KR101160016B1 - High strength hot-rolled steel for hydroforming with excellent workability and method of manufacturing the hot-rolled steel - Google Patents

Abstract

Disclosed are a hot rolled steel sheet for hydroforming having excellent workability and a method of manufacturing the same.
Method for producing a hot rolled steel sheet for hydroforming according to the present invention is carbon (C): 0.2 ~ 0.24% by weight, silicon (Si): 0.15 ~ 0.25% by weight, manganese (Mn): 1.3 ~ 1.5% by weight, phosphorus (P): 0.018% by weight or less, sulfur (S): 0.005% by weight or less, chromium (Cr): 0.05 to 0.2% by weight, and the remaining iron (Fe) and unavoidable impurities are slab of FDT (Finishing Delivery Temperature): 850 to 890 ° C. After hot rolling, the coil is wound at CT (Coiling Temperature): 640 ~ 660 ℃.

Description

High-strength hot-rolled steel sheet for hydroforming with excellent processability, and method for manufacturing the same

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for producing hot rolled steel sheet for hydroforming applied to structural members of automobiles, and more particularly, to a high strength hot rolled steel sheet for hydroforming having high strength of 540 MPa or more, and a method of manufacturing the same. .

As structural members for automobiles, hollow members having various cross-sectional shapes are used. The hollow member is mainly manufactured by joining parts formed by press working of steel sheets by spot welding.

In recent years, hollow members for structural members of automobiles are required to have a higher impact absorbing ability at the time of a collision, and thus a stronger material is required. However, in the conventional press molding method, it is increasingly difficult to produce a member having excellent shape and dimensional accuracy without forming defects.

Hydroforming has recently attracted attention as a new molding method for solving such a problem.

Hydroforming is a method of injecting a high pressure liquid into a steel pipe to form a desired shape, and by changing the cross-sectional dimension of the steel pipe by expanding the pipe, etc., it is possible to integrally form a complex-shaped member and to increase strength and resistance. It is a way.

  Such materials applied to hydroforming are required to have high formability and processability.

It is an object of the present invention to provide a high strength hot rolled steel sheet for hydroforming and a method of manufacturing the same, having a tensile strength of 540 MPa grade and excellent processability and formability through alloying and process control.

Method for producing a high strength hot rolled steel sheet for hydroforming according to an embodiment of the present invention for achieving the above one object is carbon (C): 0.2 ~ 0.24% by weight, silicon (Si): 0.15 ~ 0.25% by weight, manganese (Mn) : 1.3 to 1.5% by weight, chromium (Cr): 0.05 to 0.2% by weight and hot rolled slab plate consisting of the remaining iron (Fe) and inevitable impurities to FDT (Finishing Delivery Temperature): 850 to 890 ° C .; And cooling the hot rolled plate, and winding it at a CT (Coiling Temperature): 640 to 660 ° C.

The slab plate may include phosphorus (P): 0.018% by weight or less and sulfur (S): 0.005% by weight or less.

High-strength hot-rolled steel sheet for hydroforming according to an embodiment of the present invention for achieving the above object is carbon (C): 0.2 ~ 0.24% by weight, silicon (Si): 0.15 ~ 0.25% by weight, manganese (Mn): 1.3 ~ 1.5 Wt%, chromium (Cr): 0.05 to 0.2% by weight and the remaining iron (Fe) and inevitable impurities, characterized in that the microstructure comprises a ferrite (perrite) + pearlite (pearlite) complex structure.

The hot rolled steel sheet may include phosphorus (P): 0.018% by weight or less and sulfur (S): 0.005% by weight or less.

The hot rolled steel sheet may have mechanical properties such as tensile strength (TS) of 540 MPa or more, elongation (EL) of 28% or more, work hardening index (n-value) of 0.17 or more, and yield ratio (YP / TS) of less than 77%. .

The high strength hot rolled steel sheet for hydroforming produced by the method according to the present invention may exhibit high elongation (EL) of 28% or more and an excellent work hardening index (n-value) of 0.17 or more, even though the tensile strength is 540 MPa or more.

Accordingly, the hot rolled steel sheet prepared according to the present invention can be utilized as a material for hydroforming, which requires high strength and excellent workability and formability.

