JPH0555571B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for producing a highly ductile, high-strength, composite-structured steel sheet, which has particularly high tensile strength of 80 Kgf/mm ² or more, as well as extremely excellent ductility and spot weldability. The present invention relates to a method for manufacturing a composite steel sheet having a composite structure. (Conventional technology and problems to be solved) Thin steel sheets used as structural materials for automobiles, etc. require various properties such as workability and weldability. There is a strong demand for higher strength steel plates. That is, this strengthening is mainly aimed at increasing the tensile strength TS to 60 to 70 Kgf/mm ² , but a steel plate with high strength such as TS≧80 Kgf/mm ² is often required. By the way, as high-strength steel sheets with a grade of TS = 80 to 140 Kgf/ ^mm2 and high workability, so far, there are composite steel sheets consisting of two phases of ferrite and martensite, or two phases of bainite and martensite. is being developed. however,
In recent years, the diversity of social needs has been increasing, and the composite structure steel sheet does not necessarily satisfy the needs in terms of workability and the like. In particular, as a material for mass-produced products such as automobile steel sheets, low cost is an essential condition, and in addition, not only the strength-ductility balance but also the balance of other properties such as weldability must be sufficiently considered. Must be. From this point of view, in recent years, a highly workable, high-strength composite steel sheet consisting of ferrite, retained austenite, and martensite (including some bainite) has been disclosed (Japanese Patent Application Laid-Open No. 60-43430), but the strength - Contains a large amount of C to stabilize austenite, which is essential for improving ductility balance. For this reason, there is a problem in that spot weldability, which is a necessary property particularly for automobile steel sheets, is poor. The present invention was made to solve the problems of the above-mentioned prior art, and has a tensile strength of 80
The purpose of the present invention is to provide a method that can economically and accurately produce a composite structure steel plate having a high strength of Kgf/mm ² or more, as well as extremely excellent ductility and spot weldability. It is something to do. (Means for Solving the Problems) In order to achieve the above object, the present inventors reexamined the steel composition, hot rolling conditions, annealing conditions, etc. of the conventional manufacturing method of high C content composite structure steel sheet. I also tried to consider this. In order to obtain a composite structure steel sheet with high strength and higher ductility than that obtained by conventional methods, it is necessary to appropriately control the volume fraction of retained austenite, which has conventionally been a few percent at most. This is due to the high n-value associated with the deformation-induced transformation of austenite, and in order to obtain the expected high ductility,
It is necessary to contain 10% or more. On the other hand, as austenite stabilizing elements, C,
Mn, Ni, etc. are generally well known, and C has the greatest effect. As the amount of C increases, austenite becomes more stable, and austenite remains especially when constant temperature transformation is performed. In this way, by using high C steel and imparting an appropriate thermal history to the austenite region or the two-phase region of ferrite and martensite after reheating, a high-strength steel plate with extremely high ductility can be obtained. However, as mentioned above, the increase in C deteriorates the spot weldability, which is an essential characteristic for steel sheets for automobiles. Therefore, the present inventors have repeatedly conducted experimental research on ways to improve the spot weldability of high C steel sheets by regulating the composition, and have found that it is possible to improve the spot weldability of high C steel sheets by regulating the Si and Mn simultaneously. did. In other words, it contains more than 1.5% Si and Si + Mn.
If it is regulated within the range of 2.8 to 4.0%, ferrite will preferentially be generated during cooling in the fusion zone and heat affected zone after spot welding, and austenite will tend to remain and martensitic transformation will be suppressed.
It is thought that spot weldability is improved. Next, as a result of repeated experimental research on hot rolling conditions and annealing conditions, we found that Si and Mn
It is thought that the ductility of the thin steel sheet with a composite structure, which is simultaneously regulated, differs in ductility depending on the stability against working deformation even if the retained austenite volume fraction is the same. Wind it up. (b) Appropriately control the cooling conditions after soaking during continuous annealing.
