JPS639579B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a cold-rolled steel sheet with good surface properties, press formability, and high strength. In recent years, various high-tensile strength cold-rolled steel sheets have been developed and put into practical use for the purpose of reducing the weight of passenger cars. However, as a steel plate that can be used in the body of a passenger car to reduce the weight of the vehicle while exhibiting the same performance as conventional mild steel plates, it has the following characteristics: (1) The manufacturing cost is as low as that of mild steel plates. (2) Press workability is as good as mild steel plate. (3) Surface quality should be as good as mild steel plate. (4) The strength of the steel plate used as a panel is high. Although it is necessary to have the above four properties, the steel sheets currently in practical use do not necessarily have all of these properties. That is, for example, as a steel sheet with good press workability, so-called IF steel (Interstitial Free
Steel) is being developed. This IF steel uses carbonitride-forming elements such as
When using Ti, it is added in an amount that satisfies the following: Ti (%)≧4 {C (%) + 12/14N (%)}.
Note that the following percentages are by weight. However, this IF steel has problems such as low C, which reduces its strength and makes the steel sheet soft, which conflicts with the recent demand for high-strength steel sheets. By the way, it is generally known that the strength of steel sheets can be increased at low cost by including an appropriate amount of P in steel, but if such a method is applied to cold-rolled steel sheets, Non-uniform pickling occurs during pickling, which seriously impairs the surface quality of cold rolled steel products, and the deep drawability of cold rolled steel products deteriorates, making it impossible to obtain favorable results. This became clear through studies conducted by the present inventors. Through further research by the present inventors,
For improving surface quality and deep drawability of P-containing cold-rolled steel sheets
It turned out that adding Cu was effective. That is, by containing a specific amount of Cu,
During pickling, Cu precipitates in the area before localized overpickling occurs to prevent further overpickling, resulting in uniform pickling and the steel plate after cold rolling. In addition to significantly improving surface properties, Cu facilitates grain growth during annealing, thereby increasing the r value. This invention was made based on the above knowledge, and includes a cold-rolled steel sheet containing C: 0.001 to 0.0020%, Si: 1.0% or less, Mn: 0.01 to 1.00%, P: 0.04 to 0.20%, and Cu. : 0.05 to 0.18%, sol.Al: 0.10% or less, N: 0.008% or less, and also contains one or more of the following: Ti: 0.20% or less, Nb: 0.20% or less, Zr: 0.20% or less , and Ti equivalent = Ti (%) + 48/93Nb (%) + 48/91Zr (%)...
C equivalent = C (%) + 12/14 N (%) ... (C equivalent) - 1/4 (Ti equivalent) ≦ 0.0010 (%) ... Ti equivalent calculated by the above formula and C equivalent calculated by the above formula The composition satisfies the above formula and further contains one or more of the following, if necessary: Mo: 0.03 to 0.25%, REM: 0.03 to 0.20%, B: 0.0003 to 0.0050%. This structure improves press formability and also ensures excellent strength. Next, the reason why the composition of the cold rolled steel sheet of the present invention is limited as described above will be explained. C The C component has the effect of forming fine carbides and improving the strength of the steel sheet, but if its content is less than 0.001%, the desired effect will not be obtained and the steel plate will become too soft and the melting process will be affected. On the other hand, if the content exceeds 0.020%,
Not only is a large amount of carbonitride-forming elements required, but also the amount of precipitated carbonitrides becomes too large, deteriorating press workability, so the content was set at 0.001 to 0.020%. Si In addition to deoxidizing the steel, the Si component also has the effect of ensuring the strength of the steel plate, but if it exceeds 1.0%,
The content was set to 1.0% or less because it deteriorates the shine of the steel plate and also causes coloration on the surface of the steel plate due to scale adhesion during annealing. Mn The Mn component has the effect of improving the toughness of steel sheets, but if the content is less than 0.01%, hot brittleness will occur, while if the content exceeds 1.00%, melting will be difficult. Since the content becomes 0.01 and causes cost increase, the content is reduced to 0.01
~1.00%. PP The P component is an unavoidable alloying element for increasing the strength of steel at low cost, but if it is contained in an amount exceeding 0.20%, it significantly impairs the ductility of the steel sheet. on the other hand
If it is less than 0.04%, the curing ability is too small, so the content is set to 0.04 to 0.20%. Cu Cu is an alloying element that is the gist of the present invention.
Cu has the effect of preventing the deterioration of surface properties caused by the local overacid phenomenon that occurs on the steel plate surface during pickling due to the addition of a large amount of P, and facilitates the growth of crystal grains during annealing, increasing the r value. It has an effect. If its content is less than 0.05%, the desired effect cannot be obtained, while if it exceeds 0.18%,
During heating during hot rolling, Cu has a low melting point.
However, since it crystallizes at the grain boundaries of austenite and makes the steel brittle, causing hexagonal cracking during hot rolling, its content is set to 0.05 to 0.18%. sol.Al sol.Al is contained to improve the yield of carbide-forming elements by sufficiently deoxidizing, but sol.Al
If the content exceeds 0.10%, no further deoxidizing effect can be obtained and the cost will increase, so the upper limit was set at 0.10%. NN is preferable because the smaller the amount of N, the smaller the added content of carbonitride-forming elements. If the content exceeds 0.0080%, a decrease in r value is unavoidable even if carbonitride-forming elements are included, so the content was set to 0.0080% or less. Ti, Nb, and Zr As mentioned above, these components have the uniform effect of forming fine carbonitrides to improve press formability and strength, but each of these components has a Ti: 0.20
%, Nb: 0.20%, and Zr: 0.20%, no further improvement effect can be seen due to the above action and the cost will be high. :0.20, and
Zr: 0.20%. In addition, the above formula ~ represents the solid solution (C+N)
0.0010 (%) or less, and shows a relational expression for precipitating the remaining C+N as carbonitride. In addition, the upper limit of (C equivalent) - 1/4 (Ti equivalent)
The reason why it is set to 0.0010 (%) is that if this upper limit is exceeded, the amount of solid solution (C+N) increases and the press formability of the steel sheet deteriorates. Mo, REM, and B These components have a uniform effect of improving the secondary processing brittleness of steel sheets, so they are included as necessary, but Mo: less than 0.03%,
Contents of REM: less than 0.03% and B: less than 0.0003% do not provide the desired effect, while Mo: 0.25%, REM: 0.20% and B: respectively.
If the content exceeds 0.0050%, the weldability of the steel plate will deteriorate, so the respective contents should be set to Mo: 0.03 to 0.25% and REM: 0.03 to 0.20.
% and B: 0.0003 to 0.0050%. After continuous casting of steel having the chemical composition as described above, hot-rolled steel sheets were made from the slabs as they were or reheated, and then products were manufactured using normal cold-rolled steel sheet manufacturing processes and conditions. As a result, a high-strength cold-rolled steel sheet with good surface properties and excellent press formability was obtained. Next, the present invention will be explained using Examples and in comparison with Comparative Examples. Example 1 Steel having the composition shown in Table 1 was melted and made into a slab by continuous casting, which was then heated to 1150°C and hot rolled at a finishing temperature of 880°C and a coiling temperature of 600°C. A hot-rolled steel plate with a thickness of 4.0 m/m was used. Subsequently, the hot rolled sheet is heated to a concentration of 50 to 10%.
Immerse in hydrochloric acid heated to 90℃ and reduce the reduction rate after pickling.
Cold rolled to a thickness of 0.8m/m at 80%, then continuous annealing at 830°C for 1 minute or box annealing at 700°C for 5 hours.
Furthermore, after temper rolling with an elongation rate of 0.8%, a JIS No. 5 tensile test was conducted. Note that the Ti equivalent and C equivalent of Steels 1 and 2 in Table 1 both satisfy the above formula. The results are shown in Table 2. The surface condition was evaluated based on the appearance of the steel plate after annealing. As is clear from Table 2, steel sheets 1 and 2 of the present invention both have (C equivalent) - 1/4 (Ti equivalent) and