Therefore, the high strength hot rolled steel sheet for hydroforming according to the present invention can contribute to the weight reduction of automobile parts, and also has a useful effect that can be used as a high strength mechanical structural steel pipe.

1 is a flow chart schematically showing a method of manufacturing a high strength hot rolled steel sheet for hydroforming according to an embodiment of the present invention.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

Hereinafter, with reference to the accompanying drawings, a high-strength hot-rolled steel sheet for hydroforming and a method of manufacturing the same according to a preferred embodiment of the present invention will be described in detail.

High strength hot rolled steel sheet for hydroforming excellent workability according to the present invention is carbon (C): 0.2 ~ 0.24% by weight, silicon (Si): 0.15 ~ 0.25% by weight, manganese (Mn): 1.3 ~ 1.5% by weight, chromium (Cr ): 0.05 to 0.2 wt%. In addition, the high strength hot rolled steel sheet for hydroforming according to the present invention may include phosphorus (P): 0.018% by weight or less and sulfur (S): 0.005% by weight or less. The rest other than the alloying components consist of iron and other unavoidable impurities.

Hereinafter, the role and content of each component included in the high strength hot rolled steel sheet for hydroforming according to the present invention will be described.

Carbon (C)

Carbon (C) is an element that contributes to increasing the strength of steel. The hot rolled steel sheet according to the present invention contains a relatively large amount of carbon corresponding to the high carbon region in order to ensure sufficient strength and weldability.

The carbon is preferably added in an amount of 0.2 to 0.24% by weight of the total weight of the hot rolled steel sheet according to the present invention. When the carbon content is less than 0.2% by weight, it is difficult to secure the desired tensile strength, and there is a high possibility that a problem occurs in the weld with the parts to be welded together. On the contrary, when the carbon content exceeds 0.24% by weight, there is a problem that the moldability is lowered.

Silicon (Si)

Silicon (Si) contributes to securing strength and, in particular, serves as a deoxidizer to remove oxygen in the steel.

The silicon is preferably added in 0.15 ~ 0.25% by weight of the total weight of the hot rolled steel sheet according to the present invention. If the content of silicon is less than 0.15% by weight, the deoxidation effect is insufficient. On the contrary, when the content of silicon exceeds 0.25% by weight, there is a high possibility that the weldability is lowered, and there is a problem of lowering the surface quality by generating red scales during hot rolling.

On the other hand, it is preferable that the silicon is added so that 6 ≦ [Mn] / [Si] ≦ 9 (where [] is the weight% of each component) in relation to manganese (Mn). This is to prevent the presence of solid inclusions in the welded portion when the tube is welded using electric resistance welding (ERW). When [Mn] / [Si] is 6 to 9, the melting temperature of the Mn-Si-based oxide is the welding temperature. It is lowered and a healthy welded part can be obtained because liquid oxide is discharged out of a welded part in an ERW piping process.

Manganese (Mn)

Manganese (Mn) is an element that increases the strength and toughness of steel and increases the ingotability of steel. Addition of manganese causes less deterioration of ductility when strength is increased than that of carbon.

The manganese is preferably added at 1.3 to 1.5% by weight of the total weight of the hot rolled steel sheet according to the present invention. If the manganese content is less than 1.3% by weight, high carbon content is insufficient to secure strength. On the contrary, when the content of manganese exceeds 1.5% by weight, the amount of MnS-based non-metallic inclusions increases, so there is a problem that defects such as cracking during welding occur.

Chrome (Cr)

Chromium (Cr) is a ferrite forming element and also promotes grain growth. In general, as the grain size increases, the work hardening index (n-value) also increases, so chromium is an effective element for securing workability.

The chromium is preferably added in a content ratio of 0.05 to 0.2% by weight of the total weight of the hot rolled steel sheet according to the present invention.

When the content of chromium is less than 0.05% by weight, the effect of improving strength or workability due to the addition of chromium is insufficient. On the contrary, when the content of chromium exceeds 0.2% by weight, the brittleness of the steel may be rather increased, and the manufacturing cost may be increased.

