JPH108143A - Production of thin steel sheet excellent in workability and hardenability in coating/backing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means capable of inexpensively and stably producing a thin steel sheet showing excellent deep drawability and aging resistance and furthermore having high hardenability in coating/backing. SOLUTION: A slab contg., by weight, 0.0100 to 0.025% C, 0.05 to 0.25% Mn, 0.003 to 0.020% S, <=0.08% P, 0.005 to 0.030% Al, <=0.0030% N, 0.0015 to 0.0035% B, and the balance Fe with inevitable impurities is heated to <=1200 deg.C, is subjected to hot rolling, is subjected to finish rolling at the Ar3 transformation point or above and is coiled at 600 to 750 deg.C. Then, it is subjected to cold rolling at >=70% rolling ratio, thereafter, in the subsequent continuous annealing, after recrystallization annealing at >800 to 880 deg.C for 10 to 60sec, at first, it is slowly cooled at <=7 deg.C/s cooling rate down to the temp. range of 650 to 740 deg.C and is subsequently subjected to rapid cooling at 40 to 250 deg.C/s cooling rate to the temp. range of 350 to 400 deg.C, and thereafter, overaging treatment is executed from the rapid cooling end temp. in the above at 0 to <=0.5 deg.C/s cooling rate for 2 to 5min.

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 thin steel sheet (cold rolled steel sheet) having excellent deep drawability, aging resistance and paint bake hardenability.

[0002]

[Prior art and its problems] As a means for producing a paint-bake hardenable steel sheet with excellent deep drawability by continuous annealing, the C content in steel has been conventionally reduced to less than 0.007% (hereinafter, the component ratio is% by weight. It is generally known to control the amount of solute C by adding a carbonitride forming element such as Ti or Nb. However, this method requires “lower C content” and “expensive additional elements such as Ti and Nb”, which increases the product cost and makes it difficult to control the amount of bake hardening. Was pointed out.

Therefore, a method for obtaining a paint bake hardenable steel sheet having excellent deep drawability without using expensive additional elements such as Ti and Nb has been proposed in Japanese Patent Application Laid-Open No. Hei 6-93376.
However, in the method proposed above, the C content of the steel is reduced to 0.
Since it is necessary to reduce it to a lower level of 0015% or less, there is great industrial difficulty, and it cannot be said to be a very advantageous means from the viewpoint of product cost.

On the other hand, in order to suppress the above-mentioned increase in product cost, ordinary "low carbon Al-killed steel containing Mn, S, sol. Al" is used, and the temperature is temporarily lowered to a temperature lower than the overaging temperature in the continuous annealing process. A method of producing a non-aging bake hardenable cold-rolled steel sheet in which the density of carbides in crystal grains is controlled by incorporating a step of cooling and then reheating and performing an overaging treatment has been proposed (Japanese Patent Publication No. 6-8483). reference). However, in this method, in the continuous annealing process, before performing the overaging treatment for promoting the growth of carbides, the steel is once cooled to a temperature lower than the overaging temperature to promote nucleation of intragranular carbides and then re-cooled. Since the step of heating is necessary, not only special equipment is required for reheating after quenching, but also there is an industrially unfavorable problem that leads to an increase in energy cost.

[0005] In order to improve the deep drawability of a low carbon Al-killed thin steel sheet, high-temperature winding is performed after hot rolling to reduce “solid solution C”, “aggregate precipitation of cementite”, and “precipitation of AlN”. A method of accelerating coarsening to obtain a high r-value is generally known. However, when high-temperature winding is performed, it is caused by "a decrease in pickling properties due to formation of a thick scale" and "unevenness of the material in the coil. This leads to problems such as causing a "low yield".

Therefore, as a method of improving the deep drawability of a low carbon Al-killed steel sheet without using high-temperature winding, a steel in which a small amount of B is added and the amount of sol. Al is controlled to a low value of less than 0.010% is used. There has been proposed a method of using the material steel (see Japanese Patent Publication No. 5-49728). However, it is extremely difficult industrially to control the Al content to such a low value, and therefore, this method cannot be said to be a practical means.

In view of the above, an object of the present invention is to provide a means capable of inexpensively and stably producing a thin steel sheet having excellent deep drawability and aging resistance and having high bake hardening ability. Was to provide.

