KR20160078533A - Medium manganese steel sheet having high-elongation and high-strength and manufacturing method for the same - Google Patents

Abstract

The present invention relates to a medium manganese steel sheet having high strength and high elongation, and a manufacturing method thereof. According to an embodiment of the present invention, a medium manganese steel sheet having high strength and high elongation comprises: 0.7-1.0 wt% of carbon (C); 10-14 wt% of manganese (Mn); 0.5-2 wt% of silicon (Si); 0.1-0.6 of vanadium (V); 0.05 wt% or less of sulfur (S); 0.08 wt% or less of phosphorus (P); and the remainder consisting of Fe and inevitable impurities.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a steel sheet having a high strength and a high elongation,

The present invention relates to a medium to high strength steel sheet having high strength and high elongation, and a method for producing the same.

High-strength steel has high strength and excellent formability, and has received much attention as a next-generation automotive steel sheet and is currently used as a structural member for automobiles. However, the addition of a large amount of manganese has a disadvantage of high manufacturing cost and difficulty in manufacturing. Therefore, it is required to maintain the strength and moldability while lowering the manufacturing cost. In order to apply such a medium-thickness steel plate to a structural member for supporting a load, an additional process such as changing the shape of a part is required.

In order to meet such a requirement, extremely low carbon steel having a ferrite structure has conventionally been used. However, the ferritic ultra-low carbon steel can be assured to some degree of formability, but there is a problem that the tensile strength and the strength are not enough, the weight of the body is hardly reduced and the safety of the automobile can not be guaranteed. Further, when carbon is further added to compensate for such tensile strength and strength, the carbide or oxide may be excessively formed in the steel, resulting in deterioration of moldability.

A multiphase steel capable of compensating for the low strength of such ultra low carbon steels has been disclosed in U.S. Patent No. 4,854,976. However, this steel is poor in formability due to the influence of bainite and martensite structure , There is a disadvantage that it is used exclusively for members which do not require high moldability.

U.S. Patent No. 4,854,976

The present invention is to provide a medium-core steel sheet having a high strength and a high elongation, and a method for producing the same.

The heavy metal sheet having a high strength and a high elongation according to an embodiment of the present invention includes 0.7 to 1.0 wt% of carbon (C), 10 to 14 wt% of manganese (Mn), 0.5 to 2 wt% of silicon (Si) , Vanadium (V): 0.1 to 0.6 wt%, sulfur (S): 0.05 wt% or less, phosphorus (P): 0.08 wt% or less, the balance Fe and other unavoidable impurities.

A method for producing a high strength and high elongation medium core steel sheet according to another embodiment of the present invention comprises: 0.7 to 1.0% by weight of carbon (C), 10 to 14% by weight of manganese (Mn) (P): 0.08% by weight or less, the balance Fe and other unavoidable impurities, in the step (a) of preparing a slab comprising a slab of about 2 to about 2% by weight, a vanadium (V) of about 0.1 to about 0.6% , Heating the slab to 1200 to 1350 캜, subjecting the heated slab to finish rolling at 900 to 1000 캜, winding the hot-rolled steel sheet at 400 to 700 캜, winding the rolled steel sheet Reheating the steel sheet at 1200 to 1350 ° C, pickling the reheated steel sheet, cold rolling at a reduction ratio of 30 to 60%, and annealing the cold-rolled steel sheet at 650 to 900 ° C.

According to the embodiment of the present invention, it is possible to provide a medium-core steel sheet having high strength and high elongation, and a method of manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a microstructure of a medium-strength steel sheet having a high strength and a high elongation according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Embodiments of the present invention are also provided to more fully describe the present invention to those skilled in the art.

An object of the present invention is to provide a medium-core steel sheet having high strength and high elongation, which is mainly used for automobile materials, and a method of manufacturing the same. Therefore, the present invention can be applied to fields such as automobile materials where high strength and high elongation are important. However, the present invention is not limited thereto.

