JP3821043B2 - Hot-dip galvanized high-strength hot-rolled steel sheet with excellent weldability, manufacturing method and processing method thereof - Google Patents

Description

【００５６】
【発明の効果】
以上説明したように、本発明によれば、溶接性に優れた溶融亜鉛系めっき高張力熱延鋼板を提供することができ、自動車部材の軽量化に寄与する効果が顕著である。
【図面の簡単な説明】
【図１】本発明に係る高張力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。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-dip galvanized high-tensile-strength hot-rolled steel sheet having excellent workability and excellent weldability having a tensile strength of 550 MPa or more suitable for a material for a self-propelled machine such as an automobile, and a manufacturing method and a processing method thereof. About.
[0002]
[Prior art]
From the viewpoint of improving fuel efficiency leading to environmental conservation, there is a strong demand for reducing the strength and thickness of automotive steel sheets. Many automotive members have complicated shapes obtained by press molding, and materials that have both high strength and excellent elongation and stretch flangeability, which are indexes of workability, are required. Further, from the viewpoint of further reducing the weight of the steel sheet, further thinning is directed, and there is a high need for a hot dip galvanized high-tensile hot-rolled steel sheet to reduce the corrosion margin.
[0003]
However, in the past, various high-workability, high-tensile hot-rolled steel sheets have been proposed, but no hot-dip galvanized material with excellent workability has been developed, and hot-dip galvanized plating is possible. However, spot welding, which is frequently used for thin steel plates, evaporates zinc during welding and forms a large blow hole in the welded portion, so that the strength of the welded joint is low and is not practical.
[0004]
For example, Japanese Patent Laid-Open No. 6-172924 proposes a steel plate having excellent stretch flangeability formed by a bainitic ferrite structure having a high dislocation density. However, this steel sheet has a drawback that it has poor elongation because it contains a bainitic ferrite structure with a high dislocation density. Further, since the strength is maintained by bainitic ferrite, the strength is significantly reduced when hot dip galvanizing is performed.
[0005]
Japanese Patent Laid-Open No. 6-200351 proposes a steel plate having excellent stretch flangeability, in which most of the structure is polygonal ferrite and precipitation strengthening and solid solution strengthening are performed centering on TiC. However, to increase the tensile strength of TiC, which is a well-known precipitate used in this steel sheet, a large amount of Ti is required, TiC having a large size is easily generated, and hot dip galvanizing With this heat history, TiC is easily coarsened, and the strength reduction is remarkable in the hot dip galvanized material. Moreover, since this steel positively uses Si that increases the rolling load for improving the properties, the plating adhesion is extremely poor.
[0006]
Japanese Patent Application Laid-Open No. 7-11382 proposes a steel plate having an acicular ferrite structure in which fine TiC and / or NbC is precipitated and having excellent stretch flangeability. However, since this steel sheet is also a structure having a high dislocation density called acicular ferrite, like the steel sheet proposed in Japanese Patent Laid-Open No. 6-172924 described above, the strength reduction is remarkable due to the thermal history of the hot dip galvanizing process. At the same time, like the steel disclosed in Japanese Patent Laid-Open No. 6-200351, the adhesion of the plating is extremely poor because a large amount of Si is added.
[0007]
The present invention has been made in view of such circumstances, and is suitable for an application having a complicated cross-sectional shape at the time of pressing, such as an automobile part, and a hot-dip galvanized high-tensile hot-rolled steel sheet excellent in weldability and its It aims at providing a manufacturing method and a processing method.
[0008]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge.
(I) When a ferrite single phase structure having a low dislocation density is reinforced with fine precipitates, the strength-elongation balance is improved.
(Ii) Strengthening the single-phase structure with fine precipitates improves the strength-stretch flangeability balance.
(Iii) A carbide containing Ti and one or more of W and Mo precipitates finely. By making Ti, Mo, and W in this carbide into an atomic ratio of (Mo + W) / (Ti + Mo + W) ≧ 0.2, this carbide is less likely to be coarsened by heating.
(Iv) When the ferrite single-phase structure is strengthened with the fine precipitates, high workability can be obtained without adding Si, thereby improving the plating adhesion of hot dip galvanizing.
