JPH0320407A - Method for preventing oxidation of grain boundary in high strength cold-rolled steel sheet - Google Patents

Abstract

PURPOSE:To prevent oxidation of grain boundary and to reduce rolling load at the time of cold-rolling by hot-rolling the specific composition of high silicon steel at the specific temp. range and coiling at the specific temp. CONSTITUTION:Soaking treatment is executed to the steel composed of 0.05-0.25wt.% C, 0.5-3.0% Si, 0.5-3.0% Mn and the balance of Fe with inevitable impurities. In hot-rolling, rolling reduction is executed at >=70% in the temp. range of Ar1 to Ar3, and austenitic vol. ratio at the outlet side in the finish roll stands is made 20-70%. Successively, this is coiled at 300-650 deg.C. During coiling gamma-alpha transformation is promoted and heat-returning caused by the transformation after coiling can be made little and the oxidation of the grain boundary can be prevented. In the above steel composition if necessary, one or more kinds of 0.02-0.10% Nb, 0.01-0.10% Ti and 0.03-0.10% P, are contained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for preventing grain boundary oxidation of high-strength cold-rolled steel sheets.

[Prior Art] It is well known that grain boundary oxidation occurs on the surface of the steel plate after hot rolling in the production of high-strength cold-rolled steel plates with a high Sl content.

This grain boundary oxidation occurs at a depth of approximately 50 to 100 μm from the surface of the steel sheet, and is difficult to remove by ordinary pickling. During cold rolling, the peeled iron powder caused peeling. In high-strength steel sheets that are highly susceptible to notching, there are problems such as uneven indentations occurring on the surface of the steel sheet during continuous annealing and deterioration of workability due to microcracks in the grain boundary oxidized areas on the surface of the sheet. be.

Therefore, high-silicon steel is often used for high-strength steel plates and electric steel plates, and hot rolling of these steel plates is completed at a temperature above the Ars point in order to make the structure uniform.
Next, it is wound up using a cooling method depending on the purpose.

For example, in cold-rolled high-strength steel plates, AIN,
In order to improve the material quality by precipitating MnS etc., it is rolled to 600 ml without rapid cooling after exiting the final hot rolling stand.
It is wound at relatively high temperatures around ℃.

However, in high-silicon steel manufactured by this method, grain boundary oxidation occurs due to transformation recuperation after winding.

In addition, the present inventor has previously completed the application for a patent application filed in 1986-
As explained in the specification of No. 201765, as a method for preventing grain boundary oxidation of high-silicon steel, the time from leaving the final hot rolling stand to winding is t0, and 1/
Rapid cooling to a temperature of 700°C or less within 3 ta,
This is a method of winding at a temperature of 550°C or lower, and although grain boundary oxidation can be prevented by this method, the strength of the hot-rolled steel sheet increases, and therefore the rolling load during cold rolling is reduced. Since this increases the height, there are problems such as poor shape of the product.

Therefore, in order to prevent grain boundary oxidation in high-silicon steel, it is necessary to suppress scale formation in hot-rolled steel sheets as much as possible. Alternatively, the slab heating temperature may be lowered to suppress the oxidation of the rough rolled steel sheet as much as possible, and the grain boundary oxidation layer may be thinned to the extent that it can be removed by normal pickling. There are problems such as an increase in the load during hot rolling or an increase in the cost of heating the slab, and furthermore, grain boundary oxidation cannot be sufficiently prevented.

In the method described above, it is possible to prevent grain boundary oxidation by lowering the coiling temperature of the steel sheet after hot rolling. To prevent, 300℃
Extremely low winding temperatures are required; therefore,
The shape of the hot-rolled steel sheet deteriorates significantly, and furthermore, since the strength becomes extremely high, there arises a problem that the load during cold rolling increases.

Further, even in the invention described in the specification of Japanese Patent Application No. 60-201765 described above, there is a problem that the load during cold rolling is not sufficiently reduced and the shape of the product is poor.

