JPH06256901A - High tensile strength cold rolled steel sheet for deep drawing and its production - Google Patents

Abstract

PURPOSE:To stably provide a high tensile strength cold rolled steel sheet for deep drawing, excellent in balance between strength and ductility and also having advantages in productivity and cost. CONSTITUTION:The high tensile strength cold rolled steel sheet for deep drawing, excellent in balance between strength and ductility, has a composition consisting of 0.0005-0.0100% C, <=2.5% Si, >0.15-3.0% Mn, <=0.15% P, 0.0005-0.01% N, <=0.1% acid soluble Al, <=0.010% S, and the balance Fe with inevitable impurities, containing Ti or further containing 0.003-0.1% Nb and/or 0.0003-0.003% B, and satisfying the relations represented by three inequalities '-0.5%<=Mn-(Si+10P)<=0.5%', '-0.01%<=Ti*=0.01%', and 'Ti*X<=P0.0005%' [where Ti*=Ti-48(N/14+C/12)]. This steel sheet can be produced by subjecting the steel to hot rolling, to coiling at 600-750 deg.C, to cold rolling at 50-90% draft, and to recrystallization annealing or further to hot dip galvanizing under the condition of temp. not exceeding 550 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet for deep drawing, which has a good strength-ductility balance and is used by being pressed into various shapes, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, ultra-low carbon steel to which Ti or Nb has been added after being sufficiently decarburized at the steelmaking stage has been used.
Many proposals have been made for deep-drawing high-strength steel sheets in which Si, Mn, Cr, and P are added to increase the strength.

For example, Japanese Examined Patent Publication No. 57-57945 discloses a high-strength cold-rolled steel sheet in which a large amount of P is added to ultra-low carbon Ti-added steel to secure the strength. However, the maximum tensile strength of this cold-rolled steel sheet is only about 50 kgf / mm ² ,
Moreover, in that case, the average rank forward value (average r value)
It was hard to say that it was not so excellent in terms of deep drawability, such as below 1.7.

Further, Japanese Patent Publication No. 59-42742 also discloses that ultra-low carbon Ti-added steel is supplemented with strengthening elements of P, Mn, Si, Mo and Cr, and is further subjected to secondary processing peculiar to IF (Interstitial Free) steel. A high-strength cold-rolled steel sheet obtained by adding B to prevent brittleness is disclosed, but the strength level of this cold-rolled steel sheet is about 50 kgf / mm ² at the maximum in tensile strength. The average r value was as low as about 1.6.

On the other hand, the inventors of the present invention firstly perform hot rolling and cold rolling on an ultra-low carbon Ti-added steel to which P and Mn are added in combination, and then subject them to recrystallization annealing under specific conditions. "Strength-
We proposed a method for producing high-strength steel sheets with a good "deep drawability balance" (Japanese Patent Laid-Open No. 2-149624). It was clarified that when taken, both the ductility and the deep drawability of the steel sheet obtained by subsequent cold rolling / recrystallization annealing may be significantly deteriorated.

That is, P is added to an ultra-low carbon Ti-added IF cold-rolled steel sheet.
When a steel sheet is strengthened by adding a substitutional solid solution element such as Mn, FeTiP is formed in the steel sheet, which causes the ductility and deep drawability of the steel sheet to deteriorate.
In particular, when the steel strip is wound at a high temperature in the hot rolling process, a large amount of Fe
TiP precipitates, and ductility and deep drawability both deteriorate significantly.
Therefore, it was considered necessary to wind at low temperature in order to suppress precipitation of FeTiP and prevent its adverse effect.

