KR20150001535A

KR20150001535A - High strength cold-rolled steel sheet and method of manufacturing the same

Info

Publication number: KR20150001535A
Application number: KR20130074925A
Authority: KR
Inventors: 정준호; 김성주
Original assignee: 현대제철 주식회사
Priority date: 2013-06-27
Filing date: 2013-06-27
Publication date: 2015-01-06

Abstract

Strength cold-rolled steel sheet having a low specific gravity and excellent strength and ductility, and a method for producing the same.
The high strength cold rolled steel sheet according to the present invention comprises 10 to 15.0% of manganese (Mn), 6.0 to 9.0% of aluminum (Al), 0.5 to 2.0% of chromium (Cr) (Fe) and unavoidable impurities (Fe), 0.02 to 0.1% of vanadium (V), 0.005 to 0.015% of niobium (Nb), 0.005 to 0.02% of molybdenum Carbide ((Fe, Mn) ₃ AlC) having a mean particle size of 10 to 500 nm, which is characterized by having a composite structure comprising austenite and an average particle size of 10 to 500 nm.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength cold rolled steel sheet and a method of manufacturing the same. BACKGROUND ART [0002]

More particularly, the present invention relates to a high strength cold rolled steel sheet having a high specific gravity and a high ductility, and a method for producing the same.

The problem of environmental disasters caused by global warming and consequently climate change is getting serious today. One of the main causes of global warming is the generation of carbon dioxide due to the use of fossil fuels and the problem of air pollution. One of the main causes of carbon dioxide emission is the exhaust gas of automobiles. Accordingly, developed countries including Europe and the United States have legislated a regulation on automobile fuel consumption, and are strengthening fuel efficiency regulations.

The best way to improve the fuel efficiency of a car is to reduce the weight of the car to make it lighter. When the weight is reduced by 1000 kg, the fuel consumption can be improved by 10 km / L.

For this purpose, the steel industry is carrying out many studies to improve the properties of high strength and high ductility. In addition, there is a growing need for high strength, high ductility lightweight steel plates with high strength and high ductility characteristics and low specific gravity.

Background art related to the present invention is a high manganese steel excellent in abrasion resistance and impact resistance disclosed in Korean Patent Laid-Open Publication No. 10-2006-0071618 (published on June 26, 2006) and a method for producing the same.

It is an object of the present invention to provide a high strength cold rolled steel sheet capable of contributing to high strength and high ductility and light weight by controlling alloy components such as manganese (Mn), aluminum (Al), carbon (C), chromium (Cr), molybdenum And a method for producing the same.

In order to achieve the above object, a high strength cold rolled steel sheet according to an embodiment of the present invention includes 10.0 to 15.0% of manganese (Mn), 6.0 to 9.0% of aluminum (Al), 0.5 to 2.0% of chromium (Cr) , 0.001 to 0.02% of molybdenum (Mo), 0.001 to 0.01% of nitrogen (N), and 0.001 to 0.02% of vanadium (V) Carbide ((Fe, Mn) ₃ AlC) having a mean particle diameter of 10 to 500 nm, which is composed of Fe and unavoidable impurities.

The high-strength cold-rolled steel sheet may have a density of 7.1 g / cm ³ or less.

The high-strength cold-rolled steel sheet may exhibit a tensile strength of 1000 MPa or more and an elongation of 20% or more.

According to an aspect of the present invention, there is provided a method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: preparing a steel sheet having 10 to 15.0% manganese (Mn), 6.0 to 9.0% (N): 0.001 to 0.02%, molybdenum (Mo): 0.005 to 0.02%, nitrogen (N): 0.001 to 2.0%, carbon (C): 0.8 to 1.6%, vanadium 0.01% and the balance iron (Fe) and inevitable impurities; Cold rolling the hot rolled plate; And annealing the cold-rolled plate for 200-300 seconds in a single-phase austenite of Ac3 or higher.

