WO2022145071A1

WO2022145071A1 - Steel material

Info

Publication number: WO2022145071A1
Application number: PCT/JP2021/014788
Authority: WO
Inventors: 恭平石川; 耕平中田; 謙木村; 美百合梅原; 淳高橋; 真吾山▲崎▼
Original assignee: 日本製鉄株式会社
Priority date: 2020-12-28
Filing date: 2021-04-07
Publication date: 2022-07-07

Abstract

Provided is a steel material having a predetermined chemical composition satisfying 0.61[Zr]+0.31[Hf]-1.75[O]-3.99[N]-1.74[S]≥0.0003 (in the expression, [Zr], [Hf], [O], [N], and [S] are each the content [mass%] of an element), the steel material including grain boundaries which have a crystal orientation difference of 23-45° and in which the total segregation amount of Zr and Hf is at least 0.2 atoms/nm2.

Description

Steel material

The present invention relates to steel materials.

Elements segregated at the grain boundaries of the steel material, such as phosphorus (P), sulfur (S), carbon (C), etc., may reduce the toughness, ductility, corrosion resistance, weldability, and other properties of the steel material. Are known. It is generally considered important to reduce grain boundary segregation in order to improve these properties of steel materials.

In relation to this, Patent Document 1 describes a metal material in which crystal grains are made finer and embrittlement due to grain boundary segregation is suppressed. Patent Document 1 describes that grain boundary segregation is suppressed by miniaturizing the crystal grain size, and a metal material having an excellent balance between strength and ductility / toughness can be obtained.

Patent Document 2 describes that in the case of ferritic stainless steel containing Nb, high-temperature cracking occurs due to welding due to the influence of Nb and P segregated at the grain boundaries. In Patent Document 2, a liquid phase is generated at the grain boundaries at the time of welding due to eutectic melting of the low melting point phosphoric compound generated at the grain boundaries and the matrix, and liquefaction cracking occurs due to the applied stress. , REM promotes grain boundary segregation of P and the like, and B tends to suppress it.

Patent Document 3 describes a high-strength hot-rolled steel sheet in which an appropriate amount of C is segregated at the large-angle grain boundaries of ferrite to reduce damage to the punched end face. Patent Document 3 describes that P has an effect of embrittlement of grain boundaries, and if the segregation amount of P increases, the segregation amount of C may decrease.

Japanese Patent Application Laid-Open No. 2003-171742 Japanese Unexamined Patent Publication No. 2013-204059 Japanese Unexamined Patent Publication No. 2008-261029

An object of the present invention is to provide a steel material in which the grain boundary segregation of P in steel is reduced by a novel configuration.

In order to achieve the above object, the present inventors have investigated a new element capable of reducing the grain boundary segregation of P in steel. As a result, the present inventors secure the amount of the specific element solidly dissolved in the steel to a certain amount or more, and reduce the grain boundary segregation of P in the steel by segregating the specific element at the grain boundaries. We found that it was possible to complete the present invention.

The steel materials that have achieved the above objectives are as follows.
(1) By mass%,
C: 0.001 to 1.000%,
Si: 0.01-3.00%,
Mn: 0.10 to 4.50%,
P: 0.300% or less,
S: 0.0300% or less,
Al: 0.001-5.000%,
N: 0.2000% or less,
O: 0.0100% or less,
At least one Z element selected from the group consisting of Zr: 0 to 0.8000% and Hf: 0 to 0.8000%.
Nb: 0-3.000%,
Ti: 0 to 0.500%,
Ta: 0 to 0.500%,
V: 0 to 1.00%,
Cu: 0 to 3.00%,
Ni: 0-60.00%,
Cr: 0 to 30.00%,
Mo: 0 to 5.00%,
W: 0 to 2.00%,
B: 0-0.0200%,
Co: 0 to 3.00%,
Be: 0 to 0.050%,
Ag: 0 to 0.500%,
Ca: 0-0.0500%,
Mg: 0-0.0500%,
At least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc: 0 to 0.5000% in total.
Sn: 0 to 0.300%,
Sb: 0 to 0.300%,
Te: 0 to 0.100%,
Se: 0 to 0.100%,
As: 0 to 0.050%,
Bi: 0 to 0.500%,
Pb: 0 to 0.500%, and the balance: Fe and impurities.
It has a chemical composition that satisfies the following formula 1 and has a chemical composition.
A steel material containing grain boundaries having a crystal orientation difference of 23 to 45 ° and a total segregation amount of Zr and Hf of 0.2 atoms / nm ² or more.
0.61 [Zr] +0.31 [Hf] -1.75 [O] -3.99 [N] -1.74 [S] ≧ 0.0003 ・・・ Equation 1
Here, [Zr], [Hf], [O], [N], and [S] are the content [mass%] of each element, and are 0 when the element is not contained.
(2) The chemical composition is mass%.
Nb: 0.003 to 3.000%,
Ti: 0.005 to 0.500%,
Ta: 0.001 to 0.500%,
V: 0.001 to 1.00%,
Cu: 0.001 to 3.00%,
Ni: 0.001 to 60.00%,
Cr: 0.001 to 30.00%,
Mo: 0.001 to 5.00%,
W: 0.001 to 2.00%,
B: 0.0001-0.0200%,
Co: 0.001 to 3.00%,
Be: 0.0003 to 0.050%,
Ag: 0.001 to 0.500%,
Ca: 0.0001-0.0500%,
Mg: 0.0001-0.0500%,
At least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc: 0.0001 to 0.5000% in total.
Sn: 0.001 to 0.300%,
Sb: 0.001 to 0.300%,
Te: 0.001 to 0.100%,
Se: 0.001 to 0.100%,
As: 0.001 to 0.050%,
Bi: 0.001 to 0.500%, and Pb: 0.001 to 0.500%
The steel material according to (1) above, which comprises one or more of the above.

According to the present invention, it is possible to provide a steel material in which the grain boundary segregation of P in steel is reduced.

<Steel material>
The steel material according to the embodiment of the present invention is based on mass%.
C: 0.001 to 1.000%,
Si: 0.01-3.00%,
Mn: 0.10 to 4.50%,
P: 0.300% or less,
S: 0.0300% or less,
Al: 0.001-5.000%,
N: 0.2000% or less,
O: 0.0100% or less,
At least one Z element selected from the group consisting of Zr: 0 to 0.8000% and Hf: 0 to 0.8000%.
Nb: 0-3.000%,
Ti: 0 to 0.500%,
Ta: 0 to 0.500%,
V: 0 to 1.00%,
Cu: 0 to 3.00%,
Ni: 0-60.00%,
Cr: 0 to 30.00%,
Mo: 0 to 5.00%,
W: 0 to 2.00%,
B: 0-0.0200%,
Co: 0 to 3.00%,
Be: 0 to 0.050%,
Ag: 0 to 0.500%,
Ca: 0-0.0500%,
Mg: 0-0.0500%,
At least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc: 0 to 0.5000% in total.
Sn: 0 to 0.300%,
Sb: 0 to 0.300%,
Te: 0 to 0.100%,
Se: 0 to 0.100%,
As: 0 to 0.050%,
Bi: 0 to 0.500%,
Pb: 0 to 0.500%, and the balance: Fe and impurities.
It has a chemical composition that satisfies the following formula 1 and has a chemical composition.
It is characterized by including grain boundaries having a crystal orientation difference of 23 to 45 ° and a total segregation amount of Zr and Hf of 0.2 atoms / nm ² or more.
0.61 [Zr] +0.31 [Hf] -1.75 [O] -3.99 [N] -1.74 [S] ≧ 0.0003 ・・・ Equation 1
Here, [Zr], [Hf], [O], [N], and [S] are the content [mass%] of each element, and are 0 when the element is not contained.

