High Strength Hot Rolled Steel Sheet Containing High Mn Content with Excellent Workability and Method for Manufacturing the Same

Abstract

A hot rolled steel sheet used for a bumper reinforceing material or for an impact absorption material in a door of automobiles, and a method for manufacturing the same are disclosed. The steel sheet comprises, by weight%, C: 0.2%˜1%, Mn: 8˜15%, S: 0.05% or less, P: 0.03% or less, and the balance of Fe and other unavoidable impurities. A product of tensile strength and total elongation (TS×ToLEl) of the steel sheet is 24,000 MPa % or more. The method provides a high strength hot rolled steel sheet, which has a high strength-elongation balance value, ensuring excellent workability.

Description

TECHNICAL FIELD

The present invention relates to a hot rolled steel sheet used for a bumper reinforcing material or for an impact absorption material in a door of automobiles. More particularly, the present invention relates to a high strength hot rolled steel sheet containing a high Mn content with high elongation which ensures excellent formability, and to a method for manufacturing the same.

BACKGROUND ART

In automobiles, bumper reinforcing materials or impact absorption material inside doors are directly related to safety of passengers upon collision of the automobiles, and generally formed of super strength hot rolled steel sheets having a tensile strength of 780 MPa or more. In recent years, corresponding to gradually increasing regulations for environmental pollution, high strength steel has been increasingly used in the automobiles to increase fuel efficiency, and investigations have progressed more and more for commercial application of the high strength steel having the tensile strength of 780 MPa or more.

As for the high strength steel for the automobiles, there are dual phase (DP) steel, transformation induced plasticity (TRIP) steel, and multi-phase steel.

Typically, a method for manufacturing a hot rolled steel sheet for the automobiles comprises reheating a steel slab for allowing steel components to form a solid solution again, hot rolling the steel slab to a steel sheet having a predetermined thickness, cooling the hot rolled steel sheet at room temperature, and coiling the cooled steel sheet. Here, the dual phase steel is produced in such a way of hot rolling the reheated slab at a temperature in an austenite region, and cooling the hot rolled steel sheet to a cooling finish temperature below Ms-temperature for transformation from austenite to martensite. In the dual phase steel, an increase in ratio of the martensite structure causes an increase in strength of the steel, and an increase in ratio of the ferrite structure causes an increase in elongation of the steel.

However, the dual phase steel has a disadvantage in that a high cooling rate is required to form the martensite at low temperatures.

The TRIP steel refers to steel which has further enhanced workability by forming retained austenite in a portion of the steel structure. Since the TRIP steel has excellent uniform elongation by transformation through work hardening, and typically has an elongation of 30% for 800 MPa, which is excellent compared with that of other super strength steel, it has a high tensile strength-elongation balance value. However, high strength steel having a higher level of strength has been required to satisfy higher requirement for a light automotive body. Furthermore, machining of more complicated components with the steel is required for integration of the components, thereby requiring the steel to have an elongation 30% higher than those for the same strength level.

The multi-phase steel refers to steel which is further increased in strength and elongation at the same time than the TRIP steel by forming the austenite during hot rolling, and controlling the cooling rate and the cooling finish temperature during cooling the coiled steel sheet to form ferrite, martensite, a small fraction of bainite, and a mixed phase of martensite and austenite at room temperature. The multi-phase steel has superior weldability and high yield strength due to its smaller added amount of alloy element. However, the multi-phase steel has a disadvantage in view of formability due to a low elongation.

DISCLOSURE OF INVENTION

Technical Problem

Therefore, the present invention has been made to solve the above problems, and it is an object of the present invention to provide a high strength hot rolled steel sheet containing a high Mn content with excellent workability, which is controlled in Mn content to 8˜15 wt % to form a single phase austenite structure, ensuring an excellent elongation, and to form twins upon deformation, preventing necking which causes cracks.