The hot rolled sheet manufacturing method includes a step of reheating the slab sheet at a temperature of 1150 to 1250 占 폚, hot rolling the reheated plate to a finish rolling temperature of Ar3 or higher, cooling the hot rolled plate to a temperature of 550 占 폚 or higher As shown in FIG.

In the case of the high strength cold rolled steel sheet according to the present invention, the manganese content is remarkably lower than that of general high manganese steel containing manganese (Mn) in an amount of 20 wt% or more, so that the manufacturing cost of steel can be lowered, And it is also easy to machine.

In addition, in the case of the high-strength cold-rolled steel sheet according to the present invention, 0.5 to 2.0% by weight of chromium (Cr) is added, and vanadium, molybdenum, vanadium and the like are added to thereby improve the austenite stability and suppress coarsening of k- . Accordingly, the high strength cold rolled steel sheet according to the present invention may have a composite structure including austenite and nano-scale fine k-carbide.

The high-strength cold-rolled steel sheet according to the present invention can contribute to a low specific gravity as high as 6.0 to 9.0% by weight of aluminum and can be made of chromium, carbon, vanadium, niobium (Mn) , Molybdenum and the like, it is possible to exhibit a tensile strength of 1000 MPa or more and an elongation of 20% or more.

Fig. 1 shows a hot-rolled structure of the steel specimen 3. Fig.
Fig. 2 shows the cold-rolled structure of the steel specimen 3. Fig.
3 shows the annealed structure of the steel specimen 3.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 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.

Hereinafter, a high-strength cold-rolled steel sheet according to an embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

High strength cold rolled steel sheet

The high strength cold rolled steel sheet according to the present invention comprises 10 to 15.0% of manganese (Mn), 6.0 to 9.0% of aluminum (Al), 0.5 to 2.0% of chromium (Cr) (N): 0.005 to 0.015%, molybdenum (Mo): 0.005 to 0.02%, and nitrogen (N): 0.001 to 0.01%.

The remainder of the above components are composed of iron (Fe) and impurities inevitably included in the steelmaking process, such as phosphorus (P) and sulfur (S).

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

Manganese (Mn)

Manganese (Mn) is added for austenite stabilization.

The manganese is preferably contained in an amount of 10.0 to 15.0% by weight based on the total weight of the steel sheet. When the content of manganese is less than 10.0 wt%, the effect of the addition is insufficient, and in particular, the austenite phase may become unstable at a temperature of 800 ° C or lower. On the contrary, when the content of manganese exceeds 15.0% by weight, it may lead to an increase in the production cost and a decrease in the productivity and machinability in the steelmaking process.

Aluminum (Al)

Aluminum (Al) is a low specific gravity element and contributes to weight reduction by lowering the density of steel.

The aluminum is preferably contained in an amount of 6.0 to 9.0% by weight based on the total weight of the steel sheet. When the aluminum content is less than 6.0 wt%, it is difficult to maintain the steel density at 7.1 g / cm ³ or less. Conversely, when the content of aluminum exceeds 9.0 wt%, the elongation can be inhibited by the formation of coarse k-carbide.

Chromium (Cr)

Chromium (Cr) plays a role in stabilizing austenite and promoting the formation of fine k-carbide of about 200 nm or less, thereby simultaneously improving strength and ductility, and particularly improving elongation.

The chromium is preferably contained in an amount of 0.5 to 2.0% by weight based on the total weight of the steel sheet. When the content of chromium is less than 0.5% by weight, the k-carbide coarsening inhibiting effect is insufficient. On the contrary, when the content of chromium is more than 2.0 wt%, the effect of k-carbide refinement is large, but problems such as clogging of nozzles may occur during the performance process.

Carbon (C)

Carbon (C) is added to stabilize the austenite and increase the strength.

The carbon is preferably contained in an amount of 0.8 to 1.6% by weight based on the total weight of the steel sheet. When the content of carbon is less than 0.8% by weight, the effect of the addition is insufficient. On the contrary, when the content of carbon exceeds 1.6% by weight, the elongation of the steel sheet can be lowered by coarse k-carbide precipitation.

