WO2008078940A1

WO2008078940A1 - High manganese high strength steel sheets with excellent crashworthiness, and method for manufacturing of it

Info

Publication number: WO2008078940A1
Application number: PCT/KR2007/006780
Authority: WO
Inventors: Sung Kyu Kim; Kwang Geun Chin; Il Ryoung Sohn
Original assignee: Posco
Priority date: 2006-12-27
Filing date: 2007-12-24
Publication date: 2008-07-03
Also published as: EP2097548A4; KR20080060982A; CN101432456A; JP5393459B2; US20090074605A1; JP2009545676A; KR100851158B1; EP2097548A1

Abstract

There are provided a high-workability high strength steel sheet with excellent workability due to the high elongation and excellent crashworthiness due to the high yield strength, and a method for manufacturing of it. The high manganese steel sheet includes, by weight: carbon (C): 0.2 to 1.5%, manganese (Mn): 10 to 25%, aluminum (Al): 0.01 to 3.0%, phosphorus (P) 0.03% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.040% or less, at least one selected from the group consisting of silicon (Si): 0.02 to 2.5%, titanium (Ti): 0.01 to 0.10% and niobium (Nb): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities. The high manganese steel sheet may be a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet, and is suitable for elaborate internal sheets as well as structural members of a car body since it has press workability due to the high elongation and high strain hardening index. Also, the high manganese steel sheet may be used for parts such as a front side member of an automobile since, among its characteristics, the steel sheet has an excellent impact absorbing ability.

Description

HIGH MANGANESE HIGH STRENGTH STEEL SHEETS WITH EXCELLENT CRASHWORTHINESS, AND METHOD FOR

MANUFACTURING OF IT

Technical Field

[1] The present invention relates to a high manganese steel sheet used for a automobile steel sheet and a method for manufacturing of it, and more particularly, to a high- workability high strength steel sheet whose workability is excellent due to the high elongation and crashworthiness is excellent due to the high yield strength, and a method for manufacturing of it.

[2]

Background Art

[3] Motor companies have increasingly used lightweight and high-strength materials for the purpose of preventing environmental pollutions and improving fuel efficiency and stability, which is one of important characteristics that materials used for structural members should have in addition to the automobile parts. However, the increased strength of the materials results in deterioration in elongation of steel, and therefore a dual-phase steel, a transformation induced plasticity steel and the like, all of which have excellent formability, have been increasingly used to solve the above problems.

[4] However, high strength steel for processing, used as structural parts and internal sheets for automobiles developed thus far, does not satisfy workability that automobile parts require, and therefore it is difficult to manufacture parts with complicated structure. To solve the above problems, motor companies have used methods of simplifying shapes of automobile parts, or molding several parts separately and welding the parts. The welding process is hampered by a variety of restrictions, for example, difficulty in designing a car body since strength of a welded portion is different from that of the parent metal, and parts characteristics are deteriorated due to the increased strength in the welded portion, and the manufacturing cost is extremely high since the parts are molded separately. Accordingly, motor companies have had continuous demands for materials having a high strength and excellent workability so as to apply to parts with complicated structure and improve design flexibility.

[5]

[6] In recent years, there has been required a high strength steel sheet having excellent formability that is able to decrease the weight of automobiles for the purpose of the improvement of fuel efficiency and reduce air pollution in the field of automotive steel sheet. High strength steel, such as low carbon steel where a matrix structure is ferrite, has been used as the automotive steel sheet. However, when the high strength steel such as low carbon steel is used as the automotive steel sheet, it is impossible to ensure up to 30% elongation in high strength steels whose tensile strength is greater than 800 MPa. Accordingly, it is difficult to design parts easily, for example, make simple structures of parts, since it is difficult to apply the high strength steel of grater than 800 MPa to complicated and elaborate parts. In order to solve the above problems, there has been proposed an austenite-based high manganese steel sheet with excellent ductility and strength (Japanese Patent Laid-open Publication No. 1992-259325, International Publication No. WO02/101109). In the case of the Japanese patent, the high manganese steel sheet has a high ductility by the addition of high manganese, but may be easily broken after its processing since strain hardening is serious in its transformed region. Also, in the case of the international patent, the high manganese steel sheet has high ductility, but its electroplating and hot plating properties are low due to the addition of a large amount of silicon. Also, the steel sheets have excellent workability but deteriorated crash worthiness due to the low yield strength.

