JP3882577B2 - High-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability, and manufacturing method and processing method thereof - Google Patents

Description

【００６５】
【表２】

【００６６】
［実施例２］
表１の鋼種Ａ，Ｆ，ＭおよびＶのスラブを準備し、そのスラブを１２５０℃に加熱後、仕上熱間圧延出側温度８９０℃で熱間圧延した後、巻取温度６３０℃として板厚３．０ｍｍの鋼板を作製した。得られた鋼板からＪＩＳ５号引張試験片を採取し、引張試験を行った。また、固溶Ｃ量を内部摩擦測定により決定した。固溶Ｃ量と、引張試験により測定されたＥｌとの関係を図６に示す。
【００６７】
図６から、固溶Ｃ量を０．００２０％以下とすることで伸びが良好となることが確認される。
【００６８】
［実施例３］
表３に示す鋼を溶解し、仕上げ温度９２０℃、巻き取り温度６１０℃で熱間圧延を行い、板圧１．８ｍｍの熱延鋼板を製造した。得られた熱延鋼板を酸洗後、合金化溶融亜鉛めっきを行った。亜鉛めっき板よりＪＩＳ５号引張試験片および穴広げ試験片を採取し、引張試験および穴広げ試験を実施するとともに、光学顕微鏡および走査型電子顕微鏡（ＳＥＭ）で金属組織を確認し、析出物の成分を透過型顕微鏡（ＴＥＭ）に装備されたエネルギー分散型Ｘ線分光装置（ＥＤＸ）により分析した。その際のＴＳ、Ｅｌ、λの値を表３に併記する。なお、表３中のＡ値、Ｂ値は表１と同様である。
【００６９】
表３に示すように、本発明例であるＮｏ．２７は、合金化溶融亜鉛めっきを行っても優れた伸びおよび伸びフランジ性を示したが、比較例であるＮｏ．２８はＭｏを含有していないため、析出物がＭｏを含まないものとなり、伸びフランジ性が著しく低い値となった。
【００７０】
【表３】

【００７１】
［実施例４］
表４に示す成分組成の鋼を仕上げ温度９１０℃、巻取温度６００℃と６５０℃で熱間圧延を行った。得られた板厚３．２ｍｍの鋼板の組織を確認し、析出物の分析を行った後、鋼板よりＪＩＳ５号引張試験片を作成して引張試験を行った。６００℃巻取材と６５０℃巻取材とでＴＳの差をとり、巻き取り温度依存性を調査した。結果を表４に併記する。なお、表４に示すＮｏ．２９〜３２は、いずれもフェライト組織に本発明の炭化物が析出した組織を有していた。
【００７２】
表４に示すように、Ｂ値が０．００５％以下で上記（２）式を満たすＮｏ．２９〜３１はＴＳの差は小さく、巻取温度依存性が小さいが、上記（２）式を満たさないＮｏ．３２は巻取温度依存性が大きいことが確認された。
【００７３】
【表４】

【００７４】
【発明の効果】
以上説明したように、本発明によれば、成分組成を適切に調整することにより、微細な炭・窒化物が析出したフェライト組織を形成するので、伸びおよび伸びフランジ性に優れた高張力熱延鋼板を提供することができる。したがって、本発明は、自動車用足廻り部材の軽量化に大いに寄与するものである。
【図面の簡単な説明】
【図１】巻取温度とＴＳとの関係を示すグラフ。
【図２】Ｃ−［｛Ｔｉ−（４８／１４）Ｎ｝×（１２／４８）＋（１２／９６）×Ｍｏ］の値とＴＳとの関係を示すグラフ。
【図３】本発明に係る熱延鋼板の加工方法の作業フローの一例を示すフローチャート。
【図４】図３に示した作業を実際に行う装置と鋼板、部材、プレス成形品の流れとの関係を示すブロック図。
【図５】本発明に係る高張力熱延鋼板の金属組織を示す透過型電子顕微鏡写真。
【図６】固溶Ｃ量と、引張試験により測定されたＥｌとの関係を示すグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability suitable for a material for an automobile suspension member, and a manufacturing method and a processing method thereof.
