steel material

Abstract

The present application provides a kind of steel material, it has the prescribed chemical composition satisfying 0.40 [Pr]+0.37 [Sm]+0.37 [Eu]+0.36 [Gd]+0.35 [Tb]+0.34 [Dy]+0.34 [Ho]+0.33 [Er]+0.33 [Tm]+0.32 [Yb]+0.32 [Lu]+1.24 [Sc]-2.33 [O]-3.99 [N]-1.74 [S]≥0.0003 (in formula, [Pr], [Sm], [Eu], [Gd], [Tb], [Dy], [Ho], [Er], [Tm], [Yb], [Lu], [Sc], [O], [N] and [S] are the content of each element [mass%]).

Description

Technical Field

[0001] This invention relates to steel. Background Technology

[0002] It is generally known that refining the microstructure of a metal is effective in improving the required material properties of steel. Relatedly, conventional methods for refining the microstructure include, for example, controlling the finishing temperature during hot rolling, and more specifically, finish rolling, to suppress the recrystallization of austenite grains. This suppression of recrystallization increases the driving force for the ferrite phase transformation, resulting in the formation of more new crystals. (See, for example, Patent Documents 1 to 4)

[0003] Patent Document 1 demonstrates that by further coexisting with boron (B) in addition to Nb, the recrystallization temperature of austenite is increased by more than 50°C, and hardenability is significantly improved. Compared to the values ​​expected from Nb and B alone, the improvement in the balance of strength and toughness becomes extremely large. Patent Documents 2 and 3 describe that Nb, by raising the recrystallization temperature, is an effective element for refining the grain size of austenite at high temperatures. Patent Document 4 describes that Nb, even in trace amounts, inhibits the recrystallization of austenite, contributing to the refinement of the metal microstructure.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 58-077528

[0007] Patent Document 2: Japanese Patent Application Publication No. 63-235430

[0008] Patent Document 3: Japanese Patent Application Publication No. 63-235431

[0009] Patent Document 4: Japanese Patent Application Publication No. 2004-269924 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] While niobium (Nb) is known to be an effective element for inhibiting recrystallization, it also contributes to improved hardenability and precipitation strengthening. Therefore, increasing the Nb content to achieve a higher recrystallization inhibition effect may result in steel with excessively high strength or, consequently, reduced toughness. Thus, in this technical field, there is a demand for steels that, in addition to Nb, contain elements with recrystallization inhibition effects equal to or exceeding those of Nb, depending on the intended use of the steel or the properties required for that use.

[0012] The present invention was made in view of the above-mentioned actual situation, and its object is to provide a steel that has an improved recrystallization inhibition effect or improved recrystallization inhibition through a novel structure.

[0013] Methods for solving problems

[0014] In order to achieve the above-mentioned objective, the inventors of this invention studied elements capable of suppressing or delaying the recrystallization of austenite grains. As a result, the inventors of this invention discovered that by increasing the amount of a specific element dissolved in steel, recrystallization can be suppressed or delayed, thereby shifting the recrystallization initiation temperature (hereinafter also simply referred to as the "recrystallization initiation temperature") to a higher temperature side, thus completing this invention.

[0015] The steels that can achieve the above objectives are described below.

[0016] (1) A steel having the following chemical composition: in mass%:

[0017] C: 0.001~1.000%

[0018] Si: 0.01~3.00%

[0019] Mn: 0.10~4.50%

[0020] P: below 0.300%

[0021] S: below 0.0300%

[0022] Al: 0.001~5.000%

[0023] N: below 0.2000%

[0024] O: Below 0.0100%

[0025] At least one X element selected from Pr: 0–0.8000%, Sm: 0–0.8000%, Eu: 0–0.8000%, Gd: 0–0.8000%, Tb: 0–0.8000%, Dy: 0–0.8000%, Ho: 0–0.8000%, Er: 0–0.8000%, Tm: 0–0.8000%, Yb: 0–0.8000%, Lu: 0–0.8000%, and Sc: 0–0.8000%.

[0026] Nb: 0~3.000%

[0027] Ti: 0~0.500%

[0028] Ta: 0~0.500%

[0029] V: 0~1.00%

[0030] Cu: 0–3.00%

[0031] Ni: 0~60.00%

[0032] Cr: 0–30.00%

[0033] Mo: 0–5.00%

[0034] W: 0–2.00%

[0035] B: 0~0.0200%

[0036] Co: 0-3.00%

[0037] Be: 0~0.050%

[0038] Ag: 0-0.500%

[0039] Zr: 0~0.5000%

[0040] Hf: 0~0.5000%

[0041] Ca: 0~0.0500%

[0042] Mg: 0~0.0500%

[0043] At least one of La, Ce, Nd, Pm, and Y: totaling 0 to 0.5000%.

[0044] Sn: 0~0.300%

[0045] Sb: 0~0.300%

[0046] Te: 0~0.100%

[0047] Se: 0~0.100%

[0048] As: 0-0.050%

[0049] Bi: 0~0.500%

[0050] Pb: 0–0.500%, and

[0051] Remaining components: Fe and impurities.

[0052] Satisfy the following equation 1,

[0053] 0.40[Pr]+0.37[Sm]+0.37[Eu]+0.36[Gd]+0.35[Tb]+0.34[Dy]+0.34[Ho]+0.33[Er ]+0.33[Tm]+0.32[Yb]+0.32[Lu]+1.24[Sc]-2.33[O]-3.99[N]-1.74[S]≥0.0003 Formula 1

[0054] Wherein, [Pr], [Sm], [Eu], [Gd], [Tb], [Dy], [Ho], [Er], [Tm], [Yb], [Lu], [Sc], [O], [N] and [S] are the contents of each element [mass%], which is 0 when no element is present.

[0055] (2) The steel according to (1) above, wherein the above chemical composition, in mass percent, contains one or more of the following elements:

[0056] Nb: 0.003~3.000%

[0057] Ti: 0.005~0.500%

[0058] Ta: 0.001~0.500%

[0059] V: 0.001~1.00%

[0060] Cu: 0.001–3.00%

[0061] Ni: 0.001~60.00%

[0062] Cr: 0.001~30.00%

[0063] Mo: 0.001~5.00%

[0064] W: 0.001~2.00%

[0065] B: 0.0001~0.0200%

[0066] Co: 0.001~3.00%

[0067] Be: 0.0003~0.050%

[0068] Ag: 0.001~0.500%

[0069] Zr: 0.0001~0.5000%

[0070] Hf: 0.0001~0.5000%

[0071] Ca: 0.0001~0.0500%

[0072] Mg: 0.0001~0.0500%

[0073] At least one of La, Ce, Nd, Pm, and Y: totaling 0.0001 to 0.5000%.