[7]

[8] Materials that absorb impact energy in collision and has high yield strength to prevent transformation of automobile parts may be desirably used for the automobile parts. However, the high manganese steel sheet has low yield strength due to its austenite structure, and therefore the problems regarding the high manganese steel sheet remains to be solved.

[9]

Disclosure of Invention

Technical Problem

[10] An aspect of the present invention provides a high- workability high strength steel sheet whose workability is high due to the excellent elongation and crashworthiness is excellent due to the high yield strength, and a method for manufacturing of it.

[H]

Technical Solution

[12] According to an aspect of the present invention, there is provided a steel sheet including, by weight: carbon (C): 0.2 to 1.5%, manganese (Mn): 10 to 25%, aluminum (Al): 0.01 to 3.0%, phosphorus (P) 0.03% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.040% or less, at least one selected from the group consisting of silicon (Si): 0.02 to 2.5%, titanium (Ti): 0.01 to 0.10% and niobium (Nb): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities. The steel sheet according to the present invention has an austenite single-phase structure.

[13] [14] The steel sheet according to the present invention has a yield strength of 400 - 600

MPa or more and a tensile strength of 900 - 1000 MPa or more, and the steel sheet is subject to strain hardening through the cold rolling process to have a yield strength of 750 MPa or more and a tensile strength of 1000 MPa through the cold rolling process.

[15] The steel sheet including the components according to the present invention may be cold-rolled at a reduction ratio of 10 to 80%. Here, the rolling process may be one selected from the group consisting of temper rolling, dual rolling and hot final rolling. Also, the steel sheet may be selected from the group consisting of a hot-rolled steel sheet, a cold-rolled steel sheet and a plated steel sheet.

[16]

[17] In the method of manufacturing a cold-rolled steel sheet according to the present invention, the cold-rolled steel sheet may be prepared by homogenizing a steel including the above-mentioned components at 1050 to 1300⁰C, hot-rolling the steel at a finish rolling temperature of 850 to 1000⁰C and winding the hot-rolled steel at a temperature range of 700⁰C or below, followed by cold-rolling the hot-rolled steel sheet at a reduction ratio of 30 to 80% and annealing the cold-rolled steel sheet at 600⁰C or above.

[18]

Advantageous Effects

[19] According to the present invention, an aspect of the present invention is to provide a steel sheet that is suitable for elaborate internal sheets as well as structural members of a car body since it has high elongation and high strength. The steel sheet can be useful to be used for parts such as a front side member of an automobile since, among its characteristics, the steel sheet has an excellent impact absorbing ability.

[20]

Brief Description of the Drawings

[21] FIG. 1 is a photographic diagram illustrating a microstructure.

[22] FIG. 2 is a graph illustrating changes in tensile curve and strength vs. elongation, depending on the increasing amount of a cold-rolled steel sheet.

[23]

Best Mode for Carrying Out the Invention

[24] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[25] According to the present invention, a suitable amount of at least one selected from the group consisting of silicon, titanium and niobium is added to enhance yield strength of a steel sheet. Steels whose crash worthiness is excellent due to the high yield strength may be manufactured by cold-rolling the prepared hot-rolled, cold-rolled and plated steel sheets.

[26] The present invention is characterized in that an austenite single phase is prepared, and amounts of added manganese, carbon and aluminum are suitably adjusted to improve workability due to the presence of twins, and amounts of added silicon, titanium and niobium are also optimized to control a microstructure, thereby enhancing yield strength. Also, yield strength of the steel sheet may be enhanced through the cold-rolling process, on the basis of the fact that elongation of the steel sheet is very excellent when the steel sheet is subject to the strain hardening process using the twins, and therefore it is possible to ensure formability required for automobile parts although the elongation of the steel sheet is rather decreased in the cold-rolling process.

