WO2020208710A1 - Steel material - Google Patents

Abstract

This steel material contains, in terms of mass, 0.01-0.20% C, at most 1.00% Si, 0.1-2.5% Mn, 0.0005-0.0100% Mg, 0.015-0.500% Al, at most 0.020% P, at most 0.020% S, at most 0.0100% N, less than 0.0030% O, and a total of 0.0001-0.0100% of an X element, which is one or more of Pb, Bi, Se and Te, and optionally contains one or more of Cu, Ni, Cr, Mo, Nb, W, V, B, Ti, Zr, Ta, Ag, Hf, Ca, REM, Sn and Sb, with the remainder being Fe and impurities. The X_sol value obtained by subtracting the X_insol value, which is the total content of the Pb, Bi, Se and Te in the form of inclusions as determined by an electrolytic extraction residue technique, from the X_total value, which is the total content of the Pb, Bi, Se and Te, is 0.0001-0.0050 mass%.

Description

Steel

The present invention relates to a steel material having excellent toughness in the heat-affected zone (HAZ).

High-strength steel plates with a yield strength of about 300 to 700 MPa are used for various welded steel structures such as construction, bridges, shipbuilding, line pipes, construction machinery, marine structures, and tanks. These structures are required to have good HAZ toughness under a wide range of welding conditions from small heat input welding with a welding heat input of about 5 kJ / mm to ultra-large heat input welding with a welding heat input of more than 130 kJ / mm. Be done.

In HAZ, the heating temperature during welding becomes higher as it approaches the melting line, and austenite (γ) becomes significantly coarser especially in the region heated to 1400 ° C. or higher near the melting line, and the HAZ structure after cooling becomes coarser. And the toughness deteriorates. This tendency becomes more remarkable as the welding heat input increases.

The conventional HAZ toughness improvement technology is roughly classified based on two basic technologies. One of them is a technology to prevent the coarsening of austenite by utilizing the pinning effect of particles in steel. Fine particles that contribute to the miniaturization of HAZ crystal grains are called pinning particles. The other is a technique for refining the effective grain size by utilizing the intragranular ferrite transformation of austenite.

International Publication No. 2014/091604 (Patent Document 1), Japanese Patent Application Laid-Open No. 2013-204118 (Patent Document 2), and Japanese Patent Application Laid-Open No. 2002-3896 (Patent Document 3) describe sulfides containing fine Mg and Mn. A steel material capable of dispersing particles in steel, suppressing γ-grain growth during welding by the pinning effect of sulfide particles, and improving HAZ toughness is described.

In addition, International Publication No. 2001/027342 (Patent Document 4), JP-A-2000-80437 (Patent Document 5), JP-A-2000-80436 (Patent Document 6), JP-A-11-236645 (Patent Document 4). Document 7) describes a steel material capable of suppressing γ-grain growth during welding and improving HAZ toughness by dispersing oxide particles containing fine Ti and Mg in steel. There is.

Further, Japanese Patent Application Laid-Open No. 2001-342537 (Patent Document 8), Japanese Patent Application Laid-Open No. 2001-226739 (Patent Document 9), and Japanese Patent Application Laid-Open No. 2001-288509 (Patent Document 10) describe fine Ti, Ca, and Al. Described is a steel material in which oxide particles containing the above are dispersed in steel and these particles are used as ferrite transformation nuclei to suppress coarsening of the HAZ structure and improve toughness.

Furthermore, according to International Publication No. 2011/148754 (Patent Document 11) and JP-A-2009-174059 (Patent Document 12), fine TiN particles are dispersed in steel and welded by the pinning effect of the TiN particles. A steel material capable of suppressing the growth of γ grains at the time and improving the HAZ toughness is described.

Further, in Japanese Patent Application Laid-Open No. 2015-7264 (Patent Document 13) and Japanese Patent Application Laid-Open No. 2012-52224 (Patent Document 14), fine AlMn-based oxide particles are dispersed in steel during welding. Steel materials capable of suppressing γ-grain growth and improving HAZ toughness are described.

Furthermore, International Publication No. 2015/075771 (Patent Document 15), Japanese Patent Application Laid-Open No. 2015-98642 (Patent Document 16), and International Publication No. 2014/199488 (Patent Document 17) include TiN particles, MnS particles, and Described is a steel material in which these composite particles and Ti oxide particles are dispersed in steel and these particles are used as ferrite transformation nuclei to suppress coarsening of the HAZ structure and improve toughness.

Japanese Unexamined Patent Publication No. 2001-89925 (Patent Document 18) describes a steel material containing Bi as an optional component by utilizing the miniaturization of an oxide containing Mg in order to enhance HAZ toughness. Japanese Unexamined Patent Publication No. 2007-100203 (Patent Document 19) describes a steel material containing Mg and Ag, or further containing Bi, in order to suppress the growth of γ grains. Japanese Unexamined Patent Publication No. 2011-218370 (Patent Document 20) describes a steel material containing Bi in order to refine the solidified structure.

International Publication No. 2014/091604 Japanese Patent Application Laid-Open No. 2013-204118 Japanese Patent Application Laid-Open No. 2002-3896 International Publication No. 2001/027342 Japanese Patent Application Laid-Open No. 2000-80437 Japanese Patent Application Laid-Open No. 2000-80436 Japanese Patent Application Laid-Open No. 11-236645 Japanese Patent Application Laid-Open No. 2001-342537 Japanese Patent Application Laid-Open No. 2001-226739 Japanese Patent Application Laid-Open No. 2001-288509 International Publication No. 2011/148754 Japanese Patent Application Laid-Open No. 2009-174059 Japanese Patent Application Laid-Open No. 2015-7264 Japanese Patent Application Laid-Open No. 2012-52224 International Publication No. 2015/075771 Japanese Patent Application Laid-Open No. 2015-98642 International Publication No. 2014/199488 Japanese Patent Application Laid-Open No. 2001-89925 Japanese Patent Application Laid-Open No. 2007-100203 Japanese Patent Application Laid-Open No. 2011-218370

Even steel materials produced by these techniques can obtain the effect of miniaturizing the HAZ structure. However, there is a demand for a steel material that can obtain excellent HAZ toughness even when welding is performed under more severe welding conditions.
An object of the present invention is to provide a steel material having good HAZ toughness even after welding.

As a result of diligent studies by the present inventors, elements such as Pb, Bi, Se or Te (hereinafter, these may be referred to as X element) are contained in the steel, and the manufacturing conditions in the steelmaking process are optimized. It was found that the HAZ toughness of the steel material is improved. The gist of the present invention is as follows.