On the contrary, in the Ti-added IF cold-rolled steel sheet, if the steel strip is wound at a low temperature in the hot-rolling process for producing it, precipitation of fine TiC tends to occur, which in turn adversely affects the ductility. The inconvenience of causing

In order to suppress the precipitation of this fine TiC, it is effective to carry out high temperature winding in the hot rolling process to coarsely precipitate TiC in the hot rolled sheet, or to reduce the C concentration in the steel sheet as much as possible. . However, the high temperature winding is not preferable for the above-mentioned reason, and on the other hand, there are the following problems in the means for reducing the C concentration. In other words, in the case of "high-strength steel sheet based on ultra-low carbon steel" in which a large amount of substitutional solid solution elements such as Si, P, and Mn are contained in the steel sheet for higher strength, It is necessary to add ferrosilicon, ferrophosphorus, metal Mn, etc. as auxiliary raw materials to the molten steel that has been subjected to vacuum degassing, but this auxiliary raw material also contains a small amount of C. This results in increasing the C concentration of Therefore, it is necessary to further reduce the C concentration in the molten steel prior to the addition of the auxiliary raw material, which significantly increases the cost in the steelmaking process and prolongs the blowing time and hinders the productivity.

[0009]

[Purpose] Therefore, an object of the present invention is to provide a high-strength cold-rolled steel sheet for deep drawing, which has an excellent "strength-ductility balance" and which does not have a particular disadvantage in terms of productivity and cost. It was to establish means to provide stable supply.

[0010]

In order to achieve the above-mentioned object, the present inventor has made a special effort to obtain an ultra low carbon Ti-added IF cold rolled steel sheet.
As a result of a detailed investigation focusing on the effect of the amount of additional elements and the hot rolling coiling temperature on the mechanical properties of "high-strength cold-rolled steel sheet", the following findings were obtained. .

A) Si and P which are effective as reinforcing components of steel sheet
If the Mn content is adjusted while paying attention to the content of Mn, and if Mn, Si, and P are compounded so that their proportion meets a specific condition, not only the required strength is secured, but also deep drawability is obtained. It is possible to improve.

B) Further, when Ti is added, "Ti-48 (N / 14
+ C / 12) ”does not exceed a specific range and suppresses excessive Ti, FeTiP does not form even if the coiling temperature after hot rolling is relatively high, and precipitation occurs. It is possible to suppress the deterioration of the ductility and deep drawability of the product cold rolled steel sheet due to the material.

C) In this case, if the coiling temperature after hot rolling is high, not only TiC becomes coarse and the ductility after annealing is improved, but also a recrystallized texture which is preferable for deep drawability in the recrystallization annealing process is obtained. It is also very preferable because it promotes development.

D) Addition of Nb or B, if necessary, gives favorable results for improving the workability of the cold-rolled steel sheet. However, especially in the case of adding Nb in combination, it is necessary to reduce excessive Ti so that deep drawability is improved. I cannot hope for improvement.

The present invention has been completed on the basis of the above findings and the like. "The base material composition is C: 0.0005 to 0.0100% (hereinafter,
%, Which represents the component ratio, is weight%), Si: 2.5% or less,
Mn: over 0.15 to 3.0%, P: 0.15% or less, N: 0.
0005-0.01%, Acid-soluble Al: 0.1% or less, S:
Contains 0.010% or less and also contains Ti, or N
b: 0.003 to 0.1%, B: 0.0003 to 0.003%, including at least one of -0.5% ≤ Mn- (Si + 10P) ≤ 0.5%, -0.01% ≤ Ti ^* ≤ 0.01%, Ti ^* × P ≦ 0.0005% [however, Ti ^* = Ti−48 (N / 14 + C / 12)], and the balance is composed of Fe and unavoidable impurities. By doing so, it is possible to realize a high-strength cold-rolled steel sheet for deep drawing that has a good strength-ductility balance. ”Furthermore,“ steel of the above composition was melted into a billet. After that, hot rolling is performed at a temperature not lower than the Ar ₃ transformation point, winding is performed at 600 to 750 ° C., then cold rolling is performed at a reduction rate of 50 to 90%, and further recrystallization annealing is performed. By applying hot dip galvanizing under the condition that the maximum heating temperature does not exceed 550 ° C,
It is also characterized in that it is possible to stably manufacture a high-strength cold-rolled steel sheet for deep drawing having a good strength-ductility balance ".

