KR101406634B1 - Ultra-high strength steel sheet with excellent coating property and crashworthiness, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to an ultra-high strength steel sheet excellent in plating and impact properties and a method of manufacturing the same. More particularly, the present invention relates to an ultra high strength steel sheet excellent in platability and impact property, The ultra-high strength steel can be obtained by work-hardening by re-rolling. At the same time, the yield strength can be excellent, and the collision characteristics can be secured.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a super high strength steel sheet excellent in plating and impact properties,

The present invention relates to a super high strength steel sheet applicable to a member part at a collision site such as a bumper, a seal side and a seat rail, and a method of manufacturing the same. More particularly, the present invention relates to a super high strength steel sheet having excellent plating and impact properties, ≪ / RTI >

Automobile companies are increasingly applying automobile materials as lightweight materials and high strength materials for the purpose of improving environmental pollution, improving fuel economy, and safety. Such materials are also applied to many structural members other than automobile parts.

Conventionally, a high strength steel of a low carbon steel type having a base structure of ferrite was used in consideration of formability. However, when a high strength steel of a low carbon steel type is used as a steel sheet for an automobile, it is difficult to secure an elongation rate of 30% or more at a commercial level at a tensile strength of 800 MPa or more. Therefore, it is difficult to apply a high-strength steel of 800 MPa or more in a complicated shape component, and it is difficult to design a part freely such as simplifying the shape of a part.

In addition, it is difficult to manufacture steel capable of cold press forming or roll forming with high strength of 1300 MPa or more in tensile strength as the current steel sheet manufacturing technology.

In order to solve the above-mentioned problems, Patent Documents 1 and 2 are proposed, and these documents propose an austenitic high manganese steel excellent in ductility and strength.

However, in Patent Document 1, although the ductility is secured by adding a large amount of manganese, the steel plate easily breaks after machining hardening occurs at the deformation portion, and the high manganese steel ductility proposed in Patent Document 2 is secured However, the addition of a large amount of silicon has a disadvantage in that electrolytic electrolysis and molten electrolysis are disadvantageous. In addition, the steel sheets provided in the above Patent Documents 1 and 2 have excellent workability but have a low yield strength, resulting in poor collision characteristics.

On the other hand, recent automobile manufacturers have been expanding the use of TWIP (Twinning-Induced Plasticity) steels by the fact that high manganese steel can improve moldability by increasing the work hardening rate due to the formation of twin during plastic deformation.

However, TWIP steels having austenite structure have limitations in increasing the tensile strength, making it difficult to manufacture them as ultra high strength steels.

Japanese Patent Laid-Open No. 1992-259325 International Publication No. WO02 / 101109

An aspect of the present invention is to provide a method for controlling the content of austenite stabilizing elements and controlling the manufacturing conditions to secure excellent strength and ductility and excellent plating ability, And to obtain a super high strength steel that can be used for a complicated inner plate material.

An aspect of the present invention is a method of manufacturing a semiconductor device, which comprises 0.4 to 0.7% of carbon (C), 12 to 24% of manganese (Mn), 0.01 to 3.0% of aluminum (Al) (P): not more than 0.03%, sulfur (S): not more than 0.03%, nitrogen (N), and the like. N: not more than 0.04%, the balance being iron and other unavoidable impurities, and comprising austenite single-phase microstructure, said microstructure having an aspect ratio of 2 or more And a crystal grain size of at least 70%.

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising: homogenizing a steel ingot or a slab having a composition range as described above by heating at 1050 to 1300 캜; Subjecting the homogenized steel ingot or the performance slab to hot rolling at a final hot rolling temperature of 850 to 1000 占 폚; Rolling the hot-rolled steel sheet at 200 to 700 ° C; Cold rolling the rolled steel sheet at a cold reduction rate of 30 to 80%; Continuously annealing the cold-rolled steel sheet at 400 to 900 ° C; And a step of re-rolling the steel sheet subjected to the continuous annealing treatment. The present invention also provides a method of manufacturing an ultra-high strength steel sheet excellent in platability and impact properties.