[Problems to be Solved by the Invention] The present invention has been made in view of the many problems that have not yet been solved in various conventional measures for preventing grain boundary oxidation in high silicon steel as explained above. As a result of intensive research and repeated examinations, we have found that high-silicon steel with specific ingredients and ratios is hot-rolled in a specific temperature range and coiled at a specific coiling temperature. We have developed a method for preventing intergranular oxidation of high-strength cold-rolled steel sheets, which reduces the strength of the steel sheets, reduces the rolling load during cold rolling, and improves the product shape.

[Means for Solving the Problems] The method for preventing grain boundary oxidation of high-strength cold-rolled steel sheets according to the present invention includes: (1) C 0.05 to 0.25 wt%, Si 0
．． 5 to 3.0 wt%, Mn 0.5 to 3. After soaking a steel containing Dt% and the balance consisting of Fe and unavoidable impurities, Ar, to Ar. It is characterized by performing rolling at a temperature of 70% or more in a temperature range of 70% or more, hot rolling so that the austenite volume fraction at the exit side of the final stand is 20 to 70%, and then winding at a temperature of 300 to 650°C. The first invention is a method for preventing grain boundary oxidation of high-strength cold-rolled steel sheets, (2) C 0.05 to 0.25 wt%, S f 0
．． 5 to 3.0 wt%, Mn 0.5 to 10 wt%, and further Nb 0.02 to 0.10 wt%, Ti 0.01
~0.10wt%, P 0.03 ~0.11)wt
After soaking the steel containing one or more selected from the following % and consisting of the remainder Fe and unavoidable impurities, it is reduced by 70% or more in the temperature range of A r 1 = A r 3, and the final stand is A method for preventing intergranular oxidation of a high-strength cold-rolled steel sheet, which comprises hot rolling so that the austenite volume fraction on the exit side is 20 to 70%, and then coiling at a temperature of 300 to 650°C. This invention consists of two inventions, which are considered as the second invention.

The method for preventing grain boundary oxidation of high-strength cold-rolled steel sheets according to the present invention will be described in detail below.

In the method for preventing grain boundary oxidation of high-strength cold-rolled steel sheets according to the present invention, the present inventor found that lowering the coiling temperature after hot rolling of high-silicon steel is effective in preventing grain boundary oxidation. However, in order to prevent grain boundary oxidation by lowering the coiling temperature, it is necessary to lower the coiling temperature to about 300℃, which results in extremely poor shape of the hot rolled steel sheet. Furthermore, since the strength of the hot-rolled steel sheet becomes very high, there is a problem that the load during cold rolling becomes high.

There is a technology that reduces the load during cold rolling by annealing and softening this low-temperature coiled steel sheet before cold rolling, but this makes the process complicated, reduces productivity, and increases production costs. becomes higher.

In addition, as shown in the specification, the invention of Japanese Patent Application No. 60-201765 described above is to specify the time from the end of #1 rolling to the winding, that is, specifically, the invention of the hot rolling After leaving the final stand of
This is a method that prevents grain boundary oxidation by rapidly cooling the material, increasing the amount of γ-α transformation before winding to reduce transformation recuperation after winding, and further winding at a temperature of 550°C or less. be.

However, even in this method, the strength of the hot-rolled conditioned plate is still high, the rolling load during cold rolling becomes high, and the shape of the steel plate deteriorates.

Therefore, in the method for preventing grain boundary oxidation of high-strength cold-rolled sea bream plates according to the present invention, Ar. -Ar.
A reduction of 70% or more is carried out in the temperature range of
Therefore, grain boundary oxidation can be prevented by winding at a temperature of 300 to 650°C.

In this case, the temperature change (cooling pattern) of the plate from the time it leaves the final hot rolling stand until it is wound up is not particularly limited.

Therefore, the method for preventing grain boundary oxidation of high-strength cold-rolled steel sheets according to the present invention is different from the conventional method of controlling the temperature of steel sheets after hot rolling until coiling. The purpose is to prevent grain boundary oxidation by promoting the transformation of the steel and reducing the transformation recuperation after winding.

Next, in the method for preventing grain boundary oxidation of a high-strength cold-rolled steel sheet according to the present invention, the components and component ratios of the steel used will be explained.