In order to improve the toughness, ductility, corrosion resistance and weldability of steel materials, it is important to reduce the grain boundary segregation of P in the steel. For example, in a steel material using a structure such as martensite and bainite for high strength, tempering treatment is performed at a predetermined temperature after quenching in order to secure low temperature toughness. During such tempering treatment, P in the steel may segregate at the old austenite grain boundaries, causing so-called tempering embrittlement, and as a result, the toughness of the steel material may be lowered. In relation to this, if the P content in the steel is reduced, the amount of P segregated at the grain boundaries can be reduced accordingly, so that tempering embrittlement due to the segregation of P grain boundaries occurs. Can be suppressed. However, excessively reducing the P content has a problem that it takes time for refining, which leads to a decrease in productivity and a significant increase in production cost.

Therefore, the present inventors have investigated an element that can reduce the grain boundary segregation of P in steel. As a result, the present inventors have identified the amount of specific elements that are solid-dissolved in the steel, that is, the elements of Zr and Hf (hereinafter, also referred to as "Z element"), and the inclusions formed by these elements in the steel. More specifically, a certain amount or more is secured while considering the relationship between these elements with oxides, nitrides and sulfides (that is, the effective amount of the Z element corresponding to the left side of the formula 1 is 0.0003. % Or more), and further, it was found that the grain boundary segregation of P can be reduced by segregating the Z element at the grain boundary.

Although not intended to be bound by any particular theory, by segregating the above Z element at the grain boundaries, it is possible to reduce the number of grain boundary sites where P segregates, and as a result, the grains of P. It is considered that the boundary segregation is reduced, or the grain boundary segregation of P is reduced by the action of other specific elements that segregate at the grain boundaries accompanying the grain boundary segregation of the Z element. However, the above-mentioned element Z has the property of easily forming inclusions composed of oxides, nitrides and sulfides in combination with O (oxygen), N (nitrogen) and S (sulfur) existing in the steel. Have. If the Z element forms such inclusions in the steel, the amount of solid solution of the Z element that can be segregated at the grain boundaries is reduced, and the grain boundary segregation of P cannot be sufficiently reduced. .. In the present invention, the solid solution amount of the Z element in consideration of such inclusions is calculated as the effective amount of the Z element by the above formula 1 which will be described in detail later, and the effective amount is set to a certain amount or more, that is, 0. By securing .0003% or more, the Z element can be segregated at the grain boundary in a sufficient amount, and more specifically, the Z element is segregated at the grain boundary having a crystal orientation difference of 23 to 45 °. Segregation can be performed in an amount such that the total sum is 0.2 atoms / nm ² or more, whereby the grain boundary segregation of P can be remarkably suppressed. Therefore, according to the present invention, it is possible to obtain a steel material in which the grain boundary segregation of P in the steel is sufficiently reduced without excessively reducing the P content, which is related to the grain boundary segregation of P. It is possible to remarkably improve the characteristics of the steel material, for example, the characteristics such as toughness, ductility, corrosion resistance, and weldability. The grain boundary crystal orientation difference of 23 to 45 ° does not necessarily limit the grain boundary where the Z element segregates, and the Z element segregates to the grain boundary having a crystal orientation difference other than the crystal orientation difference of 23 to 45 °. However, it is possible to suppress the grain boundary segregation of P. The grain boundaries having a crystal orientation difference of 23 to 45 ° correspond to the former austenite grain boundaries in the case of a steel material having a martensite structure. Therefore, by promoting the segregation of the Z element at the grain boundaries having such a crystal orientation difference, P segregates into the old austenite grain boundaries, for example, during the tempering treatment of a high-strength steel material having a martensite structure. Can be suppressed. As a result, according to the present invention, it is possible to suppress the occurrence of tempering embrittlement and the like, and therefore it is possible to significantly improve the toughness and other properties of the steel material.

The Z element in the present invention easily combines with O, N and S to form inclusions as described above, and therefore it is generally difficult to secure a predetermined solid solution amount in steel. Under these circumstances, the effect of reducing the grain boundary segregation of P by the Z element has not been known so far. However, recent advances in refining technology have made it possible to reduce the content of elements such as O, N, and S, which are generally present in steel as impurities, to extremely low levels. It was possible to realize a solid solution within a predetermined range of the Z element. Therefore, the effect of reducing the grain boundary segregation of P on the Z element has been clarified for the first time by the present inventors, and is extremely surprising and surprising.

Hereinafter, the steel material according to the embodiment of the present invention will be described in more detail. In the following description, "%", which is a unit of the content of each element, means "mass%" unless otherwise specified. Further, in the present specification, "-" indicating a numerical range is used to mean that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value unless otherwise specified.

[C: 0.001 to 1.000%]
Carbon (C) is an element necessary for stabilizing hardness and / or ensuring strength. In order to sufficiently obtain these effects, the C content is 0.001% or more. The C content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if C is excessively contained, toughness, bendability and / or weldability may decrease. Therefore, the C content is 1.000% or less. The C content may be 0.800% or less, 0.600% or less, or 0.500% or less.

[Si: 0.01 to 3.00%]
Silicon (Si) is a deoxidizing element and is an element that also contributes to the improvement of strength. In order to sufficiently obtain these effects, the Si content is 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, or 0.30% or more. On the other hand, if Si is excessively contained, the toughness may be lowered or surface quality defects called scale defects may occur. Therefore, the Si content is 3.00% or less. The Si content may be 2.00% or less, 1.00% or less, or 0.60% or less.

[Mn: 0.10 to 4.50%]
Manganese (Mn) is an element effective for improving hardenability and / or strength, and is also an effective austenite stabilizing element. In order to sufficiently obtain these effects, the Mn content is 0.10% or more. The Mn content may be 0.50% or more, 0.70% or more, or 1.00% or more. On the other hand, if Mn is excessively contained, MnS harmful to toughness may be generated or the oxidation resistance may be lowered. Therefore, the Mn content is 4.50% or less. The Mn content may be 4.00% or less, 3.50% or less, or 3.00% or less.

