Steel plate

Abstract

Provided is a steel sheet which has excellent drawability and is capable of suppressing the occurrence of burrs during trimming. The steel sheet according to the present embodiment contains, in mass%, 0.010-0.100% of C, 0.350% or less of Si, 1.00% or less of Mn, 0.070% or less of P, 0.025% or less of S, 0.005-0.100% of sol.Al, 0.0060% or less of N, and 0.50-2.50 times the content of N with the balance being Fe and impurities, and the number density ND of cementite having an area of 0.30 [mu] m2 or more is 1150 pieces / m2 or more and less than 7000 pieces / mm2.

Description

Technical Field

[0001] The present disclosure relates to a steel sheet, and more particularly, to a steel sheet suitable for use in drawn cans. In this specification, a drawn can refers to a can formed by drawing and a can formed by DI (Drawing and Ironing). Background Art

[0002] Battery cans, such as secondary batteries, are manufactured by drawing or DI processing steel sheets or plated steel sheets. Therefore, the steel sheets used as blanks for these drawn cans are required to have excellent drawability.

[0003] International Publication No. 2016 / 060248 (Patent Document 1) proposes a steel sheet with excellent deep drawability. The chemical composition of the steel sheet for deep-drawn cans disclosed in Patent Document 1 includes, by mass%, C: greater than 0.150% and less than 0.260%, Sol.Al: 0.005-0.100%, B: 0.0005-0.02%, Si: 0.50% or less, Mn: 0.70% or less, P: 0.070% or less, S: 0.05% or less, N: 0.0080% or less, Nb: 0.003% or less, and Ti: 0.003% or less, with the remainder being Fe and impurities. The boron and nitrogen contents in the chemical composition of this steel sheet also satisfy, by mass%, 0.4 ≤ B / N ≤ 2.5. The microstructure of this steel sheet contains ferrite and granular cementite with an average grain size of 2.7-4.0 μm. In a tensile test conducted with the steel sheet stretched parallel to the rolling direction after aging at 100°C for one hour, the yield strength (YP) in MPa, the total elongation (EL) in %, the elongation at yield point (YP-EL) in %, and the yield ratio (YR) in % were calculated. The results are as follows: YP is 360-430 MPa, EL is 25-32%, YP-EL is 0%, and YR is 80-87%. The steel sheet disclosed in Patent Document 1 has a higher C content than conventional steel sheets and further contains B. This results in excellent deep drawability.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: International Publication No. 2016 / 060248 Summary of the Invention

[0007] Problems to be solved by the invention

[0008] On the other hand, when a steel plate is subjected to deep drawing to form a drawn can product, a flange portion is formed on the drawn can after the deep drawing. The flange portion is an unnecessary portion for the drawn can product. Therefore, after the deep drawing, the flange portion is removed by trimming. During this trimming, burrs may sometimes be generated at the cut of the drawn can after the flange portion is cut off. When a battery is manufactured using such a drawn can with burrs, the burrs may sometimes invade the interior of the drawn can. Such burrs reduce the battery characteristics. Therefore, in the trimming process after the deep drawing, it is preferable to suppress the generation of burrs. In Patent Document 1, a method for suppressing the generation of burrs during trimming is not studied.

[0009] An object of the present disclosure is to provide a steel sheet that can obtain excellent drawability and can suppress the generation of burrs during trimming.

[0010] Solutions for solving problems

[0011] The steel plate of the present disclosure contains, by mass%,

[0012] C: 0.010~0.100%,

[0013] Si: 0.350% or less,

[0014] Mn: less than 1.00%,

[0015] P: 0.070% or less,

[0016] S: 0.025% or less,

[0017] sol.Al: 0.005~0.100%,

[0018] N: 0.0060% or less, and

[0019] B: 0.50~2.50 times of N content,

[0020] The balance is Fe and impurities.

[0021] With 0.30μm 2 The number density of cementite in the area above is 1150 / mm 2 More than and less than 7000 pieces / mm 2 .

[0022] Effects of the Invention

[0023] According to the steel sheet of this embodiment, excellent drawability can be obtained and the generation of burrs during trimming can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1A This is a perspective view of a cylindrical can obtained by deep drawing in the drawability evaluation test in Examples.

[0025] Figure 1B yes Figure 1A Side view of a cylindrical tank.

[0026] Figure 2 This is a cross-sectional photograph of the end portion of the cut surface after trimming in the trimming test in the examples.

[0027] Figure 3 is with Figure 2 Differently, this is a cross-sectional photograph of the end portion of the cut surface after trimming in the trimming test of the example. DETAILED DESCRIPTION

[0028] The present inventors first studied the chemical composition of steel sheets suitable for drawn can applications. As a result, the present inventors determined that a suitable chemical composition of the steel sheet is one containing, by mass%, 0.010-0.100% C, 0.350% or less Si, 1.00% or less Mn, 0.070% or less P, 0.025% or less S, 0.005-0.100% sol. Al, and 0.0060% or less N, with the balance being Fe and impurities.

