High-tensile steel plate

Abstract

To provide high-tensile steel plates with excellent flatness and bendability. [Solution] A high-tensile steel sheet having a predetermined chemical composition, with a [Si] / [Ca] ratio of 175 or less, and a ΔHv defined by the following formula (1) of 50 or less. ΔHv = Hvmax - Hvmin···(1). Note that Hvmax is the maximum value of Vickers hardness measured at 100 points at 5 mm intervals from the center in the width direction in the rolling direction under a load of 98 N, and Hvmin is the minimum value of Vickers hardness measured at 100 points at 5 mm intervals from the center in the width direction in the rolling direction under a load of 98 N.

Description

[Technical Field]

[0001] The present disclosure relates to a high-tensile steel plate. [Background Art]

[0002] High-tensile steel plates are used as components in industrial machinery, conveying machinery and the like, and flatness and bendability are also required when assembling the components. Further, high-tensile steel plates are often required to have low-temperature toughness in consideration of severe usage environments and the like.

[0003] However, high strength and workability are conflicting properties, and it is difficult to satisfy both properties.

[0004] Therefore, conventionally, a method of refining γ grains is known as a method for improving the bendability of high-tensile steel plates. For example, Patent Document 1 discloses a method for improving bendability by making the γ crystal grain size fine and uniform through reheating quenching.

[0005] Further, as a method for improving bendability without performing heat treatment after rolling, a method of controlling the surface structure is known. For example, Patent Document 2 discloses a method for improving bending workability by providing a softened phase on the surface. [Prior Art Documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Unexamined Patent Publication No. 2009-242832 [Patent Document 2] Japanese Patent No. 4897125 [Summary of the Invention] [Problem to be Solved by the Invention]

[0007] Patent Document 1 relates to a steel with a tensile strength of 980 MPa that takes bendability into consideration. However, conventional methods such as those described in Patent Document 1 rely on refining the grain size to improve bendability, and since offline reheating and quenching are assumed in order to make the grain size fine and uniform, productivity is low.

[0008] Furthermore, while Patent Document 2 omits tempering and improves the balance between strength and flexibility, it still requires changing the rolling direction during the rolling process, making it difficult to achieve high productivity. Also, Patent Document 2 does not consider flatness at all.

[0009] This invention was made in view of these circumstances, and aims to provide a high-tensile steel sheet having excellent flatness, bendability, and low-temperature toughness. [Means for solving the problem]

[0010] This invention is based on the above findings, and its gist is as follows.

[0011] (1) A high-tensile steel sheet according to one aspect of the present invention has a chemical composition in mass% of: C: 0.060% or more, 0.120% or less, Si: 0.01% or more, 0.30% or less, Mn: 1.60% or more, 2.10% or less, P: 0.000% or more, 0.020% or less, S: 0.000% or more, 0.005% or less, Ti: 0.007% or more, 0.030% or less, Al: 0.010% or more, 0.050% or less, N: 0.0010% or more, 0.0100% or less, B: 0.0005% or more, 0.0030% or less, O: 0% or more, 0.004% or less, Ca: 0.0010% or more, 0.0060% or less, Nb: 0% or more, 0.040% or less, Cu: more than 0% and 0.50% or less, Ni: more than 0% and 1.20% or less, Cr: more than 0% and 0.50% or less, Mo: more than 0% and 0.50% or less, V : more than 0% and 0.10% or less, Mg: more than 0% and 0.0050% or less, and REM: more than 0% and 0.0200% or less is contained, with the balance being Fe and impurities, [Si] / [Ca] is 175 or less, ΔHv defined by the following formula (1) is 50 or less. ΔHv=Hvmax-Hvmin ···(1) Hvmax: the maximum value of Vickers hardness measured at 100 points at a 5 mm pitch in the rolling direction from the center in the width direction under a load of 98 N. Hvmin: the minimum value of Vickers hardness measured at 100 points at a 5 mm pitch in the rolling direction from the center in the width direction under a load of 98 N. (2) The high-tensile steel sheet according to (1) above, wherein the chemical composition further contains, by mass%, Nb: 0.040% or less, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.10% or less, Mg: 0.0050% or less, and REM: 0.0200% or less one or more selected from the group consisting of may be contained. (3) The high-tensile steel sheet according to (1) or (2) above, wherein the chemical composition further contains, by mass%, W: 0.100% or less, Co: 0.100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Te: 0.050% or less, As: 0.050% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Bi: 0.100% or less, Ta: 0.100% or less, Zn: 0.020% or less, Pb: 0.090% or less, Se:0.020% or less It may contain one or more selected from the group consisting of the following. (4) In the high-tensile steel sheet described in any of (1) to (3) above, the metal structure is It may contain ferrite in an area ratio of 10% or less. [Effects of the Invention]

[0012] According to the above embodiment of the present invention, it is possible to provide a non-heat-treated high-tensile steel sheet having excellent flatness, bendability, and low-temperature toughness. In particular, the high-tensile steel sheet of the above embodiment can achieve excellent flatness, bendability, and low-temperature toughness without heat treatment such as tempering. [Modes for carrying out the invention]

[0013] The following describes a non-heat-treated high-tensile steel sheet according to one embodiment of the present invention. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, the numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range.

[0014] <1. Results of the inventors' investigation> First, the inventors investigated the factors affecting the flatness of high-tensile steel sheets. As a result, they found that variations in the transformation temperature of the microstructure within the steel sheet lead to different local thermal expansions, which reduces flatness. They also found that aligning the transformation temperature of the microstructure results in uniform hardness, suppresses localized stress concentration, and improves bendability.

