STEEL SHEET WITH EXCELLENT BAKE HARDENING PROPERTIES AND CORROSION RESISTANCE AND METHOD FOR MANUFACTURING SAME

Abstract

Provided is a steel sheet by weight percentage (wt %), carbon (C): 0.005 to 0.08%, manganese (Mn): 1.25% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), soluble aluminum (sol.Al): 0.01 to 0.06%, chromium (Cr): 1.15 to 2.5%, antimony (Sb): 0.1% or less (excluding 0%), at least one selected from the group consisting of nickel (Ni): 0.3% or less (excluding 0%), silicon (Si): 0.3% or less (excluding 0%), molybdenum (Mo): 0.2% or less (excluding 0%), and boron (B): 0.003% or less (excluding 0%), and a remainder of iron (Fe) and other unavoidable impurities, satisfying Expression 1: 1.3≤Mn(wt %)+Cr(wt %)/1.5+Sb(wt %)≤2.7, where Mn, Cr, and Sb refer to contents (wt %) of corresponding elements, respectively; and 1 to 5% of martensite and a remainder of ferrite by an area percentage (area %).

Claims

1. A steel sheet comprising: by weight percentage (wt %), carbon (C): 0.005 to 0.08%, manganese (Mn): 1.25% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), soluble aluminum (sol.Al): 0.01 to 0.06%, chromium (Cr): 1.15 to 2.5%, antimony (Sb): 0.1% or less (excluding 0%), at least one selected from the group consisting of nickel (Ni): 0.3% or less (excluding 0%), silicon (Si): 0.3% or less (excluding 0%), molybdenum (Mo): 0.2% or less (excluding 0%), and boron (B): 0.003% or less (excluding 0%), and a remainder of iron (Fe) and other unavoidable impurities, satisfying Relational Expression 1 below,
1.3≤Mn(wt %)+Cr(wt %)/1.5+Sb(wt %)≤2.7  Relational Expression 1: where Mn, Cr, and Sb refer to contents (wt %) of corresponding elements, respectively; and 1 to 5% of martensite and a remainder of ferrite by an area percentage (area %).

2. The steel sheet of claim 1, further comprising: a hot-dip galvanized layer formed on a surface of the steel sheet.

3. The steel sheet of claim 2, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

4. The steel sheet of claim 1, further comprising: a yield strength of 210 to 270 MPa and a yield ratio (YS/TS) of 0.6 or less.

Description

MODE FOR INVENTION

Example

[0123] After preparing a steel slab having an alloying composition shown in Table 1, a hot-dip galvanized steel sheet (a GI steel sheet) or an alloyed hot-dip galvanized steel sheet (a GA steel sheet) was manufactured using the manufacturing process shown in Tables 2 and 3. In this case, hot-dip galvanization was performed using a conventional hot-dip zinc-based plating bath and an alloying heat treatment was also performed under a conventional condition (500° C. to 540° C.).

[0124] For the reference, Inventive Steels 1, 2, 4 and 5 and Comparative Steels 1 and 2 in Table 1 correspond to alloyed hot-dip galvanized steel sheets, and Inventive Steels 3, 6 and 7 in Table 1 correspond to hot-dip galvanized steel sheets. Comparative steel 1 is a BH steel using usually ultra-low carbon steel, and Comparative Steel 2 is steel of a series of high-carbon DP steels.

[0125] For each of the above-mentioned plating steel sheets, a microstructure was observed, physical properties were evaluated, and results thereof are shown in Table 4 below.

[0126] In Table 4, martensite and bainite were observed at a point of ¼t (t: steel sheet thickness (mm)) through Lepelar corrosion using an optical microscope and were re-observed using a SEM (magnification: 3,000×). Sizes and distribution amounts of the martensite and the bainite were measured using values averaged three times through a Count Point operation, and a phase, except for these structures, was estimated as the content of ferrite. In Table 4, a tensile test was performed on each specimen in a direction C using the JIS standard. In Table 4, YS denotes yield strength and YR denotes a yield ratio.

[0127] On the other hand, a low-bake hardening property (L-BH) was measured under a baking condition (170° C.×20 min) and was evaluated as a difference in yield strength after 2% pre-strain. An anti-aging property (YP-EI (%)) was measured during the tensile test after being maintained for an hour at a temperature of 100° C. When no YP-EI appeared, it was evaluated to have excellent anti-aging properties at room temperature.

