JPH0672257B2 - Method for producing cold rolled steel sheet with excellent workability by continuous annealing - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an Al-killed cold-rolled steel sheet, in which a hot-rolled sheet after hot rolling is applied with a specific cooling rate in a specific temperature range. The present invention provides a method for improving the workability of a cold-rolled steel sheet by cooling the steel sheet by cooling with a conventional method, as compared with a conventional material rolled at a low temperature.

(Prior Art) Conventionally, in order to manufacture a steel sheet with good workability by continuous annealing,
A method of fixing C in steel by reducing the amount of C in the steel to about 0.005 wt% or less and adding a carbide forming element such as Ti, Nb has been used. However, the cost increases because of the increase in the cost.

There is also a method of using Al-killed steel to reduce the material cost, but for that purpose, the hot-rolled sheet after hot rolling is 650-700 ℃.
The winding must be carried out at the above high temperature, which increases the scale thickness and increases the cost of pickling. Further, in high-temperature winding, since the cooling rate of the top and bottom portions after winding is high, the quality of the material is deteriorated and the yield is lowered.

In order to avoid this problem, it is necessary to carry out coiling at a low temperature of 650 ° C or lower, but because precipitation of AlN is insufficient or Fe ₃ C aggregation in the hot rolled sheet stage is insufficient, The workability of cold rolled steel sheet is poor.

At present, as a solution to this problem, a method has been proposed in which the C content in steel is 0.020 wt% or less as disclosed in JP-A-61-272326, but in this method, the C content is 0.015 wt% or less. However, there remains a problem that aging deterioration is likely to occur. In addition, the applicant of the present invention has filed Japanese Patent Application No. 61-249443
There is also a method of limiting the amount of Mn, S, Al, N within a certain range, as proposed in Japanese Patent No. 105931), but in order to sufficiently agglomerate Fe ₃ C in the hot rolled sheet, the amount of Mn should be 0.1 Must be below wt%. In this case, there is a drawback that cracks are likely to occur during hot rolling depending on the amount of S. Also, due to the limited amount of Al and N
Precipitation of AlN is possible even at coiling temperatures below 650 ° C, but Fe ₃ C
In order to agglomerate, the amount of C must be 0.020 wt% or less, and the problem of aging deterioration described above remains.

As described above, the conventional method for performing winding at a low temperature of 650 ° C. or lower after hot rolling using Al-killed steel is currently difficult to apply due to the strictness of the component conditions.

(Problems to be solved by the invention) Therefore, in the present invention, a low carbon Al killed steel is used, and after hot rolling.
The objective is to produce a cold-rolled steel sheet having excellent workability equivalent to that when wound at a high temperature of 650 ° C or higher, even when wound at a low temperature of 600 to 650 ° C.

(Means for Solving the Problem) The inventors of the present invention performed cooling in a temperature range of 900 to 670 ° C. of a hot rolled sheet immediately after hot rolling according to the conditions shown below,
It has been found that the above object can be achieved by winding at a temperature of ℃ or more, followed by cold rolling and continuous annealing.

1. Cool the temperature range from 900 to 670 ℃ at a cooling rate of 30 ℃ / sec or less.

2. The temperature range of 900-720 ℃ is 100 ℃ / sec. After cooling at any cooling rate, the temperature range of 720-670 ℃ is 15 ℃ / sec.
Cool at the following cooling rates.

The gist of the first invention is as follows. That is, C: 0.010-0.040%, Mn: 0.05-0.25 by weight ratio
%, S: 0.004-0.020%, Al: 0.020-0.080%, N: 0.0030
% Or less, with the balance being Fe and inevitable impurities, the slab is heated and hot-rolled to a temperature range of 900-670 ° C.
Cooling at a cooling rate of 30 ° C / sec or less, then 600
A method for producing a cold-rolled steel sheet having excellent workability, which comprises a step of winding at a temperature of ℃ or more and a step of performing cold rolling and continuous annealing thereafter.

