JP3601387B2 - High-strength hot-rolled steel sheet excellent in workability and fatigue characteristics and method for producing the same - Google Patents

Description

【００４５】
【表２】

【００４６】
【表３】

【００４７】
【表４】

【００４８】
【発明の効果】
本発明によれば、熱延鋼板の組織が主相である低Ｃ化により延性に富んだベイナイトと、固溶強化により該ベイナイトとほぼ均一な硬度としたフェライトを有し、かつＰ，Ｓｉの添加量の調整により鋼板表面の凹凸が抑制されるので、高強度で伸びフランジ性に優れ、母材、打ち抜きせん断面の疲労強度にも優れた高強度熱延鋼板が得られ、自動車用足廻り部材のようにプレス時の断面形状が複雑な部位に用いることが可能で、産業上、極めて有用である。
【図面の簡単な説明】
【図１】母材の疲労強度比（母材疲労強度／引張強さ）に及ぼす鋼板表面の最大高さＲｙの影響を示す図。
【図２】母材の疲労強度比（母材疲労強度／引張強さ）を鋼板表面の平均粗さＲａで整理した結果を示す図。
【図３】打抜き穴付き疲労試験片、母材疲労試験片の外観形状を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics suitable for materials for automobile undercarriage members, and in particular, high-strength hot-rolling excellent in stretch flangeability and fatigue characteristics of a base material and a punched shear part. It is related with a steel plate and its manufacturing method.
[0002]
[Prior art]
From the viewpoint of improving the safety of automobiles and improving fuel efficiency that leads to environmental conservation, there is a strong demand for high-strength and thin-walled hot-rolled steel sheets for automobiles. In particular, since weight reduction of wheels and suspension members is extremely effective in improving the fuel efficiency of automobiles, there is a strong demand for reducing the strength and thickness of steel materials used for these members. Since the wheel and the suspension member are a complex shape and a security strength member, it is required to have excellent fatigue characteristics of the base material and the punched shear portion as well as workability, particularly stretch flangeability.
[0003]
Japanese Patent Application Laid-Open No. 9-31534 discloses a ferritic martensitic composite steel sheet having a low N content by adding low Si, Ti and a small amount of Nb. This technology improves the surface properties by lowering the Si, secures the base metal fatigue strength by adding Ti, and secures ductility and stretch flangeability by adding low N, Ti, and a small amount of Nb, It has been proposed to secure ductility and fatigue strength of the punched shear by generating martensite.
[0004]
However, recent undercarriage members have a more complicated cross-sectional shape at the time of pressing in order to compensate for a reduction in rigidity due to thinning, and the required stretch flangeability has become increasingly severe. For this reason, in a mixed structure containing very hard martensite, the hardness difference from ferrite is large, and when subjected to stretch flange processing, voids are easily generated at the boundary between ferrite and martensite, and stretch flangeability is satisfied. I can't do it.
[0005]
In JP-A-9-137249, Ti and Nb are added so as to satisfy a predetermined relationship with C, and the metal structure is composed of ferrite as a main phase and bainite having a predetermined area ratio and martensite or residual having a predetermined area ratio. A steel sheet having a mixed structure made of austenite, a steel sheet surface roughness Ra ≦ 1.5 μm, and a ferrite grain size of the steel sheet surface layer ≦ 5 μm has been proposed.
[0006]
In this technology, Ti and Nb added so as to satisfy a predetermined relationship with C are precipitated as carbides in the ferrite to ensure workability and base metal fatigue strength, and by making the above-mentioned mixed structure, ductility and stretch flangeability are achieved. Has been proposed to ensure the fatigue strength of the base metal by defining the surface roughness and the ferrite grain size of the steel sheet surface layer. However, since martensite exists and retained austenite also undergoes martensite transformation during stretch flange processing, the recent severe stretch flange processing cannot be satisfied.
[0007]
[Problems to be solved by the invention]
The present invention has been made in view of the above points, and its object is to provide a portion having a complicated cross-sectional shape at the time of pressing, such as an automobile suspension member, and which requires excellent workability, particularly stretch flangeability. An object of the present invention is to provide a high-strength hot-rolled steel sheet that is used and excellent in fatigue characteristics of a base material and a punched shear portion.
[0008]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the present inventors diligently studied the influence of the metal structure and the surface properties of the steel sheet on the workability and fatigue characteristics, and obtained the following knowledge.
[0009]
(1) In order to improve stretch flangeability without impairing ductility with high strength, the metal structure is made as low as possible, the main phase is bainite, and solid solution strengthened ferrite is in an appropriate volume ratio.
[0010]
(2) For improving the base metal fatigue strength, it is effective to add P and Si so as to satisfy a predetermined relationship.
[0011]
(3) Avoiding pearlite or coarse carbides is effective in improving the fatigue strength of the punched shear surface.
[0012]
The present invention has been made by further studying these findings.
[0013]
1. In mass%, C: 0.02 to 0.05%, Si: 0.3 to 1%, Mn: 1.3 to 2.3%, P: 0.1% or less, S: <0.001% , Cr: 0.05 to 0.7%, Mo: 0.05 to 0.5% and satisfying the formula (1), having a bainite and ferrite structure, and having a bainite volume ratio of 60 A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics characterized by being -95%.
[0014]
(P-0.02) / Si> 1/60 (1)
However, P and Si are mass%.
[0015]
2. Furthermore, it is 1% containing 1 type or 2 types or more of Ti: 0.01-0.06%, Nb: 0.01-0.03%, V: 0.01-0.08% by the mass%. High strength hot rolled steel sheet.
[0016]