[0015] Next, the inventors investigated the causes of variations in the transformation temperature of low-temperature transformation structures and methods for suppressing them. As a result, they found that the non-uniform cooling rate of the steel sheet due to scale peeling leads to non-uniform transformation temperatures of the low-temperature transformation structure. Furthermore, they found that improving the peelability of the scale by adjusting the steel composition allows the cooling rate after rolling to become constant, thereby reducing variations in hardness.

[0016] Furthermore, we investigated components that improve scale removal properties. As a result, we found that reducing the Si content while increasing the proportion of Ca content improved scale removal properties.

[0017] Based on the above findings, the requirements of one embodiment of the present invention are described in detail below.

[0018] <2. High tensile strength steel plate> The chemical composition of the high-tensile steel sheet in this embodiment will be described below. In the following description of the chemical composition, "%" in the content means "mass%".

[0019] (C: 0.060% or more, 0.120% or less) Carbon (C) is an element that increases the strength of steel. From the viewpoint of ensuring strength, in this embodiment, the C content is 0.060% or more. Preferably, the C content is 0.080% or more. On the other hand, if the C content exceeds 0.120%, the hardness becomes excessively high, and the low-temperature toughness deteriorates. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the C content is 0.120% or less. Preferably, the C content is 0.110% or less.

[0020] (Si: 0.01% or more, 0.30% or less) Si is a deoxidizing element. From the viewpoint of ensuring the deoxidizing effect, the Si content in this embodiment is 0.01% or more. On the other hand, if the Si content exceeds 0.30%, the flatness decreases due to a decrease in scale peelability. From the viewpoint of ensuring scale peelability, the Si content in this embodiment is 0.30% or less. The Si content is preferably 0.25% or less. The Si content is even more preferably 0.20% or less.

[0021] (Mn: 1.60% or more, 2.10% or less) Mn is an element that enhances the hardenability and strength of steel. From the viewpoint of ensuring strength, in this embodiment, the Mn content is 1.60% or more. Preferably, the Mn content is 1.70% or more, or 1.80% or more. On the other hand, if the Mn content exceeds 2.10%, the low-temperature toughness decreases. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Mn content is 2.10% or less. Preferably, the Mn content is 2.00% or less.

[0022] (P: 0.000% or more, 0.020% or less) P is an impurity element. P is also an element that degrades low-temperature toughness by segregating at grain boundaries. From the viewpoint of ensuring low-temperature toughness, the P content in this embodiment is 0.020% or less. Preferably, the P content is 0.019% or less, or 0.018% or less. The lower limit of the P content is not limited and may be 0%. From the viewpoint of manufacturing cost, the P content may be 0.001% or more.

[0023] (S: 0.000% or more, 0.005% or less) S is an impurity element. S also degrades low-temperature toughness by combining with Mn to form MnS. From the viewpoint of ensuring low-temperature toughness, the S content in this embodiment is 0.005% or less. Preferably, the S content is 0.004% or less, or 0.002% or less. The lower limit of the S content is not limited and may be 0%. From the viewpoint of manufacturing cost, the S content may be 0.0001% or more.

[0024] (Ti: 0.007% or more, 0.030% or less) Ti is an element that suppresses the coarsening of γ grains by forming fine nitrides, thereby improving low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Ti content is 0.007% or more. Preferably, the Ti content is 0.011% or more, or 0.012% or more. On the other hand, if the Ti content exceeds 0.030%, coarse Ti carbonitrides precipitate, and the flexibility and low-temperature toughness deteriorate. From the viewpoint of ensuring flexibility and low-temperature toughness, in this embodiment, the Ti content is 0.030% or less. Preferably, the Ti content is 0.025% or less, or 0.020% or less.

[0025] (Al: 0.010% or more, 0.050% or less) Al is a deoxidizing element that suppresses the coarsening of γ grains by forming fine nitrides, thereby improving low-temperature toughness. From the viewpoint of ensuring deoxidizing effect and low-temperature toughness, the Al content is 0.010% or more. Preferably, the Al content is 0.020% or more. On the other hand, if the Al content exceeds 0.050%, the formation of coarse Al oxides and the increase in island-like martensites reduce wear resistance and low-temperature toughness. From the viewpoint of ensuring wear resistance and low-temperature toughness, in this embodiment, the Al content is 0.050% or less. Preferably, the Al content is 0.040% or less.

[0026] (N: 0.0010% or more, 0.0100% or less) N is an element that suppresses the coarsening of γ grains by forming fine nitrides, thereby improving low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, the N content is 0.0010% or more. Preferably, the N content is 0.0015% or more, and more preferably 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, the low-temperature toughness deteriorates due to the formation of coarse nitrides. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the N content is 0.0100% or less. Preferably, the N content is 0.0055% or less, or 0.0050% or less.

[0027] (B: 0.0005% or more, 0.0030% or less) B is an element that improves the hardenability and strength of steel. From the viewpoint of ensuring strength, the B content in this embodiment is 0.0005% or more. Preferably, the B content is 0.0007% or more, or 0.0008% or more. On the other hand, B precipitates, degrading the surface quality of steel billets produced by continuous casting and reducing low-temperature toughness. From the viewpoint of ensuring the surface quality of steel billets and ensuring low-temperature toughness, the B content in this embodiment is 0.0030% or less. Preferably, the B content is 0.0018% or less, or 0.0016% or less.