Slowly cool from 600℃ to Ar ₁ transformation point (Tq) at a cooling rate (C ₁ ) of ℃/sec or less, then rapidly cool to a temperature of 350 to 450℃ at a cooling rate (C ₂ ) of 30℃/sec or higher. . As a result, a large amount of retained austenite with appropriate stability can be obtained. That is, the present invention has clarified that the amount and stability of retained austenite that are favorable for ductility can be achieved by containing 15% or more of bainite in the structure, and furthermore, the above two conditions are 〜450℃×1〜
It was found that it is essential to generate 15% or more of bainite when held for 5 minutes. The reasons for these are not necessarily clear, but (a) The carbides in the hot-rolled steel sheet become spheroidized, so when reheated to a two-phase region of ferrite + austenite, this becomes the nucleus and becomes austenite, and in that phase This is thought to be because (b) ferrite transformation and bainite transformation during cooling are appropriately controlled because of the high degree of concentration of C, Mn, etc. in (b). Based on the above-mentioned findings, the present invention was developed after further consideration of conditions to be regulated (components, hot rolling conditions, annealing conditions) in detail, taking into account aspects such as strength-ductility balance. That is, the method for producing a high ductility high strength composite structure steel sheet with excellent spot weldability according to the present invention has a weight ratio of:
C: 0.15-0.45%, Si: more than 1.5% and less than 2.0% and
Mn≧1.1%, and Si+Mn: 2.8 to 4.0%, P: 0.02 to 0.20%, V:
0.05-0.40% and B: 1 of 0.0005-0.01%
species or two or more species, and further S≦0.005%,
For steel slabs containing solAl: 0.01~0.06%, with the remainder consisting of iron and unavoidable impurities, hot rolling is completed above the _Ar3 transformation temperature, coiled at a temperature of 650°C or above, and then continued. In annealing, after heating and holding for 4 minutes or less in the two-phase region of austenite + ferrite, it is heated to a temperature range of 350 to 450℃ for 1 to 5 minutes.
First, when quenching to hold for 30 minutes,
600℃ from the above holding temperature at a cooling rate of ℃/sec or less
By slowly cooling to the ~Ar ₁ transformation point and then rapidly cooling to a temperature of 350 to 450°C at a cooling rate of 30°C/sec or more, the volume fraction is 15% or more of bainite and the remainder is ferrite, residual austenite, and marten. This method is characterized by obtaining a composite tissue consisting of sites. The present invention will be explained in detail below based on examples. First, the components of steel targeted by the method of the present invention and the reason for limiting the range thereof will be shown. C: C is an essential element for strengthening steel, and as described below, when properly controlling hot rolling conditions and annealing conditions, it stabilizes austenite and increases the volume percentage of austenite to 10% after heat treatment.
A minimum of 0.15% is required to maintain this level. On the other hand, if it exceeds 0.45%, the retained austenite volume fraction increases and the strength-ductility balance improves, but the spot weldability, which is the main aim of the present invention,
Even if the amounts of Si and Mn are properly regulated, they will still deteriorate, so 0.45% was set as the upper limit. Si: Since Si is a ferrite former element, it does not have the function of stabilizing austenite by itself. However, in order to purify the ferrite that is generated during the maintenance of the two-phase region of austenite + ferrite or during cooling from the austenite region or the two-phase region of austenite + ferrite, it is necessary to
It contributes to the stabilization of austenite through the effect of promoting the concentration of C in untransformed austenite.