Technical Solution

In accordance with one aspect of the invention, the above and other objects can be accomplished by the provision of a high strength hot rolled steel sheet containing a high Mn content with excellent workability, comprising, by weight%: C: 0.2%˜1%, Mn: 8˜15%, S: 0.05% or less, P: 0.03% or less, and the balance of Fe and other unavoidable impurities, wherein a product of tensile strength and total elongation (TS×Tot.El) of the steel sheet is 24,000 MPa % or more.

In accordance with another aspect of the invention, a method for manufacturing a high strength hot rolled steel sheet containing a high Mn content with excellent workability comprises: reheating a steel slab at a temperature of 1,180˜1,220° C., the steel slab comprising by weight%: by weight%: C: 0.2%˜1%, Mn: 8˜15%, S: 0.05% or less, P: 0.03% or less, and the balance of Fe and other unavoidable impurities finish hot rolling the reheated steel slab at a temperature of 800° C. or more and cooling the hot rolled steel sheet at a temperature of 600° C. or more, followed by coiling the cooled steel sheet, wherein a product of tensile strength and total elongation (TS×Tot.El) of the steel sheet is 24,000 MPa % or more.

Advantageous Effects

As apparent from the above description, the present invention provides a high strength hot rolled steel sheet containing a high Mn content with excellent workability, which has a higher strength-elongation balance value than that of TRIP steel known as having excellent elongation in the art.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments will be described in detail hereinafter.

For general high strength steel having ferrite structure, deformation is caused by slip resulting from movement of dislocations, and an increase in deformation amount results in a higher degree of work hardening, which in turn results in concentration of the deformation on grain boundaries, causing necking. When the necking occurs, an increasing rate of work hardening fails to compensate a reduction rate of a cross section caused by the necking, resulting in failure of the steel.

The present invention is provided in order to solve such a typical problem of the high strength steel, and characterized by controlling an Mn content to 8˜15 wt % to form a single phase austenite structure in order to ensure excellent elongation, and by create twins to prevent generation of necking which forms cracks upon deformation. In other words, the steel of the present invention has a different deformation characteristic from that of the conventional high strength steel. Specifically, the steel according to the present invention is twin-induced plasticity steel, which has a high content of Mn so as to have the austenite structure and is formed with the twins upon application of stress to the steel. Here, unlike the dislocations causing the deformation concentration, the twins cause little deformation concentration upon work hardening, providing the excellent elongation. Meanwhile, TRIP steel is subjected to transformation upon deformation, the deformation is concentrated on martensite. However, the TWIP steel of the present invention is not subjected to the transformation upon the deformation, and maintains the austenite structure, thereby providing excellent elongation.

The present invention will be described in terms of composition and manufacturing process hereinafter.

Composition of Steel

C: 0.2˜1 wt % (hereinafter, “%”)

Carbon (C) is the most important component in steel. C has a close relationship with all physical and chemical properties of the steel, such as strength, toughness, corrosion resistance, and the like, and provides the most influential effect on the properties of the steel. If C content is less than 0.2%, not only stability of austenite but also a fraction of a secondary phase are reduced, thereby providing a problem of lowered strength. If the C content exceeds 1%, not only weldability of the steel is deteriorated, but also the fraction of the secondary phase is rapidly increased, thereby rapidly deteriorating workability. Thus, the C content is preferably in the range of 0.2˜1%.

Mn: 8˜15%

Manganese (Mn) is an austenite stabilization element, which increases the strength of steel by improving hardenability. According to the present invention, it is necessary to contain 8% or more of Mn in order to obtain a stable austenite structure. If Mn is excessively added above 15%, there are problems, such as excessive load during steel manufacturing, deterioration in weldability, formation of inclusions, and increase in manufacturing costs. Thus, the Mn content is preferably in the range of 8˜15%.

S: 0.05% or Less

Sulfur (S) is an impurity element in the steel. If S content exceeds 0.05%, coarse MnS precipitates are generated on a hot rolled steel sheet, deteriorating the workability and toughness of steel. Thus, the S content is preferably 0.05% or less.

P: 0.03% or Less

Phosphorus (P) is an impurity element in the steel. If P content exceeds 0.03%, the toughness of steel becomes deteriorated. Thus, the P content is preferably 0.03% or less.

