Method for manufacturing high manganese hot-dip galvanized steel sheet with excellent coatability and ultra-high strength, and high manganese hot-dip galvanized steel sheet manufactured by said method

Abstract

There are provided a method for manufacturing a high manganese hot-dip galvanized steel sheet having superior coatability and ultra-high strength, and a high manganese hot-dip galvanized steel sheet manufactured by the method. The method includes: cold rolling a hot-rolled steel sheet to form a cold-rolled steel sheet, the hot-rolled steel sheet including, by wt %, C: 0.3% to 1%, Mn: 8% to 25%, Si: 0.1% to 3%, Al: 0.01% to 8%, Cr: 0.1% to 2%, Ti: 0.01% to 0.2%, B: 0.0005% to 0.01%, Ni: 0.01% to 2%, Sn: 0.06% to 0.2%, and a balance of Fe and inevitable impurities; heating the cold-rolled steel sheet; cooling the heated steel sheet to, or maintaining the heated steel sheet; and performing a hot-dip galvanizing process on the cooled or temperature-maintained steel sheet by dipping the cooled or temperature-maintained steel sheet into a hot-dip galvanizing bath.

Claims

1. A high manganese hot-dip galvanized steel sheet comprising, by wt %, carbon (C): 0.3% to 1%, manganese (Mn): 8% to 25%, silicon (Si): 0.1% to 3%, aluminum (Al): 0.01% to 8%, chromium (Cr): 0.1% to 2%, titanium (Ti): 0.01% to 0.2%, boron (B): 0.0005% to 0.01%, nickel (Ni): 0.01% to 2%, tin (Sn): 0.06% to 0.2%, and a balance of iron (Fe) and inevitable impurities, wherein discontinuous particle-shaped Si oxides of a thickness of 5 m to 10 m are formed on the steel sheet before the steel sheet is hot-dip galvanized, and non-galvanized regions of the high manganese hot-dip galvanized steel sheet have an average diameter of 1 mm or less, including 0 mm, and wherein the high manganese hot-dip galvanized steel sheet has a tensile strength of 1200 MPa or greater and an elongation of 7% or greater.

2. The high manganese hot-dip galvanized steel sheet of claim 1, wherein the non-galvanized regions have an average diameter of 0.5 mm or less.

3. The high manganese hot-dip galvanized steel sheet of claim 1, wherein the non-galvanized regions have an average diameter of 0.3 mm or less.

4. A high manganese hot-dip galvanized steel sheet manufactured by a method comprising cold rolling a hot-rolled steel sheet at a reduction ratio of 30% to 60% to form a cold-rolled steel sheet; heating the cold-rolled steel sheet under conditions of a dew point temperature ranging from 30 C. to 80 C. and a heating temperature ranging from 450 C. to 650 C.; cooling the heated steel sheet to, or maintaining the heated steel sheet at, a temperature within a range of 450 C. to 500 C.; and performing a hot-dip galvanizing process on the cooled or temperature-maintained steel sheet by dipping the cooled or temperature-maintained steel sheet into a hot-dip galvanizing bath containing aluminum (Al) in an amount of 0.13 wt % to 0.25 wt % and maintained at a temperature of 450 C. to 500 C., wherein the high manganese hot-dip galvanized steel sheet comprises, by wt %, carbon (C): 0.3% to 1%, manganese (Mn): 8% to 25%, silicon (Si): 0.1% to 3%, aluminum (Al): 0.01% to 8%, chromium (Cr): 0.1% to 2%, titanium (Ti): 0.01% to 0.2%, boron (B): 0.0005% to 0.01%, nickel (Ni): 0.01% to 2%, tin (Sn): 0.06% to 0.2%, and a balance of iron (Fe) and inevitable impurities, wherein discontinuous particle-shaped Si oxides of a thickness of 5 m to 10 m are formed on the steel sheet before the steel sheet is hot-dip galvanized, and non-galvanized regions of the high manganese hot-dip galvanized steel sheet have an average diameter of 1 mm or less, including 0 mm, and wherein the high manganese hot-dip galvanized steel sheet has a tensile strength of 1200 MPa or greater and an elongation of 7% or greater.

Description

MODE FOR INVENTION

(1) Hereinafter, the present disclosure will be described more specifically through examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

EXAMPLE 1

(2) High manganese steel including 0.55% C, 15% Mn, 0.1% Si, 1.9% Al, 0.28% Cr, 0.09% Ti, 0.001% B, 0.25% Ni, and 0.06% Sn was continuously cast to form a slab, and the slab was heated to 1100 C., finish hot rolled at 950 C., and coiled at 450 C. to form a hot-rolled steel sheet having a thickness of 2.81 mm. The hot-rolled steel sheet was pickled with a 15% aqueous hydrochloric acid solution, and cold-rolled steel sheets were prepared by performing a cold rolling process on the hot-rolled steel at different reduction ratios as illustrated in Table 1 below. Tensile test samples were prepared using the cold-rolled steel sheets prepared as described above according to JIS 5, and the yield strength, tensile strength, and elongation of the samples were measured as illustrated in Table 1 below.

