KR101490560B1 - Low gravity steel material having excellent ductility and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a low specific gravity steel having excellent ductility and a method of manufacturing the same. An embodiment of the present invention is a steel sheet comprising, by weight%, 0.4 to 1.0% of C, 20 to 30% of Mn, 10 to 15% of Al, 0.5 to 3.5% of Ni, 0.01% The present invention provides a low specific gravity steel material which contains impurities and is composed of austenite having a microstructure of 40% by area or more and residual ferrite and is excellent in ductility, and a process for producing the same.
INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a steel material having a tensile strength of 1000PMa or more, a ductility at room temperature of 10% or more, and a weight reduction ratio of 15% or more and a method for producing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low specific gravity steel material having excellent ductility,

The present invention relates to a low specific gravity steel having excellent ductility and a method of manufacturing the same.

Generally, when a large amount of Al is added to an automotive steel for light weight, BCC and regular phases (B2 and DO3) are stabilized, and phase compatibility at room temperature is lowered and ductility is lowered. Therefore, in order to reduce the weight of automotive steel, it is necessary to increase the Al content in steel, but there is a limit in weight reduction by adding Al due to the problem of moldability. 1 is a state diagram of an Fe-Al system, and the crystal structure according to Al addition is as shown in FIG. As can be seen from FIG. 1, BCC is stabilized even when a trace amount of Al is added. It is known that when the Al content is 10% or more by weight in a low temperature range close to room temperature, B2 and DO3 ordered phases are formed and the brittleness is extremely high . In addition, it is known that even in the BCC, the conformity is low at the anti-phase boundary between the BCC and the B2 and DO3 phases, so that the ductility is very low and the plastic deformation does not occur.

A typical conventional technique for solving such a problem is Patent Document 1. The above-mentioned technique is to improve the stability of the austenite single phase by increasing the stacking defect energy by adding 10-20 wt% of Mn as an FCC stabilizing element, 0.5-1.0 wt% of C and controlling the addition amount of Al to 2.5-5 wt% The present invention relates to a method for producing a steel sheet having a specific gravity of 7.6 g / cm ³ or less and a uniform elongation of 70% or more. The steel sheet is subjected to hot rolling at 900 캜 or higher and then homogenized at 1000 캜 or higher, As described above, not only a complicated process of quenching after solution treatment but also a disadvantage that the effect of weight reduction by Al addition is 7.5% or less.

Another technique is Non-Patent Document 1, which is a method of constructing a duplex or triplex phase, comprising 0.5 to 2 wt% C, 18 to 35 wt% Mn, and 8 To 12% by weight of Al. In the case of the above-mentioned technique, the Al content is as high as 10% or more, but after the heat treatment at 1150 ° C for 25 minutes, the cold rolling is performed at 60% or more and then recrystallization is performed at 1050 ° C for 25 minutes. Quenched and aged at 800 to 1100 占 폚 and then water-cooled. As a result, even if the tensile strength is at least 1500 MPa and the ductility is at least 48%, commercialization is very difficult.

Korean Patent Publication No. 2011-0115651

 [G. Frommeyer and U. Brux: "Microstructures and mechanical properties of high-strength Fe-Mn-Al-C light-weight TRIPLEX steels" Int., 77 (2006), No. 9-10, pp. 627-633]

An object of the present invention is to provide a steel material having excellent room temperature ductility and a low specific gravity, even if Al is added in an amount of 10% or more, and a manufacturing method thereof, by controlling the microstructure by appropriately controlling the alloy composition and the manufacturing conditions.

An embodiment of the present invention is a steel sheet comprising, by weight%, 0.4 to 1.0% of C, 20 to 30% of Mn, 10 to 15% of Al, 0.5 to 3.5% of Ni, 0.01% And a low specific gravity steel which is excellent in ductility and is composed of an austenite having a microstructure of 40% or more by area and residual ferrite and capacarbide.

Another embodiment of the present invention is a steel sheet comprising, by weight%, 0.4 to 1.0% of C, 20 to 30% of Mn, 10 to 15% of Al, 0.5 to 3.5% of Ni, 0.01% Hot rolling the ingot containing impurities at 1100 to 1250 占 폚 to obtain a steel material by solid-solution treatment; And rapidly cooling the steel material to 550 DEG C or less. The present invention also provides a method of manufacturing a low-specific-weight steels excellent in ductility.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a steel material having a tensile strength of 1000PMa or more, a ductility at room temperature of 10% or more, and a weight reduction ratio of 15% or more and a method for producing the same.

1 is a state diagram of an Fe-Al system.
2 is an Fe-Mn system phase diagram.
3 is a microstructure photograph of Inventive Example 1. Fig.
4 is a microstructure photograph of Comparative Example 9. Fig.

