The invention relates to a high strength bake-hardenable low density steel and to a method for producing said steel.

In the continuing efforts to reduce the carbon emissions of vehicles the steel industry, together with the car manufacturers, continue to strive to steels which allow weight reduction without affecting the processability of the steels and the safety of the finished product. To meet future CO₂-emission requirements, the fuel consumption of automobiles has to be reduced. One way towards this reduction is to lower the weight of the car body. A steel with a low density and high strength can contribute to this. At the same thickness, the use of a low density steel reduces the weight of car components. A problem with known high strength steels is that their high strength compromises the formability of the material during forming of the sheet into a car component.

Ordinary high strength steels, for example dual phase steels, allow use of thinner sheets and therefore weight reduction. However, a thinner part will have a negative effect on other properties such as stiffness, crash - and dent resistance. These negative effects can only be solved by increasing the steel thickness, thus negating the effect of the downgauging, or by changing the geometry of the component which is also undesirable.

It is an object of this invention to provide a low density steel with a high strength in the finished component combined with excellent formability prior to forming the car component.

It is also an object of this invention to provide a high strength steel with excellent stiffness and dent resistance.

One or more of these objects can be reached by providing a ferritic steel strip or sheet comprising, in weight percent,

• up to 0.01 % C_total;
• at most 0.2 % Si;
• 0.1 to 1.0 % Mn;
• from 5 to up to 10 % Al;
• up to 0.010% N;
• up to 0.01 % S;
• up to 0.1 % P;

• 0.005 to 0.019% Ti;
• 0.008 to 0.08% Nb;
• 0.002 to 0.1% V;
• 0.004 to 0.1% Zr;

• optionally between 5 and 50 ppm B;
• remainder iron and inevitable impurities;

• Minimum[X,Y]
• Maximum[Z,0]
• (12/93)*Nb
• (12/91)*Zr
• (12/51)*V;

• X = 2*12/(2*32)*S;
• Y = 2*12/(4*48)*(Ti-48/14*N);
• Z = 12/48*(Ti-48/14*N-4*48/(2*32)*S);

• Minimum[X,Y] = lower value of X and Y and Minimum[X,Y] = zero if Y is negative;
• Maximum[Z,0] = higher value of zero and Z;
• and wherein C_solute is at least 0.0005 (5 ppm).

All percentages are in weight percent, unless otherwise indicated. For the sake of avoiding any misunderstanding, the formulae given above, when typed in in a commercial spreadsheet programme such as Microsoft Excel will result in the correct interpretation of the formulae. For instance 12/93*Nb is correctly interpreted as (12/93)*Nb as the skilled person will recognise the atomic masses of carbon (12) and Nb (93) in this formula. This is the same for the other numbers in the formulae (mutatis mutandis). So, superfluously: $$X=2\cdot \left(\frac{12}{2\cdot 32}\right)\cdot S$$ $$Y=2\cdot \left(\frac{12}{4\cdot 48}\right)\cdot \left(\mathit{Ti}-\left(\left(\frac{48}{14}\right)\cdot N\right)\right)$$ $$Z=\left(\frac{12}{48}\right)\cdot \left(\mathit{Ti}-\left(\left(\frac{48}{14}\right)\cdot N\right)-\left(4\cdot \frac{48}{\left(2\cdot 32\right)}\cdot S\right)\right)$$

The steel according to the invention has a tailored chemical composition to allow the steel to contain carbon in solid solution (C_solute) after the annealing and optional galvanising step. This carbon in solid solution allows the steel to be bake-hardenable e.g. in a paint-baking cycle. The car component is formed from the steel coming of the mill, and the component is painted and baked after it has been formed into its final shape.

In addition, the steel according to the invention combines the good formability prior to forming a car component, i.e. before the paint-baking operation, with a higher strength after the paint-baking operation.

The inventors found that for the steel to be bake-hardenable in a paint baking cycle at least 5 ppm of solute carbon (C_solute) must be present in steel. At lower amounts of solute carbon the effect is negligible or not reproducible.

The level of solute carbon may also not exceed a critical upper value because the steel is preferably free from natural ageing. Natural ageing is the spontaneous ageing of a supersaturated solid solution at room temperature and involves a spontaneous change in the physical properties of the steel, which occurs on being held at atmospheric temperatures after hot- or cold rolling or after a final heat treatment, e.g. during transport to or storage at a customers prior to processing the strip. This natural ageing involves changes of the mechanical properties which are considered undesirable as they lead to unpredictable variations in processability during the forming of the car components. Also, the surface quality may be adversely affected due to the formation of so-called Lüder-lines. Also, too high a carbon level in solid solution may result in a deterioration of the formability prior to bake-hardening.

