JPH0312131B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing high-strength steel sheets with good ductility and workability, and particularly to a low-cost method for manufacturing high-strength steel sheets having a tensile strength of 60 Kgf/mm ² or more. In recent years, high-strength thin steel sheets with a tensile strength of 60 kgf/mm ² or more have been increasingly used for strength members such as bumpers and door guard bars from the viewpoint of vehicle safety and weight reduction. Materials used in such applications are required to have high tensile strength, good ductility and workability, and good spot weldability when assembling a vehicle body. Recently, mixed-structure steel sheets consisting of ferrite and low-temperature transformation products mainly consisting of martensite or bainite have been widely used as steel sheets that satisfy these requirements. However, in order to increase the strength of conventional mixed structure steel sheets, Mn,
It is necessary to add large amounts of elements such as Si, Nb, and Ti, resulting in increased costs and
Addition of large amounts of Mn, Si, etc. not only increases manufacturing costs, but also tends to cause surface oxidation during continuous annealing, resulting in poor spot weldability. In addition, when a large amount of Mn or the like is contained, a band-like structure develops, which is thought to be caused by its segregation, and there is a problem in that workability such as bending, in particular, and local ductility deteriorate. An object of the present invention is to provide a method for manufacturing a high-strength steel plate that has both ductility and good workability, and is inexpensive to manufacture, solving the problems of the prior art described above. This object of the present invention is achieved by the following two inventions. The gist of the first invention is as follows. That is, C: 0.15% or less, Mn:
0.2 to 3.5%, P: 0.01 to 0.15%, Al: 0.10% or less, and the remainder consists of Fe and inevitable impurities, in a method for producing a high-strength steel plate, the steel plate being heated to a soaking temperature below the Ac ₃ transformation point. At least
The heating rate in the section from 600℃ to the Ac ₃ transformation point is 5
The average cooling rate in the temperature range of 600 ° C to 300 ° C for the step of heating at ℃ / sec or more, the soaking step of holding at the soaking temperature for 10 seconds to 10 minutes, and the cooling after the soaking step is as follows. A method for producing a high-strength steel sheet with good ductility and workability, the method comprising: cooling at a critical cooling rate CR (°C/sec) or higher calculated by equation (1). . logCR (℃/sec) = -1.73 [Mn (%) + 3.5P (%)] + 3.95 ... (1) Next, the gist of the second invention is that C, Mn, which is the same as the first invention, In addition to having the basic composition of , P, and Al, Si: 0.1-1.5% Cr: 0.1-1.0%, Mo: 0.1
Group A consisting of ~1.0%, B: 5 to 100 ppm and
Nb: 0.01~0.1%, Ti: 0.01~0.2%, V: 0.01~
In a method for manufacturing a high strength steel plate containing one or more selected from Group B consisting of 0.2% and the remainder consisting of Fe and unavoidable impurities, the steel plate is soaked at an Ac ₃ transformation point or higher. heating at a heating rate of at least 5°C/sec or more in the section from at least 600°C to the Ac ₃ transformation point;
The soaking process is held at the soaking temperature for 10 seconds to 10 minutes, and the cooling after the soaking temperature is 600~
The average cooling rate in the temperature range of 300℃ is shown below (2)
A method for producing a high-strength steel sheet with good ductility and workability, comprising the step of cooling at a critical cooling rate CR (° C./sec) or higher calculated by the formula. logCR (℃/sec) = -1.73 [Mn (%) +0.26Si (%) +3.5P (%) +1.3Cr (%) +2.67Mo (%)] +3.95 (2) However, in the case of B addition Change 3.95 in equation (2) to 3.40. As summarized above, the present invention relates to the annealing process.