[0028] (O: 0% or more, 0.004% or less) O is an impurity element and also degrades low-temperature toughness by forming coarse oxides. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the O content is 0.0040% or less. Preferably, the O content is 0.0035% or less, or 0.0030% or less. The lower limit of the O content is not limited and may be 0%, but from the viewpoint of manufacturing cost, the O content may be 0.0005% or more.

[0029] (Ca: 0.0010% or more, 0.0060% or less) Ca is an element that improves scale exfoliation. It is also an element that suppresses the formation of coarse inclusions by forming oxides, sulfides, and oxysulfides, thereby increasing low-temperature toughness. In this embodiment, from the viewpoint of ensuring scale exfoliation and low-temperature toughness, the Ca content is 0.0010% or more. Preferably, the Ca content is 0.0020% or more, or 0.0023% or more. On the other hand, if the Ca content exceeds 0.0060%, coarse Ca inclusions precipitate, reducing bendability and low-temperature toughness. From the viewpoint of ensuring bendability and low-temperature toughness, the Ca content is 0.0060% or less. Preferably, the Ca content is 0.0050% or less, or 0.0040% or less.

[0030] The remainder of the chemical composition of the high-tensile steel sheet according to this embodiment consists of iron (Fe) and impurities. Impurities are components that are mixed in during the industrial manufacturing of steel materials due to raw materials such as ore and scrap, or other factors.

[0031] The high-tensile steel sheet according to this embodiment may further contain, if necessary, one or more of the following selected elements Nb, Cu, Ni, Cr, Mo, V, Mg, and rare earth elements (REM) in place of a portion of Fe.

[0032] (Nb: 0% or more, 0.040% or less) Nb exhibits a strong γ-recrystallization inhibiting effect in TMCP (thermo-mechanical control process), improving low-temperature toughness. In this embodiment, the lower limit of the Nb content is not limited and may be 0%. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Nb content may be 0.001% or more. On the other hand, if the Nb content exceeds 0.040%, coarse Nb carbonitrides precipitate, degrading bendability and low-temperature toughness. From the viewpoint of ensuring bendability and low-temperature toughness, in this embodiment, the Nb content is 0.040% or less. Preferably, the Nb content is 0.029% or less, or 0.028% or less.

[0033] (Cu: 0% or more, 0.50% or less) Cu is an element that improves the hardenability and increases the strength of steel. In this embodiment, the lower limit of the Cu content is not limited and may be 0%. From the viewpoint of ensuring strength, in this embodiment, the Cu content may be 0.01% or more. However, if the Cu content exceeds 0.50%, it will reduce low-temperature toughness as coarse Cu precipitates form. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Cu content is 0.50% or less. Preferably, the Cu content is 0.45% or less, or 0.40% or less.

[0034] (Ni: 0% or more, 1.20% or less) Ni is an element that improves the hardenability and strength of steel, as well as its low-temperature toughness. In this embodiment, the lower limit of the Ni content is not limited and may be 0%. From the viewpoint of ensuring strength and low-temperature toughness, in this embodiment, the Ni content may be 0.01% or more. On the other hand, Ni reduces scale peelability and decreases flatness. If the Ni content exceeds 1.20%, the scale peelability deteriorates significantly. Therefore, from the viewpoint of ensuring scale peelability, in this embodiment, the Ni content is 1.20% or less. The Ni content is preferably 0.45% or less, or 0.40% or less.

[0035] (Cr: 0% or more, 0.50% or less) Cr is an element that improves the hardenability of steel and increases its strength. In this embodiment, the lower limit of the Cr content is not limited and may be 0%. From the viewpoint of ensuring strength, in this embodiment, the Cr content may be 0.01% or more. On the other hand, if the Cr content exceeds 0.50%, coarse Cr carbides precipitate, reducing bendability and low-temperature toughness. From the viewpoint of ensuring bendability and low-temperature toughness, in this embodiment, the Cr content is 0.50% or less. Preferably, the Cr content is 0.45% or less, or 0.40% or less.

[0036] (Mo: 0% or more, 0.50% or less) Mo is an element that improves the hardenability of steel and increases its strength. In this embodiment, the lower limit of the Mo content is not limited and may be 0%. From the viewpoint of ensuring strength, in this embodiment, the Mo content may be 0.01% or more. On the other hand, if the Mo content exceeds 0.50%, coarse Mo carbides precipitate, reducing bendability and low-temperature toughness. From the viewpoint of ensuring bendability and low-temperature toughness, in this embodiment, the Mo content is 0.50% or less. Preferably, the Mo content is 0.45% or less, or 0.40% or less.

[0037] (V: 0% or more, 0.10% or less) V is an element that forms carbides and nitrides, improving the strength of steel. In this embodiment, the lower limit of the V content is not limited and may be 0%. From the viewpoint of ensuring strength, in this embodiment, the V content may be 0.01% or more. On the other hand, if the V content exceeds 0.10%, coarse V carbonitrides precipitate, reducing bendability and low-temperature toughness. From the viewpoint of ensuring bendability and low-temperature toughness, in this embodiment, the V content is 0.10% or less. Preferably, the V content is 0.08% or less, or 0.06% or less.

[0038] (Mg: 0% or more, 0.0050% or less) Mg is an element that forms oxides, sulfides, and oxysulfides, suppressing the formation of coarse inclusions and improving low-temperature toughness. In this embodiment, the lower limit of the Mg content is not limited and may be 0%. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Mg content may be 0.0010% or more. On the other hand, if the Mg content exceeds 0.0050%, coarse Mg inclusions precipitate, reducing bendability and low-temperature toughness. From the viewpoint of ensuring bendability and low-temperature toughness, the Mg content is 0.0050% or less. Preferably, the Mg content is 0.0040% or less, or 0.0030% or less.

