WO2018124654A1 - High-strength medium manganese steel for warm stamping and method for manufacturing same - Google Patents

Abstract

The present invention relates to high-strength medium manganese steel, for warm stamping, containing 3-10 wt % of manganese (Mn), 0.05-0.3 wt % of carbon (C) and 0.1-1.0 wt % of silicon (Si), and comprising a balance of iron (Fe) and inevitably contained impurities. The present invention enables reduction of high thermal energy consumption of an existing hot stamping procedure by means of heat treatment at a low austenitizing temperature of medium manganese steel. Moreover, since the present invention does not require an additional tempering procedure and enables acquisition of high strength simply by means of slow cooling such as air cooling outside a mold without rapid cooling in the mold, the procedure is simplified and productivity is enhanced.

Description

High strength medium manganese steel for warm forming and its manufacturing method

The present invention relates to high strength medium manganese steel. Specifically, the present invention relates to a high strength intermediate manganese steel for warm forming and a method of manufacturing the same.

Recently, as environmental problems such as air pollution have emerged, many methods for improving fuel efficiency of automobiles have been proposed. In particular, as the weight reduction of automobiles has emerged as an important part, high strength steel sheets having high strength as well as high formability are required.

In addition, since automotive parts such as bumper reinforcements or shock absorbers in doors are directly related to passenger safety, an ultra high strength steel sheet having a tensile strength of 980 MPa or more is used, and should have high elongation as well as high strength. Similarly, research on commercialization of high strength steel is increasing as the ratio of using high strength steel increases.

In line with these social demands, research has been conducted on how to easily form high strength steel. As such a prior art, there is a hot stamping process disclosed in Korean Patent No. 10-0765723. The present prior art has been proposed a method for producing an ultra-high strength cold rolled steel sheet in a final product by performing heat treatment and press molding in a high temperature austenitic single phase region and then performing rapid quenching with a mold.

However, the prior art hot stamping process has several problems. First, there is a big problem of thermal energy consumption because of the high temperature molding of 900 ℃ or more. Next, the boron-added steel cannot obtain a hard martensite structure unless it is quenched after molding. Thus, even though the molding is finished, water is flowed into the mold to cool it rapidly while keeping the specimen as it is in the mold. This not only lowers the productivity of the process, but also causes the mold surface to suffer from heat fatigue due to repeated heating and cooling.

As a conventional technique for improving such a problem, there is Korean Patent Laid-Open No. 10-2013-0050138. In the prior art, a warm press process including heating up to a dual-phase temperature range of Ac ₁ -Ac ₃ , maintaining a temperature after heating and forming is proposed. However, due to the low molding temperature in the abnormal region, there is a problem in that the physical properties of the final product do not reach the properties comparable to those of the existing hot stamping steel. In addition, yield strength is an important physical property of an automobile body member, but is not discussed in the prior art. Therefore, it is considered that there is a limit to an alternative process of hot stamping.

[Preceding technical literature]

[Patent Documents]

(Patent Document 1) (Document 1) Korea Patent Registration No. 10-0765723 (2007.10.02)

(Patent Document 2) (Document 2) Korean Unexamined Patent Publication No. 10-2013-0050138 (2013.05.15)

The high strength medium manganese steel for warm forming and the method of manufacturing the same according to the present invention have the following problems.

First, it is intended to lower the high temperature forming temperature, which is a disadvantage of conventional hot stamping boron-added steel.

Second, we propose a composition and content of a trace alloy element-added medium manganese steel in a new iron-based warm stamping alloy that simplifies the process.

Third, after cooling, not in the mold, but slowly in the air outside the mold.

The problem of the present invention is not limited to those mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the following description.

The high strength medium manganese steel for warm forming according to the present invention contains components of manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight. It is preferred to be composed of iron and iron (Fe) and inevitably contained impurities.

In the high-strength medium manganese steel for warm forming according to the present invention, it is preferable that niobium (Nb): 0.001-0.1% by weight is further contained in the medium manganese steel.

In the high strength medium manganese steel for warm forming according to the present invention, it is preferable that the aluminum manganese steel further contains 0.001-5.0% by weight of aluminum (Al).

In the high strength medium manganese steel for warm forming according to the present invention, at least one member selected from the group consisting of chromium (Cr), molybdenum (Mo), nickel (Ni) and titanium (Ti) is 0.001-2.0 wt% It is preferable to contain more.

The high strength medium manganese steel forming member for warm forming according to the present invention contains components of manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight. , A composition containing the balance of iron (Fe) and unavoidable impurities, or a composition containing niobium (Nb): 0.001-0.1% by weight further in the composition, and after the hot rolling process, the microstructure is tempered martens It is composed of two phases of sight and bainite or three phases of tempered martensite, bainite and ferrite, and in the case of the three phases, the volume fraction of ferrite is preferably 10% or less (0% not included).

