JPS5855219B2 - Manufacturing method of low yield ratio high strength cold rolled steel sheet - 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 a low yield ratio and a strength level of 40 kg/mi7 or more and a yield ratio of 6.
The object of the present invention is to obtain a method that can appropriately produce a high-strength cold-rolled steel sheet with excellent cold workability of 0% or less.

High-strength cold-rolled steel sheets are being applied to various parts for the purpose of improving the safety of automobiles, reducing weight, or improving durability.

However, the yield ratio of ordinary high-strength cold-rolled steel sheets is 60 to 70φ.
Because of the high pressure, even if no cracks occur during processing, there are disadvantages such as large springback and easy formation of wrinkles, resulting in difficulty in shapeability and warping.

For this reason, recently, low yield ratio materials (yield ratio of 60% or less) with excellent cold workability have attracted attention, and one of the manufacturing methods with such low yield ratios is the production of martensitic phase in the ferrite phase. Alternatively, a method of forming a finely dispersed composite structure such as a bainite phase has been proposed.

That is, this composite structure can also be produced by batch annealing or continuous annealing, but in the case of batch annealing, it is necessary to add a large amount of alloying elements such as Mn, and in the case of continuous annealing, it is necessary to add a small amount of alloying elements such as Mn. However, in this case, it is necessary to strictly control the cooling rate during annealing, and if the cooling rate is as slow as air cooling, it may partially cause diffusion transformation and completely fail. Since a composite structure cannot be obtained, a large yield elongation occurs, the yield point increases, and the above-mentioned yield ratio increases.

The present invention was devised after repeated studies in view of the above-mentioned circumstances, and eliminates the above-mentioned drawbacks by adjusting the chemical composition and adopting a continuous annealing method characterized by water quenching and tempering. We succeeded in obtaining high-strength cold-rolled steel sheets.

That is, as conventional techniques for manufacturing this type of steel plate, for example, Japanese Patent Application Laid-Open Nos. 51-38219, 39524, and 420
12, JP 53-22812, JP 53-3936
8 etc. are known, and among these, JP-A-51395
24 is C20, 20%, Mn: 0.50
~1.80%, Cr: 0.03~1.1% as a basic component, Si≦1.0 width, Nb≦0.
Section 1.

■A steel containing at least one type of ≦0.1%
After heating and soaking between C1 and AC3, 5 to 100C/se
Cooling is performed by gas jet to about 400'C, followed by water quenching, air cooling or furnace cooling.
It is difficult to stably produce low yield point steel with such an annealing pattern.

That is, when C, Mn, and Cr in the basic component system are close to the upper limit values, the desired characteristics cannot be obtained even if hot water annealing is performed as described above.

In the present invention, as described above, stricter regulation of the chemical composition range and specific water quenching and tempering are used together, and C:
0.20% or less, Mn: 0.5-2.5%, C
r: 0.05 to 1.5% (however, 1.0%<Mn+
Cr < 3.5%) and Si as necessary:
1.0% or less, Mo: 1.0% or less, Nb:
Al-killed steel containing one or more of the following: 0.1% or less, ■: 0.1% or less, Ti: 0.1% or less is cold rolled by a conventional method and then heated to a temperature of AC1 point or higher for 50 minutes. Heat and soak for ~300 seconds, then heat to 400°C or higher, preferably 40°C.
It is characterized by rapid cooling from 0 to 600°C, followed by tempering at a temperature of 300°C or lower for 5 to 300 seconds.

To further explain the present invention, the reasons for limiting the chemical components in the present invention as described above are as follows.

C is an effective component for increasing strength and stably generating a composite structure, but since a large amount of C deteriorates ductility, weldability, etc., the upper limit was set at 0.2%.

Mn and Cr are the most important components, respectively, with Mn being 0.5% or less, Cr being 0.05% or less, and Mn%+C.
When r%<1.0%, a composite structure cannot be obtained and the yield ratio becomes high.

Also, if large amounts of Mn and Cr are present, they will deteriorate ductility, weldability, etc., so the upper limit of these is set to 2.5%.
, Cr: 1.5% and Mn% + Cr = 3.5%.

In addition, in the present invention, in order to improve the strength as necessary, S
One or more of i, Mo, Nb, V, and Ti can be added singly or in combination, but in order to ensure good ductility, the upper limit of each of Si is 1%.

Nb is 0.1%, ■ is 0.1 yen, and Ti is 0.1φ.

The hot rolling conditions for AA killed steel with the above composition are that the finishing temperature is A3 point or higher and the winding temperature is 7.
The usual conditions of 50°C or lower and 500'C or higher may be used.

However, the higher the winding temperature, the lower the yield ratio, so if it is particularly necessary to lower the yield ratio, it is preferable to carry out high temperature winding of 700 to 750°C.

Regarding continuous annealing performed after cold rolling of the above materials,
If the heating temperature is lower than the AC1 point, a composite structure cannot be obtained, and therefore a large yield elongation remains, making it impossible to obtain a low yield ratio material.

In order to obtain the above-mentioned composite structure, it is necessary to heat the material to a temperature of 1 or more AC and less than 3 AC to disperse the austenite phase in the ferrite phase.

In addition, it is held for more than 5 seconds in order to dissolve and diffuse iron carbides, etc., but if this holding time is long, there are disadvantages such as a decrease in production efficiency and an increase in the size of the equipment, so the maximum is set to 5 minutes (300 seconds). .