When soaking for 10 seconds to 10 minutes in the temperature range above the Ac ₃ transformation point, the section from 600℃ to the Ac ₃ transformation point is heated more rapidly than the conventionally disclosed heating rate, and This is a method for manufacturing high-strength steel sheets with good ductility and workability by controlling cooling conditions. First, the reason for limiting the components of the high-strength steel sheet of the present invention will be explained. C: C is an important element as one of the basic components of steel, and strength can be increased at low cost by increasing C, but if it exceeds 0.15%, spot weldability will deteriorate rapidly, so the upper limit should not be exceeded. Limited to 0.15%. Mn: Mn is a solid solution strengthening element and at the same time is a particularly important element for the formation of low temperature transformation products. Mn
Although 0.1% or more is required for the purpose of preventing hot embrittlement, the lower limit was set at 0.2% from the viewpoint of melting. Also, Mn
If it exceeds 3.5%, spot weldability deteriorates like C, so the upper limit was set at 3.5%. P: P is a ferrite-forming element that is inexpensive and has a large solid solution strengthening ability, and is advantageous as a reinforcing element.If it is less than 0.01%, the manufacturing cost increases and there is no particular advantage, so the lower limit was set at 0.01%. Next, 0.05%C-1.5%Mn-(0~0.2)%P steel plates were spot welded, and the ductility ratio, shear tensile strength, and cross tensile strength of the welded part were investigated, and the relationship with the P content was determined as follows. Shown in the figure. From Figure 1, P is 0.15%
The upper limit of P was limited to 0.15% because the strength and ductility ratio of the welded part will deteriorate rapidly if the P content exceeds 0.15%. Al: Al is necessary as a deoxidizing element, but if it exceeds 0.10%, it will form alumina clusters and deteriorate the surface quality, and the risk of hot cracking will increase, so the upper limit was limited to 0.10%. The above-mentioned limited amounts of C, Mn, P, and Al are the basic components of the high-strength steel sheet of the present invention, and in addition, Si,
Group A consisting of the elements Cr, Mo, and B, and Nb;
The object of the present invention can also be effectively achieved in a high-strength steel plate containing one or more selected from group B consisting of the elements Ti and V in the following limited amounts. The reasons for limiting these selected elements are as follows. Group A (Si, Cr, Mo, B): As is clear from the above equation (2), the elements of Group A lower the critical cooling rate in the cooling process during annealing necessary for forming a mixed structure, and at the same time This has the effect of increasing the amount of transformation products, resulting in increased strength. In order to effectively demonstrate this effect, Si,
Each element of Cr and Mo is at least 0.1% or more, and B is
5ppm or more is required. However, adding too much will saturate the effect and increase the cost, so the upper limit for Si is 1.5.
%, Cr, Mo are limited to 1.0%, B is limited to 100ppm, Si: 0.1 to 1.5%, Cr: 0.1 to 1.0%, Mo: 0.1, respectively.
~1.0%, B: limited to a range of 5 to 100 ppm. Group B (Nb, Ti, V): The elements Nb, Ti, and V are all carbonitride-forming elements, and are caused by grain refinement, increased strength due to precipitates, or suppression of recrystallization of the ferrite phase. It has the effect of strengthening the material. However, these effects are not fully exhibited when each element is less than 0.01%, so the lower limit was set to 0.01% for each element. Also, if excessive addition occurs, the effect will become saturated and the cost will increase, so the upper limit should be set as Nb.
is 0.1%, Ti and V are 0.2%, and each Nb: 0.01
-0.1%, Ti: 0.01-0.2%, and V: 0.01-0.2%. It should be noted that each element of Group A and Group B has its own effect when used alone, but the effects of each element are not diminished even if they are added in combination. The steel of the present invention having the above-mentioned limited composition is hot-rolled after melting.
Continuously annealed after pickling and cold rolling. Hot rolling can be carried out under normal conditions, but in order to obtain high strength,
Low-temperature winding of 600°C or less is preferred. Furthermore, by controlling heat treatment conditions in a limited manner as described below, high-strength steel plates with good ductility and workability can be manufactured at low cost. Next, the reason for limiting the annealing conditions in the present invention will be explained. The annealing conditions are the most important requirement of the present invention. In order to obtain a steel plate with high strength and excellent ductility, it is advantageous to heat the steel sheet to a temperature above the Ac ₁ transformation point and below the Ac ₃ transformation point, soak it, and rapidly cool it to form a mixed structure of ferrite and martensite. However, Mn
When the amount increases, its segregation causes Ac ₁ transformation point or higher,
Soaking below the Ac ₃ transformation point results in a band-like structure with low workability such as bending and local ductility. On the other hand, when heated in the austenite single phase region above the Ac ₃ transformation point, soaked, and rapidly cooled, the resulting structure becomes a non-band-like mixed structure mainly of ferrite and bainite, and although the ductility decreases slightly,
It is still better than recovery annealed steel, etc., and has high workability such as bending and local ductility. That is, the first

【table】

[Table] The steels with the chemical compositions shown in Table 1 are as shown in Table 2.