[0002]
[Prior art]
From the viewpoint of improving fuel efficiency leading to environmental protection, there is a strong demand for high-strength and thin-walled hot-rolled steel sheets for automobiles. In particular, the high strength and thinning of wheels and suspension members is an extremely effective means for improving the fuel efficiency of automobiles. Therefore, there is a strong demand for reduction of the strength and thickness of these members. Further, since the wheel and the suspension member have a complicated shape, a material that is excellent in both elongation and stretch flangeability in addition to high strength is required.
[0003]
Conventionally, various hot-rolled steel sheets applied to such automobile members have been proposed. For example, Japanese Unexamined Patent Publication No. 4-329848 discloses a steel sheet having a composite structure having excellent fatigue characteristics and stretch flangeability composed of ferrite and a second phase (one or more of pearlite, bainite, martensite, and retained austenite). Yes. However, since the second phase exists in the constituent elements, the stretch flangeability is not sufficient.
[0004]
Japanese Patent Laid-Open No. 6-172924 proposes a steel sheet having excellent stretch flangeability formed by a bainitic ferrite structure having a high dislocation density. However, since this steel sheet contains a bainitic ferrite structure having a high dislocation density, it cannot be said that the elongation is sufficient.
[0005]
Japanese Patent Laid-Open No. 6-200351 proposes a steel sheet having excellent stretch flangeability, in which most of the structure is polygonal ferrite and the strength is increased by precipitation strengthening and solid solution strengthening centering on TiC. However, in order to increase the tension with generally well-known precipitates used in this steel plate, it is necessary to add a large amount of Ti, and precipitates having large dimensions are likely to be formed, and the characteristics are likely to be unstable. There is a drawback.
[0006]
Japanese Patent Laid-Open No. 7-11382 proposes a steel sheet having an acicular ferrite structure in which fine TiC and / or NbC is precipitated and having excellent stretch flangeability. However, since this steel sheet also has an acicular ferrite structure with a high dislocation density, it is excellent in stretch flangeability but is not sufficiently stretched.
[0007]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and is suitable for a member having a complicated cross-sectional shape at the time of pressing, such as an automobile suspension member, and a high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability, and It aims at providing the manufacturing method and the processing method.
[0008]
[Means for Solving the Problems]
  As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.
  (1) By adding Mo, charcoal / nitride precipitates finely.
  (2) When carbon and nitride are finely precipitated by adding Mo to a ductile structure rich in ductility, excellent elongation and stretch flangeability can be obtained while being high tension.
  (3) When Ti and Nb are added excessively, precipitates become excessive, and both elongation and stretch flangeability deteriorate.
  (4) In order to obtain good elongation, the smaller the amount of dissolved C, the better.
  (5) When C, Ti, and Mo are added so that (C / 12) / {(Ti / 48) + (Mo / 96)} satisfies 0.5 to 1.5, a precipitate containing Ti and Mo Becomes finer.
  (6) The amount of C not bonded to Ti or Mo is0.005Material stability improves at less than%.
[0009]
The present invention has been made based on these findings and provides the following (1) to (11).
[0012]
  (1) By mass%, C ≦ 0.06%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.07 to 0.45%, Ti: 0.03 to 0.10%, Nb ≦ 0.08% included,The balance consists of Fe and inevitable impurities,A high-strength hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized by comprising a ferrite structure in which solid solution C is 0.0020% or less and carbide containing Ti and Mo is precipitated.
[0013]
  (2A high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized in that, in (1) above, C, Ti, and Mo are contained so as to satisfy the following expression (1).
  0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
  However, in said Formula (1), C, Ti, and Mo represent the mass% of each component.
[0014]
  (3A high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized in that in the above (1), C, Ti, N, and Mo are contained so as to satisfy the following expression (2).
  C − [{Ti− (48/14) N} × (12/48) + (12/96) × Mo] ≦ 0.005 (2)
However, in said Formula (2), C, Ti, N, and Mo represent the mass% of each component.
[0016]
  (4) A high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized in that, in (1) to (3) above, the composition further contains Cr ≦ 0.1% by mass.
  (5) Above (1)-(4), A high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized by having a hot-dip galvanized film on the surface.
