JPH06128688A - Hot rolled steel plate excellent in fatigue characteristic and it production - Google Patents

Abstract

PURPOSE:To provide a hot rolled steel plate having superior fatigue strength and suitable for machine structural use to be subjected to repeated load. CONSTITUTION:This steel plate has a composition consisting of, by weight, 0.02-0.08% C, 1.5-2.5% Si, 0.5-2.0% Mn, 0.005-0.06% P, 0.01-0.10% sol.Al, <=0.015% S, either or both of 0.2-1.0% Cr and 0.2-1.0% Mo, <=0.1%, in total, of Nb and/or Ti, and the balance iron with inevitable impurities and also has a dual-phase metallic structure consisting of, by volume ratio, 5-15% martensite and the balance essentially ferrite. Further, the value of [Vickers hardness (HV) of ferrite]/[tensile strength (MPa) of hot rolled steel plate] is regulated to >=0.27. By this method, the hot rolled steel plate excellent in fatigue strength and having 500-800MPa tensile strength can be produced.

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, and is therefore useful as a mechanical structure subjected to repeated loading such as automobile wheels, and its production. 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, and one example thereof 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. Optimal wheel design 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 may be increased to increase the fatigue strength. However, if the tensile strength is increased, the formability of the material
Workability is reduced. As a method for improving workability of high-strength steel, there is, for example, the method disclosed in JP-A-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.

After forming a material having low tensile strength into a wheel,
As a method for increasing the tensile strength and the fatigue strength by heat treatment, there is, for example, the method disclosed in 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 increase the fatigue strength of a product by changing the properties of the raw steel sheet without changing the shape of the product. That is, the optimum condition of the composition of the material hot-rolled steel sheet and the combination of rolling and heat treatment of the steel sheet should be found to enhance the fatigue strength of the product, and at that time, it is premised that no additional process that requires cost and time is added. And

[0006]

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

Therefore, prior to the completion of the present invention, the metal structure has ferrite-pearlite (FP) or ferrite-marnsite (FM), and different strengthening mechanisms (solid solution strengthening, precipitation strengthening, dislocation strengthening, 500-800 MPa reinforced by fine grain strengthening and strengthening by increasing the second phase volume ratio)
As a result of the fatigue test of the grade H hot rolled steel sheet, it was found that the durability ratio was remarkably increased by solid solution strengthening or precipitation strengthening in any hot rolled steel sheet F-P or FM. In addition, dislocation strengthening, fine grain strengthening, and strengthening by increasing the second phase volume fraction
It was also found that the tensile strength increased, but the fatigue strength hardly increased. Therefore, it is effective to lower the C content to reduce the amount of pearlite or martensite and to strengthen the soft ferrite phase by solid solution strengthening and precipitation strengthening.

As a result of further experiments, the inventors of the present invention ensured a high ductility by substantially changing the metal structure to a ferrite-martensite structure, controlled the amount of martensite, and obtained the ferrite Pickers hardness and 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: 1.5 to
2.5%, Mn: 0.5-2.0%, P: 0.005-0.0
6%, sol.Al: 0.01 to 0.10%, S: 0.015
%, Cr: 0.2-1.0% and Mo: 0.2-1.0%
1% or 2%, 1% or 2% of Nb and Ti in total of 0.1% or less, the balance is composed of iron and unavoidable impurities, and martensite is 5 to 15% in volume ratio, and the balance is substantially The ferrite has a composite metal structure, and the value of Vickers hardness (HV) of ferrite / tensile strength (MPa) of hot-rolled steel sheet is 0.27 or more, excellent fatigue characteristics, and tensile strength of 500 to 800 MPa. A hot rolled steel sheet.

(2) C: 0.02 to 0.08% by weight,
Si: 1.5-2.5%, Mn: 0.5-2.0%, P:
0.005-0.06%, sol. Al: 0.01-0.10
%, S: 0.015% or less, Cr: 0.2 to 1.0% and Mo
: 0.2 to 1.0% of 1 or 2 kinds, Nb and Ti 1 or 2 kinds in total of 0.1% or less, and the balance consisting of iron and unavoidable impurities, a steel slab immediately after casting or 1100 ℃
After reheating as described above, hot rolling is carried out, hot rolling is completed at the temperature of the final pass exit side “Ar ₃ −50 ° C.”, and then 400 to 400 ° C. at a cooling rate of 1 to 50 ° C./s. After cooling to 600 ° C, it is wound and martensite is added in a volume ratio of 5-15
%, 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 0.27 or more, which is excellent in fatigue characteristics and tensile strength. A method for manufacturing a hot-rolled steel sheet having a strength of 500 to 800 MPa.