[0029] The present inventors have further investigated methods for achieving excellent drawability and suppressing burr formation during trimming. As proposed in Patent Document 1, boron (B) randomizes the texture of the steel sheet, bringing the plastic strain ratio r (Lankford value) close to 1. This improves lug properties and drawability. Furthermore, improved lug properties allow for uniform flange formation during drawing. Consequently, it is believed that burrs are less likely to form during trimming.

[0030] Therefore, the present inventors have found that if B is further contained in the above chemical composition, and a chemical composition containing, in mass%, C: 0.010-0.100%, Si: 0.350% or less, Mn: 1.00% or less, P: 0.070% or less, S: 0.025% or less, sol. Al: 0.005-0.100%, N: 0.0060% or less, and B: 0.50 to 2.50 times the N content, with the balance being Fe and impurities, is adopted, burrs during trimming can be suppressed.

[0031] However, even the steel plates with the above chemical composition cannot fully suppress the generation of burrs during trimming. Therefore, the present inventors conducted further research. As a result, it was found that if the number density of coarse cementite in the steel plate is high, the generation of burrs during trimming can be suppressed. Therefore, the present inventors conducted further research on the relationship between the size and number density of cementite and the frequency of burr generation. As a result, it was found that in the steel plates with the above chemical composition, if the number density of coarse cementite is high, the generation of burrs during trimming can be suppressed. 2 The number density of cementite in the area above is 1150 / mm 2 As described above, when performing trimming, the generation of burrs can be sufficiently suppressed.

[0032] On the other hand, it was found that if the 2 If the number density ND of cementite in an area of ​​0.30 μm or more is too high, the drawability will be reduced. 2 The relationship between the number density ND of cementite in the area above and the deep drawing workability was found. 2 The number density of cementite in the area above ND is less than 7000 / mm 2 , then excellent deep drawing workability can be obtained.

[0033] The gist of the steel sheet of this embodiment, which was completed based on the above technical concept, is as follows.

[0034] The steel plate of the first constitution contains, by mass%,

[0035] C: 0.010~0.100%,

[0036] Si: 0.350% or less,

[0037] Mn: less than 1.00%,

[0038] P: 0.070% or less,

[0039] S: 0.025% or less,

[0040] sol.Al: 0.005~0.100%,

[0041] N: 0.0060% or less, and

[0042] B: 0.50~2.50 times of N content,

[0043] The balance is Fe and impurities.

[0044] With 0.30μm 2 The number density of cementite in the area above is 1150 / mm 2More than and less than 7000 pieces / mm 2 .

[0045] The steel plate of the second configuration is the steel plate of the first configuration, and

[0046] One selected from the group consisting of a Ni plating layer, a Ni diffusion plating layer, a Ni alloy plating layer, and a Ni alloy diffusion plating layer is further formed on the surface of the steel sheet.

[0047] The steel sheet of this embodiment will be described in detail below. Unless otherwise specified, "%" related to an element means mass %.

[0048] [Features of the Steel Sheet of the Present Embodiment]

[0049] The steel plate of this embodiment satisfies the following characteristics 1 and 2.

[0050] (Feature 1)

[0051] The chemical composition contains, in mass%, C: 0.010-0.100%, Si: 0.350% or less, Mn: 1.00% or less, P: 0.070% or less, S: 0.025% or less, sol.Al: 0.005-0.100%, N: 0.0060% or less, and B: 0.50-2.50 times the N content, with the balance being Fe and impurities.

[0052] (Feature 2)

[0053] With 0.30μm 2 The number density of cementite in the area above is 1150 / mm 2 More than and less than 7000 pieces / mm 2 .

[0054] Hereinafter, feature 1 and feature 2 will be described.

[0055] [(Feature 1) About chemical composition]

[0056] The chemical composition of the steel plate of this embodiment contains the following elements.

[0057] C:0.010~0.100%

[0058] Carbon (C) increases the strength of steel sheets. If the C content is less than 0.010%, these effects are not fully achieved. On the other hand, if the C content exceeds 0.100%, coarse carbides are excessively formed. In this case, the machinability of the steel sheet is reduced. Therefore, the C content is preferably between 0.010% and 0.100%.

[0059] The lower limit of the C content is preferably 0.015%, more preferably 0.020%, and even more preferably 0.025%.

[0060] The upper limit of the C content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0061] The preferred range of the C content is, for example, 0.015 to 0.090%, more preferably 0.020 to 0.080%, and even more preferably 0.025 to 0.070%.

[0062] Si: 0.350% or less

[0063] Silicon (Si) is an unavoidable impurity. That is, when the Si content exceeds 0%, Si reduces the adhesion of the steel plate's plating. Si also reduces the adhesion of the steel plate's paint after canning. Therefore, the Si content is set to 0.350% or less.

[0064] The Si content is preferably as low as possible. However, excessive reduction in Si content increases manufacturing costs. Therefore, considering industrial productivity, the preferred lower limit of Si content is 0.001%, more preferably 0.002%, and even more preferably 0.005%.

