JP3775337B2 - High formability, high-tensile hot-rolled steel sheet with excellent material uniformity, manufacturing method and processing method thereof - Google Patents

Description

【００６５】
【表２】

【００６６】
（実施例２）
表１に示した鋼Ｎｏ．６の鋳造片を用いて、加熱温度１２５０℃、仕上げ圧延温度約９００℃、巻取温度約６００℃で熱間圧延を行い、板厚２．３ｍｍの熱延コイルを作製した。そのコイルから、幅方向中央部と端部から引張試験用サンプルを採取し、種々の条件で熱処理を行った。図３にその際の熱処理温度および熱処理時間における、幅方向中央部と端部の降伏強度の応力差ΔＹＳを示す。この図に示すように、熱処理温度が５５０℃ではΔＹＳが熱延ままからあまり低下していないが、６００〜８００℃ではΔＹＳが３０ＭＰａ以下となった。この結果から熱処理温度が５７５℃以上で低いΔＹＳが得られることが把握される。また、図４にその際の熱処理温度および熱処理時間における、幅方向中央部の引張強度ＴＳを示す。この図に示すように、熱処理温度が７５０℃を超えるとＴＳが大幅に低下し、炭化物による強化効果が消滅していることが把握される。
【００６７】
図３で得られた熱処理温度および時間とΔＹＳの値とから、（Ｔ＋２７３）｛ｌｏｇ（ｔ）＋１６｝とΔＹＳとの関係を求めた。その結果を図５に示す。図５に示すように、（Ｔ＋２７３）｛ｌｏｇ（ｔ）＋１６｝が１６５００以上で特にΔＹＳが低減されることが確認された。また、図４で得られた熱処理温度および時間と幅方向中央部の引張強度ＴＳの値とから、（Ｔ＋２７３）｛ｌｏｇ（ｔ）＋１６｝と幅方向中央部の引張強度ＴＳとの関係を求めた。その結果を図６に示す。図６に示すように、（Ｔ＋２７３）｛ｌｏｇ（ｔ）＋１６｝が１９５００以下で特にＴＳの低下が小さくなることが確認された。
【００６８】
【発明の効果】
以上説明したように、本発明によれば、鋼の成分組成を適切に制御し、鋳造後、熱間圧延してコイルに巻き取った後に、熱処理する工程を経ることによって、実質的にフェライト組織に、特定組成のＴｉと、ＭｏおよびＷのうち１種以上とを含む炭化物が分散析出させた構成としたので、セメンタイト等の製造熱履歴で形態が変化する粗大Ｆｅ炭化物の析出を最小限に抑えることができ、広い温度域で安定である炭化物が鋼板全域に均一微細に析出することにより、降伏強度の均一性に優れた鋼板が得られる。また、実質的にフェライト組織に上述のような微細な炭化物が分散析出するため、高成形性でかつ高強度が実現される。
【図面の簡単な説明】
【図１】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２】図１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３】種々の熱処理温度および熱処理時間における、幅方向中央部と端部の降伏強度の応力差ΔＹＳを示すグラフ。
【図４】種々の熱処理温度および熱処理時間における、幅方向中央部の引張強度ＴＳを示すグラフ。
【図５】図３で得られた熱処理温度および時間とΔＹＳの値とから求めた、（Ｔ＋２７３）｛ｌｏｇ（ｔ）＋１６｝とΔＹＳとの関係を示すグラフ。
【図６】図４で得られた熱処理温度および時間と幅方向中央部の引張強度ＴＳの値とから求めた、（Ｔ＋２７３）｛ｌｏｇ（ｔ）＋１６｝と幅方向中央部の引張強度ＴＳとの関係を示すグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-formability, high-tensile hot-rolled steel sheet excellent in material uniformity, suitable for a member used in a transport device such as an automobile, and a manufacturing method and a processing method thereof.
[0002]
[Prior art]
In recent years, in the field of transport aircraft, which is representative of automobiles, weight reduction of vehicle bodies has been studied for the purpose of improving fuel efficiency. As one of the examinations for reducing the weight of the vehicle body, increasing the strength of the steel sheet used is being promoted. In addition, since automotive members are difficult to process, workability is also required.
