Low-density hot dip galvanized steel and manufacturing method therefor

Abstract

Low-density hot dip galvanized steel, comprising a steel substrate (1) located at a core portion and a coating layer (3) located on the surface. An interface layer is disposed between the steel substrate (1) and the coating layer (3), the interface layer comprises an iron particle layer (4), iron particles dispersed on the steel substrate (1) and covering the steel substrate (1) are disposed in the iron particle layer (4), and the iron particles are covered by a first inhibition layer (5). The low-density hot dip galvanized steel contains element Al in a mass percentage of 3.0% to 7.0%. Correspondingly, the present invention also comprises a manufacturing method for the low-density hot dip galvanized steel. The low-density hot dip galvanized steel has a low density, a high strength and high galvanizability and coating layer adhesion.

Claims

1. A low-density hot dip galvanized steel, comprising a steel substrate located at a core portion of the steel and a coating layer located on a surface of the steel substrate; wherein: an interface layer is disposed between the steel substrate and the coating layer, the interface layer comprises an iron particle layer, iron particles dispersed on the steel substrate and covering the steel substrate are disposed in the iron particle layer, the iron particles are covered by a first inhibition layer; the low-density hot dip galvanized steel contains element Al in a mass percentage of 3.0% to 7.0%; a density of the low-density hot dip galvanized steel is less than 7500 kg/m.sup.3, wherein the surface of the low-density hot dip galvanized steel includes portion covered with the iron particles and portion not covered with the iron particles; and the iron particles have a particle size of 0.1˜5 μm.

2. The low-density hot dip galvanized steel according to claim 1, wherein, the steel substrate has an internal oxidized layer in a portion adjacent to the iron particle layer, the internal oxidized layer contains oxides of Al.

3. The low-density hot dip galvanized steel according to claim 2, wherein, the internal oxidized layer further contains oxides of Mn.

4. The low-density hot dip galvanized steel according to claim 2, wherein the internal oxidized layer has a thickness of 0.2˜10 μm.

5. The low-density hot dip galvanized steel according to claim 2, wherein, oxides of the internal oxidized layer exist in grain boundary and inside the grain.

6. The low-density hot dip galvanized steel according to claim 1, wherein, the interface layer has a thickness of 0.1˜5 μm.

7. The low-density hot dip galvanized steel according to claim 1, wherein, the iron particles cover 30% or more of a surface area of the steel substrate.

8. The low-density hot dip galvanized steel according to claim 1, wherein, the maximum space between adjacent iron particles is no more than 10 times the average particle size of the iron particles.

9. The low-density hot dip galvanized steel according to claim 1, wherein, a portion not covered by iron particles on the surface of the steel substrate is covered by a second inhibition layer.

10. The low-density hot dip galvanized steel according to claim 9, wherein, the thickness of the second inhibition layer is less than the thickness of the first inhibition layer.

11. The low-density hot dip galvanized steel according to claim 9, wherein, the second inhibition layer contains elements Fe, Al, and Zn.

12. The low-density hot dip galvanized steel according to claim 1, wherein, the first inhibition layer contains elements Fe, Al, and Zn.

13. The low-density hot dip galvanized steel according to claim 1, wherein, microstructures of the steel substrate are ferrite and residual austenite.

14. The low-density hot dip galvanized steel according to claim 13, wherein, a phase ratio of the residual austenite is 6˜30%.

15. The low-density hot dip galvanized steel according to claim 13, wherein, a mass percentage of element C in the residual austenite is not less than 0.8%.

16. The low-density hot dip galvanized steel according to claim 1, wherein, mass percentages of chemical elements of the steel substrate are: C: 0.25˜0.50%, Mn: 0.25˜4.0%, Al: 3.0˜7.0%, and the balance being Fe and other unavoidable impurities.

17. The low-density hot dip galvanized steel according to claim 16, wherein, the low-density hot dip galvanized steel has an elongation of more than 25% and a tensile strength of more than 800 MPa.

18. The low-density hot dip galvanized steel according to claim 1, wherein, the coating layer has a thickness of 5˜200 μm.

19. A method for manufacturing the low-density hot dip galvanized steel of claim 1, comprising the steps of: (1) manufacturing a strip steel; (2) continuous annealing of the strip steel directly: heating to a soaking temperature of 750-950° C. and then holding 30-600 s, wherein dew point of annealing atmosphere is −15° C.˜20° C.; (3) hot dipping.

20. The method for manufacturing a low-density hot dip galvanized steel according to claim 19, wherein, in the step (1), a casting slab is heated at 1000˜1250° C. with a holding time of 0.5˜3 h and a finishing rolling temperature of 800-900° C. to form a hot-rolled sheet, then the hot-rolled sheet is coiled at 500˜750° C. to form a hot-rolled coil, and the hot-rolled coil is uncoiled and then subjected to pickling and cold rolling, wherein cold rolling reduction is 30˜90%.

