JP2953323B2 - Manufacturing method of low carbon cold rolled steel sheet - Google Patents

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 low-carbon cold-rolled steel sheet by continuous annealing, and more particularly to a method for producing a soft cold-rolled steel sheet having excellent aging resistance by a very short overaging treatment. Is what you do.

[0002]

2. Description of the Related Art Generally, in order to prevent strain aging in a continuously annealed material, it is necessary to sufficiently precipitate solid solution carbon by a short overaging treatment such as continuous annealing. It is known that it is effective to finely disperse carbides serving as sites in grains to shorten the diffusion distance required for precipitation. On the other hand, since an increase in the intragranular carbide density leads to hardening of the material, efforts are being made to optimize the intragranular carbide density in the production of a soft cold-rolled steel sheet having excellent aging resistance.

In recent years, there have been many inventions for improving the aging property by reducing the amount of solid solution carbon and suppressing the aging property by optimizing the cooling and the overaging treatment in the continuous annealing process.

The precipitation of carbides in crystal grains is divided into a nucleation stage and a growth stage. For example, Japanese Patent Application Laid-Open No. Sho 60-165321 and Japanese Patent Publication No.
JP-A-63-49726, JP-A-61-276935, JP-A-Hei-2
-93025, JP-A-4-327, JP-A-5-59
As shown in JP-A-445, in order to promote nucleation of intragranular carbides after quenching after annealing, the steel is once cooled to a temperature lower than the overageing temperature, nucleation is promoted, and then reheated, A production method has been proposed in which aging treatment is performed to promote carbide growth.

[0005] However, these methods are effective for improving aging, but have the drawback that special equipment is required for reheating after quenching, which leads to an increase in energy cost. Furthermore, in these embodiments, the overaging
It takes a long time from 2.5 minutes to 10 minutes.

Japanese Patent Application Laid-Open (JP-A) No. 58-48632 discloses an example of proposing delayed aging by short-time overaging, in which water quenching is performed after recrystallization annealing, and then over 30 seconds or more. A production method for obtaining a soft steel sheet excellent in aging resistance by performing aging treatment is disclosed. However, in this case, reheating is performed from around normal temperature, which causes a remarkable increase in energy cost.

Japanese Patent Application Laid-Open No. Sho 58-52429 discloses a method for delaying aging by short-time overaging without reheating. However, the first embodiment of this publication
The sample characteristics of E in the table are sufficient since the yield point elongation (YP-El) after artificial acceleration aging at 100 ° C. for 60 minutes is 1.0%, and 0.3% or less from the viewpoint of stretcher strain resistance. It cannot be said that aging resistance has been obtained.

[0008]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the conventional method, an object of the present invention is to provide a method for producing a soft cold-rolled steel sheet having excellent aging resistance by a very short overage treatment of 1.5 minutes or more. To provide.

[0009]

The present inventors have conducted various studies on a method for producing a soft cold-rolled steel sheet having excellent aging resistance by continuous annealing. To find that significant grain growth occurs at low temperatures, thereby reducing the volume of the carbide-free zone near the grain boundaries and enabling delayed aging to be achieved by only a very short overaging treatment. And completed the present invention.

The gist of the present invention is as follows:
C: 0.010 to 0.025%, Mn: 0.05 to 0.25%, S: 0.003
-0.020%, sol.Al:0.01-0.10%, N: 0.0030% or less, B: 0.0003-0.0030% A steel having a steel composition composed of the balance of iron and unavoidable impurities is subjected to normal hot rolling-pickling-cooling. Cold rolling, followed by recrystallization annealing at 820 ° C. or more and 880 ° C. or less for 20 seconds or more and 60 seconds or less in continuous annealing to reduce the grain size to JIS G 0552.
The grain size number is 6.5 or more and 8.0 or less defined by
Cool slowly from 0 to 740 ° C at a cooling rate of 7 ° C / s or less.
After cooling to 375 ° C at a cooling rate of 80 to 250 ° C / s, perform an overaging treatment at a cooling rate of 0.5 ° C / s or less, including holding, from the cooling end point temperature, preferably for 1.5 minutes to 3.0 minutes. This is a method for producing a soft low-carbon cold-rolled steel sheet having excellent aging resistance.