(3) TABLE-US-00001 TABLE 1 Thickness of cold-rolled Yield Tensile Total Reduction steel sheet strength strength elongation No. ratio (%) (mm) (MPa) (MPa) (%) *CS 1 0 2.81 454.3 836.0 57.0 CS 2 8.5 2.57 605.3 889.4 45.2 CS 3 13.5 2.43 742.8 964.3 33.7 CS 4 20 2.25 776.5 1002.3 28.7 CS 5 24 2.14 887.9 1114.6 20.6 **IS 1 30 1.97 1017.8 1248.9 13.8 IS 2 34 1.85 1074.6 1308.6 11.2 IS 3 37 1.77 1103.8 1383.5 10.4 IS 4 40 1.69 1134.7 1419.7 8.5 IS 5 42 1.63 1179.5 1460.7 8.3 IS 6 46 1.52 1234.4 1521.4 7.7 IS 7 51 1.38 1240.2 1536.9 7.6 *CS: Comparative Sample, **IS: Inventive Sample

(4) As illustrated in Table 1 above, each of Inventive Samples 1 to 7 satisfying the reduction ratio range of the present disclosure had a tensile strength of 1200 MPa or more and an elongation of 7% or more owing to an austenite single phase and a high strain hardening rate thereof.

(5) However, Comparative Samples 1 to 5 not satisfying the reduction ratio range of the present disclosure do not have ultra-high strength due to a low strain hardening rate.

EXAMPLE 2

(6) Cold-rolled steel sheets prepared at a reduction ratio of 51% in the same manner as Example 1 were processed through a heat treatment process under a reducing atmosphere containing 5% hydrogen and the balance of nitrogen, and having a dew point temperature of 40 C. The heat treatment process was performed for 40 seconds at different heating temperatures as illustrated in Table 2 below, and then the steel sheets were cooled to 480 C. and were dipped in a hot-dip galvanizing bath containing 0.23% aluminum (Al) and having a temperature of 450 C. for 5 seconds. Thereafter, the amount of coating material on each of the coated steel sheets was adjusted to be 60 g/m.sup.2 by using an air knife. In this manner, hot-dip galvanized steel sheets were prepared. The thickness and shape of surface oxides formed on each of the hot-dip galvanized steel sheets were measured, and the coating quality of the hot-dip galvanized steel sheets was evaluated according to the following grades. The results of the measurement and evaluation are illustrated in Table 2 below. Tensile test samples were prepared using the hot-dip galvanized steel sheets according to JIS 5, and the yield strength, tensile strength, and elongation of the samples were measured as illustrated in Table 2 below.

(7) The degrees of coating failures of the hot-dip galvanized steel sheets were evaluated by capturing images of coated surfaces after the hot-dip galvanizing process and measuring the sizes of non-coated regions, and the results of the evaluation were indicated by the grades explained below. Grade 1: no non-coated regions Grade 2: average diameter of non-coated regions less than 1 mm Grade 3: average diameter of non-coated regions within a range of 1 mm to less than 2 mm Grade 4: average diameter of non-coated regions within a range of 2 mm to less than 3 mm Grade 5: average diameter of non-coated regions 3 mm or greater

(8) After performing a bending test on the hot-dip galvanized steel sheets using an overlay tester (OT), a taping test was performed on the outer sides of the bent portions of the hot-dip galvanized steel sheets to observe the separation of coating layers from the hot-dip galvanized steel sheets. The degree of separation of the coating layers was evaluated according to the following grades. Grade 1: no separation Grade 2: separation less than 5% Grade 3: separation within a range of 5% to less than 10% Grade 4: separation within a range of 10% to less than 30% Grade 5: separation 30% or more

(9) TABLE-US-00002 TABLE 2 Coating quality Mechanical properties Surface oxides Coating Adhesion of HT YS TS E T failures (non coating (good No. ( C) (MPa) (MPa) (%) (m) Shape 1 5 high) 1 5 poor) CS 6 400 1293.86 1497.61 8.34 5 P Grade 3 Grade 3 IS 8 450 1254.19 1445.09 10.65 5 P Grade 1 Grade 1 IS 9 500 1195.52 1377.25 11.23 5 P Grade 1 Grade 1 IS 10 550 1047.06 1325.54 13.94 7 P Grade 1 Grade 1 IS 11 600 1019.08 1228.63 16.95 7 P Grade 1 Grade 1 IS 12 650 1010.14 1205.24 20.35 10 P Grade 2 Grade 1 CS 7 700 669.21 997.54 37.79 30 F Grade 3 Grade 4 CS 8 720 617.66 963.5 42.09 30 F Grade 3 Grade 4 CS 9 750 567.83 933.79 49.3 50 F Grade 4 Grade 5 CS 10 780 549.41 917.26 48.53 70 F Grade 4 Grade 5 CS 11 800 530.6 907.55 55.27 70 F Grade 4 Grade 5 CS: Comparative Sample, IS: Inventive Sample, HT: Heating Temperature, YS: Yield Strength, TS: Tensile Strength, E: Elongation, T: Thickness, P: Particles, F: Film

(10) As illustrated in Table 2, in Inventive Samples 8 to 12 satisfying conditions of the present disclosure such as the heating temperature range, a thin layer of particle-shaped surface oxides was formed owing to the low-temperature heat treatment, and thus surface defects such as coating failures and separation were not observed. In addition, recrystallization did not occur in Inventive Samples 8 to 12. Thus, Inventive Samples 8 to 12 had properties of ultra-high-strength steel: an ultra-high strength of 1200 MPa or greater and an elongation of 7% or more.

(11) Although Comparative Sample 6 had superior mechanical properties, coating failures were observed because Comparative Sample 6 had a temperature lower than the temperature of the hot-dip galvanizing bath due to a significantly low heating temperature, and thus had low degrees of wettability and reactivity with molten zinc. In addition, an inhibition layer was poorly formed, and thus a coating layer was separated.

(12) Surface oxides were formed in the form of thick films on Comparative Samples 7 to 11 due to a high-temperature annealing process. Thus, coating failures occurred or coating separation was observed in a later machining process. In addition, the tensile strength and yield strength of Comparative Samples 7 to 11 were very low. That is, Comparative Samples 7 to 11 lost their ultra-high-strength properties.