JPH11229039A - Production of hot dip galvannealed steel sheet excellent in surface appearance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a hot dip galvanized steel sheet capable of preventing bamboo-leaf patterns generated, when a Ti-added extra- low carbon steel sheet having high workability is used as a parent metal. SOLUTION: In producing a hot dip galvannealed steel sheet by subjecting a steel contg., by weight, <=0.003% C, <=0.1% Si, <=0.5% Mn, <=0.03% P, <=0.03% S, <=0.1% Al, <=0.03% N and 0.02 to 0.15% Ti to continuous casting, executing hot rolling, thereafter subjecting it to cold rolling, furthermore executing annealing and subsequently subjecting it to hot dip galvannealing treatment, the slab thickness by the continuous casting is regulated to >=230 mm, the slab reheating temp. in the hot rolling is regulated to <=1150 deg.C, the rough rolling ratio is regulated to >=70%, the rough rolling finishing surface temp. is regulated to >=900 deg.C, and in the case the slab heating temp. is defined as A ( deg.C), the rough rolling finishing temp. is defined as B (±C), the draft in the rough rolling is defined as R (%), and the time till the completion of the rough rolling after extraction from a heating furnace is defined as T (sec), S=(A+B)/(T×(R)<0.17 is satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet using Ti-added ultra-low carbon steel as a base material, which is used for automobiles, electric appliances and the like, and particularly to an alloyed hot-dip galvanized steel sheet having excellent surface appearance. The present invention relates to a method for manufacturing a galvanized steel sheet.

[0002]

2. Description of the Related Art Steel sheets used for automobile outer panels are required to have high workability and high corrosion resistance, and therefore, a titanium-added ultra-low carbon steel coated with hot-dip zinc is used. In this case, a bamboo leaf-like pattern is generated on the surface, and the surface quality is deteriorated. This is because many regions having a fine grain size are generated on the surface of the cold-rolled annealed sheet of Ti-added ultra low carbon steel, and in this region having many grain boundaries, the alloying reaction starting from the grain boundaries is promoted more than other portions. This is because unevenness occurs in alloying and irregularities occur on the plating surface.

[0003] Techniques for preventing this bamboo leaf pattern include:
It is disclosed in Japanese Patent Application Laid-Open No. 2-38550. According to this publication, ultra-high-temperature annealing at 860 ° C. or more is performed by assuming that the formation of fine TiN is locally caused by solid solution of N on the slab surface from the atmosphere during slab heating. It is stated that the bamboo leaf pattern can be eliminated by making the shortest diameter of the crystal grains of the surface layer 8 μm or more by remarkably promoting the growth. However, in this method, although the bamboo leaf pattern disappears, the crystal grain size outside the fine grain region becomes coarse, and roughening occurs after molding,
The surface properties after molding deteriorate.

Japanese Patent Application Laid-Open No. 7-228944 discloses a method in which the optimum slab heating temperature is specified in accordance with the amount of Ti, and TiN deposited finely in the surface layer is coarsened to reduce the leaf pattern of bamboo grass. However, the effect varies, and the actual situation is that it is not always possible to completely prevent the bamboo leaf pattern.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to prevent a bamboo leaf pattern generated when a highly workable Ti-added ultra-low carbon steel sheet is used as a base material. It is an object of the present invention to provide a method for producing a hot-dip galvanized steel sheet having excellent surface properties.

[0006]

The cause of the bamboo leaf pattern of the hot-dip galvanized steel sheet is that the crystal grain size on the surface is non-uniform.
Are unevenly formed in the surface layer of the slab to form fine TiN, which is generated by inhibiting grain growth during cold rolling annealing. In order to prevent the generation of TiN, it is considered effective to reduce the slab heating temperature. However, even if the slab heating temperature is reduced within an industrially feasible range, the penetration of N into the slab surface layer is completely prevented. Can not do.

The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, it has been found that reducing the slab heating temperature minimizes the intrusion of N into the slab surface layer, as well as the slab thickness and the rough rolling ratio. It has been found that by adjusting the surface temperature and the rough rolling time at the end of the rough rolling, it is possible to sufficiently scale off the N intrusion layer at the rough bar stage. Furthermore, after rough rolling, it was found that the effect was promoted by continuously performing leveling, rough bar heating, and descaling by a leveler.