[0074] Sn: 0.001~0.300%

[0075] Sb: 0.001~0.300%

[0076] Te: 0.001~0.100%

[0077] Se: 0.001~0.100%

[0078] As: 0.001~0.050%

[0079] Bi: 0.001~0.500%, and

[0080] Pb: 0.001~0.500%.

[0081] Invention Effects

[0082] According to the present invention, it is possible to provide steel with improved recrystallization inhibition effect or improved recrystallization inhibition. Attached Figure Description

[0083] Figure 1 This is a diagram showing the test conditions for the compression processing test in the embodiments.

[0084] Figure 2 This is a chart representing the method for determining the softening rate, excerpted from "Naoki Maruyama et al., 'The state of Nb in the early stage of the recovery recrystallization of hot-worked austenitic structure in steel', Journal of the Japan Society for Metals, Vol. 60, No. 11 (1996), pp. 1051-1057". Detailed Implementation

[0085] <Steel>

[0086] The steel according to embodiments of the present invention is characterized by having the following chemical composition: in mass %:

[0087] C: 0.001~1.000%

[0088] Si: 0.01~3.00%

[0089] Mn: 0.10~4.50%

[0090] P: below 0.300%

[0091] S: below 0.0300%

[0092] Al: 0.001~5.000%

[0093] N: below 0.2000%

[0094] O: Below 0.0100%

[0095] At least one X element selected from Pr: 0–0.8000%, Sm: 0–0.8000%, Eu: 0–0.8000%, Gd: 0–0.8000%, Tb: 0–0.8000%, Dy: 0–0.8000%, Ho: 0–0.8000%, Er: 0–0.8000%, Tm: 0–0.8000%, Yb: 0–0.8000%, Lu: 0–0.8000%, and Sc: 0–0.8000%.

[0096] Nb: 0~3.000%

[0097] Ti: 0~0.500%

[0098] Ta: 0~0.500%

[0099] V: 0~1.00%

[0100] Cu: 0–3.00%

[0101] Ni: 0~60.00%

[0102] Cr: 0–30.00%

[0103] Mo: 0–5.00%

[0104] W: 0–2.00%

[0105] B: 0~0.0200%

[0106] Co: 0-3.00%

[0107] Be: 0~0.050%

[0108] Ag: 0-0.500%

[0109] Zr: 0~0.5000%

[0110] Hf: 0~0.5000%

[0111] Ca: 0~0.0500%

[0112] Mg: 0~0.0500%

[0113] At least one of La, Ce, Nd, Pm, and Y: totaling 0 to 0.5000%.

[0114] Sn: 0~0.300%

[0115] Sb: 0~0.300%

[0116] Te: 0~0.100%

[0117] Se: 0~0.100%

[0118] As: 0-0.050%

[0119] Bi: 0~0.500%

[0120] Pb: 0–0.500%, and

[0121] Remaining components: Fe and impurities.

[0122] Satisfy the following equation 1,

[0123] 0.40[Pr]+0.37[Sm]+0.37[Eu]+0.36[Gd]+0.35[Tb]+0.34[Dy]+0.34[Ho]+0.33[Er ]+0.33[Tm]+0.32[Yb]+0.32[Lu]+1.24[Sc]-2.33[O]-3.99[N]-1.74[S]≥0.0003 Formula 1

[0124] Wherein, [Pr], [Sm], [Eu], [Gd], [Tb], [Dy], [Ho], [Er], [Tm], [Yb], [Lu], [Sc], [O], [N] and [S] are the contents of each element [mass%], which is 0 when no element is present.

[0125] To suppress the recrystallization of austenite grains, as previously described, finishing rolling needs to be completed at a low temperature. However, this may necessitate waiting until the steel cools to a suitable temperature before finishing rolling begins, potentially leading to a decrease in productivity. This reduction in productivity is particularly significant when producing thicker steels for applications such as construction materials, where a considerable amount of time may be required to sufficiently cool the steel up to the center before finishing rolling. Therefore, to manufacture steel without compromising productivity, it is generally preferable to complete hot rolling at a higher temperature; however, on the other hand, suppressing recrystallization is required for refining the metal microstructure.

[0126] Therefore, to refine the metal microstructure and improve productivity, it is necessary to expand the non-recrystallization temperature range; specifically, the temperature at which the recrystallization of austenite grains begins needs to be increased. To explain in more detail, when steel is hot-rolled, the crystals within the steel are broken up, disrupting the orderly arrangement of Fe atoms within the crystals and creating numerous discontinuous structures called deformation bands. Furthermore, numerous step-like irregularities (grain boundary edges) are generated at the grain boundaries. However, if the rolling temperature is high, the Fe atoms themselves move in a manner that eliminates deformation bands and grain boundary edges, attempting to restore the disordered and unstable state to a stable crystal with orderly Fe atom arrangement. This is a phenomenon known as recrystallization. On the other hand, if the rolling temperature is low (e.g., below approximately 800°C), the Fe atoms cannot move, resulting in the hot rolling ending with grain boundary edges and deformation bands remaining at multiple locations within the grains and at grain boundaries.

[0127] During the cooling process after hot rolling, the metal microstructure transforms from austenite to ferrite. However, this phase transformation generally occurs at sites where the arrangement of Fe atoms in the austenite becomes disordered. Therefore, when austenite recrystallizes during hot rolling, the sites where the Fe atom arrangement is disordered become grain boundaries, allowing new ferrite crystals to form only from the austenite grain boundaries. On the other hand, for example, when hot rolling is performed at a low temperature below approximately 800°C, a large number of new ferrite crystals can be formed from grain boundary thresholds and deformation bands present in most parts of the austenite. Although austenitic and martensitic steels do not transform into ferrite, the suppression of recrystallization increases the strain accumulated in the austenite grains during hot rolling, resulting in grain refinement. Thus, while low-temperature hot rolling, and more specifically low-temperature finishing rolling, is very effective in refining the metal microstructure, as mentioned above, from a productivity point of view, it is necessary to finish hot rolling at a higher temperature. Therefore, in order to refine the metal structure and improve productivity, it is preferable to raise the recrystallization initiation temperature.

[0128] Therefore, the inventors of this invention studied elements capable of suppressing or delaying the recrystallization of austenite grains. As a result, the inventors discovered that by considering the relationship between the amount of specific elements dissolved in steel, namely Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc (hereinafter also referred to as "elements X"), and the inclusions formed in the steel by these elements, more specifically, oxides, nitrides, and sulfides of these elements, and simultaneously setting the amount within a specified range (i.e., by setting the effective amount of the element X corresponding to the left-hand side of Formula 1 to 0.0003% or more), the recrystallization of austenite grains can be suppressed or delayed. Due to this suppression or delay of recrystallization, the recrystallization initiation temperature can be shifted to a higher temperature side. Therefore, according to this invention, even when hot rolling, especially finish rolling, is performed at relatively high temperatures, steel with significantly suppressed recrystallization can be obtained, thus improving productivity and refining the metal structure in the final steel. As a result, it is possible to improve properties associated with the miniaturization of metal structures, such as toughness, and also to reduce the manufacturing cost of steel and shorten the manufacturing process.