The gist of the second invention is as follows. That is, C: 0.010-0.040%, Mn: 0.05-0.25 by weight ratio
%, S: 0.004-0.020%, Al: 0.020-0.080%, N: 0.0030
% Or less, with the balance being Fe and unavoidable impurities, the slab is heated and hot-rolled to a temperature range of 900-720 ° C.
Cooling at a cooling rate of 100 ° C / sec or less, and then 720
A step of cooling the temperature range of 670 ° C to 670 ° C at a cooling rate of 15 ° C / sec or less, and a step of winding it at a temperature of 600 ° C or higher,
And a step of performing cold rolling and continuous annealing thereafter, which is a method for producing a cold rolled steel sheet having excellent workability.

First, the reasons for limiting the chemical composition of steel to which the method of the present invention is applied will be described.

When C is less than 0.010%, aging deterioration after continuous annealing is large, which is not desirable. Further, if it exceeds 0.040%, the deep drawability of the product deteriorates. Therefore, the amount of C is limited to 0.010 to 0.040%. The range of 0.010 to 0.028% is preferable in order to more sufficiently agglomerate Fe ₃ C in the hot rolled sheet stage.

Mn is a component necessary to prevent hot brittleness, but 0.05
If it is less than%, FeS is generated and its effect is not obtained. Also, 0.25%
If it exceeds, the deep drawability deteriorates. Therefore, the Mn content is 0.05
Limited to ~ 0.25%. The range of 0.10 to 0.20% is preferable in order to make MnS, which becomes precipitation nuclei of cementite during overaging of continuous annealing, more appropriate size.

If S exceeds 0.020%, it causes hot embrittlement, so it must be less than this. In addition, during overaging of continuous annealing
In order to use MnS as cementite precipitation nuclei, 0.0
A content of 04% or more is desirable.

Al requires at least 0.020% in order to precipitate N as AlN after deoxidation and winding. However, if it exceeds 0.080%, the workability is deteriorated. 0.030 to 0.080 in order to further increase the size of precipitated AlN and improve workability.
% Range is preferred.

Also, since AlN also deteriorates the workability, it is better to have less,
The N content was 0.0030% or less.

Further, the slab used in the manufacturing method of the present invention can contain Si, P and other elements.

The inventors melted steel within the above-mentioned composition range, performed hot rolling, cooled various temperature ranges at various cooling rates, and
Winded at a temperature of ~ 850 ° C. Furthermore, cold rolling and continuous annealing with a recrystallization temperature of 800 ° C. were performed to investigate the material. Typical results are shown in FIGS. 1 and 2. Figure 1 shows the weight ratio of C: 0.020%, Si: 0.010%, Mn: 0.15%,
P: 0.007%, S: 0.011%, sol.Al:0.050%, N: 0.0015%,
Fe: Steel having a composition consisting of the balance is hot-rolled at a finishing temperature of 900 ° C., and then a temperature range of 900 to 670 ° C. is set to 5 to 100.
After cooling at a cooling rate of ℃ / sec and winding at 650 ° C, followed by cold rolling and continuous annealing, the above cooling rate r
It shows the effect on the value.

Fig. 2 shows that the steel of the above components is hot-rolled at a finishing temperature of 900 ° C, then cooled at a cooling rate of 100 ° C / sec between 900 and 720 ° C, and then at 5 ° C between 720 and 670 ° C. The range of cooling rates at which good deep drawability is obtained when the film is wound at 650 ° C. after being cooled at various cooling rates of up to 80 ° C./sec is shown by diagonal lines.

From these results, it was found that the following cooling and winding conditions must be satisfied in order to obtain a steel sheet having excellent deep drawability.

1. The hot rolled sheet after hot rolling is cooled in the temperature range of 900 to 670 ° C at a cooling rate of 30 ° C / sec or less, and then wound at a temperature of 600 ° C or more.

2. Cool the temperature range from 900 to 720 ℃ at a cooling rate of 100 ℃ / sec or less, then cool the temperature range from 720 to 670 ℃ at a cooling rate of 15 ℃ / sec or less, and wind at a temperature of 600 ℃ or more. .

