JPS59159936A - Manufacture of cold rolled steel sheet with superior press formability - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet with superior press formability by hot rolling a slab of a low-C Al killed steel, coiling it at a high temp., reheating the coil under specified conditions, and carrying out cold rolling and continuous annealing. CONSTITUTION:A slab of a low-C Al killed steel contg. 0.005-0.07% C, <=1.3% Si, <=0.8% Mn, <=0.1% P, 0.01-0.10% Al and 10-80ppm N is hot rolled in a conventional stage and coiled at >=720 deg.C. The coil is reheated at 730-880 deg.C before the temp. of the outermost part lowers to 700 deg.C. The coil is then cooled, and cold rolling and continuous annealing are carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a cold rolled steel sheet with excellent press formability.

When a low carbon aluminum killed steel is continuously annealed to produce a steel plate with excellent formability, a high-temperature coiling process is performed to soften the steel plate, increase its ductility, and increase its r-value. It is known that the purpose is to precipitate AtN and aggregate cementite.

However, when the tip and rear ends of a hot coil are wound into a coil, they are located at the inner and outer peripheries, so the cooling rate after winding is fast. The material is inferior to that of the other parts.

In order to cover this problem, a method of winding the leading end and the trailing end at a higher temperature than the center is also known. Even with these methods, it is difficult to make the material of the tip and rear ends the same as the central part, and if you are aiming for a high water content, it is undeniable that this will lead to a decrease in yield. .

The object of the present invention is to ensure that the quality of the material at the leading and trailing ends is as high as that at the center by performing heat retention treatment immediately after winding a coil that has been wound at a high temperature.

FIG. 1 shows the cooling curve of the outer periphery of a coil that was wound at 780° C. at the front end and at about 750° C. at the other center in the longitudinal direction of the coil. Approximately 20℃ up to approximately 450℃
,,4nin, after which the cooling rate tends to drop suddenly as the heat transmitted from inside the coil and the heat dissipated into the air become balanced.

In that case, if we examine the materials of the outermost and inner circumferential parts, we will find the values shown in Table 2. In particular, the r value is worse than that of the central part, and the yield point of the central part is 3 to 4 kg/ It is about as high as wn2.

In order to prevent such material deterioration at the leading and trailing ends of the coil, the inventors have conducted various studies on heat treatment of the hot-rolled coil that has been wound up, and as a result, they have arrived at the present invention.The gist of the invention is C:0 .005-0.07%, St: 1.3%
Below, Mn: 0. s% or less, P: 0.1% or less, At:
A slab of 0.01 to 0.10 inches and N: 10 to 80 ppm is hot-rolled in a normal process, and the front and rear ends of the coil are wound at 720°C or higher, and the outer periphery of the wound coil is rolled. A method for producing a cold-rolled steel sheet with excellent press formability, characterized by performing reheating treatment in a temperature range of 730°C to 880°C before the part temperature falls below 700°C, and continuously annealing after cold rolling. It is.

The inventors attempted to improve the material quality of the leading and trailing ends by performing various heat treatments on the high-temperature wound coil.

The high-temperature coil was placed in a furnace, and a gas burner was used to reheat the outer and inner circumferences of the coil to 730 to 880°C. The purpose is to precipitate AtH and agglomerate cementite.

However, it was discovered that sufficient material quality could not be obtained by simply performing reheating treatment. It has been found that it is important to start the reheating process before the leading and trailing ends of the coil wound at a temperature of 720° C. or higher are cooled to a temperature of 700° C. or lower.

Samples that were cooled to below 700°C and then reheated did not have a sufficiently high r value, whereas samples that were reheated before cooling to 700°C showed that the growth of crystal grains during continuous annealing became significantly softer. Therefore, the r value also increases. In the sample that was reheated before being cooled to 700° C., AtH was relatively sparsely precipitated, and it is presumed that this helps the growth of crystal grains and contributes to the improvement of the material quality.

On the other hand, when the sample was cooled to below 700°C in a trench and then reheated, the AAN precipitation was relatively dense. It is said that AtN precipitates at the α-γ interface during cooling from the austenite region, Q, 700 tl: When reheating after cooling once, most of the precipitated clusters generated are stabilized in size. It is thought that this causes dense precipitates to form.

On the other hand, when reheating is performed at temperatures above 700°C, the precipitates disappear and grow because not enough precipitate nuclei are formed and the critical dimension for stabilizing the generated AtN clusters is larger at higher temperatures. This seems to be because the speed is fast. In order to prevent the outermost and outer circumferential parts of the wound coil from dropping below 700°C, it is necessary to wind the leading and trailing ends of the coil at a high temperature of 780 to 800°C and immediately begin reheating. .

Since the innermost periphery is cooled by the winder, it is necessary to take measures to minimize the cooling of the innermost periphery by, for example, using a material with poor heat conductivity (ceramic, etc.). In terms of heat dissipation, the innermost periphery is also more advantageous than the outermost periphery, and since the length of the innermost periphery is shorter per round, it falls onto the end plate, so it is necessary to It has the advantage that it only needs to fall within the scope of the present invention, and in practice there are few problems in terms of yield due to material defects. The problem is the plate temperature at the outermost circumference.Since the length of the circumference is long, this part is included in the product length, so in the actual manufacturing process, the purpose of the present invention can be applied to the outermost circumferential plate temperature. It can be manufactured using

Next, the reheating treatment conditions will be described.

If heated above 880°C, it will take a long time to heat and the quality of the material (especially Et, r value) will not be improved; if heated below 730°C, the material will be insufficient for short-time processing. Therefore, in order to sufficiently improve the quality of the material, it is necessary to heat it to 730 to 880°C.