According to the present invention, by controlling the kind and content of the components to be added and further performing cold rolling or re-rolling of the galvanized steel sheet, it is possible to ensure excellent impact characteristics while ensuring ultra high strength. In addition, the content of tin, nickel, chromium and the like can be optimally optimized to ensure excellent plating performance. The ultra-high strength steel sheet can be sufficiently applied to collision members of automobiles and the like.

FIG. 1 is a result of observing crystal grains of a microstructure after re-rolling a cold-rolled steel sheet satisfying the component system according to the present invention.
Fig. 2 is a schematic diagram defining the aspect ratio of the microstructure in the rolling direction.

The inventors of the present invention have studied deeply to solve the problem that the high strength can be secured by adding a large amount of manganese in the conventional high manganese steel but it is difficult to secure the ductility. As a result, It is possible to manufacture an ultra high strength steel sheet which can be used for a product having excellent workability required for the manufacture of automobile parts by controlling the components to be added for the production of automobile parts by reworking the produced steel through re- And the present invention has been completed.

Accordingly, the present invention provides a method for controlling a component system, that is, controlling the contents of carbon, manganese, and aluminum as the elements of austenite stabilization, securing a complete austenite phase at room temperature, securing plating properties by controlling the content of tin, Strength steel sheet with excellent collision characteristics by controlling the microstructure by securing excellent strength through work hardening.

Hereinafter, the present invention will be described in more detail.

First, the reason for restricting the components in the ultra high strength steel sheet of the present invention will be described in detail. At this time, the content of the component elements means all the weight percentages.

C: 0.4 to 0.7%

Carbon (C) is an element contributing to the stabilization of the austenite phase, and therefore, as the addition amount thereof is increased, it is advantageous in forming an austenite phase. However, when the content of carbon is less than 0.4%, cracks are generated during machining because the α '(alpha re-) martensite phase is formed at the time of transformation, and the ductility is lowered. On the other hand, when the content of C is more than 0.7%, carbide is formed or the electrical resistance is increased and the weldability is deteriorated. Therefore, in the present invention, the content of C is preferably limited to 0.4 to 0.7%.

Mn: 12 to 24%

Manganese (Mn) is an essential element for stabilizing the austenite phase with carbon. However, when the content is less than 12%, an α '(alpha re-) martensite phase which damages the formability is generated and the strength is increased, but the ductility is rapidly decreased and the work hardening rate is also small. On the other hand, when the content of Mn exceeds 24%, the generation of twin is suppressed and the strength is increased but the ductility is decreased. In addition, as the amount of Mn added increases, cracks easily occur during hot rolling and the production cost is increased, which is disadvantageous from the economical point of view. Therefore, in the present invention, the content of Mn is preferably limited to 12 to 24%.

Al: 0.01 to 3.0%

Aluminum (Al) is usually added for the purpose of deoxidizing steel, but in the present invention, it is added in order to improve ductility and improve delayed fracture resistance. In other words, Al is a stabilizing element in the ferrite phase, but increases the stacking fault energy (Stacking Fault Enegy) at the slip surface of the steel to suppress the formation of the ε-martensite phase, thereby improving ductility and resistance to delayed fracture. Further, Al suppresses the formation of the? -Martensite phase even when the amount of Mn added is small, thereby contributing to improving workability while minimizing the addition amount of manganese. Therefore, when the addition amount of Al is less than 0.01%, an ε-martensite phase is generated and the strength is increased but the ductility is rapidly reduced. On the other hand, when the addition amount is more than 3.0%, the occurrence of twinning is suppressed, The casting is deteriorated during continuous casting, and the surface quality of the product is lowered due to a large amount of oxidation occurring on the surface of the steel sheet during hot rolling. Therefore, in the present invention, the content of Al is preferably limited to 0.01 to 3.0%.

