JP4258934B2 - High-strength hot-rolled steel sheet excellent in workability and fatigue characteristics and method for producing the same - Google Patents

Description

【００４３】
【表２】

【００４４】
【表３】

【００４５】
【発明の効果】
以上のように、本発明によれば、加工性、特に伸びフランジ性に優れ、かつ、母材および打抜きせん断部の疲労特性に優れた高強度熱延鋼板およびその製造方法を提供することができる。本発明により提供される熱延鋼板は、特に、厳しい伸びフランジ性の要求される自動車用足廻り部品の材料として、極めて有用である。
【図面の簡単な説明】
【図１】熱延鋼板の鋼板表面最大高さＲｙと疲労強度比（母材疲労強度／ＴＳ）の関係を示すグラフ。
【図２】熱延鋼板の鋼板表面平均粗さＲａと疲労強度比（母材疲労強度／ＴＳ）の関係を示すグラフ。
【図３】（ａ）母材疲労試験片の概略図。
（ｂ）打抜き穴付き疲労試験片の概略図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet that is suitable for a material for an underbody member for automobiles and has excellent workability and fatigue properties, and a method for producing the same.
[0002]
[Prior art]
From the viewpoint of improving the safety of automobiles and improving fuel efficiency leading to environmental conservation, there is a strong demand for hot-rolled steel sheets for automobiles with high strength and thinness. In particular, weight reduction of wheels and suspension members is an extremely effective means for improving the fuel efficiency of automobiles. Therefore, increasing the strength of steel sheets for automobiles for the purpose of reducing the weight of these members has been studied. The characteristics required for steel plates used for wheels and undercarriage members are diverse, and workability, especially stretch flangeability, as well as fatigue strength characteristics of the base metal and punched sheared parts are important because it is used for safety strength members. . For this reason, studies on high-strength hot-rolled steel sheets having both workability and fatigue characteristics have been conducted.
[0003]
For example, Japanese Patent Application Laid-Open No. 9-31534 proposes a ferrite-martensitic composite steel sheet that is made low by adding low Si, Ti, and a small amount of Nb. In the technology disclosed in this publication, the surface properties are improved by using low Si, the base metal fatigue strength is secured by adding Ti, and ductility and stretch flange are added by adding low N, Ti, and a small amount of Nb. It is trying to secure ductility and fatigue strength of the punched sheared part by securing the property and generating martensite. However, with recent suspension members, the cross-sectional shape at the time of pressing has become more complex to compensate for the reduction in rigidity accompanying thinning, and the stretch flangeability required for steel sheets has become increasingly severe. It has become a thing. For this reason, in a steel sheet with a mixed structure containing extremely hard martensite, the hardness difference between ferrite and martensite is large, so voids are likely to occur at the boundary between ferrite and martensite when subjected to severe stretch flange processing. The required stretch flangeability cannot be satisfied.
[0004]
Further, in JP-A-9-137249, Ti and Nb are added so as to satisfy a predetermined relationship with C, and the metal structure has ferrite as a main phase and bainite having a predetermined area ratio and martensite having a predetermined area ratio. Alternatively, a steel sheet having a mixed structure composed of retained austenite, a steel sheet surface roughness Ra of 1.5 μm or less, and a ferrite particle diameter of 5 μm or less on the steel sheet surface has been proposed. In the technique disclosed in this publication, 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 the metal structure is mixed with the above mixed structure. By doing this, ductility and stretch flangeability are ensured, and further, it is attempted to secure the base material fatigue strength by defining the surface roughness and the ferrite grain size of the steel sheet surface layer. However, even in this technique, since martensite is present in the steel sheet, the required stretch flangeability cannot be satisfied. In addition, even when a mixed structure containing retained austenite is used, retained austenite transforms into martensite during stretch flange processing, so that it satisfies the required stretch flangeability as in the case of a mixed structure containing martensite from the beginning. Not be able to.