[0008]

In order to achieve the above object, the present inventors have developed a yield point elongation: 0.3% or less, baking hardening amount (BH amount), especially after artificial acceleration aging at "100 ° C. × 60 minutes". : 3
After conducting various studies with the aim of establishing an industrially advantageous method for producing thin steel sheets exhibiting excellent deep drawability, aging resistance, and bake hardenability of 0 N / mm ² or more and an average r value of 1.5 or more,
"Along with regulating the chemical composition of the base steel and combining it with specific hot rolling and continuous annealing conditions, it has excellent deep drawability, aging resistance, and bake hardenability without significantly increasing the production cost. The stable production of thin steel plates becomes possible. "

[0009] The present invention has been made based on the above findings and the like, and is based on "C: 0.0100 to 0.025%, Mn: 0.
Steel containing 05 to 0.25%, S: 0.003 to 0.020%, P: 0.08% or less, Al: 0.005 to 0.030%, N: 0.0030% or less, B: 0.0015 to 0.0035%, with the balance being Fe and unavoidable impurities The piece is heated to a temperature not exceeding 1200 ° C., subjected to hot rolling, subjected to finish rolling at a temperature not lower than the Ar ₃ transformation point, wound up at 600 to 750 ° C., and then cooled at a rolling rate of 70% or more. After the cold rolling, the steel is subjected to recrystallization annealing at a temperature exceeding 800 to 880 ° C. for 10 to 60 seconds in the subsequent continuous annealing, and then cooled to 7 ° C./s or less until the temperature reaches 650 to 740 ° C. After cooling rapidly to a temperature range of 350 to 400 ° C at a cooling rate of 40 to 250 ° C / s, the cooling rate is 0 to 0.5 ° C / s or less from the quenching end temperature at that time. By performing overaging treatment for ~ 5 minutes, artificial promotion of "100 ° C x 60 minutes" Yield point elongation after aging: 0.3% or less, bake hardening amount (BH amount): 3
0N / mm ² or more, the average r value: excellent deep drawability of 1.5 or more, aging resistance, the major feature in the point "as the way can be manufactured inexpensively and stably the thin steel sheet showing a bake hardenability Yes doing.

That is, the present invention particularly relates to a low-carbon Al to which B having a high nitride forming ability is added in order to reduce solid solution N before annealing a cold-rolled sheet and obtain a texture effective for deep drawability. B using killed steel as material steel and adding annealing conditions
By setting such that the effect of the coil is maximized, it is possible to reduce the pickling property due to the formation of a thick scale in the hot rolling process and to make the winding at a relatively low temperature without concern about the unevenness of the material in the coil. In order to achieve good deep drawability of cold rolled sheet products even when the work is carried out, excellent deep drawability, aging resistance,
It has been completed in accordance with the technical idea of making it possible to combine the properties of bake hardening. Hereinafter, in the present invention, the chemical composition of the material steel and the manufacturing conditions of the thin steel sheet (cold rolled steel sheet) are limited as described above. Explain why.

[0011]

[Action]

[A] Chemical composition of base steel C: The lower limit of the C content is limited from the viewpoint of aging. That is, when the C content is lower than 0.0100%, the supersaturation of C does not increase even if the cooling rate after annealing is increased in overaging precipitation, so that the density of intragranular carbides decreases, The tendency to cause deterioration becomes significant. On the other hand, the upper limit of the C content is limited from the viewpoint of deep drawability. That is, if the C content exceeds 0.025%, the growth of a structure effective for deep drawing becomes impeded by the re-dissolution of C from carbide in the annealing process. Therefore, the C content is 0.0100 to 0.025.
%, But is preferably adjusted to 0.013 to 0.020%.

Mn and S: Mn and S are important components that form MnS and serve as precipitation nuclei for nitrides and intragranular carbides. To obtain this effect, the Mn content is 0.05% or more, and Content: 0.003% or more must be ensured. However, if the Mn content exceeds 0.25%, deep drawability is deteriorated, and S
If the content exceeds 0.020%, deterioration of hot brittleness becomes noticeable. Therefore, the Mn content is set to 0.05 to 0.25% and the S content to 0.003 to 0.020%, respectively.
Content is 0.07 to 0.015%, S content is 0.003 to 0.010%
It is better to adjust.