[0003]
Japanese Patent Application Laid-Open Nos. Hei 6-200351 and Hei 6-28785 propose a technique relating to a precipitation strengthened high strength hot-rolled steel sheet having a structure mainly composed of ferrite. In the former, most of the structure is made of polygonal ferrite to ensure the required workability, and high strength is achieved by precipitation strengthening and solid solution strengthening centering on TiC. In the latter case, the majority of the structure is made of ferrite to ensure the required workability, and by controlling the addition amount of Ti and Cu, the strength is increased by precipitation strengthening and solid solution strengthening centering on TiC and Cu. ing.
[0004]
However, in these steel sheets, the precipitation temperature of TiC is in a narrow range, and TiC is thermally unstable and easily coarsened. Therefore, the cooling history of the center portion and the end portion in the width direction from the hot rolled runout table to the coiler is reduced. There was a problem that the yield strength was uneven at the center and the end in the width direction of the steel sheet due to the change in the cooling rate of the run-out table-like strip caused by the change in the material in the width direction and the change in the rolling speed due to the difference.
[0005]
[Problems to be solved by the invention]
As described above, in the prior art, although a high-tensile hot-rolled steel sheet having relatively good workability has been obtained, since the fluctuation of the tensile properties in the coil becomes large, the shape freezing property when the steel sheet is press-formed is increased. Or the strength characteristics of the final product.
[0006]
The present invention has been made in view of such circumstances, and has high formability and high tension hot rolling with excellent material uniformity that is practically industrially practical, with little variation in the material in the coil, particularly in the yield strength in the width direction. It aims at providing a steel plate and its manufacturing method and processing method.
[0007]
[Means for Solving the Problems]
As a result of intensive studies to reduce the fluctuation of the yield strength in the coil, the present inventors have strengthened the ferrite single-phase structure with fine precipitates satisfying a predetermined atomic ratio, and thus within a desired range. It was found that the fluctuation can be reduced.
[0008]
  The present invention has been made based on such findings, and the following (1) to (10)I will provide a.
[0011]
  (1)mass%, C ≦ 0.1%, Si ≦ 0.5%, Mn ≦ 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006%, Including Cr ≦ 0.5%, Ti: 0.02 to 0.2%, and further including one or more selected from Mo: 0.05 to 0.6% and W: 0.01 to 1.5% The restFe and inevitable impuritiesAfter casting, after hot rolling and winding into a coil,At 575-750 ° C.By going through the heat treatment process,98% or more by volume (excluding 98%)In the ferrite structure, carbide containing Ti and one or more of Mo and W is dispersed and precipitated in a range satisfying the following formula (1), and the difference in yield stress between the central portion and the end portion in the steel plate width direction is A high-formability, high-tensile hot-rolled steel sheet excellent in material uniformity, characterized by being 30 MPa or less.
  0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1)
  However, in the above formula (1), Ti, Mo, and W are the respective components.mass%.
[0012]
  (2)mass%, C ≦ 0.1%, Si ≦ 0.5%, Mn ≦ 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006%, Including Cr ≦ 0.5%, Ti: 0.02 to 0.2%, Mo: 0.05 to 0.6%, W: 0.01 to 1.5% And at least one selected from Nb: 0.005 to 0.1% and V: 0.01 to 0.1%, with the remainder beingFe and inevitable impuritiesAfter casting, after hot rolling and winding into a coil,At 575-750 ° C.By going through the heat treatment process,98% or more by volume (excluding 98%)A carbide containing one or more of Mo and W and Ti is dispersed and precipitated in the ferrite structure within a range satisfying the following expression (1), and the difference in yield stress between the central portion and the end portion in the width direction of the steel sheet is 30 MPa or less. A high-formability, high-tensile hot-rolled steel sheet with excellent material uniformity.
  0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1)
  However, in the above formula (1), Ti, Mo, and W are the respective components.mass%.
[0013]
  (3) In the above (1), C, Ti, and one or more of Mo and W are contained so as to satisfy the following formula (2). High formability with excellent material uniformity High tensile hot rolled steel sheet.
  0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (2)
  However, in the above formula (2), C, Ti, Mo, W are the respective components.mass%.
[0014]
  (4) In the above (2), one or more of C, Ti, Mo and W and one or more of Nb and V are contained so as to satisfy the following expression (3): High formability, high tensile hot-rolled steel sheet with excellent material uniformity.
  0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + (Nb / 93) + (V / 51)} ≦ 1.5 (3)
  However, in the above formula (3), C, Ti, Mo, W, Nb, V are the respective components.mass%.