[P: 0.300% or less]
Phosphorus (P) is an element mixed in the manufacturing process. From the viewpoint of reducing the grain boundary segregation of P in the steel, the smaller the P, the more preferable, and the P content may be 0%. However, in order to reduce the P content to less than 0.0001%, it takes time for refining, which leads to a decrease in productivity. Therefore, the P content may be 0.0001% or more, 0.0005% or more, 0.001% or more, 0.003% or more, or 0.005% or more. The P content may be 0.007% or more from the viewpoint of manufacturing cost. On the other hand, if P is excessively contained, the grain boundary segregation amount of P may increase, and various properties of the steel material such as toughness, ductility, corrosion resistance and / or weldability may deteriorate. Therefore, the P content is 0.300% or less. The P content may be 0.100% or less, 0.030% or less, or 0.010% or less.

[S: 0.0300% or less]
Sulfur (S) is an element mixed in the manufacturing process, and is preferable from the viewpoint of reducing inclusions formed with the Z element according to the embodiment of the present invention, so that the S content is 0%. There may be. However, in order to reduce the S content to less than 0.0001%, it takes time for refining, which leads to a decrease in productivity. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if S is excessively contained, the effective amount of the Z element may decrease and the toughness may decrease. Therefore, the S content is 0.0300% or less. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, and most preferably 0.0030% or less.

[Al: 0.001-5.000%]
Aluminum (Al) is a deoxidizing element and is also an effective element for improving corrosion resistance and / or heat resistance. In order to obtain these effects, the Al content is 0.001% or more. The Al content may be 0.010% or more, 0.100% or more, or 0.200% or more. In particular, from the viewpoint of sufficiently improving the heat resistance, the Al content may be 1.000% or more, 2.000% or more, or 3.000% or more. On the other hand, if Al is excessively contained, coarse inclusions may be generated to reduce toughness, troubles such as cracking may occur in the manufacturing process, and / or fatigue resistance may be deteriorated. .. Therefore, the Al content is 5.000% or less. The Al content may be 4.500% or less, 4000% or less, or 3.500% or less. In particular, from the viewpoint of suppressing the decrease in toughness, the Al content may be 1.500% or less, 1.000% or less, or 0.300% or less.

[N: 0.2000% or less]
Nitrogen (N) is an element mixed in the manufacturing process, and is preferable from the viewpoint of reducing inclusions formed with the Z element according to the embodiment of the present invention, so that the N content is 0%. There may be. However, in order to reduce the N content to less than 0.0001%, it takes time for refining, which leads to a decrease in productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, N is also an element effective for stabilizing austenite, and may be intentionally contained if necessary. In this case, the N content is preferably 0.0100% or more, and may be 0.0200% or more and 0.0500% or more. However, if N is excessively contained, the effective amount of the Z element may decrease and the toughness may decrease. Therefore, the N content is 0.2000% or less. The N content may be 0.1500% or less, 0.1000% or less, or 0.0800% or less.

[O: 0.0100% or less]
Oxygen (O) is an element mixed in the manufacturing process, and is preferable from the viewpoint of reducing inclusions formed with the Z element according to the embodiment of the present invention, so that the O content is 0%. There may be. However, in order to reduce the O content to less than 0.0001%, it takes time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if O is excessively contained, coarse inclusions may be formed, the effective amount of the Z element may be lowered, and the formability and / or toughness of the steel material may be lowered. Therefore, the O content is 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[At least one Z element selected from the group consisting of Zr: 0 to 0.8000% and Hf: 0 to 0.8000%]
The Z element according to the embodiment of the present invention is Zr: 0 to 0.8000% and Hf: 0 to 0.8000%, and zirconium (Zr) and hafnium (Hf) are present in the steel in a solid solution state. By doing so, it is possible to exhibit the effect of reducing the grain boundary segregation of P based on the grain boundary segregation of these Z elements. By exhibiting the effect of reducing the grain boundary segregation of P, it becomes possible to remarkably improve the characteristics of the steel material related to the grain boundary segregation of P, such as toughness, ductility, corrosion resistance, and weldability. ..

As the Z element, any one element may be used alone, or both may be used. Further, the Z element may be present in an amount satisfying Equation 1, which will be described in detail later, and the lower limit thereof is not particularly limited. However, for example, the content of each Z element or the total content may be 0.0010% or more, preferably 0.0050% or more, more preferably 0.0150% or more, and even more. It is preferably 0.0300% or more, and most preferably 0.0500% or more. On the other hand, even if the Z element is excessively contained, the effect is saturated, and therefore, if the Z element is contained in the steel material more than necessary, the manufacturing cost may increase. Therefore, the content of each Z element is 0.8000% or less, for example, 0.7000% or less, 0.6000% or less, 0.5000% or less, 0.4000% or less, or 0.3000% or less. May be good. The total content of the Z element is 1.6000% or less, for example, 1.2000% or less, 1.000% or less, 0.8000% or less, 0.6000% or less, or 0.5000% or less. You may.

The basic chemical composition of the steel material according to the embodiment of the present invention is as described above. Further, the steel material may contain one or more of the following optional elements, if necessary. For example, the steel material has Nb: 0 to 3.000%, Ti: 0 to 0.500%, Ta: 0 to 0.500%, V: 0 to 1.00%, Cu: 0 to 3.00%, Ni: 0 to 60.00%, Cr: 0 to 30.00%, Mo: 0 to 5.00%, W: 0 to 2.00%, B: 0 to 0.0200%, Co: 0 to 3 It may contain one or more of 0.00%, Be: 0 to 0.050%, and Ag: 0 to 0.500%. The steel materials include Ca: 0 to 0.0500%, Mg: 0 to 0.0500%, and La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er. At least one of Tm, Yb, Lu, and Sc: one or more of 0 to 0.5000% in total may be contained. Further, the steel material may contain one or two of Sn: 0 to 0.300% and Sb: 0 to 0.300%. The steel materials are Te: 0 to 0.100%, Se: 0 to 0.100%, As: 0 to 0.050%, Bi: 0 to 0.500%, and Pb: 0 to 0.500%. One or more of them may be contained. Hereinafter, these optional elements will be described in detail.

[Nb: 0-3.000%]
Niobium (Nb) is an element that contributes to strengthening precipitation and suppressing recrystallization. The Nb content may be 0%, but in order to obtain these effects, the Nb content is preferably 0.003% or more. For example, the Nb content may be 0.005% or more or 0.010% or more. In particular, the Nb content may be 1.000% or more or 1.500% or more from the viewpoint of sufficiently strengthening precipitation. On the other hand, if Nb is excessively contained, the effect may be saturated and the processability and / or toughness may be lowered. Therefore, the Nb content is 3.000% or less. The Nb content may be 2.800% or less, 2.500% or less, or 2.000% or less. In particular, from the viewpoint of suppressing the decrease in toughness of the weld heat affected zone (HAZ), the Nb content is preferably 0.100% or less, 0.080% or less, 0.050% or less, or 0.030. It may be less than or equal to%.

