JPS63241120A - Manufacture of high ductility and high strength steel sheet having composite structure - Google Patents

Abstract

PURPOSE:To manufacture a high ductility and high strength steel sheet having superior spot weldability and a composite structure by not rolling coiling a steel slab having a specified compsn. under specified conditions and by successively carrying out heating, slow cooling an rapid cooling to form the specified composite structure. CONSTITUTION:A steel slab consisting of, by weight, 0.12-0.25% C, 1.5-3.0% Si, 1.1-2.0% Mn, <0.005% S, 0.02-0.50% sol, Al and the balance Fe with inevitable impurities is hot rolled at the Ar2 transformation temp. or above coiled at <650 deg.C. Continuous annealing is then carried out by heating, holding at >=800 deg.C in the austenite+ferrite two-phase for >=4min and cooling followed by holding at 350-450 deg.C for 1-5min. The cooling is carried out by slow cooling from the holding temp. to 600-800 deg.C at <=30 deg.C/sec cooling rate and rapid cooling to 350-450 deg.C at >=30 deg.C/sec cooling rate of form a composite structure consisting of martensite, bainite, ferrite and retained austenite.

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, particularly having a high tensile strength of 80 kgf/m-2 or more,
Moreover, the present invention relates to a method for producing a composite steel sheet having extremely excellent ductility and spot weldability.

(Conventional technology and problems to be solved) Thin steel sheets, which are widely used as structural materials for automobiles, require many properties such as workability and weldability. In response to the pursuit of improved performance, there is a strong demand for higher strength thin steel sheets. That is, this increase in strength is mainly based on a tensile strength of 60 to 70 kgf/m2, but furthermore, the tensile strength TS is TS≧80 kgf/m2.
High-strength steel plates such as "mu" are often required.

As described above, composite structure steel sheets consisting of two phases of ferrite and martensite or two phases of bainite and martensite have been developed as high-strength steel sheets with high workability of TS = 80 to 140 kgf/m ■ Class 3. ing. However, in response to the increasing diversity of social needs in recent years, composite steel sheets cannot always be satisfied in terms of workability, etc., especially as materials for mass-produced products such as steel sheets for automobiles. As such, low cost is an essential condition, and in addition, strength-ductility balance, weldability, and other property considerations must also be carefully considered.

From the above points of view, a high-strength composite steel sheet with high workability consisting of ferrite + retained austenite + martensite (including some bainite) has recently been developed (Japanese Patent Application Laid-Open No. 6O-43430), but the strength - Large amount of C to stabilize austenite, which is essential for improving ductility balance.
(≧3%). For this reason, there is a problem that spot weldability, which is a necessary property for automobile steel sheets, is poor.

The present invention was made in order to solve the problems of the prior art described above, and has a tensile strength of 80 kgfZPeng2 or more and a high strength, and has not only extremely excellent ductility but also spot weldability. The object of the present invention is to provide a method for economically producing a steel sheet having a composite structure.

(Means for solving the problem) In order to obtain a composite steel sheet with high strength and ductility higher than that achieved by conventional methods, it is necessary to appropriately reduce the volume fraction of retained austenite, which conventionally contained only a few percent at most. need to be controlled.

This is due to the high n value associated with the transformation-induced plasticity of austenite, and in order to obtain the expected high ductility, it is necessary to contain at least 10% or more.

In this regard, since C is said to have the greatest effect as an austenite stabilizing element, as the amount of C increases, austenite becomes more stable, and especially when constant temperature transformation is carried out, the austenite region or ferrite + austenite is formed. If an appropriate thermal history is given to the two-phase region after reheating, a high-strength steel plate with extremely high ductility can be obtained.

However, as pointed out above as a problem with the conventional method, increasing the carbon content deteriorates the spot weldability, which is an essential property for automobile steel sheets. Therefore, in order not to deteriorate spot weldability, it is necessary to restrict the amount of C to 0.25% or less. but.

If the amount of C is regulated in this manner, stable retained austenite as described above cannot be obtained sufficiently, resulting in a deterioration of the strength-ductility balance.

Therefore, on the premise of reducing the C content, the present inventors conducted various efforts to find ways to improve the strength-ductility balance in steel sheets with a low C content and good spot weldability by regulating the composition. As a result of repeated experimental research, it was found that this is possible by simultaneous regulation of Si%Mn and 5oQAQ. That is, in the process of reducing the C amount to 0.25% or less and performing appropriate continuous annealing, the Si content is reduced to 1.5 to 3%.
．． 0%.