[0065] The upper limit of the Si content is preferably 0.250%, more preferably 0.100%, further preferably 0.050%, further preferably 0.030%.

[0066] The preferred range of the Si content is, for example, 0.001 to 0.250%, more preferably 0.002 to 0.100%, further preferably 0.005 to 0.050%, and further preferably 0.005 to 0.030%.

[0067] Mn: 1.00% or less

[0068] Manganese (Mn) is an impurity that is inevitably contained. That is, the Mn content exceeds 0%. Mn excessively increases the number density of coarse cementite. Therefore, the steel sheet hardens. As a result, the deep drawing workability of the steel sheet decreases. Therefore, the Mn content is 1.00% or less.

[0069] The Mn content is preferably as low as possible. However, excessive reduction in the Mn content increases manufacturing costs. Therefore, considering industrial productivity, the preferred lower limit of the Mn content is 0.01%, more preferably 0.02%, and even more preferably 0.05%.

[0070] The upper limit of the Mn content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.70%.

[0071] The preferred range of the Mn content is, for example, 0.01 to 0.90%, more preferably 0.02 to 0.80%, and even more preferably 0.05 to 0.70%.

[0072] P: 0.070% or less

[0073] Phosphorus (P) is an unavoidable impurity. That is, a P content exceeding 0% will reduce the drawability of the steel sheet. Therefore, the P content is set to 0.070% or less.

[0074] The P content is preferably as low as possible. However, excessive reduction in the P content increases manufacturing costs. Therefore, considering industrial productivity, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.005%.

[0075] The upper limit of the P content is preferably 0.060%, more preferably 0.050%, and even more preferably 0.040%.

[0076] The preferred range of the P content is, for example, 0.001 to 0.060%, more preferably 0.002 to 0.050%, and even more preferably 0.005 to 0.040%.

[0077] S: 0.025% or less

[0078] Sulfur (S) is an unavoidable impurity. That is, if the S content exceeds 0%, S will reduce the hot workability of the steel sheet and may cause cracks during hot working. Therefore, the S content should be 0.025% or less.

[0079] The S content is preferably as low as possible. However, excessive reduction in the S content increases manufacturing costs. Therefore, considering industrial productivity, the preferred lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.005%.

[0080] The upper limit of the S content is preferably 0.020%, more preferably 0.015%, and even more preferably 0.012%.

[0081] The preferred range of the S content is, for example, 0.001 to 0.020%, more preferably 0.002 to 0.015%, and even more preferably 0.005 to 0.012%.

[0082] sol.Al: 0.005~0.100%

[0083] Aluminum (Al) deoxidizes steel. If the sol.Al content is less than 0.005%, the above effects cannot be fully achieved. On the other hand, if the sol.Al content exceeds 0.100%, coarse nitrides and / or oxides are formed, reducing the deep drawability of the steel sheet. Therefore, the sol.Al content is between 0.005% and 0.100%.

[0084] The lower limit of the sol.Al content is preferably 0.010%, more preferably 0.015%, and further preferably 0.020%.

[0085] The upper limit of the sol.Al content is preferably 0.090%, more preferably 0.080%, and further preferably 0.070%.

[0086] The preferred range of the sol.Al content is, for example, 0.010 to 0.090%, more preferably 0.015 to 0.080%, and even more preferably 0.020 to 0.070%.

[0087] It should be noted that sol.Al refers to "acid-soluble Al".

[0088] N: 0.0060% or less

[0089] Nitrogen (N) is an unavoidable impurity. That is, the N content exceeds 0%. N causes age hardening of the steel sheet, reducing deep drawability. N also generates tensile strain. Therefore, the N content is limited to 0.0060% or less.

[0090] The N content is preferably as low as possible. However, excessive reduction in the N content increases manufacturing costs. Therefore, considering industrial productivity, the preferred lower limit of the N content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

[0091] The upper limit of the N content is preferably 0.0055%, more preferably 0.0050%, and even more preferably 0.0045%.

[0092] The preferred range of the N content is, for example, 0.0001 to 0.0055%, more preferably 0.0005 to 0.0050%, and even more preferably 0.0010 to 0.0045%.

[0093] B: 0.50~2.50 times the N content

[0094] Boron (B) combines with N to form BN, which reduces the solid solution N in the steel plate. As a result, age hardening caused by solid solution N can be suppressed. B further randomizes the texture of the steel plate, making the r value (Lankford value) of the composition strain ratio close to 1, and improving the lug characteristics. Therefore, the deep drawing workability is improved. Furthermore, the generation of burrs during trimming processing is suppressed. If the B content is less than 0.50 times the N content, the above effect cannot be fully obtained. On the other hand, if the B content exceeds 2.50 times the N content, the solid solution B in the steel plate becomes too much. In this case, the steel plate hardens, or the lug characteristics are reduced. B further suppresses the coarsening of cementite. Therefore, the generation of burrs during trimming processing cannot be fully suppressed. Therefore, the B content is 0.50 to 2.50 times the N content.

[0095] The lower limit of the B content is preferably 0.55 times the N content, more preferably 0.60 times the N content, and even more preferably 0.65 times the N content.

