JP2016065319A - Evaluation method of surface quality of high strength steel sheet and manufacturing method of high strength steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet suitable for a raw material for automobile components without deteriorating surface appearance.SOLUTION: There is provided an evaluation method of a surface quality of a high strength steel sheet including determining that the surface quality of the high strength steel sheet containing contents of Mn, Ca and S satisfying the following formula (1) in the high strength steel sheet containing C:0.05 to 0.35 mass%, Si:0.50 to 3.00 mass%, Mn:0.50 to 3.00 mass%, P:0.050 mass% or less, S:0.0050 mass% or less, Al:less than 0.100 mass%, Ca:0.0010 mass% or more and the balance Fe with inevitable impurities is good. (Mn×Ca)/S≤3.00 (1), where symbols of element represent contents (mass%) of each element.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength steel sheet having excellent surface properties.

  In recent years, improvement in fuel efficiency of automobiles has been demanded from the viewpoint of global environmental conservation. In addition, from the viewpoint of occupant protection in the event of a collision, it is also required to improve the safety of the automobile body. Therefore, weight reduction and strengthening of automobile bodies are being actively promoted. In order to achieve both weight reduction and strengthening of the automobile body, it is effective to increase the strength of the parts. Recently, high-strength steel sheets have been actively used as automotive parts steel sheets.

  In addition, since many automobile parts made of steel plates are formed by press working, steel sheets for automobile parts are also required to have excellent press formability. In order to achieve excellent press formability, it is important to ensure high ductility.

  A typical example of a high-strength steel sheet having excellent ductility is a dual-phase steel sheet having a composite structure of ferrite and martensite. For example, as described in Non-Patent Document 1, a high ductility steel sheet using transformation induced plasticity (TRIP) due to transformation of induced austenite by adding appropriate amounts of Si and Mn to 0.11 mass% C is also available. It has reached the stage of practical application.

  Since many of these steel sheets contain a relatively large amount of Si in order to improve ductility, the oxide of Si is concentrated on the surface of the steel sheet during annealing, and plating and coating films are more easily peeled off than ordinary steel sheets. There was a problem. In order to improve the plating and paint peelability due to the surface concentration of such oxides of Si and Mn, for example, as described in Patent Document 1, the scale is removed using an acid and / or a brush. The method is taken.

JP 2003-201538 A

The Japan Iron and Steel Institute “Materials and Processes vol.4 (1991) P.1942”

  However, especially in steel sheets that have been subjected to strong pickling to remove scale, black streak defects are generated on the surface of hot-rolled sheets and annealed sheets, and the surface appearance is impaired. It was.

  This invention is made | formed in view of said actual condition, and it aims at providing the high strength steel plate suitable as a raw material for motor vehicle parts, without the surface appearance being impaired.

  As a result of intensive studies, the present inventors have found that by limiting the component composition of steel to a specific range, the surface appearance is not impaired and a high-strength steel sheet suitable as a material for automobile parts can be obtained.

The gist of the present invention is as follows.
[1] Component composition: C: 0.05 to 0.35 mass%, Si: 0.50 to 3.00 mass%, Mn: 0.50 to 3.00 mass%, P: 0.050 mass% or less, S: In a high-strength steel sheet containing 0.0050 mass% or less, Al: less than 0.100 mass%, Ca: 0.0010 mass% or more, and the balance being Fe and inevitable impurities,
A method for evaluating the surface properties of a high-strength steel plate, wherein the surface properties of a high-strength steel plate containing Mn, Ca and S satisfying the following formula (1) are determined to be good.
(Mn × Ca) /S≦3.00 (1)
In addition, the element symbol in Formula (1) shows content (mass%) of each element.
[2] The surface texture is characterized in that a product plate having a streak defect occurrence rate (the number of slabs in which defects have occurred relative to the number of manufactured slabs) determined to be 20% or less is determined as good [ 1] The evaluation method of the surface property of the high-strength steel sheet.
[3] In addition to the component composition of the high-strength steel plate, 0.001 to 0.300 mass% in total of one or more selected from Ti, Nb and V is further contained. The evaluation method of the surface property of the high-strength steel sheet according to [1] or [2].
[4] As component composition, C: 0.05 to 0.35 mass%, Si: 0.50 to 3.00 mass%, Mn: 0.50 to 3.00 mass%, P: 0.050 mass% or less, S: 0.0050 mass% or less, Al: less than 0.100 mass%, Ca: 0.0010 mass% or more, and the contents of Mn, Ca and S satisfy the following formula (1), with the balance being Fe and A method for producing a high-strength steel sheet, comprising subjecting a slab made of inevitable impurities to hot rolling, cold rolling, and then dipping in a mixed solution of HCl and HNO ₃ .
(Mn × Ca) /S≦3.00 (1)
In addition, the element symbol in Formula (1) shows content (mass%) of each element.
[5] The method for producing a high-strength steel sheet according to [4], wherein in the pickling treatment, the steel sheet is immersed in a mixed solution of HCl and HNO ₃ for 6 to 21 seconds.
In the present invention, the high strength steel sheet means a tensile strength of 780 MPa or more.

