JPS5842247B2 - Method for manufacturing high-strength cold-rolled steel sheets with excellent formability and bake hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet, and particularly to a method for manufacturing a high-strength cold-rolled steel sheet having good press formability and excellent bake hardenability.

In recent years, high-strength cold-rolled steel sheets for automobiles have been developed to meet the complex demands of increasing strength to protect occupants in automobile collisions, as well as reducing vehicle weight in line with exhaust gas regulations and improving fuel efficiency. Research is being actively carried out with the aim of making plate materials thinner.

The characteristics required of these materials include excellent workability, which allows them to be formed into complex shapes with good dimensional accuracy without causing damage, and also to be able to be assembled into welded structures. Good strength, impact resistance, fatigue strength, etc. are required for weldability.

Among other things, it is required to have excellent spot weldability.

In response to such demands, high-Mn-based high-strength cold-rolled steel sheets have been used in the past, but they were still unsatisfactory in terms of workability and balance between strength and ductility.

Therefore, the present applicant has responded to the above request by providing high-Si high-strength cold-rolled steel sheets (Japanese Patent Laid-Open Nos. 51-38220 and 1986-86919), but the strength level of these cold-rolled steel sheets is It has a tensile strength of 40 kg or more, and many of them have a tensile strength of 45 kg or more.

By the way, in addition to the formability and weldability mentioned earlier, the properties required of these materials include the special property of having a high yield point after forming and painting and baking in order to provide tent resistance. become.

Therefore, even for conventional high-strength cold-rolled steel sheets that have a tensile strength of 45 kg or more, it is possible to further promote thinning of the steel sheet by increasing the yield point after paint baking treatment, that is, improving the bake hardenability. be.

However, there has been almost no study on the bake hardenability of steel sheets with a strength of around 45 kg class or higher, and the majority of people have previously believed that this type of high-strength cold-rolled steel sheet has poor bake hardenability. Ta.

High-strength cold-rolled steel sheets obtained by the continuous annealing method, which has recently been widely used as a relatively superior method, have excellent strength and bake hardenability, but have a low Lankford value (7 values).
The deep drawability represented by is extremely poor, making it impossible to achieve the above objective.

The present inventors focused on the above-mentioned circumstances, and with the aim of developing a high-strength cold-rolled steel sheet that can satisfy all of the requirements of formability, strength, and bake hardenability, the present inventors investigated the chemical composition of the steel material and the annealing conditions. We have been conducting various research on manufacturing conditions such as

As a result, it was confirmed that the above objectives could be achieved by adopting the method shown below, and the present invention was finally completed.

That is, the method for producing a high-strength cold-rolled steel sheet according to the present invention includes C:0.
01~0.20% (weight: same below), Si: 0.7~
3.0%, Mn≦0.7%, balance: iron and unavoidable impurities, and (Si /Mn)≧1.0, the molten steel is made into a slab in an ingot-making process, a blooming process, or a continuous casting process. After that, hot rolling at a temperature above A13 transformation point to 400-750℃
The gist is that after winding, descaling and cold rolling, annealing is performed at 650 to 800°C in a batch annealing furnace, and then quenching is performed at a rate of 80 to 80°C.

At this time, if the molten steel further contains P: 0.03-0.20% and/or A1: 0.015-0.15%, the performance of the high-strength cold-rolled steel sheet can be further improved. .

C is an essential element for increasing the strength of steel, and its content is 0.
．． If it is less than 0.01%, sufficient strength and satisfactory spot weldability cannot be obtained.

On the other hand, if the content exceeds 0.20%, giant cementite will form in the steel structure, reducing press formability and spot weldability, so it should be kept below this range.

A particularly preferable C content is 0.03 to 0.15%.

Si is an indispensable main element for imparting strength, and the rate of increase in tensile strength due to Si is approximately twice the effect of adding Mn.

Therefore, compared to the case where strength is imparted by Mn, the amount of alloy added can be reduced by half, which is extremely effective in terms of material costs.

Moreover, compared to high-Mn steel sheets, high-Si steel sheets have less decrease in hardness after cold rolling and annealing compared to hot-rolled sheet hardness, and have excellent strength, a balance between strength and ductility, stretch flangeability, and deep drawability. Moreover, it is also excellent in bake hardenability, which is one of the characteristics of the present invention.

In order to exhibit these effects significantly, at least 0
．． A content of 7% or more is required; on the other hand, if it exceeds 3.0%, problems occur in steelmaking operations and cold workability is significantly degraded, so it should be kept below this range.

Mn is an element effective in improving strength and preventing red heat embrittlement during hot rolling.

Furthermore, when trying to increase the strength using only Si, the transformation point often rises excessively, causing various troubles during hot rolling or reducing spot weldability, so even if it is a high-Si steel, Although there is a limit to the content of Si, when Mn is used in combination, the strength is effectively increased through its solid solution strengthening, and furthermore, in the coexistence with C, the microstructure of the spot weld is refined.

