] containing at least one selected from among 0.50% or less.

[3] The steel composition is further expressed in mass%,

Mg: 0.0050% or less,

Zr: 0.0200% or less,

REM: The steel sheet according to [1] or [2] containing at least one selected from among 0.0200% or less.

[4] Steel composition, in mass%,

C: 0.01 to 0.07%,

Si: 0.01 to 0.20%,

Mn: 0.80 to 1.80%,

P: 0.020% or less,

S: 0.0005 to 0.0050%,

Al: 0.030 to 0.100%,

Ti: 0.005 to 0.030%,

N: 0.0045 to 0.0090%,

B: 0.0010 to 0.0030%,

Ca: 0.0005 to 0.0030%,

O: 0.0040% or less,

Nb: 0.003% or less, and contained so as to satisfy the following formulas (1) and (2),

A steel material with the balance consisting of Fe and inevitable impurities,

After heating to 1000°C to 1200°C, hot rolling is performed, the rolling end temperature in the hot rolling is 800°C or higher, air cooling is performed for 10 seconds or more after the hot rolling, and the average cooling rate after air cooling is: 30°C/ A method of manufacturing steel sheets that is cooled to a temperature range of 450°C or less for more than 2 seconds.

0＜(Ca－(0.18＋130×Ca)×O)/1.25/S＜1···(1)

Ceq.≤0.36···(2)

However, Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and in equations (1) and (2), each element symbol represents the content (% by mass) of each element. Elements not contained are 0.

[5] Additionally, the steel material is expressed in mass%,

Cu: 0.50% or less,

Ni: 1.00% or less,

Cr: 0.50% or less,

Mo: 0.30% or less,

The method for producing a steel sheet according to [4], containing one or more types selected from V: 0.50% or less.

[6] Additionally, the steel material is expressed in mass%,

Mg: 0.0050% or less,

Zr: 0.0200% or less,

REM: The method for producing a steel sheet according to [4] or [5], containing at least one selected from among 0.0200% or less.

According to the present invention, a steel sheet having excellent joint toughness at a low temperature of -60°C or lower in a high-heat input welding heat-affected zone where the welding heat input is 50 kJ/cm or more, excellent base material toughness, and high tensile strength is obtained, It is very useful in industry.

(Form for carrying out the invention)

Hereinafter, embodiments of the present invention will be described in detail. First, the chemical composition that the steel sheet and steel material of the present invention should have will be described. In the explanation, all % indications regarding chemical components mean mass %.

C: 0.01 to 0.07%

C is an element that increases the strength of steel materials, and in order to secure the strength required as structural steel, C needs to be contained at 0.01% or more. The C content is preferably 0.02% or more. The C content is more preferably 0.03% or more. On the other hand, if the C content exceeds 0.07%, a low-toughness coarse structure such as upper bainite is likely to be generated in the heat-affected zone of welding at low temperatures such as -60°C, so the C content should be 0.07% or less. do. The C content is preferably 0.06% or less. The C content is more preferably 0.05% or less.

Si: 0.01 to 0.20%

Si is an element added as a deoxidizing agent when melting steel, and Si must be contained in an amount of 0.01% or more. The Si content is preferably 0.02% or more. The Si content is more preferably 0.05% or more. However, if the Si content exceeds 0.20%, the toughness of the base material and joint may decrease due to precipitation of carbides. Therefore, the Si content is set to 0.20% or less. The Si content is preferably 0.15% or less. The Si content is more preferably 0.12% or less. The Si content is more preferably 0.10% or less.

Mn: 0.80∼1.80%

In order to ensure the strength of the base material, Mn needs to be contained in an amount of 0.80% or more. The Mn content is preferably 1.00% or more. The Mn content is more preferably 1.20% or more. The Mn content is more preferably 1.40% or more. On the other hand, if the Mn content exceeds 1.80%, the toughness of the weld heat-affected zone significantly deteriorates, so the Mn content is set to 1.80% or less. The Mn content is preferably 1.70% or less. The Mn content is more preferably 1.60% or less.

