JPH0797633A - Production of galvanized steel sheet excellent in workability - Google Patents

Abstract

PURPOSE:To provide a method for producing a galvanized cold rolled steel sheet excellent in workability. CONSTITUTION:At the time of producing a steel strip as cold-rolled by using an inline annealing type continuous galvanizing equipment, it is rapidly subjected to auxiliary annealing at 820 to 910 deg.C in a short time at 100 to 2000 deg.C/s heating rate in the optional stage in the process of heating and soaking and is successively subjected to galvanizing treatment. Thus, the galvanized cold rolled steel sheet excellent in workability can stably and efficiently be produced compared to the conventional producing method.

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 a hot-dip galvanized steel sheet having excellent workability in an in-line annealing type hot dip galvanizing equipment.

[0002]

2. Description of the Related Art As a basic technique for improving the workability of hot-dip galvanized steel sheets such as deep drawability and overhanging property, a high temperature annealing technique in an in-line annealing type continuous hot dip galvanizing facility is well known. In that case, the cold-rolled steel strip is uncoiled and inserted into a continuous hot-dip galvanizing facility and recrystallization annealed, but the pattern is basically heating, soaking,
It consists of cooling, and the annealing temperature is high in order to improve workability. The heating method is direct-fired heating with a gas burner or radiant heating with a radiant tube.

On the other hand, application of electric heating to continuous annealing is also known, and is disclosed in JP-A-56-116830 and JP-A-5-160830.
It is disclosed in JP-A-6-116831, JP-A-2-166234, and JP-A-4-154947. However, the conventional electric heating method is combined with the conventional continuous annealing furnace in such a manner that it is heated from room temperature to a temperature higher than the recrystallization temperature all at once or is completely separated and independent from the normal annealing furnace.

[0004]

When high-temperature annealing is performed in a normal continuous hot-dip galvanizing facility in order to improve the workability of hot-dip galvanized steel sheet, 1) the passability such as heat buckle and sheet breakage deteriorates. ) Surface defects occur and surface quality deteriorates, 3) energy cost rises, 4) productivity declines because it is necessary to change the annealing temperature with the production of steel sheets of different grades and grades, Such problems occur. On the other hand, in the conventional electric heating method, there is a problem that 1) the heating temperature range is wide and the electric energy cost is high, and 2) the two heating methods are used separately and independently, and the equipment cost is high.

The present invention advantageously solves the problems of the prior art as described above, and it is possible to improve workability by performing auxiliary heating rapidly and for a short time at any stage during recrystallization annealing. An object is to provide an excellent hot-dip galvanized steel sheet manufacturing method.

[0006]

Means for Solving the Problems In order to achieve the above object, the present invention provides an uncoiled cold rolled steel strip to produce a hot dip galvanized steel sheet in an in-line annealing type continuous hot dip galvanizing equipment. Rapid and short-time heating to a temperature range of 820 to 910 ° C at a heating rate of 100 to 2000 ° C / s at any stage in the process of reducing the surface in a reducing atmosphere by heating the steel strip at a recrystallization temperature or higher. The method of manufacturing a hot-dip galvanized steel sheet with excellent workability, characterized in that it is subjected to an auxiliary treatment, then cooled and subjected to hot-dip galvanizing, and if necessary, subsequently re-heated to alloy the plating layer. is there.

The present invention will be described below with reference to the drawings.
FIG. 1 is a diagram schematically showing an embodiment according to the present invention.
In the figure, a is a step of uncoiling and heating the as-rolled steel strip, and the heating pattern has a heating rate of 1 to 200 ° C.
/ s, ultimate temperature is 500 ~ 900 ℃. B and e in the figure
Is a stage of soaking the steel strip, and the soaking temperature is 500 to 9
Although the temperature is 00 ° C. and the holding time is 0 to 300 s, the temperature is not always constant. In the figure, c is the most characteristic feature of the present invention, and is a stage in which the temperature is raised rapidly and in a short time by electric heating. Heating rate is 100-200
The temperature is 0 ° C / s, and heating is performed to 820 to 910 ° C. D in the figure
Is a stage of cooling after heating, and air cooling or forced cooling is performed. In the figure, f is the stage of gradual cooling after soaking. G in the figure
Is a subsequent cooling step with an inert gas until immersed in the molten zinc bath. After hot dip galvanizing at h in the figure, it is cooled to room temperature at the stage of i. This is a method for producing a hot-dip galvanized steel sheet in which a Zn—Fe alloying reaction does not occur in the plating layer. On the other hand, the method for producing an alloyed hot-dip galvanized steel sheet is to immerse it in the hot-dip galvanized bath, reheat it at j in the figure to cause an alloying reaction in the plating layer, and then cool it to room temperature at the step k.

The steels used in the present invention are shown in Table 1.