In order to exhibit these effects, at least 0.2% is required, but it is not necessary to contain 0.7% or more.

In addition, Mn contributes to the balance of strength and ductility and bake hardenability.
The effect of Si is not as remarkable as that of Si, and the role of Mn in the present invention is only subordinate.

That is, in the present invention, it is essential to use a high-Si steel with a Si/Mn ratio of 1 or more, thereby ensuring excellent bake hardenability and mechanical properties.

In addition, in the present invention, one or both of Al and P can be contained in appropriate amounts to improve physical properties.

Conventionally, attention has been paid to only the deoxidizing effect of AI, and the effect of adding Al to high-strength cold-rolled steel sheets has hardly been discussed, nor has the effect on the material been clarified.

Further, in the above composition range of the present invention, the steel becomes substantially killed steel due to the effects of Si and Mn, and the addition of Al is unnecessary for the purpose of deoxidation.

However, when a steel material with the above composition contains an appropriate amount of AI,
It was confirmed that press formability and bake hardenability were significantly improved.

Here, it is thought that Al nitride plays an important role in improving press formability as in the case of Al-killed mild steel sheets, but the effect of AI on bake hardenability is completely opposite to that in the case of Al-killed mild steel sheets. .

Although the detailed mechanism of the effect of Al addition on the bake hardenability of such high-strength cold-rolled steel sheets is not necessarily clear, at least under the types and amounts of the contained elements explained earlier and the manufacturing conditions described later, It is clear that press formability and bake hardenability are significantly improved by addition.

In order to effectively demonstrate the effect of such AI, 0.01
It is necessary to add 5% or more, but if it is too high, surface defects such as slipper scratches will appear, so it should be added to 0.15% or less, and if the surface quality of the product is particularly strict, it should be added to 0.08% or more. It is desirable to suppress this.

P is effective in increasing the strength and workability of the steel material, as well as the bake hardenability, by including it in an appropriate amount in a substrate whose main reinforcing elements are Si and Mn, and the preferable content is 0.
The range is from 03 to 12%.

However, if it is less than 0.03%, the above effects will not be exhibited significantly, while if it exceeds 0.2%, impact secondary workability will deteriorate.

A particularly preferable content is 0.03 to 0.15%.

Although the mechanism by which bake hardenability is improved by the addition of P has not necessarily been fully elucidated, the results of confirmation experiments suggest that P influences the state (size, amount, etc.) of carbides after annealing. It is thought that this is because this carbide acts more effectively on high Si steel, which exists as fine cementite, than on high Mn steel, which exists in a pearlite-like and somewhat large shape. It will be done.

Most of the oxygen in steel exists as nonmetallic inclusions, which reduces the effects of C1Si, Mn, P, etc. that contribute to strength and bake hardenability, so the amount of oxygen is kept to 0.015% or less. It is hoped that

Impurities such as S, which are unavoidably mixed in, deteriorate material properties such as stretch flangeability, so it is better to keep them as low as possible. For example, S should be kept at 0.03% or less. .

Furthermore, what is extremely important in the present invention is that Cr in the steel components must be reduced to substantially zero.

That is, as will be made clear in the Examples below, if Cr is mixed in even a small amount, the bake hardenability will be extremely reduced, and even if the baking conditions are set appropriately or P and Al are mixed in appropriate amounts, the present invention will not work. cannot achieve the goal.

The composition of the steel used in the present invention is roughly as described above, but in order to achieve the purpose of the present invention, the processing conditions must be further set as shown below. properties and bake hardenability.

First, molten steel containing the above elements is made into an ingot by a conventional method or a method that will be developed in the future, and then Ar3
Finish hot rolling is performed at a temperature above the transformation point and a winding temperature of 400 to 750°C.

The coiling temperature range itself during hot rolling is not particularly special and is substantially the same as general coiling conditions, but in the present invention as well, while ensuring a hot rolled steel sheet with a good shape, In order to increase the pickling efficiency after hot rolling and to maintain the proper state of carbides, the coiling must be carried out within the above temperature range.

After that, descaling and cold rolling are performed. Here, the cold rolling rate is
4 to maximize tensile strength and workability
It is desirable to set it to 0% or more.

In addition, for batch annealing after cold rolling, 650℃~SOO
After annealing at 80 DEG C., it must be rapidly cooled at a rate of 80 DEG C.

However, by setting such batch annealing conditions, it is possible to ensure a crystal arrangement favorable for formability, and at the same time to make the presence of carbides, the amount of solute N, the amount of solute C, etc. in the most appropriate state. A cold rolled steel sheet with excellent tensile strength, formability and bake hardenability can be obtained.

The annealing temperature was determined as above because if it is less than 650°C, sufficient recrystallization will not occur during the annealing process, and sufficient ductility cannot be imparted to the steel plate.
This is because, if it exceeds this, the growth of crystal grains becomes insufficient and it becomes difficult to obtain a crystal alignment that ensures excellent formability.

