JPH0797632A - Method for producing cold rolled steel sheet and hot dip galvanized steel sheet excellent in workability and paint bake hardenability - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a method for producing a cold-rolled steel sheet excellent in workability and BH property. [Composition] When a cold-rolled steel strip is manufactured using continuous annealing or in-line annealing type continuous hot dip galvanizing equipment, the heating rate is 100 ~ at the last stage during heating and soaking.
Assists rapid and short-time annealing at 2000 ° C / s,
A method for producing a cold-rolled steel sheet having excellent workability and BH property, which comprises rapidly cooling and then performing hot dip galvanizing treatment if necessary, is provided. [Effect] Compared with the conventional manufacturing method, according to the present invention,
It is possible to manufacture a cold-rolled steel sheet that is stable, efficient, and low in cost and has excellent workability and BH property.

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 cold rolled steel sheet which has both workability and paint bake hardenability (BH property). Here, the cold-rolled steel sheet means both a steel sheet that is not surface-treated and a steel sheet that is surface-treated.

[0002]

2. Description of the Related Art As a basic technique for improving workability such as deep drawing property and overhanging property of cold rolled steel sheet, high temperature annealing technology in continuous annealing or in-line annealing type continuous hot dip galvanizing equipment is well known. There is. On the other hand, in order to impart BH property to an ultra-low carbon steel to which at least one of Ti and Nb is added, a method of annealing at high temperature to re-dissolve a part of TiC or NbC and leave solid solution C is well known. ing. In that case, the cold-rolled steel strip is uncoiled and continuously annealed or inserted into a continuous hot dip galvanizing facility for recrystallization annealing,
The pattern basically consists of heating, soaking, and cooling, and the heating / soaking temperature is set to a high temperature in order to improve workability and impart BH property. 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 to improve the workability of cold-rolled steel sheets and to impart BH properties, 1)
Passability such as heat buckle and plate breakage deteriorates 2)
Surface defects will occur and surface quality will deteriorate, 3) energy cost will increase, 4) productivity will decrease due to the need to change the annealing temperature when manufacturing different types and grades of steel sheet, etc. Problem arises. 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 in continuous annealing and in-line annealing type continuous hot dip galvanizing equipment, the rapid and final step during heating and soaking is performed. An object of the present invention is to provide a method for manufacturing a cold-rolled steel sheet and a hot-dip galvanized steel sheet that have both workability and BH property by performing auxiliary heating for a short time.

[0006]

In order to achieve the above-mentioned object, the present invention provides (1) the final step of soaking during heating and uncoiling of a cold-rolled steel strip for continuous annealing. In addition, the heating rate is 820 to 91 at 100 to 2000 ° C / s.
A method for producing a cold-rolled steel sheet excellent in workability and paint bake hardenability, which comprises rapidly and rapidly raising the temperature to a temperature range of 0 ° C and then cooling, and (2) cold rolling When the unrolled steel strip is uncoiled and hot-dip galvanized steel sheet is manufactured by in-line annealing continuous hot dip galvanizing equipment, the final step is to heat the steel strip above the recrystallization temperature and reduce the surface in the reducing atmosphere. , Heating rate is 100 ~ 200
A rapid and short-term temperature increase at 0 ° C / s to a temperature range of 820 to 910 ° C is assisted, followed by cooling, hot dip galvanizing, and subsequent reheating, if necessary, to alloy the plated layer. Is a method for producing a hot-dip galvanized steel sheet excellent in workability and paint bake hardenability.

The present invention will be described below with reference to the drawings.
1 and 2 are diagrams schematically showing an embodiment according to the present invention. FIG. 1 shows the thermal history of continuous annealing.
In the figure, a is a step of uncoiling a cold-rolled steel strip and heating it in a continuous annealing furnace. The heating pattern has a heating rate of 1 to 200 ° C./s and an ultimate temperature of 500 to 900 ° C. In the figure, b is a stage of soaking the steel strip, the soaking temperature is 500 to 900 ° C., and the holding time is 0 to 300 s. In the figure, c is the most characteristic feature of the present invention, in which the temperature is rapidly raised in a short time by electric heating. The heating rate is 100 to 2000 ° C / s, and heating is performed up to 820 to 910 ° C. In the figure, d is a step of immediately cooling after rapid heating for a short time. In equipment having an overaging zone or the like, the cooling curve is different from that shown in the figure, but here it is represented by the curve shown in d in the figure.