Si: not more than 0.3%

Silicon (Si) is an element to be solid solution strengthened, and is an element that increases the yield strength of a steel sheet by reducing crystal grain size by the employment effect. Generally, when Si is excessively added, it is known that a silicon oxide layer is formed on the surface to lower the meltability.

However, in a steel to which a large amount of Mn is added, when a proper amount of Si is added, a thin silicon oxide layer is formed on the surface to inhibit the oxidation of Mn, so that a thick Mn oxide layer formed after rolling in the cold- And it is possible to prevent the corrosion progressed in the cold-rolled steel sheet after annealing to improve the surface quality, and to maintain excellent surface quality as the base steel sheet of the electroplating material. However, if the addition amount of Si is excessively increased, a large amount of Si oxide is formed on the surface of the steel sheet during hot rolling, which deteriorates the pickling ability and deteriorates the surface quality of the hot-rolled steel sheet. Further, Si is concentrated on the surface of the steel sheet during high-temperature annealing in the continuous annealing step and continuous hot-dip coating step, and the wettability of molten zinc on the surface of the steel sheet is lowered when the hot-dip coating is performed. Therefore, in order to avoid the above-described problems, Si is preferably added in an amount of 0.3% or less.

P and S: 0.03% or less, respectively

Generally, phosphorus (P) and sulfur (S) are inevitably included in the production of steel, and their contents are limited to 0.03% or less. Particularly, P causes segregation to reduce the workability of the steel, S forms coarse manganese sulfide (MnS) to generate defects such as flange cracks, and decreases the hole expandability of the steel sheet, Is suppressed to the maximum.

Ni: 0.05 to 1.0%

Nickel (Ni) is an element essential for stabilizing the austenite phase and is an effective element for increasing the strength of the steel sheet. However, when the content is less than 0.05%, it is difficult to obtain the above effect, whereas when it is more than 1.0%, the production cost is uneconomical. Therefore, in the present invention, it is preferable to limit the content of Ni to 0.05 to 1.0%.

Cr: 0.05 to 1.0%

Chromium (Cr) is an effective element for improving the plating ability and increasing the strength of a steel sheet. However, when the content is less than 0.05%, it is difficult to obtain the above-mentioned effect, while when it exceeds 1.0%, the production cost is uneconomical. Therefore, in the present invention, the content of Cr is preferably limited to 0.05 to 1.0%.

Sn: 0.01 to 0.1%

Tin (Sn) is an element effective for improving the plating ability of the steel sheet together with the chromium (Cr) and increasing the strength. However, when the content is less than 0.01%, it is difficult to obtain the above effect, while when it exceeds 0.1%, the production cost is uneconomical. Therefore, in the present invention, the content of Sn is preferably limited to 0.01 to 0.1%.

N: 0.04% or less

Nitrogen (N) works with aluminum (Al) in the austenite grains during coagulation process to precipitate fine nitrides and promotes the generation of twin, which improves the strength and ductility of the steel sheet. However, when the content exceeds 0.04%, the nitride is excessively precipitated to deteriorate the hot workability and elongation, so that it is preferable to control the content of nitrogen to 0.04% or less.

The steel sheet satisfying the above-mentioned component system includes austenite single-phase structure as a microstructure, and the microstructure preferably contains 70% or more of crystal grains in which the aspect ratio of the rolling direction crystal grains becomes 2 or more due to work hardening Do.

When the aspect ratio of the crystal grains in the rolling direction of the microstructure is less than 2, it is difficult to secure the desired strength and ductility. Therefore, the aspect ratio in the rolling direction of the grains deformed by the work hardening is 2 or more. By including such a grain size of 70% or more, excellent strength and ductility can be ensured, and excellent collision characteristics can be ensured.

