JPH01272720A - Production of high ductility and high strength steel sheet with composite structure - Google Patents

Abstract

PURPOSE:To obtain a high strength and high ductility steel sheet with composite structure and ensuring superior spot weldability by specifying the contents of C, Si, Mn, etc., and carrying out properly controlled hot rolling, continuous annealing and cooling. CONSTITUTION:A steel slab consisting of 0.12-0.30% C, 1.5-3.0% Si, 1.1-2.4% Mn, 0.01-0.1% Nb, <0.005% S, 0.01-0.06% sol. Al and the balance Fe with inevitable impurities is hot rolled at a finishing temp. of the Ar3 point or above. The resulting steel sheet is coiled at <=600 deg.C, cold rolled and subjected to continuous annealing including holding in the austenite-ferrite two-phase range of the Ac1 point+30 deg.C-the Ac3 point for >=4min. It is slowly cooled to 500-800 deg.C at 5-30 deg.C/sec cooling rate, rapidly cooled to 350-450 deg.C at >=70 deg.C/sec cooling rate, held at 350-450 deg.C for 1-5min and cooled to room temp. at >=2 deg.C/sec cooling rate. The composite structure consisting of martensite, bainite, ferrite and retained austenite is formed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a highly ductile, high-strength, composite-structured steel sheet, and more specifically, has a high tensile strength of 80 kgf/mm'' or more, The present invention also relates to a method for producing a highly ductile, high-strength composite steel sheet having excellent ductility, spot weldability, and stretch flangeability.

[Prior art] In recent years, thin steel sheets for automobiles have been strongly required to have high strength from the viewpoints of energy saving and safety. This is required.

In particular, many efforts have been made to develop steel sheets that have both high ductility and high strength, which are said to be contradictory properties.
-043430, JP 60-043464, JP 61-217529), by regulating the composition and manufacturing method, a large amount of retained austenite remains, and its transformation-induced plasticity is utilized. A method has been adopted to do so.

The applicant has already developed a material with high ductility and high ductility by keeping a large amount of retained austenite by regulating the composition of components such as C, Si, and Mn, and manufacturing conditions such as hot rolling conditions, coiling conditions, and annealing conditions. We have developed a composite steel sheet with excellent strength properties. In these inventions, all
Elongation was significantly improved, and a good balance of strength and ductility was obtained.

This improvement in uniform elongation is effective in improving press formability, but when press forming, comprehensive properties are required, including not only the uniform elongation component but also stretch flangeability, bendability, etc.

However, in the above invention, local elongation (local deformability), stretch flangeability and bending formability due to the components are not necessarily good.

Therefore, there is an extremely high demand for a steel plate for automobiles that fully satisfies stretch flangeability in addition to strength-balance, and the development thereof is desired.

[Problems to be Solved by the Invention] In view of the above-mentioned characteristics of conventional steel sheets for automobiles, the present inventor has conducted intensive research and as a result of repeated examinations, the content of the steel, the content ratio, and By specifying the manufacturing method, we were able to develop a high-strength product with a tensile strength of 90 kgr/mm" or higher, as well as high ductility and high elongation flangeability, which are important properties for automotive steel sheets. They developed a method for manufacturing high-strength composite steel sheets.

[Means for Solving the Problems] The method for producing a high ductility high strength composite structure steel sheet according to the present invention is characterized by: CO, 12 to OJOwt%, Si 1.5 to 3. Ow
t%, Mn 1.1-2.0wt%, NbO, 01-
0.1wt%, S<0.005wt%, 5olAl
A steel slab containing 0.01 to 0.06 wt% with the remainder being Fe and unavoidable impurities is hot rolled at a temperature above the Art transformation temperature, coiled at a temperature below 600°C, and then cold rolled. Rolling is carried out, and then Ac++
After continuous annealing for 4 minutes or less in the two-phase region of austenite + ferrite at 30°C to Ac, 5°C/se
Slowly cool from the above holding temperature to a temperature of 500 to 800 °C at a cooling rate of 30 °C or more and 30 °C / sec, and then 70 °C
Martensite and bainite are rapidly cooled to a temperature of 350 to 450°C at a cooling rate of ℃/sec or more, held at this temperature for 1 to 5 minutes, and then cooled to room temperature at a cooling rate of 2℃/sec or more. , a composite structure consisting of ferrite and retained austenite.