[0011]

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

Si is a solid solution strengthening element and is an alloying element effective in strengthening the soft ferrite phase in which fatigue cracks occur. As a result of strengthening the ferrite phase, its Vickers hardness is increased. To obtain sufficient strength, addition of 1.5% or more is necessary, but excessive addition impairs weldability and surface quality, so the upper limit was made 2.5%.

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%.

In addition to ensuring strength, Cr and Mo are
It is indispensable for obtaining martensite without inhibiting the production 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, when the content of each exceeds 1.0%, the weldability deteriorates and the hardenability increases, so the amount of martensite increases. Therefore, the Cr content is 0.2-1.0% and the Mo content is 0.2.
It was set at ~ 1.0%.

[0015] 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 0.06%.

Al is added as a deoxidizing agent, but excessive addition increases the amount of alumina-based inclusions and deteriorates workability. Therefore, the content of sol. Al is set to 0.01 to 0.10%. Since S forms an inclusion by combining with Mn, it is preferable that S is as small as possible. 0.015% is an allowable upper limit value.

Both Ti and Nb are precipitation strengthening elements, and are alloying elements effective in strengthening the soft ferrite phase in which fatigue cracks occur. However, even if the total content of both exceeds 0.1%, the effect is saturated and it is not economical. Therefore, its content is set to 0.1% or less. Incidentally, in order to further improve the workability, Ca, Zr, or a rare earth element may be added for the purpose of spheroidizing the inclusions.

Next, the constitutional conditions of the invention of the manufacturing method of the present application will be described. When performing hot rolling, it is directly heated after casting or reheated to 1100 ° C. or higher. This is to prevent impurities from completely forming a solid solution and segregating. Also the final pass exit side temperature of hot rolling, many dislocations in the ferrite phase when lower than the temperature of the "Ar ₃ -50 ℃" is introduced. Although dislocation strengthening increases the tensile strength, it is not very effective in increasing the fatigue strength, so that the durability ratio decreases. Therefore, the temperature on the outlet side of the final pass is set to be equal to or higher than the temperature of "Ar ₃ -50 ° C".

Further, after hot rolling, 1 to 400 to 600 ° C.
Winding is carried out immediately after cooling at a cooling rate of 50 ° C./s. Generation of pearlite can be suppressed by cooling to 600 ° C. at 1 to 50 ° C./s. If the cooling rate is less than 1 ° C / s, pearlite is generated, and
When the cooling rate is more 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, untransformed austenite transforms to martensite in the cooling process after winding. Although the tensile strength monotonously increases with an increase in the martensite volume ratio, the fatigue strength saturates or reaches a maximum at a certain value, and the durability ratio does not monotonically increase. Therefore, in order to clarify the effect of the martensite volume fraction Vf (M) and the effect of the ferrite phase hardness Hv (F) on the fatigue strength, Vf (M) and Hv (V) can be changed by changing the winding temperature.
It was subjected to a fatigue test by changing (F).

After the steel slabs having the three chemical compositions shown in Table 1 were reheated to 1150 ° C., the outlet temperature of the final pass was set to 880.
After hot rolling at 300C, 300 ~ 7 at 10C / s after hot rolling
After cooling to 00 ° C., it was immediately wound up to produce a hot rolled steel sheet having a thickness of 6 mm and subjected to a fatigue test. The tensile strength was examined by collecting JIS No. 5 tensile test pieces. The fatigue test was carried out by using an axial force test piece having a parallel portion of 4φ 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 under a load of 5 gf.

[0021]

[Table 1]

FIG. 1 shows the ferrite hardness Hv of each hot rolled steel sheet.
(F), the influence of the cooling stop temperature (winding temperature) on the tensile strength TS, and FIG. 2 shows the influence of Vf (M) and Hv (F) / TS on the fatigue limit and the durability ratio. The change in Hv (F) depending on the winding temperature is different between A and B.
This is because TiC precipitates as TiC by slow cooling near 600 ° C. Vf to obtain a high durability ratio (0.52 or more)
It is understood that (M) should be 5 to 15% and Hv / TS should be 0.27 or more. For that purpose, the winding temperature should be 400 to 600 ° C. Still more preferably, in the case of no Ti, the coiling temperature is preferably 450 to 550 ° C,
In the case of adding Ti, the coiling temperature is preferably 500 to 600 ° C.