[0039] (Rare earth elements (REM): 0% or more, 0.0200% or less) Rare earth elements (REM) refer to the 17 elements in total, consisting of the two elements Sc and Y, and the 15 lanthanide elements such as La, Pr, and Nd. The REM content refers to the total content of these 17 elements. The lower limit of the REM content is not limited and may be 0%. Like Mg and Ca, REM is an element that suppresses the formation of coarse inclusions by forming oxides, sulfides, and oxysulfides, thereby increasing low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, the REM content in this embodiment may be 0.0010% or more. On the other hand, if the REM content exceeds 0.0200%, coarse REM inclusions precipitate, reducing low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, the REM content is 0.0200% or less. Preferably, the REM content is 0.0150% or less, or 0.0100% or less.

[0040] The high-tensile steel sheet according to this embodiment may further contain, if necessary, one or more of the following selected elements W, Co, Zr, Hf, Te, As, Sr, Sn, Sb, Bi, Ta, Zn, Pb, and Se in place of a portion of Fe.

[0041] (W: 0% or more, 0.100% or less) W is an element that improves the hardenability of steel and increases its strength. In this embodiment, the lower limit of the W content is not limited and may be 0%. From the viewpoint of ensuring strength, in this embodiment, the W content may be 0.010% or more. On the other hand, if the W content exceeds 0.100%, coarse W carbides precipitate and the low-temperature toughness decreases. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the W content is 0.100% or less.

[0042] (Co: 0% or more, 0.100% or less) Co is an impurity element, but it is also an element that improves the hardenability of steel and increases its strength. In this embodiment, the lower limit of the Co content is not limited and may be 0%. From the viewpoint of ensuring strength, in this embodiment, the Co content may be 0.010% or more. On the other hand, if the Co content exceeds 0.100%, coarse Co carbides precipitate and the low-temperature toughness decreases. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Co content is 0.100% or less.

[0043] (Zr: 0% or more, 0.050% or less) Zr is an impurity element, but it also forms oxides, sulfides, and oxysulfides, suppressing the formation of coarse inclusions and improving low-temperature toughness. In this embodiment, the lower limit of the Zr content is not limited and may be 0%. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Zr content may be 0.001% or more. On the other hand, if the Zr content exceeds 0.050%, coarse Zr inclusions precipitate, reducing low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Zr content is 0.050% or less. Preferably, the Zr content is 0.030% or less, or 0.010% or less.

[0044] (Hf: 0% or more, 0.020% or less) Hf is an impurity element that forms oxides, sulfides, and oxysulfides, suppressing the formation of coarse inclusions and improving low-temperature toughness. In this embodiment, the lower limit of the Hf content is not limited and may be 0%. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Hf content may be 0.001% or more. On the other hand, if the Hf content exceeds 0.020%, coarse Hf inclusions precipitate, reducing low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Hf content is 0.020% or less. The Hf content is preferably 0.010% or less, or 0.005% or less.

[0045] (Te: 0% or more, 0.050% or less) Te is an impurity element, but it also forms oxides, sulfides, and oxysulfides, suppressing the formation of coarse inclusions and improving low-temperature toughness. In this embodiment, the lower limit of the Te content is not limited and may be 0%. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Te content may be 0.001% or more. On the other hand, if the Te content exceeds 0.050%, coarse Te inclusions precipitate, reducing low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Te content is 0.050% or less. Preferably, the Te content is 0.030% or less, or 0.010% or less.

[0046] (As: 0% or more, 0.050% or less) As is an impurity element and also an element that reduces the hot workability of steel. From the viewpoint of ensuring hot workability, in this embodiment, the As content is 0.050% or less. Preferably, the As content is 0.020% or less, or 0.010% or less. In this embodiment, the lower limit of the As content is not limited and may be 0%. From the viewpoint of manufacturing cost, the As content may be 0.0001% or more.

[0047] (Sr: 0% or more, 0.020% or less) Sr is an impurity element, but it also forms oxides, sulfides, and oxysulfides, suppressing the formation of coarse inclusions and improving low-temperature toughness. In this embodiment, the lower limit of the Sr content is not limited and may be 0%. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Sr content may be 0.001% or more. On the other hand, if the Sr content exceeds 0.020%, coarse Sr inclusions precipitate, reducing low-temperature toughness. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Sr content is 0.020% or less. Preferably, the Sr content is 0.010% or less, or 0.005% or less.

[0048] (Sn: 0% or more, 0.100% or less) Sn is an impurity element, but it is also an element that enhances the corrosion resistance of steel. In this embodiment, the lower limit of the Sn content is not limited and may be 0%. From the viewpoint of improving corrosion resistance, in this embodiment, the Sn content may be 0.001% or more. On the other hand, if the Sn content exceeds 0.100%, the low-temperature toughness decreases. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Sn content is 0.100% or less. The Sn content is preferably 0.050% or less, or 0.020% or less.

[0049] (Sb: 0% or more, 0.050% or less) Sb is an impurity element, but it is also an element that enhances the corrosion resistance of steel. In this embodiment, the lower limit of the Sb content is not limited and may be 0%. From the viewpoint of improving corrosion resistance, in this embodiment, the Sb content may be 0.001% or more. On the other hand, if the Sb content exceeds 0.050%, the low-temperature toughness decreases. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Sb content is 0.050% or less. Preferably, the Sb content is 0.030% or less, or 0.010% or less.

