High-strength cold-rolled steel sheet with excellent phosphate treatability and its manufacturing method

Abstract

The present invention includes a base steel sheet and a nickel or nickel alloy coating layer formed on the base steel sheet, wherein the amount of the nickel or nickel alloy coating is 50 mg / m 2 The present invention relates to a cold-rolled steel sheet, which is provided with a nano-thick metal layer coated on a cold-rolled steel sheet and then annealed to suppress the formation of oxides of Si, Mn, etc. on the steel sheet surface, while not suppressing the dissolution of Fe, and thus provides a high-strength cold-rolled steel sheet and a method for manufacturing the same, which have improved phosphating properties.

Description

[Technical Field]

[0001] The present invention relates to a high-strength cold-rolled steel sheet with excellent phosphate treatability and a method for producing the same. [Background technology]

[0002] Recently, with the emergence of environmental regulations, the demand for ultra-high strength steel sheets is rapidly increasing as a way to comply with stricter automobile fuel economy and collision stability regulations. While fuel economy needs to be improved to achieve national carbon emission reduction targets, automobile weight is continually increasing due to higher performance and the addition of various convenience devices. To address these issues, the demand for ultra-high strength steel sheets is also increasing. Steel manufacturers are therefore focusing on the development of high-strength steel sheets such as Dual Phase (DP) steel, Transformation Induced Plasticity (TRIP) steel, and Complex Phase (CP) steel.

[0003] Generally, automotive steel sheets are phosphate-treated before electrodeposition coating to ensure paint adhesion during the painting process. Phosphate crystals formed during the phosphate treatment have a significant impact on corrosion resistance and paint adhesion after electrodeposition coating. Since phosphate crystals only provide excellent adhesion to paint when they are small and densely formed, automobile manufacturers have certain standards for the size of phosphate crystals and the amount of phosphate deposition that must be met in order to be commercialized.

[0004] To increase the strength of automotive steel sheets, it is common to add large amounts of elements such as Si, Mn, and Al to the steel to increase the strength. However, steel sheets containing these elements have the problem that these elements form oxides on the steel sheet surface during annealing heat treatment, which reduces the reactivity with phosphate during phosphating. If the reactivity between the steel sheet and phosphate decreases, the phosphate crystals on the steel sheet surface may become coarse, and the phosphate crystals may not cover the entire steel sheet. This may result in reduced paint adhesion and corrosion resistance after electrodeposition coating.

[0005] In order to improve the phosphating ability of steel sheets containing large amounts of Si, Mn, and Al, it is necessary to suppress the formation of oxides on the steel sheet surface. To achieve this, it is necessary to reduce the amounts of Si and Al added to the steel. However, in such cases, it is difficult to ensure the desired properties of the steel.

[0006] In this regard, a technique has been proposed in which trace elements such as Sb are added to steel to cause preferential concentration at grain boundaries, thereby suppressing the formation of Si oxides and the like on the surface and improving phosphate treatability and paint adhesion (Patent Document 1). However, there is still a need for development of a technique that can more reliably prevent the diffusion of alloying elements in steel during annealing heat treatment. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6222040 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been devised in view of the above-mentioned circumstances, and provides a high-strength cold-rolled steel sheet and a manufacturing method thereof, in which a metal layer is coated on a steel sheet in a nano-thickness in a pretreatment process before annealing heat treatment, and the metal layer acts as a diffusion barrier for alloying elements in the steel during annealing heat treatment, thereby suppressing the formation of oxides on the steel sheet surface, thereby improving phosphating properties. [Means for solving the problem]

[0009] According to one aspect of the present invention, there is provided a steel sheet having a nickel or nickel alloy coating layer formed on the steel sheet, the nickel or nickel alloy coating layer having a coating weight of 50 mg / m 2 There is provided a cold rolled steel sheet having the following properties (excluding 0):

[0010] The thickness of the base steel sheet may be 1.0 to 1.8 mm.

[0011] The base steel sheet may contain 0.8 to 3.0 wt % of Si and 1.0 to 3.0 wt % of Mn.

[0012] The concentration of Si element from the surface of the cold-rolled steel sheet to a depth of 0.01 μm may be 0.1 atomic % or less (however, 0 is excluded).

[0013] The corrosion current density of the above cold-rolled steel sheets is 600 to 800 μA / m 2 may be.

