JP3412243B2 - Manufacturing method of steel sheet for deep drawing with excellent formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] This invention relates to a manufacturing method for molding having excellent deep drawing for thin steel plate to be used by pressing into various shapes.

[0002]

2. Description of the Related Art At present, thin steel sheets for deep drawing are generally manufactured through hot rolling of a slab manufactured by continuous casting, and further, pickling, cold rolling and annealing. It is. However, when a steel sheet is manufactured using a continuously cast slab as a rolled material in this manner, conventionally, the continuously cast slab is once cooled to room temperature and then inserted into a heating furnace to reach a temperature of 1050 to 1250 ° C. It was usual to perform hot rolling after reheating. However, after the two oil crises, a technique called "hot piece insertion" or "hot charge" for energy saving has become mainstream. This is a method in which a continuously cast slab is not cooled to room temperature, but is inserted into a heating furnace when the continuously cast hot slab is cooled to around 800 ° C. Further, a technique called "direct-feed rolling", in which hot rolling is performed only by supplementary heating using an edge heater or the like as necessary without using a heating furnace, has come into practical use.

However, in the direct rolling process, unlike the conventional process, the slab is hot-rolled without falling below the Ar ₃ transformation point at all. Was seen. That is, according to the direct rolling process, since the time held in the γ region before hot rolling is shorter than the slab reheating method, precipitates cannot be sufficiently precipitated or coarsened, and for example, direct rolling is performed. Properties (especially elongation and r-value) of thin steel sheet
Was inferior to a thin steel plate manufactured by the reheating method.

[0004] Therefore, several proposals have been made to prevent the properties of a thin steel sheet manufactured through direct-feed rolling from deteriorating as compared with a case where a reheating method is applied. JP-A-59-
JP-A-89723 also discloses one of those proposals. According to this, a rare-earth element (REM),
It is stated that by adding one or more selected from Ca, Ti and Mg, the in-plane anisotropy of the steel sheet material is reduced and the workability is improved. In particular, Ti and Mg are more R
It is described that it is better to add EM and Ca in combination, thereby obtaining a sized structure and reducing in-plane anisotropy of mechanical properties. However, such addition of REM and Ca leads to an increase in cost, and the proposed method cannot remarkably improve the elongation and the absolute value of the r value.

Japanese Patent Application Laid-Open No. Sho 61-281824 discloses that the addition of Ti to a material steel in a range determined from the C and N contents and the direct rolling method provide a slab reheating method. There has been proposed a method for producing a thin steel sheet which enables non-aging with a small addition amount of Ti. However, even with this method, the elongation and the r value could not be particularly improved.

On the other hand, Japanese Patent Application Laid-Open No. 62-287017 proposes a method of controlling slab conveyance such that a predetermined time elapses before hot rolling is started in direct rolling after continuous casting. Have been. By holding the hot slab before hot rolling for a predetermined time or more in this way, it is described that good mechanical properties can be obtained even in the direct rolling method as in the slab reheating method. However, according to this method, equipment for temporarily stagnation of the slab between the continuous casting machine and the hot rolling mill is required, and the merit of equipment such as "elimination of a heating furnace by direct-feed rolling" is reduced. Although a relatively high elongation and r-value are obtained in the product steel sheet, the value is still not sufficiently satisfactory, and the problem that the characteristics are not stable has not been solved.

[0007] As described above, even with the proposals regarding the improvement of the properties of the thin steel sheet manufactured through the direct-feed rolling, the problem that the obtained thin steel sheet is inferior in elongation and r value to that obtained by the slab reheating process. It was still unresolved. Therefore, an object of the present invention is to provide a conventional process (reheating process) even when manufactured through direct-feed rolling.
An object of the present invention is to establish a means capable of stably providing a deep-drawing thin steel sheet having good workability (elongation, r value) comparable to a material.

