KR101989236B1 - Method for manufacturing hot rolled steel sheet and hot rolled steel sheet thereof - Google Patents

Abstract

A method of manufacturing a hot-rolled steel sheet excellent in shear characteristics and a hot-rolled steel sheet produced by the method are disclosed. In one embodiment, the hot-rolled steel sheet manufacturing method comprises the steps of: 0.13 to 0.18 weight% of carbon (C), 0.8 to 1.5 weight% of silicon (Si), 1.5 to 2.3 weight% of manganese (Mn) 0.05 to 0.10% by weight of chromium (Cr), 0.02 to 0.08% by weight of niobium (Nb) and 0.05 to 0.10% by weight of titanium (Ti) , Residual iron (Fe), and other unavoidable impurities; Hot-rolling the reheated steel slab to produce a hot-rolled steel sheet; Cooling the hot-rolled sheet to a middle temperature (MT) of 560 to 600 ° C; And cooling and hot rolling the hot-rolled sheet to a temperature of 420 to 460 占 폚.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet having excellent shear characteristics and a hot rolled steel sheet produced by the method,

The present invention relates to a method of manufacturing a hot-rolled steel sheet excellent in shear characteristics and a hot-rolled steel sheet produced thereby. More particularly, the present invention relates to a hot rolled steel sheet production method having excellent elongation and shear workability and having a very high tensile strength, and a hot rolled steel sheet produced thereby.

The increase in the fuel consumption of automobiles and the strengthening of regulations in terms of environmental protection are accelerating the increase in the strength of automobile parts in order to reduce the weight of automobiles.

On the other hand, an 80K grade material composed of a ferrite-based precipitate is applied to a chassis member such as a lower arm of a differential car. Such a chassis member is required to have excellent shear processability along with high tensile strength. In particular, when the blank is sheared during the forming process, no lateral cracks should occur in the fracture surface, and no fine cracks should be generated in the elongate forming portion and the hole expanding portion when the component is molded.

BACKGROUND ART [0002] The background art relating to the present invention is disclosed in Korean Patent Registration No. 10-1714909 (published on Mar. 31, 2017, entitled "High-Strength Hot-Rolled Steel Sheet with Excellent Surface Quality and Method for Manufacturing the Same").

According to an embodiment of the present invention, there is provided a method of manufacturing a hot rolled steel sheet having an ultra high tension.

According to one embodiment of the present invention, there is provided a method of manufacturing a hot-rolled steel sheet excellent in elongation and shearing workability.

According to an embodiment of the present invention, there is provided a hot rolled steel sheet produced by the hot rolled steel sheet producing method.

One aspect of the present invention relates to a method of manufacturing a hot-rolled steel sheet excellent in shear characteristics. In one embodiment, the hot-rolled steel sheet manufacturing method comprises the steps of: 0.13 to 0.18 weight% of carbon (C), 0.8 to 1.5 weight% of silicon (Si), 1.5 to 2.3 weight% of manganese (Mn) 0.05 to 0.10% by weight of chromium (Cr), 0.02 to 0.08% by weight of niobium (Nb) and 0.05 to 0.10% by weight of titanium (Ti) , Residual iron (Fe), and other unavoidable impurities; Hot-rolling the reheated steel slab to produce a hot-rolled steel sheet; Cooling the hot-rolled sheet to a middle temperature (MT) of 560 to 600 ° C; And cooling and hot rolling the hot-rolled sheet to a temperature of 420 to 460 占 폚.

In one embodiment, the reheating may be performed at a temperature of the slab reheating temperature of 1150 to 1250 ° C.

In one embodiment, the hot rolling may be performed at a finish rolling temperature of 870 to 920 占 폚.

In one embodiment, the primary cooling and the secondary cooling may each be at a cooling rate of 30-100 DEG C / s.