[0015]
  (5) Above (1)-(4), A high-formability, high-tensile hot-rolled steel sheet with excellent material uniformity, characterized by having a hot-dip galvanized coating on the surface.
[0016]
  (6) Above (1)-(4) After heating the slab having a composition of any of the above to the temperature of the austenite single-phase region and performing hot rolling, finish rolling is completed at 800 ° C. or higher, and winding at 675 ° C. or lower is performed. A method for producing a high-formability, high-tensile hot-rolled steel sheet excellent in material uniformity, characterized by heat treatment at 750 ° C.
[0017]
  (7)the above(6), The heat treatment after winding is performed so as to satisfy the following formula (4): A method for producing a high formability high tension hot rolled steel sheet having excellent material uniformity.
  16500 ≦ (T + 273) {log (t) +16} ≦ 19500 (4)
  T: heat treatment temperature (° C.), t: heat treatment time (seconds)
[0018]
  (8) Above (1)-(5) High formability high tension hot rolling comprising: a first step of preparing a member made of any of the steel plates; and a second step of pressing the member into a press-formed product having a desired shape. Steel plate processing method.
[0019]
  (9)the above(8), The press-formed product is a method of processing a high-formability high-tensile hot-rolled steel sheet, which is an automotive part, particularly an automobile suspension member.
[0020]
  (10) From (1) to (5) Automotive parts manufactured with any steel plate.
[0021]
According to the present invention having such a configuration, (1) a ferrite structure is formed, and there is no or minimal precipitation of coarse Fe carbide whose shape changes with the production heat history such as cementite, and (2) A carbide that is stable in a wide temperature range precipitates uniformly and finely throughout the steel plate, thereby obtaining a steel plate with excellent yield strength uniformity. In addition, since the fine carbide as described above is substantially dispersed and precipitated in the ferrite structure, high formability and high strength are realized.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, the present invention will be specifically described.
  The hot-rolled steel sheet according to the present invention ismass%, C ≦ 0.1%,Si ≦ 0.5%, Mn ≦ 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006%, Cr ≦ 0.5%,Including Ti: 0.02 to 0.2%, further including one or more selected from Mo: 0.05 to 0.6%, W: 0.01 to 1.5%,The balance consists of Fe and inevitable impurities,After casting, after hot rolling and winding into a coil,At 575-750 ° C.By going through the heat treatment process,98% or more by volume (excluding 98%)In the ferrite structure, carbide containing Ti and one or more of Mo and W is dispersed and precipitated within the range satisfying the following (1), and the yield stress at the center and end in the width direction of the steel sheet The difference is 30 MPa or less. Furthermore, 1 or more types chosen from Nb: 0.005-0.1% and V: 0.01-0.1% may be included.
  0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1)
  (However, in the above formula (1), Ti, Mo, and W represent the weight% of each component.)
[0023]
The reason for performing the heat treatment after hot rolling and winding in a coil is to make the precipitation and structure of fine carbides, which are likely to be different between the central portion and the end portion in the hot rolling state, more uniform.
[0024]
  MatrixA ferrite structure of 98% or more by volume,In the case of a ferrite structure, the composite structure must control the formation of two or more kinds of structures, and it is difficult to achieve material uniformity. On the other hand, a single structure of ferrite has multiple structures. This is because the difficulty of controlling at the same time can be eliminated. For example, Fe carbides change in shape due to the thermal history of strips and coils, and if they are contained in a large amount, they cause material fluctuations.
[0025]
  In the present invention, specifically, volume% by cross-sectional structure observation etc.98% Or more. Further, if the coarse Fe carbide other than fine precipitates is less than 1% by volume, the effect of the present invention is not impaired.
[0026]
  In the hot-rolled steel sheet of the present invention in which the matrix is substantially made of ferrite, the strength is ensured by fine precipitates. In fine precipitates composed of carbides containing Ti and one or more of Mo and W, if these elements are not in the proper ratio, the form of the precipitate is likely to change due to the thermal history of the strip or coil, The progress of coarsening by heating is fast. For this reason, strength fluctuations in the coil due to the difference in cooling rate that occurs between the center and the end in the width direction seen in the coil as it is hot-rolled or the difference in cooling rate that occurs between the outer periphery and the center of the coil Even if heat-treated, it is large. This is because the progress of coarsening due to heating is fast, so fine carbides were precipitated by the heat treatment when the fine carbides were precipitated at the time of winding after hot rolling, and fine carbides were formed by the heat treatment when they were not precipitated at the time of winding. This is because the strength changes in the coil. On the other hand, when it is a carbide containing Ti and one or more of Mo and W and these elements are in an appropriate ratio, the progress of coarsening by heating is slow. For this reason, keep the coil in the hot rolled state so that the inside of the coil has a homogeneous ferrite structure.At 575-750 ° C.When the heat treatment is performed, fine carbides are uniformly deposited throughout the coil, and the strength fluctuation in the coil is reduced. And the effect with respect to the fluctuation | variation of yield strength is remarkable among intensity fluctuations.
