Cold-rolled high-strength steel plate having excellent phosphating performance and formability and manufacturing method therefor

Abstract

A cold-rolled high-strength steel plate having excellent phosphating performance and formability and a manufacturing method therefor. The chemical composition of the steel plate is, in percentage by weight, C 0.01-0.20%, Si 1.50-2.50%, Mn 1.50-2.50%, P≤0.02%, S≤0.02%, Al 0.03-0.06%, N≤0.01%, the remainder being Fe and impurities. The surface layer of the steel plate has an inner oxide layer with a thickness of 1-5 μm, and there is no enrichment of Si and Mn on the surface of the steel plate. The steel plate has tensile strength of ≥980 MPa and an elongation of ≥20%. The structure at the room temperature contains retained austenite, ferrite, and martensite and/or bainite.

Claims

1. A cold-rolled high-strength steel plate having excellent phosphatability and formability, comprising chemical elements in percentage by mass of: C 0.10 to 0.20%, Si 1.50 to 2.50%, Mn 1.50 to 2.50%, P 0.02%, S 0.02%, Al 0.03 to 0.06%, N 0.01%, and a balance of Fe and unavoidable impurity elements, wherein a surface layer of the cold-rolled high-strength steel plate comprises an inner oxide layer having a thickness of 1 to 5 μm; the inner oxide layer comprises iron as a matrix; the matrix comprises oxide particles which are at least one of Si oxides, composite oxides of Si and Mn; and no Si or Mn element is enriched in the surface of the steel plate; wherein the oxide particles have an average diameter of 50 to 200 nm and an average spacing A between the oxide particles satisfying the following relationship:
A=0.115×(0.94×[Si]+0.68×[Mn]).sup.1/2×d
B=1.382×(0.94×[Si]+0.68×[Mn]).sup.1/2×d
A≤λ≤B wherein [Si] is the content % of Si in the steel; [Mn] is the content % of Mn in the steel; and d is the diameter of the oxide particles in nm; wherein the cold-rolled high-strength steel plate has a tensile strength ≥980 MPa, and an elongation ≥20%, wherein a room temperature structure of the cold-rolled high-strength steel plate has a residual austenite content of 5-15%, a ferrite content of ≤35%, and a balance of martensite and/or bainite, and wherein the steel plate comprises a surface covered with phosphate crystals in a coverage area exceeding 80%.

2. The cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 1, wherein the oxide particles are at least one of silicon dioxide (S.sub.iO.sub.2), manganese silicate, iron silicate and ferromanganese silicate.

3. A manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 1, comprising the following steps: 1. Smelting and casting Smelting and casting the chemical composition of claim 1 to form a slab; 2. Hot rolling and coiling Heating the slab to 1170-1300° C.; holding for 0.5-4h; rolling, with a final rolling temperature 850° C.; and coiling at a coiling temperature of 400-600° C. to obtain a hot rolled coil; 3. Pickling and cold rolling Uncoiling the hot rolled coil, pickling at a speed of 80-120 m/min, and cold rolling with a cold rolling reduction of 40-80% to obtain a rolled hard strip steel; 4. Continuous Annealing Uncoiling and cleaning the resulting rolled hard strip steel; Heating to a soaking temperature of 800-930 ° C., and holding for 30-200 s, wherein a heating rate is 1-20 ° C/s, and an atmosphere of the heating and holding stage is a N.sub.2-H.sub.2 mixed gas, wherein a H.sub.2 content is 0.5-20%; wherein a dew point of an annealing atmosphere is from −25° C. to 10° C.; Then rapid cooling to 180-280° C. at a cooling rate 50° C/s; Then reheating to 350-450° C. and holding for 60-250 s to obtain the cold-rolled high-strength steel plate having excellent phosphatability and formability.

4. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 3, wherein when the hot rolling in step 2) is performed, the temperature for reheating the slab is 1210-1270° C.

5. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 3, wherein the coiling temperature in step 2) is 450-520° C.

6. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 3, wherein the dew point of the annealing atmosphere is from −15° C. to 0° C.

7. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 3, wherein oxide particles of the cold-rolled high-strength steel plate having excellent phosphatability and formability are at least one of silicon oxide, manganese silicate, iron silicate and ferromanganese silicate.

8. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 7, wherein when the hot rolling in step 2) is performed, the temperature for reheating the slab is 1210-1270° C.

9. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 7, wherein the coiling temperature in step 2) is 450-520° C.

10. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 7, wherein the dew point of the annealing atmosphere is from −15° C. to 0° C.

11. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 7, wherein a room temperature structure of the cold-rolled high-strength steel plate has a residual austenite content of 5-15%, a ferrite content of no more than 35% and a balance of martensite and/or bainite.

12. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 10, wherein the cold-rolled high-strength steel plate has a tensile strength ≤980 MPa, and an elongation ≤20%.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing an inner oxide layer in a surface of a cold-rolled high-strength steel plate according to an embodiment of the present disclosure, wherein 1 represents a steel plate, 2 represents oxide particles, and 3 represents an inner oxide layer.

(2) FIG. 2 is an SEM (scanning electron microscopy) backscattered electron image of a cross-section of a cold-rolled high-strength steel plate according to an embodiment of the present disclosure, wherein 1 represents an inner oxide layer in the surface layer of the steel plate, and the arrows indicate oxide particles.

(3) FIG. 3 is an SEM secondary electron image of a surface of a phosphated cold-rolled high-strength steel plate according to an embodiment of the present disclosure.

(4) FIG. 4 is an SEM backscattered electron image of a cross-section of a cold-rolled high-strength steel plate of Comparative Example 1, in which the surface layer of the steel plate has no internal oxide layer.

(5) FIG. 5 is an SEM secondary electron image of a surface of a phosphated cold-rolled high-strength steel plate of Comparative Example 1.

DETAILED DESCRIPTION

(6) The cold-rolled high-strength steel plate having excellent phosphatability and formability and the manufacturing method thereof according to the disclosure will be further explained and illustrated with reference to the accompanying drawings and the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

EXAMPLES AND COMPARATIVE EXAMPLES

(7) Cold-rolled high-strength steel plates having excellent phosphatability and formability in Examples 1-20 according to the present disclosure and steel plates in Comparative Examples 1-6 were obtained by the following steps:

(8) Table 1 lists the mass percentages (%) of the chemical elements in the steel of Examples 1-20 and Comparative Examples 1-6.

(9) A steel material having a composition shown in Table 1 was smelted and cast to form a slab. The slab was heated at a heating temperature of 1250° C. for 1 h, and then hot rolled. Finish rolling was fulfilled at a final rolling temperature of 900° C. or higher. The hot-rolled steel plate had a thickness of about 2.5 mm. The hot-rolled steel plate was coiled at 450° C., pickled and cold-rolled with a cold rolling reduction of 60%. The final thickness of the rolled hard strip steel was 1.0 mm.

(10) The resulting rolled hard strip steel was uncoiled, cleaned, annealed, and then evaluated for mechanical properties, residual austenite content, ferrite content, inner oxide layer thickness in the surface layer, average diameter of oxide particles, average spacing between particles and phosphatability of the cold-rolled high-strength steel plate after the annealing. The annealing process and atmosphere conditions employed in the Examples and Comparative Examples are shown in Table 2, and the evaluation results are shown in Table 3.

(11) As can be seen from Table 3, all the Examples with the annealing process of the present disclosure used had a tensile strength of 980 MPa or higher, an elongation of 20% or higher, and a residual austenite content of 5-15% and a ferrite content of no higher than 50% in the room temperature structure. At the same time, by controlling the dew point of the annealing atmosphere, an inner oxide layer existed in the surface layer of the steel plate. The characteristics of the inner oxide layer are shown in FIGS. 1-2. After phosphating, the phosphated crystals covered the surface of the steel plate uniformly, and the crystal size was small, wherein the coverage area exceeded 80%, indicating excellent phosphatability, as shown by FIG. 3.