[0017]
  (6)% By mass, C ≦ 0.06%, Si ≦ 0.3%, Mn: 1-2%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0 0.006%, Mo: 0.07 to 0.45%, Ti: 0.03 to 0.10%, Nb ≦ 0.08% includedThe remainder consists of Fe and inevitable impuritiesSteel is melted and formed into a steel slab by continuous casting. Immediately or after cooling, the steel is heated to 1150 ° C. or higher and then hot-rolled to a finish hot rolling outlet temperature of 850 ° C. or higher. A method for producing a high-strength hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized by winding at a temperature of from 700C to 700C.
[0018]
  (7) The method for producing a high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized in that, in the above (6), the steel contains mass% and further contains Cr ≦ 0.1%.
  (8) From (1) to (5) Processing of a high-tensile hot-rolled steel sheet having a first step of preparing a member made of any one of the steel plates and a second step of pressing the member into a press-formed product having a desired shape Method.
[0019]
  (9)the above(8), The press-formed product is a method for processing a high-tensile hot-rolled steel sheet, which is an automobile part, particularly an automobile suspension member.
[0020]
  (10) From (1) to (5) An automotive part manufactured from the steel sheet according to any of the above.
[0021]
In the present invention, carbon / nitride refers to any of carbide, nitride and carbonitride, or a combination of two or more of these.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be specifically described by dividing it into component composition, metal structure and production conditions.
[0023]
  [Ingredient composition]
  The steel plate in the present invention ismass%, C ≦ 0.06%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006 %, Mo: 0.07 to 0.45%, Ti: 0.03 to 0.10%, Nb ≦ 0.08%Including. Further, the solid solution C is 0.0020% or less.. Hereinafter, the reason why each component is defined in this way will be described.
[0024]
C ≦ 0.06%
C forms carbides and is effective in strengthening steel. However, if added over 0.06%, pearlite is formed, and the elongation and stretch flangeability are impaired.
[0025]
Si ≦ 0.3%
Si is effective as a solid solution strengthening element. However, if added over 0.3%, C discharge from the ferrite is promoted, and coarse iron carbide tends to precipitate at the grain boundaries, and the stretch flangeability deteriorates.
[0026]
Mn: 1-2%
Mn is added at 1% or more to strengthen the steel. However, if added over 2%, segregation occurs, a hard phase is formed, and both elongation and stretch flangeability deteriorate. Therefore, the amount of Mn is made into the range of 1-2%.
[0027]
P ≦ 0.06%
P is effective for solid solution strengthening. However, if added over 0.06%, segregation occurs and elongation and stretch flangeability both decrease, so the content is made 0.06% or less.
[0028]
S ≦ 0.005%
The smaller the S, the better. If it exceeds 0.005%, the stretch flangeability is deteriorated, so the content is made 0.005% or less.
[0029]
Al ≦ 0.06%
Al is added as a deoxidizer. However, if it exceeds 0.06%, both elongation and stretch flangeability deteriorate, so the content is made 0.06% or less.
[0030]
N ≦ 0.006%
The smaller N, the better. If it exceeds 0.006%, coarse nitrides increase, and both elongation and stretch flangeability deteriorate, so the content is made 0.006% or less.
[0031]
Mo: 0.07 to 0.45%
Mo is an important element in the present invention. Addition of 0.07% or more suppresses pearlite transformation, refines the carbon / nitride containing Ti or Ti and Nb, and also forms a carbide and precipitates Mo itself, thereby obtaining high strength. Stretch flangeability is improved. However, if added over 0.45%, a hard phase is formed, and stretch flangeability deteriorates. Therefore, the Mo amount is set to 0.07 to 0.45% or less.
[0032]
Ti: 0.03-0.10%
Ti is an important element in the present invention. By adding 0.03% or more, the effect of carbon / nitride precipitation is obtained, and the elongation and stretch flangeability are improved while being high strength. However, if Ti is added in a large amount exceeding 0.10%, coarse TiC is likely to be generated even if Mo is added together, and stretch flangeability is deteriorated. Therefore, the Ti content is 0.03 to 0.10%.
[0033]
Nb ≦ 0.08%
Nb improves elongation and stretch flangeability while being high strength by precipitation of charcoal and nitride. However, if added over 0.08%, the amount of precipitates becomes excessive, and elongation and stretch flangeability deteriorate, so the content is made 0.08% or less.