[0005]
[Problems to be solved by the invention]
As described above, the conventional technology is excellent in workability, meets the demands of strict stretch flangeability required for the materials of recent wheels and suspension members, and is excellent in fatigue characteristics of the base material and punched shear part. It is difficult to provide a high strength hot rolled steel sheet.
[0006]
The present invention has been made in view of such circumstances, and provides a high-strength hot-rolled steel sheet having excellent workability, particularly stretch flangeability, and excellent fatigue characteristics of a base material and a punched shear portion, and a method for producing the same. The purpose is to do.
[0007]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have obtained the following findings (1) to (4).
(1) In order to improve stretch flangeability without impairing ductility with high strength, the C content is lowered as much as possible, the main phase is made a bainite structure, and a solid solution strengthened or precipitation strengthened ferrite structure is contained in an appropriate volume ratio. To be effective.
(2) Strength and stretch flangeability can be further improved by adding Cu in addition to the low C of (1).
(3) In order to improve the base metal fatigue strength, it is effective to contain P so that P and Si satisfy a predetermined relationship.
(4) In order to improve the fatigue strength of the punched shear surface, it is effective to avoid pearlite or coarse carbides.
[0008]
The reason for (1) above is that it is effective to use bainite as the main phase in order to obtain high strength. Further, by lowering C, the hardness of bainite is reduced, the ductility of bainite is improved, and This is because stretch flangeability can be improved by reducing the difference in hardness from the melt strengthened ferrite. The reason for the above (2) is not necessarily clear, but it is thought that by adding Cu, Cu precipitates in the cooling process after hot rolling and strengthens ferrite and improves stretch flangeability. It is done. The reason for the above (3) is that when Si is added to enhance the ductility and stretch flangeability without damaging, the irregularities on the steel sheet surface increase as the irregularities increase at the interface between the scale and the steel. When the stress concentrates in the concave portion, the base metal fatigue strength decreases. However, by incorporating P so as to satisfy a predetermined relationship with Si, the unevenness can be reduced, and the base metal fatigue strength is reduced. This is because it can be prevented. The reason for the above (4) is that when pearlite or coarse carbide is present, voids are generated around cementite and coarse carbide in extremely hard pearlite during punching shearing. This is because stress concentrates and fatigue strength decreases.
[0009]
This invention is made | formed based on the said knowledge, Comprising: The following (1)-(5) is provided.
(1) In mass %,
C: 0.02 to 0.05%,
Si: 0.3 to 1.5%,
Mn: 1.3 to 2.3%
P: 0.1% or less,
S: less than 0.001%,
Cr: 0.05 to 0.7%,
Mo: 1% or less,
Ni: 1% or less, satisfying the relationship of (P−0.02) / Si> 0.01 and Si <3 (Mo + Ni), the balance being Fe and inevitable impurities, and the metal structure being bainite and ferrite A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics, characterized in that the bainite volume fraction is 60 to 95%.
[0010]
(2) In the hot-rolled steel sheet according to (1), further, in mass %,
Ti: 0.01 to 0.08%,
Nb: 0.01-0.05%
V: 0.01 to 0.1%
A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics, characterized by containing one or more of them.
[0011]
(3) In the hot-rolled steel sheet according to (1) or (2), further, in mass %,
Cu: 0.1 to 1.0%
A high-strength hot-rolled steel sheet with excellent workability and fatigue characteristics characterized by containing
[0012]
(4) In the hot-rolled steel sheet according to any one of (1) to (3), the maximum height Ry of the steel sheet surface is 20 μm or less, and excellent workability and fatigue characteristics are provided. High strength hot rolled steel sheet.
[0013]
(5) In producing the hot-rolled steel sheet according to any one of (1) to (4), after casting, after performing hot rolling including finish rolling with a rolling end temperature of Ar ₃ or more, Excellent workability and fatigue characteristics, characterized by cooling to 500-600 ° C at a cooling rate of 30-60 ° C / s, then cooling at a cooling rate of 2-20 ° C / s, and winding at 300-475 ° C Manufacturing method of high-strength hot-rolled steel sheet.
