JP3821042B2 - High-formability high-tensile steel sheet with excellent strength stability and method for producing and processing the same - Google Patents

Description

【００６７】
【表２】

【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、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を分散析出した構成としたので、γ→α変態直後の多量の固溶Ｃと偏析するＭｎの相互作用によるセメンタイトまたはパーラ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、また、ＭｏまたはＷまたはＭｏ＋Ｗの作用によりランナウトテーブル上のγ→α変態が遅延され、広い温度域で安定的に析出するＴｉと、ＭｏおよびＷのうち１種以上とを含む微細炭化物が巻取り時に進行するフェライト変態とともに析出するようになるので、ランナウトテーブル上の温度変化やコイル内変動が生じても組織変動が抑えられ、材質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Ｔｉと、ＭｏおよびＷのうち１種以上とを含む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】Ｍｎ含有量とＥｘ．Ｃ量の適正範囲を示す図。
【図２】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３】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。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-formability, high-tensile steel sheet suitable for members used in transportation equipment such as automobiles and having excellent strength stability with a tensile strength of 550 MPa or more, 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. Conventionally, as a high-strength steel plate, a steel plate having a low production cost in which solid solution strengthening of C, Mn, Si and precipitation strengthening of Ti, Nb has been used, but C is 0.12 to 0.15%. As a result, a large amount of cementite is precipitated in the steel, resulting in poor ductility, and press-cracking has occurred in difficult-to-process materials such as automobile members.
[0003]
As a high formability high tensile hot rolled steel sheet that avoids such press cracks, JP-A-6-172924 discloses that carbide precipitation is suppressed by Si, and low-temperature transformation phase generation is suppressed by limiting the amount of Cr added. Has disclosed a Ti-added high-stretch flangeability high-tensile hot-rolled steel sheet using Bainitic ferrite single phase and Ni and Mo as solid solution strengthening elements. However, in the bainitic ferrite structure, which is a lath-like structure that does not cause carbide precipitation between the laths that form the basis of this technology, the Ti addition amount is limited to the carbonitride formation limit or less, so it does not bond with Ti. C must be supersaturated in bainitic ferrite. For this reason, since the carbide precipitation driving force is high, the carbide precipitation is extremely sensitive to subtle increases and decreases in additive components and hot rolling conditions. In the present state, the workability is abruptly deteriorated partially in the width direction.
[0004]
In JP-A-7-11382, the amount of Ti and Nb bonded to C is added in an atomic ratio of 0.5 or more to C, and ferrite nucleation after hot rolling with solute Ti and Nb is performed. A high-stretch flanged hot-rolled steel sheet is disclosed in which the structure is changed to acicular ferrite and the strength is adjusted by solid solution strengthening of Cr and Mo. However, the hot rolled steel sheet having an acicular ferrite structure in this technique has a strength of 843 MPa and an elongation of 15%, and the ductility is not yet sufficient for stretch forming that requires high elongation. When the hot-rolled steel sheet is actually stretched and formed, cracking occurs. Furthermore, the addition of Ti and Nb to ultra-low carbon steel and the acicular effect of the structure by Ti and Nb is extremely high so that an acicular ferrite structure cannot be obtained even if sufficient amounts of solute Ti and Nb are secured. With this technique, acicular ferrite cannot be obtained with a slight change in manufacturing conditions.
[0005]
On the other hand, Japanese Patent Application Laid-Open No. 11-152544 discloses a method for realizing high workability and high strength by the effect of refinement of Ti, Nb, V, and Mo. However, since this technique makes the particle diameter 2 μm or less, deterioration of elongation is unavoidable, and cracks are also generated in the overhang molding. In addition, since the grain size is too fine, grain growth is extremely large, and grains that exceed 2 μm are partly formed due to fluctuations in the widthwise hot rolling conditions that usually occur, resulting in a mixed grain structure, and workability is drastically increased. The current situation is that it deteriorates.
