JPH036323A - Press forming method for steel sheet excellent in formability - Google Patents

Abstract

PURPOSE:To properly form a steel sheet having superior formability and to increase the degree of freedom of formed parts by subjecting a steel in which trace amounts of Nb are added to hot rolling and to annealing at specific temp. and also controlling forming velocity at the time of press forming after cold rolling. CONSTITUTION:A slab of a steel having a composition consisting of, by weight, <=0.001 5% C, 0.03-0.60% Mn, <=0.1% solAl, 0.003-0.015% Nb, <=0.0020% N, and the balance Fe with inevitable impurities is hot-rolled, annealed, at 680-950 deg.C, and subjected to a repetition of cold rolling and annealing, and further, at the time of press forming, forming velocity is controlled to regulate equivalent strain rate in a zone with a risk of fracture to <=0.25<-1>, or, <=0.008% P is incorporated to the above steel slab and coiling is performed at 620-800 deg.C in stead of annealing.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to press forming of a steel plate with excellent formability, which can be formed with high precision and is suitable as a material for molded parts having a complex shape. Regarding the method.

(Prior Art) Conventionally, ultra-deep drawn steel sheets used as materials for automobile body panels and other molded parts with complex shapes have a drawing strength of 0.03 to 0.
．． Funatsuri was manufactured by cold rolling low carbon Af2 killed steel containing about 0.6% by weight of carbon and then decarburizing and annealing it in an open coil annealing furnace.

Recently, due to remarkable progress in refining technology, it has become easy to manufacture ultra-low carbon steels containing carbide-forming elements such as Nb, and ultra-deep drawn steel sheets made from these steels have also begun to be used. For example, in Japanese Patent Publication No. 61-32S75, the C content is 0.007% with composite addition of Ti and Nb.
The following method for producing ultra-deep drawn steel plates has been proposed, which consists of cold rolling and annealing the ultra-low carbon steel, and there are many other proposals regarding this type of ultra-deep drawn steel plates.

However, due to design and functional requirements, there is a strong tendency for modern automotive molded parts to have very complex shapes and sharp-angled shapes, and the ultra-deep-drawn steel plates mentioned above are no longer able to meet these demands. It has occurred.

Therefore, it has been considered to use plastic for such parts, and it is actually used. However, in addition to high costs, plastics have disadvantages in use, such as being easily scratched, difficult to repair, and having low heat resistance, so they have not been widely used across the board.

(Problem to be solved by the invention) Therefore, if plastic-like "formability" could be obtained for steel plates, it would be inexpensive, highly productive, and greatly improve design freedom, which would be extremely desirable from an industrial perspective. That is clear.

In general, deep drawability refers to the ability to form an overall shape, such as being able to draw deeper. However, if the formability of a local shape, such as the roundness of a corner, is very small, deformation will concentrate there and breakage will occur, making it impossible to fully utilize the original deep drawability. .

Therefore, in order to obtain the above-mentioned formability, it is not enough just to have excellent deep drawability, but also to have sufficient tenacity to withstand acute local deformation. Furthermore, in order to actually create plastic-like shapes, in addition to using excellent materials, appropriate molding methods are extremely important.

Here, the object of the present invention is to create a material that can be molded with high precision and is suitable as a material for molded parts having a complicated shape.
An object of the present invention is to provide a method for press forming a steel plate with excellent formability.

(Means for Solving the Problems) It has been theoretically and experimentally demonstrated that deep drawability can be determined by the r value (plastic anisotropy) generally measured in a tensile test. Therefore, steel sheets for ultra-deep drawing have a high r value.
It is characterized by.

However, there is still no well-established method for evaluating resistance to local deformation concentration, which is the method provided by the present invention. This was thought to be a major obstacle in the development of highly formable steel sheets.

Therefore, the present inventors formed a steel plate 2 into a conical shape using a pointed conical punch 1 as shown in FIG. An attempt was made to evaluate the molding height. In the figure, the punch diameter is 50mm, the apex radius is 1cm, and the standard apex angle is 95°.
The diameter of the die is 52.5 mm, and the steel plate 2 is completely clamped around it with a temporary holding force of 5 Ton so as not to move.