[0050] (Bi: 0% or more, 0.100% or less) Bi is an impurity element and also an element that reduces the hot workability of steel. From the viewpoint of ensuring hot workability, in this embodiment, the Bi content is 0.100% or less. Preferably, the Bi content is 0.050% or less, or 0.020% or less. In this embodiment, the lower limit of the Bi content is not limited and may be 0%. From the viewpoint of manufacturing cost, the Bi content may be 0.0001% or more.

[0051] (Ta: 0% or more, 0.100% or less) Ta is an impurity element, but it also forms oxides, sulfides, and oxysulfides, suppressing the formation of coarse inclusions and improving low-temperature toughness. In this embodiment, the lower limit of the Ta content is not limited and may be 0%. From the viewpoint of improving low-temperature toughness, in this embodiment, the Ta content may be 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, coarse Ta inclusions precipitate and low-temperature toughness decreases. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Ta content is 0.100% or less. Preferably, the Ta content is 0.050% or less, or 0.020% or less.

[0052] (Zn: 0% or more, 0.020% or less) Zn is an impurity element, but it is also an element that enhances the corrosion resistance of steel. In this embodiment, the lower limit of the Zn content is not limited and may be 0%. From the viewpoint of improving corrosion resistance, in this embodiment, the Zn content may be 0.001% or more. On the other hand, if the Zn content exceeds 0.020%, the low-temperature toughness decreases. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Zn content is 0.020% or less. The Zn content is preferably 0.010% or less, or 0.005% or less.

[0053] (Pb: 0% or more, 0.090% or less) Pb is an impurity element and also an element that reduces the hot workability of steel. From the viewpoint of ensuring hot workability, in this embodiment, the Pb content is 0.090% or less. Preferably, the Pb content is 0.050% or less, or 0.020% or less. In this embodiment, the lower limit of the Pb content is not limited and may be 0%. From the viewpoint of manufacturing cost, the Pb content may be 0.0001% or more.

[0054] (Se: 0% or more, 0.020% or less) Se is an impurity element. Se is also an element that degrades low-temperature toughness by combining with Mn to form MnSe. From the viewpoint of ensuring low-temperature toughness, in this embodiment, the Se content is 0.020% or less. Preferably, the Se content is 0.010% or less, or 0.005% or less. The lower limit of the Se content is not limited and may be 0%. From the viewpoint of manufacturing cost, the Se content may be 0.0001% or more.

[0055] ([Si] / [Ca]:175 or less) In the chemical composition of this embodiment, the ratio of [Si] / [Ca] is 175 or less. By setting the ratio of [Si] / [Ca] to 175 or less, unevenness in scale peeling on the steel sheet surface can be suppressed. As a result, uneven cooling of the surface can be suppressed during the cooling process, and good flatness can be obtained. Preferably, the ratio of [Si] / [Ca] is 100 or less from the viewpoint of scale peelability. The lower limit of [Si] / [Ca] is not particularly limited, but it may be 5 or more, or 10 or more.

[0056] Next, the microstructure of the high-tensile steel sheet in this embodiment will be described. In this embodiment, the microstructure at a depth of 0.7 mm from the surface of the high-tensile steel sheet in the thickness direction may include ferrite, pearlite, upper bainite, lower bainite, and martensite.

[0057] (Ferrite area ratio) The microstructure of the high-tensile steel sheet according to this embodiment is a mixed structure of ferrite, pearlite, upper bainite, lower bainite, and martensite. From the viewpoint of ensuring sufficient strength, it is preferable that the area ratio of ferrite, which is the soft phase, be low, preferably 10% or less. The area ratio of ferrite is more preferably 8% or less, and even more preferably 5% or less. The area ratio of ferrite may be 0%.

[0058] In addition to ferrite, pearlite, upper bainite, lower bainite, and martensite, the microstructure of the high-tensile steel sheet according to this embodiment may also contain trace amounts of precipitates, inclusions, etc. However, in the microstructure of the high-tensile steel sheet according to this embodiment, the area ratio of precipitates, inclusions, etc. is negligibly small compared to ferrite, pearlite, upper bainite, lower bainite, and martensite.

[0059] In this embodiment, the area ratio of ferrite can be determined by the following method. A test specimen for microstructural observation is prepared from the steel sheet according to this embodiment. The test specimen is prepared so that the observation surface is perpendicular to the thickness direction (including the rolling direction and the width direction) at a position 0.7 mm below the surface of the steel sheet in the thickness direction. After electropolishing the observation surface of the test specimen, analysis is performed by electron backscatter diffraction (EBSD). The measurement magnification is 300x, and measurements are performed in a 300 μm × 300 μm area at a measurement pitch of 0.3 μm. The confidence index (CI) is set to 0.1 or higher. The determination of ferrite, pearlite, upper bainite, lower bainite, and martensite is performed by setting the threshold of KAM (Kernel Average Misorientation) with respect to the third nearest neighbor to 1.0. Regions where KAM is 1.0 or less are ferrite, and regions where KAM is greater than 1.0 are pearlite, upper bainite, lower bainite, or martensite.

[0060] <Hardness distribution> In the high-tensile steel sheet according to this embodiment, ΔHv, as defined by the following formula (1), is 50 or less. In this embodiment, ΔHv is an indicator of hardness variation. By keeping ΔHv below 50, the bendability can be improved.