[0006]
Japanese Patent Laid-Open No. 2000-328186 discloses that the average grain size is set to 2.0 to 10 μm and the ferrite area ratio is set to 95% or more by reducing the austenite of Ti and Nb and suppressing recrystallization. An ultrafine ferritic steel sheet having excellent flangeability is disclosed. However, since this technique mainly focuses on transformation from unrecrystallized austenite to ferrite, the material change is large because the recrystallization recovery rate of austenite greatly changes due to temperature fluctuations between rolling and winding. Further, since fine grains have a large driving force for grain growth in the coil, partial grain growth, that is, mixed grains are likely to occur, and the strength is not stable. Thus, it is impossible to obtain a product with stable quality. Furthermore, this method inevitably increases the dislocation density, and a reduction in elongation is unavoidable. In addition, to obtain a strength of at most 685 MPa, Ti must be added in an amount of 0.32%, the rolling load increases, and the shape of the plate becomes inferior compared to a steel plate of the same strength.
[0007]
Furthermore, Japanese Patent Laid-Open No. 6-200351 has a structure in which TiC is dispersed in polygonal ferrite with an area ratio of pearlite and low-temperature transformation phase to polygonal ferrite of 15% or less, and solid solution strengthening of Mo Discloses a high-strength hot-rolled steel sheet having excellent stretch flangeability after strength adjustment. However, in this steel sheet, since the precipitation temperature of TiC is in a narrow range, even if the workability is good at the center in the width direction, the edge is below the specified strength and the ductility is significantly deteriorated.
[0008]
[Problems to be solved by the invention]
In this way, in the conventional technology, although a high-tensile steel plate excellent in workability has been proposed, the variation in tensile properties in the coil is large, the workability deteriorates due to variations in production conditions that can normally occur, etc. Clearly it is not suitable for industrial production.
[0009]
The present invention has been made in view of such circumstances. Naturally, fluctuations in the strength in the coil caused by fluctuations in manufacturing conditions that can occur naturally, especially fluctuations in strength in the longitudinal direction, are minimal, and can be used for industrially stable strength. An object of the present invention is to provide a high-formability high-tensile steel sheet having excellent properties and a method for producing and processing the same.
[0010]
[Means for Solving the Problems]
The inventors of the present invention have not been solved by the prior art, such as a change in the cooling rate of the strip on the runout table caused by a change in material in the width direction or a change in rolling speed due to a difference in cooling history between the center and the edge from the hot rolled runout table to the coiler. As a result of intensive studies to reduce fluctuations in the longitudinal direction of the material in the coil, particularly strength fluctuations, the lath structure and bainitic ferrite shown in the above prior art are not fine grains of 2 μm or less, In a steel in which a ferrite single-phase structure is strengthened with fine precipitates containing Ti and one or more of Mo and W, Ex. C is 0.015% or less, and Mn is 0.2 ≦ Mn1.7-30 × Ex. It was found that the material variation, particularly strength variation, is reduced by setting C. In addition, it has also been found that such a structure makes it possible to reduce the addition of Si, which has been indispensable in the past, as much as possible, and to suppress the generation of red scale due to Si that deteriorates the surface properties. .
[0011]
  The present invention has been made based on such findings, and the following (1) to (7)I will provide a.
[0014]
  (1) By mass%, C: 0.03 to 0.15%, Mn ≧ 0.2%, Si ≦ 0.3%, P ≦ 0.02%, Al ≦ 0.1%, S ≦ 0. 01%, N ≦ 0.01%, Ti: 0.05 to 0.35%, and one or more selected from Mo ≦ 0.6% and W ≦ 1.5%, and Mo and W are When each of them is contained alone, Mo ≧ 0.1% and W ≧ 0.2%, the balance is made of Fe and inevitable impurities, and Ex. C is 0.015% or less, and Mn ≦ 1.7-30 × Ex. C is satisfied, and 95% or more by volume% is a ferrite structure, and precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed in the ferrite structure. A high-formability, high-tensile steel plate with excellent strength stability with a tensile strength of 550 MPa or more.