The high strength medium manganese steel forming member for warm forming according to the present invention contains components of manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight. , A composition containing a balance of iron (Fe) and unavoidable impurities, or a composition further containing 0.001-0.1% by weight of niobium (Nb) in the composition, and ferrite and austenite in the Ac ₁ -Ac ₃ or higher region Ac ₃ at a temperature of 1: 1 (T ₅₀ ) After the austenizing process in the first temperature section up to the temperature, the sum of the volume fractions of tempered martensite, bainite and ferrite is preferably 50% or more.

The high strength medium manganese steel forming member for warm forming according to the present invention contains components of manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight. in the composition or the composition containing glass denied iron (Fe) and inevitable impurities, niobium (Nb): 0.001 to 0.1 of a composition further contains the components of the% by weight of ferrite and austenite or more reverse Ac ₃ At temperature, Ac ₃ After the austenizing process in the second temperature section from the temperature to 100 ℃, it is preferable that the volume fraction of martensite is 90% or more.

The yield strength of the high strength medium manganese steel forming member for warm forming according to the present invention is preferably 1.0 GPa or more, and tensile strength is 1.5 GPa or more.

The method for manufacturing a high strength medium manganese steel forming member for warm forming according to the present invention comprises manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight S3 step of preparing a hot-rolled steel sheet or a cold-rolled steel sheet containing a composition containing the remaining iron (Fe) and unavoidable impurities, or the composition further contains niobium (Nb): 0.001-0.1% by weight of the component; Ac ₃ at a temperature which is 1 (T _{50): 1} and Ac ₃ -Ac than the ferrite and austenite in the first station 1st temperature range up to temperature or Ac ₃ It is preferable to include the step S4 of performing austenizing while maintaining a predetermined time after heating the hot-rolled steel sheet or the cold-rolled steel sheet in a second temperature section from a temperature to 100 ℃.

In the method for manufacturing a high strength intermediate manganese steel forming member for warm forming according to the present invention, before the step S3, the intermediate manganese steel slab having the composition is reheated for a predetermined time at 1000-1200 ° C., which is a temperature section of the austenitic single phase region, and Ac ₃ Ms temperature ~ Ac ₃ after hot finish rolling at temperature above 1000 ℃ It is preferable to further include an S1 step of producing a hot rolled steel sheet by winding at a temperature.

In the method for manufacturing a high strength medium manganese steel forming member for warm forming according to the present invention, after the step S1 and before the step S3, it is preferable to further include the step S2 of performing cold rolling at room temperature to produce a cold rolled steel sheet.

In the method for manufacturing a high strength medium manganese steel forming member for warm forming according to the present invention, it is preferable to further include a step S3-1 before annealing the hot rolled steel sheet or the cold rolled steel sheet.

The method for manufacturing a high strength intermediate manganese steel forming member for warm forming according to the present invention preferably includes a step S5-1 of performing warm forming in a temperature section of step S4 after the austenizing.

The method for manufacturing a high strength intermediate manganese steel forming member for warm forming according to the present invention preferably includes a step S5-2 after performing austenizing and performing warm forming at a temperature lower than 10-300 ° C. than the austenizing temperature.

The method for manufacturing a high strength medium manganese steel forming member for warm forming according to the present invention is heated to 1-100 ° C./sec to the warm forming temperature, and is press-molded after holding at 10-1000 sec, followed by a speed of 1 to 30 ° C./sec. Slow cooling is preferred.

In the method for producing a high strength medium manganese steel forming member for warm forming according to the present invention, it is preferable to obtain a martensite structure by slow cooling after warm forming.

The high-strength medium manganese steel for warm forming according to the present invention and its manufacturing method have the following effects.

First, compared with conventional hot stamping boron-added steel, there is an effect of replacing the low content of manganese steel of 3-10% by weight of manganese (Mn) and 0.05-0.3% by weight of carbon (C). Furthermore, a small amount of Nb is added, which further has the effect of improving the strength.

Second, the heat energy is reduced by heat treatment of the high heat energy consumption of the existing hot stamping process at low austenizing temperature of the middle manganese steel.

Third, high strength is obtained even by slow cooling such as air cooling outside the mold without cooling the mold at a high speed, thereby simplifying the process and improving productivity.

The effects of the present invention are not limited to those mentioned above, and other effects that are not mentioned will be clearly understood by those skilled in the art from the following description.

1 is a graph showing the tensile properties according to the heat treatment temperature after cold rolling of the specimen of Example 1 according to the present invention.

