KR101586901B1 - Hot-rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a hot-rolled steel sheet for automobile wheel rim and a method for manufacturing the same, which can ensure excellent durability and weldability through control of alloy components and process conditions.
The hot-rolled steel sheet for automobile wheel rims according to the present invention contains 0.050 to 0.075% of C, 0.05 to 0.15% of Si, 0.7 to 0.9% of Mn, 0.018% or less of P, 0.003% or less of S, (YS) of not less than 235 MPa and an elongation (EL) of not less than 34%; a tensile strength (TS) of not less than 370 MPa; a yield strength (YS) of not less than 235 MPa; .

Description

TECHNICAL FIELD [0001] The present invention relates to a hot-rolled steel sheet,

The present invention relates to a hot-rolled steel sheet and a method of manufacturing the same, and more particularly, to a hot-rolled steel sheet for automobile wheel rim and a method of manufacturing the same, capable of securing excellent durability and weldability through control of alloy components and process conditions.

At present, the automobile wheel material with a tensile strength of 350 MPa is applied only to the disc use, and it is necessary to develop it for the automobile wheel rim.

These parts for automobile wheel rims are welded using flash butt welding. The flash welding method is a method in which the joints of two base materials are sufficiently heated and then the opposite side of the base material is pressed and bonded. In this case, the proportion of the chemical components greatly affects the weldability.

A related prior art document is Korean Patent Registration No. 10-0352587 (issued on December 26, 2002).

An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet for automobile wheel rim that can ensure excellent durability and weldability through alloy component control and process condition control.

Another object of the present invention is to provide a hot-rolled steel sheet for automobile wheel rim having a tensile strength (TS) of 370 MPa or more, a yield strength (YS) of 235 MPa or more, and an elongation (EL) of 34% or more.

In order to achieve the above object, according to an embodiment of the present invention, there is provided a hot-rolled steel sheet for automobile wheel rim comprising: (a) 0.050 to 0.075% of C, 0.05 to 0.15% of Si, 0.7 to 0.9% of Mn, (Slab Reheating Temperature): 1150 占 폚 to 20 占 폚. The slab plate is composed of Al, S: not more than 0.003%, S: 0.01 to 0.05%, Nb: Reheating; (b) subjecting the reheated plate to finishing hot rolling under the condition of Finish Delivery Temperature (FDT): 870 占 20 占 폚; (c) cooling the finished hot-rolled plate to 580 占 폚 to 20 占 폚.

According to another aspect of the present invention, there is provided a hot-rolled steel sheet for automobile wheel rims, comprising: 0.050 to 0.075% of C, 0.05 to 0.15% of Si, 0.7 to 0.9% of Mn, 0.018% (YS): 235 MPa or more; S: not more than 0.003%; S: 0.01 to 0.05%; Nb: more than 0 and not more than 0.007%; and the balance of Fe and unavoidable impurities. And an elongation (EL) of 34% or more.

The present invention relates to a method for manufacturing a hot-rolled steel sheet for automobile wheel rim which can secure superior durability and weldability by controlling the content of P and S and the ratio of Mn / Si to 5 to 15 in addition to adding Si .

Thus, the car wheel rimyong hot-rolled steel sheet produced by the above method is the tensile strength (TS): 370MPa or more, the yield strength (YS): 235MPa or more, an elongation (EL): number of more than 38% and mm ² per MnS: more than 500 .

1 is a flow chart showing a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and how to accomplish them, will become apparent by reference to the embodiments described in detail below with reference to the accompanying drawings. However, it should be understood that the present invention is not limited to the embodiments disclosed in the following description, but may be embodied in various forms, and that the present invention is not limited thereto and the present invention may be embodied in other forms without departing from the technical scope of the present invention Is provided to fully convey 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a hot-rolled steel sheet for automobile wheel rims and a method of manufacturing the same according to preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Hot-rolled steel sheet for automobile wheel rim

The hot-rolled steel sheet for automobile wheel rims according to the present invention has a tensile strength (TS) of not less than 370 MPa, a yield strength (YS) of not less than 235 MPa, an elongation (EL) of not less than 34%, and a number of MnS per mm ² of not more than 500 We aim.

For this purpose, the hot-rolled steel sheet for automobile wheel rims according to the present invention comprises 0.050 to 0.075% of C, 0.05 to 0.15% of Si, 0.7 to 0.9% of Mn, 0.018% or less of P, 0.01 to 0.05% of Al, more than 0 to 0.007% of Nb, and the balance of iron (Fe) and unavoidable impurities.

At this time, it is preferable that the steel sheet includes the manganese (Mn) and the silicon (Si) in the range including the formula 1 below.

Equation 1: 5? [Mn] / [Si]? 15

(Where [] is the weight percentage of each element)

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

Carbon (C)

Carbon (C) is added to ensure strength.