Cooling after reheating is an average of 10°C until it reaches nearly 600°C due to shrinkage of enlarged austenite and precipitation of solid solution C at grain boundaries.
It is desirable to cool at a cooling rate of /min or less.

The reheating treatment described in detail above can be carried out by i) covering the high-temperature coiled coil with a well-insulated cover and transporting it on a conveyor while being reheated; or 11) installing it on a conveyor or cart carrying the hot-rolled coil. This process involves reheating in a tunnel furnace, followed by cooling.

In addition to such continuous heat treatment, the coil wound at high temperature may be placed in a patch type furnace for heat treatment.

Next, the composition range of steel processed according to the present invention will be described.

The amount of C is 0.005 to 0.07%. C is 0.005
If it is less than %, the formation of cementite is small and the precipitation of AtN is easy, so the present invention does not need to be particularly applied.

If C exceeds 0.07%, the amount of cementite will be too large and the effect of the wood invention will not be achieved.

Depending on the strength level of the steel sheet, Si, Mn, and P do not fundamentally affect the effects of the present invention even if they are contained.

However, as a lead plate used for press working, the St is 1.3.
%v, Mn is 0.8% or less, and P is 0.1 or less. At and N are At: 0, 01~0.1, N: 10~8
It may be within the normal range of 0 ppm.

However, in order to obtain a soft steel plate with a particularly high r value, the above At
, N: At: 0.01 to 0.05%, N: 10
A lower value of ~40 ppm is preferable.

The steel sheet manufactured by the present invention can be applied to the manufacture of cold-rolled steel sheets, hot-dip galvanized steel sheets, hot-dip aluminized steel sheets, etc., which are processed by a continuous annealing type annealing treatment line.

Next, embodiments of the present invention will be described in detail.

Example I C: 0.04%, Si: 0.02%, Mn: 0.18%
, P: 0.018%, S: 0.010%, At: 0.0
After heating a slab of low carbon aluminum killed steel with 4-chi, N: 35 ppm to 1200°C, it was finish rolled into a coil with a thickness of 3.2 mm at A3 point or above of 890-920°C, and the center part in the longitudinal direction was 750 mm thick. ℃, approximately 30m at the tip and rear end of the coil is 7
It was wound up between 80 and 800°C. The coil was placed in a preheated box furnace, the temperature at the outermost periphery was measured as shown in Table 1 and FIG. 2, and various heat cycles were performed.

The investigation was conducted by varying the air cooling time after winding so that the temperature at the outermost periphery immediately before entering the furnace varied.

In the furnace, the outermost periphery was heated to 780 μ by gas heating, then furnace-cooled to 500° C. at a rate of 10° C./min, and then air-cooled.

After pickling, it was cold rolled to a thickness of 0.8 mm and subjected to continuous annealing. Annealing temperature is 800℃ x 30 seconds to 400t: 50℃
/see, and over-aging treatment was performed at 400° C. for 3 minutes. The material results after a 1.2% skin pass are shown in Table 2.

The average material of the central part in the longitudinal direction is YP: 18k9/rm
n2, TS: 33kg/mm2, Et: 45%, r value:
1.68 Example 2 C: 0.03%, StO. 015%, Mn: 0.18%
,P:0. O15 Chi, S: 0.010, At: 0.0
Section 38, after heating a slab of low carbon aluminum killed steel with N: 35 ppm to 1200°C,
Finish rolling was carried out at three or more points to form a coil with a thickness of 3.2 m, and the lengthwise central portion was wound at 750°C, and the coil tip and rear ends about 30 m were wound at a temperature of 800 to 830°C without pouring water.

The coil was immediately placed in a preheated box-shaped furnace, and the heat treatment cycle shown in Table 3 and Figure 2 was performed, varying the maximum temperature reached by reheating, and the investigation was conducted by subjecting it to a variety of heat cycles. .

Plate temperature was controlled by measuring the temperature at the outermost part of the coil. The plate temperature at the outermost part of the coil just before entering the furnace is 700-73
It was between 0°C.

The coil was heated by gas heating, and the coil was cooled after heating at a rate of 10° C./min to 500° C., followed by air cooling. After pickling, it was cold rolled to a thickness of 0.8 mm and subjected to continuous annealing. Annealing temperature is 800℃ x 30 seconds until 400℃ at 50℃/sec
The sample was cooled at 400° C. for 3 minutes and subjected to an overaging treatment. Table 4 shows the results of the material after the skin-nosed yarn in section 1.2.

The average material of the central part in the longitudinal direction is YP: 18k! ? /m
m2, TS:321Q? /an2, Et: 45%, r value: 1.66 O As described above, according to the present invention, it is possible to ensure that the material of the tip and rear ends of the coil is as high as the center part. This is extremely advantageous as it leads to a possible improvement in yield.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a temperature curve at the outermost circumference of a conventional coil, and FIG. 2 is an explanatory diagram showing a temperature curve at the outermost circumference of a coil according to the present invention.

Claims (1)

[Claims] C: 0.005 to 0.07%, Si: 1.3% or less, M
n: 0.8% or less, P: 0.1 inch or less, At: 0.01
~0.10%, N: 10 to 80 ppm slab is hot rolled in a normal process, and the coil is wound at 720°C or higher, and the temperature of the outer peripheral part of the wound coil does not fall below 700°C. A method for producing a cold-rolled steel sheet with excellent press formability, characterized by performing reheating treatment at a temperature range of 730°C to 880°C, followed by continuous annealing after cold rolling.