[0129] While not intended to be bound by any particular theory, it is believed that the X element described above in the embodiments of the present invention is anchored at lattice defects such as dislocations introduced into the steel during hot rolling, for example, suppressing recrystallization by hindering the rearrangement of these dislocations and their movement toward a stable configuration. It is believed that all the aforementioned X elements have larger atomic radii than Nb used in conventional techniques; therefore, by anchoring elements with such larger atomic radii at lattice defects such as dislocations, the effect of hindering dislocation rearrangement is enhanced. As a result, at least equal or higher recrystallization suppression effects can be achieved compared to conventional steels using Nb. Therefore, in the present invention, it is extremely important to dissolve a large amount of elements with such larger atomic radii in the steel.

[0130] However, these X elements have the following problem: they readily combine with O (oxygen), N (nitrogen), and S (sulfur) present in steel to form inclusions containing oxides, nitrides, and sulfides. If X elements form such inclusions in steel, the amount of X elements that contribute to recrystallization suppression becomes less, making it impossible to fully obtain the recrystallization suppression effect obtained by anchoring the X elements at lattice defects such as dislocations. In this invention, by using Formula 1 described in detail below to calculate the amount of X elements that takes such inclusions into account as the effective amount of X elements, and then setting this effective amount to a specified range, i.e., 0.0003% or more, a higher recrystallization suppression effect can be achieved.

[0131] As mentioned above, element X in this invention readily combines with O, N, and S to form inclusions, making it generally difficult to ensure a specified solid solution content in steel. Because of this, the recrystallization inhibition effect of element X was previously unknown. However, with recent advancements in refining technology, it has become possible to reduce the content of elements such as O, N, and S, which are generally present as impurities in steel, to very low levels, thus enabling the achievement of a specified solid solution content for element X. Therefore, the recrystallization inhibition effect resulting from the solid solution of element X is something the inventors of this invention have unexpectedly and surprisingly discovered for the first time.

[0132] The steel according to embodiments of the present invention will now be described in more detail. In the following description, the unit for the content of each element, "%", refers to "mass %" unless otherwise specified. Furthermore, in this specification, the "~" indicating a numerical range is used to mean the lower and upper limits of the values ​​described before and after it, unless otherwise specified.

[0133] [C: 0.001~1.000%]

[0134] Carbon (C) is an element required for stabilizing hardness and / or ensuring strength. To achieve these effects sufficiently, the C content is 0.001% or more. The C content can also be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the C content is excessive, toughness, flexibility, and / or weldability may decrease. Therefore, the C content is 1.000% or less. The C content can also be 0.800% or less, 0.600% or less, or 0.500% or less.

[0135] [Si: 0.01~3.00%]

[0136] Silicon (Si) is a deoxidizing element and also contributes to increased strength. To fully achieve these effects, the Si content is 0.01% or more. The Si content can also be 0.05% or more, 0.10% or more, or 0.30% or more. On the other hand, if the Si content is excessive, it may reduce toughness or produce poor surface quality known as oxide scale defects. Therefore, the Si content is 3.00% or less. The Si content can also be 2.00% or less, 1.00% or less, or 0.60% or less.

[0137] [Mn: 0.10~4.50%]

[0138] Manganese (Mn) is an effective element for improving hardenability and / or strength, and also an effective austenite stabilizing element. To fully obtain these effects, the Mn content is 0.10% or more. The Mn content can also be 0.50% or more, 0.70% or more, or 1.00% or more. On the other hand, if the Mn content is excessive, it may generate MnS, which is detrimental to toughness, or reduce oxidation resistance. Therefore, the Mn content is 4.50% or less. The Mn content can also be 4.00% or less, 3.50% or less, or 3.00% or less.

[0139] [P: below 0.300%]

[0140] Phosphorus (P) is an element introduced during the manufacturing process. The P content can also be 0%. However, refining to reduce the P content to below 0.0001% requires time, leading to reduced productivity. Therefore, the P content can also be 0.0001% or more, 0.0005% or more, 0.001% or more, 0.003% or more, or 0.005% or more. From a manufacturing cost perspective, the P content can also be 0.007% or more. On the other hand, excessive P content can potentially reduce the workability and / or toughness of the steel. Therefore, the P content is 0.300% or less. The P content can also be 0.100% or less, 0.030% or less, or 0.010% or less.

[0141] [S: below 0.0300%]

[0142] Sulfur (S) is an element introduced during the manufacturing process. From the viewpoint of reducing inclusions formed between the sulfur content and element X in embodiments of the present invention, the lower the sulfur content, the better. Therefore, the sulfur content can also be 0%. However, refining requires time to reduce the sulfur content to below 0.0001%, resulting in a decrease in productivity. Therefore, the sulfur content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the sulfur content is excessive, the effective amount of element X may be reduced, and the toughness may decrease. Therefore, the sulfur content is 0.0300% or less. The sulfur content is preferably 0.0100% or less, more preferably 0.0050% or less, and most preferably 0.0030% or less.

[0143] [Al: 0.001~5.000%]

[0144] Aluminum (Al) is a deoxidizing element and is also effective in improving corrosion resistance and / or heat resistance. To achieve these effects, the Al content is 0.001% or more. The Al content can also be 0.010% or more, 0.100% or more, or 0.200% or more. In particular, from the viewpoint of sufficiently improving heat resistance, the Al content can also be 1.000% or more, 2.000% or more, or 3.000% or more. On the other hand, if the Al content is excessive, coarse inclusions may form, reducing toughness, causing defects such as cracking during manufacturing, and / or reducing fatigue resistance. Therefore, the Al content is 5.000% or less. The Al content can also be 4.500% or less, 4.000% or less, or 3.500% or less. In particular, from the viewpoint of suppressing the reduction of toughness, the Al content can also be 1.500% or less, 1.000% or less, or 0.300% or less.

[0145] [N: below 0.2000%]

[0146] Nitrogen (N) is an element introduced during the manufacturing process. From the viewpoint of reducing inclusions formed between the nitrogen and element X in embodiments of the present invention, the less nitrogen, the better; therefore, the nitrogen content can also be 0%. However, refining takes time to reduce the nitrogen content to below 0.0001%, resulting in a decrease in productivity. Therefore, the nitrogen content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, nitrogen is also an element effective in stabilizing austenite and can be intentionally included as needed. In this case, the nitrogen content is preferably 0.0100% or more, and can also be 0.0200% or more, or 0.0500% or more. However, if nitrogen is excessively present, the effective amount of element X may decrease, and toughness may decrease. Therefore, the nitrogen content is 0.2000% or less. The nitrogen content can also be 0.1500% or less, 0.1000% or less, or 0.0800% or less.