[0034]
  In the present invention, by optimizing the steel composition, in particular, the amount of C, Ti, and Mo, as described later, most of the C in the steel is precipitated as fine carbon / nitride. There is very little melt C. In order to obtain good elongation, the smaller the amount of dissolved C, the better. The amount of dissolved C exceeds 0.0020%, and the elongation tends to decrease. More preferably, it is 0.0010% or less.
[0035]
  When the content of C, Ti, and Mo is controlled so that the atomic ratio between C and (Ti + Mo) in the steel is 0.5 to 1.5, carbides containing Ti and Mo precipitate finely. It becomes easy. As a result, excellent elongation and stretch flangeability can be easily obtained while having high tension. Therefore, it is desirable that the value of (C / 12) / {(Ti / 48) + (Mo / 96)} satisfies the following expression (1).
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in said Formula (1), C, Ti, and Mo represent the mass% of each component.
The value of (C / 12) / {(Ti / 48) + (Mo / 96)} more preferably satisfies 0.8 to 1.3.
[0036]
  When the amount of C that does not form a carbide containing Ti and Mo exceeds 0.005%, the material variation tends to increase. Further, Ti is bonded to C by the remainder after bonding with N. Therefore, it is preferable to satisfy the following expression (2).
C − [{Ti− (48/14) N} × (12/48) + (12/96) × Mo] ≦ 0.005 (2)
However, in said Formula (2), C, Ti, N, and Mo represent the mass% of each component.
  This will be explained by experimental results.
  Steel A containing C = 0.050%, Si = 0.21%, Mn = 1.61%, Ti = 0.083%, N = 0.636%, Mo = 0.20% (above (2) C − [{Ti− (48/14) N} × (12/48) + (12/96) × Mo] in the formula (hereinafter referred to as B value) is 0.0073), and C = 0. Steel B containing 040%, Si = 0.03%, Mn = 1.34%, Ti = 0.095%, N = 0.040%, Mo = 0.24% (B value is 0 or less) is used. A hot-rolled steel sheet was prepared. The finishing temperature was 910 ° C. and the winding temperature was changed from 550 to 700 ° C., and the relationship between the winding temperature and the strength was investigated. Strength (TS) was measured with a JIS No. 5 test piece. Thereby, it can be seen whether or not the strength changes when the winding temperature changes in the coil. That is, when the dependency on the coiling temperature is strong, the strength changes remarkably with a small variation in the coiling temperature.
[0037]
The survey results are shown in FIG. As shown in FIG. 1, it can be seen that in steel A in which C (A value) not bonded to Ti and Mo exceeds 0.005%, the winding temperature dependence of TS is remarkable.
[0038]
Next, C = 0.040-0.055%, Si = 0.2%, Mn = 1.6%, Ti = 0.080-0.090%, Mo = 0.10-0.20%, Using N = 0.020% steel, hot rolling was performed at a constant coiling temperature of 650 ° C., and the relationship between the B value and TS was determined. The result is shown in FIG. As shown in FIG. 2, it can be seen that when the B value is 0.005% or more, TS tends to decrease.
[0039]
In addition, since Cr has the effect | action which suppresses a pearlite transformation and improves stretch flangeability, you may add in 0.1% or less of range. The reason why the content is 0.1% or less is that when the content exceeds 0.1%, a hard phase is formed and stretch flangeability is deteriorated.
[0040]
  Also,The balance is Fe and inevitable impurities.
[0041]
[Metal structure]
In the present invention, in order to ensure excellent elongation and stretch flangeability, a ferrite structure is formed in which Ti or Ti and Nb carbon / nitride and Mo carbide are precipitated individually and / or in combination. That is, it is possible to achieve both excellent elongation and stretch flangeability by precipitating an appropriate amount of carbon / nitride in a ferrite structure rich in ductility.
[0042]
As a form in which Ti or Ti and Nb carbon / nitride and Mo carbide precipitate each independently and / or in combination, Ti carbide and Mo carbide precipitate each independently and / or combine and precipitate. Those that do are preferred. In particular, it is desirable that a carbide containing Ti and Mo in which these are combined is precipitated. Since carbides containing Ti and Mo are finely and uniformly dispersed and precipitated, it is easy to achieve both high tension and excellent elongation and stretch flangeability.