Another aspect of the present invention relates to a hot-rolled steel sheet produced by the hot-rolled steel sheet producing method. In one embodiment, the hot-rolled steel sheet comprises 0.13 to 0.18 wt% of carbon (C), 0.8 to 1.5 wt% of silicon (Si), 1.5 to 2.3 wt% of manganese (Mn) (S): 0 to 0.008 wt% or less, chromium (Cr): 0.5 to 1.0 wt%, niobium (Nb): 0.02 to 0.08 wt%, titanium (Ti): 0.05 to 0.10 wt% Iron (Fe) and other inevitable impurities, and the microstructure has bainite and retained austenite structure.

In one embodiment, the hot-rolled steel sheet may have a tensile strength (TS) of 980 MPa or more, a yield strength (YS) of 800 MPa or more, and an elongation (El) of 10% or more.

The hot-rolled steel sheet produced according to the present invention has an ultra-high tensile strength and is excellent in elongation so that cracks do not occur in the cross-section of the fracture surface during shearing, and fine cracks do not occur in the elongate forming portion and hole- The workability can be excellent.

1 shows a method of manufacturing a hot-rolled steel sheet according to one embodiment of the present invention.
2 shows the microstructure of a hot-rolled steel sheet according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to be exemplary, self-explanatory, allowing for equivalent explanations of the present invention.

Method of manufacturing hot-rolled steel sheet excellent in shear characteristics

One aspect of the present invention relates to a method of manufacturing a hot-rolled steel sheet excellent in shear characteristics. 1 shows a method of manufacturing a hot-rolled steel sheet according to one embodiment of the present invention. Referring to FIG. 1, the hot-rolled steel sheet manufacturing method includes: (S10) a steel slab reheating step; (S20) hot rolling step; (S30) a primary cooling step; And (S40) a second cooling and winding step. More specifically, the hot-rolled steel sheet manufacturing method comprises the steps of: (S10) providing 0.13 to 0.18 wt% of carbon (C), 0.8 to 1.5 wt% of silicon (Si), 1.5 to 2.3 wt% of manganese (Mn) : 0 to 0.05% by weight, S: 0 to 0.008% by weight, Cr: 0.5 to 1.0%, Nb: 0.02 to 0.08%, Ti: Reheating the steel slab containing the weight percent and the balance iron (Fe) and other unavoidable impurities; (S20) hot-rolling the reheated steel slab to produce a hot-rolled steel sheet; (S30) cooling the hot-rolled sheet to a middle temperature (MT) of 560 to 600 占 폚; And (S40) cooling the hot rolled sheet by second cooling to a temperature of 420 to 460 占 폚 and winding.

Hereinafter, constituent components of the steel slab will be described in detail.

Carbon (C)

The carbon (C) is an element contributing to an increase in the strength of the steel. In one embodiment, the carbon is included in an amount of 0.13 to 0.18 wt% based on the total weight of the steel slab. When the amount of carbon added is less than 0.13 wt%, it is difficult to secure strength, and it is difficult to realize austenite microstructure at room temperature, which is a stabilized high temperature phase. When the carbon content exceeds 0.18 wt%, carbide formation in the steel is promoted, The shearing characteristics may be degraded.

silicon( Si )

The silicon (Si) is an element that acts as a deoxidizer and can increase the strength without lowering the ductility. Further, after the hot-rolled steel sheet of the present invention is cooled and wound, carbide formation can be suppressed. In one embodiment, the silicon is included in an amount of 0.8 to 1.5 wt% based on the total weight of the steel slab. When the amount of silicon is less than 0.8 wt%, it is difficult to suppress the formation of cementite during isothermal transformation, and the effect of improving the elongation can not be obtained. When the content exceeds 1.5 wt%, the rolling load may increase or segregation may occur.

Manganese (Mn)

The manganese (Mn) is an element for securing strength and workability of the steel sheet. In one embodiment, the manganese is contained in an amount of 1.5 to 2.3 wt% based on the total weight of the steel slab. When the content of manganese is less than 1.5% by weight, the effect of the addition is insignificant, and when it exceeds 2.3% by weight, the weldability may be deteriorated.

