SE512626C2 - Austenitic, high manganese steel for electronic panels and automobile mfr. - Google Patents

Abstract

An austenitic, high Mn steel comprises less than 0.7 wt.% C, with Mn and Al added in amts. within the range defined by the pts. A, B, C, D and E and the balance Fe. Alternatively, the steel comprises (wt.%): less than 1.5 C, 15-35 Mn, 0.1-6 Al,and opt. one or more of up to 0.6 Si, up to 5 Cu, up to 1 Nb, up to 0.5 V, up to 0.5 Ti, up to 9 Cr, up to 4 Ni and up to 0.2 N. Both steels are prepd. by hot rolling and cold rolling a steel slab, and annealing the resulting sheet at 550-1000 deg.C for 5 secs. to 20 hrs. The resulting austenite grains are less than 40 microns in size.

Description

15 20 25 30 512 626 2 use in car parts which do not require high compatibility.

At the same time, steel plates, which are used for the outer panel in electronic devices, must be of non-magnetic material, which is not affected by magnetic fields and have high strength and formability. Therefore, mainly austenitic steels are used for this purpose, but this steel contains up to approx. 8% precious nickel, while its magnetic susceptibility becomes unstable due to voltage-induced α-martensites formed during the manufacturing process.

The present applicant has for many years studied how to overcome the disadvantages of conventional automotive steel sheets and steel plates for electronic panels, and has successfully developed an austenitic steel with a high content of manganese, with superior formability and durability. So far, no example has been found in which a high impurity steel is used to provide high formability and high strength.

At present, high manganese steels are used in nuclear fusion reactors, in magnetic levitation rails to prevent the generation of electrostatic charge, and as non-magnetic construction materials in transformers (Japanese Patent Publications Sho-63-35758, 64-17819, 61-288052 and 60- 36647). In addition, this material has been used as non-magnetic steel in some parts for VTRs and electronic audio systems (Japanese Patent Publication Sho-62-136557).

However, in this high manganese non-magnetic steel, Al is not added as a component of the alloy, or it is added only in an amount up to 4% for deoxidation, oxidation resistance, corrosion resistance, solid solution hardening and grain purification (Japanese Patent Publications Sho-60 -36647, 63-35758 and 62-136557).

At the same time, the alloy of the same compositional system, which is related to the present invention, is described in Korean Patent Publication 29304 (corresponding to US-P-4 847 046 and JP-1 631 935) by the same inventor.

However, the alloy system described in Korean Patent Publication 29304 is considered based on its strength and toughness at ultra-low temperatures and is therefore intended for cryogenic applications. It therefore differs substantially from the steel of the present invention, which is intended to improve the formability, strength and weldability.

SUMMARY OF THE INVENTION An object of the present invention is to provide an austenitic manganese steel and a process for its production, in which it takes advantage of the fact that an austenitic Fe-Mn-Al-C steel having a surface-centered cubic lattice has high extensibility, to produce a suitable number of deformation pairs, thereby improving the formability, strength and weldability.

Another object of the present invention is to provide an austenitic steel having a high manganese content and a process for producing the same, in which a hardening element of solid solution is added to an austenitic Fe-Mn-Al-C alloy with surface-centered cubic poison, so that the deformation pairs further improve the formability, strength and weldability.

Brief Description of the Drawings This object and other advantages of the present invention will become more apparent when a suitable embodiment of the present invention will be described in detail with reference to the accompanying drawings, in which: Eig. 1 is a Greek illustration of the addition intervals for Mn and Al; Fig. 2 is a graphical illustration of the limits of formability according to experiments performed; Fig. 3 is an image taken with an electron microscope showing the formation of deformation pairs in the steel of the present invention; Fig. 4 is an image taken with an electron microscope showing the formation of deforneton pairs in another embodiment of the present invention; Fig. 5 is a graphical representation showing the limit of formability according to the experiments; and Fig. 6 is a graph showing the variation in hardness of a weld according to the experiments.

Description of a Suitable Embodiment The steel of the present invention contains less than 0.70% by weight of C, and Mn and Al are added in amounts within the range defined by A, B, C, D and E in Pig. The residue consists of Fe and other indispensable impurities, forming an austenitic steel with a high manganese content, which has superior malleability, strength and weldability.

After extensive studies and experiments, it has been found that a steel with a high manganese content and superior formability, strength and weldability can be obtained even if the proportion of C, Mn and Al in the austenitic steel with a high manganese content is varied to a certain extent and also if a solid solution curing agent is added.

A new invention was devised based on this fact and this new invention ll ll lll ll l ll l '| llll l' lllll HI Il ii li lnil 10 15 20 25 30 512 626 will be described in detail in the following.

The steel of the present invention consists, by weight percent, of less than 1.5% C, 15.0-35.0% Mn and 0.1-6.0% Al, the remainder being Fe and indispensable impurities. The grain size is 40.0 glues and formability, strength and weldability are superior.

In another embodiment, the steel of the present invention, by weight percent, consists of less than 1.5% C, 15.0-35.0% Mn, 0.1-6.0% Al and one or more of the following: less less than 0.60% Si, less than 5.0% Cu, less than 1,0% Nb, less than 0,5% V, less than 0,5% Ti, less than 9,0% Cr, less than 4 .0% Ni and less than 0.12% N. The residue comprises Fe and other indispensable impurities while the grain size is less than 40 .mu.m, giving an austenitic steel with a high manganese content and superior formability, strength and weldability. The high manganese steel of the present invention is hot rolled and cold rolled sequentially.

The manufacturing process of the steel according to the present invention consists in producing a steel sheet blank which contains, calculated in weight percent, less than 1.5% C, 15.0-35.0% Mn, 0, 1-6.0% Al and the residue Fe and other unavoidable contaminants and this sheet metal blank are rolled into a hot-rolled sheet in a known manner. Alternatively, the hot-rolled steel sheet is cold-rolled, after which it is annealed at a temperature of 500-1000 ° C for 5 seconds to 20 hours, obtaining an austenitic steel with a high manganese content and excellent ductility, strength and weldability.

