Steel sheet and method for manufacturing the same

Abstract

Disclosed is a steel sheet, as a steel sheet capable of reducing damage to a die at the time of cold press, having a prescribed chemical composition and steel structure, and a plurality of steps having a height difference exceeding 5.0 μm are present on the sheet surface at intervals of 2.0 mm or less.

Description

Technical Field

[0001] This application relates to steel plates and methods for manufacturing them. Background Technology

[0002] In recent years, to improve fuel efficiency, lightweighting of car bodies has been promoted through the use of high-strength steel. Furthermore, to ensure passenger safety, high-strength steel is increasingly being used in car bodies instead of mild steel. Going forward, to further advance lightweighting of car bodies, the strength grade of high-strength steel must be increased even further than before.

[0003] Automotive parts are formed by die stamping, using both cold and hot dies. In cold pressing, the increased strength of the steel sheet leads to higher surface pressure during stamping, reducing die life. While existing technologies have explored improving the processability of steel sheets by softening them (see Patent Documents 1-3 below), there is still room for improvement in reducing die damage and extending die life during cold pressing.

[0004] Patent Document 1 discloses the following method: hot-rolled steel strip containing 0.3-1.3% C, 0.03-0.35% Si, 0.20-1.50% Mn, with the remainder consisting substantially of Fe and unavoidable impurities, is cold-rolled with a reduction rate of 20% or more and 85% or less. Then, it is repeatedly heated to Ac1 point to Ac1 point +50°C at a heating rate of 20-100°C / hour in a bell-shaped intermittent annealing furnace with a gas atmosphere consisting of 75% or more hydrogen and the remainder consisting substantially of nitrogen and unavoidable impurities. After maintaining the heating for 8 hours or less, it is cooled to below Ar1 point at a cooling rate of 50°C / hour or less. This method produces high-carbon cold-rolled steel strip that is softened and has excellent workability by preventing sintering damage at a low cost.

[0005] Patent document 2 discloses a steel plate for processing with excellent coating vividness, characterized in that the surface of the steel plate is formed into a rough surface with concave and convex patterns, such that the wavelength λ of the concave and convex patterns in the rough surface is less than 500 μm and the average roughness Ra of the center line is in the range of 1 to 5 μm.

[0006] Patent Document 3 discloses a steel plate and a manufacturing method thereof. The steel plate has a specified chemical composition, and the metal structure contains, in terms of area ratio, 40.0% or more and less than 60.0% polygonal ferrite, 30.0% or more bainitic ferrite, 10.0% or more and less than 25.0% retained austenite, and less than 15.0% martensite. The proportion of retained austenite with an aspect ratio of 2.0 or less, a major axis length of 1.0 μm or less, and a minor axis length of 1.0 μm or less is 80.0% or more. The proportion of bainitic ferrite with an aspect ratio of 1.7 or less and an average crystal orientation difference of 0.5° or more and less than 3.0° in the region surrounded by grain boundaries with a crystal orientation difference of 15° or more is 80.0% or more. The connectivity D value of the martensite, the bainitic ferrite, and the retained austenite is 0.70 or less.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 10-204540

[0010] Patent Document 2: Japanese Patent Application Publication No. 4-253503

[0011] Patent Document 3: Japanese Patent No. 6791838 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] In view of the above-mentioned actual situation, this application discloses a steel plate that can reduce mold damage during cold pressing and improve mold life, and a method for manufacturing the same.

[0014] Methods for solving problems

[0015] The inventors of this invention carefully studied methods to solve the above-mentioned problems and confirmed that by increasing the surface roughness of the steel sheet compared to existing materials, oil can be carried into the surface of the steel sheet during cold pressing, thereby improving lubricity and reducing die damage during cold pressing under high surface pressure. Therefore, by increasing the surface roughness of the steel sheet, the life of the stamping die can be improved.

[0016] Furthermore, the inventors of this invention have discovered that the aforementioned steel sheet can be manufactured by a consistent manufacturing method characterized by improving the surface roughness of the hot-rolled sheet under hot-rolling conditions and performing an annealing process without completely smoothing the roughness.

[0017] Furthermore, the inventors of this invention have accumulated various studies and discovered that steel plates with surface irregularities such as those described above, which reduce damage to stamping dies and improve die life, are difficult to manufacture simply by working on hot rolling conditions, annealing conditions, etc., and can only be manufactured by optimizing so-called integrated processes such as hot rolling / annealing.

[0018] The main points of this invention are as follows.

[0019] (1) A steel plate having the following chemical composition:

[0020] By mass%, it contains

[0021] C: 0.15–0.35%

[0022] Si: 0.01~2.00%

[0023] Mn: 0.10~4.00%

[0024] P: below 0.0200%

[0025] S: Below 0.0200%

[0026] Al: 0.001~1.000%

[0027] N: below 0.0200%

[0028] Ti: 0~0.500%

[0029] Co: 0~0.500%

[0030] Ni: 0~0.500%

[0031] Mo: 0–0.500%

[0032] Cr: 0–2.000%

[0033] O: 0~0.0100%

[0034] B: 0~0.0100%

[0035] Nb: 0~0.500%

[0036] V: 0~0.500%

[0037] Cu: 0~0.500%

[0038] W: 0~0.1000%

[0039] Ta: 0~0.1000%

[0040] Sn: 0~0.0500%

[0041] Sb: 0~0.0500%

[0042] As: 0~0.0500%

[0043] Mg: 0~0.0500%

[0044] Ca: 0~0.0500%

[0045] Y: 0~0.0500%

[0046] Zr: 0~0.0500%

[0047] La: 0~0.0500%, and

[0048] Ce: 0~0.0500%,

[0049] The remaining part consists of Fe and impurities.

[0050] The steel microstructure comprises a total of 90.0% or more martensite and tempered martensite by area ratio, a total of 0% or more and 10.0% or less ferrite, pearlite and bainite by area ratio, and a total of 0% or more and 5.0% or less retained austenite.

[0051] On the surface of the plate, there are multiple steps with a height difference of more than 5.0 μm at intervals of less than 2.0 mm.

[0052] (2) The steel plate according to (1) above has the following chemical composition:

[0053] It contains, by mass%, the following:

[0054] Ti: 0.001~0.500%

[0055] Co: 0.001~0.500%

[0056] Ni: 0.001~0.500%

[0057] Mo: 0.001~0.500%

[0058] Cr: 0.001~2.000%

[0059] O: 0.0001~0.0100%

[0060] B: 0.0001~0.0100%

[0061] Nb: 0.001~0.500%

[0062] V: 0.001~0.500%

[0063] Cu: 0.001~0.500%

[0064] W: 0.0001~0.1000%

[0065] Ta: 0.0001~0.1000%

[0066] Sn: 0.0001~0.0500%

[0067] Sb: 0.0001~0.0500%

[0068] As: 0.0001~0.0500%

[0069] Mg: 0.0001~0.0500%

[0070] Ca: 0.0001~0.0500%

[0071] Y: 0.0001~0.0500%

[0072] Zr: 0.0001~0.0500%

[0073] La: 0.0001~0.0500%, and

[0074] Ce: 0.0001 to 0.0500% of one or more of the following.

[0075] (3) A method for manufacturing a steel plate, comprising:

[0076] A hot-rolled plate is obtained by hot rolling a steel billet having the chemical composition described in (1) or (2) above;

[0077] Roll up the above-mentioned hot-rolled sheet;

[0078] Pickling of the above-mentioned hot-rolled plates; and

[0079] The hot-rolled plates described above are either annealed without cold rolling, or annealed after cold rolling.

[0080] The aforementioned hot rolling is performed in the stand preceding the final stand of the finishing mill, where lubricant is supplied between the rolling rolls and the plate while rolling at a reduction rate exceeding 30% and below 70%.

[0081] The temperature during the winding of the above-mentioned hot-rolled sheet is below 700℃.

[0082] In the case of the above-mentioned cold rolling, the reduction rate in the cold rolling is 0.1% to 20%.

[0083] Invention Effects

[0084] According to the steel sheet disclosed herein, mold damage during cold pressing can be reduced, thereby increasing mold life. In other words, the steel sheet disclosed herein is suitable as a steel sheet for cold pressing. Attached Figure Description

[0085] Figure 1 The diagram schematically illustrates the shape of the steps on the surface of the steel plate.

[0086] Figure 2 This is a simplified diagram used to illustrate the difference between "maximum height roughness Rz" and the "step" referred to in this application.