[0011]

The reasons for limiting the numerical values of the C, Mn, S, sol. Al, N and B components of the present invention will be described.

C: The lower limit of C is limited from the viewpoint of aging. That is, if the amount is too small, the supersaturation degree of C does not increase even if the cooling rate after annealing is increased in overaging precipitation, so that a driving force for precipitation of intragranular carbides cannot be obtained.
The lower limit is 010%. Further, the upper limit is limited from the viewpoint of grain growth, and if it exceeds 0.025%, the grain growth is impaired in the annealing process. Therefore, even if high temperature annealing at 820 ° C. or more is performed, the crystal grain size number cannot be less than 8.0. It will be difficult. Preferably, it is 0.012 to 0.020.

Mn, S: Since Mn and S are important as precipitation nuclei of intragranular carbides, Mn is 0.05% or more and S is 0.00%.
3% or more. Further, when Mn is large, it causes hardening of the material, so that it is made 0.25% or less, and S is made 0.020% or less from the viewpoint of hot brittleness. Preferably, Mn is between 0.08 and 0.15
% And S are 0.005 to 0.010%.

Al: Al is necessary to promote the formation of AlN, so the lower limit is made 0.01%. If the content is too large, the material becomes hard, so the upper limit is made 0.10%. Preferably, it is 0.015 to 0.04%.

N: N is preferably small from the viewpoint of workability, but the upper limit is made 0.0030% in order to prevent an increase in steelmaking costs. Preferably, it is 0.0010 to 0.0020%.

B: B is a main constituent factor of the present invention, and Al
Residual solid N before annealing since it has higher nitride forming ability and forms BN
, The grain growth after annealing is improved. Therefore, to obtain this effect, the lower limit is 0.0003%
And However, excessive B becomes solid solution B and conversely inhibits grain growth during annealing, so the upper limit is made 0.0030%. Preferably, it is 0.0005 to 0.0020%.

Next, the reasons for limiting the continuous annealing conditions of the present invention will be described. Recrystallization annealing: In the quenching process of continuous annealing, solid solution C near the grain boundaries decreases with diffusion and carbide precipitation at the grain boundaries. As a result, in the vicinity of the grain boundary, a sufficient supersaturation degree is not achieved, and a carbide-free precipitation zone is generated. The solid solution C in the carbide-free precipitation zone increases the distance to the precipitation site,
A long overaging process is required.

Therefore, in order to reduce the volume of the carbide-free precipitation zone to realize delayed aging by, for example, overaging treatment for about 1.5 minutes, recrystallization annealing at 820 ° C. to 880 ° C. for 20 seconds to 60 seconds is performed. The crystal grain size number (JIS G 0552)
The grain boundary area is reduced to 6.5 or more and 8.0 or less.

The reason why the upper limit of the annealing temperature is set to 880 ° C. is that if the annealing temperature is set to be higher than 880 ° C., the characteristic will be degraded due to randomization of the crystal orientation. Further, the upper limit of the annealing time is set to 60 seconds in order to prevent an increase in the length of the annealing equipment and an increase in energy cost. Although it is desirable that the crystal grains be large, the crystal grain number should be 6.5 or more in order to prevent rough skin due to coarsening.

Primary cooling treatment: 650 after completion of recrystallization treatment
Slowly cooling at a cooling rate of 7 ° C / S or less to 740 ° C. This increases the amount of solid solution of C, and increases the degree of supersaturation of C required for precipitation of intragranular carbide in rapid cooling and overaging. If the quenching start temperature is lower than 650 ° C or higher than 740 ° C, sufficient supersaturation of solid solution C cannot be obtained due to the solid solubility limit of C in ferrite. Requires a longer overaging treatment. Therefore, preferably, the quenching start temperature is 650 to 740 ° C.

