Hot dipped high manganese steel and manufacturing method therefor

Abstract

A hot dipped high manganese steel and a manufacturing method therefor. The high manganese steel comprises a steel base plate and a coating on the surface of the steel base plate. The core of the steel base plate is austenite. The surface layer of the steel base plate is a ferrite fine grain layer. The ferrite fine grain layer comprises an oxide of Al. Furthermore, the steel base plate of the hot dipped high manganese steel comprises, in mass percentages, 10 to 30% of Mn element, 1 to 2% of Al element, and 0.4 to 0.8% of C element. The manufacturing method comprises: 1) manufacturing strip steel; 2) primary annealing and acid washing; 3) secondary annealing and hot dipping.

Claims

1. A hot dipped high manganese steel, comprising a steel substrate and a coating on a surface of the steel substrate, wherein the steel substrate has a core structure of austenite; the steel substrate has a skin layer which is a fine ferrite grain layer; the fine ferrite grain layer comprises an Al oxide, wherein the steel substrate comprises 10 to 30% Mn, 1 to 2% Al and 0.4 to 0.8% C by mass, and a balance Fe and unavoidable impurities.

2. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a thickness of 0.2-5 μm.

3. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a grain size of ≤5 μm.

4. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a grain size smaller than a grain size of the austenite in the steel substrate.

5. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a Mn content lower than the Mn content in the steel substrate.

6. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a Mn content of ≤5%.

7. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a Mn content of ≤2%.

8. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has an Al content higher than the Al content in the steel substrate.

9. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has an Al content of >1%.

10. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has an Al content of <5%.

11. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a C content lower than the C content in the steel substrate.

12. The hot dipped high manganese steel according to claim 1, wherein the fine ferrite grain layer has a C content of 0.2%.

13. The hot dipped high manganese steel according to claim 1, wherein the coating has a thickness of 5-200 μm.

14. The hot dipped high manganese steel according to claim 1, wherein the hot dipped high manganese steel has a yield strength of 450-650 MPa, a tensile strength of 950-1100 MPa, and an elongation at break of at least 50%.

15. A method for manufacturing the hot dipped high manganese steel of claim 1, comprising the following steps: 1) Manufacturing a strip steel; 2) Primary annealing and pickling wherein the strip steel is heated on a continuous annealing production line to a soaking temperature of 600 to 750° C. for a soaking time of 30 to 600 s, wherein a mixed gas of N.sub.2 and H.sub.2 is used as an annealing atmosphere which has a H.sub.2 content of 0.5-10% by volume and a dew point of −20 to +20° C.; subsequently, the strip steel is cooled to below 100° C. after the annealing, and pickled with an acid solution having a hydrogen ion concentration of 0.1-5%, wherein the acid solution has a temperature of 50-70° C., and a pickling time is 1 to 10 s; then, the strip steel is rinsed, dried and coiled; 3) Secondary annealing and hot dipping wherein the strip steel obtained in step (2) is subjected to secondary annealing and accomplishes hot dipping on a hot dipping production line, wherein the secondary annealing is performed at a soaking temperature of 600-850° C. for a soaking time of 60-360 s in an annealing atmosphere of a mixed gas of N.sub.2 and H.sub.2, wherein the annealing atmosphere has a H.sub.2 content of 2-10% by volume, and a dew point of −60 to +10° C.; subsequently, the strip steel is cooled to 380 to 500° C., and then immersed in a plating bath to perform the hot dipping.

16. The method for manufacturing the hot dipped high manganese steel according to claim 15, wherein the soaking temperature in step (2) is 600-700° C.

17. The method for manufacturing the hot dipped high manganese steel according to claim 15, wherein the soaking time in step (2) is 30-180 s.

18. The method for manufacturing the hot dipped high manganese steel according to claim 15, wherein the annealing atmosphere in step (2) has a dew point of −10 to +10° C.

19. The method for manufacturing the hot dipped high manganese steel according to claim 15, wherein the plating bath in step (3) comprises, in mass percentage, 0.1≤Al≤6%, 0<Mg≤5%) and a balance of Zn and unavoidable impurities.

20. The method for manufacturing the hot dipped high manganese steel according to claim 15, wherein the annealing atmosphere in step (3) has a dew point of −60 to −20° C.