[0087] Figure 3 This is a simplified diagram used to illustrate the measurement conditions for sliding friction resistance. Detailed Implementation

[0088] Hereinafter, embodiments of the present invention will be described. However, these descriptions are merely illustrative of embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0089] <Steel Plate>

[0090] The steel plate of this embodiment is characterized by having the following chemical composition:

[0091] It contains, by mass%, the following:

[0092] C: 0.15–0.35%

[0093] Si: 0.01~2.00%

[0094] Mn: 0.10~4.00%

[0095] P: below 0.0200%

[0096] S: Below 0.0200%

[0097] Al: 0.001~1.000%

[0098] N: below 0.0200%

[0099] Ti: 0~0.500%

[0100] Co: 0~0.500%

[0101] Ni: 0~0.500%

[0102] Mo: 0–0.500%

[0103] Cr: 0–2.000%

[0104] O: 0~0.0100%

[0105] B: 0~0.0100%

[0106] Nb: 0~0.500%

[0107] V: 0~0.500%

[0108] Cu: 0~0.500%

[0109] W: 0~0.1000%

[0110] Ta: 0~0.1000%

[0111] Sn: 0~0.0500%

[0112] Sb: 0~0.0500%

[0113] As: 0~0.0500%

[0114] Mg: 0~0.0500%

[0115] Ca: 0~0.0500%

[0116] Y: 0~0.0500%

[0117] Zr: 0~0.0500%

[0118] La: 0~0.0500%, and

[0119] Ce: 0~0.0500%,

[0120] The remaining part consists of Fe and impurities.

[0121] The steel microstructure comprises a total of 90.0% or more martensite and tempered martensite by area ratio, a total of 0% or more and 10.0% or less ferrite, pearlite and bainite by area ratio, and a total of 0% or more and 5.0% or less retained austenite.

[0122] On the surface of the plate, there are multiple steps with a height difference of more than 5.0 μm at intervals of less than 2.0 mm.

[0123] First, the reasons for defining the chemical composition of the steel plate involved in the embodiments of the present invention will be explained. Here, "%" of the composition refers to mass percentage. Furthermore, in this specification, "~" indicating a numerical range is used to mean the lower limit and upper limit of the value described before and after it unless otherwise specified.

[0124] (C: 0.15-0.35%)

[0125] Carbon (C) is an element that increases tensile strength at a low cost. It is crucial for controlling the strength of steel by inhibiting the phase transformation from austenite to ferrite, bainite, and pearlite during continuous annealing. This effect is easily achieved when the C content is 0.05% or higher, and is particularly pronounced when the C content is 0.15% or higher. The C content can also be 0.20% or higher. On the other hand, excessive C content deteriorates elongation and porosity, makes it difficult to achieve the desired surface texture during hot rolling, and sometimes promotes die damage during cold pressing of the steel sheet. These problems are easily avoided when the C content is 0.35% or lower. The C content can also be 0.30% or lower.

[0126] (Si: 0.01~2.00%)

[0127] Si acts as a deoxidizer, suppressing the precipitation of carbides during the cooling process in cold rolling annealing. This effect is easily achieved when the Si content is 0.01% or higher. The Si content can also be 0.10% or higher. On the other hand, excessive Si content leads to increased steel strength and decreased workability, resulting in coarse oxide dispersions on the surface of the hot-rolled sheet. This makes it difficult to achieve the desired surface texture after cold rolling annealing, and sometimes promotes die damage during cold pressing of the steel sheet. This problem is easily avoided when the Si content is 2.00% or lower. The Si content can also be 1.60% or lower.

[0128] (Mn: 0.10~4.00%)

[0129] Mn is a factor affecting the ferrite phase transformation of steel and is an effective element for increasing strength. This effect is easily achieved when the Mn content is 0.10% or higher. The Mn content can also be 0.60% or higher. On the other hand, excessive Mn content leads to increased steel strength but reduced workability, resulting in coarse oxide dispersions on the surface of hot-rolled sheets. This makes it difficult to achieve the desired surface texture after cold rolling and annealing, and sometimes promotes die damage during cold pressing. This problem is easily avoided when the Mn content is 4.00% or lower. The Mn content can also be 3.00% or lower.

[0130] (P: below 0.0200%)

[0131] Phosphorus (P) is an element that promotes the concentration of manganese (Mn) in the unsolidified portion during the solidification process of molten steel. It reduces the Mn concentration in the negative segregation zone and promotes an increase in the area fraction of ferrite; a lower content is preferred. However, excessive P content leads to increased steel strength but also causes brittle fracture, and sometimes deteriorates formability such as elongation and porosity. The P content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0200% or less, or 0.0180% or less.

[0132] (S: below 0.0200%)

[0133] Sulfur (S) is an element that forms non-metallic inclusions such as MnS in steel, leading to reduced ductility of steel components; a lower content is preferred. However, excessive S content deteriorates formability, such as elongation and porosity, and makes it difficult to achieve the desired surface texture on cold-rolled and annealed steel sheets. Therefore, it can sometimes promote die damage during cold pressing of the steel sheet. The S content can be 0%, 0.0001% or more, 0.0005% or more, or 0.0200% or less, or 0.0180% or less.

[0134] (Al: 0.001~1.000%)

[0135] Al acts as a deoxidizer in steel, stabilizing ferrite, and is added as needed. This effect is easily achieved when the Al content is 0.001% or higher. The Al content can also be 0.010% or higher. On the other hand, excessive Al content can sometimes excessively promote ferrite and bainite phase transformations during annealing, reducing the strength of the steel sheet. Furthermore, excessive Al content raises concerns about the formation of large amounts of coarse Al oxides on the steel sheet surface during hot rolling, making it difficult to achieve the desired surface texture. This problem is easily avoided when the Al content is 1.000% or lower. The Al content can also be 0.800% or lower.

[0136] (N: below 0.0200%)

[0137] Nitrogen (N) is an element that forms large nitrides in steel sheets, reducing their machinability. Additionally, N contributes to porosity during welding. Furthermore, excessive N combines with Al and Ti to form large amounts of AlN or TiN. These nitrides inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. Sometimes, it can even promote die damage during cold pressing. The N content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0200% or less, or 0.0160% or less.

[0138] The basic chemical composition of the steel plate in this embodiment is as described above. Furthermore, the steel plate in this embodiment may, as needed, contain at least one of the following selected elements. Since these elements may also be omitted, the lower limit is 0%.

[0139] (Ti: 0~0.500%)

[0140] Ti is a strengthening element. It contributes to increased steel sheet strength through precipitation strengthening, fine-grain strengthening due to grain growth inhibition, and dislocation strengthening through recrystallization inhibition. On the other hand, excessive Ti content leads to the precipitation of coarse carbides, which inhibit the contact between the steel sheet surface and the rolls during hot rolling. This makes it difficult to achieve the desired surface texture in cold-rolled and annealed steel sheets, and sometimes promotes die damage during cold pressing. Ti content can be 0%, 0.001% or more, 0.005% or more, or 0.500% or less, or 0.400% or less.

[0141] (Co: 0-0.500%)

[0142] Co is an effective element for controlling the morphology of carbides and increasing strength; it is added as needed to control strength. On the other hand, excessive Co content leads to the precipitation of a large number of fine Co carbides, which inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture on the cold-rolled and annealed steel sheet. Sometimes, it can also promote die damage during cold pressing of the steel sheet. The Co content can be 0%, 0.001% or more, 0.500% or less, or 0.400% or less.

[0143] (Ni: 0~0.500%)

[0144] Ni is a strengthening element and is effective in improving hardenability. Furthermore, it can be added to improve the wettability of the steel sheet and coating, and to promote alloying reactions. On the other hand, excessive Ni content affects the peeling properties of oxide scale during hot rolling, promotes scratches on the steel sheet surface, makes it difficult to achieve the desired surface texture after cold rolling and annealing, and sometimes promotes die damage during cold pressing of the steel sheet. The Ni content can be 0%, 0.001% or more, or 0.500% or less, or 0.400% or less.

[0145] (Mo: 0~0.500%)

[0146] Mo is an effective element for increasing the strength of steel sheets. Furthermore, Mo has the effect of suppressing the ferrite phase transformation that occurs during heat treatment in continuous annealing or continuous hot-dip galvanizing equipment. On the other hand, if Mo is excessive, a large amount of fine Mo carbides precipitate. These carbides inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture on the steel sheet after cold rolling and annealing. Sometimes, they even promote die damage during cold pressing of the steel sheet. The Mo content can be 0%, 0.001% or more, or 0.500% or less, or 0.400% or less.

[0147] (Cr: 0~2.000%)

[0148] Cr, like Mn, is an element that inhibits pearlite phase transformation and is effective in increasing the strength of steel; it is added as needed. On the other hand, excessive Cr content can sometimes promote the formation of retained austenite, leading to a decrease in porosity due to the presence of excess retained austenite. The Cr content can be 0%, 0.001% or more, or 2.000% or less, or 1.500% or less.

