KR101193718B1 - Nitrogen-added high manganese steel having high strength and large ductility and method for manufacturing the same - Google Patents

Abstract

The high manganese nitrogen-containing steel sheet according to the present invention comprises the steps of placing the electrolytic iron, electrolytic manganese and carbon powder in the chamber; Filling the chamber with an argon-nitrogen atmosphere; And arcmelting the electrolytic iron, manganese and carbon powder. The steel sheet formed by the method includes 0.5 wt% to 1.0 wt% carbon, 10 wt% to 20 wt% manganese, 0.02 wt% to 0.2 wt% nitrogen, the balance iron and inevitable impurities.
The high manganese nitrogen-containing steel sheet according to the present invention replaces manganese with an inexpensive element, but has higher strength and ductility than a steel sheet containing a large amount of manganese, and is easy to process.

Description

Nitrogen-added high manganese steel having high strength and large ductility and method for manufacturing the same

The present invention provides a high manganese nitrogen-containing steel sheet having high strength and high ductility, more specifically, a steel sheet for automobiles requiring high formability, and a high manganese nitrogen-containing steel sheet usable as an impact absorber such as a bumper reinforcement for automobiles, and the manufacture thereof. It is about a method.

In the case of steel sheets used in the body of an automobile, high formability is basically required. In order to meet these demands, the ultra low carbon steel having a low tensile strength of 200 to 300 MPa but excellent moldability has been widely used as an automotive steel sheet. However, in recent years, as environmental problems such as air pollution are highlighted, many methods for increasing fuel economy of automobiles have been proposed. In particular, as weight reduction of automobiles is important for improving fuel efficiency, automobile steel sheets are required to have high strength as well as high formability.

In addition, since automotive parts such as bumper reinforcement for automobiles or shock absorbers in doors are directly related to passenger safety, high strength steels, such as steel sheets having a high elongation at the same time as the tensile strength of 780 MPa or more, need to be used. The need for commercialization is greatly increasing.

Such high-strength steel for automobiles include, for example, a dual phase (DP) steel, a TRans (Tansformation Induced Plasticity) steel, and a Twin Wind Induced Plasticity (TWIP) steel.

First, in the course of hot-rolling steel and cooling the steel to room temperature, the cooling end temperature is lower than the martensite transformation start temperature (Ms), and a part of the austenite is transformed into martensite, and the austenite is removed from the austenite at room temperature. It has a malformed martensite and ferrite abnormal tissue. The abnormal tissue steel (DP steel) can obtain a variety of mechanical properties by controlling the fraction of martensite and ferrite.

On the other hand, the metamorphic organic plastic steel (TRIP steel) is to form a part of the structure of the retained austenite, and improved the workability of the steel by using a phase transformation from austenite to martensite during part molding. This, TRIP steel has the advantage of high strength due to the large work hardening by martensite transformation, but has the disadvantage that the elongation is too low.

In other words, in the case of DP steel and TRIP steel, the work hardening mechanism mainly uses martensite, which is a hard phase. Such martensite shows a high increase in work hardening during plastic deformation, and thus a high strength hot rolled steel sheet can be manufactured. It is difficult to secure an elongation of more than%.

On the other hand, twin eutectic steel (TWIP steel) contains a large amount of manganese, has a stable austenitic single phase at room temperature, and increases work hardening by forming mechanical twins in the austenite structure during component processing. That is, the TWIP steel is austenite rather than ferrite, and the mechanical twins are continuously generated in the austenite grains during plastic deformation, thereby preventing dislocations, thereby additionally obtaining work hardening, thereby obtaining excellent elongation. In addition, since TWIP steels form mechanical twins that cause high work hardening, high elongation as well as high tensile strength can be obtained. In particular, the TWIP steel has an excellent elongation ratio of 50% or more, which is higher than that of conventional DP steel or TRIP steel, and is excellent for use in automotive steel plates and the like.

However, the TWIP steels developed to date have a high manganese content of 18-30% for securing austenite stability and controlling stacking fault energy, and a large amount of aluminum or silicon is added in addition to manganese. And there is a disadvantage that the manufacturing cost is greatly increased. In addition, there is a disadvantage in that additional manufacturing cost burden due to volatilization of manganese or temperature reduction during the steelmaking process or the playing process, the situation is required to develop a TWIP steel with reduced manganese content. In addition, in terms of mechanical properties, TWIP steels developed so far have a yield strength of about 300 MPa and a tensile strength of not more than 1 GPa. Therefore, it is necessary to provide a steel sheet having higher strength while maintaining elongation. .