(12) As known from a combination of Tables 2 and 3, the dew point of Comparative Example 1 was −40° C., far lower than the lower limit designed by the present disclosure, and no inner oxide layer was formed in the surface (see FIG. 4). Instead, Si and Mn were enriched in the surface of the steel plate. Therefore, after phosphating of the steel plate, phosphated crystals only appeared in local areas of the surface, the crystal size was large, and most of the surface was not covered by phosphated crystals, indicating poor phosphatability, as shown by FIG. 5.

(13) The rapid cooling temperature of Comparative Example 2 was 100° C., far lower than the designed lower limit. The austenite was all transformed into martensite, and thus there was no residual austenite. Therefore, the strength of the steel plate was rather high, and the elongation was rather low.

(14) The soaking temperature of Comparative Example 3 was 770° C., lower than 800° C. required by the design, and the ferrite content during annealing was rather high. Therefore, the strength of the material was rather low.

(15) The reheating temperature of Comparative Example 4 was 500° C. which exceeded the designed upper limit, and the martensite in the steel plate was significantly tempered and softened, resulting in a decrease in strength and elongation.

(16) In Comparative Example 5, due to the use of a dew point exceeding the upper limit designed by the present disclosure, the inner oxide layer in the surface of the steel plate was rather thick, which affected the tensile strength and elongation of the material. At the same time, the excessively high dew point caused reenrichment of Si and Mn elements in the surface of the steel plate. As a result, the phosphatability of the steel plate began to deteriorate again.

(17) As known from a combination of Tables 1 and 3, the silicon content of Comparative Example 6 was less than 1.5%, and its elongation was unable to reach 20%. This is because the Si content did not reach the designed lower limit. Therefore, during the annealing process, the content of the residual austenite was insufficient, resulting in a low elongation.

(18) Tensile test method was as follows: A No. 5 tensile test specimen under JIS was used, and the tensile direction was perpendicular to the rolling direction.

(19) Method of measuring a residual austenite content: A specimen of 15×15 mm in size was cut from a steel plate, ground, polished, and tested quantitatively using XRD.

(20) Method of measuring a ferrite content: A specimen of 15×15 mm in size was cut from a steel plate, ground, polished, and analyzed quantitatively using EBSD.

(21) Method of measuring a thickness of an oxide layer in a surface layer of a steel plate: Steel plates were sampled along their cross-sections. After grinding and polishing, the cross-sectional morphologies were observed for all the steel plate samples at a magnification of 5000 times under a scanning electron microscope.

(22) Method of measuring an average diameter and an average spacing of oxide particles in an oxide layer: A steel plate was sampled along its cross-section. After grinding and polishing, 10 fields of view were observed randomly at a magnification of 10000 times under a scanning electron microscope, and an image software was used to calculate the average diameter and average spacing of the oxide particles.

(23) Method of evaluating phosphatability of a steel plate: An annealed steel plate was subjected to degreasing, water washing, surface conditioning and water washing in order, and then phosphated, followed by water washing and drying. The phosphated steel plate was observed in 5 random fields of view at a magnification of 500 times under a scanning electron microscope, and an image software was used to calculate the area not covered by the phosphated film. If the uncovered area was less than 20%, the phosphatability was judged to be good (OK); and conversely, the phosphatability was judged to be poor (NG).