Vanadium (V)

Vanadium (V) forms a vanadium-based carbonitride and contributes to the strength improvement.

The vanadium is preferably added in an amount of 0.02 to 0.1% by weight based on the total weight of the steel sheet. When the addition amount of vanadium is less than 0.02% by weight, the effect of addition thereof is insufficient. On the other hand, when the addition amount of vanadium exceeds 0.1% by weight, cracking of the slab is caused and the rolling property is deteriorated.

Niobium (Nb)

Niobium (Nb) forms a precipitate and contributes greatly to the improvement of strength.

The niobium is preferably added in an amount of 0.005 to 0.015% by weight based on the total weight of the steel sheet. When the addition amount of niobium is less than 0.005% by weight, the effect of addition thereof is insufficient. On the other hand, when the addition amount of niobium exceeds 0.2 wt%, the performance is deteriorated and the yield ratio of the steel sheet may become excessively high.

Molybdenum (Mo)

Molybdenum (Mo) contributes to the stabilization of austenite, and is an element effective for improving strength and toughness.

The molybdenum is preferably added in an amount of 0.005 to 0.02% by weight based on the total weight of the steel sheet. If the addition amount of molybdenum is less than 0.005% by weight, the effect of the addition is insufficient. On the contrary, when the addition amount of molybdenum exceeds 0.02% by weight, the ductility of the cold-rolled steel sheet to be produced deteriorates.

Nitrogen (N)

Nitrogen (N) contributes to the stabilization of austenite and also contributes to the strength improvement by the formation of carbonitride.

The nitrogen is preferably contained in an amount of 0.001 to 0.01% of the total weight of the steel sheet. When the content of nitrogen is less than 0.001% by weight, it is difficult to exhibit the above effect. On the contrary, when the content of nitrogen exceeds 0.01% by weight, coarse AlN may be formed to cause problems such as nozzle clogging.

In the case of the high strength steel sheet according to the present invention having the above alloy composition, it is possible to have a composite structure containing austenite and fine k-carbide ((Fe, Mn) ₃ AlC) having an average particle diameter of 10 to 500 nm have. In the composite structure, ferrite may be included in an area ratio of about 0.5 to 5%.

Further, the high strength cold rolled steel sheet according to the present invention and fine austenite through k- carbide alloy composition of the above _{((Fe, Mn) 3 AlC} ) low specific gravity of 7.1g / cm ³ or less by having a composite structure containing, 1000MPa Or more and an elongation of 20% or more.

Method of manufacturing high strength cold rolled steel sheet

The high-strength cold-rolled steel sheet manufacturing method according to the present invention can use a conventional cold-rolled steel sheet manufacturing method including a hot rolling step, a cold rolling step and an annealing step.

More specifically:

First of all, in terms of% by weight, manganese (Mn): 10.0 to 15.0%, aluminum (Al): 6.0 to 9.0%, chromium (Cr): 0.5 to 2.0%, carbon (C) (Fe) and inevitable impurities, in the range of 0.02 to 0.1% of niobium (Nb), 0.005 to 0.015% of molybdenum (Mo), 0.005 to 0.02% A plate material is produced.

The hot rolled plate is preferably manufactured by reheating the slab plate having the alloy composition at 1150 to 1250 캜, hot rolling the reheated plate to a finish rolling temperature condition of Ar 3 or higher, cooling the hot rolled plate to 550 Lt; 0 > C or more.

Next, the hot-rolled sheet is cold-rolled at a reduction ratio of about 40 to 80%, and is processed to a final thickness of the product.

Next, the cold-rolled plate is subjected to annealing for 200 to 300 seconds at a single-phase austenite of Ac3 or higher. If the annealing treatment time is less than 200 seconds, austenite formation may be insufficient. On the other hand, when the annealing treatment time exceeds 300 seconds, austenite and fine k-carbide are coarsened and strength and elongation can be lowered.

