JPH0753890B2 - Manufacturing method of high strength cold rolled steel sheet for deep drawing - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-strength cold-rolled steel sheet having good deep drawability.

<Prior art and its problems> Conventionally, in the production of high-strength steel sheets for deep drawing, ordinary hot rolling was applied to low carbon aluminum killed steel to which Si, Mn, P, etc. were added as strengthening elements, and this was further cold rolled. It was general to adopt the method of continuous annealing or box annealing after that. But S
When i, Mn, P, etc. are added as strengthening elements, there is a problem that the development of recrystallized texture, which is preferable for deep drawability, is hindered as the amount of addition of them increases. I had to limit the amount. Therefore, it has been strongly desired to realize a high-strength steel sheet having a desired strength and exhibiting sufficient deep drawability.

In addition, under such circumstances,
A high-strength cold-rolled steel sheet for deep drawing in which Si, Mn, P, etc. are added as a strengthening element has also been proposed (JP-A-56-142852), but as described above, extremely low C which originally has good deep drawability. Even if the Ti-added steel is used as a base, when the amount of the reinforcing element added reaches a certain level, the deep drawability is also deteriorated, and there is a limit to achieving both sufficient strength and deep drawability.

On the other hand, until now, in the production of steel sheets for deep drawing, it has been essential to carry out steel strip rolling in the hot rolling step in the austenite region. This is because when the above hot rolling is performed in the ferrite region, Goss which is disadvantageous to deep drawability near the surface layer of the steel sheet.
This is because the ({110} <001>) orientation develops. However, this Goss orientation temporarily changes to another orientation due to subsequent cold rolling, but it appears again in the cold-rolled sheet as Goss orientation after annealing, which significantly deteriorates the deep drawability of the annealed sheet. .

However, in the actual hot rolling process, the edges of the steel strip are likely to be cooled, and therefore the steel strip is rolled at a lower temperature than the center of the steel strip. Therefore, it tends to be hot rolled in the ferrite region. Therefore, the material at the end of the steel strip tends to be inferior to that at the center of the steel strip, and there is a risk that product yield may be reduced unless auxiliary heating is applied to the end of the steel strip during hot rolling. It was

Moreover, in case that Si addition is made as a strengthening element in the hot-rolled material, Si is a ferrite forming element A ₃
To act to raise the transformation point, finishing hot-rolling completion temperature of the steel strip ends to be greater than or equal to A ₃ transformation point it is necessary to increase the slab heating temperature, by increasing or scale loss of energy consumption I could not ignore the problem of yield loss.

Therefore, the object of the present invention is to solve the above-mentioned problems pointed out in the production of a high-strength cold-rolled steel sheet for deep drawing, and to provide a "high deep-drawing strength having excellent deep drawability and sufficient strength". It was placed in establishing a means by which a "tension cold-rolled steel sheet" can be manufactured at a low cost with stable workability.

<Means for Solving the Problems> The present inventors, in order to achieve the above-mentioned object, particularly from various viewpoints about the composition of the steel and the steel plate manufacturing conditions that affect the development of the hot rolled texture of the ultra-low carbon steel. As a result of conducting the research and earnestly researching based on the results, the following new findings were obtained. That is, (a) in the production of a cold-rolled steel sheet for deep drawing, when ultra-low C-Ti-added steel is finish hot-rolled below the A ₃ transformation point unlike the conventional case, as the Si addition amount increases, it follows that In the center layer of the steel sheet after cold rolling and recrystallization annealing, there is a tendency that the recrystallization orientation preferable for deep drawability increases. (B) This is because strain remains in the steel sheet due to low temperature finish hot rolling This is because "the same effect as increasing the rolling reduction" is brought about, and it is considered that the above-mentioned strain promotes the development of "a preferable recrystallization texture for deep drawability" during recrystallization annealing.

(C) Further, during hot rolling, which is a problem from the viewpoint of deep drawability.
The development of Goss orientation is not only caused by the additional shear deformation due to friction, but easily recrystallizes during hot rolling in the region where the strain is considerably increased by the additional shear deformation (that is, the layer near the surface layer). Occurs, and this recrystallization orientation is caused by rolling again.

(D) However, the recrystallization during the hot rolling can be suppressed as much as possible through the addition of Si, and the addition of Si in a specific amount or more can suppress the development of the Goss orientation near the surface layer of the steel sheet, (e) However, the effect of improving the deep drawability cannot be obtained by simply adding Si to an ultra-low C steel containing no Ti, but N, S, C in the steel can be nitrided or sulfided by adding Ti. ， The above-mentioned effect is sufficiently secured only by using so-called “interstitial free steel” fixed as carbide. (F) However, when interstitial free steel is used, the strength of the grain boundary decreases. However, when B is added, the secondary work embrittlement can be suppressed without any particular adverse effect on the deep drawability.

