JPH07242996A - Steel sheet for deep drawing, excellent in formability, and its production - Google Patents

Abstract

PURPOSE:To stably produce a steel sheet for deep drawing, having superior ductility equal to that of a material prepared by the conventional reheating and rolling process, even if this steel sheet is produced by means of direct rolling. CONSTITUTION:The steel sheet for deep drawing is constituted so that it has a composition consisting of <=0.01% C, <=0.1% Si, 0.01-2% Mn, <=0.01% P, <=0.005% S, <=0.1% Ti, 0.005-0.1% acid soluble Al, <=0.005% N, and the balance Fe with inevitable impurities or further containing Nb and/or B and satisfying the relations represented by three inequalities of [P+S<0.012%], [Ti*>0.01%], and [Ti*(%)XP(%)<=0.0002], where Ti*=Ti-48(C/12+N/14+S/32) is satisfied. Further, a continuously cast slab of this composition is used and hot rolling is started while regulating the temp. to a figure not lower than 950 deg.C and finished at >=850 deg.C. The resultant steel plate is coiled at 400-700 deg.C, cold-rolled at 60-95% draft, and then annealed at a temp. in the region between the recrystallization temp. and the Ac3 point.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a deep-drawing thin steel sheet having excellent formability, which is used by press working into various shapes, and a method for producing the same.

[0002]

2. Description of the Related Art At present, a thin steel sheet for deep drawing is generally manufactured by hot rolling a slab manufactured by continuous casting, and then subjecting it to pickling, cold rolling and annealing. Is. However, in the case of producing a steel sheet using the continuously cast slab as a rolling material in this way, conventionally, the continuously cast slab is once cooled to room temperature and then inserted into a heating furnace to a temperature of 1050 to 1250 ° C. It was common to reheat and then hot roll. However, after the two oil crises, a technique called "heat piece insertion" or "hot charge" for the purpose of energy saving has become mainstream. This is a method in which the continuously cast slab is not cooled to room temperature but is inserted into a heating furnace when the continuously cast thermal slab is cooled to around 800 ° C. Further, a technique called "direct feed rolling", in which hot rolling is performed without using a heating furnace and only by supplementary heating with an edge heater or the like, has come into practical use.

However, in the direct-feed rolling process, unlike the conventional process, the slab is hot-rolled without falling below the Ar ₃ transformation point, so that the mechanical properties are deteriorated due to the difference in the morphology of precipitates. I was seen. That is, according to the direct rolling process, since the time held in the γ region before hot rolling is shorter than that in the slab reheating method, the precipitates cannot be sufficiently precipitated or cannot be coarsened. The characteristics (especially elongation and r value) of the thin steel sheet manufactured by
Was inferior to the thin steel sheet manufactured by the reheating method.

Therefore, some proposals have been made in order to prevent the characteristics of the thin steel sheet manufactured by direct rolling from being deteriorated as compared with the case of applying the reheating method. JP-A-59-
One of those proposals is also published in Japanese Patent Publication No. 89723, which discloses that a rare steel element (REM) is added to a material steel,
It is said that the addition of one or more selected from Ca, Ti and Mg reduces the in-plane anisotropy of the steel sheet material and improves the workability. Especially, Ti and Mg are more
It is said that it is preferable to add it together with EM and Ca, and by this, a grain size control structure can be obtained and the in-plane anisotropy of mechanical properties becomes small. However, such addition of REM and Ca leads to an increase in cost, and the absolute values of elongation and r-value could not be remarkably improved by the method proposed.

Further, in Japanese Patent Laid-Open No. 61-281824, by adding Ti in a range determined by the C and N contents to the raw steel and combining the direct feed rolling method, a slab reheating method is used. A method of manufacturing a thin steel sheet that enables non-aging with a small Ti addition amount has been proposed. However, even with this method, the elongation and the r value could not be improved significantly.