(V) Deformation resistance in the austenite region is reduced by setting the component of the steel to 4.8C + 4.2Si + 0.4Mn + 2Ti ≦ 2.5 in mass %.
(Vi) The blowhole of the weld metal at the time of spot welding is reduced by combining the above heat-resistant precipitate and the deformation resistance reduction of austenite. This is because the steel plate strength is maintained in the intermediate temperature range where the zinc-based plating melts when the temperature of the welding is increased, and because the deformation resistance of the steel plate is large, the applied pressure is transferred to the plating, and the molten galvanizing is efficiently performed between the plates. When the steel sheet is melted and the zinc-based plating is evaporated, the deformation resistance is reduced and the steel sheet can be deformed, and it is possible to promote efficient discharge of zinc vapor between the plates. It is.
[0009]
The present invention has been completed based on these findings and provides the following (1) to (7).
[0010]
(1) In mass %, C: 0.01 to 0.1%, Si ≦ 0.3%, Mn: 0.2 to 2.0%, P ≦ 0.04%, S ≦ 0.02%, Al ≦ 0.1%, N ≦ 0.006%, Ti: 0.03 to 0.2%, and one or more of Mo ≦ 0.5% and W ≦ 1.0%, the balance Consists of Fe and inevitable impurities ,
4.8C + 4.2Si + 0.4Mn + 2Ti ≦ 2.5% by mass
And the structure is ferrite with an area ratio of 98% or more (excluding 98%) ,
Atomic ratio, (Mo + W) / (Ti + Mo + W) ≧ 0.2
A hot dip galvanized high-tensile hot-rolled steel sheet excellent in weldability, characterized in that precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed within a range satisfying the above.
[0011]
(2) By mass %, C: 0.01 to 0.1%, Si ≦ 0.3%, Mn: 0.2 to 2.0%, P ≦ 0.04%, S ≦ 0.02%, Al ≦ 0.1%, N ≦ 0.006%, Ti: 0.03 to 0.2%, and one or more of Mo ≦ 0.5% and W ≦ 1.0%, One or more of V ≦ 0.15% and Nb ≦ 0.08%, with the balance being Fe and inevitable impurities ,
4.8C + 4.2Si + 0.4Mn + 2Ti + 3.5Nb ≦ 2.5 by mass %
And the structure is ferrite with an area ratio of 98% or more (excluding 98%) ,
At atomic ratio, (Mo + W) / (Ti + Mo + W + V + Nb) ≧ 0.2
In the range satisfying the above, melting with excellent weldability, characterized in that precipitates of less than 10 nm including Ti, one or more of V and Nb, and one or more of Mo and W are dispersed. Zinc-based high-strength hot-rolled steel sheet.
[0012]
(3) In the above (1) or (2), in mass %, B ≦ 0.001%, Cr ≦ 0.5%, Cu ≦ 0.5%, Ni ≦ 0.5 % , Ca ≦ 0 Hot-dip galvanized high-tensile hot-rolled steel sheet excellent in weldability, characterized by containing 0.01% and a total of REM ≦ 0.1%.
[0013]
(4) Hot-rolling the steel having the composition of any one of (1) to (3) above in an austenite single-phase region, winding it at 550 ° C. or more, and producing a ferrite single-phase hot-rolled steel sheet, Further, a method for producing a hot-dip galvanized high-tensile hot-rolled steel sheet excellent in weldability, characterized in that scale removal or hot-dip galvanizing is performed as it is.
[0014]
(5) A first step of preparing a member made of the high-tensile hot-rolled steel sheet according to any one of (1) to (3) above, and press-molding the member to process a press-formed product having a desired shape. A processing method of a high-tensile hot-rolled steel sheet having the second step.
[0015]
(6) In the above (5), the press-formed product is a processing method for a high-tensile hot-rolled steel sheet which is an automotive part.
[0016]
(7) An automotive part manufactured from the high-tensile hot-rolled steel sheet according to any one of (1) to (3).
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be specifically described by dividing it into a metal structure, a chemical component composition, and a production method.