In (P)

The phosphorus (P) contributes partly to the improvement in strength. In one embodiment, the phosphorous is included in an amount of 0 to 0.05 wt% or less based on the total weight of the steel slab. When the phosphorus is contained in an amount exceeding 0.05 wt%, the weldability is deteriorated, and center segregation or micro segregation is formed and the material may be deteriorated.

Sulfur (S)

The sulfur (S) inhibits toughness and weldability and increases MnS non-metallic inclusions, thereby inhibiting bending workability. In one embodiment, the sulfur is present in an amount of greater than 0 to 0.008 weight percent based on the total weight of the steel slab. If the sulfur content exceeds 0.008 wt%, the number of MnS non-metallic inclusions increases, and the workability may deteriorate.

chrome( Cr )

The chrome (Cr) is an element which increases the temperature range in which the bainite microstructure is formed in the present invention. In one embodiment, the chromium is included in an amount of 0.5 to 1.0 wt% based on the total weight of the steel slab. If the content of chromium is less than 0.5% by weight, the effect of the addition is insignificant. If the content of Cr exceeds 1.0% by weight, balance of strength and ductility of the hot-rolled steel sheet may be broken and brittleness of steel may increase.

Niobium ( Nb )

The niobium (Nb) is an element which improves the strength of steel through grain refinement and precipitation hardening. In one embodiment, the niobium is included in an amount of 0.02 to 0.08 wt% based on the total weight of the steel slab. When the content of niobium is less than 0.02% by weight, the effect of the addition is insignificant, and when it is more than 0.08% by weight, the moldability may be lowered.

titanium( Ti )

The titanium (Ti) is an element that improves the strength of steel through fine grain refinement and formation of precipitates with excellent high-temperature stability. In one embodiment, the titanium is included in an amount of 0.05 to 0.10 wt% based on the total weight of the steel slab. When the amount of titanium is less than 0.05% by weight, the effect of the addition is insignificant. When the amount of titanium exceeds 0.10% by weight, coarse precipitates are formed and the impact characteristics of the steel may be deteriorated.

In one embodiment, the reheating may be performed by heating at a slab reheating temperature of 1150 to 1250 ° C. When reheating to the above-mentioned temperature, the segregation component can be sufficiently reused and the product quality can be excellent.

(S20) Hot rolling  step

The step is a step of hot-rolling the reheated steel slab to produce a hot-rolled steel sheet. One embodiment of the hot rolling may be performed by hot-rolling the steel slab at a finishing rolling temperature of 870 to 920 占 폚. The grain refining effect is excellent in the above range, and the desired strength can be secured.

(S30) The primary cooling step

The above step is a step of first cooling the hot rolled plate to an intermediate temperature (MT): 560 to 600 ° C. When the hot-rolled sheet is subjected to the primary cooling at a temperature of less than 560 캜, the retained austenite microstructure is hardly formed in the hot-rolled steel sheet of the present invention, so that it is difficult to secure the elongation. When the primary cooling is completed at a temperature exceeding 600 캜 , The amount of austenite is increased under a high temperature condition. When secondary cooling of the hot-rolled steel sheet according to the present invention, components such as carbon and manganese are less concentrated in the austenite and the austenite stability is lowered. So that moldability and mechanical strength may be lowered.

(S40) secondary cooling and Coiling  step

The above step is a step of cooling the hot-rolled sheet to a temperature of 420 to 460 占 폚 for coiling. When the coiling temperature is lowered to a temperature lower than 420 캜 and the coiling is carried out, martensite microstructure is formed on the hot-rolled steel sheet to lower the elongation. When the coiling is carried out at a temperature exceeding 460 캜, a ferrite microstructure is formed So that the strength may be lowered.

