steel

Abstract

We provide steel materials with excellent corrosion resistance. [Solution] A steel material having a chemical composition in mass%, C: 0.001-0.20%, Si: 1.00% or less, Mn: 0.01-2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.06% or less, the remainder being Fe and impurities, wherein the metal structure in a cross section perpendicular to the rolling direction of the steel material contains 90.0% or more ferrite by area ratio, and when observing the region from the surface of the steel material to a depth of 500 μm in the depth direction in the cross section, the area ratio of ferrite grains having a (111) crystal orientation is 20.0% or more.

Description

Technical Field

[0001] The present invention relates to steel materials.

Background Art

[0002] Steel materials used in coastal areas and the like are subjected to anticorrosion treatments such as painting in order to improve the corrosion resistance against flying salts. However, if there are parts where the painting is insufficient or parts where the paint film has deteriorated (hereinafter also referred to as "painting defect parts"), corrosion due to chlorides will occur and progress from these parts. Therefore, maintenance such as periodically repainting the steel material is required, increasing the cost. Against this background, the development of steel materials with excellent corrosion resistance that can extend the interval between repaintings is desired.

[0003] Patent Document 1 discloses a steel material containing alloy elements such as Cu, Ni, and W and used in a corrosive environment such as the sea where the amount of flying salts is large.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, the corrosion due to flying salts in the painting defect part is considered to occur and progress by the following mechanism. When flying salts adhere to the steel material and the humidity becomes high, the salt deliquesces and forms a liquid film containing salt on the steel material surface. If there is a painting defect part, corrosion occurs when this liquid film and the base material come into direct contact. Also, during the process of the liquid film drying, chloride ions concentrate on the surface of the painting defect part, and locally the liquid film becomes acidic. As a result, it is considered that the corrosion of the base material progresses severely.

[0006] Patent Document 1 describes a method for suppressing corrosion by forming a rust layer containing alloying elements, but it does not consider the perspective of suppressing the occurrence of corrosion. Furthermore, since the invention described in Patent Document 1 requires the inclusion of alloying elements, sufficient consideration has not been given to the cost aspect.

[0007] The present invention aims to solve the above problems and provide a steel material with excellent corrosion resistance without actively incorporating alloying elements. [Means for solving the problem]

[0008] This invention was made to solve the above problems, and its essence is the following steel material.

[0009] (1) The chemical composition of the steel material is, in mass%, C: 0.001~0.20%, Si: 1.00% or less, Mn: 0.01~2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.06% or less, The remainder consists of Fe and impurities. In a cross-section perpendicular to the rolling direction of the steel material, The metal structure contains 90.0% or more ferrite by area ratio. In the aforementioned cross-section, when observing the region from the surface of the steel material to a depth of 500 μm, the area ratio of ferrite grains having a (111) crystal orientation is 20.0% or more. Steel material.

[0010] (2) The chemical composition of the steel material is, in mass%, C: 0.001~0.20%, Si: 1.00% or less, Mn: 0.01~2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.06% or less, Furthermore, it contains one or more selected from the following groups A, B, and C: The remainder consists of Fe and impurities. In a cross-section perpendicular to the rolling direction of the steel material, The metal structure contains 90.0% or more ferrite by area ratio. In the aforementioned cross-section, when observing the region from the surface of the steel material to a depth of 500 μm, the area ratio of ferrite grains having a (111) crystal orientation is 20.0% or more. Steel material. [Group A] One or more elements selected from the group consisting of Cr: ≤1.00%, Cu: ≤1.00%, Ni: ≤2.00%, Mo: ≤1.00%, Co: ≤1.00%, Sn: ≤0.20%, W: ≤1.00%, Sb: ≤0.20%, In: ≤0.20%, As: ≤0.10%, Bi: ≤0.10%, Te: ≤0.10%, Pb: ≤0.10%, Zn: ≤0.10%, Ga: ≤0.10%, Ge: ≤0.10%, and Hf: ≤0.10%. [Group B] One or more selected from the group consisting of Nb: ≤0.050%, V: ≤0.050%, Ti: ≤0.050%, B: ≤0.005%, Zr: ≤0.100%, and Ta: ≤0.10%. [Group C] One or more selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less.

[0011] (3) The steel material according to (2) above, wherein the chemical composition contains one or more elements selected from group A.

[0012] (4) The steel material according to (2) above, wherein the chemical composition contains one or more elements selected from group B.

[0013] (5) The steel material according to (2) above, wherein the chemical composition contains one or more elements selected from the C group.

