NL1013776C2 - Ultra Low Carbon steel and method for its manufacture. - Google Patents

Description

ULTRA LOW CARBON STEEL AND METHOD FOR THE MANUFACTURE THEREOF

The invention relates to Ultra Low Carbon (ULC) steel, and to a method of manufacturing ULC steel.

ULC steel usually has a composition with 10 to 50 ppm C, 10 to 50 ppm N and 0 to 20 ppm B. In addition, Mn, Si and P may be added as alloying elements. Thereby, Mn can be present between 0.1 and 1 wt%, of P between 0.001 and 0.1 wt% and of Si between 0.01 and 0.2 wt%. The lower limit wound is determined by the amount in which these elements are present as usual impurity; the upper limit is determined by undesirable effects which would produce even greater amounts of these elements in the steel. In addition, in the ULC steel unavoidable impurities are present, and an amount of Ti and Nb is added such that Ti / 48 + Nb / 93> (C - 0.0015) / 12 + 15 N / 14 + S / 32 (in wt% ), to bind C, N and S. S is a contaminant the amount of which must be kept as low as possible. The balance is formed by Fe.

ULC steel with titanium and niobium is widely used as a superdeforming steel in the automotive industry, for example as sheet material 20 for the hood, roof, trunk and doors of a car. The automotive industry is continuously striving to reduce weight with a view to reducing fuel consumption. Therefore, there is a demand for ULC steel sheet which is thinner. As a result, the steel must be stronger.

It is known that a stronger steel can be obtained by adding alloying elements. However, in most cases alloying elements are expensive, so adding alloying elements is not desirable.

However, the alloying element phosphorus (P) has a very large influence on the strength of ULC steel, while it is not particularly expensive. It is known that adding 0.01 wt% phosphorus to the ULC steel increases the yield strength of the steel by approximately 10 MPa. Manganese is another widely used alloying element for steel. For an equal yield strength of 10 MPa, about 0.25 wt% manganese must be added. The yield strength of ULC steel is a good measure of strength.

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However, adding phosphorus also has disadvantages. When more phosphorus is added, the casting speed of the slabs from which the ULC strip steel is rolled must be lower. This increases the cost of the final ULC sample. In addition, it has been found that surface rolling occurs during the hot rolling of the strip due to, inter alia, oxidation of the phosphorus in the hot strip. These surface defects cause rejection and loss of efficiency. Finally, much ULC strip steel is galvanized at higher temperatures to obtain an intermetallic bond between the zinc protection layer and the steel. It has been found that when more phosphorus is added, this treatment must be performed more slowly. This also increases the cost price of the steel strip.

A disadvantage of manganese is that it has to be added in large quantities, since manganese has no great effect on strength. However, the manganese-containing additives are always contaminated with carbon, so that the carbon content of the ULC steel will increase too much when much manganese is used. Thus, alloying with large amounts of steel is undesirable.

It is an object of the invention to provide a ULC steel which has a higher than usual strength with a usual amount of alloying elements, including phosphorus.

According to a first aspect of the invention, as a result of a large number of experiments, this object has been achieved with an Ultra Low Carbon steel with the composition: C <40ppm N <40ppm 3 ppm <B <20 ppm 25 Mn, Si and P inevitable impurities as an alloying element; an amount of Ti and Nb for which applies:

Ti / 48 + Nb / 93> (C - 0.0015) / 12 + N / 14 + S / 32 (in weight%); balance Fe; 30 with a yield strength Rp between 180 MPa and 400 MPa, where Rp> 160 + 40 Mn + 80 Si + 1000 P (in weight%).

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This provides a ULC steel with a yield strength and therefore with a strength which is clearly higher than is usual for the added amounts of alloying elements. The higher yield strength is obtained by an adapted process control during the hot rolling of the strip steel and during the annealing and galvanizing of the strip steel respectively.