The specimen was annealed at a seed temperature and corroded with nital, the micrographs of which are shown in Figures 2A and B. In Fig. 2 A and B, Fig. 2 A was annealed at 725°C, which is above the Ac ₁ transformation point and below the Ac ₃ transformation point, and a strong band-like structure remains, whereas Fig. 2 B
is annealed at 870℃ above the Ac ₃ transformation point,
The band-like tissue has disappeared. As for bendability, as shown in Table 2, the critical bending radius is 6 mm when annealed at 725°C.
It is 0 mm when annealed at 870°C, and annealing at the Ac ₃ transformation point or higher is better. Based on these results, in the present invention, the annealing temperature was limited to the Ac ₃ transformation point or higher to improve workability at the expense of some ductility. Next, regarding the heating rate, it is not difficult to achieve an average heating rate of 5°C/sec or more from room temperature to the soaking temperature in an actual annealing furnace. However, it is a well-known fact, as shown in FIG. 3, that the heating rate of a steel plate decreases as the temperature increases. In FIG. 3, the solid line indicates the steel plate temperature, and the dotted line indicates the average heating rate. Therefore, even if an average heating rate of 5°C/sec or more is achieved when heating a steel plate from a low temperature near room temperature to a soaking temperature above the Ac ₃ transformation point, for example, from 500 to 600°C to the target Ac ₃ Heating rate 5℃/sec in the high temperature range up to the soaking temperature above the transformation point
This tendency becomes more pronounced as the temperature increases. The present inventors focused on the influence of the heating rate in such a high temperature section on the tensile properties after annealing and conducted the following basic experiment. 1.2 of the chemical composition shown in Table 1
The heating rate that seems to be sufficient for a cold-rolled steel plate with a thickness of 600°C using the normal continuous annealing method is about 10°C.
℃/sec, then heated at a temperature of 30℃/sec after soaking at 850℃ for 1 minute until the soaking temperature reaches 850℃, which is a single-phase austenite with an Ac ₃ transformation point or higher. The tensile properties were investigated by short-time annealing by cooling at a cooling rate, and the results are shown in FIG. In FIG. 4, both tensile strength and yield stress increase as the heating rate increases, but elongation hardly decreases. Moreover, since this effect is particularly remarkable when the heating rate is 5° C./sec or higher, the heating temperature in the section from 600° C. to the Ac ₃ transformation point was limited to 5° C./sec or higher. Next, we investigated the starting temperature T _H that requires rapid heating in the high temperature range. That is, a cold-rolled steel sheet having the chemical composition shown in Table 1 is heated at a rate of 10/sec from room temperature to the rapid heating start temperature T _H as shown in Figure 5, and the rapid heating start temperature T _H is changed from this temperature to 850 sec. ℃, rapidly heated at 5℃/sec to 850℃.
After soaking at ℃ for 1 minute, short-time annealing was performed by cooling at 30℃/sec to investigate the tensile strength, and FIG. 6 shows the influence of the rapid heating start temperature T _H on the tensile strength. From Figure 6, 5 in the temperature range of 600℃ or higher.
