JPS59219415A - Manufacture of cold rolled steel sheet with superior workability - Google Patents

Abstract

PURPOSE:To manufacture a homogeneous cold rolled steel sheet with superior workability by subjecting a slab contg. specified percentages of C, Si, Mn, P, Al and N to hot rolling, coiling, cooling, a cold rolling ad continuous annealing under specified conditions. CONSTITUTION:A slab contg. 0.005-0.07% C, <=1.3% Si, <=0.8% Mn, <=0.1% P, 0.01-0.1% Al and 10-80ppm N is heated to 1,000-1,180 deg.C, hot rolled, and coiled at >=680 deg.C. This hot rolled coil is slowly cooled at 1.3-5 deg.C/sec averge cooling rate while the temp. of the outside of the coil falls from 650 deg.C to 550 deg.C. The coil is then cold rolled at 70-85% draft, and continuous annealing is carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a cold-rolled steel sheet that has excellent workability in consideration of cooling the hot-rolled coil and has excellent uniformity of material in the width direction and width direction of the coil. .

When manufacturing steel sheets with excellent formability by continuously annealing low-carbon aluminum guild steel, etc., high-temperature winding treatment is performed to soften the steel sheets, increase their ductility, and increase their r-value. It is known that the purpose is to precipitate AtN and agglomerate cementite. However, when the tip and rear ends of a hot-rolled coil are wound into a coil, they become the inner and outer periphery, so the cooling rate after winding is fast, so even when high-temperature winding is performed, The material is inferior to the central part in the longitudinal direction. In order to overcome this problem, a method is known in which the tip and rear ends are heated to a higher temperature than the center.

Even with these methods, it is difficult to make the material quality of the leading end and the trailing end the same as that of the central part, and when aiming for a high material quality level, it is undeniable that this leads to a decrease in yield.

The purpose of the present invention is to ensure that the fibers at the tip and rear ends are at a high level comparable to that at the center by performing heat retention treatment immediately after winding the coil that has been rolled up. .

FIG. 1 shows the cooling curve of the outer circumferential portion of a coil that was wound at a temperature of 800° C. at the front and rear ends and at about 750° C. at the other central portion in the longitudinal direction of the coil. Approximately 20℃/up to approximately 450℃
The coil is cooled at a minimum speed of 150 min, after which the cooling rate tends to drop rapidly as the heat transmitted from inside the coil and the heat generated in the air become balanced. In this case, the materials of the outer and inner circumferential portions have poorer resistance, especially in terms of r value, than the central portion9, and also have a higher yield point.

The present invention was developed by the inventors as a result of various studies regarding the heat treatment of the hot-rolled coil that has been wound up in order to prevent material deterioration of the outer and inner circumferential parts.
When heat-treating a hot-rolled coil obtained by hot rolling a slab heated to a low temperature of 0°C to 1180°C, 6
The cooling rate of the outer circumference of the coil from 50℃ to 550℃ is 1
．． After cooling at a rate of 3°C/min to 5°C/min, pickling, 70°C
~85 inches of cold rolling, continuous annealing, etc. are performed to obtain a cold rolled steel sheet with excellent workability.

The inventors attempted to improve the material quality of the leading and trailing ends by performing various heat treatments after the entire coil was wound up at a high temperature. The purpose of this is to precipitate AtN and agglomerate cementite, but as shown in the examples described later, if the coil is simply air-cooled after high-temperature winding, it may be possible to soften the coil.
You can't raise the value high enough. To increase the r value, increase the temperature from 650°C to 550'C1 by 5°C/min or more at 1.3°C/min.
It has been found that it is important to perform slow cooling (hereinafter referred to as heat retention treatment) so that the cooling rate is less than 1 minute. In particular, the slow cooling start temperature must be 650° C. or higher, and if the material is cooled to a temperature lower than this at a rate faster than the above-mentioned cooling rate, the material will deteriorate significantly. The temperature at which the slow cooling ends is 550°C, preferably 500°C.

The reason why the r value improves with slow cooling in this temperature range is not clear, but it is presumed that it is probably related to the precipitation of solid solution carbon in the cementite at the grain boundaries.

There is a tendency that the slower the cooling rate is compared to natural cooling, the better the quality of the material in the inner and outer circumferential parts of the coil is, but in order to achieve the same level as the central part of the coil, a cooling rate of 5° C./min or less is required. Further, slow cooling at a rate of less than 1.3° C./min does not improve the quality of the material, increases scale thickness on the coil surface, deteriorates pickling properties, and lengthens heat treatment time, disrupting logistics. In addition, cementite aggregates more than necessary in the central portion in the longitudinal direction of the coil, resulting in a significant deterioration of ductility. This heat treatment not only improves the uniformity of the material in the longitudinal direction of the coil, but also improves the uniformity of the material in the IJ force direction.

