JPS5849624B2 - Method for manufacturing high-strength cold-rolled steel sheets with excellent drawability and shapeability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet with excellent drawability and shapeability.

Recently, the use of high-strength steel sheets for the purpose of improving the strength or reducing the weight of automobile bodies has been promoted, and there is a desire to develop high-strength cold-rolled steel sheets suitable for this type of use.

Since steel sheets for automobiles are generally used after being press-formed, they must have properties suitable for this press-forming.

In particular, parts that undergo severe processing such as car body panels require excellent drawability and shapeability.

The higher the r-value (Lankford value) of a steel sheet, the better the drawability is, and various types of steel sheets with higher r-values have been developed by improving manufacturing conditions for conventional soft steel sheets.

However, the r value tends to decrease as the strength of the steel plate increases, and the r value of the high tensile strength steel plate was around 1.0.

The lower the yield strength and yield ratio of the steel plate, the better the shape properties will be. However, in general steel with a ferrite + pearlite structure, the yield ratio is 0.65 to 0.85, so as the tensile strength of the steel increases, the yield ratio becomes better. The strength also increased, and generally speaking, the shapeability of high-tensile steel sheets was extremely poor, so they were not used much for automobiles.

In recent years, a steel with finely dispersed martensite in ferrite has been developed, but in this steel, the martensite becomes a source of dislocations and uniform deformation occurs relatively easily, so the yield strength is low and 0. This results in a low yield ratio of .60 or less.

The results of various press formability tests conducted on steel with such a low yield ratio confirmed that it has better formability than conventional steel sheets with a ferrite + pearlite structure (yield ratio of 0.65 to 0.85). There is.

Now, in order to obtain a steel plate with such a low yield ratio, the method of continuous annealing after cold rolling was considered to be the most advantageous.

The metallurgical background is that the steel plate is heated to the coexistence temperature of ferrite (ct) + austenite (r) and then rapidly cooled to transform the γ phase into martensite and obtain the low yield ratio. The amount of C, Mn, Si, etc. in the steel is adjusted depending on the conditions.

The low yield ratio high tensile strength steel sheet manufactured by this manufacturing method has good shape properties as mentioned above, but has a low r value of 1.0 or less.In order to obtain a martensitic structure, a large amount of Mn or S is required.
There were two problems with the necessity of adding i.

As a result of various experiments and research, the inventors found that Mn in steel
By adding appropriate amounts of , SolAl, and N and box annealing in a temperature range of 650 to 800°C before heat treatment in the continuous annealing furnace, the yield ratio is 0.60 or less and the r value is 1.2 or more after continuous annealing. It was found that a high tensile strength steel plate of 100% can be easily obtained.

In other words, when a cold-rolled steel sheet is box annealed, Ruhe precipitates during the heating process, forming a recrystallized texture favorable to the r value, which is not destroyed even in the next step of continuous annealing, so it is 1.2
A high r value as above can be obtained.

Further, in the soaking process of box annealing, the steel sheet is heated to 650 to 800°C, so that C atoms and Mn atoms are significantly polarized into the γ phase existing at that time.

However, since the cooling rate in box annealing is slow, the γ phase transforms into pearlite and a low yield ratio is not achieved.

In the next continuous annealing process, the steel plate is heated to 600 to 850°C, so the pearlite phase becomes γ phase, and then 1
Since it is cooled at a fast cooling rate of 0 to 100'C/SeC, the γ phase becomes a martensitic phase.

The cooling rate is preferably 20 to ioo°C/
It is desirable to set it to sec.

In this way, a ferrite+martensite structure is obtained.

Since this manufacturing method utilizes the polarization of Mn, the average amount of Mn in the steel can be small, which contributes to lowering costs and improving the r value.

More specifically, the test steels (1), CB), and (C) containing the components shown in Table 1 were melted into slabs in a converter, and then heated at a heating temperature of 1220°C and a finishing temperature of 86°C, respectively.
3. Hot rolled under temperature conditions of 0°C and coiling temperature of 560°C.
Finished with a 2-inch thick steel plate.

After pickling, this was subjected to normal cold rolling. After finishing a cold-rolled steel plate with a thickness of 8 mm, the heating rate was 40°C/hr, the cooling rate was 40'C/hr, the soaking time was 16 hr, and H2 and N were heated.
Soaking temperature is 500~8 under the annealing condition of mixed gas atmosphere in 2.
After box annealing at a temperature of 0.0°C, the heating rate was increased to 2.
A heat treatment test was conducted in which continuous annealing was performed using a heat pattern of 0'C/sec, soaking at 750°C for 1 min, and cooling to room temperature at a cooling rate of 10°C/sec.