[0023]

[Example] Steel having the chemical composition shown in Table 2 was melted in a vacuum melting furnace of 50 kg, and then a slab having a thickness of 60 mm was manufactured by hot forging.
After carrying out the hot rolling and cooling conditions shown in the same table, it was immediately wound into a hot rolled steel sheet having a thickness of 6 mm. JIS from the obtained steel sheet
A No. 5 tensile test piece was sampled and examined for mechanical properties and fatigue strength. Table 3 shows the results of the mechanical properties and the fatigue strength test. The fatigue test was carried out by using a test piece with an axial force of 4φ in the parallel portion and repeating the load at a repetition rate of 20 Hz under load control. The fatigue limit was defined as the stress amplitude at which the number of repeated fractures was 10 ⁷ .
Thus, it can be seen that the hot rolled steel sheet produced by the method of the present invention exhibits a high fatigue limit. 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.

[0024]

[Table 2]

[0025]

[Table 3]

All the test numbers (hereinafter abbreviated as No) 1 to 9 satisfy the claims, and the durability ratios are all 0.5.
3 or more. No. 10 lacked C and the tensile strength TS did not reach 500 MPa. In No. 11, since C is excessive, TS is high instead of the hardness of the ferrite phase and the durability ratio is low. In No. 12, the ferrite phase hardness Hv (F) is small because Si is too low, and therefore Hv (F) / TS
Is low and the durability ratio is low.

No. 13 has too much Si. Fatigue limit is N
o. Although not lower than those of 1 to 9, the addition of 3% is useless because it significantly impairs weldability and surface properties. No. 14 has an excessive amount of Mn, No. 15 has an excessive amount of Cr, and No. 16 has an excessive amount of Mo. Therefore, the martensite volume ratio Vf (M) becomes too high.
Although the ferrite phase is not strengthened despite the increase in S, the durability ratio is low. In No. 17, the final hot rolling temperature was 70 ° C. lower than the Ar ₃ point (about 910 ° C.), and not only the ferrite phase but also the martensite phase were dislocation-strengthened, so that TS was extremely high and the durability ratio was low.

In No. 18, since the cooling rate after the final hot rolling is low, a ferrite-martensite structure is not formed and TS is insufficient. No. 19 has a ferrite-martensite structure because the coiling temperature is too high, and TS is insufficient. In No. 20, since the winding temperature is too low, TS becomes too high, and therefore Hv (F) / TS is low and the durability ratio is low. In addition, in order to cool at a cooling rate of 50 ° C./s, it is necessary to perform quench quenching in water, but it is difficult to stop cooling when the coiling temperature reaches within the scope of the present claims, and it is extremely difficult in practical use.

[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 a graph showing a relationship between a winding temperature, a ferrite phase hardness Hv (F) and a tensile strength TS in a method for manufacturing a hot rolled steel sheet according to the present invention.

FIG. 2 is a graph showing the relationship between the winding temperature and Hv (F) / TS, martensite volume ratio, fatigue limit, and durability ratio in the method for manufacturing a hot-rolled steel sheet of the present invention.

Claims (2)

[Claims] 1. C: 0.02 to 0.08% by weight, Si
: 1.5-2.5%, Mn: 0.5-2.0%, P: 0.0
05-0.06%, sol.Al: 0.01-0.10%,
S: 0.015% or less, Cr: 0.2 to 1.0% and Mo:
0.2 to 1.0% of one or two kinds, Nb and Ti of one or two kinds in total of 0.1% or less, the balance consisting of iron and inevitable impurities, and martensite in a volume ratio of 5 ~ 1
It has a composite metal structure of 5% and the balance is essentially ferrite, and the value of Vickers hardness (HV) of ferrite / tensile strength (MPa) of hot-rolled steel sheet is 0.27 or more, which is excellent in fatigue properties. A hot-rolled steel sheet having a tensile strength of 500 to 800 MPa. 2. C: 0.02 to 0.08% by weight, Si
: 1.5-2.5%, Mn: 0.5-2.0%, P: 0.0
05-0.06%, sol. Al: 0.01-0.10%,
S: 0.015% or less, Cr: 0.2 to 1.0% and Mo:
0.2 to 1.0% of one or two kinds, Nb and Ti of one or two kinds in total of 0.1% or less, and the balance consisting of iron and unavoidable impurities, a steel slab immediately after casting or at 1100 ° C. After reheating as above, hot rolling is carried out, and hot rolling is finished when the final pass outlet temperature is “Ar ₃ −50 ° C.” or higher,
400-600 ° C at a cooling rate of 1-50 ° C / s
After cooling until wound, martensite in volume ratio of 5 to 15
%, The balance has a composite metal structure consisting essentially of ferrite, the value of Vickers hardness (HV) of ferrite / tensile strength (MPa) of hot-rolled steel sheet is 0.27 or more, excellent fatigue properties, A method for manufacturing a hot-rolled steel sheet having a strength of 500 to 800 MPa. [0001]