[0008]

Means for Solving the Problems The present inventors have conducted detailed studies on the chemical composition and manufacturing conditions of steel sheets in order to achieve the above object, and have obtained the following new findings as shown in a) to d) below. I got it. a) In the case of manufacturing a steel sheet for deep drawing by a process that adopts a direct rolling method, the content of S that inevitably penetrates into the steel has a great effect on the elongation of a product that can be obtained. In order to secure an excellent elongation of more than 51% in steel sheets, it is essential to reduce the S content to 0.005% or less (hereinafter,% representing component ratio is referred to as% by weight).

[0009] Fig. 1 is a graph showing "S content, ductility and ductility" obtained by examining "cold rolled steel sheet manufactured through a process corresponding to direct-feed rolling" and "cold rolled steel sheet manufactured through a reheating process". Are arranged and shown. Here, the above survey was conducted as follows. That is, first, the steel shown in Table 1 was melted in an experimental furnace and cast into a split mold to form a slab. Then, as a direct rolling simulation, the mold was removed and cooled to 1050 ° C., followed by hot rolling. And the slab reheating rolling simulation
The slab removed from the mold was taken out, cooled to room temperature, heated again in a furnace at 1200 ° C. for 60 minutes, and taken out.
After cooling to 0 ° C., hot rolling was started. After the hot rolling is completed at a finishing temperature of 920 ° C. and a finished 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 the rolling reduction is 82%. % Cold rolling to a sheet thickness of 0.8 mm. Further, heat treatment corresponding to continuous annealing was performed using an infrared heating furnace at a heating rate of 10 ° C./sec, a soaking temperature of 860 ° C., a soaking time of 30 sec, and a cooling rate of 10 ° C./sec. Temper rolling at an elongation of 0.3%, a JIS No. 5 test piece was sampled from the cold-rolled steel sheet thus obtained, and a tensile test was performed.
L) was investigated.

[0010]

[Table 1]

From the results of the investigation shown in FIG. 1, the elongation (EL) decreases with an increase in the S content of the steel sheet. In particular, when the S content exceeds 0.005%, the material equivalent to the direct-rolling process does It can be seen that the elongation is extremely deteriorated as compared with the material corresponding to the heating process.

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

C) As described above, in Ti-added steel, P
Precipitates as FeTiP, the elongation of the steel sheet deteriorates. Therefore, in order to produce a thin steel sheet exhibiting good elongation, P
The absolute value of the content must be reduced to a specific value or less, and the regulation of the P content in relation to the Ti content is indispensable.

D) However, when the steel whose composition is adjusted as described above is treated according to predetermined conditions, a deep drawing thin steel sheet having high elongation and high r value and excellent formability can be obtained even by a process employing a direct rolling method. Being able to manufacture stably.

The present invention has been made based on 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% In the following, Ti: 0.1% or less, acid-soluble Al: 0.005 to 0.1%, N: 0.005% or less, or Nb: 0.003 to 0.05%, B: 0.0003 to 0.0030% It contains, and P + S <0.012%, Ti ^* > 0.01%, Ti ^* (%) × P (%) ≦ 0.0002 [However, Ti ^* = Ti−48 (C / 12 + N / 14 + S / 32)] Satisfies the relationship expressed by the following three equations, and the balance is Fe
And after the slab component couples formed of steel consisting of unavoidable impurities by continuous casting, to complete the hot rolling at 850 ° C. or higher, or the temperature of the slab surface so as not to fall below 950 ° C. The hot rolling Started and hot rolled 8
Completed at 50 ° C or higher, and 400-700 ° C (650 ° C
(Except for the above) , after winding in the temperature range
%, And then annealed in the temperature range between the recrystallization temperature and the Ac _three- point temperature to ensure stable production of deep-drawing thin steel sheets with excellent formability. " To
It is an feature.

Hereinafter, the reason why the composition of the steel sheet and the production conditions thereof are limited as described above in the present invention will be described together with the operation thereof.

[Action]

A) Ingredient ratio <C> C is an impurity element inevitably mixed into steel, but it has a bad influence on the formability of the steel sheet, so the smaller the content, the better. However, as a means of avoiding the adverse effects of C, which is inevitable to be mixed, "fixing C as a carbide by adding Ti" is effective. However, as the C content increases, this is fixed. Increases the required amount of Ti, which increases the hardening of the steel and increases the production cost.
The upper limit of the content was determined.