2 is a graph showing the tensile properties according to the heat treatment temperature after cold rolling annealing the specimen of Example 1 according to the present invention.

3 is a graph showing the tensile properties according to the heat treatment temperature after cold rolling of the specimen of Example 2 according to the present invention.

Figure 4 is a photograph showing the microstructure between the tempered martens formed during slow cooling after the hot rolling and winding process of Example 1 according to the present invention.

5 is a graph showing the hardness value according to the presence or absence of the winding process after hot rolling of Example 1 according to the present invention.

Figure 6 shows a method of manufacturing a high-strength medium manganese steel forming member for warm molding according to the present invention.

7-10 illustrate various embodiments of austenizing and warm forming in accordance with the present invention.

The high strength medium manganese steel for warm forming according to the present invention contains components of manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight. It is preferred to be composed of iron and iron (Fe) and inevitably contained impurities.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. As those skilled in the art can easily understand, the embodiments described below may be modified in various forms without departing from the concept and scope of the present invention. Where possible, the same or similar parts are represented using the same reference numerals in the drawings.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” include plural forms as well, unless the phrases clearly indicate the opposite.

As used herein, the meaning of "comprising" embodies a particular property, region, integer, step, operation, element, and / or component, and other specific properties, region, integer, step, operation, element, component, and / or It does not exclude the presence or addition of groups.

All terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Terms defined in advance are additionally interpreted to have a meaning consistent with the related technical literature and the presently disclosed contents, and are not interpreted in an ideal or very formal sense unless defined.

In the present specification, the content of the composition is described using weight percent.

The present specification provides (1) mid-manganese alloy design and (2) manufacturing method to replace the existing hot stamping process. EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

(1) medium manganese alloy design

The alloy of the present invention comprises Mn: 3-10% by weight, C: 0.05-0.3% by weight, Si: 0.1-0.5% by weight and is composed of the remaining Fe and other unavoidable impurities. In order to further improve the strength of the present invention, a small amount of alloying elements Nb is included at 0.001-0.1% by weight. The steel sheet may additionally contain 0.001-5.0 wt% of one or more selected from the group consisting of Cr, Mo, Ni, Al, and Ti.

Hereinafter, the reason for limitation of the chemical component range of said steel is demonstrated.

Manganese (Mn): 3-10 wt%

The 3-10% by weight Mn content improves the stability of austenite at high temperatures, suppresses ferrite transformation during cooling, and thus martensite structure can be obtained even in slow cooling. In addition, the press molding temperature can be lowered by lowering the dual-phase zone temperature.

If the Mn content is less than 3% by weight, austenite stability is poor, and ferrite may be generated during cooling after hot rolling, and may exhibit martensite-ferrite abnormality at room temperature. On the other hand, if the Mn content exceeds 10% by weight, not only the increase in raw material cost and manufacturing cost but also a problem in that the weldability is lowered and a large amount of manganese sulfide (MnS) is formed. Therefore, in the present invention, it is preferable to limit the Mn content to 3-10% by weight.

Carbon (C): 0.05-0.3 wt%

The C content of 0.05-0.3% by weight can ensure the stability of the austenite at high temperatures, and can be dissolved in martensite at room temperature to improve strength.

If the C content is less than 0.05% by weight, there is a possibility that austenite is less stable and ferrite is formed during cooling, and the strength of the martensite internal solid solution C is lowered. On the other hand, if the C content is more than 0.3% by weight, there is a possibility that the cold rolling property after the hot rolling is less than the strength, the weldability may be lowered. Therefore, in the present invention, it is preferable to limit the C content to 0.05-0.3% by weight.

Silicon (Si): 0.1-1.0 wt%

Although Si is a ferrite stabilizing element, it increases the austenite hardenability at high temperatures to suppress ferrite transformation during cooling. It also inhibits carbide formation during cooling and accelerates C segregation to austenite in ferrite-austenite anomalies.

If Si is less than 0.1% by weight, the solid solution strengthening effect of Si is reduced and the diffusion of carbon into austenite becomes difficult. On the other hand, when the Si content exceeds 1.0% by weight, problems such as an increase in raw material costs and manufacturing costs, continuous casting, difficulty in welding and plating, and the like may occur. Therefore, in the present invention, it is preferable to limit the Si content to 0.1-1.0% by weight.

Niobium (Nb): 0.001-0.1 wt%

Nb is an element that enhances the strength of steel sheet, refines grains, and improves heat treatment properties by depositing a solid solution effect and solid solution carbon. When the content of the element is less than 0.001% by weight, the amount of precipitated niobium carbide (NbC) is small and it is difficult to expect the effect of improving the strength. When the content of the element exceeds 0.1% by weight, the excessive manufacturing cost rises. Therefore, it is preferable to limit the content to 0.001-0.1% by weight.