Hereinafter, the most preferable method derived by the present inventors for producing an ultra-high strength steel sheet satisfying the above-mentioned component system will be specifically described below.

The present invention is characterized in that a steel ingot or a slab composed of the same composition and composition range as described above is heated and homogenized and then subjected to hot rolling and hot rolling to obtain a hot rolled steel sheet, and the hot rolled steel sheet is subjected to cold rolling and annealing, Or the cold-rolled steel sheet may be manufactured from an electro-galvanized or hot-dip galvanized steel sheet. In the present invention, the steel ingot or the performance slab is simply referred to as a slab.

Hereinafter, each of the manufacturing conditions for the manufacturing process of the steel sheet will be described in detail.

Heating step (homogenization treatment): 1050 to 1300 DEG C

In the present invention, when the slab of high manganese steel is heated and homogenized, the heating temperature is preferably set to 1050 to 1300 ° C.

When the slab is homogenized by heating, the grain size increases as the heating temperature is increased, surface oxidation may occur and the strength may decrease or the surface may become dull. Further, since a liquid film is formed on the columnar phase boundary of the slab, there is a possibility that cracking occurs during hot rolling. Therefore, the upper limit of the heating temperature is preferably limited to 1300 캜. On the other hand, when the heating temperature is lower than 1050 DEG C, it is difficult to secure the temperature at the time of finish rolling, so that the rolling load increases due to the decrease in temperature and rolling can not be sufficiently performed to a predetermined thickness. Do.

Hot rolling step: Finishing Hot rolling temperature 850 ~ 1000 ℃

The slabs homogenized by the above heating are subjected to hot rolling to obtain steel sheets. At this time, it is preferable to set the temperature of the finish hot rolling to 850 to 1000 占 폚.

If the finish hot rolling temperature is less than 850 캜, the rolling load becomes high and the quality of the inside of the steel sheet may deteriorate as well as the rolling machine becomes rough. On the other hand, when the finish hot rolling temperature is excessively high, Surface oxidation may occur. Therefore, it is preferable to limit the temperature of the finish hot rolling to 850 to 1000 占 폚, more preferably to 900 to 1000 占 폚.

Coiling step: 200 to 700 ° C

The hot-rolled steel sheet is hot rolled, and the coiling temperature is preferably 700 ° C or less.

When the coiling temperature is higher than 700 ° C during hot rolling, hot rolled steel sheet may have a thick oxide film and internal oxidation on its surface, so that it is difficult to remove the oxide layer during the pickling process. desirable. However, in order to make the coiling temperature lower than 200 캜, a lot of cooling water must be sprayed after hot rolling, and in this case, the coil is difficult to advance and the workability is lowered. Therefore, it is preferable to set the lower limit of the coiling temperature range at 200 占 폚.

Cold rolling step: cold reduction 30 to 80%

After completion of hot rolling under the above-described conditions, cold rolling can be carried out under ordinary conditions to control the shape and thickness of the steel sheet. At this time, the cold rolling reduction ratio is preferably 30 to 80% for the purpose of controlling the strength and elongation while manufacturing the steel to meet the thickness required by the customer.

Continuous annealing step: 400 to 900 ° C

The cold-rolled steel sheet is continuously annealed. At this time, in order to ensure excellent plating and high strength at the same time, the continuous annealing temperature is preferably set to 400 to 900 占 폚.

More specifically, when the annealing temperature is too low at the time of continuous annealing, it is difficult to secure sufficient workability and the austenite transformation sufficient to maintain the austenite phase at a low temperature is not sufficiently carried out. However, if the annealing temperature is too high, the strength may be lowered to 1000 MPa or less through over-recrystallization or grain growth, and in particular, it is difficult to obtain excellent plating property due to increased amount of oxide on the surface during hot-

Since the high manganese steel according to the present invention is an austenitic steel in which no phase transformation takes place, sufficient workability can be secured by heating at a temperature higher than the recrystallization temperature. Therefore, it is preferable to produce by performing annealing under ordinary annealing conditions.

