JPS60159120A - Steel sheet having composite structure, superior bendability and low yield ratio and its manufacture - Google Patents

Abstract

PURPOSE:To obtain the titled steel sheet by providing a structure of a single ferrite phase in which a phase formed by transformation at a low temp. is not practically present to the prescribed surface layers of a steel sheet having a low yield ratio and a composite structure. CONSTITUTION:A hot rolled steel sheet is heated to a temp. in the range in which ferrite and austenite phases coexist with each other, and it is cooled to obtain a hot rolled steel sheet having a low yield ratio and a composite metallic structure contg. a phase formed by transformation at a low temp. and dispersed in the matrix of a ferrite phase. In this method, the surfaces of the hot rolled steel sheet are decarburized before said heating and cooling to form decarburized surface layers of 20-100mum thickness. A structure of a single ferrite phase in which a phase formed by transformation at a low temp. is not practically present can be provided to the surface layers of 20-100mum thickness, so the bendability of the steel sheet can be considerably improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a low yield ratio composite set NJi with excellent bending workability.
(Hot-rolled) steel plate and its manufacturing method.

Recently, high-strength hot-rolled steel sheets have been increasingly adopted in fields where hot-rolled plain steel sheets were used in the past, such as for the purpose of reducing the weight of automobiles. This is attracting attention on the back of ρi tension hot-rolled steel sheets, which are being considered for adoption.

There is a low yield ratio composite structure hot rolled steel sheet. This is a hot-rolled steel sheet with a mixed structure of a ferrite phase and a low-temperature transformation phase.
It is attracting particular attention because it has properties such as a+ strength, good ductility, and low yield ratio, which are convenient for forming processing.

This low yield ratio composite structure steel sheet exhibits a metal structure in which a low-temperature transformed phase mainly consisting of a hard martensitic phase is dispersed in a soft Ferra C1-phase matrix.

Compared to high-strength steel plates that utilize solid solution strengthening and precipitation strengthening,
In general, it has a large work hardening coefficient and exhibits large elongation in a tensile test. Generally speaking, if the growth is in the opposite direction, it is 1.
Normally, it is expanded to improve bending workability, but the conventional low yield ratio composite set! Even though the elongation is large in the case of 1lli steel plate.

It was hard to say that its bending workability was particularly superior compared to solid solution strengthened or precipitation strengthened high tensile strength steel plates. this is,
In low-yield-ratio composite steel sheets, the uniformity of
Although the elongation is large due to the large work hardening coefficient,
It seems that the local elongation after necking is related to the fact that it is not very sensitive to people.

By the way, in automobile parts, etc., for which this low yield ratio composite structure steel sheet is used, complex forming processes including stretching and bending are usually performed, so if the bending workability could be improved. It can be said that the formability of the steel sheet is further improved.

The present invention was made for the purpose of improving the bending workability of this low yield ratio composite structure steel plate. The present inventors aimed to improve the bending workability of this low-yield-ratio composite-structure steel sheet (as a result of various test studies, it was found that a low-temperature transformed phase is substantially present in the surface layer from the surface of this steel sheet to a depth of 20μ to 100μ). It has been found that the bending workability of this low-yield-ratio composite-structure steel sheet can be greatly improved if a ferrite single-phase structure, which does not exist, is formed and stored in a shop.

5. When producing a hot-rolled steel sheet with a composite structure that has good bending workability, a surface decarburized layer of 20 to 100 μm is formed on the surface of the hot-rolled steel sheet, and then this hot-rolled steel sheet is divided into ferrite phase and austenite phase. It can be advantageously produced by heating to a temperature in the two-phase coexistence region and then cooling to perform a heat treatment to obtain a metal structure in which a low-temperature transformed phase is dispersed in a ferrite phase matrix. As a method for forming a surface decarburized layer of 20 to 100 microns on the surface of a hot rolled steel sheet before heat treatment to obtain this composite structure, (1) an oven coil of a hot rolled steel sheet that has been descaled is heated with N2. A method of box annealing and forming in the presence of a mixed gas consisting of N20 and N2, (21,
A method of box annealing a tie 1 coil of a hot rolled steel sheet without descaling to form it by reaction with surface scale, or (3) a method of forming the tie 1 coil of a hot rolled steel sheet by reaction with surface scale, or (3) a method of forming the tie 1 coil of a hot rolled steel sheet without descaling;
It is preferable to use a method in which the film is formed by reaction with surface sgale during slow cooling after one winding at 00°C.