Alternatively, the manufacturing process for the steel of the present invention consists in producing a steel sheet blank which contains, in percentage by weight, less than 1.5% C, 15.0-35.0% Mn, 0.1-6.0% Al and one or more of the following elements: less than 0,60% Si, less than 5,0% Cu, less than 1,0% Nb, less than 0,5% V, less than 0,5% Ti, less than 9.0% Cr, less than 4.0% Ni, and less than 0.2% N. The residue consists of Fe and other unavoidable impurities, and this sheet blank is rolled into a hot-rolled steel sheet as a final product. Alternatively, the hot-rolled steel sheet is cold-rolled, after which it is annealed at a temperature of 550-1000 ° C for 5 seconds to 20 hours, whereby an austenitic steel with a high manganese content and excellent formability, strength and weldability is obtained.

Now the choice of alloy-forming substances and the addition intervals will be discussed.

Carbon (C) inhibits the formation of e-martensites by increasing the lattice error energy and thereby improving the stability of the austenite. However, if the amount is above 1.5% by weight (in the following%), the lattice error energy becomes too high, resulting in no pairs can be formed.

In addition, the solubility limit for carbon in the austenite is exceeded, resulting in highly precipitated carbides and impairing extensibility and malleability. Therefore, the amount of carbon should suitably be less than 1.5%.

Manganese (Mn) is an indispensable substance for improving the strength and for stabilizing the austenite phase. However, if the amount is less than 15.0%, an α 'martensite phase is formed, while if the amount is above 35.0%, the formation of pairs is inhibited because the addition effect is nullified. Therefore, the amount of manganese should be suitably limited to between 15.0 and 35.0%.

Aluminum (Al) raises the lattice fault energy, just like carbon, which stabilizes the austenite phase and does not form e-martensite even during strong deformation, e.g. cold rolling, but contributes to the formation of pairs. Therefore, aluminum is an important substance for improving cold formability and press formability. However, if the amount is less than 0.1%, e-martensites are formed which deteriorate the extensibility, although the strength is enhanced, which results in the cold formability and the press formability deteriorating. At the same time, if the amount exceeds 6.0%, the grating error energy is amplified too much, so that sliding deformation occurs due to a perfect dislocation. The amount of aluminum should therefore suitably be 0.1-6.0%.

As previously described, the addition of manganese and aluminum inhibits the formation of α-martensites, and excludes the possibility of e-martensites being formed, or slip deformation occurring due to perfect dislocation. At the same time, the amount of the two substances is limited to enable the formation of pairs through partial dislocations.

Silicon (Si) is a substance that is added to deoxidize and improve the strength by solution curing. If its content is greater than 0.6%, the deoxidizing effect becomes saturated and the painting properties of the sheet deteriorate during car production and cracks occur during welding. Therefore, the amount of Si should be suitably limited to less than 0.60%.

Copper (Cu) is a substance that is added to improve the corrosion resistance and increase the strength by curing in solid solution. If the amount is more than 5.0%, red embrittlement occurs, which causes inconvenience during hot rolling. The amount of Cu should therefore suitably be limited to less than 5.0%.

Niobium (Nb), tungsten (V) and titanium (Ti) are substances that are added to improve the strength by solid solution curing. If the amount of Nb is greater than 1.0%, cracks are formed during hot rolling, while in the case of the amount V is greater than 0.5%, chemical compounds with a low melting point are formed, which deteriorates the hot rolling quality. At the same time, Ti reacts with nitrogen inside the steel and precipitates nitrides, whereupon pairs are formed and strength and formability are improved. However, if the amount is above 0.5%, precipitates are formed in large amounts, so that small cracks occur during cold rolling, and formability and weldability deteriorate. Therefore, the amounts of Nb, V and Ti should be limited to 1.0%, 0.5% and 0.5% respectively.

Chromium (Cr) and nickel (Ni) are substances which are added to prevent the formation of amaritensite by stabilizing the austenite phase, and to improve the strength by solid solution curing. If the amount of Cr is less than 9.0%, the austenite phase is stabilized and the occurrence of cracks during heating of the sheet metal and hot rolling is prevented, thereby improving the hot rolling properties. However, if the amount is greater than 9.0%, oN-martensites are formed in large amounts, thereby degrading the formability. Therefore, the amount of Cr should be suitably limited to less than 9.0%. You improve the extensibility and also improve mechanical properties, e.g. impact resistance. However, if the amount exceeds 4.0%, the effect of this additive is saturated and therefore the amount should be appropriately limited to 4.0%, taking into account the economic aspect.

Nitrogen (N) precipitates nitrides when it reacts with Al during solidification, during hot rolling and during annealing after cold rolling and is therefore of central importance in the formation of pairs during the press forming of steel sheets, thereby improving formability and durability. However, if the amount exceeds 0.2%, the nitrides precipitate in a large amount and impair the extensibility and weldability. Therefore, the amount of N should suitably be below 0.2%.

The present invention will now be described with respect to the conditions of manufacture.

The steel with the previously described composition undergoes a number of production steps, Lex. melting, continuous casting (or block casting) and hot rolling. As a result, a hot-rolled steel sheet with a thickness of 1.5 - 8 mm is obtained, for use in trucks, buses and other large vehicles.

This hot-rolled steel sheet is cold-rolled and annealed to a cold-rolled sheet with a thickness of less than 1.5 mm for use primarily in motor vehicles. As far as the incandescent heat treatment is concerned, both continuous incandescent heat treatment and coffin annealing are possible. However, the continuous annealing heat treatment is preferable due to its economic advantages in mass production.

Hot rolling of the steel according to the present invention is carried out in the usual way, and suitably the reheating temperature of the blank is 1100-1250 ° C while the temperature after completion of hot rolling should be 700-1000 ° C. This water rolling temperature of 1100-1250 ° C is applied so that the sheet blank is uniformly heated within a short period of time to improve the energy yield. If the final temperature of the roll is too low, productivity decreases and therefore the lower limit should be 700 ° C. The upper limit of the final temperature of the hot rolling should be 1000 ° C as more than 10 rolling must be performed during the hot rolling process.