The cold-rolled steel sheet produced according to the above-described manufacturing conditions may be immersed in a plating bath to produce a hot-dip galvanized steel sheet, or electroplated to produce an electroplated steel sheet or an alloyed hot-dip galvanized steel sheet by alloying hot-dip galvanizing treatment.

In order to produce the electroplated steel sheet, it is possible to perform electroplating under ordinary methods and conditions. Further, the cold-rolled steel sheet subjected to the continuous annealing can be subjected to a conventional alloying hot-dip coating treatment to produce an alloyed hot-dip coated steel sheet.

Usually, the heat treatment conditions in the electroplating or alloyed hot dip galvanizing process affect most of the generalized textured steel, so that appropriate heat treatment conditions are often required. However, the high manganese steel according to the present invention has austenite single- There is no significant difference in mechanical properties even if there is no special heat treatment condition. Therefore, the steel sheet can be produced by plating under normal conditions.

The steel sheet produced as described above, such as a cold rolled steel sheet, a hot-dip galvanized steel sheet, a galvannealed steel sheet or an electroplated steel sheet produced by the above-described conditions is subjected to a skin pass mill, a double reduction, By performing re-rolling in one of the hot rolling and continuous rolling, the strength can be increased through work hardening.

At this time, the re-rolling rate is preferably 30% or more for the purpose of efficiently increasing the tensile strength and preventing the rolling load from becoming large. More preferably, rolling is performed at a re-rolling rate in the range of 30 to 50% for securing the strength.

 As shown in FIG. 1, the grain size of the microstructure after re-rolling was observed by Electron Back Scattered Diffraction (EBSD). As a result, it was confirmed that the aspect ratio of the grain in the rolling direction after re-rolling was 2 or more, The twinning fraction was also increased. Therefore, the high manganese steel of the present invention can secure ultra high strength by re-rolling. Therefore, in the present invention, it is preferable that the crystal grains having an aspect ratio of 2 or more in the rolling direction after re-rolling is 70% or more.

Here, the aspect ratio of the crystal grains means the value (b / a) of the ratio of the width (a) and the length (b) of the crystal grains as shown in Fig.

The impact characteristics are related to the mechanical properties of the inner metal layer, unlike the corrosion resistance of the plated layer, and since the heat treatment conditions for plating do not affect the mechanical properties of the high manganese steel having austenite single phase structure, do.

As described above, the steel sheet satisfying the component system and the manufacturing conditions proposed in the present invention is an ultra-high strength steel sheet having a tensile strength of 1300 MPa or more and a yield strength of 1000 MPa or more.

That is, the present invention secures excellent not only tensile strength but also yield strength, so that excellent collision characteristics can be secured.

Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

( Example )

A steel ingot having a constituent system as shown in the following Table 1 was held in a heating furnace at 1,200 占 폚 for one hour and subjected to hot rolling. At this time, the hot rolling finishing temperature was set at 900 캜, and the hot rolling was performed at 650 캜 after hot rolling. Thereafter, pickling was performed using the hot-rolled steel sheet, and cold rolling was performed at a cold rolling reduction of 50%. Thereafter, the cold-rolled specimen was subjected to continuous annealing heat treatment at an annealing temperature of 800 캜 and an overhanging temperature of 400 캜. Further, the cold-rolled steel sheet was subjected to a continuous annealing heat treatment under the same conditions as above, and then a hot-dip galvanizing simulation test was conducted by setting the temperature of the molten zinc bath at 460 캜. Then, the steel sheet subjected to continuous annealing in the same manner as above was subjected to re-rolling with different re-rolling ratios shown in Table 2 below.