  ADVANTAGE OF THE INVENTION According to this invention, the surface external appearance is not impaired and the high strength steel plate suitable as a raw material for motor vehicle parts can be provided.

It is a graph which shows the relationship between MnxCa and S.

  Hereinafter, the present invention will be specifically described.

  First, the component composition that the high-strength steel sheet of the present invention should have will be specifically described.

C: 0.05-0.35 mass%
C is an element indispensable for increasing the strength of steel. However, if C is less than 0.05 mass%, a desired high strength cannot be obtained. On the other hand, when C exceeds 0.35 mass%, the weldability is deteriorated. For this reason, C is set to a range of 0.05 to 0.35 mass%. More preferably, it is 0.10 to 0.25 mass%.

Si: 0.50 to 3.00 mass%
Si is an element that not only contributes to strengthening steel by solid solution strengthening but also improves the ductility of steel. However, if Si is less than 0.50 mass%, the content effect is poor. on the other hand,
When Si exceeds 3.00 mass%, the transformation behavior is changed and ductility is deteriorated. For this reason, Si is made into the range of 0.50-3.00 mass%. More preferably, it is 1.00 to 2.00 mass%.

Mn: 0.50 to 3.00 mass%
Mn not only strengthens steel by solid solution strengthening, but also improves the hardenability of the steel and improves the mechanical properties. Such an effect is recognized when the amount of Mn is 0.50 mass% or more. On the other hand, if it exceeds 3.00 mass%, the effect reaches saturation, and an effect corresponding to the content cannot be expected, but rather the cost increases. For this reason, Mn is taken as 0.50 to 3.00 mass%. More preferably, it is 1.00 to 2.00 mass%.

P: 0.050 mass% or less P is a solid solution strengthening element, and is usually an element useful for obtaining a high-strength steel sheet. However, if it exceeds 0.050 mass%, the spot weldability is reduced. Therefore, in this invention, it is set as 0.050 mass% or less. More preferably, it is 0.020 mass% or less.

S: 0.0050 mass% or less S is a harmful element that forms MnS in steel and lowers the toughness of the steel sheet. For this reason, it is preferable to reduce the mixing of S as much as possible, but it is acceptable if it is 0.0050 mass% or less. More preferably, it is 0.0030 mass% or less.

Al: Less than 0.100 mass% Al is an element necessary for separating non-metallic inclusions, which effectively contribute as a deoxidizing agent in the steelmaking stage and reduce hole expandability, into the slag. However, the content of 0.100 mass% or more increases the alloy cost. Therefore, in this invention, it is set as less than 0.100 mass%. More preferably, it is 0.020-0.090 mass%.

Ca: 0.0010 mass% or more Ca is an element necessary for producing the high-strength steel sheet of the present invention, and is contained in 0.0010 mass% or more.

  On the other hand, Ca contributes to the generation of sulfide inclusions, and Ca is an element that easily forms CaS by combining with S, and also easily forms oxysulfides and oxides in a situation where oxygen easily coexists. is there. Therefore, some control is required for Ca. Therefore, the present inventors have conducted intensive studies on the causes of black streak defects on the surface of hot-rolled sheets and annealed sheets in steel sheets subjected to strong pickling with strong hydrochloric acid or nitric acid for scale removal. It was confirmed that black streak-like defects are likely to occur when pickling is performed on a steel sheet to which a relatively large amount of Si is added. And when this defect was analyzed in detail, many inclusions (precipitates) were generated on the surface of the defective steel plate, and most of the precipitates were oxides or sulfides or oxysulfides containing Ca and Mn. It was found that these precipitates are the cause of defects due to the formation of these precipitates on the surface layer of the steel sheet. Further, although MnS is usually generated as a sulfide in steel, CaS is a compound that is thermodynamically stable as compared with MnS, so CaS is generated only by mixing a small amount. Therefore, it is considered that Ca must be reduced as much as possible.

