JPH06264185A - Hot rolled steel plate excellent in fatigue property and its production - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel plate having excellent fatigue properties and suitable for a machine structure applied with repeated loads. CONSTITUTION:A hot rolled steel plate having a compsn. contg., by weight, 0.02 to 0.08% C, <1.5% Si, 0.5 to 2.0% Mn, 0.02 to 0.20% Ti, 0.005 to 0.06% P, 0.01 to 0.10% sol.Al, <=0.015% S and one or more kinds of 0.2 to 1.0% Cr and 0.2 to 1.0% Mo, and the balance iron with inevitable impurities, having a composite metallic structure in which the volumetric ratio of martensite is regulated to 5 to 15%, and the balance substantial ferrite, furthemore, in which the value of the Vickers hardness (HV) of the ferrite/the tensile strength (MPa) of the hot rolled steel plate is regulated to >=0.28 and having excellent fatigue properties and 500 to 900MPa tensile strength is prepd. In this way, the objective hot rolled steel plate having a high endurance ratio can be obtd.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a very excellent fatigue strength as compared with conventional materials, which makes it useful for machine structures subject to repetitive loading such as automobile wheels, and the production thereof. Regarding the method.

[0002]

2. Description of the Related Art Very many products made of hot-rolled steel sheet have a problem of fatigue strength, one of which is an automobile wheel. In order to improve the durability of the wheel, it is necessary to (1) optimize the shape of the wheel and (2) increase the fatigue strength of the material. The optimal design of the wheel shape has already been done and there is no room for improvement. Therefore, a material approach is needed to achieve the above objectives.

The properties required for hot-rolled steel sheets for machine structures include not only high fatigue strength but also excellent formability and workability. Generally, the fatigue strength corresponds to the tensile strength of the material, and the tensile strength can be increased to increase the fatigue strength. However, if the tensile strength is increased, the formability and workability of the material will decrease. An example of high-strength steel with improved workability is disclosed in Japanese Patent Laid-Open No. 55-38980. In this method, a ferrite-martensite composite steel sheet is used to prevent deterioration of ductility due to high strength. However, the fatigue strength of the steel sheet manufactured by this method is still insufficient.

As a method for increasing tensile strength and fatigue strength by heat-treating a material having low tensile strength after forming a wheel, there is, for example, Japanese Patent Laid-Open No. 2-66116. This method is intended to improve fatigue strength by press-forming a steel sheet containing copper while suppressing carbon to an extremely low amount and then performing press forming, and strengthening precipitation of copper. However, this method requires heat treatment, resulting in high cost and limited applications.

[0005]

The problem to be solved by the present invention is to change the properties of the raw steel sheet without changing the shape of the current product and without adding an extra process that requires cost and labor. By increasing the fatigue strength of the product. That is, in producing a material hot rolled steel sheet with high fatigue strength, what kind of composition is selected,
The point is to find out the optimum rolling condition under which it is manufactured.

[0006]

Generally, ductility decreases in inverse proportion to tensile strength. Therefore, the durability ratio (fatigue strength / tensile strength) may be increased to increase the fatigue strength without lowering the workability. The durability ratio changes depending on the composition and the morphology of the structure.

Therefore, prior to the completion of the present invention, ferrite-pearlite (FP) or ferrite-martensite (FM) is used as the metal structure, and different strengthening mechanisms (solid solution strengthening, precipitation strengthening, dislocation) are provided. Strengthened, fine grain strengthened, strengthened by increasing the volume fraction of the second phase) 50
As a result of performing a fatigue test on a hot-rolled steel sheet of 0 to 900 MPa class, it was found that the durability ratio significantly increases in both the FP and FM hot-rolled steel sheets due to solid solution strengthening or precipitation strengthening. It was also found that the tensile strength is increased but the fatigue strength is hardly increased by the dislocation strengthening, the fine grain strengthening, and the strengthening by increasing the volume fraction of the second phase. Therefore, in order to obtain excellent fatigue strength, it is effective to lower the C content to reduce the amount of pearlite or martensite, and to solid solution strengthen and precipitation strengthen the soft ferrite phase.

As a result of further experiments, the present inventors ensured high ductility by substantially changing the metal structure to a ferrite-martensite structure, controlled the amount of martensite, and increased the Vickers hardness of ferrite. It has been found that controlling the tensile strength ratio of the hot rolled steel sheet produces a hot rolled steel sheet having a high durability ratio. The metal structure is 5
It is preferable that all ferrites except for 15% martensite are ferrites, but if the total volume ratio is 10% or less,
Cementite, pearlite and bainite may be mixed.