Preferably, a ULC steel with a relatively high percentage of Mn and P according to any one of claims 2, 3 or 4 is used, since this achieves a higher yield strength for the ULC steel.

It is possible to make a more precise determination of the minimum value for the yield strength if more elements are involved. This is the case with Ultra Low Carbon steel with the composition: C <40ppm N <40ppm Mn <0.8 wt% 15 P <0.075 wt%

Si <0.5 wt% 3 ppm <B <20 ppm unavoidable impurities; an amount of titanium and niobium for which holds: 20 Ti / 48 + Nb / 93> (C - 0.0015) / 12 + N / 14 + S / 32 (in wt%); balance Fe; with a yield strength between 180 and 400 MPa, where:

Rp> 160 + 15Mn + 45Si + 1200P + 20000B (in wt%).

An even more precise determination of the minimum yield strength value is possible if the elements Mo and Cr are also included in the valuation, for Ultra Low Carbon steel with the composition: C <40 ppm N <40 ppm Mn <0.8 wt. % 30 P <0.075 wt%

Si <0.5 wt%. _ «V ·) ·· * ··»> ***> 10137 fo -4-

Mo <0.2 wt%

Cr <0.2 wt% 3 ppm <B <20 ppm unavoidable impurities; 5 an amount of titanium and niobium for which:

Ti / 48 + Nb / 93> (C - 0.0015) / 12 + N / 14 + S / 32 (in wt%); balance Fe; with a yield strength between 180 and 400 MPa, where:

Rp> 160 + 15Mn + 45Si + 1293P + 22540B + 54Mo - 84Cr (in wt%).

In the process of obtaining the ULC steel according to the invention, the steel is subjected, inter alia, to a post-rolling treatment after an annealing treatment. A certain post-rolling percentage is achieved thereby, for example a post-rolling percentage of 1%. However, it is not possible to measure the strength of the material after the material has been rolled.

However, experiments have shown that there is a direct and reproducible relationship between the post-roll percentage and the microstrain for ULC steel. The ULC steel according to the invention is therefore preferably characterized in that the steel has a microstrain of less than 0.05%, preferably less than 0.03%. A high re-rolling percentage and a high microstrain lead to an undesirably sharp decrease in the deformation properties of the strip steel. The microstrain is determined by means of a standardized measuring method, in which the lattice disturbance of the iron atoms in the steel is measured by X-ray diffraction. The deviation from the standard spread in grid spacing of pure, undisturbed iron is a measure of the microstrain. The microstrain is determined on the basis of the broadening of the (211) reflection of Fe, measured on a plane, plan parallel to the rolling plane and half the thickness of the material.

According to an advantageous feature of the invention, the ULC steel has a Lankford value r for which rgem> 2.8 - 0.004Rp applies. The Lankford value is a measure of the anisotropy of the material. A high Lankford value is favorable for sheet material, since the sheet material becomes thinner less when deformed. Therefore, when pressing and deep drawing parts, whereby the material is extended in the plane of the plate, the material will not succumb quickly.

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A known from the literature for the Lankford value is that rgem = 2.6 -0.004Rp. The ULC steel according to the invention therefore has a significantly higher Lankford value at the same yield strength.

According to a preferred embodiment, the ULC steel according to the invention has a percentage of recrystallized grains which is greater than 95% and less than 99.7%, and which is preferably 98% to 99%, and more preferably about 98%. Recrystallization takes place during the annealing of the strip steel. The percentages given are achieved by heating the steel to a lower than usual temperature during annealing.

The ULC steel according to the invention is preferably provided in the form of strip steel or galvanized strip steel.

According to a second aspect of the invention, there is provided a method for obtaining a high strength Ultra Low Carbon steel, wherein Ultra Low Carbon steel is subjected to a annealing treatment in a continuous annealing line 15, the maximum annealing temperature being less than 760 ° C , and is at least 650 ° C.