[0018]
[Metal structure]
The hot-dip galvanized high-strength hot-rolled steel sheet according to the present invention is a ferrite whose structure has an area ratio of 98% or more , and in the range that satisfies (Mo + W) / (Ti + Mo + W) ≧ 0.2 by atomic ratio therein, A carbide containing Ti and one or more of Mo and W is dispersed. The size of this carbide is less than 10 nm. In addition to these, the carbide may contain one or more of Nb and V. Hereinafter, these will be described.
[0019]
・Ferrite with an area ratio of 98% or more :
The reason why the matrix is made of ferrite with an area ratio of 98% or more and a ferrite single phase structure is that ferrite with a low dislocation density is effective for improving elongation, and single phase for improving stretch flangeability. This is because it is effective to form a structure, and the effect is particularly remarkable in a ferrite single-phase structure rich in ductility.
[0020]
A precipitate containing Ti satisfying (Mo + W) / (Ti + Mo + W) ≧ 0.2 in atomic ratio and one or more of Mo and W:
Inclusion of Ti and one or more of Mo and W in the carbide as a precipitate is an important requirement of the present invention. To make the hot-rolled sheet structure a single phase of ferrite, disperse fine precipitates in the grains, and suppress the formation of coarse carbides exceeding 1 μm at the grain boundaries, W or Mo is required, and carbides containing these are formed. I have to let it. In steels with precipitation of TiC and NbC in the prior art, coarse carbides are inevitably produced when the matrix is a ferrite single phase. Moreover, TiC and NbC are easily coarsened and it is difficult to obtain high strength. For this reason, they are unsuitable for increasing the strength, and in order to obtain the required strength, it is necessary to add them up to the deterioration of workability. On the other hand, carbides containing Ti and one or more of Mo and W and satisfying (Mo + W) / (Ti + Mo + W) ≧ 0.2 in atomic ratio are finely precipitated and stable, so that the strength is improved. Therefore, sufficient strength can be obtained with the amount of precipitates that can maintain good workability. Furthermore, favorable intensity | strength can be obtained by the magnitude | size being less than 10 nm. The size of the precipitate is preferably 5 nm or less. These fine precipitates maintain high strength of the steel in the middle temperature range at the initial stage of spot welding and promote discharge from the galvanized plate to plate.
[0021]
-A precipitate containing one or more of V and Nb in addition to Ti and one or more of Mo and W:
Even if the carbide as a precipitate contains at least one of Ti and Mo and W, and at least one of V and Nb, the atomic ratio of (Mo + W) / (Ti + Mo + W + V + Nb) ≧ 0.2 is satisfied. If it is satisfied, the carbide can exist stably and finely like the carbide containing Ti and one or more of Mo and W. For this reason, as a carbide | carbonized_material, in addition to Ti and 1 or more types of Mo and W, 1 or more types of Nb and V may be contained. However, the Nb and V amounts are preferably less than half of the Ti amount.
[0022]
[Chemical composition]
In this invention, in order to implement | achieve the said metal structure, the component composition is the mass %, C: 0.01-0.1%, Si <= 0.3%, Mn: 0.2-2.0% P ≦ 0.04%, S ≦ 0.02%, Al ≦ 0.1%, N ≦ 0.006%, Ti: 0.03 to 0.2%, and Mo ≦ 0.5% One or more of W ≦ 1.0%, the balance being made of Fe and inevitable impurities , satisfying 4.8C + 4.2Si + 0.4Mn + 2Ti ≦ 2.5 by mass %, and the balance being substantially made of Fe And Furthermore, in the case where one or more of Nb and V are contained in the composite precipitate as described above, in addition to the above components, one or more of Nb ≦ 0.08% and V ≦ 0.15% are contained, 4.8C + 4.2Si + 0.4Mn + 2Ti + 3.5Nb ≦ 2.5 is satisfied, and the balance is made of Fe and inevitable impurities . Hereinafter, each of these components will be described.