It is clear that high strength can be obtained without deteriorating ductility by heating at a rapid heating rate of ℃/sec or higher, so the rapid heating start temperature T _H
The temperature was limited to 600℃ or higher. It is obvious that the faster the heating rate is in both the low temperature range and the high temperature range, the better the quality of the material will be obtained. As mentioned above, the reason why the balance between strength and ductility is improved by heat treatment at a heating rate of 5°C/sec or higher in a high temperature range of 600°C or higher to a soaking temperature of Ac ₃ transformation point or higher can be estimated as follows. . That is, when considering the annealing of the steel with limited components of the present invention, the temperature of 600°C approximately corresponds to the recrystallization start temperature of ferrite grains. By increasing the heating rate in the region above that temperature and shortening the residence time between the recrystallization start temperature and the Ac ₁ transformation point after cold rolling, very fine recrystallized grains or complete recrystallization can be achieved. The Ac ₁ transformation point is reached and austenite transformation begins without completing the process. By further heating to a temperature above the Ac ₃ transformation point in a short time, the austenite grain size that exists during soaking becomes smaller, and after cooling, this fine grain size is reduced. As austenite transforms, a microstructure of ferrite and bainite (some of which includes martensite) is finally obtained. It is thought that this microstructural refinement is effective in improving the balance between strength and ductility. As mentioned above, high-strength steel with good ductility is heated at a heating rate of 5°C/sec or more from the recrystallization start temperature of about 600°C to at least the Ac ₃ transformation point, preferably to the soaking temperature. This is one of the important requirements for obtaining In addition, the soaking time must be held for 10 seconds or more to complete the austenite transformation, and holding it for more than 10 minutes will cause the austenite grains to coarsen, so the soaking time should be limited to 10 seconds to 10 minutes. did. The cooling rate after soaking is determined in order to obtain high strength and good ductility. In other words, the cooling rate is the critical cooling rate CR determined by equation (1) or (2) below.
(°C/sec) or higher. (a) When only basic components of C, Mn, P, and Al are contained in limited amounts (first invention) logCR (℃/sec) = -1.73 [Mn (%) + 3.5P (%)] + 3.95... …(1) (b) In addition to the basic components of C, Mn, P, and Al, Si, Cr,
When containing each limited amount of one or more selected from group A consisting of Mo and B and group B consisting of Nb, Ti, and V (second invention) logCR (℃/sec) = - 1.73 [Mn% +0.26Si (%) +3.5P (%) +1.3Cr (%) +2.67Mo (%)] +3.95...(2) However, in the case of B addition, 3.95 of equation (2) is Change to 3.40. The reason for limiting the cooling rate as above is the cooling rate.
Critical cooling rate CR (℃/
If the cooling rate is below CR (℃/
sec) or higher, the structure usually consists of ferrite and bainite (including some martensite), which provides high strength, good ductility, and workability, so the cooling rate can be calculated using equation (1) or (2). The cooling rate was limited to above the critical cooling rate. Next, we specified the cooling rate in the range of 600 to 300℃, when cooling from the soaking temperature,
If the cooling rate between 600℃ and 300℃, which is sufficiently lower than the _M ), critical cooling rate CR determined by equation (2)
(℃/sec) or more. As described above, the present invention limits the basic composition and the composition of selectively added elements, and annealing from 600℃
Heating at a heating rate of 5℃/sec or higher to the Ac ₃ transformation point, and at a soaking temperature of Ac ₃ transformation point or higher for 10 seconds to
By soaking for 10 minutes and then rapidly cooling between 600 and 300℃ at a rate higher than the critical cooling rate determined by equation (1) or (2), fine particles consisting of ferrite and bainite containing some martensite are produced. tissue is obtained,
As a result, it was possible to obtain a high-tensile steel plate with high strength, good ductility, and workability. Example For steel having the four types of components shown in Table 3, the finish rolling temperature was 830 to 870°C, and the coiling temperature was 500 to 550°C.
Hot rolled at ℃, followed by the same as shown in Table 3.
Heating rate from 600℃ to Ac ₃ transformation, soaking temperature,
Annealing was performed under heat treatment conditions such as a cooling rate of 600°C to 300°C. The yield stress (YS), tensile strength (TS), elongation and bendability of these annealed steel plates were investigated, and the results are also shown in Table 3.

【table】

It is shown in [Table]. The bendability was expressed using the critical bending radius shown below. Critical bending radius = bending radius without cracking/plate thickness In Table 3, sample material No. 1 of the invention example, sample material No. 5 of the comparative example, and specimen material No. 2 of the invention example. Sample material No. 7 of the comparative example has the same composition and the same soaking temperature, but the inventive example differs from the comparative example in that the heating rate is 5°C/sec or more, so the strength is improved without deterioration in elongation. It can be seen that the amount can be increased. In addition, sample material No. 1 of the present invention example and sample material No. 1 of the comparative example.