[0043]
  Also,As mentioned above,Since the steel sheet of the present invention optimizes the steel composition, especially the amount of C, Ti, and Mo, and precipitates most of the C in the steel as fine carbon / nitride, there is very little solute C.But,In order to obtain good elongation, the amount of dissolved C isThanThe smaller the amount, the better. The amount of solute C exceeds 0.0020%, and the elongation tends to decrease.,Preferably 0.0010% or lessTo.
[0044]
The high-tensile hot-rolled steel sheet of the present invention includes a hot-dip galvanized steel sheet that has a hot-dip galvanized film formed on the surface. Since the high-tensile hot-rolled steel sheet of the present invention has good elongation and stretch flangeability, good stretch and stretch flangeability can be maintained even when a hot dip galvanized film is formed. Here, the hot dip galvanizing is hot dip plating mainly composed of zinc and zinc, and includes those containing alloy elements such as Al and Cr in addition to zinc. The high-tensile hot-rolled steel sheet of the present invention subjected to such hot-dip galvanizing may be subjected to an alloying treatment after plating or as it is plated.
[0045]
[Production conditions]
The hot-rolled steel sheet of the present invention is a steel slab produced by melting a steel having the above-mentioned composition, and immediately or immediately after cooling to 1150 ° C. or higher, and then hot rolling and finishing. It is manufactured by setting the hot rolling outlet temperature to 850 ° C. or higher and winding at 550 ° C. to 700 ° C.
[0046]
  ・ Steel slab is immediately or once cooled and then heated to 1150 ℃ or higher, then hot rolled
  In order to deposit Ti or Ti and Nb charcoal / nitrides and Mo carbides individually and / or in combination after hot rolling,In particular, in order to precipitate a carbide containing Ti and Mo in which Ti carbide and Mo carbide are combined,Since it is necessary to dissolve Ti, Nb and Mo in the slab stage before hot rolling, the steel slab is immediately hot rolled, or once cooled, heated to 1150 ° C. or higher and then hot rolled. In other words, since Ti, Nb and Mo are in solid solution after slab production, the solid solution state is maintained when hot rolling is performed immediately, but coarse precipitates are formed once cooled. It is necessary to re-dissolve Ti, Nb and Mo by heating to 1150 ° C. or higher.
[0047]
-Finishing hot rolling delivery temperature: 850 ° C or higher
In order to ensure elongation and stretch flangeability, the finish hot rolling outlet temperature is set to 850 ° C. or higher. When it is less than 850 ° C., the surface layer becomes coarse particles, and elongation and stretch flangeability are impaired.
[0048]
-Winding temperature: 550 ° C to 700 ° C
In order to obtain a ferrite structure, the coiling temperature is set to 550 ° C to 700 ° C. If the temperature is lower than 550 ° C., the structure has a high dislocation density, so that the elongation is lowered. More preferably, it is 600-660 degreeC. When the coiling temperature is 600 to 660 ° C., the balance between strength, elongation and stretch flangeability is further improved.
[0049]
When hot-dip steel sheet is subjected to hot dip galvanizing treatment, it may be carried out in accordance with a conventional method. However, as for the pre-plating annealing temperature, plating is not possible at less than 450 ° C., and strength reduction is likely to occur at temperatures exceeding 750 ° C. . Therefore, the annealing temperature is preferably 450 ° C. or higher and 750 ° C. or lower.
[0050]
In addition, the hot-rolled steel sheet of the present invention has no difference in characteristics between the scaled state and the pickling material. There is no problem with temper rolling as long as it is a condition normally performed. In addition, the hot dip galvanizing may remain scaled even after pickling. Electroplating may be performed, and even chemical conversion treatment is not particularly problematic. The effect of the present invention is not affected even if the direct feed rolling is performed in which hot rolling is performed as it is immediately after casting or after heating for supplementary heat. In addition, even if the rolled material is heated or heated after rough rolling and before or during finish rolling, continuous rolling performed by joining the rolled material after rough rolling, or heating and continuous rolling of the rolled material Even if it performs simultaneously, the effect of this invention is not impaired.