C is an effective element for making high-strength steel sheets by strengthening the transformation structure. If the content is less than 0.05 wt%, this effect will be small, and if the content exceeds 0.25 wt%, the spot weldability will be reduced. deteriorates. Therefore, C content?
; t 0.05 to 0.25 wt%.

Although Si is harmful to grain boundary oxidation, it is effective in improving the ductility of steel, so it is a necessary element for high ductility and high strength steel sheets, and if the content is less than 0.5 wt%, grain boundary oxidation will not occur. There is no problem because it is not present, and if the content exceeds 3.0 wt%, cold workability deteriorates.

Therefore, the St content is set to 0.5 to 3.0 wt%.

Mn is an indispensable element for strengthening the transformed structure, and if the content is less than 0.5 wt%, this effect cannot be obtained.
Furthermore, if the content exceeds 3.0 wt%, the ductility of the steel sheet will deteriorate. Therefore, the Mn content is 0.5 to 3. Qwt
%.

In addition, in the method for preventing grain boundary oxidation of a high-strength cold-rolled steel sheet according to the present invention, one or more selected components described below can be included in addition to the above-mentioned essential components.

Nb is an element necessary to obtain precipitation strengthening and grain refinement strengthening, and if the content is less than 0.02 wt%, such effects will be small; If the content exceeds 10 wt%, not only the effect will be saturated, but also the ductility will deteriorate. Therefore,
The Nb content is 0.02 to 0.10 wt%.

Ti, like Nb, is an element necessary for precipitation strengthening and grain refinement strengthening, and if the content is less than 0.011 t%, this effect will be small, and if it is contained in an amount exceeding 0.10 wt%, it will deteriorate the strength of the steel sheet. Ductility deteriorates. Therefore, the Ti content is 0. O
I~0.10wt%.

Like S, P is an element that solid-solution strengthens ferrite and is effective in producing high-ductility, high-strength steel sheets, and its content is 0.03w.
This effect is small below t%, and o. totwt
If the content exceeds %, spot weldability will deteriorate. Therefore, the P content is 0.03 to 1.10 wt%.

The heat treatment of the method for preventing grain boundary oxidation of high-strength cold-rolled steel sheets according to the present invention will be explained.

The hot rolling performed after soaking is performed using the conventional 0. l7wt%
When using C-1.5wt%St-2.0wt%Mn=Fe residual steel and finishing it at a temperature above the Ars point, even if it is wound at a temperature of 300 to 650℃, it will not be as shown in Figure 1. As shown, the 1-α transformation requires a very long time, and as a result, the heat recovery due to the transformation after winding becomes significant, making it impossible to prevent grain boundary oxidation.

However, Ar. ~Ars temperature range, the austenite volume fraction at the exit side of the final stand is 20 to 70%, and then winding is performed at a temperature of 300 to 650 ° C., until winding. The γ-α transformation of the steel is promoted, the recuperation due to the transformation after winding can be reduced, and grain boundary oxidation can therefore be prevented.

Art~Ar. Reduction of 70% or more in the temperature range suppresses the occurrence of ferrite bands that are likely to occur during two-phase rolling, and produces a hot rolled steel sheet with a uniform structure.
This is a necessary condition to ensure the ductility of the product.

In addition, the reason why the austenite volume fraction is set to 20 to 70% is to reduce the transformation recuperation after coiling, and the austenite volume fraction is 20% at the exit side of the final stand of hot rolling.
If it is less than 70%, the effect will be saturated, and furthermore, the deformation resistance during hot rolling will increase and the shape will deteriorate.
If the value exceeds 1, transformation recuperation increases and grain boundary oxidation occurs.

[Example] An example of the method for preventing grain boundary oxidation of a high-strength cold-rolled steel sheet according to the present invention will be described.

Example Steel having the components and proportions shown in Table 1 is melted in a converter, a slab is manufactured by continuous casting, the slab is heated in a normal atmosphere under the conditions shown in Table 2, and 2.
．． It was hot rolled to a thickness of 5 mm, then cold rolled to a thickness of 1.2 mm after pickling, and then heat treated by continuous annealing in the water quenching type shown in FIG.