【table】

[Table] Steel plate 3 has a high strength and good r-value because its contents such as copper and copper are within the range of the present invention, and the surface quality is also good. The surface of the steel plate after annealing has minute irregularities (a type of rash) and the appearance is poor. Example 2 Si: 0.01%, Mn: 0.18%, P: 0.080%, Cu:
Contains 0.10%, S: 0.006%, sol.Al: 0.21% as basic components, C 0.001 to 0.020%, N 0.0020%.
~0.0050%, Ti varying within the range of 0.04 to 0.10%, and the rest made into slabs by continuous casting. Slab heating temperature: 1200℃, finishing temperature: 740℃, winding temperature: 400
A hot-rolled steel sheet with a thickness of 4.0 m/m was obtained by hot rolling at ℃, and then cold-rolled at a reduction rate of 80% to obtain a cold-rolled steel sheet with a thickness of 0.8 m/m. Subsequently, continuous annealing was performed at a temperature of 800° C. for 90 seconds to produce cold rolled steel sheets with various Ti equivalents and C equivalents. The r value of this cold-rolled steel sheet was determined, and this result was calculated as the first
Shown in the figure. As shown in Figure 1, the value of the above formula is 0.0015%
It is clear that the following cases exhibit high r values. Example 3 Steels having the respective compositions shown in Table 3 were melted and made into slabs by continuous casting. Each slab was heated to a slab temperature of 1200°C, and then a finishing temperature of approximately 800°C. and coiling temperature: After hot rolling at approximately 600℃ to obtain a hot-rolled sheet with a thickness of 4.0m/m, pickling with hydrochloric acid was performed, followed by rolling reduction:
Invention steel plates 1 to 17 and comparison steel plates 18 to 22, each having a thickness of 0.8 m/m, were obtained by cold rolling at 80% and continuous annealing at a temperature of 800°C for 90 seconds. cold-rolled steel sheets were manufactured. In addition, all of the cold-rolled steel sheets of Comparative Steel Sheets 18 to 22 have compositions outside the range of the present invention.