In one embodiment, the primary cooling and the secondary cooling may each be at a cooling rate of 30-100 DEG C / s. Under the above conditions, the microstructure containing the bainite and retained austenite of the present invention can be secured, and the strength, elongation, and shearing characteristics can be excellent.

The hot-rolled steel sheet produced by the hot-

Another aspect of the present invention relates to a hot-rolled steel sheet produced by the hot-rolled steel sheet producing method. Wherein the hot rolled steel sheet comprises 0.13 to 0.18 weight% of carbon (C), 0.8 to 1.5 weight% of silicon (Si), 1.5 to 2.3 weight% of manganese (Mn) (S): more than 0 to 0.008% by weight, chromium (Cr): 0.5 to 1.0% by weight, niobium (Nb): 0.02 to 0.08% ) And other unavoidable impurities, and the microstructure has bainite and retained austenite structure. As for the components of the hot-rolled steel sheet, the same components and contents as those of the above-described steel slabs can be applied, so that a detailed description thereof will be omitted.

In one embodiment, the hot-rolled steel sheet may have a tensile strength (TS) of 980 MPa or more, a yield strength (YS) of 800 MPa or more, and an elongation (El) of 10% or more. For example, tensile strength (TS): 980 to 1150 MPa, yield strength (YS): 800 to 1000 MPa, and elongation (El): 10 to 15%.

The hot-rolled steel sheet produced according to the present invention secures full-bainite microstructure, has an ultra-high tensile strength, and is excellent in elongation, so that cracks are not generated in the cross section of the fracture surface during shearing, So that fine cracks do not occur in the portion, and shear workability can be excellent.

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.

Example

A steel slab containing the components as shown in Table 1 and the balance of iron (Fe) and other unavoidable impurities was prepared and reheated at a slab reheating temperature of 1200 캜, followed by rolling at a finish rolling temperature of 900 캜, A plate material was prepared. Next, the hot-rolled sheet was first cooled to a middle temperature (MT) of 580 占 폚 at a cooling rate of 30 to 100 占 폚 / s and then cooled at a cooling rate of 30 to 100 占 폚 / Cooled and rolled to obtain a hot-rolled steel sheet.

Comparative Example

A hot-rolled steel sheet was prepared in the same manner as in the above example except that the hot-rolled sheet was first cooled to a middle temperature (MT) of 580 ° C, and then cooled to a coiling temperature of 400 ° C.

The yield strength (MPa), the tensile strength (MPa) and the elongation percentage (%) of the hot-rolled steel sheets of the examples and comparative examples were measured and the results are shown in Table 2 below.

2 shows the microstructure of the hot-rolled steel sheet produced according to the above embodiment. Referring to Table 2 and FIG. 2, it can be seen that the hot-rolled steel sheet according to the present invention has microstructures including bainite and retained austenite, and has high tensile strength and an elongation of 10% or more there was. On the other hand, in the case of the comparative example in which the secondary cooling end temperature of the present invention is lower than that of the present invention, the yield strength and elongation are lower than those of the above examples.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (6)

(S): 0.1 to 0.18 wt% of carbon (C), 0.8 to 1.5 wt% of silicon (Si), 1.5 to 2.3 wt% of manganese (Mn) (Fe), and other unavoidable elements, such as iron (Fe), iron (Fe) and other inevitable elements, Reheating the steel slab containing impurities;
Hot-rolling the reheated steel slab to produce a hot-rolled steel sheet;
Cooling the hot-rolled sheet to a middle temperature (MT) of 560 to 600 ° C; And
And cooling the hot-rolled sheet material to a second temperature of 420 to 460 占 폚 to wind up the hot-rolled sheet material.
The method according to claim 1,
Wherein the reheating is performed at a slab reheating temperature of 1150 to 1250 占 폚.
The method according to claim 1,
Wherein the hot rolling is performed at a finish rolling temperature of 870 to 920 占 폚.
The method according to claim 1,
Wherein the primary cooling and the secondary cooling are respectively performed at a cooling rate of 30 to 100 占 폚 / s.


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