As can be easily guessed, in such a pointed cone, intense deformation concentration occurs at the apex of the tip, so the ultra-deep drawn steel plate breaks at the step where the forming height is shallow.

In such a pointed cone test, the forming height is determined by the resistance force against local deformation concentration at the apex, so it is found to be appropriate for evaluating formability.

Therefore, more specifically in this specification, "formability" refers to the properties of a cold rolled steel sheet that are experimentally evaluated at the critical overhang height of the above-mentioned pointed cone overhang test.

Now, in pointed cone forming, the strain at the apex becomes extremely large, so it is first important that the material has a sufficiently high deformability (ductility). For this purpose, it is necessary to appropriately control the crystal grain size of the material. That is, when the crystal grains become coarse, the steel sheet becomes noticeably rough during forming, resulting in unevenness on the surface. It is known that these irregularities promote the so-called plastic instability phenomenon and reduce deformability.

On the other hand, in this test, it was completely fixed around the steel plate and did not flow into the die hole. Many studies have revealed that the r value has no effect on the molding height under such conditions. Regarding this point, the present inventors have further investigated and obtained the impression that the n value (work hardening index) has an influence. The reason is that the higher the n value, the greater the hardening of the deformation concentrated portion, which suppresses deformation there, resulting in less deformation concentration and improved formability.

Based on the above knowledge, several of the present inventors came up with a method for manufacturing a steel plate with excellent formability, and filed a patent application in 1983.
-270537 and Japanese Patent Application No. 63-311600.

Indeed, as shown in Fig. 2, an example of this is shown in the steel plate produced by the above method, which means that the apex fracture disappears at an appropriate apex angle (95°), and a complete conical shape is formed.
An amazing molding height of m is obtained.

However, as a result of further detailed study by the present inventors regarding the forming of the highly formable steel sheet manufactured in this way,
In this way, even with a steel plate having a high n value, high formability cannot always be obtained, and the inventors discovered a completely new finding that the effect of suppressing deformation concentration varies greatly depending on the deformation rate, leading to the present invention.

Here, the gist of the present invention is, in weight %, C: 0.0015% or less, Mn: 0.03 to 0.
60%, sol, AQ: 0.1% or less, Nb: 0.
003 to 0.015%, N: 0.0020% or less, balance: After hot rolling a steel piece having a steel composition consisting of Fe and inevitable impurities, 6
Annealing is performed in a temperature range of 80 to 950°C, and then cold rolling and annealing are performed to obtain a cold rolled steel plate, and when press forming it, the equivalent strain rate of the fracture risk area is 0.25-'
This is a press forming method for a steel plate with excellent formability, which is characterized by controlling the forming speed as follows.

From another aspect, the present invention provides the above-mentioned press forming method for a steel plate with excellent formability, in which the steel piece further contains 0.008% by weight of P and a temperature of 680 to 950°C. This is a press-forming method for a steel plate with excellent shapeability, which is characterized by performing coiling at a temperature range of 620 to 800°C instead of annealing at a temperature range of 620 to 800°C.

(Function) Although there are many unknown points regarding the mechanism by which high formability is expressed by the present invention, it is possible to significantly reduce the amount of carbon or phosphorus in the material steel and add a small amount of Nb, making it an extremely low nitrogen steel. Therefore, if annealing is performed after hot rolling or coiling at high temperature, the crystal grain size of the steel sheet becomes appropriate, and the presence of appropriate solid solution carbon and Nb is combined, resulting in high formability. considered to be a thing.

Specifically, ultra-low carbonization is related to high purity steel and grain size adjustment, and addition of a small amount of Nb and annealing after hot rolling are related to grain size adjustment. It is surmised that this is more strongly related to quantity.

Next, in the present invention, the reason why the chemical composition, annealing temperature, and strain rate of the steel slab used are limited will be explained.

(1) Component composition C: When the amount of C exceeds 0.0015%, cementite is formed and inhibits the ductility of the steel sheet, making it impossible to ensure desired formability. Therefore, the C content is 0.0015
% or less.