A method for manufacturing a high ductility high strength composite structure steel sheet according to the present invention will be described in detail below.

Most of the steel sheets used for automobiles are press-formed, and in addition to elongation (ductility), local deformability is a factor that greatly controls press-formability, especially stretch flangeability. In steel sheets that utilize the transformation-induced plasticity of retained austenite as in the present invention, --like elongation is greatly improved, but local elongation due to local transformation ability is not improved much, and stretch flangeability and bendability are reduced. This is a concern. Therefore, in order to improve the stretch flangeability of composite steel sheets, the second phase (martensite and retained austenite) is uniformly and finely dispersed in the matrix to alleviate local stress concentration. and can improve stretch flangeability, that is, Nb
content 0. By containing 1 to 0.1 wt% of O, fine carbonitrides precipitate during hot rolling, and by delaying recrystallization, the structure becomes finer and the finally generated second phase becomes uniform, sharp, and fine. can be dispersed into

In addition, in order to fully exhibit the effects of Nb, it is necessary to set the heating temperature before hot rolling to 1100°C or higher to sufficiently dissolve Nb as a solid solution and to uniformly disperse carbonitrides that will precipitate later. There is.

Next, in order to produce a composite structure steel sheet with high strength and high ductility, it is effective to utilize the transformation-induced plasticity of retained austenite. It is necessary to contain it.

And, as an austenite stabilizing element, C5Mn
, Ni, etc. are known, and a particularly effective element is C.
Due to this increase in C content, if an appropriate thermal history is performed after annealing, a large amount of retained austenite can be obtained, making it possible to produce high-strength steel sheets that are extremely ductile and have a good strength-ductility balance. be. Furthermore, by specifying the regulation of the content ratio of Si and Mn and the manufacturing conditions,
The effect of C above becomes even more effective.

That is, the content ratio of C15t and Mn is 0.
．． 12 to 12% to OJOwt%, and Si content to 1.5 to 3%.
When Mn is in the range of 0 wt% and Mn is in the range of 1.1 to 2.4 wt%, stable retained austenite can be sufficiently obtained, and the strength-ductility balance is significantly improved.

The components and content ratios of the steel used in the method for manufacturing a high-ductility, high-strength composite steel sheet according to the present invention will be explained.

C is an essential element that allows a large amount of austenite to remain after annealing, and by appropriately controlling heat treatment conditions and annealing conditions, it is possible to stabilize austenite and ensure that 10% or more of austenite remains after heat treatment in terms of volume percentage. The content is required to be 0.12 wt%, and
As the C content increases, the retained austenite volume fraction increases and the strength-ductility balance is improved, but at 0.30 wt.
If the content exceeds %, spot weldability will deteriorate. Therefore,
The C content is 0.12 to 0.30 wt%.

Si is a ferrite-forming element, and although Si itself does not have the effect of stabilizing austenite, it is a ferrite that is formed during the maintenance of the two-phase region of ferrite-decaustenite or during cooling from the austenite region or the two-phase region of ferrite-decaustenite. It is an element that contributes to the stabilization of austenite through the action of promoting the concentration of C in untransformed austenite, and by including Si, it is extremely effective in improving both strength and ductility. This effect is small when the content is less than 1.5wt%, and when the content is increased to 3.0wt%,
If it exceeds %, problems such as melting cracks and deterioration of chemical conversion properties will occur. Therefore, the Si content is set to 1.5 to 3 Owt%.

Mn is an important austenite-forming element, and in order to obtain a good strength-ductility balance, a retained austenite volume fraction of 105 or more is required.
If the content is less than 1.1 wt%, this effect cannot be obtained, and if the content increases and exceeds 2.4 wt%, martensitic transformation becomes more likely to occur in the process after continuous annealing, and eventually The volume fraction of martensite increases, resulting in a significant increase in strength and a significant deterioration in ductility, which becomes an obstacle to improving the strength-ductility balance. Therefore, the Mn content is set to 1.1 to 2.4 wt%.

Nb is an element that improves stretch flangeability through microstructural refinement, that is, it is an element that delays recrystallization by forming fine carbonitrides and is effective for microstructural refinement.
If the content is less than 0.01 wt%, such an effect cannot be expected, and if the content exceeds 0.1 wt%, the strength will increase due to the precipitation effect, but the ductility will deteriorate significantly, and the formation of carbides will cause austenite to deteriorate. The absolute amount of solid solution C that concentrates in the solid solution decreases. Therefore, the Nb content is 0.01 to 0.
Let it be lwt%. In addition, in order to fully demonstrate the effect of Nb, it is necessary to set the heating temperature before hot rolling to 1100°C or higher to sufficiently dissolve Nb as a solid solution, and to uniformly disperse Nb carbonitrides that will precipitate later. be.