[0014] (6) A corrosion-resistant coating is provided on the surface, The steel material described in any of (1) to (5) above. [Effect of the Invention]

[0015] According to the present invention, a steel material with excellent corrosion resistance can be obtained. [Embodiments for Carrying Out the Invention]

[0016] In order to solve the above-described problems, the present inventors conducted a detailed investigation of the corrosion resistance of a steel material that does not actively contain alloy elements, and as a result, obtained the following findings.

[0017] As is generally said, on the surface of a steel material, a portion where the crystal structure such as grain boundaries is disordered serves as a starting point for corrosion. This is because a portion where the crystal structure is disordered is energetically unstable and the bond between Fe atoms is also weak, so it is considered that the dissolution reaction of Fe easily proceeds.

[0018] Based on such a background, the present inventors focused on the crystal structure and carefully studied a method capable of suppressing the occurrence of corrosion. As a result, when observing the surface layer of the steel material in a cross section perpendicular to the rolling direction of the steel material, it was found that the larger the area ratio of the crystal grains of ferrite having a (111) plane (hereinafter, also simply referred to as "area ratio of the (111) plane"), the more effectively the occurrence of corrosion can be suppressed. This is considered to be due to the following reasons. In the crystal grains having a (111) plane, Fe atoms are more densely packed than in crystal grains having other crystal orientations. Therefore, it is considered that the bond between atoms is strong and the dissolution reaction of Fe hardly occurs. Thus, by increasing the area ratio of the (111) plane, the corrosion resistance can be improved even in a steel material that does not actively contain alloy elements.

[0019] Furthermore, the inventors also investigated preferred manufacturing conditions for increasing the area ratio of (111) planes. Normally, in cold rolling, rolling rolls are placed above and below the steel material, but in the present invention, the rolls are adjusted and positioned so that the diameters of these upper and lower rolls differ within a predetermined range. Then, cold rolling is performed using such rolls at a reduction ratio within a predetermined range. In this way, not only is the reduction ratio of cold rolling controlled, but the difference in diameter between the upper and lower rolls used in cold rolling is also controlled to strain the surface layer of the steel material and ensure that the (111) planes are aligned. As a result, the area ratio of (111) planes on the surface layer of the steel material can be increased.

[0020] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0021] (A) Chemical composition The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0022] C: 0.001~0.20% Carbon (C) is an element that improves the strength of steel. However, if C is present in excess, the ferrite area ratio cannot be made 90.0% or more. Therefore, the C content should be between 0.001% and 0.20%. A C content of 0.005% or more is preferable, and 0.01% or more is more preferable. Furthermore, a C content of 0.18% or less is preferable, and 0.15% or less is more preferable.

[0023] Si: 1.00% or less Si is an element that contributes to deoxidation and strength improvement, and controls the morphology of oxides. However, if Si is present in excess, the area ratio of ferrite cannot be made 90.0% or more. It also reduces toughness. Therefore, the Si content should be 1.00% or less. A Si content of 0.90% or less is preferable, and 0.80% or less is more preferable. To obtain the above effects, a Si content of 0.10% or more is preferable, and 0.20% or more is more preferable.

[0024] Mn: 0.01~2.00% Mn is an element that improves strength and toughness. However, if Mn is present in excess, the ferrite area ratio cannot be made 90.0% or more. Also, the hot workability deteriorates. Therefore, the Mn content should be 0.01 to 2.00%. A Mn content of 0.05% or more is preferable, and 0.10% or more is more preferable. Furthermore, a Mn content of 1.80% or less is preferable, and 1.60% or less is more preferable.

[0025] P:0.030% or less P is an impurity that reduces the hot workability and productivity of steel. It also reduces the area ratio of ferrite. Therefore, an upper limit is set on the P content to 0.030% or less. Preferably, the P content is 0.025% or less, and more preferably 0.020% or less. It is preferable to reduce the P content as much as possible, meaning that the content may be 0%, but extreme reduction will lead to an increase in steelmaking costs. Therefore, the P content may be 0.001% or more, or 0.003% or more.

[0026] S: 0.030% or less S is an impurity that reduces the hot workability and productivity of steel. Therefore, an upper limit is set on the S content to 0.030% or less. Preferably, the S content is 0.025% or less, and more preferably 0.020% or less. It is preferable to reduce the S content as much as possible, meaning that the content may be 0%, but extreme reduction will lead to an increase in steelmaking costs. Therefore, the S content may be 0.001% or more, or 0.003% or more.