(1) The steel material according to one aspect of the present invention has C: 0.01 to 0.20%, Si: 1.00% or less, Mn: 0.1 to 2.5%, Mg: 0 in mass%. .0005 to 0.0100%, Al: 0.015 to 0.500%, P: 0.020% or less, S: 0.020% or less, N: 0.0100% or less, O: less than 0.0030% , X element Pb, Bi, Se, Te, one or more total: 0.0001 to 0.0100%, Cu: 0 to 2.0%, Ni: 0 to 2.0%, Cr : 0 to 2.0%, Mo: 0 to 1.0%, Nb: 0 to 0.10%, W: 0 to 2.0%, V: 0 to 0.20%, B: 0 to 0. 010%, Ti: 0 to 0.100%, Zr: 0 to 0.10%, Ta: 0 to 0.10%, Ag: 0 to 0.10%, Hf: 0 to 0.10%, Ca: It contains 0 to 0.0100%, REM: 0 to 0.010%, Sn: 0 to 0.50%, Sb: 0 to 0.50%, and the balance is composed of Fe and impurities, and the Pb and Bi , Se, from the total X _total content of Te, obtained by subtracting the Pb of state of forming inclusions obtained by electrolytic extraction residue method, Bi, Se, and X _insol is the sum of the content of Te X _{The sol} is 0.0001 to 0.0050% in mass%.
(2) The steel material according to (1) above has Cu: 0.02 to 2.0%, Ni: 0.02 to 2.0%, Cr: 0.02 to 2.0%, in mass%. Mo: 0.02 to 1.0%, Nb: 0.01 to 0.10%, W: 0.01 to 2.0%, V: 0.01 to 0.20%, B: 0.0003 to 0.010%, Ti: 0.005 to 0.100%, Zr: 0.01 to 0.10%, Ta: 0.01 to 0.10%, Ag: 0.01 to 0.10%, Hf : 0.01 to 0.10% of 1 type or 2 or more types may be contained.
(3) The steel material according to (1) or (2) above contains one or both of Ca: 0.0001 to 0.0100% and REM: 0.001 to 0.010% in mass%. May be good.
(4) The steel material according to any one of (1) to (3) above contains one or both of Sn: 0.01 to 0.50% and Sb: 0.01 to 0.50% in mass%. It may be contained.
(5) The steel material according to any one of (1) to (4) above has 1.00 to 1.00 × 10 ⁴ particles / mm ² particles having a diameter equivalent to a circle of 0.5 to 5.0 μm. Among the particles, the number ratio of the particles containing the X element of 1% or more in atomic% to the total of Ca, Mg, Mn, S, and the X element is 30% or more. May be good.

According to the steel material of the present invention, good HAZ toughness can be obtained even after welding.

It is a figure which shows the relationship between the solid solution amount X _sol of the X element, and the toughness after a reproduction thermal cycle test. The equivalent circle diameter at a position 1/4 of the thickness from the surface is 0.5 to 5.0 μm, and contains 1 atomic% or more of X element with respect to the total of Ca, Mg, Mn, S, and X elements. It is a figure which shows the relationship between the number ratio of particles, and toughness after a reproduction thermal cycle test.

It is assumed that the steel material according to the present embodiment is a steel material manufactured by a manufacturing method including deoxidation with Al and Mg. The present inventors conducted a detailed investigation and study on the relationship between the structure of HAZ and toughness. As a result, it was found that it is effective to remarkably refine (fine grain) the austenite particles of HAZ in order to improve the HAZ toughness. It is effective to utilize the pinning effect of the particles in steel for the miniaturization of austenite particles. However, depending on the amount of heat input to welding and the temperature used as the member, the effect of improving the toughness by miniaturizing the austenite grains of HAZ by utilizing the pinning effect has been limited.

In view of the above circumstances, the present inventors have added one or more "X elements" selected from the group consisting of Pb, Bi, Se or Te to the steel to obtain the X element and HAZ toughness. We examined the relationship between. As a result, it was newly found that the HAZ toughness can be further improved by optimizing the manufacturing conditions in the steelmaking process and controlling the solid solution amount of the X element within a predetermined range.
Further, on that, particles of a predetermined size are generated in the steel so as to have a number density in a predetermined range, and among these particles, Ca, Mg, Mn, S, and the total of the X elements are added. On the other hand, it was newly found that the HAZ toughness can be further improved by setting the number ratio of particles containing 1% or more of X element in atomic% to 30% or more.

The present inventors used steel materials having various chemical components to obtain "solid solution amount of element X (X _sol )" and "ca, among particles having a circle equivalent diameter of 0.5 to 5.0 μm. A study was conducted to clarify the relationship between "the ratio of the number of particles containing 1% or more of X element in atomic% to the total of Mg, Mn, S, and X elements" and the toughness of HAZ. The solid solution amount of element X was determined by inductively coupled plasma mass spectrometry (sometimes referred to as ICP mass spectrometry) and electrolytic extraction residue method. Further, the equivalent circle diameter of the particles, the number density, and the number ratio of the particles containing the X element were determined by an electron microscope as described later.
The toughness of HAZ was evaluated by performing a regenerative heat cycle test in which a sample collected from a steel material was given a thermal history (corresponding to a welding heat input of 450 kJ / cm) to reproduce welding. Specifically, after the reproducible heat cycle test, the Charpy absorbed energy was measured at −20 ° C. with the number of tests set to 3 in accordance with JIS Z 2242: 2005, and the HAZ toughness was evaluated at the lowest value. As a result, as shown in FIG. 1, it was found that the HAZ toughness was improved when X _sol was in the range of 0.0001 to 0.0050% (1 to 50 ppm).
Further, as shown in FIG. 2, among the particles having a circle equivalent diameter of 0.5 to 5.0 μm at a position 1/4 of the thickness from the surface, the total of Ca, Mg, Mn, S, and X elements is added. On the other hand, it was found that the HAZ toughness was further improved when the number ratio of the particles containing 1 atomic% or more of the X element was 30% or more.

Hereinafter, the steel material according to this embodiment will be described in detail.

First, the chemical composition of the steel material according to the present embodiment will be described.
The steel material according to the present embodiment has C: 0.01 to 0.20%, Si: 1.00% or less, Mn: 0.1 to 2.5%, Mg: 0.0005 to 0% in mass%. Contains 0100%, Al: 0.015 to 0.500%, P: 0.020% or less, S: 0.020% or less, N: 0.0100% or less, O: less than 0.0030%, and further. , Pb: 0.0100% or less, Bi: 0.0100% or less, Se: 0.0100% or less, Te: 0.0100% or less, 1 type or 2 or more types of X elements in total from 0.0001 to 0.0100%, Cu: 0 to 2.0%, Ni: 0 to 2.0%, Cr: 0 to 2.0%, Mo: 0 to 1.0%, Nb: 0 to 0.10%, W: 0 to 2.0%, V: 0 to 0.20%, B: 0 to 0.010%, Ti: 0 to 0.100%, Zr: 0 to 0.10%, Ta: 0 to 0 .10%, Ag: 0 to 0.10%, Hf: 0 to 0.10%, Ca: 0 to 0.0100%, REM: 0 to 0.010%, Sn: 0 to 0.50%, Sb : Contains 0 to 0.50%, and the balance consists of Fe and impurities.