The cold rolling is also performed in the usual way. In the manufacture of Fe-Mn-Al-C steel, deformed austenitic grains cannot be recrystallized sufficiently if the annealing temperature is below 500 ° C. In this case, further rolled stretched grains remain and the extensibility therefore becomes too small, even though the strength is high. On the other hand, if the annealing temperature is above 1000 ° C, the austenitic grain grows to above 40.0 μm, which results in a decrease in formability. Therefore, the annealing temperature should be suitably limited to the range 500 - 1000 ° C.

If the annealing time is shorter than 5.0 seconds, the heat does not reach the inner parts of the cold-rolled sheet, resulting in complete recrystallization not forming.

In addition, in this case the cold-rolled grains remain, which reduces the permeability.

On the other hand, the time limit for the formation of coarse carbides is violated in cases where the annealing time exceeds 20 hours, and strength and durability are reduced. Therefore, the annealing time must be limited to the interval 5 seconds to 20 hours.

In the case where the Fe-Mn-Al-C steel is prepared by adding a solid solution hardener, it is desirable that the annealing temperature and time be limited to 500-1000 ° C and 5.0 seconds to 20 hours, respectively, of previous described causes.

The hot-rolled steel sheet produced by the various steps of alloying - melting - continuous casting - hot rolling according to the present invention, cold rolled and annealed, so that the size of the austenite grains is less than 40 μm, the tensile strength greater than 50 kg / mm 2 and the extensibility over 40%.

In the steel of the present invention, the formability deteriorates if the grain size is above 40 μm and therefore the annealing should be adjusted so that the grain size is reduced to below 40 μm.

The present invention will now be described in more detail by means of current examples.

Example 1 A steel having the composition of Table 1 below was melted in vacuo and 30 kg steel ingots were formed. Then a solution treatment was performed and then a rolling to form sheet blanks with a thickness of 25 mm.

The sheet blank prepared according to the previously described method was heated to 1200 ° C and hot rolling was performed, the final rolling temperature being 900 ° C.

A hot-rolled steel sheet with a thickness of 2.5 mm was produced in this hot-rolling process, and then the hot-rolled sheet was cold-rolled to a thickness of 0.8 mm.

The cold-rolled sheet was annealed at 1000 ° C for 15 minutes and an X-ray diffraction test was performed for each specimen. Then, the volume fraction of the phases was examined at room temperature and the results are shown in Table 1 below. Furthermore, the pencilability of each specimen was measured. The results are also shown in Table 1 below.

Furthermore, tensile tests were performed with the test pieces to determine tensile strength, yield strength and elongation. In addition, the evenly extended portion of the tensile specimen was cut out after the tensile tests were performed, and X-ray diffraction tests were performed with the specimen to measure the volume fractions in the stressed phase. The results are shown in Table 2 below. 512 626 ann .. 9 Table I .___ u 145111152 kmpositim 1011434) Ffšas fi šçå fi eßmm 'ïëfabí' ššäl- C Mn PS Al Ti Cr Ni (auïšte- marEten-ímaoí-égn- (H = 1ÛÛÛÛe) nit) sit 1 - 3.0 - - - 100 - - 1.0003 2 0.33 11.3 - - 3.3 - - - 100 - - 1.0003 3 0.21 13.1 - - 3.2 - _ - 100 - ~ 1.0003 4 0.33 13.1 - - 3.1; - - - 100 - - 1.0003 5 0.13 22.1 - - 1.3 - - - 100 - _. k - 1.0003 å 50.13 23.0 - - 4.0 - - - 100 - I - 1.0003 * g 10.4123.1 - - 3.5 - - - 100 - - 1.0003 ä: 30.0123.3 - - 1.1 - - - 100 - - 1.0003 J, 3 0.34 24.3 - - 1.3 - - - 100 - - 1.0003 g 10 0.13 25.3 - - 0.3 - - - 100 - - 1.0003 11 0.12 21.2 - - 3.1 - - - »100 - - 1.0003 ä 12 0.43 23.1 - - 0.5 - - - 100 - - 1.0003 13 0.05 14.4 - - 2.3 - - - 51.4 10.3 13.3 13 14 0.22 15.5 - - 0.5 - - - 11.3 12.3 15.3 ss 15 0.13 13.3 - - 0.01 - - - 31.5 3.4 - 1.0003 15 0.10 20.3 - - 3.1 - _ - 15 - 25 34 I 11011223 - - 0.01 - '- _ 33.1 1.3 - 1.0003 i 13 0.11 23.1 - - 4.3 - - - 100 -' - 1.0003 _ 13 0.15 32.2 - - 3.2 - - - 100 - _ 1.0003 Så 200.04 1.2 0.02 0.003 - - 13.3 3.3 100 - - 1.02 21 0.002 0.50 0.03 0.010 0.035 0.045 - - - - 1003 300 M 512 626 10 Table 2 Steel- * thick- Draærov ågïrsiäjrrrmâñerxšoå / é) fractions after I type lek sträck- Dfaghå11_ 1101- TE _ a - gïíïïmf »fíïšffín» "131 03:00" 23: 32- 20: 15: 2- 1 00 24.5 54.8 50.0 100 _ _ 2 10.1 50.4 51.4 100 '_ _ 0 22.0 50.0 01.1 100 _ _ 4 20.0 50 2 01. 2 100 _ _ 5 10.0 50.0 40.0 100 _ _ '0'. 10.4 40.0 40.0 100 _ 'Û- _ 1' 24.1 55.2 40.5 100 _ _ _ C 0 '10.0 50.5 50.0 100 _ _ .ä 0'. 22.0 05.4 50.0 '100 _ _ "g 10' 10.0 50.4 52.0 100 _ _ 11 '20.0 50.1 42.4 100 _ _ 12' 20.4 55.1 40.0 100 _ _ 10 '21.0 00.1 20.4 40.0 25.0 25.0 14' 20.0 00.0 14.0 44.1 10.1 42.2 15 022 01.1 101 0 «1 10 0 - 10 '25.5 51.5 01.0 52.4 _ 41.0 11 201 024 20.1 05 0 04 2 V' - 10 '21.5 50.0 01.2 100 _ _ 10' 10.0 40.0 00.0 100 _ - 20 20 5 05.5 10 2 00 _ 20 LÉÉÉ 21 10 00 42 _ _ 10021 10 15 20 25 30 512 626 11 As can be seen from Table 1 above, the steels 1 to 12 according to the present invention formed no e-martensites or of-martensites, but only one austenite phase, so they should be non-magnetic steels.