[0027]
  Of Ti, Mo, W in fine carbidemassThe value of Ti / [48 {(Mo / 96) + (W / 184)}] calculated by% is 0.1 to 3.5 when less than 0.1 or more than 3.5 This is because the fine carbides are not homogeneously precipitated and the material fluctuation cannot be reduced. Desirably, it is 0.7-1.5.
[0028]
Next, the composition will be described.
C:
C is fixed as a carbide containing Ti, Mo, and W, and is an indispensable element to bear the strength of steel. However, if its content exceeds 0.1%, the ductility deteriorates due to the formation of coarse Fe carbides and the formation of island martensite. Therefore, the upper limit of the C amount is set to 0.1%. From the viewpoint of reducing the amount of Fe carbide produced, 0.08% or less is desirable. On the other hand, in order to maintain the strength of 540 MPa or more, it is desirable to contain 0.01% or more.
[0029]
Ti:
Ti forms fine carbide together with Mo and W, and bears the strength of the steel sheet. However, if its content is less than 0.02%, it is insufficient to ensure the required strength. On the other hand, if it exceeds 0.2%, the transformation point is significantly increased and finish rolling is finished in the austenite region. It becomes difficult to make it difficult to process. Therefore, the Ti content is set to 0.02 to 0.2%.
[0030]
Mo, W:
Since both Mo and W are effective in delaying the coarsening of fine carbide due to heat and reducing the strength fluctuation in the coil through a heat treatment step after hot rolling, at least one kind is added. If Mo is less than 0.05%, it cannot be sufficiently precipitated as carbide. On the other hand, if it exceeds 0.6%, the hot strength becomes high and hot rolling becomes difficult. Therefore, the Mo content is set to 0.05 to 0.6%. Desirably, it is 0.5% or less. On the other hand, if W is less than 0.01%, it cannot be sufficiently precipitated as carbide, while if it exceeds 1.5%, the hot strength becomes high and hot rolling becomes difficult. Therefore, the W content is set to 0.01 to 1.5%.
[0031]
Nb, V:
Nb and V both form carbides and are effective in bearing the strength of the steel sheet, and at least one of these can be added. However, if Nb is less than 0.005%, the effect of Nb carbide precipitation cannot be obtained, and if it exceeds 0.1%, the hot strength becomes high and hot rolling becomes difficult. Further, if V is less than 0.01%, the effect of V carbide precipitation cannot be obtained, and if it exceeds 0.1%, the effect is saturated. Accordingly, when Nb is added, the content is 0.005 to 0.1%, and when V is added, the content is 0.01 to 0.1%.
[0032]
  Next, Si, Mn, P, S, Al, N, and Cr other than C, Ti, Mo, W, Nb, and V will be described.
[0033]
Si:
Si has often been used as a solid solution strengthening element. However, Si produces a red scale and deteriorates the surface properties. Accordingly, the Si content is preferably 0.5% or less. Furthermore, 0.2% or less is desirable.
[0034]
Mn:
Mn is used as a solid solution strengthening element. However, if it exceeds 2%, segregation during casting tends to occur. Therefore, the Mn content is preferably 2% or less.
[0035]
P:
P is a solid solution strengthening element, but if added over 0.06%, it causes significant segregation to the grain boundary and deteriorates ductility, so 0.06% or less is preferable.
[0036]
S:
S is fixed as MnS and TiS. For this reason, S is preferably 0.01% or less because it reduces the amount of Mn and Ti that effectively act on the material properties and lowers the ductility. More preferably, it is 0.005% or less.
[0037]
Al:
Al in steel is used as a deoxidizer. However, if its content exceeds 0.1%, the ductility of the steel is reduced, so 0.1% or less is preferable.
[0038]
N:
N is an impurity in the steel. If its content exceeds 0.006%, coarse nitride formation that lowers ductility is caused, so 0.006% or less is preferable.