Phosphorus (P)

Phosphorus (P) is an element that has a high possibility of segregation in the production of steel sheet, not only the center segregation but also fine segregation, which adversely affects the material, and also causes delayed fracture that occurs after a certain time after forming.

Therefore, the content of phosphorus (P) is preferably limited to 0.018% by weight or less of the total weight of the hot rolled steel sheet according to the present invention.

Sulfur (S)

Sulfur (S) combines with manganese to form non-metallic inclusions such as MnS, causing defects such as hook cracks during the tubing process.

Therefore, the content of sulfur (S) is preferably limited to 0.005% by weight or less of the total weight of the hot rolled steel sheet according to the present invention.

Hereinafter, a method of manufacturing a high strength hot rolled steel sheet for hydroforming according to the present invention will be described.

1 is a flow chart schematically showing a method of manufacturing a high strength hot rolled steel sheet for hydroforming according to an embodiment of the present invention.

Referring to FIG. 1, the illustrated method for manufacturing a high strength hot rolled steel sheet for hydroforming according to the present invention includes a hot rolling step (S110) and a winding step (S120).

In the hot rolling step (S110), carbon (C): 0.2 to 0.24 wt%, silicon (Si): 0.15 to 0.25 wt%, manganese (Mn): 1.3 to 1.5 wt%, chromium (Cr): 0.05 to 0.2 wt% And the slab plate made of the remaining iron (Fe) and inevitable impurities hot rolling FDT (Finishing Delivery Temperature): 850 ~ 890 ℃.

The slab sheet may include phosphorus (P): 0.018 wt% or less and sulfur (S): 0.005 wt% or less. Further, silicon (Si) and manganese (Mn) may be included in a range that satisfies 6 ≦ [Mn] / [Si] ≦ 9, where [] is a weight percent of each component.

Finish rolling temperature (FDT) in the hot rolling step (S110) is preferably 850 ~ 890 ℃. When the finish rolling temperature exceeds 890 ° C in the hot rolling step, the grains of austenite after rolling are coarsened. As a result, the crystal grains of the ferrite after transformation are coarsened, which lowers toughness and adversely affects strength. On the contrary, if the finishing rolling temperature is too low below 850 ° C, problems such as generation of a mixed structure due to abnormal reverse rolling may occur.

On the other hand, the present invention may further include a step of reheating the slab plate material SRT (Slab Reheating Temperature): 1150 ~ 1250 ℃ before hot rolling. Components that are segregated during casting by slab reheating can be reused.

Next, in the winding step (S120) to cool the hot-rolled sheet, it is wound at CT (Coiling Temperature): 640 ~ 660 ℃.

In the winding-up step S120, the winding temperature CT is preferably 640 to 660 ° C. In the present invention, the coiling temperature corresponds to a relatively high temperature range. As the coiling temperature increases, the ferrite grain size increases and the work hardening index (n-value) increases. However, when the coiling temperature exceeds 660 ℃, the grain size becomes very large, resulting in a decrease in strength and an elongation (EL) is also reduced.

On the contrary, when the coiling temperature is lower than 640 ° C, the ferrite grain size decreases, and the carbon solubility in the ferrite increases. Also, as the shape of the fine grains changes into an acicular type, the strength increases and thus the elongation decreases. .

The final microstructure of the hot rolled steel sheet produced through the alloying component and hot rolling conditions presented above may include a ferrite + pearlite composite structure.

In addition, the manufactured hot rolled steel sheet may have a tensile strength (TS) of about 540 MPa or more, about 540 to 650 MPa, and an elongation (EL) of 28% or more. In addition, the hot rolled steel sheet manufactured may have a work hardening index (n-value) of about 0.17 or more, about 0.17 to about 0.20, and a yield ratio (YP / TS) of less than 77%.

As described above, the hot rolled steel sheet according to the present invention has a tensile strength of 540 MPa or more and has a high elongation and work hardening index through setting of a finish rolling temperature (FDT) and a winding temperature (CT) during the adjustment of the alloy component system and hot rolling. It may be due to the microstructure control.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Preparation of Hot Rolled Specimens

Specimens 1 to 5 having the compositions shown in Table 1 were melted in a vacuum melting furnace to produce an ingot, and then hot rolled to a finish rolling temperature of 870 ° C., and then cooled to 650 ° C.