[0027] Al: 0.06% or less Al is added as a deoxidizing agent. However, if Al is present in excess, the ferrite area ratio cannot be made 90.0% or more. It also degrades the toughness of the weld metal. Therefore, the Al content should be 0.06% or less. Preferably, the Al content is 0.05% or less, and more preferably 0.04% or less. To obtain the above effects, the Al content is preferably 0.01% or more, and more preferably 0.03% or more.

[0028] In the chemical composition of the steel material of the present invention, the remainder is Fe and impurities. Here, impurities refer to components that are mixed in during the industrial production of steel material due to raw materials such as ore and scrap, or other factors, and are acceptable within a range that does not adversely affect the properties of the steel material of the present invention.

[0029] In the chemical composition of the steel of the present invention, in order to improve corrosion resistance, one or more elements selected from Cr, Cu, Ni, Mo, Co, Sn, W, Sb, In, As, Bi, Te, Pb, Zn, Ga, Ge, and Hf may be included within the ranges shown below. Note that these elements are not necessarily essential in the steel of the present invention, so the lower limit of their content is 0%. The reasons for limiting each element are explained below.

[0030] Cr:1.00% or less Cr is an element that contributes to improved corrosion resistance. It is also an element that enhances hardenability and thus improves strength. Therefore, Cr may be included as needed. However, excessive Cr content may reduce acid resistance, and corrosion resistance may decrease in environments with high chloride content. Therefore, the Cr content should be 1.00% or less. Preferably, the Cr content is 0.90% or less, and more preferably 0.80% or less. If the above effects are to be obtained more reliably, the Cr content is preferably 0.01% or more, and more preferably 0.05% or more.

[0031] Cu:1.00% or less Cu is an element that exhibits remarkable corrosion resistance. Therefore, Cu may be included as needed. However, if Cu is included in excess, hot workability will decrease, impairing productivity. For this reason, the Cu content should be 1.00% or less. Preferably, the Cu content is 0.80% or less, and more preferably 0.60% or less. Furthermore, if the above effects are to be obtained more reliably, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more.

[0032] Ni: 2.00% or less Ni, like Cu, is an element that improves corrosion resistance. Therefore, Ni may be included as needed. However, since Ni is an expensive element, excessive inclusion will lead to a decrease in economic efficiency. Accordingly, the Ni content should be 2.00% or less. Preferably, the Ni content is 1.80% or less, and more preferably 1.60% or less. Furthermore, if the above effects are to be obtained more reliably, the Ni content is preferably 0.01% or more, and more preferably 0.05% or more.

[0033] Mo: 1.00% or less Mo is an element that improves corrosion resistance. Therefore, Mo may be included as needed. However, since Mo is an expensive element, excessive inclusion will lead to a decrease in economic efficiency. Therefore, the Mo content should be 1.00% or less. Preferably, the Mo content is 0.80% or less, and more preferably 0.60% or less. Furthermore, if the above effects are to be obtained more reliably, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

[0034] Co: 1.00% or less Co is an element that forms oxides and improves corrosion resistance. Therefore, Co may be included as needed. However, since Co is an expensive element, excessive inclusion leads to a decrease in economic efficiency. Accordingly, the Co content should be 1.00% or less. A Co content of 0.80% or less is preferable, and 0.60% or less is more preferable. Furthermore, if the above effects are to be obtained more reliably, a Co content of 0.01% or more is preferable, and 0.05% or more is more preferable.

[0035] Sn: 0.20% or less Sn is an element that improves corrosion resistance. Therefore, Sn may be included as needed. However, if Sn is included in excess, the hot workability will decrease. For this reason, the Sn content should be 0.20% or less. Preferably, the Sn content is 0.18% or less, and more preferably 0.16% or less. Furthermore, if the above effects are to be obtained more reliably, the Sn content is preferably 0.005% or more, and more preferably 0.01% or more.

[0036] W: 1.00% or less W is an element that improves corrosion resistance. Therefore, W may be included as needed. However, since W is an expensive element, excessive inclusion will lead to a decrease in economic efficiency. Accordingly, the W content should be 1.00% or less. Preferably, the W content is 0.80% or less, and more preferably 0.60% or less. Furthermore, if the above effects are to be obtained more reliably, the W content is preferably 0.01% or more, and more preferably 0.05% or more.

[0037] Sb: 0.20% or less Sb is an element that exhibits remarkable corrosion resistance. Therefore, Sb may be included as needed. However, if Sb is included in excess, hot workability will decrease, impairing productivity. For this reason, the Sb content should be 0.20% or less. Preferably, the Sb content is 0.18% or less, and more preferably 0.16% or less. Furthermore, if the above effects are to be obtained more reliably, the Sb content is preferably 0.005% or more, and more preferably 0.01% or more.

