JPS5824490B2 - Manufacturing method of soft cold-rolled steel sheet with excellent formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a cold-rolled steel sheet that has few surface defects and excellent deep drawability, and at the same time is made extremely soft and has excellent stretchability and shape fixability.

Cold-rolled steel sheets for press forming generally undergo severe deformation during use, but recent trends in the design and forming technology of automobile parts and electrical appliances have resulted in parts that undergo severe deformation such as deep drawing and overhanging, and relatively less deformation. Increasingly, molding involves small parts where flatness and accuracy of curved surface shape are important.

Such forming requires a soft steel sheet that has excellent deep drawing and stretchability as well as good shape fixability.

Conventionally, when manufacturing cold-rolled steel sheets that are soft and have excellent formability,
Like ordinary cold-rolled steel sheets, they are broadly divided into killed steel sheets and rimmed steel sheets, depending on the deoxidation method used in the steel ingot manufacturing process.

Killed steel usually fixes and removes O in the molten steel as oxides, and at the same time produces cold-rolled steel sheets, it fixes the N in the steel sheets and forms a good texture of the steel sheets, increasing the r value ( It is easy to design materials that increase the Lankford value or plastic strain ratio.

As mentioned above, killed steel sheets can easily improve deep drawability and have a fixed N content compared to rimmed steel sheets, making it easier to lower the yield point, but they have the disadvantage that it is difficult to significantly improve elongation properties.

In addition, the tensile strength of the steel sheet is higher than that of the decarburized rimmed steel sheet described later, and cannot be used satisfactorily in cases where shape fixability in secondary forming is required.

On the other hand, rimmed steel sheets are not only relatively hard due to the fact that C is forced to remain in the manufacturing process, but also generally
It is difficult to increase the value.

Decarburization is a technology for softening rimmed steel sheets and increasing r-value.
Denitrification treatment is performed.

Decarburization and denitrification technologies can produce materials with low yield points, low tensile strength, and relatively high r-values, but processing conditions that take into account high r-values tend to result in coarsening of crystal grains. be.

Excessive coarsening of crystal grains tends to cause roughness in parts that are highly deformed, and may also lower the general deformation limit itself.

The present inventors conducted the following experiments on cold-rolled steel sheets using deoxidized and desulfurized steel using a special molten steel processing technology in order to improve these properties and solve the problems in the manufacturing process all at once.

In Figures 1 and 2, the amount of S is 0.011% and 0.0007%.
In two levels of deoxidized steel (C) (0.004%), Mn
The amounts of other components (C, Si.

N, AA, P) are within the range that does not vary significantly (0,015%
<c〈o, os%, Si〈0.01%, N〈0.006
5%, 0.015%, An<・0.06%, P.

,. 18%), the ingot-formed steel ingot is bloomed, hot-rolled at a temperature higher than A3 transformation temperature, coiled at a coiling temperature between 570°C and 670°C to form a copper strip, and further heated to 65% It shows the relationship between the yield point and tensile strength, as well as the elongation value and the Mn content in a tensile test of a cold rolled steel sheet produced by cold rolling at a rolling reduction of ~80% and then recrystallization annealing at 650° C. for 5 hours.

In steel sheets with a normal S content, the dependence of tensile strength and elongation on the Mn content is small, whereas in steel sheets with extremely low S content, as the Mn decreases, the tensile strength decreases and the elongation value increases significantly. It can be clearly seen that there is an improvement in

In particular, the increase in elongation value is extremely remarkable when the Mn content reaches 0.05%.

However, in the lower Mn content region (Mn <0.0
1%), care must be taken to ensure that the variation in elongation values becomes large as seen in the figure.

FIG. 3 shows the relationship between the r value and the amount of Mn.

There is also a clear tendency for the r value to increase as the amount of Mn decreases.

Attempts to improve the material quality of cold-rolled steel sheets by lowering Mn have already been made in Japanese Patent Publication No. 47-30803 and Japanese Patent Publication No. 48-7.
565, but considering the examples and the level of desulfurization technology at the time of the invention, the remarkable material softening and ductility improvement in the ultra-low sulfur and ultra-low manganese regions attempted in this experiment were not expected at that time. It is clear that this was not what was expected.

The Δ marks in FIGS. 1 and 2 are the Mn and S
This is an experimental example in which the effect of further reducing P in steel with reduced P content.

The cold-rolled steel plate manufacturing conditions are the same as in the previous example.