21. The method for manufacturing a low-density hot dip galvanized steel according to claim 19, wherein, in the step (2), atmosphere of a heating section and a holding section is a mixed gas of N.sub.2 and H.sub.2, wherein volume content of H.sub.2 is 0.5˜20%.

22. The method for manufacturing a low-density hot dip galvanized steel according to claim 19, wherein, in the step (3), the strip steel after continuous annealing is cooled to a temperature of strip steel entering into a molten zinc bath, wherein cooling rate is 1˜150° C./s, and the temperature of strip steel entering into a molten zinc bath is 0˜20° C. higher than a temperature of a zinc bath; the strip steel is then immersed in the zinc bath of the molten zinc bath for hot dipping; wherein, the temperature of the zinc bath is 30˜60° C. higher than the melting point of the components of the selected zinc bath.

23. The method for manufacturing a low-density hot dip galvanized steel according to claim 22, wherein, in the step (3), the mass percentage of the components in the zinc bath is: 0.10≤Al≤6%, 0≤Mg≤5%, and the balance being Zn and other unavoidable impurities.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram showing the structure of the low-density hot dip galvanized steel of the present invention.

(2) FIG. 2 shows the cross-sectional metallographic structure of the low-density hot dip galvanized steel of the present invention.

(3) FIG. 3 is a graph showing the depth distribution curve of element Al in the range of 0˜5 μm of the steel surface before hot dipping after annealing in hard-rolled steel, Comparative Example B1 and Example A4.

(4) FIG. 4 is a graph showing the depth distribution curve of element Fe in the range of 0˜5 μm of the steel surface before hot dipping after annealing in hard-rolled steel, Comparative Example B1 and Example A4.

(5) FIG. 5 is a graph showing the depth distribution curve of three elements of Al, Fe, and Zn of Comparative Example B1 and Example A4 after the hot dipping of Zn-0.2% Al.

DETAILED DESCRIPTION OF THE INVENTION

(6) The low-density hot dip galvanized steel and manufacturing method thereof of the present invention will be further explained and illustrated with reference to Drawings and specific Examples. However, the explanation and illustration do not constitute undue limitations of the technical solutions of the present invention.

(7) FIG. 1 shows the structure of the low-density hot dip galvanized steel of the present invention. As shown in FIG. 1, the low-density hot dip galvanized steel of the present invention comprises a steel substrate 1 and a coating layer 3 on the surface, wherein an interface layer is disposed between the steel substrate 1 and the coating layer 3, the interface layer comprises an iron particle layer 4. Wherein, iron particles are covered by a first inhibition layer 5, and a second inhibition layer 6 does not cover iron particles. The steel substrate 1 further has an internal oxidized layer 2 adjacent to the iron particle layer 4.

(8) FIG. 2 shows the cross-sectional metallographic structure of the low-density hot dip galvanized steel of the present invention. As shown in FIG. 2, in the low-density hot dip galvanized steel of the present invention, the formation of the external oxidation of the iron particle layer 4 on the surface of Al.sub.2O.sub.3 is suppressed by controlling the dew point of the annealing atmosphere and it is converted to internal oxidation of the internal oxidized layer 2; the first inhibition layer and the second inhibition layer are composed of Fe, Al, and Zn; the first inhibition layer covers a surface of the iron particles contacting with the coating layer, and the second inhibition layer is the portions at which the surface of the steel substrate is not covered by the iron particles. This is because when the steel sheet is immersed in the zinc bath, element Al and a small amount of element Zn in the zinc bath firstly react with the steel sheet Fe covered by the iron particle layer to form a first inhibition layer; and a small amount of a second inhibition layer containing Fe, Al, and Zn may be formed on the surface of the substrate at the portion not covered by the iron particles on the substrate surface or at the position of the gap between the iron particles, but the thickness thereof is thinner than the first inhibition layer on the surface of the iron particles. Wherein, the thickness of the internal oxidized layer 2 is 0.2˜10 μm, and the oxides of the internal oxidized layer 2 exist in the grain boundary and inside the grain, the thickness of interface layer is 0.1˜5 μm.

Examples A1-A16 and Comparative Examples B1-B6

(9) Table 1 lists the mass percentages of the chemical elements in components of the low-density hot dip galvanized steels of Examples A1-A16 and of the conventional steel sheets of Comparative Examples B1-B6.

(10) TABLE-US-00001 TABLE 1 (wt %, the balance is Fe) C Mn Al Si N S P Component I 0.37 1.1 4.1 0.31 0.0025 0.002 0.004 Component II 0.45 2 6.1 — 0.0040 0.003 0.007 Component III 0.34 2.8 5.2 — 0.0027 0.003 0.007

(11) As can be seen from Table 1, the mass percentage ranges of chemical elements in components I, II, and III are controlled as follows: C: 0.25˜0.50%, Mn: 0.25˜4.0%, Al: 3.0˜7.0%, P≤0.02%, S≤0.01%, N≤0.01%, and Si is added to the component I.

