ArcelorMittal S.A.
A MANUFACTURING PROCESS OF HOT PRESS FORMED ALUMINIZED STEEL PARTS

Abstract:

A manufacturing process of a press hardened coated part including providing a furnace containing N zones, each furnace zone being respectively heated at a setting temperature .THETA..sub.1F, .THETA..sub.2F, . . . .THETA..sub.iF, . . . , .THETA..sub.NF, implementing the following successive steps: providing a steel sheet with thickness th between 0.5 and 5 mm, the steel sheet covered by an aluminium alloy precoating with a thickness between 15 and 50 .mu.m, the emissivity coefficient being equal to 0.15(1+.alpha.), a being between 0 and 2.4, then cutting said steel sheet to obtain a precoated steel blank, then placing the precoated steel blank in furnace zone 1 for a duration t.sub.1 between 5 and 600 s, wherein .THETA..sub.1F and t.sub.1 are such that: .THETA..sub.1Fmax>.THETA..sub.1F>.THETA..sub.1Fmin with: .THETA..sub.1Fmax=(598+A e.sup.Bt1+Ce.sup.Dt1) and .THETA..sub.1Fmin=(550+A' e.sup.B't1+C'e.sup.D't1), A, B, C, D, A', B', C', D' being such that A=(762 e.sup.0.071 th-426 e.sup.-0.86 th) (1-0.345.alpha.), B=(-0.031 e.sup.-2.151 th-0.039 e.sup.-0.094 th) (1+0.191.alpha.), C=(394 e.sup.0.193 th-434.3 e.sup.-1.797 th) (1-0.364.alpha.), D=(-0.029 e.sup.-2.677 th-0.011 e.sup.-0.298 th) (1+0.475.alpha.), A'=(625 e.sup.0.123 th-476 e.sup.-1.593 th) (1-0.345.alpha.), B'=(-0.059 e.sup.-2.109 th-0.039 e.sup.-0.091 th) (1+0.191.alpha.), C'=(393 e.sup.0.190 th-180 e.sup.-1.858 th) (1-0.364.alpha.), D'=(-0.044 e.sup.-2.915 th-0.012 e.sup.-00.324 th) (1+0.475.alpha.), wherein .THETA..sub.1F, .THETA..sub.1Fmax .THETA..sub.1Fmin are in .degree. Celsius, t.sub.1 is in s., and th is in mm, then transferring the precoated steel blank in the furnace zone 2 heated at a setting temperature .THETA..sub.2F=.THETA..sub.1B and maintaining isothermally the precoated steel blank for a duration t.sub.2, .theta..sub.2F and t.sub.2 being such that: t.sub.2min.gtoreq.t.sub.2.gtoreq.t.sub.2max with: t.sub.2min=0.95 t.sub.2* and t.sub.2max=1.05 t.sub.2*with: t.sub.2=t.sub.1.sup.2 (-0.0007 th.sup.2+0.0025 th-0.0026)+33952-(55.52.times..THETA..sub.2F) wherein .THETA..sub.2F is in .degree. Celsius, t.sub.2, t.sub.2min, t.sub.2max, t.sub.2* are in s., and th is in mm, then transferring the precoated steel blank in further zones (3, . . . i, . . . , N) of the furnace, so to reach a maximum blank temperature .THETA..sub.MB between 850.degree. C. and 950.degree. C., the average heating rate V.sub.a of the blank between .THETA..sub.2F and .THETA..sub.MB being between 5 and 500.degree. C./s, then transferring the heated steel blank from the furnace into a press, then hot forming the heated steel blank in said press so as to obtain part, then cooling the part at a cooling rate in order to obtain a microstructure in the steel substrate containing at least one constituent chosen among martensite or bainite.