KR101185218B1 - High strength hot-rolled steel for hydroforming with excellent workability and method of manufacturing the hot-rolled steel - Google Patents

Abstract

A method for producing a hot rolled steel sheet for hydroforming having a high strength of 370 MPa or more and having a work hardening index (n-value) of 0.23 or more is disclosed.
The method for manufacturing a hot rolled steel sheet for hydroforming according to the present invention is carbon (C): 0.05 to 0.08 wt%, silicon (Si): 0.030 wt% or less, manganese (Mn): 0.50 to 0.80 wt%, aluminum (Al): 0.020 ~ 0.050% by weight, nitrogen (N): 40ppm or less, and hot-rolled slab plate consisting of the remaining iron (Fe) and inevitable impurities to FDT (Finishing Delivery Temperature): 870 ~ 910 ℃; And cooling the hot rolled sheet, and winding it at a CT (Coiling Temperature): 620 to 660 ° C.

Description

High-strength hot-rolled steel sheet for hydroforming with excellent formability and manufacturing method thereof {HIGH STRENGTH HOT-ROLLED STEEL FOR HYDROFORMING WITH EXCELLENT WORKABILITY AND METHOD OF MANUFACTURING THE HOT-ROLLED STEEL}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydroformed hot rolled steel sheet and a method for manufacturing the same, which are applied to structural members of automobiles, and more particularly, having a high strength of 370 MPa or more and a work hardening index (n-value) of 0.23 or more. The excellent high-strength hot-rolled steel sheet for hydroforming and its manufacturing method.

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.

One object of the present invention is to provide a method for producing a high strength hot rolled steel sheet for hydroforming having an excellent formability while having a tensile strength of 370 MPa grade through alloying and process control.

In addition, another object of the present invention is to improve the strength and elongation at the same time, to lower the yield strength to secure the workability for hydroforming to provide a high-strength hot rolled steel sheet for hydroforming high workability index 0.23 or more.

High-strength hot-rolled steel sheet for hydroforming according to an embodiment of the present invention for achieving the above object is carbon (C): 0.05 ~ 0.08% by weight, silicon (Si): 0.030% by weight or less, manganese (Mn): 0.50 ~ 0.80% by weight, Aluminum (Al): 0.020 ~ 0.050% by weight, Nitrogen (N): 40ppm or less and slab plate consisting of remaining iron (Fe) and unavoidable impurities FDT (Finishing Delivery Temperature): 870 Hot rolling to 910 ° C .; And cooling the hot rolled sheet, and winding it at a CT (Coiling Temperature): 620 to 660 ° C.

In addition, the slab plate may include phosphorus (P): 0.015% by weight or less and sulfur (S): 0.007% by weight or less.

High strength hot rolled steel sheet for hydroforming according to an embodiment of the present invention for achieving the above another object is carbon (C): 0.05 ~ 0.08% by weight, silicon (Si): 0.030% by weight or less, manganese (Mn) : 0.50 ~ 0.80% by weight, aluminum (Al): 0.020 ~ 0.050% by weight, nitrogen (N): 40ppm or less and the remaining iron (Fe) and inevitable impurities, the microstructure is ferrite (pearl) + pearlite (pearlite) Characterized in that it comprises a complex tissue.

In addition, the steel sheet may include phosphorus (P): 0.015% by weight or less and sulfur (S): 0.007% by weight or less.

The high strength hot rolled steel sheet for hydroforming having excellent moldability manufactured by the method according to the present invention may exhibit high elongation (EL) of 40% or more and an excellent work hardening index (n-value) of 0.23 or more with high tensile strength of 370 MPa or more. have.

Accordingly, the hot rolled steel sheet of the present invention may be used as a material for hydroforming, which requires high strength and excellent workability and formability.

Therefore, the high strength hot rolled steel sheet for hydroforming excellent in formability according to the present invention can contribute to the weight reduction of automobile parts and can be utilized 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.
FIG. 2 is a graph illustrating changes in time and temperature for each process of FIG. 1.
3 is a view showing an application example of 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 will be described in detail with respect to the high-strength hot-rolled steel sheet for hydroforming and its manufacturing method excellent in formability according to an embodiment of the present invention.

High strength hot rolled steel sheet

High strength hot rolled steel sheet for hydroforming excellent in formability according to the present invention is carbon (C): 0.05 ~ 0.08% by weight, silicon (Si): 0.030% by weight or less, manganese (Mn): 0.50 ~ 0.80% by weight, aluminum (Al ): 0.020 ~ 0.050% by weight, nitrogen (N): 40ppm or less and the remaining iron (Fe) and inevitable impurities, the microstructure consists of ferrite (pearlite)-pearlite (complex).