[0149] (O: 0~0.0100%)

[0150] Oxide (O) forms oxides, which degrade processability, thus its content needs to be controlled. In particular, oxides often exist as inclusions. If coarse granular oxides are present on the steel sheet surface, they can cause surface cracks and the formation of fine iron powder during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. Furthermore, if present on the punching or cutting face, they can form cut-like scratches and large indentations, sometimes leading to reduced porosity. O content can be 0.0100% or less, or 0.0080% or less. While O content can be 0%, controlling it to less than 0.0001% raises concerns about increased refining time and manufacturing costs. To prevent increased manufacturing costs, O content can be 0.0001% or more, or 0.0010% or more.

[0151] (B: 0~0.0100%)

[0152] Boron (B) is an element that inhibits the formation of ferrite and pearlite during the cooling process of austenite, while promoting the formation of low-temperature phase transformation structures such as bainite or martensite. Additionally, B is beneficial for increasing the strength of steel and is added as needed. On the other hand, excessive B content leads to the formation of coarse B oxides in the steel. These oxides inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. Sometimes, they also promote die damage during cold pressing. Furthermore, these oxides become initiation points for porosity formation, easily causing damage and sometimes leading to reduced porosity. The B content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0100% or less, or 0.0080% or less.

[0153] (Nb: 0~0.500%)

[0154] Nitrogen (Nb) is an effective element for controlling the morphology of carbides, and its addition refines the microstructure, thus also contributing to improved toughness. However, excessive Nb content leads to the precipitation of large amounts of fine and hard Nb carbides. These carbides inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. Sometimes, they even promote die damage during cold pressing. Furthermore, these carbides become the starting point for damage, sometimes resulting in reduced porosity. The Nb content can be 0%, 0.001% or more, or 0.500% or less, or 0.400% or less.

[0155] (V: 0~0.500%)

[0156] V is a strengthening element. It contributes to increased steel sheet strength through precipitation strengthening, fine-grain strengthening caused by inhibited ferrite grain growth, and dislocation strengthening via recrystallization inhibition. On the other hand, excessive V content leads to increased precipitation of carbonitriding compounds. These compounds inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. Sometimes, they even promote die damage during cold pressing. Furthermore, these carbides become the starting point for damage, sometimes resulting in reduced porosity. V content can be 0%, 0.001% or more, or 0.500% or less, or 0.400% or less.

[0157] (Cu: 0~0.500%)

[0158] Cu is an effective element for increasing the strength of steel sheets. On the other hand, if the Cu content is excessive, the steel becomes brittle during hot rolling and cannot be hot rolled. Moreover, the Cu layer concentrated on the surface of the steel sheet inhibits the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture of the steel sheet after cold rolling and annealing, and sometimes promoting die damage during cold pressing of the steel sheet. The Cu content can be 0%, 0.001% or more, or 0.500% or less, or 0.400% or less.

[0159] (W: 0~0.1000%)

[0160] Besides its effectiveness in increasing the strength of steel sheets, W-containing precipitates and crystals also act as hydrogen trapping sites. On the other hand, excessive W content leads to the formation of coarse carbides, which inhibit the contact between the steel sheet surface and the rolls during hot rolling. This makes it difficult to achieve the desired surface texture in cold-rolled and annealed steel sheets, and sometimes promotes die damage during cold pressing. Furthermore, damage is easily initiated from coarse carbides, sometimes resulting in reduced porosity. The W content can be 0%, 0.0001% or more, 0.0010% or more, or 0.1000% or less, or 0.0800% or less.

[0161] (Ta: 0~0.1000%)

[0162] Like Nb, V, and W, Ta is an effective element for controlling the morphology of carbides and increasing their strength, and should be added as needed. However, excessive Ta content leads to the precipitation of numerous fine Ta carbides, which inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. This can sometimes promote die damage during cold pressing. Furthermore, damage can easily occur starting from these carbides, sometimes resulting in reduced porosity. The Ta content can be 0%, 0.0001% or more, 0.0010% or more, or 0.1000% or less, or 0.0800% or less.

[0163] (Sn: 0~0.0500%)

[0164] Sn is an element present in steel when scrap iron is used as a raw material, and the lower the better. Excessive Sn content leads to surface cracks and the formation of fine iron powder during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. It can also sometimes promote die damage during cold pressing. Furthermore, it can reduce porosity due to embrittlement of the steel. Sn content can be below 0.0500% or 0.0400%. While Sn content can be 0%, controlling it to less than 0.0001% raises concerns about increased refining time and manufacturing costs. To prevent increased manufacturing costs, Sn content can be above 0.0001% or 0.0010%.

[0165] (Sb: 0~0.0500%)

[0166] Like Sn, Sb is an element present when scrap iron is used as the raw material for steelmaking. Sb strongly segregates at grain boundaries, leading to grain boundary embrittlement and reduced ductility; therefore, a lower content is better. Furthermore, excessive Sb content can cause surface cracks and the formation of fine iron powder during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. Sometimes, it can also promote die damage during cold pressing. Additionally, embrittlement of the steel sheet can sometimes reduce its porosity. The Sb content can be 0.0500% or less, or 0.0400% or less. While an Sb content of 0% is acceptable, controlling the Sn content to less than 0.0001% raises concerns about increased refining time and manufacturing costs. To prevent increased manufacturing costs, the Sb content can be 0.0001% or more, or 0.0010% or more.

[0167] (As: 0~0.0500%)

[0168] As, like Sn and Sb, is an element that is present when scrap iron is used as a steel raw material and strongly segregates at grain boundaries; the lower the content, the better. Furthermore, excessive As content leads to surface cracks and the formation of fine iron powder during hot rolling, making it difficult to achieve the desired surface texture after cold rolling and annealing. Sometimes, it can also promote die damage during cold pressing. Additionally, it can reduce the porosity due to embrittlement of the steel. The As content can be 0.0500% or less, or 0.0400% or less. While the As content can be 0%, controlling it to less than 0.0001% raises concerns about increased refining time and manufacturing costs. To prevent increased manufacturing costs, the As content can be 0.0001% or more, or 0.0010% or more.

[0169] (Mg: 0~0.0500%)

[0170] Mg is an element whose sulfide morphology can be controlled by adding trace amounts, and it is added as needed. On the other hand, if Mg is present in excess, coarse inclusions are formed, which inhibit the contact between the steel sheet surface and the rolls during hot rolling. This makes it difficult to obtain the desired surface texture of the steel sheet after cold rolling and annealing, and sometimes promotes die damage during cold pressing of the steel sheet. In addition, it can sometimes lead to a decrease in porosity due to embrittlement of the steel sheet. The Mg content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0500% or less, or 0.0400% or less.

[0171] (Ca: 0~0.0500%)

[0172] Besides its useful role as a deoxidizing element, Ca also plays a role in controlling the morphology of sulfides. On the other hand, excessive Ca content can lead to surface cracks and the formation of fine iron powder in hot rolling, making it difficult to achieve the desired surface texture in cold-rolled and annealed steel sheets. Sometimes, it can even promote die damage during cold pressing. The Ca content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0500% or less, or 0.0400% or less.

[0173] (Y: 0~0.0500%)

[0174] Like Mg and Ca, yttrium (Y) is an element whose sulfide morphology can be controlled by adding it in trace amounts, and it is added as needed. On the other hand, if y is present in excess, coarse yttrium oxides are formed. These oxides inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture on the steel sheet after cold rolling and annealing. Sometimes, they can even promote die damage during cold pressing of the steel sheet. Furthermore, these oxides can become the starting point for damage, thus sometimes leading to a decrease in porosity. The yttrium content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0500% or less, or 0.0400% or less.

[0175] (Zr: 0~0.0500%)

[0176] Like Mg, Ca, and Y, Zr is an element whose sulfide morphology can be controlled by adding it in trace amounts, and it should be added as needed. On the other hand, if Zr is present in excess, coarse Zr oxides are formed. These Zr oxides inhibit the contact between the steel sheet surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture on the steel sheet after cold rolling and annealing. Sometimes, they can even promote die damage during cold pressing of the steel sheet. In addition, these oxides become the starting point for damage, thus sometimes leading to a decrease in porosity. The Zr content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0500% or less, or 0.0400% or less.

[0177] (La: 0~0.0500%)

[0178] La (La) is an element that effectively controls the morphology of sulfides through trace addition, and should be added as needed. On the other hand, excessive La content leads to the formation of La oxides, which inhibit the contact between the steel sheet surface and the rolls during hot rolling, thus making it difficult to obtain the desired surface texture on the steel sheet after cold rolling and annealing, and sometimes promoting die damage during cold pressing of the steel sheet. In addition, these oxides become the starting point for damage, thus sometimes leading to a decrease in porosity. The La content can be 0%, or more than 0.0001%, or more than 0.0010%, or less than 0.0500% or less than 0.0400%.

