Press hardened part with high resistance to delayed fracture and a manufacturing process thereof

20:

• A press hardened coated steel part with high resistance to delayed fracture, comprising a base of steel and a coating, the coating containing (Fex-Aly) intermetallic compounds resulting from the diffusion of iron into an aluminum or an aluminum-based alloy, or an aluminum alloy of a precoating, wherein the chemical composition of the steel comprises, in weight:
• 0.16%≤C≤0.42%
• 0.1%≤Mn≤3%
• 0.07%≤Si≤1.60%
• 0.002%≤Al≤0.070%
• 0.02%≤Cr≤1.0%,
• 0.0005≤B≤0.005%
• 0.002%≤Mg≤0.007%
• 0.002%≤Ti≤0.110%
• 0.0008%≤0≤0.005%
• wherein (Ti)×(O)2×107≤2
• 0.001%≤N≤0.007%
• 0.001%≤S≤0.005%
• 0.001%≤P≤0.025%
• and optionally one or more elements selected from:
• 0.005%≤Ni≤0.23%, and
• 0.005%≤Nb≤0.060%,
• a remainder being Fe and unavoidable impurities,
• and wherein a microstructure of the steel includes at least 95% martensite.

24:

• The press hardened coated steel part as recited in claim 20 wherein an average size dav of oxides, carbonitrides, sulfides and oxisulfides is less than 1.7 μm and wherein at least one of the conditions (C1) or (C2) is fulfilled:
• (C1):
• the sum N(MgO+MgO-Al2O3) of the numbers of MgO and MgO-Al2O3 particles per area unit is higher than 90 per mm2,
• (C2):
• the number N(MgO-TixOy) of MgO-TixOy particles per area unit is higher than 100 per mm2 with the average size lower than 1 m.

29:

• A process for manufacturing a press hardened coated steel part with high resistance to delayed fracture, comprising the following and successive steps:
• providing liquid steel comprising by weight:
• 0.16%≤C≤0.42%, 0.1%≤Mn≤3%, 0.07%≤Si≤1.60%, 0.002%≤Al≤0.070%, 0.02%≤Cr≤1.0%, 0.0005≤B≤0.005%, 0.002%≤Ti≤0.11%, 0.001%≤O≤0.008% wherein (Ti)×(O)2×107≤2, 0.001%≤N≤0.007%, and optionally:
• 0.005%≤Ni≤0.23%, 0.005%≤Nb≤0.060%, 0.001%≤S≤0.005%, and 0.001%≤P≤0.025%, a remainder being Fe and unavoidable impurities, then;
• adding Mg or Mg-alloy at a temperature Taddition so as to obtain a further liquid steel with a chemical composition by weight:
• 0.16%≤C≤0.42%, 0.1%≤Mn≤3%, 0.07%≤Si≤1.60%, 0.002%≤Al≤0.070%, 0.02%≤Cr≤1.0%, 0.0005≤B≤0.005%, 0.002%≤Mg≤0.007%, 0.002%≤Ti≤0.11%, 0.0008%≤O≤0.005%, wherein (Ti)×(O)2×107≤2, 0.001%≤N≤0.007%, 0.001%≤S≤0.005%, 0.001%≤P≤0.025%, and optionally one or more elements selected from:
• 0.005%≤Ni≤0.23%, and 0.005%≤Nb≤0.060%, a remainder being Fe and unavoidable impurities, the temperature Taddition being between Tliquidus and (Tliquidus+70° C.);
• then
• casting the liquid steel in the form of a semi-product, a duration tD elapsing between the addition of Mg or Mg alloy and the solidification start of the liquid steel being less than 30 minutes;
• then
• heating the semi-product at a temperature between 1250 and 1300° C. so to obtain a heated semi-product;
• then
• rolling the semi-product so to obtain a rolled steel sheet;
• then
• precoating the rolled steel sheet with aluminum or aluminum-based alloy, or aluminum alloy so to obtain a precoated steel sheet;
• then
• cutting the precoated steel sheet so to obtain a precoated steel blank;
• then
• heating the precoated steel blank so to obtain a heated blank with a full austenitic structure;
• then
• hot press forming the heated blank so to obtain a hot press formed part, then
• cooling the hot press formed part while maintaining the hot press formed part in a press tooling so as to obtain a press hardened coated steel part with a microstructure comprising at least 95% martensite.