P: 0.020% or less

P promotes the formation of MA (Martensite-Austenite Constituent or island martensite) in the HAZ near the bond portion and significantly reduces toughness, so the P content needs to be 0.020% or less. The P content is preferably 0.015% or less, and more preferably 0.012% or less. The lower limit of P is not particularly limited, but excessive reduction causes a sharp increase in refining costs, so the P content is preferably 0.005% or more.

S: 0.0005 to 0.0050%

S is an element necessary to form MnS or CaS, which acts as a nucleation site for ferrite. For this reason, it is necessary to contain S in an amount of 0.0005% or more. The S content is preferably 0.0008% or more. The S content is more preferably 0.0010% or more. However, if it is contained excessively, it will cause a decrease in the toughness of the base material, so the S content is set to 0.0050% or less. The S content is preferably 0.0040% or less. The S content is more preferably 0.0020% or less.

Al: 0.030∼0.100%

Al is an element added to deoxidize steel, and Al needs to be contained at 0.030% or more. The Al content is preferably 0.035% or more. However, if Al is contained in excess of 0.100%, not only the toughness of the base metal but also the toughness of the weld metal decreases. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.090% or less. The Al content is more preferably 0.080% or less. The Al content is more preferably 0.070% or less.

Ti: 0.005 to 0.030%

Ti precipitates in the base material as TiN when the molten steel solidifies, and contributes to improving base material toughness by suppressing coarsening of austenite grains. Additionally, during welding, TiN suppresses the coarsening of the structure in the weld heat-affected zone and becomes a transformation nucleus of ferrite, contributing to increased toughness. In order to obtain this effect, Ti must be contained in an amount of 0.005% or more. The Ti content is preferably 0.008% or more. The Ti content is more preferably 0.010% or more. On the other hand, if Ti is contained in excess of 0.030%, the precipitated TiN becomes excessively coarse and the above effect is not obtained. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably 0.025% or less. The Ti content is more preferably less than 0.025%.

N: 0.0045 to 0.0090%

The N content is set to 0.0045% or more in order to suppress the coarsening of the structure in the weld heat-affected zone during welding and to generate TiN, which becomes a transformation nucleus of ferrite and contributes to increased toughness. The N content is preferably 0.0050% or more. On the other hand, if it exceeds 0.0090%, there is concern that when TiN is dissolved by the welding heat cycle, dissolved N in the raw tissue becomes excessive and deteriorates HAZ toughness. From the above, the N content is set to 0.0090% or less. The N content is preferably 0.0080% or less. The N content is preferably 0.0075% or less.

B: 0.0010 to 0.0030%

B generates BN in the weld heat-affected zone, reduces dissolved N, and also becomes a ferrite transformation nucleus to generate ferrite to improve toughness. In order to stably obtain this effect, B must be contained in an amount of 0.0010% or more. The B content is preferably 0.0012% or more. The B content is more preferably 0.0015% or more. However, if B is contained in excess of 0.0030%, it causes a decrease in the toughness of the base material and HAZ. For this reason, the B content is set to 0.0030% or less. The B content is preferably 0.0028% or less. The B content is more preferably 0.0025% or less.

Ca: 0.0005 to 0.0030%

Since Ca fixes S and improves toughness, it is set at 0.0005% or more to obtain the effect. The Ca content is preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.0030%, the effect is saturated, so the Ca content is set to 0.0030% or less. The Ca content is preferably 0.0025% or less.

O: 0.0040% or less

Since O indirectly affects the formation of complex sulfides in which MnS precipitates on CaS, the O content is set to 0.0040% or less, preferably 0.0030% or less. The lower limit of O is not particularly limited, but excessive reduction causes an increase in refining costs, so the O content is preferably 0.0010% or more.