Meanwhile, the steel of the present invention may further comprise at least one of Al, Ni and Cu.

Al: 0.3˜3%

Aluminum (Al) is a ferrite stabilization element, which resides in a solid solution state in ferrite. Al serves to enhance the strength of steel, and is generally added to the steel as a deoxidation agent. Meanwhile, in the present invention, Al is effective to continuously generate twins during deformation by increasing stacking fault energy. If Al content is less than 0.3%, the effect of increasing the stacking fault energy is small. If the Al content exceeds 3%, there can be problems of increasing nozzle clogging and inclusions during the steel manufacturing or continuous casting process. Thus, the Al content is preferably in the range of 0.3˜3%.

Ni: 2˜7%

Nickel (Ni) is the austenite stabilization element, and advantageous in terms of properties of the steel when it is added to the steel as much as possible. If Ni content is less than 2%, the effect by addition of Ni cannot be obtained. If the Al content exceeds 7%, there is a problem of significantly increasing the manufacturing costs. Thus, the Ni content is preferably in the range of 2˜7%.

Cu: 2˜5%

Copper (Cu) is an element which is solid-soluted in the austenite or forms a precipitation phase, thereby decreasing an amount of crystal grains of the austenite and refining crystal grains of the ferrite. In order to obtain these solid solution and precipitation effects, it is necessary to have Cu content of 2% or more. If the Cu content exceeds 5%, there are problems of significantly increasing a reheating temperature while increasing the manufacturing costs. Thus, the Cu content is preferably in the range of 2˜5%.

With the composition described as above, the hot rolled steel sheet of the present invention has a single phase austenite structure, and exhibits an elongation of 30% or more. For the steel sheet of the present invention, a tensile strength-total elongation balance value (TS×Tot.El) is of 24,000 MPa % or more. Such a balance value is very high when considering that the TRIP steel known as having excellent elongation has a tensile strength-total elongation balance value less than 24,000 MPa %.

Manufacturing Method

A method of manufacturing a steel sheet according to the present invention comprises reheating a steel slab having the composition as described above for allowing components segregated during casting to form a solid solution thereof again, hot rolling the reheated steel slab to a steel sheet having a desired thickness, cooling and coiling the hot rolled steel sheet to ensure desired properties of the steel. These steps will be described in detail hereinafter.

First, in the method of manufacturing the steel sheet, the steel slab having the composition described as above is reheated. Reheating of the steel slab is performed for the purpose of solid solutioning the components segregated during the casting, and preferably at a temperature of 1,180˜1,220° C. If a reheating temperature is less than 1,180° C., the components segregated during the casting are not solid soluted again. If the reheating temperature exceeds 1,220° C., the grain size of austenite increases, and grains of ferrite become coarsened, thereby decreasing the strength of steel.

Then, the reheated steel slab is subjected to hot rolling, preferably, at a finish hot rolling temperature of 800° C. or more. If the finish hot rolling temperature is less than 800° C., a great amount of dislocations are induced in the ferrite formed during the hot rolling, and the ferrite is grown during cooling or coiling, forming coarse surface grains.

Next, the hot rolled steel sheet is cooled, and coiled. Since a final structure of the hot rolled steel sheet according to the present invention is a single phase austenite, the hot rolled steel sheet does not suffer from phase transformation during the cooling. Thus, the present invention is not limited to a specific cooling rate, and may have a typical cooling pattern including air cooling and water cooling. In this regard, if a cooling finish temperature is less than 600° C., there is a problem of generating excessive retained stress. Thus, the cooling finish temperature is preferably 600° C. or more.

MODE FOR THE INVENTION

The present invention will be described in detail by reference to examples.

EXAMPLE

After reheating a steel slab having the composition as shown in Table 1 at 1,200° C. for one hour, the steel slab was subjected to hot rolling at a finish hot rolling temperature of 860° C. Then, after cooling the hot rolled steel sheet to a temperature of 680° C., samples were manufactured using the steel sheet, and measured for strength and elongation, results of which are shown in Table 2.