As is clear from the figure, in ultra-low sulfur, low manganese steel, the effect of lowering P was clear, with tensile strength softening by more than 1 kg/- and elongation improving by about 1.5%.

Now, as mentioned above, Mn, S.

Due to the synergistic effect of the reduction, the aluminum killed steel sheet with significantly reduced 0 and P has low yield point, low tensile strength, high elongation value, and high r value, so it has good deep drawability in normal press forming. In addition to being excellent in stretchability, it also has extremely good stretch flangeability and shape fixability, so it can be said to be a steel sheet with comprehensive formability.

The present invention was made based on the completely new knowledge as described above, and the gist thereof is that Mn: 0.01 to 0
．． 08%, S; 0.003% or less, 0; 0.004% or less, N; 0.006% or less, Al; 0.015 to 0.0
8% or additionally P 0.005
% or less, the hot-rolled finishing temperature is A3 point or higher and the coiling temperature is 65% using normal hot rolling equipment.
A method for producing a soft cold-rolled steel sheet with excellent formability, characterized by hot rolling at 0°C or lower, pickling cold rolling, box annealing in a range of 570°C to 670°C, and skin pass rolling. .

Below, the reason for limiting the content of chemical components of steel in the manufacturing method of the present invention and the manufacturing conditions necessary to maintain high properties will be described in detail.

First, it is necessary to treat molten iron and molten steel to reduce sulfur; however, the upper limit of sulfur content was set at 0.003% because of the strength associated with lowering Mn as shown in Figures 1 and 2. This is because a reduction in yield point and tensile strength and a significant improvement in elongation value cannot be achieved when the S content exceeds 0.003%.

It is desirable to reduce the oxygen content as much as possible to take advantage of the low sulfur content. The upper limit is set at 0.004% based on hot rolling results.

The required amount of Mn in molten steel varies depending on the amount of sulfur and oxygen present in the steel, but if ultra-low sulfidation and low oxygen treatment is performed and the sulfur and oxygen are within the above range,
The lower limit of the Mn content is determined by the rapid increase in the variation in properties seen in the low sulfur material shown in FIG.

This includes steel sheets with a Mn content of less than 0.01%, which have low elongation values and low r values.

Furthermore, since the amount of Mn contained in general hot metal considerably exceeds 0.01%, it is necessary to remove Mn from the strength of the molten steel in order to achieve a content of 0.01% or less.

For the above reasons, the lower limit of the Mn content is set to 0.01%.

Further, the upper limit of Mn is determined by the content at which the elongation value and r value rapidly deteriorate, and when the Mn content exceeds 0.08%, no significant difference from the characteristic values of ordinary steel sheets is observed.

Based on the above, the limit range of Mn content is 0.01 to 0.08
%.

A7 and N basically follow the contents of conventional aluminum killed steel sheets.

However, low N is desirable for the production of soft materials, which is the gist of the present invention, and it is sufficient to have a minimum amount of AA that is sufficient to fix this N and increase the r value. A higher amount of Al is required than in conventional component steels.

For the above reasons, N is 0.006% or less and A7 is 0.0%.
1 to 0.06%.

Next, hot rolling conditions and annealing conditions, which are conditions that greatly influence material properties among the steel plate manufacturing conditions in the manufacturing method of the present invention using ultra-low sulfur and ultra-low manganese steel, will be described.

Of the hot rolling conditions, finishing temperature and coiling temperature have a large influence on material properties, especially r-value, but even in the steel types targeted by the present invention, the level of r-value is lower than that of conventional aluminium-killed steel plate kettle or In order to keep the value higher than that, it is necessary that the finishing temperature does not fall below point A3 and that the winding temperature is 650° C. or lower.

Figure 4 shows an example of the relationship between the r-value and the coiling temperature of a cold-rolled steel sheet made of steel with the above-mentioned components. is relaxed toward the high temperature side.

Next, in order to obtain the soft characteristics that are the gist of the present invention, it is best to rely on box annealing, but the annealing temperature range is 570°.
It is necessary to limit the temperature to C to 670°C.

This middle and lower limit temperature is the temperature at which complete recrystallization is completed plus an additional temperature as a safety factor to account for variations, etc., and at the same time it almost coincides with the temperature at which the high level r value mentioned above is achieved. There is.

Further, the upper limit temperature is determined by the grain size of the steel sheet to be manufactured.

In other words, steel sheets annealed at temperatures exceeding this temperature have large crystal grain sizes and are highly susceptible to rough skin (orange peel) during forming.

