Method of shaping an article from a zinc or zinc alloy coated steel blank

Abstract

A method of shaping an article from a zinc or zinc alloy coated steel blank, including the steps of: a) providing a blank of the zinc or zinc alloy coated steel; b) reheating of the blank obtained in step a) to a reheating temperature T.sub.RH in the range Ac3-200° C. of the steel; c) soaking the blank for a time up to 3 minutes at the reheating temperature T.sub.RH; d) shaping the article in a press; and e) cooling the article. The steel includes (in wt. %) C: 0.01-0.2; Mn: 3.1-9.0; Al: 0.5-3.0; and optionally further alloying elements selected from Si, Cr, V, Nb, Ti and Mo; inevitable impurities and the balance is Fe.

Claims

1. A method of shaping an article from a zinc or zinc alloy coated steel blank, the steel consisting of in wt. %: C: 0.01-0.2; Mn: 3.1-9.0; Al: 0.5-3.0; optionally one or more further alloying elements: Si: less than 1.5; Cr: less than 2.0; V: less than 0.1; Nb: less than 0.1; Ti: less than 0.1; Mo: less than 0.5; inevitable impurities; and the remainder being Fe; the method comprising the steps of: a) annealing a strip of the steel and providing a blank of the zinc or zinc alloy coated steel obtained from the annealed strip; b) reheating of the blank obtained in step a) to a reheating temperature T.sub.RH in the range Ac3-200° C. to Ac3 of the steel; c) soaking the blank for a time up to 3 minutes at the reheating temperature T.sub.RH; d) shaping the article in a press; and e) cooling the article, wherein reheating step b) comprises raising the temperature at a rate of 30° C./s or more to the reheating temperature T.sub.RH, wherein between steps c) and d) the heated article is transferred to the press and a temperature drop does not exceed 150° C., wherein the resulting shaped article has a microstructure comprising in vol. % ferrite: 30% or more; austenite: 20% or more; martensite: 50% or less including 0%.

2. The shaping method according to claim 1, wherein the reheating temperature T.sub.RH is in the range Ac3-100° C. to Ac3.

3. The shaping method according to claim 1, wherein in step c) the reheated blank is soaked for a time of less than 2 minutes.

4. The shaping method according to claim 1, wherein step e) comprises press quenching to a temperature in the range of 100-250° C. at a quenching rate of at least 3° C./s.

5. The shaping method according to claim 1, wherein step e) comprises air cooling to ambient temperature.

6. The shaping method according to claim 1, wherein in step a) the blank is obtained from an intercritically annealed, zinc or zinc alloy coated cold rolled or hot rolled steel strip.

7. The shaping method according to claim 6, wherein the reheating temperature T.sub.RH is lower than an intercritical annealing temperature.

8. The shaping method according to claim 1, wherein the steel in wt. % satisfies at least one elemental content selected from the group consisting of C content in the range of 0.05-0.20, Mn content in the range of 3.5-9.0, and Al content in the range of 0.6-2.9.

9. The shaping method according to claim 1, wherein the resulting shaped article as obtained has the following properties: yield strength: 800 MPa or more; tensile strength: 980 MPa or more; total elongation: 15% or more; minimum bending angle at 1.0 mm thickness: 90° or more.

10. The shaping method according to claim 1, wherein the reheating temperature T.sub.RH is in an intercritical temperature range Ac1 to Ac3.

11. The shaping method according to claim 1, wherein reheating step b) comprises raising the temperature at a rate of 100° C./s or more to the reheating temperature T.sub.RH.

12. The shaping method according to claim 1, wherein in step c) the reheated blank is soaked for a time of less than 30 seconds.

13. The shaping method according to claim 1, wherein in step a) the blank is obtained from an intercritically annealed, zinc or zinc alloy coated cold rolled or hot rolled steel strip, which has been subjected to an intercritical annealing at a annealing temperature of less than 700° C.

14. The shaping method according to claim 1, wherein the steel in wt. % satisfies: C content in the range 0.1-0.19, Mn content in the range of 7.2-8.8, and Al content in the range of 1.0-2.25.

15. The shaping method according to claim 1, wherein the resulting shaped article has a microstructure comprising in vol. % ferrite: 40% or more; austenite: 30% or more; martensite: 30% or less including 0%.

16. The shaping method according to claim 1, wherein the resulting shaped article as obtained has the following properties: yield strength: 900 MPa or more; tensile strength: 1000 MPa or more; total elongation: 25% or more; minimum bending angle at 1.0 mm thickness: 100° or more.

17. The shaping method according to claim 1, wherein the steel consists of in wt. %: C: 0.094-0.13; Mn: 7.15-7.32; Al: 1.0-2.25; optionally one or more further alloying elements: Si: 0.15-1.0; Cr: 0.003-2.0; V: 0.001-0.1; Nb: 0.0007-0.1; Ti: 0.001-0.1; Mo: 0.001-0.5; inevitable impurities; and the remainder being Fe.

Description

(1) The invention will be elucidated with reference to the examples described below.

(2) FIG. 1 shows cross-sections of hot formed components.

(3) Steel samples were made on laboratory scale with the composition as shown in Table 1. Carbon, manganese, silicon and aluminium are elements added to the steel. The other elements are unavoidable impurities in the steel.

