JPH0146583B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is a method for manufacturing high-strength hot-rolled steel sheets with excellent formability, resistance weldability, and fatigue properties, which are suitably used for wheel rims, discs, bumpers, and other automobile parts. Regarding. (Prior Art) As a measure to reduce vehicle weight in order to save fuel consumption, attempts have been made to reduce the size of vehicle bodies and to change materials by using high-strength steel materials. Among these, reducing the weight of wheels is considered to be extremely effective in reducing fuel consumption, and the application of high-strength hot-rolled steel sheets to wheel rims, disks, etc. is being actively studied. For example, in the United States, ferrite and
Martensitic composite structure hot-rolled steel sheets (dual phase steel sheets) have been deemed optimal, and prototype tests are underway. However, problems with material properties have become apparent, and it has not yet been put into practical use. That is, when the above-mentioned dual phase steel plate is applied to wheel discs, there are problems such as cracks occurring at the hole enlarged part during forming, poor formability, especially stretch flangeability, and poor fatigue properties. Not enough. Furthermore, when applied to wheel rims, there is a problem in that the weld heat affected zone becomes soft during flash butt welding, which causes cracks to occur in this area during subsequent molding. (Object of the Invention) The present invention was made in order to solve the problems of the above-mentioned dual phase steel as a high-strength hot-rolled steel sheet applied to wheel rims, disks, etc. , stretch flangeability,
The object of the present invention is to provide a method for manufacturing high-strength hot-rolled steel sheets with excellent resistance weldability and fatigue properties. (Structure of the Invention) The method for producing a high-strength hot-rolled steel sheet with excellent formability, resistance weldability, and fatigue properties according to the present invention includes
After hot rolling a steel consisting of 0.01~0.12% C, 0.1~1.6% Si, and 0.7~2.5% Mn, the balance being iron and unavoidable impurities, the steel is rolled at 5~35℃/sec to a temperature of 700~500℃. The area ratio of bainite is reduced by cooling at an average cooling rate of It is characterized by a composite structure consisting of 5 to 60% polygonal ferrite and bainite. The method of the present invention will be explained in detail below. First, the reason why the chemical components of the steel are limited in the present invention will be explained. C has the effect of strengthening steel and improving hardenability. In order to effectively obtain such effects, it is necessary to add 0.01% or more. However, if too much is added, the hardness will decrease due to decarburization of the joint surface during flash butt welding, and the difference in hardness between the weld line and its vicinity will become large, so the upper limit of the amount added is 0.12
%, preferably 0.09%. Mn is an essential element for compensating for the decrease in strength of steel due to low carbon content and for obtaining a bainitic structure.
In order to effectively obtain such effects, it is necessary to add 0.7% or more in the present invention. That is, when the amount added is less than 0.7%, the required strength and structure cannot be obtained. However, when adding too much, it becomes difficult to melt and the ductility of the steel deteriorates, so the upper limit of the amount added is set at 2.5%. Si promotes the formation of polygonal ferrite,
It is an effective element for obtaining a proper structure, and is also a suitable element for obtaining high strength and high ductility. In order to effectively obtain such effects, the present invention requires addition of 0.1% or more. However, when adding too much, it causes embrittlement of the welded part, that is, an increase in the transition temperature, so the upper limit of the amount added is
The rate shall be 1.6%. In the present invention, in addition to the above-mentioned components, at least one member selected from the group consisting of Nb 0.01 to 0.08%, V 0.02 to 1.5%, Ti 0.01 to 0.08%, and Zr 0.02 to 0.18% is added to the steel as necessary. Seed elements can be added. Nb, V, Ti, and Zr are all effective elements for preventing decomposition of the bainite structure and reduction in hardness in the heat affected zone during flash butt welding. Furthermore, since these elements have a precipitation strengthening effect, they are also effective as auxiliary elements for increasing strength. However, if excessive amounts are added to increase the amount of precipitation strengthening, not only will the ductility of the steel decrease, but the precipitates will re-dissolve in the weld heat affected zone, causing softening. ,
It is desirable that the amount added be within the above range. Furthermore, in the present invention, P 0.02 to 0.15% and/or Al 0.005 to 0.06% can be added to the steel together with the above elements or independently of the above elements. P is an element effective in increasing strength without impairing ductility, and is added in a range of 0.02 to 0.15% in order to effectively obtain this effect. Also,
Al is added as a deoxidizer in a range of 0.005 to 0.06%. Furthermore, in the present invention, at least one member selected from the group consisting of REM 0.005 to 0.1%, Ca 0.0005 to 0.01%, and Mg 0.0005 to 0.01% is added to the steel together with or independently of the above elements. elements can be added. REM (rare earth element), Ca, and Mg each have the effect of rendering inclusions harmless and improving formability through the shape control effect of sulfide. To effectively obtain this effect, REM must be 0.005 to 0.1