[Ti: 0 to 0.500%]
Titanium (Ti) is an element that contributes to improving the strength of steel materials by strengthening precipitation. The Ti content may be 0%, but in order to obtain such an effect, the Ti content is preferably 0.005% or more. The Ti content may be 0.010% or more, 0.050% or more, or 0.080% or more. On the other hand, if Ti is excessively contained, a large amount of precipitates may be formed and the toughness may be lowered. Therefore, the Ti content is 0.500% or less. The Ti content may be 0.300% or less, 0.200% or less, or 0.100% or less.

[Ta: 0 to 0.500%]
Tantalum (Ta) is an element effective in controlling the morphology of carbides and increasing their strength. The Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if Ta is excessively contained, a large amount of fine Ta carbides may be deposited, which may lead to an excessive increase in strength of the steel material, resulting in a decrease in ductility and a decrease in cold workability. Therefore, the Ta content is 0.500% or less. The Ta content may be 0.300% or less, 0.100% or less, or 0.080% or less.

[V: 0 to 1.00%]
Vanadium (V) is an element that contributes to improving the strength of steel materials by strengthening precipitation. The V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more. The V content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, if V is excessively contained, a large amount of precipitates may be formed and the toughness may be lowered. Therefore, the V content is 1.00% or less. The V content may be 0.80% or less, 0.60% or less, or 0.50% or less.

[Cu: 0 to 3.00%]
Copper (Cu) is an element that contributes to the improvement of strength and / or corrosion resistance. The Cu content may be 0%, but in order to obtain these effects, the Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more, 0.10% or more, 0.15% or more, 0.20% or more, or 0.30% or more. On the other hand, if Cu is excessively contained, the toughness and weldability may be deteriorated. Therefore, the Cu content is 3.00% or less. The Cu content may be 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[Ni: 0-60.00%]
Nickel (Ni) is an element that contributes to the improvement of strength and / or heat resistance, and is also an effective austenite stabilizing element. The Ni content may be 0%, but in order to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.10% or more, 0.50% or more, 0.70% or more, 1.00% or more, or 3.00% or more. In particular, from the viewpoint of sufficiently improving the heat resistance, the Ni content may be 30.00% or more, 35.00% or more, or 40.00% or more. On the other hand, if Ni is excessively contained, the deformation resistance during hot working increases in addition to the increase in alloy cost, which may increase the equipment load. Therefore, the Ni content is 60.00% or less. The Ni content may be 55.00% or less or 50.00% or less. In particular, from the viewpoint of economic efficiency and / or from the viewpoint of suppressing deterioration of weldability, the Ni content is 15.00% or less, 10.00% or less, 6.00% or less, or 4.00% or less. You may.

[Cr: 0 to 30.00%]
Chromium (Cr) is an element that contributes to the improvement of strength and / or corrosion resistance. The Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.05% or more, 0.10% or more, or 0.50% or more. In particular, from the viewpoint of sufficiently improving the corrosion resistance, the Cr content may be 10.00% or more, 12.00% or more, or 15.00% or more. On the other hand, if Cr is excessively contained, the toughness may decrease in addition to the increase in alloy cost. Therefore, the Cr content is 30.00% or less. The Cr content may be 28.00% or less, 25.00% or less, or 20.00% or less. In particular, from the viewpoint of suppressing deterioration of weldability and / or processability, the Cr content may be 10.00% or less, 9.00% or less, or 7.50% or less.

[Mo: 0 to 5.00%]
Molybdenum (Mo) is an element that enhances the hardenability of steel and contributes to the improvement of strength, and is also an element that contributes to the improvement of corrosion resistance. The Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.50% or more, or 1.00% or more. On the other hand, if Mo is excessively contained, the deformation resistance during hot working increases, and the equipment load may increase. Therefore, the Mo content is 5.00% or less. The Mo content may be 4.50% or less, 4.00% or less, 3.00 or less, or 1.50% or less.

[W: 0 to 2.00%]
Tungsten (W) is an element that enhances the hardenability of steel and contributes to the improvement of strength. The W content may be 0%, but in order to obtain such an effect, the W content is preferably 0.001% or more. The W content may be 0.01% or more, 0.02% or more, 0.05% or more, 0.10% or more, or 0.50% or more. On the other hand, if W is excessively contained, ductility and weldability may decrease. Therefore, the W content is 2.00% or less. The W content may be 1.80% or less, 1.50% or less, or 1.00% or less.

[B: 0 to 0.0200%]
Boron (B) is an element that contributes to the improvement of strength. The B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more. The B content may be 0.0003% or more, 0.0005% or more, or 0.0007% or more. On the other hand, if B is excessively contained, toughness and / or weldability may decrease. Therefore, the B content is 0.0200% or less. The B content may be 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Co: 0 to 3.00%]
Cobalt (Co) is an element that contributes to the improvement of hardenability and / or heat resistance. The Co content may be 0%, but in order to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.02% or more, 0.05% or more, 0.10% or more, or 0.50% or more. On the other hand, if Co is excessively contained, the hot workability may be lowered, which leads to an increase in raw material cost. Therefore, the Co content is 3.00% or less. The Co content may be 2.50% or less, 2.00% or less, 1.50% or less, or 0.80% or less.

[Be: 0 to 0.050%]
Beryllium (Be) is an element effective for increasing the strength of the base metal and refining the structure. The Be content may be 0%, but in order to obtain such an effect, the Be content is preferably 0.0003% or more. The Be content may be 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, if Be is excessively contained, the moldability may be deteriorated. Therefore, the Be content is 0.050% or less. The Be content may be 0.040% or less, 0.030% or less, or 0.020% or less.

[Ag: 0 to 0.500%]
Silver (Ag) is an element effective for increasing the strength of the base material and refining the structure. The Ag content may be 0%, but in order to obtain such an effect, the Ag content is preferably 0.001% or more. The Ag content may be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, if Ag is excessively contained, the moldability may be deteriorated. Therefore, the Ag content is 0.500% or less. The Ag content may be 0.400% or less, 0.300% or less, or 0.200% or less.

[Ca: 0-0.0500%]
Calcium (Ca) is an element that can control the morphology of sulfides. The Ca content may be 0%, but in order to obtain such an effect, the Ca content is preferably 0.0001% or more. On the other hand, even if Ca is excessively contained, the effect is saturated, and therefore, if Ca is contained in the steel material more than necessary, the manufacturing cost may increase. Therefore, the Ca content is 0.0500% or less.