[0061] ΔHv = Hvmax - Hvmin ···(1) Hvmax: The maximum Vickers hardness measured at 100 points at 5mm intervals from the center in the width direction in the rolling direction under a load of 98N. Hvmin: The minimum Vickers hardness measured at 100 points at 5mm intervals from the center in the width direction in the rolling direction under a load of 98N.

[0062] In this embodiment, the hardness distribution is determined by the average Vickers hardness measured at 100 points at 5 mm intervals from the center in the width direction in the rolling direction, under a load of 98 N, after grinding down to 0.3 mm below the surface.

[0063] <Mechanical properties> The high-tensile steel sheet in this embodiment has excellent strength, bendability, and low-temperature toughness.

[0064] (Tensile strength, yield strength) The tensile strength and yield strength of the high-tensile steel plate in this embodiment can be determined by the following method. A plate-shaped test specimen for tensile testing is prepared from the center of the thickness of the high-tensile steel sheet according to this embodiment. At this time, the axial direction of the plate-shaped test specimen is perpendicular to the rolling direction. The plate-shaped test specimen is JIS No. 14B as specified in JIS Z 2241:2022. A tensile test is performed on the tensile test specimen at room temperature and in air, in accordance with JIS Z 2241:2022. From the results of the tensile test, the tensile strength TS (MPa) and yield strength YS (MPa) are determined. The yield strength YS is defined as the 0.2% proof stress. The high-tensile steel sheet according to this embodiment preferably has a tensile strength of 930 MPa to 1200 MPa and a yield strength of 800 MPa or more.

[0065] (Bendability) The bendability is evaluated using the metal bending test method specified in JIS Z 2248:2022. A No. 1 test piece is taken from a predetermined position (1 / 4 of the width) of the obtained steel sheet so that the longitudinal direction of the test piece is in the L direction (rolling direction) and the T direction (perpendicular to the rolling direction). Then, the No. 1 test piece is bent 180 degrees in the L direction (rolling direction) and the T direction (perpendicular to the rolling direction) with a bending radius (3.0t) that is 3.0 times the sheet thickness t. It is preferable that no cracks or other defects occur on the outside of the curved portion after the bending test in either direction.

[0066] (flatness) Flatness is evaluated according to the provisions of JIS G 3193:2019. For steel plates with a width of less than 2000 mm, the maximum upper limit for flatness shall be 20 mm for plate thicknesses less than 3.15 mm, 16 mm for plate thicknesses between 3.15 mm and 4.00 mm, 14 mm for plate thicknesses between 4.00 mm and 5.00 mm, 13 mm for plate thicknesses between 5.00 mm and 8.00 mm, and 12 mm for plate thicknesses of 8.00 mm or more. For steel plates with a width of 2000 mm or more, the maximum upper limit for flatness shall be 30 mm for plate thicknesses less than 3.15 mm, 27 mm for plate thicknesses between 3.15 mm and 4.00 mm, 24 mm for plate thicknesses between 4.00 mm and 5.00 mm, 21 mm for plate thicknesses between 5.00 mm and 8.00 mm, and 16 mm for plate thicknesses of 8.00 mm or more. More preferably, for board widths less than 2000 mm, the maximum upper limit of flatness is set to 16 mm for board thicknesses less than 3.15 mm, 14 mm for board thicknesses between 3.15 mm and 4.00 mm, 13 mm for board thicknesses between 4.00 mm and 5.00 mm, 12 mm for board thicknesses between 5.00 mm and 8.00 mm, and 9 mm for board thicknesses of 8.00 mm or more. Furthermore, for board widths of 2000 mm or more, more preferably, the maximum upper limit of flatness is set to 27 mm for board thicknesses less than 3.15 mm, 24 mm for board thicknesses between 3.15 mm and 4.00 mm, 21 mm for board thicknesses between 4.00 mm and 5.00 mm, 16 mm for board thicknesses between 5.00 mm and 8.00 mm, and 13 mm for board thicknesses of 8.00 mm or more.

[0067] (Low temperature toughness) In this embodiment, the low-temperature toughness of the high-tensile steel sheet is indicated by the Charpy impact absorption energy at -40°C. The Charpy impact absorption energy is measured at -40°C using an impact blade with a radius of 2 mm, in accordance with the provisions of JIS Z 2242:2023. The Charpy impact test is performed using three specimens, and the result is calculated by averaging them. The specimens used are V-notch specimens, taken from the center of the rolled plate thickness, with the longitudinal direction parallel to the rolling direction and notches etched in the width direction in which cracks propagate. The thickness of the specimen is t. s This varies depending on the thickness t of the rolled sheet. For 3.0mm ≤ t ≤ 6.0mm, t s 2.5mm, 6.0mm <t≦11.0mmではt s 5.0mm, 11.0mm <tではtsは10.0mmとし、t s For the absorbed energy of 2.5 mm and 5.0 mm test specimens, the obtained absorbed energy is corrected to the full-size absorbed energy by multiplying it by 4 and 2, respectively, before evaluation. The Charpy impact absorbed energy of three test specimens is measured at -40°C, and the average value of these measurements is taken as the Charpy impact absorbed energy of the high-tensile steel sheet of this embodiment at -40°C. From the viewpoint of ensuring low-temperature toughness, the Charpy impact absorbed energy of the high-tensile steel sheet of this embodiment at -40°C is preferably 47 J or more.

[0068] <plate thickness> The thickness of the high-tensile steel sheet according to this embodiment is not particularly limited, but for example, it is 3 mm or more and less than 16 mm. If the high-tensile steel sheet has a thickness of 3 mm or more and less than 16 mm, a non-heat-treated high-tensile steel sheet with excellent flatness, bendability, and low-temperature toughness can be obtained stably by the preferred manufacturing method described later.