High-strength steel has high strength and excellent formability, and has received much attention as a next-generation automotive steel sheet and is currently used as a structural member for automobiles. However, the addition of a large amount of manganese has a disadvantage of high manufacturing cost and difficulty in manufacturing. Therefore, it is required to maintain the strength and moldability while lowering the manufacturing cost.

As described above, generally, moldability according to high strength and high elongation is one of the most important characteristics in automotive steel sheets and the like, and is required to manufacture a product having various designs and ensuring the safety of passengers.

In order to produce a steel sheet having such properties, a method of improving the strength and elongation at the same time is generally a method of adding manganese in an amount of 15 wt% or more and adding carbon in an amount of 0.5 wt% or more. However, when a large amount of manganese is added, not only the manufacturing cost is increased but also the damage of the transformer and the like is increased due to the increase of the amount of the alloy, and the quality of the cast steel is lowered. Further, when the content of manganese increases, the rolling load during hot rolling increases, and the line weldability decreases in continuous rolling and annealing processes.

The present invention provides a medium-core steel sheet having a high strength and a high elongation although the manganese content is low by decreasing the amount of manganese added and increasing the amount of carbon added, and a method for producing the same.

The heavy metal sheet having a high strength and a high elongation according to an embodiment of the present invention includes 0.7 to 1.0 wt% of carbon (C), 10 to 14 wt% of manganese (Mn), 0.5 to 2 wt% of silicon (Si) , Vanadium (V): 0.1 to 0.6 wt%, sulfur (S): 0.05 wt% or less, phosphorus (P): 0.08 wt% or less, the balance Fe and other unavoidable impurities.

The high-strength and high-elongation medium-strength steel sheet may have austenite single phase. By having an austenite single phase, high elongation can be obtained at room temperature, and moldability is improved. In order to secure the stability of austenite, instead of reducing manganese, which is an austenite stabilizing element, the amount of carbon added is increased, and the amount of silicon, which is a ferrite stabilizing element, is adjusted to secure austenite single phase structure. In order to increase the strength of the steel, the addition amount of vanadium was appropriately controlled.

Component  And composition range

Hereinafter, the composition of the high-manganese steel sheet having high porosity and high strength according to the embodiment of the present invention and the range of the composition will be described.

Carbon (C): 0.7 to 1.0 wt%

Carbon (C) plays a role in increasing strength by being dissolved in the steel or by forming a precipitation phase.

Carbon (C) is an element that replaces manganese (Mn). In order to maintain the strength while reducing the content of manganese (Mn), carbon (C) should be added in an amount of 0.7 wt% or more. When the content of carbon (C) is excessive, the fraction and size of the carbide increase, and since the carbonaceous material is not decomposed by heat treatment, the addition amount of carbon (C) is limited to 1.0 wt%.

manganese( Mn ): 10 to 14 wt%

Manganese (Mn) serves to stabilize the austenite structure.

Manganese (Mn) is added in an amount of 10 wt% or more to obtain an austenite single phase. When the content of manganese (Mn) exceeds 14% by weight, problems such as an increase in production cost and an increase in the amount of ferroalloy cause damage to equipment such as a converter and a quality deterioration in the case of producing a cast steel. Further, there is a problem in that the rolling load increases during hot rolling and the line weldability is lowered in the continuous rolling or annealing step. Therefore, the content of manganese (Mn) is adjusted to 10 to 14% by weight.

silicon( Si ): 0.5 to 2 wt%

Silicon (Si) is an austenite stabilizing element and has a large influence on the austenite fraction which increases the yield strength. It is known as an effective element for preventing the formation of carbides in high-strength steels for automobiles. Silicon (Si) is used as a deoxidizing agent in steel, and when added excessively, oxides are formed on the surface to reduce acidity, cracks, and the like, which are known to lower the surface quality.