Nb: 0.003% or less

Nb is an effective element in securing the strength of the base material, but in high-heat input welded joints, it lowers the melting point of TiN and contributes to the creation of a low-toughness structure such as upper bainite. Since the above appears significantly when the Nb content exceeds 0.003%, from the viewpoint of ensuring joint toughness at low temperatures, it is preferable not to contain Nb, or when it is contained, Nb is set to 0.003% or less. For the reason that it improves the tensile strength of the base material, when containing Nb, it is preferable that the Nb content is 0.001% or more. It is more preferable that the Nb content is 0.002% or more.

0＜(Ca－(0.18＋130×Ca)×O)/1.25/S＜1···(1)

Here, Ca, O, and S are the content of each component (mass%)

This parameter equation improves the toughness of the weld heat-affected zone when steel with the above composition range is welded with high heat input. If the contents of Ca, O, and S are specified to satisfy this equation, MnS on CaS The precipitated composite grains are generated and finely dispersed to improve the toughness of the weld heat-affected zone.

When the value of (Ca - (0.18 + 130 It elongates and reduces the toughness of the base material. Additionally, because MnS melts in the weld heat-affected zone, excellent toughness cannot be obtained. Therefore, the value of A is set to exceed 0. Preferably, the A value is 0.1 or more. More preferably, the A value is 0.2 or more.

On the other hand, when the A value is 1 or more, S is almost fixed by Ca, and MnS, which becomes a ferrite formation nucleus, does not precipitate on CaS, so ferrite is not formed in the weld heat-affected zone, and the toughness improvement effect is not obtained. No. Therefore, the value of A is set to less than 1. Preferably, the A value is 0.9 or less. More preferably, the A value is 0.8 or less.

Ceq.≤0.36···(2)

However, Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and each element symbol represents the content (% by mass) of each element. Elements not contained are 0.

This parameter equation serves as an index for ensuring the low-temperature toughness of the joint when high-heat input welding of steel with the above composition range. By satisfying the above equation in addition to the containing range of each element, a good joint can be obtained even at -60°C or lower. Humanity can be achieved. If Ceq. exceeds 0.36, low-toughness structures such as upper bainite begin to be partially generated, making it difficult to achieve stable joint toughness. Therefore, Ceq. is set to 0.36 or less. In order to secure the yield stress and tensile strength of the base material, Ceq. is preferably set to 0.30 or more.

The above is the basic component composition of the present invention, with the balance being Fe and inevitable impurities. In addition to the above components, the steel sheet and steel material of the present invention may contain at least one selected from among Cu, Ni, Cr, Mo, and V as an optional element in the following range for the purpose of improving strength, etc. You can.

Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.30% or less, V: 0.50% or less

Cu: 0.50% or less

Cu is an element effective in increasing the strength of steel sheets, but if it is contained in excess, there are concerns that it may promote cracking of cast steel ingots and reduce the toughness of steel sheets. Therefore, when it contains Cu, the Cu content is set to 0.50% or less. Additionally, if the Cu content is less than 0.01%, the effect of high strength cannot be obtained, so it is preferable that the Cu content is 0.01% or more. More preferably, the Cu content is 0.10% or more. More preferably, the Cu content is 0.20% or more.

Ni: 1.00% or less

Ni improves the toughness of the steel sheet and also increases its strength, but excessive inclusion reduces the toughness and also puts pressure on manufacturing costs. Therefore, when containing Ni, the Ni content is set to 1.00% or less. Preferably, the Ni content is 0.80% or less. More preferably, the Ni content is 0.50% or less. Additionally, if the Ni content is less than 0.01%, the effect of high strength cannot be obtained, so it is preferable that the Ni content is 0.01% or more. More preferably, the Ni content is 0.10% or more. More preferably, the Ni content is 0.20% or more.