[0069] <3. Manufacturing method for high-tensile steel sheets> Next, a preferred method for manufacturing high-tensile steel sheets in this embodiment will be described.

[0070] The method for manufacturing the high-tensile steel sheet in this embodiment is not particularly limited, but for example, after melting steel that satisfies the aforementioned chemical structure, steel billets are produced by continuous casting. The steel billets are heated, hot-rolled, and then cooled by water cooling. The high-tensile steel sheet in this embodiment may also be manufactured by winding it into a coil after hot-rolling.

[0071] The cast slab, produced by the above-mentioned chemical composition and molten steel treatment, is air-cooled to (Ar3 -200°C) or below, and then reheated to a temperature range of 1000°C to 1300°C. "Ar3" is represented by the following formula (2). From the viewpoint of carbide solution treatment, the reheating temperature Tc is preferably 1050°C or higher. On the other hand, from the viewpoint of suppressing γ grain coarsening and improving low-temperature toughness, the reheating temperature Tc is preferably 1250°C or lower.

[0072] Ar3(℃)=868-396×[C]+24.6×[Si]-68.1×[Mn] -20.7×[Cu]-36.1×[Ni]-24.8×[Cr] +29.1 × [Mo] ... Formula (2) Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] represent the mass percentages of C, Si, Mn, Cu, Ni, Cr, and Mo, respectively.

[0073] After reheating, hot rolling is performed to refine the austenite structure. In hot rolling, an elongated γ-grain structure can be obtained by increasing the cumulative reduction ratio at temperatures below 950°C. By maintaining the elongated γ-grain structure of the hot-rolled steel sheet, the bainite and martensite structures after transformation are refined, resulting in excellent low-temperature toughness. To obtain a sufficiently elongated γ-grain structure, the cumulative reduction ratio at temperatures below 950°C may be 60% or more. If the cumulative reduction ratio at temperatures below 950°C is less than 60%, the bainite and martensite structures after transformation may not be sufficiently refined, potentially impairing low-temperature toughness. The cumulative reduction ratio at temperatures below 900°C is preferably 70% or more.

[0074] The end temperature of hot rolling is preferably Ar3(°C) or higher, from the viewpoint of ending the hot rolling process in the temperature range where the microstructure of the high-tensile steel sheet is austenite. If the end temperature of hot rolling is below Ar3(°C), ferrite transformation may proceed before cooling begins, potentially impairing the strength.

[0075] In this embodiment, the high-tensile steel is subjected to accelerated cooling after hot rolling. Accelerated cooling is preferably started at a temperature of Ar3(°C) or higher. The average cooling rate of accelerated cooling is preferably 20°C / s or higher from the viewpoint of ensuring strength. There is no particular upper limit to the average cooling rate of accelerated cooling, but from the viewpoint of ensuring low-temperature toughness, it is 100°C / s or less, preferably 80°C / s or less.

[0076] Accelerated cooling is preferably carried out until the surface temperature of the hot-rolled steel sheet drops below 250°C, at which point cooling is stopped. If cooling is stopped midway, the strength may decrease due to carbide precipitation, etc. The high-tensile steel sheet according to this embodiment is manufactured through the above process. [Examples]

[0077] Slabs were manufactured from molten steel having the chemical composition shown in Table 1. Note that blank spaces in Table 1 indicate that the content of each element is at impurity levels.

[0078] The manufactured slabs were subjected to a hot rolling process. Each slab with a test number was heated in a heating furnace at the heating temperatures shown in Table 2.

[0079] A hot rolling process was carried out on the heated slab. As shown in Table 2, the cumulative reduction ratio at temperatures below 950°C during hot rolling was as shown in Table 2.

[0080] Steel sheets that had undergone hot rolling were cooled under the conditions described in Table 2 to produce steel sheets of the thicknesses described in Table 2.

[0081] [Evaluation Test] The following evaluation tests were performed on the steel materials of each test number produced. (Test 1) Tensile test (Test 2) Microtissue observation test (Test 3) Hardness measurement test (Test 4) Bending Test (Test 5) Flatness Test (Test 6) 2mm V-notch Charpy impact test

[0082] [(Test 1) Tensile Test] Tensile tests were performed on the steel plates of each test number using the method described above to determine the tensile strength TS (MPa) and yield strength YS (MPa). The obtained tensile strength TS (MPa) and yield strength (MPa) for each test number are shown in Table 3.

[0083] [(Test 2) Microtissue observation test] Microstructural observation tests were performed on the steel plates for each test number. Test specimens were prepared using the method described above, and EBSD analysis was performed. Ferrite was identified from KAM, and its area percentage was determined. The area percentage (%) of ferrite for each test number is shown in Table 3.

[0084] [(Test 3) Hardness Measurement Test] Hardness testing was performed on the steel plates for each test number. The hardness was measured using the method described above, and ΔHv was calculated from the maximum and minimum values. The obtained ΔHv for each test number is shown in Table 3.

[0085] [(Test 4) Bending Test] A bending test was performed on the steel plate for each test number. Test specimens were prepared using the method described above, and their bendability was evaluated. A specimen was considered acceptable if no tears or other defects occurred on the outside of the curved portion after the bending test in any direction. The evaluation results for the bendability of each test number are shown in Table 3, with ○ indicating acceptance and × indicating rejection.