Furthermore, the austenite phase thus generated transforms into a second phase such as a martensite phase or a bainite phase by cooling at a speed higher than air cooling and water cooling from a temperature of 400° C. or higher and lower than AC3 to form a composite structure. , then 30
Tempering is performed for 5 seconds to 5 minutes at a temperature of 0'C or lower to improve ductility, but after water quenching at a temperature of 400'C or higher, tempering at a temperature of 300'C or lower is not possible. One of the important constituent elements of the present invention is that such water quenching treatment expands the tolerance range for the cooling rate after heating, and in the absence of this water quenching treatment, it is difficult to obtain the desired low yield material. It is possible to obtain a stable target steel sheet even at a relatively slow cooling rate that would otherwise be difficult to achieve.

This is because the water quenching treatment prevents diffusion transformation from occurring during cooling and makes it possible to obtain a safe composite structure; however, if the water quenching temperature is less than 400°C, this effect will not occur.

Moreover, if the temperature exceeds 600'C, the ductility decreases to some extent, so it is desirable that the water quenching temperature is in the range of 400 to 600°C.
The thus obtained steel sheet does not have a poor strength-ductility balance compared to conventional high-strength cold-rolled steel sheets, and does not lose its characteristics of low yield ratio and excellent cold workability.

In fact, it even has the advantage of being able to reduce the amount of alloying elements added to improve strength.

However, in order to form a composite structure, it is necessary to set the water quenching temperature to a temperature range below the AC3 point where two phases of ferrite and austenite coexist.

Tempering treatment after water quenching has the effect of improving elongation.

However, if this tempering temperature exceeds 300°C, the strength decreases and the yield ratio increases significantly, so the tempering temperature is set to 300°C or less.

Regarding this tempering time, if it is less than 5 seconds, it will not be effective, and if it is more than 5 minutes, it will cause a decrease in production efficiency, so it is undesirable as described in the case of tempering.

The high-strength cold-rolled steel sheet with a composite structure obtained as described above has no yield elongation as annealed, and can be used in a no-skin pass process.

In addition, the yield elongation does not recover due to aging, so it is slow aging.
At the same time, since the amount of solid solute C can be increased by the water quenching treatment, so-called bake hardenability, in which the strength can be significantly increased by the painting process after processing, can be expected.

*Specific examples of the present invention are explained below.

Example IC: 0.05%, Mn: 1.0%, Cr:
0.50%.

Si: 0.05%, P: 0.010%, S: 0.003
%, sol, Al: 0.030% Al killed steel was tapped in a converter, finishing temperature 870°C, winding temperature 620°C
The sample was rolled into a hot-rolled plate with a thickness of 28 mm, and then, after pickling, cold rolling was performed in 71 steps to produce a cold-rolled plate with a thickness of 0.8 mm.

As shown in the attached drawing, this cold-rolled slab l-IJ tube is
C., soaked for 2 minutes, water quenched at various temperatures as shown in this drawing, and then continuously annealed according to a thermal cycle of tempering at 300.degree. C. or lower for 2 minutes.

The results of measuring the mechanical properties of the obtained steel plates were as shown in Table 1 below (the steel plates with numbers inside ○ are those according to the present invention).

It was also confirmed that elongation was improved by tempering at 300'C or less after quenching.

Example 2 Al-killed** steel having the composition shown in Table 2 below was prepared.

(Fragrances 1 and 2 have Mn+Cr that does not reach the lower limit of the present invention, and perfumes 6 and 7 have Mn or Cr that exceeds the upper limit of the present invention.

) These steels are finished at a finishing temperature of 900'C and a rolling temperature of 620'C.
C. to a thickness of 28 mrn/l, and then, after pickling, cold rolling of 71 steps was performed to obtain a cold rolled sheet with a thickness of 0.8 mm.

However, these cold-rolled sheets were heated at 750°C for 2 minutes, air cooled, water quenched at 500°C, and then heated at 200°C.
The mechanical test values of the specimens subjected to continuous annealing at 20° C. for 2 minutes are shown in Table 3 below.

Note: Perfumes within the range of 20 are within the scope of the present invention.

In other words, the yield ratio is high for perfumes of 1°2 in which Mn + Cr is below the lower limit of the present invention, and steel plates 6 and 7 in which Mn or Cr exceeds the upper limit of the present invention have a significant decrease in EAI'. I learned something.

According to the present invention as explained above, the intensity level is 4.
0kg/mt? By using high-strength cold-rolled steel sheets with a yield ratio of 60 coefficients or less and excellent cold workability, we have achieved safety, light weight, and punt resistance by using this product in automobiles and other applications. This invention has great industrial effects because it can appropriately obtain various desirable products that satisfy the following requirements and have excellent shape properties.

[Brief explanation of the drawing]

The drawing is an illustration of an annealing cycle for one embodiment of the method of the present invention.

Claims (1)

[Claims] IC: 0.20% or less, Mn: 0.5 to 2.5
%. Cr: 0.05-1.5% (However, 1.0%
<Mn+Cr <3.5%) is cold rolled by a conventional method, then heated and soaked to a temperature of AC1 to AC3 for 5 to 300 seconds, and then heated to a temperature of 400 °C or higher and lower than AC3. Rapid cooling from temperature, then 300°C
A method for producing a low yield ratio, high strength cold-rolled steel sheet, which comprises tempering for 5 to 300 seconds at the following temperature: 2C: 0.20% or less, Mn: 0.5 to 2.5
%. Cr: 0.05 to 1.5% (however, 1.0% <M
n+Cr <3.5%) and Si: 1
．． 0% or less, Mo: 1.0% or less, Nb: 0.1%
Hereinafter, AA killed steel containing one or more types of collapse with V: 0.1φ or less and Ti: 0.1% or less is cold rolled by a conventional method and then heated to a temperature of AC1 to AC3 for 5 to 300 seconds. Soaking is carried out, followed by rapid cooling from a temperature of 400'C or more and less than AC3, followed by 5-300℃ at a temperature of 300'C or less.
A method for producing a low yield ratio, high strength cold-rolled steel sheet, which is characterized by performing tempering for seconds.