[0023]
C: 0.01 to 0.1%
C forms carbides and is effective for strengthening steel. However, if the content is less than 0.01%, the steel is not sufficiently strengthened. If the content exceeds 0.1%, pearlite tends to be formed, and the stretch flangeability deteriorates. For this reason, C content was made into 0.01 to 0.1%.
[0024]
Si: 0.3% or less Si is an effective element for solid solution strengthening, but if added over 0.3%, the plating adhesion of galvanizing deteriorates. Further, since C is discharged from α to γ during the γ → α transformation, the structure becomes difficult to become a ferrite single phase when the addition amount is large, and the coarsening of precipitates that play an important role in the present invention is caused. It causes a decrease in strength. For these reasons, the Si content is set to 0.3% or less.
[0025]
Mn: 0.2 to 2.0%
Mn is required to be 0.2% or more from the viewpoint of strengthening the steel by solid solution strengthening, but if added over 2.0%, segregation occurs and pearlite is easily generated. For this reason, the Mn content is set to 0.2 to 2.0%. From the viewpoint of strengthening steel, 0.5% or more is preferable, and when importance is placed on stability of strength, less than 1.5% is preferable.
[0026]
P: 0.04% or less P is effective for solid solution strengthening, but if added over 0.04%, it segregates at the grain boundaries and deteriorates the elongation, so 0.04% or less.
[0027]
S: 0.02% or less S is preferably as small as possible. In a ferrite single phase structure, if it exceeds 0.02%, the stretch flangeability is deteriorated. Preferably it is 0.01% or less, More preferably, it is 0.005% or less, Desirably 0.003% or less.
[0028]
Al: 0.1% or less Al is added as a deoxidizer. However, if it exceeds 0.1%, both stretch flangeability and elongation tend to decrease.
[0029]
N: 0.006% or less N is preferably as small as possible. If it exceeds 0.006%, coarse nitrides increase and stretch flangeability is deteriorated, so the content is made 0.006% or less.
[0030]
Mo: 0.5% or less Mo is an important element in the present invention, and suppresses pearlite transformation while containing fine carbides with Ti or one or more of Nb and V, in addition to Ti, Nb and By forming a fine carbide containing one or more of V and making the hot-rolled sheet into a ferrite single phase, excellent elongation and stretch flangeability can be secured, and the steel can be strengthened. However, if it exceeds 0.5%, a hard phase is formed and the stretch flangeability tends to be lowered. For this reason, Mo content was 0.5% or less. When W is not added, Mo is required to be 0.05% or more in order to obtain such an effect, so the Mo content is set to 0.05% or more.
[0031]
W: 1.0% or less W, like Mo, is an important element in the present invention, and contains fine composite precipitates with Ti or one or more of Nb and V while suppressing pearlite transformation. Can form fine carbides containing at least one of Nb and V in addition to Ti, ensuring excellent elongation and stretch flangeability, and strengthening the steel. However, if added over 1.0%, a hard phase is formed and the stretch flangeability tends to be lowered. For this reason, the W content is set to 1.0% or less. When Mo is not added, in order to obtain such an effect, W is required to be 0.1% or more, so the W content is set to 0.1% or more.
[0032]
Ti: 0.03-0.2%
Ti is an important element in the present invention. By forming fine carbides with Mo and W, the structure becomes a ferrite single phase and further strengthened. However, if it is less than 0.03%, the effect of strengthening steel is insufficient, and if it exceeds 0.2%, bainite and bainitic ferrite tend to be generated in the hot-rolled sheet. Therefore, the Ti content is set to 0.03 to 0.2%.
[0033]
Nb: 0.08% or less Nb is effective for refining the structure and forms fine carbides with Ti, Mo and / or W, and is added as necessary. However, if the amount of Nb exceeds 0.08%, the crystal grains extend in one direction and a structure unfavorable for stretch flangeability develops. Therefore, when Nb is contained, the content is made 0.08% or less. From the viewpoint of obtaining the Nb microstructure refinement effect, 0.005% or more is preferable.
[0034]
V: 0.15% or less Since precipitation of V is slow, V is added as necessary to adjust the precipitation timing of the carbide, which is a precipitate. However, if the V content exceeds 0.15%, a hard phase is formed in the hot-rolled sheet and the stretch flangeability deteriorates. Therefore, when V is contained, the content is made 0.15% or less.