  Ex. C = C- {Ti-N * (48/14) -S * (48/32)} * (12/48) -Mo * (12/96) -W * (12/184) (1)
  (However, the element symbol in the formula (1) indicates mass% of each element.)
[0015]
  (2) By mass%, C: 0.03-0.15%, Mn ≧ 0.2%, Si ≦ 0.3%, P ≦ 0.02%, Al ≦ 0.1%, S ≦ 0. 01%, N ≦ 0.01%, Ti: 0.05 to 0.35%, and one or more selected from Mo ≦ 0.6% and W ≦ 1.5%, and Mo and W are When each is included alone, Mo ≧ 0.1%, W ≧ 0.2%, and Cr ≦ 0.5%, V ≦ 0.1%, Nb ≦ 0.08%, B ≦ Ex. 0.001% containing one or more, the balance consisting of Fe and unavoidable impurities, Ex. C is 0.015% or less, and Mn ≦ 1.7-30 × Ex. C is satisfied, and 95% or more by volume% is a ferrite structure, and precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed in the ferrite structure. A high-formability, high-tensile steel plate with excellent strength stability with a tensile strength of 550 MPa or more.
  Ex. C = C- {Ti-N * (48/14) -S * (48/32)} * (12/48) -Mo * (12/96) -W * (12/184) (1)
  (However, the element symbol in the formula (1) indicates mass% of each element.)
[0016]
  (3) Above (1)Or (2)A high-formability, high-tensile steel sheet excellent in strength stability with a tensile strength of 550 MPa or more, wherein the plate thickness is less than 2.0 mm.
[0017]
  (4) Above (1)Or (2)In the hot rolling after heating the steel having the component composition to the temperature of the austenite single phase region, the finish rolling is completed at 800 ° C. or higher, and the tensile strength is 550 MPa. The manufacturing method of the high formability high tension steel plate excellent in the above strength stability.
[0018]
  (5) Above (1)-(3A high-formability high-tensile steel plate 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. Processing method.
[0019]
  (6)the above(5), The press-formed product is a method for processing a high-formability high-tensile steel sheet, which is an automobile part, particularly an automobile suspension member.
[0020]
  (7) From (1) to (3) An automotive part manufactured from the steel sheet according to any of the above.
[0021]
According to the present invention having such a configuration, (1) a ferrite structure is formed, and the form is formed by a production heat history of cementite or pearlite due to the interaction between a large amount of solute C immediately after the γ → α transformation and segregated Mn. There is no or minimal precipitation of changing coarse Fe carbide, and (2) γ → α transformation on the run-out table is delayed by the action of Mo or W or Mo + W, and stable over a wide temperature range As the fine carbides containing Ti and one or more of Mo and W are precipitated together with the ferrite transformation that proceeds during winding, solid solution C is reduced by consuming C in the steel. Furthermore, even if a temperature change on the run-out table or a fluctuation in the coil occurs, the structural fluctuation is suppressed, and a steel sheet having excellent material uniformity can be obtained. In addition, since fine carbides containing Ti and one or more of Mo and W are substantially dispersed and precipitated in the ferrite structure, high formability and high strength are realized.
[0022]
Furthermore, in the present invention, it is possible to reduce Si as much as possible to obtain a desired characteristic in the prior art as much as possible, and suppress the generation of red scale due to Si that deteriorates the surface properties. Can do.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, the present invention will be specifically described.
  The steel sheet according to the present invention is in mass%, C: 0.03 to 0.15%, Mn ≧ 0.2%, Si ≦ 0.3%, P ≦ 0.02%, Al ≦ 0.1%, Including S ≦ 0.01%, N ≦ 0.01%, Ti: 0.05 to 0.35%, and including one or more selected from Mo ≦ 0.6% and W ≦ 1.5%, When Mo and W are contained alone, Mo ≧ 0.1% and W ≧ 0.2%, the balance is made of Fe and inevitable impurities, and Ex. C is 0.015% or less, and Mn ≦ 1.7-30 × Ex. C is satisfied, and 95% or more by volume% is a ferrite structure, and precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed in the ferrite structure.