3. Further, the high-strength hot-rolled steel sheet according to 1 or 2, wherein the maximum height Ry of the steel sheet surface is ≦ 20 μm.
[0017]
After finishing and rolling the steel having the chemical composition described in 4.1 or 2 at a rolling end temperature Ar3 or higher, the steel is cooled to 500 to 600 ° C. at a cooling rate of 35 to 65 ° C./S, and then cooled to 2 to 20 ° C. / The method for producing a high-strength hot-rolled steel sheet according to any one of 1 to 3, which is wound at 300 to 475 ° C. after cooling with S.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The chemical component, metal structure and production conditions of the present invention will be described in detail.
[0019]
[Chemical composition]
C: 0.02-0.05%
C forms bainite and is effective for strengthening steel, and 0.02% or more is added to form bainite. However, if added over 0.05%, the bainite is hardened, ductility is lowered, the difference in hardness between ferrite and bainite is increased, and stretch flangeability is impaired, so the content is made 0.02 to 0.05% or less.
[0020]
Si: 0.3 to 1%
Si enhances the strength of the solid solution by strengthening the ferrite while suppressing the decrease in ductility, reduces the hardness difference between bainite and ferrite, and improves the stretch flangeability, so 0.3% or more is added. However, if added over 1%, the ferrite volume fraction becomes too high, and it is out of the structure ratio required for high strength and high stretch flangeability, so 0.3 to 1% is added.
[0021]
Mn: 1.3 to 2.3%
Mn is added in an amount of 1.3% or more in order to strengthen the steel. However, if added over 2.3%, segregation is likely to occur during casting, and the structure of bainite and ferrite after hot rolling becomes band-like and non-uniform, reducing workability, so 1.3 to 2.3%. .
[0022]
P: 0.1% or less P is important in the present invention, and has the effect of increasing the allowable amount of Si that can reduce the unevenness of the scale-base metal interface that affects the base metal fatigue strength. Strengthens by solid solution strengthening, and stretch flangeability is improved by reducing the hardness difference from bainite. However, if it exceeds 0.1%, cracks occur during hot rolling, so 0.1% or less.
[0023]
S: <0.001%
S is important for improving stretch flangeability, and is made less than 0.001% in order to reduce sulfide as much as possible and avoid voids generated during stretch flange processing.
[0024]
Cr: 0.05-0.7%
Cr suppresses the deterioration of workability and the formation of pearlite that lowers the fatigue strength of the punched shear part, and by strengthening the solid solution of ferrite, the hardness difference from bainite is reduced and the stretch flangeability is improved. Add at least 05%. In order to further improve the fatigue strength and stretch flangeability of the punched sheared portion, it is desirable that the content be 0.2% or more. However, if added excessively, martensite is generated, and the stretch flangeability is extremely lowered.
Note that the fatigue strength of the punched shear part is estimated to decrease due to the formation of voids around cementite and coarse carbides in extremely hard pearlite during processing, and stress concentration as microcracks appearing on the end face. The
[0025]
Mo: 0.05-0.5%
Mo suppresses the deterioration of workability and the formation of pearlite which lowers the fatigue strength of the punched shear part, and also strengthens ferrite to reduce the hardness difference from bainite and improve stretch flangeability. Add at least 05%. In order to further improve the fatigue strength and stretch flangeability of the punched sheared portion, it is desirable to set it to 0.1% or more. However, even if added over 0.5%, the effect is saturated and the cost is increased, so the content is made 0.05 to 0.5%.
[0026]
(P-0.02) / Si> 1/60
However, P and Si are mass%.
[0027]
This parameter defines the amount of P and Si added to suppress the unevenness of the steel sheet surface that reduces the fatigue strength of the base metal. When P and Si are added to satisfy this parameter, Unevenness on the surface of the steel sheet caused by an increase in the unevenness of the scale-base metal interface is suppressed.
[0028]
One or more of Ti, Nb, and V These elements have the effect of refining the ferrite produced from austenite after hot rolling and reducing the hardness difference between ferrite and bainite, thus further improving stretch flangeability In some cases, one or more are added. Addition amounts are Ti: 0.03% or more and less than 0.06%, Nb: 0.01% or more and less than 0.03%, V: 0.01% or more and less than 0.08%. The lower limit of the addition amount of each element obtains the effect, and the upper limit is specified because the effect is saturated and the cost is increased.
[0029]
[Metal structure]
In the present invention, in order to ensure the metal structure with high strength and high stretch flangeability, the metal structure is defined to have a bainite and ferrite structure and satisfy a bainite volume ratio of 60 to 95%. By making a bainite structure having a hardness difference from ferrite smaller than martensite as a main phase having a volume ratio of 60 to 95% and including a ferrite phase, a structure having high strength and good workability is obtained.
[0030]
That is, bainite effective for increasing the strength is used as the main phase, but the hardness is reduced by lowering C. And the hardness of a ferrite is increased by solid solution strengthening, the hardness difference of a bainite and a ferrite is made small, and stretch flangeability is improved.
[0031]
In the present invention, “bainite and ferrite” is defined as a structure excluding martensite and retained austenite, bainite includes granular bainitic ferrite and bainitic ferrite, and ferrite includes polygonal ferrite and pseudopolygonal ferrite. Shall be included.