Figure 1 is a chart showing the relationship between the yield ratio and r value obtained from the characteristic values of the tensile test in the L direction based on the JI No. 85 tensile test piece and the box annealing temperature of the test steel plate obtained in the above heat treatment test. .

In the figure, the ● mark and characteristic curve P indicate the test steel (4), the ○ mark and the characteristic curve The relationship is shown in which the ● mark and characteristic curve S represent the test steel ω, the ○ mark and the characteristic curve T the test steel (B), and the ▲ mark and the characteristic curve V the yield ratio and box annealing temperature of the test steel (C). shows the relationship between

As shown in FIG. 1, the steel (B) of the present invention satisfies both the r value of 1.20 or more and the yield ratio of 0,460 or less when the box annealing temperature is in the range of 650 to 800°C.

Next, C0.04 to 0.0 s%, Soi3All
0. 0 3 to 0.08%, NO. 003~0.0
Section 08, containing P0.010~0.020φ and having a low Si content with Si0.05~0.25 added,
SiO. Various test steels were prepared by adding Mn in the range of 0.2 to 3.3% to two types of steel, one with a high Si content of 6 to 0.8%, and the other with a high Si content of 6 to 0.8%. After finishing a cold-rolled steel plate with a thickness of 0.8 mπ under the same conditions as described in Figure 1, the heating rate was 40°C/hr, the soaking temperature was 700°C, and the soaking time was 16.
hr, box annealing under the conditions of a mixed gas atmosphere of H2 and N2, and this was performed using the same heat pattern as shown in Fig. 1, that is, a temperature increase rate of 2.
0℃/Sec, soaking temperature 750℃ 1mi! A heat treatment test was conducted in which continuous annealing was performed at a cooling rate of L% of 10° C./SeC.

FIG. 2 is a chart showing the relationship between the yield ratio and r value determined from the characteristic values of the L-direction tensile test based on the JI No. 85 tensile test piece, which is a representative example obtained in the above heat treatment test, and the Mn content.

In the figure, ○ marks indicate low-Si test steel sheets, ● marks show high-Si test steel sheets, characteristic curve F shows the relationship between the r value and the amount of Mn, and characteristic curve G shows the relationship between the yield ratio and the amount of M.

As shown in Figure 2, both the high-Si test steel sheet and the low-Si test steel sheet satisfy both the r value of 1.20 or more and the yield ratio of 0.60 or less when the Mn content is in the range of 0.8 to 2.5. Show your grades.

The present invention provides a new method for producing high-strength cold-rolled steel sheets based on the above findings, with improvements made in terms of both alloy content and heat treatment. 0% or less, M n 0. 8 ~2. 5%, SoAA70.
01 to 0.20%, NO. 00 1 5 ~0.01
After hot rolling and cold rolling, a steel containing 50% P, 0.10% or less of P, and the remainder substantially Fe, was heated to 650
The gist is to perform box annealing in a temperature range of ~800°C, cool it, and then heat and cool it in a continuous annealing furnace with no heating zone at 60°C or higher.

The reason why the components of the steel are limited as described above in the present invention will be explained.

C: If it exceeds 0.20%, spot weldability deteriorates and the r value also decreases.

Si:1. If it exceeds 0%, the surface quality of the cold rolled steel sheet deteriorates and the r value also decreases.

SoAAl: 0.0 If it is less than 1%, the cleanliness of the steel decreases and the r value also decreases.

0. If it exceeds 20%, it becomes difficult to melt the steel.

Mn: If it is less than 0.80%, the Mn concentration in the Mn enriched section will be insufficient and the effect of lowering the yield ratio will be insufficient;
If the value exceeds 1.20, the r value decreases to 1.20 or less.

If N: 0.0015φ or less, the amount of A7N precipitated during box annealing is small and a high r value cannot be obtained.

If it is more than 0.015, the ductility decreases.

P: Although it is an inexpensive hardening element, adding 0.10% or more tends to cause brittle fracture.

It may be added up to 0.10 Yu if necessary.

In the method of the present invention, the soaking temperature range of box annealing is 650 to 8
The temperature was limited to 00°C because a yield ratio of 0.60 or less cannot be obtained if the temperature is less than 650°C or exceeds 800°C, and the temperature of the heating zone of the continuous annealing furnace was limited to 600 to 850°C. The reason is that below 600° C., the pearlite portion in which C and Mn are concentrated does not change to the austenite phase, and as a result, martensite is not formed during the cooling process, resulting in a high yield ratio.

