JP2840479B2 - Manufacturing method of high strength hot rolled steel sheet with excellent fatigue strength and fatigue crack propagation resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-rolled steel sheet having both excellent fatigue strength and fatigue crack propagation resistance, and more particularly, to a hot-rolled steel sheet suitable for applications such as undercarriage parts of automobiles and structural members of vehicle bodies. The present invention relates to a method for producing a high-tensile hot-rolled steel sheet of 50 kgf / mm ² or more excellent in both fatigue durability limit and fatigue crack propagation resistance.

[0002]

2. Description of the Related Art In recent years,
In the automotive industry, the use of high-tensile thin steel sheets is increasing in order to improve fuel efficiency, and particularly in undercarriage parts and structural members of vehicle bodies, there is an extremely high demand for thinning by increasing the tensile strength.

[0003] However, even if the tensile strength and the proof stress are improved, the fatigue strength, which is an important characteristic in automobiles, is not sufficiently improved.
There were problems such as lowering the resistance to fatigue crack propagation from the structural discontinuity.

Conventionally, techniques for improving fatigue strength have been disclosed in JP-A-58-123823 and JP-A-63-2822.
As seen in No. 40 etc., the ferrite grains in the entire steel sheet or in the surface layer are refined, but the grain refinement has the disadvantage of deteriorating the fatigue crack propagation characteristics. There is a problem of deteriorating fatigue characteristics including welding defects and welding defects.

[0005] Even in other prior arts, there is no technology that can improve both the fatigue endurance limit and the fatigue crack propagation characteristics.

As described above, in the prior art, the improvement in fatigue strength relies exclusively on the refinement of crystal grains, and as a result, there has been a problem that the fatigue crack propagation resistance is deteriorated.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a method for obtaining a high-strength hot-rolled steel sheet having both contradictory characteristics of fatigue durability and fatigue crack propagation resistance. Is what you do.

[0008]

In order to solve the above-mentioned problems, the present inventor has made an attempt to obtain a high-strength hot-rolled steel sheet having both of these contradictory characteristics, that is, excellent fatigue durability limit and fatigue crack propagation resistance. The present invention has been completed here as a result of extensive and intensive studies on the structure, manufacturing conditions, and the like.

That is, according to the present invention, C: 0.03-0.
15%, Si: 0.10 to 1.5%, Mn: 0.10
2.0%, Al: 0.01 to 0.10% and P: 0.0
It contains 3 to 0.15%, and further contains Cu: 0.
03-1.5% and Ni: 0.03-0.5% (however,
14P + Cu <2%) or a steel further containing 10 to 60 ppm of Ca in these Cu and Ni, with the balance being Fe and inevitable impurities, finish rolling in a temperature range of 780 to 900 ° C., After cooling to 680 to 720 ° C. at 40 ° C./sec or more, then air cooling for 2 to 8 seconds, cooling at 40 ° C./sec or more, and winding at a temperature of 500 ° C. or less, the final tissue becomes 5 μm ≦ ferrite grain size ≦ 25 μm, 10% ≦ second phase (martensite and / or bainite and / or retained austenite)
The main object of the present invention is to provide a method for producing a high-strength hot-rolled steel sheet having excellent fatigue strength and fatigue crack propagation resistance, characterized in that a structure controlled to a volume fraction of ≦ 30% is obtained.

Hereinafter, the present invention will be described in more detail.

[0011]

According to the present invention, the final structure of a hot-rolled steel sheet is a second phase (martensite and / or bainite and / or retained austenite) (hereinafter referred to as "second phase").
And the carbon content is regulated to 0.03 to 0.15% so as not to generate a large amount of the second phase, and the lack of strength due to the solid solution strengthening of P and / or the precipitation of Cu The necessary fatigue strength is ensured by supplementing with reinforcement. In addition, by adding an appropriate amount of Ca and spheroidizing the inclusions, high fatigue strength by P and Cu is made more effective, and at the same time, good workability is given. In addition, by controlling the hot rolling finish temperature, cooling rate, and winding temperature, the ferrite grains are controlled to 5 to 25 μm, and at the same time, the volume fraction of the second phase is controlled to 10 to 30%. Thus, the fatigue crack propagation resistance is improved.

First, the reasons for limiting the chemical components of steel in the present invention will be described.

C: C is an element necessary for obtaining the second phase, and for that purpose, 0.03% or more is required. However, if it is added in excess of 0.15%, the volume fraction of the second phase increases, and the fatigue crack propagation resistance deteriorates as described later. Therefore, the C content is in the range of 0.03 to 0.15%.