In the following explanation of chemical components, mass% is expressed as%. Further, in the following description, when the upper limit value and the lower limit value of the element content are connected by "-" and displayed in a range, the range including the upper limit value and the lower limit value is meant unless otherwise specified. Therefore, when expressed as 0.01 to 0.20% in mass%, the range means a range of 0.01 mass% or more and 0.20 mass% or less.

C: 0.01 to 0.20%
C is an element that increases the strength of the base metal. If the C content is less than 0.01%, the effect of improving the strength of the base metal is small, so 0.01% or more is set as the lower limit. The lower limit of the more preferable C content is 0.06% or more. On the other hand, when the C content exceeds 0.20%, the amount of cementite or a mixture of martensite and austenite (referred to as Martinite-Austenite Constituent: MA), which is the starting point of brittle fracture, increases, so that HAZ toughness decreases. .. Therefore, the upper limit of the C content is set to 0.20% or less. In particular, with respect to the HAZ toughness and low temperature toughness of high heat input welding, even a relatively small amount of small cementite or MA is likely to be the starting point of brittle fracture and may reduce the HAZ toughness. Therefore, it is preferable to strictly regulate the upper limit of the C content. The upper limit of the C content is preferably 0.15% or less, more preferably 0.13% or less, even more preferably 0.10% or less, still more preferably 0.08% or less. ..

Si: 1.00% or less Si is an element that functions as an antacid and contributes to an increase in strength, but if it is contained in excess, MA, which is a hard embrittled structure, is formed in the microstructure of HAZ. It will be easier to do. Since this MA deteriorates the toughness of HAZ, it is desirable to limit the Si content, but if it is 1.00% or less, Si may be intentionally contained. The Si content is preferably 0.50% or less, more preferably 0.30% or less. Since it is desirable that the Si content is low in order to improve the HAZ toughness, it is not necessary to particularly limit the lower limit value, and the lower limit value is 0%. However, reducing the Si content to less than 0.03% may accompany an increase in cost, in which case it is desirable to set the lower limit to 0.03% or more.

Mn: 0.1-2.5%
Mn needs to be contained in an amount of 0.1% or more as an effective component for ensuring the strength and toughness of the base material. In order to secure the strength, the Mn content is more preferably 0.3% or more, further preferably 0.4% or more, and even more preferably 0.5% or more. The inclusion of a large amount of Mn leads to segregation and formation of a hard phase, which lowers HAZ toughness. The upper limit was set to 2.5% or less within an acceptable range. A more preferable upper limit of the Mn content is 2.3% or less, more preferably 2.0% or less.

P: 0.020% or less P is an element that causes intergranular embrittlement and is harmful to toughness. Therefore, it is desirable that the P content is low. If P of more than 0.020% is contained, the HAZ toughness is lowered even if the austenite grains of HAZ are refined, so the P content is limited to 0.020% or less. It is preferably 0.010% or less, more preferably 0.008% or less. It is not necessary to limit the lower limit of the P content in particular, but since it is not technically easy to set the P content to 0%, the lower limit may be set to more than 0%. The P content may be 0.001% or more.

S: 0.020% or less S is an element that forms pinning particles containing Mg and contributes to the improvement of HAZ toughness. If S of more than 0.020% is contained, the stability of the pinning particles at a high temperature is lowered, and the effect of improving the HAZ toughness may not be sufficiently obtained. Therefore, the upper limit of the S content is set to 0.020% or less. The upper limit of the preferable S content is 0.015% or less. In order to improve HAZ toughness, the upper limit of the S content may be 0.010% or less and 0.008% or less. It is not necessary to particularly limit the lower limit of the S content, but since it is not technically easy to set the S content to 0%, the lower limit may be set to more than 0%. On the other hand, in order to increase the amount of particles that contribute to pinning, the S content is preferably 0.0020% or more. In order to generate a larger amount of particles, the S content may be 0.0025% or more, or 0.0030% or more.

Mg: 0.0005-0.0100%
Mg is an important element that forms pinning particles and contributes to the improvement of HAZ toughness. If the Mg content is less than 0.0005%, a sufficient number of pinning particles may not be obtained, so the lower limit is set to 0.0005% or more. In order to generate a larger amount of particles, the Mg content is preferably 0.0007% or more, more preferably 0.0008% or more, and even more preferably 0.0010% or more. On the other hand, even if the Mg content exceeds 0.0100%, the effect of improving HAZ toughness is saturated and the economic efficiency is impaired. Therefore, the upper limit of the Mg content is set to 0.0100% or less. The upper limit of the Mg content may be 0.0080% or less or 0.0050% or less.

Al: 0.015 to 0.500%
Al is an element that functions as an antacid and reduces the amount of dissolved oxygen in molten steel. The lower limit of the Al content is 0.015% or more in order to promote the formation of pinning particles. The Al content is preferably 0.020% or more, more preferably 0.030% or more. However, if Al is excessively contained, the HAZ toughness deteriorates, so the Al content is set to 0.5500% or less. The upper limit of the preferable Al content is 0.300% or less. In order to improve HAZ toughness, the upper limit of Al content may be 0.170% or less, 0.10% or less, or 0.080% or less.

N: 0.0100% or less N is an element that forms a nitride, and when the N content is large, coarse nitrides such as AlN and TiN are likely to be produced. These coarse particles serve as a starting point for brittle fracture and may lead to a decrease in HAZ toughness. Therefore, the upper limit of the N content is set to 0.0100% or less. The upper limit of the N content is preferably 0.0007% or less, more preferably 0.0050% or less. It is desirable that the N content is small, but reducing the N content to less than 0.0020% may accompany an increase in cost, and therefore the lower limit may be 0.0020% or more. The N content may be 0.0030% or more.

O: Less than 0.0030% O is an element that forms an oxide, and if the content is large, a coarse oxide is likely to be formed. The coarse oxide becomes the starting point of fracture and lowers the HAZ toughness, so the O content is set to less than 0.0030%. The upper limit of the preferable O content is 0.0028% or less, more preferably 0.0025% or less, and even more preferably 0.0023% or less. On the other hand, reducing the O content to less than 0.0001% leads to an increase in cost, and in order to generate fine particles described later, it is preferable to contain O content of 0.0001% or more. The O content may be 0.0005% or more, or 0.0010% or more in order to generate finer particles.