The present invention was made based on the above findings, and the gist of the present invention is as follows. (1) By weight%, C: 0.02 to 0.08%, Si: 1.
Less than 5%, Mn: 0.5 to 2.0%, Ti: 0.02 to 0.0.
20%, P: 0.005 to 0.06%, sol.Al: 0.0
1 to 0.10%, S: 0.015% or less, Cr: 0.2 to
1.0% and one or more of Mo: 0.2 to 1.0%, the balance consisting of iron and unavoidable impurities, the martensite being 5 to 15% by volume, and the balance being substantially ferrite. It has a structure and the value of ferrite Vickers hardness (HV) / tensile strength (MPa) of hot-rolled steel sheet is 0.2.
8 or more, excellent fatigue properties, and tensile strength of 500-9
Hot-rolled steel sheet having a pressure of 00 MPa.

(2) C: 0.02 to 0.08% by weight
%, Si: less than 1.5%, Mn: 0.5 to 2.0%, T
i: 0.02 to 0.20%, P: 0.005 to 0.06%,
sol.Al: 0.01 to 0.10%, S: 0.015% or less, Cr: 0.2 to 1.0% and Mo: 0.2 to 1.0%, one or more, the balance being iron and A slab of unavoidable impurities,
Immediately after casting or after reheating to a temperature of 1100 ° C. or higher, hot rolling is performed and the final pass outlet temperature is Ar ₃ −5.
The hot rolling is completed at a temperature of 0 ° C or higher, and then 1 to 50
The martensite has a volume ratio of 5 and is wound up after cooling to a temperature of 550 to 650 ° C at a cooling rate of ° C / s.
It has a composite metal structure of ˜15% and the balance is substantially ferrite, and the value of Vickers hardness (HV) of ferrite / tensile strength (MPa) of hot rolled steel sheet is 0.28 or more, which is excellent in fatigue properties. A method for producing a hot-rolled composite structure steel sheet having a tensile strength of 500 to 900 MPa.

[0011]

[Working] The composition of the hot-rolled steel sheet of the present invention will be described.
C must have a content of 0.02% or more in order to secure the strength required for high-strength steel and to generate martensite. However, even if the content exceeds 0.08%, even if the tensile strength increases due to the increase in the amount of martensite, the amount of increase in fatigue strength is small and, as a result, the durability ratio decreases. The preferred amount of martensite is 5-15
%, So the C content was set to 0.02 to 0.08%.

Si is a solid solution strengthening element and is an alloying element effective in strengthening the soft ferrite phase in which fatigue cracking occurs. As a result of the strengthening of the ferrite phase, its Vickers hardness increases. If the amount of Si added increases, the weldability and surface properties are impaired, so it is necessary to set the upper limit so that it does not exceed 1.5%. It is preferable to add 0.5 to less than 1.5% when the matrix is particularly required to be strengthened by Si, and 1.0 to 1. It is necessary to add less than 5%.

Ti is a precipitation strengthening element and is an element effective for increasing the Vickers hardness of ferrite. The effect cannot be obtained with less than 0.02%. Moreover, even if the content exceeds 0.20%, the effect is saturated and it is not economical. Therefore, its content is set to 0.02 to 0.20%.

Mn is indispensable for obtaining the martensite by suppressing the pearlite transformation in addition to securing the strength. On the other hand, if it exceeds 2.0%, the weldability is deteriorated, and ferrite is not sufficiently generated, resulting in deterioration of workability, which is not preferable. Therefore, the amount of Mn is set to 0.5 to 2.0%.
It is preferably 1.0 to 2.0%.

Cr and Mo not only secure the strength,
It is essential in order to obtain martensite by suppressing the formation of polygonal ferrite and suppressing the pearlite transformation. Therefore, Cr: 0.2% or more and Mo: 0.2% or more are determined. On the other hand, if each exceeds 1.0%, the weldability deteriorates and the hardenability increases, so the amount of martensite increases. Therefore, the Cr content is 0.2 to 1.0% and the Mo content is 0.2%.
It was set to 2 to 1.0%.

[0016] P is effective for strengthening the steel sheet by solid solution strengthening, but if it is too much, workability and toughness deteriorate. Therefore, its content is set to 0.005 to 0.03%.
Al is added as a deoxidizing agent, but excessive addition increases the amount of alumina-based inclusions and reduces workability. Therefore
The sol.Al content was set to 0.01 to 0.10%.

