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

Abstract

A cold-rolled steel plate (1) and a manufacturing method therefor. The chemical composition of the steel plate (1) in percentage by weight is: C 0.15-0.25%, Si 1.50-2.50%, Mn 2.00-3.00%, P≤0.02%, S≤0.01%, Al 0.03-0.06%, N≤0.01%, with the balance being Fe and impurities. The surface layer has an inner oxide layer (2) with a thickness of 1-5 μm, and there is no enrichment of Si or Mn on the surface. The steel plate (1) has good phosphating performance and formability, with a tensile strength of ≥1180 MPa and an elongation of ≥14%, and has a complex-phase structure of ferrite, martensite, and retained austenite, the content of the retained austenite being not lower than 5%. A dew point is at −25° C. to 10° C. in continuous annealing, such that external oxidation transitions to internal oxidation.

Claims

1. A cold-rolled high-strength steel plate having excellent phosphatability and formability, comprising chemical elements in percentage by mass of: C 0.15 to 0.25%, Si 1.50 to 2.50%, Mn 2.00 to 3.00%, P≤0.02%, S≤0.01%, Al 0.03 to 0.06%, N≤0.01%, and a balance of Fe and unavoidable impurity elements, wherein a surface layer of the 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 oxides of Si, composite oxides of Si and Mn; no Si or Mn element is enriched in the surface; the oxide particles have an average diameter of 50 to 200 nm and an average spacing λ between the oxide particles satisfying the following relationship:
A=0.247×(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 having excellent phosphatability and formability comprises a room temperature structure consisting of a composite structure of ferrite, martensite and residual austenite, and wherein the residual austenite has a content of no less than 5%; wherein after phosphating, crystals resulted from the phosphating covered the surface of the steel plate uniformly, and the crystal size is less than 10 μm, wherein the coverage area exceeds 80%; and wherein the cold-rolled high-strength steel plate has a tensile strength ≥1180 MPa, and an elongation ≥14%.

2. The cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 1, wherein the steel plate further comprises at least one of Cr 0.01 to 1.0%, Mo 0.01 to 0.5% and Ni 0.01 to 2.0%, and/or further comprises at least one of Ti 0.005 to 0.05%, Nb 0.005 to 0.1% and V 0.005 to 0.1%.

3. 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 (SiO.sub.2), manganese silicate, iron silicate and ferromanganese silicate.

4. 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 according to said chemical composition to form a slab; 2) Hot rolling and coiling Heating the slab to 1170-1300° C.; holding for 0.5-4 h; rolling, with a final rolling temperature ≥850° C.; and coiling at a coiling temperature of 400-700° C. to obtain a hot rolled coil; 3) Pickling and cold rolling Uncoiling the hot rolled coil, pickling at a speed ≤150 m/min, and cold rolling with a cold rolling reduction of 40-80% to obtain a rolled hard strip steel; 4) Continuous Annealing Uncoiling the resulting rolled hard strip steel, cleaning, heating to a soaking temperature of 790-920° C., and holding for 30-200 s, wherein a heating rate is 1-20° C./s, and an atmosphere of the heating and holding stages 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 200-300° C. at a cooling rate ≥30° 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.

5. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 4, wherein when the hot rolling in step 2) is performed, the temperature for reheating the slab is 1210-1270° C., and the coiling temperature is 450-550° C.

6. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 4, wherein in step 4), the soaking temperature is 810-870° C., and the dew point of the annealing atmosphere is from −10° C. to 5° C.

7. The cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 2, wherein the oxide particles are at least one of silicon dioxide (SiO.sub.2), 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 5, wherein in step 4), the soaking temperature is 810-870° C., and the dew point of the annealing atmosphere is from −10° C. to 5° C.

9. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 4, wherein the steel plate further comprises at least one of Cr 0.01 to 1.0%, Mo 0.01 to 0.5% and Ni 0.01 to 2.0%, and/or further comprises at least one of Ti 0.005 to 0.05%, Nb 0.005 to 0.1% and V 0.005 to 0.1%.

10. The manufacturing method for the cold-rolled high-strength steel plate having excellent phosphatability and formability according to claim 4, wherein the oxide particles are at least one of silicon oxide, manganese silicate, iron silicate and ferromanganese silicate.

Description

DESCRIPTION OF THE DRAWING

(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 the present disclosure, wherein 1 represents a steel plate, 2 represents an inner oxide layer, and 3 represents oxide particles.

(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 a steel plate, and 2 represents an inner oxide layer in the surface layer of the steel plate.

(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.

(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 present 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 present disclosure.

EXAMPLES AND COMPARATIVE EXAMPLES

(7) Cold-rolled high-strength steel plates having excellent phosphatability and formability in Examples 1-16 according to the present disclosure and steel plates in Comparative Examples 1-5 were obtained by the following steps:

(8) Table 1 lists the mass percentages (%) of the chemical elements in Examples 1-16 and Comparative Examples 1-5, with the rest being Fe.

(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. and held for 1 h, followed by hot rolling. 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 500° C., pickled and cold-rolled with a cold rolling reduction of 52%. The final thickness of the rolled hard strip steel was 1.2 mm.