[0147] [O: below 0.0100%]

[0148] Oxygen (O) is an element introduced during the manufacturing process. From the viewpoint of reducing inclusions formed between the oxygen and element X in embodiments of the present invention, the less oxygen, the better; therefore, the O content can also be 0%. However, refining requires time to reduce the O content to below 0.0001%, resulting in a decrease in productivity. Therefore, the O content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the O content is excessive, coarse inclusions may form, the effective amount of element X is reduced, and the formability and / or toughness of the steel are reduced. Therefore, the O content is 0.0100% or less. The O content can also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0149] [Selected from at least one element X selected from Pr: 0–0.8000%, Sm: 0–0.8000%, Eu: 0–0.8000%, Gd: 0–0.8000%, Tb: 0–0.8000%, Dy: 0–0.8000%, Ho: 0–0.8000%, Er: 0–0.8000%, Tm: 0–0.8000%, Yb: 0–0.8000%, Lu: 0–0.8000%, and Sc: 0–0.8000%]

[0150] In the embodiments of the present invention, the X element is Pr: 0-0.8000%, Sm: 0-0.8000%, Eu: 0-0.8000%, Gd: 0-0.8000%, Tb: 0-0.8000%, Dy: 0-0.8000%, Ho: 0-0.8000%, Er: 0-0.8000%, Tm: 0-0.8000%, Yb: 0 ~0.8000%, Lu: 0~0.8000%, and Sc: 0~0.8000%. Praseodymium (Pr), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and scandium (Sc), existing in austenite in a solid solution state, can exhibit a recrystallization inhibition effect. By exhibiting this recrystallization inhibition effect, even under hot rolling at relatively high temperatures, especially finish rolling, the metal structure in the final steel can be refined, thus improving toughness, as evaluated by Charpy impact characteristics, and significantly improving productivity.

[0151] The aforementioned element X can be used alone, or in any specific combination of two or more of the aforementioned elements. Furthermore, the element X only needs to be present in an amount satisfying Formula 1, which will be described in detail later; its lower limit is not particularly limited. However, for example, the content of each element X or the total content can be 0.0010% or more, preferably 0.0050% or more, more preferably 0.0150% or more, even more preferably 0.0300% or more, and most preferably 0.0500% or more. On the other hand, even if the element X is excessively present, the effect will saturate, and thus, including more than the necessary amount of element X in the steel may lead to an increase in manufacturing costs. Therefore, the content of each element X is 0.8000% or less, for example, it can also be 0.7000% or less, 0.6000% or less, 0.5000% or less, 0.4000% or less, or 0.3000% or less. In addition, the total content of element X is 9.6000% or less, for example, it can also be 6.0000%, 5.0000%, 4.0000%, 2.0000%, 1.0000%, or 0.5000%.

[0152] The basic chemical composition of the steel according to embodiments of the present invention is as described above. Furthermore, the steel may also contain one or more of the following optional elements as needed. For example, the steel may contain one or more of the following: Nb: 0–3.000%, Ti: 0–0.500%, Ta: 0–0.500%, V: 0–1.00%, Cu: 0–3.00%, Ni: 0–60.00%, Cr: 0–30.00%, Mo: 0–5.00%, W: 0–2.00%, B: 0–0.0200%, Co: 0–3.00%, Be: 0–0.050%, and Ag: 0–0.500%. In addition, the steel may contain one or more of the following: Zr: 0–0.5000%, Hf: 0–0.5000%, Ca: 0–0.0500%, Mg: 0–0.0500%, and at least one of La, Ce, Nd, Pm, and Y, totaling 0–0.5000%. Furthermore, the steel may contain one or more of Sn: 0–0.300% and Sb: 0–0.300%. Additionally, the steel may contain one or more of Te: 0–0.100%, Se: 0–0.100%, As: 0–0.050%, Bi: 0–0.500%, and Pb: 0–0.500%. These optional elements are described in detail below.

[0153] [Nb: 0~3.000%]

[0154] Niobium (Nb) is an element that contributes to precipitation strengthening and suppression of recrystallization. The Nb content can be 0%, but to achieve these effects, the Nb content is preferably 0.003% or more. For example, the Nb content can also be 0.005% or more, or 0.010% or more. In particular, from the viewpoint of fully maximizing precipitation strengthening, the Nb content can also be 1.000% or more, or 1.500% or more. On the other hand, if the Nb content is excessive, the effect may saturate, reducing processability and / or toughness. Therefore, the Nb content is 3.000% or less. The Nb content can also be 2.800% or less, 2.500% or less, or 2.000% or less. In particular, from the viewpoint of suppressing the reduction of toughness in the weld heat-affected zone (HAZ), the Nb content is preferably 0.100% or less, and can also be 0.080% or less, 0.050% or less, or 0.030% or less.

[0155] [Ti: 0~0.500%]

[0156] Titanium (Ti) is an element that contributes to the strength of steel through precipitation strengthening and other methods. The Ti content can be 0%, but to achieve this effect, a Ti content of 0.005% or more is preferred. The Ti content can also be 0.010% or more, 0.050% or more, or 0.080% or more. On the other hand, if the Ti content is excessive, a large amount of precipitates may form, reducing toughness. Therefore, the Ti content is 0.500% or less. The Ti content can also be 0.300% or less, 0.200% or less, or 0.100% or less.

[0157] [Ta: 0~0.500%]

[0158] Tantalum (Ta) is an effective element for controlling the morphology of carbides and increasing strength. The Ta content can be 0%, but to achieve these effects, a Ta content of 0.001% or more is preferred. The Ta content can also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if the Ta content is excessive, a large amount of fine Ta carbides may precipitate, leading to an excessive increase in the strength of the steel, resulting in reduced ductility and cold workability. Therefore, the Ta content is 0.500% or less. The Ta content can also be 0.300% or less, 0.100% or less, or 0.080% or less.

[0159] [V: 0~1.00%]

[0160] Vanadium (V) is an element that contributes to the strength of steel through precipitation strengthening and other methods. The V content can be 0%, but to achieve this effect, a V content of 0.001% or more is preferred. The V content can also be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, if the V content is excessive, a large amount of precipitates may be formed, reducing toughness. Therefore, the V content is 1.00% or less. The V content can also be 0.80% or less, 0.60% or less, or 0.50% or less.