<Si> Si is also an impurity element unavoidably mixed into steel, and the lower the content, the better.
That is, when the Si content is high, a specific pickling failure occurs or the plating property is deteriorated. Therefore, the allowable limit of 0.1% is set as the upper limit of the Si content.

<Mn> Mn has the effect of strengthening the steel sheet.
It is a component that can be appropriately added for this purpose. However, when the content exceeds 2%, the transformation point is remarkably lowered, and TiC is refined to cause deterioration of elongation, which is not preferable. However, molten iron usually contains about 0.01 to 0.05% of Mn, and it is not preferable to lower the Mn because it leads to an increase in cost. Therefore, the Mn content is determined to be 0.01 to 2%.

<P> P is an impurity element inevitably mixed into steel. And, as described above, this P is FeTi
Since it precipitates as P and deteriorates the elongation of the steel sheet, the lower the P content, the better from the viewpoint of securing 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 a range of less than. However,
In order to sufficiently suppress the precipitation of FeTiP and suppress the adverse effect on elongation, the relationship with the effective Ti amount (Ti ^* ) described later is “Ti ^*
(%) × P (%) ≦ 0.0002 ”. Further, the P content needs to be limited in relation to the S content, which has a significant adverse effect on elongation, and in order to secure a desired elongation value, it is necessary to satisfy a condition represented by the expression “P + S <0.012%”. There is.

<S> In the present invention, S is an element having an important meaning beyond 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 direct-rolling steel having a high S content is reduced,
Conversely, steel with a low S content has a small decrease in elongation. " The mechanism has not been fully elucidated yet, but is presumed as follows. That is,
S is an element that easily forms sulfides. However, in a hot rolling method in which the holding time in the γ region is short, such as direct rolling, precipitation of sulfides is not sufficient and steel having a high S content (S content Quantity
In the case of steel exceeding 0.005%), solid solution S remains when TiC is precipitated. Since S exerts an interaction attracting each other to Ti which easily forms a sulfide, the activity of Ti is reduced due to the remaining solid solution S, and the precipitation of TiC shifts to a lower temperature side. Therefore, the precipitates (TiC) cannot be coarsened and are dispersed as fine TiC in the hot-rolled sheet, so that the elongation is deteriorated. On the other hand, in low S steels (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, so that 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 obtained cold-rolled steel sheet significantly deteriorates.
It is determined as follows. However, the S content needs to be limited in relation to the P amount which has a significant adverse effect on elongation, and in order to secure a desired elongation value, it is necessary to satisfy a condition represented by the expression “P + S <0.012%”. Is as described above.

<Ti> In order to improve the r-value of a cold-rolled steel sheet and to suppress strain aging, it is important that C and N do not exist in the hot-rolled sheet, and Ti is an interstitial solid solution element. Is a component added for the purpose of fixing C and N. And enough r
To increase the value and suppress the strain aging, the effective Ti given by
The amount (Ti ^* ) must be greater than 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 needs to be 0.1%, and the effective Ti content ( Even when the amount of Ti ^* is excessive, FeTiP precipitates and deteriorates elongation. Therefore, in order to secure good elongation, as described above, the expression “Ti ^* (%) × P (%) ≦ 0.0002” is used. Requirements must be satisfied.

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

<N> N is also an unavoidable impurity element in steel, and the lower the content, the better from the viewpoint of improving ductility. If the N content exceeds 0.005%, the amount of Ti added to fix the N content increases, which not only causes an increase in cost, but also increases the amount of TiN deposited and deteriorates ductility. Therefore, the N content was determined to be 0.005% or less.

<Nb and B> Each of these components has an effect of improving the workability of the cold-rolled steel sheet, and therefore, if necessary, a single or two or more kinds of additives are added. However, each content was determined for the following reasons. Nb: Refines the crystal grain size of the hot-rolled sheet to promote the development of a recrystallized texture preferable for deep drawability. However, if the content is less than 0.003%, such effects do not appear, while
If the content exceeds 05%, the recrystallization temperature rises and annealing becomes difficult.