(10) The resulting rolled hard strip steel was uncoiled, cleaned, and annealed, wherein the annealing process and atmosphere conditions employed in the Examples and Comparative Examples are shown in Table 2. Then, the annealed, cold-rolled high-strength steel plates were evaluated for mechanical properties, residual austenite content, inner oxide layer thickness in the surface layer, average diameter of oxide particles, average spacing between particles and phosphatability, 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 1180 MPa or higher, an elongation of 14% or higher, and a residual austenite content of no less than 5% in the room temperature structure and had good formability. At the same time, by controlling the dew point of the annealing atmosphere, a 1-5 μm 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 less than 10 μm, 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., wherein 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 755° C., lower than 790° C. required by the design. In the soaking process, austenization was not sufficient. In the subsequent cooling and heating processes, residual austenite couldn't be stabilized in a sufficient amount. Therefore, the strength and elongation of the material were rather low.

(15) In Comparative Example 4, 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.

(16) As known from a combination of Tables 1 and 3, the silicon content of Comparative Example 5 was rather low, and its elongation was unable to reach 14%. 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.

(17) 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.

(18) 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.

(19) 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.

(20) 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.

(21) 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% and the phosphated crystal size was less than 10 μm, the phosphatability was judged to be good (OK); and conversely, the phosphatability was judged to be poor (NG).

(22) 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.

(23) TABLE-US-00001 TABLE 1 No. C Si Mn P S Al N Cr Mo Ti Nb V A 0.16 1.6 2.5 0.009 0.003 0.045 0.0057 0.5 — 0.02  — — B 0.23 1.5 2.9 0.015 0.004 0.033 0.0037 — 0.1 — 0.03  — C 0.18 1.7 2.5 0.01 0.006 0.04 0.0065 0.2 0.15 — — 0.05  D 0.2  1.8 2.3 0.008 0.007 0.052 0.0043 — 0.2 0.015 0.015 — E 0.14 1.2 2.3 0.011 0.002 0.032 0.0023 — 0.05 — 0.015 0.025

(24) 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 −15 840 120 250 375 240 Ex. 2 A −10 875 100 220 400 60 Ex. 3 A  10 822 55 280 420 120 Ex. 4 A  3 800 150 200 393 170 Ex. 5 B  7 902 60 260 405 150 Ex. 6 B −11 834 100 240 390 103 Ex. 7 B  −2 796 180 292 430 208 Ex. 8 B  0 850 120 245 410 180 Ex. 9 C −10 810 125 235 403 140 Ex. 10 C −14 869 84 275 442 220 Ex. 11 C  5 893 105 290 385 167 Ex. 12 C  10 827 200 228 400 160 Ex. 13 D  0 805 140 210 405 100 Ex. 14 D −10 904 79 240 394 235 Ex. 15 D −10 845 104 283 420 127 Ex. 16 D  −5 820 197 255 368 80 Comp. Ex. 1 A −40 832 90 270 410 100 Comp. Ex. 2 B −20 840 100 150 390 90 Comp. Ex. 3 C −10 755 120 260 375 170 Comp. Ex. 4 D  15 900 105 280 425 20 Comp. Ex. 5 E  0 850 60 240 405 200

(25) TABLE-US-00003 TABLE 3 Mechanical Thickness Oxide Average Residual Properties of Inner Particle Interparticle Austenite YS TS TEL Oxide Layer Diameter Spacing Content No. Composition (MPa) (MPa) (%) (μm) (nm) (nm) (%) Phosphatability Ex. 1 A 920 1212 16.2 1.5  50  73 10  OK Ex. 2 A 975 1244 14.1 3.1 168 245 7 OK Ex. 3 A 830 1206 18.1 2.9 152 222 12  OK Ex. 4 A 817 1195 17.4 2.3 148 216 8 OK Ex. 5 B 1127 1370 14.5 4.2 191 286 6 OK Ex. 6 B 1038 1289 15.3 1.8 140 210 7 OK Ex. 7 B 806 1211 14.7 2.3 135 202 5 OK Ex. 8 B 1079 1293 15.1 2.1 114 171 7 OK Ex. 9 C 872 1191 17   1.7 128 189 9 OK Ex. 10 C 1010 1203 15.8 1.6 150 222 10  OK Ex. 11 C 1050 1237 14.6 3.7 185 274 8 OK Ex. 12 C 903 1196 17.2 3.5 178 263 9 OK Ex. 13 D 880 1224 15.2 2.4 110 162 8 OK Ex. 14 D 1083 1258 14.5 2.5 167 245 6 OK Ex. 15 D 975 1243 16.1 1.9  80 118 10  OK Ex. 16 D 902 1219 17.2 2.3 121 178 11  OK Comp. Ex. 1 A 850 1172 15.2 0    0  0 6 NG Comp. Ex. 2 B 1142 1407 11.6 1.5  61 127 0 OK Comp. Ex. 3 C 790 1162 13.1 1.1  72  73 3 OK Comp. Ex. 4 D 1082 1279 12.8 8.2 389  32 5 NG Comp. Ex. 5 E 976 1177 10.9 2.2 102  87 3 OK