At the same time, the steels 13 to 17, which are included for comparison and which differ in manganese and aluminum content from the composition according to the present invention, formed oß-martensites which gave magnetic properties and / or e-martensitic ICT.

The conventional steel 20 and the comparative steels 18 and 19, which contain larger amounts of manganese and aluminum compared to the composition of the present invention, exhibited austenitic single phase and no magnetic properties. The conventional steel 21, which is usually an extra low carbon content, exhibited a ferrite phase (a) and was magnetic. On the other hand, the comparative steels 13 to 15 and 17 had high tensile strength but very low elongation. This is because the levels of manganese and aluminum were too low, with e-martensites and cß-martensites being formed by load-induced conversion.

The comparative steel 16 showed low elongation and this is due to the fact that the aluminum content was too high (although the manganese content was relatively low), whereby o / martensites were formed by load-induced conversion, without the presence of pairs.

Comparative steels 18 and 19 exhibited low tensile strength and low elongation, due to the addition of too much manganese and aluminum, which resulted in no martensite being formed by load-induced conversion, nor any pairs. At the same time, the conventional steel 20 which is an ordinary stainless steel exhibited a high tensile strength and high extensibility. However, it had magnetic properties due to the formation of af-martensites by load-induced conversion. At the same time, the conventional steel 21, which is a steel with an extra low carbon content, exhibited a considerably lower tensile strength than the steels 1 to 12 according to the present invention, and this is due to the fact that the conventional steel 21 has a ferrite phase.

Example 2 Compatibility green diagrams were made by experiments with steels 2 and 9 according to the present invention, the comparative steels 14 and 18 and the conventional steel 21 from Example 1. The results are shown in Fig. 2.

As can be seen from Fig. 2, the steels 2 and 9 according to the present invention show superior formability compared to the conventional steel 21 with extra low carbon content, since pairs were formed in the former. Comparative steels 14 and 18 show no acceptable compatibility because they do not form pairs.

At the same time, the steels 1 to 12 of the present invention, which met the composition range of the present invention, exhibited a yield strength of 19 to 26 kg / mm 2. a tensile strength of 50 to 70 kg / mmz and an elongation of 40 to 68%. The results are given in Table 2. In particular, the high extensibility of steels 1 to 12 according to the present invention is due to the formation of pairs by load deformation. This fact is confirmed by the electron micrograph of the steel 5 according to the present invention, shown in Fig. 3.

In Fig. 3, the white areas indicate pairs, while the black areas (matrix) indicate the austenite.

Example 3 A steel with the composition according to Table 3 was melted under vacuum, whereupon steel ingots of 30 kg were prepared. Then a solution treatment and then rolling was performed to form sheet blanks with a thickness of 25 mm. This sheet blank was heated to 1200 ° C, whereupon a hot rolling was performed with the final rolling temperature of 900 ° C, which gave rise to hot rolled sheets with a thickness of 2.5 mm. The microstructure of these hot-rolled sheets was examined to measure the size of the austenite grains, and the results of these tests are shown in Table 3-A below.

Then the hot-rolled sheets were subjected to the following examinations; yield strength, tensile strength and extensibility. After these examinations, an evenly stretched part of the tensile specimen was cut out and examined by X-ray diffractometry, whereby the volume fractions of the phases were measured. The results are shown in Table 3-A below. 512 626 13 Table3 Conical 1 Steel- T type c 1111 A1 s 11 22 0.01 15.5 0.0 _ 23 0.00 11.0 00.0 _ 24 0.21 10.1 0.2 _ 25 0.11 20.1 0.5 _ å 26 0.01 20.0 1.1 _: ä 27 1.40 25.1 0.0 _ É; 28 0.10 25.0 0.0 _ få 29 * 0.00 20.5 0.0 _ å 30 0.10 20.1 0.5 _ ä 31. 1.12 '04.1 2.5 _ 32 0.00 11.4 2.0 _ 33 010 10.0 0.01 _ 34' 0.10 20.0 0.1 _ g 35 0.11 22.0 0.02 _ ä 361.00 00.1 1.1 _ 37 '0.00 01.0 0.0 _ -åâï- 100 0.002 j 0.50 j 0.005 .00 .010 0.01? 14 512 626 NN. - - NN_ _: N NNN N_NN NN. .NN mNwN NN. - - NN_ NNN NNN NNN NN _ NN min.

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NN __ - - _. N._N NNN NNN NN _ NN NN __ - - __ N._N: N N._N NN __ NN NN .NNE - - NN_ NNN NNN ... NN NN NN NN NEÉN NNNN l: _ w 2: 5 N. .z ... Ü W: .èï WE .. Û _: NZ m: dv AßNVC wcwnßv? Élv BNC Q: -_ __ -N N ABN ÉNNNÜNNNNNN. »Šwbm NENQNÉNOV. 1.2 _ N fl a å fi m Go Nmšoäömä âümwwpo LÉBwD / N šooÜ å fi w .öäm NmcoDxmfCÉNo> wmEmmæN NN NBNE ___ _ _ _ _ _ _ _ _ __ 14, 10 15 20 25 512 626 15 As shown in Table 3-A in i. the hot-rolled steel sheets 22 to 31, which are produced according to the composition range and the hot-rolling conditions covered by the present invention, have superior properties. That is, they exhibited a tensile strength of 54 to 70 kg / mm 2 and an extensibility over 40% and this is due to the fact that deformation pairs were formed as a result of load deformation.

After the tensile strength tests, all steels 21 to 31 exhibited an austenitic single phase, and the lattice structure of the deposition pairs was a surface-centered cubic structure corresponding to the structure of the austenite phase, which resulted in it not being distinguishable by X-ray diffraction. On the other hand, in the case of the hot-rolled comparative steels 32, 33 and 35, their tensile strength was high but the extensibility was low. This is because the manganese and aluminum contents were too low, which resulted in the formation of e-martensites and of-martensites by load-induced conversion.

