KR20150112512A - High strength hot-rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

Disclosed are a high-strength hot-rolled steel sheet having transformation-induced plasticity grain in a hot-rolled state, and a manufacturing method thereof. According to the present invention, the method to manufacture a hot-rolled steel sheet comprises the steps of: hot-rolling, at a temperature higher than or equal to Ar3, a slab plate composed of 0.14-0.17 wt% of carbon (C), 1.40-1.60 wt% of silicon (Si), 1.40-1.60 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.003 wt% or less of sulfur (S), 0.045-0.055 wt% of titanium (Ti), 0.45-0.55 wt% of chromium (Cr), and the remainder consisting of iron (Fe) and inevitable impurities; primarily cooling the hot-rolled plate to a ferrite region; keeping the primarily cooled plate cooled by air in the ferrite region for 5 seconds or longer; secondarily cooling the air-cooled plate to a bainite region; and winding the secondarily cooled plate in the bainite region.

Description

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

More particularly, the present invention relates to a high-strength hot-rolled steel sheet excellent in elongation and having a TRIP (Transformation Induced Plasticity) structure in a hot rolled state and a method for manufacturing the same.

Transformation Induced Plasticity (TRIP) steel is a steel containing residual austenite structure in the microstructure, mainly developed as a cold rolled steel sheet, and is applied to automobile parts.

In the case of TRIP steel, it is mainly manufactured in the form of a cold-rolled steel sheet, because the cold-rolled steel sheet production process includes an annealing heat treatment process that facilitates formation of retained austenite and control of fraction.

Since a conventional hot rolled steel sheet manufacturing process does not include annealing heat treatment, it is difficult to produce a TRIP steel in the hot rolled steel sheet condition.

As a background technique related to the present invention, Korean Patent Laid-Open Publication No. 10-2012-0121810 (published on November 11, 2012) discloses a high-strength steel sheet on which a plated layer is formed on the surface of a hot-rolled steel sheet and a manufacturing method thereof.

An object of the present invention is to provide a high strength hot-rolled steel sheet excellent in elongation and having a TRIP structure in a hot rolled state through an alloy component and process control, and a method for producing the same.

In order to accomplish the above object, the present invention provides a method for manufacturing a high strength hot-rolled steel sheet, comprising: 0.14 to 0.17% of carbon (C), 1.40 to 1.60% of silicon (Si) (P): 0.02% or less, S: 0.003% or less, Ti: 0.045-0.055%, and Cr: 0.45-0.55% ) And unavoidable impurities at a temperature of Ar3 or higher; Firstly cooling the hot-rolled plate to a ferrite region; Maintaining the primary cooled plate in the ferrite region for at least 5 seconds; Cooling the air-cooled plate material to a bainite region; And winding the secondary cooled plate material in the bainite region.

At this time, it is more preferable that the start temperature is 670 to 690 DEG C in the air-cooling holding.

Further, the secondary cooling can be performed at an average cooling rate of 50 DEG C / sec or more.

In order to achieve the above object, a high strength hot-rolled steel sheet according to an embodiment of the present invention comprises 0.14 to 0.17% of carbon (C), 1.40 to 1.60% of silicon (Si), 1.40 to 1.60% of manganese (Mn) (Fe): 0.02% or less, S: 0.003% or less, Ti: 0.045-0.055%, and Cr: 0.45-0.55% And is composed of unavoidable impurities and has a composite structure including ferrite, bainite and retained austenite.

At this time, in the high-strength hot-rolled steel sheet, the area percentage of the retained austenite may be 3 to 20%.

The high-strength hot-rolled steel sheet may exhibit a tensile strength of 780 MPa or more and an elongation of 20% or more.

According to the present invention, it is possible to provide a method of manufacturing a high strength hot-rolled steel sheet having a TRIP structure including retained austenite through process control such as control of alloy components such as silicon and chromium, cooling and the like.

The high-strength hot-rolled steel sheet according to the present invention has the above-mentioned TRIP structure and can exhibit a high elongation of 20% or more while exhibiting a tensile strength of 780 MPa or more.

1 schematically shows a method of manufacturing a high-strength hot-rolled steel sheet according to an embodiment of the present invention.
Fig. 2 shows the microstructure of the specimen according to Example 1. Fig.
Fig. 3 shows the tensile curves of the specimen according to Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a hot-rolled steel sheet according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

High-strength hot-rolled steel sheet

The high-strength hot-rolled steel sheet according to the present invention is characterized in that it comprises 0.14 to 0.17% of carbon (C), 1.40 to 1.60% of silicon (Si), 1.40 to 1.60% of manganese (Mn) ): 0.003% or less, titanium (Ti): 0.045-0.055%, and chromium (Cr): 0.45-0.55%.

The rest of the alloy components are composed of Fe and unavoidable impurities in the steelmaking process.

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

Carbon (C)

Carbon (C) is an element contributing to the strength of steel. Carbon also serves to stabilize the austenite phase according to the amount that is concentrated in the austenite phase.