This method ensures that the steel does not recrystallize completely, but that a small percentage of grains do not recrystallize. This small percentage of non-recrystallized grains gives the steel an extra strength, without having to use a larger than usual amount of alloying elements and without negatively affecting the other properties of the steel. Too high a percentage of non-recrystallized granules will also increase the microstrain. As mentioned before, too high a microstrain strongly decreases the deformation properties.

The invention will be elucidated with reference to the figures.

Figure 1 shows a graph with values for the yield strength Rp from the literature and according to the ULC sample of the invention.

Figure 2 shows a graph with measured values of the microstrain of ULC steel plotted against the elongation at break.

An exemplary embodiment is an Ultra Low Carbon Steel with the composition: '1 C13 / -70 -6- C <40 ppm N <40 ppm Mn <0.8 wt% P <0.075 wt% 5 Si <0.5 wt%

Mo <0.2 wt%

Cr <0.2 wt% 3 ppm <B <20 ppm unavoidable impurities; 10 an amount of titanium and niobium for which:

Ti / 48 + Nb / 93> (C - 0.0015) / 12 + N / 14 + S / 32 (in wt%); balance Fe.

This ULC steel has been subjected to the usual treatment for producing strip steel, however, during the annealing treatment in the continuous annealing line, the annealing temperature is a maximum of 760 ° C. The annealing temperature must not be lower than 650 ° C. After annealing, a post-rolling treatment follows, whereby a post-rolling percentage of about 1% is achieved.

Due to this treatment, the ULC strip steel has a yield strength Rp between 180 and 400 MPa, where: 20 Rp> 160 + 15Mn + 45Si + 1293P + 22540B + 54Mo - 84Cr (in wt%).

The yield strength thus obtained is clearly higher than the known values for steel from the literature, as can also be seen from the graph of figure 1. This graph shows two measurements of the ULC steel according to the invention with open circles; known values are shown with closed circles and diamonds. The measured value Rp-B is shown on the vertical axis; the calculated value Rp-A is shown on the horizontal axis. In determining this, with the known amounts of alloying elements for each known steel, the known yield strength is used to calculate the constant C in the formula

Rp = C + 15Mn + 45Si + 1293P + 22540B + 54Mo - 84Cr (in wt%) 30. For all known types of steel, the mean value of the constant C appears to be Cgem = 143.8. With this average value Cgem, the calculated value Rp-A was then determined for each known type of steel.

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The value Rp-A = Rp-B = 143.8 + 15Mn + 45Si + 1293P + 22540B + 54Mo -84Cr, which on average applies to all known steel types, is the drawn oblique line in the graph. A deviation from this line up or down means that the steel type is stronger or less strong, respectively, than might be expected on the basis of the formula for the average of the steel types.

The graph clearly shows that for the ULC steel according to the invention the calculated value Rp-A with Cgem = 143.8 (the oblique line) is significantly lower than the measured value Rp-B (the open circles). For the ULC steel according to the invention, therefore, a conctant C which is significantly higher applies in the formula. In any case, this constant 10 is higher than 160, and for the measurements shown in the graph even higher than 165 or 170 and even higher than 175.

The re-rolling percentage cannot be measured on the product itself. However, the retreading percentage is important as it should not exceed about 1.5%; at a re-rolling percentage of 3%, the desired yield strength can be achieved, but other properties, such as deformability, deteriorate sharply.

A good, measurable measure of the re-rolling percentage is the microstrain of the material. It has been found that the microstrain should be less than 0.05%, preferably less than 0.03%. Figure 2 shows the microstrain on the horizontal axis plotted against the breaking strain A80 on the vertical axis. The graph clearly shows that if the 20 microstrain remains low enough, the breaking elongation, and therefore the deformability, remains high enough. The microstrain is determined by the X-ray diffraction broadening of the (211) reflection of Fe.