The hot rolled comparative steels 34 and 37 exhibited low tensile strength and low extensibility, and this is because the manganese and aluminum content was too high, so that the formation of martensite by load-induced conversion could not take place and pairs could not be formed.

At the same time, the hot-rolled comparative steel sheet 36 exhibited a high yield strength and high tensile strength, but low extensibility, and this is because the carbon content was too high with respect to any major precipitation of carbides.

Furthermore, the hot-rolled steel sheets were cold-rolled to a thickness of 0.8 mm, and these cold-rolled steel sheets were annealed at 1000 ° C for 15 minutes. Then, a microstructure test is performed on each sample piece to measure the grain size of the austenite. Tensile tests were performed to measure the yield strength, tensile strength and extensibility. Furthermore, a test piece was cut from the evenly extended part of the tensile test pieces after the tensile tests were performed, in order to subject this test piece to X-ray diffraction. In this way, the volume fractions of the phases were measured and the results of the measurements are shown in fl Table 3-B 'below.

Furthermore, the steel 24 according to the invention as shown in Table 3-B was also examined with an electron microscope, and the result of the examination is shown in Fig. ..4. 16 512 626 .ßm - - z z z z. z www müw z - z. zz zz N.NN _ z. z. z .N 2: N.NN f: ... z z ._ z: w z z zz _.z: N .z z ._ z z - z. ... z f; zz z. z k. w z N: __; Nä Nä Nz z. z »N z z Nä: N fz MIN z _. z u w z,, mi: N: N z, z ß z -. : N Nz: N z, z m ... w fi z -. ... NN Nz: N z. z m. z -. : m ... z: _ z. z 1 W NN -, ___:: N zz z, _ NN W, z - __ zz: z: _ z. z .w W .. z - _. : N Nz Nz z. z m z -, Nä zz: N z, z z -. f: zz I: z. z NN - 2: ... z zz: N z Nz NN: m: m .Éšzs šv råšv 259: 23 LÄN Name Läfwf: ß _ m5: _ fi wbmmß wcmam -monïw .NWÜM QS _ AE -Nëumaa -vambw ÉNBÉQV. -N82 -Nå uwzö fl xmf fi cï fi nš mwcumww; z ün ünömpo l fi cmuwëq. mmm: BE _. : ___ Now ... As shown in Table 3-B above, the steels 22 to 31 of the invention, which meet the composition of the present invention, had a tensile strength of 50 to 70 kg / mm 2 which is approx. twice more than that of the conventional steel 38 which had a tensile strength of 38 kg / mm 2. At the same time, the extensibility of the steels 22 to 31 was found to be over 40%, while the phase after the tensile tests turned out to be an austenitic single phase. On the other hand, the comparative steels '32, 33 and 35 showed a high tensile strength but low extensibility. This is because the levels of manganese and aluminum were too low, which resulted in e-martensites and of-martensites being formed by load-induced conversion.

At the same time, the comparative steels 34 and 37 exhibited both low tensile strength and low extensibility, and this is because the content of manganese and aluminum was too high, so that no martensite phase or pair could be formed by load-induced conversion.

At the same time, the comparative steel 36 showed a high yield strength and tensile strength, but low elongation, due to the fact that the carbon content was too high, which led to the precipitation of too many carbides.

At the same time, the conventional steel 38, which is an extra low carbon steel, exhibited a tensile strength appreciably lower than that of steel of the present invention, and this is because the steel 38 had a ferrite structure.

As previously described, the steels 22 to 31 of the present invention which meet the composition of the present invention exhibited a yield strength of 19 to 31 kg / mm 2, a tensile strength of SO to 70 kg / mm 2 and an elongation of 40 to 68%. . In particular, the high extensibility of the steels 22 to 31 of the present invention is due to the formation of pairs by tensile deformation. This can be confirmed by the electron microscope photograph of the steel 24 of the present invention, as shown in Fig. 4.

In Fig. 4, the white areas indicate pairs, while the black area indicates the austenite structure (matrix).

Example 4 The feasibility limit tests were performed with the steels 23 and 26, the comparative steel 35 and the conventional steel 38 from Example 3, and the results are shown in Fig. 5.

As can be seen from Fig. 5, the steels 23 and 26 exhibited a formability superior to that of the conventional steel 38, which was an extra low carbon content, while the comparative steel 35 exhibited a formability inferior to that of the conventional steel 38.

This is because the comparative steel 35 forms e-martensites, which degrade forrnbarll ll! Lli'l | ll Il l Il l 'tt MI! v i i: 10 15 512 626 18 while the steels 23 and 26 of the present invention exhibit superior formability due to the formation of pairs.

Example 5 A steel having the composition of Table 4 below was melted and 30 kg steel ingots were prepared. Then a solution treatment was performed, and then they were rolled into sheet blanks with a thickness of 25 mm.

In this case, the steels 39 to 40 according to the present invention and the comparative steels 54 to 60 (see Table 4) were melted under vacuum, while the ferrous steels 61 and the steels 50 to 53, which contained a large amount of nitrogen (N), were melted under ordinary atmosphere.

The sheet blank prepared as described above was heated to 1200 ° C and hot rolled to a completion temperature of 900 ° C to give 2.5 mm thick hot rolled steel sheets. These hot-rolled steel plates were examined for their microstructure, after which the size of the austenite grains was measured. The results are shown in Table 4-A below.