A total of 0.0001 to 0.0100% of X elements of Pb, Bi, Se, and Te
The steel material according to the present embodiment contains one or more of the X elements Pb, Bi, Se, and Te as essential components, and as will be described later, from the total X _total of the contents of these X elements, Pb in a state of forming the inclusions obtained by electrolytic extraction residue method, Bi, Se, X _sol obtained by subtracting the X _insol is the sum of the content of Te is, in mass%, 0.0001 to 0.0050 %. The amount of element X that dissolves in steel can be measured by the electrolytic extraction residue method. As described above, when a predetermined amount of X element is dissolved in solid solution, the grain growth of austenite grains in HAZ can be suppressed and the HAZ toughness can be improved, although the reason is unknown.
In the steel material according to the present embodiment, in order to secure X _sol , the content of X element (total content of Pb, Bi, Se, and Te: X _total ) needs to be 0.0001% or more. The total content of the X element is preferably 0.0005% or more, more preferably 0.0010% or more, still more preferably 0.0020% or more.
Further, the steel material according to the present embodiment, at the position of 1/4 of the thickness from the surface, the particles of 0.5 ~ 5.0 .mu.m equivalent circle diameter of ^{1.00 (1.00 × 10 0) ~} 1. The number of particles existing at a number density of 00 × 10 ⁴ / mm ² and containing 1% or more of the X element in atomic% with respect to the total of Ca, Mg, Mn, S and the X element among the particles. The ratio is preferably 30% or more. When the number of particles containing 1% or more of X element in atomic% is increased, the grain growth of austenite particles in HAZ is suppressed, and the HAZ toughness is further improved. In order to increase the number ratio of particles containing 1% or more of X element in atomic%, the content of X element (content of one or more of Pb, Bi, Se, and Te: X _total) ) Must be 0.0001% or more. The content of the X element is preferably 0.0005% or more, more preferably 0.0010% or more, still more preferably 0.0020% or more. The effect of the X element is not always clear, but it is possible that the formation of particles containing the X element contributes to the improvement of the pinning effect of the fine particles in the steel.
On the other hand, if these X elements are excessively contained, the HAZ toughness is lowered. Therefore, the upper limit of the content of each of the X elements is 0.0100% or less, and the upper limit of the total content of the X elements is 0.0100% or less. The total content of the X element is more preferably 0.0080% or less, further preferably 0.0050% or less, and most preferably 0.0030% or less.

The balance of the chemical components of the steel material according to this embodiment is iron (Fe) and impurities. Impurities are components that are mixed in by raw materials such as ores and scraps and other factors when steel materials are industrially manufactured, and are allowed as long as they do not adversely affect the steel materials according to the present embodiment. To do. However, among impurities, it is necessary to limit the upper limit values for P, S, O and N as described above.

The steel material according to the present embodiment basically contains the above-mentioned chemical components, but in order to improve the mechanical properties and HAZ toughness of the steel material (base material), it may be replaced with a part of Fe as necessary. Further, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 1.0% or less, Nb: 0.10% or less, W: 2.0% or less, V: 0.20% or less, B: 0.010% or less, Ti: 0.100% or less, Zr: 0.10% or less, Ta: 0.10% or less, Ag: 0.10% or less, Hf: It may contain 1 type or 2 or more types of 0.10% or less. However, since the content of these elements is not essential, the lower limit is 0%.

Cu: 0-2.0%
Cu is an element effective for increasing the strength of the base material, and Cu may be contained. However, if Cu is contained in excess of 2.0%, HAZ toughness may decrease. Therefore, the Cu content is limited to 2.0% or less. The Cu content is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.5% or less. Cu may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. In order to improve the strength of the base material, the Cu content is preferably 0.02% or more. More preferably, the Cu content is 0.1% or more.

Ni: 0-2.0%
Ni is an element effective for improving toughness and strength, and Ni may be contained. However, even if Ni is contained in excess of 2.0%, the effect is saturated. Therefore, the Ni content is limited to 2.0% or less from the viewpoint of economy. The Ni content is preferably 1.5% or less, more preferably 1.0% or less, and even more preferably 0.7% or less. Ni may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. In order to improve the strength of the base metal, the Ni content is preferably 0.02% or more. More preferably, the Ni content is 0.1% or more.

Cr: 0-2.0%
Cr is an element that increases the strength of the base metal by improving hardenability and strengthening precipitation, and Cr may be contained. However, if Cr is contained in excess of 2.0%, MA is likely to be generated in HAZ, and HAZ toughness is lowered. Therefore, the Cr content is limited to 2.0% or less. The Cr content is preferably 1.0% or less, more preferably 0.5% or less. Cr may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. In order to improve the strength of the base metal, the Cr content is preferably 0.02% or more. More preferably, the Cr content is 0.1% or more.

Mo: 0-1.0%
Mo is an element that improves hardenability and increases the strength of the base material, and Mo may be contained. However, if Mo is contained in excess of 1.0%, a hard structure may be formed in the HAZ and the HAZ toughness may decrease. Therefore, the Mo content is limited to 1.0% or less. The Mo content is preferably 0.5% or less, more preferably 0.3% or less. Mo may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited and may be 0%. The Mo content is preferably 0.02% or more in order to improve the strength of the base material. More preferably, the Mo content is 0.1% or more.

Nb: 0 to 0.10%
Nb is an element that improves hardenability, and also contributes to the miniaturization of the structure by suppressing the formation of precipitates and recrystallization. Nb may be contained in order to increase the strength of the base material and improve the toughness and productivity of the base material. However, if Nb is contained in excess of 0.10%, a hard structure or inclusions may be formed in HAZ, and HAZ toughness may decrease. Therefore, the Nb content is limited to 0.10% or less. The Nb content is preferably 0.05% or less, more preferably 0.04% or less. Nb may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. The Nb content is preferably 0.01% or more in order to improve the strength and toughness of the base metal and to make it economical.

W: 0-2.0%
W is an element that contributes to the improvement of hardenability and the strengthening of precipitation. W may be contained in order to increase the strength of the base metal and improve the toughness. However, if W is contained in excess of 2.0%, a hard structure may be formed in HAZ and the HAZ toughness may decrease. Therefore, the W content is limited to 2.0% or less. The W content is preferably 1.0% or less, more preferably 0.5% or less. W may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. The W content is preferably 0.01% or more in order to improve the strength and toughness of the base metal.

V: 0 to 0.20%
Since V is an element that improves hardenability and is an element that forms carbides and nitrides and is effective in increasing the strength of the base metal, V may be contained. However, if V is contained in excess of 0.20%, the precipitation of carbonitride in HAZ becomes remarkable, and HAZ toughness may decrease. Therefore, the V content is limited to 0.20% or less. The V content is preferably 0.10% or less. V may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. In order to improve the strength of the base material, the V content is preferably 0.01% or more.

B: 0 to 0.010%
B is an element that remarkably enhances hardenability and improves the strength and toughness of the base material and HAZ, and B may be contained. However, if B is contained in excess of 0.010%, HAZ toughness and weldability may deteriorate. Therefore, the B content is limited to 0.010% or less. The preferred B content is 0.007% or less, more preferably 0.005% or less. The lower limit of the B content may be 0%, but the B content is preferably 0.0003% or more in order to obtain the effect of increasing the strength. The B content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

Ti: 0 to 0.100%
Ti is an element that forms TiN and contributes to the refinement of crystal grains. Ti may be included to improve strength and toughness. However, if Ti is contained in excess of 0.100%, TiC may be excessively generated and HAZ toughness may decrease. Therefore, the Ti content is limited to 0.100% or less. The Ti content is preferably 0.050% or less, more preferably 0.030% or less. Ti may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. The Ti content is preferably 0.005% or more, more preferably 0.010% or more.