[0035]
In addition to the above definition, it is necessary to satisfy 4.8C + 4.2Si + 0.4Mn + 2Ti ≦ 2.5 by mass %. When Nb is contained, it is necessary to satisfy 4.8C + 4.2Si + 0.4Mn + 2Ti + 3.5Nb ≦ 2.5. In the present invention, at the time of spot welding, the deformation resistance must be high at the ferrite region temperature and the deformation resistance must be low at the austenite region, while being high strength. This ferrite, hard, austenitic, and soft property promotes the elimination of zinc plating and zinc vapor during welding. Since the strength of austenite is affected by the composition of the steel, the deformation resistance of austenite is made low by defining it according to the above formula depending on the component that affects the deformation resistance. If the value of 4.8C + 4.2Si + 0.4Mn + 2Ti exceeds 2.5, the deformation resistance of austenite increases and zinc vapor is not smoothly discharged, resulting in blowholes in the weld. Since the conventional steel sheet has a high value of 4.8C + 4.2Si + 0.4Mn + 2Ti, the generation of blow holes could not be sufficiently prevented. Thus, although the value of 4.8C + 4.2Si + 0.4Mn + 2Ti is 2.5 or less, the effect of discharging zinc vapor can be obtained, but it is preferably 2.0 or less, and more preferably 1.5 or less. Similarly, when Nb is contained, the value of 4.8C + 4.2Si + 0.4Mn + 2Ti + 3.5Nb is 2.5 or less, preferably 2.0 or less, and more preferably 1.5 or less.
[0036]
Here, the blowhole in question significantly reduces the joint strength, and there is no problem even if there is a minute blowhole that does not greatly affect the strength.
[0037]
Since discharge of such zinc vapor proceeds more smoothly when the plate thickness is small, the plate thickness of the steel plate is preferably 3 mm or less from such a viewpoint. Since such an effect becomes remarkable when the plate thickness is 2 mm or less, the thickness is desirably 2 mm or less.
[0038]
In the present invention, B ≦ 0.001%, Cr ≦ 0.5 % , Cu ≦ 0.5%, Ni ≦ 0.5%, depending on purposes such as secondary work brittleness resistance and corrosion resistance improvement. Even if one or more of Ca ≦ 0.01% and REM (total) ≦ 0.1% are included, there is no problem in characteristics.
[0039]
[Production method]
In the present invention, when producing the above hot-dip galvanized high-tensile hot-rolled steel sheet, it is hot-rolled in an austenite single-phase region, wound at 550 ° C. or more, and after producing a ferrite single-phase hot-rolled steel sheet, the scale is further removed. Alternatively, hot dip galvanizing is applied as it is.
Hereinafter, these conditions will be described.
[0040]
-Finish rolling end temperature Finish rolling end temperature is important for elongation and stretch flangeability. When rolling is performed at a temperature at which the austenite single phase region cannot be maintained, coarse grains are generated and stretch flangeability is impaired. Therefore, the austenite single phase region is maintained until the end of finish rolling.
[0041]
The coiling temperature is 550 ° C. or higher so that the microstructure is ferrite and fine carbides containing Ti and one or more of Mo and W are precipitated in the ferrite. Desirably, it is 600 degreeC or more. Moreover, since it becomes easy to produce | generate pearlite when it exceeds 675 degreeC, 675 degreeC or less is desirable. More preferably, it is 650 degrees C or less.
[0042]
-Hot-dip galvanized plating In the present invention, a hot-dip galvanized coating film is formed on the surface of a high-tensile hot-rolled steel plate to obtain a hot-dip galvanized steel plate. An alloyed hot dip galvanized steel sheet that has been subjected to an alloying reaction after hot dip galvanizing is also included. The hot dip galvanizing in the present invention is hot dip plating in which the plating film is substantially made of Zn or hot dip plating mainly composed of Zn, and may contain alloy elements such as Cr and Mn in addition to zinc. .