[Mg: 0 to 0.0500%]
Magnesium (Mg) is an element that can control the morphology of sulfides. The Mg content may be 0%, but in order to obtain such an effect, the Mg content is preferably 0.0001% or more. The Mg content may be greater than 0.0015%, greater than 0.0016%, greater than or equal to 0.0018% or greater than or equal to 0.0020%. On the other hand, even if Mg is excessively contained, the effect is saturated, and cold formability and / or toughness may decrease due to the formation of coarse inclusions. Therefore, the Mg content is 0.0500% or less. The Mg content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.

[At least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc: 0 to 0.5000% in total]
Lantern (La), Cerium (Ce), Neogym (Nd), Yttrium (Y), Promethium (Pm), Placeozim (Pr), Samarium (Sm), Europium (Eu), Gadrinium (Gd), Terbium (Tb), Dysprosium (Dy), formium (Ho), elbium (Er), yttrium (Tm), yttrium (Yb), lutetium (Lu), and scandium (Sc) can control the morphology of sulfides as well as Ca and Mg. It is an element. Even if the total content of at least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc is 0%. However, in order to obtain such an effect, it is preferably 0.0001% or more. The total content of at least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc is 0.0002% or more. It may be 0.0003% or more or 0.0004% or more. On the other hand, even if these elements are excessively contained, the effect is saturated, and therefore, including these elements in the steel material more than necessary may lead to an increase in manufacturing cost. Therefore, the total content of at least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc is 0.5000%. It may be less than or equal to 0.4000%, 0.3000% or less, or 0.2000% or less.

[Sn: 0 to 0.300%]
Tin (Sn) is an element effective for improving corrosion resistance. The Sn content may be 0%, but in order to obtain such an effect, the Sn content is preferably 0.001% or more. The Sn content may be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, excessive inclusion of Sn may lead to a decrease in toughness, particularly low temperature toughness. Therefore, the Sn content is 0.300% or less. The Sn content may be 0.250% or less, 0.200% or less, or 0.150% or less.

[Sb: 0 to 0.300%]
Antimony (Sb) is an element effective for improving corrosion resistance like Sn, and the effect can be increased by including it in combination with Sn. The Sb content may be 0%, but in order to obtain the effect of improving the corrosion resistance, the Sb content is preferably 0.001% or more. The Sb content may be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, excessive content of Sb may lead to a decrease in toughness, particularly low temperature toughness. Therefore, the Sb content is 0.300% or less. The Sb content may be 0.250% or less, 0.200% or less, or 0.150% or less.

[Te: 0 to 0.100%]
Tellurium (Te) is an element effective for improving the machinability of steel because it forms a low melting point compound with Mn, S and the like to enhance the lubricating effect. The Te content may be 0%, but in order to obtain such an effect, the Te content is preferably 0.001% or more. The Te content may be 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, even if Te is excessively contained, the effect is saturated and the alloy cost increases. Therefore, the Te content is 0.100% or less. The Te content may be 0.090% or less, 0.080% or less, or 0.070% or less.

[Se: 0 to 0.100%]
Selenium (Se) is an effective element for improving the machinability of steel because the selenium produced in the steel changes the shear-plastic deformation of the work material and the chips are easily crushed. .. The Se content may be 0%, but in order to obtain such an effect, the Se content is preferably 0.001% or more. The Se content may be 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, even if Se is excessively contained, the effect is saturated and the alloy cost increases. Therefore, the Se content is 0.100% or less. The Se content may be 0.090% or less, 0.080% or less, or 0.070% or less.

[As: 0 to 0.050%]
Arsenic (As) is an element effective in improving the machinability of steel. The As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more. The As content may be 0.005% or more or 0.010% or more. On the other hand, if As is excessively contained, the hot workability may be deteriorated. Therefore, the As content is 0.050% or less. The As content may be 0.040% or less, 0.030% or less, or 0.020% or less.

[Bi: 0 to 0.500%]
Bismuth (Bi) is an element effective in improving the machinability of steel. The Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.001% or more. The Bi content may be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, even if Bi is excessively contained, the effect is saturated and the alloy cost increases. Therefore, the Bi content is 0.500% or less. The Bi content may be 0.400% or less, 0.300% or less, or 0.200% or less.

[Pb: 0 to 0.500%]
Lead (Pb) is an element effective for improving the machinability of steel because it melts when the temperature rises due to cutting and promotes the growth of cracks. The Pb content may be 0%, but in order to obtain such an effect, the Pb content is preferably 0.001% or more. The Pb content may be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, if Pb is excessively contained, the hot workability may be deteriorated. Therefore, the Pb content is 0.500% or less. The Pb content may be 0.400% or less, 0.300% or less, or 0.200% or less.

In the steel material according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. Impurities are components that are mixed in by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel materials are industrially manufactured.

[Effective amount of Z element]
According to the embodiment of the present invention, the effective amount of the Z element consisting of Zr and Hf is determined by the left side of the following formula 1, and the value thereof satisfies the following formula 1.
0.61 [Zr] +0.31 [Hf] -1.75 [O] -3.99 [N] -1.74 [S] ≧ 0.0003 ・・・ Equation 1
Here, [Zr], [Hf], [O], [N], and [S] are the content [mass%] of each element, and are 0 when the element is not contained.

By making the effective amount of the Z element satisfy the above formula 1, the amount of these elements existing in the solid solution state in the steel can be increased, so that the Z element can be sufficiently applied to the grain boundary. It is possible to segregate in a large amount, and as a result, it is possible to reduce the grain boundary segregation of P. More specifically, these Z elements (hereinafter, also simply referred to as “Z”) are combined with O (oxygen), N (nitrogen) and S (sulfur) present in steel to form an oxide (ZO ₂ ). , Tends to form inclusions consisting of nitrides (ZN) and sulfides (ZS). Once the inclusions are formed, at least the Z element in these inclusions cannot segregate at the grain boundaries. Therefore, in order to promote the segregation of Z element into the grain boundary and reduce the segregation of P in the steel, the Z element existing in the steel in a solid solution state without forming inclusions is formed. It is necessary to increase the amount (that is, the solid solution amount of the Z element in the steel).

Here, the solid solution amount of the Z element in the steel is obtained by subtracting the maximum amount that can be consumed to form inclusions (oxides, nitrides and sulfides) from the amount of the Z element contained in the steel. It is possible to make an approximation. Therefore, in the embodiment of the present invention, the amount of Z element effective for reducing the grain boundary segregation of P in the steel estimated in this way (that is, "effective amount of Z element") is concrete. Is defined by the following formula A.
Effective amount of Z element [atomic%] = Σ (M _[Fe] / M _[Z] ) x [Z]-(M _[Fe] / M _[O] ) x [O] x 1 / 2- (M _[ ] _Fe] / M _[N] ) x [N]-(M _[Fe] / M _[S] ) x [S] ... Equation A
Here, Z represents each Z element of Zr and Hf, M _[Z] is the atomic weight of the Z element, M _[Fe] is the atomic weight of Fe, M _[O] is the atomic weight of O, and M _[N] is N. Atomic weight, M _[S] represents the atomic weight of S, and [Z], [O], [N], and [S] are the corresponding element content [mass%], respectively, when no element is contained. Is 0.