Cr: 0.50% or less

Cr is an element advantageous for increasing the strength of steel sheets, but its excessive inclusion reduces toughness. Therefore, when Cr is contained, the Cr content is set to 0.50% or less. Preferably, the Cr content is 0.40% or less. More preferably, the Cr content is 0.30% or less. Additionally, if the Cr content is less than 0.01%, the effect of increasing strength cannot be obtained, so it is preferable that the Cr content is 0.01% or more. More preferably, the Cr content is 0.10% or more. More preferably, the Cr content is 0.20% or more.

Mo: 0.30% or less

Mo is an element advantageous for increasing the strength of steel sheets, but its excessive inclusion reduces toughness. Therefore, when Mo is contained, the Mo content is set to 0.30% or less. Preferably, the Mo content is 0.20% or less. Additionally, if the Mo content is less than 0.002%, the effect of high strength cannot be obtained, so it is preferable that the Mo content is 0.002% or more. It is more preferable that the Mo content is 0.01% or more.

V: 0.50% or less

V is an element advantageous for increasing the strength of steel sheets, but excessive inclusion reduces toughness. Therefore, when it contains V, V is set to 0.50% or less. Preferably, the V content is 0.30% or less. Additionally, if the V content is less than 0.002%, the effect of increasing strength cannot be obtained, so it is preferable that the V content is 0.002% or more. It is more preferable that the V content is 0.01% or more.

In addition to the above components, the steel sheet and steel material of the present invention may further contain at least one selected from Mg, Zr, and REM as an optional element in the following range for the purpose of improving toughness.

At least one selected from Mg: 0.0050% or less, Zr: 0.0200% or less, REM: 0.0200% or less

Mg, Zr, and REM are all elements that have the effect of improving toughness by dispersing oxides. In order to achieve this effect, when Mg is contained, it is preferable to contain Mg in an amount of 0.0005% or more. When containing Zr, it is preferable to contain 0.0010% or more of Zr. When containing REM, it is preferable to contain 0.0010% or more of REM. On the other hand, even if Mg contains more than 0.0050% and Zr and REM each contain more than 0.0200%, the effect is only saturated. Therefore, when it contains Mg, the Mg content is set to 0.0050% or less. When containing Zr, the Zr content is set to 0.0200% or less. When containing REM, the REM content is set to 0.0200% or less.

The microstructure of the present invention will be described.

The area fraction of grains with an average local orientation difference exceeding 1° is 50% or less.

The local orientation difference average is a value obtained by averaging the orientation differences within the same crystal grain, which can be measured using EBSD (Electron BackScatter Diffraction), as will be described later. As the number of grains with an average local orientation difference exceeding 1° increases, the ductility and low-temperature toughness of the steel decrease. In order to ensure good base material toughness, grains with an average local orientation difference exceeding 1° are limited to 50% or less as an area fraction for all grains. Preferably, the area fraction of grains with an average local orientation difference exceeding 1° is 45% or less with respect to all grains. Since the lower limit can increase the yield stress and tensile strength of the base material, the grains with an average local orientation difference exceeding 1° are preferably 10% or more, more preferably 20% or more, in terms of area fraction, with respect to all grains, 30% or more is more preferable.

Average crystal grain size is 50㎛ or less

If the crystal grains become coarse, the toughness of the base material decreases, so the average grain size is set to 50 μm or less. Preferably, the average crystal grain size is 40 μm or less. More preferably, the average crystal grain size is 35 μm or less.

The lower limit is for the reason that if the crystal grains become excessively fine, the strength of the base material becomes excessively high and the toughness actually decreases. Therefore, the average grain size is preferably 10 ㎛ or more, more preferably 15 ㎛ or more, and even more 20 ㎛ or more. desirable.

The manufacturing method of the present invention will be described. The molten steel of the above composition is melted by a conventional method such as a converter, electric furnace, or vacuum melting furnace, used as a rolling material such as a slab by a conventional casting method such as a continuous casting method or an ingot method, and then heated and hot rolled. Then cool. In addition, descriptions of the steel sheet temperature in explaining the manufacturing process all refer to the temperature of the steel sheet surface.