21. The method for manufacturing the hot dipped high manganese steel according to claim 15, wherein the plating bath in step (3) has a temperature of 420 to 480° C.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing a structure of a hot dipped high manganese steel according to the present disclosure.

(2) FIG. 2 shows a structure of the hot dipped high manganese steel according to the present disclosure before the hot dipping.

(3) FIG. 3 is a metallographic photograph showing a cross section of Example 2 according to the present disclosure.

(4) FIG. 4 is a metallographic photograph showing a cross section of Comparative Example 2.

(5) FIG. 5 is a metallographic photograph showing a cross section of Comparative Example 6.

(6) FIG. 6 shows maps of the O, Al, Mn elements in the cross-sectional metallographic phase of the skin layer after the primary annealing in Example 2.

(7) FIG. 7 shows maps of the O, Al, Mn elements in the cross-sectional metallographic phase of the skin layer after the primary annealing in Comparative Example 6.

(8) FIG. 8 shows distribution curves of the surface Mn element as a function of depth for Example 2 after the primary annealing, after the primary annealing and pickling, and after the secondary annealing, and Comparative Example 2 after the primary annealing.

DETAILED DESCRIPTION

(9) The hot dipped high manganese steel and the manufacture method thereof according to the present disclosure will be further explained and illustrated with reference to the accompanying drawings and the examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the present disclosure.

(10) FIG. 1 shows the structure of the hot dipped high manganese steel according to the present disclosure. As shown in FIG. 1, the hot dipped high manganese steel according to the present disclosure comprises a steel substrate 1 and a coating 2 on the surface of the steel substrate 1, wherein the core structure 11 of the steel substrate 1 is austenite, and the skin layer 12 of the steel substrate 1 is a fine ferrite grain layer.

(11) FIG. 2 shows a structure of the hot dipped high manganese steel according to the present disclosure before the hot dipping. As shown in FIG. 2, the core structure 11 of the steel substrate 1 is austenite, and the skin layer 12 of the steel substrate 1 is a fine ferrite grain layer, wherein the grain size of the ferrite is smaller than the grain size of the austenite in the steel substrate.

(12) Table 1 lists mass percentages of the chemical components in the hot dipped high manganese steels of Examples 1 to 20 and the conventional steel plates of Comparative Examples 1-12, wherein the balance is Fe and unavoidable impurities.

(13) As can be seen from Table 1, the mass percentage contents of the chemical components in Compositions I, II and III are controlled in the ranges of C: 0.4 to 0.8%, Mn: 10 to 30%, and Al: 1.0 to 2.0%, Si≤0.5%, P≤0.02%, S≤0.01%, N≤0.01%. The C and Mn contents in Composition IV are outside the above ranges.

(14) TABLE-US-00001 TABLE 1 (unit: wt %) C Mn Al Si N P S I 0.6 16 1.5 0.09 0.02 0.007 0.006 II 0.4 28 1.6 0.5 0.021 0.017 0.005 III 0.8 12 1.2 0.13 0.018 0.005 0.005 IV 0.3 7 1 0.2 0.011 0.008 0.01

(15) The following steps were employed for the hot dipped high manganese steels in Examples 1-20:

(16) (1) Manufacturing a strip steel;

(17) (2) Primary annealing and pickling: The strip steel was heated on a continuous annealing production line to a soaking temperature of 600 to 750° C. for a soaking time of 30 to 600 s, wherein the annealing atmosphere was a mixed gas of N.sub.2 and Hz, the H.sub.2 content was 0.5-10% by volume, and the dew point was −20 to +20° C.; the annealed strip steel was cooled to below 100° C., and pickled with an acid solution having a hydrogen ion concentration of 0.1-5%, wherein the temperature of the acid solution was 50-70° C., and the pickling time was 1 to 10 s; then, the strip steel was rinsed, dried and coiled;

(18) (3) Secondary annealing and hot dipping: the strip steel obtained in step (2) was subjected to secondary annealing and accomplished hot dipping on a hot dipping production line, wherein the soaking temperature in the secondary annealing was 600-850° C., the soaking time was 60-360 s, and the annealing atmosphere was a mixed gas of N.sub.2 and Hz, wherein the H.sub.2 content was 2-10% by volume, and the dew point was −60 to +10° C.; subsequently, the strip steel was cooled to 380 to 500° C., and then immersed in a plating bath to perform the hot dipping.