[0008] The present invention has been made based on these findings, and in terms of% by weight, C ≦ 0.003% and Si ≦
0.1%, Mn ≦ 0.5%, P ≦ 0.03%, S ≦ 0.
03%, Al ≦ 0.1%, N ≦ 0.003%, 0.02
% ≦ Ti ≦ 0.15% steel is continuously cast, hot-rolled, cold-rolled, further annealed, and then subjected to alloying hot-dip galvanizing to produce a hot-dip galvanized steel sheet. The slab thickness by continuous casting
mm or more, the slab reheating temperature of hot rolling is 1150 ° C or less,
Coarse rolling rate 70% or more, rough rolling end surface temperature 900 ° C. or more, slab heating temperature A (° C.), rough rolling end temperature B (° C.), rough rolling reduction R (%), heating furnace extraction When the time until the end of the post-rough rolling is T (sec), the parameter S defined by the following equation satisfies the following expression: S = (A + B) / (T × R) <0.17 It is intended to provide a method for producing a galvannealed steel sheet having excellent heat resistance.

Further, the present invention relates to a method for producing a rubber composition, comprising:
3%, Si ≦ 0.1%, Mn ≦ 0.5%, P ≦ 0.03
%, S ≦ 0.03%, Al ≦ 0.1%, N ≦ 0.003
%, 0.02% ≦ Ti ≦ 0.15% steel is continuously cast, hot rolled, cold rolled, further annealed, and then alloyed by hot dip galvanizing. In producing a hot-dip galvanized steel sheet, the slab thickness by continuous casting is 230 mm or more, the slab reheating temperature of hot rolling is 1150 ° C or less, the rough rolling ratio is 70% or more, and the rough rolling end surface temperature is 900 ° C or more. The present invention provides a method for producing an alloyed hot-dip galvanized steel sheet having excellent surface appearance, characterized in that straightening by a leveler, heating at a temperature of 30 ° C. or more by coarse bar heating, and descaling are continuously performed before finishing rolling after rolling. is there.

[0010] Further, the present invention provides the method according to any of the above, wherein the steel further comprises 0.0020% by weight.
An object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet having excellent surface appearance, characterized by containing one or two kinds of the following B and 0.04% or less of Nb.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. First, the steel composition will be described. The base steel sheet used in the present invention is a Ti-added ultra-low carbon steel. Specifically, in terms of% by weight, C ≦ 0.003%, Si ≦ 0.1%, M
n ≦ 0.5%, P ≦ 0.03%, S ≦ 0.03%, Al
≦ 0.1%, N ≦ 0.003%, 0.02% ≦ Ti ≦
It contains 0.15%. Also, in weight%, 0.0
It may contain one or two of 020% or less of B and 0.04% or less of Nb.

(1) C, NC, and N are fixed as precipitates by Ti. However, if the amount of precipitates is large, workability is deteriorated. Therefore, in the present invention, the upper limits of both C and N are set to 0.003%.

(2) Si, Mn, PSi, Mn, and P are desirably reduced as much as possible because they form a solid solution to strengthen the steel and deteriorate workability. From such a viewpoint, the present invention sets the upper limit of Si to 0.1.
1%, the upper limit of Mn is 0.5%, and the upper limit of P is 0.03%.

(3) SS S is fixed as a precipitate by Ti, like C, but if the precipitate is large, the workability is reduced. Therefore, in the present invention, the upper limit of S is set to 0.03%.

(4) Al Since Al is a deoxidizer, it is contained to some extent,
If contained in a large amount, it hardens, so the upper limit is made 0.1%.

(5) Ti Ti fixes C, N and S as precipitates and improves workability by improving EL and r values. From such a viewpoint, in the present invention, the content of Ti is set to 0.02% or more as an amount sufficient to fix C, N, and S. However, if the content exceeds 0.15%, a large amount of solid-solution Ti is generated and the steel is hardened.
15%.