[0051]
Since the hot-rolled steel sheet of the present invention is excellent in stretch and stretch flangeability, when it is press-molded, its characteristics are utilized, and a cross section at the time of pressing such as an automobile member, particularly a suspension member such as a suspension arm. A member having a complicated shape can be manufactured with good quality, and in particular, can contribute to weight reduction of a press-formed product. Hereinafter, a method for processing a hot-rolled steel sheet according to the present invention, in other words, a method for manufacturing a press-formed product will be described.
[0052]
FIG. 3 is a flowchart showing an example of a work flow of the method for processing a hot-rolled steel sheet according to the present invention. This work flow usually has a pre-process of manufacturing a steel plate according to the present invention or transporting the manufactured steel plate to a destination place as a coil, for example. First, a hot-rolled steel plate according to the present invention is prepared. (S0, S1). Before pressing the steel sheet, the steel sheet may be pre-processed (S2), or may be processed into a predetermined size or shape by a cutting machine (S3). In the former step S2, for example, cutting or drilling is performed at a predetermined position in the width direction of the steel sheet, and a press-formed product having a predetermined size and shape or pressed processing is performed at the stage where the subsequent press processing is completed or in the process of the press processing. It can be separated as a member. In the latter step of S3, the final press-molded product is processed (and thus cut) into a steel plate member having a predetermined size and shape in consideration of the size and shape of the final press-formed product in advance. Thereafter, the member that has undergone the steps S2 and S3 is subjected to press working, and finally a desired press-formed product having a desired size and shape is manufactured (S4). This press working is usually performed in multiple stages, and often has 3 stages or more and 7 stages or less.
[0053]
The step S4 may include a step of further cutting the member that has passed through the steps S2 and S3 into a predetermined size and shape. The operation of “cutting” in this case is, for example, an operation of cutting an unnecessary portion in a final press-formed product such as an end portion of a member that has passed through steps S2 and S3 at least in the process of pressing. Alternatively, it may be an operation of cutting the member to be pressed along the cutting or perforation in the width direction of the steel plate provided in the step S2.
[0054]
In FIG. 3, N1 to N3 may be a work of conveying a steel plate, a member, or a press-formed product mechanically or by an operator.
[0055]
The press-formed product manufactured in this way is sent to the next step as necessary. As the next process, for example, a process for further machining the press-molded product to adjust dimensions and shape, a process for transporting and storing the press-molded product to a predetermined place, a process for performing a surface treatment on the press-molded product, a press There is an assembly process for assembling an object such as an automobile using a molded product.
[0056]
FIG. 4 is a block diagram showing the relationship between the apparatus that actually performs the operation shown in FIG. 3 and the flow of the steel plate, member, and press-formed product. In this figure, the hot-rolled steel sheet according to the present invention is prepared in a coil shape, and a press-formed product is manufactured by a press machine. The press machine is of a type that performs multi-stage pressing, but the present invention is not limited to this.
[0057]
In some cases, a cutting machine or other pre-processing machine is installed in the front stage of the press machine (FIG. 4A), and in some cases, it is not installed (FIG. 4B). When a cutting machine is installed, a member having a required size or shape is cut from a long steel sheet according to the present invention supplied from a coil, and this member is pressed by a press machine, and a predetermined press It becomes a molded product. In the case where a pre-processing machine that performs notches and perforations in the width direction of the steel sheet is installed, the press machine may cut along the notches and perforations. When the pretreatment machine is not installed, cutting is performed in the process of pressing the steel plate in the press machine, and finally a press-formed product having a predetermined size and shape is manufactured. The meaning of “cutting” in FIG. 4 is the same as the cutting in FIG.
[0058]
The press-formed product manufactured in this way uses the steel sheet according to the present invention which is excellent in elongation and stretch flangeability as its raw material, so it is manufactured with good quality even if the cross-sectional shape at the time of pressing is complicated Can be lightweight. Such a feature is particularly useful when the press-formed product is a member for an automobile, particularly a suspension member such as a suspension arm.
[0059]
【Example】
[Example 1]
Steel having the chemical components shown in Table 1 was used as a hot-rolled steel plate having a thickness of 3.2 mm using the manufacturing conditions shown in Table 2. In Table 1, the A value is the value of (C / 12) / {(Ti / 48) + (Mo / 96)} in the above formula (1), and the B value is C-[{Ti− in the above formula (2). (48/14) N} × (12/48) + (12/96) × Mo]. In Table 2, SRT is the slab heating temperature, FT is the finish hot rolling outlet temperature, and CT is the coiling temperature.