TABLE 1
Sample	Composition (wt %)
No.	C	Mn	S	P	Al	Ni	Cu
IS1	0.45	14.9	0.003	0.02	—	—	—
IS2	0.51	9.98	0.003	0.02	1.00	2.00	2.00
IS3	0.51	9.98	0.003	0.02	1.00	2.00	3.00
IS4	0.51	9.98	0.003	0.02	1.00	5.00	2.00
IS5	0.51	9.98	0.003	0.02	2.00	2.05	2.00
IS6	0.51	9.98	0.003	0.02	3.00	2.05	2.00
IS7	0.79	9.98	0.003	0.02	1.02	2.05	2.00
CS1	0.70	8.02	0.003	0.02	1.05	2.05	2.00
CS2	0.2	4.0	0.003	0.02	0.11	—	—
IS: Inventive steel
CS: Comparative steel

TABLE 2
Sample	YS	TS	Uni. El	Tot. El	TS × Tot. El
No.	(MPa)	(MPa)	(%)	(%)	(MPa %)
IS1	395.0	933.0	38.2	37.5	34987
IS2	401.7	752.3	31.7	32.1	24149
IS3	497.0	793.6	33.7	34.8	27617
IS4	412.0	760.1	48.9	57.4	43630
IS5	464.9	763.5	41.8	46.2	35274
IS6	519.7	768.9	31.2	37.2	28603
IS7	384.4	829.3	30.6	30.6	25377
CS1	362.6	637.3	13.8	15.9	10133
CS2	497.8	628.9	14.3	27.8	17455
IS: Inventive steel
CS: Comparative steel

As can be appreciated from Table 2, Inventive Samples 1˜7 satisfying the conditions of the present invention have excellent strength, elongation, and strength-elongation balance value. In particular, the Inventive Samples 1˜7 have strength-elongation balance values of 24,000 MPa % or more, thereby securing excellent workability along with high strength.

On the contrary, Comparative Samples 1 and 2 do not satisfy the condition of the present invention in terms of Mn content, and have low strength-elongation balance values.

It should be understood that the embodiments and the accompanying drawings have been described for illustrative purposes, and the present invention is limited only by the following claims. Further, those skilled in the art will appreciate that various modifications, additions and substitutions are allowed without departing from the scope and spirit of the invention according to the accompanying claims.

Claims

1. A high strength hot rolled steel sheet containing a high Mn content with excellent workability, comprising, by weight %:

C: 0.2%˜1%, Mn: 8˜15%, S: 0.05% or less, P: 0.03% or less, and the balance of Fe and other unavoidable impurities,

wherein a product of tensile strength and total elongation (TS×Tot.El) of the steel sheet is 24,000 MPa % or more.

2. The steel sheet according to claim 1, further comprising: at least one selected from the group consisting of Al: 0.3˜3%, Ni: 2˜7% and Cu: 2˜5%.

3. The steel sheet according to claim 1, wherein the hot rolled steel sheet has a single phase austenite structure.

4. A method for manufacturing a high strength hot rolled steel sheet containing a high Mn content with excellent workability, comprising:

reheating a steel slab at a temperature of 1,180˜1,220° C., the steel slab comprising, by weight %: C: 0.2%˜1%, MN: 8˜15%, S: 0.05% or less, P: 0.03% or less, and the balance of Fe and other unavoidable impurities;

finish hot rolling the reheated steel slab at a temperature of 800° C. or more; and

cooling the hot rolled steel sheet at a temperature of 600° C. or more, followed by coiling the cooled steel sheet,

wherein a product of tensile strength and total elongation (TS×Tot.El) of the steel sheet is 24,000 MPa % or more.

5. The method according to claim 4, wherein the steel slab comprises at least one selected from the group consisting of Al: 0.3˜3%, Ni: 2˜7% and Cu: 2˜5%.

6. The method according to claim 4, wherein the hot rolled steel sheet has a single phase austenite structure.

7. The steel sheet according to claim 2, wherein the hot rolled steel sheet has a single phase austenite structure.

8. The method according to claim 5, wherein the hot rolled steel sheet has a single phase austenite structure.