[0038] In:0.20% or less In corrosive environments. 3+ It dissolves as an inhibitor and has the effect of suppressing corrosion through its inhibitory action in acidic chloride solutions. Furthermore, In 3+ Because UPD significantly suppresses the anodic dissolution reaction of steel, even a small amount can greatly improve corrosion resistance. Therefore, In may be included as needed. However, if In is included in excess, the above effect will not only saturate, but the toughness will also deteriorate. Therefore, the In content should be 0.20% or less. The In content is preferably 0.15% or less, and more preferably 0.10% or less. Furthermore, if the above effect is to be obtained more reliably, the In content is preferably 0.01% or more, and more preferably 0.02% or more.

[0039] As: 0.10% or less Although As does not have a significant effect compared to Sb, it is an effective element for improving corrosion resistance. Therefore, As may be included as needed. However, if As is included in excess, toughness will decrease. For this reason, the As content should be 0.10% or less. Preferably, the As content is 0.080% or less, and more preferably 0.060% or less. Furthermore, if the above effects are to be obtained more reliably, the As content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.040% or more.

[0040] Bi:0.10% or less Bi is an effective element for improving corrosion resistance. Therefore, Bi may be included as needed. However, if Bi is included in excess, toughness will decrease. For this reason, the Bi content should be 0.10% or less. Preferably, the Bi content is 0.080% or less, and more preferably 0.050% or less. If the above effects are to be obtained more reliably, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0041] Te: 0.10% or less Te is an effective element for improving corrosion resistance. Therefore, Te may be included as needed. However, if Te is included in excess, toughness will decrease. For this reason, the Te content should be 0.10% or less. A Te content of 0.080% or less is preferable, and 0.050% or less is more preferable. Furthermore, if the above effects are to be obtained more reliably, a Te content of 0.001% or more is preferable, 0.002% or more is more preferable, and 0.005% or more is even more preferable.

[0042] Pb: 0.10% or less Lead (Pb) forms sulfides with sulfur (S), making it an effective element for improving corrosion resistance. Therefore, Pb may be included as needed. However, if Pb is included in excess, toughness deteriorates. For this reason, the Pb content should be 0.10% or less. Preferably, the Pb content is 0.080% or less, and more preferably 0.050% or less. If the above effects are to be obtained more reliably, the Pb content is preferably 0.005% or more, and more preferably 0.010% or more.

[0043] Zn: 0.10% or less Ga: 0.10% or less Zn and Ga form sulfides with S, and are elements that are effective in improving corrosion resistance. Therefore, Zn and / or Ga may be included as needed. However, if Zn and Ga are included in excess, toughness deteriorates. For this reason, the content of Zn and Ga should be 0.10% or less each. Preferably, the content of Zn and Ga should be 0.080% or less each, and more preferably 0.050% or less each. If the above effect is to be obtained more reliably, the content of Zn and Ga should be 0.005% or more each, and more preferably 0.010% or more each.

[0044] Ge: 0.10% or less Ge forms sulfides with S and is an effective element for improving corrosion resistance. Therefore, Ge may be included as needed. However, if Ge is included in excess, toughness deteriorates. For this reason, the Ge content should be 0.10% or less. Preferably, the Ge content is 0.080% or less, and more preferably 0.050% or less. If the above effect is to be obtained more reliably, the Ge content is preferably 0.005% or more, and more preferably 0.010% or more.

[0045] Hf: 0.10% or less Hf is an element that forms oxides and improves corrosion resistance. Therefore, Hf may be included as needed. However, if Hf is included in excess, the economic efficiency decreases. For this reason, the Hf content should be 0.10% or less. Preferably, the Hf content is 0.080% or less, and more preferably 0.060% or less. Furthermore, if the above effects are to be obtained more reliably, the Hf content is preferably 0.005% or more, and more preferably 0.010% or more.

[0046] In the chemical composition of the steel of the present invention, one or more elements selected from Nb, V, Ti, B, Zr, and Ta may be further included within the ranges shown below in order to improve mechanical properties, etc. Note that these elements are not necessarily essential in the steel of the present invention, so the lower limit of their content is 0%. The reasons for limiting each element are explained below.