[0086] [(Test 5) Flatness Test] A flatness test was conducted on the steel plates for each test number. Flatness was evaluated using the method described above, and a plate was deemed to have passed if it met the criteria. The evaluation results for the flatness of each test number are shown in Table 3, with ○ indicating a pass and × indicating a fail.

[0087] [(Test 6) 2mm V-notch Charpy impact test] A 2mm V-notch Charpy impact test was performed on the steel plates of the test numbers using the method described above. Test specimens were prepared using the method described above, and a 2mm V-notch Charpy impact test was performed at -40°C to measure the absorbed energy. The absorbed energy for each test number obtained was converted to the full-size value according to the plate thickness of the test specimen as described above, and the values ​​are shown in Table 3.

[0088] [Evaluation Results] Referring to Tables 1, 2, and 3, the steel sheets for test numbers 1 to 14 had chemical compositions within the scope of the present invention and were manufactured using a favorable method, resulting in tensile strength (TS) and yield strength (YS) being within a favorable range, and a ΔHv of 50 or less. As a result, they exhibited excellent flatness and low-temperature toughness.

[0089] The steel plate in test number 15 had too low a carbon content. As a result, it lacked sufficient strength.

[0090] The steel plate in test number 16 had too high a carbon content. As a result, it lacked flexibility, flatness, and low-temperature toughness.

[0091] The steel plate in test number 17 had too high a Si content. As a result, its hardness was uneven, and its bendability, flatness, and low-temperature toughness were insufficient.

[0092] The steel plate in test number 18 had too low a calcium content. As a result, its hardness was uneven, and its bendability and flatness were insufficient.

[0093] The steel plate in test number 19 had an excessively high [Si] / [Ca] ratio. As a result, it exhibited uneven hardness and insufficient flatness.

[0094] The steel plate in test number 20 had too low a manganese content. As a result, it lacked hardenability and strength.

[0095] The steel plate in test number 21 had too high a manganese content. As a result, it lacked flexibility and low-temperature toughness.

[0096] The steel plate in test number 22 had too high an aluminum content. As a result, it lacked flexibility and low-temperature toughness.

[0097] The steel plate in test number 23 had too high a Ti content. As a result, it lacked flexibility and low-temperature toughness.

[0098] The steel plate in test number 24 had too high a copper content. As a result, it lacked flexibility and low-temperature toughness.

[0099] The steel plate in test number 25 had too high a chromium content. As a result, it lacked flexibility and low-temperature toughness.

[0100] The steel plate in test number 26 had too high a molybdenum (Mo) content. As a result, it lacked flexibility and low-temperature toughness.

[0101] The steel plate in test number 27 had too high a V content. As a result, it lacked flexibility and low-temperature toughness.

[0102] [Table 1A]

[0103] [Table 1B]

[0104] [Table 2]

[0105] [Table 3] [Industrial applicability]

[0106] According to this disclosure, it is possible to provide a non-heat-treated high-tensile steel sheet with excellent flatness and bendability without the need for heat treatment such as tempering. Therefore, the high-tensile steel sheet of this disclosure is suitable as a welded structural steel material for construction machinery, industrial machinery, etc.

Claims

1. The chemical composition is expressed in mass percent. C: 0.060% or more, 0.120% or less, Si: 0.01% or more, 0.30% or less, Mn: 1.60% or more, 2.10% or less, P: 0.000% or more, 0.020% or less, S: 0.000% or more, 0.005% or less, Ti: 0.007% or more, 0.030% or less, Al: 0.010% or more, 0.050% or less, N: 0.0010% or more, 0.0100% or less, B: 0.0005% or more, 0.0030% or less, O: 0% or more, 0.004% or less, Ca: 0.0010% or more, 0.0060% or less, Nb: 0% or more, 0.040% or less, Cu: 0% or more, 0.50% or less, Ni: 0% or more, 1.20% or less, Cr: 0% or more, 0.50% or less, Mo: 0% or more, 0.50% or less, V: 0% or more, 0.10% or less, Mg: 0% or more, 0.0050% or less, REM: 0% or more, 0.0200% or less It contains, with the remainder being Fe and impurities. [Si] / [Ca] is 175 or less, A high-tensile steel plate in which ΔHv, defined by the following formula (1), is 50 or less. ΔHv=Hvmax-Hvmin...(1) Hvmax: The maximum Vickers hardness value measured at 100 points at 5 mm intervals from the center in the width direction in the rolling direction under a load of 98 N. Hvmin: The minimum Vickers hardness value measured at 100 points at 5 mm intervals from the center in the width direction in the rolling direction under a load of 98 N.

2. The aforementioned chemical composition is further expressed in mass%, Nb: 0.040% or less, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, Mo: 0.50% or less V: 0.10% or less, Mg: 0.0050% or less, REM: 0.0200% or less The high-tensile steel sheet according to claim 1, comprising one or more selected from the group consisting of the following.

3. The aforementioned chemical composition is further expressed in mass%, W: 0.100% or less, Co: 0.100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Te: 0.050% or less, As: 0.050% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Bi: 0.100% or less, Ta: 0.100% or less, Zn: 0.020% or less, Pb: 0.090% or less, Se: 0.020% or less The high-tensile steel sheet according to claim 1 or 2, comprising one or more selected from the group consisting of the following.

4. In metallographic structures, The high-tensile steel sheet according to claim 1 or 2, wherein the area ratio of ferrite is 10% or less.

5. In metallographic structures, The high-tensile steel sheet according to claim 3, wherein the area ratio of ferrite is 10% or less.

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