[0179] (Ce: 0~0.0500%)

[0180] Like La, Ce is an element that can be added in trace amounts to form sulfides, and the amount added is as needed. On the other hand, if Ce is present in excess, Ce oxides are formed. These Ce oxides inhibit the contact between the steel sheet surface and the rolls during hot rolling, thus making it difficult to obtain the desired surface texture on the steel sheet after cold rolling and annealing. Sometimes, they can even promote die damage during cold pressing of the steel sheet. In addition, these oxides become the starting point for damage, thus sometimes leading to a decrease in porosity. The Ce content can be 0%, 0.0001% or more, 0.0010% or more, or 0.0500% or less, or 0.0400% or less.

[0181] Furthermore, in the steel plate of this embodiment, the remaining components described above are Fe and impurities. Impurities refer to components that are mixed in during the industrial manufacturing of the steel plate involved in this embodiment, primarily from raw materials such as ore and scrap iron, due to various factors in the manufacturing process.

[0182] Next, the characteristics of the structure and properties of the steel plate involved in the embodiments of the present invention will be described.

[0183] (The total area ratio of martensite and tempered martensite is 90.0% or more.)

[0184] The combined area fraction of martensite and tempered martensite is an effective microstructure for improving the strength of the steel sheet. However, if the area fraction of the softer microstructure than martensite and tempered martensite increases, the areas with large hardness differences between the microstructures increase, thus deteriorating porosity. The area fraction of martensite and tempered martensite can be 90.0% or more, preferably 95.0% or more. There is no specific upper limit, but it can also be 100%.

[0185] (The total area ratio of ferrite, pearlite, and bainite: 0% or more and 10.0% or less)

[0186] Ferrite, pearlite, and bainite are microstructures softer than martensite and tempered martensite. These microstructures are effective in improving the strength-ductility balance of steel sheets, but because they are softer than martensite and tempered martensite, they have a large hardness difference, making them prone to creating porosity at these interfaces during deformation, thus reducing porosity. Therefore, the lower the total area fraction of ferrite, pearlite, and bainite, the better. The total area fraction of ferrite, pearlite, and bainite can be 0%, 1.0% or more, 10.0% or less, 5.0% or less, or 3.0% or less. Furthermore, although productivity may decrease slightly, by precisely controlling consistent manufacturing conditions, it is possible to achieve a total area fraction of ferrite, pearlite, and bainite of 0%.

[0187] (Area percentage of retained austenite: 0% or more and 5.0% or less)

[0188] The area ratio of retained austenite is an effective microstructure for balancing the strength and ductility of steel sheets. On the other hand, if the area ratio of retained austenite is too large, the proportion of chemically unstable austenite increases, which can lead to processing-induced phase transformations during deformation, sometimes resulting in reduced porosity. The area ratio of retained austenite can be 0%, 1.0% or more, 5.0% or less, or 3.0% or less.

[0189] (Surface unevenness)

[0190] The distribution spacing of steps with a height difference exceeding 5.0 μm on the steel sheet surface helps to increase the amount of oil carried in during stamping, which is important for suppressing die damage and improving die life during stamping. A shorter distribution spacing is better, but with a spacing less than 0.01 mm, the steel sheet surface sometimes becomes serrated. Therefore, the spacing can be 0.01 mm or more, or 0.05 mm or more. On the other hand, if it exceeds 2.0 mm, it is sometimes difficult to suppress the aforementioned die damage, making it difficult to improve die life. Therefore, the spacing can be 2.0 mm or less, 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, 1.0 mm or less, 0.7 mm or less, or 0.4 mm or less. Furthermore, in the steel sheet of this embodiment, it is necessary for multiple steps with a height difference exceeding 5.0 μm to be distributed at the aforementioned spacing on the steel sheet surface. This height difference can be 7.0 μm or more, or 10.0 μm or more. There is no particular upper limit to the height difference of the steps; for example, it can be 20.0 μm or less, 15.0 μm or less, or 10.0 μm or less. Regarding the steel plate involved in this embodiment, multiple steps with a height difference exceeding 5.0 μm can exist at intervals of 2.0 mm or less on the surface of the steel plate, covering 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more of the surface area.

[0191] Figure 1 The image shows an example of a "step with a height difference of more than 5.0 μm". Figure 1 The morphology of the step is shown when observing a cross-section along the thickness direction of the steel plate. For example... Figure 1As shown, unevenness can be repeatedly formed on the surface of the steel plate along the rolling direction, with the height difference of each step defined by the unevenness exceeding 5.0 μm, and multiple steps included within a range of 2.0 mm, i.e., the interval between steps is less than 2.0 mm. In this invention, at least a portion of the multiple steps may also have a so-called negative angle portion (undercut portion). Furthermore, in this invention, the individual heights of the multiple steps may differ from each other; for example, the heights may differ irregularly (randomly). Additionally, the shapes of the multiple steps may differ from each other. Furthermore, the interval between the multiple steps may not be constant, but rather irregular (random). Such a step shape can be formed by the method described later.

[0192] Furthermore, the "step with a height difference exceeding 5.0 μm" mentioned in this application is a concept different from general surface roughness such as maximum height roughness Rz and arithmetic mean roughness Ra. For example, as Figure 2 As shown in (A), "maximum height roughness Rz" refers to the distance between the most convex and the most concave parts of the surface roughness (the maximum height difference). Furthermore, the distribution (interval) of the surface roughness cannot be determined based on "maximum height roughness Rz". Similarly, "arithmetic mean roughness Ra" is merely the average surface roughness; its maximum value is unknown. Furthermore, the distribution (interval) of the surface roughness cannot be determined based on "arithmetic mean roughness Ra". In contrast, the "step with a height difference exceeding 5.0 μm" mentioned in this application... Figure 2 As shown in (B), it means that the height difference of "a step" exceeds 5.0 μm, and the step needs to exist in multiple places with an interval of less than 2.0 mm.

[0193] (tensile strength)

[0194] To improve the lightweighting of structures using steel as a raw material and the resistance of the structure during plastic deformation, steel materials with high work hardening capacity and maximum strength are preferred. On the other hand, if the tensile strength is too high, failure may easily occur during plastic deformation due to low energy, and sometimes the formability may be reduced. The tensile strength of the steel plate is not particularly limited, but it can be above 1300 MPa, above 1400 MPa, or below 2100 MPa, below 2000 MPa, or below 1900 MPa.

[0195] (Total elongation)

[0196] When cold-forming steel sheets into structures, elongation is required to form complex shapes. If the total elongation is too low, the material may crack during cold forming. On the other hand, while higher total elongation is generally better, excessively increasing it requires a large amount of retained austenite in the steel microstructure, which can sometimes reduce porosity. There is no particular limitation on the total elongation of the steel sheet, but it can be 5% or more, 8% or more, or 18% or less, or 15% or less.

[0197] (Porosity)

[0198] When cold-forming steel sheets into structures, both elongation and porosity are required to achieve complex shapes. Insufficient porosity can sometimes cause the material to crack during cold forming. There is no particular limitation on the porosity of the steel sheet, but it can be above 20% or 25%, or below 90% or 80%.

[0199] (Sliding friction resistance)

[0200] To suppress die damage during cold pressing of steel sheets, the sliding friction resistance of the steel sheet is preferably below 1.0. If the sliding friction resistance is too high, the friction during stamping will increase, potentially shortening the die life. The sliding friction resistance can be below 0.8 or below 0.6. There is no particular limitation on the lower limit of the sliding friction resistance.

[0201] (plate thickness)

[0202] Plate thickness is a factor affecting the stiffness of the formed steel component; the greater the plate thickness, the higher the stiffness of the component. If the plate thickness is too small, it can sometimes lead to a decrease in stiffness and reduced stamping formability due to unavoidable non-ferrous inclusions within the steel plate. On the other hand, if the plate thickness is too large, the stamping load increases, leading to die wear and reduced productivity. The plate thickness is not particularly limited and can be 0.2 mm or more, or 6.0 mm or less. Furthermore, the "steel plate" mentioned in this application can also be a single-layer steel plate. Here, "single-layer steel plate" means not a multi-layer steel plate, but rather a steel plate whose cross-section does not show the interface between the parent steel plates in the thickness direction. For example, a steel plate formed from a single slab. The "plate thickness" of the aforementioned steel plate can also be the thickness of a single-layer steel plate. Additionally, a surface treatment layer such as a coating can be formed on the surface of a single-layer steel plate. That is, the "steel plate" mentioned in this application can also have both a single-layer steel plate and a surface treatment layer.

[0203] Next, the methods for observing and measuring the organizations specified above, as well as the methods for measuring and evaluating the characteristics specified above, will be described.

