FIELD: metallurgy.
SUBSTANCE: invention relates to metallurgy, particularly, to production of steel plate with zinc or zinc alloy coating used in automotive industry. Cold-rolled steel sheet has the following composition, wt. %: 0.07 ≤ C ≤ 0.5, 0.3 ≤ Mn ≤ 5, 0.010 ≤ Al ≤ 1, 0.010 ≤ Si ≤ 2.45, 0.35 ≤ (Si + Al) ≤ 2.5, 0.001 ≤ Cr ≤ 1.0, 0.001 ≤ Mo ≤ 0.5, optionally 0.005 ≤ Nb ≤ 0.1, 0.005 ≤ V ≤ 0.2, 0.005 ≤ Ti ≤ 0.1, 0.0001 ≤ B ≤ 0.004, 0.001 ≤ Cu ≤ 0.5 and 0.001 ≤ Ni ≤ 1.0, the rest is iron and unavoidable impurities, the composition of which is as follows: S < 0.003, P < 0.02 and N < 0.008, is heated to temperature T1 ranging from 550 °C to Ac1 + 50 °C in the furnace zone with atmosphere (A1) containing 2–15 vol. % hydrogen (H2) and the rest - nitrogen and unavoidable impurities, so that iron is not subjected to oxidation. Water vapor or oxygen is added into the furnace atmosphere at temperature T ≥ T1 at the input rate Q exceeding (0.07 %/h × α), where α is equal to 1 in case of water vapor addition or equal to 0.52 in case of oxygen addition, to obtain atmosphere (A2) with DP2 dew point temperature making from -15 °C to temperature Te dew point equilibrium iron/iron oxide. Feed rate Q is an input volume of steam or oxygen per hour, divided by the volume of the furnace between the inlet of steam or oxygen and the end of the furnace section, heated to holding temperature T2. Sheet is heated from said temperature T1 to temperature T2 of 720–1000 °C, in furnace zone with atmosphere (A2) of nitrogen containing 2–15 % of hydrogen, more than 0.1 vol. % CO, with oxygen partial pressure of higher than 10-21 atm, the sheet is kept at T2, wherein the duration tD of said heating plate from the temperature T1 to the end of exposure at the temperature T2 is from 100–500. Sheet is cooled at rate of 10–400 °C/s. Sheet is coated with zinc or zinc alloy.
EFFECT: sheet has ultimate tensile strength of more than 900 MPa and enables to obtain welded joints by contact spot welding with average content of not more than two cracks caused by liquid metal embrittlement, having depth of 100 mcm or more, to weld point.
19 cl, 5 tbl, 1 ex