  Ex. C = C- {Ti-N * (48/14) -S * (48/32)} * (12/48) -Mo * (12/96) -W * (12/184) (1)
  (However, the element symbol in the formula (1) indicates mass% of each element.)
[0024]
  More than 95% by volume is ferrite structure and ferrite single phase structure substantiallyThis is because, in the composite structure, two or more types of structures must be controlled and it is difficult to achieve material uniformity, whereas in the ferrite single phase, it is difficult to control multiple structures simultaneously. It is because it 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]
  IngredientsPhysically, cross-sectional structure observation etc.RIt is only necessary that 95% or more by volume is ferrite. Preferably it is 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 order to transform the ferrite during winding, it is necessary to delay the γ → α transformation that normally occurs during run-out cooling until the winding. Therefore, in the present invention, Mo or W is added, or W and Mo are added in combination to delay the ferrite transformation.
[0027]
In the steel sheet of the present invention in which the matrix is substantially composed of ferrite, the strength is ensured by fine precipitates. In this case, if the strength of the steel sheet is 550 MPa or more, the effect of the present invention can be obtained. However, the effect is further enhanced at 780 MPa or more, and the effect is further enhanced at 980 MPa or more. In general, Mo forms Mo carbide and W forms W carbide, which contributes to precipitation strengthening. However, because the precipitation rate of Mo carbide and W carbide is slow, it is difficult to achieve high strength of 550 MPa or more with Mo or W alone. In addition, since it precipitates when the coil is cooled after winding, the strength changes between the outer peripheral portion and the central portion of the coil where the cooling rate is fast. Therefore, in order to promote precipitation of carbides containing Mo or / and W before and after winding, Ti is added and the precipitation rate of the precipitates is controlled to an appropriate value.
[0028]
The reason why fine precipitates are dispersed in the matrix of ferrite single phase structure is that when the dislocation density is increased and high strength is achieved, the total elongation is reduced. Because. Moreover, the reason why the particle size of the precipitate is less than 10 nm is that it becomes difficult to obtain a strength of 550 MPa or more when the particle diameter is 10 nm or more. In addition, when trying to strengthen with precipitates of 10 nm or more, the volume fraction of the precipitates must be increased, and the amount of the precipitate-forming elements must be increased. The strength is maintained by making the ferrite finer with increasing the amount of added, and the total elongation is reduced. From the above, in the present invention, the particle size of the fine precipitate is set to less than 10 nm, but is preferably 5 nm or less, and more preferably 3 nm or less when higher strength is required.
[0029]
Next, the component composition will be described.
C: 0.03-0.15%
C is fixed as a carbide containing Ti and one or more of Mo and W, and is an element indispensable for bearing the strength of steel. However, if the content is less than 0.03%, sufficient strength of 550 MPa class or more cannot be obtained. On the other hand, if the content exceeds 0.15%, coarse Fe carbide is generated and ductility deteriorates. Therefore, the C content is set to 0.03 to 0.15%. In order to obtain a strength of 780 MPa or more, it is preferably 0.035% or more.
[0030]
N ≦ 0.01%
N is an impurity in the steel. If the content exceeds 0.01%, coarse nitride formation that lowers the ductility is caused, so the content is made 0.01% or less. If it exceeds 0.006%, a large amount of Ti must be added, so 0.006% or less is preferable.
[0031]
Ti: 0.05 to 0.35%
Ti forms carbide with Mo or / and W, and bears the strength of steel. However, if it is less than 0.05%, the amount of fine precipitates decreases, and high strength cannot be realized. On the other hand, if it exceeds 0.35%, bainitic ferrite that lowers the elongation (EL) tends to be generated. Therefore, the Ti content is set to 0.05 to 0.35%.