(24) TABLE-US-00001 TABLE 1 C Si Mn P S Al N A 0.15 2.1 2.0 0.012 0.002 0.037 0.0031 B 0.12 1.7 2.5 0.010 0.005 0.053 0.0035 C 0.18 1.7 2.3 0.013 0.006 0.040 0.0041 C 0.16 1.9 2.1 0.008 0.009 0.032 0.0043 E 0.20 1.5 1.8 0.009 0.010 0.060 0.0052 F 0.17 1.2 2.2 0.015 0.006 0.045 0.0037

(25) TABLE-US-00002 TABLE 2 Annealing Process Dew point of annealing Soaking Soaking Rapid cooling Reheating Reheating atmosphere temperature time temperature temperature time No. Composition (° C.) (° C.) (s) (° C.) (° C.) (s) Ex. 1 A −15 840 120 240 375 240 Ex. 2 A −10 875 100 220 400 60 Ex. 3 A  10 822 55 280 420 120 Ex. 4 A  5 800 150 180 393 170 Ex. 5 B  5 902 60 260 405 150 Ex. 6 B −10 834 100 240 390 103 Ex. 7 B  0 805 180 280 430 208 Ex. 8 B  10 850 120 210 410 180 Ex. 9 C −10 810 125 215 403 140 Ex. 10 C −15 869 84 275 442 220 Ex. 11 C  5 893 105 240 385 167 Ex. 12 C  10 827 200 200 400 160 Ex. 13 D  0 805 140 210 405 100 Ex. 14 D −10 904 79 240 394 235 Ex. 15 D −10 845 104 280 420 127 Ex. 16 D  −5 820 197 255 368 80 Ex. 17 E −15 889 115 240 405 100 Ex. 18 E  5 860 75 180 412 175 Ex. 19 E  0 850 129 220 385 130 Ex. 20 E  10 820 102 260 430 110 Comp. Ex. 1 A −40 830 90 270 410 80 Comp. Ex. 2 B −15 820 150 100 400 140 Comp. Ex. 3 C −10 770 120 260 375 170 Comp. Ex. 4 D  0 850 75 250 500 110 Comp. Ex. 5 E  15 900 105 280 425 120 Comp. Ex. 6 F −15 830 120 260 410 210

(26) TABLE-US-00003 TABLE 3 Thickness of Average Residual Mechanical Properties Inner Oxide Oxide Particle Interparticle Austenite Ferrite YS TS TEL Layer Diameter Spacing Content Content No. (MPa) (MPa) (%) (μm) (nm) (nm) (%) (%) Phosphatability Ex. 1 678 1047 22.7 1.5  84 115 13  30 OK Ex. 2 770 1067 21.1 2.3  99 135 9 20 OK Ex. 3 622 1009 23.1 4.3 187 256 12  35 OK Ex. 4 579 1035 20.1 2.8 112 153 7 40 OK Ex. 5 800 1054 20.5 3.1 147 200 5 10 OK Ex. 6 671 1012 22.1 2.2 107 145 11  28 OK Ex. 7 592  987 24.1 2.7 132 179 15  31 OK Ex. 8 775 1032 20.6 3.9 173 235 9 26 OK Ex. 9 622 1017 25.2 2.1  79 105 14  25 OK Ex. 10 710 1089 20.6 1.7  64  85 6 18 OK Ex. 11 830 1092 20   2    70  93 5 15 OK Ex. 12 724 1053 20.7 3.9 171 228 8 27 OK Ex. 13 602  986 23.6 2.6 134 180 13  35 OK Ex. 14 830 1089 20   2.5 120 161 5  5 OK Ex. 15 730 1028 21.5 2.2 120 161 10  20 OK Ex. 16 654 1011 20.9 2.6 129 173 9 25 OK Ex. 17 810 1058 20.7 1.9 101 123 7 10 OK Ex. 18 800 1072 20.1 3.4 155 188 7 20 OK Ex. 19 793 1047 20.5 2.9 127 154 8 22 OK Ex. 20 702  994 21.3 4.2 180 219 10  27 OK Comp. Ex. 1 626 1044 21.8 0    0  0 10  30 NG Comp. Ex. 2 800 1161 13.4 1.5  90 122 0 30 OK Comp. Ex. 3 526  971 20.1 2.3 105 140 6 60 OK Comp. Ex. 4 605  962 19.3 3.1 150 201 8 25 OK Comp. Ex. 5 830 1131 13.7 7.6 275  23 5  5 NG Comp. Ex. 6 740 1018 17.3 1.3  87 102 4 35 OK