On the other hand, if the temperature exceeds 850°C, the Mn concentrated in the austenite will diffuse and become uniform again during box annealing, and the desired martensite will not be obtained during the cooling process, resulting in a low r value.

Also, if the cooling rate after heating is less than 10°C/SeC,
A ferrite or pearlite structure is formed, which increases the yield ratio, which is undesirable.

On the other hand, if it exceeds 100'C/sec, a large amount of martensite is formed too much, the r value of the steel sheet decreases, and the desired effect cannot be obtained.

Next, examples of the present invention will be described.

Test steel (4), (B), with the components shown in Table 1 above.
After melting each of (C) into a slab in a converter, the heating temperature is 1220°C, the finishing temperature is 860°C, and the winding temperature is 560°C.
After hot rolling at a temperature of 3.2°C and finishing pickling, a steel plate with a thickness of 3.2°C is subjected to normal cold rolling. A cold-rolled steel plate with a thickness of 8 mm was finished and heated at a heating rate of 40'C/hr, a cooling rate of 40'C/hr, a soaking temperature of 700°C, and a soaking time of 1.
6 hr. After box annealing and cooling under the annealing conditions of a mixed gas atmosphere of H2 and N2, the heating rate was 20°C/sec, and the soaking temperature was 75°C.
00Cl11li! A heat treatment test was conducted in two ways: one in which continuous annealing was performed using a heat pattern with a cooling rate of 10°C/SeC, and the other in which continuous annealing was performed without the box annealing.

Test steel plates (4), (B), obtained in the above heat treatment test
Table 2 shows each characteristic value and r value of the tensile test of (C).

The above tensile property values are the results of a tensile test in the L direction using a JI No. 85 tensile test piece.

As shown in Table 2, box annealed steels having the composition range of the present invention have a yield ratio of 0. 6. It shows a precept that satisfies both the r value of 1.20 or less, but the steel without box annealing does not satisfy the required level for both the yield ratio and the r value, and all comparison steels have a yield ratio and an r value of 1.20 or less. Indicates that one or both do not meet the required level.

C0.04%, sio. ot%, Mn0.85%, PO
．． 006%, SolAA' 0. 1 2 1%, N
O. Steel consisting of 0.088%, balance Fe and unavoidable impurities is melted in a converter to form a slab, and then heated to a temperature of 122
0'C. It was hot rolled at a finishing temperature of 830°C and a winding temperature of 560°C to produce a finished steel plate with a thickness of 3.2 mm.

After pickling and finishing it into a 0.8 mm thick cold-rolled steel plate by ordinary cold rolling, the temperature was raised at 20'C/hr, the cooling rate was 40'C/hr, the soaking temperature was 750°C, and the steel was soaked at 750°C. time 24
**hr, after box annealing in a mixed gas atmosphere of H2 and N2, heating rate 40℃/SeC, soaking time 9QSe
Continuous annealing was carried out under conditions C, changing the soaking temperature and the cooling rate from the soaking temperature to room temperature as shown in Table 3.

Table 3 shows each characteristic value and r value of the tensile test of the test steel sheets obtained in the above heat treatment test.

As shown in Table 3, /I61 has a high yield ratio because the soaking temperature during continuous annealing is lower than the range of the present invention, so the pearlite phase does not become the γ phase and no martensite is formed after cooling.

/l6.4 has a high soaking temperature during continuous annealing and, contrary to AI, a large amount of martensite is formed, resulting in a low RF directivity.

/465, since the cooling rate of continuous annealing is slower than the range of the present invention, the structure becomes pearlite transformed and the yield ratio becomes high.

Contrary to /l65, /l6IO has a cooling rate that is faster than the range of the present invention, so a large amount of martensite is formed, resulting in a high yield ratio and poor r value.

On the other hand, heat treatment conditions within the range of the method of the present invention show results satisfying a yield ratio of 0.6 or less and an r value of 1.20 or more.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the relationship between the yield ratio and r value and the box annealing temperature in a heat treatment test of test steel sheets, and FIG. 2 is a chart showing the relationship between the yield ratio and r value and the Mn content.

Claims (1)

[Claims] I C0.20% or less, Sil. 0% or less, Mn0.
8-2.5%, SolAll0.01-0.2 o%,
No. 0015 to 0.0150 excellent, P0.10 or less,
After hot-rolling and cold-rolling, a steel containing Fe, the remainder being substantially Fe, is box annealed in a temperature range of 650 to 800°C, cooled, and then heated to 6oo to 850°C in a continuous annealing furnace. and then cooling at a cooling rate of 10 to 100'C/SeC.