[0032]
Mo ≦ 0.6%
As described above, Mo suppresses ferrite transformation on the runout table, reduces the influence of thermal history on the runout table on ferrite structure formation, suppresses pearlite transformation, and forms fine carbide with Ti. This contributes to increasing the strength of steel. However, when 0.6% or more is added, a large amount of low-temperature transformation phase is generated, and the stability of the material is deteriorated. Therefore, the Mo content is set to 0.6% or less. When W is included together, there is no lower limit of the Mo content. However, when W is not included, in order to form a carbide containing a sufficient amount of Ti and Mo, the Mo content is set to 0.1%. % Or more.
[0033]
W ≦ 1.5%
As described above, W suppresses the ferrite transformation on the runout table and reduces the influence of the thermal history on the runout table on the ferrite structure formation. In addition, fine carbides are formed together with Ti, contributing to high strength of steel. However, if W exceeds 1.5%, a low temperature transformation phase is likely to be generated, and the stability of the material is deteriorated. Therefore, the W content is set to 1.5% or less. When Mo is included together, there is no lower limit of W. However, when Mo is not included, it is impossible to suppress precipitation of Fe carbide if W is less than 0.2%. Is 0.2% or more.
[0034]
C, Ti, Mo, and W preferably satisfy 0.5 ≦ (C / 12) / {(Ti / 48) + (W / 184) + (Mo / 96)} ≦ 1.5. This is the atomic ratio between C and (Ti + Mo + W) in steel (the atomic ratio between C and (Ti + Mo) when W is not included, and the atomic ratio between C and (Ti + W) when Mo is not included. Number ratio) is 0.5 to 1.5, and by adjusting the content of C, Ti, Mo or / and W so as to be in such a range, Ti and Mo And carbides containing one or more of W easily precipitate finely.
[0035]
In the present invention, for example, when forming a precipitate with Ti and Mo, when obtaining a strength of 780 MPa class, C: 0.035 to 0.055%, Ti: 0.07 to 0.09%, Mo: 0.15 to 0.25%, and when obtaining a strength of 980 MPa class, C: 0.070 to 0.080%, Ti: 0.16 to 0.2%, Mo: 0.30 to 0.40 % Is preferred.
[0036]
  Ex. C ≦ 0.015%
  Ex. C is C which is not bonded to Ti, Mo, W, Ex. C = C- {Ti-N * (48/14) -S * (48/32)} * (12/48) -Mo * (12/96) -W * (12/184)(However, the element symbols in the formula indicate the mass% of each element.)It is represented by Ex. That does not bond to such Ti, Mo, W. When C is present in a large amount, pearlite is generated. That is, Ti that is not bonded to Mo, W, and N or S binds to C and consumes C along with precipitation of precipitates, but does not bond to Ti, Mo, and W. Ex. C concentrates to untransformed γ to form pearlite. Therefore, Ex. C was set to 0.015% or less at which pearlite was not generated. Preferably it is 0.010% or less, More preferably, it is 0.005% or less.
[0037]
Mn: 0.2 to 1.7-30 × Ex. C
Mn is a solid solution strengthening element, and its content is 0.2% or more in order to ensure sufficient strength. On the other hand, in the present invention, Mn has been actively used as a solid solution strengthening element. By limiting the amount of Mn that has been produced, we succeeded in reducing fluctuations in strength. That is, since Mn segregates during solidification, C tends to collect in the Mn concentrated portion. Even if C is 0.015% or less, if the amount of Mn is large, pearlite is generated in the Mn-concentrated portion, and the strength stability against fluctuations in the winding temperature cannot be maintained. For this reason, in the present invention, the Mn content is set to Ex. In association with C amount, 1.7-30 × Ex. C or less. Ex. When the calculated value of the C amount is a negative value, the Mn amount is 1.7% or less.
[0038]
This will be explained by experimental results.