[0014]
In the present invention, bainite and ferrite refer to a structure or phase excluding martensite and retained austenite. More specifically, bainite includes granular bainitic ferrite and bainitic ferrite. Refers to polygonal ferrite and pseudopolygonal ferrite.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be specifically described.
1. Steel composition The hot-rolled steel sheet of the present invention is in mass %, C: 0.02 to 0.05%, Si: 0.3 to 1.5%, Mn: 1.3 to 2.3%, P: 0. 0.1% or less, S: less than 0.001%, Cr: 0.05 to 0.7%, Mo: 1% or less, Ni: 1% or less, and (P-0.02) / Si > 0.01 and Si <3 (Mo + Ni), which consists of the remainder Fe and inevitable impurities, and if necessary, Ti: 0.01 to 0.08%, Nb: 0.01 to 0.00. 05%, V: contains one or more of 0.01 to 0.1%. These components will be specifically described.
[0016]
C: 0.02-0.05%
C has an effect of strengthening steel by forming bainite. However, when the C content exceeds 0.05%, the bainite is hardened and the ductility is lowered, and the hardness difference between ferrite and bainite is increased and the stretch flangeability is impaired. On the other hand, when the C content is less than 0.02%, sufficient bainite is not generated to ensure the strength of the steel sheet. For this reason, C content shall be 0.02-0.05%.
[0017]
Si: 0.3 to 1.5%
Si enhances the strength of the ferrite by solid-solution strengthening and suppresses the decrease in ductility, and improves the stretch flangeability by reducing the hardness difference between bainite and ferrite. However, if the Si content exceeds 1.5%, the ferrite volume becomes too high and deviates from the structure ratio for obtaining high strength and high stretch flangeability, and even if P is contained in the range described later. Unevenness at the scale-base metal interface cannot be reduced. On the other hand, if the Si content is less than 0.3%, the solid solution strengthening necessary for obtaining sufficient stretch flangeability cannot be obtained. For this reason, Si content shall be 0.3 to 1.5%.
[0018]
Mn: 1.3 to 2.3%
Mn has an effect of strengthening steel. However, if the content exceeds 2.3%, segregation occurs during casting, and the structure of bainite and ferrite after hot rolling becomes non-uniform in a band shape, thereby reducing workability. Further, at least 1.3% is necessary to ensure the strength. For this reason, Mn content shall be 1.3 to 2.3%.
[0019]
P: 0.1% or less P increases the range of the amount of Si that can be added without increasing the unevenness of the interface between the scale and the base iron, which affects the fatigue strength of the base metal. It is an important element that increases the strength by solid solution strengthening and decreases the hardness difference between ferrite and bainite and improves stretch flangeability. However, if the P content exceeds 0.1%, cracking occurs during hot rolling, so the P content is 0.1% or less.
[0020]
S: Less than 0.001% It is important to keep S low in order to improve stretch flangeability. In order to reduce sulfides as much as possible and avoid voids generated during stretch flange processing, the S content is limited. Less than 0.001%.
[0021]
Cr: 0.05-0.7%
Cr suppresses the production | generation of the pearlite which reduces workability deterioration and the fatigue strength of a punching shear part. However, if the Cr content exceeds 0.7%, martensite is generated and stretch flangeability is extremely lowered. On the other hand, if the Cr content is less than 0.05%, the effect of suppressing the formation of pearlite and improving stretch flangeability is not sufficient. For this reason, the Cr content is set to 0.05 to 0.7%. In order to obtain a further excellent fatigue strength and stretch flangeability of the punched shear portion, the Cr content is desirably 0.2% or more.
[0022]
Mo: 1% or less Mo suppresses ferrite that is easily formed when Si and P are added, optimizes the structure ratio of ferrite and bainite, and strengthens the generated ferrite by solid solution strengthening to expand flange It is an especially important element in the present invention for improving the properties. However, even if Mo is contained in excess of 1%, the effect is saturated and the cost is increased.