The above-mentioned limited temperature range is shifted to the lower temperature side compared to a normal aluminum-killed steel sheet, and the superiority of the steel sheet of the present invention can also be seen from the viewpoint of energy saving during annealing.

Another feature of the steel sheet of the present invention is that the temper rolling is extremely light (1% or less).

Here, the steps of the method of the present invention will be briefly described below.

After preliminary desulfurization of molten steel, the molten steel treated in a converter or open hearth is further deoxidized and desulfurized by the LF method, AOD method, and A7 addition, and the carbon content is 0.0.004% or less, SO, 0.003% or less, and the carbon content is the product. In order not to impair the moldability, the CO should be kept at 0.05% or less, and Mn should be reduced to 0.01 to 0.01% by removing Mn or adding Mn.
It is contained in a range of 0.08% and made into a slab by ordinary ingot formation or continuous casting.

Next, using this as a starting material, A3
The rolling process is completed at a temperature of 650°C or higher, and the hot-rolled steel sheet is rolled at a temperature of 650°C or lower.
After recrystallization annealing by box annealing at a temperature in the range of 70° C. to 670° C., a cold rolled steel sheet is manufactured by performing skin pass rolling.

If further softening is required, the molten steel is further dephosphorized during the process to reduce the Po to 0.05% or less, and the same steps as above are followed to produce a cold rolled steel sheet.

The effects of the present invention will be illustrated in more detail with reference to Examples below.

Example After hot metal desulfurization treatment by KR method, it is made into molten steel in a converter and Al is added.
Components are adjusted by AOD treatment etc., C: 0.01-0.0
5%, O: 0.004% or less, S: 0.0015% or less, N: 0.0013 to 0.0070%, AA; 0.
012-0.060%, Mn; 0.01-0.06
5%, P; 0.008 to 0.015%, and then add a total of seven 20-ton steel ingots to the above composition range using the normal ingot pouring method. 0.005
Two 20-ton steel ingots were manufactured.

These steel ingots are heated to a temperature of 1285℃ and a finishing temperature of approximately 9℃.
The steel strip is hot-rolled at a temperature range of 00 °C and a coiling temperature of 560 °C to 630 °C, and after pickling, the steel strip is cold rolled to 0.70 or 0.75 mm at a rolling rate of 66 to 79%. This was annealed at 630° C. for 5 hours and subjected to approximately 1.0% skin pass rolling to obtain cold rolled steel sheets having chemical compositions as shown in Table 1.

(a) ~ (ω is extremely low sulfur and extremely low manganese material, (h),
(i) is a steel sheet that has been further reduced in phosphorus.

Also, l) to (n) are general aluminum killed steel plates manufactured using conventional ingredients and manufacturing methods.

From the comparison of the tensile property values listed in Table 1, the soft properties and excellent formability of the steel sheets obtained by the method of the present invention are clearly recognized.

[Brief explanation of the drawing]

Figure 1 shows that the tensile strength (T, S, ) and yield point (Y
, P, ), Figure 2 shows the elongation value (E
Figure 3 shows the relationship between r value and Mn content.
Figure 3 is a diagram showing the relationship between r value and Mn content. Figure 4 is the effect of hot rolling winding on the r value of the final cold rolled steel sheet product made of ultra-low sulfur and low manganese steel. FIG. 3 is a diagram comparing the influence of temperature with an actual example of conventional component steel.

Claims (1)

[Claims] I Mn 0.01-0.08%, 80.003% or less, 00.004% or less, NO, 0.06% or less, A
A mild steel slab whose composition is restricted to 11O, OI ~ 0.06% is hot rolled using normal hot rolling equipment at a hot rolling finish temperature of A3 or higher and a coiling temperature of 650°C or lower, pickled, and cold rolled. A method for producing a soft cold-rolled steel sheet with excellent formability, characterized by box annealing in the range of 570°C to 670°C and skin pass rolling. 2 Mn 0.01-0.08%, 80.003% or less, 00.004% or less, NO, 006% or less, A70
．． Ingredients are limited to 01 to 0.06%, and Po, 005
% or less is hot rolled using normal hot rolling equipment at a finishing temperature of A3 or higher and a coiling temperature of 650°C or lower, pickled, and then box annealed in the range of 570°C to 670°C after cold rolling. A method for producing a soft cold-rolled steel sheet with excellent formability, characterized by subjecting it to cold-rolled steel sheets and temper rolling.