Furthermore, in the present invention, Si is regulated from a more important viewpoint. In other words, when the amount of Si is small, the upper limit of the amount of C that satisfies spot weldability is 0.2%, but if Si is contained in excess of 1.5%, the upper limit of C increases to 0.45%, and is the strength −
Good ductility balance. From this point of view, the amount of Si is set to exceed 1.5%, and on the other hand, if it exceeds 2.0%, these effects are saturated and the scale properties deteriorate, so the upper limit is set to 2.0%. Mn: Mn is important as an austenite forming element, and from the viewpoint of obtaining a good strength-ductility balance, a minimum content of 1.1% or more is required in order to have a retained austenite volume fraction of 10% or more. Furthermore, Mn is regulated from the viewpoint of improving spot welding properties due to Si. In high C-Si steel with a relatively large amount of C, which is the target of the present invention, austenite is stabilized, so martensitic transformation is less likely to occur during the cooling process after spot welding, and a certain amount of Si
When containing Mn and Mn, on the contrary, the spot weldability becomes better. From this point of view, Si + Mn should be 2.8% or more,
4.0% or less. Note that the upper limit of the amount of Mn is regulated due to necessity. Furthermore, the regulation ranges for Si and Mn are illustrated in FIG. 1. S: Since S deteriorates workability, it is desirable to have as little S as possible. In particular, since the steel targeted by the present invention has a small distribution coefficient of S during solidification, the amount of sulfide inclusions is greater than that of ordinary steel. Therefore, it is necessary to regulate S to an even lower level, which is 0.005% or less. solAl: solAl is effective as a deoxidizing agent for steel, but if its content is less than 0.01, no deoxidizing effect can be expected. On the other hand, if the content exceeds 0.06%, the deoxidizing effect will be saturated and no further effect can be expected, so it was limited to 0.01 to 0.06%. In addition, each of the above components is an essential component that should be regulated in the steel targeted by the present invention, but the following P and B
In order to further improve the strength-ductility balance, one type or two or more types of V and V can be contained as necessary. P: Like Si, P is a ferrite former element and further stabilizes austenite through its effect of promoting the concentration of C in untransformed austenite. Therefore, even if the P content is at a normal level, there is no problem in terms of properties such as strength-ductility balance, but if P is contained in an amount of 0.02% or more as required, an even better strength-ductility balance can be obtained. On the other hand, 0.2
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 %. B: B is an element that improves hardenability and is advantageous in obtaining a desired structure without adding expensive elements such as Cr. In other words, when B is contained in an amount of 0.0005% or more, the hardness of the martensite produced is increased, and the necessary strength can be obtained with a small martensite volume fraction, so it is possible to increase the ferrite and austenite volume fraction that improves ductility. . The lower limit is based on the amount that can produce the effect,
In addition, the upper limit was limited to 0.0005 to 0.01% because the effect reaches saturation and becomes uneconomical. V: V is originally a precipitation-strengthening element and prevents a decrease in hardness of the heat-affected zone during spot welding, thereby improving spot weldability. Further, when 0.05% or more of V is added, austenite is stabilized and the strength-ductility balance is improved. From this perspective, the amount is regulated,
Set at 0.05-0.4%. After soul forming or continuous casting, the steel having the chemical composition shown above is hot rolled at a temperature higher than the Ar ₃ transformation point, and coiled at 650°C or higher, preferably 650 to 700°C.
In particular, it is necessary to set the coiling temperature to 650°C or higher in order to spheroidize the carbides in the hot rolled steel sheet and to stably obtain a large amount of retained austenite during subsequent continuous annealing. In the subsequent continuous annealing, first, ₄
This is held for a minute or less, but as a result, the spheroidized carbide becomes austenite as a nucleus, and the degree of concentration of C, Mn, etc. in the austenite phase is increased. The subsequent cooling mode is 30℃/30℃ from the above holding temperature.
Slow cooling from 600℃ to Ar ₁ transformation point (Tq) at a cooling rate (C 1 ) of sec or less, then to a temperature of 350 to 450℃ ( _{T 2} ) at a cooling rate (C ₂ ) of 30℃/sec or more. Cool quickly and hold for 1-5 minutes. These annealing conditions, in combination with the hot rolling conditions described above, appropriately control ferrite transformation and bainite transformation, resulting in a 15%
The above bainite content ensures that the amount and stability of retained austenite favorable for ductility is ensured. (Example) Thirteen kinds of test steels having chemical components as shown in Table 1 were melted. Test steel B, E, F, G, K, L
and M satisfy the scope of the present invention, and the others are comparative steels. Each steel is hot-rolled at a hot-rolling temperature of 650-700℃,
Furthermore, a test material with a thickness of 1.0 mm was obtained by cold rolling.
Next, after continuous annealing under the conditions of F2 in Section 2 (range of the present invention), a JIS No. 5 tensile test piece with a gauge length of 50 mm was prepared and a tensile test was conducted. In addition, in order to determine the suitability of the structure, the structure was observed and the volume fractions of bainite and austenite were measured. The results are shown in Table 1. As is clear from Table 1, sample steels B, E,
F, G, K, L and M steels subject to the present invention have a TS of 80.
In addition to having high strength of Kgf/mm ² or more, TS×El is also high.
