JPS63247338A - High tensile strength steel plate for deep drawing having excellent secondary workability and baking hardenability - Google Patents

Abstract

PURPOSE:To produce the titled plate by incorporating specific ratios of C, Si, B, Al, P, N, Mn, S and Ti into Fe. CONSTITUTION:The steel plate contg., by weight, 0.005-0.005% C, <=0.7% Si, 0.0003-0.003% B, 0.005-0.100% Al, 0.04-0.15% P, <=0.004% N, <=0.5% Mn and <=0.006% S, contg. Ti in the range of 48/14(N%)+48/32(S%)<=Ti%<=48/14(N%)+48/32(S%)+3.48/12(C)%) and consisting of the balance Fe with inevitable impurities is prepd. The steel plate having the excellent secondary workability and showing low yield stress, good deep drawability and baking hardenability while having the high tensile strength is thereby obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) In recent years, the automobile industry has been working to reduce the weight of vehicle bodies with the aim of improving fuel efficiency, and the demand for high-strength thin steel sheets is increasing.

On the other hand, when press forming a steel plate into the shape of a car body, good deep drawability is required.

The present invention relates to a steel plate that has the above two properties and also has the property of increasing yield strength and tensile strength when baking the paint after press forming, that is, has bake hardenability.

(Prior art) In addition to reducing the thickness of steel plates for automobiles and improving deep drawability, Punto resistance is another issue. In addition, as car bodies are increasingly being molded into one piece, conflicting properties such as strength, deep drawability, and Punto resistance are required for a single steel plate.

For example, it is advantageous to lower the yield point for press formability, and to increase the yield point for Punt resistance.

Recently, after press forming, bake hardenability (hereinafter referred to as BH
(abbreviated as gender) is attracting attention.

As a technique for imparting BH properties to steel sheets for deep drawing, JP-A-59-31827 discloses the use of Ti in ultra-low carbon steel.

There is a disclosure about adding Nb.

That is, N is completely fixed by Ti, the amount of Nb added is adjusted in relation to the C content, leaving an appropriate amount of solid solution C, and B
This is intended to ensure H properties.

Further, JP-A-59-38337 discloses that Ti, Nb, and B are added to ensure BH properties by remaining solid solution C and solid solution B.

However, when adjusting solid solution Cl with Nb, Nb
reacts quantitatively with C and fixes it, so BH properties are directly affected by variations in C content, making it difficult to ensure stable BH properties.

Also, as a technology to obtain stable BH properties, JP-A-61-267
Publication No. 57 describes not only the amount of Ti added to fix C, but also the amount of S in ultra-low carbon steel containing Ti.

It is described that the amount of N is limited. Here S;0
．． 0.003% or less, N; 0.004% or less, and [S
%]+[N%]≦0.005, but
It is difficult to stably produce S and Njl within the above-mentioned ranges, and especially when a large amount of reinforcing elements are added, these elements are likely to be mixed in as impurities, which tends to increase costs.

On the other hand, steel sheets with extremely low carbon and improved deep drawability tend to undergo brittle fracture when subjected to secondary processing after deep drawing.
When BH properties are imparted, the grain boundaries are strengthened by solid solution C, and this tendency can be alleviated. However, P.
When a large amount of reinforcing elements such as St and Mn are added, the tendency for embrittlement becomes significant, and it is difficult to improve this problem with solid solution Cl that imparts good H properties.

To improve secondary processing brittleness, Japanese Patent Application Laid-Open No. 59-1932
As shown in Publication No. 21, it is known that B addition is effective. This effect is due to the solid solution B concentrating in the grain boundaries and increasing the strength of the grain boundaries. However, B has a strong tendency to form nitrides and easily becomes BN, and in the above-mentioned technology, Ti is added to allow solid solution B to remain, and the N in the steel is replaced by Ti.
Nb is fixed as Nb, and Nb is added in combination to reduce solid solution C in steel to the point where it becomes virtually non-aging, but as mentioned above, when C is fixed with Nb, Nb is quantitatively added. Because it reacts with C, it is difficult to stably leave solid solution C.