[0047] Nb: 0.050% or less Nb is an element that forms nitrides and contributes to grain refinement and improved strength. Therefore, Nb may be included as needed. However, if Nb is included in excess, the nitrides become coarse, and the mechanical properties deteriorate. For this reason, the Nb content should be 0.050% or less. Preferably, the Nb content is 0.045% or less, more preferably 0.040% or less, and even more preferably 0.035% or less. If the above effects are to be obtained more reliably, the Nb content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

[0048] V:0.050% or less V is an element that forms nitrides and contributes to grain refinement and improved strength. Therefore, V may be included as needed. However, if V is included in excess, the nitrides become coarse, and the mechanical properties deteriorate. For this reason, the V content should be 0.050% or less. Preferably, the V content is 0.045% or less, and more preferably 0.040% or less. If the above effects are to be obtained more reliably, the V content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

[0049] Ti: 0.050% or less Ti is an element that forms nitrides, contributing to grain refinement and improved strength. Therefore, Ti may be included as needed. However, if Ti is included in excess, the nitrides become coarse, and the mechanical properties deteriorate. For this reason, the Ti content should be 0.050% or less. A Ti content of 0.045% or less is preferable, and 0.040% or less is more preferable. Furthermore, if the above effects are to be obtained more reliably, a Ti content of 0.005% or more is preferable, and 0.010% or more is more preferable.

[0050] B: 0.005% or less B is an element that improves hardenability and increases strength. Therefore, B may be included as needed. However, if B is included in excess, the effect may saturate, and the toughness of the base material and HAZ may decrease. For this reason, the B content should be 0.005% or less. A B content of 0.004% or less is preferable, and 0.003% or less is more preferable. Furthermore, if the above effects are to be obtained more reliably, a B content of 0.0001% or more is preferable, and 0.0005% or more is more preferable.

[0051] Zr: 0.100% or less Zr, like Ti, is an element that forms nitrides and contributes to grain refinement and improved strength. Therefore, Zr may be included as needed. However, if Zr is included in excess, the nitrides become coarse, and the mechanical properties deteriorate. For this reason, the Zr content should be 0.100% or less. A Zr content of 0.090% or less is preferable. Furthermore, if the above effects are to be obtained more reliably, a Zr content of 0.005% or more is preferable, and 0.010% or more is more preferable.

[0052] Ta: 0.10% or less Ta is an element that contributes to improving strength. Therefore, Ta may be included as needed. However, Ta is an expensive element, and including a large amount will lead to an increase in steelmaking costs. For this reason, the Ta content should be 0.10% or less. A Ta content of 0.090% or less is preferable, 0.080% or less is more preferable, and 0.060% or less, 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less is even more preferable. Furthermore, if the above effects are to be obtained more reliably, a Ta content of 0.001% or more is preferable, and 0.005% or more is more preferable.

[0053] In the chemical composition of the steel of the present invention, one or more elements selected from Ca, Mg, and REM may be further included within the ranges shown below for the purpose of deoxidation and control of inclusions. Since these elements are not necessarily essential in the steel of the present invention, the lower limit of their content is 0%. The reasons for limiting each element are explained below.

[0054] Ca: 0.020% or less Ca has the effect of forming fine oxides. Therefore, Ca may be included as needed. However, adding too much Ca will increase steelmaking costs. For this reason, the Ca content should be 0.020% or less. Preferably, the Ca content is 0.015% or less, and more preferably 0.010% or less. If the above effect is to be obtained more reliably, the Ca content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0055] Mg: 0.020% or less Mg has the effect of forming fine oxides. Therefore, Mg may be included as needed. However, adding too much Mg will increase steelmaking costs. For this reason, the Mg content should be 0.020% or less. Preferably, the Mg content is 0.015% or less, and more preferably 0.010% or less. If the above effect is to be obtained more reliably, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0056] REM: 0.020% or less Rare earth elements (REMs) are primarily used for deoxidation and have the effect of forming fine oxides. Therefore, REMs may be included as needed. However, adding excessive amounts of REMs will increase steelmaking costs. Accordingly, the REM content should be 0.020% or less. Preferably, the REM content is 0.015% or less, and more preferably 0.010% or less. If the above effects are to be obtained more reliably, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0057] Here, REM is a collective term for 17 elements including Sc, Y, and lanthanides, and the REM content represents the total amount of these elements. Note that lanthanides are added industrially in the form of mischmetal.

[0058] (B) Area ratio of ferrite The steel material according to the present invention contains 90.0% or more ferrite in area ratio in a cross section perpendicular to the rolling direction of the steel material. As described later, corrosion resistance can be improved by setting the area ratio of (111) planes to 20.0% or more. If the area ratio of ferrite is less than 90.0%, the area ratio of (111) planes necessary for improving corrosion resistance cannot be secured. Therefore, the area ratio of ferrite is set to 90.0% or more. Preferably, the area ratio of ferrite is 92.0% or more, and more preferably 94.0% or more.