Secondary cooling treatment: Next, rapid cooling is performed to achieve a supersaturation rate enabling precipitation of intragranular carbide, but if the cooling rate is lower than 80 ° C./s, the density of intragranular carbide decreases. Therefore, a long overage processing time is required. On the other hand, if the temperature exceeds 250 ° C./s, intragranular carbides are finely dispersed, so that the yield strength increases and the ductility decreases. Therefore, the cooling rate is 80 to 250 ° C / s. Preferably, the cooling rate is between 100 and 180 ° C / S.

Overaging treatment: The cooling end point temperature and the overaging temperature of the secondary cooling treatment determine the distribution and growth of intragranular carbide. When the cooling end point temperature is higher than 375 ° C., the diffusion of solid solution C can be sufficiently ensured, but the solubility of solid solution C is large, so that the solid solution C cannot be sufficiently reduced. If the temperature is lower than 325 ° C., the diffusion of solid solution C becomes insufficient and a large amount of solid solution C remains. From this, the quenching end point temperature is 325 to 375 ° C.

If the cooling rate in the overaging process is larger than 0.5 ° C./s, the diffusion of solid solution C to the precipitation site becomes insufficient and the aging property is deteriorated. Therefore, the gradient overaging or isothermal aging at 0.5 ° C./s or less is performed. Apply That is, in the present invention, the overage processing includes the retention processing.

In the case of the present invention, it is sufficient that the time required for the overaging treatment is 1.5 minutes or more and 3.0 minutes or less. In other words, according to the present invention, excellent aging resistance is realized even with a short overage treatment of 1.5 to 3.0 minutes. Of course, the overaging treatment may be performed for a longer time, but it is unnecessary from an economic viewpoint.

The reason for limiting the continuous annealing conditions in the present invention as described above will be further supplemented based on experimental examples as follows.

FIG. 1 is a graph showing the general relationship between yield point elongation and bake hardenability for steel within the range specified in the present invention. Aging is evaluated by elongation at yield after artificial aging and bake hardenability. That is, 100 ℃ x 60
Yield point elongation (YP-El) after artificial acceleration aging for 170 minutes and artificial acceleration aging at 170 ° C x 20 minutes after 2% prestrain, and bake hardenability defined by the increase in yield strength before and after this aging
(Hereinafter referred to as BH), and it is necessary to make YP-El 0.3% or less after aging from the viewpoint of stretcher strain resistance. From this point of view, the results in FIG. 1 indicate that it is necessary to set the BH to 35 N / mm ² or less in order to consider it as late aging.

Next, in order to examine the relationship between BH between the annealing temperature and the grain size number, the grain growth and aging of steels c and g having the chemical components shown in Table 1 described below were examined. The annealing and soaking time was 30 seconds, and the sample was gradually cooled to 700 ° C at 4 ° C / s, rapidly cooled at 100 ° C / s, and then overaged at 350 ° C for 1.5 minutes.

The results are shown in FIG. 2. From this, it can be seen that when the components are in the above range, such as steel c, remarkable grain growth occurs around 800 ° C., and the crystal grain size number in a relatively low temperature range of 820 ° C. or higher. It can be seen that it is possible to realize 8.0 or less and to reduce the BH to 35 N / mm ² or less. Steel g is in the above component range except for B, but no remarkable grain growth is confirmed, and it is necessary to perform high-temperature annealing at more than 880 ° C. to make the crystal grain size number 8.0 or less, which is substantially difficult.

Further, in order to determine an appropriate overaging treatment time, the steel c in Table 1 was also annealed at an annealing temperature of 830 ° C., a soaking time of 30 seconds, and gradually cooled to 700 ° C. at 4 ° C./s. ° C
Quenching was performed at 100 ° C / s until the overaging treatment temperature was 350 ° C.

The results are shown in FIG. 3. As can be seen, in the overaging treatment for less than 1.5 minutes, the BH is larger than 35 N / mm ² , and the improvement of the aging property is insufficient. It is understood that the overaging time is required. Also, even if the overaging treatment is performed for more than 3 minutes, the effect of improving the aging effect is small
The upper limit of the overaging treatment time was set to 3 minutes in order to prevent the overaging treatment equipment from becoming longer and energy costs from increasing. Next, the operation and effect of the present invention as described above will be described more specifically with reference to examples.