The microstructure is a microstructure mainly composed of ferrite and bainite, and it is desirable that the total of ferrite and bainite be 90% or more in terms of area fraction. The upper limit may be 100%.

Tensile strength is over 450 MPa

From the viewpoint of securing load capacity by reducing the weight of the hull by using high strength, the tensile strength of the steel plate is set to 450 MPa or more. On the other hand, if the tensile strength is too high, it may become a problem in terms of bending processing precision and reduced elongation, so it is preferable to set it to 650 MPa or less.

Slab heating temperature: 1000℃∼1200℃

It is heated to 1000°C or higher from the viewpoint of homogenizing the structure before transformation and reducing the equipment load during rolling processing. More preferably, the heating temperature is 1030°C or higher. More preferably, the heating temperature is 1050°C or higher. On the other hand, heating to a temperature exceeding 1200°C causes deterioration of toughness due to coarsening of the structure and reduces manufacturing efficiency. For the above reasons, the heating temperature is set to 1200°C or lower. More preferably, the heating temperature is 1150°C or lower. More preferably, the heating temperature is 1100°C or lower.

The rolling end temperature in hot rolling is 800℃ or higher.

Hot rolling and subsequent cooling are performed to improve the toughness of the base material by refining the microstructure of the steel sheet and to secure a certain level of strength required for the structure. Normally, in order to secure the toughness of the base material, low-temperature rolling, which is the basic principle of the Thermo-Mechanical Control Process (hereinafter referred to as TMCP), is performed. However, in the composition system in this application, low Ceq. The ferrite main structure generated as a result of design becomes partially coarse due to the suppression of nucleation sites by solid solution B, making it difficult to secure the low-temperature toughness of the base material below -60°C. Here, in the present application, BN precipitation is promoted by setting an air cooling time of 10 seconds or more after the end of finish rolling, and nucleation is generated from this as a starting point, thereby creating a homogeneous and fine ferrite structure.

If the rolling end temperature is lower than 800°C, the temperature of the steel sheet decreases during the air cooling period, transformation begins before sufficient BN precipitation occurs, and sufficient effects cannot be obtained. Therefore, the rolling end temperature is 800°C or higher. More preferably, the rolling end temperature is 820°C or higher. More preferably, the rolling end temperature is 850°C or higher. Since the upper limit imparts processing distortion that leads to the formation of ferrite nuclei to the material, the rolling end temperature is preferably 900°C or lower, and it is more preferable that the rolling end temperature is 880°C or lower.

After hot rolling, air cooling for more than 10 seconds, and average cooling rate after air cooling: 30℃/sec or more, cooling to a temperature range of 450℃ or less.

Likewise, when the air cooling time is less than 10 seconds, BN precipitation becomes insufficient and the base material toughness becomes unstable. Therefore, after hot rolling, the air cooling time is 10 seconds or more. The air cooling time after hot rolling is preferably 12 seconds or more. Since prolonged air cooling does not significantly increase BN precipitation and actually causes a decrease in production efficiency, it is preferable that the air cooling time after hot rolling is 30 seconds or less. It is preferable that the air cooling time is 20 seconds or less. The average cooling rate in air cooling is usually in the range of 5°C/sec or less. In addition, this patent also includes cases where the temperature is lowered to a temperature range of 450°C or lower by air cooling, then reheated, and cooled again to a temperature range of 450°C or lower at an average cooling rate of 30°C/sec or more, but the number of steps increases. Because this reduces production efficiency, it is basically desirable that the temperature reached for air cooling is 750°C or higher. The upper limit temperature to reach air cooling is preferably 800°C or lower.