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. Manufacture of steel specimens

Ingot-form steel specimens 1 to 6 having alloy compositions shown in Table 1 were prepared.

[Table 1]

The ingot steel samples 1 to 6 were reheated at 1200 ° C for 2 hours, hot-rolled at a finish rolling temperature of 880 ° C, cooled to 600 ° C at 20 ° C / sec, and then cooled to room temperature. Thereafter, the steel sheet was cold-rolled at a reduction ratio of 50%, annealed at 860 ° C for 250 seconds, cooled to 400 ° C at a cooling rate of 10 ° C / sec, and then cooled to room temperature.

2. Evaluation of mechanical properties

(1) Density

For the density measurements of specimens 1 ~ 6, specimens were collected from the center of the specimen and the density of specimens was measured using Archimedes' principle. An indium (In) ingot (7.31 g / cm ³ ) having a purity of 99.8% was used as a standard sample. As shown in Table 1, the densities of specimens 1 to 6 were less than 7.1 g / cm ³ , and these densities vary depending on the aluminum content.

(2) Tensile test

Tensile tests were performed on the specimens produced by each method. Tensile specimens were processed to ASTM E8 standard. The tensile test was carried out at a cross-head speed of 0.5 mm / min at room temperature. This rate corresponds to an initial strain of 3.3 x 10 < -4 > s-1.

(3) Results of physical property evaluation

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

[Table 2]

Referring to Table 2, the specimens 2 to 3 satisfying the steel composition shown in the present invention exhibited a tensile strength of 1000 MPa or more and an elongation of 20% or more. This is because, in the case of the cold-rolled steel sheet produced by the method according to the present invention, the k-carbide is finely granulated together with the austenite.

However, in the case of specimens 1 and 4 to 6, which did not satisfy the steel composition shown in the present invention, fracture occurred or elongation was less than 20%.

Figs. 1 to 3 show the hot-rolled structure (Fig. 1), the cold-rolled structure (Fig. 2) and the annealed structure (Fig. 3) of the steel specimen 3.

Referring to Figs. 1 to 3, in the case of specimen 3, it can be seen that fine k-carbide is dispersed in the austenite base.

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

(Mn): 10.0-15.0%, aluminum (Al): 6.0-9.0%, chromium (Cr): 0.5-2.0%, carbon (C): 0.8-1.6%, vanadium (V): 0.02 (Fe) and unavoidable impurities, and the amount of nitrogen (N) is in the range of 0.001 to 0.1%, 0.005 to 0.015% of niobium, 0.005 to 0.02% of molybdenum (Mo)
Austenite, and fine k-carbide ((Fe, Mn) ₃ AlC) having an average particle diameter of 10 to 500 nm.

The method according to claim 1,
The high strength cold rolled steel sheet
Cm < ³ > or less.

The method according to claim 1,
The high strength cold rolled steel sheet
A tensile strength of 1000 MPa or more, and an elongation of 20% or more.

(Mn): 10.0-15.0%, aluminum (Al): 6.0-9.0%, chromium (Cr): 0.5-2.0%, carbon (C): 0.8-1.6%, vanadium (V): 0.02 (Fe) and unavoidable impurities, and the hot-rolled sheet material is formed from a slab sheet made of a steel sheet having a thickness of 0.1 to 0.1%, niobium (Nb) of 0.005 to 0.015%, molybdenum (Mo) of 0.005 to 0.02%, nitrogen (N) Producing;
Cold rolling the hot rolled plate; And
And annealing the cold-rolled plate for 200-300 seconds in a single-phase austenite of Ac3 or higher.

5. The method of claim 4,
The hot-rolled sheet manufacturing step
Reheating the slab plate at 1150 to 1250 ° C,
Hot-rolling the reheated plate to a finishing rolling temperature condition of Ar3 or higher;
And cooling the hot-rolled plate material and winding the hot-rolled plate material at a temperature of 550 DEG C or higher.