[27]

[28] According to the present invention, amounts of austenite stabilizers such as manganese and carbon are optimized to ensure a complete austenite phase at room temperature, and the complete austenite phase is transformed by these components to form a twin. Also, an amount of aluminum is adjusted to control a twin-forming rate, thereby improving tensile properties. It is important to minimize an amount of manganese (Mn) added to lower the manufacturing cost, and also to add a portion of carbon to reduce the amount of the added manganese. Amounts of added carbon and aluminum are also adjusted suitably to facilitate twin transformation during the processing of steel. Meanwhile, it is preferred to reduce grain sizes of the components so as to increase yield strength of the steel sheet. For this purpose, it is possible to further add at least one selected from the group consisting of silicon, titanium and niobium.

[29] According to the present invention, the steel sheet includes a hot-rolled steel sheet, a cold-rolled steel sheet and a plated steel sheet.

[30]

[31] Hereinafter, the grounds for selection of the above-mentioned components of the steel and limitation of content ranges of the components will be described in detail, as follows.

[32] A content of carbon (C) is preferably in a range from 0.2 to 1.5%.

[33] It is desirable to increase an amount of added carbon since the carbon contributes to stabilizing an austenite phase. Because an α'-martensite phase is formed during the phase transformation when the amount of the added carbon is less than 0.2%, cracks may occur during processing and ductility may be deteriorated. And, when the amount of the added carbon exceeds 1.5%, stability of the austenite phase is highly enhanced, and, thus, workability is deteriorated due to the transition of deformation behaviors by slip deformation.

[34] A content of manganese (Mn) is preferably in a range from 10 to 30%, and more preferably from 10 to 25%.

[35]

[36] Manganese (Mn) is also an essential element that stabilizes an austenite phase.

However, an α'-martensite phase that adversely affects formability is formed when the content of the manganese (Mn) is less than 10%, and therefore strength is increased but ductility is seriously decreased. And, twin formation is suppressed when the content of the added manganese exceeds 30%, which leads to the increased strength but the decreased ductility. And, as the content of the added manganese increases, cracks easily occurs during the hot rolling process, and the manufacturing cost of the steel sheet is increased due to the use of a large amount of the expensive manganese. Therefore, the manganese is preferably added at a content of 25% or less.

[37]

[38] A content of aluminum (Al) is preferably in a range from 0.01 to 3.0%.

[39] Aluminum (Al) is usually added to deoxidize steel, but added to improve ductility in the present invention. That is, the aluminum (Al) is an element that stabilizes a ferrite phase, but increases stacking fault energy in a slip surface of steel to prevent transformation into an ε-martensite phase, which leads to the improved ductility. In addition, the aluminum contributes significantly to minimizing an amount of the added manganese and improving workability since it suppresses the transformation into the ε- martensite phase even when the manganese is present at a low content. Accordingly, because ε-martensite is formed when the amount of the added aluminum (Al) is less 0.01%, strength is increased but ductility is seriously decreased. And, because twin formation is suppressed when the amount of the added aluminum (Al) exceeds 3.0%, ductility is deteriorated, castability is poor in a continuous casting process, and a steel surface is seriously oxidized during a hot rolling process, which lead to the deteriorated quality in a surface to the product.

[40]

[41] Contents of phosphorus (P) and sulfur(S) are preferably in a range of 0.03% or less.

[42] Phosphorus (P) and sulfur(S) are inevitably present in the manufacture of a steel sheet, and therefore their contents are adjusted to a content range of 0.03% or less. In particular, the phosphorus (P) causes slabs to arise, which deteriorates workability of steel. And, the sulfur (S) reacts to form coarse manganese sulfide (MnS) which cause defects such as flange cracks, and deteriorates hole expansibility of a steel sheet. Therefore, it is preferred to use the minimum content of the components.

[43] A content of nitrogen (N) is preferably in a range of 0.04% or less.

[44] Nitrogen reacts with aluminum during a coagulation process to extract fine nitrides from austenite grains, which facilitate twin formation, and the nitrogen (N) improves strength and ductility of steel in molding a steel sheet. However, hot workability and elongation are deteriorated since the excessive nitrides are extracted when the content of the added nitrogen exceeds 0.04%.

[45]

[46] The steel with the above composition includes at least one selected from the group consisting of silicon, titanium and niobium.

[47] A content of silicon (Si) is preferably in a range of 0.02 to 2.5%.