Not only does it have an excellent TS-El balance of over 2300, but its cross tensile strength during spot welding is significantly superior to comparative steels. On the other hand, although the comparative steels do not have a particularly poor TS-El balance (sample steel C is good), they do not have good spot weldability as intended by the present invention. Further, using the above-mentioned test steel F, hot rolling and continuous annealing were performed under the conditions shown in Table 2. Test steels F2, F3, F6, and F7 have conditions within the scope of the present invention, and the others are outside the scope. The results are shown in Table 2. In addition, T ₁ , C ₁ , Tq, in the same table
C ₂ and T ₂ respectively indicate the conditions of the continuous annealing cycle shown in FIG. As is clear from Table 2, the test steel of the invention example
F2, F3, F6, and F7 all contain 15% or more of bainite (and 10% or more of austenite) in their composite structures, and have an excellent TS-El balance of 2300 or more in TS×El. There is. On the other hand, in the sample steel of the comparative example, the bainite transformation does not proceed while the intermediate temperature is maintained, C does not sufficiently transfer to the remaining austenite, and the final austenite amount is small. For this reason, desired properties cannot be obtained. In addition, although the above-mentioned example showed the case where cold rolling was performed after hot rolling, the same effect can be obtained even if cold rolling is not performed.

【table】

[Table] *** See Figure 2 (Effects of the Invention) As detailed above, while the conventional high ductility high strength composite structure steel sheet deteriorates spot weldability due to high carbon content, the present invention For example, Si
The steel has a specific composition with +Mn regulated in the range of 2.8 to 4.0%, and is hot-rolled and continuously annealed under specific conditions to obtain a composite structure containing 15% or more of bainite. Well balanced with ductility
It becomes possible to accurately and inexpensively produce a composite structure steel plate having a high strength of 80 Kgf/mm ² or more and excellent spot weldability, and is particularly suitable for producing steel plates for automobiles.

[Brief explanation of the drawing]

Figure 1 shows the amount of Si in steel to which the method of the present invention is applied.
FIG. 2 is a diagram showing the relationship between the Mn content and the heat cycle conditions for continuous annealing in an embodiment of the present invention.

Claims (1)

[Claims] 1. C: 0.15 to 0.45%, Si: more than 1.5% and 2.0% or less, and Mn≧1.1%, and Si+
Mn: 2.8 to 4.0%, and S≦0.005%,
For steel slabs containing solAl: 0.01~0.06%, with the remainder consisting of iron and unavoidable impurities, hot rolling is completed above the _Ar3 transformation temperature, coiled at a temperature of 650°C or above, and then continued. In annealing, after heating and holding for 4 minutes or less in the two-phase region of austenite + ferrite, it is heated to a temperature range of 350 to 450℃ for 1 to 5 minutes.
First, when quenching to hold for 30 minutes,
600℃ from the above holding temperature at a cooling rate of ℃/sec or less
By slowly cooling to the ~Ar ₁ transformation point and then rapidly cooling to a temperature of 350 to 450°C at a cooling rate of 30°C/sec or more, the volume fraction is 15% or more of bainite and the remainder is ferrite, residual austenite, and marten. A method for producing a highly ductile, high-strength composite structure steel sheet with excellent spot weldability, which is characterized by obtaining a composite structure consisting of sites. 2 In terms of weight percentage, C: 0.15 to 0.45%, Si: more than 1.5% and 2.0% or less, and Mn≧1.1%, and Si+
Mn: 2.8-4.0%, further P: 0.02-0.20%,
Contains one or more of V: 0.05 to 0.40% and B: 0.0005 to 0.01%, and further S≦0.005
%, solAl: 0.01 to 0.06%, and the balance consists of iron and unavoidable impurities, hot rolling is completed at a temperature of _Ar3 transformation temperature or higher, coiling is performed at a temperature of 650°C or higher, and then In continuous annealing, after heating and holding in the two-phase region of austenite + ferrite for 4 minutes or less, when rapidly cooling to hold in the temperature range of 350 to 450 °C for 1 to 5 minutes, first, cooling at 30 °C / sec or less Slowly cool from the above holding temperature to 600℃ ~ Ar ₁ transformation point at a speed of 30℃.
By rapidly cooling to a temperature of 350 to 450℃ at a cooling rate of ℃/sec or more, bainite becomes 15% by volume.
A method for producing a high-ductility, high-strength composite-structure steel sheet with excellent spot weldability, characterized in that the remainder is a composite structure consisting of ferrite, retained austenite, and martensite.