[0023]
Ni: 1% or less Ni, as well as Mo, optimizes the structure ratio of ferrite and bainite and strengthens the solid solution of ferrite, but the effect is saturated and the cost rises even if it exceeds 1%. Is less than 1%.
[0024]
(P-0.02) / Si> 0.01, where P and Si are weight%
This parameter defines the amount of P and Si added to suppress unevenness on the surface of the steel sheet, which 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.
[0025]
Si <3 (Mo + Ni), where Si, Mo, Ni are weight%
This parameter is specified in order to optimize the structure ratio of ferrite and bainite while solidifying and strengthening ferrite. The structure ratio is appropriate by adding Si, Mo and Ni to satisfy this parameter. Can be
[0026]
Ti: 0.01 to 0.08%
Nb: 0.01 to 0.05%
V: 0.01 to 0.1%
Ti, Nb, and V are used to refine ferrite produced from austenite after hot rolling to reduce the hardness difference between ferrite and bainite, and to further improve stretch flangeability. Added. However, when the contents of Ti, Nb, and V are each less than 0.1%, the above effects cannot be obtained, and Ti: more than 0.08%, Nb: more than 0.05%, V: 0.1% In the case of exceeding, these effects are saturated and only increase the cost. Therefore, when adding these, Ti: 0.01 to 0.08%, Nb: 0.01 to 0.05%, V: The range is 0.01 to 0.1%.
[0027]
Cu: 0.1 to 1.0%
Cu suppresses the formation of pearlite to improve the fatigue strength of the punched sheared part, and after winding during hot rolling, when the coil is cooled down, the ferrite is precipitated and hardened, resulting in a hardness difference between bainite and ferrite. Stretch flangeability is improved by reducing. If the Cu content is less than 0.1%, this effect cannot be obtained, and even if the Cu content exceeds 1.0%, the effect is saturated, so the Cu content is set to 0.1 to 1.0%.
[0028]
In order to investigate the influence of Cu on stretch flangeability, the present inventors conducted a hole expansion test on steel containing 0.5% Cu (tensile strength 721 MPa) and steel not containing Cu (tensile strength 719 MPa). I did it. As a result, the hole expansion ratio of steel containing no Cu is 72%, whereas the steel sheet containing Cu is almost equal in tensile strength is 119%. It was confirmed that the property, that is, the stretch flangeability is improved.
[0029]
2. The structure of the metal structure is important in order to obtain a hot-rolled steel sheet with high strength and high stretch flangeability and excellent ductility. To achieve high strength and high stretch flangeability, high strength is required. However, it is effective to contain bainite having a smaller hardness difference from ferrite than martensite as the main phase and to contain ferrite in order to ensure ductility. And the steel plate with high intensity | strength and favorable workability can be obtained by the volume ratio of bainite being 60 to 95%. Therefore, the metal structure is made of bainite and ferrite, and the volume fraction of bainite is 60 to 95%.
[0030]
3. The surface roughness of steel sheets increases the notch sensitivity as the strength increases, but the unevenness on the surface of the steel sheet and the unevenness at the interface between the scale and the ground iron play the same role as the notch, so suppressing this unevenness increases fatigue. It is effective for strengthening. Specifically, the maximum height Ry of the steel sheet surface is preferably 20 μm or less in order to increase the fatigue strength. FIG. 1 shows the relationship between the maximum height Ry of the steel sheet surface and the fatigue strength ratio (base material fatigue strength / TS). FIG. 1 shows that the fatigue strength ratio (base material fatigue strength / TS) is higher when the maximum height Ry of the steel sheet surface is 20 μm or less.
[0031]
Here, the reason why the preferable range of the surface roughness is defined by the maximum height Ry is that if there is a place where stress is concentrated even at one place on the surface of the steel sheet, fatigue fracture occurs from there. FIG. 2 shows the relationship between the average roughness Ra expressed by averaging the properties of the steel sheet surface and the fatigue strength ratio (base material fatigue strength / TS). FIG. 2 shows that the fatigue strength ratio may not increase even when the average roughness Ra is low. This is because the maximum height Ry is large even if the average roughness Ra is small, and the fatigue strength ratio is high. It is thought that there was not.