The inventors of the present invention conducted research to solve the problem that the ductility is lowered due to the lowering of the phase-to-phase consistency caused by the stabilization of the regulations when a large amount of Al is added in the production of a low specific gravity steel. Phase boundary between the phase and the BCC is improved and the ductility of the steel is improved, thereby completing the present invention.

Hereinafter, the present invention will be described.

First, the alloy composition of the steel of the present invention will be described.

C: 0.4 to 1.0 wt%

In order to obtain an elongation at room temperature of 10% or more to be obtained in the present invention, a certain amount of austenite must be present in the microstructure. For this purpose, it is necessary to add 0.4% or more of C contributing to FCC stabilization. However, if it exceeds 1.0%, the hardness of kappa carbide is formed at the grain boundaries to deteriorate the ductility, and the strength is rapidly increased in the mouth to decrease the ductility, so that the content of C is satisfied in the range of 0.4 to 1.0 wt% .

Mn: 20 to 30 wt%

Mn is an element which stabilizes the FCC while being effective for strengthening solid solution and hardening ability, and is an element for inhibiting the formation of BCC and B2 / DO3 by Al addition and securing a proper austenite fraction. In order to secure a certain amount of austenite or more, Mn should be contained in an amount of 20% or more, but if it exceeds 30%, as shown in FIG. 2, which is an Fe-Mn system state diagram, Phase may be formed to lower ductility and excessively increase the strength. Therefore, the content of Mn is preferably in the range of 20 to 30% by weight.

Al: 10 to 15 wt%

Al is added for the purpose of weight reduction and strength enhancement. Al bonds with Fe, Mn and C to form a cappadovite precipitate or form Fe / Al phase B2 / DO3 phase dissolved in Fe. The solubility of Al in Fe is much higher than that of Mg or Ti in lightweight elements, so that it can be effectively utilized as a lightweight element. In order to utilize it effectively, Al is preferably added in an amount of 10% or more. However, when it exceeds 15%, the problem of ductility reduction due to the rule occurs, and therefore it is preferable that the Al content is in the range of 10-15% by weight.

Ni: 0.5 to 3.5 wt%

Ni is an element that stabilizes the FCC and secures a certain amount of austenite. In the present invention, since the austenite fraction is insufficient only by Mn addition, it is preferable that the content of Ni is 0.5% or more in order to secure an austenite of 40% or more by utilizing the effect of the Ni addition. However, if it exceeds 3.5%, the ductility may be lowered and the cost may increase. Therefore, it is preferable that the content of Ni is in the range of 0.5 to 3.5% by weight.

N: not more than 0.01% by weight

The lower the amount of N is, the better, but it is inevitably an impurity remaining in the steel. When the content of N is more than 0.01%, the number of precipitates increases, so that the strength and ductility may be lowered. Therefore, the content of N is preferably 0.01 wt% or less.

The steel material provided by the present invention preferably has the following microstructure while satisfying the alloy composition as described above.

The steel of the present invention is preferably composed of austenite having a surface area of 40% by area or more, residual ferrite and capacarbide. In the present invention, Al is added in order to reduce the weight of the steel, which inevitably forms ferrite. Accordingly, in the present invention, ductility is improved by securing austenite as much as possible. Further, in order to increase the austenite fraction, an appropriate amount of C is added. When C is excessively added, a precipitate of capalcarbide may be formed and a problem may occur. That is, the present invention aims at securing mechanical properties by increasing FCC fraction instead of ferrite and kapacarbide which are unavoidably formed. By securing at least 40% of austenite as described above, BCC and B2 easily formed by Al at room temperature, It is possible to prevent the DO3 from being formed excessively and to improve the ductility. Further, in order to improve the weight reduction ratio, it is impossible to secure a ductility of 10% or more by stabilizing the BCC as the amount of Al increases, but when the austenite fraction is secured by 40% or more through addition of Ni as in the present invention, . The higher the austenite fraction is, the more advantageous it is in ensuring ductility, and therefore the upper limit of the austenite fraction is not particularly limited.

The steel material provided by the present invention may be a hot-rolled steel sheet, a cold-rolled steel sheet or a coated steel sheet.

The steel material of the present invention satisfying the above conditions has a tensile strength of 1000 MPa or more, a ductility (elongation) of 10% or more, and a weight reduction ratio of 15% or more of that of ordinary carbon steel, Can be preferably applied.

Hereinafter, the production method of the present invention will be described.

First, a steel ingot satisfying the above alloy composition is hot-rolled at 1100 to 1250 占 폚 to obtain a steel material. If the hot-rolling temperature is lower than 1100 ° C, the hot-rolling property deteriorates. If the hot-rolling temperature is higher than 1250 ° C, the melting point of Al is lowered to cause grain segregation and the like, . Therefore, the hot rolling temperature is preferably 1100 to 1250 ° C. On the other hand, one side of the steel has a shape of a steel plate.