(Problem to be solved by the invention) In a copper plate that has excellent secondary processing resistance and BH properties, when controlling the BH properties with Nb, it is difficult to maintain stable BHI because the C content cannot be kept constant. In order to control S and Nil, it is necessary to reduce them extremely, which leads to an increase in cost.Furthermore, adding a large amount of reinforcing elements and increasing the tensile strength may cause defects in secondary workability. was there.

The purpose of this invention is to provide stable BH properties at a low cost while maintaining press formability to a high-strength steel plate for deep drawing that has excellent secondary workability in order to solve the above problems. be.

(Means for solving the problem) In order to develop a steel plate that satisfies properties such as secondary workability, strength, deep drawability, and punt resistance, the inventors added Ti, B is added, and P is further added as a reinforcing element.
, Si, and Mn were added to high-strength m-plates for deep drawing, and the relationship between the amount of each addition and the material quality showed that when S was lowered, the BH strength was less affected by the relationship between the amount of Mn and the amount of S. I discovered that.

This invention is based on the above findings.

That is, in this invention, C: 0.0005 to 0.005 wtχSi: 0.
7ga t% or less B: 0.0003 to 0.003 Tsuba tχAll:
0.005-0.100 wtχP: 0.04-0
．． 15wtX and N: 0.004 wt2 or less, Mn: 0.5wt% or less, S: 0.006
This is a high-strength steel sheet for deep drawing that has excellent secondary workability and bake hardenability, and contains Ti in a range of wtχ with the balance being iron and unavoidable impurities.

In this invention, P and Si are added to the steel, which has extremely low carbon content and ensures good deep drawability, to impart strength.

At this time, it is necessary to contain B in order to ensure secondary processability. B in steel has a strong tendency to form nitrides and easily generates BN, but in order to improve secondary workability, it is necessary to concentrate it in the grain boundaries as solid solution B. Therefore, by incorporating Ti, which has a stronger tendency to form nitrides than B, N is fixed as TiN, and a part of the solid solution C is fixed as TiC, and by slow aging, B
The amount of solid solute C is reduced to a range that provides good H properties.

Even in conventional steels containing only Ti, the Ti content was reduced to N.

When sold in equal quantities with S and C, no solid solution C remains as with Nb-containing steel, and it has been considered difficult to adjust the amount of BH to an appropriate range. However, according to experiments conducted by the inventors, it is possible to reduce S to an extent that does not lead to an extreme increase in cost.
Furthermore, by setting Mn/S≧50 in relation to the Mn content, even if Ti is contained commercially in equal amounts with S, N, and C, TiC will precipitate later than the Ti and C contents. Therefore, it was found that solid solution C remained stably and BH properties could be ensured.

This is because TiC precipitates using TiS as a precipitation nucleus.
This is considered to be because TiS decreases by reducing N, and the precipitation nucleus density decreases, causing a delay in TiC precipitation. Furthermore, the BH properties are not only influenced by sl as described above, but also greatly influenced by the amount of Mn/S, and the BH properties can be improved by adding an excessive amount of Mn compared to the amount of S. The experiment that yielded this knowledge will be specifically described below.

C; 0.0025wtχ (hereinafter simply indicated as %),
Si: 0.1%, p; 0.08%, AIo,
04%, N: 0.0025%.

Vacuum melted steel containing 0.0010% B, 0.0035% S, and varying amounts of Mn and Ti was melted in a laboratory, hot rolled to a thickness of 3.5 mm, and further 0.0035%.
It was cold rolled to a thickness of 71 mm.

This cold-rolled sheet was annealed in a continuous annealing cycle of soaking at 850°C for 40 seconds, and then temper-rolled at a rolling reduction of 0.8%, and then subjected to a tensile test as a JISS No. test piece. investigated. The BH property was evaluated by measuring the amount of increase in yield point by aging treatment equivalent to baking at 170° C. for 20 minutes after applying 2% prestrain.