[0128] An unplated evaluation was made by observation with naked eyes, and a relative evaluation was made to have grades 1 to 5 based on the degree of occurrence of an unplated one. The grades 1 and 2 mean that the quality of external panel materials is secured.

[0129] A corrosion resistance evaluation was made by cutting a steel plate in a size of 75×150 mm and then masking edges with a tape for a salt pray corrosion resistance evaluation. Then, prepared specimens entered a salt spray tester having a spray pressure of 0.098±0.0025 MPa and a spraying amount of 1.0 to 2.0 ml per hour. At a point in time since 1,000 hours have passed, the degrees of generation of red rust were compared with each other. The standards for the evaluation were given as below.

[0130] ⊚: less than 5% of a red rust generation area, ◯: 5 to 20% of the red rust generation area, Δ: 21 to 50% of the red rust generation area, and x: more than 50% of the red rust generation area

TABLE-US-00001 TABLE 1 Type of Composition of Cold-Rolled Steel Sheet (wt %) Steel C Mn Si Cr P S N Sol. Al Ni Sb Mo B IS1 0.006 0.33 0.003 2.31 0.003 0.006 0.003 0.021 0.005 0.005 — 0.0004 IS2 0.007 0.55 0.004 2.18 0.003 0.005 0.003 0.034 0.01 0.009 — 0.0006 IS3 0.010 0.72 0.003 1.75 0.003 0.004 0.002 0.045 — 0.018 0.07 — IS4 0.012 0.75 0.021 1.82 0.002 0.004 0.003 0.043 — 0.053 — 0.0021 IS5 0.024 0.82 0.022 1.56 0.001 0.006 0.004 0.052 — 0.061 0.18 0.0028 IS6 0.031 0.93 0.008 1.69 0.003 0.004 0.005 0.026 — 0.01 0.08 0.0008 IS7 0.070 1.15 0.007 1.15 0.005 0.006 0.003 0.041 — 0.04 0.03 0.0009 CS1 0.0018 0.15 0.003 — 0.006 0.004 0.002 0.035 — — — — CS2 0.09 1.85 1.2 — 0.005 0.007 0.004 0.045 — — 0.21 0.0018 IS: Inventive Steel/CS: Comparative Steel

TABLE-US-00002 TABLE 2 Cold Rolling Initial Hot Rolling Cold Finish Cooling Rolling Hot Rate Cold Stand Reheating Rolling Coiling before Reduction Reduction Type of Temperature Temperature Temperature Coiling Ratio Ratio Steel (° C.) (° C.) (° C.) (° C./sec) (%) (%) IS1 1185 886 569 25 54 29 1186 892 552 33 53 26 IS2 1187 904 655 27 65 28 1187 908 646 31 63 26 IS3 1214 895 717 43 73 30 1212 896 715 55 76 33 IS4 1195 887 534 81 38 31 1196 928 584 19 75 34 IS5 1224 846 634 18 57 21 1228 914 637 8 42 39 IS6 1187 894 676 37 83 35 1194 895 653 36 74 34 IS7 1206 899 674 45 34 36 1207 888 652 56 63 33 CS1 1189 915 672 38 74 45 CS2 1201 891 535 34 63 37 1207 898 674 28 63 35 IS: Inventive Steel/CS: Comparative Steel