The favorable properties of the material according to the invention partly depend on the percentage of non-crystallized granules. Usually, a 100% recrystallization is obtained by annealing. By annealing at a slightly lower temperature than usual, and a temperature of up to 760 ° C, a percentage of recrystallized grains is greater than 95% and less than 99.7%. Preferably, the percentage of recrystallized granules is 98% to 99%, and more preferably about 98%. To obtain such a percentage, the annealing process must be carefully controlled. The annealing temperature should not fall below 650 ° C.

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It has been found that the ULC steel according to the invention has a Lankford value r, where rgem> 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 types. A high Lankford value is favorable for sheet material, since the thickness reduction with material extension is lower the higher the Lankford value.

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Claims (15)

1. Ultra Low Carbon steel with the composition: -9- C <40 ppm Ultra Low Carbon steel according to claim 1, wherein: Mn <0.2 wt% P <0.05 wt% with a yield strength> 220 MPa. Ultra Low Carbon steel according to claim 1, wherein: Mn <0.5 wt% P <0.075 wt% with a yield strength> 260 MPa. Ultra Low Carbon steel according to claim 1, wherein: Mn <0.8 wt% P <0.075 wt% with a yield strength> 300 MPa. 5. Ultra Low Carbon steel with the composition: C <40ppm 1 0 13 T> O - ΙΟΝ <40 ppm Μη <0.8 wt% Ρ <0.075 wt% Si <0.5 wt% 5. ppm <B <20 ppm inevitable impurities; an amount of titanium and niobium for which holds: Ti / 48 + Nb / 93> (C - 0.0015) / 12 + N / 14 + S / 32 (in wt%); balance Fe; 10 with a yield strength between 180 and 400 MPa, where: Rp> 160 + 15Mn + 45Si + 1200P + 20000B (in wt%). 5 N <40 ppm 3 ppm <B <20 ppm Mn, Si and P as alloying element inevitable impurities; an amount of Ti and Nb for which applies: 6. Ultra Low Carbon steel with the composition: C <40 ppm 15 N <40 ppm Mn <0.8 wt% P <0.075 wt% Si <0.5 wt% Mo <0.2 wt% 20 Cr <0.2 wt% 3 ppm <B <20 ppm inevitable impurities; an amount of titanium and niobium for which holds: Ti / 48 + Nb / 93> (C - 0.0015) / 12 + N / 14 + S / 32 (in wt%); Balance Fe; with a yield strength between 180 and 400 MPa, where: Rp> 160 + 15Mn + 45Si + 1293P + 22540B + 54Mo - 84Cr (in wt%). Ultra-Low Carbon steel according to any one of the preceding claims, wherein the steel has a microstrain of less than 0.05%. 1013776 -11- Ultra Low Carbon steel according to any one of the preceding claims, wherein the steel has a microstrain of less than 0.03%. Ultra-low carbon steel according to any one of the preceding claims, wherein the steel has a Lankford value r for which rgem> 2.8 - 0.004Rp applies. Ultra Low Carbon steel according to any one of the preceding claims, wherein the percentage of recrystallized grains is greater than 95% and less than 99.7%. 10 10 Ti / 48 + Nb / 93> (C - 0.0015) / 12 + N / 14 + S / 32 (by weight%); balance Fe; with a yield strength Rp between 180 MPa and 400 MPa, where Rp> 160 + 40 Mn + 80 Si + 1000 P (in weight%). Ultra Low Carbon steel according to any one of the preceding claims, wherein the percentage of recrystallized grains is 98% to 99%. Ultra-low carbon steel according to any one of the preceding claims, wherein the percentage of recrystallized grains is about 98%. Ultra Low Carbon Steel according to any one of the preceding claims in the form of strip steel. Ultra Low Carbon Steel according to any one of the preceding claims in the form of galvanized steel strip. 15. A method for obtaining a high strength Ultra Low Carbon steel, wherein Ultra Low Carbon steel is subjected to a annealing treatment in a continuous annealing line, the maximum annealing temperature being less than 760 ° C and a minimum of 650 ° C. 1013778