Aluminum (Al): 0001-5.0 wt%

Al deoxidizes and is added as an element which improves the cleanliness of steel and suppresses carbide generation. As the addition amount increases, the abnormal region may be expanded to perform uniform heat treatment, but if it exceeds 5.0% by weight, the abnormal region temperature is increased, and the low austenizing temperature of the present invention is increased again. Higher stability may result in the presence of ferrite at room temperature after molding. In addition, the plating property of the steel sheet is lowered and the manufacturing cost is increased. Therefore, in the present invention, it is preferable to limit the Al content to 0.001-5.0% by weight.

Sum of at least one of chromium (Cr), molybdenum (Mo), titanium (Ti) and nickel (Ni): 0.001-2.0 wt%

Cr, Mo, Ti, and Ni are hardening elements and precipitation strengthening effects, and are elements having a large effect to further secure high strength. If it is less than 0.001 weight%, sufficient hardenability and precipitation strengthening effect are hard to expect, and when it exceeds 2.0 weight%, manufacturing cost will rise. Therefore, in the present invention, it is preferable to limit the content to 0.001-2.0% by weight.

(2) manufacturing method

Niobium (Nb) addition manganese steel manufacturing method for warm stamping includes hot rolling and cold rolling conditions, blank forming step, blank heating and forming step, blank cooling step. do.

① Hot rolling and cold rolling conditions

After dissolving the composition alloy of the present invention, the cast ingot is homogenized for 12 hours at 1000-1400 ℃ for uniform alloy element distribution in the steel. If the said heating temperature is less than 1000 degreeC, the homogenization of performance structure will not fully be ensured. If it exceeds 1400 ° C., an increase in manufacturing cost occurs.

Thereafter, reheating is performed for 1 hour in the austenite single phase region of 1000-1200 ° C. Hot finish rolling shall be performed at the temperature of Ac ₃ , which is the transformation critical temperature, at 1000 ° C. or lower during heating. Thereafter, the annealed wound in more than Ac ₃ temperature below Ms (martensite start temperature generated).

In slow cooling, austenite begins to transform martensite as it passes Ms. At this time, martensite is a body-centered tetragonal (bct) crystal structure with many dislocations formed therein, and carbon diffusion is accelerated. Therefore, cementite precipitates inside martensite during slow cooling. At room temperature, the final microstructure consists of two phases of tempered martensite and bainite, or three phases of tempered martensite, bainite and ferrite. In the three phases, the volume fraction of ferrite is preferably 10% or less (0% not included). The tissue photograph and hardness test results of the three phases are shown in FIG. 4.

In the austenizing heat treatment, ferrite remains at a temperature of Ac ₃ or lower, and the final normal temperature strength is lowered. Through the above process, it is possible to improve the cold rolling property by lowering the strength of the hot rolled steel sheet.

 Thereafter, the hot rolled plate may be additionally cold rolled at room temperature to produce a cold rolled steel sheet. The cold rolled steel sheet may then comprise an annealing heat treatment at 550-750 ° C. for 1 minute-5 minutes.

② blank forming step

In the blank forming step, the steel sheet is cut to form a blank. This blank is designed to fit the mold shape.

③ Blank heating (Austenitizing) and forming step

Heating the blank to the blank heat treatment in the step ₁ Ac -Ac ₃ or higher (dual-phase) ferrite and austenite in the first area: Ac ₃ at a temperature (T ₅₀₎ which is 1 The abnormal region temperature section up to the temperature is referred to as 'first temperature section' in the present invention. In addition, Ac ₃ Ac ₃ at temperature The temperature section from the temperature to 100 ° C. is referred to as the “second temperature section” in the present invention. Hold for about 1-15 minutes after heating in the first or second temperature zone. This process is called an austenitizing process (see FIGS. 7 to 10).

In Korea Patent Publication No. 10-2013-0050138, the heat treatment temperature was proposed as an Ac ₁ -Ac ₃ or higher region temperature. In the present invention, although a low molding temperature, strength corresponding to a hot stamping steel was not obtained. The reason for this is that the composition and content of the present invention, such as essentially containing aluminum (Al), does not produce sufficient austenite at the heat treatment temperature, and thus sufficient martensite is not produced during cooling after molding, thereby securing a target strength. It is judged that it was difficult.

Therefore, in the present invention, the high-strength steel having a high elongation in the first temperature range from the temperature (T ₅₀ ) at which the volume fraction of ferrite and austenite becomes 1: 1 in the Ac ₁ -Ac ₃ or higher region to Ac ₃ temperature and a second temperature range of from Ac ₃ temperature to 100 ℃ a temperature above the Ac ₃ temperature is designated the ultra-high strength in the age ranging austenite temperature range of interest.