(19) Table 2 lists the specific process parameters for the hot dipped high manganese steels of Examples 1 to 20 and the conventional steel plates of Comparative Examples 1-12.

(20) FIG. 3 shows the cross-sectional metallographic phase of the hot dipped high manganese steel in Example 2 according to the present disclosure. As shown in FIG. 3, the hot dipped high manganese steel comprises a steel substrate 1, a fine ferrite grain skin layer 2 on the steel substrate and a coating 1 covering 2.

(21) FIG. 4 shows the cross-sectional metallographic phase of Comparative Example 2 in which the method for manufacturing the hot dipped high manganese steel according to the present disclosure was not utilized. As shown in FIG. 4, when the dew point of the annealing atmosphere is −40° C., a fine ferrite grain layer was not formed in the skin layer of the steel substrate. Although a coating was plated on the surface of the steel substrate, the coating adhesion was poor.

(22) FIG. 5 shows the cross-sectional metallographic phase of Comparative Example 6 in which the method for manufacturing the hot dipped high manganese steel according to the present disclosure was not utilized. As shown in FIG. 5, when the primary annealing temperature was 800° C. and the dew point of the annealing atmosphere was +10° C., although a fine ferrite grain layer was formed in the skin layer of the steel substrate, coarse oxide particles appeared in this layer, affecting the formability of the skin layer.

(23) FIG. 6 shows maps of the O, Al, Mn elements in the cross-sectional metallographic phase of the skin layer of the hot dipped high manganese steel in Example 2 after the primary annealing according to the present disclosure. As shown in FIG. 6, when the annealing temperature was 680° C., the dew point of the annealing atmosphere was 0° C., and the hold time was 170 s, after the primary annealing, a manganese oxide layer was formed on the surface of the steel plate; a manganese-lean, fine ferrite grain layer was formed under the manganese oxide layer; Al in the fine ferrite grain layer formed aluminum oxide which was mainly distributed along the ferrite grain boundary in a flake form, and the length direction of the flake was nearly perpendicular to the surface of the steel plate. Meanwhile, the oxide of Mn was not obvious in the fine ferrite grain layer.

(24) FIG. 7 shows maps of the O, Al, Mn elements in the cross-sectional metallographic phase of the skin layer of Comparative Example 6 after the primary annealing, wherein the method for manufacturing the hot dipped high manganese steel according to the present disclosure was not utilized. As shown in FIG. 7, when the annealing temperature was 800° C., the dew point of the annealing atmosphere was +10° C., and the hold time was 180 s, although a fine ferrite grain layer was formed in the skin layer of the steel substrate, the oxide of aluminum was distributed randomly in the ferrite layer, and the main morphologies were granules and strips. At the same time, Mn oxide of a large size appeared in the ferrite layer. The above features had a negative influence on the formability of the skin layer of the steel plate.

(25) FIG. 8 shows distribution curves of the surface Mn element as a function of depth for Example 2 after the primary annealing, after the primary annealing and pickling, and after the secondary annealing, and Comparative Example 2 after the primary annealing, wherein:

(26) A represents an annealed steel plate obtained in Comparative Example 2 wherein the primary annealing atmosphere had a dew point of −40° C. The manganese oxide on the surface of the steel substrate was thin, and the manganese-lean skin layer of the steel substrate was not noticeable.

(27) B represents a steel plate obtained after primary annealing in Example 2 wherein the dew point was 0° C. Manganese oxide of about 0.5 μm in thickness was present on the surface of the steel plate, and a manganese-lean skin layer of about 1 μm in thickness was present in the steel substrate. B1 was the distribution of the surface Mn element as a function of depth for the primarily annealed strip steel B after pickling, wherein the manganese oxide on the surface of the steel substrate was washed away with an acid, while the manganese-lean skin layer of the steel substrate was retained.