Since S deteriorates workability, it is desirable that it be as small as possible, and the content is regulated to 0.005 wt% or less.

5olAl is an effective element as a deoxidizing agent for steel, and if the content is less than 0.01 wt%, no deoxidizing effect can be expected.
Furthermore, if the content exceeds 0.06 wt%, the deoxidizing effect will be saturated and no further effect can be expected.

Therefore, 5olA] content is 0.01 to 0.06 wt%
shall be.

The manufacturing method for steel with such content components and content ratios is to perform cold rolling after coiling at a temperature of 600°C or less as a condition after hot rolling, and then perform continuous annealing at a temperature lower than the maximum heating temperature. 5°C/secC or more 30° (:
Cool to a temperature of 500 to 800°C at a cooling rate of 70°C/sec or less, and then cool to 35°C at a cooling rate of 70°C/sec or more.
Various mechanical properties can be improved by rapidly cooling to a temperature of 0 to 450°C and holding at this temperature for 1 to 5 minutes.

Furthermore, in the method for manufacturing a high ductility high strength composite steel sheet according to the present invention, ■Nb O is set at 01 to 0.1 wt%, ■G O
, 12-0.30 wt%, Si 1.5-3. Owt
%, Mn should be 1.1 to 2.0 wt%, ■ The steel plate should be coiled at a temperature of 600°C or less after hot rolling, and ■ Continuous annealing and subsequent cooling should be appropriately processed according to specific manufacturing conditions. This is important, and by selecting such conditions, strength - ductility -
The balance and press formability can be improved, and the reason for this is not necessarily clear, but it is thought to be explained below.

Regarding ■, fine carbonitrides precipitate during hot rolling,
By delaying recrystallization, the structure becomes finer, and by uniformly and finely dispersing the finally generated second phase, the difference in hardness between the matrix (ferrite) and the martensite in the second phase is reduced. This improves stretch flangeability.

Regarding (①), by limiting the contained components and content ratios in this way, proper continuous annealing can reduce C.
The concentration of the residual austenite into the austenite is promoted and the residual austenite is stabilized.

Regarding ■, the lamella spacing of carbonitrides becomes finer,
This is heated to the two-phase region of austenite + ferrite of Ac and +30°C to Aca, and the austenite form and Ca degree when the pearlite as a whole becomes austenite influence the structure after cooling.

Regarding ■, in the primary cooling after heating to the two-phase region of austenite + ferrite of Ac, +30℃ to Ac3,
As ferrite transformation progresses, C is concentrated in austenite, and pearlite transformation is avoided in the subsequent secondary cooling.
When kept at a temperature of 50 to 450° C., bainite transformation progresses, and as a result, C concentration in austenite progresses. The heating temperature in the α+72 phase region was set to AC++30°C to Ac because it was necessary to sufficiently dissolve the carbide into the γ phase before annealing.

[Example] Next, an example of the method for manufacturing a high ductility, high strength composite structure steel sheet according to the present invention will be described.

Example 1 Eleven types of - having the components and content ratios shown in Table 1 were melted and slabs were manufactured by continuous casting.

The test steel, FSH, satisfies the content components and content ratios of steel used in the manufacturing method of high ductility, high strength composite steel sheet according to the present invention (sometimes simply referred to as the manufacturing method of the present invention). Yes, and the others are comparative steels. Note that the slab may be produced by a normal ingot-forming method.

Next, each of these steels was hot rolled and coiled at a temperature of 450°C.
The sample steel was wound up and further cold-rolled to obtain a test steel having a thickness of 0.85 mm.

Next, a tensile test was conducted under conditions D5 in Table 2 (manufacturing method of the present invention) using a JISS No. tensile test piece having a gauge length of 50 mm after continuous annealing. In addition, to determine the suitability of the structure, the structure was observed and the volume fraction of austenite was measured. Spot weldability was evaluated by measuring cross tensile strength and shear tensile strength. In addition, the hole expansion limit (λ), which is an index of stretch flangeability, is determined from the hole diameter d when a punched hole of d0 = 7.5φ is expanded with a conical punch with an apex angle of 30° and a crack penetrates through the plate thickness. It was calculated using the following formula.