In the second temperature section, when the temperature is lower than Ac ₃ , ferrite is present in the final microstructure, thereby lowering the yield strength. Ac ₃ When the temperature exceeds 100 deg. C from the temperature, the reverse transformation austenite becomes coarse and the strength may be lowered, which is similar to the austenizing temperature of hot forming. Therefore, the manufacturing cost rises again.

After the austenizing process, the heated steel sheet is transferred to a mold for warm forming at a temperature 10-300 ° C. lower than the austenizing temperature. When transferring to a mold, it is inevitable that the temperature of the heated steel sheet falls below 10 ° C., and when the temperature falls below 300 ° C., the yield strength of the steel sheet increases, and a large load is applied to the mold during molding. This leads to an increase in the life of the mold and the manufacturing cost.

④ blank cooling stage

In the blank cooling step, the heated blank is transferred to a press die for press molding, and then the molded parts are taken out and cooled in air. In the existing hot stamping process, quenching is required in a mold to obtain martensite, but the inventive steel can obtain martensite structure even at a slow cooling rate such as air cooling.

The microstructure of the formed steel sheet exhibits martensite, bainite, ferrite, austenite multiphase structure. Martensite and bainite in the first temperature range. Ferrites, the sum of their volume fractions is at least 50%. When the austenite volume fraction is T50, the stability of the austenite becomes 20% or more. In the second temperature section, the steel sheet has a volume fraction of martensite of 90% or more.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

Steel slabs, as shown in Table 1 below, were heated at a reheating temperature of 1100-1250 ° C. for 1 hour and subjected to hot rolling. At this time, hot rolling was finished hot rolling at 900-1000 ℃ and air-cooled to room temperature. The prepared hot rolled sheet was 55% at the cold rolling rate to prepare a cold rolled sheet.

The cold rolled sheet thus manufactured was used to simulate the heat treatment conditions of the warm stamping process. In the abnormal heat treatment region, the ferrite and austenite were 1: 1 at a temperature _ranging from T ₅₀ to austenite single phase region temperature range of 50 ° C. higher than A ₃ . At this time, the temperature increase rate is 3 ℃ / second and the cooling rate is 10 ℃ / second.

Steel grade	Chemical content (% by weight)						Abnormal zone temperature ( o C)		Remarks
	C	Mn	Si	Nb	B	Other	A 1	A 3
A	0.15	5.9	0.38	0.05	-	-	610	715	Invention steel
B	0.16	5.15	0.37	-			625	735	Invention steel
C	0.093	7.22	0.49	-	-	-	580	700	Invention steel
D	0.22	1.29	0.28	-	0.003	Ti: 0.039 Cr: 0.193 Ni: 0.013			Comparative Steel (Hot Forming Boron Added Steel)

In addition, as shown in Table 2, the inventive steels A-1 to A-3 exhibit excellent room temperature tensile properties when the heat treatment is performed at a temperature of about 10-50 degrees or more than the A ₃ temperature. On the other hand, invented steels A-4 to A-7, which are lower than A ₃ temperature, do not reach the yield strength and tensile strength (TS) required as a hot stamping replacement steel. Invented steel A-7 can be applied to parts requiring high yield strength and elongation index (EI). The tensile curve for each heat treatment temperature for the inventive steel A is shown in FIG.

Steel grade	Heat treatment temperature (actual process application temperature) ( o C)	Yield strength (MPa)	Tensile Strength (MPa)	% Total elongation	T.S. * El. (MPa%)
A-1	770	1040	1680	9.3	15624
A-2	750	1080	1765	9.7	17120
A-3	730	1030	1725	9.1	15697
A-4	710	870	1855	8.9	16509
A-5	690	480	1800	12.1	21780
A-6	670	775	1515	15.1	22876
A-7	650	1180	1200	22.8	27360
D	900	1000	1500	8	12,000

Mechanical properties after warm stamping simulation heat treatment using medium manganese steel without Nb are shown in Tables 3 and 4. Inventive steels B and C also exhibit excellent physical properties when subjected to austenizing heat treatment above the A ₃ temperature. Tensile curves according to the respective heat treatment temperatures of the inventive steels B and C are shown in FIGS. 2 and 3, respectively. The austenitic fraction of the final microstructure at room temperature after air cooling according to temperature is shown in Table 5 below. The lower the temperature, the higher the stability of the reverse-transformed austenite and the residual austenite fraction increased. Inventive steel A-3 heat-treated above the A3 temperature indicates that no residual austenite is present, and all of the reverse austenite is transformed into martensite. Therefore, room temperature yield and tensile strength are excellent.