  As a result of conducting particle analysis of precipitates using SEM, it was found that the ratio of CaS and MnS (CaS / MnS) was changed and correlated with the analysis results of defects. Therefore, the present inventors thought that this defect could be improved epoch-makingly by reducing the production of CaS. Focusing on MnS / CaS, the production of CaS was considered to be affected by the contents of Mn and S, and the relationship between the amount of Ca present in steel and the amounts of Mn and S was examined.

First, molten steel having the component composition shown in Table 1 was melted in a converter and made into a slab by continuous casting. The obtained slab was hot-rolled to a thickness of 3.0 mm, then pickled, and then cold-rolled to obtain a cold-rolled steel plate having a thickness of 1.6 mm. Next, these cold-rolled steel sheets are heated and held in a continuous annealing line, cooled, and after cooling, an acid immersed in a (0.6% HCl + 26% HNO ₃ ) mixed solution at 53 ° C. for 6 to 21 seconds. Washing was performed, then water washing and drying, and then subjected to temper rolling with a rolling reduction of 0.5% to obtain a steel sheet. The steel sheet was visually inspected for the presence of streak defects. Here, when the incidence of streak defects on the product plate (the number of slabs with defects generated relative to the number of manufactured slabs) is 20% or less, the surface quality is good, and conversely, the surface quality is 20% or more belongs to. The results are shown in FIG.

  From the results of FIG. 1, the surface properties are satisfied by satisfying the above component composition and satisfying (Mn × Ca) /S≦3.00 with respect to the element content (mass%) of Mn, Ca, and S. It turns out that it is favorable. On the other hand, in (Mn × Ca) / S> 3.00, it can be seen that the surface properties are poor. This is considered to be because generation of CaS can be reduced by generation of MnS and black streak-like defects can be suppressed. From the results in FIG. 1, the production of CaS can be controlled by appropriately controlling the amounts of Mn and Ca.

From the above, in the present invention, the above-described component composition is satisfied, and the contents of Mn, Ca, and S
(Mn × Ca) /S≦3.00 (1)
(The element symbols in the formula (1) indicate the content (mass%) of each element.)
The production of CaS can be suppressed by controlling in a range that satisfies the above. And the surface property of a high-strength steel plate can be kept favorable by controlling so that the said component composition may be satisfied and Formula (1) may be satisfied.

  In the present invention, in addition to the above component composition, one or two elements selected from Nb, Ti and V can be contained in the following ranges.

Total content of one or more selected from Nb, Ti and V: 0.001 to 0.300 mass%
All of Ti, Nb and V precipitate carbide (Ti, Nb, V) C, suppress the growth of the ferrite phase during the heating stage during continuous annealing, refine the steel structure, and increase strength and hole expandability. In addition to being an element useful for remarkably improving the content, the solid solution C that affects the TRIP effect is changed. In order to exhibit such an effect, the content is at least 0.001 mass%. On the other hand, if it exceeds 0.300 mass%, the tensile strength YS increases due to precipitation strengthening, and not only the workability decreases, but also there is a disadvantage that the retained austenite for expressing the TRIP effect is reduced. Therefore, in the present invention, the total content of Ti, Nb, and V is preferably 0.001 to 0.300 mass%, regardless of whether they are contained alone or in combination. More preferably, it is 0.001 to 0.100 mass%.

  The balance is Fe and inevitable impurities.

  Below, the manufacturing method of the high strength steel plate of this invention is demonstrated. Molten steel having the above component composition is melted in a generally known converter or electric furnace, and is made into a slab by continuous casting. Next, the obtained slab is heated and hot-rolled to obtain a hot-rolled sheet. The slab heating temperature for hot rolling is preferably set to a temperature range of 1100 to 1250 ° C. from the viewpoint of material stability. Moreover, it is preferable that the finishing temperature of hot rolling shall be 950-1000 degreeC from the point of sulfide suppression. The hot-rolled sheet can be subjected to hot-rolled sheet annealing as necessary for the purpose of improving workability in a subsequent process. (When performing hot-rolled sheet annealing, it is preferable to perform box annealing at 600 to 800 ° C. for 5 hours or more, or to perform continuous annealing for a short time in the temperature range of 600 to 800 ° C.) The hot-rolled sheet can be descaled to obtain a product as it is, or a cold-rolled material. The hot-rolled sheet of the material for cold rolling is subjected to cold rolling at a cold rolling reduction ratio of 40% or more to obtain a cold-rolled sheet. The cold rolling reduction ratio is preferably 55 to 60%. Moreover, in order to provide further workability of the cold-rolled sheet, recrystallization annealing can be performed at 400 ° C. or higher, preferably 500 to 600 ° C. Moreover, you may perform cold rolling-annealing twice or more. Furthermore, when glossiness is required, a skin pass or the like may be applied. In addition, it is preferable to perform a pickling process after hot rolling or after annealing. The scale formed on the surface in the pickling process is removed, and then cold rolled. In addition, although it does not specifically limit as pickling conditions, Strict pickling conditions, such as sulfuric acid hydrochloric acid washing, may be sufficient. Further, the steel sheet after cold rolling can be subjected to temper rolling for the purpose of shape correction, adjustment of surface roughness, and the like.