The deep-drawing high-tensile cold-rolled steel sheet according to the present invention may be surface-treated as it is (bare steel sheet) as long as the composition of the base material satisfies the above conditions. It goes without saying that even if it is applied, it can sufficiently exhibit the required characteristics. Hereinafter, in the present invention, the reason why the component composition of the steel sheet and the manufacturing conditions thereof are limited as described above will be explained together with its action.

[0017]

[Action]

A) Component ratio <C> C is an element that is inevitably accompanied in steel, but the smaller the content, the better. However, if the C content is 0.
It is not possible to easily and stably set the content to be less than 0005% by the current steelmaking technology. However, the C content is 0.0100%
If it exceeds, not only the required addition amount of Ti increases and the cost rises, but also the precipitation amount of TiC increases and the ductility is impaired. Therefore, the C content is set to 0.0005 to 0.0100%.

<Si> Si is also an element necessarily contained in steel, but it is desirable to contain up to 2.5% as a solid solution strengthening element. However, if the content exceeds 2.5%, precipitates such as SiN and SiO ₂ increase in the steel sheet, and ductility deteriorates. Further, when Si alone is added, the hot-rolled sheet is coarsened and the deep drawability after cold rolling / annealing is deteriorated, but by adding it in combination with Mn, the adverse effect can be suppressed. However, the above effect in the case of adding Mn in combination is not sufficient unless the condition represented by the formula −0.5% ≦ Mn− (Si + 10P) ≦ 0.5% is satisfied.

<Mn> Mn forms MnS by combining with S, which is an unavoidable impurity, and suppresses the adverse effects of S. In addition, solid addition strengthens by adding Si and P that are solid solution strengthening elements. Since the element has the effect of preventing the deep drawability from being adversely affected, it is necessary to secure a content exceeding 0.15%. That is, when the Mn content is 0.15% or less, S cannot be sufficiently captured and TiS precipitates, so that solid solution C that cannot be fixed by Ti remains in the steel and is also added together with P and Si. The effect is lost and a recrystallization texture suitable for deep drawability cannot be obtained after cold rolling and annealing. On the other hand, when Mn is contained in excess of 3.0%, the steel sheet becomes too hard, which adversely affects the deep drawability. Therefore, the Mn content is 0.
Determined to be over 15 to 3.0%. Further, even if Mn is added alone, a recrystallization texture preferable for deep drawability cannot be obtained after cold rolling / annealing, and it is always necessary to add Mn in combination with Si. And
The good deep drawability can be stably realized in the case of compound addition with Si and P when the condition expressed by the above-mentioned formula −0.5% ≦ Mn− (Si + 10P) ≦ 0.5% is satisfied. .

<P> P is also an element necessarily contained in steel, and it is desirable to contain up to 0.15% as a solid solution strengthening element. However, if the content exceeds 0.15%, the steel sheet becomes brittle and is not suitable for plastic working such as pressing. Further, even if P is added alone, the hot-rolled sheet becomes coarse grains and deteriorates the deep drawability after cold rolling / annealing, so in order to prevent this, it is necessary to always add it in combination with Mn. However, the effect of the composite addition is not sufficient unless the condition represented by the formula −0.5% ≦ Mn− (Si + 10P) ≦ 0.5% is satisfied.

<S> S is an unavoidable impurity element that is necessarily accompanied in the steel, and the lower the content, the better. And, if the S content is high, the required amount of Mn is increased correspondingly and the cost increases, so the S content is 0.010%.
The following was set.

<Acid-soluble Al> Acid-soluble Al contained in the steel sheet
Is a part of Al added as a deoxidizer in the steelmaking stage. And usually, to ensure sufficient deoxidation
A guideline is to contain 0.001% or more. However, if the content of acid-soluble Al exceeds 0.1%, Al ₂ O ₃
Therefore, the content is determined to be 0.1% or less.