On the other hand, Japanese Patent Laid-Open No. 62-287017 proposes a method of controlling the slab transportation so that a predetermined time elapses before the start of hot rolling in the case of direct rolling after continuous casting. Has been done. By holding the hot slab before hot rolling for a predetermined time or more in this way, it is said that good mechanical properties can be obtained even in the direct rolling method, as in the slab reheating method. However, according to this method, a facility for temporarily stagnating the slab between the continuous casting machine and the hot rolling mill is required, and the facility merit of "eliminating the heating furnace by direct rolling" is reduced. Although the product steel sheet has a relatively high elongation and r-value, it cannot be said that the value is still sufficiently satisfactory, and the problem that the properties are not stable has not been solved.

As described above, even the proposals made so far regarding the improvement of the properties of the thin steel sheet produced through direct rolling have a problem that the obtained thin steel sheet is inferior to that obtained by the slab reheating process in terms of elongation and r value. It was still unsolved. Therefore, the object of the present invention is to provide a conventional process (reheating process) even when manufactured through direct rolling.
It is to establish means for stably providing a deep-drawing thin steel sheet having good workability (elongation, r value) similar to that of the material.

[0008]

Means for Solving the Problems The inventors of the present invention conducted a detailed study on the chemical composition and manufacturing conditions of a steel sheet in order to achieve the above-mentioned object, and as a result, obtained new findings as shown in the following a) to d). I was able to get it. a) When manufacturing a thin steel sheet for deep drawing by a process incorporating a direct rolling method, in particular, the content of S that inevitably penetrates into the steel has a great influence on the elongation of the obtained product. In order to secure an excellent elongation of more than 51% in a steel sheet, it is an essential requirement to reduce the S content to 0.005% or less (hereinafter,% representing the composition ratio is% by weight).

[0009] Fig. 1 shows "S content and ductility," which was obtained by investigating "a cold rolled steel sheet manufactured through a process corresponding to direct rolling" and "a cold rolled steel sheet manufactured through a reheating process". Is a graph showing the “relationship of”. Here, the above survey was conducted as follows. That is, first, the steel shown in Table 1 is melted in an experimental furnace and cast into a split mold to form a slab, and then, as a direct-feed rolling simulation, the mold is removed and waiting for cooling to 1050 ° C. is followed by hot rolling. As a slab reheating rolling simulation,
After removing the mold and cooling the slab taken out to room temperature, the slab was heated again in a furnace at 1200 ° C. for 60 minutes and taken out.
After cooling to 0 ° C., hot rolling was started. Then, after finishing the hot rolling at a finishing temperature of 920 ° C. and a finishing plate thickness of 5 mm, the surface of the hot rolled plate obtained by winding at 450 ° C. is ground to remove the scale, and a rolling reduction of 82 % Cold-rolled to a plate thickness of 0.8 mm. Further, using an infrared heating furnace, a heat treatment corresponding to continuous annealing at a temperature rising rate: 10 ° C / sec, a soaking temperature: 860 ° C, a soaking time: 30 sec, and a cooling rate: 10 ° C / sec is performed, and then this is performed. Elongation rate: 0.3% after temper rolling, JIS 5 test pieces are taken from the cold-rolled steel sheet thus obtained and subjected to a tensile test to determine elongation (E
L) was investigated.

[0010]

[Table 1]

From the survey results shown in FIG. 1, the elongation (EL) decreases as the S content of the steel sheet increases, and especially when the S content exceeds 0.005%, the material equivalent to the direct rolling process is re-slab-recycled. It can be seen that the deterioration of elongation is extremely remarkable as compared with the material equivalent to the heating process.

B) On the other hand, Ti has an action of increasing the r value of the steel sheet to improve the formability, so that it is necessary to produce a deep-drawing thin steel sheet showing excellent formability by a process incorporating the direct rolling method. Is a component that must be contained by all means. However, if its content becomes excessive, P existing in the steel and FeTiP are formed to precipitate and deteriorate the elongation. Therefore, it is necessary to contain Ti according to strictly controlled conditions.

C) As described above, in the Ti-added steel, P in the steel is
Since the precipitation of FeTiP deteriorates the elongation of the steel sheet, P
The absolute value of the content must be reduced below a specific value, and the regulation of the P content associated with the Ti content is also essential.