[0096] The upper limit of the B content is preferably 2.45 times the N content, more preferably 2.40 times the N content, and even more preferably 2.35 times the N content.

[0097] The preferred range of the B content is, for example, 0.55 to 2.45 times the N content, more preferably 0.60 to 2.40 times the N content, and even more preferably 0.65 to 2.35 times the N content.

[0098] The balance of the chemical composition of the steel sheet of this embodiment is Fe and impurities. Here, impurities refer to substances that are mixed from raw materials such as ore and scrap, or from the manufacturing environment during industrial production of steel sheets, and are allowed within a range that does not adversely affect the steel sheet of this embodiment.

[0099] [About impurities]

[0100] The chemical composition of the steel sheet of this embodiment may further contain one or more elements selected from the group consisting of Nb: 0-0.003%, Ti: 0-0.003%, Cu: 0-0.50%, Ni: 0-0.50%, Cr: 0-0.30%, and Sn: 0-0.05% to replace part of the Fe. Each of these elements may be 0%. These elements are so-called impurity elements that may be contained in the scrap used as the raw material and are all impurities.

[0101] [Method for measuring the chemical composition of steel plates]

[0102] The chemical composition of the steel plate of this embodiment can be measured by a known component analysis method in accordance with JIS G0321:2017. Specifically, cuttings are collected from the steel plate using a cutting tool such as a drill. The collected cuttings are dissolved in acid to obtain a solution. The solution is subjected to ICP-MS (Inductively Coupled Plasma Mass Spectrometry) to perform elemental analysis of the chemical composition. The C content and S content are determined using the known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the known inert gas fusion-thermal conductivity method.

[0103] It should be noted that the content of each element is rounded off to the nearest decimal place based on the significant figures specified in this embodiment. For example, the C content of the steel material in this embodiment is specified to the third decimal place. Therefore, the C content is the value obtained by rounding off the fourth decimal place of the measured value to the third decimal place.

[0104] The same also applies to the contents of other elements in the steel sheet of the present embodiment except for the C content. The measured values ​​are rounded off to the smallest digit specified in the present embodiment as the corresponding element contents.

[0105] It should be noted that rounding means that if the decimal is less than 5, it is discarded, and if the decimal is greater than 5, it is rounded up.

[0106] [Number density of coarse cementite ND]

[0107] In the steel sheet of this embodiment, further, it has a thickness of 0.30 μm 2 The number density of cementite in the area above is 1150 / mm 2 More than and less than 7000 pieces / mm 2 In the following description, there will also be 0.30μm 2 Cementite with an area greater than 100 μm is called "coarse cementite". It should be noted that the upper limit of the area of ​​coarse cementite is not particularly limited. From the perspective of crystal growth, the area of ​​coarse cementite can be, for example, 4.00 μm. 2 the following.

[0108] Coarse cementite improves the chip separation during trimming. Therefore, it suppresses the generation of burrs during trimming. In order to fully suppress the generation of burrs, it is effective to increase the number density of coarse cementite. In the steel plate that meets the first characteristic, if the number density ND of coarse cementite is 1150 pieces / mm 2As described above, the generation of burrs during the trimming process can be sufficiently suppressed.

[0109] On the other hand, if the number density ND of coarse cementite is too high, the drawability is reduced. Therefore, the number density ND of coarse cementite is 1150 pieces / mm 2 More than and less than 7000 pieces / mm 2 .

[0110] The preferred lower limit of the number density ND of coarse cementite is 1400 pieces / mm 2 , more preferably 1500 pieces / mm 2 , more preferably 1600 pieces / mm 2 .

[0111] The upper limit of the number density ND of coarse cementite is preferably 6100 pieces / mm 2 , more preferably 5500 pieces / mm 2 .

[0112] The preferred range of the number density ND of coarse cementite is, for example, 1400 to 6100 pieces / mm 2 , more preferably 1500~5500 pieces / mm 2 , more preferably 1600~5500 pieces / mm 2 .

[0113] [Method for measuring the number density ND of coarse cementite]

[0114] The number density ND of coarse cementite can be obtained by the following method.

[0115] Collect samples from any position of the steel plate. The cross section of the surface of the sample perpendicular to the rolling direction of the steel plate is defined as the observation surface. After polishing the observation surface of the sample, use 6% picric alcohol etching solution (etching solution prepared by 6g of picric acid relative to 94mL of ethanol) to etch the observation surface for 20 seconds. Use a scanning electron microscope (SEM: Scanning Electron Microscope) to observe any 5 observation areas (61μm×43μm) of the etched observation surface at an observation magnification of 2000 times, photograph the particles (precipitates and inclusions) in the observation area, and generate a photographic image as a secondary electron image.

[0116] Based on the contrast of the photographic image, cementite is identified in the five observation areas. The area of ​​each identified cementite is calculated. Contrast-based identification of cementite and the area of ​​each cementite can be calculated using known image processing software. For example, image processing software is available under the trade name LUZEX AP MQ3104.0 from NIRECO Co., Ltd.