P: P has the effect of increasing the strength of the steel sheet. It is sufficient that P contains an arbitrary amount in order to secure the desired strength. Therefore, the upper limit of the P content was determined to be 0.08%.

Al: Al has the effect of forming AlN and promoting the capture of solid solution N. However, if its content is less than 0.005%, the desired effect cannot be obtained by the above-mentioned effect.
If the content exceeds 0.030%, the addition of B
The precipitation rate of N decreases, and the precipitation of fine AlN during the annealing process is promoted, resulting in deterioration of deep drawability. Therefore, Al
The content was determined to be 0.005 to 0.030%.

N: N, which is one of the inevitable impurities, is preferably as low as possible from the viewpoint of workability, but the upper limit of the N content is set to 0.0030% in view of steelmaking costs.

B: B has a higher nitride-forming ability than Al.
The effect of improving the grain growth after annealing is exhibited by forming N to reduce the residual solid solution N before annealing, but if the content is less than 0.0015%, the desired effect by the above-mentioned effect may be obtained. On the other hand, if the B content exceeds 0.0035%, excessive B becomes solid solution B and conversely inhibits grain growth during annealing. Therefore, the B content is 0.0015 to 0.00
Although set to 35%, it is preferable to adjust it to 0.00018 to 0.0025%.

[B] Manufacturing Conditions a) Hot Rolling Conditions A steel slab (slab) having the above chemical composition obtained by melting and continuous casting in the usual manner is first subjected to hot rolling. If the slab heating temperature is too high, a part of "nitride" and "MnS, which is a precipitation site of intragranular carbide after annealing" will form a solid solution, and deep drawability and aging resistance due to a decrease in the precipitation rate of BN Therefore, it is necessary to take care that heating of the slab during hot rolling does not exceed 1200 ° C. The finish rolling is performed at a temperature equal to or higher than the Ar ₃ transformation point, in order to prevent a decrease in the r value of the cold-rolled annealed sheet due to coarsening of the hot-rolled sheet.

The winding temperature after finish rolling is set at 600 ° C. or higher to promote carbide agglomeration and precipitation, to enhance the effect of the addition of B and to improve the grain growth during continuous annealing to obtain a high r value. It is necessary to On the other hand, the upper limit of the winding temperature is set to 750 ° C. from the viewpoint of preventing the pickling property from being deteriorated due to the increase in the scale thickness. However, it is more preferable to set the winding temperature to 630 to 720 ° C.

B) Cold Rolling Conditions When the rolling reduction in the cold rolling is small, a sufficient r value cannot be obtained. Therefore, it is necessary to carry out the cold rolling at a rolling reduction of 70% or more. Need not be particularly limited. In addition, more preferable cold rolling rate is 75-85%.
It is.

C) Conditions for Continuous Annealing In the present invention, the annealing conditions are defined in order to achieve sufficient grain growth and to obtain a high r value in a cold-rolled sheet of B-added low carbon Al-killed steel. In the present invention, the cold-rolled sheet is first subjected to recrystallization annealing at a temperature exceeding 800 ° C. for 10 seconds or more. In this case, if the annealing temperature is too high, the characteristics will be degraded due to randomization of the crystal orientation, so the upper limit of the annealing temperature is 880 ° C. 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.

Subsequently, the cold-rolled sheet is gradually cooled from the time when the recrystallization annealing is completed to a quenching start temperature (650 to 740 ° C.). In order to effectively generate intragranular carbides in the rapid quenching and overaging processes that are continued with increasing the amount, the temperature is set to 7 ° C / s or less (preferably 5 ° C / s or less). When 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 ferrite in the ferrite, and the carbide density in the grains is low. The quenching start temperature is set to 650 to 740 ° C. since the aging property is deteriorated due to the decrease in quenching.

The quenching process following the slow cooling is necessary to secure the driving force for the precipitation of intragranular carbides. However, under the production conditions according to the present invention, MnS, which is a precipitation site of intragranular carbides, precipitates coarsely. Precipitation of intragranular carbide is promoted. for that reason,
Sufficiently good aging resistance can be realized without performing the conventionally proposed treatment of “supercooling to a temperature lower than the overaging temperature and performing nucleation and then reheating”. The cooling rate during this quenching process is 40 ° C / s
If it is lower than this, the intragranular carbide density decreases, so that a long overaging treatment time is required, while 250 ° C / s
If it is higher than this, the intragranular carbide is finely dispersed, so that the yield strength increases and the ductility decreases. Therefore, the cooling rate in the rapid cooling process is set to 40 to 250 ° C./s, but is preferably set to 60 to 150 ° C./s.