[0039]
Cr:
Cr has an effect of suppressing ferrite transformation. When ferrite transformation proceeds at 750 ° C. or higher on the run-out table after finish rolling, coarse carbides containing Ti, Mo, W are formed, and once produced coarse carbides are refined again by the subsequent thermal history. However, the steel cannot be effectively strengthened, but this can be suppressed by adding Cr. However, if Cr exceeds 0.5%, the surface properties deteriorate, so 0.5% or less is preferable. In order to exhibit such an effect effectively, 0.04% or more is preferable.
[0040]
  In the present invention, in addition to the above composition, the following formula (2) is preferably satisfied.
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (2)
  (However, in the above formula (2), C, Ti, Mo and W aremass%. )
  This is because the atomic ratio between C and (Ti + Mo + W) in the steel, that is, (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} is 0.5. By adjusting the contents of C, Ti, Mo, and W so as to be ~ 1.5, carbides containing Ti, Mo, and W are easily and finely dispersed and precipitated effectively for ensuring the strength. Because. If the value of the above formula (2) is less than 0.5, the strength of the entire coil cannot be made uniform, and if it exceeds 1.5, carbite is coarsened and a pearlite that impairs stretch flangeability is formed. Degradation of workability.
[0041]
  Moreover, when adding 1 or more types of Nb and V in addition to C, Ti, Mo, and W, it is preferable to satisfy | fill the following (3) Formula.
  0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + (Nb / 93) + (V / 51)} ≦ 1.5 (3)
(However, in the above formula (3), C, Ti, Mo, W, Nb, V aremass%. )
  In this case as well, the atomic ratio between C and (Ti + Mo + W + Nb + V) in the steel, that is, (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + By adjusting the contents of C, Ti, Mo, W, Nb, and V so that the value of (Nb / 93) + (V / 51)} is 0.5 to 1.5, the carbide is homogeneous. This is because it becomes easy to finely disperse and precipitate. If the value of the above formula (3) is less than 0.5, the strength of the entire coil cannot be made uniform, and if it exceeds 1.5, pearlite is formed which coarsens carbides and impairs stretch flangeability. Degradation of workability.
[0042]
The value of (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} and (C / 12) / {(Ti / 48) + (Mo / 96) + ( The value of (W / 184) + (Nb / 93) + (V / 51)} is more preferably 0.8 to 1.3.
[0043]
In addition, elements other than those described above may be added in minute amounts within the range in which the effects of the present invention are maintained, and other inevitable impurity elements may be contained.
[0044]
Next, preferable production conditions for the hot-rolled steel sheet of the present invention as described above will be described.
Here, in hot rolling after heating the steel having the above composition to the temperature of the austenite single phase region, finish rolling is completed at 800 ° C. or higher, and after winding at 675 ° C. or lower, the temperature is 575 to 750 ° C. Heat treatment.
[0045]
Bring the slab to austenite single phase temperature:
Before hot rolling, the slab is brought to the temperature of the austenite single phase region in order to dissolve the carbides of Ti, Nb, V, Mo, and W. Desirably, the temperature is 1200 ° C. or higher.
[0046]
Finish rolling temperature of 800 ° C or higher:
The finishing rolling temperature is important for making the material uniform. Below 800 ° C, strain-induced precipitation of carbide occurs during rolling, but since this precipitation occurs at a high temperature, the carbide is coarsened and is not refined again in the subsequent thermal history, so that the strength of the carbide increases. Does not work effectively. Therefore, the finish rolling temperature is set to 800 ° C. or higher.
[0047]
The winding temperature is 675 ° C. or lower:
In the steel of the present invention, it is necessary that the carbide is not precipitated after the hot rolling or the carbide is finely precipitated. When the coiling temperature exceeds 675 ° C., the carbide after the hot rolling is coarsened and is not refined again by the subsequent thermal history, and therefore the steel cannot be effectively strengthened. For this reason, the coiling temperature was set to 675 ° C. or less.
[0048]
Heat treatment at 575-750 ° C .:
In the present invention, the coil wound by hot rolling is further heat-treated. This improves the material uniformity across the entire coil. If the heat treatment temperature is less than 575 ° C., homogenization of the ferrite structure and carbide precipitation cannot be achieved throughout the coil, and if it exceeds 750 ° C., the carbide becomes coarse and the strength decreases.