[0161] [Cu: 0~3.00%]

[0162] Copper (Cu) is an element that contributes to improved strength and / or corrosion resistance. The Cu content can be 0%, but to achieve these effects, a Cu content of 0.001% or more is preferred. The Cu content can also 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, excessive Cu content can lead to deterioration of toughness and weldability. Therefore, the Cu content is 3.00% or less. The Cu content can also be 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0163] [Ni: 0~60.00%]

[0164] Nickel (Ni) is an element that contributes to improved strength and / or heat resistance, and is also an effective austenite stabilizing element. The Ni content can be 0%, but to achieve these effects, the Ni content is preferably 0.001% or more. The Ni content can also 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 heat resistance, the Ni content can also be 30.00% or more, 35.00% or more, or 40.00% or more. On the other hand, if the Ni content is excessive, not only will the alloy cost increase, but the deformation resistance during hot working will also increase, potentially increasing the equipment load. Therefore, the Ni content is 60.00% or less. The Ni content can also be 55.00% or less, or 50.00% or less. In particular, from the point of view of economy and / or suppressing the reduction of weldability, the Ni content may also be less than 15.00%, less than 10.00%, less than 6.00%, or less than 4.00%.

[0165] [Cr: 0~30.00%]

[0166] Chromium (Cr) is an element that contributes to improved strength and / or corrosion resistance. The Cr content can be 0%, but to achieve these effects, a Cr content of 0.001% or more is preferred. The Cr content can also 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 corrosion resistance, the Cr content can also be 10.00% or more, 12.00% or more, or 15.00% or more. On the other hand, if the Cr content is excessive, not only will the alloy cost increase, but the toughness may also decrease. Therefore, the Cr content is 30.00% or less. The Cr content can also be 28.00% or less, 25.00% or less, or 20.00% or less. In particular, from the viewpoint of suppressing a decrease in weldability and / or workability, the Cr content can also be 10.00% or less, 9.00% or less, or 7.50% or less.

[0167] [Mo: 0~5.00%]

[0168] Molybdenum (Mo) is an element that improves the hardenability of steel, contributes to increased strength, and also improves corrosion resistance. The Mo content can be 0%, but to achieve these effects, a Mo content of 0.001% or more is preferred. The Mo content can also be 0.01% or more, 0.02% or more, 0.50% or more, or 1.00% or more. On the other hand, excessive Mo content may increase deformation resistance during hot working, leading to a greater load on equipment. Therefore, the Mo content is 5.00% or less. The Mo content can also be 4.50% or less, 4.00% or less, 3.00% or less, or 1.50% or less.

[0169] [W: 0~2.00%]

[0170] Tungsten (W) is an element that improves the hardenability of steel and contributes to increased strength. The W content can be 0%, but to achieve this effect, a W content of 0.001% or more is preferred. The W content can also 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, excessive W content may reduce ductility and weldability. Therefore, the W content is 2.00% or less. The W content can also be 1.80% or less, 1.50% or less, or 1.00% or less.

[0171] [B: 0~0.0200%]

[0172] Boron (B) is an element that contributes to increased strength. The B content can be 0%, but to achieve this effect, a B content of 0.0001% or more is preferred. The B content can also be 0.0003% or more, 0.0005% or more, or 0.0007% or more. On the other hand, excessive B content may reduce toughness and / or weldability. Therefore, the B content is 0.0200% or less. The B content can also be 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0173] [Co: 0-3.00%]

[0174] Cobalt (Co) is an element that contributes to improved hardenability and / or heat resistance. The Co content can be 0%, but to achieve these effects, a Co content of 0.001% or more is preferred. The Co content can also 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, excessive Co content may reduce hot workability and increase raw material costs. Therefore, the Co content is 3.00% or less. The Co content can also be 2.50% or less, 2.00% or less, 1.50% or less, or 0.80% or less.

[0175] [Be: 0~0.050%]

[0176] Beryllium (Be) is an effective element for increasing the strength of the base material and refining its microstructure. The Be content can be 0%, but to achieve this effect, a Be content of 0.0003% or more is preferred. The Be content can also be 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, excessive Be content may reduce formability. Therefore, the Be content is 0.050% or less. The Be content can also be 0.040% or less, 0.030% or less, or 0.020% or less.

[0177] [Ag: 0~0.500%]

[0178] Silver (Ag) is an effective element for increasing the strength of the base material and refining its microstructure. The Ag content can be 0%, but to achieve this effect, an Ag content of 0.001% or more is preferred. The Ag content can also be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, excessive Ag content may reduce formability. Therefore, the Ag content is 0.500% or less. The Ag content can also be 0.400% or less, 0.300% or less, or 0.200% or less.

[0179] [Zr: 0~0.5000%]

[0180] Zirconium (Zr) is an element that can control the morphology of sulfides. While the Zr content can be 0%, it is preferable to have a Zr content of 0.0001% or more to achieve this effect. On the other hand, even with excessive Zr content, the effect becomes saturated, and including more Zr than necessary in steel could lead to increased manufacturing costs. Therefore, the Zr content is 0.5000% or less.

[0181] [Hf: 0~0.5000%]

[0182] Hafnium (Hf) is an element that can control the sulfide morphology. While the Hf content can be 0%, it is preferable to have an Hf content of 0.0001% or more to achieve this effect. On the other hand, even with excessive Hf content, the effect saturates, and including more Hf than necessary in the steel could lead to increased manufacturing costs. Therefore, the Hf content is 0.5000% or less.

[0183] [Ca: 0~0.0500%]

[0184] Calcium (Ca) is an element that can control the form of sulfides. The Ca content can be 0%, but to achieve this effect, the Ca content is preferably 0.0001% or more. On the other hand, even with excessive Ca content, the effect saturates, so including more Ca than necessary in the steel may lead to increased manufacturing costs. Therefore, the Ca content is 0.0500% or less.

[0185] [Mg: 0~0.0500%]

[0186] Magnesium (Mg) is an element that can control the form of sulfides. The Mg content can be 0%, but to achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content can also be more than 0.0015%, 0.0016%, 0.0018%, or 0.0020%. On the other hand, even with excessive Mg content, the effect saturates, and the formation of coarse inclusions may lead to a decrease in cold formability and / or toughness. Therefore, the Mg content is 0.0500% or less. The Mg content can also be 0.0400%, 0.0300%, or 0.0200% or less.

[0187] [At least one of La, Ce, Nd, Pm, and Y: totaling 0 to 0.5000%]

[0188] Lanthanum (La), cerium (Ce), neodymium (Nd), promethium (Pm), and yttrium (Y) are elements that, like Ca and Mg, can control the sulfide form. The total content of at least one of La, Ce, Nd, Pm, and Y can be 0%, but to achieve this effect, it is preferable to be 0.0001% or more. The total content of at least one of La, Ce, Nd, Pm, and Y can also be 0.0002% or more, 0.0003% or more, or 0.0004% or more. On the other hand, even if these elements are excessively present, the effect saturates, and coarse oxides may form, thus reducing cold formability. Therefore, the total content of at least one of La, Ce, Nd, Pm, and Y is 0.5000% or less, or it can be 0.4000% or less, 0.3000% or less, or 0.2000% or less.