However, even with these treatments for hot-rolled coils, if the heating temperature of the slab exceeds 1180°C, the r value will not rise above 1.55, making it difficult to fully utilize the effects of the present invention. Can not. However, since it is difficult to achieve a heating temperature of less than 1000°C, the heating temperature of the slab is set to 1180°C or less and 100°C or more.

Next, regarding high-temperature rolled material and scale, it is known that high-temperature rolled material has the disadvantage that the scale becomes thicker and pickling time becomes longer. Heat retention treatment tends to further increase scale thickness9 and deteriorate pickling properties. To improve the pickling properties of heat-retaining treated materials 1) Heat-retaining treated cover (
02 in the atmospheric gas by blowing N2 gas into the
A method of suppressing the increase in scale thickness by lowering the concentration to 1r, 3chi or less, 1l) Rapid cooling from a temperature of 400°C or higher, for example by immersing in a water bath, and FeO generated at high temperature decomposes into 4FeO- + Fe + Fe3O4. There are methods to improve pickling properties by inhibiting the reaction that occurs. The fact that the lower limit temperature of the slow cooling temperature range according to the present invention is 550°C makes it possible to improve pickling properties by rapidly cooling from a temperature of 400 to 550°C, improving material quality and pickling properties. It is possible to perform processing that allows for improvements in both.

In the embodiment of the slow cooling according to the present invention described in detail above, 1) the coil that has been wound at a high temperature is covered with a cover having good heat insulation properties;

Alternatively, the hot-rolled coil conveyor is passed through a well-insulated tunnel, etc., and the heat dissipated from the coil is used to slowly cool the coil without heating it using fuel or electricity from the outside (in this case, the inner and outer peripheries of the coil are cooled once). Even if the temperature drops below 650℃, it will still reach 650℃ due to internal heat due to insulation power/4- etc.
11) There is a method in which the hot-rolled coil is placed in a furnace that can be heated from the outside and the inner and outer circumferences are heated to 650° C. or higher and the cooling rate is controlled.

Further, as a method of quenching from 400 to 550° C. to improve pickling properties, there are a method of immersing the coil in a water tank and a method of quenching the coil by spraying water on it.

Furthermore, the crystal grains of the p-hot rolled sheet tend to increase due to the heat retention treatment. It is known that as the crystal grains of a hot rolled sheet become larger, sufficient crystal rotation does not occur at a low rolling reduction rate, making it impossible to obtain a high r value.

The present inventors have found that a high r-value can be obtained by applying a high-pressure rolled metal with a cold rolling reduction of 70 to 85 inches in addition to this heat retention treatment. In particular, in order to obtain a steel plate with a high r value, a high cold rolling rate of 76 to 85 inches is better even within the above range of cold rolling.

Next, the composition range of the steel used in the present invention will be explained.

C is 0.005 to 0.07. When C is less than 0.005 inch, little cementite is formed, and there is no problem in terms of material quality even if the present invention is not applied. Also, C is 0.07
%, the amount of cementite is too large and the effect of the present invention cannot be achieved.

However, in order to obtain a soft steel plate with a particularly high r value, the above C
Even within this range, it is desirable that C be 0.04% or less.

Even if Si+Mn+P is contained depending on the strength level of the steel sheet, it basically does not affect the effects of the present invention. However, for steel sheets used for press forming, the Sl content is 1.3% or less.
Mn is 0.8% or less, and P is 0.1% or less.

M and N (dAz: 0.01-0.1%, N is 10-80
Any normal range of ppm is sufficient. However, in order to obtain a soft steel plate with a particularly high r value, it is better to use a lower value of Aj = 0.01 to 005% and N-10 to 40 ppm even in the above-mentioned At and N ranges.

The steel sheet manufactured according to the present invention can be applied to the manufacture of cold-rolled steel sheets, hot-dip galvanized steel sheets, molten 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.

C=0.028%, 5i=0.02%, Mn=0
．． 17chi, P=0.010%, 5=O1010chi, Aa
A slab of low carbon aluminum guild steel with = 0.035% and N = 33 ppm was heated to tiiso°C, and then finish rolled into a coil of 3.2 mm and 2.59 thickness at point A3 above 890 °C, and the length was The center part of the coil was wound at 730°C, and the front and rear ends of the coil were wound at a temperature of 770 to 780°C. (However, in Example 3, heating was performed at 1100°C.) Immediately place the coil in a preheated box furnace or a cover with good insulation properties (hereinafter referred to as a heat insulating cover), and
, i2 Various thermal cycles shown in Fig. 2 were performed. Since the inner circumference has better heat recovery and less heat radiation than the outer circumference, the material should be better than the outer circumference, so it is sufficient to manage the temperature history of the outer circumference.