C: 0.03 to 0.1%, Mn: 0.25 to 1.8%, and after changing the amount of Ti and Mo to 1250 ° C, the finishing temperature is 910 ° C and the winding temperature is 650 ° C. Hot rolling was performed at 600 ° C. to produce a 1.6 mmt 980 MPa class hot rolled sheet. The obtained hot-rolled sheet was subjected to a tensile test to evaluate the strength. At this time, the strength difference between the strength of 650 ° C. winding and the strength of 600 ° C. winding was calculated. This shows the material fluctuation when the winding temperature fluctuates between 600 ° C. and 650 ° C. The result is shown in FIG. In FIG. 1, the case where the strength difference between the 650 ° C. winding and the 600 ° C. winding is 30 MPa or less is indicated by “◯”, and the case where the strength difference exceeds 30 MPa is indicated by “X”. It can be seen that when the Mn content is in the above range, the difference in strength between the 650 ° C. winding and the 600 ° C. winding is small.
[0039]
  next,Si, P, Al, and S other than C, N, Ti, Mo, W, and Mn will be described. Further, the steel sheet of the present invention may further contain Cr ≦ 0.5%, V ≦ 0.1%, and Nb ≦ 0.08%, which will be described below.
[0025]
[0040]
Si ≦ 0.3%
Si has often been used as a solid solution strengthening element. However, Si generates a red scale, which deteriorates the surface properties and promotes the concentration of C into the untransformed γ during the γ → α transformation, thereby facilitating the formation of a low temperature transformation phase. Therefore, the Si content is preferably 0.3% or less. Furthermore, 0.2% or less is more desirable, More preferably, it is 0.1% or less.
[0041]
P ≦ 0.02%
P is a solid solution strengthening element, but if added over 0.02%, significant segregation to the grain boundary is caused and the ductility deteriorates, so 0.02% or less is preferable.
[0042]
S ≦ 0.01%
S is fixed as TiS. For this reason, S is preferably 0.01% or less because it reduces the amount of Ti that effectively acts on the material properties and also reduces the ductility. More preferably, it is 0.005% or less, More preferably, it is 0.003% or less.
[0043]
Al ≦ 0.1%
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. More preferably, it is 0.06% or less.
[0044]
Cr ≦ 0.5%
When Cr is added together with Mo and W, the effect of suppressing ferrite transformation at a high temperature becomes remarkable. When the ferrite transformation after rolling proceeds remarkably on the runout table, the temperature variation in the width direction of the strip causes the deviation of the ferrite transformation (position on the runout table) as it is, and affects the ferrite structure after the transformation. That is, the mechanical properties greatly differ between the central portion and the end portion in the width direction. Cr promotes the effect of suppressing such fluctuations in the mechanical properties in the width direction. However, if Cr exceeds 0.5%, a low-ductility low-temperature transformation phase is likely to be generated as in the case of Mn. Therefore, the Cr content is preferably 0.5% or less. In addition, in order to make such an effect more remarkable, 0.04% or more is preferable.
[0045]
V ≦ 0.1%
V forms carbides and nitrides and strengthens the steel. However, if the amount exceeds 0.1%, coarse carbides and nitrides are formed and ductility deteriorates, so 0.1% or less is preferable.
[0046]
Nb ≦ 0.08%
Nb is added for the purpose of appropriately refining the steel and adjusting the crystal grain shape. However, if it exceeds 0.08%, the crystal grains are extremely refined and uniform elongation tends to decrease, so 0.08% or less is preferable.
[0047]
  In addition,In addition to the above ingredients,B ≦ 0.001%, Ni ≦ 0.5%, Cu ≦ 0.5%, Ca ≦ 0.005%, Sb ≦ 0.005%, Co: 0.01%May be added for the purpose of corrosion resistance, prevention of grain boundary oxidation, prevention of vertical cracks, and the like.
[0048]
In the present invention, the plate thickness is preferably less than 2.0 mm. This is because the effect of the present invention becomes significant when the plate thickness is less than 2.0 mm. This is because in steel sheets of less than 2 mm, the strength at the front and rear ends of the coil is likely to be different due to the progress of the strip, especially due to the fact that there is a limit to water cooling at the front end of rolling, and further that accelerated rolling is required. .