(4) TABLE-US-00001 TABLE 1 Composition of the steel in wt. % Steel C Mn Si Al P S Cr Mo S1 0.13 7.32 0.22 1.57 0.001 0.002 0.003 0.010 S2 0.094 7.15 0.20 1.54 0.001 0.0014 0.004 0.002 Steel Nb Ti V N Fe S1 0.001 0.001 0.002 0.0009 Balance S2 0.0007 0.001 0.001 0.0010 Balance

(5) The steel samples were made using the following process steps. Ingots of 200 mm×100 mm×100 mm were made in vacuum induction furnace. They were reheated for 2 hours at 1250° C., and rough-rolled to 25 mm thickness. Then, the strips were reheated again for 30 minutes at 1250° C., and hot rolled to 3 mm thickness. Thereafter, the hot rolled steels were air cooled to room temperature.

(6) After that the strips were annealed for 96 hours at 650° C. and air cooled to room temperature. Then the strips were pickled in HCl acid to remove the oxides, and cold rolled to 1.5 mm thickness. Thereafter the samples were continuously annealed at 675° C. for 5 minutes and hot-dip galvanised in a Zn-0.4Al wt. % bath at 460° C. using a Rhesca annealing/hot dip galvanising simulator.

(7) Then the Zn-coated strips of dimensions 200 mm×100 mm were hot-formed in a hot forming press supplied by Schuler SMG GmbH & Co. KG using the thermal cycles given in Table 2. Two types of for forming tools were used—a flat tool for obtaining tensile, bending and microstructure specimens, and a hat-top tool to obtain omega-shaped profiles for micro-cracking investigation.

(8) TABLE-US-00002 TABLE 2 Reheating time and temperatures Reheating Temperature, Reheating Time Steel (° C.) (second) S1 675 180 S2 617 46 675 57

(9) The reheating time in Table 2 is the time needed to heat the strip from room temperature to the reheating temperature plus the soaking time. The time for heating the strip from room temperature up to the reheating temperature is approximately 8 seconds. After reaching the reheating temperature, the strip is transferred to the hot forming press in 3 seconds and then hot pressed and cooled down to a temperature below 200° C., removed from the press and further cooled down in air.

(10) Tensile tests were performed in accordance with NEN10002 standard. Tensile specimens had a 50 mm gauge length and 20 mm width. Bending tests were in accordance with VDA 238-100 standard on 40 mm×30 mm×1.5 mm specimens.

(11) Table 3 shows the results of the mechanical properties before hot forming, and Table 4 after hot forming. In the Tables 3 and 4, Rp=yield strength, Rm=ultimate tensile strength, Ag=uniform elongation, At=total elongation. BA=bending angle, L=longitudinal specimen where bending axis is parallel to the rolling direction, T=transversal specimen where bending axis is perpendicular to the rolling direction. The measured bending angles at 1.5 mm thickness were converted to 1 mm equivalents using the following formula: BA @ 1.0 mm thickness=measured BA×Square root of thickness.

(12) TABLE-US-00003 TABLE 3 Mechanical properties of the Zn-coated blanks before hot press forming R.sub.m A.sub.g A.sub.t BA-L BA-T Steel R.sub.p, MPa (MPa) (%) (%) (°) (°) S1 919 989 25.9 31.4 125.0 153.9 S2 970 997 9.5 15.1 103.6 137.7

(13) TABLE-US-00004 TABLE 4 Mechanical properties of the Zn-coated steel sheets after hot press forming Reheating Temperature R.sub.p R.sub.m A.sub.g A.sub.t BA-L BA-T Steel (° C.) (MPa) (MPa) (%) (%) (°) (°) S1 675 895 1005 26.1 30.5 126.6 154.9 S2 617 930 1037 15.2 15.4 93.6 136.2 675 906 1001 15.1 15.6 97.1 139.0

(14) The microstructure of the samples was determined as follows. The amount of retained austenite has been determined by X-ray diffraction (XRD) at ¼ thickness location of the samples. The XRD patterns were recorded in the range of 45 to 165° (2Θ) on a Panalytical Xpert PRO standard powder diffractometer (CoK.sub.α-radiation). Quantitative determination of phase proportions was performed by Rietveld analysis using Bruker Topas software package for Rietveld refinement. Martensite content was determined from the peak-split at the ferrite diffraction locations in the diffractograms.

(15) The microstructural components are shown in Table 5 for the blanks before hot forming, and in Table 6 for the components after hot forming.

(16) TABLE-US-00005 TABLE 5 Microstructural components of the Zn- coated blanks before hot press forming Retained Austenite Ferrite Martensite Steel (vol. %) (vol. %) (vol. %) S1 40.0 45.3 14.7 S2 33.8 50.2 16.0

(17) TABLE-US-00006 TABLE 6 Microstructural components of the steels after hot press forming Reheating Retained Temperature, Austenite Ferrite Martensite Steel ° C. (vol. %) (vol. %) (vol. %) S1 675 43.0 44.1 12.9 S2 617 34.2 53.5 12.3 675 30.3 52.4 17.3

(18) The components made from steel S2 at the reheating temperatures as given in Table 3 that were formed in a hat-top tool were examined by making a cross-section of the component. The side portion of the hat-top shape was examined under the microscope. Photographs of the cross-section are shown in FIG. 1 for both substrate composition S2. Both photographs show that the substrate is covered by a zinc coating having no cracks at all. Thus, these components will have a good corrosion resistance.

(19) In comparison, the top photograph in FIG. 1 shows a cross-section of a hat-top made from zinc coated 22MnB5 substrate, which was hot formed using the standard hot forming cycle by heating a blank up to approximately 850° C., austenizing during approximately 5 minutes, and transferring to the press and hot pressing as described above. The 22MnB5 sample clearly shows many micro-cracks in the cross-section.