[0049]
More preferable heat treatment conditions after winding the hot-rolled coil are shown in the following formula (4).
16500 ≦ (T + 273) {log (t) +16} ≦ 19500 (4)
(Where T: heat treatment temperature (° C.), t: heat treatment time (seconds))
Thus, when the value of (T + 273) {log (t) +16} is 16500 or more, the uniformity of the yield strength is further enhanced. However, if the value exceeds 19500, the coarsening of the carbide proceeds, and it becomes difficult to effectively exert the strengthening effect by the carbide that is the feature of the present invention.
[0050]
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 workability, good workability 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. As for the pre-plating annealing temperature, if the temperature is lower than 450 ° C., the plating cannot be applied, and if it exceeds 750 ° C., the strength tends to decrease. Therefore, the annealing temperature is preferably 450 ° C. or higher and 750 ° C. or lower.
[0051]
In addition, the hot-rolled steel sheet of the present invention has no difference in its characteristics whether it is black or pickled. There is no particular restriction on temper rolling as long as it is usually performed. Moreover, the hot dip galvanization has no problem even after pickling or as it is black. For galvanization, electroplating is also possible. There is no particular problem with chemical conversion treatment. The effect of the present invention is not affected even if direct feed rolling, in which hot rolling is performed directly after casting or after heating for the purpose of supplementary heating, is performed. Furthermore, even if the rolled material is heated after the rough rolling and before the finish rolling, the continuous rolling performed by joining the rolled material after the rough rolling may be performed, or the heating and continuous rolling of the rolled material may be performed simultaneously. The effect of the present invention is not impaired.
[0052]
The hot-rolled steel sheet of the present invention is excellent in formability and has little variation in the material in the coil. Therefore, when it is press-molded, its characteristics are utilized, and when it is pressed like an automobile member, particularly a suspension member such as a suspension arm. A member having a complicated cross-sectional shape can be manufactured with good quality, and in particular, it 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.
[0053]
FIG. 1 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. 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.
[0054]
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.
[0055]
In FIG. 1, N1 to N3 may be a work of conveying a steel plate, a member, or a press-formed product mechanically or by an operator.
[0056]
The press-formed product manufactured in this way is sent to the next step as necessary. As the next process, for example, a further process is performed on the press-molded product to adjust dimensions and shape, a process of transporting and storing the press-molded product to a predetermined place, a process of subjecting the press-molded product to surface treatment, a press There is an assembly process for assembling an object such as an automobile using a molded product.
[0057]
FIG. 2 is a block diagram showing the relationship between the apparatus that actually performs the operation shown in FIG. 1 and the flow of steel plates, members, and press-formed products. 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.
[0058]
In some cases, a cutting machine or other pre-processing machine is installed in the front stage of the press machine (FIG. 2A), and in some cases, it is not installed (FIG. 2B). 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. 2 is the same as the cutting in FIG.
[0059]
The press-molded product manufactured in this way uses the steel sheet according to the present invention, which has excellent surface properties and ductility as the raw material, and has little fluctuation in the material in the coil, so that it has good and uniform quality. The production yield of goods is also high. 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.
[0060]
【Example】
(Example 1)
Steels having the chemical components shown in Table 1 were melted and hot-rolled at a heating temperature of 1250 ° C., a finish rolling temperature of about 900 ° C. and a winding temperature of about 610 ° C. to produce a steel plate having a thickness of 2.3 mm. . A sample for tensile test and a sample for hole expansion test were collected from the coil, and then the hot-rolled coil was heat-treated with a continuous annealing facility. At this time, the conditions for heating in the continuous annealing facility were 700 ° C. and 2 minutes. A sample for tensile test and a sample for hole expansion test are collected from the obtained final heat-treated steel sheet, and a thin film is collected. Observation of precipitates with a transmission electron microscope (TEM) and Ti, Mo, W in the precipitates, The composition of Nb and V was analyzed with an energy dispersive X-ray spectrometer (EDX) equipped with TEM. Moreover, the structure | tissue observation of the matrix was performed with the scanning electron microscope (SEM). These results are also shown in Table 1. In Table 1, A value indicates the value of Ti / [48 {(Mo / 96) + (W / 184)}], and B value is (C / 12) / {(Ti / 48) +. (Mo / 96) + (W / 184)} or (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + (Nb / 93) + (V / 51)} Indicates the value of.