[0032]
4). Manufacturing method The hot-rolled steel sheet of the present invention is subjected to hot rolling including finish rolling at a rolling end temperature of Ar ₃ or higher after casting, and then cooled to 500 to 600 ° C at a cooling rate of 30 to 60 ° C / s. Then, it can be manufactured by cooling at a cooling rate of 2 to 20 ° C./s and winding up at 300 to 475 ° C.
[0033]
In hot rolling, the finish rolling temperature of finish rolling is set to Ar ₃ points or higher because if the rolling finish temperature is lower than Ar ₃ points, finish rolling ends with a two-phase structure of ferrite and austenite. This is because of remaining and impairing ductility and stretch flangeability.
[0034]
The reason for cooling to 500 to 600 ° C. at a cooling rate of 30 to 60 ° C./s after finish rolling is to adjust the volume fraction of ferrite produced from austenite to more than 5% to less than 40%. When cooling in the range of the cooling rate is higher than 600 ° C., the volume ratio of ferrite becomes 40% or more during the subsequent cooling even when the maximum cooling rate is 60 ° C./s, and the above-described metal structure is obtained. I can't. Further, when the cooling within the range of the cooling rate is performed to a temperature of less than 500 ° C., the volume ratio of ferrite becomes 5% or less and the above-described metal structure cannot be obtained even when the cooling rate is set to the minimum of 30 ° C./s.
[0035]
The reason why the cooling rate is a slow cooling rate of 2 to 20 ° C./s after cooling in the above cooling rate range is to improve the ductility or stretch flangeability of bainite produced from the remaining austenite. When the cooling rate exceeds 20 ° C./s, dislocations in the bainite are insufficiently released, resulting in bainite having a low ductility. When the cooling rate is less than 2 ° C./s, agglomeration of carbides in the bainite. This is because the coarse carbides are generated due to excessive progress and the stretch flangeability is impaired.
[0036]
The reason for setting the coiling temperature to 300 to 475 ° C. is to obtain a bainite structure. Further, if the coiling temperature is lower than 300 ° C., martensite that lowers the stretch flangeability is generated, and if it is higher than 475 ° C., segregation of P becomes remarkable, and both ductility and stretch flangeability are impaired. In order to obtain a further excellent stretch flangeable steel sheet, it is desirable that the coiling temperature be 400 ° C. or lower.
[0037]
【Example】
After casting the steel having the components shown in Table 1, after heating to 1180-1280 ° C. and rough rolling, finish rolling finish temperature FT shown in Table 2, cooling rate CR1 in the first stage cooling, first stage According to the conditions of the cooling stop temperature T1, the cooling rate CR2 in the second stage cooling, and the coiling temperature CT, the hot rolled steel was finished and then cooled and wound to obtain a hot rolled steel sheet having a thickness of 3.6 mm. After pickling the hot-rolled steel sheet, a JIS No. 5 tensile test piece (in the vertical direction of rolling), a hole expansion test piece, a base material fatigue test piece shown in FIG. 3A and fatigue with a punched hole shown in FIG. Specimens (both in the vertical direction of rolling) were collected, and each specimen was subjected to a test to measure a characteristic value.
[0038]
In the hole expansion test to evaluate stretch flangeability, a test piece having a hole punched with a clearance of 12.5% by a 10 mmφ punch at the center of a 130-square steel plate was prepared, and the hole was punched by a 60 ° conical punch. The method was to push up from the opposite side of the burr side. In order to perform a more objective measurement, for the crack determination in the hole expansion test, the thermography during the test is used, the moment when the temperature starts to decrease at the time of crack generation is regarded as crack generation, and the hole expansion ratio λ (= [ (Hole diameter at the time of crack occurrence−initial hole diameter) / initial hole diameter] × 100).
[0039]
3A and 3B are drawings showing the shapes of the base material and the fatigue test piece with punched holes. The punched hole of the fatigue test piece with a punched hole was formed by punching with a clearance of 15% using a 10 mmφ punch. These test pieces were subjected to a unidirectional tensile fatigue test, and the maximum stress that would not break even after repeated loading 10 ⁷ times was defined as fatigue strength.