[0049]
Next, preferable production conditions for the steel sheet of the present invention as described above will be described.
Here, when the steel having the above composition is heated to the temperature of the austenite single phase region and then hot-rolled, finish rolling is completed at 800 ° C. or higher and wound at 575 to 675 ° C.
[0050]
Finish rolling temperature: 800 ℃ or more
The finishing rolling temperature is important for making the material uniform. If the temperature is less than 800 ° C., the recrystallization rate of the processed γ changes due to the temperature change in the width direction, and fluctuations are observed in the transformation structure. Therefore, the finish rolling temperature is set to 800 ° C. or more.
[0051]
Winding temperature: 575-675 ° C
In the steel of the present invention, the coiling temperature at which strength is ensured by precipitation of carbide containing Ti and one or more of Mo and W was set to 575 to 675 ° C. at which the carbide is likely to precipitate. In the steel of the present invention, ferrite transformation is suppressed by Mo and W, and Ex. Since C and Mn are controlled, the structure becomes uniform in the width direction regardless of the coiling temperature variation in the coil, and carbides containing Ti and one or more of Mo and W are precipitated immediately after the ferrite transformation. To do. For this reason, both strength and ductility are stable.
[0052]
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.
[0053]
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.
[0054]
The hot-rolled steel sheet of the present invention is excellent in surface properties and ductility and has little fluctuation in the material in the coil, so when it is press-molded, its characteristics are utilized, such as automobile parts, particularly suspension members such as suspension arms. A member having a complicated cross-sectional shape at the time of pressing 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.
[0055]
FIG. 2 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.
[0056]
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.
[0057]
In FIG. 2, N1 to N3 may be a work of conveying a steel plate, a member, or a press-formed product mechanically or by an operator.
[0058]
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.
[0059]
FIG. 3 is a block diagram showing the relationship between the apparatus that actually performs the operation shown in FIG. 2 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.
[0060]
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. 3 is the same as the cutting in FIG.
[0061]
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.
[0062]
【Example】
Steel having the chemical components shown in Table 1 is melted and hot-rolled at a heating temperature of 1250 ° C., a finish rolling temperature of about 890 ° C., and a winding temperature of about 620 ° C., and the plate thickness is 1.4 mm and 1.2 mm. A steel plate was produced. The thin film produced from the obtained steel plate was observed with a transmission electron microscope (TEM). The composition of Ti, W, Mo, etc. was grasped from analysis by an energy dispersive X-ray spectrometer (EDX) equipped in the TEM. Moreover, the structure | tissue observation of the matrix was performed with the scanning electron microscope (SEM).
[0063]
Furthermore, JIS No. 5 test specimens were collected from two places, one central part in the width direction and one end part in the width direction, of the central part in the longitudinal direction of the obtained steel sheet, and the tensile properties and changes thereof were investigated. The tensile strength (TS) and elongation (El) at the center in the width direction were obtained as tensile properties, and the change in tensile properties was evaluated by the absolute value of the material difference between the center and the end in the width direction. 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 characteristic results are shown in Table 2.
[0064]
As shown in Table 1, no. 1-7 are Ex. The amount of C, the amount of Mn, and other component compositions are within the scope of the present invention, the matrix structure is ferrite, and precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed. These are examples of the present invention within the scope of the present invention, and as shown in Table 2, good strength stability could be obtained.
[0065]
On the other hand, No. of a comparative example out of the scope of the present invention. 8 and 9, Ex. The amount of C and the amount of Mn deviate from the scope of the present invention. Compared with 1-7, the strength was low and the variation in tensile properties was also large.