[0061]
The tensile test was performed by collecting JIS No. 5 test pieces in the coil longitudinal direction from the center and the end in the width direction. The comparison of the yield stress in the width direction was made with the absolute value of the strength difference. Moreover, the hole expansion test for evaluating the stretch flangeability (λ) of the steel sheet was performed according to the Japan Iron and Steel Federation standard. These characteristics are shown in Table 2.
[0062]
As shown in Table 1, no. In Nos. 1 to 13, the chemical composition is within the range of the present invention, the matrix structure is ferrite, and the A value of the precipitate is 0.1 or more and 3.5 or less. As shown in the figure, good material properties and material uniformity were obtained.
[0063]
On the other hand, No. containing a large amount of C. In No. 14, pearlite is generated, the stretch flangeability is lowered, and the yield strength at the center and end in the width direction after heat treatment is also non-uniform. A pearlite is formed and the A value is less than 0.1. No. 15, the stretch flangeability is particularly low, and the yield strength at the center and end in the width direction after heat treatment is also non-uniform. No. A value is less than 0.1 and B value is less than 0.5. 16 does not eliminate the uneven yield strength at the center and end in the width direction after heat treatment. No. A value is less than 0.1 and B value is more than 1.5. In No. 17, since pearlite is generated, the stretch flangeability is low, the strength decrease in the center portion in the width direction is large after the heat treatment, and the uneven yield strength at the center portion and the end portion in the width direction is not eliminated. No. A value exceeding 3.5. No. 18 does not eliminate the uneven yield strength in the width direction after the heat treatment.
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
(Example 2)
Steel No. shown in Table 1 Using the cast piece No. 6, hot rolling was performed at a heating temperature of 1250 ° C., a finish rolling temperature of about 900 ° C., and a winding temperature of about 600 ° C. to produce a hot rolled coil having a plate thickness of 2.3 mm. From the coil, samples for a tensile test were collected from the center and end in the width direction, and heat-treated under various conditions. FIG. 3 shows the stress difference ΔYS of the yield strength between the central portion and the end portion in the width direction at the heat treatment temperature and heat treatment time at that time. As shown in this figure, when the heat treatment temperature is 550 ° C., ΔYS is not lowered so much as it is hot rolled, but at 600 to 800 ° C., ΔYS is 30 MPa or less. From this result, it is understood that a low ΔYS is obtained when the heat treatment temperature is 575 ° C. or higher. FIG. 4 shows the tensile strength TS at the center in the width direction at the heat treatment temperature and heat treatment time. As shown in this figure, it is understood that when the heat treatment temperature exceeds 750 ° C., TS is greatly reduced, and the strengthening effect due to the carbide disappears.
[0067]
From the heat treatment temperature and time obtained in FIG. 3 and the value of ΔYS, the relationship between (T + 273) {log (t) +16} and ΔYS was determined. The result is shown in FIG. As shown in FIG. 5, it was confirmed that (Y + S) is particularly reduced when (T + 273) {log (t) +16} is 16500 or more. Further, the relationship between (T + 273) {log (t) +16} and the tensile strength TS at the center in the width direction is obtained from the heat treatment temperature and time obtained in FIG. 4 and the value of the tensile strength TS at the center in the width direction. It was. The result is shown in FIG. As shown in FIG. 6, it was confirmed that when (T + 273) {log (t) +16} is 19500 or less, the decrease in TS is particularly small.
[0068]
【The invention's effect】
As described above, according to the present invention, the composition of steel is appropriately controlled, and after casting, it is hot-rolled and wound into a coil, and then undergoes a heat treatment step, thereby substantially forming a ferrite structure. In addition, since a carbide containing Ti having a specific composition and one or more of Mo and W is dispersed and precipitated, the precipitation of coarse Fe carbide whose form changes with the production heat history of cementite and the like is minimized. The carbide that can be suppressed and is stable in a wide temperature range is uniformly and finely precipitated throughout the steel plate, thereby obtaining a steel plate having excellent yield strength uniformity. In addition, since the fine carbide as described above is substantially dispersed and precipitated in the ferrite structure, high formability and high strength are realized.
[Brief description of the drawings]
FIG. 1 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.
FIG. 2 is a block diagram showing the relationship between an apparatus that actually performs the work shown in FIG. 1 and the flow of steel plates, members, and press-formed products.
FIG. 3 is a graph showing the stress difference ΔYS of the yield strength between the central portion and the end portion in the width direction at various heat treatment temperatures and heat treatment times.