When the inventive steels A and B were compared, the yield and tensile strength of the inventive steel A increased without decreasing the elongation of the inventive steel B at the same heat treatment temperature. It can be confirmed that the strength due to the solid solution strengthening and precipitation strengthening by adding Nb is improved.

Steel grade	Heat treatment temperature (actual process application temperature) ( o C)	Yield strength (MPa)	Tensile Strength (MPa)	% Total elongation	T.S. * El. (MPa%)
B-1	770	1045	1530	9.2	14076
B-2	750	1040	1535	10	15350
B-3	730	1000	1580	9.6	15168
B-4	700	770	1660	4.5	7470
B-5	690	620	1640	11.5	18860
B-6	680	670	1410	13.5	19035
B-7	660	1000	1240	24.5	30380
D	900	1000	1500	8	12,000

Steel grade	Heat treatment temperature (actual process application temperature) ( o C)	Yield strength (MPa)	Tensile Strength (MPa)	% Total elongation	T.S. * El. (MPa%)
C-1	700	1000	1775	10	17,750
C-2	680	1000	1650	12	19,800
C-3	670	600	1580	17	26,860
C-4	650	1020	1400	23	32,200
D	900	1000	1500	8	12,000

Steel grade	Residual Austenite Volume Fraction (%)
A-3	0
A-4	10
A-5	32
A-6	38

Example 2

Inventive steel A, which was cold rolled in Example 1, was further subjected to annealing heat treatment (CA). The heat treatment conditions were performed while changing the temperature to 650-750 ^° C 3 minutes. The cold-rolled steel sheet was processed by subdividing the heat treatment conditions of the warm stamping process into an austenizing temperature (T _A ) and a molding temperature (T _S ). The austenizing temperature was 650-750 ^o C for 5 minutes and the molding temperature was 650-750 ^o C for 1 minute. After heat treatment, air was cooled to room temperature. At this time, the temperature increase rate is 3 ^o C / sec and the cooling rate is 10 ^o C / sec.

Steel grade	Heat treatment temperature ( o C)			Yield strength (MPa)	Tensile Strength (MPa)	% Total elongation
	CA	T A	T S
A-8	750	750	700	1085	1810	11.6
A-9			650	1070	1820	11.6
A-10			600	1040	1740	10.2
A-11			550	1065	1780	11.2
A-12		700	650	530	1630	14.0
A-13			600	510	1650	16.5
A-14			550	500	1630	14.6
A-15		650	600	950	1210	32.1
A-16			550	980	1160	32.5
A-17			500	970	1180	32.7
A-18	700	750	700	1090	1780	11.4
A-19			650	1010	1720	10.8
A-20			600	1000	1690	10.0
A-21			550	1040	1760	10.6
A-22		700	650	350	1640	14.3
A-23			600	360	1670	14.1
A-24			550	370	1650	14.4
A-25		650	600	1050	1350	22.7
A-26			550	1030	1330	22.4
A-27			500	1080	1380	22.8
A-28	650	750	700	1040	1750	11.7
A-29			650	1065	1800	10.4
A-30			600	1010	1760	10.3
A-31			550	1060	1780	10.3
A-32		700	650	500	1710	8.2
A-33			600	350	1680	14.8
A-34			550	340	1740	14.2
A-35		650	600	1155	1280	21.2
A-36			550	1180	1260	22.6
A-37			500	1175	1260	22.0

In addition, as shown in Table 6, the annealing temperature (CA) and the molding temperature (T _S ) was confirmed that does not significantly affect the room temperature tensile properties. Similar to Example 1, the austenizing temperature (T _A ) has a large effect. Inventive steel A-8 to invention steel A-11, invention steel A-18 to invention steel A-21, and invention steel A-28 to invention steel A-31 all exhibited superior physical properties than hot stamping steel. have. At low T _A , the target tensile temperature was not reached.

Example 3

In Example 3, unlike Example 1 and Example 2, the inventive steel A was advanced to a hot rolled steel sheet without cold rolling. Invented steel A-38 and Invented steel A-39 were subjected to annealing heat treatment at 750 ° C. for 3 minutes. Thereafter, the steel sheet was further divided into heat treatment conditions of a warm stamping process into an austenizing temperature (T _A ) and a molding temperature (T _S ). The austenizing temperature was varied by changing the temperature to 650-750 ° C. for 5 minutes and the molding temperature to 600 ° C. for 1 minute. After heat treatment, air was cooled to room temperature. At this time, the temperature increase rate is 3 ℃ / second and the cooling rate is 10 ^° C / second.

