JPH02428B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a hot rolled steel sheet for wheels such as wheel rims and wheel discs of automobiles etc., which has excellent flash butt weldability and formability. As a measure to reduce vehicle weight and reduce fuel consumption, attempts have been made to reduce the size of vehicle bodies and change materials by using high-strength steel. 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 and discs is being actively studied. For example, in the United States, hot-rolled steel sheets with a composite structure are considered to be the most suitable material for these parts, and prototype tests are underway. However, problems with the material became apparent,
It has not yet been put into practical use. A problem with using high-strength steel plates for wheel rims is the occurrence of cracks during roll forming after flash butt welding, and it is said that the incidence of cracking from the heat-affected zone during forming is as high as 50%. This is the biggest reason why the composite structure type steel has not been put into practical use in the United States despite intense consideration of its application. In view of these circumstances, the present inventors have previously determined that a steel plate having a bainitic structure and an appropriate chemical composition is extremely effective as a steel plate for relatively high strength wheel rims of 60 kg/mm ² or more. I know that. (Patent Application No. 54-171594, etc.) By the way, this steel plate for wheel rims has a strength level of 50 to 60 kg/mm ² depending on the car model (manufacturer), and has excellent ductility during processing, compared to current mild steel. It is desired to be able to stably obtain a low defect rate during processing comparable to that of plate rims. In order to meet such demands, the present invention has been made with the object of providing a hot-rolled steel sheet for wheels that maintains the advantages of the previously proposed bainitic steel and has excellent formability. That is, the hot rolled steel sheet for wheels according to the present invention contains (a) C0.01~0.12%, Mn0.8~2.5%, Si0.01~1.5%, and Cr0.01~1.5% in weight percent; , (b) contains one or more of Nb0.01~0.08%, V0.02~1.5%, and Ti0.01~0.08%, and is a composite structure consisting of polygonal ferrite and bainite, and It is characterized in that the area ratio of bainite is 5% or more and less than 60%. In addition to the above-mentioned elements, the hot-rolled steel sheet according to the present invention can also contain 0.0005 to 0.005% of B, 0.0005 to 0.01% of Ca, 0.0005 to 0.01% of Mg, and 0.005 to 0.005 of rare earth elements. It can contain 0.1% of one or more types. In addition, in the present invention, bainite structure includes the so-called bainite phase, a structure called achiular ferrite, etc., which is metallographically not clearly distinguished from bainite, and can be considered to be substantially the same structure as bainite. This is a collective term for organizations. The present invention will be explained in more detail below. First, regarding the chemical components, C is set at 0.01% or more in order to exhibit the effect of strengthening and improving hardenability. However, if it is too large, 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 is 0.12%, preferably about 0.09%. Mn is an essential element for compensating for the decrease in strength due to lower carbon content and for obtaining a bainitic structure.
If the content is less than 0.8%, the required strength and structure cannot be obtained, while if it exceeds 2.5%, there will be difficulties in melting technology as well as deterioration in ductility. accordingly
It is added in a range of 0.8-2.5%. Si is an effective element for promoting the formation of polygonal ferrite and obtaining an appropriate structure, and is also an element suitable for imparting high strength and high ductility. Therefore, it is necessary to add about 0.01% or more. However, if added in excess, it will cause embrittlement (increase in transition temperature) of the weld, so the upper limit is set at 1.5%. Cr has the function of increasing hardenability and facilitating the formation of bainite structure, and to obtain this effect,
It is necessary to add approximately 0.01% or more. But about 1.5%
The effect reaches almost saturation when it exceeds 1.5%.
is the upper limit. Nb, V and Ti are all effective elements for preventing decomposition of the bainitic structure and reduction in hardness in the heat-affected zone during flash butt welding.
It is an essential element in the present invention. In addition, these elements have a precipitation-strengthening effect, so they have meaning as auxiliary elements for increasing strength, but if they are added in excess to increase the amount of precipitation strengthening, not only will ductility decrease, but heat Nb0.01~0.08%, V0.02~1.5%, Ti0.01~0.
Contain one or more types within the range of 0.08%. In addition to these common actions, Nb influences the transformation behavior of the structure after hot rolling, and is the most effective element for forming the bainite structure. Ti is also effective in controlling the shape of sulfides, which are harmful to ductility, and V is an effective element in making the welding center moderately hard (Hv approximately 25) compared to the base metal. In the present invention, in addition to the above-mentioned components, the following elements can be included as necessary. Rare earth elements (REM), Ca or Mg, render these inclusions harmless through the shape control effect of sulfide.
It has the effect of increasing moldability. In order to effectively obtain this effect, REM must be 0.005-0.1% and Ca 0.0005%.
~0.01%, Mg is added in each range from 0.0005 to 0.01%. These elements may be added alone or in combination of two or more of them. However, if the amount added is too large, these elements will actually impair the cleanliness and worsen the ductility, so it is desirable that the upper limit of the total amount added be about 0.1%. Note that A is contained in an amount of 0.06% or less as a deoxidizing agent during melting. Furthermore, it is desirable to regulate S to 0.02% or less. Next, the structure of the steel of the present invention will be described. In the case of a structure mainly composed of bainite structure, it is recognized in steel with a composite structure of ferrite and martensite. As previously proposed, there is no softening in the weld heat affected zone after flash butt welding, and therefore there is no problem of breakage from this area during subsequent processing into the wheel rim by roll forming, etc. There is. (Japanese Patent Application No. 54-171594, etc.) In the present invention, in order to improve the formability of ferrite + bainite composite structure steel while maintaining the properties of bainite steel, the area ratio of both structures is varied. As a result of the test, the area ratio of bainite was 50, as is known from Figure 1 of the example described later.
When it becomes ~60% or less, the increase in elongation becomes remarkable and the strength-elongation balance improves. On the other hand, in the case of ordinary ferrite-pearlite structure steel that does not contain bainite, the strength-elongation balance decreases and the elongation at yield point becomes extremely large. Therefore, in the steel of the present invention, the area ratio of bainite is set to 5% or more.
It is defined as a ferrite-bainite structure with less than 60%, preferably 10 to 50%. Next, examples of the present invention will be shown together with comparative examples. Example 1 Steel having the composition shown in Table 1 was melted and hot rolled (finishing temperature 850°C) to a thickness of 3.2 mm.
It was cooled at a cooling rate of 10-20°C/second and wound up at 550-610°C. Note that Nos. 10, 15, and 16 were manufactured under hot rolling and winding conditions other than those described above. Table 2 shows the results of measuring the structure, mechanical properties, and toughness and hardness values of these hot rolled sheets after flash butt welding under the following welding conditions. Welding conditions ● Flashing distance: 3 mm ● 〃 Time: 3 seconds ● Setup distance: 3 mm ● 〃 Time: 2/60 seconds ● 〃 Speed: 150 mm/sec ● Base plate shape: 30 mm ^w × 75 mm × 3.2 mm ^t