The above formula A will be described in detail below. First, although the steel material according to the embodiment of the present invention contains various alloying elements, the steel material as a whole is almost composed of Fe or is an optional element. When a certain Ni and / or Cr is contained in a relatively large amount (the maximum contents are 60.00% and 30.00%, respectively), it is clear that it is almost composed of Ni and / or Cr in addition to Fe. Is. On the other hand, it is well known that the atomic weights of Ni and Cr are equivalent to the atomic weights of Fe. Therefore, even when the steel material contains a relatively large amount of Ni and / or Cr, the atomic% of each Z element of Zr and Hf is approximately Fe in the content [mass%] of each Z element. It can be calculated by multiplying the atomic weight of each Z element by the ratio of the atomic weight of each Z element, that is, (M _[Fe] / M _[Z] ) × [Z]. Therefore, by summing up the amounts of each Z element calculated by (M [ _Fe ] / M [Z _] ) × [Z] (that is, Σ (M _[Fe] / M _[Z] ) × [Z]. By calculation), the atomic% of the entire Z element can be calculated.

Next, by subtracting the maximum amount (atomic%) that can be consumed to form oxides (ZO ₂ ), nitrides (ZN) and sulfides (ZS) from the atomic% of the total Z element, P It is possible to calculate the amount of Z element in the steel that can effectively act to reduce the grain boundary segregation. Here, the maximum amount (atomic%) of Z element that can be consumed to form oxides (ZO ₂ ), nitrides (ZN) and sulfides (ZS) is for the same reasons as described above. Approximately, using the atomic weights of Fe, O, N and S and the contents of O, N and S in the steel, (M _[Fe] / M _[O] ) × [O] × 1/2, respectively. It can be calculated as (M _[Fe] / M _[N] ) × [N] and (M _[Fe] / M _[S] ) × [S]. Therefore, the effective amount of the Z element for reducing the grain boundary segregation of P can be defined by the following formula A.
Effective amount of Z element [atomic%] = Σ (M _[Fe] / M _[Z] ) x [Z]-(M _[Fe] / M _[O] ) x [O] x 1 / 2- (M _[ ] _Fe] / M _[N] ) x [N]-(M _[Fe] / M _[S] ) x [S] ... Equation A

Here, the atomic weights of Fe, O, N and S and each Z element are Fe: 55.845, O: 15.9994, N: 14.0069, S: 32.068, Zr: 91.224, Hf, respectively. It is 178.49. Therefore, by substituting the atomic weight of each element into the above formula A and rearranging it, the effective amount of the Z element in terms of atomic% can be approximately expressed by the following formula B.
Effective amount = 0.61 [Zr] +0.31 [Hf] -1.75 [O] -3.99 [N] -1.74 [S] ... Expression B
Here, [Zr], [Hf], [O], [N], and [S] are the content [mass%] of each element, and are 0 when the element is not contained.

In the embodiment of the present invention, in order to reduce the grain boundary segregation of P, it is necessary that the effective amount of the Z element determined by the above formula B is 0.0003% or more, that is, at least satisfy the following formula 1. ..
0.61 [Zr] +0.31 [Hf] -1.75 [O] -3.99 [N] -1.74 [S] ≧ 0.0003 ・・・ Equation 1
The effective amount of the Z element may be, for example, 0.0005% or more or 0.0007% or more, preferably 0.0010% or more, more preferably 0.0015% or more, still more preferably 0.0030%. The above is most preferably 0.0050% or more or 0.0100% or more. Further, as is clear from the above formula 1, in order to stably secure the effective amount, it is preferable to reduce the contents of O, N and S in the steel as much as possible. Here, the upper limit of the effective amount of the Z element is not particularly limited, but even if the effective amount of the Z element is excessively increased, the effect is saturated and the manufacturing cost increases (alloy cost due to the increase in the content of the Z element). And / or an increase in refining cost for O, N and S), which is not always preferable. Therefore, the effective amount of Z element is 2.000% or less, for example, even if it is 1.8000% or less, 1.5000% or less, 1.2000% or less, 1.000% or less, or 0.8000% or less. good.

[The total amount of segregation of Zr and Hf at grain boundaries with a crystal orientation difference of 23 to 45 ° is 0.2 atoms / nm ² or more]
In the embodiment of the present invention, in order to reduce the grain boundary segregation of P, it is necessary that the Z element segregates at the grain boundary in a predetermined amount. In the embodiment of the present invention, Z elements, that is, Zr and Hf, are segregated at grain boundaries having a crystal orientation difference of 23 to 45 ° at an amount such that the total segregation amount is 0.2 atoms / nm ² or more. If this is the case, the effect of the Z element can be sufficiently exerted, so that the grain boundary segregation of P can be reliably reduced. The total segregation amount of the Z element is preferably 0.3 atoms / nm ² or more, more preferably 0.4 atoms / nm ² or more, still more preferably 0.6 atoms / nm ² or more, and most preferably 0.7 atoms. / Nm ² or more or 1.0 atoms / nm ² or more. The upper limit of the total sum of segregation amounts is not particularly limited, but even if the segregation amount is excessively increased, the effect is saturated and the manufacturing cost increases (the alloy cost increases due to the increase in the content of Z element and / or O). , Increase in refining cost for N and S), which is not always preferable. Therefore, the total segregation amount is 5.0 atoms / nm ² or less, for example, 4.5 atoms / nm ² or less, 4.0 atoms / nm ² or less, 3.5 atoms / nm ² or less, 3.2 atoms / nm ² or less. Alternatively, it may be 3.0 atoms / nm ² or less.

[Measurement of grain boundary segregation amount of element Z]
The amount of segregation of the Z element at the grain boundaries having a crystal orientation difference of 23 to 45 ° is determined as follows using the three-dimensional atom probe method (for example, J. Takahashi et al., "Atomic-scale study". on segmentation behave or austenite grain boundates in boron-and molybdenum-added steels ”, Acta Matter. Vol. 133, pp. 41-54, 2017). First, by EBSD (electron backscatter diffraction method), grain boundaries having a crystal orientation difference of 23 to 45 ° are selected from grain boundaries in a sample collected from a 1 / 4t portion of a steel material. Next, a minute block is cut out from this grain boundary by a lift-out method of FIB (focused ion beam processing). The block is attached to the needle pedestal by a Pt vapor deposition method or the like, and FIB processing is performed so that the grain boundaries in the block are located at the needle tip. This needle sample is measured by the three-dimensional atom probe method. The three-dimensional atom probe measurement may be either a normal electrode type or a local electrode type, and the measurement mode may be either a voltage mode or a laser mode. In the voltage mode, the sample temperature is 50 to 70 K, the applied voltage is 3 to 15 kV, and the pulse ratio indicating the ratio of the pulse voltage to the DC voltage is 15 to 25%. In the laser mode, the sample temperature is 30 to 60 K, and the laser pulse energy is a value corresponding to a pulse ratio of 10 to 30%. From the data acquired by the 3D atom probe measurement, a 3D element map including the grain boundary can be obtained by using the dedicated 3D construction software. The grain boundary positions can be recognized from the positions where the elements are concentrated in a planar manner and the atomic planes are discontinuous.