<N> N, like C, is an unavoidable impurity element necessarily contained in steel, and the lower the content, the better. However, in the current steelmaking technology, N is 0.0005%.
It cannot be easily and stably reduced to less than 1. When the N content exceeds 0.01%, not only the required addition amount of Ti increases and the cost increases, but also the precipitation amount of TiN increases and the ductility deteriorates. Therefore, the N content is 0.0005.
~ 0.01%.

<Ti> Ti means C and N in the steel sheet respectively
It has an action of fixing as C and TiN and promoting the development of a recrystallization texture preferable for deep drawability, and is an important constituent component of the steel sheet according to the present invention. When the Ti content is insufficient and the value of Ti ^* represented by “Ti ^* = Ti−48 (N / 14 + C / 12)” is lower than −0.01%, the desired effect due to the above action can be secured. On the other hand, if the amount of excess Ti increases, FeTiP is generated and the ductility deteriorates after cold rolling and annealing.
Since it comes to inhibit the development of recrystallization texture which is preferable for deep drawability, the Ti content is expressed by the formula "Ti ^* = Ti-48 (N / 1
4 + C / 12) ", the value of Ti ^* is" -0.01% ≤ Ti ^* ≤
It was limited to the range of "0.01%" and "Ti ^* x P ≤ 0.0005%".

<Nb and B> Since all of these components have 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 crystal grain size of the hot-rolled sheet is made fine, and carbide is formed similarly to Ti to promote the development of a recrystallization texture preferable for deep drawability. However, if the content is less than 0.003%, such an effect does not appear. On the other hand, if the content exceeds 0.1%, the recrystallization suppressing effect by the solid solution Nb is too large and the deep drawability is rather deteriorated. B: Si, M in ultra-low carbon steel with little excess Ti (that is, Ti ^* )
When a large amount of n and P are added, precipitation of TiC is suppressed and solid solution C remains in the steel sheet, so that the secondary work embrittlement resistance is improved. However, in a high P-added steel sheet, the effect of inhibiting the secondary work embrittlement resistance due to solid solution P may not be negligible this time, but the adverse effect can be mitigated by adding a small amount of B in such a case. However, if the B content is less than 0.0003%, this effect cannot be expected. On the other hand, if the B content exceeds 0.003%, the above effect is saturated and the development of a recrystallization texture preferable for deep drawability is inhibited. Become so.

B) Manufacturing Conditions In the method of the present invention, steel ingot having a predetermined composition is first made into a slab by continuous casting or the like, and this is hot-rolled at a temperature not lower than the Ar ₃ transformation point. In this case, when the rolling temperature is lower than the Ar ₃ transformation point, a rolling texture called “goss orientation” develops on the surface layer of the hot rolled sheet and does not disappear even after cold rolling and annealing, which impairs the deep drawability of the final product. Not preferred because of Incidentally, the hot rolling is preferably rolling to without cooling to "temperature below the Ar ₃ transformation point" to heat billet after casting is good by "direct rolling". Of course, even in this case, the hot steel billet can be heated in a heating furnace or an end heating device as required.

The coiling temperature in the hot rolling process is 600 to 7
Although it was set to 50 ° C., if the coiling temperature is lower than 600 ° C., not only is TiC coarsened insufficiently to give good ductility to the product steel sheet, but also it is preferable for deep drawability in recrystallization annealing after cold rolling. Recrystallized texture is not fully developed. On the other hand, when wound at a temperature higher than 750 ° C., coarse crystal grains are likely to be formed, which leads to a reduction in the deep drawability of the cold rolled / annealed sheet. The upper limit of the winding temperature is 700 if possible.
It is preferable to keep the temperature at ℃.

Cold rolling is carried out after descaling by pickling or the like according to a conventional method, and the reduction ratio of cold rolling is 50-9.
0% The lower limit of the rolling reduction is set to 50% because if the rolling reduction is lower than this, the recrystallization texture that is preferable for deep drawability cannot be sufficiently formed by the recrystallization annealing. Basically, the higher the rolling reduction is, the more preferable it is, but if the rolling reduction is 90% or more, it is difficult to perform rolling in a continuous rolling mill, so the upper limit of the rolling reduction is set to 90%.