[Table 1]

That is, in% by weight, C is 0.0003-
It is 0.05%, and if it is less than 0.0003%, the cost is remarkably increased, and if it exceeds 0.05%, the formability is deteriorated. Si is 0.005 to 1.5%, and 0.00
If it is less than 5%, the cost will increase, and if it exceeds 1.5%, there will be problems in workability and surface treatment. Mn is 0.03 to 3%
If it is less than 0.03%, hot embrittlement occurs, and if it exceeds 3%, there is a problem in workability. P is 0.001 to
It is 0.2%, and if it is less than 0.001%, a significant cost increase occurs, and if it exceeds 0.2%, embrittlement occurs. S is 0.001 to 0.02%. 0.001
If it is less than%, the cost increases significantly. 0.02
When it is more than 0.1%, hot embrittlement occurs. Al is added in 0.005 to 0.2% for deoxidation. If it exceeds 0.2%, workability deteriorates. Ti and Nb are added as needed to fix C and N. The amount added is 0 to 0.
1% If it exceeds 0.1%, the workability deteriorates. N is 0.0003 to 0.0050%,
Less than 0.0003% is difficult to reach, while 0.00
If it exceeds 50%, the workability is significantly deteriorated. B is added as necessary, and the addition amount is 0.0040% or less.
If it exceeds 0.0040%, the workability deteriorates.

The above-mentioned steel is heat-treated in the pattern shown in FIG. 1. The main heat history condition ranges are as shown in Table 2.

[Table 2]

In FIG. 1, a, if the heating rate is less than 1 ° C./s, the productivity is poor, and in the case of direct heating or radiant heating, 200 ° C./s.
Super becomes difficult. Also, if the reached temperature is less than 500 ° C,
It is in a non-recrystallized state, and on the other hand, if it exceeds 900 ° C., there arises a problem of high temperature annealing, that is, a problem of threadability and surface defects. If the retention time at the ultimate temperature exceeds 300 s, the productivity will be poor. The rapid heating for a short time in c in the figure is a feature of the present invention. If the heating rate is less than 100 ° C./s, it takes too long to heat, while if it exceeds 2000 ° C./s, control is difficult. In the figure, d indicates a process in which the electric power is turned off after the rapid heating and the steel strip temperature approaches the furnace temperature. The temperature and time of the subsequent annealing in the soaking pit are shown in Table 2.
If it is out of the range, productivity will deteriorate,
There may be a problem with the plateability. Further, the thermal history after f in the figure is the same as the normal one, and includes both cases with and without alloying treatment.

2 and 3 show the influence of the rapid short-time annealing temperature on the Rankford value (r value) and elongation of the product sheet. The steel used is C: 0.0028%, Si:
0.02%, Mn: 0.15%, P: 0.011%,
S: 0.009%, Al: 0.045%, Ti: 0.0
The chemical composition is 15%, Nb: 0.008%, N: 0.0018%. Slab heating temperature: 1180 ° C, hot rolling finishing temperature: 920 ° C, after hot rolling to a thickness of 4.0 mm,
It was wound at 710 ° C. Then, after pickling, it was cold-rolled to 0.8 mm and subjected to in-line annealing type hot dip galvanizing under the following conditions. Here, in FIG. 1, a (heating rate): 10 ° C./s, b: 770 ° C. × 20 s, c: 700 ° C.
/ s to various temperatures, d: air cooling, e: 770 ℃,
f: 7 ° C / s (up to 650 ° C), g: 40 ° C / s, h: 47
Immerse in a molten zinc bath (containing 0.1% Al) at 0 ° C for 2s,
j: heating to 520 ° C. at 20 ° C./s and soaking for 20 s; k: cooling to room temperature at an average cooling rate of 20 ° C./s. afterwards,
It was subjected to a tensile test after temper rolling of 0.8%.

The relationship between the maximum temperature reached and the average r value in FIG. 2c is shown, and the relationship between the temperature and the elongation (El) is shown in FIG. As is clear from FIGS. 2 and 3, it can be seen that the supplementary heating for a short time significantly improves the r value and the elongation. That is, when manufactured in the range of the present invention where the ultimate temperature is 820 to 910 ° C, the r value is 1.6 to 2.4, the elongation is 44 to 53%, and the alloyed hot dip galvanized cold rolled steel sheet is extremely excellent in workability. Can be manufactured.

[0014]

EFFECTS OF THE INVENTION By applying the present invention, 1) there is no heat buckle, plate breakage, etc.
2) The surface quality of the steel sheet improves without the occurrence of surface defects, 3) The productivity can be improved because the furnace temperature can be kept constant even when manufacturing steel sheets of different grades, and 4) the temperature range for electric heating can be improved. Since it is narrow, it is possible to reduce the electric energy consumption. Further, the present invention can be applied to the production of cold-rolled steel sheet such as in-line annealing type aluminum plating, and its application range is wide. Therefore, the industrial significance of the present invention is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a heat treatment pattern of the present invention.

FIG. 2 is a graph showing the relationship between the maximum temperature reached and the average r value in the example of the present invention.

FIG. 3 is a graph showing the relationship between the maximum temperature reached and the elongation in the example of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Tezaba 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Stock of Kimitsu Steel Co., Ltd.

Claims (2)

[Claims] 1. When a cold rolled steel strip is uncoiled to produce a hot dip galvanized steel sheet in an in-line annealing type continuous hot dip galvanizing equipment, the steel strip is heated at a recrystallization temperature or higher and the surface is kept in a reducing atmosphere. At any stage in the middle of reducing, a supplementary rapid and short time heating up to a temperature range of 820 to 910 ° C at a heating rate of 100 to 2000 ° C / s, and then cooling and hot dip galvanizing And a method for producing a hot-dip galvanized steel sheet having excellent workability. 2. The method for producing a hot-dip galvanized steel sheet having excellent workability according to claim 1, wherein after the hot-dip galvanizing, re-heating is performed to alloy the plated layer.