%, Ca in the range of 0.0005 to 0.01%, and Mg in the range of 0.0005 to 0.01%. Next, the steel produced by the method of the present invention has the above-mentioned chemical composition, has a steel structure consisting of polygonal ferrite and bainite, and has an area ratio of bainite in the range of 5 to 60%. In this way, according to the present invention, by making the steel structure a ferrite-bainite having bainite in a predetermined area ratio, the steel structure is made to have a ferrite-bainite structure with bainite in a predetermined area ratio. As is clear from FIG. 4 in the example, resistance weldability is excellent, and in particular, there is no softening of the heat affected zone. However, ferrite/bainite steel with a bainite area ratio of over 60% is
Similar to martensitic steel, softening occurs in the weld heat affected zone. Furthermore, as is clear from FIG. 5, the fatigue properties are also excellent. In addition, regarding formability, stretch flangeability is
As shown in FIG. 2, the ferrite according to the present invention
Bainite steel is superior to both ferrite-martensitic steel and ferrite-pearlite steel, and furthermore, when looking at the strength-elongation balance, as shown in Figures 1 and 6, even the same ferrite-bainite steel has Steels produced by the process of the invention with a bainite area ratio in the range of 5 to 60% are superior to ferrite-bainitic steels with a bainite area ratio of more than 60%. That is, according to the present invention, by setting the area ratio of bainite in the ferrite-bainite steel to a range of 5 to 60%, it is possible to particularly improve the strength-elongation balance and to prevent softening in the weld heat affected zone. It can be prevented. In addition, the bainite referred to in the present invention refers to lower bainite and upper bainite (B, B) that involve a carbide precipitation reaction, as well as B, Widmanstätten, or ashkyula ferrite that involves a carbide reaction. This shall also include organizations that do not. Next, the manufacturing method of the present invention will be explained in detail. In the method of the present invention, a steel slab having the above-mentioned chemical composition is hot rolled according to a conventional method. After completion of this hot rolling, first, in order to generate ferrite with a desired area ratio, the temperature is 700 to 500℃, preferably 600℃, at an average cooling rate of 5 to 35%/second (primary Cooling. Cooling rate 5
It is practically technically difficult to make the cooling rate smaller than 35° C./second, and on the other hand, if the cooling rate is faster than 35° C./second, sufficient ferrite will not be produced, so the cooling rate is limited to the above range. Further, the temperature at which the cooling rate is changed is also determined from the same purpose. In other words, at high temperatures of 700°C or higher, ferrite is not sufficiently produced, and at temperatures lower than 500°C,
This is because it leads to a decrease in productivity. Following the above primary cooling, in order to transform untransformed austenite into bainite, cooling is performed to 575-250°C at an average cooling rate higher than the above-mentioned cooling rate and in the range of 25-80°C/sec. (secondary cooling). Then, it is wound at that temperature. If this cooling rate is slower than 25°C/sec, there is a risk that pearlite will appear, while on the other hand, a cooling rate faster than 80°C/sec is difficult to employ in practice. (Effects of the Invention) As described above, according to the method of the present invention, steel having a predetermined chemical composition is hot-rolled, cooled under predetermined conditions, and rolled to form polygonal ferrite and bainite. It has a composite structure of
Compared to conventional dual-phase steel with a ferrite-martensitic composite structure, formability,
In particular, it not only has excellent stretch flangeability, but also excellent welding resistance, particularly no softening in the weld heat affected zone, and furthermore, excellent fatigue properties. Furthermore, in contrast to composite structure steels in which the area ratio of bainite is outside the above range, softening is inevitable in the weld heat affected zone and the elongation-strength balance is poor, according to the steel of the present invention, the welding heat affected zone There is no softening in the affected area and the elongation-strength balance is excellent. (Examples) The present invention will be explained below by giving Examples together with Comparative Examples, but the present invention is not limited by these Examples in any way. Steel with the chemical composition shown in Table 1 is melted and hot rolled (finishing temperature 800-855°C) to a thickness of 3.2 mm, and then heated according to the cooling and coiling conditions shown in Table 2. A rolled steel plate was manufactured. Table 3 shows the results of microscopic measurements and mechanical property measurements of the hot rolled steel sheets thus obtained.
In addition, Figure 1 shows the strength-elongation balance of these hot rolled steel sheets, Figure 2 shows the stretch flangeability, and Figure 3 shows the toughness.
Fig. 4 shows the hardness distribution of the flash butt weld, and Fig. 5 shows the Sienck fatigue test results. Furthermore, for steel R shown in Table 1, after hot rolling,
By varying the cooling rate and coiling temperature, hot-rolled steel sheets made of ferrite-bainite steel with different area ratios of these structures were manufactured, and their mechanical properties were investigated. The results are shown in Figure 6. In the figure, (F+P) indicates ferrite/pearlite steel. The flash butt welding conditions in FIG. 4 are as follows. Flashing distance: 3mm Flashing time: 3 seconds Set-up distance: 3 mm Set-up time: 2/60 seconds Set-up speed: 150 mm/second Base plate shape: Width 30 mm, length 75 mm, thickness
Below 3.2 mm, F indicates pearlite, B indicates bainite, and M indicates martensite. Table 3 and 1
As is clear from the figure, the invention steel (F+5~60