Figure 1 shows [Mn%
]/(S%) and BH properties.

From the same figure, when [Mn%]/[S%]≧50, the atomic equivalent of Ti with C is 1 or more, and the S content is 0.003%.
It can be seen that despite the above, it shows good H properties.

The reason for this is not necessarily clear, but the presence of Mn in excess of S may cause a delay in the precipitation of TiS, or Mn may destabilize TiS by forming a composite precipitate with TiS. There are many possible reasons.

(Function) Next, the reason for limiting the component composition range in this invention will be explained.

C: 0.0005 to 0.005% The lower the C content, the better the quality of the material.If the C content is high, the content of Ti, which fixes with C, must also be increased, and the amount of TiC precipitated increases, causing C is 0.0 to deteriorate the drawability.
0.5% or less.

On the other hand, in order to obtain BH properties, a C content of 0.0005% or more is required. Therefore, the amount of C is o,ooos~0.00
The range shall be 5%.

St: 0.7% or less Si is included to adjust the strength level, but 0.7% or less.
If it exceeds 7%, the surface quality will deteriorate, so the content should be 0.7% or less.

B: 0.0003 to 0.003% B is necessary to improve secondary processing brittleness. In this invention, P and St are contained as reinforcing elements to increase the strength of the steel sheet, but if these elements are contained in large amounts, brittle fracture is likely to occur when subjected to secondary processing after deep drawing. . B is effective in improving secondary processing brittleness because it strengthens grain boundaries, and its content is 0.0003%.
The effect appears when the content exceeds 0.003%, while the content exceeding 0.003% deteriorates deep drawability.
% range.

A 170.005 to 0.100% Since octaβ is a helic acid, it is necessary to contain it at 0.005 wt% or more, but if it exceeds o or ioo%, it will have a negative effect on the surface quality, so it is 0.005 to 0. .100%.

P: 0.04-0.15% P is an element contained in order to strengthen the steel plate and is also advantageous for deep drawability. If it is less than 0.04%, the strengthening effect is insufficient (on the other hand, If the content exceeds 0.04% to 0.15%, the spot weldability will deteriorate.

N: 0.004% or less N is inevitably mixed, and the lower the content, the better the quality of the material. That is, when the amount of N is large, it is necessary to increase the Ti content in order to fix the solid solution N, leading to an increase in cost, and when a large amount of TiN is generated, the deep drawability is deteriorated, which is not preferable. However, since an extreme reduction involves an increase in cost, the upper limit is set at 0.004%.

Mn: 0.5% or less If Mn exceeds 0.5%, the deep drawability deteriorates significantly, so a lower value is preferable, but as shown in Figure 1, from the viewpoint of BH properties, Mn/S ≥ 50. There is a need to. Therefore M
The upper limit of the amount of n added is 0.5%, and the lower limit is [Mn%]/[S%]≧50 in relation to the S content.

S: 0.006% or less The amount of S has a great effect on the BH properties, and the lower the S content, the more advantageous it is to the BH properties, but an extreme reduction will lead to an increase in cost. In this invention, by setting Mn/S≧50, the restriction on reducing the amount of S is loosened, and B is stabilized at as low a cost as possible.
Trying to get sex. However, if the S content exceeds 0.006%, good H properties cannot be obtained even if Mn/S≧50. In other words, the absolute amount of TiS increases, and T
Since there are sufficient iC precipitation nuclei, the upper limit of the S content is set to 0.006%.

Ti: When Ti is contained to fix S, N and C,
The contained N cannot be fixed, and B, which is intended to improve secondary processing brittleness, becomes BN and precipitates, and its effect is no longer exhibited. Moreover, when Ti [C%] is exceeded, Mn
Even when /S≧50, good H properties cannot be obtained.