FIG. 2 shows the heat history of in-line annealing continuous hot dip galvanizing. In FIG. 2, a is a step of uncoiling and heating the as-cold rolled steel strip. Heating rate is 1 ~ 200 ℃ / s, ultimate temperature is 500
~ 900 ° C. In the figure, b is a stage of soaking the steel strip, the soaking temperature is 500 to 900 ° C., and the holding time is 0 to 30.
It is 0 s. In the figure, c is the most characteristic feature of the present invention, in which the temperature is rapidly raised in a short time by electric heating. The heating rate is 100 to 2000 ° C / s and is 820 to 9
Heat to 10 ° C. In the figure, d is a stage in which after rapid heating for a short time, cooling is performed until it is immersed in a molten zinc bath with an inert gas. After hot dip galvanizing at e in the figure, it is cooled to room temperature at the stage of f. 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 the galvannealed steel sheet is to immerse it in a hot dip galvanized bath, reheat it at g in the figure to cause an alloying reaction in the plating layer, and then cool to room temperature at the stage of h.

The steels used in the present invention are listed in Table 1, and at least one of Ti and Nb is added.

[Table 1]

That is, in% by weight, C is 0.0003 to
It is 0.005%, and if it is less than 0.0003%, a significant cost increase is caused, and if it exceeds 0.005%, the formability is deteriorated. Si is 0.005 to 1.5%, and 0.
If it is less than 005%, the cost increases, and if it exceeds 1.5%, there is a problem in workability and surface treatment. Mn is from 0.03
If it is less than 0.03%, hot embrittlement occurs, and if it exceeds 3%, there is a problem in workability. P is 0.0
It is from 0.1 to 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%. 0.0
If it is less than 01%, the cost increases. 0.02%
If it becomes the above, hot embrittlement will occur. Al is added in 0.005 to 0.2% for deoxidation. If it exceeds 0.2%, workability deteriorates. At least one of Ti and Nb is added to fix C and N. The addition amount is 0.00
2 to 0.1%. If less than 0.002%, Ti or N
There is no effect of adding b and the workability is poor. On the other hand, 0.1%
If it exceeds, the workability deteriorates again. N is 0.000
It is 3 to 0.0050%, and it is difficult to achieve less than 0.0003%, and if it exceeds 0.0050%, workability is significantly deteriorated. B is added as necessary, and the addition amount is 0.
It is 0040% or less. If it exceeds 0.0040%, the workability deteriorates.

The above steel is heat-treated in the pattern shown in FIG. 1 or FIG. 2, and the condition range of the heat history is as follows. In Fig. 1 and Fig. 2a, if the heating rate is less than 1 ° C / s, the productivity is poor, and in the case of direct heating or radiant heating,
It becomes difficult to exceed 00 ° C / s. Also, in b in the figure, the reached temperature is 5
If it is less than 00 ° C, it is in a non-recrystallized state. On the other hand, if it exceeds 900 ° C, there arises a problem of high temperature annealing, that is, problems such as stripability and surface defects. If the retention time at the ultimate temperature exceeds 300 s, the productivity will be poor. Rapid and short-time heating of c in the figure
The feature of the present invention is that the heating rate is 100 ° C.
If it is less than / s, it takes too long to heat, while 2000 ° C / s
When it becomes super, it is difficult to control. In the figure, d is after rapid heating
It is a stage where the electric power is turned off and immediately cooled. In addition, the thermal history after e in FIG. 2 is the same as the normal one, and includes both cases with and without alloying treatment.

