COMPLEX-PHASE STEEL HAVING HIGH HOLE EXPANSIBILITY AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed in the present invention is complex-phase steel having high hole expansibility. The complex-phase steel has a microstructure of ferrite and bainite. The complex-phase steel having high hole expansibility comprises the following chemical elements in percentage by mass: C: 0.06-0.09%, Si: 0.05-0.5%, Al: 0.02-0.1%, Mn: 1.5-1.8%, Cr: 0.3-0.6%, Nb≤0.03%, Ti: 0.05-0.12%, and the balance of Fe and inevitable impurities. In addition, also disclosed in the present invention is a manufacturing method for the foregoing complex-phase steel having high hole expansibility. The method comprises the following steps: (1) smelting and casting; (2) heating; (3) hot-rolling; (4) phosphorous removal; (5) laminar cooling: a relaxation time period is controlled to be 0-8 s, and a laminar cooling rate is 40-70° C./s; (6) coiling; (7) leveling; and (8) pickling. The complex-phase steel having high hole expansibility can simultaneously satisfy the requirements for hole expansibility and good plasticity.

Claims

1. A complex-phase steel having high hole expansibility, wherein the microstructure of the complex-phase steel having high hole expansibility is ferrite+bainite, and mass percentages of chemical elements of the complex-phase steel having high hole expansibility are: C: 0.06-0.9%, Si: 0.05-0.5%, Al: 0.02-0.1%, Mn: 1.5-1.8%, Cr: 0.3-0.6%, Nb≤0.03%, Ti: 0.05-0.12%, and a balance of Fe and inevitable impurities.

2. The complex-phase steel having high hole expansibility according to claim 1, wherein the Nb content is 0.015-0.03%.

3. The complex-phase steel having high hole expansibility according to claim 1, wherein in the inevitable impurities, P≤0.03%, S≤0.02%, and N≤0.005%.

4. The complex-phase steel having high hole expansibility according to claim 1, wherein the mass percentage contents of chemical elements satisfy one of the following formulas:
0.2%≤Cr−0.5(Si+Al)≤0.42%;
0.08%≤3.3Nb+Ti≤0.20%.

5. The complex-phase steel having high hole expansibility according to claim 1, wherein the microstructure has microalloy precipitates, which include (Ti, Nb)C and NbN.

6. The complex-phase steel having high hole expansibility according to claim 1, wherein a tensile strength and the mass percentage contents of chemical elements satisfy: tensile strength Rm=343+789×C+170×Si+132×Mn+195×Cr+843×(Nb+Ti)−207×Al, wherein the dimension of the tensile strength Rm is Mpa.

7. The complex-phase steel having high hole expansibility according to claim 6, wherein the complex-phase steel having high hole expansibility has a transverse tensile strength of ≥780Mpa, a yield strength of ≥700Mpa, an elongation rate A.sub.50 of ≥5%, and a punching hole expansion rate of ≥50%.

8. The complex-phase steel having high hole expansibility according to claim 1, wherein the complex-phase steel having high hole expansibility has a transverse tensile strength of ≥800Mpa, a yield strength of ≥730Mpa, an elongation rate A.sub.50 of ≥15%, and a punching hole expansion rate of ≥70%.

9. A method for manufacturing the complex-phase steel having high hole expansibility of claim 1, comprising the following steps: (1) Smelting and casting; (2) Heating; (3) Hot rolling: a total reduction rate is controlled to be ≥80%, a rough rolling is controlled to be rolled in a recrystallization area, and a rough rolling outlet temperature is 1020-1100° C.; a quasi constant speed rolling process is adopted in a finish rolling process, a finish rolling speed is controlled at 6-12 m/s, and a steel rolling acceleration is controlled to be ≤0.005 m/s.sup.2; a finish rolling temperature is controlled at 840-900° C.; (4) Phosphorus removal; (5) Laminar cooling: a relaxation time is controlled at 0-8 s and a cooling rate of laminar cooling is controlled at 40-70° C./s; (6) Coiling; (7) Flattening; (8) Pickling.

10. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein in step (2), a heating temperature is 1200-1260° C.

11. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein in step (4), a phosphorus removal pressure is controlled to be 15-35Mpa.

12. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein in step (6), a coiling temperature is 480-560° C.

13. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein in step (7), a flattening rolling force is controlled to be 100-800 tons, and a flattening elongation rate meets 1.5%.

14. The complex-phase steel having high hole expansibility according to claim 4, wherein a tensile strength and the mass percentage contents of chemical elements satisfy: tensile strength Rm=343+789×C+170×Si+132×Mn+195×Cr+843×(Nb+Ti)−207×Al, wherein the dimension of the tensile strength Rm is MPa.

15. The complex-phase steel having high hole expansibility according to claim 5, wherein a tensile strength and the mass percentage contents of chemical elements satisfy: tensile strength Rm=343+789×C+170×Si+132×Mn+195×Cr+843×(Nb+Ti)−207×Al, wherein the dimension of the tensile strength Rm is MPa.

16. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein the Nb content is 0.015-0.03%.

17. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein the mass percentage contents of chemical elements satisfy one of the following formulas: 0.2%≤Cr−0.5(Si+Al)≤0.42%, and 0.08%≤3.3Nb+Ti≤0.20%.

18. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, a tensile strength and the mass percentage contents of chemical elements satisfy: tensile strength Rm=343+789×C+170×Si+132×Mn+195×Cr+843×(Nb+Ti)−207×Al, wherein the dimension of the tensile strength Rm is MPa.

19. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein the complex-phase steel having high hole expansibility has a transverse tensile strength of ≥780 MPa, a yield strength of ≥700 MPa, an elongation rate A.sub.50 of ≥15%, and a punching hole expansion rate of ≥50%.

20. The method for manufacturing the complex-phase steel having high hole expansibility according to claim 9, wherein the complex-phase steel having high hole expansibility has a transverse tensile strength of ≥800 MPa, a yield strength of ≥730 MPa, an elongation rate A.sub.50 of ≥15%, and a punching hole expansion rate of ≥70%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] FIG. 1 is a metallographic microstructure photo of the complex-phase steel having high hole expansibility of Example 1.

[0061] FIG. 2 is an SEM microstructure photo of the complex-phase steel having high hole expansibility of Example 1.

[0062] FIG. 3 is a schematic diagram of the surface morphology of the surface oxide scale of the strip steel with good surface.

[0063] FIG. 4 is a schematic diagram of the surface morphology of the surface oxide scale of the strip steel with surface NG1.

[0064] FIG. 5 is a schematic diagram of the change of mechanical properties of the complex-phase steel having high hole expansibility of Example 3 under different flattening deformation.

DETAILED DESCRIPTION

[0065] The complex-phase steel having high hole expansibility of the present disclosure and its manufacturing method will be further explained and illustrated with reference to the drawings and specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the present disclosure.

Examples 1-7 and Comparative Examples 1-6

[0066] The complex-phase steels having high hole expansibility of Examples 1-7 and its manufacturing method and the comparison steel plates of Comparative Examples 1-6 are prepared by the following steps:

[0067] (1) Smelting and casting were carried out according to the chemical composition shown in Table 1. Converter steelmaking was adopted. The molten steel was subject to RH vacuum degassing treatment and LF furnace desulfurization treatment, wherein P≤0.015% and S≤0.005% were controlled. During continuous casting, the degree of superheat, secondary cooling water and appropriate soft reduction were controlled to control the central segregation of continuous casting slab.

[0068] (2) Heating: the heating temperature is 1200-1260° C.

[0069] (3) Hot rolling: the total reduction rate was controlled to be ≥80%, the rough rolling was controlled to be rolled in the recrystallization zone, and the rough rolling outlet temperature was 1020-1100° C.; quasi constant speed rolling process was adopted in the finish rolling process, the finish rolling speed was controlled at 6-12 m/s, and the steel rolling acceleration was controlled to be ≤0.005 m/s.sup.2; the finish rolling temperature was controlled at 840-900° C.