λ=(d-do)/d.

Table 1 shows these results.

From this Table 1, according to the manufacturing method of the high ductility high strength composite structure steel sheet according to the present invention, F, HSK has a TS of 100 kg.
It has high strength with f/m of over 2500, and has an excellent strength-ductility balance of over 2500, and furthermore,
It can be seen that the hole expansion limit (λ) is good at 45%, and the cross tensile strength and shear tensile strength for spot welding are superior to the comparative steel.

However, comparison w4A has a good hole expansion limit (λ) but a poor strength-ductility balance, and comparison RB has a good strength-ductility balance but has a good spot weldability and hole expansion limit (λ).
In addition, when comparing Comparative DC and D produced by the production method of the present invention, Comparison 14E and F1 produced by the production method of the present invention, Comparative steel G and H produced by the production method of the present invention, the hole expansion limit ( It can be seen that λ) has been improved. Other comparative steels also do not satisfy the strength-ductility balance, spot weldability, and stretch flangeability at the same time.

Example 2 Hot rolling and continuous annealing were performed under 12 types of conditions shown in Table 2 using test steels having the components and content ratios used in the production method of the present invention. In addition, the sample w4D
Samples 2 to D5 and D8 were processed according to the production method of the present invention, and the others were processed under conditions other than the production method of the present invention.

Also, in Table 2, T1, C3, Tq, Ct, T1
, [and C3 respectively indicate the conditions of the continuous annealing cycle shown in FIG. 1, T is the annealing temperature, Tq is the cooling start temperature of C7, T is the intermediate holding temperature, C1 is the cooling rate of Tt-Tq, C3 is the cooling rate of 'rq→T (where T, >T
Q>Tt), t is the holding time at T, and C3 is the cooling rate from T to room temperature.

As in Example 1, each test steel was subjected to a tensile test, structure observation, measurement of the austenite volume fraction, and hole expansion limit (
λ) was measured.

Table 2 shows the results.

From this Table 2, test steels D2 to D5 manufactured by the manufacturing method of the present invention
and D8, both of which contain austenite with a volume fraction of 15% or more in their structures, and a TS of 95 kgr/mm"
It has a high strength and TSXEI of 2500 or more, which is an excellent strength-ductility balance, and the hole expansion limit (λ) is 4.
It can be seen that it is 5% or more.

Therefore, comparative steels DI, D6, D7 and D9 to DI2
The samples processed under conditions other than the production method of the present invention had a small amount of retained austenite, and good mechanical properties could not be obtained.

[Effects of the Invention] As explained above, since the method for manufacturing a high ductility high strength composite steel sheet according to the present invention has the above configuration, it is possible to reduce the content of steel with low carbon content. By specifying the components and content ratios and performing appropriately controlled hot rolling and continuous annealing, it is possible to ensure excellent spot weldability and achieve a 90%
It has high strength of more than kgf/mo+” and high ductility,
Furthermore, a composite structure steel sheet having an excellent balance of strength and ductility and press formability can be produced, and in particular, an excellent steel sheet for automobiles can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a heat cycle of continuous annealing in one embodiment of the method for manufacturing a high-ductility, high-strength, composite-structure steel sheet according to the present invention. Spear 1

Claims (1)

[Claims] C0.12-0.30wt%, Si1.5-3.0wt
%, Mn1.1-2.4wt%, Nb0.01-0.1
wt%, S<0.005wt%, solAl0.01~
A steel slab containing 0.06 wt% and the balance consisting of Fe and unavoidable impurities is hot rolled at a temperature higher than the Ar_3 transformation temperature, coiled at a temperature lower than 600°C, and then cold rolled, Then, Ac_1+30℃～A
After continuous annealing for 4 minutes or less in the two-phase region of austenite + ferrite of c_3, 5℃/sec or more 3
500°C from the above holding temperature at a cooling rate of 0°C/sec or less
Slow cooling to a temperature of ~800°C, then 70°C/sec
Rapid cooling to a temperature of 350 to 450°C at the cooling rate above,
After maintaining this temperature for 1 to 5 minutes, return to room temperature at 2℃/sec.
A method for producing a high-ductility, high-strength composite-structure steel sheet, characterized in that a composite structure consisting of martensite, bainite, ferrite, and retained austenite is formed by cooling at the above cooling rate.