(6) Nb Nb, like Ti, fixes C, N, and S as precipitates and improves the workability. Therefore, Nb is added as necessary for the purpose of further improving the workability. be able to. However, if added in excess of 0.04%, the effect is saturated, and a large amount of solute Nb is generated to harden the steel and increase the recrystallization temperature. %.

(7) B In the Ti-added ultra-low carbon steel sheet, C is fixed as a precipitate and does not exist in a solid solution state, so that secondary working embrittlement occurs. On the other hand, B is added to increase the grain boundary strength and prevent such secondary working embrittlement, so that it can be added as necessary. However, if the amount exceeds 0.0020%, the r value is lowered or hardened by solid solution B, so the upper limit is made 0.0020%.

Next, the manufacturing conditions will be described. In the present invention, after continuously casting steel having the above composition and hot rolling,
Cold-rolled, further annealed, and then subjected to hot-dip galvanizing treatment and alloying treatment to produce an alloyed hot-dip galvanized steel sheet, the slab thickness by continuous casting is 230 mm or more, hot-rolled slab reheating Temperature 1150
℃ or less, rough rolling rate 70% or more, rough rolling end surface temperature 9
The slab heating temperature is A (° C.), the rough rolling end temperature is B (° C.), the rough rolling reduction R (%), and the time from the heating furnace extraction to the rough rolling end is T (sec). When
The parameter S = (A + B) / (T × R) <0.17 is satisfied. Also, instead of defining this parameter S,
After rough rolling and before finish rolling, straightening by a leveler, heating at 30 ° C. or more by rough bar heating, and descaling are performed successively.

(1) Thickness of slab If the slab thickness is less than 230 mm, the temperature of the surface layer drops remarkably, and the surface reheating before rough rolling and after descaling during rough rolling is not promoted. Therefore, the scale-off amount is reduced, and the N invasion layer cannot be sufficiently removed. Therefore, the lower limit of the slab thickness is set to 230 mm.

(2) Coarse Rolling Ratio If the rough rolling ratio is lower than 70%, the thickness of the N infiltration layer after the rough rolling does not become thin, so that it can be sufficiently removed by scale-off with a coarse bar. Therefore, it is not possible to suppress the occurrence of leaf pattern of bamboo grass. Therefore, the lower limit of the rough rolling ratio is set to 70%.

(3) Slab heating temperature If the slab heating temperature exceeds 1150 ° C., the penetration depth of N into the slab surface layer becomes deep, and it cannot be sufficiently removed by scale-off with a coarse bar. The occurrence of bamboo leaf pattern cannot be suppressed under any conditions.

This will be described with experimental results. Almost C: 0.002%, Si: 0.01%, Mn: 0.2% by weight.
%, P: 0.015%, S: 0.01%, Al: 0.0
45%, N: 0.002%, Ti: 0.08%,
Steel whose balance is Fe and unavoidable impurities has a thickness of 25
It was cast into a 0 mm slab and hot rolled. The slab heating temperature was changed with the time from the heating furnace extraction to the end of the rough rolling being 170 seconds, the rough rolling outlet temperature kept constant at 950 ° C., and the rough bar thickness being 40 mm. After that, finish rolling is performed, and 62
Winding was performed at 0 ° C. to prepare a 3.2 mm-thick hot-rolled sheet. Cold rolling to 0.8 mm after pickling, annealing at 850 ° C,
Alloyed hot-dip galvanizing was performed. The presence or absence of a bamboo leaf pattern on the surface was visually evaluated. Table 1 shows the results.

[0024]

[Table 1]

As shown in Table 1, the heating temperature was 1150 ° C.
Is exceeded, a bamboo leaf pattern is generated regardless of the value of the parameter S. Therefore, the upper limit of the slab heating temperature is set to 1
The temperature is set to 150 ° C or lower.

(4) Termination Temperature of Rough Rolling In the present invention, the termination temperature of the rough rolling must be a temperature at which the scale is sufficiently formed in order to remove the N-infiltration layer on the surface. From such a viewpoint, the lower limit of the rough rolling end temperature is set to 900 ° C. or more.

(5) Parameter S <0.17 Parameter S is the slab heating temperature A (° C.), the rough rolling end temperature B (° C.), the rolling reduction R (%) of the rough rolling, When the time until the end of rolling is T (sec),
S = (A + B) / (T × R)
<0.17 is specified.