On the other hand, as a invention to solve the problems of the conventional DP steel, TRIP steel and TWIP steel, Korean Patent Application No. 2009-36963 proposes a high manganese steel that improves the strength and elongation by increasing the content of nitrogen. That is, nitrogen is a very useful element in the production of TWIP steel because it can stabilize austenite even in a small amount, and also easily control stacking defect energy. However, since nitrogen exists as a gas at room temperature, it is very difficult to contain a large amount of nitrogen in the alloy in a certain amount, and Korean Patent Application No. 2009-36963 contains more nitrogen than is normally possible by adding chromium. Successfully manufactured TWIP steel. However, the present invention has been successful in providing high strength and high ductility at the same time by increasing the content of nitrogen, but for this purpose, it contains chromium, which is an expensive metal, chromium is also an expensive metal, the cost is expensive when manufacturing steel Is still not solved.

Accordingly, an object of the present invention is to provide a steel sheet that can solve the problems of conventional DP steel, TRIP steel and TWIP steel.

Specifically, an object of the present invention is to provide a steel sheet having a high strength and high ductility at the same time while reducing the content of manganese.

In addition, an object of the present invention is to provide a steel sheet having a higher strength and ductility than a steel sheet containing a large amount of manganese while being easy to process while replacing manganese with an inexpensive element.

High manganese nitrogen-containing steel sheet according to the present invention for achieving the above object is:

0.5 wt% to 1.0 wt% carbon, 10 wt% to 20 wt% manganese, 0.02 wt% to 0.2 wt% nitrogen, balance iron and inevitable impurities.

Or from 0.5% to 1.0% by weight of carbon, from 10% to 20% by weight of manganese, from 4.0% by weight of chromium, from 0.02% to 0.3% by weight of nitrogen, the balance of iron and unavoidable impurities. It is characterized by including.

In this case, at least a part of the nitrogen is preferably formed by an arc-melting method.

In addition, the high manganese nitrogen-containing steel sheet is preferably a product of the tensile strength and the total elongation (TS × El) of 50,000MPa% or more.

In addition, the manganese is more preferably contained in 15 to 18% by weight.

In addition, the nitrogen is preferably contained in 0.10% to 0.3% by weight.

In addition, the steel sheet may be a hot rolled steel sheet.

Alternatively, the steel sheet may be a cold rolled annealing steel sheet.

Method for producing a high manganese nitrogen-containing steel sheet according to the present invention is:

Placing electrolytic iron, manganese and carbon powder in the chamber;

Filling the chamber with an argon-nitrogen atmosphere; And

It characterized in that it comprises the step of arcmelting the electrolytic iron, electrolytic manganese and carbon powder.

In addition, the arc melting is preferably performed repeatedly a plurality of times.

In addition, the nitrogen-argon atmosphere preferably has a nitrogen fraction of 0.2 to 0.8.

In addition, the method comprises the steps of hot rolling the high nitrogen steel sheet at 900 ℃ or more; And air cooling or forced cooling the hot rolled steel sheet.

In addition, the method comprises the steps of cold rolling the cooled steel sheet at room temperature with a thickness reduction rate of 50% or more; Annealing the cold rolled steel sheet at 800 ° C. or higher; And it is preferable to further include the step of air cooling or forced cooling the annealing heat-treated steel sheet.

In addition, the steel sheet formed by the method comprises 0.5% to 1.0% by weight of carbon, 10% to 20% by weight of manganese, 0.02% to 0.2% by weight of nitrogen, balance iron and inevitable impurities It is desirable to.

In addition, the raw material disposed in the chamber in the method preferably further comprises chromium.

In this case, the steel sheet formed by the above method comprises 0.5 wt% to 1.0 wt% carbon, 10 wt% to 20 wt% manganese, 4.0 wt% or less chromium, 0.02 wt% to 0.2 wt% nitrogen, It is preferable to contain the balance of iron and unavoidable impurities.

The high manganese nitrogen-containing steel sheet according to the present invention replaces manganese with an inexpensive element, but has higher strength and ductility than a steel sheet containing a large amount of manganese, and is easy to process.

1 is a diagram showing a tension curve of steel formed according to Experimental Example 3. FIG.

Preferred embodiments of the high manganese nitrogen-containing steel sheet according to the present invention will be described below.

According to this embodiment, the arc melting method was used to add more than 0.02% by weight of nitrogen. That is, since nitrogen is present as a gas at room temperature, it is difficult to contain a large amount of nitrogen in the alloy in a large amount or more when using the steelmaking method by conventional dissolution. The present invention provides arc melting in a nitrogen atmosphere, preferably a nitrogen-argon atmosphere. This could increase the nitrogen content of the steel.