(12) The low-density hot dip galvanized steels of Examples A1-A16 and the conventional steel sheets of Comparative Examples B1-B6 were prepared by the following steps:

(13) (1) manufacturing strip steel: the steel was smelted according to the composition of Table 1, the casting slab was heated at 1000˜1250° C. with a holding time of 0.5˜3 h and a finishing rolling temperature is controlled to 800˜900° C., and then the hot-rolled sheet was coiled at 500˜750° C., and the hot-rolled coil was uncoiled and then subjected to pickling and cold rolling, wherein the cold rolling reduction was 30˜90%.

(14) (2) continuous annealing of the strip steel: heating to a soaking temperature of 750˜950° C. and then holding 30˜600 s, wherein heating rate was 1˜20° C./s, the atmosphere of the heating section and the holding section was a mixed gas of N.sub.2 and H.sub.2, wherein the volume content of H.sub.2 was 0.5˜20%, dew point of annealing atmosphere was −15° C.˜20° C.

(15) (3) hot dipping: the strip steel after continuous annealing was cooled to the temperature of the strip steel entering into the molten zinc bath, wherein the cooling rate was 1˜150° C./s, and the temperature of the strip steel entering into the molten zinc bath was 0˜20° C. higher than the temperature of zinc bath; the strip steel was then immersed in the zinc bath of a molten zinc bath for hot dipping; wherein the temperature of zinc bath was 30˜60° C. higher than the melting point of the components of the selected zinc bath. Wherein, mass percentages of components in zinc bath are: 0.10≤Al≤6%, 0<Mg≤5%, and the balance is Zn and other unavoidable impurities.

(16) Table 2 lists the specific process parameters of the low-density hot dip galvanized steels of Examples A1-A16 and of the conventional steel sheets of Comparative Examples B1-B6.

(17) TABLE-US-00002 TABLE 2 Step (1) Step (2) Finishing Cold Holding Heating rolling Coiling rolling Soaking time of temperature Holding temperature temperature reduction temperature soaking Component (° C.) time (h) (° C.) (° C.) (%) (° C.) (s) A1 I 1178 2.0 807 659 60 776 267 A2 I 1178 2.0 807 659 60 815 356 A3 I 1178 2.0 807 659 60 932 103 A4 I 1178 2.0 807 659 60 837 135 A5 I 1178 2.0 807 659 60 900 32 A6 I 1178 2.0 807 659 60 833 129 A7 I 1232 1.6 830 621 60 815 30 A8 I 1232 1.6 830 621 60 792 289 A9 I 1232 1.6 830 621 45 812 287 A10 I 1161 1.7 817 729 45 867 189 A11 I 1039 0.6 801 521 45 868 157 A12 I 1150 0.5 898 647 45 817 221 A13 II 1116 1.8 854 516 60 790 281 A14 II 1232 0.6 830 621 60 850 191 A15 III 1208 0.8 828 656 60 814 40 A16 III 1179 2.1 888 594 60 827 303 B1 I 1178 2.0 807 659 60 837 135 B2 I 1070 3.0 835 545 60 815 248 B3 I 1246 1.4 830 663 60 700 72 B4 I 1134 2.8 900 738 60 960 164 B5 II 1145 1.6 817 547 60 913 215 B6 III 1233 2.2 809 681 60 780 293 Step (2) Step (3) Dew Temperature Mass Mass point of Volume of the strip percentage percentage annealing content Cooling steel into Ternperature of Al in of Mn in atmosphere of H.sub.2 rate molten zinc of galvanizing galvanizing galvanizing (° C.) (%) (° C./s) bath (° C.) zinc (° C.) zinc (%) zinc (%) A1 −15 5 32 465 460 0.2 0 A2 −10 5 35 465 460 0.2 0 A3 −5 5 50 465 460 0.2 0 A4 0 5 32 465 460 0.2 0 A5 10 5 43 465 460 0.2 0 A6 20 10 38 465 460 0.2 0 A7 −10 5 35 465 460 0.2 0 A8 −10 2.5 25 465 460 0.2 0 A9 −10 15 22 480 460 0.2 0 A10 −10 11 68 470 470 6 3 A11 −10 5 53 450 438 2 1.5 A12 −5 5 52 450 445 1 1 A13 0 3 23 465 460 0.2 0 A14 −10 3 64 465 460 0.2 0 A15 −10 3 62 470 460 0.2 0 A16 −5 3 61 470 460 0.2 0 B1 −40 5 52 465 460 0.2 0 B2 −20 5 43 465 460 0.2 0 B3 −10 5 88 465 460 0.2 0 B4 −5 10 72 465 460 0.2 0 B5 −40 5 31 470 460 0.2 0 B6 −30 5 73 470 460 0.2 0