[48] Silicon is a solid-solution strengthening element that increases yield strength by reducing grain sizes through a solid- solution strengthening effect. It has been known that, when the excessive silicon (Si) is present, a silicon oxide layer is formed on a surface of a steel sheet to deteriorate a hot plating property. However, when a suitable amount of the silicon is added to the steel including a large amount of the added manganese, a thin silicon oxide layer is formed on a surface of the steel to prevent oxidation of manganese. Therefore, the formation of a thick manganese oxide layer on a cold-rolled steel sheet is prevented after the rolling of the steel sheet, the corrosion in the cold-rolled steel sheet may be prevented after an annealing process to improve surface quality of the cold-rolled steel sheet, and it is possible to maintain an excellent surface quality as a substrate steel sheet of electroplating materials.

[49] However, the increased content of the added silicon makes it possible to form silicon oxides on a surface of a steel sheet when the steel sheet is hot-rolled, which leads to the deteriorated pickling property and the poor surface quality of the hot-rolled steel sheet. And, the silicon is condensed on a surface of the steel sheet when the steel sheet is annealed at high temperature in the continuous annealing process and the continuous hot plating process, and wetability of molten zinc on the surface of the steel sheet is low when the steel sheet is hot-plated, which leads to the poor plating property. Furthermore, the addition of a large amount of the silicon results in the deteriorated weldability of the steel. Accordingly, the maximum content of the silicon is preferably 2.5%, based on the total content of the steel sheet. Crashworthiness is associated with mechanical properties of an inner metal seed layer but not associated with cor- rosiveness of the plating layer, and a heat treatment conditions for plating a steel sheet does not affect the mechanical properties of the high manganese steel sheet with an austenite single phase structure. Therefore, the preventive product has crashworthiness of a plated product.

[50]

[51] A content of titanium (Ti) is preferably in a range from 0.01 to 0.1%.

[52] Titanium is a strong carbide-forming element that reacts with carbon to form a carbide. In this case, the resultant carbide has an effect on miniaturization of grain size since it functions to suppress grain growth. However, the effect on miniaturization of grain size does not appear when the content of the titanium (Ti) is less than 0.005%, whereas the excessive titanium (Ti) is slabbed in grain boundaries to cause grain boundary embrittlement, or a coarse precipitate phase is excessively formed when the content of the titanium (Ti) exceeds 0.10%, which leads to the poor effect on the grain growth.

[53]

[54] A content of niobium (Nb) is preferably in a range from 0.005 to 0.1%, and more preferably from 0.01 to 0.1%.

[55] Niobium is a strong carbide-forming element that binds to carbon in the same manner as the titanium to form a carbide. Also, the resultant carbide is an element that has an effect on miniaturization of grain size since it functions to suppress grain growth, and has a high precipitation strengthening effect by the miniaturization of grain size and the formation of the precipitate phase since a precipitate phase is formed at a lower temperature than the conventional titanium. However, the precipitation strengthening effect does not appear when the content of the niobium (Nb) is less than 0.005%, whereas the excessive niobium (Nb) is slabbed in grain boundaries to cause grain boundary embrittlement, or a coarse precipitate phase is excessively formed when the content of the niobium (Nb) exceeds 0.10%, which leads to the poor effect on the grain growth. Therefore, a preferable content of the added niobium is in a range from 0.01 to 0.1%.

[56] Hereinafter, a method for manufacturing a high manganese steel sheet will described in detail.

[57]

[58] In general, the manufacture of a high manganese hot-rolled steel sheet may be carried out using the continuous casting method as in the manufacture of conventional steels. The above-mentioned composition is homogenized in the similar manner to the general conditions of steel, finish-rolled and wound to prepare a hot-rolled steel sheet.