The carbon (C) is preferably added in an amount of 0.050 to 0.075% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of carbon (C) is less than 0.050% by weight, it may be difficult to secure sufficient strength. On the other hand, if the content of carbon (C) exceeds 0.075% by weight, the toughness may be lowered and weldability may be deteriorated during the electrical resistance welding (ERW).

Silicon (Si)

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel in the steelmaking process. Silicon also has a solid solution strengthening effect.

The silicon (Si) is preferably added in an amount of 0.05 to 0.15% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of silicon (Si) is less than 0.05% by weight, the effect of adding silicon can not be exhibited properly. On the other hand, when the content of silicon (Si) exceeds 0.15% by weight, the nonmetal inclusions are excessively formed on the surface of the steel sheet, thereby deteriorating toughness.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element and serves to improve the strength and toughness by reducing the Ar ₃ point to expand the control rolling temperature range, thereby finer crystal grains by rolling.

The manganese (Mn) is preferably added in an amount of 0.7 to 0.9% by weight based on the total weight of the hot rolled steel sheet according to the present invention. When the content of manganese (Mn) is less than 0.7% by weight, the fraction of the second phase structure is lowered and it may be difficult to secure the strength. On the other hand, when the content of manganese (Mn) exceeds 0.9% by weight, the sulfur dissolved in the steel precipitates into MnS, which lowers impact toughness at low temperatures.

On the other hand, it is more preferable to add manganese (Mn) and silicon (Si) to the hot-rolled steel sheet according to the present invention within the content range of the formula 1 below.

Equation 1: 5? [Mn] / [Si]? 15

(Where [] is the weight percentage of each element)

This is because the Mn / Si ratio within the range of 5 to 15 in the flash welding for manufacturing the automobile wheel rim should be within a certain range to facilitate the formation of the low melting point compound, thereby advantageously releasing the MnS inclusions and pores during the pressing of the back surface, The occurrence can be significantly reduced. If the Mn / Si ratio is less than 5 or more than 15, MnO ₂ and SiO ₂ oxide which are stable at high temperature are generated, thereby causing hook crack in the flash welding, thereby deteriorating the weld quality.

Phosphorus (P), sulfur (S)

Phosphorus (P) contributes partly to the strength improvement, but it is a representative element that lowers impact toughness at low temperatures. The lower the content is, the better. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.018% by weight or less based on the total weight of the hot-rolled steel sheet.

Sulfur (S) together with phosphorus (P) is an element that is inevitably contained in the production of steel, and forms MnS to lower impact toughness at low temperatures. Therefore, in the present invention, the content of sulfur (S) is limited to 0.003% by weight or less based on the total weight of the steel sheet.

Aluminum (Al)

Aluminum (Al) acts as a deoxidizer to remove oxygen in the steel.

The aluminum (Al) is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.01% by weight, the above deoxidation effect can not be exhibited properly. On the contrary, when the content of aluminum (Al) exceeds 0.05% by weight, it is difficult to perform, resulting in a decrease in productivity and formation of a compound causing a pinning effect such as Al ₂ O ₃ , thereby causing finer austenite crystal grains .

Niobium (Nb)

Niobium (Nb) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. Niobium carbide or nitride improves the strength and low-temperature toughness of a steel sheet by suppressing crystal grain growth during rolling and making crystal grains finer.

However, when a large amount of niobium (Nb) is added in an amount exceeding 0.007% by weight of the total weight of the hot-rolled steel sheet in the present invention, the weldability of the steel sheet is lowered and the strength and low-temperature toughness due to the increase in niobium content are not further improved. There is a risk that the impact toughness is lowered. Therefore, it is preferable that niobium (Nb) is added in a content ratio of 0 to 0.007% by weight or less based on the total weight of the hot-rolled steel sheet according to the present invention.

Manufacturing method of hot-rolled steel sheet for automobile wheel rim

1 is a flow chart showing a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, a method for manufacturing a hot-rolled steel sheet for automobile wheel rims according to an embodiment of the present invention includes a slab reheating step S110, a hot rolling step S120, and a cooling step S130. At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to carry out the reheating step to obtain effects such as reuse of precipitates.

In the method for manufacturing a hot-rolled steel sheet for automobile wheel rims according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process may contain 0.050 to 0.075% of C, 0.05 to 0.15% of Si, 0.7 to 0.9% of Mn, P: not more than 0.018%, S: not more than 0.003%, Al: 0.01 to 0.05%, Nb: more than 0 and not more than 0.007%, and the balance of iron (Fe) and unavoidable impurities.

Reheating slabs

In the slab reheating step S110, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1150 占 폚 to 20 占 폚. Here, the slab plate can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. At this time, in the slab reheating step (S110), the slab plate obtained through the continuous casting process is reheated to reuse the segregated components during casting.

