Steel plate for producing light structures and method for producing said plate

A steel plate, the chemical composition of which comprises steel, the contents being expressed by weight:

• 0.010%≦C≦0.05%
• 0.06%≦Mn≦3%
• Si≦1.5%
• 0.005%≦Al≦1.5%
• S≦0.030%
• P≦0.040%,
• titanium and boron in amounts such that:
• 4.6%≦Ti≦6%
• (0.45×Ti)−0.35%≦B≦(0.45×Ti)+0.70%,
• the balance of the composition consisting of iron and inevitable impurities resulting from the smelting,
• a mean grain size of said steel being equal to or less than 15 microns.

A manufacturing process comprising the steps of:

• supplying a steel plate, the chemical composition of which comprises steel, the contents being expressed by weight:
• 0.010%≦C≦0.05%
• 0.06%≦Mn≦3%
• Si≦1.5%
• 0.005%≦Al≦1.5%
• S≦0.030%
• P≦0.040%,
• titanium and boron in amounts such that:
• 4.6%≦Ti≦6%
• (0.45×Ti)−0.35%≦B≦(0.45×Ti)+0.70%,
• the balance of the composition consisting of iron and inevitable impurities resulting from the smelting,
• casting the steel in a form of a semifinished product, the casting temperature not exceeding more than 40° C. above the liquidus temperature of said steel;
• reheating said semi-finished before it is hot-rolled, the temperature and the duration of said reheat both being chosen in such a way that a density of TiB2 and optionally Fe2B eutectic precipitates, with a maximum size lmax greater than 15 microns and an aspect ratio f>5, is less than 400/mm2;
• and
• hot rolling said semi-finished product.

A manufacturing process comprising the steps of:

• supplying a steel plate, the chemical composition of which comprises steel, the contents being expressed by weight:
• 0.010%≦C≦0.05%
• 0.06%≦Mn≦3%
• Si≦1.5%
• 0.005%≦Al≦1.5%
• S≦0.030%
• P≦0.040%,
• titanium and boron in amounts such that:
• 4.6%≦Ti≦6%
• (0.45×Ti)−0.35%≦B≦(0.45×Ti)+0.70%,
• the balance of the composition consisting of iron and inevitable impurities resulting from the smelting,
• casting the steel in a form of a semifinished product, the casting temperature not exceeding more than 40° C. above the liquidus temperature of said steel;
• hot-rolling said semifinished product;
• and a cold-rolling operation and an annealing operation, the rolling and annealing conditions being adjusted in such a way that a steel plate with a mean grain size equal to or less than 15 microns is obtained.

A process for manufacturing a structural part, comprising the steps of:

• supplying a steel plate, the chemical composition of which comprises steel, the contents being expressed by weight:
• 0.010%≦C≦0.05%
• 0.06%≦Mn≦3%
• Si≦1.5%
• 0.005%≦Al≦1.5%
• S≦0.030%
• P≦0.040%,
• titanium and boron in amounts such that:
• 4.6%≦Ti≦6%
• (0.45×Ti)−0.35%≦B≦(0.45×Ti)+0.70%,
• the balance of the composition consisting of iron and inevitable impurities resulting from the smelting,
• cutting at least one blank from the steel plate;
• and
• deforming said at least one blank within a temperature range from 20° to 900° C., the deforming condition being adjusted in such a way that a steel plate with a mean grain size equal to or less than 15 microns is obtained.