B: Secondary processing embrittlement may occur in a part where severe molding is performed. B segregates at the crystal grain boundaries to strengthen the grain boundaries, and thus has the function of preventing secondary working embrittlement. However, if the B content is less than 0.0003%, this effect cannot be expected. On the other hand, if the B content exceeds 0.0030%, the effect is saturated.

B) Manufacturing Conditions In the method of the present invention, first, a steel smelted to the above-mentioned predetermined composition is formed into a slab by continuous casting, which is then hot-rolled.
(For example, direct rolling ) . The steel whose composition has been adjusted as described above is a steel suitable for direct rolling, but it is a matter of course that the same performance is exhibited even if hot rolling is performed by the slab reheating method. Hot rolling is preferably performed in the γ region. If the finishing temperature is lower than 850 ° C., abnormal grain growth occurs during winding and the surface properties deteriorate. Therefore, 85
In order to ensure a finishing temperature of 0 ° C. or higher, it is necessary to start hot rolling of a continuously cast slab such that the temperature of the slab surface does not fall below 950 ° C.

In the steel having the composition according to the present invention, the influence of the winding temperature on the mechanical properties is small. However, if the winding temperature is high, the winding shape will be lost or the scale loss will increase, so the upper limit of the winding temperature is set to 700 ° C (however,
And, a range of more than 650 ° C. Except claims). On the other hand, if the winding temperature becomes too low, the load on the down coiler increases, so the lower limit of the winding temperature is set to 400 ° C. When higher formability is required by completely suppressing the precipitation of FeTiP, the winding temperature is preferably set to 400 to 500 ° C.

In cold rolling, which is performed after removing the scale by performing pickling or the like after the winding, if the rolling reduction is less than 60%, the r value of the obtained cold rolled steel sheet becomes low, which is not preferable. On the other hand, the fact that the rolling reduction exceeds 95% means that the thickness of the hot-rolled sheet is correspondingly large, and since such a hot-rolled sheet is difficult to manufacture, the rolling reduction of the cold rolling is difficult. The rate was defined as 60-95%.

If the annealing temperature is lower than the recrystallization temperature, elongation cannot be expected because the steel sheet is hard and not softened if the annealing temperature is lower than the recrystallization temperature. On the other hand, if the annealing is performed at a temperature of _three or more Ac, it is high. Since the texture exhibiting the r value is destroyed by the α → γ transformation, the r value decreases. Therefore, the annealing temperature was determined to be equal to or higher than the recrystallization temperature and equal to or lower than the temperature at the _three Ac points. In addition, in the above-mentioned annealing applied to the steel sheet having the component composition according to the present invention, since the rate of temperature rise does not affect the mechanical properties, the annealing method may be either continuous annealing or box annealing. Alternatively, hot-dip galvanizing can be performed after annealing using a continuous 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, cast into a split mold, formed into a slab, removed from the mold, and cooled to 1050 ° C. After waiting, hot rolling was started. Then, hot rolling was completed at 920 ° C. to form a hot-rolled sheet having a thickness of 5 mm, and the sheet 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 a sheet thickness of 0.8 mm. Furthermore, using an infrared heating furnace, the heating rate: 10 ° C./sec, soaking temperature: 840 ° C., soaking time: 30
After performing a heat treatment corresponding to continuous annealing at 10 sec / sec and a cooling rate of 10 ° C./sec, 0.3% temper rolling was subsequently performed.

From each of the cold-rolled steel sheets thus obtained,
IS5 test pieces were sampled and subjected to a tensile test to determine their elongation (E
L) was investigated. The results of the elongation (EL) survey are shown in FIG. 2 in relation to the S content and the P content of the cold-rolled steel sheet. 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 shows good elongation and will exhibit excellent formability.

Example 2 A steel having the composition shown in Table 3 was melted, and a cold-rolled steel sheet was manufactured under the same conditions as in Example 1.

[0035]

[Table 3]

From each of the cold-rolled steel sheets thus obtained,
IS5 test pieces were sampled and subjected to a tensile test to determine their elongation (E
L) was investigated. The results of the elongation (EL) investigation are shown in FIG. 3 in relation to the P content and the effective Ti content (Ti ^* ) of the cold-rolled steel sheet. 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 shows good elongation and will exhibit excellent formability.