[0070] (4) Phosphorus removal: the phosphorus removal pressure was controlled at 15-35 MPa.

[0071] (5) Laminar cooling: the relaxation time was controlled at 0-8 s and the cooling rate of laminar cooling was controlled at 40-70° C./s.

[0072] (6) Coiling: the coiling temperature is 480-560° C.

[0073] (7) Flattening: the flattening rolling force was controlled to be 100-800 tons, and the flattening elongation met ≤1.5%.

[0074] (8) Pickling: the pickling speed was controlled at 60-100m/min, and the temperature of the last pickling tank in the pickling process was controlled at 80-90° C. and the iron ion concentration was controlled at 30-40 g/L.

[0075] Table 1 shows the mass percentage of each chemical element of the complex-phase steels having high hole expansibility of Examples 1-7 and its manufacturing method and the steel plates of Comparative Examples 1-6.

TABLE-US-00001 TABLE 1 (unit: weight %, and a balance of Fe and inevitable impurities other than P, S and N) Cr-0.5 3.3Nb + Predicted No C Si Mn Cr Nb Ti Al (Si + Al) Ti Rm/MPa Example 1 0.07 0.25 1.65 0.45 0.015 0.08 0.03 0.31 0.13 820 Example 2 0.07 0.2  1.77 0.45 0 0.1  0.03 0.34 0.10 832 Example 3 0.09 0.12 1.65 0.42 0.03 0.05 0.02 0.35 0.15 797 Example 4 0.06 0.5  1.53 0.6  0 0.08 0.08 0.31 0.08 845 Example 5 0.07 0.12 1.79 0.3  0.02 0.12 0.04 0.22 0.19 823 Example 6 0.06 0.05 1.79 0.45 0.015  0.075 0.02 0.42 0.12 795 Example 7 0.07 0.45 1.68 0.55 0  0.075 0.1 0.28 0.08 846 Comparative 0.07 0.8  1.65 0.45 0.015 0.08 0.03 0.14 0.12 914 Example 1 Comparative 0.07 0.25 1.65 0.22 0.015 0.08 0.03 0.08 0.13 775 Example 2 Comparative 0.07 0.2  1.77 0.45 0 0.04 0.03 0.34 0.04 781 Example 3 Comparative 0.07 0.25 1.65 0.45 0.015 0.08 0.03 0.31 0.13 820 Example 4 Comparative 0.07 0.25 1.65 0.45 0.015 0.08 0.03 0.31 0.13 820 Example 5 Comparative 0.07 0.25 1.65 0.45 0.015 0.08 0.03 0.31 0.13 820 Example 6

[0076] Table 2 shows the specific process parameters of the complex-phase steels having high hole expansibility of Examples 1-7 and its manufacturing method and the steel plates of Comparative Examples 1-6.

TABLE-US-00002 TABLE 2 Rough Finish Phos- Heating Rolling Steel Rolling phorus Cooling Coiling Flat- Tem- Outlet Finish Rolling Tem- Re- Re - Rate of Tem- Flat- tening pera- Tem- rolling Acceler- Re- pera- moval lax- Laminar pera- tening Elon- ture/ pera- speed/ ation/m/ duction ture/ Pressure/ ation Cooling ture/ Rolling gation No. ° C. ture/° C. m/s s.sup.2 Rate/% ° C. MPa time/s ° C./s ° C. Force/ton Rate/% Example 1230 1060 9 0.003 98.4 880 20 4 50 520 148 0.2 1 Example 1250 1070 11 0.003 99.0 890 20 3 60 520 223 0.3 2 Example 1200 1020 6 0.003 97.3 840 20 8 50 540 574 0.8 3 Example 1220 1030 7 0.003 97.5 850 35 7 40 560 706 1.0 4 Example 1260 1100 12 0.003 99.2 900 30 0 60 500 154 0.2 5 Example 1230 1050 8 0.003 98.2 860 15 5 70 480 146 0.2 6 Example 1250 1080 10 0.003 98.5 890 30 2 50 500 451 0.6 7 Com- 1230 1060 9 0.003 98.4 880 20 4 50 520 136 0.2 parative Example 1 Com- 1230 1060 9 0.003 98.4 880 20 4 50 520 136 0.2 parative Example 2 Com- 1250 1070 11 0.003 98.4 890 20 3 50 520 198 0.3 parative Example 3 Com- 1150 1060 9 0.003 98.4 880 20 4 50 520 131 0.2 parative Example 4 Com- 1230 1060 9 0.003 98.4 880 20 4 50 430 160 0.2 parative Example 5 Com- 1230 1060 9 0.003 98.4 880 20 4 50 520 785 1.8 parative Example 6