 [Table 1] (unit:% by weight)

2. Evaluation of mechanical properties

Table 2 shows the results of evaluation of the mechanical properties of each of the prepared hot-rolled specimens 1 to 5.

[Table 2]

Referring to Table 2, for specimens 1 and 2 corresponding to the examples, tensile strength: 540 MPa or more, elongation (EL): 28% or more, work hardening index (n-value): 0.17 or more, yield ratio (YP / TS) can be seen to satisfy all less than 77%.

On the other hand, in the case of specimen 3 corresponding to the comparative example, chromium (Cr) was not added, so the work hardening index (n-value) did not reach the target value.

In addition, in the case of specimen 4 corresponding to the comparative example, the tensile strength exceeded the target value as the carbon (C) content exceeded 0.24% by weight, but the elongation (EL) did not reach the target value, and the yield ratio (YP / TS) was also 79.3%, which was not good for processing due to high yield ratio.

In addition, in the case of the specimen 5 corresponding to the comparative example, the manganese (Mn) content is less than 1.3% by weight, did not reach the target strength.

As described above, the hot rolled steel sheet for hydroforming according to the present invention has a tensile strength of 540 MPa or more, and is excellent in workability and formability, thereby contributing to the weight reduction of automotive parts to which hydroforming is applied, and also a high strength machine. It can be used as structural steel pipe.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: hot rolling stage
S120: winding step

Claims (9)

Carbon (C): 0.2 to 0.24% by weight, Silicon (Si): 0.15 to 0.25% by weight, Manganese (Mn): 1.3 to 1.5% by weight, Chromium (Cr): 0.05 to 0.2% by weight and the remaining iron (Fe) Hot-rolling the slab plate made of unavoidable impurities to a Finishing Delivery Temperature (FDT): 850 to 890 ° C; And
Cooling the hot-rolled sheet, CT (Coiling Temperature): winding at 640 ~ 660 ℃; high-strength hot-rolled steel sheet manufacturing method comprising a.
The method of claim 1,
Reheating the slab plate SRT (Slab Reheating Temperature): 1150 ~ 1250 ℃ before the hot rolling; Hydro-forming high-strength hot-rolled steel sheet manufacturing method further comprising.
The method of claim 1,
The slab plate
Phosphorus (P): 0.018% by weight or less and sulfur (S): 0.005% by weight or less, characterized in that a high strength hot rolled steel sheet manufacturing method for hydroforming.
The method of claim 1,
The slab plate is
High strength hot rolled steel for hydroforming, characterized in that silicon (Si) and manganese (Mn) are contained in a range satisfying 6 ≤ [Mn] / [Si] ≤ 9 (where [] is a weight% of each component). Steel plate manufacturing method.
Carbon (C): 0.2 to 0.24% by weight, Silicon (Si): 0.15 to 0.25% by weight, Manganese (Mn): 1.3 to 1.5% by weight, Chromium (Cr): 0.05 to 0.2% by weight and the remaining iron (Fe) Made of inevitable impurities,
High-strength hot-rolled steel sheet for hydroforming, characterized in that the microstructure comprises a ferrite (pearlite) + pearlite composite structure.
The method of claim 5,
The hot rolled steel sheet
A high strength hot rolled steel sheet for hydroforming, comprising: phosphorus (P): 0.018 wt% or less and sulfur (S): 0.005 wt% or less.
The method of claim 5,
The hot rolled steel sheet
6 ≤ [Mn] / [Si] ≤ 9 (wherein [] is the weight percent of each component) the high strength hot rolled for hydroforming, characterized in that the silicon (Si) and manganese (Mn) is included Grater.
The method of claim 5,
The hot rolled steel sheet
A high strength hot rolled steel sheet for hydroforming, characterized by having a tensile strength (TS) of at least 540 MPa and an elongation (EL) of at least 28%.
The method of claim 5,
The hot rolled steel sheet
A high strength hot rolled steel sheet for hydroforming, having a work hardening index (n-value) of 0.17 or more and a yield ratio of less than 77% (YP / TS).

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