When the precipitation state of Fe ₃ C in the hot-rolled sheet was examined by an optical microscope, those according to the conditions of the present invention were sufficiently aggregated and their distribution intervals were wide, whereas cooling after hot-rolling was performed. It was found that when one or both of the conditions and the coiling temperature conditions deviate from the conditions of the present invention, the cohesion degree is insufficient and the distribution intervals are close.

The cooling reduction ratio may be the same as that usually used, but a high-pressure cold rolling reduction ratio of 70% or more is preferable in order to develop the (111) texture after continuous annealing and improve the deep drawability.

Next, the continuous annealing conditions will be described. The heating temperature needs to be higher than the recrystallization temperature. As long as hot rolling, cold rolling and the like are performed according to the present invention, sufficient deep drawability can be provided even by low temperature annealing. Therefore, at present, the temperature is mainly heated to 700 to 830 ° C., but annealing at a temperature higher than this temperature does not impair the deep drawability. In order to improve the aging characteristics, the primary cooling rate should be 50 ° C / sec or more, then 250
It is better to perform overaging treatment in the temperature range of ~ 350 ° C. This is a distribution state in which MnS in steel acts as a precipitation nucleus of cementite during overaging due to the slab temperature history condition, and the action is particularly effectively exhibited by this overaging condition. Because.

The cold-rolled sheet produced as described above has excellent deep drawability as compared with the conventional material that is directly hot-rolled.

Example 1 C: 0.022%, Si: 0.013%, Mn: 0.15%, P: 0.008%, S: 0.012
%, Sol.Al:0.040%, N: 0.0020%, Fe: Steel with the composition of the balance is melted and hot rolled at a finishing temperature of 900 ℃
After being made into mm, it was cooled in various temperature ranges at various cooling rates and then wound at a temperature of 500 to 730 ° C. Furthermore, cold rolling and continuous annealing with a recrystallization temperature of 800 ° C. were performed, and the materials were investigated. Table 1 shows the cooling conditions after the hot rolling, the winding conditions, and the material test results.

Samples A, B, C, D and E were cooled in the temperature range of 900 to 670 ° C at a cooling rate within the range of the present invention. Samples F, G and H are 900-
After cooling the temperature range of 720 ° C at 30, 50, 80 ° C / sec, respectively, the temperature range of 720 to 670 ° C was cooled at 10 ° C / sec.

Samples A, B and C were all wound at 650 ° C for 1 hour and then cooled at 30 ° C / hr. Samples D and E are
The winding temperatures were 600 ° C and 730 ° C, respectively.

Samples I, J and K have faster cooling rates in the temperature range of 900 to 670 ° C than the present invention.

Samples L, M and N have a cooling rate in the temperature range of 900 to 670 ° C within the range of the present invention, but a coiling temperature lower than that in the range of the present invention.

Further, N dissolved in the steel after the completion of winding is precipitated as AlN and coarsened in all the samples, and there is no adverse effect of the dissolved N and AlN on the workability of the product sheet.

These hot-rolled sheets were pickled, cold-rolled to a thickness of 0.8 mm at a rolling reduction of 80%, and further, this cold-rolled sheet was continuously annealed at a temperature of 800 ° C. for 1 minute.

The cold-rolled steel sheet obtained as a result was subjected to a tensile test using a JIS No. 5 tensile test piece to measure r value, yield strength and elongation. The results of these measurements are also shown in Table 1.

From the results shown in Table 1, when the cooling condition in the temperature range of 900 to 670 ° C is out of the range of the present invention, any or all of r value, yield strength and elongation are materials sufficient for deep drawing. On the other hand, when it is within the range of the present invention, it is clear that the r value, the yield strength and the elongation are all materials sufficient for deep drawing.

An investigation of the distribution of Fe ₃ C in the hot rolled steel sheet revealed the following.