D) However, when the steel whose composition has been adjusted as described above is processed according to predetermined conditions, a thin steel sheet for deep drawing which has a high elongation and a high r value and is excellent in formability can be obtained by a process including a direct rolling method. Be able to manufacture stably.

The present invention has been made on the basis of the above findings and the like. "C: 0.01% or less, Si: 0.1% or less, Mn: 0.
01 to 2%, P: 0.01% or less, S: 0.005% or less,
Ti: 0.1% or less, acid-soluble Al: 0.005 to 0.1%, N: 0.005% or less, or at least one of Nb: 0.003 to 0.05% and B: 0.0003 to 0.0030%. However, P + S <0.012%, Ti ^* > 0.01%, Ti ^* (%) x P (%) ≤ 0.0002 [However, Ti ^* = Ti-48 (C / 12 + N / 14 + S / 32)] 3 Satisfy the relationship expressed by two equations, with the balance being Fe
In addition, by constructing a thin steel sheet for deep drawing into a component composition consisting of unavoidable impurities, it is possible to stably impart excellent formability. After the steel is continuously cast into a slab, hot rolling is started while the temperature of the slab surface does not fall below 950 ° C, and hot rolling is completed at 850 ° C or higher,
Furthermore, after rolling in the temperature range of 400 to 700 ° C, cold rolling is performed at a rolling reduction of 60 to 95%, and then the recrystallization temperature is higher than A.
c It is possible to stably manufacture a thin steel sheet for deep drawing with excellent formability by annealing in a temperature range of ₃ points or less ”.

In the following, the reason why the component composition of the steel sheet and the manufacturing conditions thereof are limited as described above in the present invention will be explained together with its action.

[Action]

A) Component ratio <C> C is an impurity element that is inevitably mixed in the steel, but since it adversely affects the formability of the steel sheet, the smaller the content, the better. However, “fixing C as a carbide by adding Ti” is effective as a means for avoiding the adverse effect of C, which is unavoidable to be mixed, but it is fixed as the C content increases. The amount of addition of Ti required for steel increases, which causes the steel to harden and raises the manufacturing cost.
It was set as the upper limit of the content.

<Si> Si is also an impurity element that is inevitably mixed in steel, and the lower the content, the better.
That is, if the Si content is high, a specific pickling failure may occur or the plating property may be deteriorated. Therefore, 0.1%, which is the allowable limit, is set as the upper limit of the Si content.

<Mn> Mn has a function of strengthening a steel plate,
For this purpose, it is a component that can be added appropriately without any problem. However, if its content exceeds 2%, the transformation point is remarkably lowered, and TiC becomes finer to cause deterioration of elongation, which is not preferable. However, molten iron usually contains about 0.01 to 0.05% of Mn, and lowering Mn below that is not preferable because it leads to cost increase. Therefore, the Mn content is set to 0.01 to 2%.

<P> P is an impurity element that is inevitably mixed into the steel. And, as mentioned above, this P is FeTi
Since it precipitates as P and deteriorates the elongation of the steel sheet, the lower the P content is, the more preferable from the viewpoint of ensuring good elongation. However, since P also has the effect of increasing the r value of the steel sheet, In the present invention, the adverse effect on elongation is allowed 0.01%
It was determined that P may be contained in the range below. However,
In order to sufficiently suppress the precipitation of FeTiP and suppress the adverse effect on the elongation, the relationship with the effective Ti amount (Ti ^* ) described below is "Ti ^*
(%) × P (%) ≦ 0.0002 ”must be satisfied. Furthermore, it is necessary to limit the P content in association with the amount of S that has a significant adverse effect on elongation, and in order to secure the desired elongation value, it is necessary to satisfy the condition represented by the formula “P + S <0.012%”. There is.