[0077] According to the test method of hole expansion rate specified in ISO/DIS16630 standard, the size of the experimental sample was 150×150 mm, punching size was Φ10 mm, the clearance was set as 12.5%, the hole was punched from the shear plane with a 60° conical heavy head, and the inner diameter d was calculated when the crack passed through the plate thickness. If the inner diameter before punching is set to do, the limit hole expansion value λ% is calculated from the following formula. Limit hole expansion value λ%=(d−d.sub.0)/d.sub.0×100%. Tensile standard: the transverse JIS 5 #tensile sample was taken to measure the mechanical properties; 180° bending performance was conducted according to GB/T232-2010 standard.

[0078] Table 3 shows the mechanical property test results of the complex-phase steels having high hole expansibility of Examples 1-7 and its manufacturing method and the steel plates of Comparative Examples 1-6.

TABLE-US-00003 TABLE 3 Thickness/ Predicted Rp0.2/ Rm/ A50/ λ/ 180° Cold mm Rm/MPa Mpa Mpa % % Bending Example 1 3.5 820 742 824 17.5 88 1.5 a Example 2 2.2 832 743 833 16.2 87 1.5 a Example 3 6.0 797 718 793 18.5 82 1.5 a Example 4 5.5 845 706 789 20.1 58 1.5 a Example 5 1.8 823 771 859 15.6 78 1.5 a Example 6 4.0 795 732 812 15.1 76 1.5 a Example 7 3.2 846 752 845 16.8 85 1.5 a Comparative 3.5 914 812 895 11.2 54 2.5 a Example 1 Comparative 3.5 775 678 765 17.5 53 1.5 a Example 2 Comparative 3.5 781 661 759 16.8 76 1.5 a Example 3 Comparative 3.5 820 653 768 17.6 82 1.5 a Example 4 Comparative 3.5 820 798 923 11.4 34 2.5 a Example 5 Comparative 3.5 820 785 863 14.2 68 2.0 a Example 6

[0079] It can be seen from Table 3 that the transverse tensile strength of the complex-phase steel having high hole expansibility of each example of the disclosure is ≥780Mpa, the yield strength is ≥700Mpa, the elongation rate A50≥15%, and the punching hole expansion rate is ≥50%.

[0080] It can be seen combined with Table 1 that Cr−0.5(Si+Al) in Comparative Example 1 does not meet the requirement of 0.2%≤Cr−0.5(Si+Al)≤0.42%. Compared with Example 1, the two adopted the same process system, but the Si content is higher in Comparative Example 1, so it is easy to form fayalite (2FeO—SiO.sub.2) iron oxide scale, which is difficult to remove, and it is difficult to obtain strip steel with high-grade surface. At the same time, because the red iron scale on the surface is difficult to control, it is difficult to measure accurately in the process of hot rolling temperature measurement, resulting in unstable product performance. The strength of the region having fayalite (2FeO—SiO.sub.2) is too high and the elongation is low. In Table 1, Cr−0.5(Si+Al) in Comparative Example 2 does not meet the requirement of 0.2%≤Cr−0.5(Si+Al)≤0.42%. Compared with Example 1, the two adopted the same process system, but Comparative Example 2 is not conducive to the transformation of bainite structure, and a large number of polygonal ferrite and pearlite exists in the structure, which is not conducive to the improvement of strength and hole expansion rate. In Table 1, comparing Comparative Example 3 with Example 2, it can be found that the Ti content of Comparative Example 3 is low, which does not meet the requirement of 0.08%≤3.3Nb+Ti≤0.20%. The two adopted the same process system, but the grain refinement effect is less and the precipitation strengthening effect is weak in Comparative Example 3, and the tensile strength can not reach more than 780Mpa.