The plating properties of the hot-dip galvanized steel sheet were measured and are shown in Table 2 below. At this time, the plating of the steel sheet was carried out by setting the temperature of the molten zinc bath to 460 DEG C and placing the steel sheet in the molten zinc bath. Then, the appearance of the plated steel sheet was visually observed to evaluate the plating ability. In this case, 'good' when the plating layer is formed uniformly and 'bad' when the plating layer is formed unevenly is shown in Table 2 below.

In addition, when the cold-rolled steel sheet thus prepared was subjected to re-rolling, the mechanical properties according to the re-rolling rate, that is, the strength and elongation rate were evaluated through a tensile test. At this time, the re-rolled steel sheet was subjected to a tensile test using a universal tensile tester after processing the tensile specimen according to JIS No. 5 standard.

Psalter C Al Mn P S Si Ni Cr Sn N division One 0.35 1.48 12.00 0.01 0.01 0.01 0.255 0.31 0.03 0.0080 Comparative steel 2 0.59 0.00 14.92 0.02 0.01 0.01 0.004 0.30 0.00 0.0080 Comparative steel 3 0.75 1.01 15.24 0.02 0.01 0.01 0.004 0.31 0.00 0.0088 Comparative steel 4 0.59 2.02 15.18 0.01 0.00 0.01 0.009 0.31 0.00 0.0077 Comparative steel 5 0.51 1.31 15.42 0.02 0.01 0.01 0.255 0.31 0.03 0.0078 Invention river 6 0.50 1.79 15.23 0.01 0.00 0.01 0.253 0.31 0.03 0.0083 Invention river 7 0.62 1.60 18.20 0.01 0.01 0.01 0.210 0.20 0.03 0.0078 Invention river 8 0.60 0.05 24.00 0.01 0.01 0.06 0.000 0.00 0.00 0.0068 Comparative steel

Steel grade Plating property Reduction rate YS TS T-El division 1-1 Good 20.1 654.9 1078.6 40.1 Comparative Example 1-2 29.9 802.1 1249.5 31.2 Comparative Example 1-3 39.7 949.3 1420.3 22.3 Comparative Example 2-1 Bad 20.1 1154.0 1480.0 16.2 Comparative Example 2-2 30.9 1324.0 1730.0 6.3 Comparative Example 3-1 Bad 34.5 1300.0 1655.0 12.4 Comparative Example 4-1 Bad 36.4 1260.0 1604.0 10.9 Comparative Example 4-2 36.4 1226.0 1546.0 8.7 Comparative Example 4-3 40.8 1271.0 1615.0 10.4 Comparative Example 4-4 43.4 1287.0 1633.0 10.3 Comparative Example 5-1 Good 32.4 1178.0 1498.0 11.8 Honor 5-2 36.9 1233.0 1563.0 10.3 Honor 5-3 38.2 1262.0 1594.0 10.0 Honor 5-4 41.9 1325.0 1666.0 9.3 Honor 6-1 Good 18.0 918.0 1240.0 20.2 Comparative Example 6-2 30.5 1088.0 1390.0 12.2 Honor 6-3 36.7 1188.0 1499.0 10.7 Honor 6-4 39.6 1231.0 1541.0 10.4 Honor 6-5 44.7 1294.0 1613.0 8.0 Honor 7-1 Good 20.1 858.9 1286.3 41.5 Comparative Example 7-2 31.2 1004.6 1452.0 32.8 Honor 7-3 39.7 1153.3 1621.2 24.0 Honor 8-1 Bad 19.9 651.9 1111.9 27.2 Comparative Example 8-2 29.8 800.6 1281.0 18.4 Comparative Example 8-3 39.9 952.3 1453.6 5.4 Comparative Example

The results of the plating performance in Table 2 are the results of measurement of the galvanic property of the cold-rolled steel sheet subjected to the hot-dip galvanizing test before re-rolling the specimen of Table 1. The results of the strength measurement are the results of evaluating the strength of the steel sheet obtained by subjecting the steel ingot having the component system shown in Table 1 to hot rolling and cold rolling, re-rolling, and work-hardening.