Furthermore, the hot-rolled steel sheets were subjected to tensile tests to determine the tensile strength, tensile strength and elongation. After performing the tensile tests, the evenly stretched portion of the tensile sample was excised and examined by X-ray diffraction, whereby the volume fractions of the phases were estimated. The results are shown in Table 4-A below. 512 626 19 ïli (Erhet: vjkt-íåš) c s1 110 141 cr 111 Cu Nb V Ti N pp 39 .13 - 10.155 - 3.3 - - - - 0.005 40 _34 _ 13.131 1.2 _ - - - - 0.005 41 .44 - 20.3 5.0. _ - - 0.2 0.4 - 0.000 .42 .35 .35 _ 22.5 1.0 _ _ _ 0.3 _ 0.010.003 43 _00 - 24.0 3.0 _ _ _ _ 0.3 0.140.003 44 .10 0.10 21.4 1.5 _ _ _ _ _ 0.150.000 45 - _35 _ 21.0 2.2 _ _ 2.1 _ _ _ _ 0.000 45 _31 _ 23.5 3.3 1.2 1.4 _ 0.1 _ _ 0.001 47_ _23 _ 32.3 2.1 _ _ 0.4 0.1 _ _ 0.000 É 4.8 _03 0.03 32.11 0.34 _ _ _ _ _ _ 0.000 å 49 .13 0.22 33.5 1.2 _ _ 2.0 _ _ _ 0.005 å: 50 _53 0.05 20.4 3.1 ._ _ _ _ _ _ _ 0.13 fi 51 .45 0.05 21.4 1.2 _ - - '- _ _ 0.03 w 52 0.35 0.01 25.0 1.2 _ _ _ _ _ _ 0.00 53 .40 0.20 20.5 2.3 _ _ _ _ _ _ _ 0.10 | 54 1.12 - 10.1 2.1 10.2 f '- - 0.01 _0.030.000 55 .13 - 10.3 1.4 - _ - - - 0.01 0.440. 001 _50 10 _ 24.4 5.4 - 1.0 - - -6 0.510.001 57 .24 _ 21.4 4.1 _ 0.4 - 1.3 _ _ 0.000 i: 58 .13 0.10 30.1 0.3 _ _ 0.4 _ _ _ 0.000 å 53 .15 0.35 32.1 3.3 1.0 _ 2.5 1.1 _ _ 0.003 50 .21 0.31 30.0 5.2 0.5 _ _ _ _ _ 0001; 610.44 0.05 27.23 - _ _ _ _ _ 023 111111051 1.. 512 626 20 Table 4-A Pšíål- Thick- Auste- Dragmnv Fasemas volymfraktiuwer plåt lek nit- _ KUTIYEHÉBPBI 'm, (m) kom tfäck- Dragwån- röj- T a _ a -_ (steel type) 3313114151 íäš fi š 12 “S5“ tw- "2113-. 4 Steel enl. 33 2.5 32 21.2 53.4 43.5 100 _ _ fimmgen 335 40 '35 25. 4 53.0 44.1 - _ ._ - 40 41' 34 21.3 51.1 40.4 '_ _' 41 42 '32 23.1 55.4 43.3 - _ '42 I få 43' 31 25.4 53. 5 44.2 _ _ '43 -š 44' 33 24.3 53.3 53.3 _ _ _ 44 _ 3- 45 '35 23. 3 50.2 40.2' _ _ _ '45 å 45 '23 25.1 50.5 42.1 _ _' 45 3 41 '34 23.2 50.3 44. 4 _ _ 41: g 43' 30 24.1 51.5 40.3 _ _ '43 Ii, 43' 33 25.2 50. 4 43. 5 _ _ '43 få 50 '35 23.1 51.1 43.1 _ _' 50 å 51 '31 23.3 53.5 45.4 _ _ 51 å 52' 30 21.4 53.0 45.0 _ _ '52 “> ° 53' 34 23. 3 55.1 45.5 _ _ '53 _., JänFdrelse- å 54 '35 33.1 30.1 15.4 33 _ 11 steel 54: ä 55' 34 21.5 53.3 11.3 100 _ _ '55 å 55' 32 25.5 54.5 23.5 100 _ _ ~ 51; å 51 '32 24.1 51.5 25.3 100 - _ '- 51 53' 31 23.4 50.3 35. 3 100 - _ - 511 Éq 53 '30 21.5 52.3 30.1 100 _ _ ° 53 É 50 '35 20.1 53.4 25.2 100 _ _' 50 å! 51 '34 25.3 53.1 25.5 100 _ - - Ü I ïi H E11 10 15 20 25 30 512 626 vi As can be seen from Table 4-A, the hot-rolled steel sheets 39 to 53 according to the present invention showed a yield strength of 22 to 30 kg / mm 2 , a tensile strength of 60 to 70 kg / mmz and an elongation of 40 to 60%.

Furthermore, the hot-rolled steel sheets 39 to 53 of the present invention contained fine austenitic grains down to 40 microns in size, while not forming e-martensites or of-martensites even after being subjected to tensile deformation, but retaining a completely austenitic phase. The reason why the steels 39 to 53 according to the present invention showed such a high elongation of over 40% is that pairs were formed during the tensile deformation.

Among the steels of the present invention, the hot rolled steels exhibited 39 to 46 and 48 to 53, in which large amounts of solid solution hardeners, e.g. Cr, Ni, Cu, Nb, V, Ti, N and the like had been added, high yield strengths and tensile strengths, higher than those of the hot rolled steel sheet 47 according to the present invention, in which the solid solution hardeners were added in smaller amounts. This is because the addition of solid solution hardeners results in increased strength.

Furthermore, among the steels of the present invention, the hot-rolled steel sheets 50 to 53 of the present invention to which nitrogen was added in large quantities exhibited higher yield strengths and higher tensile strength than the hot-rolled steel sheets 39 to 49 to which nitrogen was added in smaller amounts. This is because fine pairs are formed during the deformation caused by the aluminum nitrides formed in the solidification stage, during the hot rolling stage and during the glowing heat treatment after the cold rolling.

At the same time, the hot-rolled comparative steel plates 58 and 60, to which Cu and Si were added in a larger amount than in the composition according to the invention, showed an austenitic single phase, but their extensibility was too low. This is because non-metallic impurities and cracks formed during rolling, which helped to lower the extensibility.

Furthermore, the hot-rolled comparative steel sheets 55 to 57 and 59, to which Nb, V and Ti were added in a larger amount than in the composition according to the invention, showed low extensibility, and this is due to the fact that carbides were formed to a large extent in the steel, which lowers the extensibility.

The hot-rolled comparative steel plate 54, which contained Cr in an amount outside the composition range of the present invention, exhibited high strength but its extensibility was too low. This is because a large amount of o / -manensites were formed after the tensile deformation.