[0081] In addition, it can be seen combined with table 2 that in Comparative Example 4, the heating temperature is relatively low, which is not conducive to the solid solution of Ti and Nb, the precipitation of fine carbides of Nb and Ti in the subsequent cooling and coiling process, and the improvement of strength. A Low coiling temperature is adopted in Comparative Example 5, and there will be a certain amount of martensite in the undercooled structure, which is not conducive to the improvement of elongation and hole expansion rate. A large amount of flatness is adopted in Comparative Example 6, and the elongation loss is 3.4% compared with Example 1.

[0082] Comparing the effects of different surface states of hot rolling on the uniformity of mechanical properties, the composition and process of Example 4 were adopted, and steel strips with different surface states were obtained by setting different phosphorus removal pressure. The worse the surface treatment effect, the greater the surface roughness, the higher the corresponding strength and the lower the elongation.

[0083] Table 4 lists the effects of different surface states on mechanical properties. In addition, FIGS. 3 and 4 show the morphology of different surface states respectively. Among them, FIG. 3 shows the surface morphology of the surface oxide scale of the strip steel with good surface, and FIG. 4 shows the surface morphology of the surface oxide scale of the strip steel with surface “NG1”.

TABLE-US-00004 TABLE 4 Phosphorus Thick- Removal Surface ness/ Pressure/ Roughness/ Rp0.2/ Rm/ A50/ mm Mpa μm Mpa Mpa % Good 3.5 20 1.33 706 789 20.1 Surface Surface 3.5 8 4.78 835 897 13.5 NG1 Surface 3.5 5 5.34 864 937 11.8 NG2 Surface 3.5 9 3.15 760 856 14.5 NG3

[0084] FIG. 1 is the metallographic microstructure photo of the complex-phase steel having high hole expansibility of Example 1.

[0085] FIG. 2 is the SEM microstructure photo of the complex-phase steel having high hole expansibility of Example 1.

[0086] It can be seen combined with FIGS. 1 and 2 that the microstructure of the complex-phase steel having high hole expansibility of the present disclosure is ferrite+bainite, and the microstructure has microalloy precipitates, which include (Ti, Nb)C and NbN.

[0087] FIG. 5 illustrates the change of mechanical properties of the complex-phase steel having high hole expansibility of Example 3 under different flattening deformation.

[0088] As shown in FIG. 5, the strength tends to rise with the increase of flatness.

[0089] In conclusion, the complex-phase steel having high hole expansibility of the present disclosure can simultaneously satisfy the requirements for good hole expansibility and plasticity, and compared with traditional material like low-alloy high-strength steel and ferrite-martensite dual-phase steel, the two phases of the complex-phase steel having high hole expansibility of the present disclosure are ferrite and bainite, so the hardness difference is small, making the steel have good hole expansibility and cold formability. In addition, the manufacturing method of the present disclosure also has the above advantages and beneficial effects.

[0090] It should be noted that the prior art part of the protection scope of the present disclosure is not limited to the embodiments given in the present disclosure, and all prior technologies that do not conflict with the solution of the present disclosure, including but not limited to prior patent documents, prior public publications, prior public use, etc., can be included in the protection scope of the present disclosure.

[0091] In addition, the combination mode of the technical features in the present disclosure is not limited to the combination mode recorded in the claims or the combination mode recorded in the specific embodiment of the present disclosure. All the technical features recorded in present disclosure can be combined or integrated in any way, unless there is a contradiction between them.

[0092] It should also be noted that the embodiments listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above embodiments, and the subsequent similar changes or deformations can be directly obtained or easily thought of by those skilled in the art from the contents disclosed in the present disclosure, which should belong to the protection scope of the present disclosure.