[0066]
[Table 1]

[0067]
[Table 2]

[0068]
【The invention's effect】
As described above, according to the present invention, the steel component composition is appropriately controlled, and a carbide containing a specific composition of Ti and one or more of Mo and W is substantially dispersed and precipitated in the ferrite structure. Because of the structure, it is possible to minimize the precipitation of coarse Fe carbides whose morphology changes due to the production heat history of cementite or pearlite due to the interaction between a large amount of solute C immediately after the γ → α transformation and segregated Mn. Further, the γ → α transformation on the run-out table is delayed by the action of Mo or W or Mo + W, and Ti that precipitates stably in a wide temperature range, and fine carbide containing at least one of Mo and W are wound. Steel that has excellent material uniformity because it will precipitate together with the ferrite transformation that progresses at the time of machining, and even if temperature changes on the runout table or fluctuations in the coil occur, structural changes are suppressed. It is obtained. In addition, since fine carbides containing Ti and one or more of Mo and W are substantially dispersed and precipitated in the ferrite structure, high formability and high strength are realized.
[Brief description of the drawings]
FIG. 1 shows Mn content and Ex. The figure which shows the appropriate range of C amount.
FIG. 2 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. 3 is a block diagram showing the relationship between the apparatus that actually performs the work shown in FIG. 1 and the flow of steel plates, members, and press-formed products.

Claims (7)

In mass%, C: 0.03 to 0.15%, Mn ≧ 0.2%, Si ≦ 0.3%, P ≦ 0.02%, Al ≦ 0.1%, S ≦ 0.01%, N ≦ 0.01%, Ti: 0.05 to 0.35%, and one or more selected from Mo ≦ 0.6% and W ≦ 1.5%, and Mo and W are each independently When it is contained, Mo ≧ 0.1%, W ≧ 0.2%, the balance is made of Fe and inevitable impurities, and Ex. C is 0.015% or less, and Mn ≦ 1.7-30 × Ex. C is satisfied, and 95% or more by volume% is a ferrite structure, and precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed in the ferrite structure. A high-formability, high-tensile steel plate with excellent strength stability with a tensile strength of 550 MPa or more.
Ex. C = C- {Ti-N * (48/14) -S * (48/32)} * (12/48) -Mo * (12/96) -W * (12/184) (1)
(However, the element symbol in the formula (1) indicates mass% of each element.) In mass%, C: 0.03 to 0.15%, Mn ≧ 0.2%, Si ≦ 0.3%, P ≦ 0.02%, Al ≦ 0.1%, S ≦ 0.01%, N ≦ 0.01%, Ti: 0.05 to 0.35%, and one or more selected from Mo ≦ 0.6% and W ≦ 1.5%, and Mo and W are each independently When included, Mo ≧ 0.1%, W ≧ 0.2%, and Cr ≦ 0.5%, V ≦ 0.1%, Nb ≦ 0.08%, B ≦ 0.001 %, And the balance consists of Fe and unavoidable impurities, and is represented by the following formula (1). C is 0.015% or less, and Mn ≦ 1.7-30 × Ex. C is satisfied, and 95% or more by volume% is a ferrite structure, and precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed in the ferrite structure. A high-formability, high-tensile steel plate with excellent strength stability with a tensile strength of 550 MPa or more.
Ex. C = C- {Ti-N * (48/14) -S * (48/32)} * (12/48) -Mo * (12/96) -W * (12/184) (1)
(However, the element symbol in the formula (1) indicates mass% of each element.)   The high-formability high-tensile steel sheet excellent in strength stability with a tensile strength of 550 MPa or more according to claim 1 or 2, wherein the plate thickness is less than 2.0 mm.   In carrying out hot rolling after heating the steel having the component composition of claim 1 or claim 2 to the temperature of the austenite single phase region, finish rolling is completed at 800 ° C. or higher and winding at 575 to 675 ° C. A method for producing a high-formability, high-tensile steel sheet having excellent strength stability with a characteristic tensile strength of 550 MPa or more.   A first step of preparing a member made of a steel plate according to any one of claims 1 to 3, 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 steel sheet.   The method for processing a high-formability high-tensile steel sheet according to claim 5, wherein the press-formed product is an automotive part.   The automotive part manufactured with the steel plate in any one of Claims 1-3.

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