[0204] (Method for determining the total area ratio of ferrite, pearlite, and bainite)

[0205] Tissue observation was performed using a scanning electron microscope (SEM). Prior to observation, the sample was polished to a mirror finish using wet abrasive with sandpaper and diamond abrasive grains with an average particle size of 1 μm. The tissue was then etched using a 3% nitric acid alcohol solution. The magnification was set to 3000x, and 10 images were randomly taken at 1 / 4 thickness points from the surface of the steel plate, covering a 30 μm × 40 μm field of view. The tissue ratio was calculated using an integral method. For each obtained tissue image, 100 grid points were identified at 3 μm vertical and 4 μm horizontal intervals. The tissue structure beneath these grid points was determined, and the tissue ratio in the steel was calculated based on the average of the 10 images. Ferrite consisted of massive grains and did not contain iron-based carbides with a length diameter greater than 100 nm. Bainite is an aggregate of lath-shaped grains that does not contain iron-based carbides with a major diameter greater than 20 nm, or contains iron-based carbides with a major diameter greater than 20 nm that belong to a single variant, i.e., a group of iron-based carbides extending in the same direction. Here, a group of iron-based carbides extending in the same direction means that the difference in the extension direction of the iron-based carbides is within 5°. Bainite is counted as a single bainite grain when surrounded by grain boundaries with an orientation difference of 15° or more. Here, the "grain boundaries with an orientation difference of 15° or more" are determined using SEM-EBSD in the following order. Before measurement using SEM-EBSD, the observation surface of the test sample is polished to a mirror finish by grinding. After further removing the strain caused by grinding, similar to the SEM observation described above, a field of view of 30 μm × 40 μm at 1 / 4 of the thickness from the surface of the steel plate is set as the measurement range, and the crystal orientation data of BCC iron is obtained by SEM-EBSD. Measurements using EBSD were performed using an EBSD detector attached to the SEM, with a measurement interval (STEP) set to 0.05 μm. In this invention, software such as "OIMData Collection TM (ver. 7)" manufactured by TSL Solutions, Ltd., was used as the data acquisition software for crystal orientation. In the BCC iron crystal orientation MAP data obtained under these measurement conditions, boundaries with a crystal orientation difference of 15° or more, excluding regions with a reliability value (CI value) less than 0.1, were defined as grain boundaries. Furthermore, bainite can also be described as a mixed structure of bainitic ferrite formed by the body-centered cubic structure of iron and iron-based carbides (Fe3C). Bainitic ferrite is distinct from the ferrite described above. Pearlite is a structure containing rows of precipitated cementite; the area fraction was calculated based on the region captured with bright contrast in a second electron image.

[0206] (Method for determining the area ratio of martensite and tempered martensite)

[0207] Martensite and tempered martensite were observed using scanning electron microscopy and transmission electron microscopy. Those containing Fe-based carbides were identified as tempered martensite, while those containing almost no carbides were identified as martensite. Regarding Fe-based carbides, various crystal structures were reported, but they could also contain any type of Fe-based carbide. Depending on the heat treatment conditions, sometimes multiple Fe-based carbides were present.

[0208] (Method for determining the area ratio of retained austenite)

[0209] The area fraction of retained austenite was determined by X-ray determination as follows. First, a portion from the surface of the steel plate to one-quarter of its thickness was removed by mechanical and chemical polishing. The chemically polished surface was then measured using MoKα rays as characteristic X-rays. Next, based on the integral intensity ratio of the diffraction peaks of the body-centered (bcc) phases (200) and (211) and the face-centered (fcc) phases (200), (220), and (311), the area fraction of retained austenite at the center of the plate thickness was calculated using the following formula.

[0210] Sγ=(I200f+I220f+I311f) / (I200b+I211b)×100

[0211] (Sγ is the area fraction of the retained austenite in the center of the plate thickness, I200f, I220f and I311f represent the intensities of the (200), (220) and (311) diffraction peaks of the fcc phase, respectively, and I200b and I211b represent the intensities of the (200) and (211) diffraction peaks of the bcc phase, respectively.)

[0212] The sample for X-ray diffraction is prepared by mechanically grinding a steel plate to reduce its thickness from the surface to a specified thickness. Then, strain is removed by chemical grinding or electrolytic grinding. Simultaneously, the sample is adjusted according to the above method within a thickness range of 1 / 8 to 3 / 8, ensuring a suitable surface is used for measurement. Of course, by satisfying the aforementioned X-ray intensity limit not only around 1 / 4 of the thickness but also across as much thickness as possible, material anisotropy is further reduced. However, by performing measurements from 1 / 8 to 3 / 8 of the distance from the surface of the steel plate, the overall material properties of the steel plate can be roughly represented. Therefore, 1 / 8 to 3 / 8 of the thickness is defined as the measurement range.

[0213] (Method for measuring the intervals of surface irregularities (steps with a height difference exceeding 5.0 μm))

[0214] The unevenness and distribution interval of the steel plate surface were measured using a field-emission scanning electron microscope (FE-SEM). Before SEM observation, samples with a length exceeding 20 mm in the rolling direction were embedded in resin. The surface parallel to the rolling direction and perpendicular to the thickness direction (TD surface) was polished to a mirror finish by grinding. The SEM magnification was set to 1000x, and a field of view was obtained along the entire 20 mm rolling length, encompassing the steel plate and resin within an observation range exceeding 110 μm in the rolling direction and 70 μm in the thickness direction, to obtain continuous photographs showing the unevenness of the steel plate surface. In this series of photographs, a location on the steel plate surface with a height difference exceeding 5 μm within a 20 μm length in the rolling direction is defined as a "step with a height difference exceeding 5.0 μm on the steel plate surface." The average interval between the tops of this height difference within the 20 mm length in the rolling direction, which is the shooting range of the series of photographs, is set as "the interval of a step with a height difference exceeding 5.0 μm on the steel plate surface." Furthermore, in this application, minute unevenness with a height difference of less than 1.0 μm is not considered a "step."

[0215] Furthermore, even after the steel plate is formed / processed into a certain component, it is possible to obtain a portion of the formed / processed component (e.g., a flat portion) and analyze its surface condition to determine whether the component, in the state of the steel plate before forming / processing, has steps with a height difference exceeding 5.0 μm at intervals of less than 2.0 mm.

[0216] (Methods for determining tensile strength and total elongation)

[0217] The tensile test used to determine tensile strength and total elongation is conducted according to JIS Z 2241, with JIS No. 5 test pieces collected from a direction parallel to the rolling right angle of the steel strip along the length of the test piece.

[0218] (Method for determining porosity)

[0219] Hole expansion performance was evaluated by punching a 10mm diameter circular hole with the flange on the die side using a 60° conical punch, under a clearance of 12.5%. The hole expansion rate λ (%) was used for evaluation. Five hole expansion tests were conducted under each condition, and the average value was taken as the hole expansion rate.

[0220] (Methods for determining sliding friction resistance)

[0221] Sliding friction resistance μ through Figure 3The results were obtained from the plate pull-out test shown. A 10mm wide test piece coated with lubricating oil was clamped in a mold with a pressure of 20MPa. The average value of the sliding friction resistance when the plate was pulled out 100mm at a sliding speed of 100mm / s was set as μ. If the pushing force was set as P and the pulling load was set as F, the sliding friction resistance could be calculated as μ = F / 2P. Furthermore, a lubricating oil with a kinematic viscosity of 10mm was used. 2 / s of ordinary lubricating oil. The coating amount needs to be tested with lubricating oil stored in the unevenness of the steel plate surface, therefore it is set at 3.0 g / m. 2 .

[0222] <Methods for Manufacturing Steel Plates>

[0223] The steel plate manufacturing method according to this embodiment is characterized by using materials within the above-described composition range and performing consistent management of hot rolling, cold rolling, and annealing. Specifically, the steel plate manufacturing method according to this embodiment is characterized by including the following steps: Regarding the steel plate, a steel billet (steel slab) having the same chemical composition as described above is hot-rolled and coiled at a predetermined reduction rate using a lubricant on the preceding mill of the final finishing mill; the resulting hot-rolled steel plate is pickled, cold-rolled, and then annealed. More specifically, the steel plate manufacturing method according to this embodiment is characterized by including:

[0224] Hot-rolled plates are obtained by hot rolling a steel billet having the above-mentioned chemical composition;

[0225] Roll up the above-mentioned hot-rolled sheet;

[0226] Pickling of the above-mentioned hot-rolled plates; and

[0227] The hot-rolled sheet is annealed without cold rolling. Alternatively, it can be annealed after cold rolling.

[0228] The aforementioned hot rolling is performed in the stand preceding the final stand of the finishing mill, where lubricant is supplied between the rolling rolls and the plate while rolling at a reduction rate exceeding 30% and below 70%.