The quenching end point temperature and the overaging temperature determine the distribution and growth of intragranular carbides, affect the residual amount of dissolved C, and greatly affect the aging resistance. Rapid cooling end point temperature is 4
When the temperature is higher than 00 ° C., the diffusion of the solid solution C can be sufficiently ensured, but the solubility of the solid solution C is large, so that the solid solution C is not sufficiently reduced as a result. On the other hand, the quenching end point temperature is 350
If the temperature is lower than ℃, the diffusion of solid solution C becomes insufficient and a large amount of solid solution C remains. Therefore, the quenching end point temperature is set to 350 to 400 ° C. in order to sufficiently reduce the solid solution C and secure excellent aging resistance.

The overaging treatment is carried out by isothermal overaging in which the above-mentioned quenching end point temperature is maintained or by gradient overaging in which the temperature is gradually lowered from the quenching end point temperature at a sufficiently low cooling rate. That is, the cooling rate in the overaging process is 0.5 ° C
If it is greater than / s, the diffusion of solid solution C to the precipitation site becomes insufficient and the aging property deteriorates. Therefore, a gradient overaging or isothermal overaging of 0.5 ° C / s or less is applied. In any case, the overaging treatment time is 2 to 5 minutes.

By subjecting the steel slab having the chemical composition specified in the present invention to treatment according to the above conditions, a thin steel sheet excellent in deep drawability, aging resistance and bake hardenability can be obtained. The production method will be described more specifically with reference to examples.

[0026]

【Example】

[Example 1] First, steels having the chemical compositions shown in Tables 1 and 2 were smelted and tapped to form a slab by a continuous casting method.
The slab is heated to 1100 to 1150 ° C. and then subjected to hot rolling.
(Temperature higher than the Ar ₃ transformation of any steel) was finish-rolled to a thickness of 4.0 mm and wound around a coil at 640 to 660 ° C.

[0027]

[Table 1]

[0028]

[Table 2]

Next, the hot-rolled coil is unwound, pickled, and then cold-rolled to a thickness of 0.8 mm (a cold-rolling ratio of 80).
%), Followed by continuous annealing. In the continuous annealing, the annealing temperature was set to 820 ° C., the soaking time was set to 30 seconds, and after the soaking, 7
Slowly cool to 00 ° C at a cooling rate of 4 ° C / s, then 380 ° C
Was rapidly cooled at a cooling rate of 80 ° C./s, and then over-aged from 380 ° C. to 330 ° C. over 2.5 minutes (cooling rate: 0.3 ° C./s). Then, the steel plate after the overaging treatment was subjected to skin pass rolling at an elongation of 1.2% in accordance with a conventional method.

The mechanical properties of each of the thus produced cold rolled steel sheets after the artificial acceleration aging of “100 ° C. × 60 minutes” were determined. The results are shown in Tables 1 and 2.

In general, in order to be regarded as delayed aging, the yield point elongation (YP-El) must be 0.3% or less after artificial acceleration aging at "100 ° C. × 60 minutes" from the viewpoint of stretcher strain resistance. Further, in order to use the steel sheet for deep drawing, the average r value in three directions needs to be 1.5 or more. However, as can be seen from the results shown in Tables 1 and 2, the thin steel sheets manufactured according to the conditions specified in the present invention (the steel sheets according to Inventive Examples 1 to 16) have excellent deep drawability and aging resistance. Properties and paint baking properties are realized.

On the other hand, in Comparative Examples 17 and 24, since the carbon content of the base steel was too low, the precipitation of intragranular carbide during the continuous annealing process was insufficient, and solid solution C remained. As a result, the aging property of the product steel sheet is inferior. On the other hand, in Comparative Examples 18 and 25, since the C content of the base steel was too high, the deep drawability of the product steel sheet was inferior.