The hot-rolled comparative steel sheet 61, which contained nitrogen (N) in an amount greater than the composition range of the invention exhibited low extensibility, and this ll "l ll li -Il I ll lll l ill i 'nu ii ll llll l ri i * 10 512 626 22 may be due to too much nitrides falling out.

The hot-rolled steel sheets prepared as described above were cold-rolled to a thickness of 0.8 mm and then annealed at 100 ° C for 15 minutes.

Then an examination of their structure was performed with a microscope, to determine the size of the austenite grains, and then tensile tests were performed to determine e.g. yield strength, tensile strength and extensibility. The evenly stretched part of the tensile sample was then cut out to determine the volume fractions of the phases. Then a deep compression test was performed with a punch with a diameter of 33 mm to determine the limit drawing ratio (LDR). The results of these experiments are shown in Table 4-B below.

In Table 4-B below, the LDR value is defined as LDR = [round diameter] / [punch diameter]. The standard value for LDR for car plates, "for which good formability is required, is 1,94. On the basis of this standard, the formability was assessed according to whether a steel plate had an LDR value above or below 1.94. 512 626 23 Table 4-B r s- - AUSÜ "I Form- Fasefß vol Fraktí r: ååïïï- 0222000 0520212- ym me 25mm f 2 2222 g“ g <^ 112 ~ “: 1: °" ““: 12 “" (m) (kz / rruf) (h / mf) (%) 22 0.2 24 20.2 02.2 42.4 elnäënæ fl oo - - 22 40 22 24.2 21.2 42.5 'l 100 - - 40 41 i 21 20.2 52.1 40.2' 100 - - 41 42 22 21.2 04.2 45.0 100 - - 42 42 25 24.1 20.2 45.0 100 - - 42 å ç 44 24 22.0 25.2 21.1 100 - - 44 -š 45 21 22.0 52.4 40.2 'l 100 - - 45 ef. å, 42 22 22.1 52.2 42.5 '100 - - 40% ê 41 22 21.2 51.1 45.2' 100 - - 41 'få 42 24 22.2 52.2 42.4' l 100 - - 42 É É 42 m '22 20.4 52.2 42.2 | 100 - - 42: ä 50 ¶ 'i 21 20.5 25.1 44.0' | 100 - - 502.9; 51 22 22.2 21.1 44.2 '100 - - 51 gg 52 22 25.1 00.5 42.2 100 - - 52 ä 52 25 25.2 02.2 41.1' l 100 - - 52 š 54 25 22.1 21.2 14.0 1.24 l 21 - 12 54;, -; eller rqindre "a 55 22 22.1 01.2 12.1 '| 100 - - 55-272; 52 22 24.2 22.2 20.4 100 - _- 50 .ä _., S7 38 24.2 S fl fl 27.5 lÛÛ - - 57 iš: å 52 24 22.2 52.2 21.1 100 _ - 52 ig 52 25 20.2 22.2 21.2 100 - - sf fi ë 20 22 12.4 01.2 22.2 100 _ 100 20: å ä 20, '22 22.4 01.2 21.5 100 _ 100 513 2- 1311431 - _' Punsers diarreter lllll 10 15 20 25 As shown in Table 4-B, the steels 39 to 53 of the present invention have a yield strength of 20 to 27 kg / mm 2, a tensile strength of 57 to 66 kg / mm 2 and an extensibility of 40 to 60%.

Furthermore, the steels 39 to 49 of the present invention did not form e-martensites or of-martensites, but exhibited an austenitic single-phase structure, forming very stable steels. In addition, they showed an extensibility of over 40% and thus had an excellent formability. This is because pairs are formed during the tensile deformation.

Among the steels of the present invention, the steels exhibited 39 to 46 and 48 to 53, to which the solid solution hardeners, e.g. Cr, Ni, Cu, Nb, V, Ti, N and the like were added in large amounts, high yield strength and tensile strength compared to the steel 47 according to the present invention, to which the solid solution hardeners were added in smaller amounts. This is because the solid solution hardeners resulted in increased strength.

Furthermore, the steels 50 to 53, according to the present invention, to which nitrogen was added in large amounts, had a higher yield strength and tensile strength than the steels 39 to 49 of the present invention, to which nitrogen was added in smaller amounts. This is because the nitrides precipitated in a reaction with Al during the solidification stage, during the hot rolling stage and during the annealing after the cold rolling, and thawing pairs were formed during the deformation, caused by the aluminum nitrides.

At the same time, the iron-bearing steels 58 and 60, to which Cu and Si were added in excess of the composition range of the present invention, exhibited an austenitic single phase, but their formability was not acceptable. This is because the formability is impaired by non-metallic impurities and fine cracks formed during rolling.

Furthermore, the comparative steels 55 to 57 and 59, to which Nb, V and Ti were added in excess compared to the composition range of the invention, showed unacceptable malleability. This is because carbides formed in the steel degraded the formability.

The comparative steel 54, to which excess Cr was added compared to the composition range of the present invention, exhibited high strength but low extensibility and malleability. This is because large amounts of of-martensites were formed after the tensile deformation.

The comparative steel 61, to which nitrogen (N) was added in excess compared with the composition range of the present invention, showed deteriorated extensibility and malleability, and this is due to the fact that nitrides precipitated in large amounts. Example 6 The steel 44 of the present invention, having a composition shown in Table 4, Example 5, was hot rolled and cold rolled in the same manner as in Example 5. Then, the cold rolled steel was annealed under the annealing conditions of Table 5 in the following.

After annealing, the microstructure of the cold-rolled steel sheets was examined and tensile tests were then performed to determine the yield strength, tensile strength and extensibility. A deep pressing test was performed with a punch, 33 mm in diameter, to determine the formability. The results are shown in Table 5 below.