In addition, the high-strength hot-rolled steel sheet for hydroforming according to the present invention may include phosphorus (P): 0.015% by weight or less and sulfur (S): 0.007% by weight or less.

Hereinafter, the role and content of each component included in the high strength hot rolled steel sheet for hydroforming having excellent moldability 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 small amount of carbon corresponding to the low carbon region in order to secure sufficient strength and formability.

The carbon is preferably added in a content ratio of 0.05 to 0.08% by weight of the total weight of the hot rolled steel sheet according to the present invention. When the carbon content is less than 0.05% 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, if the carbon content exceeds 0.08% 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 less than 0.030% by weight of the total weight of the hot rolled steel sheet according to the present invention. If the silicon content exceeds 0.030% by weight, the weldability is likely to be lowered, and there is a problem of deteriorating the surface quality by generating red scales during hot rolling.

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 in 0.50 ~ 0.80% by weight of the total weight of the hot rolled steel sheet according to the present invention. When manganese is added in an amount less than 0.50% by weight, high carbon content is insufficient to secure strength. On the contrary, when the content of manganese is added in excess of 0.80% by weight, the amount of MnS-based non-metallic inclusions increases, there is a problem that defects such as crack generation during welding.

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 0.015% 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 0.007% by weight or less of the total weight of the hot rolled steel sheet according to the present invention.

Aluminum (Al)

Aluminum (Al) has an excellent deoxidation ability compared to silicon (Si) or manganese (Mn) and is an effective element for removing oxygen in molten steel during the steelmaking process.

The aluminum is preferably added in a content ratio of 0.020 to 0.050% by weight of the steel according to the present invention. If aluminum is added at less than 0.020% by weight, the deoxidation effect is not sufficient. Conversely, if aluminum is added at more than 0.050% by weight, it reduces the machinability when manufacturing parts using steel sheets manufactured by preventing the spheroidization of cementite during pearlite transformation. There is.

Nitrogen (N)

Nitrogen (N) is also an unavoidable impurity, and there is a problem in that elongation of solid solution nitrogen increases when a large amount is added, thereby reducing elongation and formability of steel. Therefore, the content of nitrogen in the steel sheet according to the present invention is preferably limited to 40 ppm or less.

Manufacturing method of high strength hot rolled steel sheet

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, Figure 2 is a graph showing the time and temperature changes for each process of FIG.

1 and 2, the illustrated method for producing 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): 0.05 to 0.08 wt%, silicon (Si): 0.030 wt% or less, manganese (Mn): 0.50 to 0.80 wt%, aluminum (Al): 0.020 to 0.050 wt%, Nitrogen (N): Below 40ppm and the slab plate consisting of the remaining iron (Fe) and inevitable impurities hot-rolled to a Finishing Delivery Temperature (FDT): 870 ~ 910 ℃.

At this time, the slab plate may include phosphorus (P): 0.015% by weight or less and sulfur (S): 0.007% by weight or less.

When the finish rolling temperature FDT exceeds 910 ° C in the hot rolling step S110, the grains of austenite after rolling are coarsened. As a result, the crystal grains of the ferrite after transformation are also coarsened, which lowers toughness and adversely affects strength. On the contrary, if the finishing rolling temperature is too low, below 870 ° C., problems may arise such as a mixed structure caused by abnormal reverse rolling.

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.

If the slab reheating temperature is less than 1150 ℃ segregated components are not reusable, on the contrary, when the slab reheating temperature exceeds 1250 ℃ austenite grain size is increased and the ferrite grain size is coarse to reduce the strength.

Next, in the winding step (S120) to cool the hot-rolled sheet, it is wound at CT (Coiling Temperature): 620 ~ 660 ℃.

The cooling of the hot rolled sheet may be performed by quenching and air cooling the quenched steel. At this time, the cooling middle termination temperature (CMT) in which the process of quenching by water cooling is completed is preferably 670 to 710 ° C.

If the cooling intermediate termination temperature (CMT) exceeds 710 ° C., the strength may decrease due to an increase in ferrite grain size. On the contrary, if the cooling intermediate termination temperature (CMT) is less than 670 ° C., the ferrite grain size decreases. The strength is increased, but the elongation is reduced.

On the other hand, the winding temperature (CT) in the winding step (S120) is preferably 620 ~ 660 ℃. 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 winding temperature exceeds 660 ° C., the grain size becomes very large and the strength may be lowered.

On the contrary, when the coiling temperature is less than 620 ° C, the carbon solubility in the ferrite increases with the decrease of the ferrite grain size, and the strength increases with the change of the shape of the fine grain into the acicular type, thereby decreasing the elongation. .

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 hot rolled steel sheet manufactured by the above-described process may have a tensile strength (TS) of 370 MPa or more and an elongation (EL) of 40% or more. In addition, the hot rolled steel sheet manufactured by the above-described process may have a work hardening index (n-value) of 0.23 or more and a uniform elongation (U-EL) of 20% or more.