[0189] [Sn: 0~0.300%]

[0190] Tin (Sn) is an effective element for improving corrosion resistance. The Sn content can be 0%, but to achieve this effect, a Sn content of 0.001% or more is preferred. The Sn content can also be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, excessive Sn content can lead to a decrease in toughness, especially low-temperature toughness. Therefore, the Sn content is 0.300% or less. The Sn content can also be 0.250% or less, 0.200% or less, or 0.150% or less.

[0191] [Sb: 0~0.300%]

[0192] Antimony (Sb) is an element as effective as Sn in improving corrosion resistance, especially when contained in combination with Sn, the effect is amplified. The Sb content can be 0%, but to achieve the desired improvement in corrosion resistance, the Sb content is preferably 0.001% or more. The Sb content can also be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, excessive Sb content may lead to a decrease in toughness, especially low-temperature toughness. Therefore, the Sb content is 0.300% or less. The Sb content can also be 0.250% or less, 0.200% or less, or 0.150% or less.

[0193] [Te: 0~0.100%]

[0194] Tellurium (Te) is an effective element for improving the machinability of steel because it forms low-melting-point compounds with Mn or S, thereby enhancing lubrication. The Te content can be 0%, but to achieve this effect, a Te content of 0.001% or more is preferred. The Te content can also be 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, even with excessive Te content, the effect saturates, leading to an increase in alloy cost. Therefore, the Te content is 0.100% or less. The Te content can also be 0.090% or less, 0.080% or less, or 0.070% or less.

[0195] [Se: 0~0.100%]

[0196] Selenium (Se) is an effective element for improving the machinability of steel because the selenides formed in steel alter the shear plastic deformation of the cutting material, making the chips easier to break. The Se content can be 0%, but to achieve this effect, the Se content is preferably 0.001% or more. The Se content can also be 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, even with excessive Se content, the effect saturates, leading to an increase in alloy cost. Therefore, the Se content is 0.100% or less. The Se content can also be 0.090% or less, 0.080% or less, or 0.070% or less.

[0197] [As: 0~0.050%]

[0198] Arsenic (As) is an effective element for improving the machinability of steel. The As content can be 0%, but to achieve this effect, the As content is preferably 0.001% or more. The As content can also be 0.005% or more, or 0.010% or more. On the other hand, excessive As content may reduce hot workability. Therefore, the As content is 0.050% or less. The As content can also be 0.040% or less, 0.030% or less, or 0.020% or less.

[0199] [Bi: 0~0.500%]

[0200] Bismuth (Bi) is an effective element for improving the machinability of steel. The Bi content can be 0%, but to achieve this effect, a Bi content of 0.001% or more is preferred. The Bi content can also be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, even with excessive Bi content, the effect saturates, leading to an increase in alloy cost. Therefore, the Bi content is 0.500% or less. The Bi content can also be 0.400% or less, 0.300% or less, or 0.200% or less.

[0201] [Pb: 0~0.500%]

[0202] Lead (Pb) is an effective element for improving the machinability of steel because it melts due to the temperature rise caused by cutting and promotes crack propagation. The Pb content can be 0%, but to achieve this effect, a Pb content of 0.001% or more is preferred. The Pb content can also be 0.010% or more, 0.020% or more, 0.030% or more, or 0.050% or more. On the other hand, excessive Pb content may reduce hot workability. Therefore, the Pb content is 0.500% or less. The Pb content can also be 0.400% or less, 0.300% or less, or 0.200% or less.

[0203] In the steel of the embodiments of the present invention, the remaining portion other than the elements described above is composed of Fe and impurities. Impurities refer to components that are introduced during the industrial manufacturing of steel through various factors in the manufacturing process, such as raw materials like ores and waste.

[0204] [Effective quantity of element X]

[0205] According to an embodiment of the present invention, the effective amount of element X, which is composed of Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Sc, is obtained by the left side of the following equation 1, and then the value is made to satisfy the following equation 1.

[0206] 0.40[Pr]+0.37[Sm]+0.37[Eu]+0.36[Gd]+0.35[Tb]+0.34[Dy]+0.34[Ho]+0.33[Er ]+0.33[Tm]+0.32[Yb]+0.32[Lu]+1.24[Sc]-2.33[O]-3.99[N]-1.74[S]≥0.0003 Formula 1

[0207] Wherein, [Pr], [Sm], [Eu], [Gd], [Tb], [Dy], [Ho], [Er], [Tm], [Yb], [Lu], [Sc], [O], [N] and [S] are the contents of each element [mass%], which is 0 when no element is present.

[0208] By ensuring that the effective amount of element X satisfies Equation 1 above, the amount of these elements existing in the steel in a solid solution state can be increased. This allows for the suppression or delay of austenite grain recrystallization, which in turn shifts the recrystallization initiation temperature to a higher temperature. More specifically, these elements X (hereinafter also simply "X") tend to combine with O (oxygen), N (nitrogen), and S (sulfur) present in the steel to form inclusions containing oxides (X₂O₃), nitrides (XN), and sulfides (XS). If such inclusions are formed, at least the X elements in these inclusions cannot contribute to suppressing the recrystallization of austenite grains. Therefore, to suppress the recrystallization of austenite grains, it is necessary to increase the amount of element X existing in the steel in a solid solution state without forming inclusions (i.e., the solid solution amount of element X in the steel).

[0209] Here, the amount of element X dissolved in the steel can be estimated by subtracting the maximum amount that can be consumed to form inclusions (oxides, nitrides, and sulfides) from the amount of element X contained in the steel. Therefore, in embodiments of the present invention, the amount of element X dissolved in the steel calculated in this way is taken as the amount of element X effective in suppressing the recrystallization of austenite grains (i.e., the "effective amount of element X"), specifically defined by the following formula A.

[0210] Effective amount of element X [atomic %] = Σ(M [Fe] / M [X] )×[X]-(M [Fe] / M [O] )×[O]×2 / 3-(M [Fe] / M [N] )×[N]-(M [Fe] / M [S] )×[S]Form A

[0211] Where X represents each of the X elements of Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc, and M [X] M represents the atomic weight of element X. [Fe] M represents the atomic weight of Fe. [O] M represents the atomic weight of O. [N] M represents the atomic weight of N. [S] [X] represents the atomic weight of S, and [X], [O], [N] and [S] represent the content [mass%] of the corresponding elements, which is 0 when no element is present.

[0212] The above formula A will be explained in detail below. First, although the steel in the embodiments of the present invention contains various alloying elements, it is essentially composed of Fe as a whole, or, in the case of containing relatively large amounts of Ni and / or Cr as optional elements (with maximum contents of 60.00% and 30.00% respectively), it is essentially composed of Ni and / or Cr except for Fe, which is obvious. 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 contains relatively large amounts of Ni and / or Cr, the atomic percentage of each of the X elements Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc can be approximately expressed by multiplying the ratio of the atomic weight of Fe to the atomic weight of each X element by the content [mass percentage] of that X element, i.e., (M [Fe] / M [X] )×[X] is used to calculate it. Therefore, it is calculated by (M)×[X]. [Fe] / M [X] The quantities of each element X calculated by Σ(M) are summed (i.e., by calculating Σ(M)). [Fe] / M [X] )×[X]) can be used to calculate the total atomic percentage of element X.