Si: Si is an element required to increase the static strength, and it is necessary to add 0.10% or more to exhibit its effect. However, if added in excess of 1.5%, red scale is generated to impair the surface properties, and as a result, the fatigue strength as rolled is reduced. Therefore, the Si content is 0.1.
The range is 10 to 1.5%.

Mn: Mn is added in an amount of 0.10% or more to prevent hot brittleness. However, when the content exceeds 2.0%, a large amount of the second phase is generated, so that the Mn content is in the range of 0.10 to 2.0%.

Al: Al is added as a deoxidizing agent, but if it is less than 0.01%, the deoxidizing effect is small, and if it exceeds 0.10%, inclusions increase, and if the tensile strength is increased, fatigue cracking may occur. The starting point. Therefore, the content of Al is set in the range of 0.01 to 0.10%.

P: P is high static strength and fatigue endurance limit ratio (fatigue endurance limit /
It is the most important solid solution strengthening element for imparting tensile strength.
Only by regulating the amounts of C, Si and Mn described above, the strength of the ferrite grains is not sufficient, and a high tensile strength of 50 kgf / mm ² or more cannot be achieved. P is an indispensable element necessary for that, and addition of 0.03% or more is necessary to exhibit solid solution strengthening effective for improving the fatigue durability limit ratio. However, if added in excess of 0.15%, hot workability and weldability deteriorate. Therefore, the P content is 0.04 to
The range is 0.15%.

In addition to the above elements, Cu and Ni can be added in appropriate amounts as necessary, as shown below.
In addition, Ca can be added in an appropriate amount in addition to Cu and Ni.

Cu: Cu is an element that can provide the same effect as P, and thus can be added in addition to P. P improves fatigue strength using solid solution strengthening, whereas Cu improves fatigue strength by precipitating fine precipitates (ε-Cu) during winding after the hot rolling process. is there. These precipitates are significantly smaller than carbides such as cementite in conventional low carbon steels, and therefore have a large dispersion strengthening effect and are effective in improving fatigue strength. To precipitate ε-Cu, 0.03% or more is required.
%, Hot brittleness tends to occur. Therefore, the Cu content is in the range of 0.03 to 1.5%.

However, in the case where P and Cu are added in combination, the effect is saturated when the total amount of 14P + Cu exceeds 2%, so that 14P + Cu <2%.

Ni: Ni is added for the purpose of preventing hot brittleness when Cu is added. The Ni content is required to be approximately 1/3 of the Cu content, and is therefore in the range of 0.03 to 0.5%.

Ca: Ca spheroidizes sulfides as inclusions and reduces stress concentration, so that high fatigue strength can be more effectively achieved by P and Cu, and good cold workability is imparted. 10 ppm or more. However, if it is added in excess of 60 ppm, the content of Ca-based inclusions is increased, so that fatigue strength and cold workability deteriorate. Therefore, the Ca content is 10 to
The range is 60 ppm.

Next, the reasons for limiting the volume fraction of crystal grains and the second phase in the hot-rolled steel sheet obtained by the present invention will be described.

As shown in FIG. 1, from the viewpoint of fatigue strength, the finer the crystal grain, the better, but the lower limit of fatigue crack propagation (ΔK
th) decreases conversely. In a steel having a lower limit of P and Cu specified in the present invention, if the crystal grain exceeds 25 μm, the tensile strength becomes 50 kg / mm ² or less, and the required strength cannot be secured. In addition, 50k tensile strength without P and Cu
In a conventional hot-rolled low carbon steel of g / mm ² or more, crystal grains are refined for higher strength, and ΔKth is at most 12 kg / mm ^3/2 . At least this conventional Δ
In order to achieve the Kth value, it is necessary to set the ferrite grain size to 5 μm or more. Therefore, the ferrite grain size is 5
25 μm.

Next, as for the volume fraction of the second phase (martensite and / or bainite and / or retained austenite), the highest ΔKth is obtained in the range of 10 to 30% as shown in FIG. 10% as hot rolled steel
If the volume fraction is less than 10%, the second phase does not become completely martensite, and if the volume fraction is higher than 30%, the carbon concentration of martensite decreases. Hardness decreases. Decreasing the hardness is not preferable because it reduces the resistance to fatigue crack propagation. Therefore, the volume fraction of the second phase is in the range of 10 to 30%.

The manufacturing conditions in the present invention will be described.

The steel having the above-mentioned chemical composition is melted and cast by a conventional method, and is subjected to hot rolling. The finishing temperature of the hot rolling is set at 780 to satisfy the above-mentioned predetermined conditions for the crystal grain size.
To 900 ° C.