Steel grade	Heat treatment temperature ( o C)			Yield strength (MPa)	Tensile Strength (MPa)	% Total elongation
	CA	T A	T S
A-38	X	750	600	1025	1630	10.3
A-39		650		950	1100	23.3
A-40	750	750		1050	1670	7.4
A-41		650		900	1150	25.4

As shown in Table 7, the annealing temperature (CA) and the forming temperature (T _S ) was confirmed that does not significantly affect the room temperature tensile properties. Similar to Examples 1 and 2, the austenizing temperature (T _A ) has a large effect. Invented steel A-38 and invented steel A-40 all exhibit superior physical properties than hot stamping steel. At low T _A , the target tensile temperature was not reached. Through the above embodiment, it was confirmed that the hot stamping steel sheet can be applied as a hot stamping steel sheet to replace the hot stamping steel sheet without the hot rolling process.

Hereinafter, various embodiments of a method for manufacturing a high strength medium manganese steel forming member for warm forming according to the present invention will be described.

Figure 6 shows a method of manufacturing a high strength intermediate manganese steel forming member for warm molding according to the present invention. One manufacturing method according to the invention comprises at least S3 step and S4 step.

Step S3 according to the present invention contains components of manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight, and the balance of iron (Fe) ) And a hot rolled steel sheet or a cold rolled steel sheet having a composition containing an unavoidable impurity or a composition further containing a component of niobium (Nb): 0.001-0.1% by weight. In step S3 all compositions of FMS according to the present invention can be applied.

In addition, a cold rolled steel sheet subjected to hot rolling and cold rolling may be applied, or a hot rolled steel sheet roughly subjected to hot rolling may be applied.

In the step S1 according to the present invention, the intermediate manganese steel slab having the composition is reheated for a predetermined time at 1000-1200 ° C., which is a temperature section of the austenitic single phase region, and subjected to hot finishing rolling at an Ac ₃ temperature or more and 1000 ° C. or lower, and then, an Ms temperature. ~ Ac ₃ It can be wound at a temperature to produce a hot rolled steel sheet.

S2 step according to the present invention can be produced by cold rolling at room temperature cold rolling.

In the present invention, it may further comprise a S3-1 step of annealing the hot rolled steel sheet or the cold rolled steel sheet.

Step S4 according to the present invention is Ac ₁ -Ac ₃ above reverse the ferrite and austenite 1 at a temperature (T ₅₀₎ is the first Ac ₃ 1st temperature range up to temperature or Ac ₃ After heating the hot rolled steel sheet or the cold rolled steel sheet in a second temperature section from a temperature to 100 ° C., austenizing may be performed while maintaining a predetermined time.

In the present invention, after the austenizing may include a step S5-1 performing the warm molding in the temperature section of the step S4. FIG. 7 shows that steps S4 and S5-1 are performed in the second temperature section, and FIG. 8 shows that steps S4 and S5-1 are performed in the second temperature section.

In the present invention, after the austenizing, it may include a step S5-2 to perform the warm molding at a temperature 10-300 ℃ lower than the austenizing temperature. 9 shows that step S4 is performed in the first temperature section, step S5-2 is performed in the lower temperature section, and FIG. 10 shows step S4 is performed in the first temperature section, and in the lower temperature section. Indicates that step S5-2 is performed.

Furthermore, the present invention can obtain martensite structure by slow cooling such as air cooling after warm forming.

Major examples of the production method according to the present invention are shown in Table 8 below, but are not limited to the examples shown in Table 8.

S1-> S2-> S3-1-> S3-> S4-> (S5-1 or S5-2)

S1 -------> S3-1-> S3-> S4-> (S5-1 or S5-2)

S1 ---------------> S3-> S4-> (S5-1 or S5-2)

S2-> S3-1-> S3-> S4-> (S5-1 or S5-2)

S2 ---------> S3-> S4-> (S5-1 or S5-2)

S1-> S2 ---------> S3-> S4-> (S5-1 or S5-2)

The embodiments described in the present specification and the accompanying drawings merely illustrate some of the technical ideas included in the present invention. Therefore, since the embodiments disclosed herein are not intended to limit the technical spirit of the present invention but to explain, it is obvious that the scope of the technical spirit of the present invention is not limited by these embodiments. Modifications and specific embodiments that can be easily inferred by those skilled in the art within the scope of the technical idea included in the specification and drawings of the present invention should be construed as being included in the scope of the present invention.