Regarding cooling after air cooling, in order to obtain sufficient strength, for example, tensile strength of TS450 MPa or more, even in the ferrite-based structure, the sheet is cooled at an average cooling rate of 30°C/sec or more in the center of the sheet thickness. Accordingly, the ferrite structure becomes finer and the hardness of the second phase structure increases, allowing sufficient strength to be obtained. When the average cooling rate is less than 30°C/sec, the strength of the base material decreases. In addition, even in cases where the cooling stop temperature exceeds 450°C, the transformation structure is refined and the second phase structure is insufficiently hardened, making it impossible to obtain the desired steel strength. Therefore, after air cooling, it is cooled to a temperature range of 450°C or lower at an average cooling rate of 30°C/sec or more. The average cooling rate after air cooling is preferably 40°C/sec or more, and more preferably 45°C/sec or more. For the reason of preventing the formation of a martensite structure with low toughness, it is preferable to cool to a temperature range of 450°C or less with an average cooling rate of 150°C/sec or less after air cooling. The average cooling rate after air cooling is more preferably 100°C/sec or less, and even more preferably 80°C/sec or less.

As described above, using steel of the above-mentioned composition, hot rolling is performed at a slab heating temperature of 1000 to 1200°C, the rolling end temperature in the hot rolling is set to 800°C or higher, and after the hot rolling, Air cooling for 10 seconds or more and average cooling rate after air cooling: 30°C/sec or more. By cooling to a temperature range of 450°C or less, this manufacturing method has high manufacturing efficiency by eliminating the need for intermediate reheating or second-stage cooling. is being achieved. In addition, for the purpose of further improving the properties, it is possible to undergo processes such as reheating quenching-tempering and reheating normalizing-tempering after completion of the cooling.

In addition, the range of plate thickness manufactured here is 5 mm to 40 mm.

Example

Examples of the present invention will be described below. In addition, the steel plate of the present invention and its manufacturing method are not limited to the examples.

Using a high-frequency vacuum melting furnace, steels Nos. 1 to 21 having the component compositions shown in Table 1 were melted, casted to form steel ingots, and then hot rolled to obtain steel sheets with a thickness of 20 mm.

Next, a No. 1A test piece described in JISZ2201 was taken from the steel plate so that the length direction of the test piece coincided with the width direction, and the yield stress: YS (MPa) and tensile strength: TS (MPa) were measured. It is preferable that YS is 325 MPa or more, and that TS is 450 MPa or more was considered acceptable.

Additionally, a test piece for microstructure observation was taken from a position that was 1/4 of the plate thickness, and after mirror polishing, EBSD measurement was performed to determine the local average misorientation (GAM) for each grain. The target surface was a cross section (C cross section) perpendicular to the rolling direction, and measurements were made at a step interval of 0.4 μm in an area of 300 μm × 300 μm, and the average of three views was evaluated. n orientation differences β _i (i is a number from 1 to m) between measurement points in the same crystal grain were determined, and the average value for each grain obtained from the equation below was taken as the local orientation difference average.

In addition, the crystal grain size is calculated by converting the area of the area surrounded by diagonal grain boundaries (diagonal grain boundaries refer to grain boundaries where the orientation difference with adjacent grains is determined to be 15° or more by EBSD measurement) into the equivalent circle diameter, and the average grain size is saved.

Additionally, a V-notch Charpy impact test piece as described in JISZ2202 was taken from a position that is 1/4 of the plate thickness, and a Charpy impact test was performed appropriately at a test temperature in the range of -120 to 40°C to determine the fracture transition with a ductile fracture ratio of 50%. The temperature vTrs (°C) was determined to evaluate the base material toughness, and those with vTrs (°C) of -60°C or lower were considered acceptable.

In order to evaluate the toughness (joint toughness) of the welded heat-affected zone, a test piece measuring 80 mm wide After performing a cooling reheat cycle, 2 mmV notched Charpy test pieces were collected from these test pieces. The obtained Charpy test piece was subjected to a Charpy impact test at a test temperature of -60°C to evaluate its toughness. The average shock absorption value of the three test results exceeded 100J, and the lowest value exceeded 50J was judged to have passed. The above reheat cycle conditions correspond to the heat cycle in the case of submerged arc welding with a heat input of 100 kJ/cm simulating one-pass welding with a plate thickness of 20 mm.