【table】

【table】

[Table] In addition, various steel plates with a ferrite-bainite structure and different area ratios of both structures were manufactured by changing the hot rolling conditions, cooling rate, and coiling temperature for the steel No. 1 in Table 1, and the machine We investigated the properties of The results are shown in FIG. As is known from Figure 1, the area ratio of bainite (relative to the whole) is 50 to 60%.
When it becomes below, the improvement in elongation becomes remarkable and the strength-elongation balance (TS×E) improves. In addition, in ordinary ferrite-pearlite structure steel that does not contain bainite, the strength-elongation balance deteriorates. Next, hot rolled sheets No. 1, 13, and 15 in Table 2 were examined for the hardness distribution of the welded area after flash butt welding, and a tensile test was conducted on this area to examine the strain distribution and fracture position. The results are shown in FIG. Second
As can be seen from the figure, F-M composite structure steel No.15
In No. 13, the softening (due to the decomposition of the second phase martensite) in the HAZ part is significant, and in No. 13, which is a B-F steel, due to the excessive Nb content, NbC re-dissolves in the HAZ part, resulting in softening as well. Significant. As a result these
In Nos. 13 and 15, fracture occurred at this HAZ portion. On the other hand, in the case of Invention Steel No. 1, there is no softened portion as described above, and the fracture position is also on the base metal side. Example 2 Wheel rims were manufactured using hot-rolled sheets (3 mm thick) of steel No. 1 in Table 1 with various textures by changing hot-rolling conditions, etc., by forming by flat butt welding → roll forming. . Table 3 shows the results.

[Table] As stated above, the present invention is a hot-rolled steel sheet with excellent properties for use in wheel rims.
Furthermore, since it has an excellent strength-elongation balance, it can also be used as molding members other than wheel rims, especially wheel disc materials.

[Brief explanation of drawings]

Figure 1 shows the bainite area ratio, tensile strength (TS), and elongation (E) of ferrite-bainite steel.
Figure 2 shows the hardness distribution after flat butt welding of the inventive steel and comparative steel, and the strain distribution in the tensile test of this part.
It is a figure showing a fracture position.

Claims (1)

[Claims] 1% by weight contains (a) 0.01-0.12% C, 0.8-2.5% Mn, 0.01-1.5% Si, and 0.01-1.5% Cr, furthermore, (b) Nb0 .01~0.08%, V0.02~1.5%, and Ti0.01~0.08%, and has a composite structure consisting of polygonal ferrite and bainite, and the area ratio of bainite is A hot-rolled steel sheet for wheels, characterized in that it has a carbon content of 5% or more and less than 60%. 2. Contains (a) C0.01~0.12%, Mn0.8~2.5%, Si0.01~1.5%, and Cr0.01~1.5% in weight%, and (b) Nb0.01~0.08%, A composite structure containing one or more of V0.02~1.5% and Ti0.01~0.08%, and (c) 0.0005~0.005%, and comprising polygonal ferrite and bainite, and bainite A hot-rolled steel sheet for wheels, characterized in that the area ratio of is 5% or more and less than 60%. 3. Contains (a) C0.01-0.12%, Mn0.8-2.5%, Si0.01-1.5%, and Cr0.01-1.5% in weight%, and (b) Nb0.01-0.08%, (c) contains one or more of V0.02~1.5% and Ti0.01~0.08%, and (c) one or more of Ca0.0005~0.01% and rare earth element 0.005~0.1%, and polygonal ferrite. 1. A hot-rolled steel sheet for wheels, which has a composite structure consisting of bainite and bainite, and has an area ratio of bainite of 5% or more and less than 60%.