For that reason a maximum value of 50 ppm of solute carbon is preferable. A more suitable maximum is 40 ppm of solute carbon (i.e. 0.004%).

In an embodiment of the invention C_solute is at least 0.0010 (10 ppm) and/or at most 0.0030 (30 ppm). This achieves a stable process and reproducible properties.

Nitrogen, in particularly free nitrogen (i.e. nitrogen in solid solution), is not desirable but unavoidable in steel making. Titanium can be optionally added to bound nitrogen into TiN. The large amount of aluminium in the steel can also ensure that all nitrogen is bound. This means that the matrix is substantially free of nitrogen in solid solution.

Boron is optionally added to the steel. Its presence is not mandatory, but it may help to suppress any tendency for secondary work embrittlement. If added, a minimum amount of 5 ppm boron is required.

The manganese content is at least 0.1%. In another embodiment the aluminium content is at least 6 % and/or at most 9%, preferably at most 8%.

The steel is preferably calcium treated. The chemical composition may therefore also contain calcium in an amount consistent with a calcium treatment.

In the steels according to the invention the amount of carbon in solid solution is controlled by the addition of microalloying elements (Ti, Nb, V, Zr) in combination with excellent control of the total carbon content in the steel.

The amount of Ti or Nb should be strictly controlled. Too much titanium or niobium will combine with carbon to form carbides or, in the presence of sulphur, carbosulphides. As a consequence of this, no solute carbon is available and no bake-hardenability.

The amount of carbon in solid solution according to this invention is calculated by subtracting from the total carbon content C_total the precipitates comprising carbon as follows: $$\mathrm{C}\_\mathrm{solute}=\mathrm{C}\_\mathrm{total}$$

• Minimum[X,Y]
• Maximum[Z,0]
• (12/93)*Nb
• (12/91)*Zr
• (12/51)*V;

$$\mathrm{X}=2*12/\left(2*32\right)*\mathrm{S};$$

$$\mathrm{Y}=2*12/\left(4*48\right)*\left(\mathrm{Ti}-48/14*\mathrm{N}\right);$$

$$\mathrm{Z}=12/48*\left(\mathrm{Ti}-48/14*\mathrm{N}-4*48/\left(2*32\right)*\mathrm{S}\right);$$

• Minimum[X,Y] = lower value of X and Y and Minimum[X,Y] = zero if Y is negative;
• Maximum[Z,0] = higher value of zero and Z.

For the interpretation of these formulae see herein above. The addition of Ti is beneficial for binding nitrogen, but not strictly necessary. Up to 0.019% Ti can be added into the steel, mainly to bind nitrogen into TiN and secondarily to control the amount of solute carbon. The titanium content must 0.019% or lower, e.g. at most 0.018% or 0.015% or even at most 0.012%.

If titanium is added as an alloying element, a suitable minimum value for the titanium content is 0.005%. If added, then a suitable minimum value for Nb is 0.008%. If added, then for V and Zr suitable minimum values are 0.002 and 0.004 respectively.

According to a preferable embodiment the composition of the ferritic steel according to the invention has a base composition where no titanium is added to the steel, and any titanium present is an unavoidable impurity.

Titanium, as an alloying element or as an inevitable impurity, will first form TiN. If there is excess nitrogen, then the remaining nitrogen will be bound to aluminium. If there is excess titanium, then the remaining titanium will form Ti₄C₂S₂ until all titanium is consumed. The factor Minimum[X,Y] calculates how much carbon is consumed by the formation of Ti₄C₂S₂ after all free nitrogen was bound to TiN. If the calculation results in a negative value for Y, then the factor is to be set to zero.

If there is no titanium at all, no TiN or Ti₄C₂S₂ will be formed and then Minimum[X,Y] amounts to zero. The factor Maximum[Z,0] determines how much carbon is bound to titanium after accounting for the formation of TiN and Ti₄C₂S₂.

The other three factors account for the formation of NbC, ZrC and VC, and thereby together with the factors Minimum[X,Y] and Maximum[Z,0] determine the amount of solute carbon in the steel.

By adding no or only small amounts of titanium and/or a specified amount of Nb, there will be sufficient solute carbon available for bake hardening. By controlling the level of solute carbon below 50 ppm, and preferably below 40 ppm, the steel according to the invention is bake hardenable and nature-aging resistant.

According to a second aspect, a method for producing a ferritic steel strip is provided comprising the steps of:

• providing a steel slab or thick strip by:
  • ∘ continuous casting, or
  • ∘ by thin slab casting, or
  • ∘ by belt casting, or
  • ∘ by strip casting;
• optionally followed by reheating the steel slab or strip at a reheating temperature of at most 1250°C;
• hot rolling the slab or thick strip and finishing the hot-rolling process at a hot rolling finishing temperature of at least 850°C;
• coiling the hot-rolled strip at a coiling temperature of between 550 and 750°C.