Cooling in the furnace was controlled by gas heating to a predetermined cycle. During the heat retention treatment, N2 gas was blown into some of the coils to reduce the 02 concentration in the heat retention cover and the heat retention furnace to 3- or less.

After some of the coils were removed from the furnace, they were immersed in an adjacent water tank for about 30 minutes to improve pickling properties. After pickling, it was cold rolled to a thickness of 0.8 tmrr and subjected to continuous annealing.

The annealing temperature is 800℃ x 30 seconds to 400℃' at 50℃/
It was cooled at seC and over-aged at 400°C for 3 minutes. The material results after the 1.2% skin pass are shown in Table ■.

The average material of the central part in the longitudinal direction of the coil has a hot rolled finish thickness of 3.
．． YP = 17 Kyf for 2 m (cold rolling rate 75 inches)
/wl, TS -32h Roux2, zt = 4'6%, r value = 1.72, hot rolled finish thickness is 2.5 wn (cold rolling rate 68 inches), YP = 16 Kff/1an2, T.S.
= 32 Kr 2, Et = 46%, r1 straight - 1,50.

As these Examples show, the temperature range in which slow cooling of 1°3°C/M to 5°C/K is required is between 650°C and 550°C. In addition, the cooling rate is 1.5℃/foot.

At a cooling rate of 3°C/black, it is possible to obtain the material of the center plate in the longitudinal direction, but at a cooling rate of 1.3°C/black, the slow cooling time becomes considerably longer. This unnecessarily worsens the flow rate of the silica, which has deteriorated the pickling property. Above all, the major problem is that the central part in the longitudinal direction of the coil is kept at a high temperature for a long time, so cementite aggregates more than necessary. For this reason, during deformation, Kuntsuk enters the cementite, causing a significant deterioration of ductility (see Table ■).

As for the pickling property, since the thickness of the scale is reduced by blowing N2 gas, the pickling efficiency in Examples 2 and 4 is improved to the same level as the high-temperature rolled material without heat retention treatment (Examples 8 and 9). In addition, Example 1 of the coil was immersed in a water tank from about 500°C.
, 2 results in further improved pickling properties.

As detailed above, the present invention provides a method for manufacturing deep drawing steel sheets with a high r value using the most common low carbon aluminum killed steel by a continuous annealing method with good yield and at low cost. However, it is an extremely practical value Ivj.

[Brief explanation of the drawing]

FIG. 1 shows the air-cooling temperature curve of the outer circumference of the hot-rolled coil, and FIG. 2 shows the thermal history after winding of the coil shown in the embodiment of the present invention. Patent applicant Nippon Steel Corporation Shason 2 Figure 1 Riki Hayashi/Kenro Wakunori

Claims (2)

[Claims] (1) C: 0.005-0.07%, Si: 1.
3% or less, Mn: 0.8% or less, P: 0.1
% or less, At: 0, 01~0.1%, N: 10~
Hot rolling is performed by heating an 80 ppm slab to 1000°C to 1180°C, and the coil is wound up at 680°C or higher, and the temperature of the outer circumference of the wound hot-rolled coil reaches 650°C to 550°C. A cold-rolled steel sheet with excellent workability characterized by being slowly cooled at an average cooling rate of 1.3°C/min or more and 5°C/min or less, followed by cold rolling of 70 to 85 inches and continuous annealing. Production method. (2) c: o, o O5~0.07%,
81: 1.3 or less, Mn: 0.8% or less, p:o
, i% or less, M:o, o i ~o, i%, N
: 10~80 ppm slab at 100℃~118℃
Hot rolling is performed by heating to 0°C, the coil is wound at 680°C or higher, and the wound hot-rolled coil is placed in an N2 atmosphere (02°C).
The temperature of the outer periphery of the coil should be between 650°C and 55°C by placing it in a heat insulating cover or placing it in a furnace to keep the temperature at 3°C.
Average cooling rate to 0℃: 1.3℃/min or more 5℃
After slow cooling at less than 1 minute, immerse it in a water tank and then cool it for 70 minutes.
A method for producing a cold-rolled steel sheet with excellent workability, characterized by cold rolling of ~85 inches and continuous annealing.