Example 3 A steel having the composition shown in Table 4 was melted, and a cold-rolled steel sheet was manufactured under the same conditions as in Example 1.

[0038]

[Table 4]

From each of the cold-rolled steel sheets thus obtained,
IS5 specimens were collected and subjected to a tensile test to determine the tensile strength,
In addition to examining the elongation and the r value, a separately sampled test piece was formed into a cup having a drawing ratio of 1.8, and then subjected to a drop weight test to determine a brittle transition temperature, which was used as an index of secondary work brittle resistance. Table 5 shows the results.

[0040]

[Table 5]

The following can be seen from the results shown in FIG. That is, the cold-rolled steel sheets obtained under the conditions specified in the present invention (the steel sheets obtained by the test types A, D, I, K, L, N and O) have high elongation and high r-value, and have excellent forming properties. It has nature. On the other hand, in the cold-rolled steel sheets obtained in 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 in the present invention. , The tensile strength is high and the elongation and r-value are small, resulting in poor moldability.

The transition temperature of the cold-rolled steel sheet obtained from the test type J using steel having a high B content as a material is lower than -100 ° C. If B is contained within the specified range of the invention, a sufficiently satisfactory transition temperature (−90 ° C.) is practically achieved, and the test type J is excessively added with B, leading to a high cost. It can be said. Further, in the cold-rolled steel sheet obtained in the test type M, since the Nb content is higher than the range specified in the present invention, the recrystallization temperature becomes higher, the grain growth becomes insufficient, and the elongation and the r value become lower. It is lower.

[0043]

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 at low energy cost and equipment cost. In addition, industrially useful effects such as a significant contribution to the production of an integrally molded material of an automobile part can be brought about.

[Brief description of the drawings]

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

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

FIG. 3 is a graph showing the effect of the P content and the effective Ti content on the elongation of a cold-rolled steel sheet when direct rolling is performed, as a result of an example.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-177322 (JP, A) JP-A-1-191748 (JP, A) JP-A-63-86819 (JP, A) JP-A-2- 30719 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/48 C22C 38/00-38/60

Claims (2)

(57) [Claims] At 1. A weight ratio, C: 0.01% or less, Si: 0.1% or less, Mn: 0.01~2%, 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 , or Nb: 0.003 to 0.05%, B: 0.0003 to 0.0030% , and P + S <0.012%, Ti ^{* > 0.01%, Ti * (%} ) × P (%) ≦ 0.0002 ^{[However, Ti * = Ti- 48 (C} / 12 + N / 14 + S / 32) ] satisfied the following relationship represented by the three formulas And the rest is Fe
After the steel of the composition consisting of and unavoidable impurities is made into a slab by continuous casting, hot rolling is completed at 850 ° C. or more,
Further, after winding in a temperature range of 400 to 700 ° C. (excluding 650 ° C. or more) , cold rolling is performed at a rolling reduction of 60 to 95%, and thereafter, annealing is performed in a temperature range of a recrystallization temperature or higher and a temperature of _three points Ac or lower. A method for producing a deep-drawing thin steel sheet having excellent formability, characterized in that: 2. In a weight ratio, 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 , or Nb: 0.003 to 0.05%, B: 0.0003 to 0.0030% , and P + S <0.012%, Ti ^{* > 0.01%, Ti * (%} ) × P (%) ≦ 0.0002 ^{[However, Ti * = Ti- 48 (C} / 12 + N / 14 + S / 32) ] satisfied the following relationship represented by the three formulas And the rest is Fe
And after making a steel having a composition consisting of unavoidable impurities into a slab by continuous casting, hot rolling is started so that the temperature of the slab surface does not fall below 950 ° C., and the hot rolling is completed at 850 ° C. or more; 400-700 ° C (65
(Excluding 0 ° C or higher)) , and then the rolling reduction is 60
Perform cold rolling at 95%, then recrystallization temperature or higher Ac ₃
A method for producing a steel sheet for deep drawing having excellent formability, characterized by annealing in a temperature range not higher than a point temperature.