As described above, the hot rolled steel sheet according to the present invention has a tensile strength of 370 MPa or more and has a high elongation and work hardening index through adjustment of the alloy component system and setting of finish rolling temperature (FDT) and winding temperature (CT) during hot rolling. It may be due to the microstructure control.

On the other hand, Figure 3 is a view showing an application example of a high-strength hot-rolled steel sheet for hydroforming according to an embodiment of the present invention.

Referring to FIG. 3, in the case of a high strength hot rolled steel sheet for hydroforming according to an embodiment of the present invention, it may be applied to an engine cradle 100 among automotive components. As such, when manufacturing the engine cradle 100 using a high-strength hot rolled steel sheet according to an embodiment of the present invention, tensile strength (TS) of 370 MPa or more, elongation (EL) of 40% or more, work hardening index of 0.23 or more (n-value) ) And 20% or more of uniform elongation (U-EL), excellent moldability, easy to process and contribute to the weight reduction of automotive parts.

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 3 having the compositions shown in Table 1 were dissolved in a vacuum melting furnace to produce an ingot, and then hot-rolled to a finish rolling temperature of 890 ° C., followed by water cooling to 690 ° C., followed by air cooling at 620 ° 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 3.

[Table 2]

Referring to Table 2, for specimen 1 corresponding to the example, yield strength (YS): 270 MPa or less, tensile strength (TS): 370 MPa or more, elongation (EL): 40% or more, work hardening index (n-value) : 0.23 or more and uniform elongation (U-EL): 20% or more are seen to be satisfied.

On the other hand, in the case of specimens 2 and 3 corresponding to Comparative Examples 1 and 2, it can be seen that the elongation (EL), the work hardening index (n-value) and the uniform elongation (U-EL) do not reach the target values.

In the case of specimen 2 corresponding to Comparative Example 1, it can be seen that a relatively large amount of nitrogen (N) was added in the component relationship with specimen 1 corresponding to the example. As described above, when a large amount of nitrogen (N) is added, the tensile strength (TS) exceeds the target value, but it can be seen that the elongation (EL) and the work hardening index (n-value) decrease rapidly.

In addition, in the case of specimen 3 corresponding to Comparative Example 2, the content of carbon (C) is relatively small and the amount of manganese (Mn) is relatively high You can see that. As such, when a small amount of carbon (C) is added and a large amount of manganese (Mn) is added, the tensile strength (TS) reaches the target value, but the elongation (EL) and the work hardening index (n-value) It can be seen that did not reach the target value, due to which the workability was not excellent.

As described above, the hot rolled steel sheet for hydroforming according to the present invention has a tensile strength of 370 MPa or more, and also has excellent 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): 0.05 to 0.08 wt%, Silicon (Si): more than 0 wt% to 0.030 wt% or less, manganese (Mn): 0.50 to 0.80 wt%, aluminum (Al): 0.020 to 0.050 wt%, phosphorus ( P): more than 0% by weight to 0.015% by weight, sulfur (S): more than 0% by weight to 0.007% by weight, nitrogen (N): 40ppm or less and slab plate consisting of remaining iron (Fe) and unavoidable impurities (Finishing Delivery Temperature): hot rolling to 870 ~ 910 ℃; And
Cooling the hot rolled plate, CT (Coiling Temperature): comprising a step of winding at 620 ~ 660 ℃,
The cooling includes the step of quenching the hot-rolled plate, and the step of air-cooling the quenched plate,
Cooling middle end temperature of the quenching process (Cool Middle Temperature: CMT) is 670 ~ 710 ℃ high strength hot rolled steel sheet manufacturing method characterized in that the.
delete The method of claim 1,
Before the hot rolling
Method for producing a high strength hot rolled steel sheet for hydroforming, characterized in that it further comprises the step of reheating the slab plate SRT (Slab Reheating Temperature): 1150 ~ 1250 ℃.
delete delete Carbon (C): 0.05 to 0.08 wt%, Silicon (Si): more than 0 wt% to 0.030 wt% or less, manganese (Mn): 0.50 to 0.80 wt%, aluminum (Al): 0.020 to 0.050 wt%, phosphorus ( P): more than 0 wt% to 0.015 wt% or less, sulfur (S): more than 0 wt% to 0.007 wt% or less, nitrogen (N): 40 ppm or less and the remaining iron (Fe) and unavoidable impurities,
The microstructure comprises a ferrite + pearlite complex tissue,
A high strength hot rolled steel sheet for hydroforming having a tensile strength (TS) of 370 MPa or more and a yield strength (YS) of 270 MPa or less, and having a work hardening index (n-value) of 0.23 or more and an elongation of at least 40%. delete delete delete

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