Mn: 1.1-2.0%, go Q A Q 0.02
By regulating each within the range of ~0.50%,
It has been found that sufficient stable retained austenite can be obtained and the strength-ductility balance can be significantly improved*
The reason why the strength-ductility balance improves due to the regulation of Si, Mn, and AQ is not necessarily clear, but in the process of proper continuous annealing, C preferentially concentrates in austenite and stabilizes the retained austenite. This is thought to be due to the presence of

That is, in the present invention, C: 0.12 to 0.25%.

Si: 1.5-3.0%, Mn: 1.1-2.0%, S
0.005% and 5o (IAQ: 0.02-0.50
%, and if necessary P: 0.02 to 0.20
%, Cr: 0.1-0.5%, B: O, 0005-0.
For steel slabs containing one or more of Ti: 0.01% and Ti: 0.01 to 0.05%, with the remainder consisting of Fe and unavoidable impurities, hot rolling is completed above the Ar transformation temperature. The austenite +
After being held in the two-phase region of ferrite for 4 minutes or less, 350~
When rapidly cooling to hold in the temperature range of 450°C for 1 to 5 minutes, first, slowly cool from the above holding temperature to 600 to 800"C at a cooling rate of 30°C/sec or less, and then A high ductility high strength composite structure steel sheet with excellent spot weldability characterized by obtaining a composite structure consisting of rutinsite, bainite, ferrite and retained austenite by rapidly cooling to a temperature of 350 to 450°C at a cooling rate. Manufacturing method.

The main points are as follows.

The present invention will be explained in detail below based on examples.

First, the reason for limiting the composition of steel targeted by the method of the present invention will be explained. Of course, this restriction of the components is made in conjunction with optimization of process conditions (hot rolling conditions and continuous annealing conditions), which will be described later.

C: C is an essential element for strengthening steel, and it stabilizes austenite by properly controlling hot rolling conditions and annealing conditions, and after heat treatment, it stabilizes austenite at a volume fraction of 10.
% or more, a minimum of 0.15% is required.

On the other hand, increasing the amount of C increases the retained austenite volume fraction and improves the strength-ductility balance, but 0.25
%, the spot weldability, which is the main objective of the present invention, deteriorates, so 0.25% was set as the upper limit.

Si: Si is a ferrite former element, so it does not have the function of stabilizing austenite by itself, but it can be used to maintain the two-phase region of austenite in ferrite, or to stabilize the two-phase region of austenite in ferrite. In order to purify the ferrite produced during cooling, C is an element that contributes to the stabilization of austenite through the effect of promoting the concentration of C into untransformed austenite.

Thus, the addition of SL is very effective in improving both strength and ductility, and for this purpose it is necessary to add at least 1
．． 5% is necessary, and the lower limit was set at 1.5%. On the other hand, if the amount of Si increases, problems such as melting cracks and deterioration of chemical conversion treatment properties will occur, so the upper limit was set at 3.0%.

Mn: Mn is important as an austenite former element, and from the viewpoint of obtaining a good strength-ductility balance, a minimum content of 1.1% is required in order to contain a retained austenite volume fraction of 10% or more. On the other hand, when the amount of Mn increases,
Martensitic transformation tends to occur during the cooling process after continuous annealing.

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 upper limit of the Mn content was set at 2.0% as the upper limit at which this effect does not appear.

5o12AQ: 5offAa is effective as a deoxidizing agent for steel, but
Adding an appropriate amount also improves the strength-ductility balance. This is a ferrite similar to AQ-t+si.
It is a former element and is thought to have the same effect as Si. The upper limit is the amount at which this effect is saturated,
In addition, the lower limit value is 0, from the amount at which the deoxidizing effect can no longer be expected.
．． It was limited to a range of 0.02 to 0.50%.

In the present invention, the above elements are essential components.

The following elements can be contained as necessary.

P: P is a ferrite former element like Si,
Austenite can be further stabilized through the effect of promoting concentration of C in untransformed austenite. Therefore, even if the amount of P is at a normal level, there will be no problem in terms of properties such as strength-ductility balance, but if the amount of P is 0.02% or more,
By containing , a better strength-ductility balance can be obtained.

However, if it exceeds 0.2%, the effect not only becomes saturated, but also causes the steel to become brittle due to grain boundary segregation.
．． The upper limit is 2%.