[0229] The temperature during the winding of the above-mentioned hot-rolled sheet is below 700℃.

[0230] In the case of the above-described cold rolling, the reduction rate during cold rolling is 0.1% to 20%. Hereinafter, each process will be described in detail, focusing on the key points of this embodiment.

[0231] (Reduction rate in the stand preceding the final stand of the finishing mill)

[0232] The reduction rate in the stand preceding the final stand of the finishing mill is a factor affecting the surface condition of the steel sheet. Here, a lubricant (e.g., a water-soluble mixture of lubricant) is supplied to the workpiece (plate) before rolling in the stand preceding the final stand. High surface pressure is applied while the lubricant remains on the plate surface during rolling. This intermittently applies partial sliding and contact between the plate and the roll surfaces during rolling, thereby improving the surface roughness of the plate. If the reduction rate is too low, the surface pressure between the plate and the rolls is insufficient during rolling, making it impossible to form the desired surface roughness in the steel sheet. Conversely, if the reduction rate is too high, the surface pressure generated between the plate and the rolls during rolling becomes excessively high, and the frequency of contact between the plate and the rolls is higher than that of sliding, making it difficult to apply the desired surface roughness to the final steel sheet. Based on the above considerations, in this embodiment, the reduction rate in the stand preceding the final stand of the finishing mill during hot rolling is more than 30% and less than 70%, preferably more than 35% and less than 60%. Furthermore, in the final stand of the finishing mill, it is difficult to perform large reductions in order to correct the shape of the plate. The reduction rate in the final stand of the finishing mill can, for example, be less than 20%.

[0233] Furthermore, in the stand preceding the final stand, a reduction rate of 30% or more is applied while lubricant is supplied, thereby forming a step on the plate surface. Then, by controlling the cumulative reduction rate up to the final stand to achieve a light reduction (e.g., a cumulative reduction rate of less than 20%), the desired surface unevenness of the hot-rolled steel plate after finishing can also be formed. From this perspective, a large reduction to improve the surface unevenness of the plate can also be performed in a stand upstream of the final stand. However, on the upstream side of finishing, the plate temperature is high, and the surface shape of the plate is easily altered by the reduction. That is, after a large reduction, the cumulative reduction rate needs to be controlled while taking into account the effect of temperature. From this perspective, on the downstream side of finishing, especially based on a large reduction of 30% or more while lubricant is supplied in the stand preceding the final stand, a light reduction in the final stand makes it easier to adjust the plate shape and form the desired surface unevenness of the steel plate.

[0234] Various types of lubricants can be used as described above. For example, the components of a lubricant may include esters, mineral oils, polymers, fatty acids, sulfur-based additives, and calcium-based additives. The viscosity of the lubricant can also be 250 mm. 2 / s or less. As mentioned above, the lubricant can also be mixed with water for use. There is no particular limitation on the amount of lubricant supplied; for example, 0.1 g / m can be applied to the surface of the steel plate. 2 Above or 1.0g / m 2 Above, 100.0g / m 2 Below or 50.0g / m 2The following lubricants are available. There are no particular limitations on the method of supplying the lubricant; for example, it can also be supplied by spraying the lubricant onto the board surface.

[0235] (Roll-up temperature of the roll material)

[0236] The temperature during hot-rolled sheet winding (winding temperature of hot-rolled coil) is a factor controlling the oxide scale formation state of the hot-rolled sheet and affecting its strength. To maintain the surface unevenness produced by hot rolling, a thin oxide scale on the hot-rolled sheet surface is preferable; therefore, a low winding temperature is preferred. Furthermore, special equipment is required to achieve extremely low winding temperatures. Conversely, if the winding temperature is too high, as mentioned above, the oxide scale formed on the surface of the hot-rolled sheet becomes significantly thicker. Consequently, the raised portions of the unevenness formed on the surface of the hot-rolled sheet during hot rolling are incorporated into the oxide scale. Subsequent pickling removes the oxide scale, making it difficult to form the desired unevenness on the surface of the hot-rolled sheet. Based on these considerations, the winding temperature of the hot-rolled sheet is 700°C or below, or 680°C or below, or 0°C or above, or 20°C or above.

[0237] (Reduction rate in cold rolling)

[0238] The reduction ratio in cold rolling is a crucial factor for controlling the shape and surface roughness of hot-rolled sheets. If the reduction ratio is too low during cold rolling, shape defects in the hot-rolled sheet cannot be corrected, leaving residual bending in the strip, which can sometimes lead to reduced manufacturability in subsequent annealing processes. On the other hand, if the reduction ratio is too high, the protrusions formed on the surface of the hot-rolled steel sheet during rolling are flattened by cold rolling, making it difficult to achieve the desired surface roughness after subsequent annealing. Based on these considerations, the reduction ratio in cold rolling is typically between 0.1% and 20%, preferably between 0.3% and 18.0%.

[0239] Alternatively, hot-rolled sheets can be annealed directly without cold rolling. In this case, it is also easy to obtain a steel sheet with the desired surface texture.

[0240] The preferred embodiment of a method for manufacturing steel sheets with minimal die damage during cold pressing will be described in detail below. The following description illustrates preferred embodiments of heat treatment during hot rolling and annealing, as well as plating treatments, and does not limit the steel sheet manufacturing method described in this embodiment in any way.

[0241] (Finishing temperature of hot rolling)

[0242] The finishing temperature of hot rolling (hot rolling finishing temperature) is a factor that affects the texture control of the original austenite grain size. From the viewpoint that the developed rolling texture of austenite leads to anisotropy of steel properties, the finishing temperature is preferably 650°C or higher. In addition, in order to suppress the texture shift caused by abnormal austenite grain growth, the finishing temperature is preferably, for example, 940°C or lower.

[0243] (Holding temperature during annealing)

[0244] To fully obtain the combined area fraction of martensite and tempered martensite, it is important to control the maximum heating temperature above Ac3 point -20°C regarding the annealing holding temperature. If it is below Ac3 point -20°C, the combined area fraction of martensite and tempered martensite decreases, making it difficult to ensure a tensile strength of 1300 MPa or higher. On the other hand, excessively high-temperature heating leads to increased costs, making it economically undesirable, and can also cause problems such as deterioration of the sheet shape during high-temperature plate running or reduced roll life. Therefore, the upper limit of the maximum heating temperature is preferably 900°C. Furthermore, Ac3 point is calculated using a small piece pre-sampled from the cold-rolled steel sheet, based on the thermal expansion curve when heated to 900°C at 10°C / s.

[0245] (Annealing holding time)

[0246] During annealing, it is preferable to hold the heating temperature described above for at least 5 seconds. This is because if the holding time is too short, the austenitic phase transformation of the base steel sheet may become insufficient, resulting in a significant decrease in strength. Furthermore, insufficient recrystallization of the ferrite structure leads to a larger deviation in hardness, thus deteriorating porosity. From these perspectives, a holding time of 10 seconds or more is more preferable. More preferably, 20 seconds or more is preferable.

[0247] (Cooling rate after annealing)

[0248] Regarding cooling after annealing, it is preferable to cool from 750°C to the cooling stop temperature at an average cooling rate of 10°C / s or more but less than 100°C / s. The reason for setting the lower limit of the average cooling rate to 10°C / s is to suppress the formation of ferrite, pearlite, and bainite during cooling, which would soften the steel sheet. When the average cooling rate is less than 10°C / s, the strength decreases significantly. More preferably, it is 15°C / s or more, further preferably 30°C / s or more, and even more preferably 50°C / s or more. At temperatures above 750°C, it is difficult for ferrite phase transformation to occur significantly, therefore the cooling rate is not limited. At temperatures below 150°C, martensite is sufficiently formed, therefore the cooling rate is not limited. If cooling is performed at a rate faster than 100°C / s, the shape of the steel sheet is prone to deterioration, therefore, a rate of 100°C / s or less is preferred. More preferably, it is 90°C / s or less, and even more preferably 80°C / s or less.

[0249] (Cooling stop temperature after annealing)

[0250] The annealing (cooling stop temperature) of cold-rolled steel sheet is set to 250°C or below. The cooling stop temperature is important for ensuring the total area ratio of martensite and tempered martensite. When the upper limit of the cooling stop temperature is above 250°C, the martensitic transformation is not fully completed during cooling, resulting in a total area ratio of less than 90% for martensite and tempered martensite, and a significant reduction in strength. Preferably, it is 200°C or below, more preferably 100°C or below. The lower limit of the cooling stop temperature is not particularly limited, but in practice it is above 20°C.