[0059] In addition, while the steel material according to the present invention may contain carbides, inclusions, etc., in addition to ferrite, these structures are acceptable as long as their total area ratio is 10.0% or less.

[0060] The area fraction of ferrite is measured by the following method. First, scale is removed from the steel material, and a sample is taken so that the cross section perpendicular to the rolling direction of the steel material becomes the observation surface. Wet polishing is performed on the observation surface with emery paper, and then buff polishing is performed using diamond abrasive grains with an average diameter of 1 μm to finish the observation surface to a mirror finish. After that, electrolytic polishing is performed at a voltage of 40 V using A3 electrolyte (methanol:2-n-butoxyethanol:perchloric acid = 10:6:1). Then, on the observation surface, a region of 500 μm in depth and 200 μm in width from the surface of the steel material is observed in 1 μm steps using backscattered electron crystal orientation analysis (SEM-EBSD: Scanning Electron Microscope-Electron Back Scattering Diffraction). The observation magnification is set to 3000x. Then, the BCC phase is identified from the measurement results, and its area fraction is determined, which is then used as the area fraction of the ferrite phase.

[0061] (C)(111) Area ratio of surface The steel material according to the present invention has a surface area ratio of 20.0% or more when the region from the surface of the steel material to a depth of 500 μm in the depth direction (in the description of the present invention, this region is also referred to as the "surface layer") is observed in a cross section perpendicular to the rolling direction of the steel material. If the surface area ratio of 20.0% or more is less than 20.0%, the corrosion resistance cannot be improved. The surface area ratio of 25.0% or more is preferable, and 30.0% or more is preferable.

[0062] There is no particular upper limit on the area ratio of (111) surfaces. Under the manufacturing conditions described later, the area ratio of (111) surfaces will be 50.0% or less, 40.0% or less, or 35.0% or less.

[0063] (111) The area ratio of the surface is measured by the following method. First, scale is removed from the steel material, and a sample is taken so that the cross section perpendicular to the rolling direction of the steel material becomes the observation surface. The region from the surface of the steel material up to 500 μm is made the observation surface and is mirror polished, and electropolishing is performed at a voltage of 40 V using A3 electrolyte. On the observation surface, the crystal orientation of the region with a depth of 500 μm and a width of 500 μm from the surface of the steel material is observed at intervals of 200 nm using the EBSD method, and the crystal orientation information of this region is obtained.

[0064] Based on the obtained crystal orientation information, an IPF (Inverse Pole Figure) map is created that can represent the crystal orientation by color. In the created IPF map, using image processing software such as Photoshop (Adobe), the blue of the obtained IPF map is treated as the (111) plane and processed into three colors: red, blue, and green, and the area ratio of the (111) plane is calculated.

[0065] Furthermore, the allowable azimuth difference is set to 10°, and data with a Confidence Index (CI value) greater than 0.1 will be selected. In addition, the analysis software (OIM Analysis Ver. 7.3.1: manufactured by TSL Solutions Co., Ltd.) will be used to create the IPF-map.

[0066] (D) Corrosion prevention treatment The steel material of the present invention described above exhibits good corrosion resistance even when used as is. However, in order to further improve corrosion resistance in strongly acidic and weakly acidic environments, acid-resistant paint, heat-resistant paint, heat-resistant and acid-resistant paint, etc., may be applied to the surface of the steel material.

[0067] Examples of corrosion-resistant coatings made from organic resins include resin coatings made from vinyl resin, epoxy resin, silicone resin, and phenolic resin acid-resistant paints.

[0068] (E) Manufacturing method The steel material according to the present invention can be stably manufactured by, for example, the following manufacturing method. The steel material according to the present invention includes steel plates. While the plate thickness is not particularly limited, when the steel plate according to the present invention is used as a structure, for example, the plate thickness may be 3.0 to 30 mm.

[0069] <Hot rolling process> Steel having the chemical composition described above is melted and cast to obtain an ingot, which is then subjected to hot rolling. There are no particular restrictions on the heating conditions during hot rolling; normal conditions can be used. For example, the ingot can be heated to a temperature range of 1050-1100°C for 0.5-2.0 hours before hot rolling, and the finishing rolling can be completed at a temperature range of 920-1000°C. After hot rolling, the cooling method is air cooling to increase the ferrite area ratio in the final product.