At this stage, when the slab reheating temperature (SRT) is less than 1130 DEG C, there is a problem that the reheating temperature is low and the rolling load becomes large. In addition, since the Nb-based precipitates NbC and NbN can not reach the solid solution temperature, they can not be precipitated as fine precipitates upon hot rolling, and the austenite grain growth can not be suppressed, resulting in a rapid coarsening of the austenite grains. On the other hand, when the slab reheating temperature is higher than 1170 ° C, the austenite grains are rapidly coarsened and it is difficult to secure the strength and low temperature toughness of the steel sheet to be produced.

Hot rolling

In the hot rolling step (S120), the reheated plate is subjected to finishing hot rolling under the condition of FDT (Finish Delivery Temperature): 870 占 20 占 폚. At this time, the hot rolling can utilize a plurality of rolling passes so that controlled rolling is applied.

In this step, when the hot rolling end temperature is lower than 850 占 폚, abnormal reverse rolling occurs to form a non-homogeneous structure, thereby causing a problem that the low temperature impact characteristic is rapidly deteriorated. On the other hand, when the hot rolling end temperature exceeds 890 DEG C, ductility and toughness are increased, but it may be difficult to secure sufficient strength.

At this time, the hot rolling may be performed so that the cumulative rolling reduction in the austenite non-recrystallized region is 40 to 60%. When the cumulative rolling reduction is less than 40%, it is difficult to obtain a uniform but fine structure, and the structure of the central portion is coarsened, which causes a problem that the low temperature impact property is deteriorated. On the other hand, when the cumulative rolling reduction exceeds 60%, there is a problem that the rolling process time is prolonged and the fishy property is deteriorated.

Cooling

In the cooling step (S130), the finished hot rolled plate is cooled to 580 占 폚.

At this time, when the cooling end temperature is less than 560 DEG C, a large amount of low-temperature transformed structure is formed, which lowers the low-temperature toughness. On the other hand, when the cooling end temperature exceeds 600 캜, there is a problem that strength is lowered due to formation of coarse microstructure.

After that, air cooling can be performed up to room temperature.

In the hot-rolled steel sheet for automobile wheel rim manufactured in the above-described processes (S110 to S130), the content of P and S is strictly limited and the ratio of Mn / Si is controlled to be between 5 and 15, And weldability can be ensured.

Therefore, the car wheel rimyong hot-rolled steel sheet produced by the above method is the tensile strength (TS): 370MPa or more, the yield strength (YS): 235MPa or more, an elongation (EL): number of more than 38% and mm ² per MnS: more than 500 .

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. 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.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

The specimens according to Examples 1 to 3 and Comparative Examples 1 to 3 were produced with the compositions shown in Table 1 and the process conditions shown in Table 2. At this time, in the case of the hot-rolled samples according to Examples 1 to 3 and Comparative Examples 1 to 3, ingots having respective compositions were prepared and subjected to heating, hot rolling and cooling using a rolling simulation tester.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Table 3 shows the results of evaluation of mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 to 3.

[Table 3]

(TS) of 370 MPa or more, yield strength (YS) of 235 MPa or more and elongation (EL) of the specimens prepared according to Examples 1 to 3, 38% or more. In particular, in the case of the specimens according to Examples 1 to 3, it can be confirmed that the number of MnS inclusions per mm ² was measured to be 500 or less.

On the other hand, the tensile strength (TS), yield strength (YS) and elongation (EL) of the specimens according to Comparative Examples 1 to 3 satisfied the target value, but the number of MnS inclusions per mm ² did not satisfy the target value Can be confirmed.

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: Slab reheating step
S120: Hot rolling step
S130: cooling step

Claims (5)

(a) 0.05 to 0.10% of C, 0.05 to 0.15% of Si, 0.7 to 0.9% of P, 0.018% or less of P, 0.003% or less of S, 0.01 to 0.05% of Al, (0.007% or less) and remaining iron (Fe) and inevitable impurities to a slab reheating temperature (SRT) of 1150 占 폚 to 20 占 폚;
(b) subjecting the reheated plate to finishing hot rolling under the condition of Finish Delivery Temperature (FDT): 870 占 20 占 폚;
(c) cooling the finished hot rolled plate to 580 占 폚 to 20 占 폚.
The method according to claim 1,
In the step (a)
The slab plate
(Mn) and silicon (Si) in a range satisfying the following formula (1).

Equation 1: 5? [Mn] / [Si]? 15
(Where [] is the weight percentage of each element)
P: 0.018% or less; S: 0.003% or less; Al: 0.01 to 0.05%; Nb: more than 0 to 0.007% (Fe) and inevitable impurities,
A tensile strength (TS) of 370 MPa or more, a yield strength (YS) of 235 MPa or more, and an elongation (EL) of 34% or more.
The method of claim 3,
The steel sheet
Wherein the hot-rolled steel sheet comprises manganese and silicon in a range including the following formula (1).

Equation 1: 5? [Mn] / [Si]? 15
(Where [] is the weight percentage of each element)
5. The method of claim 4,
The steel sheet
mm ² And the number of MnS per unit of 500 or less.

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