[0251] (Tempering)

[0252] After the aforementioned cooling, the steel plate can be held at a temperature range of 150°C to 400°C for at least 2 seconds. This process tempers the martensite formed during cooling, creating tempered martensite, thereby improving resistance to hydrogen embrittlement. During the tempering process, if the holding temperature is too low or the holding time is too short, the martensite cannot be fully tempered, resulting in almost no change in microstructure and mechanical properties. On the other hand, if the holding temperature is too high, the dislocation density in the tempered martensite decreases, leading to a reduction in tensile strength. Therefore, during tempering, it is preferable to hold the plate at a temperature range of 150°C to 400°C for at least 2 seconds. Tempering can be performed in a continuous annealing apparatus or offline after continuous annealing using other equipment. In this case, the tempering time varies depending on the tempering temperature; that is, the lower the temperature, the longer the tempering time, and the higher the temperature, the shorter the tempering time.

[0253] (Surface smoothing reduction rate)

[0254] Furthermore, surface finishing can be performed with the aim of improving ductility by correcting the shape of the steel sheet and introducing movable dislocations. The reduction rate of surface finishing after heat treatment is preferably in the range of 0.1% to 1.5%. If it is less than 0.1%, the effect is small and it is difficult to control, so this is set as the lower limit. If it exceeds 1.5%, the productivity decreases significantly, so this is set as the upper limit. Surface finishing can be performed online or offline. In addition, surface finishing with the target reduction rate can be performed in one go or in multiple stages. Furthermore, the strength of the annealed steel sheet is higher than that of the hot-rolled sheet, so the change in surface unevenness when rolling with the same reduction rate is different. However, from the perspective of maintaining the unevenness formed by the hot-rolled sheet, the total cold rolling rate and surface finishing rate is preferably 20% or less.

[0255] According to the manufacturing method described above, the steel plate involved in the above-described embodiments can be obtained.

[0256] Example

[0257] The following illustrates embodiments of the present invention. The present invention is not limited to this single example. Various conditions can be employed to achieve the purpose of the invention without departing from its spirit and essence.

[0258] Steel billets are produced by melting steels with various chemical compositions. These billets are then inserted into a furnace heated to 1220°C and held for 60 minutes for homogenization before being removed and exposed to atmosphere for hot rolling to obtain a steel sheet with a thickness of 1.8 mm. During hot rolling, lubricant is supplied between the rolls and the sheet in the stand preceding the final stand. The reduction rate in the stand preceding the final stand of the finishing mill, the finishing temperature, and the coiling temperature of the hot-rolled coil are the values ​​shown in Tables 2-1 to 2-3 below. Next, the oxide scale of the hot-rolled steel sheet is removed by pickling, and cold rolling is performed at the reduction rates shown in Tables 2-1 to 2-3 below to reduce the sheet thickness to 1.4 mm. Furthermore, the cold-rolled steel sheet is annealed and tempered under the conditions shown in Tables 2-1 to 2-3 below. Finally, the cold-rolled steel sheet is surface-finished with the reduction rates (%) shown in Tables 2-1 to 2-3 below. The chemical compositions of the samples collected from the various steel plates are shown in Tables 1-1 to 1-6. In addition to the components shown in Tables 1-1 to 1-6, the remaining components are Fe and impurities.

[0259] The evaluation results of the properties of the various steel plates manufactured as described above are shown in Tables 3-1 to 3-3 below. Furthermore, the methods for measuring the "area ratio of the microstructure of the cold-rolled annealed sheet" and "properties (tensile strength, total elongation, porosity, spacing of steps with a height difference exceeding 5.0 μm on the sheet surface, sliding friction resistance)" in Tables 3-1 to 3-3 are as described above.

[0260]

[0261]

[0262]

[0263]

[0264]

[0265]

[0266]

[0267]

[0268]

[0269] Table 3-1

[0270]

[0271] Table 3-2

[0272]

[0273] Table 3-3

[0274]

[0275] The following information can be obtained from Tables 1-1 to 3-3.

[0276] Compared to other examples, No. 53 has a lower carbon content in its steel, resulting in a slight decrease in steel strength.

[0277] No. 54 Due to the excessive carbon content in the steel, its strength increases, but its porosity decreases. Furthermore, it is believed that significant decarburization occurs on the steel sheet surface during hot rolling. In this decarburization reaction, carbon atoms released from the steel surface inhibit partial deposition between the roll surface and the steel sheet surface, making it difficult to obtain the desired unevenness. As a result, the desired unevenness cannot be formed on the surface of the final steel sheet, and the sliding friction resistance increases.

[0278] It is believed that No. 55 steel has an excessive Si content, which causes coarse oxides to easily disperse on the surface of the hot-rolled sheet, making it difficult to obtain the desired texture during hot rolling. As a result, the desired texture cannot be formed on the surface of the final steel sheet, and the sliding friction resistance increases.

[0279] It is believed that No. 56 steel has excessive Mn content, which reduces its workability. Furthermore, the coarse oxides tend to disperse on the surface of the hot-rolled sheet, making it difficult to achieve the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel sheet, leading to increased sliding friction resistance.

[0280] No. 57 Due to excessive phosphorus content in the steel, it leads to brittle fracture, reduced elongation and porosity.

[0281] No. 58 Due to the excessive sulfur content in the steel, its elongation and porosity are reduced. Furthermore, cracks originating from non-metallic inclusions are prone to occur during hot rolling, breaking off midway through the process and peeling away from the steel sheet. The micronized iron powder then grinds the steel sheet surface during hot rolling, making it difficult to achieve the desired surface texture. Consequently, the desired surface texture cannot be formed on the final steel sheet, resulting in increased sliding friction resistance.

[0282] No. 59 Due to the excessive Al content in the steel, the ferrite and bainite phase transformations are promoted during the annealing cooling process, thus reducing the steel's strength. Furthermore, it is believed that the large amount of coarse Al oxides formed on the steel surface during hot rolling grinds the steel sheet surface, making it difficult to achieve the desired texture and deformation during hot rolling. As a result, the desired texture cannot be formed on the surface of the final steel sheet, leading to increased sliding friction resistance.

[0283] It is believed that No. 60 steel has an excessive nitrogen content, leading to the excessive formation of nitrides. These nitrides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0284] It is believed that No. 61 steel has an excessive Ti content, resulting in the excessive formation of coarse carbides. These carbides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0285] It is believed that No. 62 steel has an excessive Co content, resulting in the over-formation of Co carbides. These Co carbides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0286] It is believed that No. 63 steel has an excessive Ni content, which affects the peelability of oxide scale during hot rolling and promotes the formation of scratches on the plate surface. As a result, it is impossible to form the desired unevenness on the surface of the final steel plate, and the sliding friction resistance increases.

[0287] It is believed that No. 64 steel has an excessive Mo content, resulting in the excessive formation of Mo carbides. These Mo carbides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0288] No. 65 steel has an excessive Cr content, which promotes the formation of retained austenite. The presence of excessive retained austenite reduces the porosity.

[0289] It is believed that No. 66 steel has reduced porosity due to excessive oxygen content. Furthermore, the formation of coarse granular oxides on the steel surface leads to surface cracks and the formation of fine iron powder during hot rolling, making it difficult to achieve the desired surface texture during hot rolling. Consequently, the desired surface texture cannot be formed on the final steel sheet, resulting in increased sliding friction resistance.

[0290] It is believed that No. 67 steel has an excessive boron content, resulting in the formation of boron oxides within the steel. These boron oxides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. Consequently, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0291] It is believed that No. 68 steel has an excessive Nb content, resulting in the formation of a large amount of Nb carbides. These Nb carbides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0292] It is believed that No. 69 steel has an excessive V content, resulting in the formation of a large amount of carbonitriding compounds. These compounds inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0293] It is believed that No. 70 steel has an excessive Cu content, causing Cu to concentrate on the plate surface. This concentrated Cu inhibits the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0294] It is believed that No. 71 steel has an excessive W content, which leads to the formation of carbides in the steel. These carbides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0295] It is believed that No. 72 steel has an excessive Ta content, which leads to the formation of carbides in the steel. These carbides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0296] It is believed that No. 73 steel, due to its excessive Sn content, causes surface cracks and the formation of fine iron powder during hot rolling, making it difficult to achieve the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel sheet, increasing sliding friction resistance. Furthermore, the excessive Sn content leads to embrittlement of the steel sheet and reduced porosity.

[0297] No. 74 Due to the excessive Sb content in the steel, its porosity is reduced. Furthermore, hot rolling leads to surface cracks and the formation of fine iron powder, making it difficult to achieve the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel sheet, and sliding friction resistance increases.

[0298] No. 75 steel suffers from reduced porosity due to excessive As content. Furthermore, it is believed that hot rolling leads to surface cracking and the formation of fine iron powder, making it difficult to achieve the desired surface texture. Consequently, the desired surface texture cannot be formed on the final steel sheet, resulting in increased sliding friction resistance.