Next, the amount of grain boundary segregation is quantified (see, for example, Takahashi et al., "Quantitative observation of the amount of grain boundary segregated carbon in coated and baked hardened steel sheets", Nippon Steel Technical Report, No. 381, 26-30, 2004). .. A selection box larger than 10 nm × 10 nm × 10 nm is set so as to cross perpendicular to the grain interface, for example, a selection box of 20 nm × 20 nm parallel to the grain boundary and 30 nm perpendicular to the grain boundary is set and perpendicular to the grain interface. A ladder chart is drawn for each segregating element in the direction (see, for example, FIG. 2 of JP-A-2008-261029). Since the increase in the integrated atomic number with the grain boundary as the boundary indicates the segregated atomic number, divide this value by the grain boundary area (for example, 20 nm × 20 nm) and further divide by the ion detection rate of the three-dimensional atom probe. It is possible to obtain the Interfacial Excess value indicating the amount of grain boundary segregation. The arithmetic average of the total value of the interfacial excess values indicating the amount of grain boundary segregation of the Z element (Zr and Hf) obtained by performing the same measurement at 5 points in the steel material is "a crystal orientation difference of 23 to 45 °." It is determined as "the total amount of segregation of Z elements (Zr and Hf) at the grain boundary". Therefore, the steel material according to the embodiment of the present invention has a grain boundary with a crystal orientation difference of 23 to 45 ° in the arithmetic mean when five points in the steel material are measured by the three-dimensional atom probe method, and Zr and Hf. The total segregation amount of Zr and Hf in the steel material containing grain boundaries with a total segregation amount of 0.2 atoms / nm ² or more, or the grain boundaries with a crystal orientation difference of 23 to 45 ° is 0.2 atoms / nm ² or more. It can also be said that it is a steel material.

The steel material according to the embodiment of the present invention may be any steel material in which the Z element is segregated at the grain boundaries, and is not particularly limited. The steel material according to the embodiment of the present invention includes, for example, thick steel plates, thin steel plates, steel bars, wire rods, shaped steels, steel pipes, and the like.

The steel material according to the embodiment of the present invention can be manufactured by any suitable method known to those skilled in the art, depending on the form of the final product and the like. For example, when the steel material is a thick steel plate, the manufacturing method includes a step generally applied when manufacturing the thick steel plate, for example, a step of casting a slab having the chemical composition described above, and casting. It includes a step of hot rolling the slab and a step of cooling the obtained rolled material, and may further include a quenching step, a tempering step and the like, if necessary. The steel material manufacturing process according to the embodiment of the present invention may be a thermal processing control process (TMCP) that combines controlled rolling and accelerated cooling.

Further, when the steel material is a thin steel plate, the manufacturing method includes a step generally applied when manufacturing the thin steel plate, for example, a step of casting a slab having the chemical composition described above, and casting. It may further include a step of hot rolling the slab, a step of cooling and winding the obtained rolled material, a cold rolling step, a baking step and the like, if necessary. Similarly, in the method for producing steel bars and other steel materials, a step generally applied when manufacturing steel bars and other steel materials is included, and for example, a steelmaking process for forming molten steel having the chemical composition described above is formed. It includes a step of casting slabs, billets, blooms, etc. from molten steel, a step of hot rolling the cast slabs, billets, blooms, etc., and a step of cooling the obtained rolled material, and other steps include those steps. Appropriate steps known to those skilled in the art for producing steel materials can be appropriately selected and carried out.

The segregation of the Z element into the grain boundaries can be controlled by the heat history in the hot rolling process or the heat treatment after the hot rolling process. When the Z element is segregated at the grain boundary in the heat history in the hot rolling step, the hot rolling may be performed in consideration of the temperature and time range in which each Z element segregates, and then the cooling may be performed in the cooling step. Although not particularly limited, for example, such a thermal history includes setting the holding time in the temperature range of 850 to 1100 ° C. for 5 seconds or more after rough rolling and / or finish rolling. The term "retention" includes a case where the temperature gradually decreases due to cooling, air cooling, or the like within the above temperature range. The upper limit of the holding time is not particularly limited, but may be, for example, 1800 seconds or less.

Further, after the hot rolling step and the cooling step, the Z element can be segregated at the grain boundary by performing a heat treatment of heating, holding and cooling to a temperature at which each Z element segregates. When heat treatment is performed, the holding time in the temperature range of 850 to 1100 ° C. may be 5 seconds or longer, preferably 10 seconds or longer, and cooling may be performed. The upper limit of the holding time is not particularly limited, but may be, for example, 1800 seconds or less or 600 seconds or less. When the heating temperature of the heat treatment is high, for example, if it is heated to 1200 ° C. or higher and cooled without holding it in the temperature range of 850 to 1100 ° C., the distribution of each Z element becomes uniform and the grain boundary segregation may decrease. be.

Since the Z element can be sufficiently segregated to the grain boundaries by the thermal history of the hot rolling process or the heat treatment after the hot rolling process, P is segregated to the grain boundaries even by the subsequent tempering treatment or the like. It can be suppressed. As a result, it is possible to suppress the occurrence of tempering embrittlement and the like, and therefore it is possible to significantly improve the toughness and other properties of the steel material. Further, in the production of the steel material according to the embodiment of the present invention, it is important to secure an effective amount of Z element for reducing the grain boundary segregation of P, and for that purpose, Z element and inclusions in the steel are used. It is extremely important that the contents of O, N and S that can be formed are sufficiently reduced in the refining step.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[Examples 1 to 83 and Comparative Examples 84 to 90]
First, slabs having various chemical compositions were cast, and then hot rolling was carried out at a rolling reduction of 50% or more and cooled. Next, the obtained rolled material was heated, held at a predetermined temperature in the range of 850 to 1100 ° C. for 20 to 50 seconds, and then rapidly cooled to obtain a steel material in which the Z element was segregated at the grain boundaries.

[Comparative Example 91]
Hot rolling was carried out in the same manner as in Example 1, and after cooling, the rolled material was heated to 1200 ° C. and held for 50 seconds, and then rapidly cooled to obtain a steel material. The chemical compositions obtained by analyzing the samples collected from the steel materials obtained in Examples and Comparative Examples are as shown in Table 1 below. The grain boundary segregation amount of the Z element in each of the obtained steel materials was measured by the following method.