The present invention has been made based on the above findings and the like, and "C: 0.0050% or less (hereinafter,% representing a component ratio is% by weight).
, Si: 0.5 to 3.5%, Mn: 0.001 to 0.5%, P: 0.050% or less, S: 0.02% or less, N: 0.0050% or less, acid soluble Al: 0.001 to 0.1%, Ti: oxide Except those included as [4 x C (%)
+ 3.4 × N (%) + 1.3 × S (%)] value (%) or more, or B: 0.0030% or less, with the balance being Fe and inevitable impurities After soaking the steel slabs of 1000 ℃ or more and performing rough hot rolling,
₃ Reduction temperature: 500 ° C or higher Temperature reduction: 50% or higher finish hot rolling, coiling at 600 ° C or lower, then descaling, Cold reduction: 40-95% cold rolling Then, when the austenite phase appears at the recrystallization temperature or higher and 950 ° C. or lower or the composition causes annealing by heating to a temperature range of Ac ₃ transformation point or lower, excellent deep drawability and high strength are obtained. The feature is that it is possible to stably manufacture the provided high-strength cold-rolled steel sheet ”.

That is, the method for producing a deep-drawing high-strength cold-rolled steel sheet according to the present invention, Ti, N, S, C in the steel is sufficient to fix as nitrides, sulfides, carbides, and ferrite hot rolling during hot rolling. Ultra-low carbon steel with sufficient Si added to suppress recrystallization is finish hot-rolled below the Ar ₃ transformation point, and descaling treatment such as pickling,
Although cold rolling and annealing are the main points, the reason why the composition of the raw material steel and the manufacturing conditions of the steel sheet (hot rolling conditions, cold rolling conditions, annealing conditions) are limited as described above in the present invention is as follows. The operation will be described below.

<Action> (A) Component composition of raw steel a) Cc is an element that is inevitably introduced into the steel, and the lower the content, the more desirable. If the C content exceeds 0.0050%, the required addition amount of Ti increases and the manufacturing cost increases, so the C content was limited to 0.0050% or less.

b) Si Si is an element that plays an important role in the present invention, and at the same time it is added as a strengthening element of the steel sheet, the hot rolling workability is facilitated by raising the Ar ₃ transformation point and raising the finish hot rolling completion temperature. Further, it exerts the effect of ensuring excellent deep drawability through the action of suppressing the development of the Goss orientation during hot rolling and the action of increasing the preferred recrystallization orientation after cold rolling / recrystallization annealing. However, 0.5% to obtain these effects
It is necessary to secure the above Si amount. On the other hand, if it is added in excess of 3.5%, the cold workability of steel may be significantly reduced and the steel strip may crack during cold rolling. ~ 3.5
Defined as%.

c) Mn Mn acts to prevent hot embrittlement of steel by combining with S, so 0.001% is added to ensure the desired effect due to the action.
The above additions are made. However, on the other hand, Mn has an action of lowering the Ar ₃ transformation point because it is an austenite-forming element, so if it is contained in excess of 0.5%, the content of Mn is less than “Ar ₃ transformation point” However, there is a risk that the hot rolling of the hot rolling mill would have to be performed at an unnecessarily low temperature, and the hot rolling mill would be overloaded. Therefore, Mn
The content was set to 0.001 to 0.5%.

d) P P is an element that is inevitably introduced into steel, and current steelmaking technology cannot inexpensively and stably reduce it to less than 0.001%. However, P may be added as a strengthening element of steel if necessary, but in Ti-added steel
Since Ti phosphide is formed, the amount of Ti added necessary for forming an interstitial free steel increases, which causes an increase in cost. Further, as the P content increases, the secondary work brittleness of the cold rolled steel sheet becomes remarkable. Therefore, the above inconvenience can be tolerated.
The P content was limited to 0.050% or less.

e) S S is also an element that is inevitably introduced into the steel, and the smaller the content, the better. When the S content exceeds 0.02%, the required addition amount of Ti increases and the cost becomes remarkable, so the S content was set to 0.02% or less, but it is preferable to suppress it to 0.005% or less. .