S: Since S deteriorates workability, it is desirable to have as little S as possible, and it is regulated to 0.005% or less.

Cr: Cr is an element that improves hardenability and is an advantageous element in obtaining the desired structure in the present invention. That is,
When Crtto is contained in an amount of 1% or more.

Since the hardness of the martensite that is produced is increased and the necessary strength can be obtained with a small volume fraction of martensite, it is possible to increase the volume fraction of ferrite and austenite, which improve ductility. Therefore, the lower limit of the amount of Cr is limited based on the amount that can exhibit its effect, and is set to 0.1% or more.

On the other hand, as the amount of Cr increases, the volume fraction of martensite increases, and if it is added in excess of 0.5%, the ductility deteriorates on the contrary. For this reason, the upper limit is set to 0.5%.

B and TI= B is also an element that improves hardenability, and is an advantageous element in obtaining a desired structure without adding expensive elements such as Cr. B has the function of lowering the free energy of austenite grain boundaries, and is useful for transforming austenite and obtaining stable retained austenite. The lower limit value is regulated from the amount that can produce the effect, and the upper limit value is regulated from the amount that becomes uneconomical, and the amount of B is 0.
．． 0005 to 0.01%. However, B exerts this effect only when it is in a solid solution state.

However, B has a strong bonding force with N, and even if there is a very small amount of N, B and N will bond and the desired effect of B cannot be obtained. Therefore, Ti, which has a stronger bonding force with N than B, Add T
By preferentially combining 1 and N, the lower limit of the amount of aTx that can fully exhibit the effect of B is regulated based on the amount that can exhibit that effect, and the upper limit is regulated from the amount that becomes uneconomical. The amount of T1 is limited to 0.01 to 0.05%.

In composite steel sheets having the above chemical components, even if the retained austenite volume fraction is the same, it is thought that differences in ductility will occur depending on the stability against processing deformation.
In the present invention, based on the results of various experimental studies, in particular (a) during hot rolling, winding is performed at a temperature of less than 650°C; (b) during continuous annealing, cooling after soaking is carried out gradually or rapidly under specific conditions. Control as follows.

This makes it possible to obtain a large amount of retained austenite with appropriate stability.

The specific conditions for this are as follows.

First, in hot rolling, it is necessary to finish hot rolling at Ar, 1 transformation temperature or higher, and to wind up at a temperature lower than 650°C.

In continuous annealing, after heating and holding for 4 minutes or less in the two-phase region of ferrite in austenite at 800°C or higher, first 60°C from the above holding temperature at a cooling rate of 30°C/sec or less.
Slowly cool to 0 to 800°C, then rapidly cool to a temperature of 350 to 450°C at a cooling rate of 30°C/sec or more.
It is necessary to hold the temperature in the range of 50-450°C for 1-5 minutes.

The reason why the above effects are obtained by regulating the hot rolling and continuous annealing conditions in this way is not necessarily clear, but it is thought to be as follows.

In other words, because the carbides in the hot-rolled steel sheet are dispersed due to the regulations in (,) above, austenite grains grow with these carbides as nuclei when heated to a two-phase region of ferrite + austenite at 800°C or higher during continuous annealing. is widely distributed, and finally retained austenite is also widely distributed. Also,
Due to the regulation (b) above, ferrite transformation occurs during cooling in the continuous annealing process.

Bainite metamorphosis is properly controlled. Moreover, these effects are greatest when the chemical components are regulated as described above.

Next, examples of the present invention will be shown.

Twelve types of steel having the chemical components (wt%) shown in Table 1 were melted and slabs were obtained by continuous casting. Test steel, E,
F.1. J and L satisfy the chemical composition range of the present invention, and the others are comparative steels.

Note that the steel ingot may be obtained by a normal ingot-forming method.

Next, each of the above-mentioned steels was hot-rolled at a hot-rolling temperature of 520 to 730°C, and further cold-rolled to obtain a test material with a thickness of 0.811 m.First, the process of D2 in Table 2 described below was performed. conditions(
Gauge length 50mm after continuous annealing (within the scope of the present invention)
A JISS No. tensile test piece was prepared and a tensile test was conducted.

In addition, in order to determine the suitability of the structure, the structure am and the volume fraction of austenite were measured. These results are the first
As shown in the table.