[0213] Next, by subtracting the maximum amount (atomic %) of element X that can be consumed in the formation of oxides (X₂O₃), nitrides (XN), and sulfides (XS) from the total atomic percentage of element X, the amount of element X in the steel that can effectively suppress the recrystallization of austenite grains can be calculated. Here, the maximum amount (atomic %) of element X that can be consumed in the formation of oxides (X₂O₃), nitrides (XN), and sulfides (XS) can be approximated by the atomic weights of Fe, O, N, and S in the steel, as well as the contents of O, N, and S, respectively, expressed in terms of (M [Fe] / M [o] )×[O]×2 / 3、(M [Fe] / M [N] )×[N] and (M [Fe] / M [S] The effective amount of element X used to suppress the recrystallization of austenite grains can be defined by the following formula A.

[0214] Effective amount of element X [atomic %] = Σ(M [Fe] / M [X] )×[X]-(M [Fe] / M [O] )×[O]×2 / 3-(M [Fe] / M [N] )×[N]-(M [Fe] / M [S])×[S]Form A

[0215] Here, the atomic weights of Fe, O, N, S, and each of the X elements are Fe: 55.845, O: 15.9994, N: 14.0069, S: 32.068, Pr: 140.908, Sm: 150.36, Eu: 151.964, Gd: 157.25, Tb: 158.925, Dy: 162.500, Ho: 164.930, Er: 167.259, Tm: 168.934, Yb: 173.045, Lu: 174.967, and Sc: 44.9559. Therefore, if the atomic weights of each element are substituted into Equation A above and rearranged, the effective amount of element X based on atomic percent can be approximately represented by Equation B below.

[0216] Effective quantity = 0.40[Pr] + 0.37[Sm] + 0.37[Eu] + 0.36[Gd] + 0.35[Tb] + 0.34[Dy] + 0.34[Ho] + 0.33[Er] + 0.33[Tm] + 0.32[Yb] + 0.32[Lu] + 1.24[Sc] - 2.33[O] - 3.99[N] - 1.74[S] Equation B

[0217] Wherein, [Pr], [Sm], [Eu], [Gd], [Tb], [Dy], [Ho], [Er], [Tm], [Yb], [Lu], [Sc], [O], [N] and [S] are the contents of each element [mass%], which is 0 when no element is present.

[0218] In an embodiment of the present invention, in order to suppress the recrystallization of austenite grains, the effective amount of element X obtained from the above formula B needs to be 0.0003% or more, that is, to satisfy the following formula 1.

[0219] 0.40[Pr]+0.37[Sm]+0.37[Eu]+0.36[Gd]+0.35[Tb]+0.34[Dy]+0.34[Ho]+0.33[Er ]+0.33[Tm]+0.32[Yb]+0.32[Lu]+1.24[Sc]-2.33[O]-3.99[N]-1.74[S]≥0.0003 Formula 1

[0220] The effective amount of element X can be, for example, 0.0005% or more or 0.0007% or more, preferably 0.0010% or more, more preferably 0.0015% or more, even more preferably 0.0030% or more, and most preferably 0.0050% or more or 0.0100% or more. Furthermore, as shown in Formula 1 above, to stably ensure this effective amount, it is preferable to minimize the content of O, N, and S in the steel. Here, there is no particular upper limit to the effective amount of element X, but even if the effective amount of element X is excessively increased, the effect saturates, and it becomes a factor leading to an increase in manufacturing costs (an increase in alloy costs associated with the increase in the content of element X and / or an increase in refining costs related to O, N, and S), which is not necessarily preferred. Therefore, the effective amount of element X is preferably 2.0000% or less, for example, it can also be 1.8000% or less, 1.5000% or less, 1.2000% or less, 1.0000% or less, or 0.8000% or less.

[0221] The steel used in embodiments of the present invention can be any type of steel, without particular limitation. The steel used in embodiments of the present invention includes, for example, steel before it exhibits the recrystallization inhibition effect, such as steel before hot rolling (i.e., slabs, billets, ingots), or steel after it exhibits the recrystallization inhibition effect, such as hot-rolled steel. Hot-rolled steel is not particularly limited, and may include, for example, thick steel plates, thin steel plates, and further include bars, wire rods, sections, and steel pipes.

[0222] The steel products of the embodiments of the present invention can be manufactured by any suitable method known to those skilled in the art, depending on the shape of the final product. For example, when the steel product is a thick steel plate, the manufacturing method includes steps generally used in the manufacture of thick steel plates, such as: a step of casting a slab having the chemical composition described above; a step of hot rolling the cast slab, which includes finishing rolling at a temperature lower than the recrystallization start temperature; and a step of cooling the resulting rolled material. Suitable heat treatment and tempering steps may also be included as needed. For example, the steel products of the embodiments of the present invention are particularly suitable for application of a thermal processing control process (TMCP) that combines controlled rolling and accelerated cooling.

[0223] Furthermore, when the steel is a thin steel sheet, the manufacturing method includes the steps generally used in manufacturing thin steel sheets, such as: casting a slab with the chemical composition described above; hot rolling the cast slab, which includes finishing rolling at a temperature lower than the recrystallization start temperature; and cooling and coiling the resulting rolled material. Cold rolling and annealing steps may also be included as needed. Similarly, the manufacturing methods for bars and other steel products also include the steps generally used in manufacturing bars and other steel products, such as: a steelmaking process to form molten steel with the chemical composition described above; casting slabs, billets, ingots, etc., from the formed molten steel; hot rolling the cast slabs, billets, ingots, etc., which includes finishing rolling at a temperature lower than the recrystallization start temperature; and cooling the resulting rolled material. Other steps may be appropriately selected from suitable steps known to those skilled in the art for manufacturing the aforementioned steel products. There are no particular limitations on the specific conditions for each of the above-mentioned processes; appropriate conditions can be selected according to the steel grade, type, and shape. In the manufacture of the steel according to the embodiments of the present invention, ensuring the effective amount of element X is important. Therefore, it is extremely important to sufficiently reduce the content of O, N, and S, which can form inclusions with element X in the steel, during the refining process.

[0224] The present invention will now be described in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0225] Example

[0226] In this embodiment, firstly, molten steel with various chemical compositions is melted in a vacuum melting furnace, and steel ingots of approximately 50 kg are manufactured by casting. The chemical compositions obtained by analyzing samples collected from each of the resulting steel ingots are shown in Table 1 below. Next, a compression processing test is conducted using cylindrical test specimens (φ8 mm × height 12 mm) obtained from the steel ingots, and the recrystallization inhibition effect of the steel is evaluated based on the softening rate calculated from the results of the test.