f) N N is also an unavoidable impurity element, and it is difficult to easily and stably make it less than 0.0005% by the current steelmaking technology, but the smaller the content, the better. When the N content exceeds 0.0050%, the necessary addition amount of Ti also increases and the cost becomes remarkable, so the N content is set to 0.0050% or less, but preferably 0.0020% or less is suppressed. good.

g) Acid-soluble Al Acid-soluble Al is added for deoxidation adjustment prior to adding Ti to molten steel. Therefore, it is shown that deoxidation was sufficiently performed if it was contained in the steel as much as possible, and a sufficient effect is brought if the content of 0.001% or more can be confirmed. However, if the content exceeds 0.1%, the steel becomes hard and the ductility deteriorates, so the content of acid-soluble Al was set to 0.001 to 0.1%.

h) Ti Ti is also an element that plays an important role in the present invention, and Ar ₃
In order to perform the finish hot rolling below the transformation point to ensure the excellent deep drawability in the cold rolled steel sheet, it is essential to use the interstitial free steel by adding Ti together with the addition of Si. Therefore, Ti is [4 x C (%) + 3.4 x N
(%) + 1.3 × S (%)] value (%) or more.

i) B When interstitial-free steel is used, the strength of the grain boundary is reduced and secondary work embrittlement of the cold-rolled steel sheet easily occurs.
Has the effect of suppressing this. However, the ductility-brittleness transition temperature at the time of secondary working depends on the workability of the cold-rolled steel sheet in the primary working, so it may be added only when the workability of the primary working is severe. However, if contained in a large amount, the Ar ₃ transformation point is lowered, and the finish hot rolling below the Ar ₃ transformation point must be performed at a lower temperature than necessary, which may lead to a situation where the hot rolling mill is overloaded. is there. Therefore, when B is added, its content is specified to be 0.0030% or less.

(B) Steel plate manufacturing conditions a) Slab heating temperature during hot rolling When the slab heating temperature is less than 1000 ° C, temperature unevenness easily occurs in the slab, and it becomes difficult to complete hot rolling at the Ar ₃ transformation point or higher. Therefore, the soaking temperature of the slab during hot rolling was determined to be 1000 ° C or higher, but preferably 1050 to 1150
It is good to set it to ℃.

If b) finishing heat finishing temperature of rolling conditions hot rolling is A ₃ transformation point or higher, cold-rolled, the the increase in deep drawability "preferred recrystallization orientation in deep drawability" the cold-rolled steel sheet after recrystallization annealing It is not possible to secure the effect of improving it. Further, in order to obtain the effect of improving the deep drawability of the cold-rolled steel sheet by finish hot rolling below the Ar ₃ transformation point, the cumulative rolling reduction in the ferrite region must be 50% or more. On the other hand, finishing hot rolling at less than 500 ° C. is not realistic because the hot rolling mill is overloaded. Therefore, finish hot rolling is performed in the temperature range of 500 ° C or higher below the Ar ₃ transformation point,
Furthermore, it was decided that the reduction rate at that time should be 50% or more.

If lubrication is applied by finish hot rolling, the Goss orientation in the vicinity of the surface of the steel strip is further suppressed.
If possible, finish hot rolling is preferably lubrication rolling.

c) Winding temperature When winding is performed at a temperature over 600 ° C, it is difficult to retain the strain caused by finish hot rolling below the Ar ₃ transformation point in the steel sheet, and finish hot rolling below the Ar ₃ transformation point. The effect of improving the deep drawability of the cold-rolled steel sheet cannot be obtained sufficiently. Therefore, the winding temperature is set to 600 ° C or lower, but preferably 500 ° C or lower. Further, from the same viewpoint, quenching the steel strip in the process from completion of hot rolling to winding of the finish enhances the deep drawability improvement effect of the cold rolled steel sheet by finish hot rolling below the Ar ₃ transformation point. This is preferable from the above.

d) Cold rolling reduction If the cold rolling reduction is less than 40%, a recrystallization texture suitable for deep drawability does not develop. On the other hand, it is preferable that the rolling reduction is high, but if it exceeds 95%, the deep drawability deteriorates.

e) Annealing condition Needless to say, it is necessary to heat above the recrystallization temperature because it is recrystallization annealing, but if the heating temperature exceeds 950 ° C or the austenite phase appears depending on the composition, it exceeds the Ac ₃ transformation point. When heated to temperature α → γ → α
Since the "recrystallization texture formed in the recrystallization process that is preferable for deep drawability" is erased by the transformation, the heating temperature is 950 ° C or lower, or when the austenite phase appears, the Ac ₃ transformation It is necessary to keep it below the point.