Therefore, the steel according to the invention is produced by conventional means, even though the Ti content is low. For example, after forming a slab by continuous casting, it is subjected to hot rolling, cold rolling, continuous annealing, and then temper rolling to obtain a product. In continuous annealing, it is advantageous to cool the steel to a temperature of about 1° C./s or higher to leave solid solution C and ensure BH properties. - In slow cooling of several degrees to several tens of degrees per hour, such as the power tube annealing method, the solid solution C becomes Ti during cooling.
C precipitates, making it impossible to obtain BH properties. Therefore, the continuous annealing method is optimal.The temperature during annealing should be above the recrystallization temperature from the perspective of ensuring workability, and from the perspective of ensuring deep drawability. The A3 transformation point or lower is desirable.

(Example) Steel with the composition shown in Table 1 was melted using a converter-RH degassing method, made into a slab using a continuous casting method, then subjected to normal hot rolling, and cold rolled after pickling. After forming a cold-rolled plate with a thickness of 0.8 mm, continuous annealing was performed at a soaking temperature of 820 to 850°C, and then skin pass rolling was performed at a reduction rate of 0.3 to 0.7%. The obtained steel plate was processed into a JISS No. test piece, and the mechanical properties of the test piece were investigated. Table 2 shows the results.

As mentioned above, after 2% pre-strain, the hardness is 20°C at 170°C.
Evaluated by the amount of increase in yield point after 20 minutes of baking equivalent treatment, and 2
Next, try a conical cup on the steel plate for workability! Aperture ratio 1 with Sa machine
．． A cup was molded using 88 and a 5 kg weight was dropped on the cup at 0° C., and quality was judged based on the presence or absence of cracks.

In addition, the YS, 4S, El, and r values shown in Table 2 are in the rolling direction (x
o) Average of test results taken in the direction of 45° (X'S) from the rolling direction and 90° (X9°) from the rolling direction The results are based on test pieces taken parallel to the rolling direction.

From Table 2, for the invented steel, TS = 35 to 43 kgf/mmt
, YS≦25 kgf/mm”+Ef≧41%, r value≧2, and BH≧3.0 kgf/mm”, and it can be seen that there is no problem in secondary workability.

However, in Comparative Steel 7 with a C content outside the compatible range, Comparative Steel 8 with a Mn content outside the compatible range, and Comparative Steel 10 with a B content outside the compatible range, the r value and E2 deteriorate. Also, a comparison where Mn/S is less than 50! Comparison steel 11 with i49, Ti''/C exceeding 12, that is, Ti% exceeding , and comparison w where sl is outside the conforming range.
413, the BH amount is 0.9 kgf/mm" and 0.413, respectively.
5kgf/mm”.

Comparative steel 12, which shows a low value of 0.8 kgf/mm'' and has a low Bi, has secondary work cracking.

(Effects of the Invention) According to the present invention, it is possible to obtain a steel plate that has excellent secondary workability, high tensile strength but low yield stress, good deep drawability, and good BH properties, and is useful for lightweight car bodies in the automobile industry. It can be used in a wide range of applications such as molding and integral molding, and its industrial value is great. In particular, stable B that is not affected by changes in C content.
H properties can be imparted at low cost, and the effect is great.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the amount (Mn%/S%) in steel and BH properties.

Claims (1)

[Claims] 1. C: 0.0005 to 0.005 wt%, Si: 0.
7 wt% or less, B: 0.0003-0.003 wt% Al: 0.005-0.100 wt% P: 0.04-0.15 wt% and N: 0.004 wt%
% or less, Mn: 0.5 wt% or less, S: 0.
006wt% or less and [Mnwt%]/[Swt%]
Satisfying the condition of ≧50, and further adding Ti (48/14) [Nwt%] + (48/32) [Swt
%]≦[Tiwt%]≦(48/14)[Nwt%]+
(48/32) [Swt%] +3・(48/12) [C
A high-strength steel sheet for deep drawing that has excellent secondary workability and bake hardenability and has a composition of iron and unavoidable impurities.