[59]

[60] According to the present invention, a heating temperature of a casting slab is preferably in a range from 1050 to 1300 ⁰C when the high manganese steel sheet is hot-rolled. The maximum heating temperature is limited to a temperature range of 1300⁰C. This is why a grain size increases with the increasing temperature, surface oxidation results in the decrease in strength of steel, or a surface of a steel sheet has poor physical properties. Also, a liquid-phase layer is formed in columnar crystal grain boundaries of the casting slab when the high manganese steel sheet is heated to greater than 1300⁰C, which leads to the cracks during the hot rolling process. Meanwhile, the minimum heating temperature is limited to a temperature range of 1050⁰C. This is why it is difficult to ensure a temperature required for finish-rolling a steel sheet when the heating temperature is low, and the increase in rolling load due to the decreased temperature makes it possible to sufficiently roll a steel sheet to a predetermined thickness. That is, since the conventional finish rolling temperature is at least 85O⁰C or above, and preferably about 900⁰C during the hot rolling process, the rolling load increases with the decreasing finish rolling temperature, and therefore the unreasonable load is inflicted on a rolling machine, and also has a bad effect on the quality of an internal steel sheet. And, the excessively high finish rolling temperature facilitates oxidation in a surface of the steel sheet during the hot rolling process, and therefore the finish rolling temperature is limited to a temperature range of 1000⁰C.

[61]

[62] The hot rolling process is carried out at a coiling temperature of 700⁰C or below.

An oxide layer is not easily removed off during the pickling process since a thick oxide layer is formed on a surface of the hot-rolled steel sheet and the inner part of the hot- rolled steel sheet is oxidized when the coiling temperature exceeds 700⁰C. Accordingly, the hot-rolled steel sheet is preferably hot-rolled at a low coiling temperature.

[63]

[64] The resultant hot-rolled steel sheet may be manufactured into a cold-rolled steel sheet, when necessary.

[65] The cold-rolled steel sheet is prepared by cold-rolling a steel sheet so as to meet the shape and thickness of the steel sheet, and a preferable cold rolling is carried out at a reduction ratio of 30 to 80%.

[66] The cold-rolled steel sheet is continuously annealed at a temperature of 600⁰C or above. At this time, when the annealing temperature is too low, it is difficult to ensure sufficient workability and a sufficient level of austenite is not formed during the phase transformation to maintain an austenite phase at a low temperature. Accordingly, it is preferred to perform the annealing process at an annealing temperature of 600⁰C or above. Because an austenite steel whose phase transformation does not occur easily is used in the present invention, it is possible to ensure sufficient workability when the steel is heated to a temperature greater than its recrystallization temperature. Therefore, the cold-rolled steel sheet may be annealed under conventional annealing conditions.

[67]

[68] The annealed steel sheet, as prepared thus, is plated when necessary. Here, the plating may be selected from hot plating, electroplating and deposition processes, and the hot plating process is preferred. The method for manufacturing a plated steel sheet includes: continuously annealing a cold-rolled steel sheet at 600⁰C or above and manufacturing a hot-plated, electroplated or deposited steel sheet. The conventional heat treatment affect a transformation induced plasticity steel sheet during the electroplating or hot plating process, but it is possible to plate the inventive steels under the con- ventional conditions since the inventive steels have an austenite single phase and a low difference in mechanical properties due to the lack of the phase transformation.

[69] [70] According to the present invention, one of the above-mentioned high manganese steel sheets satisfying the components according to the present invention, for example a hot-rolled steel sheet, a cold-rolled steel sheet and a plated steel sheet may be cold- rolled again at a reduction ratio of 10 ~ 80% to enhance yield strength. Here, the rolling process may be carried out using one of a temper rolling process, a dual rolling process and a hot coil process used in steel mills.

[71]

Mode for the Invention

[72] Hereinafter, exemplary embodiments of the present invention will be described in detail.

[73] [Example] [74] The inventive steels and the comparative steels are listed in the following Table 1, and molten steel ingots were kept in a 1200⁰C heating furnace for one hour, and then hot-rolled. At this time, a hot rolling finish temperature was 900⁰C, and a coiling temperature was 65O⁰C. Some of the hot-rolled steel sheets were processed into tensile test samples according to the JIS5 standard, and the tensile test samples were tested for tensile strength using a conventional universal testing machine. And, the hot-rolled steel sheets were pickled and cold-rolled at a cold reduction ratio of 50%. The cold- rolled test samples were continuously annealed, simulated and heat-treated at an annealing temperature of 800⁰C and an over-aging temperature of 400⁰C. After the continuous annealing, simulation and heat-treatment processes, the cold-rolled test samples were tested for tensile strength using a conventional universal testing machine. Meanwhile, the cold-rolled test samples were continuously annealed, simulated and heat-treated at an annealing temperature of 800⁰C in a 46O⁰C hot galvanizing bath.