[0040]
Table 3 shows the values obtained in these tests. Table 3 also shows the fatigue strength ratio (fatigue strength / tensile test) for the base material fatigue strength. Furthermore, in Table 3, TS × El of 12000 or more is good (evaluation ○) for ductility, and less than 12000 is evaluated as bad (evaluation ×), and the hole expansion rate is 100 or more for evaluation of stretch flangeability (evaluation ○). Less than 100 is evaluated as defective (evaluation x), and the fatigue strength ratio of the base metal fatigue strength is 0.70 or more as good (evaluation ○), and less than 0.70 is evaluated as defective (evaluation x). As for the strength, the results of evaluation of hole fatigue strength of 190 or more as good (evaluation o) and evaluation of less than 190 as poor (evaluation x) are also shown.
[0041]
From Table 3, No. of steel composition and manufacturing conditions according to the present invention. 1-5, 10-16, 21-44 are excellent in all of ductility, stretch flangeability, base metal fatigue strength, and punched shear fatigue strength, and are suitable characteristics as materials for automobile suspension members and the like. It turns out that the hot-rolled steel plate which has this is obtained. On the other hand, the manufacturing conditions deviate from the scope of the present invention. Nos. 6-9, 17-20, and steel compositions deviating from the scope of the present invention. The hot rolled steel sheet obtained under the conditions of 45 to 52 is inferior in any of the characteristics.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
[Table 3]

[0045]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a high-strength hot-rolled steel sheet excellent in workability, particularly stretch flangeability, and excellent in fatigue characteristics of a base material and a punched shear portion, and a method for producing the same. . The hot-rolled steel sheet provided by the present invention is extremely useful as a material for automobile undercarriage parts that are required to have severe stretch flangeability.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the maximum steel surface height Ry and the fatigue strength ratio (base metal fatigue strength / TS) of a hot-rolled steel plate.
FIG. 2 is a graph showing the relationship between the steel sheet surface average roughness Ra and the fatigue strength ratio (base material fatigue strength / TS) of a hot-rolled steel plate.
FIG. 3A is a schematic view of a base metal fatigue test piece.
(B) Schematic of fatigue test piece with punched holes.

Claims (5)

% By mass
C: 0.02 to 0.05%,
Si: 0.3 to 1.5%,
Mn: 1.3 to 2.3%
P: 0.1% or less,
S: less than 0.001%,
Cr: 0.05 to 0.7%,
Mo: 1% or less,
Ni: 1% or less, satisfying the relationship of (P−0.02) / Si> 0.01 and Si <3 (Mo + Ni), the balance being Fe and inevitable impurities, and the metal structure being bainite and ferrite A high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics, characterized in that the bainite volume fraction is 60 to 95%. Furthermore, in mass %,
Ti: 0.01 to 0.08%,
Nb: 0.01-0.05%
V: 0.01 to 0.1%
The high-strength hot-rolled steel sheet having excellent workability and fatigue characteristics according to claim 1, wherein one or more of them are included. Furthermore, in mass %,
Cu: 0.1 to 1.0%
The high-strength hot-rolled steel sheet having excellent workability and fatigue characteristics according to claim 1 or 2, characterized by comprising:   The high-strength hot-rolled steel sheet excellent in workability and fatigue characteristics according to any one of claims 1 to 3, wherein the maximum height Ry of the steel sheet surface is 20 µm or less. In producing the hot-rolled steel sheet according to any one of claims 1 to 4, 500 to 600 after casting and hot rolling including finish rolling with a rolling end temperature of Ar ₃ or higher after casting. High strength heat excellent in workability and fatigue characteristics characterized by cooling to 30 ° C. at a cooling rate of 30 to 60 ° C./s, then cooling at a cooling rate of 2 to 20 ° C./s and winding at 300 to 475 ° C. A method for producing rolled steel sheets.

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