The method of forming a steel sheet according to the first embodiment of the present invention is as follows. First, electrolytic iron, electrolytic manganese, and carbon powder are disposed in the chamber. At this time, it is possible to control the composition of the steel sheet as a finished product by adjusting the amount of each material introduced into the chamber. The chamber interior is then evacuated and then filled with an argon-nitrogen atmosphere. At this time, argon and nitrogen are all 1 atm, and the partial pressure of nitrogen is preferably maintained in the range of 0.2 to 0.8 atm. If the ratio of nitrogen is less than 0.2 atm relative to the total pressure of 1 atm, the amount of nitrogen added to the steel during arcmelting is too small and the efficiency is lowered. In addition, when the ratio of nitrogen exceeds 0.8 atm relative to the total pressure of 1 atm, the pressure of argon, which is an inert gas, is too low, so that fumes by manganese are severely generated and the inside of the chamber is severely polluted. In addition, when the nitrogen ratio is too high, the scattering of the raw material is severe due to the melting of the tungsten electrode bar has a disadvantage that the surface state of the steel sheet after the arc melting is very rough. Subsequently, the materials inside the chamber are arc melted using electrodes, and cooled for a suitable time to complete the steel sheet. In addition, the arc melting-cooling process may be completed once, but is preferably repeated a plurality of times. In addition, the content of nitrogen is higher as the arc melting-cooling process is repeated.

The high manganese nitrogen-containing steel sheet formed by the above method comprises 0.5 wt% to 1.0 wt% carbon, 10 wt% to 20 wt% manganese, 0.02 wt% to 0.3 wt% nitrogen, and the balance of iron and unavoidable impurities. Include.

In the present invention, the austenitic single phase was obtained at room temperature by adding carbon and nitrogen instead of lowering the manganese content to 10 to 20% compared to conventional twin-twist organic steel (TWIP) steel containing 20% by weight of manganese. In particular, nitrogen affects stacking defect energy in addition to the effect of solid solution strengthening, leading to the formation of mechanical twins.

Therefore, the high manganese nitrogen-containing steel sheet according to the present invention includes the alloying elements, while the content of expensive alloying elements such as manganese or aluminum is less than that of conventional TWIP steel, while the elongation is maintained at 50% or more, It can have higher yield strength and tensile strength than TWIP steel.

Specifically, the high manganese nitrogen-containing steel sheet according to the first embodiment of the present invention contains 10 to 20% by weight of manganese. That is, since TWIP steel is a mechanical twin formed during plastic deformation at the austenite matrix at room temperature, an alloying element must be added to extend the hot austenite region to the room temperature on the iron-carbon state diagram. This example used manganese as the austenite stabilizing element for this purpose.

Further, in this embodiment, the content of manganese is more preferably 15% to 18% by weight. That is, when the content of manganese is 15% by weight, austenite stability can be secured, and the tensile strength x elongation is also excellent because the formation of mechanical twins during the plastic deformation is reduced by effectively lowering the lamination defect energy.

On the other hand, when the content of manganese is less than 10% by weight, the stability of the austenite phase is greatly degraded, so that a ferrite or martensite phase may occur during cooling in the austenite region after hot rolling. In addition, when the content of manganese is less than 10% by weight, there is a disadvantage that the stacking defect energy of the austenite phase is too high to form a mechanical twin.

In addition, when the content of manganese exceeds 20% by weight, the lamination defect energy is too large, so that no twin is formed and plastic deformation of the austenite phase occurs, resulting in poor mechanical properties.

In addition, the high manganese nitrogen-containing steel sheet according to this embodiment contains 0.5% by weight to 1.0% by weight of carbon. That is, the iron-manganese binary alloy having a manganese content of 20% by weight or less cannot obtain austenite single phase at room temperature, and ε martensite or α ′ martensite are partially formed. Therefore, according to this embodiment, in order to obtain austenite single phase structure at room temperature, carbon, which is a cheap and powerful austenite stabilizing element, was added.

On the other hand, if the carbon content is less than 0.5% by weight, the austenite stability is still not enough to obtain austenite single phase during cooling after hot rolling, or even if austenite single phase is obtained at room temperature, Phase transformation from knight to martensite occurs, resulting in metamorphic organic plastic steel (TRIP steel), and twin-organic plastic steel (TWIP steel) cannot be obtained.