[0032]
In the present invention, it suffices if the main structure is composed of bainite and ferrite, and a structure other than bainite and ferrite is allowed in the range in which the effect can be obtained.
[0033]
[Surface properties]
The higher the strength of the steel sheet, the higher the notch sensitivity. The unevenness on the surface of the steel sheet acts in the same way as the notch, stress concentrates in the recesses, and reduces the fatigue strength of the steel sheet (base material). Therefore, the maximum height Ry in the surface roughness is set to 20 μm or less. FIG. 1 shows the influence of the maximum height Ry of the steel sheet surface on the fatigue strength ratio (base metal fatigue strength / TS). When the maximum height Ry is within 20 μm, the fatigue strength ratio (base material fatigue strength / TS) is improved.
[0034]
In the present invention, the maximum height Ry is used as a guide for the surface property of the steel sheet because fatigue failure is caused by the stress concentration part from one place. In the case of the average roughness Ra expressed by averaging the roughness of the steel sheet surface, there are cases where Ry is large and Ra is small. Therefore, as shown in FIG. 2, the average roughness Ra is not a guideline for fatigue strength. Further, the surface roughness is determined according to JISB0601.
[0035]
[Production conditions]
Finish rolling end temperature: Ar3 or higher The finish rolling end temperature is set to Ar3 or higher in order to ensure ductility. If it is less than Ar3, finish rolling is completed with a two-phase structure of ferrite and austenite, so that the processed ferrite remains and the ductility is impaired.
[0036]
Cooling conditions: After finish rolling, after cooling at 500 to 600 ° C. at a cooling rate of 35 to 65 ° C./S, subsequently at 2 to 20 ° C./S, the cooling and cooling conditions adjust the volume fraction of ferrite and the ductility of bainite or Specified to improve stretch flangeability. When the stop temperature of the first stage cooling after finish rolling is higher than 600 ° C., even if the cooling rate is 65 ° C./S, the volume fraction of ferrite exceeds 40% in the subsequent cooling pattern, and lower than 500 ° C., Even if the cooling rate is 35 ° C./S, the volume fraction of ferrite is less than 5%.
[0037]
Further, in the subsequent cooling, if the cooling rate is higher than 20 ° C / S, the dislocation in the bainite is not sufficiently released and the ductility is lowered. If it is lower than 2 ° C / S, the agglomeration of carbides in the bainite proceeds and becomes coarse. Stretch flangeability is impaired. Accordingly, in the present invention, the cooling conditions are defined so that after finish rolling, the cooling is performed at a cooling rate of 35 to 65 ° C./S to 500 to 600 ° C., and then the cooling is performed at 2 to 20 ° C./S.
[0038]
Winding temperature: 300-475 ° C
The coiling temperature is set to 300 to 475 ° C. in order to obtain a bainite structure. When the temperature is lower than 300 ° C., martensite is generated, and the stretch flangeability is lowered. When the temperature is higher than 475 ° C., segregation of P becomes remarkable and both ductility and stretch flangeability are deteriorated. In order to obtain even more excellent stretch flangeability, it is desirable that the winding temperature be 400 ° C. or lower.
[0039]
【Example】
After heating the steel which has a chemical component shown in Table 1 to 1180-1280 degreeC, it was set as the hot-rolled steel plate of plate thickness 3.6mm using the manufacturing conditions shown in Table 2. In Table 2, finish rolling finish temperature (FT), cooling rate in first stage cooling after finish rolling (CR1), stop temperature in first stage cooling (T1), cooling rate after T1 (CR2), and The winding temperature (CT) is assumed.
[0040]
After pickling from the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece (rolling vertical direction), a hole expansion test piece, a base material fatigue test piece, and a fatigue test piece with punched holes (both in the rolling vertical direction) were collected. In the hole expansion test, a test piece having a hole punched at a clearance of 12.5% by a punch of 10 mmφ at the center of a 130 mm square steel plate is pushed up by a 60 ° conical punch from the opposite direction on the burr side of the punch hole. did.
[0041]
A base metal fatigue test piece and a fatigue test piece with punched holes are shown in FIG. The punched holes were punched with a clearance of 15% using a 10 mmφ punch. The fatigue test was a one-sided tensile fatigue test, and the fatigue strength was the maximum stress that would not break at 10 ⁷ times repeated loading. Table 3 shows the results of these tests. As for the base metal fatigue strength, the fatigue limit ratio (fatigue strength / tensile strength) is also shown. In Table 3, V fB indicates a bainite volume fraction (%).
[0042]
As for the test results, TS × El of 12000 or more is good (evaluation ○) for ductility, and less than 12000 is bad (evaluation ×), and the hole expansion rate is good when evaluation is 80 or more (evaluation ○), less than 80 For the base metal fatigue strength, a fatigue strength ratio of 0.70 or more is good (evaluation ○), and less than 0.70 is a failure (evaluation ×). A strength of 190 or higher was evaluated as good (evaluation ◯), and a strength of less than 190 was determined as poor (evaluation x).
[0043]
From Table 3, the hot-rolled steel sheet according to the present invention has excellent results in all of ductility, stretch flangeability, base metal fatigue strength, and punched shear portion fatigue strength. The above characteristics are inferior.
[0044]
[Table 1]