The thermal cycle conditions for this continuous annealing were the same for all steels of the same R type, and the temperature at which the strength-elongation balance was highest for that steel type was used.

Table 2 shows the results.

In Table 2, 2, 7, 9, 1 0.1 1 means 1.3, 4, 5, 6 of the present invention manufactured by the method for preventing grain boundary oxidation of high-strength cold rolled steel sheets according to the present invention. .8 is comparative steel.

In Table 2, Steels 1 and 2.3 are all steel types with the same composition of high silicon #:a.

Although the comparative AT was rolled at a low temperature of 550°C, the austenite volume fraction on the exit side of the final hot rolling stand was as high as 90%, so grain boundary oxidation occurred due to transformation recuperation after winding. .

Comparative FG43 is able to prevent grain boundary oxidation, but the austenite volume fraction at the exit side of the final hot rolling stand is as low as 15%, so the shape of the hot rolled steel sheet is poor, and as a result, the shape of the cold rolled aFi, The shape has deteriorated, scratches, etc. have occurred, and the quality has deteriorated.

Inventive Steel 2 has appropriate austenite volume fraction and coiling temperature at the exit side of the final hot rolling stand, so the shape of the cold rolled steel sheet is also good and grain boundary oxidation does not occur.

Comparison [4 and 5.6 are both high-silicon steels with the same composition.

In comparison [4], since the winding temperature was high, grain boundary oxidation occurred even though the austenite volume fraction on the final exit side of hot rolling was within an appropriate range.

In Comparison 115, the rolling reduction of the hot-rolled steel sheet is as low as 60%, so ferrite bands are formed, and the elongation is significantly lower than that of the other grades.

Since the winding temperature of Comparative Grade 6 is low, the strength of the hot-rolled steel sheet is extremely high, and the shape of the cold-rolled steel sheet is poor.

Inventive steels 7, 9, and 10.11 have austenite volume fraction on the exit side of the final hot rolling stand, hot rolling reduction ratio, and coiling temperature within appropriate ranges, all have good shapes, and grain boundary oxidation occurs. Moreover, it shows a good strength-elongation balance.

Comparatively, since aS is a low-silicon steel, grain boundary oxidation does not occur even when hot rolling is performed under conditions outside the range of the method for preventing grain boundary oxidation of a high-strength cold rolled steel sheet according to the present invention.

[Effects of the Invention] As explained above, since the method for preventing intergranular oxidation of high-strength cold-rolled steel sheets according to the present invention has the above-mentioned configuration, occurrence of intergranular oxidation is not observed even in high-silicon steel. Pickling can be carried out at the same speed as ordinary steel sheets, it can be manufactured without reducing productivity, and it has the excellent effect of being able to be applied to as-hot-rolled materials. It is something.

[Brief explanation of drawings]

FIG. 1 is a transformation curve diagram of the steel after hot rolling. spear 1

Claims (2)

[Claims] (1) C0.05-0.25wt%, Si0.5-3.
After soaking a steel containing 0 wt%, Mn 0.5 to 3.0 wt%, and the balance consisting of Fe and unavoidable impurities, 70% in the temperature range of Ar_1 to Ar_3
High-strength cold rolling characterized by carrying out the above rolling, hot rolling so that the austenite volume fraction at the exit side of the final stand is 20 to 70%, and then coiling at a temperature of 300 to 650°C. Method for preventing grain boundary oxidation in steel sheets. (2) C0.05-0.25wt%, Si0.5-3.
0wt%, Mn0.5-3.0wt%, and further contains Nb0.02-0.10wt%, Ti0.01-0.1
0 wt%, P0.03 to 0.10 wt%, and the balance is Fe and unavoidable impurities.
High-strength cold rolling characterized by carrying out the above rolling, hot rolling so that the austenite volume fraction at the exit side of the final stand is 20 to 70%, and then coiling at a temperature of 300 to 650°C. Method for preventing grain boundary oxidation in steel sheets.