[0117] Based on the 0.30 μm 2 The total number of coarse cementite in the area above and the total area of ​​the five observation fields were used to calculate the number density ND (number / mm 2 ).

[0118] [Effects of the Steel Sheet of the Present Embodiment]

[0119] The steel sheet of the present embodiment satisfies characteristics 1 and 2. Therefore, in the steel sheet of the present embodiment, excellent drawability can be obtained, and the generation of burrs during trimming can be sufficiently suppressed.

[0120] [Regarding the Preferred Thickness of the Cold-Rolled Steel Sheet of the Present Embodiment]

[0121] The thickness of the cold-rolled steel sheet of the present embodiment is not particularly limited, and is, for example, 0.15 to 1.00 mm.

[0122] [Microstructure of the Steel Sheet According to the Present Embodiment]

[0123] The microstructure of the steel plate of this embodiment is mainly composed of ferrite and cementite. Specifically, in the microstructure of the steel plate of this embodiment, the total area ratio of ferrite and cementite is 95.0% or more. In the microstructure, the structure other than ferrite and cementite is composed of one or more selected from the group consisting of pearlite, hard structure (martensite and bainite), retained austenite, precipitates other than cementite, and inclusions. In the microstructure, the total area ratio of the structure other than ferrite and cementite is 5.0% or less.

[0124] [Method for observing the microstructure of steel plates]

[0125] The total area ratio of ferrite and cementite in the steel plate of the present embodiment is determined by the following method.

[0126] Samples were collected from the center of the steel plate's width. A cross-section of the sample's surface perpendicular to the steel plate's rolling direction was defined as the observation surface. The observation surface of the sample was mirror-polished. The mirror-polished observation surface was etched with 3% Nital (Nital etching solution). Five random observation fields (61 μm × 43 μm) were observed on the etched observation surface using a 2000x SEM.

[0127] In the observation field, ferrite and cementite can be easily distinguished from other structures by contrast. Ferrite is observed as a white area. Cementite is observed as a grain with a brightness lower than that of ferrite. Based on the total area of ​​ferrite and cementite in the five observation fields and the total area of ​​the five observation fields, the area ratio (%) of ferrite and cementite is calculated.

[0128] [About Ni-based plating]

[0129] The steel sheet of the present embodiment can be formed with a Ni-based coating on the surface of the steel sheet. The Ni-based coating is, for example, one selected from the group consisting of a Ni coating, a Ni diffusion coating, a Ni alloy coating, and a Ni alloy diffusion coating. The Ni coating is a coating made of Ni. The Ni diffusion coating is a coating in which Fe that moves from the steel sheet to the coating is diffused in a portion or all of the Ni coating. That is, the Ni diffusion coating is a coating containing Ni and Fe. The Ni alloy coating is a coating in which a portion or all of the Ni coating is replaced by an alloy layer of Ni and a known metal element other than Fe. The Ni alloy diffusion coating is a coating in which Fe from the steel sheet is diffused in a portion or all of any one of the layers in which a portion of the Ni alloy coating or the Ni coating is replaced by a layer consisting of a known metal element other than Fe.

[0130] In addition, the steel sheet of this embodiment does not need to have a Ni-based plating layer formed on the surface of the steel sheet.

[0131] [Method for Manufacturing Steel Sheet According to the Present Embodiment]

[0132] An example of a method for manufacturing a steel plate according to this embodiment will be described. A steel plate satisfying characteristics 1 and 2 may also be manufactured by a method other than the method described below. However, the method described below is a preferred example of a method for manufacturing a steel plate according to this embodiment.

[0133] An example of the method for manufacturing a steel plate according to the present embodiment includes the following steps.

[0134] (Process 1) Blank preparation process

[0135] (Process 2) Hot rolling process

[0136] (Process 3) Cold rolling process

[0137] (Process 4) Continuous annealing process

[0138] (Process 5) BAF annealing process

[0139] Hereinafter, steps 1 to 5 will be described.

[0140] [(Process 1) Blank preparation process]

[0141] In the billet preparation step, molten steel meeting characteristic 1 is produced. The produced molten steel is used to produce billets (slabs) through a casting process. For example, slabs can be produced using the molten steel through a known continuous casting process. Alternatively, ingots can be produced using the molten steel through an ingot casting process. Through the above manufacturing steps, slabs or ingots serving as billets for steel plates are prepared.

[0142] [(Process 2) Hot rolling process]

[0143] In the hot rolling process, slabs or ingots are hot rolled to produce hot-rolled steel sheets. The hot rolling process includes a rough rolling process in which the slabs or ingots are roughly rolled to produce a rough strip (intermediate steel sheet), and a finish rolling process in which the rough strips are finish rolled to produce steel sheets.

[0144] In the rough rolling process, the billet (slab or ingot) is heated and rolled using a rough rolling mill to produce a rough strip. The billet is heated to a temperature of, for example, 1000 to 1300°C.