Mn: Mn is basically S, which is inevitably included in steel.
It is included in order to prevent embrittlement caused by. If the content is less than 0.03%, the effect of preventing embrittlement will not be sufficient, while if it exceeds 0.60%, the effect will be saturated, which will only lead to an increase in cost. Therefore, the Mn content was limited to 0.03% or more and 0.60% or less.

sol, M: Since Al1 is used for deoxidizing steel, some amount remains in the steel as sol, M. However, since sol and AQ precipitate AQN and inhibit ductility,
If it exceeds 0.1%, desired formability cannot be obtained. Therefore, the AQC content was limited to 0.1% or less.

Nb: Nb is added to coarsen the crystal grains of the final product and form NbC, thereby ensuring high formability. However, if the amount of Nb is less than 0.003%, the desired effect cannot be obtained, and on the other hand, if the amount of Nb is more than 0.015%, the solid solution Nb will increase, the steel will harden, and the formability will deteriorate. Therefore, the amount of Nb added should be 0.003% or more.
．． It was limited to 0.015% or less.

N: Like C, N is also an element that is better for formability as it is less. If the N content exceeds 0.0020%, a large amount of NbN is formed, inhibiting ductility and making it impossible to ensure desired formability. Therefore, the amount of N added is 0.0
It was limited to 0.020% or less.

(2) Reason for limiting the annealing temperature In the usual manufacturing process of cold rolled steel sheets, hot rolled steel sheets are descaled by pickling and then cold rolled as they are. However, a major feature of the present invention is that the hot rolled steel sheet is annealed at a high temperature.

This annealing is performed using NbC and AQN in the steel sheet before cold rolling.
It is important as a preparatory step to adjust the precipitation state, crystal grains, and texture, such as, so that the composition after cold rolling and annealing is favorable for formability.

In other words, in hot-rolled steel sheets, Nb is precipitated in the form of NbC combined with C, but moderate high-temperature annealing prevents coarsening of NbC and makes NbC precipitate sufficiently, and the Ensure fine grain size and high r value.

Furthermore, most of the sol and AQ in the steel precipitate as AQN, reducing the solid solution N.

However, at an annealing temperature of less than 680°C, such precipitation of NbC and MN does not occur sufficiently, resulting in a low r value and elongation, making it impossible to obtain high formability. On the other hand, 950℃
At an annealing temperature higher than 100%, the precipitated NbC becomes coarser, resulting in significant coarsening of crystal grains. As a result, the surface of the molded product becomes rough, lowering the deformation limit and deteriorating formability. For the above reasons, the annealing temperature is 680-950℃
It was determined that

This high-temperature annealing may be performed by either batch annealing with the coil as it is or continuous annealing while loosening the coil. Further, this annealing may be applied to the scaled hot rolled sheet as it is, or may be applied to the steel sheet which has been descaled. In the method of the present invention, the C content of the steel plate is 1
Since the amount is 5 ppm or less, decarburization due to scale is not a problem.

(3) Reasons for limiting the strain rate During press forming of a steel plate, the steel plate generally undergoes biaxial deformation within its plane. In order to uniformly express the strain state at this time, equivalent strain in plastic mechanics is used. In other words, the in-plane maximum strain is ε8, and the principal strain perpendicular to it is ε.
, the plate thickness strain is ε5,2, the equivalent strain is ε, and the in-plane average r value of the steel plate is r, then 3r(1+2r)2.

According to plasticity theory, the equivalent strain is the value that defines work hardening due to deformation, so it is the most suitable parameter to express the effect of suppressing local deformation.

Now, as mentioned above, when the strain state is expressed by equivalent strain, if the equivalent strain rate exceeds 0.2S-', the effect of suppressing local concentration of deformation decreases, and formability sharply decreases, so 0.2S-'' was set as the upper limit.

The reason why formability changes depending on the strain rate of deformation is not yet known in detail, but when the deformation rate is slow, minute amounts of carbon in the steel diffuse during deformation, increasing the work hardening properties of the steel sheet. It is presumed that this is because of this.

Generally, the strain amount and strain rate of a press-formed product differ depending on its location, but since the current issue is fracture, the strain rate of the fracture-prone area should be taken into consideration.

Note that the fracture-dangerous parts may be identified in advance by measuring the strain distribution of each part of the molded product.