Comparing 6, the soaking temperature is 870℃ and 750℃ respectively.
The example of the present invention, in which the soaking temperature is above the Ac ₃ transformation point, has an excellent critical bending radius of 1;
The comparative example whose Ac is less than ₃ transformation point has a significantly poor bendability of 5. As is clear from the above examples, the high-strength steel plate with good ductility and workability according to the present invention has an Ac ₃ transformation point or higher when annealing a steel plate obtained by hot rolling and cold rolling a steel slab with a limited chemical composition in a conventional manner. When heating to the _soaking temperature of
Soak for 10 minutes and then cool down to 600~
The 300℃ section was cooled at a cooling rate higher than the critical cooling rate CR (℃/sec) determined by the inventors as a function of the steel composition, and the steel structure was changed to a microstructure of bainite containing ferrite and some martensite. Good strength of ductility and workability by 600Kg
We have established a method for manufacturing high-strength steel plates that ensure f/mm ² or higher. Further, the present invention has the advantage that the manufacturing cost is low and the spot weldability is excellent as a characteristic of steel, so that it can be widely used as a strength member for automobiles and the like.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between the P content and the tensile test results of spot welds in the experiment to obtain the present invention, and Fig. 2 A and B are diagrams showing the relationship between the annealing soaking temperature and Ac ₃ in the experiment to obtain the present invention, respectively. Micrographs showing metal structures below the transformation point and above the Ac ₃ transformation point, 3rd
The figure shows the heating and cooling pattern of a steel plate during normal annealing, and shows that the higher the steel plate temperature, the lower the heating rate. Figure 5 is a diagram showing the heating and cooling pattern of the annealing method in the annealing experiment to obtain the present invention, and Figure 6 is a diagram showing the relationship between the heating rate and the tensile test results. FIG. 2 is a diagram showing the relationship between rapid heating start temperature T _H and tensile test results.

Claims (1)

[Claims] 1. C: 0.15% or less, Mn: 0.2 to 3.5 by weight
%, P: 0.01 to 0.15%, Al: _0.10 % or less, and the balance is Fe and inevitable impurities. Both 600℃
The heating rate in the section from to Ac ₃ transformation point was set to 5℃/
The average cooling rate in the temperature range of 600 to 300 ° C for the step of heating at sec or more, the soaking step of holding at the soaking temperature for 10 seconds to 10 minutes, and the cooling after the soaking step is as follows (1) A step of cooling at a critical cooling rate CR (℃/sec) calculated by the formula,
A method for producing a high-strength steel plate with good ductility and workability, the method comprising: logCR (℃/sec) = -1.73 [Mn (%) + 3.5P (%)] + 3.95 ... (1) 2 Weight ratio: C: 0.15% or less, Mn: 0.2 to 3.5
%, P: 0.01 to 0.15%, Al: 0.10% or less,
Furthermore, Si: 0.1~1.5%, Cr: 0.1~1.0%, Mo: 0.1~
Group A consisting of 1.0%, B: 5 to 100 ppm and
Nb: 0.01~0.1%, Ti: 0.01~0.2%, V: 0.01~
In a method for manufacturing a high strength steel plate containing one or more selected from Group B consisting of 0.2% and the remainder consisting of Fe and unavoidable impurities, the steel plate is soaked at an Ac ₃ transformation point or higher. heating at a heating rate of at least 5°C/sec or more in the section from at least 600°C to the Ac ₃ transformation point;
The soaking process is held at the soaking temperature for 10 seconds to 10 minutes, and the cooling after the soaking temperature is 600~
The average cooling rate in the temperature range of 300℃ is shown below (2)
A method for producing a high-strength steel sheet with good ductility and workability, comprising the step of cooling at a critical cooling rate CR (°C/sec) or higher calculated by the formula. logCR (℃/sec) = -1.73 [Mn (%) +0.26Si (%) +3.5P (%) +1.3Cr (%) +2.67Mo (%)] +3.95...(2) However, B is added In this case, change 3.95 in equation (2) to 3.40.