[Measurement of grain boundary segregation amount of element Z]
The amount of segregation of the Z element at the grain boundaries having a crystal orientation difference of 23 to 45 ° was determined by using the three-dimensional atom probe method. More specifically, first, a grain boundary having a crystal orientation difference of 23 to 45 ° was selected from the grain boundaries in the sample collected from the 1 / 4t portion of the steel material by EBSD. Next, a minute block was cut out from this grain boundary by a lift-out method of FIB (focused ion beam processing). The block was attached to the needle pedestal by the Pt vapor deposition method, and FIB processing was performed so that the grain boundaries in the block were located at the needle tip. This needle sample was measured by a three-dimensional atom probe method (normal electrode type, measurement mode: voltage mode, sample temperature: 50 to 70 K, applied voltage: 3 to 15 kV, and pulse ratio: 15 to 25%). From the data acquired by the 3D atom probe measurement, a 3D element map including the grain boundary is obtained using the dedicated 3D construction software, and 20 nm in the direction parallel to the grain boundary so as to cross perpendicular to the grain interface. A selection box of × 20 nm and 30 nm in the direction perpendicular to the grain boundary was set, and a ladder chart was drawn for each segregated element in the direction perpendicular to the grain interface. Since the increase in the cumulative number of atoms at the boundary of 10 grains indicates the number of segregated atoms, divide this value by the grain boundary area (20 nm x 20 nm) and further divide by the ion detection rate of the three-dimensional atom probe. The interfacial excess value indicating the amount of grain boundary segregation was determined by. The arithmetic mean of the total value of the interfacial excess values indicating the amount of grain boundary segregation of Z element obtained by performing the same measurement at 5 points in the steel material is "Z element at the grain boundary with a crystal orientation difference of 23 to 45 °". (At least one of Zr and Hf) was determined as the total amount of segregation.

[Evaluation of P grain boundary segregation]
In order to verify the effect of the grain boundary segregation of the Z element on the reduction of the grain boundary segregation of P, the grains of P after each of the steel materials obtained above was tempered under the conditions of 550 ° C. and 1200 seconds. The amount of field segregation was measured using the same three-dimensional atom probe method as for element Z. The results are shown in Table 1.

In this example, the case where the grain boundary segregation amount of P after the tempering treatment was 0.4 atoms / nm ² or less was evaluated as a steel material in which the grain boundary segregation of P in the steel was reduced. Referring to Table 1, in Comparative Examples 84 to 90, since the effective amount of the Z element composed of Zr and Hf was low, the Z element could not be segregated at the grain boundaries, and as a result, the grains of P in the steel material could not be segregated. The field segregation could not be reduced. On the other hand, in Comparative Example 91, the effective amount of the Z element was higher than 0.0003%, and therefore satisfied the formula 1, but the Z element could be sufficiently segregated at the grain boundaries in the manufacturing process. Therefore, it was not possible to reduce the grain boundary segregation of P in the steel material. In contrast, in all the embodiments of the present invention, the effective amount of the Z element is 0.0003% or more, and the Z element is sufficiently segregated at the grain boundaries to allow P in the steel material. It was possible to reduce the grain boundary segregation.

The steel material according to the embodiment of the present invention is a steel material after hot rolling in which the Z element is segregated at the grain boundary, for example, thick steel sheets used for bridges, construction, shipbuilding, pressure vessels, etc., automobiles, home appliances, etc. It also includes thin steel plates used in the above, as well as steel bars, wire rods, shaped steels, steel pipes, and the like. When the steel material according to the embodiment of the present invention is applied to these materials, the grain boundary segregation of P in the steel is sufficiently reduced, so that the characteristics of the steel material related to the grain boundary segregation of P, for example, , Toughness, ductility, corrosion resistance, weldability and other properties can be significantly improved.

Claims

By mass%,
C: 0.001 to 1.000%,
Si: 0.01-3.00%,
Mn: 0.10 to 4.50%,
P: 0.300% or less,
S: 0.0300% or less,
Al: 0.001-5.000%,
N: 0.2000% or less,
O: 0.0100% or less,
At least one Z element selected from the group consisting of Zr: 0 to 0.8000% and Hf: 0 to 0.8000%.
Nb: 0-3.000%,
Ti: 0 to 0.500%,
Ta: 0 to 0.500%,
V: 0 to 1.00%,
Cu: 0 to 3.00%,
Ni: 0-60.00%,
Cr: 0 to 30.00%,
Mo: 0 to 5.00%,
W: 0 to 2.00%,
B: 0-0.0200%,
Co: 0 to 3.00%,
Be: 0 to 0.050%,
Ag: 0 to 0.500%,
Ca: 0-0.0500%,
Mg: 0-0.0500%,
At least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc: 0 to 0.5000% in total.
Sn: 0 to 0.300%,
Sb: 0 to 0.300%,
Te: 0 to 0.100%,
Se: 0 to 0.100%,
As: 0 to 0.050%,
Bi: 0 to 0.500%,
Pb: 0 to 0.500%, and the balance: Fe and impurities.
It has a chemical composition that satisfies the following formula 1 and has a chemical composition.
A steel material containing grain boundaries having a crystal orientation difference of 23 to 45 ° and a total segregation amount of Zr and Hf of 0.2 atoms / nm 2 or more.
0.61 [Zr] +0.31 [Hf] -1.75 [O] -3.99 [N] -1.74 [S] ≧ 0.0003 ・・・ Equation 1
Here, [Zr], [Hf], [O], [N], and [S] are the content [mass%] of each element, and are 0 when the element is not contained.
The chemical composition is by mass%.
Nb: 0.003 to 3.000%,
Ti: 0.005 to 0.500%,
Ta: 0.001 to 0.500%,
V: 0.001 to 1.00%,
Cu: 0.001 to 3.00%,
Ni: 0.001 to 60.00%,
Cr: 0.001 to 30.00%,
Mo: 0.001 to 5.00%,
W: 0.001 to 2.00%,
B: 0.0001-0.0200%,
Co: 0.001 to 3.00%,
Be: 0.0003 to 0.050%, and Ag: 0.001 to 0.500%
The steel material according to claim 1, which comprises one or more of the above.
The chemical composition is by mass%.
Ca: 0.0001-0.0500%,
Mg: 0.0001 to 0.0500%, and at least one of La, Ce, Nd, Y, Pm, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc. : 0.0001 to 0.5000% in total
The steel material according to claim 1 or 2, which comprises one or more of the above.
The chemical composition is by mass%.
Sn: 0.001 to 0.300%, and Sb: 0.001 to 0.300%
The steel material according to any one of claims 1 to 3, which comprises one or two of the above.
The chemical composition is by mass%.
Te: 0.001 to 0.100%,
Se: 0.001 to 0.100%,
As: 0.001 to 0.050%,
Bi: 0.001 to 0.500%, and Pb: 0.001 to 0.500%
The steel material according to any one of claims 1 to 4, which comprises one or more of the two or more.