Silicon (Si) is added in an amount of 0.5 wt% or more in order to maintain the austenite single phase and increase the strength while at the same time preventing ferrite formation. However, when the content of silicon (Si) is more than 2% by weight, an oxide is formed to form a crack or the like, and the ferrite fraction increases and the elongation decreases. Therefore, the addition amount of silicon (Si) do.

Vanadium (V): 0.1 to 0.6 wt%

Vanadium (V) is an element that tends to form a carbonitride.

When the precipitation phase is formed, the strength is increased by the precipitation phase, and the grain size is finely formed to increase the yield strength. Therefore, vanadium (V) is added in an amount of 0.1 wt% or more for effective precipitation. However, when vanadium (V) is added in an amount exceeding 0.6% by weight, the cost of the alloy is increased, and coarse precipitation phase is formed and the formability is lowered.

Sulfur (S): not more than 0.05% by weight

Sulfur (S) is added in an amount of 0.05% by weight or less for the control of the inclusions. If the amount of sulfur (S) exceeds 0.05% by weight, there arises a problem of hot brittleness.

Phosphorus (P): 0.08% by weight or less

Phosphorus (P) is an element that easily segregates and causes cracking during casting. To prevent this, the content of phosphorus (P) should be controlled to 0.08 wt% or less. If the amount of phosphorus (P) exceeds 0.08%, the main constitution may deteriorate.

In the heavy metal sheet having high strength and high elongation according to the present invention, the remaining component except for the above components is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are self-evident to those of ordinary skill in the art of manufacturing, and therefore, not all thereof are specifically referred to herein.

The structure of the heavy metal sheet may be austenitic single phase structure. In addition, the above-mentioned heavy-walled steel sheet can have high strength and high elongation. Preferably, the heavy-walled steel sheet may have a tensile strength of 1300 MPa or more and a tensile strength x elongation value of 30000 MPa ·% or more.

Manufacturing method

Hereinafter, a method for manufacturing a high manganese steel sheet having high porosity and high strength according to an embodiment of the present invention will be described.

Heating step

The slabs having the above-described composition and composition range are heated at a temperature of 1200 to 1350 캜. By heating the slab in this manner, the rolling load is reduced, the casting structure formed during the casting is made uniform, and the carbide is reused.

If the heating temperature is lower than 1200 deg. C, there is a problem that coarse carbides are formed because the carbides formed during casting are not sufficiently reused. In addition, if the heating temperature is higher than 1350 DEG C, there is a problem that the internal oxidation is severely generated due to the heating at a high temperature and the surface quality is deteriorated.

Step of hot rolling

The slab having been subjected to the heating process is subjected to a hot rolling step of finishing rolling to 900 to 1000 ° C.

When the hot rolling finishing temperature is lower than 900 占 폚, there is a problem that the load becomes large during rolling and deformation resistance is high. If the hot rolling finishing temperature is higher than 1000 占 폚, there is a problem that the surface quality is lowered.

Winding  step

The hot rolled steel sheet is wound into a coil shape. After the hot rolling, for example, the hot-rolled steel sheet is cooled to the winding temperature by the cooling water.

At this time, the coiling temperature is set to 400 to 700 占 폚. If the coiling temperature is lower than 400 占 폚, a large amount of cooling water is required for cooling, and there is a problem that the load is largely affected during winding.

On the other hand, if the coiling temperature is higher than 700 캜, the reaction between the oxide film on the surface of the steel sheet and the steel sheet matrix proceeds during the cooling process after winding, which deteriorates the acidity.

Steps to reheat

The rolled steel sheet is reheated at a temperature of 1200 to 1350 占 폚 before cold rolling.

The present process serves to decompose the carbide to improve the moldability. In particular, the reheating process helps to homogenize microstructures, since the carbides crushed through hot rolling are easily decomposed through reheating processes.

If the heating temperature is lower than 1200 deg. C, there is a problem that coarse carbides are formed because the carbides formed during casting are not sufficiently reused. In addition, if the heating temperature is higher than 1350 DEG C, there is a problem that the internal oxidation is severely generated due to the heating at a high temperature and the surface quality is deteriorated.