In addition, the cooling conditions after annealing are extremely important in achieving the purpose of the present invention, and in order to leave solute N and C in the steel to improve formability and bake hardenability, the cooling conditions should be at least 80%.
It must be rapidly cooled at a rate of at least ℃/hour.

On the other hand, there is no particular upper limit to the cooling rate, but if equipment constraints are taken into consideration, the upper limit is considered to be about 150° C./hour.

Next, the structure and effects of the present invention will be explained with reference to Examples, but the following does not limit the present invention, and the amount of contained elements, annealing conditions, etc. are changed in accordance with the spirit of the above and below. It is also possible to do so, all of which are within the scope of the present technology.

Example A test steel having the chemical composition shown in Table 1 was hot rolled at a finishing temperature of 900°C after the usual ingot making and blooming process, and was rolled at a coiling temperature of 550°C.
A hot-rolled sheet with a thickness of 2.7 mm was obtained.

Next, the steel plate was pickled and then cold rolled to obtain a steel plate having a thickness of 0.8 mm.

The steel plate was annealed at 660°C for 3 hours, cooled at a rate of 100°C/hour, and further subjected to 1% temper rolling to obtain a cold rolled steel plate.

The mechanical properties and bake hardening performance (BH value) of each steel plate before processing are summarized in Table 2 and Figures 1 to 3.

However, each symbol in Table 2 is as follows.

BH value: After applying a 2% tensile prestrain to a tensile test piece of JI85, the test piece was heat-treated at 170°C for 30 minutes (in order to correspond to the baking conditions), and the tensile test was performed again, and the yield at this time was The value obtained by subtracting the deformation stress at 2% tension from the stress is used as a measure of bake hardenability.

Figure 1 is a graph of the relationship between each test steel plate and the BH value, dividing each test steel plate into AI, non-AI, high-Si, high-Mn, and high-Mn/high-Si systems. Figure 3 shows the relationship between the presence/absence of Al and the BH value.
It is a graph showing a comparison of the effects on the BH value depending on whether it is a high-Mn type or a high-Mn type.

The following considerations can be made from the results shown in Table 2 and Figures 1 to 3.

(2) As is clear from FIG. 1, the BH value can be improved by containing an appropriate amount of P or by increasing the amount of P.

These effects of adding P are particularly noticeable in high-Si steels.

However, if a small amount of Cr is mixed, the BH value decreases extremely in either case.

■ Figure 2 uses materials with similar components except for AI.
The data compares the presence and absence of Al, and the BH value is improved in both cases by including Al.

However, the effect is much greater for high-Si steel than for high-altitude steel.

■ Figure 3 shows data comparing the relationship between the addition of P and the BH value for high-Si steel or high-Mn steel. This figure also shows that high-Si steel has a much better BH value. It shows.

■ Table 2 shows the measurement results of the 〒 values for some of the test steel sheets, but when comparing similar component steels based on the presence or absence of Al, for example, A and F, B and G, D and J It is clear from the comparison that <7 value is higher when AI is included (
Therefore, Al is extremely effective in improving not only bake hardenability but also deep drawing workability.

Comparative Example Steel types F, G, H, J, K, and L shown in Table 1 were selected, and cold-rolled steel sheets were prepared in the same manner as in the example except that the cooling rate after annealing was 20°C/hour. Obtained.

Table 3 shows the results of measuring the mechanical properties and bake hardening performance of each steel plate before processing.

As is clear from the results in Table 3, the cooling rate was increased to 80℃/
If it is less than 1 hour, the physical properties before processing are excellent, but the bake hardenability (BH value) is low, and the object of the present invention cannot be achieved.

[Brief explanation of the drawing]

1 to 3 are graphs showing experimental results.

Claims (1)

[Claims] IC: 0.01 to 0.20% (weight %: the same below)
, Si: 0.7 to 3.0%, Mn≦0.7%, balance: iron and inevitable impurities, and (S i /M
n)≧1.0 is made into a slab in the ingot making process, blooming process or continuous casting process, then hot rolled at a temperature above the Ar3 transformation point, coiled at a temperature of 400 to 750°C, and then descaled. and a method for producing a high-strength cold-rolled steel sheet, which is annealed at 650-800°C in a batch annealing furnace after cold rolling, and then has excellent 80-b properties and bake hardenability. 2 C: 0.01 to 0.20% (weight: same below),
Si: 0.7 to 3.0%, 0.7%, P and/or At (P is 0.03 to 0.20%, Al is 0.015 to
0.15%), balance: molten steel consisting of iron and unavoidable impurities and (S i /Mn )≧1.0 is made into a slab in an ingot making process, blooming process or continuous casting process, and then the Ar3 transformation point or higher is obtained. After hot rolling at a temperature of 400 to 750°C, descaling and cold rolling were carried out in a batch annealing furnace for 6
A method for producing a high-strength cold-rolled steel sheet with excellent formability and bake hardenability, characterized by annealing at 50 to 800°C and then annealing at 80 to 80°C.