(28) B2 was the distribution of the surface Mn element as a function of depth for the pickled strip steel B1 after secondary annealing in step (3), wherein a small amount of Mn was enriched in the surface of the strip steel B2, but far less than that in the surface of the strip steel A. As indicated by FIG. 8 showing the variation of the surface state of the strip steel at different stages, since the enrichment of the Mn element in the surface of the strip steel B2 was far less than that in Comparative Example 2, the platability of the strip steel B2 was improved greatly.

(29) Table 3 lists the various property parameters and structural features of the hot dipped high manganese steel plates of Examples 1 to 20 and the conventional steel plates of Comparative Examples 1-12.

(30) The platability was judged by directly observing the appearance of the strip steel after plating with naked eyes. If no iron was exposed obviously on the surface, the platability was good (indicated by ∘); and if iron was exposed obviously on the surface, the platability was poor (indicated by x).

(31) The coating adhesion was tested by taking a sample having a length of 200 mm and a width of 100 mm from a strip steel, bending it to an angle of 180 degree, flattening it, and adhering an adhesive tape to the bent position. If no zinc layer was peeled off by the tape or the bent surface of the bent coating to which the tape was once adhered did not pill, it suggested that the coating adhesion was good (indicated by ∘); if the coating was peeled off by the tape or the bent surface of the bent coating to which the tape was once adhered pilled, it suggested that the coating adhesion was poor (indicated by x).

(32) As shown by Table 3, the yield strength of Examples 1-20 was 450-650 MPa, the tensile strength was 950-1100 MPa, and the elongation at break was >50%. The thickness of the fine ferrite grain layer in Example 1-20 was 0.2-5 μm, the grain size of the fine ferrite grain layer was ≤5 μm, and both the platability and the coating adhesion were superior to those of Comparative Examples 1-10.

(33) The reason is that a fine ferrite grain layer was formed on the surface of the steel substrate in step (2) in the Examples, so that the diffusion of Mn from the steel substrate to the surface of the steel plate was suppressed in step (3). This was advantageous for the formation of an effective Fe—Al barrier layer from Al and the fine ferrite grain layer in the plating bath, thereby providing good platability and coating adhesion.

(34) In addition, since the steel substrate composition and the manufacturing method defined by the present disclosure were not used for Comparative Example 11-12, despite their good platability and coating adhesion, the steel plates of Comparative Example 11-12 were not characterized by a structure in which the steel substrate was austenite, and the skin layer of the steel substrate was a fine ferrite grain layer.

(35) It is to be noted that there are listed above only specific examples of the invention. Obviously, the invention is not limited to the above examples. Instead, there exist many similar variations. All variations derived directly or envisioned from the present disclosure by those skilled in the art should be all included in the protection scope of the present disclosure.

(36) TABLE-US-00002 TABLE 2 Step (2) Step (3) Soaking Soaking H.sub.2 Dew Acid Sol. Acid Sol. Pickling Soaking Soaking H.sub.2 Dew Temp Time content Point Conc. Temp. Time Temp Time Content Point No. Composition (° C.) (s) (%) (° C.) (%) (° C.) (s) (° C.) (s) (%) (° C.) Ex. 1 I 700 290 7 −15 4 64 9 750 160 8 −42 Ex. 2 I 680 170 9 0 5 69 10 750 120 8 −56 Ex. 3 I 640 500 6 20 4 55 9 800 120 4 4 Ex. 4 I 640 590 10 −4 2 51 2 670 90 4 −3 Ex. 5 I 720 40 4 −19 3 58 4 650 140 7 2 Ex. 6 I 680 370 4 5 4 66 9 740 90 6 10 Ex. 7 I 710 370 1 10 1 63 5 680 110 4 −35 Ex. 8 I 670 140 9 12 3 61 9 840 290 7 −39 Ex. 9 I 630 350 7 20 3 50 3 850 100 4 −13 Ex. 10 I 620 180 7 8 5 59 2 620 70 2 10 Ex. 11 I 680 490 3 15 1 53 9 800 330 6 −53 Ex. 12 I 640 400 8 −13 3 58 5 700 200 7 −58 Ex. 13 II 690 180 10 11 5 61 2 620 170 6 −49 Ex. 14 II 740 110 6 −8 1 68 2 810 150 10 −55 Ex. 15 II 720 260 7 19 5 55 5 790 320 8 0 Ex. 16 II 710 420 7 −20 2 58 5 660 90 9 5 Ex. 17 III 640 530 9 −16 4 66 4 810 220 4 −52 Ex. 18 III 660 140 2 −4 1 64 7 780 360 6 −46 Ex. 19 III 610 370 5 17 4 53 5 720 120 6 −39 Ex. 20 III 660 210 5 −20 4 66 8 740 260 10 −17 Comp. I / / / / / / / 800 140 6 −10 Ex. 1 Comp. I / / / / / / / 710 60 6 −51 Ex. 2 Comp. I / / / / / / / 690 230 5 −18 Ex. 3 Comp I / / / / / / / 780 90 6 −19 Ex. 4 Comp. I 690 490 4 −30 3 59 10 770 260 2 −46 Ex. 5 Comp. I 800 130 7 10 4 60 1 640 200 2 −27 Ex. 6 Comp. II / / / / / / / 600 300 8 −43 Ex. 7 Comp. II 750 550 7 −40 5 65 8 640 80 8 −20 Ex. 8 Comp. III / / / / / / / 850 260 10 −40 Ex. 9 Comp. III 820 30 8 10 5 63 8 600 340 9 −44 Ex. 10 Comp. IV / / / / / / / 690 190 10 −20 Ex. 11 Comp. IV 600 580 2 −17 5 66 4 820 160 9 −8 Ex. 12