The present invention will be described in detail above based on test examples.

After melting steel with the chemical composition shown in Table 1 in a 90-ton converter, the continuously cast slab was hot-rolled under normal conditions to a thickness of 3.
A hot-rolled steel plate with a thickness of 6 mm and a thickness of 3 mm was prepared by grinding the front and back surfaces by 0.3 mm to make the surface condition of the steel plate constant.
Omm steel plate test was set to 1. A portion of this sample was subjected to one surface decarburization treatment. Surface decarburization treatment is H2/N
The test was carried out by heating to 700°C in an atmosphere with a mixed gas having a volume ratio of 3/19 and a dew point of 50°C. At that time, this 700℃
The depth of the surface decarburized layer was adjusted to a range of 20 to 250 μm by changing the time for which the sample was held.

The as-ground sample and the two-surface decarburized sample were heat-treated to form an 8-composite structure. This heat treatment heats the sample to 800°C, which is the two-phase coexistence region of ferrite and austenite.
After heating to 10°C, it is air cooled at an average cooling rate of about 10°C/s, ec, and when the temperature has decreased to about 500°C, it is water cooled to form a composite structure of a ferrite phase and a martensitic phase. The obtained composite structure steel plate was subjected to a tensile test and a bending test, and the results shown in Table 2 were obtained.

In addition, the lkl sample in Table 2 (which was subjected to heat treatment for composite texture while being ground) and the N03 sample (which was subjected to heat treatment for composite texture after applying a surface decarburization layer of 80μ).
Photographs of the r-plane metallographic structure of the surface layer are shown in Figures 1 and 2. As seen in this photograph, the full tensile 1-phase exists in the surface layer of the No. 1 sample, but the steel plate surface layer of the No. 3 sample is covered with a soft ferrite phase that does not contain the martinsite phase.

As is clear from the results in Table 2, compared to those without two-surface decarburization, those with one surface decarburized have improved elongation in the tensile test and a reduced limit bending radius in the bending test, improving bending workability. are doing. However, for samples No. 4 with a surface decarburized layer of 240μ, a clear decrease in tensile strength is seen compared to the sample without surface decarburization, indicating that problems will occur in terms of strength if the surface decarburized layer is too deep. .

As shown in the above test results, when manufacturing a steel sheet with a composite structure in which a low-temperature transformed phase is separated into a ferrite base by applying heat treatment to the two-phase coexistence region of ferrite + austenite by heating and then cooling, this composite structure is If a suitable surface decarburization layer is formed on the hot-rolled steel sheet before the heat treatment, bending workability can be improved without causing a decrease in strength. More specifically, a surface decarburized layer with a thickness of less than 20 μm has little effect on improving bending workability, and a surface decarburized layer with a thickness of more than 100 μm causes a decrease in strength. It can be said that it is within the range.

In this way, by forming an appropriate surface decarburized layer and then performing a heat treatment for composite structure, the surface layer becomes a single layer of ferrite with virtually no low-temperature transformed phase.
It has become clear that the bending workability, which is a drawback of composite structure steel, can be improved without impairing the properties of strength and low yield ratio, which are the characteristics of composite structure steel. The reason can be considered as follows.

That is, according to the bending test of a low yield ratio composite structure steel plate conducted by the present inventors, when the state of the crank during bending of a conventional composite structure steel plate was observed, it was found that, similar to the crank that occurs in a tensile test, The voids generated at the interface between the ferrite phase and the low-temperature transformed phase were connected to form cracks. On the other hand, in one bending process, the largest strain is applied to the surface layer outside the bend. Therefore, if a hard, low-temperature transformed phase is dispersed in this surface layer, the above-mentioned thickening is likely to occur, and the dirt becomes a notch, leading to deterioration in bending workability. but.

In the case of the steel sheet of the present invention in which there is no low-temperature transformed phase on the surface of the steel sheet, the surface of the steel sheet is covered only with the ferrite phase, so the generation of cracks on the surface that will become the notches is suppressed, and as a result 9 This will probably improve bending workability. It is believed that the formation of a surface decarburized layer should be prevented as much as possible, but in the case of the present invention, the surface decarburized to an appropriate degree without significantly affecting the tensile strength and fatigue strength. Since the surface layer is covered with a single layer of ferrite by heat treatment for texturing, it is possible to obtain the advantage of improved bending workability without losing the characteristics of steel with three composite structures.