[0060]
After pickling the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece and a hole-expansion test piece were collected and subjected to a tensile test and a hole-expansion test, and the metal structure was measured with an optical microscope and a scanning electron microscope (SEM). confirmed. The hole expansion test is performed by preparing a test piece with a hole punched at a clearance of 12.5% with a 10mmφ punch at the center of a 130mm square steel plate and pushing it up in the opposite direction on the burr side of the punched hole with a 60 ° conical punch. It was. Moreover, the thin film produced from the steel plate was observed with the transmission electron microscope (TEM), and the deposit was confirmed. Moreover, the component of the deposit was analyzed by an energy dispersive X-ray spectrometer (EDX) equipped in the TEM.
[0061]
The metal structure and test results are also shown in Table 2. Regarding the evaluation of the test results, the tensile strength (TS) was 780 MPa or more, the elongation (El) was 20% or more, and the hole expansion ratio (λ) was 70% or more.
[0062]
As shown in Table 2, no. The steel sheets 1 to 3 and 5 to 15 were ferrite structures in which carbides containing appropriate amounts of Ti and Mo having a component composition within the scope of the present invention and a metal structure within the scope of the present invention were precipitated. For this reason, the result which was excellent in both elongation and stretch flangeability has been obtained. In FIG. The transmission electron micrograph of the steel plate of No. 2 is shown. From this photograph, it can be seen that carbides containing fine Ti and Mo are uniformly dispersed in the ferrite single phase structure.
[0063]
In contrast, no. No. 4 steel sheet has a low dislocation density and has a high dislocation density, resulting in a bainite structure with a low El. The hot rolled steel sheets of 16 to 18 and 20 are formed with pearlite or martensite as the second phase, and have low elongation and stretch flangeability. Steel No. 19 has a small addition amount of Mo, so that Ti and Nb charcoal / nitrides are not fine, and there is almost no precipitation of Mo carbides, so λ is low. Steel No. 21 has a low TS and λ due to a small amount of Ti addition and a small amount of Ti charcoal / nitride. The steel sheets 22 to 26 have too much added amount of Ti or Nb or both, so that the amount of precipitates is excessive and both El and λ are low.
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
[Example 2]
After preparing slabs of steel types A, F, M and V in Table 1, heating the slabs to 1250 ° C, hot rolling at the finish hot rolling exit temperature of 890 ° C, and then setting the coiling temperature to 630 ° C A 3.0 mm steel plate was produced. A JIS No. 5 tensile test piece was collected from the obtained steel plate and subjected to a tensile test. Further, the amount of solute C was determined by internal friction measurement. FIG. 6 shows the relationship between the amount of solute C and El measured by a tensile test.
[0067]
From FIG. 6, it is confirmed that the elongation becomes good when the solid solution C amount is 0.0020% or less.
[0068]
[Example 3]
The steel shown in Table 3 was melted and hot rolled at a finishing temperature of 920 ° C. and a winding temperature of 610 ° C. to produce a hot-rolled steel plate having a plate pressure of 1.8 mm. The obtained hot-rolled steel sheet was pickled and then galvannealed. JIS No. 5 tensile test piece and hole expansion test piece were collected from the galvanized plate, and the tensile test and hole expansion test were conducted. The metal structure was confirmed with an optical microscope and a scanning electron microscope (SEM). Was analyzed by an energy dispersive X-ray spectrometer (EDX) equipped with a transmission microscope (TEM). The values of TS, El, and λ at that time are also shown in Table 3. The A value and B value in Table 3 are the same as in Table 1.
[0069]
As shown in Table 3, No. 1 as an example of the present invention. No. 27 showed excellent elongation and stretch flangeability even when alloyed hot dip galvanizing was performed. Since 28 did not contain Mo, the precipitates did not contain Mo, and the stretch flangeability was extremely low.