Pickling and Cold  Step of rolling

The rolled steel sheet is pickled and cold rolled at a reduction ratio of 30 to 60%.

In the present invention, the pickling process can be carried out using any method well known in the art, and is not particularly limited. The oxide formed on the surface of the wound steel sheet can be removed with an ordinary acid solution. For example, using a hydrochloric acid solution at a concentration of 5% to 15%. When the concentration of the acid is lower than 5%, it is difficult to completely remove the oxide film on the surface, whereas when it exceeds 15%, the acid is over-acidated. The picking time is determined by the operating speed and is not particularly limited. The pickling temperature is also not particularly limited. When the pickling process is carried out at a high temperature, there is a problem that the corrosion of the equipment is promoted and toxic gas is generated, and when it is carried out at a low temperature, the pickling reaction is slow and the operation time is long. Thus, the pickling temperature can be determined in consideration of these points.

However, in the case of a high manganese steel sheet according to an embodiment of the present invention, since the recrystallization proceeds in the heat treatment process after the cold rolling, the driving force of the recrystallization is well controlled , So that the reduction rate should be adjusted to 30 to 60%.

If the reduction rate is lower than 30%, the strength of the product is lowered. If the reduction rate is higher than 60%, the load of the rolling mill is increased.

Annealed  step

Anneal the cold-rolled steel sheet. The annealing can be performed by, for example, a continuous annealing method.

The annealing temperature is set to 650 to 900 占 폚, and preferably to 800 to 900 占 폚. When the annealing temperature is lower than 650 ° C., recrystallization does not occur sufficiently. If the annealing temperature is higher than 900 ° C., oxides are formed on the surface of the steel sheet and the workability with the pre- have. Therefore, the annealing temperature is adjusted to 650 to 900 占 폚.

Example

Hot-rolled steel sheets were produced by heating, hot rolling, coiling, reheating, cold rolling and annealing steel slabs having the composition shown in Table 1 below. In the following Tables 1 and 2, the examples correspond to the composition of the present invention, and the comparative examples deviate from any one or more of the compositions of the present invention.

The examples and comparative examples in the following Table 1 are prepared by the same process except that the composition and the annealing temperature are according to Table 1. Specifically, in the internal combustion steel sheets of Examples and Comparative Examples, a slab having the composition shown in Table 1 was heated at 1300 占 폚, finishing rolled at 950 占 폚, rolled at 550 占 폚, reheated at a temperature of 1300 占 폚, And then subjected to continuous annealing in accordance with the temperature shown in Table 1.

The yield strength, tensile strength, elongation, yield ratio, tensile strength x elongation of the cold-rolled steel sheet thus prepared were measured, and the results are shown in Table 2 below. The microstructure of the steel sheet of Example 2 shown in Table 2 was observed, and the results are shown in Fig.

carbon
(C) silicon
(Si) manganese
(Mn) sign
(P) sulfur
(S) aluminum
(Al) Niobium
(Nb) vanadium
(V) Annealing temperature
(° C) Comparative Example 1 0.15 0.42 2.50 0.010 0.0060 0.05 0.026 - 800 Comparative Example 2 0.87 0.95 11.80 0.010 0.0080 - - - 700 Comparative Example 3 0.84 1.05 11.10 0.009 0.0050 - - 0.7 700 Comparative Example 4 0.87 0.95 11.80 0.008 0.0090 - - - 800 Example 1 0.90 1.10 11.60 0.027 0.0085 - - 0.2 800 Example 2 0.86 1.20 11.20 0.009 0.0080 - - 0.5 800 Example 3 0.90 1.10 11.60 0.010 0.0050 - - 0.2 900 Example 4 0.86 1.20 11.20 0.009 0.0090 - - 0.5 900 Comparative Example 5 0.87 0.95 11.80 0.009 0.0080 - - - 900 Comparative Example 6 0.84 1.05 11.10 0.008 0.0085 - - 0.7 900 Comparative Example 7 0.45 0.01 12.00 0.012 0.0110 1.48 - - 800 Comparative Example 8 0.44 0.01 14.80 0.012 0.0090 1.40 - - 800 Comparative Example 9 0.43 0.01 15.00 0.009 0.0050 0.05 - - 800 Comparative Example 10 0.15 0.02 15.00 0.010 0.0050 1.50 - - 800

Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Yield Ratio (%) Tensile Strength X Elongation
(MPa 占%) Comparative Example 1 520.0 800.0 23.0 0.65 18400 Comparative Example 2 793.0 1261.0 14.6 0.63 18411 Comparative Example 3 1156.0 1299.0 5.6 0.89 7274 Comparative Example 4 524.0 1308.0 15.5 0.40 20274 Example 1 755.0 1386.0 23.3 0.54 32294 Example 2 859.0 1316.0 23.5 0.65 30926 Example 3 609.0 1325.0 37.7 0.46 49953 Example 4 710.0 1305.0 24.8 0.54 32364 Comparative Example 5 429.0 1164.0 44.9 0.37 52264 Comparative Example 6 733.0 1259.0 21.6 0.58 27194 Comparative Example 7 339.0 678.0 40.3 0.50 27323 Comparative Example 8 341.0 862.0 63.2 0.40 54478 Comparative Example 9 373.0 978.0 37.0 0.38 36186 Comparative Example 10 377.0 1019.0 52.5 0.37 53498

Referring to Tables 1 and 2, Examples 1 to 4 include the case where the composition includes a component system and a component range according to the embodiment of the present invention, and the tensile strength is 1,300 MPa or more and the tensile strength x elongation value is 30000 MPa ·% or more Able to know.

Further, as shown in FIG. 1, it can be seen that the steel sheet of Example 2 according to the present invention includes a single-phase structure of austenite.

On the other hand, Comparative Examples 1 to 10, which are devoid of the embodiment of the present invention, have a tensile strength x elongation value lower than 30000 MPa.% Even if the tensile strength is lower than 1300 MPa or the tensile strength is higher.

Thus, it can be seen that a middle-length steel sheet having a high elongation and a high strength can be obtained by manufacturing a middle-length steel sheet according to an embodiment of the present invention.

Claims (6)

(Si): 0.5 to 2 wt%, vanadium (V): 0.1 to 0.6 wt%, sulfur (S): 0.05 to 0.5 wt%, carbon (C): 0.70 to 1.0 wt%, manganese (P): 0.08% by weight or less, the balance Fe and other unavoidable impurities, and a high elongation.
The method according to claim 1,
Medium strength steel sheet having a high strength and high elongation with austenite single phase structure.
The method according to claim 1,
High strength and high elongation with tensile strength of 1300 MPa or more.
The method according to claim 1,
High strength and high elongation with tensile strength x elongation value of 30000 MPa ·% or more.
(Si): 0.5 to 2 wt%, vanadium (V): 0.1 to 0.6 wt%, sulfur (S): 0.05 to 0.5 wt%, carbon (C): 0.70 to 1.0 wt%, manganese By weight or less, phosphorus (P): 0.08% by weight or less, the balance Fe and other unavoidable impurities;
Heating the slab to 1200 to 1350 占 폚;
A hot rolling step of finishing the heated slab to 900 to 1000 占 폚;
Rolling the hot-rolled steel sheet at 400 to 700 占 폚;
Reheating the wound steel sheet to 1200 to 1350 占 폚;
Picking up the rolled steel sheet and cold rolling the steel sheet at a reduction ratio of 30 to 60%; And
And annealing the cold-rolled steel sheet at 650 to 900 占 폚.
6. The method of claim 5,
Wherein the annealing step has a high strength and a high elongation at an annealing temperature of 800 to 900 占 폚.

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