Recrystallization annealing may be performed by any of box annealing, continuous annealing, and annealing treatment before plating in the continuous hot dip galvanizing process. However, needless to say, the annealing temperature is higher than the recrystallization temperature, and in order to obtain a good deep drawability in the annealed sheet, the temperature must not exceed the Ac ₃ transformation point. The annealed plate is temper-rolled if necessary, further subjected to surface treatment such as electroplating, and then shipped.

When hot-dip galvanizing or alloying hot-dip galvanizing is applied to the annealed steel sheet, the maximum heating temperature of the steel sheet must be suppressed to 550 ° C. This is because the deep drawability deteriorates when heated to a temperature higher than 550 ° C. during plating.

Further, since the steel sheet having the above-mentioned composition contains Si and Mn having a strong oxidation tendency at a high level, it is difficult to stably form a hot-dip galvanized layer or an alloyed hot-dip galvanized layer. , The following methods are effective. That is, prior to the plating process, after recrystallization annealing in a "low dew point annealing furnace" such as a hydrogen annealing furnace or a "continuous annealing furnace having steam cooling or water quenching type cooling and a pickling device", continuous melting This is a method in which plating is performed at a maximum heating temperature of 550 ° C. or less in a galvanizing line, and alloying treatment is performed if necessary.

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

EXAMPLE First, a steel having the composition shown in Table 1 (all Ar ₃ transformation points are 930 ° C. or lower) was melted in an experimental vacuum melting furnace, and these were hot forged to form a 25 mm thick experimental slab. And

[0033]

[Table 1]

Then, the obtained slab is heated to 125 in an electric furnace.
After heating at 0 ° C. for 1 hour, it was rolled for 3 passes in a temperature range of 1150 ° C. to 930 ° C. by an experimental hot rolling mill to obtain a hot rolled sheet having a thickness of 5 mm. Then, as a simulation of winding, the steel sheet is cooled to a temperature of 450 to 800 ° C. by forced air cooling or water spray cooling immediately after hot rolling, and then inserted into an electric furnace maintained at that temperature, and 1 at that temperature
After holding for a time, the furnace was cooled at a cooling rate of 20 ° C./hr.

Next, the hot-rolled sheets after furnace cooling were surface-ground
A cold rolled base material having a thickness of 3.2 mm was cold-rolled to a thickness of 0.8 mm (reduction ratio: 75%). Furthermore, the obtained cold-rolled sheet,
Imitation of recrystallization annealing process, heating rate 10 ℃ in infrared heating furnace
After heating to 820 ° C. at / s, the temperature was maintained for 40 seconds, then gradually cooled to 700 ° C. at a cooling rate of 3 ° C./s, and then cooled to room temperature at a cooling rate of 50 ° C./s.

Each of the cold-rolled sheets that had been subjected to the recrystallization annealing was subsequently subjected to temper rolling with an elongation rate of 1.2%, formed into JIS No. 5 tensile test pieces, and subjected to a tensile test. In addition, for some, a tensile test was performed after performing recrystallization annealing and performing hot dip galvanizing in a hot dip zinc bath at 530 ° C. These results are shown in Table 2, and "average r value" and "strength-ductility balance" arranged by hot rolling temperature are shown in FIG. In Table 2 and FIG. 1, “product of tensile strength and total elongation” is shown as an index showing “strength-ductility balance”.

[0037]

[Table 2]

As is clear from the results shown in these, the "strength-ductility balance" and "average r value" of the steel sheets of the present invention obtained in Test Nos. 2 and 3 are very high values. However, when the hot rolling coiling temperature deviates from the conditions specified in the present invention, it is understood that the above properties are inferior. On the other hand, even in the comparative steel sheets whose component compositions do not satisfy the conditions specified in the present invention (steel sheets obtained by test numbers 5 to 8), the lower the hot rolling coiling temperature is, the "strength-ductility balance" and the "average r value". , ”But still falls short of the steel sheet produced according to the method of the present invention.