[0227] Specifically, according to Figure 1 The test conditions for the compression processing test shown are as follows: First, the cylindrical test material is heated to 950–1300°C, followed by processing at a temperature of 950°C, a true strain ε = 0.4, and a strain rate ε / t = 5 s. -1 Two compression tests were conducted under the condition that the time between passes was 10 seconds. Figure 1 The softening rate is determined by the stress-strain curves obtained from processing 1 and processing 2. A more detailed explanation is provided below. Figure 2(Excerpted from "Naoki Maruyama et al., 'The State of Nb in the Early Stage of Restored Recrystallization of Hot-Worked Austenite Structure in Steel', Journal of the Japan Institute of Metals, Vol. 60, No. 11 (1996), pp. 1051-1057") As shown, the yield stresses during the first and second compression processing were set to σ1 and σ2, respectively, and the maximum stress during the first compression processing was set to σ m In the case of softening rate X s It can be calculated using the following formula.

[0228] X s =(σ m -σ2) / (σ m -σ1)

[0229] If recrystallization occurs sufficiently between the first and second compression processes, the stress-strain curves measured by processes 1 and 2 will show the same behavior, thus σ2 becomes close to the value of σ1, and therefore the softening rate X... s It gradually approaches 1. On the other hand, if recrystallization is suppressed between the first and second compression processes, the dislocation density increases during the second compression process, resulting in work hardening. Consequently, the yield stress σ² becomes higher, and the softening rate X... s It gradually approaches 0. Therefore, by measuring the softening rate X of the steel... s This allows for the evaluation of the recrystallization inhibition effect of the steel. In this embodiment, the softening rate X... s When the value is below 0.20, the steel is evaluated as having an improved recrystallization inhibition effect. The results are shown in Table 1 below.

[0230]

[0231]

[0232]

[0233]

[0234]

[0235]

[0236] Referring to Table 1, in Comparative Examples 84-91, the effective amount of element X, composed of Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Sc, was low, thus failing to exhibit a sufficient recrystallization inhibition effect. More specifically, in Comparative Example 84, since element X was not present, a sufficient recrystallization inhibition effect was not exhibited. Furthermore, in Comparative Examples 85-91, although element X was included, its content was low relative to O, N, and / or S; in other words, the content of O, N, and / or S was excessive relative to element X. Therefore, it is believed that a relatively large number of inclusions were formed between element X and these elements. As a result, the effective amount of element X was low, and a sufficient recrystallization inhibition effect could not be exhibited. In contrast, in all embodiments of the present invention, by setting the effective amount of element X to 0.0003% or more, a high recrystallization inhibition effect could be exhibited.

[0237] Industrial availability

[0238] The steel used in embodiments of the present invention can be, for example, steel before hot rolling, i.e., slabs, billets, ingots, or hot-rolled steel. Examples of hot-rolled steel include, for instance, thick steel plates used in applications such as bridges, construction, shipbuilding, and pressure vessels; thin steel plates used in applications such as automobiles and home appliances; and further include bars, wire rods, sections, and pipes. When the steel of the embodiments of the present invention is used in these materials, the recrystallization suppression effect allows for the manufacture of steel without compromising productivity. Furthermore, the metal structure in the steel can be refined, thus significantly improving properties associated with such metal structure refinement, such as toughness.

Claims

1. A type of steel having the following chemical composition: (in mass%): C：0.001～1.000%、 Si: 0.01~3.00% Mn: 0.10~4.50%, P: Below 0.300% S: Below 0.0300% Al：0.001～5.000%、 N: below 0.2000% O: Below 0.0100% At least one X element selected from Pr: 0–0.8000%, Eu: 0–0.8000%, Gd: 0–0.8000%, Tb: 0–0.8000%, Dy: 0–0.8000%, Ho: 0–0.8000%, Er: 0–0.8000%, Tm: 0–0.8000%, Yb: 0–0.8000%, Lu: 0–0.8000%, and Sc: 0–0.8000%. Nb: 0~3.000%, Ti: 0~0.500%, Ta: 0~0.500%, V：0～1.00%、 Cu: 0–3.00% Ni: 0~60.00% Cr：0～10.00%、 Mo: 0–5.00%, W：0～2.00%、 B：0～0.0200%、 Co: 0-3.00% Be: 0~0.050%, Ag: 0~0.500%, Zr：0～0.5000%、 Hf: 0~0.5000% Ca: 0~0.0500%, Mg: ≥0% and ≤0.0001% or ≥0.0015% and ≤0.0500% At least one of La, Ce, Nd, Pm, and Y: totaling 0 to 0.5000%. Sn: 0~0.300% Sb: 0~0.300%, Te: 0~0.100%, Se: 0~0.100% As: 0~0.050%, Bi: 0~0.500%, Pb: 0–0.500%, and Remaining components: Fe and impurities. Satisfy the following equation 1, 0.40[Pr]+0.37[Eu]+0.36[Gd]+0.35[Tb]+0.34[Dy]+0.34[Ho]+0.33[Er]+0 .33[Tm]+0.32[Yb]+0.32[Lu]+1.24[Sc]-2.33[O]-3.99[N]-1.74[S]≥0.0003 Formula 1 in, [Pr], [Eu], [Gd], [Tb], [Dy], [Ho], [Er], [Tm], [Yb], [Lu], [Sc], [O], [N] and [S] represent the content of each element in terms of mass%, which is 0 if the element is not present.

2. The steel according to claim 1, wherein, The chemical composition, expressed as a percentage by mass, contains one or more of the following elements: Nb: 0.003~3.000% Ti: 0.005~0.500%, Ta: 0.001~0.500% V：0.001～1.00%、 Cu: 0.001~3.00% Ni: 0.001~60.00% Cr：0.001～10.00%、 Mo: 0.001~5.00%, W：0.001～2.00%、 B：0.0001～0.0200%、 Co: 0.001~3.00% Be: 0.0003~0.050%, and Ag: 0.001~0.500%.

3. The steel according to claim 1 or 2, wherein, The chemical composition, expressed as a percentage by mass, contains one or more of the following elements: Zr：0.0001～0.5000%、 Hf: 0.0001~0.5000% Ca: 0.0001~0.0500%, Mg: exceeding 0.0015% but less than 0.0500%, and At least one of La, Ce, Nd, Pm and Y: totaling 0.0001 to 0.5000%.

4. The steel according to any one of claims 1 to 3, wherein, The chemical composition, expressed as a percentage by mass, contains one or two of the following elements: Sn: 0.001~0.300%, and Sb: 0.001~0.300%.

5. The steel according to any one of claims 1 to 4, wherein, The chemical composition, expressed as a percentage by mass, contains one or more of the following elements: Te: 0.001~0.100% Se: 0.001~0.100% As: 0.001~0.050% Bi: 0.001~0.500%, and Pb: 0.001~0.500%.

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