The annealing method may be any of continuous annealing, box annealing, and continuous hot dip galvanizing.

Since the steel sheet produced in this way is an interstitial free steel, temper rolling is not required in principle, but it is temper rolled after annealing if necessary for the purpose of flattening or the like and then shipped. In some cases, the products are shipped after being subjected to surface treatment such as zinc plating.

Next, the present invention will be described more specifically by way of examples.

<Examples> First, ultra-low carbon steels having various component compositions shown in Table 1 were melted in a laboratory vacuum melting furnace.

Next, these were divided into hot-rolled samples by hot forging, hot-rolled, cold-rolled and annealed under the conditions shown in Tables 2 and 3, and further temper-rolled. In addition, about some samples, beef tallow was apply | coated to the hot rolling roll and the lubrication hot rolling was performed.

Subsequently, a JIS No. 5 test piece was cut out from the obtained cold rolled steel sheet and a tensile test was conducted.

The test results are shown in Tables 2 and 3 together, the results shown in Table 2 are summarized in FIG. 1, and the results shown in Table 3 are summarized in FIG. Although these are respectively illustrated in Fig. 6, it can be confirmed from these results that the cold-rolled steel sheet manufactured under the conditions specified in the present invention has both excellent deep drawability and strength.

In addition, from the results shown in Table 2 and FIG. 1, when the Si addition amount of the material steel is 0.5% or more, the total elongation decreases with the increase of strength, but the r value of r, which is an index of deep drawability, decreases. You can see that it will improve.

Furthermore, the Ti addition amount is [4 × C (%) + 3.4 × N (%) + 1.3
XS (%)] value (%) or the amount of Mn added is
It can also be seen that the r value is low when it exceeds 0.5%.

On the other hand, from the results shown in Table 3 and FIG. 2, although the r value improves as the reduction amount in the α-region hot rolling increases, and the r value increases as the cold rolling reduction increases. However, it can be confirmed that when the temperature exceeds a certain level, it decreases. The cold rolling reduction ratio at which the r value is the highest depends on the steel composition, but is 95
It is also clear that there is no problem if it is less than%.

It can also be confirmed that the r value after annealing is improved by performing the hot rolling lubrication even under the same hot rolling conditions.

<Summary of Effects> As described above, according to the present invention, it is possible to stably mass-produce a high-strength cold-rolled steel sheet for deep drawing having excellent deep drawability and strength with good workability. , An extremely useful effect is brought about in the industry.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of steel components on the mechanical properties of cold-rolled steel sheet. FIG. 2 is a graph showing the influence of manufacturing conditions on the mechanical properties of cold-rolled steel sheet.

Claims (2)

[Claims] 1. By weight ratio, C: 0.0050% or less, Si: 0.5 to 3.5%, Mn: 0.001 to 0.5%, P: 0.050% or less, S: 0.02% or less, N: 0.0050% or less, acid-soluble Al: 0.001 to 0.1%, Ti: Excluding those included as oxides [4 x C (%)
+3.4 × N (%) + 1.3 × S (%)] value (%) or more, with the balance being Fe and inevitable impurities. After hot rolling, Ar
₃ Reduction temperature: 500 ° C or higher Temperature reduction: 50% or higher finish hot rolling, coiling at 600 ° C or lower, then descaling, Cold reduction: 40-95% cold rolling After that, when the austenite phase is higher than the recrystallization temperature and lower than 950 ° C. or the composition, it is heated to a temperature range lower than the Ac ₃ transformation point and annealed, and cold-rolled for deep drawing. Steel plate manufacturing method. 2. By weight ratio, C: 0.0050% or less, Si: 0.5 to 3.5%, Mn: 0.001 to 0.5%, P: 0.050% or less, S: 0.02% or less, N: 0.0050% or less, acid soluble Al: 0.001 to 0.1%, Ti: Excluding those included as oxides [4 x C (%)
+ 3.4 × N (%) + 1.3 × S (%)] value (%) or more, B: 0.0030% or less, the balance is Fe and inevitable impurities. After soaking and rough hot rolling,
₃ Reduction temperature: 500 ° C or higher Temperature reduction: 50% or higher finish hot rolling, winding at 600 ° C or lower, then descaling, Cold reduction: 40 to 95% cold rolling After that, when the austenite phase is higher than the recrystallization temperature and lower than 950 ° C. or the composition, it is heated to a temperature range lower than the Ac ₃ transformation point and annealed, and cold-rolled for deep drawing. Steel plate manufacturing method.