FIG. 4 is a graph showing the tensile strength TS at the center in the width direction at various heat treatment temperatures and heat treatment times.
5 is a graph showing the relationship between (T + 273) {log (t) +16} and ΔYS, obtained from the heat treatment temperature and time obtained in FIG. 3 and the value of ΔYS.
6 shows (T + 273) {log (t) +16} and the tensile strength TS at the center in the width direction, obtained from the heat treatment temperature and time obtained in FIG. 4 and the value of the tensile strength TS at the center in the width direction. The graph which shows the relationship.

Claims (10)

% By mass
C ≦ 0.1%,
Si ≦ 0.5%,
Mn ≦ 2%,
P ≦ 0.06%,
S ≦ 0.01%,
Al ≦ 0.1%,
N ≦ 0.006%,
Cr ≦ 0.5%,
Ti: 0.02 to 0.2%
In addition, Mo: 0.05-0.6%,
W: 0.01 to 1.5%
Including at least one selected from the group consisting of Fe and inevitable impurities ,
After casting, after hot rolling and winding in a coil, by passing through a step of heat treatment at 575 to 750 ° C., a ferrite structure having a volume% of 98% or more (however, excluding 98%) has the following ( 1) In a range satisfying the formula, carbide containing Ti and one or more of Mo and W is dispersed and precipitated.
A high formability high tensile hot-rolled steel sheet excellent in material uniformity, characterized in that the difference in yield stress between the center and the end in the width direction of the steel sheet is 30 MPa or less.
0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1)
However, in said Formula (1), Ti, Mo, and W represent the mass % of each component. % By mass
C ≦ 0.1%,
Si ≦ 0.5%,
Mn ≦ 2%,
P ≦ 0.06%,
S ≦ 0.01%,
Al ≦ 0.1%,
N ≦ 0.006%,
Cr ≦ 0.5%,
Ti: 0.02 to 0.2%,
In addition, Mo: 0.05-0.6%,
W: 0.01 to 1.5%
Including one or more selected from Nb: 0.005 to 0.1%,
V: 0.01 to 0.1%
Including at least one selected from the group consisting of Fe and inevitable impurities ,
After casting, after hot rolling and winding in a coil, by passing through a step of heat treatment at 575 to 750 ° C., a ferrite structure having a volume% of 98% or more (however, excluding 98%) has the following ( 1) A carbide containing one or more of Mo and W and Ti is dispersed and precipitated within a range satisfying the formula,
A high formability high tensile hot-rolled steel sheet excellent in material uniformity, characterized in that the difference in yield stress between the center and the end in the width direction of the steel sheet is 30 MPa or less.
0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1)
However, in said Formula (1), Ti, Mo, and W represent the mass % of each component. C, Ti, and one or more of Mo and W are contained so as to satisfy the following formula (2): High formability and excellent material uniformity according to claim 1 Tensile hot-rolled steel sheet.
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (2)
However, in said Formula (2), C, Ti, Mo, and W represent the mass % of each component. The material according to claim 2, comprising at least one of C, Ti, Mo and W, and at least one of Nb and V so as to satisfy the following expression (3). High formability, high-tensile hot-rolled steel sheet with excellent uniformity.
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + (Nb / 93) + (V / 51)} ≦ 1.5 (3)
However, in said Formula (3), C, Ti, Mo, W, Nb, and V represent the mass % of each component. High moldability high-tensile hot-rolled steel sheet excellent in material homogeneity according to any one of claims 1 to 4, characterized in that it comprises a hot-dip galvanized coating on the surface. In carrying out hot rolling after heating the slab having the component composition of any one of claims 1 to 4 to a temperature of an austenite single phase region, finish rolling is completed at 800 ° C or more and winding at 675 ° C or less. The manufacturing method of the high formability high tension hot rolled steel plate excellent in material uniformity characterized by heat-processing at 575-750 degreeC after taking. The method for producing a high-formability, high-tensile hot-rolled steel sheet with excellent material uniformity according to claim 6 , wherein the heat treatment after winding is performed so as to satisfy the following expression (4).
16500 ≦ (T + 273) {log (t) +16} ≦ 19500 (4)
T: heat treatment temperature (° C.), t: heat treatment time (seconds) 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 formability, high-tensile hot-rolled steel sheet. The method for processing a high-formability, 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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