<S> In the present invention, S is an element having an important meaning beyond the conventional recognition. That is, as described above, according to the study by the present inventors, "the elongation of the cold rolled steel sheet obtained when the steel having a high S content is directly rolled and reduced,
On the contrary, in steels with a low S content, the decrease in elongation is small. " The mechanism has not been fully clarified yet, but it is presumed as follows. That is,
S is an element that easily forms sulfides, but a hot rolling method such as direct rolling in which the holding time is short in the γ region does not sufficiently precipitate sulfides, so that a steel with a high S content (S-containing Quantity
In steels exceeding 0.005%), solid solution S remains when TiC is precipitated. Then, S exhibits an interaction that attracts each other with Ti that easily forms a sulfide, so that the activity of Ti decreases due to the remaining solid solution S, and the precipitation of TiC shifts to the low temperature side. Therefore, the precipitates (TiC) cannot be coarsened and are dispersed as fine TiC in the hot-rolled sheet, so that the elongation deteriorates. On the other hand, in low S steel (steel with S content of 0.005% or less), the amount of solid solution S is originally small, so Ti
The precipitation of C starts at a higher temperature than that of the high S material, and therefore the precipitate is sufficiently coarsened so that the elongation does not deteriorate. As described above, when the S content exceeds 0.005%, the elongation of the cold-rolled steel sheet obtained is significantly deteriorated.
The following was set. However, it is necessary to limit the S content in association with the P content, which has a significant adverse effect on elongation, and to ensure the desired elongation value, the condition represented by the formula “P + S <0.012%” must be satisfied. The fact that there is is as described above.

<Ti> In order to improve the r value of the cold rolled steel sheet and suppress strain aging, it is important that solid solution C and N do not exist in the hot rolled sheet. Ti is an interstitial solid solution element. It is a component added for the purpose of fixing C and N. And enough r
To improve the value and suppress strain aging, the effective Ti given by
The amount (Ti ^* ) must be above 0.01%. Ti ^* = Ti-48 (C / 12 + N / 14 + S / 32) However, if Ti is added excessively, the steel becomes hard, so the upper limit of the Ti content must be 0.1%, and the effective Ti amount ( Even if Ti ^* ) is excessive, FeTiP precipitates and deteriorates the elongation. Therefore, in order to secure good elongation, use the formula “Ti ^* (%) × P (%) ≦ 0.0002” as described above. It is necessary to satisfy the conditions to be met.

<Acid-Soluble Al> In order for the steel to be sufficiently deoxidized, the content of acid-soluble Al must be 0.005% or more.
However, if the content exceeds 0.1%, the steel becomes hard and the elongation decreases, which is not preferable.

<N> N is also an impurity element that is unavoidably contained in steel, and the lower the content, the better in terms of improving ductility. When the N content exceeds 0.005%, the amount of Ti added to fix this increases, which not only causes an increase in cost, but also increases the amount of TiN precipitated and deteriorates ductility. Therefore, the N content was set to 0.005% or less.

<Nb and B> Since any of these components has the effect of improving the workability of the cold-rolled steel sheet, they may be added alone or in combination of two kinds if necessary. However, the content of each was determined for the following reasons. Nb: The grain size of the hot-rolled sheet is reduced to promote the development of a recrystallized texture which is preferable for deep drawability. However, if the content is less than 0.003%, such an effect does not appear, while on the other hand,
If the content exceeds 05%, the recrystallization temperature rises and annealing becomes difficult.

B: Secondary processing brittleness may occur at a site where severe molding is performed. B segregates at the crystal grain boundaries and strengthens the grain boundaries, and therefore has a function of preventing secondary work embrittlement. However, if the B content is less than 0.0003%, this effect cannot be expected, while if the B content exceeds 0.0030%, the above effect is saturated.

B) Manufacturing Conditions In the method of the present invention, first, the steel melted to have the above-mentioned predetermined composition is made into a slab by continuous casting, which is directly fed and rolled. The steel whose components have been adjusted as described above is a steel suitable for direct-feed rolling, but it goes without saying that the same performance is exhibited even if hot rolling is performed by the slab reheating method. The hot rolling is preferably performed in the γ range. If the finishing temperature is lower than 850 ° C, abnormal grain growth will occur during winding and the surface properties will deteriorate. Therefore, in order to secure a finishing temperature of 850 ° C. or higher, it is necessary to start hot rolling of the continuously cast slab so that the temperature of the slab surface does not fall below 950 ° C.