[0043]
The hot dip galvanized high-tensile hot-rolled steel sheet of the present invention is excellent in workability, particularly in stretch flangeability, so when it is press-molded, its characteristics are utilized, such as automotive parts, particularly suspension arms. A member having a complicated cross-sectional shape at the time of pressing, such as a suspension member or a reinforcement, can be manufactured with good quality, and in particular, can contribute to weight reduction of a press-formed product. The processing method of the high-tensile steel plate according to the present invention, in other words, the manufacturing method of the press-formed product will be described below specifically.
[0044]
FIG. 1 is a flowchart showing an example of a work flow of a method for processing a high-strength 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 high-tensile 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.
[0045]
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.
[0046]
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.
[0047]
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.
[0048]
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 high-tensile steel plate 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.
[0049]
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.
[0050]
The press-formed product produced in this way uses the high-tensile steel plate according to the present invention that is excellent in workability as a raw material, particularly excellent in stretch flangeability, so even if the cross-sectional shape at the time of pressing is complicated Can be manufactured with good quality and light weight. Such a feature is particularly useful when the press-formed product is a member for automobiles, in particular, a suspension member such as a suspension arm or a reinforcement component such as a reinforcement member.
[0051]
【Example】
Steel having the chemical components shown in Table 1 was heated to 1250 ° C., and hot-rolled at a finishing temperature of 920 ° C. and a coiling temperature of 600 to 650 ° C. (see Table 1). The obtained steel sheet was pickled and then hot dip galvanized. After plating, alloying treatment was performed. The thin film produced from the obtained steel plate was observed with a transmission electron microscope (TEM), and the precipitated carbide dimensions were measured. Moreover, the elemental analysis which comprises a carbide | carbonized_material was performed using the energy dispersive X-ray spectrometer (EDX). Furthermore, the concentration of each element was calculated from the peak value.
[0052]
Further, a JIS No. 5 tensile test piece and a hole expansion test piece were collected from the plated plate. Tensile test pieces were taken from the vertical direction of rolling and subjected to a tensile test. The hole expansion test was conducted in accordance with the Japan Iron and Steel Federation standard. Subsequently, resistance spot welding was performed according to the WES standard, and the tensile shear strength of the welded joint was measured. In addition, the cross-sectional structure of the weld was observed and the occurrence of blowholes was ascertained.
[0053]
Table 1 also shows the coiling temperature, structure, size of precipitates, mechanical properties, and joint strength.
In Table 1, No. 1 to 7 are examples of the present invention. 8 to 10 are comparative examples. Among the examples of the present invention, No. 1, 3, 5, 6, and 7 are examples of hot-rolled steel sheets of 780 MPa class. All of these have a ferrite single phase structure. 1 is a carbide containing Ti and Mo; 3 is a carbide containing Ti, Mo, W, No. 3; No. 5 is a carbide containing Ti, Mo, W, Nb, No. 5; No. 6 is a carbide containing Ti, Mo, W, V, No. 6; In No. 7, carbides containing Ti and Mo are precipitated, and each carbide has a particle size of less than 10 nm, and (Mo + W) / (Ti + Mo + W) ≧ 0.2 or (Mo + W) / (Ti + Mo + W + V + Nb) ≧ 0.2. All of them had excellent elongation and stretch flangeability. No. 7 is an example in which B is added for resistance to secondary work brittleness and Ca is added for improving stretch flangeability, but the effect of the present invention is obtained. No. Nos. 2 and 4 are examples of 980 MPa class hot-rolled steel sheets, both of which have a ferrite single phase structure. 2 is a carbide containing Ti and Mo; In No. 4, carbides containing Ti, Mo, and W are precipitated in the ferrite, and all the carbides have a particle size of less than 10 nm and satisfy (Mo + W) / (Ti + Mo + W) ≧ 0.2. And stretch flangeability. No. The weld joint strengths of 1 to 7 all exceeded 20 kN, indicating good joint strength. Also, no blowholes that could affect the joint strength were found in the weld.