[0045]
[Table 2]

[0046]
[Table 3]

[0047]
[Table 4]

[0048]
【The invention's effect】
According to the present invention, the hot rolled steel sheet has a bainite rich in ductility due to low C, which is the main phase of the structure, and a ferrite having a substantially uniform hardness with the bainite by solid solution strengthening, and P and Si. By adjusting the addition amount, unevenness on the surface of the steel sheet is suppressed, resulting in a high-strength hot-rolled steel sheet with high strength, excellent stretch flangeability, and excellent fatigue strength of the base metal and punched shear surface. It can be used for a part having a complicated cross-sectional shape at the time of pressing, such as a member, and is extremely useful industrially.
[Brief description of the drawings]
FIG. 1 is a diagram showing the influence of the maximum height Ry of a steel sheet surface on the fatigue strength ratio (base material fatigue strength / tensile strength) of a base material.
FIG. 2 is a diagram showing a result of arranging the fatigue strength ratio (base material fatigue strength / tensile strength) of the base material by the average roughness Ra of the steel sheet surface.
FIG. 3 is a diagram showing the external shape of a fatigue test piece with punched holes and a base metal fatigue test piece.

Claims (4)

In mass%, C: 0.02 to 0.05%, Si: 0.3 to 1%, Mn: 1.3 to 2.3%, P: 0.1% or less, S: <0.001% , Cr: 0.05 to 0.7%, Mo: 0.05 to 0.5%, and steel satisfying the formula (1), having a bainite and ferrite structure, and having a bainite volume ratio of 60 A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics characterized by being -95%.
(P-0.02) / Si> 1/60 (1)
However, P and Si are mass%. Furthermore, by mass%, Ti: 0.01-0.06%, Nb: 0.01-0.03%, V: 0.01-0.08% of 1 type or 2 types or more are contained. The high-strength hot-rolled steel sheet described in 1. The high-strength hot-rolled steel sheet according to claim 1, wherein the maximum height Ry of the steel sheet surface is ≦ 20 μm. The steel having the chemical component according to claim 1 or 2 is finish-rolled at a rolling finish temperature Ar3 or higher, and then cooled to 500 to 600 ° C at a cooling rate of 35 to 65 ° C / S, and thereafter, a cooling rate of 2 to 20 ° C / The method for producing a high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the steel sheet is wound at 300 to 475 ° C after being cooled with S.

Images