TABLE-US-00003 TABLE 3 Cooling Annealing Primary Secondary Final Annealing Concentration Primary Cooling Secondary Cooling Final Cooling Temperature of Hydrogen Cooling Termination Cooling Termination Cooling Termination Type of (° C.) in Furnace Rate Temperature Rate Temperature Rate Temperature Steel (° C.) (%) (° C./sec) (° C.) (° C./sec) (° C.) (° C./sec) (° C.) Note IS1 782 5.1 2.6 640 4.6 460 4.3 Room IE1 Temp. 782 4.5 2.5 640 4.6 460 5.1 Room IE2 Temp. IS2 776 5.2 3.5 640 4.1 460 6.2 Room IE3 Temp. 790 6.1 3.8 640 4.1 460 6.9 Room IE4 Temp. IS3 813 5.8 4.3 640 5.2 460 9.8 Room IE5 Temp. 812 5.2 9.3 640 6.1 460 9.3 Room IE6 Temp. IS4 675 6.1 5.6 640 9.1 460 5.3 Room CE1 Temp. 831 3.5 5.8 640 8.4 460 7.8 Room IE7 Temp. IS5 680 3.5 11.2 640 11.4 460 2.5 Room CE2 Temp. 833 3.1 8.5 640 12.1 460 6.4 Room CE3 Temp. IS6 841 2.5 7.7 640 8.9 460 5.3 Room CE4 Temp. 834 38.1 15.5 640 7.2 460 8.1 Room CE5 Temp. IS7 845 3.5 6.7 640 2.8 460 27 Room CE6 Temp. 835 3.2 15.5 640 21 460 8.4 Room CE7 Temp. CS1 843 3.6 4.8 640 6.5 460 5.2 Room CE8 Temp. CS2 815 4.4 4.9 640 6.5 460 5.3 Room CE9 Temp. 781 4.5 4.1 640 7.8 460 3.8 Room CE10 Temp. IS: Inventive Steel/CS: Comparative Steel IE: Inventive Example/CE: Comparative Example

TABLE-US-00004 TABLE 4 {circle around (5)} Type of YP-EI L-BH YS YR {circle around (4)} Corrosion Steel {circle around (1)} {circle around (2)} {circle around (3)} (%) (MPa) (MPa) (MPa) Unplated Resistance IS1 2.5 0 1.88 0 38 221 0.54 1 ⊚ IE1 2.2 0 1.88 0 42 213 0.52 1 ⊚ IE2 IS2 3.1 0 2.01 0 42 224 0.53 1 ⊚ IE3 3.2 0 2.01 0 38 222 0.57 1 ⊚ IE4 IS3 3.2 0 1.90 0 45 232 0.54 1 ⊚ IE5 4.3 0 1.90 0 47 234 0.52 1 ⊚ IE6 IS4 3.2 0 2.02 0.38 53 224 0.52 3 ◯ CE1 2.1 0 2.02 0 51 223 0.57 1 ⊚ IE7 IS5 67 04 1.92 0.35 48 256 0.66 3 ◯ CE2 4.5 07 1.92 0.32 46 255 0.66 3 ◯ CE3 IS6 4.6 0.6 2.07 0.31 42 261 0.68 3 ◯ CE4 67 0 2.07 0.41 47 263 0.62 4 ◯ CE5 IS7 1.8 0.3 1.96 0.28 47 287 0.58 4 ◯ CE6 2.1 12 1.96 0.18 33 283 0.56 5 Δ CE7 CS1 0 0 0.15 0 0 181 0.73 2 ⊚ CE8 CS2 8.3 37 1.85 0 45 287 0.65 4 Δ CE9 9.1 2.5 1.85 0 39 291 0.66 4 Δ CE10 IS: Inventive Steel/CS: Comparative Steel IE: Inventive Example/CE: Comparative Example (In Table 4, {circle around (1)} refers to a martensite area ratio (%), {circle around (2)} refers to a bainite area ratio (%), {circle around (3)} refers to a value of Relation Expression 1: Mn(wt %) + Cr(wt %)/1.5 + Sb(wt %), {circle around (4)} refers to an unplated evaluation (grades 1 and 2: excellent, grades 3 and 4: average, and grade 5: poor), and {circle around (5)} refers to a result of salt spray corrosion resistance evaluation).

[0131] As can be seen from Tables 1 to 4, in the case of Inventive Examples 1 to 7 satisfying the alloy composition and manufacturing conditions of the present disclosure, yield strength has a range of 210 to 270 MPa, no yield point elongation (YP-EI) appeared during a tensile test after performing a heat treatment on specimens under the condition of 100° C.×60 min, and thus, anti-aging properties and baking hardening property were excellent, a yield ratio (YS/TS) was 0.6 or less, grades thereof were grades 1 and 2, the level of an external panel, during an unplated determination, and corrosion resistance was evaluated to be in the best grade.

[0132] Meanwhile, as can be seen from Tables 1 to 4, in Comparative Examples 1 to 10, not satisfying at least one of the alloying composition and the manufacturing conditions of the present disclosure, at least one of the physical properties such as yield strength, yield ratio, baking hardening property, corrosion resistance, and anti-aging property was deteriorated or insufficient.