Thereafter, the steel material is preferably subjected to solid-solution treatment in the hot rolling temperature range, whereby the precipitated kapacaride precipitated during rolling and cooling can be solidified in the substrate. If the solid solution temperature is lower than 1100 ° C, the precipitated kappa carbide hardly solidifies in the matrix. If the solid solution temperature exceeds 1250 ° C, the melting point of Al is lowered to cause grain boundary segregation and the like.

At this time, it is preferable that the heating is performed for 1 to 2 hours. If the heating is carried out for less than 1 hour, the cappaccar precipitate may not be sufficiently solidified. If the heating time is more than 2 hours, The amount of C, which is an advantageous element, sharply decreases in the entire steel material and in the surface portion, thereby decreasing the austenite fraction and possibly deteriorating the ductility at room temperature.

Thereafter, it is preferable to quench the heated steel to 550 ° C. or less. The quenching may reduce the amount of precipitated capasilicate formed in the FCC granules and the grain boundaries, and the final product may contain 40% or more of austenite . This makes it possible to secure a room temperature ductility of 10% or more in the final product. When the quenching-stop temperature is higher than 550 ° C, the capacarbide solidified in the matrix may be precipitated again or the amount of BCC formed may increase, which may lower the toughness at room temperature. On the other hand, if the quenching is stopped at a temperature of room temperature or more, the properties desired to be obtained by the present invention can be obtained. Therefore, the lower limit of the quenching stop temperature is not particularly limited.

The cooling rate during the quenching is preferably in the range of 100 to 500 ° C / s. When the cooling rate is lower than 100 ° C / s, the diffusion phenomenon occurs due to the slow cooling rate and the dissolved cappuccardite is precipitated again or the BCC transformation amount is increased There is a concern. If it exceeds 500 DEG C / s, thermal stress due to quenching may occur, which may lower the toughness at room temperature.

The quenching step may further include maintaining the quenched quenching temperature for 1 to 3 hours for constant temperature transformation of the steel material. If the above-mentioned constant temperature transformation time is less than 1 hour, it is insufficient to solve the thermal stress generated during quenching. If it exceeds 3 hours, cappuccarde is re-precipitated due to prolonged diffusion or BCC transformation occurs, .

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

(Example)

A steel ingot having the alloy composition shown in the following Table 1 was manufactured to a thickness of 60 mm and a width of 175 mm through vacuum induction melting and the steel ingot was reheated at 1,250 DEG C for one hour and then hot rolled at 1100 DEG C to produce a hot- Respectively. Thereafter, the hot-rolled steel sheet was subjected to solid-state treatment for 1 hour under the conditions shown in Table 2, followed by quenching and holding through the heat treatment of the salt bath, and the holding time was 2 hours. ASTM subsize tensile specimens were subjected to a tensile test at room temperature, and the tensile strength and elongation were shown in Table 2 below. After measuring the specific gravity of the specimen, the weight reduction ratio is shown in Table 2 as compared with carbon steel having a specific gravity of 7.83 g / cm < ^{3 &} gt ;. On the other hand, among the measured microstructural fractions, the ferrite fraction includes a fraction of the capacarbide phase, because it is difficult to accurately measure the fraction because the capacarbide phase is present in the austenite and ferrite base and its grain boundaries.

division Chemical composition (% by weight) C Mn Al Ni N Inventive Steel 1 0.80 27.5 12.1 1.04 0.0045 Comparative River 1 0.79 27.7 12.1 0.2 0.0084 Comparative River 2 0.78 28.0 12.1 0.4 0.0087 Invention river 2 0.81 28.0 12.0 0.6 0.0078 Invention steel 3 0.81 27.8 12.0 1.0 0.0041 Inventive Steel 4 0.82 27.9 12.0 2.1 0.0023 Invention steel 5 0.78 28.1 11.9 3.2 0.0065 Comparative Steel 3 0.79 28.1 12.2 3.6 0.0042 Comparative Steel 4 0.77 27.5 12.1 5.1 0.0090 Comparative Steel 5 0.80 27.6 8.2 1.05 0.0056 Comparative Steel 6 0.79 27.7 9.1 1.11 0.0025 Invention steel 6 0.83 27.9 10.2 0.98 0.0068 Invention steel 7 0.79 28.1 12.5 1.03 0.002 Comparative Steel 7 0.82 27.5 15.2 1.2 0.0089 Comparative Steel 8 0.79 27.9 17.5 0.97 0.0077