【table】

【table】

【table】

【table】

[Table] %B) is for comparative steel (F+P), comparative steel (F+75~
It has an excellent strength-elongation balance compared to 90% B), and is equal to or better than the comparative steel (F+M). In addition, as shown in Figure 6, the bainite area ratio in ferrite-bainite steel has a large effect on the strength-elongation balance.
By setting the bainite area ratio in the range of 5 to 60%, the strength-elongation balance can be significantly improved. On the other hand, the bainite area ratio is 60%
When it exceeds this, the strength-elongation balance is poor. Moreover, as is clear from FIG. 2, the F+B steel according to the present invention has significantly improved stretch flangeability compared to the F+M steel. Also, regarding toughness,
As shown in FIG. 3, F+B steel has excellent toughness. Next, regarding flash butt weldability, which is a problem when applied to wheel rims, etc., and in particular the hardness distribution of the welded part, as shown in Figure 4, softening (hardness decrease) in the heat affected zone, which is a problem with F + M steel, is a problem. depression)
is not observed in the F+B steel according to the present invention. However, comparative steel A with a bainite area ratio of 75%
-2 shows obvious softening in the weld heat affected zone. Furthermore, as shown in Figure 5, fatigue characteristics, which are a problem when applied to wheel discs, etc., are
The F+B steel according to the invention is superior to the F+M steel.

[Brief explanation of drawings]

Fig. 1 is a graph showing the strength-elongation balance of the invention steel and comparative steel, Fig. 2 is a graph showing the stretch flangeability (hole expansion ratio), and Fig. 3
Figure 4 is a graph showing the toughness, Figure 4 is a graph showing the hardness distribution of the welded part in flash butt welding, and Figure 5 is a graph showing the Sienck fatigue test results, where a is the raw plate and b is the 9% tensile strength. Deformation and 5mm
Diameter punched hole material is shown. FIG. 6 is a graph showing the relationship between bainite area ratio and mechanical properties in ferrite-bainite steel.

Claims (1)

[Claims] After hot rolling a steel consisting of 1% by weight of C 0.01-0.12%, Si 0.1-1.6%, and Mn 0.7-2.5%, the balance iron and unavoidable impurities, the steel is heated at 700-500°C. temperature at an average cooling rate of 5 to 35°C/sec, then cooled at an average cooling rate of 25 to 80°C/sec, and then rolled at a temperature of 575 to 250°C. Production of a high-strength hot-rolled steel sheet with excellent formability, resistance weldability, and fatigue properties, which is characterized by forming a composite structure consisting of polygonal ferrite and bainite with an area ratio of 5 to 60% of bainite. Method. Contains (a) C 0.01-0.12%, Si 0.1-1.6%, and Mn 0.7-2.5% in 2% by weight, and (b) Nb 0.01-0.08%, V 0.02-1.5%, Ti 0.01-0.08%, After hot rolling a steel containing at least one element selected from the group consisting of The area ratio of bainite is reduced by cooling at an average cooling rate of A method for producing a high-strength hot-rolled steel sheet with excellent formability, resistance weldability, and fatigue properties, characterized by forming a composite structure consisting of 5 to 60% polygonal ferrite and bainite. 3. Contains (a) C 0.01-0.12%, Si 0.1-1.6%, and Mn 0.7-2.5% in weight percent, and (b) REM 0.005-0.1%, Ca 0.0005-0.01%, and Mg 0.0005-0.01%. After hot rolling a steel containing at least one element selected from the group consisting of iron and unavoidable impurities, the steel is cooled to a temperature of 700 to 500°C at an average cooling rate of 5 to 35°C/sec. Then, the area ratio of bainite is 5 to 60% by cooling at an average cooling rate of 25 to 80 °C/sec and winding at a temperature of 575 to 250 °C. A method for producing a high-strength hot-rolled steel sheet with excellent formability, resistance weldability, and fatigue properties, which is characterized by having a composite structure consisting of polygonal ferrite and bainite.