The carbon is preferably added in an amount of 0.14 to 0.17% by weight based on the total weight of the hot-rolled steel sheet. When the amount of carbon added is less than 0.14% by weight, it is difficult to secure a desired strength. On the other hand, when the amount of carbon added is more than 0.17 wt%, the elongation and bending properties are deteriorated.

Silicon (Si)

Silicon (Si) mostly acts as a deoxidizer. However, in the present invention, silicon promotes the austenite-ferrite transformation to increase the ferrite fraction, and helps the formation of retained austenite by rapidly concentrating and stabilizing carbon (C) with austenite. The silicon is preferably contained in an amount of 1.40 to 1.60 wt% of the total weight of the hot-rolled steel sheet. When the content of silicon is less than 1.40 wt%, the effect of the addition is insufficient. On the other hand, when the addition amount of silicon exceeds 1.60% by weight, there is a problem that the quality of the surface of the steel sheet deteriorates and the plating property, weldability, and the like decrease.

Manganese (Mn)

Manganese (Mn) contributes to the improvement of the strength of the steel through solid solution strengthening and penetration. In addition, manganese stabilizes austenite to facilitate the formation of retained austenite and martensite.

The manganese is preferably added in an amount of 1.40 to 1.60 wt% of the total weight of the hot-rolled steel sheet. When the content of manganese is less than 1.40 wt%, the addition effect is insufficient. On the contrary, when the addition amount of manganese exceeds 1.60% by weight, a manganese band structure is formed and segregation increases sharply, which deteriorates the workability and weldability of steel.

In (P)

Phosphorus (P) contributes to strength improvement, but if it contains a large amount, fine segregation is formed as well as center segregation, which adversely affects the material and may deteriorate the weldability.

Therefore, in the present invention, the content of phosphorus is limited to 0.02 wt% or less of the total weight of the hot-rolled steel sheet.

Sulfur (S)

Sulfur (S) combines with manganese to form nonmetallic inclusions, and these nonmetallic inclusions are a factor to deteriorate toughness, weldability and the like.

Accordingly, the content of sulfur in the present invention is limited to 0.003% by weight or less of the total weight of the hot-rolled steel sheet.

Titanium (Ti)

Titanium (Ti) forms a carbonitride precipitate. Such a titanium-based precipitate contributes to enhancement of strength and elongation through grain refinement.

The titanium is preferably added in an amount of 0.045 to 0.055% by weight based on the total weight of the hot-rolled steel sheet. When the addition amount of titanium is less than 0.045 wt%, the effect of the addition is insufficient and it is difficult to expect improvement in strength and the like. On the contrary, when the addition amount of titanium exceeds 0.055% by weight, the titanium-based precipitates are coarsened and the workability and formability are lowered.

Chromium (Cr)

Chromium (Cr) together with silicon (Si) contributes to ferrite formation and also contributes to increase the amount of retained austenite.

The chromium is preferably added in an amount of 0.45 to 0.55% by weight based on the total weight of the hot-rolled steel sheet. When the addition amount of chromium is less than 0.45 wt%, the effect of addition is insufficient. On the contrary, when the addition amount of chromium exceeds 0.55% by weight, the rolling property may be lowered.

The high-strength hot-rolled steel sheet according to the present invention having the alloy composition described above may have a composite structure including ferrite, bainite and retained austenite by the manufacturing process control described below. More specifically, the microstructure of the high-strength hot-rolled steel sheet according to the present invention may also include ferrites in an area ratio of 40% or more, particularly 3 to 20% in area percentage of retained austenite. The area of bainite is about 20%.

The high-strength hot-rolled steel sheet according to the present invention can exhibit a tensile strength of 780 MPa or more and an elongation of 20% or more in terms of mechanical properties.

Due to the microstructure and mechanical properties as described above, the high strength hot-rolled steel sheet according to the present invention has excellent strength and elongation as well as excellent impact absorbing ability by retained austenite, so that it can be applied to automobile parts and the like.

Method of manufacturing high strength hot-rolled steel sheet

Hereinafter, a method for manufacturing a hot-rolled steel sheet according to the present invention will be described.

1 schematically shows a method of manufacturing a high-strength hot-rolled steel sheet according to an embodiment of the present invention.

1, a method of manufacturing a high strength hot-rolled steel sheet according to the present invention includes a hot rolling step S110, a primary cooling step S120, an air cooling maintaining step S130, a secondary cooling step S140, ).

In the hot rolling step (S110), the slab plate is hot-rolled at a predetermined reduction ratio.

Prior to hot rolling, titanium and the like may be further re-heated and a slab reheating step for hot rolling may be further included. The slab reheating step may be performed at about 1100 to 1300 ° C for about 1 to 4 hours.

The hot rolling is preferably performed at a temperature equal to or higher than Ar3 so as to prevent abnormal reverse rolling, and can be finishing rolling at about 860 to 900 ° C.