[0299] No. 76 Due to the excessive Mg content in the steel, its porosity is reduced. Furthermore, large inclusions form in the steel, which inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0300] It is believed that No. 77 steel, due to its excessive Ca content, causes surface cracks and the formation of fine iron powder during hot rolling, making it difficult to achieve the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel sheet, leading to increased sliding friction resistance.

[0301] It is believed that No. 78 steel has an excessive amount of γ (yellow) in its composition, leading to the formation of γ oxides within the steel. These γ oxides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to achieve the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0302] It is believed that No. 79 steel has an excessive Zr content, resulting in the formation of Zr oxides within the steel. These Zr oxides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. Consequently, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0303] It is believed that No. 80 steel has an excessive La content, which leads to the formation of La oxides within the steel. These La oxides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. As a result, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0304] It is believed that No. 81 steel has an excessive Ce content, resulting in the formation of Ce oxides within the steel. These Ce oxides inhibit the contact between the plate surface and the rolls during hot rolling, making it difficult to obtain the desired surface texture during hot rolling. Consequently, the desired surface texture cannot be formed on the final steel plate, and the sliding friction resistance increases.

[0305] It is believed that in No. 82, the reduction rate in the stand preceding the final stand of the hot rolling mill was too small, resulting in insufficient surface pressure between the plate and the rolls during hot rolling, making it difficult to form the desired unevenness. As a result, the desired unevenness could not be formed on the surface of the final steel plate, and the sliding friction resistance increased.

[0306] It is believed that in No. 83, the excessive reduction rate in the stand preceding the final stand of the hot-rolling finishing mill resulted in excessive surface pressure between the plate and rolls during rolling, leading to a higher frequency of contact between the plate and rolls compared to sliding. Consequently, the desired unevenness could not be formed on the surface of the final steel plate, and the sliding friction resistance increased.

[0307] It is believed that in No. 84, the oxide scale formed on the surface of the hot-rolled sheet was significantly thicker due to the excessively high temperature during coiling. The unevenness formed on the surface of the hot-rolled sheet during hot rolling was incorporated into the oxide scale, which was removed by subsequent pickling, thus eliminating the unevenness. As a result, the desired unevenness could not be formed on the surface of the final steel sheet, leading to increased sliding friction resistance.

[0308] It is believed that in No. 85, due to excessive reduction during cold rolling, the raised areas on the surface of the sheet formed during hot rolling were flattened during cold rolling. As a result, the desired raised areas could not be formed on the surface of the final steel sheet, and the sliding friction resistance increased.

[0309] Although No. 86 can create the desired surface irregularities on the steel plate surface and reduce sliding friction resistance, the annealing holding temperature after cold rolling is too low. As a result, the area ratio of martensite and tempered martensite in the steel plate becomes smaller, and the strength of the steel plate is greatly reduced.

[0310] It is believed that in No. 87, due to the lack of lubricant supplied in the stand preceding the final stand of the finishing mill during hot rolling, sliding between the plate and the rolls became difficult. As a result, the desired unevenness could not be formed on the surface of the final steel plate, and the sliding friction resistance increased.

[0311] For No.1 to 52 and 88, which are manufactured under specified manufacturing conditions with the content of each element within the specified range, the desired microstructure is obtained in the final steel sheet, and the desired unevenness is formed on the surface of the steel sheet, resulting in increased sliding friction resistance.

[0312] Based on the above results, steel plates that meet the following requirements (I) to (III) can be said to have low sliding friction resistance, reduced mold damage during cold pressing, and thus improved mold life.

[0313] (I) It has the following chemical composition: by mass%, it contains C: 0.15–0.35%, Si: 0.01–2.00%, Mn: 0.10–4.00%, P: less than 0.0200%, S: less than 0.0200%, Al: 0.001–1.000%, N: less than 0.0200%, Ti: 0–0.500%, Co: 0–0.500%, Ni: 0–0.500%, Mo: 0–0.500%, Cr: 0–2.000%, O: 0–0.0100%, B: 0–0.0 100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0-0.0500%, As: 0-0.0500%, Mg: 0-0.0500%, Ca: 0-0.0500%, Y: 0-0.0500%, Zr: 0-0.0500%, La: 0-0.0500%, and Ce: 0-0.0500%, with the remainder consisting of Fe and impurities.

[0314] (II) A steel microstructure consisting of a total of 90.0% or more of martensite and tempered martensite in terms of area ratio, a total of 0% or more and 10.0% or less of ferrite, pearlite and bainite in terms of area ratio, and a total of 0% or more and 5.0% or less of retained austenite.

[0315] (III) Multiple steps with a height difference of more than 5.0 μm exist on the plate surface at intervals of less than 2.0 mm.

[0316] Furthermore, it is known that steel plates satisfying the above requirements (I) to (III) can be manufactured by a conventional manufacturing method, characterized by improving the surface roughness of the hot-rolled plate under hot-rolling conditions and undergoing an annealing process without completely smoothing the roughness. Specifically, the steel plate can be manufactured by the following manufacturing method.

[0317] A method for manufacturing a steel plate, comprising:

[0318] A hot-rolled plate is obtained by hot rolling a steel billet having the chemical composition described above (I).

[0319] Roll up the above-mentioned hot-rolled sheet;

[0320] Pickling of the above-mentioned hot-rolled plates; and

[0321] The hot-rolled plates described above are either annealed without cold rolling, or annealed after cold rolling.

[0322] The aforementioned hot rolling is performed in the stand preceding the final stand of the finishing mill, where lubricant is supplied between the rolling rolls and the plate while rolling at a reduction rate exceeding 30% and below 70%.

[0323] The temperature during the winding of the above-mentioned hot-rolled sheet is below 700℃.

[0324] In the case of the above-mentioned cold rolling, the reduction rate in the cold rolling is 0.1% to 20%.

Claims

1. A steel plate, characterized in that, It has the following chemical composition: (by mass%), it contains: C：0.15～0.35％、 Si: 0.01~2.00% Mn: 0.10~4.00% P: below 0.0200% S: Below 0.0200% Al：0.001～1.000％、 N: below 0.0200% Ti: 0~0.500% Co: 0~0.500% Ni: 0~0.500% Mo: 0–0.500% Cr：0～2.000％、 O：0～0.0100％、 B：0～0.0100％、 Nb: 0~0.500% V：0～0.500％、 Cu: 0~0.500% W：0～0.1000％、 Ta: 0~0.1000% Sn: 0~0.0500% Sb: 0~0.0500% As: 0~0.0500% Mg: 0~0.0500% Ca: 0~0.0500% Y：0～0.0500％、 Zr：0～0.0500％、 La: 0~0.0500%, and Ce: 0~0.0500%, The remaining part consists of Fe and impurities. The steel microstructure comprises a total of 90.0% or more martensite and tempered martensite by area ratio, a total of 0% or more and 10.0% or less ferrite, pearlite and bainite by area ratio, and a total of 0% or more and 5.0% or less retained austenite. On the surface of the plate, there are multiple steps with a height difference of more than 5.0 μm at intervals of more than 0.01 mm and less than 2.0 mm.

2. The steel plate according to claim 1, characterized in that, It has the following chemical composition: It contains, by mass%, the following: Ti: 0.001~0.500% Co: 0.001~0.500% Ni: 0.001~0.500% Mo: 0.001~0.500% Cr：0.001～2.000％、 O：0.0001～0.0100％、 B：0.0001～0.0100％、 Nb: 0.001~0.500% V：0.001～0.500％、 Cu: 0.001~0.500% W：0.0001～0.1000％、 Ta: 0.0001~0.1000% Sn: 0.0001~0.0500% Sb: 0.0001~0.0500% As: 0.0001~0.0500% Mg: 0.0001~0.0500% Ca: 0.0001~0.0500% Y：0.0001～0.0500％、 Zr：0.0001～0.0500％、 La: 0.0001~0.0500%, and Ce: 0.0001 to 0.0500% of one or more of the following.

3. The steel plate according to claim 1 or 2, characterized in that, The tensile strength of the steel plate is above 1300MPa.

4. A method for manufacturing a steel plate, characterized in that, Include: A hot-rolled plate is obtained by hot rolling a steel billet having the chemical composition of any one of claims 1 to 3; The hot-rolled sheet is wound up; Pickling is performed on the hot-rolled plate; and The hot-rolled sheet may be annealed without cold rolling, or annealed after cold rolling. The hot rolling is performed in the stand preceding the final stand of the finishing mill, where lubricant is supplied between the rolling rolls and the plate while the plate is rolled at a reduction rate exceeding 30% and below 70%. The temperature during the winding of the hot-rolled sheet is below 700°C. In the case of cold rolling, the reduction rate during cold rolling is 0.1% to 20%.

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