Table5 siödqwiriosbesingeise fi äïzfgåíïekl SträCk-Dïâïåïšll TW leaves- 9125- 54 .. 44.444 "lnïf 100-441, item.: Rid Wmäimi ikgfmv: tig / mi | 14 20 Sec 4 50.0 11.5 41.0 2.os s2 so0 ° c 1. us 42.0 2.os 20 hours s 411.4 02.0 40.1 2.os 20 sec io 40.0 11.1 53.5 2.os ss s00 ° c 1 min io 40.0 12.5 51.5 2.os å »20 sim. 15 55.9 12.4 51.1 2. os 20 sec 10 50.2 14.5 54.0 2.os É 54 s00 ° c 1 min 20 50.0 15.5 55.1 2.os få; 20 tim- 24 34.0 10.5 51.2 2.os É 20 sec 51 211.1 s5.s sofo 1.94 55 1o00 ° ci min ao 23.1 54.4 51.2 1.94 20 tim 54 25.0 55.2 51.1 1.94 88 SNC 15 mi fl- - 91.5 105.6 11.7 1.54 305101- - 05.2 101.2 0.2 HS? Som 4 sec. - 54.4 101.2 1.4: ä ao sim. 20 24.2 51.3 52.2 ä 20 sec. 5: 4 20.1 55.2 51.1 'få ss 1o5o ° c 1 min. Ss 20.4 51.0 5114 Lä 20 tim. 51 21.0 1 55.4 53.5 ._.... = "| As shown in Table 5, the steels 62 to 65 of the present invention meet the annealing conditions and composition of the present invention. The following properties: the size of the austenite grains after annealing was reduced to less than 40 .mu.m, the tensile strength, the tensile strength and extensibility were high and the formability was excellent. That is, when the annealing temperature was lower than the annealing temperature range of the present invention or when the annealing time was short, the austenitic structure would not have recrystallized to give high strength, but the extensibility and malleability were too low.In case the annealing temperature was too high or the annealing time too long, the austenitic coma was coarse, which improved extensibility but degradability due to the formation of carbides in the steel.

Example 7 The steel 44 of the present invention and the conventional steel 38 of Table 4 in Example 5 were hot rolled and cold rolled in the same manner as in Example 6, whereupon annealing was carried out at 1000 ° C for 15 minutes.

Then spot welding of the annealed steel sheets was performed as follows: a pressure of 300 kgf, a welding current of 10 kA and a current conduction time of 30 cycles (60 Hz).

The hardness tests were performed on the welded part at 0.1 mm intervals with a weight of 100 g. The results are shown in Fig. 6.

As shown in Fig. 6, the weld metal, the zone affected by the heat and the base material, the steel 44 according to the present invention, has a Vickers hardness of 250 in all three parts and this proves that the steel 44 according to the present invention has a superior weldability.

The reason why the steel 44 according to the present invention has such a superior weldability is that no brittle structural layer is formed on the zone affected by the friend. On the other hand, it was found for the conventional steel 38, that the weld metal and the zone affected by the heat had a Vickers hardness around 500, which is much higher than for the base material. This shows that its weldability is unacceptable, as brittle phases are formed on the weld metal and the zone affected by the heat.

According to the present invention, as described above, the steel according to the invention has a tensile strength of 50 to 70 kg / mm 2, which is twice higher than 512 626 27 for extra low carbon steels. This can reduce the car's weight and increase its safety. In addition, the solubility limit is very high and thus the carbon content can be increased to 1.5% so that no special treatment is necessary and no special care to increase the formability is needed during the cold rolling process. Accordingly, an austenitic steel having a high manganese content and superior formability, strength and weldability can be produced. l I llll

Claims (5)

10 15 20 25 30 512 626 m: Patent claim A high manganese austenitic steel alloy and in the form of a sheet product having either a hot-rolled or a sequential hot-rolled, cold-rolled and annealed final structure, said high-manganese steel alloy sheet comprising: a) a composition, by weight: less than 1.5% C, 15.0-35.0% Mn, 0.1-6.0% Al, more than 0% to less than 0.2% N, residual iron and unavoidable impurities, and possibly one or more elements selected from the group of less than 0.60% Si, less than 5.0% Cu, less than 1.0% Nb, less than 0.5% V, less than 0.5% Ti, less than 9.0 % Cr and less than 4.0% Ni, and b) a microstructure consisting of 100% austenite grain with a grain size below 40.0 μm and a regulated lattice error energy such that the austenitic microstructure forms deformation pairs upon deformation at room temperature but excludes image strain-induced s- and oC-martensite phases, and c) and a final tensile strength above 490.5 N / mmz [50 kg / mmz] and an elongation at break more than 40% at room temperature. A high manganese austenitic steel alloy sheet according to claim 1, comprising less than 0.7% by weight C and with additions of Mn and Al within the ranges enclosed by the ABCDEA diagram in fi g. 1. A high manganese austenitic steel alloy sheet according to claim 1, comprising less than 0.12% N and one or fl era of less than 0.60% Si, less than 5.0% Cu, less than 1.0% Nb, less than 0.5% V, less than 0.5% Ti, less than 9.0% Cr and less than 4.0% Ni. A process for the production of high manganese austenitic steel alloy sheet, comprising the steps of: a sheet metal blank having a composition of, by weight: less than 1.5% C, 15.0-35.0% Mn, 0.1-6.0% Al, more than 0% to less than 0.2% N, the residue Fe and unavoidable impurities, and optionally one or fl elements selected from the group consisting of: less than 0.60 Si, less than 5.0% Cn, less than 1.0% Nb, less than 0.5% V, less than 0.5% Ti, less than 9.0% Cr and less than 4.0% Ni; O: \ users \ MKL \ DOK \ word-dok \ 103261600.P.doc 10 512 626 27 said steel sheet blank is heated to 1000-1250 ° C; and said steel sheet is hot rolled to form a hot rolled sheet having a final temperature at the hot rolling of 700-1000 ° C, and the hot rolled sheet is cold rolled to give a cold rolled sheet, and the cold rolled sheet is annealed at a temperature of 500-1000 ° C for 5 seconds to 20 hours; said step resulting in a microstructure consisting of 100% austenite grain with a grain size below 40.0 μm in the hot rolled, cold rolled and annealed plate, and the austenite grains form deformation pairs upon deformation at room temperature but exclude the formation of elongation-induced s and ot'- martensite phases. Use of a sheet metal product according to any one of claims 1-3 or produced by the method according to claim 4, as car sheet metal or sheet metal for electronic panels. O: \ users \ MKL \ DOK \ word-dok \ 103261600.P.do <: lll ll ll ll