Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60

Definitions

• the invention relates to ultra-low carbon (ULC) steel and to a process for producing ULC steel.
• ULC steel usually has a composition containing from 10 to 50 ppm C, 10 to 50 ppm N and 0 to 20 ppm B.
• Mn, Si and P may be added as alloying elements. If so, there may be between 0.1 and 1% by weight Mn, between 0.001 and 0.1% by weight P and between 0.01 and 0.2% by weight Si.
• the bottom limit is defined by the amount of these elements which is present as a usual impurity; the upper limit is determined by undesirable effects which would result from higher quantities of these elements in the steel.
• the ULC steel contains inevitable impurities, and an amount of Ti and Nb is added such that Ti/48 + Nb/93 > (C - 0.0015)/12 + N/14 + S/32 (in % by weight), in order to bind C, N and S.
• S is an impurity of which the amount must be kept as low as possible. The remainder is formed by Fe.
• ULC steel containing titanium and niobium is in widespread use as a superformable steel in the automotive industry, for example as sheet material for the bonnet, the roof, the boot lid and the doors of a car.
• sheet material for the bonnet, the roof, the boot lid and the doors of a car.
• An object of the invention is to provide a ULC steel which, with a standard amount of alloying elements, including phosphorus, has a higher strength than usual.
• this object as a result of a large number of experiments, has been achieved using an ultra-low carbon steel having the following composition: C ⁇ 40 ppm
• the object of the invention is achieved using an ultra-low carbon steel having the following compostion:
• the compositon of this ultra-low carbon steel is the same as the former steel, but instead of the percentage of recrystallised grains the steel is characterised by a microstrain of less than 0.05%.
• a high microstrain leads to an undesirably great reduction in the forming properties of the strip steel.
• the microstrain is determined via a standardised measurement method, in which X-ray diffraction is used to measure the disturbance to the lattice of the iron atoms in the steel. The deviation from the standard distribution in lattice spacing of pure, undisturbed iron is a measure of the microstrain.
• the microstrain is determined with reference to the widening of the (211) reflection of Fe, measured on a plane parallel to the rolling surface and at half the thickness of the material.
• the steel can also be characterised by a microstrain of less than 0.05% instead of by the percentage of recrystallised grains.
• the minimum level of the yield point can be defined even more accurately if the elements Mo and Cr are also included in the definition, for ultra-low carbon steel having the following composition:
• the steel can also be characterised by a microstrain of less than
• the above steel for which the percentage of recrystallised grains is given also has a microstrain of less than 0.05%.
• the ULC steel according to the invention has a microstrain of less than 0.03%.
• the forming properties of the steel are now very good.
• the steel is subjected, inter alia, to a re-rolling treatment after an annealing treatment.
• a defined re-rolling percentage is achieved, for example a re-rolling percentage of 1%.
• the extent to which the material produced has been re- rolled cannot be measured.
• experiments have shown that there is a direct and reproducible relationship between the re-rolling percentage and the microstrain for ULC steel.
• the ULC steel has a Lankford value r for which r ave > 2.8 - 0.004Rp.
• the Lankford value is a measure of the anisotropy of the material.
• a high Lankford value is beneficial for sheet material, since the sheet material becomes less thin when deformed. Therefore, the material will not rupture quickly during pressing and deep-drawing of components involving the material being lengthened in the plane of the sheet.
• a Lankford value given in the known literature indicates that r ave ⁇ 2.6 - 0.004Rp.
• the ULC steel according to the invention therefore has a significantly higher Lankford value at the same yield point.
• the ULC steel according to the invention has a percentage of recrystallised grains which is greater than 98% and less than 99%, and which is preferably approximately 98%.
• the recrystallisation takes place during the annealing treatment of the strip steel. The percentages indicated are reached by heating the steel to a lower temperature than usual during the annealing.
• the ULC steel according to the invention is preferably provided in the form of strip steel or galvanised strip steel.
• a process for obtaining a high- strength, ultra-low carbon steel in which ultra-low carbon steel is subjected to an annealing treatment in a continuous annealing line in which the maximum annealing temperature is lower than 760°C and at least 650°C.
• This process ensures that the steel does not recrystallise completely, but rather a small percentage of grains do not recrystallise.
• This small percentage of grains which have not recrystallised provides the steel with additional strength without there being any need to use a greater amount of alloying elements than usual and without having an adverse effect on the other properties of the steel.
• An excessively high percentage of grains which have not recrystallised will also cause the microstrain to increase. As stated above, an excessively high microstrain causes the forming properties to deteriorate considerably.
• Figure 1 shows a graph illustrating values for the yield point Rp from the literature and for the ULC steel according to the invention.
• Figure 2 shows a graph plotting measured values of the microstrain of ULC steel against the elongation at break.
• the starting material was an ultra-low carbon steel of the following composition: C ⁇ 40 ppm
• This ULC steel is subjected to the standard treatment for the production of strip steel, except that during the annealing treatment in the continuous annealing line the annealing temperature is at most 760°C.
• the annealing temperature must be no lower than 650°C.
• the annealing is followed by a re-rolling treatment, resulting in a re-rolling percentage of approximately 1%.
• the ULC strip steel has a yield point Rp of between 180 and 400 MPa, where:
• the yield point obtained in this way is considerably higher than the values for steel which are known from the literature, as can also be seen from the graph illustrated in Figure 1.
• This graph plots two measurements carried out on the ULC steel according to the invention by means of open circles; known values are indicated by closed circles and diamond shapes.
• the measured value Rp-B is plotted on the vertical axis; the calculated value Rp-A is plotted on the horizontal axis.
• the re-rolling percentage cannot be measured on the product itself. However, the re-rolling percentage is of importance, since it must be no higher than approximately 1.5%; at a re-rolling percentage of 3%, although the desired yield point can be achieved, other properties, such as the deformability, are considerably impaired.
• microstrain of the material A good, measurable standard for the re-rolling percentage is the microstrain of the material. It has been found that the microstrain must be less than 0.05%, preferably less than 0.03%.
• Figure 2 shows the microstrain plotted on the horizontal axis against the elongation at break A80 on the vertical axis. The graph clearly shows that if the microstrain remains sufficiently low the elongation at break, and therefore the deformability, remains sufficiently high. The microstrain is determined on the basis of the widening of the (211) reflection of Fe measured using X-ray diffraction.
• the favourable properties of the material according to the invention are partly dependent on the percentage of grains which have not recrystallised.
• the annealing usually produces recrystallisation of 100%.
• the percentage of recrystallised grains obtained is greater than 95% and lower than 99.7%.
• the percentage of recrystallised grains is 98% to 99%, and more preferably approximately 98%.
• the annealing temperature must not fall below 650°C. It has been found that the ULC steel according to the invention has a Lankford value r for which r ave > 2.8 - 0.004Rp.
• the Lankford value of the material according to the invention is therefore at least 0.2 higher than the Lankford value for the known ULC steel grade.
• a high Lankford value is beneficial for sheet material, since the reduction in thickness during elongation of the material falls as the Lankford value increases.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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Citations (7)

• 1999
  • 1999-12-07 NL NL1013776A patent/NL1013776C2/nl not_active IP Right Cessation
• 2000
  • 2000-05-26 WO PCT/NL2000/000363 patent/WO2000075382A1/en active Application Filing
  • 2000-05-26 AU AU52548/00A patent/AU5254800A/en not_active Abandoned

Patent Citations (7)

Non-Patent Citations (2)

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