[0145] In the finishing rolling process, the rough strip produced in the rough rolling process is further rolled (finish rolling) using a finishing mill to produce a hot-rolled steel sheet. In the finishing rolling process, the finishing rolling temperature is set to, for example, 800-1000°C, and the coiling temperature is set to, for example, 500-720°C.

[0146] [(Process 3) Cold Rolling Process]

[0147] In the cold rolling process, the hot-rolled steel sheet is cold-rolled to produce the cold-rolled steel sheet. During the cold rolling process, the cumulative reduction ratio is set to, for example, 80% or greater. There is no particular upper limit on the cumulative reduction ratio. However, considering industrial productivity, the preferred upper limit of the cumulative reduction ratio is 90%.

[0148] [(Step 4) Continuous Annealing Step]

[0149] In the continuous annealing process, the cold-rolled steel sheet after the cold rolling process or the cold-rolled steel sheet after the plating process described below and having a Ni-based plating layer formed thereon is subjected to continuous annealing using a continuous annealing line. Continuous annealing is performed under the following conditions.

[0150] (Condition 1) Annealing temperature T0: 710~830℃

[0151] (Condition 2) Holding time t0 at annealing temperature T0: 6.5 to 32.0 seconds

[0152] If the annealing temperature T0 is 710-830°C and the holding time t0 is 6.5-32.0 seconds, recrystallization is promoted in the cold-rolled steel sheet and cementite is generated. As a result, the total area ratio of ferrite and cementite in the microstructure of the manufactured steel sheet is 95.0% or more. Furthermore, by pre-generating a certain amount of cementite in the continuous annealing process, the BAF annealing process in the next process is carried out, so that the manufactured steel sheet has a microstructure of 0.30μm. 2The number density of cementite in the area above is 1150 / mm 2 More than and less than 7000 pieces / mm 2 .

[0153] If the BAF annealing step, described later, is performed without a continuous annealing step, the amount of cementite generated in the cold-rolled steel sheet before the BAF annealing step is insufficient. Therefore, even after the BAF annealing step, the number density ND of coarse cementite is insufficient. Therefore, in the manufacturing method of this embodiment, a continuous annealing step is performed before the BAF annealing step. It should be noted that in the continuous annealing step, the cold-rolled steel sheet after the continuous annealing step is coiled and cooled to below 100°C.

[0154] [(Step 5) BAF annealing step]

[0155] BAF (Box Annealing Furnace) annealing is performed on the cold-rolled steel sheet at a temperature of 100°C or less after the continuous annealing process. BAF annealing is also known as box annealing (or batch annealing). In BAF annealing, annealing is performed under the following conditions.

[0156] (Condition 3) Heating rate HR1: 0.90~4.80℃ / min

[0157] (Condition 4) Annealing temperature T1: 440~460℃

[0158] (Condition 5) Holding time t1: 460~490 minutes

[0159] (Condition 6) Cooling rate CR1: 0.30°C / min or less

[0160] Conditions 3 to 6 are described below.

[0161] [(Condition 3) Regarding the temperature rise rate HR1]

[0162] During the heating process of the BAF annealing step, cementite formation sites are ensured. If the heating rate HR1 is too fast, the ferrite structure in the steel sheet will coarsen and the cementite formation sites will decrease. If the heating rate HR is below 4.80°C / min, recrystallization will proceed during the heating process, and a sufficient number of cementite formation sites can be ensured. Therefore, in the manufactured steel sheet, there is a 0.30μm 2 The number density of cementite in the area above is 1150 / mm 2 More than and less than 7000 pieces / mm 2 Note that there is no particular lower limit for the heating rate HR1. However, if the heating rate HR1 is too slow, productivity decreases. Therefore, the preferred lower limit for the heating rate HR1 is 0.90°C / min.

[0163] [(Conditions 4 and 5) Regarding Annealing Temperature T1 and Holding Time t1]

[0164] By setting the annealing temperature T1 to 440-460°C and the holding time t1 to 460-490 minutes, a sufficient number density of cementite is generated. If the annealing temperature T1 is lower than 440°C or the holding time t1 is lower than 460 minutes, the solid solution C remains in the steel plate. In this case, the number density ND of coarse cementite cannot be sufficiently obtained in the manufactured steel plate. On the other hand, if the annealing temperature T1 exceeds 460°C or the holding time t1 exceeds 490 minutes, the ferrite structure coarsens. In this case, the formation sites of cementite are reduced. As a result, the number density ND of coarse cementite cannot be sufficiently obtained.

[0165] [(Condition 6) Regarding Cooling Rate CR1]

[0166] During the cooling process in the BAF annealing step, the generated cementite grows coarsely. If the cooling rate CR1 exceeds 0.30°C / min, the cooling rate is too fast. In this case, the cementite cannot grow sufficiently. Therefore, the number density ND of coarse cementite cannot be fully obtained in the manufactured steel plate. Therefore, the cooling rate CR1 is set to 0.30°C / min or less. The lower limit of the cooling rate CR1 is not particularly limited. However, if the cooling rate CR1 is too slow, productivity decreases. Therefore, the preferred lower limit of the cooling rate CR1 is 0.10°C / min.