Next, in the present invention, as an alternative method, the steel billet used further contains P, 50.08%, and 680 to 9
Instead of annealing at 50°C, winding at 620 to 800°C may be performed.

The reason for this limitation is as follows.

(4) P≦o, oos% P segregates at grain boundaries and inhibits formability, but when the P content is particularly reduced to 0.008% or less, the negative effects of segregation are significantly reduced. , high formability can be stably obtained after cold rolling and winding. if,
If the P content exceeds 0.008%, the adverse effect on formability cannot be ignored, and desired characteristics cannot be obtained. Therefore, the P content in the steel slab used is limited to 0.008% or less.

(5) Reason for limiting the coiling temperature When coiling after hot rolling, N
It is important as a preparatory step for adjusting the precipitation state, crystal grains, and agglomerated structure of bC, TiN, etc., and obtaining a preferable structure after cold rolling and annealing. In normal hot rolling, winding is carried out at a temperature range of 600 to 500°C, but in the present invention it is carried out at a higher temperature than conventionally.

That is, by winding at an appropriate high temperature, enough NbC can be produced without coarsening the NbC. This improves the r value and elongation of the steel plate after cold rolling and annealing, and ensures high formability. However, such an effect is 620'
A winding temperature of less than 620°C will not be sufficient.
was set as the lower limit.

On the other hand, in high-temperature winding exceeding 800° C., NbC precipitation becomes too coarse and the structure becomes coarse-grained. As a result, surface roughness occurs during forming and lowers the deformation limit of the steel plate, making it impossible to obtain high formability.

Note that in the present invention, since the P content is low, precipitation of FeTiP can be suppressed even during high-temperature winding.

Therefore, in the above-mentioned alternative method, instead of performing annealing in the temperature range of 680 to 950°C, the winding temperature is limited to 620°C or more and 800°C or less.

5 Furthermore, the present invention will be explained in detail using Examples, but these are illustrative of the present invention and the present invention is not limited thereby.

Example 1 Steel having the components shown in Table 1 was melted into a slab, hot-rolled at a heating temperature of 1100°C and a rolling finishing temperature of 910''C, and then annealed at 680-950°C. A hot rolled steel plate having a thickness of .2 mm was obtained.

Next, after pickling this, cold rolling was performed at a reduction rate of 75%, and then by holding at 850 ° C. for 40 seconds,
Samples 1 to 9, which were cold-rolled steel plates with a thickness of 0.8 m+n, were obtained.

Each sample thus obtained was subjected to a pointed cone stretching test with an apex angle of 95° and vertices R1 and 5R as shown in Fig. 1 at equivalent strain rates of 0.05 s-' and 0.3 s-' at the apex. The critical molding height was measured. In this case, since the vertex is in an equibiaxial tensile stress state, ε8−εa and ε8 obtained from the law of volume invariance, which is a fundamental property of plastic strain.
Considering +εa+ε2−0, 6ε−[(r+2)/3]・ε, so from the above formula, measure the plate thickness strain ε and convert it to the equivalent strain,
The punching speed was adjusted to obtain a predetermined strain rate.

As shown in Table 1, the results show that a perfect cone of 27 m can be formed in pointed cone forming only when the ingredients and manufacturing conditions are within the range of the present invention, and the equivalent strain rate is within the range of the present invention. It can be seen that the above molding height can be obtained and excellent high moldability is achieved.

Example 2 Among the samples shown in Table 1, sample Nα4 and sample Nα8
(all ultra-low carbon steels) and sample No. 9 (open coil decarburized annealed steel), the height of the pointed cone formed was measured by changing the strain rate. The method of determining the strain rate is the same as in Example 1.

The results are shown in Figure 3. As can be seen from FIG. 3, in the case of sample N[L4 according to the present invention, when the equivalent strain rate at the apex becomes 0.2 S-' or less, the critical forming height increases greatly and a perfect cone is formed. In other words, the desired high formability can be obtained in this area. On the other hand, in samples No. 8, which are ultra-low carbon steels, the forming height tends to increase when the strain rate is lowered, but the increase is small. Sample No. is also a decarburized annealed steel.

In No. 9, the speed effect itself was small, and good formability could not be obtained in any speed range.