[0031]

【Example】

(Example 1) Steel having the chemical components shown in Table 1 was melted, and a slab was manufactured by a continuous casting method.
Finish rolling at ~ 920 ° C to a plate thickness of 3.5 mm, steel ah
At 630 ° C. and at 720 ° C. for steel i. Thereafter, it was pickled, cold rolled to 0.8 mm, and continuously annealed.

The continuous annealing conditions are as follows: annealing temperature is 830 ° C., soaking time is 30 seconds, the temperature is gradually cooled to 700 ° C. at 4 ° C./s, and 350 ° C.
It was rapidly cooled at 100 ° C / s and overaged at 350 ° C for 1.5 minutes. Thereafter, skin pass rolling was performed at an elongation of 1.2%. As a test, the grain size number and BH value were determined. Table 2 shows the results.

Steels e and f are within the composition range of the present invention except for C. However, in the case of steel e, precipitation of intragranular carbide due to low C becomes insufficient. , The grain growth becomes insufficient because of a slightly high aging, and the aging properties are deteriorated.

Steel g and steel h are within the composition range of the present invention except for B, but in the case of steel g, grain growth was insufficient because B was not added, and in the case of steel h, the amount of B added was low. Is too large, grain growth is inhibited and aging is poor. In steel i, C
In addition, since B is not added, grain growth is poor and aging is poor.

The components of the steels a to d are within the range of the present invention, and have been reduced to BH35N / mm ² or less by overaging treatment for 1.5 minutes due to sufficient grain growth, and the aging properties are sufficiently improved.

Example 2 The effect of continuous annealing conditions on the aging properties of steel c shown in Table 1 was investigated using cold rolled steel sheets obtained under the same manufacturing conditions as in Example 1. Table 3 shows the annealing conditions and their properties. The test method is the same as in Example 1.

Under the heat treatment condition 1, grain growth is insufficient due to a low annealing temperature, and deterioration of aging is recognized. Under the heat treatment conditions 2 to 5, the aging property is poor because the cooling end point temperature and the overaging treatment temperature are out of appropriate temperatures. Under the heat treatment condition 6, the overaging treatment time is too short, so that the improvement of the aging property is insufficient.

The heat treatment conditions 7 to 9 are the annealing conditions of the present invention, and are set to BH 35 N / mm ² or less by a very short overage treatment of 1.5 minutes or more due to sufficient grain growth, and delayed aging is realized.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

The present invention makes it possible to produce a soft cold-rolled steel sheet having excellent aging resistance by short-time overaging by adjusting the steel composition and optimizing the continuous annealing heat cycle.
This is an excellent invention having the effect of preventing an increase in equipment required for reheating and the like and an increase in cost, thereby improving productivity.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between BH and YP-EL after aging treatment.

FIG. 2 is a graph showing the effect of annealing temperature on grain size number and BH for steels of each component.

FIG. 3 is a graph showing the effect of overaging time on BH.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C21D 8/00-9/48 C22C 38/00-38/06

Claims (1)

(57) [Claims] C .: 0.010 to 0.025%, Mn: 0.05 to 0.25%, S: 0.003% by weight.
~ 0.020%, sol.Al:0.01~0.10%, N: 0.0030% or less, B: 0.0003
~ 0.0030% Steel having a steel composition consisting of the balance of iron and unavoidable impurities is subjected to normal hot rolling, pickling and cold rolling, followed by continuous annealing at 820 ° C to 880 ° C for 20 seconds to 60 seconds. The crystal grain size is reduced by JIS G 0552 by the following recrystallization annealing.
The crystal grain size number is defined as 6.5 or more and 8.0 or less, and then gradually cooled from 650 to 740 ° C at a cooling rate of 7 ° C / s or less,
Furthermore, after cooling to 325 to 375 ° C at a cooling rate of 80 to 250 ° C / s, over-aging is performed at a cooling rate of 0.5 ° C / s or less, including retention from the cooling end point temperature. Method of producing soft low-carbon cold-rolled steel sheet excellent in quality.