[0167] The cold-rolled steel sheet after BAF annealing is cooled to 100° C. or lower. Through the above-described manufacturing steps, the steel sheet of the present embodiment satisfying the first and second characteristics is manufactured.

[0168] [About any manufacturing process]

[0169] The method for producing the steel sheet of the present embodiment may further include a temper rolling step and / or a plating step.

[0170] [Surface temper rolling process]

[0171] The temper rolling step is an optional step and may not be performed. If performed, the temper rolling step involves temper rolling (skin pass rolling) of the annealed steel sheet after the BAF annealing step to produce the steel sheet. The cumulative reduction ratio during temper rolling is, for example, 0.5 to 5.0%. During the temper rolling step, the surface roughness of the steel sheet or the hardness of the steel sheet is adjusted.

[0172] [Plating process]

[0173] The plating step is an optional step and may not be performed. If performed, the plating step may be performed after the BAF annealing step or after the temper rolling step. The plating step may be performed after the cold rolling step and before the continuous annealing step.

[0174] In the plating process, a Ni-based plating layer is formed on the surface of the steel sheet. The plating layer is formed using a known method. For example, the cold-rolled steel sheet is immersed in a plating bath for forming the Ni-based plating layer and subjected to electroplating or chemical plating. Through the above process, a Ni-based plating layer or a Ni alloy plating layer is formed on the cold-rolled steel sheet.

[0175] When the plating step is performed after the BAF annealing step or after the temper rolling step, the produced steel sheet has a Ni plating layer or a Ni alloy plating layer.

[0176] On the other hand, when the plating step is performed after the cold rolling step and before the continuous annealing step, the Fe in the steel sheet diffuses into the Ni plating layer or Ni alloy plating layer formed on the cold-rolled steel sheet during the continuous annealing step. Therefore, in this case, the manufactured steel sheet has a Ni diffusion plating layer or a Ni alloy diffusion plating layer.

[0177] Example

[0178] Hereinafter, the effects of one aspect of the steel sheet according to the present embodiment will be described in more detail with reference to examples.

[0179] Cold-rolled steel sheets having the chemical compositions shown in Table 1 were produced.

[0180] [Table 1]

[0181]

[0182] Specifically, molten steel is continuously cast to produce slabs. The slabs are then subjected to hot rolling processes (roughing and finishing). The slabs are heated to 1100-1250°C. The heated slabs are rolled in a roughing mill to produce rough strips. Furthermore, the rough strips are rolled in a finishing mill to produce hot-rolled steel sheets. The finishing rolling temperature is 850-940°C, and the coiling temperature is 500-720°C.

[0183] Hot-rolled steel sheets were cold-rolled to produce cold-rolled steel sheets. The cumulative reduction ratio was 80-90%. The cold-rolled steel sheets were continuously annealed. The annealing temperature T0 (°C) and holding time t0 (seconds) during the continuous annealing process are shown in Table 2. After continuous annealing, the cold-rolled steel sheets were coiled and cooled to room temperature (25°C).

[0184] [Table 2]

[0185]

[0186] The cooled steel sheet was subjected to a BAF annealing step. The heating rate HR1 (°C / min), annealing temperature T1 (°C), holding time t1 (min) at annealing temperature T1, and cooling rate CR1 (°C / min) during BAF annealing are shown in Table 2. After BAF annealing, the steel sheet was cooled to room temperature.

[0187] The cooled steel sheets were temper rolled. The cumulative reduction during temper rolling was set at 0.5 to 5.0%. Through the above manufacturing process, steel sheets with different test numbers were produced. The thickness of the steel sheets was 0.30 mm.

[0188] [Evaluation test]

[0189] The following evaluation tests were performed on the steel plates of each test number.

[0190] (Test 1) Coarse cementite number density ND measurement test

[0191] (Test 2) Microstructure observation test

[0192] (Test 3) Drawing workability evaluation test

[0193] (Test 4) Trimming test

[0194] Hereinafter, Experiments 1 to 4 will be described.

[0195] [(Test 1) Coarse Cementite Number Density ND Measurement Test]

[0196] The number density ND (number / mm2) of the coarse cementite of the steel plate of each test number was determined according to the method described in the above-mentioned [Method for measuring the number density ND of coarse cementite]. 2 ). The obtained number density ND (pieces / mm 2 ) are shown in Table 3.

[0197] [Table 3]

[0198]

[0199] [(Test 2) Microstructure Observation Test]

[0200] The total area ratio (%) of ferrite and cementite in the steel plate for each test number was determined using the method described in the [Method for Observing the Microstructure of Steel Plate] above. The obtained total area ratio (%) is shown in Table 3. In all test numbers, the total area ratio of ferrite and cementite was 95.0% or greater.