[0054]
On the other hand, the comparative example No. No. 8 has a large amount of Si and poor plating adhesion, and the value of 4.8C + 4.2Si + 0.4Mn + 2Ti + 3.5Nb which determines the deformation resistance of austenite is as high as 3.68, the precipitates are coarse, and the welding strength is below 20 kN. The value was low. Coarse blowholes were generated in the weld, which reduced the joint strength. The strength of the C No. Compared to 5, it was extremely low. No. No. 9 was obtained by adding Si and making the structure an acicular ferrite, but the size of the precipitate was coarse and the strength was low. The plating adhesion was poor and the weld joint strength was low. No. Since 10 also has a large amount of Si, the structure of the plated steel sheet became ferrite + pearlite and the precipitates became coarse. The plating adhesion was poor, remarkable blowholes were generated in the weld, and the joint strength was also reduced. Thus, it was confirmed that a good weld joint can be obtained by employing the present invention.
[0055]
[Table 1]

[0056]
【The invention's effect】
As described above, according to the present invention, a hot-dip galvanized high-tensile hot-rolled steel sheet excellent in weldability can be provided, and the effect of contributing to weight reduction of automobile members is remarkable.
[Brief description of the drawings]
FIG. 1 is a flowchart showing an example of a work flow of a method for processing a high-tensile steel plate 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.

Claims (7)

In mass %, C: 0.01 to 0.1%, Si ≦ 0.3%, Mn: 0.2 to 2.0%, P ≦ 0.04%, S ≦ 0.02%, Al ≦ 0 0.1%, N ≦ 0.006%, Ti: 0.03 to 0.2%, and one or more of Mo ≦ 0.5% and W ≦ 1.0%, with the balance being Fe and Consisting of inevitable impurities ,
4.8C + 4.2Si + 0.4Mn + 2Ti ≦ 2.5% by mass
And the structure is ferrite with an area ratio of 98% or more (excluding 98%) ,
Atomic ratio, (Mo + W) / (Ti + Mo + W) ≧ 0.2
A hot dip galvanized high-tensile hot-rolled steel sheet excellent in weldability, characterized in that precipitates of less than 10 nm containing Ti and one or more of Mo and W are dispersed within a range satisfying the above. In mass %, C: 0.01 to 0.1%, Si ≦ 0.3%, Mn: 0.2 to 2.0%, P ≦ 0.04%, S ≦ 0.02%, Al ≦ 0 0.1%, N ≦ 0.006%, Ti: 0.03 to 0.2%, and one or more of Mo ≦ 0.5% and W ≦ 1.0%, and further V ≦ 0 .15%, including one or more of Nb ≦ 0.08%, the balance being Fe and inevitable impurities ,
4.8C + 4.2Si + 0.4Mn + 2Ti + 3.5Nb ≦ 2.5 by mass %
And the structure is ferrite with an area ratio of 98% or more (excluding 98%) ,
At atomic ratio, (Mo + W) / (Ti + Mo + W + V + Nb) ≧ 0.2
In the range satisfying the above, melting with excellent weldability, characterized in that precipitates of less than 10 nm including Ti, one or more of V and Nb, and one or more of Mo and W are dispersed. Zinc-based high-strength hot-rolled steel sheet. In addition, B ≦ 0.001%, Cr ≦ 0.5%, Cu ≦ 0.5%, Ni ≦ 0.5 % , Ca ≦ 0.01%, and the total of REM ≦ 0.1%. The hot-dip galvanized high-tensile hot-rolled steel sheet having excellent weldability according to claim 1 or 2.   The steel having the composition according to any one of claims 1 to 3 is hot-rolled in an austenite single-phase region, wound at 550 ° C or higher, and a ferritic single-phase hot-rolled steel sheet is manufactured, and further scale-removed. Or the manufacturing method of the hot dip galvanized high-tensile-strength hot-rolled steel plate excellent in the weldability characterized by performing hot dip galvanizing as it is.   A first step of preparing a member made of the high-tensile hot-rolled steel sheet 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-tensile hot-rolled steel sheet comprising the steps of:   The method for processing a high-tensile hot-rolled steel sheet according to claim 5, wherein the press-formed product is an automotive part.   An automotive part manufactured from the high-tensile hot-rolled steel sheet according to any one of claims 1 to 3.

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