(37) TABLE-US-00003 TABLE 3 Thickness of Grain Size of Yield Tensile Elongation Fine Ferrite Fine Ferrite Strength Strength at Break Grain Layer Grain Layer Platability Coating No. Composition (MPa) (MPa) (%) (μm) (μm) (appearance) Adhesion Ex. 1 I 582 1056 56 0.4 0.3 ∘ ∘ Ex. 2 I 506 974 57 1.7 1.0 ∘ ∘ Ex. 3 I 645 1062 57 3.8 1.1 ∘ ∘ Ex. 4 I 473 972 58 2.3 2.2 ∘ ∘ Ex. 5 I 489 1087 61 3.4 2.2 ∘ ∘ Ex. 6 I 630 980 65 4.0 1.4 ∘ ∘ Ex. 7 I 634 1068 52 2.1 1.2 ∘ ∘ Ex. 8 I 600 957 58 4.3 3.2 ∘ ∘ Ex. 9 I 469 957 58 3.6 3.0 ∘ ∘ Ex. 10 I 532 1093 56 3.0 2.7 ∘ ∘ Ex. 11 I 578 971 53 2.5 2.2 ∘ ∘ Ex. 12 I 615 984 50 4.5 2.4 ∘ ∘ Ex. 13 II 575 960 63 1.2 1.0 ∘ ∘ Ex. 14 II 596 986 58 4.2 3.8 ∘ ∘ Ex. 15 II 529 952 62 2.9 2.5 ∘ ∘ Ex. 16 II 622 990 54 1.8 1.8 ∘ ∘ Ex. 17 III 492 975 52 0.8 1.2 ∘ ∘ Ex. 18 III 549 959 62 4.5 2.7 ∘ ∘ Ex. 19 III 458 1056 51 3.7 1.3 ∘ ∘ Ex. 20 III 537 1027 57 1.2 1.0 ∘ ∘ Comp. I 642 1100 54 0.0 / x x Ex. 1 Comp. I 624 1070 56 0.0 / x x Ex. 2 Comp. I 590 994 52 0.0 / x x Ex. 3 Comp. I 464 950 60 0.0 / x x Ex. 4 Comp. I 576 1028 50 0.0 / x x Ex. 5 Comp. I 634 983 64 1.3 1.0 ∘ x Ex. 6 Comp. II 453 1049 62 0.0 / x x Ex. 7 Comp. II 641 1045 63 0.0 / x x Ex. 8 Comp. III 622 1002 51 0.0 / x x Ex. 9 Comp. III 501 1090 52 4.0 3.0 ∘ x Ex. 10 Comp. IV 450 778 25 0.0 / ∘ ∘ Ex. 11 Comp. IV 440 720 20 0.0 / ∘ ∘ Ex. 12