[0014] When the cold-rolled steel sheet is phosphating, the phosphate coverage according to the following formula 1 may be 95% or more. [Formula 1] Phosphate coverage = (area of ​​phosphate-forming regions / total area) x 100

[0015] According to another aspect of the present invention, there is provided a method for manufacturing a steel sheet, the method comprising the steps of hot rolling and cold rolling a base steel sheet, and depositing nickel or a nickel alloy on the hot-rolled and cold-rolled base steel sheet in an amount of 50 mg / m 2There is provided a method for manufacturing a cold-rolled steel sheet, which includes the steps of forming a metal coating layer having the following properties (excluding 0), and subjecting the steel slab on which the metal coating layer has been formed to an annealing heat treatment.

[0016] The step of forming the metal coating layer may be performed by electroplating. [Effects of the Invention]

[0017] According to the present invention, a high-strength cold-rolled steel sheet and a manufacturing method thereof are provided, which are produced by coating a cold-rolled steel sheet with a nano-thick metal layer and then annealing the steel sheet to suppress the formation of oxides of Si, Mn, etc. on the steel sheet surface, while not suppressing the dissolution of Fe, thereby improving phosphating properties. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram illustrating the process in which oxides of Si, Mn, etc. are formed on the surface of a steel sheet when the steel sheet on which no metal coating layer is formed is heat-treated and then phosphate-treated. [Figure 2] FIG. 1 is a schematic diagram illustrating a process in which the formation of oxides of Si, Mn, etc. on the surface of a steel sheet is suppressed when the steel sheet having a metal coating layer formed thereon is heat-treated and then phosphate-treated, according to an embodiment of the present invention. [Figure 3] 1 is a scanning electron microscope (SEM) image of the surface of a steel sheet according to Example 3 of the present invention. [Figure 4] This is a scanning electron microscope (SEM) image of the surface of a steel sheet without a metal coating layer. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, preferred embodiments of the present invention will be described with reference to various embodiments. However, the present invention may be embodied in many different forms, and the scope of the present invention is not limited to the embodiments described below.

[0020] The present invention relates to a high-strength cold-rolled steel sheet with excellent phosphate treatability and a method for producing the same.

[0021] Fig. 1 is a schematic diagram illustrating the process by which oxides of Si, Mn, etc. are formed on the surface of a conventional cold-rolled steel sheet when the surface is heat-treated and then phosphating. Referring to Fig. 1, large amounts of alloying elements such as Si and Mn in the steel sheet diffuse to the surface of the steel sheet during the annealing heat treatment process and form oxides. When phosphating a cold-rolled steel sheet on which a large amount of oxides has formed, the oxides can reduce the area over which phosphate crystals cover the steel sheet surface, resulting in reduced phosphate coverage. This can lead to problems such as reduced paint adhesion and corrosion resistance after electrodeposition coating.

[0022] Therefore, the present inventors have conducted extensive research into methods for improving phosphating properties, and have found that excellent phosphating properties can be ensured by forming a diffusion barrier film, which can prevent alloying elements such as Si and Mn in steel from diffusing to the steel sheet surface during heat treatment, using an electroplating method in a pretreatment process prior to heat treatment, and coating the steel with a metal layer having a coating weight within a range that does not prevent the elution of Fe, thereby achieving the present invention.

[0023] According to one aspect of the present invention, there is provided a steel sheet having a nickel or nickel alloy coating layer formed on the steel sheet, the nickel or nickel alloy having a coating weight of 50 mg / m 2 The following cold-rolled steel sheet is provided: Figure 2 is a schematic diagram illustrating a process in which the formation of oxides of Si, Mn, etc. on the steel sheet surface is suppressed when a steel sheet having a metal coating layer formed thereon is heat-treated and then phosphating, according to one embodiment of the present invention. The present invention will be described in more detail below with reference to Figure 2.

[0024] The base steel sheet is not particularly limited, but may contain 0.8 to 3.0% by weight of Si and 1.0 to 3.0% by weight of Mn, and may contain, for example, by weight, C: 0.05 to 0.30%, Si: 0.05 to 3.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.008% or less, and Sb: 0.01 to 0.10%, with the balance being Fe and unavoidable impurities.

[0025] Si is an important element that contributes to improving strength through solid solution strengthening, and plays a role in improving strength while suppressing deterioration of workability. However, if the Si content exceeds 3.0%, not only does the effect of improving strength saturate, but there is also the problem of workability deteriorating, so the Si content is preferably 0.05 to 3.0%, and more preferably 0.1 to 2.0%.

[0026] Mn contributes to improving strength through solid solution strengthening and is an element that improves the hardenability of the austenite phase, effectively contributing to stabilizing strength. To stably obtain the desired strength, the Mn content must be 1.0% or more. However, if the Mn content exceeds 3.0%, workability deteriorates, so the Mn content is preferably in the range of 1.0 to 3.0%, and more preferably in the range of 1.5 to 2.5%.