In addition, when the carbon content exceeds 1.0% by weight, it is possible to obtain stable austenite at room temperature, but there is a problem in that cementite precipitation occurs to reduce elongation or deteriorate weldability. In addition, when the carbon content exceeds 1.0% by weight, the lamination defect energy becomes too large, which makes it difficult to generate mechanical twins during deformation.

In addition, the high manganese nitrogen-containing steel sheet according to the present embodiment contains 0.02% to 0.30% by weight of nitrogen. Specifically, nitrogen is an invasive element that stabilizes the austenite phase. When the amount is increased, nitrogen increases the austenite stability and increases the strength by solid solution strengthening. In addition, nitrogen does not increase the stacking defect energy even if the content is increased may facilitate the creation of mechanical twins.

On the other hand, according to this embodiment, when the content of nitrogen is 0.10% by weight or more, the solid solution strengthening effect is increased, and ultimately the yield strength is significantly higher, it is more preferable.

In addition, when the nitrogen content is less than 0.02% by weight, it is difficult to achieve the improvement of the austenite stability as the degree of addition as an impurity when forming a conventional steel sheet, and a ferrite or martensite phase may be formed at room temperature after hot rolling, and the lamination defect energy may be increased. The disadvantage is that it is difficult to obtain the function to adjust. In addition, it is difficult to include nitrogen in an amount of 0.2% by weight or more even if arc-melting is not added unless additional elements such as chromium are added.

Next, except that the high manganese nitrogen-containing steel sheet according to the second embodiment of the present invention further includes chromium in addition to electrolytic iron, electrolytic manganese, and carbon powder as a raw material in the chamber, the method of forming the steel sheet is similar to that of the first embodiment. Since the same, detailed description is omitted.

In this case, the high manganese nitrogen-containing steel sheet according to the second embodiment is 0.5 wt% to 1.0 wt% carbon, 10 wt% to 20 wt% manganese, 4.0 wt% or less chromium, and 0.02 wt% to 0.3 By weight nitrogen and up to 4% by weight chromium, balance iron and inevitable impurities.

Chromium not only improves the corrosion resistance of the steel, but also increases the solubility of nitrogen. In addition, chromium reduces the deposition defect energy that is increased due to the addition of carbon to promote the formation of mechanical twins. However, if chromium is added in excess of 4.0% by weight as a ferrite stabilizing element, some ferrite may be produced during hot rolling. In addition, chromium has a disadvantage in that the manufacturing cost is too large when used too much as a high-quality material, it is preferable to limit the content to 4% by weight or less.

Experimental Example

First, the method of forming the steel sheet according to Experimental Example 1 to Experimental Example 3 is as follows. Electrolytic iron, manganese, and carbon powder were placed in various proportions inside the chamber, and the chamber was vacuumed, and then the inside of the chamber was filled with an argon-nitrogen atmosphere. At this time, argon and nitrogen are all 1 atm, and the partial pressure of nitrogen is preferably maintained in the range of 0.2 to 0.8 atm. In this experiment, the arc electrodes were spaced 2 to 5 cm apart from the specimens at 400A for 30 minutes and then arcmelted and cooled for 30 minutes. In addition, the arc melting and cooling process was repeated three times.

Next, the steel sheet according to Experimental Example 4 is an iron-melted steel sheet containing chromium in addition to electrolytic iron, electrolytic manganese, and carbon powder in the chamber. The following steel sheet forming method was performed in the same manner as in Experimental Example 1 to Experimental Example 3.

Next, the steel sheet according to the comparative example is to form a steel by melting the raw material in a nitrogen atmosphere without performing the arc melting according to the method.

Specific compositions of the high manganese steel sheet according to Experimental Examples 1 to 4 and Comparative Examples formed through the above method are as follows.

division Composition (% by weight) Remarks C Mn Cr N Experimental Example 1 0.003 11.95 - 0.093 Arc Melting Experimental Example 2 0.760 14.27 - 0.109 Arc Melting Experimental Example 3 0.570 16.47 - 0.090 Arc Melting Experimental Example 4 0.004 14.42 1.99 0.141 Arc Melting Comparative example 0.618 15.03 1.82 0.086 Nitrogen atmosphere melting

As in Experimental Examples 1 to 3, the steel containing no chromium could be formed in the argon-nitrogen atmosphere by arcmelting to form a steel having a high nitrogen content. In addition, it was possible to form a steel with a higher nitrogen content by acmelting the steel containing chromium in an argon-nitrogen atmosphere as in Experimental Example 4.