Zr: 0 to 0.10%
Since Zr is an element that forms carbides and nitrides and is effective for increasing the strength of the base material and refining the structure, Zr may be contained. However, if Zr is contained in excess of 0.10%, coarse nitrides may be formed and the toughness may decrease. Therefore, the Zr content is limited to 0.10% or less. The Zr content is preferably 0.05% or less. The lower limit of the Zr content does not have to be particularly limited and may be 0%, but the Zr content is preferably 0.01% or more in order to improve the strength of the base metal.

Ta: 0 to 0.10%
Ta is an element effective for ensuring the strength and toughness of the base material, and Ta may be contained. However, if Ta is contained in excess of 0.10%, HAZ toughness may decrease. Therefore, the Ta content is limited to 0.10% or less. The Ta content is preferably 0.05% or less. Ta may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof need not be particularly limited and may be 0%. The lower limit of the Ta content may be 0.01% or more.

Ag: 0 to 0.10%
Ag is an element effective for increasing the strength of the base material and making the structure finer, and may contain Ag. However, if Ag is contained in excess of 0.10%, HAZ toughness may decrease. Therefore, the Ag content is limited to 0.10% or less. The Ag content is preferably 0.05% or less. Ag may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited and may be 0%. The lower limit of Ag content may be 0.01% or more.

Hf: 0 to 0.10%
Hf is an element that contributes to the formation of pinning particles, and Hf may be contained. However, if Hf is contained in excess of 0.10%, coarse nitrides may be formed in HAZ and the HAZ toughness may decrease. Therefore, the Hf content is limited to 0.10% or less. The Hf content is preferably 0.05% or less. The lower limit of the Hf content need not be particularly limited and may be 0%. The lower limit of the Hf content may be 0.01% or more.

Further, in the steel material according to the present embodiment, in order to control the form of inclusions, Ca: 0.0100% or less and REM: 0.010% or less are further replaced with a part of Fe as necessary. One or both may be contained.

Ca: 0 to 0.0100%
Ca is an element that forms oxides and sulfides, and may be contained in order to control the morphology of inclusions. In this case, the Ca content is preferably 0.0001% or more. Further, since reducing the Ca content to less than 0.0001% may accompany an increase in cost, the Ca content may be 0.0001% or more from that viewpoint as well. However, if Ca is contained in excess of 0.0100%, coarse oxides are likely to be produced, so the Ca content is limited to 0.0100% or less. The Ca content is preferably 0.0060% or less, more preferably 0.0050% or less, even more preferably 0.0040% or less, still more preferably 0.0030% or less. In order to promote the formation of pinning particles, it is preferable to limit the Ca content to 0.0015% or less. It is not necessary to particularly limit the lower limit of the Ca content, and it may be 0%.

REM: 0 to 0.010%
REM is an element that forms oxides and sulfides, and REM may be contained in order to control the morphology of inclusions. However, if the REM content is high, coarse oxides are likely to be formed and the HAZ toughness may decrease. Therefore, the REM content is limited to 0.010% or less. The REM content is preferably 0.005% or less, more preferably 0.004% or less. In order to generate pinning particles, it is preferable to limit the REM content to 0.0005% or less. The lower limit of the REM content need not be particularly limited and may be 0%. The REM content may be 0.001% or more. In the present embodiment, REM is a general term for a total of 17 elements including lanthanoid elements such as La and Ce and Sc and Y. That is, the REM content is the total content of these elements. When adding these elements, the effect does not change even if a misch metal containing these elements is used.

Further, in order to improve the corrosion resistance, the steel material according to the present embodiment further replaces a part of Fe with one or both of Sn: 0.50% or less and Sb: 0.50% or less, if necessary. May be contained.

Sn: 0 to 0.50%, Sb: 0 to 0.50%
Sn and Sb may be contained from the viewpoint of corrosion resistance and the like, but if they are contained in excess, HAZ toughness may be impaired. Therefore, the contents of Sn and Sb are 0.50% or less, more preferably 0.20% or less, and even more preferably 0.10% or less. The lower limit of these elements does not need to be particularly limited and may be 0%. The contents of Sn and Sb may be 0.01% or more, respectively.

From the viewpoint of HAZ toughness, the chemical composition of the steel material according to the present embodiment preferably has a carbon equivalent Ceq represented by the following formula in the range of 0.25 to 0.50. When Ceq is 0.30 or more, the steel material has more excellent HAZ toughness. Further, when Ceq is 0.45 or less, the formation of MA is suppressed and the HAZ toughness is improved, which is more preferable. It is more preferable that Ceq is 0.40 or less.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5+ (Cu + Ni) / 15
[C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] in the formula are the contents of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. (Mass%), and if it is not contained, 0 is substituted.

Next, the X element existing in the steel will be described.
As described above, the steel material according to the present embodiment contains one or more of the X elements (Pb, Bi, Se, Te). This element X exists in steel in a solid solution state or in a state where particles (inclusion particles) are formed with other elements. In the steel according to the present embodiment, the content X _sol of the element X in solid solution state, when the content of X _insol the X element in a state of forming the inclusions, and the content of the sum of these was X _total , X _sol obtained by subtracting the X _insol from X _total is, by mass%, 0.0001 to 0.0050%. When X _sol is 0.0001 to 0.0050%, coarsening of austenite grains due to the heat effect of welding is suppressed, and HAZ toughness of the steel material is improved.

If X _sol is less than 0.0001%, the effect of suppressing the coarsening of austenite grains cannot be sufficiently obtained. Therefore, X _{sol is set} to 0.0001% or more. X _sol is preferably 0.0002% or more, more preferably 0.0003% or more.
On the other hand, when X _sol exceeds 0.0050%, HAZ toughness deteriorates, although the cause is not clear. Therefore, X _{sol is set} to 0.0050% or less. The X _sol is preferably 0.0040% or less, more preferably 0.0030% or less. As the content of Ca, Al, O and S increases, X _sol may decrease.
The reason why the coarsening of austenite particles is suppressed by the solid solution of element X is not clear, but the causes are the effect of making pinning particles finer and the effect of improving pinning force, and the γ grain boundary easiness due to segregation of element X. The effect of reducing mobility can be considered.

X _total , X _insol , and X _sol may be obtained by the following methods, respectively.
For X _total , the content of each X element may be determined by inductively coupled plasma mass spectrometry, and the total content of these elements may be X _total .
X _insol can be determined by the electrolytic extraction residue method. Specifically, a sample collected from the steel material according to the present embodiment is electrolyzed and dissolved in a non-aqueous solvent. Then, the residue in the solution is recovered by a filter having a pore size of 0.2 μm, and the total content of each X element contained in the residue is determined by inductively coupled plasma mass spectrometry. In carrying out the electrolytic extraction residue method, 4% methyl salicylate-1% salicylic acid-1% tetramethylammonium chloride-methanol was adopted as the electrolytic solution so that pinning particles containing element X were included in the residue after electrolysis. Then, electrolytic extraction is performed at an electrolytic potential of −100 mV.
X _sol can be determined by subtracting the X _insol from X _total obtained by the aforementioned method. Therefore, the X element contained in the particles significantly finer than the pore size of the filter used in the electrolytic extraction residue method may be contained in X _sol , but it is a small amount and does not affect the HAZ toughness. You don't have to.