[0201] [(Test 3) Drawing workability evaluation test]

[0202] A disc-shaped test piece was collected from each test number steel plate. The disc-shaped test piece had a thickness of 0.30 mm and a diameter of 36.00 mm. A cylindrical deep drawing was performed on the disc-shaped test piece using a punch with a diameter of 21.55 mm to produce Figure 1A The cylindrical tank 1 is shown. Figure 1B yes Figure 1A The side view of the cylindrical tank 1 is shown. Figure 1B For the cylindrical can 1 obtained, the maximum value Hmax of the peak height (peak height of the lug) of the can sidewall of the cylindrical can 1 and the minimum value Hmin of the valley height of the can sidewall of the cylindrical can 1 are determined. Furthermore, the can sidewall height of the cylindrical can 1 is measured at 0.8° intervals over 360°, and the average value Have is determined. Using the obtained values ​​Hmax, Hmin, and Have, the lug ratio is calculated using the following formula.

[0203] Lug rate = (Hmax-Hmin) / Have

[0204] When the obtained lug ratio was 3.0% or less, it was determined that excellent drawability was obtained (indicated by "EX (Excellent)" in the "Drawability" column in Table 3). On the other hand, when the obtained lug ratio exceeded 3.0%, it was determined that sufficient drawability was not obtained (indicated by "B (Bad)" in the "Drawability" column in Table 3).

[0205] [(Test 4) Trimming test]

[0206] For the steel plates of each test number, a trimming test was performed by the following method to evaluate the presence or absence of burrs after trimming. Specifically, a disc-shaped test piece was collected from the steel plates of each test number. The diameter of the disc-shaped test piece was 36.00 mm and the thickness was 0.30 mm. The disc-shaped test piece was subjected to flanged cylindrical drawing to produce a flanged cylinder with a flange width of 5.00 mm. The drawing ratio was 2.0. The obtained flanged cylinder was trimmed to trim the entire flange portion. A tool blade was used for trimming. The trimming amount was set to 5.00 mm. In addition, during the trimming process, a plate force of 5% of the maximum shear load was applied.

[0207] Cut the cylinder with the flange trimmed parallel to the axial direction. Observe the end of the cut surface formed by trimming in the cross section with a scanning electron microscope (SEM) at 200 times magnification to check whether there are burrs on the end. Figure 2 If burrs 10 are confirmed in the end section as shown in FIG, it is determined that the burrs are not sufficiently suppressed during the trimming process (marked as "Yes" in the "Presence of Burr Generation" column in Table 3). Figure 3When no burr is observed in the end cross section as shown, it is determined that the burr is sufficiently suppressed during the trimming process (marked as "None" in the "Presence or Absence of Burr Generation" column in Table 3).

[0208] [Evaluation results]

[0209] Referring to Tables 1 to 3, the steel sheets of Test Nos. 1 to 15 satisfied Characteristics 1 and 2. Therefore, the generation of burrs was not confirmed in the trimming test, indicating that the generation of burrs was sufficiently suppressed during the trimming process.

[0210] On the other hand, in Test No. 16, the B content was too low. Therefore, sufficient drawability was not obtained. In addition, burrs were observed in the trimming test.

[0211] In test number 17, the B content was too high, so burrs were observed in the trimming test.

[0212] In Test No. 18, the annealing temperature T0 in the continuous annealing step was too low. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0213] In Test No. 19, the annealing temperature T0 in the continuous annealing step was too high. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0214] In Test No. 20, the holding time t0 in the continuous annealing step was too short. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0215] In Test No. 21, the holding time t0 in the continuous annealing step was too long. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0216] In Test No. 22, the heating rate HR1 in the BAF annealing step was too high. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0217] In Test No. 23, the annealing temperature T1 in the BAF annealing step was too low. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0218] In Test No. 24, the annealing temperature T1 in the BAF annealing step was too high. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0219] In Test No. 25, the holding time t1 in the BAF annealing step was too short. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0220] In Test No. 26, the holding time t1 in the BAF annealing step was too long. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0221] In Test No. 27, the cooling rate CR1 in the BAF annealing step was too high. Therefore, the number density ND of coarse cementite was too low. As a result, burrs were observed in the trimming test.

[0222] In test number 28, the Mn content was too high. Therefore, the number density ND of coarse cementite was too high. As a result, the lug ratio in the drawability evaluation test was too high, and sufficient drawability was not obtained.

[0223] The above describes the embodiments of the present disclosure. However, the above embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above embodiments, and the above embodiments can be implemented by appropriately modifying them within the scope of the present disclosure.

Claims

1. A steel plate comprising, by mass%, C：0.010~0.100%、 Si: 0.350% or less, Mn: less than 1.00%, P: 0.070% or less, S: 0.025% or less, sol.Al: 0.005~0.100%, N: 0.0060% or less, and B: 0.50~2.50 times of N content, The balance is Fe and impurities. With 0.30μm 2 The number density of cementite in the area above is 1150 / mm 2 More than and less than 7000 pieces / mm 2 .

2. The steel plate according to claim 1, wherein A layer selected from the group consisting of a Ni plating layer, a Ni diffusion plating layer, a Ni alloy plating layer, and a Ni alloy diffusion plating layer is further formed on the surface of the steel plate.

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