[0070] <Cold rolling process> In the cold rolling process, the hot-rolled steel material is cold-rolled at a reduction ratio of 50-70% using a rolling mill with a difference in the diameter of a pair of rolls (hereinafter also simply referred to as "roll diameter difference") of 0.1-1.0%.

[0071] During cold rolling, steel is typically passed between a pair of rolls for rolling. If the roll diameter difference is less than 0.1%, it is not possible to impart distortion to the steel surface, and the (111) plane cannot be made 20.0% or more. On the other hand, if the roll diameter difference exceeds 1.0%, the steel sheet warps, resulting in an undesirable product. Therefore, the roll diameter difference should be between 0.1% and 1.0%.

[0072] When rolling is performed by passing the material through multiple pairs of rolls, the difference in roll diameter between one or more pairs of rolls should be 0.1 to 1.0%. Similarly, when using a rolling mill consisting of multiple rolling stands, the difference in roll diameter between one or more stands should be 0.1 to 1.0%.

[0073] Furthermore, the difference in roll diameter is calculated using the following formula (i). (R1-R2) / R1×100 ···(i) R1: The diameter (mm) of the larger of the two rolls positioned above and below the steel sheet during cold rolling. R2: The diameter (mm) of the smaller of the two rolls positioned above and below the steel sheet during cold rolling.

[0074] If the reduction ratio is less than 50%, it is not possible to impart distortion to the steel surface, and the area ratio of the (111) surface cannot be made 20.0% or more. On the other hand, if the reduction ratio exceeds 70%, the load on the rolling mill becomes too great. Therefore, the reduction ratio should be between 50% and 70%. From the viewpoint of increasing the area ratio of the (111) surface, a reduction ratio of 60% or more is preferable.

[0075] <Cold-rolled sheet annealing process> In the cold-rolled sheet annealing process, the cold-rolled steel sheet is annealed at a temperature range of 600 to 750°C. If the annealing temperature is below 600°C, the strain applied during the cold-rolling process cannot be sufficiently relieved, which may degrade the mechanical properties of the steel. On the other hand, if the annealing temperature exceeds 750°C, depending on the chemical composition of the steel, it may enter the austenite region, and after cooling, the amount of metal structure other than ferrite may increase by more than 10.0%. Therefore, the annealing temperature is set between 600 and 750°C.

[0076] While there are no specific restrictions on the annealing time, 5 to 15 seconds is generally acceptable. Annealing for 5 seconds or longer allows for sufficient removal of the strain applied during the cold rolling process, minimizing the deterioration of the steel's mechanical properties. Conversely, annealing for 15 seconds or less suppresses grain coarsening and further reduces the deterioration of mechanical properties. After holding the material under the above conditions, it should be water-cooled to room temperature.

[0077] The process of covering the material with the aforementioned corrosion-preventive coating can be carried out using the usual methods. Furthermore, it is not always necessary to apply the corrosion-preventive coating to the entire surface of the steel material; it is sufficient to treat only one side of the steel material that is exposed to the corrosive environment, or in the case of steel pipes, only the outer or inner surface, i.e., at least a portion of the steel material's surface.

[0078] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0079] Steel with the chemical compositions shown in Tables 1 and 2 was melted and formed into 50 kg ingots. These ingots were then heated under the conditions shown in Table 3 and hot-rolled to a thickness of 30 mm. After air cooling, the ingots were cold-rolled and cold-rolled sheet annealed under the conditions shown in Table 3, and then water-cooled to room temperature to obtain steel plates (Test Nos. 1-38).

[0080] [Table 1]

[0081] [Table 2]

[0082] [Table 3]

[0083] <Area ratio of ferrite> The ferrite area fraction was measured using the following method. First, the scale of the steel material was removed, and a sample was taken so that the cross-section perpendicular to the rolling direction of the steel material became the observation surface. The observation surface was wet polished with emery paper, and then buffed with diamond abrasive grains with an average diameter of 1 μm to finish the observation surface to a mirror surface. After that, electropolishing was performed at a voltage of 40 V using A3 electrolyte. Then, on the observation surface, a region of 500 μm in the depth direction and 200 μm in the width direction from the surface of the steel material was observed in 1 μm steps using backscattered electron crystal orientation analysis (SEM-EBSD). The observation magnification was set to 3000x. The BCC phase was identified from the measurement results, and its area fraction was determined, which was then used as the area fraction of the ferrite phase.