In the primary cooling step (S120), the hot-rolled plate is primarily cooled to the ferrite region. The primary cooling can be performed at an average cooling rate of about 10-100 DEG C / sec.

Next, in the air-cooling maintaining step (S130), the ferrite fraction is controlled by keeping the ferrite region for a predetermined time. By this air-cooling and maintaining step, ferrite, bainite and retained austenite fraction can be controlled.

The air cooling time is preferably 5 seconds or more, and more preferably 5 to 10 seconds. When the air cooling time is less than 5 seconds, the ferrite fraction becomes too low to secure an elongation of 20% or more.

On the other hand, it is more preferable that the starting temperature is 670 to 690 캜 for air-cooling maintenance. It is possible to control the ferrite fraction while suppressing the formation of carbide when the air cooling maintenance is started in the above-mentioned temperature range, and it is also possible to secure bainite of about 20% in area ratio.

Next, in the secondary cooling step (S140), the air-cooled plate material is cooled to the bainite region.

The secondary cooling is preferably carried out at an average cooling rate of 50 DEG C / sec or more to prevent pearlite transformation.

In the winding step (S150), the secondary cooled plate is wound in the bainite area. In this process, austenite is transformed into bainite, and a part of austenite remains. At this time, in the case of the present invention, the transformation from austenite to bainite is retarded by chromium, silicon, or the like, and 3% or more of retained austenite can be secured at an area ratio.

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 hot-rolled steel specimen

An ingot having the composition shown in Table 1 was prepared and then reheated at 1150 ° C for 120 minutes. Thereafter, hot rolling was performed at a finish rolling temperature of 880 캜. Thereafter, the sample was first cooled to 680 ° C. at an average cooling rate of 30 ° C./sec, held for 6 seconds, and then cooled to 400 ° C. at an average cooling rate of 60 ° C./sec to prepare test pieces 1, 3 to 6 Respectively.

In the case of specimen 2, it was maintained for 4 seconds after primary cooling and then cooled to 400 캜.

[Table 1] (unit:% by weight)

2. Property evaluation

Tensile test and buckling (hole expansion ratio) were evaluated for the prepared specimens, and the retained austenite fraction was measured by microstructural analysis.

The tensile test was carried out on the JIS No. 5 test piece.

The hole expandability evaluation was performed by forming a perforation hole having an initial diameter (d ₀ : 10 mm), then expanding it with a 60 ° conical punch to measure the hole expansion rate (d) from the hole diameter d at the time when the crack penetrated the plate (dd ₀ ) / d ₀ X 100).

The evaluation results of the physical properties are shown in Table 2.

[Table 2]

Referring to Table 2, in the case of Specimen 1 and Specimen 3 satisfying the alloy component and process conditions proposed in the present invention, the retained austenite fraction was 3% or more in area ratio, elongation of 20% or more with tensile strength of 780 MPa or more Respectively.

On the other hand, the tensile strength of specimen 2 with relatively short air - hold time was high but the elongation was less than 20%.

Further, in the case of Specimens 4 to 6, in which the alloy component was out of the range suggested in the present invention, the retained austenite fraction was below the target value or the strength was below the target value.

Fig. 2 shows the microstructure of specimen 1. Fig.

In Fig. 2, the bright portion corresponds to ferrite, the black portion corresponds to bainite, and the gray portion corresponds to retained austenite.

Referring to Fig. 2, it can be seen that bismuth and retained austenite are formed in the ferrite base structure in the case of specimen 1.

3 shows the results of tensile test of the specimen 1. Fig.

Referring to FIG. 3, the tensile test results of the specimen 1 are as shown in Table 1, and it can be seen that residual austenite is formed through the "A" portion.

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.

Claims (6)

(Si): 1.40 to 1.60%, manganese (Mn): 1.40 to 1.60%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% Hot rolling at least a slab plate containing 0.045 to 0.055% of titanium (Ti) and 0.45 to 0.55% of chromium (Cr) and the remainder consisting of iron (Fe) and unavoidable impurities;
Firstly cooling the hot-rolled plate to a ferrite region;
Maintaining the primary cooled plate in the ferrite region for at least 5 seconds;
Cooling the air-cooled plate material to a bainite region; And
And winding the secondary cooled plate material in the bainite region.
The method according to claim 1,
Wherein the air-cooled holding is performed at a starting temperature of 670 to 690 占 폚.
The method according to claim 1,
Wherein the secondary cooling is performed at an average cooling rate of 50 DEG C / sec or more.
(Si): 1.40 to 1.60%, manganese (Mn): 1.40 to 1.60%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% Of ferrite, bainite and retained austenite, and the balance comprising 0.045 to 0.055% of titanium (Ti) and 0.45 to 0.55% of chromium (Cr) And a composite structure.
5. The method of claim 4,
Wherein the high-strength hot-rolled steel sheet contains 3 to 20% by area of the retained austenite.
5. The method of claim 4,
Wherein the high-strength hot-rolled steel sheet has a tensile strength of 780 MPa or more and an elongation of 20% or more.

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