[0070]
[Table 3]

[0071]
[Example 4]
Steels having the component compositions shown in Table 4 were hot-rolled at a finishing temperature of 910 ° C. and winding temperatures of 600 ° C. and 650 ° C. After confirming the structure of the obtained steel plate having a thickness of 3.2 mm and analyzing the precipitate, a JIS No. 5 tensile test piece was prepared from the steel plate and subjected to a tensile test. The difference in TS was measured between the 600 ° C winding material and the 650 ° C winding material, and the winding temperature dependency was investigated. The results are also shown in Table 4. In addition, No. shown in Table 4 Each of 29 to 32 had a structure in which the carbide of the present invention was precipitated in a ferrite structure.
[0072]
As shown in Table 4, when the B value is 0.005% or less, No. 2 satisfying the above formula (2) is satisfied. Nos. 29 to 31 have a small TS difference and a small dependency on the winding temperature, but No. 29 does not satisfy the above equation (2). It was confirmed that No. 32 has a large coil temperature dependency.
[0073]
[Table 4]

[0074]
【The invention's effect】
As described above, according to the present invention, by appropriately adjusting the component composition, a ferrite structure in which fine carbon / nitride is precipitated is formed. A steel plate can be provided. Therefore, the present invention greatly contributes to the weight reduction of the automobile suspension member.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between winding temperature and TS.
FIG. 2 is a graph showing a relationship between a value of C − [{Ti− (48/14) N} × (12/48) + (12/96) × Mo] and TS.
FIG. 3 is a flowchart showing an example of a work flow of a method for processing a hot-rolled steel sheet according to the present invention.
4 is a block diagram showing the relationship between an apparatus that actually performs the work shown in FIG. 3 and the flow of a steel plate, member, and press-formed product. FIG.
FIG. 5 is a transmission electron micrograph showing the metal structure of a high-tensile hot-rolled steel sheet according to the present invention.
FIG. 6 is a graph showing the relationship between the amount of solute C and El measured by a tensile test.

Claims (10)

% By mass, C ≦ 0.06%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0. 006%, Mo: 0.07 to 0.45%, Ti: 0.03 to 0.10%, Nb ≦ 0.08%, the balance is made of Fe and inevitable impurities, and the solid solution C is 0.8. A high-strength hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized by being composed of a ferrite structure in which carbides containing Ti and Mo are precipitated at 0020% or less. C, Ti, and Mo are contained so that the following (1) Formula may be satisfied, The high-tensile-strength hot-rolled steel plate excellent in the elongation and stretch flangeability of Claim 1 characterized by the above-mentioned.
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in said Formula (1), C, Ti, and Mo represent the mass% of each component. C, Ti, N, and Mo are contained so that the following formula (2) may be satisfied, The high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability according to claim 1.
C − [{Ti− (48/14) N} × (12/48) + (12/96) × Mo] ≦ 0.005 (2)
However, in said Formula (2), C, Ti, N, and Mo represent the mass% of each component. The high-tensile hot-rolled steel sheet excellent in elongation and stretch flangeability according to any one of claims 1 to 3, further comprising Cr ≤ 0.1% by mass. The high-tensile hot-rolled steel sheet having excellent elongation and stretch flangeability according to any one of claims 1 to 4 , wherein the surface has a hot-dip galvanized coating film. % By mass, C ≦ 0.06%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0. 006%, Mo: 0.07~0.45%, Ti: 0.03~0.10%, see containing the Nb ≦ 0.08%, and steels the balance being Fe and unavoidable impurities, A steel slab is formed by continuous casting. Immediately or after being cooled, the steel slab is heated to 1150 ° C or higher and then hot-rolled to a finish hot rolling exit temperature of 850 ° C or higher and wound at 550 ° C to 700 ° C. A method for producing a high-strength hot-rolled steel sheet excellent in elongation and stretch flangeability, characterized by being taken. The method for producing a high-tensile hot-rolled steel sheet having excellent elongation and stretch flangeability according to claim 6, wherein the steel is in mass% and further contains Cr ≦ 0.1%. A first step of preparing a member made of the steel sheet according to any one of claims 1 to 5 , and a second step of subjecting the member to press forming and processing into a press-formed product having a desired shape. A method for processing a high-tensile hot-rolled steel sheet. The method for processing a high-tensile hot-rolled steel sheet according to claim 8 , wherein the press-formed product is an automotive part. The automotive part manufactured with the steel plate in any one of Claims 1-5 .

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