From the same table, steels of the present invention, E, F, 1. J and L have high strength with a TS of 90 kg f/**” or more,
It can be seen that TSxEQ not only has an excellent strength-ductility balance of 2400 or more, but also has a much better cross tensile strength after spot welding than the comparative steel. On the other hand, comparative steels B and C have a good strength-ductility balance but poor single-point weldability, and none of the other comparison steels have the same strength-ductility balance and spot weldability. do not have.

Using the above-mentioned test steel F having chemical components within the range of the present invention, hot rolling and continuous annealing were performed under the conditions shown in Table 2. In addition, test steel D2-3. D5-7. D
Samples Nos. 9 to 10 and F1 to 3 were processed under process conditions within the scope of the present invention, and the others were processed under process conditions outside the scope of the present invention. In addition, T in the same table, C engineering, Tq, C2, T
, and t respectively indicate the conditions of the continuous annealing cycle shown in FIG.
2 is the cooling rate of Tq−)T (where T□>T q
>Ts).

Tensile tests were conducted on each steel sample in the same manner as in Example 1.

Structure [5] was carried out and the austenite volume fraction was measured. The results are shown in Table 2.

From the same table, test steels D2-3, D5-7, D9 of the invention examples
~10. F1-3 all have a volume fraction of 15 in one tissue.
Contains more than % austenite, TS is 90 kg f
It can be seen that the steels have a high strength of 2300 or more and a TSXEQ of 2300 or more, and have an excellent strength-ductility balance.In addition, the test steels F1 to 3 of the present invention example have a high strength of 2300 or more.
However, the value of TSXEQ is further improved by the addition of soQAn, and there is almost no effect on TSXEQ due to the difference in annealing temperature T.

In contrast, sample steels D1, D4, D8, and D11 of comparative examples
Samples 1 to 14 treated under conditions outside the scope of the present invention had less than 1 ounce of retained austenite, and good mechanical properties were not obtained.

[Blank below] (Effects of the invention) As detailed above, according to the present invention, the chemical components are regulated at a low C content, and hot rolling and continuous annealing are performed under appropriate conditions. Not only can it ensure excellent spot weldability, but it can also produce composite structure steel sheets that have high strength of 80 kgf/mm2 or more, high ductility, and an excellent strength-ductility balance, and are particularly suitable for automotive steel sheets. Suitable for manufacturing.

[Brief explanation of drawings]

FIG. 1 is a diagram showing heat cycle conditions for continuous annealing in an embodiment of the present invention. Patent applicant: Kobe Steel, Ltd. Patent attorney Hisashi Nakamura Figure 1

Claims (2)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.12 to 0.2
5%, Si: 1.5-3.0%, Mn: 1.1-2.0
%, S<0.005% and solAl: 0.02-0.
For steel slabs containing 50% iron and the remainder consisting of iron and unavoidable impurities, hot rolling is completed at Ar_3 transformation temperature or higher, coiling is performed at a temperature lower than 650°C, and then continuous annealing is performed at 800°C or higher. After heating and holding for 4 minutes or less in the two-phase region of austenite + ferrite, 350 ~
When rapidly cooling to hold in the temperature range of 450°C for 1 to 5 minutes, first slowly cool from the above holding temperature to 600 to 800°C at a cooling rate of 30°C/sec or less, then 3
High ductility with excellent spot weldability characterized by obtaining a composite structure consisting of martensite, bainite, ferrite and retained austenite by rapidly cooling to a temperature of 350 to 450 °C at a cooling rate of 0 °C / sec or more. Manufacturing method for high-strength composite structure steel plate. (2) C: 0.12-0.25%, Si: 1.5-3.
0%, Mn: 1.1-2.0%, S<0.005% and solAl: 0.02-0.50%, and further P:
0.02-0.20%, Cr: 0.1-0.5%, B:
0.0005-0.01% and Ti: 0.01-0.0
For steel slabs containing one or more of 5% and the remainder consisting of Fe and unavoidable impurities, hot rolling is completed above the Ar_3 transformation temperature, coiled at a temperature below 650°C, and then In the subsequent continuous annealing, after heating and holding in the two-phase region of austenite + ferrite at 800℃ or higher for 4 minutes or less,
When rapidly cooling to hold for a minute, first, 30℃/
600 to 80% from the above holding temperature at a cooling rate of sec or less
A composite structure consisting of martensite, bainite, ferrite, and retained austenite is obtained by slowly cooling to 0°C and then rapidly cooling to a temperature of 350 to 450°C at a cooling rate of 30°C/sec or more. A method for manufacturing high-ductility, high-strength, composite-structured steel sheets with excellent weldability.