In preferable embodiment the coiling temperature is at least 600°C and/or the hot rolling finishing temperature is at least 900°C.

This hot-rolled strip can be subsequently further processed in a process comprising the steps of:

• cold-rolling the hot-rolled strip at a cold-rolling reduction of from 40 to 90% to produce a cold-rolled strip;
• annealing the cold-rolled strip in a continuous annealing process with a peak metal temperature of between 700 and 900°C;
• optionally galvanising the annealed strip in a hot-dip galvanising or electro-galvanising or a heat-to-coat process.

The hot-rolled strip is usually pickled and cleaned prior to the cold-rolling step. In an embodiment the peak metal temperature in the continuous annealing process is at least 750°C, preferably at least 800°C.

In an embodiment the cold rolling reduction is at least 50%.

In an embodiment the thickness of the hot-rolled strip is between 1 and 5 mm and/or the thickness of the cold-rolled strip is between 0.4 and 2 mm.

In an embodiment of the invention the hot-rolled strip is annealed in a continuous annealing step and optionally galvanised in a hot-dip galvanising step. The annealing may also take place in a so called heat-to-coat cycle. In a heat-to-coat cycle the hot-rolled steel is reheated to a temperature sufficient for performing the hot-dip galvanising, but not to a temperature as high as the conventional continuous annealing step. During the reheating the carbon, which may have precipitated during the slow cooling of the hot rolled coil after hot rolling is brought into solid solution again. After annealing and/or galvanising the steel has to be fast cooled to avoid precipitation of the carbon in solid solution. When using this galvanised steel sheet for producing a car component or other product by forming, followed by painting and baking, then the paint-baking also ensures the strength increase associated with the paint-baking cycle.

The invention is now further explained by means of the following, non-limiting examples.

Steels were produced and processed into cold-rolled steel sheets having a thickness of 1 mm. The hot rolled strip had a thickness of 3.0 mm. The chemical composition of the steels is given in Table 1. Steel C Al Mn N Ti Nb S C_solute 1 0.0020 7.0 0.20 0.0035 0.000 0.000 0.004 0.0020 R 2 0.0020 7.0 0.20 0.0030 0.010 0.000 0.004 0.0020 I 3 0.0040 7.0 0.20 0.0030 0.000 0.020 0.004 0.0014 I 3a 0.0040 6.9 0.20 0.0025 0.005 0.010 0.001 0.0031 I 4 0.0030 8.0 0.20 0.0030 0.010 0.010 0.004 0.0017 I 5 0.0040 7.5 0.20 0.0040 0.000 0.020 0.004 0.0014 I 6 0.0050 6.5 0.25 0.0030 0.010 0.020 0.004 0.0024 I 7 0.0050 6.0 0.20 0.0030 0.010 0.040 0.005 0.0000 R 8 0.0050 6.8 0.20 0.0030 0.100 0.000 0.005 0.0000 R 9 0.0050 7.0 0.20 0.0030 0.010 0.050 0.005 0.0000 R

The steels were produced by casting a slab and reheating the slab at a temperature of at most 1250°C. This temperature is the maximum temperature, because at higher reheating temperatures excessive grain growth may occur. The finishing temperature during hot rolling was 900°C, coiling temperature 650°C followed by pickling and cold rolling (67%) and continuous annealing at a peak metal temperature of 800°C and hot-dip-galvanising. Steel 3a also contained 16 ppm B. As-produced After 2% + 170°C/20min steel YLD (MPa) UTS (MPa) A80 (%) YLD WH(MPa) BH (MPa) 1 340 460 32 420 35 45 2 345 465 31 425 35 45 3 351 470 30 426 36 39 4 420 530 17 498 34 44 5 408 518 18 483 35 40 6 349 468 29 424 35 40 7 295 420 34 330 35 0 8 359 475 29 394 35 0 steel YLD (MPa) UTS (MPa) A80 (%) YLD WH (MPa) BH (MPa) 9 362 480 29 398 36 0 3a 371 460 27 457 34 52 WH= workhardening due to 2% prestrain BH = Bake-hardening due to 20min at 170°C

The results presented in Table 2 clearly demonstrate that the presence of solute carbon at levels of 14 to 24 or to 31 ppm results in an increase of about 40 MPa on top of the work-hardening and the base strength of the steel. The inventors found this effect to be present at solute carbon levels between 5 and 50 ppm.