[0027] Meanwhile, the base steel sheet preferably has a thickness of 1.0 to 1.8 mm for use as an automotive steel sheet for impact structural members. A nickel or nickel alloy coating layer is formed on the base steel sheet, which serves as a diffusion barrier that can prevent elements in the steel, such as S and Mn, from diffusing to the steel sheet surface during heat treatment. The nickel or nickel alloy coating layer can be formed by, but is not limited to, an electroplating method.

[0028] Furthermore, metal plating using nickel and nickel alloys increases the rate of Fe dissolution due to the formation of local cells, thereby increasing the generation and growth of phosphate crystals, so it is preferable to form a nickel and nickel alloy coating layer on the base steel sheet.

[0029] The amount of nickel or nickel alloy coating is 50 mg / m 2 It is preferably 5 to 50 mg / m or less (excluding 0). 2 More preferably, it is 50 mg / m 2 If the amount of nickel or nickel alloy coating exceeds 50 mg / m, it may hinder the surface elution of Fe. 2 It is preferable that:

[0030] The concentration of Si element from the surface to a depth of 0.01 μm of the above cold-rolled steel sheet is preferably 0.1% or less (excluding 0). If the concentration of Si element concentrated on the steel sheet surface exceeds 0.1%, a film-like Si-rich oxide is formed on the steel sheet surface, which inhibits the elution of Fe from the steel sheet during the phosphating process, thereby resulting in poor phosphating ability.

[0031] The corrosion current density of the above cold-rolled steel sheets is 600 to 800 μA / m 2 The corrosion current density may be 600 μA / cm 2 If the temperature is less than 800μA / cm, Fe elution does not occur smoothly, making it difficult to form a phosphate film, and the amount of film attached decreases, and the corrosion current density reaches 800μA / cm 2 If the temperature exceeds this value, the deposition reaction of the phosphate film does not occur, and the etching reaction in which only Fe dissolution occurs becomes dominant, resulting in poor phosphating properties.

[0032] The cold-rolled steel sheet according to the present invention has significantly improved phosphating properties, and thus, when the cold-rolled steel sheet is phosphated, the phosphate coverage according to the following formula 1 can be 95% or more. [Formula 1] Phosphate coverage = (area of ​​phosphate-forming regions / total area) x 100

[0033] According to another aspect of the present invention, there is provided a method for manufacturing a steel sheet, the method comprising the steps of hot rolling and cold rolling a base steel sheet, and depositing nickel or a nickel alloy on the hot-rolled and cold-rolled base steel sheet in an amount of 50 mg / m 2 There is provided a method for manufacturing a cold-rolled steel sheet, which includes the steps of forming a metal coating layer having the following properties (excluding 0), and subjecting the steel slab on which the metal coating layer has been formed to an annealing heat treatment.

[0034] The hot rolling and cold rolling of the base steel sheet may be performed according to methods and conditions commonly used in the art. For example, the method may involve heating the base steel sheet (slab) to a temperature of 1100 to 1300°C, hot rolling it at a finish rolling temperature of 800 to 1000°C, coiling it, and cold rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet. Thereafter, degreasing and water washing processes may be further performed.

[0035] Then, using an electroplating method, a coating weight of nickel or nickel alloy of 50 mg / m 2 The following (excluding 0) configurations for forming the metal coating layer have been described above, and therefore detailed explanations will be omitted here.

[0036] As described above, according to the present invention, a high-strength cold-rolled steel sheet can be manufactured by degreasing and rinsing a cold-rolled steel sheet in a pretreatment step, depositing nickel by electroplating to form a high-strength steel sheet covered with a nano-thick coating layer, and imparting ductility through an annealing heat treatment process. [Example]

[0037] The present invention will be described in more detail below with reference to specific examples. The following examples are merely illustrative examples to aid in understanding the present invention, and are not intended to limit the scope of the present invention.

[0038] Example 1 A slab with a total Mn and Si content of 3.3 wt% was manufactured to a thickness of 1.4 mm through hot rolling and cold rolling, and the surface was electroplated to a nickel coating amount of 43 mg / m 2 A coating layer having the following composition was formed.

[0039] After that, the steel sheet underwent degreasing, water washing, and surface conditioning processes, followed by immersion in a phosphoric acid-zinc solution for approximately 90 seconds to form a phosphate film. To observe whether the phosphate film had been successfully formed, three locations on the steel sheet surface were observed at approximately 500x magnification using a scanning electron microscope, and the average area fraction (coverage) of phosphate crystals formed was calculated and listed in Table 1.