Since this equation has been obtained empirically, this equation will be explained based on experimental results. Almost C: 0.002%, Si: 0.01%, Mn: 0.2% by weight.
%, P: 0.015%, S: 0.01%, Al: 0.0
45%, N: 0.002%, Ti: 0.08%,
Steel whose balance is Fe and unavoidable impurities has a thickness of 25
It was cast into a 0 mm slab, heated at a slab heating temperature of 1110 ° C., and then subjected to hot rolling. The time from the extraction in the heating furnace to the end of the rough rolling was 150 seconds, and the outlet temperature of the rough rolling was constant at 970 ° C., and the thickness of the rough bar was changed. Thereafter, finish rolling was performed, and the resultant was rolled at 620 ° C. to prepare a hot-rolled sheet having a thickness of 2.8 mm. Cold rolling to 0.8 mm after pickling, 850
After annealing at ℃, galvannealing was performed. The presence or absence of a bamboo leaf pattern on the surface was visually evaluated. Table 2 shows the results.

[0029]

[Table 2]

As shown in Table 2, when the value of the parameter S is smaller than 0.17, it is understood that the bamboo leaf pattern does not occur. Therefore, in the present invention, the parameter S is set as S <
It is specified as 0.17.

Further, in the present invention, the bamboo leaf pattern can be obtained by performing leveler straightening, coarse bar heating, and descaling after rough rolling, as shown below, regardless of the parameters shown in (5). Can be suppressed.

(6) Straightening by leveler, coarse bar heating,
Descaling In the present invention, the main point is to remove the N intrusion layer on the surface of the rough bar whose thickness has been reduced by rough rolling. for that reason,
After removing scale by coarse bar straightening by leveler,
The generation of scale is promoted by rough bar heating, and the occurrence of bamboo leaf pattern can be efficiently suppressed by combining descaling. Although scale-off is possible by de-scaling instead of a leveler, the coarse bar surface temperature decreases, and the coarse bar is thin, so the recuperation is not enough, and the scale growth is suppressed, which is not preferable. . In order to obtain a remarkable effect by the rough bar heating, the temperature rise by the rough bar heating is set to 30 ° C. or more. The rough bar heating method may be induction heating, gas heating, heating by a furnace such as a tunnel furnace, or heating by winding up a coil box, but induction heating which can raise the surface temperature in a short time is preferable. By completely removing the scale generated by coarse bar heating due to de-scaling after coarse bar heating and performing finish rolling, it is possible to completely suppress the occurrence of bamboo leaf pattern, scale biting in the finishing mill, etc. Can be prevented.

The steps of cold rolling, annealing and galvannealing may be performed according to a conventional method, and the conditions are not particularly limited. Usually, cold rolling is performed at a rolling reduction of 60% or more. The subsequent annealing is performed at 780 ° C. or higher.

In the present invention, the components may be adjusted using a converter or an electric furnace. Also, there is no problem if scrap is used as a raw material, and Cu, Sn, S
Even if elements such as b, As, and Co are mixed, the effects of the present invention will not be impaired if the amount is small. However, the mixing of these elements is desirably 1% or less in total. There is no problem even if a small amount of Ni or Cr is added for improving the corrosion resistance, but the addition of these elements causes hardening of the steel, so that the total added amount is desirably 1% or less. Further, there is no particular limitation on the winding temperature of hot rolling. Regarding plating, electroplating, chemical conversion treatment, N
Even if i-plating or the like is performed, the effects of the present invention are not impaired. The maintenance of the continuous casting slab does not affect the effect of the present invention. That is, even if the continuous casting slab is inserted into the heating furnace without care, for example, 1 mm or more to remove steel-made inclusions, 3 to 10 mm to remove surface segregation, and partially remove the surface to remove surface defects. There is no problem even if you care for.

[0035]

Embodiments of the present invention will be described below. (Example 1) Steel having the chemical composition shown in Table 3 was continuously cast into a slab having the thickness shown in the table, and hot-rolled under the conditions shown in Table 4. The obtained steel sheet was cold-rolled, annealed at 820 ° C., and then subjected to an alloyed hot-dip galvanizing treatment to produce an alloyed hot-dip galvanized steel sheet having a thickness of 1.0 mm. The presence or absence of bamboo leaf pattern on the surface of the obtained plated steel sheet was observed. Table 4 shows the results.
It is described together.