3 and 4 and FIGS. 5 and 6, respectively, show the rapid shortening effect on the Rankford value (r value) and BH property of cold rolled steel sheet without surface treatment and alloyed hot dip galvanized cold rolled steel sheet. The effect of time annealing temperature is shown. The steel used is C:
0.0028%, Si: 0.02%, Mn: 0.15
%, P: 0.011%, S: 0.009%, Al: 0.
045%, Ti: 0.015%, Nb: 0.008%,
N: 0.0018% chemical composition, slab heating temperature: 1180 ° C, hot rolling finishing temperature: 920 ° C, 4.0 mm
After hot rolling to a thickness, it was wound at 710 ° C. Subsequently, it was pickled, cold-rolled to 0.8 mm, and annealed under the following conditions.

That is, in the case of continuous annealing, in the heat cycle shown in FIG. 1, a (heating rate): 10 ° C./s,
b: 770 ° C. × 40 s, c: 700 ° C./s in FIGS. 3 and 4
In the figure, it was heated to various temperatures (maximum temperature reached) and d: air-cooled to room temperature. Then, after 0.8% temper rolling, it was subjected to a tensile test. In the case of in-line alloyed hot dip galvanizing, in the heat cycle shown in FIG. 2, a (heating rate) in the figure: 10 ° C./s, b: 770 ° C. × 40 s,
c: 700 ° C./s, heated to various temperatures (maximum temperature reached) in FIGS. 5 and 6, d: 40 ° C./s, e: 470 ° C. molten zinc bath (containing 0.1% Al) ) Soak for 2s, g: 2
The sample was heated to 520 ° C. at 0 ° C./s, soaked for 20 s, and h: cooled 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%.

As can be seen from the diagrams of FIGS. 3 to 6, a short time of supplementary heating significantly improves the r-value,
It can be seen that it exerts an effect in imparting H property. Where BH
As for the property, the amount of increase in stress when the tensile pre-strained material of 2% is subjected to heat treatment equivalent to paint baking at 170 ° C. for 20 minutes and then subjected to the tensile test again (2% from the lower yield stress during the re-tension test)
Evaluation was made by subtracting the deformation stress). The ultimate temperature is 82
When manufactured in the range of the present invention of 0 to 910 ° C, in the case of a cold rolled steel sheet, the average r value is 1.8 to 2.5 and the BH property is 25 MPa.
In the case of hot dip galvanized steel sheet, the average r value is 1.7 to 2.3 and the BH property is 20 MPa.
Since it is up to 50 MPa, a steel sheet excellent in workability and BH property can be manufactured.

[0015]

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 produce cold rolled steel sheets and hot-dip galvanized steel sheets having excellent workability and BH, as well as having excellent manufacturing effects such as reduced 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 an example of a heat treatment pattern of a cold rolled steel sheet according to the present invention.

FIG. 2 is a diagram schematically showing an example of a heat treatment pattern in hot dip galvanizing of the present invention.

FIG. 3 is a diagram showing the relationship between the maximum temperature reached in heat treatment of a cold-rolled steel sheet and paint bake hardenability (BH).

FIG. 4 is a diagram showing the relationship between the maximum ultimate temperature and the average r value in heat treatment of a cold rolled steel sheet.

FIG. 5 is a diagram showing the relationship between the highest temperature reached in heat treatment of hot dip galvanizing and the baking bake hardenability (BH).

FIG. 6 is a diagram showing the relationship between the maximum attainable temperature and the average r value in the heat treatment of hot dip galvanizing.

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

Claims (3)

[Claims] 1. The unstretched and continuously annealed steel strip as cold-rolled is heated to 820 to 910 at a heating rate of 100 to 2000 ° C./s at the final stage during soaking.
A method for producing a cold-rolled steel sheet excellent in workability and paint bake hardenability, which comprises rapidly and quickly raising the temperature to a temperature range of ℃, and then cooling. 2. 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. In the final stage of reducing, the heating rate is 100 to 2000 ° C / s, and the temperature is rapidly and rapidly increased to a temperature range of 820 to 910 ° C, and then cooled and hot dip galvanized. A method for producing a hot-dip galvanized steel sheet having excellent workability and paint bake hardenability. 3. A method for producing a hot-dip galvanized steel sheet having excellent workability and paint bake hardenability, which comprises performing hot-dip galvanizing and then reheating to alloy the plated layer.