[0040] On the other hand, after the heat treatment at 800°C, the depth profile was measured by GDOES (Glow Discharge Optical Emission Spectroscopy) to measure the amount of Si alloying element concentrated on the steel sheet surface, and the integrated value up to a depth of 0.01 μm is shown in Table 1.

[0041] Furthermore, to confirm whether or not a phosphate film was formed on the surface due to the promotion of phosphate nucleation by the nickel coating layer, and to compare the rate of Fe dissolution associated with the formation of the phosphate film, the linear polarization resistance was measured to calculate the corrosion current density, and the results are listed in Table 1.

[0042] Example 2 Nickel deposition amount: 5mg / m 2 The same method as in Example 1 was carried out, except that the heat treatment temperature was 804°C.

[0043] Example 3 Nickel deposition amount: 16mg / m 2 The same method as in Example 1 was carried out, except that the heat treatment temperature was 785°C.

[0044] Example 4 Nickel deposition amount: 25mg / m 2The same method as in Example 1 was carried out, except that the heat treatment temperature was 802°C.

[0045] Example 5 Nickel deposition amount: 12 mg / m 2 The same method as in Example 1 was carried out, except that the heat treatment temperature was 793°C.

[0046] Comparative Example 1 Nickel deposition amount: 152 mg / m 2 The same method as in Example 1 was carried out, except that the heat treatment temperature was 830°C.

[0047] Comparative Example 2 Nickel deposition amount: 240mg / m 2 The same method as in Example 1 was carried out, except that the heat treatment temperature was 815°C.

[0048] Comparative Example 3 Nickel deposition amount: 365mg / m 2 The same method as in Example 1 was carried out, except that the heat treatment temperature was 798°C.

[0049] Comparative Example 4 Nickel deposition amount: 504mg / m 2 The same method as in Example 1 was carried out, except that the heat treatment temperature was 806°C.

[0050] [Table 1]

[0051] Table 1 shows that Examples 1 to 5, which satisfy the nickel deposition amount, surface segregation integral, and corrosion current density conditions specified in the present invention, achieve phosphate coverage of 95% or more and exhibit excellent phosphate treatability. However, Comparative Examples 1 to 4, which have nickel deposition amounts exceeding the conditions of the present invention, and Comparative Example 5, which does not have a nickel coating layer, not only fail to suppress Si surface segregation, but also suppress Fe elution, which prevents smooth phosphate film formation and leads to deterioration of phosphate coverage and phosphate treatability.

[0052] Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and it is obvious to those skilled in the art that various modifications and variations are possible within the scope that does not deviate from the technical idea of ​​the present invention described in the claims.

Claims

1. A base steel sheet, a nickel or nickel alloy coating layer formed on the base steel sheet, The amount of nickel or nickel alloy deposited is 50 mg / m ２ A cold-rolled steel sheet having the following characteristics: The thickness of the base steel sheet is 1.0 to 1.8 mm, The corrosion current density of the cold-rolled steel sheet is 600 to 800 μA / cm ２ and The cold-rolled steel sheet has a phosphate coverage of 95% or more according to the following formula 1 when the cold-rolled steel sheet is phosphate-treated: [Formula 1] Phosphate coverage = (area of ​​phosphate-forming regions / total area) x 100

2. The cold-rolled steel sheet according to claim 1, wherein the base steel sheet contains 0.05 to 3.0 wt % of Si and 0.1 to 3.0 wt % of Mn.

3. The cold-rolled steel sheet according to claim 1, wherein the concentration of Si element from the surface to a depth of 0.01 μm of the cold-rolled steel sheet is 0.1% or less.

4. A step of hot rolling and cold rolling the slab; The hot-rolled and cold-rolled base steel sheet is coated with nickel or a nickel alloy in an amount of 50 mg / m ２ forming a metal coating layer, and heat treating the base steel sheet on which the metal coating layer is formed, The thickness of the base steel sheet is 1.0 to 1.8 mm, The corrosion current density of the cold-rolled steel sheet is 600 to 800 μA / cm ２ and The method for producing a cold-rolled steel sheet, wherein during the phosphating treatment of the cold-rolled steel sheet, a phosphate coverage according to the following formula 1 is 95% or more. [Formula 1] Phosphate coverage = (area of ​​phosphate-forming regions / total area) x 100

5. The method of claim 4, wherein the forming of the metal coating layer is performed by electroplating.

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