The steel material according to the present embodiment has a position of 1/4 of the thickness from the surface of the steel material (when the steel material is a steel plate, a position of 1/4 depth of the plate thickness in the plate thickness direction from the surface, and the steel material has a circular cross section. When it has a shape, particles having a diameter equivalent to a circle of 0.5 to 5.0 μm are 1.00 to 1.00 × 10 ⁴ particles / mm ² at a position (1/4 of the diameter from the surface toward the center). Among these particles, the number ratio of particles containing X element of 1% or more in atomic% to the total of Ca, Mg, Mn, S, and the X element is 30% or more. It is preferable to have. The reason is now that the equivalent circle diameter is 0.5 to 5.0 μm and the ratio of the number of particles containing 1 atomic% or more of X element to the total of Ca, Mg, Mn, S, and X elements increases. Although it is unknown at this point, the effect of improving HAZ toughness is enhanced. In the present embodiment, particles having a circle-equivalent diameter of 0.5 to 5.0 μm are targeted, but particles having a circle-equivalent diameter of less than 0.5 μm and more than 5.0 μm may be present. If the number density of particles having a circle-equivalent diameter of 0.5 to 5.0 μm is 1.00 particles / mm ² or more, the effect of suppressing the grain growth of austenite becomes remarkable. On the other hand, when the number density of particles having a circle-equivalent diameter of 0.5 to 5.0 μm exceeds 1.00 × 10 ⁴ particles / mm ² , the HAZ toughness decreases. In the present embodiment, the equivalent circle diameter refers to the diameter of a circle having an area equal to the projected area of the measured particles, and is specifically derived by the following equation.
Circle equivalent diameter = √ {4 × (area of the particle) ÷ π}

Percentage of particles containing 1 atomic% or more of X element among the particles having a circle equivalent diameter of 0.5 to 5.0 μm and existing at a number density of 1.00 to 1.00 × 10 ⁴ particles / mm ^2. Is important, and the larger the ratio, the greater the effect of improving HAZ toughness. In the present embodiment, particles having a concentration of element X of 1 atomic% or more are defined as particles containing element X. Among these particles, the number ratio of particles containing 1% or more of X element in atomic% with respect to the total of Ca, Mg, Mn, S, and X elements is preferably 30% or more, more preferably 40% or more. It is preferable, and more preferably 50% or more. Further, if the concentration of the X element is 1 atomic% or more, it can be reliably detected by an analytical instrument, so that particles containing an X element of 1 atomic% or more can be measured.

The circle-equivalent diameter, the number density, and the number ratio of the particles containing 1 atomic% or more of the X element in the steel material according to the present embodiment are determined by elemental analysis and image analysis using an electron microscope. Specifically, among the particles observable with a field emission scanning electron microscope (FE-SEM), the number ratio of particles containing the X element (Pb, Bi, Se, Te) is measured. .. Whether or not the particles contain 1 atomic% or more of X elements may be determined by an energy dispersive X-ray element analyzer (Energy Dispersive X-ray Spectrometry, EDS). At that time, the elements to be analyzed are Mn, Mg, Ca, S, and X elements.

For the number density of particles contained in the steel material, a sample is taken from the steel material, the cross section in the thickness direction is mirror-polished, and the position of 1/4 of the thickness is observed from the surface of the steel material with an FE-SEM with EDS. Can be measured. It is obtained by measuring the number of particles having a diameter equivalent to a circle and having a size of 0.5 to 5.0 μm for an area of at least 25,000 μm ² or more and converting it into a number density per unit area. For particles having a circle-equivalent diameter of less than 0.5 μm, the number of particles is insufficiently measured by FE-SEM observation, so particles having a diameter of 0.5 μm or more are measured. On the other hand, large particles have a small pinning effect, and particles having a circle-equivalent diameter of more than 5.0 μm have a small contribution to the growth inhibition of austenite particles. Therefore, we focused on the number of particles with a circle-equivalent diameter of 5.0 μm or less. The number is measured, for example, at a magnification of 2000 times, one field of view is 50 μm × 50 μm, and at least 10 fields of view are observed. If 250 at this time of 0.5 ~ 5.0 .mu.m number 10 field of view of the particle (25000μm ^2), the number of particles can be converted with 1.00 × 10 ⁴ per 1 mm ^2. However, when the number observed in one field of view is small, specifically 20 or less, the field of view is expanded to a maximum of 20 mm ² (5 mm × 4 mm), and the number is confirmed.

Next, among the particles whose numbers have been measured, it is measured how many particles contain 1% or more of X element in atomic% with respect to the total of Ca, Mg, Mn, S, and X elements. When the number of particles is large, for example, the number of particles may be 1000 or more, so it is a difficult task to identify all the particles one by one. Therefore, it is sufficient to identify whether or not at least 20 or more particles contain 1 atomic% or more of X element under the following conditions, and determine the abundance ratio thereof. For particles having a concentration of element X of 1 atomic% or more, elements other than element X may be detected. The concentration of element X in the particles is determined by quantifying the average of the entire particles by surface analysis of EDS. The electron beam diameter used for this quantification is 0.01 to 1.0 μm, and the magnification of SEM observation is 1000 to 10000 times.

The number of particles may be measured by preparing a sample from a steel material obtained by heating a steel material to 1400 ° C. and holding it for about 3 seconds to quench it. This is because, for example, when cementite or alloy nitride is produced, it is difficult to measure the number of particles having a circle-equivalent diameter of 0.5 to 5.0 μm to be observed. By heating to a high temperature to dissolve the precipitates other than the observation target and then quenching them, or by applying a thermal cycle in which ferrite is generated during quenching, it is possible to prepare materials with less cementite and carbonitrides. it can. Since the particles containing Mg are stable even when heated to a high temperature and their morphology hardly changes during cooling, the measurement result of the number of particles hardly changes even if such a heat cycle is applied.

Next, a method for manufacturing a steel material according to the present embodiment will be described.
When controlling the state of existence of element X in steel, it is effective to control the melting process. Specifically, as a method for melting steel, for example, with the molten steel temperature set to 1650 ° C. or lower and the O concentration of the molten steel controlled to 0.0100% or less, a deoxidizing element such as Al is added, and then Mg is added. And element X are added. The addition of element X is performed at the same time as the addition of Mg or before and after the addition of Mg, and no other steps are included between them.
At a position 1/4 of the thickness from the surface of the steel material, particles having a diameter equivalent to a circle of 0.5 to 5.0 μm are present at a number density of 1.00 to 1.00 × 10 ⁴ particles / mm ^2. When the number ratio of particles containing 1% or more of X element in atomic% to the total of Ca, Mg, Mn, S, and X elements is 30% or more, for example, the molten steel temperature is 1650. With the O concentration of the molten steel controlled to 0.0100% at ° C or lower, a deoxidizing element such as Al is added, and Mg is added at the same time as the addition of the X element, or Mg is added after the X element is added. Is added, and no other steps are included in the meantime.
In order to control the O concentration of the molten steel to 0.0100% or less, preliminary deoxidation with Si, Mn, Al or the like may be performed. After adding Mg and X element, the content of other elements may be adjusted within a predetermined range. It is preferable to adopt continuous casting for casting. As a result, a slab having an X _sol of 0.0001 to 0.0050% can be obtained.