【table】

[Table] (Applicable items are marked with * in Table 3.) Next, the steel sheets 1 to 1 of the present invention obtained from this result are
Table 3 shows the results of measuring the tensile properties and r values of cold-rolled steel sheets No. 17 and Comparative Steel Sheets 18 to 22, as well as the results of investigating the surface properties of the annealed sheets. As shown in Table 3, the cold rolled steel sheets of the present invention steel sheets 1 to 15 have good surface properties, high strength and high r value, that is, good press formability, whereas the comparative steel sheets 18~19 cold rolled steel plate is Cu
Since the amount is outside the range of the present invention, a kind of rash appears on the surface of the annealed plate, resulting in poor surface quality and poor r value. In addition, since the cold-rolled steel sheet of comparative steel sheet 20 does not contain carbonitride-forming elements, (C equivalent) - 1/4 (Ti equivalent)
Since P is outside the range of the present invention, the r value is extremely low, and furthermore, the cold rolled steel sheet of comparative steel plate 21 has low strength because P is outside the lower limit of the range of the present invention. In addition, in the cold-rolled steel sheet of comparative steel sheet 22, since the amount of Cu was outside the upper limit of the range of the present invention, hexagonal cracking occurred from the stage of hot-rolling, and a pattern remained on the surface of the steel sheet after cold-rolling and annealing. It's bad. As mentioned above, the cold-rolled steel sheet according to the present invention has good press formability in terms of surface properties and high strength.If this cold-rolled steel sheet is used in the bodies of automobiles and other vehicles, it will be possible to reduce the weight and will be used in industrial applications. Moreover, it brings about useful effects.

[Brief explanation of the drawing]

Figure 1 shows the C equivalent of the steel plate -1/4 (Ti equivalent) and r
It is a curve diagram showing the relationship with the value.

Claims (1)

[Claims] 1 C: 0.001 to 0.020%, Si: 1.0% or less, Mn: 0.01 to 1.00%, P: 0.04 to 0.20%, Cu: 0.05 to 0.18%, Sol.Al: 0.10% or less, N : 0.008% or less, contains one or more of the following: Ti: 0.20% or less, Nb: 0.20% or less, Zr: 0.20% or less, and Ti equivalent = Ti (%) + 48/93Nb ( %) +48/91Zr(%)...
C equivalent = C (%) + 12/14 N (%) ... (C equivalent) - 1/4 (Ti equivalent) 0.0010 (%) ... Ti equivalent calculated by the above formula and C equivalent calculated by the above formula A high-strength cold-rolled steel sheet with good surface quality, the relationship of which satisfies the above formula, and has a composition (the above weight %) consisting of: Fe + unavoidable impurities: the remainder. 2 C: 0.001-0.020%, Si: 1.0% or less, Mn: 0.01-1.00%, P: 0.04-0.20%, Cu: 0.05-0.18%, sol.Al: 0.10% or less, N: 0.008% or less, Contains one or more of the following: Mo: 0.03 to 0.25%, REM: 0.03 to 0.20%, B: 0.0003 to 0.0050%, and furthermore, Ti: 0.20% or less, Nb: 0.20% or less, Zr: 0.20% or less, also contains one or more of the following, and Ti equivalent = Ti (%) + 48/93Nb (%) + 48/91Zr (%)...
C equivalent = C (%) + 12/14 N (%) ... (C equivalent) - 1/4 (Ti equivalent) 0.0010 (%) ... Ti equivalent calculated by the above formula and C equivalent calculated by the above formula A high-strength cold-rolled steel sheet with good surface quality, the relationship of which satisfies the above formula, and has a composition (the above weight %) consisting of: Fe + unavoidable impurities: the remainder.