After finish rolling, it is heated up to 680-720 ° C.
After cooling at a temperature of at least C./sec.
Cool at 0 ° C / sec or more. The reason why the air cooling time after reaching 680 to 720 ° C. is 2 to 8 seconds is to make the volume fraction of the second phase 10 to 30% as described above. The reason for setting each cooling rate to 40 ° C./sec or more is that
This is for preventing coarsening of ferrite crystal grains after hot rolling, preventing segregation of P, and changing the second phase to martensite and / or bainite and / or retained austenite.

After the above-mentioned cooling, the film is wound.
In order to maximize the precipitation of u, the temperature is set to 500 ° C. or lower.

In the present invention, as rolled
However, it is also possible to obtain a steel sheet having a volume fraction of the crystal grains and the second phase by adjusting the annealing temperature in the continuous annealing process.

Next, an embodiment of the present invention will be described.

[0032]

EXAMPLES Hot-rolled steel sheets were manufactured from steel having the chemical components shown in Table 1 under the hot-rolling conditions shown in the same table. In addition, No. 1 to N
o. No. 8 is an example of the present invention. 9-No. No. 12 is an example of a conventional low carbon steel. 13-No. 14 is an example of steel whose chemical composition is out of the range of the present invention.

The obtained hot-rolled steel sheet was subjected to a tensile test, a plane bending fatigue test, a fatigue crack propagation test with a stress ratio of 0.1, and the ferrite crystal grain size and the volume fraction of the second phase were measured. The results are shown in Tables 1 and 2 and FIG.

[0034]

[Table 1]

[0035]

[Table 2]

From the tables and figures, the hot-rolled steel sheet of the present invention is
Satisfies tensile strength of 50 kg / mm ² or more, 14P + Cu
It can be seen that within the range satisfying <2%, the durability limit ratio (fatigue limit / tensile strength) shows a higher value than the conventional steel. Further, in the present invention example in which the ferrite crystal grain size and the volume fraction of the second phase are controlled in the ranges of 5 to 25 μm and 10 to 30%, respectively, the lower limit of fatigue crack propagation (ΔKth) is higher than that of the conventional steel. It turns out that it shows.

[0037]

As described in detail above, according to the present invention,
A high strength hot rolled steel sheet with a high fatigue endurance limit ratio and excellent fatigue crack propagation characteristics can be obtained, so there are structural and structural discontinuities such as notches and welds as well as simple smooth members. It can be used for various structural strength members, and the effect is great.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between ferrite crystal grain size, tensile strength (σB), and lower limit of fatigue crack propagation (ΔKth).

FIG. 2 is a graph showing a relationship between a volume fraction (VF) of a second phase and a lower limit of fatigue crack propagation (ΔKth).

FIG. 3 shows tensile strength (σB) and fatigue durability limit ratio (σW / σ).
It is a figure which shows the relationship of B).

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-149646 (JP, A) JP-A-3-82708 (JP, A) JP-A-4-337037 (JP, A). Toshinori Yokomaku, Takafumi Iwai, Yasuo Minokata "Effects of Microscopic Strengthening Mechanism on Fatigue and Fatigue Crack Propagation Properties in Ferritic Steel", Preprints, Academic Lecture Meeting of the Japan Society for Materials Science, Vo 1.39th, P178 −180 (58) Field surveyed (Int.Cl. ⁶ , DB name) C21D 9/46-9/48 C21D 8/02-8/04

Claims (3)

(57) [Claims] C: 0.03% by weight (hereinafter the same)
0.15%, Si: 0.10 to 1.5%, Mn: 0.1 to 0.15%.
10 to 2.0%, Al: 0.01 to 0.10% and P:
Finish rolling of a steel containing 0.03 to 0.15% and the balance consisting of Fe and unavoidable impurities is completed in a temperature range of 780 to 900 ° C, and then 40 to 680 to 720 ° C.
After cooling at a temperature of at least C / sec, and then air cooling for 2 to 8 seconds,
By cooling at a temperature of 40 ° C./sec or more and further winding at a temperature of 500 ° C. or less, the final structure is 5 μm ≦ ferrite crystal grain size ≦ 25 μm, 10% ≦ second phase (martensite and / or bainite and / or bainite and / or A method for producing a high-strength hot-rolled steel sheet having excellent fatigue strength and fatigue crack propagation resistance, characterized in that a structure controlled to a volume fraction of retained austenite ≦ 30% is obtained. 2. The steel according to claim 1, further comprising Cu: 0.03 to 1.5.
% And Ni: 0.03 to 0.5% (however, 14P + Cu
The method of claim 1 containing <2%). 3. The method according to claim 2, wherein the steel further contains 10 to 60 ppm of Ca.