In Comparative Examples 19 and 26, the solute N was left because the Al content of the base steel was too small, resulting in inferior aging properties of the product steel sheet. Comparative Examples 20 and 27
In this case, since the Al content of the base steel is excessive, the deep drawability of the product steel sheet is inferior.

In Comparative Examples 21 and 28, the N
Since the content is too high, the deep drawability of the product steel sheet is inferior. In Comparative Examples 22 and 29, the grain growth was poor because B was not added to the base steel, and the deep drawability of the product steel sheet was poor. On the contrary, in Comparative Examples 23 and 30, the B content of the material steel is excessive, so that the grain growth is hindered and the deep drawability of the product steel sheet is inferior.

Example 2 First, steels A to Z having a chemical composition satisfying the specified conditions of the present invention shown in Table 3 were melted and tapped, and slabs were formed by continuous casting.

[0036]

[Table 3]

Next, each of the slabs was hot-rolled under the conditions shown in Tables 4 and 5 (the finish rolling was carried out at a temperature of Ar ₃ transformation or higher). At a temperature shown in FIG.

[0038]

[Table 4]

[0039]

[Table 5]

Next, the hot-rolled coil is rewound, pickled, and then cold-rolled to a thickness of 0.8 mm (a cold-rolling ratio of 80).
%), Followed by continuous annealing. In the continuous annealing, the conditions shown in Tables 4 and 5 were applied except that the soaking time in the annealing process was set to 30 seconds.

The mechanical properties of each of the thus produced cold-rolled steel sheets after the artificial acceleration aging of “100 ° C. × 60 minutes” were determined. The results are shown in Tables 4 and 5.

As is clear from the results shown in Tables 4 and 5, the thin steel sheets manufactured according to the conditions specified in the present invention (the steel sheets according to Examples 31 to 35 and 44 to 48 of the present invention) were excellent. Deep drawability, aging resistance and paint seizure are realized.

On the other hand, in Comparative Examples 36 and 49, the deep drawing property of the product steel sheet was inferior because the heating temperature of the slab was too high. In Comparative Examples 37 and 50, the winding temperature was too low, resulting in inferior deep drawability of the product steel sheet. In Comparative Examples 38 and 51, the grain growth was insufficient because the annealing temperature was too low, and the deep drawability of the product steel sheet was inferior.
On the other hand, in Comparative Examples 39 and 52, since the annealing temperature was too high, the crystal orientation was randomized, resulting in inferior deep drawability of the product steel sheet.

In Comparative Examples 40, 41, 53 and 54, the overageing temperature was out of the proper temperature, resulting in poor aging of the product steel sheet. Further, in Comparative Examples 42 and 55, the aging property of the product steel sheet was deteriorated because the gradient of the overaging treatment temperature was large. In Comparative Examples 43 and 56, the aging property of the product steel sheet was poor because the overaging treatment time was too short.

[0045]

As described above, according to the present invention, it is possible to adjust the chemical composition of the base steel, optimize the hot rolling conditions and optimize the continuous annealing heat cycle without applying any means that causes extra cost. Industrially useful effects such as stable production of thin steel sheets having excellent deep drawability, aging resistance and paint bake hardenability are provided.

Claims (1)

[Claims] C: 0.0100 to 0.025% by weight,
Mn: 0.05 to 0.25%, S: 0.003 to 0.020%, P: 0.
08% or less, Al: 0.005 to 0.030%, N: 0.0030
%, B: 0.0015 to 0.0035%, and the balance is heated to a temperature not exceeding 1200 ° C. by heating a slab consisting of Fe and unavoidable impurities, and finish rolling at a temperature not lower than the Ar ₃ transformation point. And then winding at 600-750 ° C,
Next, after performing cold rolling at a rolling rate of 70% or more, recrystallization annealing is performed at more than 800 to 880 ° C. for 10 to 60 seconds in the subsequent continuous annealing, and then the temperature range reaches 650 to 740 ° C. Until it cools down slowly at a cooling rate of 7 ° C / s or less, and then 350-400 ° C at a cooling rate of 40-250 ° C / s.
After rapid cooling to the temperature range of, the over-aging treatment is performed for 2 to 5 minutes at a cooling rate of 0 to 0.5 ° C / s or less from the quenching end point temperature at that time, deep drawability, aging resistance And a method for producing a thin steel sheet having excellent paint bake hardenability.