Table 2 also shows the manufacturing conditions of the rolled sheet and the test results of the mechanical properties evaluated by the above-mentioned method.

Steel plates No. 1 to 10, 23, and 25, which are inventive examples, showed excellent base material toughness, tensile strength of the base material, and joint toughness, while steel plates No. 11 to 22 and 24 whose steel composition or manufacturing conditions were outside the scope of the present invention In this case, the base material toughness, heat-affected zone toughness, or tensile strength are low compared to the invention example.

Claims (6)

Steel composition, in mass%,
C: 0.01 to 0.07%,
Si: 0.01 to 0.20%,
Mn: 0.80 to 1.80%,
P: 0.020% or less,
S: 0.0005 to 0.0050%,
Al: 0.030 to 0.100%,
Ti: 0.005 to 0.030%,
N: 0.0045 to 0.0090%,
B: 0.0010 to 0.0030%,
Ca: 0.0005 to 0.0030%,
O: 0.0040% or less,
Nb: 0.003% or less, and contained so as to satisfy the following formulas (1) and (2),
The balance is Fe and inevitable impurities,
A steel plate that has a microstructure in which grains with an average local orientation difference exceeding 1° are 50% or less as an area fraction of all grains, and an average grain size is 50 μm or less, and a tensile strength of 450 MPa or more.
0＜(Ca－(0.18＋130×Ca)×O)/1.25/S＜1···(1)
Ceq.≤0.36···(2)
However, Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and in equations (1) and (2), each element symbol represents the content (% by mass) of each element. Elements not contained are 0. According to paragraph 1,
The steel composition is further expressed in mass%,
Cu: 0.50% or less,
Ni: 1.00% or less,
Cr: 0.50% or less,
Mo: 0.30% or less,
V: A steel sheet containing at least one selected from the group consisting of 0.50% or less. According to claim 1 or 2,
The steel composition is further expressed in mass%,
Mg: 0.0050% or less,
Zr: 0.0200% or less,
REM: Steel sheet containing at least one selected from 0.0200% or less. Steel composition, in mass%,
C: 0.01 to 0.07%,
Si: 0.01 to 0.20%,
Mn: 0.80 to 1.80%,
P: 0.020% or less,
S: 0.0005 to 0.0050%,
Al: 0.030 to 0.100%,
Ti: 0.005 to 0.030%,
N: 0.0045 to 0.0090%,
B: 0.0010 to 0.0030%,
Ca: 0.0005 to 0.0030%,
O: 0.0040% or less,
Nb: 0.003% or less, and contained so as to satisfy the following formulas (1) and (2),
A steel material with the balance consisting of Fe and inevitable impurities,
After heating to 1000°C to 1200°C, hot rolling is performed, the rolling end temperature in the hot rolling is 800°C or higher, air cooling is performed for 10 seconds or more after the hot rolling, and the average cooling rate after air cooling is: 30°C/ A method of manufacturing steel sheets that is cooled to a temperature range of 450°C or less for more than 2 seconds.
0＜(Ca－(0.18＋130×Ca)×O)/1.25/S＜1···(1)
Ceq.≤0.36···(2)
However, Ceq. = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and in equations (1) and (2), each element symbol represents the content (% by mass) of each element. Elements not contained are 0. According to paragraph 4,
Additionally, the steel material is expressed in mass%,
Cu: 0.50% or less,
Ni: 1.00% or less,
Cr: 0.50% or less,
Mo: 0.30% or less,
V: A method of producing a steel sheet containing at least one selected from the group consisting of 0.50% or less. According to clause 4 or 5,
Additionally, the steel material is expressed in mass%,
Mg: 0.0050% or less,
Zr: 0.0200% or less,
REM: A method of producing a steel sheet containing at least one selected from 0.0200% or less.