With the O concentration of the molten steel controlled to 0.0100% or less, deoxidizing elements such as Al are added, and by adding the X element and Mg in an appropriate order, particles containing Mg can be found in the molten steel. It is formed and finely dispersed in the cast steel. Since these fine particles are stable at high temperatures, coarsening of γ particles heated by welding can be suppressed. On the other hand, when a deoxidizing element such as Al is added while the O concentration of the molten steel is more than 0.0100%, inclusions such as acid sulfide take in the X element and coagulate and float, so that the X element is also discharged. Will be done. Therefore, it can be inferred that by controlling the O concentration of the molten steel before adding a deoxidizing element such as Al to 0.0100% or less, the emission of the X element can be suppressed and it can be effectively utilized.

More preferably, as a method for melting steel, for example, a deoxidizing element such as Al is added in a state where the O concentration of the molten steel is controlled to 0.0100% or less and the S concentration of the molten steel is controlled to 0.0200% or less. Further, Mg and X element are added. More preferably, for example, in a state where the O concentration of the molten steel is controlled to 0.0100% or less and the S concentration of the molten steel is controlled to 0.0200% or less, a deoxidizing element such as Al is added, and then the X element is added. , Mg is added. In this way, by adding the deoxidizing element, the X element, and Mg while controlling the O concentration and the S concentration of the molten steel, the formation of coarse inclusions and the emission of the X element can be suppressed.

If deoxidation with Al or the like is insufficient, Ca or REM may be added as an element that promotes deoxidation. In order to promote the formation of particles containing Mg, it is preferable to limit the amount of Ca and the amount of REM in the molten steel before adding Mg to 0.0005% or less. By limiting the contents of Ca and REM that form sulfides, S can be used for the formation of pinning particles. Even if Ca and REM are not intentionally added, they may be mixed into the molten steel from refractories used in molten steel pots, fluxes and slags added for the purpose of desulfurization, and alloy raw materials. In order to suppress the content of Ca and REM to 0.0005% or less, the amount of Ca and REM contained in refractories, flux, slag, alloy raw materials and the like may be controlled. The form and shape of Ca and REM in the molten steel may be controlled so as to be a stable oxide or the like that is difficult to be mixed in the molten steel.

The mechanism by which the HAZ toughness is improved by the X element is not clear, but it is possible to make the X element uniformly present in the steel by ensuring the amount of the X element (X _sol ) present in the steel in the solid solution state. Presumed to be important. Further, it is considered that a synergistic effect is exhibited by securing X _sol and forming particles containing Mg in the steel, and an excellent pinning effect can be obtained.

As described above, by defining the O concentration of molten steel and the order of addition of element X, deoxidizing elements such as Al, and Mg, the number density of particles with a diameter equivalent to a circle of 0.5 to 5.0 μm can be controlled. Explain why you can. Simply adding element X to steel does not produce particles containing element X. The reason is not clear, but it is presumed that element X has a high vapor pressure in the molten steel and is unlikely to remain in the molten steel even if a large amount is added. Therefore, it is presumed that by controlling the deoxidized product that is the core of the particles containing the X element, the X element is captured by the inclusions and particles that contribute to the improvement of the toughness of HAZ are generated.

The heating, rolling, and heat treatment conditions after casting are appropriately set according to the target mechanical properties of the steel material, for example, controlled rolling / controlled cooling, direct quenching / tempering after rolling, quenching / tempering after cooling once after rolling, and the like. You can select it.

Hereinafter, the steel material according to this embodiment will be specifically described with reference to examples. However, the conditions in the following examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to the following examples.

(Example 1)
The slab obtained by melting and casting steel was hot-rolled to obtain a steel plate having a thickness of 25 mm.
In the melting step, Mg and element X were added at the same time with the molten steel temperature set to 1650 ° C. or lower and the molten steel O concentration set to 0.0100% or lower. Further, the content of other elements was adjusted to a predetermined range, and casting was performed by continuous casting to obtain a slab.

A sample was taken from the obtained steel sheet, and the components of the steel sheet were analyzed using a fluorescent X-ray analysis method, a combustion-infrared absorption method, an inert gas melting method, an ICP mass spectrometry method, and the like. The content of element X (Pb, Bi, Se, Te) contained in the steel sheet was determined by ICP mass spectrometry. The analysis results of the steel sheet components are shown in Tables 1 to 4.

The carbon equivalent Ceq shown in Tables 2 and 4 was calculated by the following formula.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5+ (Cu + Ni) / 15
[C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] in the formula are the contents of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. (Mass%), and if it is not contained, 0 is substituted.

The resulting sample was taken from the steel plate, the X _insol determined by electrowinning residue method to determine the X _sol by subtracting the X _insol from X _total measured by ICP mass spectrometry. In addition, a regenerative thermal cycle test was conducted in which small pieces were given a thermal history to reproduce welding. Specifically, the reproduction heat cycle test was carried out under the condition that the temperature was maintained at 1400 ° C. for 23 s and the temperature from 800 ° C. to 500 ° C. was cooled at 300 s (corresponding to welding heat input 450 kJ / cm).
Then, a V-notch test piece was prepared from the sample after the reproduction thermal cycle test with the position of 1/4 of the thickness from the surface of the steel plate as the center of the thickness of the test piece, and the Charpy test was performed in accordance with JIS Z 2242: 2005. Was done. The Charpy test was carried out at a test temperature of −20 ° C. with a number of tests of 3, and was evaluated by the lowest value of the measured Charpy absorption energy (vE- ₂₀ ). When the minimum value of the Charpy absorption energy of the three test pieces was 100 J or more, it was judged that the HAZ toughness was excellent. The results are shown in Tables 5 and 6.

As shown in Table 5, the steel materials (No. 1 to 25) having a steel component and X _sol (%) within the range of the present invention have high Charpy absorption energy at −20 ° C. after the regenerative heat cycle test. I understand. On the other hand, as shown in Table 6, the steel materials (No. 101 to 110) having a steel component or X _sol (%) outside the range of the present invention have a Charpy absorption energy at −20 ° C. after the regenerative heat cycle test. It can be seen that it is lower than that of the invention example.

No. In 101, the X element was not contained, the X _sol (%) became 0%, and the Charpy absorption energy decreased. No. 102-No. Each of 105 had a large Pb content, Bi content, Se content, and Te content, and X _sol (%) exceeded the upper limit in each case, so that the Charpy absorption energy decreased.

No. No. 106 does not contain Mg and is No. Since 107 has a low Al content, the Charpy absorption energy is reduced. No. Since 108 had a large O content, the Charpy absorption energy decreased. No. In 109, X _sol (%) became 0%, and the Charpy absorption energy decreased. No. In 110, the Charpy absorbed energy decreased because X _sol (%) exceeded the upper limit.