[0084] <(111) Area ratio of surface> (111) The area ratio of the surface was measured by the following method. First, the scale of the steel material was removed, and a sample was taken so that the cross section perpendicular to the rolling direction of the steel material would be the observation surface. The region from the surface of the steel material up to 500 μm was made the observation surface, and electropolishing was performed using A3 electrolyte at a voltage of 40 V. On the observation surface, the crystal orientation of the region with a depth of 500 μm and a width of 500 μm from the surface of the steel material was observed at intervals of 200 nm using the EBSD method, and the crystal orientation information of this region was obtained.

[0085] Based on the obtained crystal orientation information, an IPF-map was created that can represent crystal orientations by color. Using the image processing software Photoshop (Adobe), the blue in the obtained IPF-map was treated as the (111) plane and processed into three colors: red, blue, and green, and the area ratio of the (111) plane was determined.

[0086] Furthermore, a tolerance of azimuth was set to 10°, and data with a reliability index (CI) greater than 0.1 were selected. In addition, the IPF-map was created using analysis software (OIM Analysis Ver. 7.3.1: manufactured by TSL Solutions Co., Ltd.).

[0087] <Corrosion resistance> Test specimens measuring 3 mm in thickness, 10 mm in width, and 60 mm in length were taken from each steel plate, from the surface in the thickness direction, and finished with wet #600 polishing to serve as test specimens for corrosion resistance evaluation. These test specimens were immersed for 24 hours in a solution simulating the environment to which the painted defect area would be exposed. The solution was a hydrochloric acid aqueous solution adjusted to pH 1.0 at 30°C. The corrosion rate was then calculated from the weight change of the test specimen before and after immersion. Specifically, the corrosion rate was calculated by substituting the weight change Δm of the test specimen before and after immersion, the surface area S of the test specimen before immersion, the density ρ of the test specimen, and the test time t into the following equation (ii). Δm / (S×ρ×t) ···(ii)

[0088] In Table 4, if the corrosion rate was 15 mm / y or less, the material was judged to have excellent corrosion resistance and was marked with a "○" for corrosion resistance effectiveness. On the other hand, if the corrosion rate was greater than 15 mm / y, the material was judged to have poor corrosion resistance and was marked with a "×" for corrosion resistance effectiveness effectiveness.

[0089] [Table 4]

[0090] As shown in Table 4, tests No. 1 to 30, which satisfied all the provisions of the present invention, showed excellent corrosion resistance. In contrast, comparative examples No. 31 to 38 showed deteriorated corrosion resistance. [Industrial applicability]

[0091] According to the steel material of the present invention, a steel material with excellent corrosion resistance can be obtained.

Claims

1. The chemical composition of the steel material, in mass percent, C: 0.001-0.20%, Si: 1.00% or less, Mn: 0.01 to 2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.06% or less, The remainder consists of Fe and impurities. In a cross-section perpendicular to the rolling direction of the steel material, The metal structure contains 90.0% or more ferrite by area ratio. In the aforementioned cross-section, when observing the region from the surface of the steel material to a depth of 500 μm, the area ratio of ferrite grains having a (111) crystal orientation is 20.0% or more. Steel material.

2. The chemical composition of the steel material, in mass percent, C: 0.001-0.20%, Si: 1.00% or less, Mn: 0.01 to 2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.06% or less, Furthermore, it contains one or more selected from the following groups A, B, and C: The remainder consists of Fe and impurities. In a cross-section perpendicular to the rolling direction of the steel material, The metal structure contains 90.0% or more ferrite by area ratio. In the aforementioned cross-section, when observing the region from the surface of the steel material to a depth of 500 μm, the area ratio of ferrite grains having a (111) crystal orientation is 20.0% or more. Steel material. [Group A] One or more elements selected from the group consisting of Cr: 1.00% or less, Cu: 1.00% or less, Ni: 2.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Sn: 0.20% or less, W: 1.00% or less, Sb: 0.20% or less, In: 0.20%, As: 0.10% or less, Bi: 0.10% or less, Te: 0.10% or less, Pb: 0.10% or less, Zn: 0.10% or less, Ga: 0.10% or less, Ge: 0.10% or less, and Hf: 0.10% or less. [Group B] One or more selected from the group consisting of Nb: 0.050% or less, V: 0.050% or less, Ti: 0.050% or less, B: 0.005% or less, Zr: 0.100% or less, and Ta: 0.10% or less. [Group C] One or more substances selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less.

3. The steel material according to claim 2, wherein the chemical composition contains one or more elements selected from group A.

4. The steel material according to claim 2, wherein the chemical composition contains one or more elements selected from group B.

5. The steel material according to claim 2, wherein the chemical composition contains one or more elements selected from the C group.

6. It has a corrosion-resistant coating on its surface. The steel material according to any one of claims 1 to 5.

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