Molten steel having the composition shown in Table 2 was melted in a converter and made into a slab by continuous casting. The obtained slab was hot-rolled to a thickness of 3.0 mm, then pickled, and then cold-rolled to obtain a cold-rolled steel plate having a thickness of 1.6 mm. Next, these cold-rolled steel sheets are heated and held in a continuous annealing line, cooled, and after cooling, an acid immersed in a (0.6% HCl + 26% HNO ₃ ) mixed solution at 53 ° C. for 6 to 21 seconds. Washing treatment was performed, followed by washing with water and drying, followed by temper rolling with a rolling reduction of 0.5%.

The obtained steel sheet was examined for surface appearance and mechanical properties. The measurement method and evaluation criteria are as follows.
<Surface appearance>
The surface appearance is judged visually, and the rate of occurrence of streak defects on the product plate surface is the rate of occurrence of streak defects on the product plate (number of slabs with defects relative to the number of manufactured slabs). Asked. A defect occurrence rate of 20% or less was judged as good appearance, and a defect occurrence rate of 20% or more was judged as poor appearance.
<Mechanical properties>
Tensile strength, yield strength, and ductility were evaluated by conducting a tensile test. The tensile test was performed according to JIS Z 2241 using a JIS No. 5 test piece, and the tensile strength TS (MPa), the yield strength YP (MPa), and the total elongation EL (%) were measured. The tensile strength was 780 MPa or more. A yield strength of 380 MPa or more was considered acceptable. For the ductility, a total elongation of 30% or more was regarded as acceptable.

  The results are shown in Table 3.

  From the results of Table 3, all of the inventive examples have good surface appearance and excellent mechanical properties. On the other hand, the comparative example could not obtain a satisfactory surface appearance and mechanical properties. In addition, as another strong pickling condition, the case where the pickling treatment was performed by immersing in a 5% HCl solution at 55 ° C. for 17 seconds was also examined. As a result, the present invention example passed both the surface appearance and the mechanical properties. Satisfies the criteria.

Claims (5)

As component composition, C: 0.05 to 0.35 mass%, Si: 0.50 to 3.00 mass%, Mn: 0.50 to 3.00 mass%, P: 0.050 mass% or less, S: 0.0050 mass % Or less, Al: less than 0.100 mass%, Ca: 0.0010 mass% or more, with the balance being high strength steel plate made of Fe and inevitable impurities,
A method for evaluating the surface properties of a high-strength steel plate, wherein the surface properties of a high-strength steel plate containing Mn, Ca and S satisfying the following formula (1) are determined to be good.
(Mn × Ca) /S≦3.00 (1)
In addition, the element symbol in Formula (1) shows content (mass%) of each element.   The surface texture is determined to be good when the rate of occurrence of streak defects on the product plate (the number of slabs with defects relative to the number of manufactured slabs) is 20% or less. The evaluation method of the surface property of the high strength steel plate of description.   2. In addition to the component composition of the high-strength steel sheet, further, 0.001 to 0.300 mass% in total of one or more selected from Ti, Nb and V is contained. Or the evaluation method of the surface property of the high strength steel plate of 2. As component composition, C: 0.05 to 0.35 mass%, Si: 0.50 to 3.00 mass%, Mn: 0.50 to 3.00 mass%, P: 0.050 mass% or less, S: 0.0050 mass % Or less, Al: less than 0.100 mass%, Ca: 0.0010 mass% or more, and the contents of Mn, Ca and S satisfy the following formula (1), the balance being Fe and inevitable impurities A method for producing a high-strength steel sheet, comprising subjecting a slab made of the above to hot rolling, cold rolling, and then dipping in a mixed solution of HCl and HNO ₃ .
(Mn × Ca) /S≦3.00 (1)
In addition, the element symbol in Formula (1) shows content (mass%) of each element. 5. The method for producing a high-strength steel sheet according to claim 4, wherein in the pickling treatment, the steel sheet is immersed in a mixed solution of HCl and HNO ₃ for 6 to 21 seconds.

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