(Example 2)
The slab obtained by melting and casting steel was hot-rolled to obtain a steel plate having a thickness of 25 mm.
In the melting step, Al, X element, and Mg were added in the order shown in Table 9 in a state where the molten steel temperature was 1650 ° C. or lower and the molten steel O concentration was 0.0100% or less. Further, the content of other elements was adjusted to a predetermined range, and casting was performed by continuous casting to obtain a slab.

Samples are taken from the obtained steel plate and used by fluorescent X-ray analysis, combustion-infrared absorption, inert gas melting, inductively coupled plasma mass spectrometry (ICP mass spectrometry), etc. The composition of the steel plate was analyzed. The content of element X (Pb, Bi, Se, Te) contained in the steel sheet was determined by ICP mass spectrometry. The analysis results of the steel sheet components are shown in Tables 7 to 8.

The carbon equivalent Ceq shown in Table 8 was calculated by the following formula.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5+ (Cu + Ni) / 15
[C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] in the formula are the contents of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. (Mass%), and if it is not contained, 0 is substituted.

A sample was taken from the obtained steel sheet, heated and held at 1400 ° C. for 3 seconds, rapidly cooled, mirror-polished, and observed with an FE-SEM equipped with EDS. The number of particles having a circle-equivalent diameter of 0.5 to 5.0 μm was measured for an area of 25,000 μm ² or more, and converted into the number per unit area. Next, among the measured number of particles having a circle-equivalent diameter of 0.5 to 5.0 μm, 20 or more particles are mapped for the entire particles by EDS, and the concentration of the X element is determined. The number ratio of particles containing 1 atomic% or more of X element to the total of Mg, Mn, S and X elements was determined. The number density of these particles and the number ratio of particles containing the X element were evaluated according to the criteria shown below. The results are shown in Table 9.

(Particle number density standard)
OK: The number density of particles having a circle equivalent diameter of 0.5 to 5.0 μm is 1.00 to 1.00 × 10 ⁴ particles / mm ² at a position 1/4 of the plate thickness from the surface of the steel material.
NG: The number density of particles having a circle-equivalent diameter of 0.5 to 5.0 μm is less than 1.00 particles / mm ² .

(Based on the number ratio of particles containing element X)
OK: The number ratio of particles containing 1 atomic% or more of X element is 30% or more.
NG: The number ratio of particles containing 1 atomic% or more of X element is less than 30%.

In addition, a regenerative thermal cycle test was conducted in which small pieces were given a thermal history to reproduce welding. Specifically, the reproducible heat cycle test was carried out under the condition of holding at 1400 ° C. for 23 s and cooling from 800 ° C. to 500 ° C. at 300 s (corresponding to welding heat input 450 kJ / cm).
Then, from the sample after the reproduction thermal cycle test, a V-notch test piece was prepared with the position of 1/4 of the plate thickness from the surface of the steel plate as the center of the thickness of the test piece, and Charpy was prepared in accordance with JIS Z 2242: 2005. The test was conducted. The Charpy test was carried out at a test temperature of −20 ° C. with a number of tests of 3, and was evaluated by the lowest value of the measured Charpy absorption energy (vE- ₂₀ ).

As shown in Table 9, the number density of particles having a circle equivalent diameter of 0.5 to 5.0 μm is 1.00 to 1.00 × 10 ⁴ particles / mm at a position 1/4 of the plate thickness from the surface of the steel material. Steel materials (No. 201 to 225) having a number of ² and having a number ratio of particles containing 1 atomic% or more of X element among the particles of 30% or more absorb Charpy at −20 ° C. after the reproducible thermal cycle test. It can be seen that the energy is 150 J or more and the HAZ toughness is further excellent.

According to the present invention, it is possible to provide a steel material having good HAZ toughness after welding. In particular, the steel material of the present invention is suitable for various welded steel structures such as construction, bridges, shipbuilding, line pipes, construction machinery, marine structures, tanks, etc., which require a high tension with a yield strength of about 300 to 700 MPa. Can be used for.

Claims (5)

1. By mass%
   C: 0.01 to 0.20%,
   Si: 1.00% or less,
   Mn: 0.1-2.5%,
   Mg: 0.0005-0.0100%,
   Al: 0.015 to 0.500%,
   P: 0.020% or less,
   S: 0.020% or less,
   N: 0.0100% or less,
   O: Less than 0.0030%,
   Total of X elements Pb, Bi, Se, and Te: 0.0001 to 0.0100%,
   Cu: 0-2.0%,
   Ni: 0-2.0%,
   Cr: 0-2.0%,
   Mo: 0-1.0%,
   Nb: 0 to 0.10%,
   W: 0-2.0%,
   V: 0 to 0.20%,
   B: 0 to 0.010%,
   Ti: 0 to 0.100%,
   Zr: 0 to 0.10%,
   Ta: 0 to 0.10%,
   Ag: 0 to 0.10%,
   Hf: 0 to 0.10%,
   Ca: 0-0.0100%,
   REM: 0-0.010%,
   Sn: 0 to 0.50%,
   Sb: 0 to 0.50%,
   Containing, the balance consists of Fe and impurities,
   From the total X _total of the contents of Pb, Bi, Se, and Te, subtract X _insol , which is the total content of Pb, Bi, Se, and Te in the state where the inclusions obtained by the electrolytic extraction residue method are formed. A steel material having a mass% of X _sol of 0.0001 to 0.0050%.
2. By mass%
   Cu: 0.02-2.0%,
   Ni: 0.02-2.0%,
   Cr: 0.02 to 2.0%,
   Mo: 0.02 to 1.0%,
   Nb: 0.01 to 0.10%,
   W: 0.01-2.0%,
   V: 0.01 to 0.20%,
   B: 0.0003 to 0.010%,
   Ti: 0.005 to 0.100%,
   Zr: 0.01-0.10%,
   Ta: 0.01-0.10%,
   Ag: 0.01-0.10%,
   Hf: 0.01 to 0.10%,
   The steel material according to claim 1, wherein the steel material contains one or more of the above.
3. By mass%
   Ca: 0.0001 to 0.0100%,
   REM: 0.001 to 0.010%
   The steel material according to claim 1 or 2, wherein the steel material contains one or both of them.
4. By mass%
   Sn: 0.01 to 0.50%,
   Sb: 0.01 to 0.50%
   The steel material according to any one of claims 1 to 3, wherein the steel material contains one or both of them.
5. The O content is 0.0001% or more and less than 0.0030%.
   Particles with a diameter equivalent to a circle of 0.5 to 5.0 μm exist at a number density of 1.00 to 1.00 × 10 ⁴ particles / mm ² .
   Among the particles, the number ratio of the particles containing the X element in atomic% of 1% or more with respect to the total of the Ca, the Mg, the Mn, the S and the X element is 30% or more. The steel material according to any one of claims 1 to 4, which is characteristic.

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