As shown in Table 2, the steel types 1-1 to 1-3 are cases where the test piece 1 shown in Table 1 is used. Depending on the content of Ni, Cr, or Sn affecting the plating ability, But the content of C affecting the strength of the steel sheet is less than the content proposed in the present invention, so that tensile strength and yield strength after work hardening can not be secured. In particular, the strength was lower when the re-rolling rate was less than 30% as compared with 30% or more.

The specimens 2 to 4 of Table 1 show the case where Sn which affects the plating performance is not added. The steel types 2-1 and 2-2, the steel types 3-1 and 4-1 to 4-4 used in the tests It was confirmed that the plating ability was for heat.

The steel types 8-1 to 8-3 using the specimen 8 of Table 1 were observed to be very poor in plating ability when any one of Ni, Cr, and Sn, which influenced the plating performance, was not added.

On the other hand, the steel types 5-1 to 5-4, 6-2 to 6-5, and 7-2 to 7-3 using the specimens 5 to 7 satisfying all of the component systems proposed in the present invention are not only plated The yield strength and the tensile strength were both excellent. However, in the steel types 6-1 and 7-1, re-rolling was performed at a re-rolling rate of less than 30%. In this case, the tensile strength and yield strength did not satisfy the present invention. That is, the yield strength and the tensile strength were increased as the re-rolling rate was increased, specifically, when the re-rolling rate was 30% or more. Accordingly, it can be seen that the above-mentioned results suggest that application of a re-rolling rate of 30% or more during re-rolling is desirable for ensuring excellent yield strength and tensile strength.

In addition, in order to examine the influence of the microstructure on yield strength and tensile strength increase by re-rolling, the microstructure change after re-rolling using Inventive Steel 5 according to the present invention was observed with Electron Back Scattered Diffraction (EBSD) This is shown in Fig.

As shown in FIG. 1, it was confirmed that the aspect ratio of the crystal grains in the rolling direction after re-rolling was 2 or more, and that the grain size was 70% or more.

As described above, it can be interpreted that the tensile strength and the yield strength after re-rolling increase as the aspect ratio of the crystal grain in the rolling direction increases and the formation of twin crystal increases by re-rolling. As a result, the tensile strength and the yield strength after re-rolling are increased in the case of the other inventive examples.

Therefore, the high manganese steel of the present invention can secure ultra high strength by re-rolling, and can secure excellent collision characteristics.

Claims (7)

delete delete delete (Si): 0.3% or less; nickel (Ni): 0.05% or less; carbon (C): 0.4 to 0.7%, manganese (Mn): 12 to 24% (P): 0.03% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.04% or less , The remainder iron and other unavoidable impurities, or the performance slab is heated to a temperature of 1050 to 1300 캜 to homogenize the steel slab;
Subjecting the homogenized steel ingot or the performance slab to hot rolling at a final hot rolling temperature of 850 to 1000 占 폚;
Rolling the hot-rolled steel sheet at 200 to 700 ° C;
Cold rolling the rolled steel sheet at a cold reduction rate of 30 to 80%;
Continuously annealing the cold-rolled steel sheet at 400 to 900 ° C; And
Rolling the continuously annealed steel sheet at a re-rolling rate of 30 to 50%
Lt; / RTI >
Wherein the microstructure includes austenite single phase structure and the microstructure includes at least 70% of grains having an aspect ratio of 2 or more in the rolling direction grain by work hardening, a tensile strength of 1300 MPa or more, a yield strength A steel sheet having a strength of 1000 MPa or more A method for producing an ultra high strength steel sheet excellent in plating and impact properties.
5. The method of claim 4,
Wherein the re-rolling step is performed by one of a skin pass mill, a double reduction, a hot rolling correction and a continuous rolling.
delete 5. The method of claim 4,
Further comprising performing an electroplating or a hot-dip plating step after the annealing treatment step.

Images