(18) FIG. 3 is a graph showing the depth distribution curve of element Al in the range of 0˜5 μm of the steel surface before hot dipping after annealing in hard-rolled steel, Comparative Example B1 and Example A4. As can be seen from the Figure, the strip surface of Comparative Example B1 is rich in Al, corresponding to the Al.sub.2O.sub.3 film on the strip steel surface after annealing; the Al enrichment in the strip steel surface of Example A4 disappeared, while the element Al enrichment is present in the internal oxidized layer, indicating that the external oxidation has been converted to internal oxidation.

(19) FIG. 4 is a graph showing the depth distribution curve of element Fe in the range of 0˜5 μm of the steel surface before hot dipping after annealing in hard-rolled steel, Comparative Example B1 and Example A4, wherein the strip steel surface of Comparative Example B1 has a low Fe content, while the iron particle layer of the Example has a sharp Fe peak.

(20) FIG. 5 is a graph showing the depth distribution curve of three elements of Al, Fe, and Zn of Comparative Example B1 and Example A4 after the hot dipping of Zn-0.2% Al zinc bath, wherein, in the Comparative Example B1, element Al in the coating layer/substrate interface smoothly transitions without peak change, while in Example A4, element Al in the coating layer/substrate interface shows a peak, indicating that Comparative Example B1 did not form the first inhibition layer and the second inhibition layer on the coating layer/substrate interface, while Example A4 formed effective first inhibition layer and second inhibition layer on the coating layer/substrate interface, resulting in poor galvanizability and coating layer adhesion of Comparative Example B1.

(21) Table 3 lists the performance parameters of the low-density hot dip galvanized steels of Examples A1-A16 and of the conventional steel sheets of Comparative Examples B1-B6.

(22) Wherein, the galvanizability is determined by directly observing the appearance of the strip steel after cladding with the naked eye. If the surface has no obvious uncoated iron, the galvanizability is good (indicated by ◯); if the surface has obvious uncoated iron, the galvanizability is poor (indicated by X).

(23) The adhesion of the coating layer is determined by the following method: taking a sample having a length of 200 mm and a width of 100 mm from the strip steel, bending it 180 degrees, then flattening, and sticking the bending position with a tape. If no zinc layer is adhered on the tape or the surface of the coating layer on the bending surface adhered by the tape is not fluffed, the coating layer adhesion is good (indicated by ◯); If the coating layer is adhered on the tape or the surface of the coating layer on the bending surface adhered by the tape is fluffed, the coating layer adhesion is poor (indicated by X).

(24) TABLE-US-00003 TABLE 3 Tensile Coating Density Elongation strength layer (kg/m.sup.3) (%) (MPa) Galvanizability adhesion A1 7340 25 838 ◯ ◯ A2 7340 32 831 ◯ ◯ A3 7340 33 844 ◯ ◯ A4 7340 25 823 ◯ ◯ A5 7340 28 858 ◯ ◯ A6 7340 34 852 ◯ ◯ A7 7340 29 843 ◯ ◯ A8 7340 33 828 ◯ ◯ A9 7340 29 830 ◯ ◯ A10 7340 27 851 ◯ ◯ A11 7340 27 821 ◯ ◯ A12 7340 26 848 ◯ ◯ A13 7150 27 839 ◯ ◯ A14 7150 28 850 ◯ ◯ A15 7280 33 850 ◯ ◯ A16 7280 26 836 ◯ ◯ B1 7340 28 825 X X B2 7340 27 851 X X B3 7340 32 848 X X B4 7340 35 849 ◯ X B5 7340 30 836 X X B6 7280 27 836 X X

(25) As can be seen from Table 3, all of the Examples A1-A16 have densities of less than 7500 kg/m.sup.3, elongations of more than 25% and tensile strengths of more than 800 MPa, and the galvanizability and coating layer adhesion of Examples A1-A16 are superior to those of Comparative Examples B1-66.

(26) The reasons are as follows: since the surface of the substrate has an iron particle layer in the Examples, when the strip steel is immersed in the zinc bath, Al and Fe in the coating layer firstly react to form a inhibition layer; on the contrary, since the surface of the substrate does not form an effective layer of iron particles but a continuous dense Al.sub.2O.sub.3 oxide film in the Comparative Examples, which hinders the reaction of Al in the zinc bath with the Fe of the substrate, and thus an effective inhibition layer is not formed.

(27) It is to be noted that the above description is only specific Examples of the present invention, and it is obvious that the present invention has many similar modifications and is not limited to the above Examples. All modifications derived or conceived by those skilled in the art from the disclosure of the present invention should fall within the scope of the present invention.