Claims (16)

1. Manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight, the remainder of iron (Fe) and inevitable impurities High strength medium manganese steel for warm forming, characterized in that the composition.
2. The method according to claim 1,

   Niobium (Nb) in the medium manganese steel: high strength medium manganese steel for warm forming, characterized in that it further contains 0.001-0.1% by weight of the component.
3. The method according to claim 1 or 2,

   The high-strength medium manganese steel for warm forming, characterized in that the middle manganese steel further contains 0.001-5.0% by weight of aluminum (Al).
4. The method according to claim 1 or 2,

   The high-strength medium manganese steel for warm forming, characterized in that the middle manganese steel contains at least 0.001-2.0% by weight of at least one selected from the group consisting of chromium (Cr), molybdenum (Mo), nickel (Ni) and titanium (Ti). .
5. Manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight, containing the balance of iron (Fe) and unavoidable impurities Niobium (Nb): 0.001-0.1% by weight of the composition further contained in the composition or the composition,

   After the hot rolling process, the microstructure consists of two phases of tempered martensite and bainite or three phases of tempered martensite, bainite and ferrite, and in the three phases, the volume fraction of ferrite is 10% or less ( High strength intermediate manganese steel forming member for warm forming.
6. Manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight, containing the balance of iron (Fe) and unavoidable impurities Niobium (Nb): 0.001-0.1% by weight of the composition further contained in the composition or the composition,

   After the austenizing process in the first temperature range from the temperature (T ₅₀ ) at which the ferrite and austenite become 1: 1 in the Ac ₁ -Ac ₃ region (T ₅₀ ) to the Ac ₃ temperature, the tempered martensite, bainite and A high strength intermediate manganese steel forming member for warm forming, characterized in that the sum of the volume fractions of ferrite is 50% or more.
7. Manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight, containing the balance of iron (Fe) and unavoidable impurities Niobium (Nb): 0.001-0.1% by weight of the composition further contained in the composition or the composition,

   Ac _{3 in} ferrite and austenite anomalies At temperature, Ac ₃ A high-strength medium manganese steel forming member for warm forming, wherein the volume fraction of martensite is 90% or more after the austenizing process in a second temperature section from temperature to 100 ° C.
8. The method according to claim 6 or 7,

   Yield strength of the molded member is 1.0 GPa or more, high strength intermediate manganese steel forming member for warm forming, characterized in that the tensile strength is 1.5 GPa or more.
9. Manganese (Mn): 3-10% by weight, carbon (C): 0.05-0.3% by weight and silicon (Si): 0.1-1.0% by weight, containing the balance of iron (Fe) and unavoidable impurities S3 step of preparing a hot-rolled steel sheet or a cold-rolled steel sheet having a composition or a composition further containing niobium (Nb): 0.001-0.1% by weight of the composition; And

   In the Ac ₁ -Ac ₃ or higher region, the first temperature range from the temperature (T ₅₀ ) at which ferrite and austenite becomes 1: 1 (T ₅₀ ) to Ac ₃ temperature or Ac ₃ S4 step of austenizing is performed while maintaining the predetermined time after heating the hot-rolled steel sheet or the cold-rolled steel sheet in a second temperature section from the temperature to 100 ° C Manufacturing method.
10. The method according to claim 9, before step S3,

    The intermediate manganese steel slab having the above composition is reheated for a predetermined time at 1000-1200 ° C., which is a temperature section of the austenitic single phase region, and hot finished rolling is carried out at an Ac ₃ temperature or more and 1000 ° C. or less, and then Ms temperature to Ac _3. The method of manufacturing a high strength intermediate manganese steel forming member for warm forming, characterized in that it further comprises the step S1 of producing a hot rolled steel sheet by winding at a temperature.
11. The method according to claim 9, after step S1 before step S3,

    The method of manufacturing a high strength intermediate manganese steel forming member for warm forming, further comprising the step S2 of performing cold rolling at room temperature to produce a cold rolled steel sheet.
12. The method according to claim 10 or 11, before step S3,

    The method of manufacturing a high strength intermediate manganese steel forming member for warm forming, further comprising the step S3-1 of annealing the hot rolled steel sheet or the cold rolled steel sheet.
13. The method according to claim 9,

    The method of manufacturing a high strength intermediate manganese steel forming member for warm forming, characterized in that it comprises a step S5-1 performing the warm forming in the temperature section of step S4 after the austenizing.
14. The method according to claim 9,

    After the austenizing, S5-2 step of performing a warm molding at a temperature 10-300 ℃ lower than the austenizing temperature, the method of manufacturing a high-strength medium manganese steel forming member for warm forming.
15. The method according to claim 13 or 14,

    The temperature is raised to 1-100 ° C./sec to the warm forming temperature, press-molded after holding at 10-1000 sec, followed by slow cooling at a rate of 1 to 30 ° C./sec. Way.
16. The method according to claim 15,

    A method for producing a high strength intermediate manganese steel forming member for warm forming, characterized by obtaining a martensite structure by slow cooling after warm forming.

Images