In the steel having the chemical composition according to the present invention, the effect of the coiling temperature on the mechanical properties is small. However, if the coiling temperature is high, the coiled shape may collapse and the scale loss may increase, so the upper limit of the coiling temperature is set to 700 ° C. On the other hand, if the winding temperature becomes too low, the load of the down coiler increases, so the lower limit of the winding temperature was set to 400 ° C. When the precipitation of FeTiP is completely suppressed and higher formability is required, the coiling temperature is preferably 400 to 500 ° C.

In cold-rolling rolling which is carried out after pickling and the like to remove the scale after winding, if the rolling reduction is less than 60%, the r value of the cold-rolled steel sheet obtained will be low, which is not preferable. On the other hand, a rolling reduction of more than 95% means that the thickness of the hot-rolled sheet becomes correspondingly thick, and since such a hot-rolled sheet is difficult to manufacture, the reduction of the cold rolling is performed. The rate was defined as 60-95%.

In the annealing after cold rolling, if the annealing temperature is lower than the recrystallization temperature, the steel sheet is not softened while being hard, so that elongation cannot be expected. On the other hand, if annealing is performed at a temperature of Ac ₃ or higher, it is high. Since the texture that develops the r value is destroyed by the α → γ transformation, the r value becomes low. Therefore, the annealing temperature was determined to be higher than the recrystallization temperature and lower than the temperature at the Ac ₃ point. In the above annealing applied to the steel sheet having the composition according to the present invention, the temperature rising rate does not affect the mechanical properties, so the annealing method may be either continuous annealing or box annealing. It is also possible to perform hot dip galvanizing after annealing using a hot dip galvanizing line.

Next, the present invention will be described with reference to examples.

【Example】

Example 1 First, simulating continuous casting, steel having the composition shown in Table 2 was melted in an experimental furnace and cast into a split mold to form a slab, and then the mold was removed and cooled to 1050 ° C. After waiting, hot rolling was started. Then, hot rolling was finished at 920 ° C to obtain a hot-rolled sheet having a plate thickness of 5 mm, which was wound at 450 ° C.

[0031]

[Table 2]

Next, the surface of the obtained hot-rolled sheet was ground to remove the scale, and cold-rolled at a rolling reduction of 82% to give a sheet thickness of 0.8 mm. Furthermore, using an infrared heating furnace, the rate of temperature rise: 10 ° C / sec, soaking temperature: 840 ° C, soaking time: 30
After performing a heat treatment corresponding to continuous annealing at sec and cooling rate: 10 ° C./sec, a temper rolling of 0.3% was subsequently performed.

From each cold-rolled steel sheet thus obtained, J
The IS5 test piece was sampled and subjected to a tensile test to determine its elongation (E
L) was investigated. The results of this elongation (EL) investigation are arranged in relation to the S content and P content of the cold rolled steel sheet and are shown in FIG. As is clear from the results shown in FIG. 2, it is understood that any cold-rolled steel sheet satisfying the conditions specified in the present invention will exhibit good elongation and will exhibit excellent formability.

Example 2 Steels having the compositions shown in Table 3 were melted and cold-rolled steel sheets were manufactured under the same conditions as in Example 1.

[0035]

[Table 3]

From each cold rolled steel sheet thus obtained, J
The IS5 test piece was sampled and subjected to a tensile test to determine its elongation (E
L) was investigated. The results of this elongation (EL) investigation are arranged in relation to the P content and effective Ti content (Ti ^* ) of the cold rolled steel sheet and are shown in FIG. As is clear from the results shown in FIG. 3, it is understood that any cold-rolled steel sheet satisfying the conditions specified in the present invention will exhibit good elongation and exhibit excellent formability.

Example 3 Steels having the compositions shown in Table 4 were melted and cold-rolled steel sheets were manufactured under the same conditions as in Example 1.