On the other hand, the comparative example illustrates the case where the steel is formed by melting in accordance with a conventional steelmaking method containing chromium and in a nitrogen atmosphere. It may be confirmed that the nitrogen content of the steel is smaller than that of the steel according to the present experimental example even if chromium is included and dissolved in a nitrogen atmosphere as in the comparative example. That is, when forming a steel containing 1.73% by weight of chromium by dissolution as in Comparative Example, the nitrogen content was 0.086% by weight, compared to 0.141% by weight of the nitrogen content of Experimental Example 4 containing a similar amount of chromium. It can be seen that the content is significantly less.

1 is a figure which shows the tension curve of the steel formed according to Experimental Example 3. FIG. As shown in Figure 1, the steel according to Experimental Example 3 was very excellent in strength and ductility of 985MPa, 56%, respectively, the product of strength and elongation is also about 55,000MPa%, which is 20% by weight of manganese It is a very high value compared to high manganese steels which do not contain chromium and which is below, and is very useful because it is similar to that of high manganese steels containing more than 20% by weight of manganese or further containing expensive metals such as chromium. .

The manufacturing method of the high manganese nitrogen-containing steel sheet and the high manganese nitrogen-containing steel sheet according to a preferred embodiment of the present invention has been described in detail. However, it will be understood by those skilled in the art that various modifications and variations can be made to the embodiment. Therefore, the scope of the present invention is to be determined only by the claims which will be described later.

Claims (14)

High manganese nitrogen content comprising 0.5 to 1.0 weight percent carbon, 10 to 20 weight percent manganese, 0.02 to 0.2 weight percent nitrogen, balance iron and unavoidable impurities Grater. 0.5% to 1.0% by weight of carbon, 10% to 20% by weight of manganese, more than 0% to 4.0% by weight of chromium, 0.02% to 0.3% by weight of nitrogen, balance iron A high manganese nitrogen-containing steel sheet comprising inevitable impurities. The high manganese nitrogen-containing steel sheet according to claim 1 or 2, wherein at least a part of the nitrogen is included in the steel sheet by an arc-melting method. 4. The high manganese nitrogen-containing steel sheet according to claim 3, wherein the high manganese nitrogen-containing steel sheet has a product (TS x El) of tensile strength and total elongation of 50,000 MPa% or more. The high manganese nitrogen-containing steel sheet according to claim 3, wherein the manganese is contained in an amount of 15 wt% to 18 wt%. 3. The high manganese nitrogen-containing steel sheet according to claim 2, wherein the nitrogen is contained in an amount of 0.10% to 0.3% by weight. Placing electrolytic iron, manganese and carbon powder in the chamber;
Filling the chamber with an argon-nitrogen atmosphere; And
Method for producing a high manganese nitrogen-containing steel sheet, comprising the step of arc-melting the electrolytic iron, electrolytic manganese and carbon powder. Placing electrolytic iron, manganese, chromium and carbon powder in the chamber;
Filling the chamber with an argon-nitrogen atmosphere; And
Method for producing a high manganese nitrogen-containing steel sheet, comprising the step of arc-melting the electrolytic iron, electrolytic manganese, chromium and carbon powder. 10. The method of claim 7 or 8, wherein the arc melting is repeated a plurality of times. The method for producing a high manganese nitrogen-containing steel sheet according to claim 7 or 8, wherein the nitrogen-argon atmosphere has a nitrogen fraction of 0.2 to 0.8 atm based on 1 total of nitrogen-argon. The method according to claim 7 or 8, wherein the method comprises: hot rolling a high manganese nitrogen-containing steel sheet formed by the arc melting at 900 ° C or higher; And air cooling or forcibly cooling the hot rolled steel sheet. The method of claim 11, wherein the method further comprises: cold rolling the air-cooled or forced-cooled steel sheet at room temperature with a thickness reduction rate of 50% or more; Annealing the cold rolled steel sheet at 800 ° C. or higher; And air-cooling or forcibly cooling the annealing heat-treated steel sheet. 8. The steel sheet formed by the method comprises: 0.5 wt% to 1.0 wt% carbon, 10 wt% to 20 wt% manganese, 0.02 wt% to 0.2 wt% nitrogen, and the balance iron; A method for producing a high manganese nitrogen-containing steel sheet, which comprises inevitable impurities. The steel sheet formed by the method according to claim 8, wherein the steel sheet formed by the method comprises 0.5 wt% to 1.0 wt% carbon, 10 wt% to 20 wt% manganese, more than 0 wt% to 4.0 wt% chromium, and 0.02 wt% A method for producing a high manganese nitrogen-containing steel sheet, characterized in that it comprises nitrogen of% to 0.3% by weight, balance iron and inevitable impurities.

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