Further, as is clear from the result of test No. 9, even if the steel sheet contains a large amount of Si, if it is within the range specified by the present invention, "strength-ductility balance" is obtained by winding at 700 ° C. It can be seen that a product having a high “average r value” can be obtained. Further, from the results of Test Nos. 10 and 11, it can be seen that the deep drawability is further enhanced by the combined inclusion of Nb.
From the results of Test Nos. 11 and 12, it can be confirmed that the inclusion of a trace amount of B does not hinder the deep drawability.

However, as shown by the results of Test Nos. 13 and 14, when the relational expression containing Mn and P is out of the specified range of the present invention, the deep drawability is remarkably lowered, and the ductility is also deteriorated. Is clear.

On the other hand, from the test Nos. 15 and 16, the characteristics of the hot-dip galvanized steel sheet of the present invention can be confirmed. In both cases, a surface-treated steel sheet having high "strength-ductility balance" and "average r value" can be obtained. It is shown that.

[0042]

[Summary of Effects] As described above, according to the present invention, a high-strength steel sheet having a good strength-ductility balance and being extremely excellent for deep drawing can be realized at low cost,
For example, in recent years, social demands for improving fuel economy of automobiles have increased due to global environmental problems, and the present invention can bring about great industrial advantages, such as being able to greatly contribute to weight reduction of vehicle bodies.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between “hot rolling coiling temperature”, “strength-ductility balance”, and “deep drawability” for each steel type when the results of Examples are arranged.

Claims (4)

[Claims] 1. A base material composition having a weight ratio of C: 0.0005 to.
0.0100%, Si: 2.5% or less, Mn: over 0.15 to 3.0
%, P: 0.15% or less, N: 0.0005 to 0.01%, acid-soluble Al: 0.1% or less, S: 0.010% or less, and
It also contains Ti and -0.5% ≤ Mn- (Si + 10P) ≤ 0.5%, -0.01% ≤ Ti ^* ≤ 0.01%, Ti ^* × P ≤ 0.0005% [where Ti ^* = Ti-48 (N / 14 + C / 12)], and the balance is Fe and unavoidable impurities, and the balance is strength and ductility. steel sheet. 2. A base material composition having a weight ratio of C: 0.0005 to.
0.0100%, Si: 2.5% or less, Mn: over 0.15 to 3.0
%, P: 0.15% or less, N: 0.0005 to 0.01%, acid-soluble Al: 0.1% or less, S: 0.010% or less, and
Furthermore, Ti and Nb: 0.003-0.1%, B: 0.0003-
It also contains at least one of 0.003%, and -0.5% ≤ Mn- (Si + 10P) ≤ 0.5%, -0.01% ≤ Ti ^* ≤ 0.01%, Ti ^* × P ≤ 0.0005% [where Ti ^* = Ti−48 (N / 14 + C / 12)] satisfying the relations represented by the three formulas, the balance being Fe and unavoidable impurities, and having a good strength-ductility balance. High tensile cold rolled steel sheet for drawing. 3. A steel having the composition described in claim 1 or 2 is melted to form a billet, which is hot-rolled at a temperature not lower than the Ar ₃ transformation point and wound at 600 to 750 ° C. Reduction rate 50-
A method for producing a high-strength cold-rolled steel sheet for deep drawing having good strength-ductility balance, which comprises cold rolling at 90% and further performing recrystallization annealing. 4. A steel having the composition as set forth in claim 1 or 2 is melted into a slab, which is hot-rolled at a temperature not lower than the Ar ₃ transformation point and wound at 600 to 750 ° C. Reduction rate 50-
Cold rolling at 90%, recrystallization annealing, and then hot dip galvanizing under the condition that the maximum heating temperature does not exceed 550 ° C. Manufacturing method of tension cold-rolled steel sheet.