[75] [76] Table 1

[77] [78]

[79] [80] Changes in the mechanical properties of the inventive steels and the comparative steels according to the manufacturing conditions of the steel sheets are listed Table 2. In the Note, the steel sheets according to the present invention represented by the inventive steels represents hot-rolled steel sheets, and, when the tensile tests were carried out after the continuous annealing, simulation and heat-treatment processes, the hot-rolled steel sheets had a tensile strength of 700 MPa or more, an elongation of 40% or more and a yield strength of 500 MPa or more. Therefore, materials that are suitable for structural members such as automobile members and fillers were found.

[81] [82] In the case of the steel sheets of test sample Nos. 1 and 2, it is impossible to secure sufficient strength and ductility due to the low content of the added manganese.

[83] The steel sheets of test sample Nos. 3 to 6, 8 to 9, 14 to 15 and 19 were not suitable as the structural members since the elongation was poor, and the yield strength was low at 500 MPa or less due to the insufficient contents of the added carbon, manganese, silicon and aluminum.

[84] The steel sheets of test sample Nos. 7, 10 to 13 and 16 to 18 were suitable as the structural members since the contents of the added carbon, manganese and aluminum were appropriate, and the yield strength is desirable due to the addition of the silicon, titanium and niobium.

[85] [86] [Example 2] [87] The high strength steel sheets with high workability and high manganese having an austenite single phase, prepared in Example 1, were cold-rolled again, and measured for mechanical properties. The results are listed in the following Table 3.

[88] Table 3

[89] [90] As shown in Table 3, it was revealed that the yield strengths of the high strength steel sheets are enhanced. It was usually seen that the yield strengths are increased to 750MPa or more in the 10% phase transformation, and the elongation is excellent at 44%, indicating that the high strength steel sheets have excellent formability and crash- worthiness as the structural members.

[91] While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

[1] A high manganese high strength steel sheet with excellent crashworthiness, comprising, by weight: carbon (C): 0.2 to 1.5%, manganese (Mn): 10 to 25%, aluminum (Al): 0.01 to 3.0%, phosphorus (P) 0.03% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.040% or less, at least one selected from the group consisting of silicon (Si): 0.02 to 2.5%, titanium (Ti): 0.01 to 0.10% and niobium (Nb): 0.01 to 0.10%, and the balance of Fe and other inevitable impurities.

[2] The high manganese high strength steel sheet of claim 1, having an austenite single-phase structure.

[3] The high manganese high strength steel sheet of claim 1, having a yield strength of 600 MPa or more and a tensile strength of 900 MPa or more.

[4] A method for manufacturing a high manganese high strength steel sheet with excellent crashworthiness, comprising: cold-rolling the steel sheet of claim 1 at a reduction ratio of 10 to 80%.

[5] The method of claim 4, wherein the rolling is selected from the group consisting of temper rolling, dual rolling and hot final rolling.

[6] The method of claim 4, wherein the steel sheet is selected from the group consisting of a hot-rolled steel sheet, a cold-rolled steel sheet and a plated steel sheet.

[7] The method of claim 4, wherein the steel sheet is selected from the group consisting of a hot-rolled steel sheet, a cold-rolled steel sheet and a plated steel sheet prepared according to at least one operation selected from the group consisting of the following operations (a), (b) and (c);

(a) homogenizing a steel slab at 1050 to 1300⁰C, followed by hot-rolling the steel slab at a finish rolling temperature of 850 to 1000⁰C and winding the hot-rolled steel slab at a temperature range of 700⁰C or below,

(b) cold-rolling the hot-rolled steel sheet at a reduction ratio of 30 to 80% and annealing the resultant steel sheet at 600⁰C or above to prepare a cold-rolled steel sheet, and

(c) plating the hot-rolled steel sheet or the cold-rolled steel sheet using a process selected from the group consisting of hot plating, electroplating and deposition to prepare a plated steel sheet.