Grade Nr. division Heat treatment condition Microstructure Mechanical properties Heat treatment temperature (캜) Quench-stop
Temperature (℃) Austenite
(area%) ferrite
(area%) The tensile strength
(MPa) Elongation
(%) Lightweight
(%) Inventive Steel 1 Comparative Example 1 903 502 15 85 1072 3 18.2 Comparative Example 2 902 551 19 81 1055 4 18.2 Comparative Example 3 905 603 16 84 1103 4 18.2 Comparative Example 4 903 651 13 87 1154 2 18.2 Comparative Example 5 1004 503 22 78 1072 6 18.2 Comparative Example 6 1002 548 25 75 1055 4 18.2 Comparative Example 7 1003 601 23 77 1103 5 18.2 Comparative Example 8 1002 652 21 79 1154 3 18.2 Inventory 1 1102 502 45 55 1072 12 18.2 Inventory 2 1104 547 43 57 1054 11 18.2 Comparative Example 9 1103 601 35 65 1054 4 18.2 Comparative Example 10 1107 657 22 78 1054 6 18.2 Comparative River 1 Comparative Example 11 1109 520 29 71 998 3 18.2 Comparative River 2 Comparative Example 12 1108 523 31 69 1022 7 18.2 Invention river 2 Inventory 3 1104 520 41 59 1052 11 18.0 Invention steel 3 Honorable 4 1106 519 45 55 1059 13 18.0 Inventive Steel 4 Inventory 5 1112 515 47 53 1102 12 18.0 Invention steel 5 Inventory 6 1108 520 44 56 1150 10 17.9 Comparative Steel 3 Comparative Example 13 1105 525 42 58 1201 6 18.3 Comparative Steel 4 Comparative Example 14 1102 543 33 67 1250 2 18.2 Comparative Steel 5 Comparative Example 15 1113 533 65 35 950 51 12.3 Comparative Steel 6 Comparative Example 16 1106 530 56 44 982 44 13.7 Invention steel 6 Honorable 7 1109 526 49 51 1044 32 15.3 Invention steel 7 Honors 8 1107 529 43 57 1056 14 18.8 Comparative Steel 7 Comparative Example 17 1113 531 32 68 1107 6 22.8 Comparative Steel 8 Comparative Example 18 1108 527 21 79 1140 3 26.3

As shown in Tables 1 and 2, in the case of Comparative Examples 1 to 8 produced using Inventive Steel 1 satisfying the alloy composition of the present invention, the solid solution temperature was lower than 1100 ° C, It is difficult to secure an austenite structure at 40% or more even if the (constant temperature transformation retaining) condition is applied, and it can be understood that an elongation of 10% or more is not obtained.

However, in Inventive Examples 1 and 2 where the solid-state quenching temperature also satisfied the conditions proposed by the present invention, the austenite fraction increased to 40% or more, and the ductility at room temperature increased to 10% or more .

However, even in the case of satisfying the solid solution treatment conditions of 1100 占 폚 or more, the austenite phase fraction decreases and the elongation decreases in Comparative Examples 9 and 10 where the quenching stop temperature exceeds 550 占 폚.

Fig. 3 is a microstructure photograph of Inventive Example 1, and Fig. 4 is a microstructure photograph of Comparative Example 9. Fig. As shown in Fig. 3, a large amount of austenite was obtained in Inventive Example 1, but a relatively small fraction of austenite was formed in Comparative Example 4, as shown in Fig.

In Examples 3 to 6 and Comparative Examples 11 to 14, the results of measurement of mechanical properties according to changes in Ni content were examined. In Comparative Examples 11 and 12 in which the Ni content was less than 0.5%, the elongation was low have. However, in Inventive Examples 3 to 6, which satisfy the Ni content proposed by the present invention, not only an elongation of 10% or more is obtained but also a high strength of 1000 MPa or more is secured. However, in the case of Comparative Examples 13 and 14 exceeding 3.5%, it can be seen that the elongation was rather lowered.

Inventive Examples 7 and 8 and Comparative Examples 15 to 18 are for evaluating the results of measurement of mechanical properties according to changes in Al content. In Comparative Examples 15 and 16 in which the Al content is less than 10%, the ductility at room temperature is excellent, It is low. However, in Examples 7 and 8 which satisfy the Al content range proposed by the present invention, excellent mechanical properties such as a lighter weighting rate of 15% or more, a tensile strength of 1000 MPa or more, and an elongation of 10% or more are secured. However, It was found that the ductility decreased sharply in the case of Comparative Examples 17 and 18.

Claims (6)

delete delete A steel ingot containing C: 0.4 to 1.0%, Mn: 20 to 30%, Al: 10 to 15%, Ni: 0.5 to 3.5%, N: 0.01% or less and the balance Fe and other unavoidable impurities to 1100 Hot rolling at ~ 1250 占 폚 and solid-processing for 1 to 2 hours to obtain a steel material; And
And rapidly cooling the steel material to 550 DEG C or less. delete The method of claim 3,
Wherein the quenching is performed at a rate of 100 to 500 ° C / s. The method of claim 3,
Further comprising the step of maintaining the steel material for 1 to 3 hours after the quenching.

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