Since S combines with Mn to form inclusions, it is preferable that S is as small as possible. 0.015% is an allowable upper limit value. In addition, in order to further improve the workability, a small amount of Ca, Zr, or a rare earth element may be added for the purpose of spheroidizing the inclusions.

Next, the constituent features of the present invention regarding the manufacturing method will be described. When performing hot rolling, it is heated directly after casting or at a temperature of 1100 ° C. or higher. This is to prevent impurities from completely forming a solid solution and segregating. If the final pass exit temperature of hot rolling is set to less than Ar ₃ -50 ° C, the amount of dislocation-strengthened ferrite, which is not so effective in increasing the fatigue strength, becomes too large and the durability ratio decreases. Therefore, the temperature at the exit side of the final pass was set to Ar ₃ -50 ° C or higher.

Further, after hot rolling, coiling is carried out immediately after cooling to a temperature of 550 to 650 ° C. at a cooling rate of 1 to 50 ° C./s. 1 to 50 up to a temperature range of 550 to 650 ° C
By cooling at a cooling rate of ° C / s, it is possible to suppress the generation of pearlite and further suppress the coarse precipitation of TiC to generate a sufficient amount of ferrite. If the cooling rate is less than 1 ° C / s, pearlite is generated, and
When the cooling rate is higher than ° C / s, it is difficult to control the coiling temperature, and the amount of ferrite produced is insufficient, so that the desired metallographic structure cannot be obtained. In addition, by rapidly winding after cooling to a temperature of 550 to 600 ° C., TiC is precipitated in the generated ferrite and the hardness of the ferrite increases,
As a result, the durability ratio is improved.

Further, untransformed austenite transforms to martensite in the cooling process after winding. If the cooling stop temperature exceeds 650 ° C, TiC becomes coarse, and if the cooling stop temperature is lower than 550 ° C, Ti cannot sufficiently diffuse and precipitation of TiC on the ferrite base is insufficient. Hardness increase is not sufficient. In the present invention, since the Vicker hardness of ferrite is increased by the precipitation of TiC, the winding temperature is important together with the composition. Therefore, in order to clarify the influence of the winding temperature on the fatigue strength, the following tests were conducted.

[0021]

[Table 1]

Three compositions shown in Table 1 (steel types 1, 2,
After reheating the steel slab having 3) to a temperature of 1150 ° C., hot rolling is completed at the exit side temperature of 880 ° C. in the final pass, and after hot rolling 10
After cooling to a temperature range of 500 to 700 ° C at a cooling rate of ° C / s, it was immediately wound up to produce a hot rolled steel sheet having a thickness of 6 mm and subjected to a fatigue test. Of the steel types 1, 2, and 3, only steel type 1 has a composition within the scope of the present invention. The tensile strength is JIS5.
No. 10 tensile test pieces were collected and investigated. Fatigue test is parallel section 4φ
The test was carried out by using the axial force test piece of No. 2 and repeating the load at a repetition rate of 20 Hz and completely swinging. The fatigue limit was defined as the stress amplitude at which the number of repeated fractures was 10 ⁷ . The Vickers hardness of ferrite was measured with a load of 5 gf.

FIG. 1 shows the tensile strength and fatigue limit of each hot rolled steel sheet.
Cooling stop temperature (winding temperature) affecting ferrite hardness
Shows the effect of. Only in steel type A containing Ti, the ferrite hardens at the cooling stop temperature of 550 to 650 ° C, and as a result, the fatigue strength increases. Figure 2 shows the durability ratio and ferrite hardness (H
V) / tensile strength (MPa), the ferrite hardness (HV) / tensile strength (MPa) is 0.28 or more and the durability ratio is 0.60 or more.

[0024]

[Example] Steel having the chemical composition shown in Table 2 was melted in a vacuum melting furnace of 50 kg and then hot-forged to manufacture a slab having a thickness of 60 mm, and the hot rolling and cooling conditions shown in Table 3 were carried out. It was rolled into a 6 mm thick hot rolled steel sheet. From the obtained steel plate J
The IS5 tensile test piece was sampled and examined for mechanical properties. The fatigue test uses an axial force test piece with a parallel section of 4φ and a repetition rate of 2
0Hz load control It was carried out with full swing. Here, the fatigue limit was defined as the stress amplitude at which the number of repeated fractures was 10 ⁷ . Ar ₃ point of the steel type A in about 760 ° C., also the Ar ₃ point in any other steels at 800 ° C. or higher. Thus, it can be seen that the hot-rolled steel sheet manufactured by the method of the present invention exhibits high fatigue strength. It is obvious to those skilled in the art that the scope of the claims of the present invention is not limited by the present embodiment.