In addition, in order to form a surface decarburized layer on a hot rolled steel sheet,
Methods (11. This is what accelerates the decarburization reaction.
By controlling the gas inflow time and the box annealing temperature, the depth of the surface decarburized layer can be adjusted within the range of 20 to 100 microns. In addition, method (2) causes a decarburization reaction by using the scale attached to the surface of the steel sheet as an oxidizing agent, and the depth of the surface decarburization layer is controlled in the range of 20 to 100μ by controlling the salinity of one box annealing. can be adjusted to (
In the method of controlling the coiling temperature in the hot rolling process of 3), (2)
Similar to the above method, this method uses scale as an oxidizing agent to advance the decarburization reaction during slow cooling after winding into a coil, and in this case, annealing for decarburization is not particularly necessary. do not. In order to promote the decarburization reaction during the slow cooling after winding, it is necessary to make the salinity of the second winding relatively high.
To obtain a surface decarburized layer of 100 μm, a coiling temperature of 600° C. or higher is required. However, if the winding temperature exceeds 800°C, the scale will become thick and the descaling performance will deteriorate, so it is preferable to set the winding temperature to 800°C or less.

Example Steel with the chemical composition shown in Table 3 was melted in a 90-ton converter, then continuously cast into a slab with a thickness of 250 cm and heated at 1.250°C.
After heating, it was hot-rolled under normal conditions to a plate thickness of 3.0 mm. The coiling temperature after hot rolling is 520 to 73 as shown in Table 4.
Do not change in the range of 0℃ - 8 for the obtained hot rolled coil
Box 4 The surface decarburization treatment shown in Table 4 was performed.

Thereafter, heat treatment was performed by heating to 780° C. and cooling in air to obtain a hot-rolled steel sheet with a low yield ratio composite structure.

Table 5 shows the mechanical properties of each steel plate after this heat treatment.

As is clear from the results in Table 5, the steel sheet of the present invention, which has been subjected to surface decarburization of 20 to 100μ to form a low-temperature transformed phase in the surface layer of 20 to 100μ, has the strength and low The bending properties are improved without losing the yield ratio properties.

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[Brief explanation of the drawing]

Figure 1 is a micrograph showing the metallographic structure of the surface layer of a composite texture steel sheet that was heat-treated to create a composite texture without surface decarburization, and Figure 2 is a composite texture steel plate that was heat-treated to create a composite texture after surface decarburization. 2 is a micrograph showing the metallographic structure of the surface layer. Applicant Nisshin Steel Co., Ltd. No. 1 flashing rod Figure 2 (X2.,,l(),)

Claims (4)

[Claims] (1) In a low-yield-ratio composite steel sheet with a metal structure in which a low-temperature transformed phase is dispersed in a ferrite phase matrix. A low-yield-ratio composite-structure steel plate with excellent bending workability formed by forming a layer of ferrite single-phase structure in which there is virtually no low-temperature transformed phase in the surface layer from the surface of the 1st to 7th plates to a depth of 20μ to 100μ. . (2) A hot-rolled steel sheet is heated to a temperature in the two-phase coexistence region of ferrite and austenite phases, and then cooled to disperse low-temperature transformed phases in the ferrite phase matrix, resulting in a low yield ratio composite structure. A3 B for the manufacturing method of hot-rolled steel sheet A low yield with excellent bending workability characterized by forming a surface decarburized layer of 20 to 100μ on the surface of the hot-rolled mesh sheet before heating and cooling. A method for manufacturing composite composite 111i hot-rolled steel sheet. (3) If there are 8 surface decarburized layers, an oven coil made of a descaled hot rolled steel sheet is heated to an N2. The manufacturing method according to claim 2, wherein the manufacturing method is formed by box annealing in the presence of a mixed gas consisting of N20 and N2. (4) 9 surface decarburized layer is 1 m hot rolled without scaling
3. The manufacturing method according to claim 2, wherein the tight coil of the plate is formed by box annealing and reaction with surface scale. (4) 6 surface j ground coal layer, hot rolling coiling temperature 600~80
The manufacturing method according to claim 2, wherein the manufacturing method is formed by reaction with surface scale during slow cooling after S removal at 0°C.