[0036]

[Table 3]

[0037]

[Table 4]

As shown in Table 4, according to the present invention, it was confirmed that a galvannealed steel sheet having a parameter S value of less than 0.17 and having good surface properties without bamboo leaf pattern can be produced.

(Example 2) Steel having the chemical composition shown in Table 5 was continuously cast into a slab having the thickness shown in the table, and hot-rolled under the conditions shown in Table 6. For hot rolling, leveling was performed before heating the coarse bar, and descaling was performed after heating the coarse bar. The obtained steel sheet was cold-rolled, annealed at 820 ° C., and subjected to an alloyed hot-dip galvanizing treatment to produce a 0.7 mm-thick galvannealed steel sheet. The presence or absence of bamboo leaf pattern on the surface of the obtained plated steel sheet was observed. Table 6 shows the results.
It is described together.

[0040]

[Table 5]

[0041]

[Table 6]

As shown in Table 6, in accordance with the present invention, after continuous straightening by a leveler, coarse bar heating, and descaling, an alloyed hot-dip galvanized steel sheet with good surface properties without bamboo leaf pattern was obtained. It was confirmed that it could be manufactured.

[0043]

As described above, according to the present invention,
A hot-dip galvanized steel sheet having excellent surface properties and capable of preventing a bamboo leaf pattern generated when a highly workable Ti-added ultra-low carbon steel sheet is used as a base material can be manufactured.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Junichi Inagaki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Yoko Muramatsu 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Sun Honko Co., Ltd.

Claims (3)

[Claims] 1. The composition according to claim 1, wherein C ≦ 0.003%, Si ≦
0.1%, Mn ≦ 0.5%, P ≦ 0.03%, S ≦ 0.
03%, Al ≦ 0.1%, N ≦ 0.003%, 0.02
% ≦ Ti ≦ 0.15% steel is continuously cast, hot-rolled, cold-rolled, further annealed, and then subjected to an alloyed hot-dip galvanizing process to obtain an alloyed hot-dip galvanized steel sheet. In manufacturing, the slab thickness by continuous casting is 230mm or more, and the slab reheating temperature of hot rolling is 1150 ° C.
Hereinafter, the rough rolling rate is 70% or more, and the surface temperature at the end of the rough rolling is 90.
0 ° C. or higher, the slab heating temperature is A (° C.), the rough rolling end temperature is B (° C.), the rough rolling reduction R (%), and the time from the heating furnace extraction to the rough rolling end is T (sec). A parameter S defined by the following equation: S = (A + B) / (T × R) <0.17. A method for producing an alloyed hot-dip galvanized steel sheet having excellent surface appearance, characterized in that: 2. In% by weight, C ≦ 0.003%, Si ≦
0.1%, Mn ≦ 0.5%, P ≦ 0.03%, S ≦ 0.
03%, Al ≦ 0.1%, N ≦ 0.003%, 0.02
% ≦ Ti ≦ 0.15% steel is continuously cast, hot-rolled, cold-rolled, further annealed, and then subjected to an alloyed hot-dip galvanizing process to obtain an alloyed hot-dip galvanized steel sheet. In manufacturing, the slab thickness by continuous casting is 230mm or more, and the slab reheating temperature of hot rolling is 1150 ° C.
Hereinafter, the rough rolling rate is 70% or more, and the surface temperature at the end of the rough rolling is 90.
Manufacture of alloyed hot-dip galvanized steel sheet with excellent surface appearance characterized by continuously performing leveling, roughing bar heating, heating at 30 ° C or higher, and descaling at a temperature of 0 ° C or higher, before roughing and finishing rolling. Method. 3. The steel according to claim 1, further comprising:
3. The composition according to claim 1, wherein the composition contains one or two of B at 20% or less and Nb at 0.04% or less.
3. A method for producing an alloyed hot-dip galvanized steel sheet having excellent surface appearance according to item 1.