[0025]

[Table 2]

[0026]

[Table 3]

Test Nos. 1 to 6 produced by the method of the present invention have a composite structure of ferrite and martensite and have a volume ratio of 5 to 15% martensite, and thus are parameters of workability (tensile strength x elongation). ) The value exceeds 20000. The Vickers hardness / tensile strength of ferrite is 0.2
Since the ferrite was higher than 8 and the ferrite was sufficiently strengthened, a high fatigue strength with a durability ratio of 0.6 or more was exhibited.

Test No. 7, which has a low heating temperature, has a large amount of undissolved Ti during heating, the amount of TiC precipitated in the ferrite after winding is small, the ferrite strengthening is small, and the durability ratio is low. In test number 8 in which the temperature at the exit side of the final pass of hot rolling is low, ferrite is dislocation-strengthened, and the ductility and durability ratio are low. Further, in Test No. 8 having a low cooling rate after hot rolling, ferrite is dislocation-strengthened, and the ductility and the durability ratio are low. Test number 9 having a slow cooling rate after hot rolling and test number 10 having a high cooling stop temperature caused coarse precipitation of TiC, and test number 11 having a low cooling stop temperature caused insufficient precipitation of TiC, and Is not strengthened and the durability ratio is low. Test No. 16 in which the amount of C exceeded the range of the present invention had a large amount of martensite, and the strengthening by martensite, which is not effective for fatigue strength, was too large and the durability ratio was low. Test No. 17, which does not contain Ti, has a low endurance ratio because the ferrite strengthening is small. In the test number 18 with a small amount of Cr and Mo, martensite was not formed and ductility (elongation, tensile strength × elongation) was low.

[0029]

As described in detail above, the hot-rolled steel sheet produced according to the present invention has a high durability ratio, and is therefore most suitable for parts such as wheels around automobiles such as automobiles. The significance is extremely remarkable.

[Brief description of drawings]

FIG. 1 is the tensile strength of steel types 1 to 3 listed in Table 1,
It is a graph which shows the influence of the cooling stop temperature which affects fatigue strength and ferrite hardness.

FIG. 2 is a graph showing the relationship between durability ratio and ferrite hardness (HV) / tensile strength (MPa) in steel types 1 to 3 listed in Table 1.

Claims (2)

[Claims] 1. C: 0.02 to 0.08% by weight, S
i: less than 1.5%, Mn: 0.5 to 2.0%, Ti: 0.0
2 to 0.20%, P: 0.005 to 0.06%, sol.A
1: 0.01 to 0.10%, S: 0.015% or less, C
One or more of r: 0.2 to 1.0% and Mo: 0.2 to 1.0%, the balance consisting of iron and unavoidable impurities, and martensite in a volume ratio of 5 to 15%, and the balance substantially. Has a composite metal structure of ferrite, and further has Vickers hardness (HV) of ferrite / tensile strength (MPa) of hot-rolled steel sheet.
Value is 0.28 or more, the fatigue property is excellent, and the hot-rolled steel sheet has a tensile strength of 500 to 900 MPa. 2. C: 0.02 to 0.08% by weight, S
i: less than 1.5%, Mn: 0.5 to 2.0%, Ti: 0.0
2 to 0.20%, P: 0.005 to 0.06%, sol.A
1: 0.01 to 0.10%, S: 0.015% or less, C
Immediately after casting or after reheating a steel slab containing one or more of r: 0.2 to 1.0% and Mo: 0.2 to 1.0%, and the balance of iron and unavoidable impurities, at a temperature of 1100 ° C. or higher. , Hot rolling is carried out, the hot rolling is completed when the temperature of the exit side of the final pass is equal to or higher than the temperature of Ar ₃ -50 ° C, and subsequently 1 to 50 ° C / s.
It is cooled to a temperature of 550 to 650 ° C. at a cooling rate of 5 to 15 and is wound up after being cooled to a temperature of 550 to 650 ° C.
%, The balance has a composite metal structure consisting essentially of ferrite, and the value of Vickers hardness (HV) of ferrite / tensile strength (MPa) of hot rolled steel sheet is 0.28 or more. A method for producing a hot-rolled composite structure steel sheet having a pressure of 500 to 900 MPa. [0001]