As described above, formability is very sensitive to strain rate, and it is clear that selecting an appropriate strain rate is important in order to fully express this ability.

Example 3 Steel having the components shown in Table 2 was melted into a slab, hot-rolled at a heating temperature of 1100°C and a rolling finishing temperature of 910°C, and then coiled to form a hot-rolled steel plate with a thickness of 3.2111 m. I got it.

Next, after pickling this, cold rolling was performed at a reduction rate of 75%, and then held at 850 ° C. for 40 seconds to obtain cold rolled steel sheets with a thickness of 0.8 mm Sample No. 10 to Sample No.
I got 18.

Each sample thus obtained was subjected to a tensile test and a pointed cone limit forming height test in exactly the same manner as in Example 1. The results are shown in Table 2.

The results are shown in Table 2. Only when the ingredients and manufacturing conditions are within the range of the present invention and the equivalent strain rate is within the range of the present invention, a perfect cone of 27 mm can be formed in pointed cone molding. It can be seen that the above molding height can be obtained and excellent high moldability is achieved.

Example 4 Among the steel plates shown in Table 2, sample No. 13 and sample N
.

Using Sample No. 16 (extremely low carbon steel), Sample No. 18 (open coil decarburized annealed steel), the height of pointed cone forming was measured by changing the strain rate. The method of determining the strain rate is the same as in Example 1.

The results are shown in Figure 4. As can be seen from Figure 4, in the case of sample No. 13, which is an example of the present invention, when the equivalent strain rate at the apex becomes 0.28-' or less, the critical forming height increases greatly and a perfect cone is formed. . In other words, the desired high formability can be obtained in this area. On the other hand, in sample No. 16, which is made of ultra-low carbon steel, although the forming height tends to increase when the strain rate is lowered, the increase is slight. In addition, in sample No. 18, which is decarburized annealed steel, the speed effect itself was small, and formability could not be obtained in any speed range.

As described above, formability is very sensitive to strain rate, and it is clear that selecting an appropriate strain rate is important in order to fully express this ability.

Example 5 A steel containing a maximum of 39 ppm of carbon and other components within the range of the present invention was annealed under the same conditions as in Example 1, with the annealing temperature varied from 650 to 1000°C, and the strain rate was reduced to 0.
The overhang height was measured using IS-'. As shown in FIG. 5, the results show that good formability can be obtained at temperatures between 680 and 950°C.

Example 6 A steel containing a maximum of 50 ppm of carbon and other components within the range of the present invention was rolled up under the same conditions as in Example 3, with the coiling temperature varied from 500 to 820°C, and the strain rate was reduced to 0. 1
The overhang height was measured at 5-'. As shown in FIG. 6, the results show that good formability is obtained between 620 and 800°C.

90 (Effects of the Invention) As explained above, according to the present invention, a steel plate having excellent formability can be appropriately formed, and the formability can be expressed in actual forming.

This increases the degree of freedom in the design of parts obtained by press-forming steel plates, and greatly improves the productivity of forming, which is extremely useful in industry.

[Brief explanation of drawings]

Fig. 1 is a schematic cross-sectional view of the point-to-cone limit forming height test device; Fig. 2 is a graph showing the relationship between the apex angle of the punch and the forming limit height; and Figs. It is a graph showing the results of an example of the invention.

Claims (2)

[Claims] (1) In weight%, C: 0.0015% or less, Mn: 0.03 to 0.60%
, sol, Al: 0.1% or less, Nb: 0.003-0
．． After hot rolling a steel piece having a steel composition consisting of 0.015%, N: 0.0020% or less, remainder: Fe and inevitable impurities, 6
Annealing is performed in a temperature range of 80 to 950°C, followed by cold rolling and annealing to obtain a cold rolled steel plate, and when press forming it, the equivalent strain rate at the fracture risk area is 0.2S^-
A press forming method for a steel plate with excellent formability, characterized by controlling the forming speed so that the forming speed is ^1 or less. (2) In the method for press forming a steel plate with excellent formability according to claim 1, the steel billet further contains P≦0.008% by weight, and is suitable for annealing in a temperature range of 680 to 950°C. A press-forming method for a steel plate with excellent formability, which is characterized in that winding is performed at a temperature range of 620 to 800°C.