When the cooling condition or the coiling temperature in the temperature range of 900 to 670 ° C. after hot rolling is out of the range of the present invention (Samples I, J, K, L, M and N), the Fe ₃ C distribution is a grain boundary. In the case of being within the scope of the present invention (samples A, B, C, D, E, F, G, and H), Fe ₃ C aggregates in a point sequence in a large number and is not aggregated. Are widely distributed and the distribution intervals are wide and the number is small.

That is, by setting the cooling condition in the temperature range of 900 to 670 ° C. after hot rolling within the range of the present invention, agglomeration of Fe ₃ C in the hot rolled steel sheet when performing low temperature winding, which was conventionally difficult. This makes it possible to manufacture cold-rolled steel sheets with excellent deep drawability.

Example 2 A steel having the composition shown in Table 2 was melted and the finishing temperature was 900 ° C.
After being hot-rolled to a thickness of 4.0 mm, it was cooled in a temperature range of 900 to 670 ° C at a cooling rate of 30 ° C / sec and then wound at a temperature of 650 ° C. Furthermore, cold rolling and continuous annealing with a recrystallization temperature of 800 ° C. were performed, and the materials were investigated. The results of this material test are shown in Table 3.

The compositions of Samples O, P and Q are within the scope of the invention, while Samples R, S, T, U and V are all outside the scope of the invention for the underlined components.

These hot-rolled sheets were pickled, cold-rolled to a thickness of 0.8 mm at a rolling reduction of 80%, and further, this cold-rolled sheet was continuously annealed at a temperature of 800 ° C. for 1 minute.

The cold-rolled steel sheet obtained as a result was subjected to a tensile test using a JIS No. 5 tensile test piece to measure r value, yield strength and elongation. The results of these measurements are shown in Table 3.

From the results shown in Table 3, it can be seen that when the composition is out of the range of the present invention, any or all of the r value, the yield strength and the elongation cannot obtain a sufficient material for deep drawing.

(Effects of the Invention) As is apparent from the above description, according to the method of the present invention,
After hot rolling of Al killed steel, the temperature is lower than before 600 ~ 650
A cold-rolled steel sheet having excellent deep drawability can be produced even if it is wound at ℃. In addition, according to this method, since it is wound at a low temperature,
Compared to high temperature winding, the scale thickness is thinner, good pickling property is obtained, and the cost of pickling is reduced. In addition, the material of the top and bottom parts after winding has little deterioration and the yield is good.

[Brief description of drawings]

Fig. 1 shows 900-670 ℃ after hot rolling which gives good deep drawability.
The range of the cooling rate between them is shown by diagonal lines. Figure 2 shows that after hot rolling, it was cooled between 900 and 720 ℃ at a cooling rate of 100 ℃ / sec, then between 720 and 670 ℃ at various cooling rates of 5 to 80 ℃, and then 650 ℃. The range of cooling speeds in which a good deep drawability is obtained when wound at a temperature is shown by diagonal lines.

Claims (2)

[Claims] 1. A weight ratio of C: 0.010 to 0.040%, Mn: 0.05 to
0.25%, S: 0.004-0.020%, Al: 0.020-0.080%, N: 0.0
A slab containing 030% or less of which the balance is Fe and unavoidable impurities is heated, hot-rolled, and then cooled in a temperature range of 900 to 670 ° C at a cooling rate of 30 ° C / sec or less, and then 600 ° C or more. A method for producing a cold rolled steel sheet having excellent workability by continuous annealing, which comprises winding at a temperature, followed by cold rolling and continuous annealing. 2. A weight ratio of C: 0.010 to 0.040%, Mn: 0.05 to
0.25%, S: 0.004-0.020%, Al: 0.020-0.080%, N: 0.0
A slab containing 030% or less of which the balance is Fe and inevitable impurities is heated and hot-rolled, and then cooled in a temperature range of 900 to 720 ° C at a cooling rate of 100 ° C / sec or less, and further 720 to 67
After cooling the temperature range of 0 ℃ at a cooling rate of 15 ℃ / sec or less, 6
A method for producing a cold rolled steel sheet having excellent workability by continuous annealing, which comprises winding at a temperature of 00 ° C or higher, followed by cold rolling and continuous annealing.