[0038]

[Table 4]

From each cold-rolled steel sheet thus obtained, J
The IS5 test piece was sampled and subjected to a tensile test to determine the tensile strength,
The elongation and the r-value were examined, and a separately collected test piece was formed into a cup having a drawing ratio of 1.8, and then a drop weight test was performed to obtain a brittle transition temperature, which was used as an index of secondary work embrittlement resistance. The results are shown in Table 5.

[0040]

[Table 5]

The following can be seen from the results shown in FIG. That is, the cold-rolled steel sheets (steel sheets obtained by the test types A, D, I, K, L, N, and O) obtained under the conditions specified in the present invention have both high elongation and r value and are excellent in forming. Have sex. On the other hand, in the cold-rolled steel sheets obtained by the test types B, C, E, F, G and H, the content of C, Si, Mn, sol.Al, N or Ti is within the range specified by the present invention. Therefore, the tensile strength is high, and the elongation and r value are small, resulting in poor moldability.

Further, the cold rolling steel sheet obtained by the test type J using a steel having a high B content as a raw material has a transition temperature lower than −100 ° C. If B is contained within the specified range of the invention, a transition temperature (−90 ° C.) that is practically sufficiently satisfactory is achieved, and in the test type J, the addition of B is excessive and the cost increases. Can be said. Furthermore, in the cold-rolled steel sheet obtained by the test type M, the Nb content is higher than the range specified in the present invention, so the recrystallization temperature becomes high and the grain growth becomes insufficient. It's getting low.

[0043]

[Summary of Effects] As described above, according to the present invention, a cold rolled steel sheet having high elongation and r value and exhibiting excellent formability can be provided stably and easily under low energy cost and equipment cost. It is possible to do so, and industrially useful effects are brought about, such as being able to greatly contribute to the production of integrally molded materials for automobile parts.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of S content on the elongation of a cold rolled steel sheet when direct rolling and slab reheating are performed.

FIG. 2 is a graph showing the effects of the S content and the P content on the elongation of the cold-rolled steel sheet when direct rolling is performed, as a result of the example.

FIG. 3 is a graph showing the effects of the amount of P and the amount of effective Ti on the elongation of the cold rolled steel sheet when direct rolling is performed as a result of the example.

Claims (3)

[Claims] 1. A weight ratio of C: 0.01% or less, Si:
0.1% or less, Mn: 0.01 to 2%, P: less than 0.01%,
S: 0.005% or less, Ti: 0.1% or less, acid-soluble Al: 0.005 to 0.1%, N: 0.005% or less, and P + S <0.012%, Ti ^* > 0.01%, Ti ^* (%) x P ( %) ≤ 0.0002 [however, Ti ^* = Ti-48 (C / 12 + N / 14 + S / 32)], the relation expressed by the three formulas is satisfied, and the balance is Fe.
And an unavoidable impurity, which is a thin steel sheet for deep drawing having excellent formability. 2. A weight ratio of C: 0.01% or less, Si:
0.1% or less, Mn: 0.01 to 2%, P: 0.01% or less,
S: 0.005% or less, Ti: 0.1% or less, acid-soluble Al: 0.005 to 0.1%, N: 0.005% or less, and further, one of Nb: 0.003 to 0.05%, B: 0.0003 to 0.0030%. It also contains the above, and P + S <0.012%, Ti ^* > 0.01%, Ti ^* (%) x P (%) ≤ 0.0002 [where Ti ^* = Ti-48 (C / 12 + N / 14 + S / 32) ] The relation expressed by the following three equations is satisfied, and the balance is Fe
And an unavoidable impurity, which is a thin steel sheet for deep drawing having excellent formability. 3. A steel having the composition according to claim 1 or 2 is continuously cast into a slab, and hot rolling is started while the temperature of the slab surface does not fall below 950 ° C. and the hot rolling is performed. Is completed at 850 ° C or higher, and 400 ~
After rolling in the temperature range of 700 ° C, the rolling reduction is 60 to 95%
A method for producing a deep-drawing thin steel sheet having excellent formability, which comprises performing cold rolling in the following, and then annealing in a temperature range from a recrystallization temperature to a temperature of Ac ₃ point or less.