Ultra-low carbon interstitial-free steel

Abstract

The present invention relates to an extra-low carbon interstitial-free steel with an improved composition of Ti+Nb+V up to 0.10% by weight, which steel has very good ductility.

Description

[Technical Field]

[0001] The present invention relates to an ultra low carbon interstitial free steel. [Background technology]

[0002] Interstitial-free steel (IF steel) is a mild steel with excellent formability because the interstitial elements carbon, nitrogen, and boron are all bound by, for example, titanium, niobium, or aluminum to form carbides, nitrides, and borides. Therefore, the interstitial elements do not impede the movement of dislocations within the iron grains. Due to the stabilization of carbon, nitrogen, and boron, IF steel can be stored almost indefinitely, in contrast to bake-hardenable steel.

[0003] Due to its high formability, IF steel is primarily used in the automotive industry to manufacture body-in-white parts, which are often complex to press-form. Therefore, high formability is required. Due to its low carbon content, IF steel is a mild steel with low yield strength and high ductility.

[0004] For example, IF steels are standardized in EN 10346:2015. In this standard, low-carbon steels for cold forming are listed in Tables 1 and 7. They are designated DX51 to DX57, with higher numbers indicating better formability. The best formability is provided by DX57. According to this standard, these IF steels contain a maximum of 0.30% titanium by mass.

[0005] German standard VDA 239-100 (2016) also provides cold-rolled mild steels in Tables 6 and 24. These steels are designated CR1 to CR5 and have nearly identical compositions and mechanical properties to the DX types of norm EN 10346:2015. CR6 cold-rolled mild steel with improved ductility is listed as a draft standard in the May 2019 revision of VDA 239-100.

[0006] It is known to be advantageous to use ultra-low carbon steels (ULC steels) as IF steels, since the lower the amount of carbon (and nitrogen) in the steel, the less titanium or niobium is needed to bind the carbon and nitrogen, and therefore in practice less of these elements are used than the maximum amounts given in the norms.

[0007] On the other hand, ULC steels must have reasonable yield and tensile strength as specified by the norm, which means that there must be sufficient presence of strength-providing elements such as manganese, phosphorus and / or silicon. Summary of the Invention [Problem to be solved by the invention]

[0008] It is an object of the present invention to provide an extra low carbon IF steel strip, sheet or blank with improved formability.

[0009] Another object of the present invention is to provide an extra low carbon IF steel strip, sheet or blank having an improved plastic strain ratio.

[0010] It is a further object of the present invention to provide an extra low carbon IF steel strip, sheet or blank having an improved strain hardening exponent.

[0011] Furthermore, it is an object of the present invention to provide strip, sheet or blank of an extra-low carbon IF steel that meets the requirements set out in the draft VDA 239-100 (May 2019) norm. [Means for solving the problem]

[0012] The present invention has the following elemental composition in weight percent: C: Maximum 0.005 Mn: max. 0.20 Si: max 0.10 Al: 0.01 to 0.20 Ti: max 0.10 Nb: max 0.10 V: max. 0.10 P: maximum 0.02 S: Maximum 0.02 N: Maximum 0.01 Depending on the situation, Cr: Max 0.10 Ni: max. 0.10 B: Maximum 0.0005 Ca: max 0.01 Cu: max. 0.10 Mo: Max 0.10 Sn: max 0.05 Fe and unavoidable impurities: balance and the sum of Ti+Nb+V is a maximum of 0.10; Steel, Transverse yield strength: 110-170 MPa; Tensile strength in the transverse direction: 250-330 MPa Elongation in the transverse direction A80: at least 42%; Average plastic strain ratio r: at least 2.00, and Strain hardening exponent n90 in the transverse direction: at least 0.22 The present invention relates to an extra low carbon interstitial free steel strip, sheet or blank having

[0013] The average plastic strain ratio r or average r value is calculated using the following formula: (r0+(2×r45)+r90) / 4 [In the formula, r0 is the plastic strain ratio in the rolling or longitudinal direction; r45 is the plastic strain ratio in the diagonal direction, r90 is the plastic strain ratio in the transverse direction.] is defined as:

[0014] Because measurements of the elongation A80 (in all three directions), plastic strain ratio r, and strain hardening exponent n values ​​may be imprecise and all measured values ​​may vary slightly for hot-rolled strip from one cast, the values ​​of A80, r, and n used in the claims are the average of at least three measurements performed on at least three different strips from one cast, thus statistically reducing imprecision.

[0015] The inventors have found that strip, sheet or blank of ultra-low carbon interstitial-free steel having this composition can have the above mechanical properties, where the elongation A80 in the transverse direction, the plastic strain ratio r and the strain hardening exponent n90 are particularly good indicators of the ductility required for IF steel. The inventors have also found that it is particularly necessary to limit the sum of Ti, Nb and V to 0.10 wt. % in order to obtain the required mechanical properties and ductility.

[0016] Carbon is normally present to give steel strength, but too much carbon will form more carbides and hinder ductility. Therefore, carbon is present in a maximum amount of 0.005% by weight, preferably a maximum of 0.003% by weight. More preferably, the maximum amount is 0.0022% by weight. Since some carbon is unavoidable and necessary for strength, the minimum amount is preferably 0.0001% by weight, more preferably 0.0005% or 0.0010% by weight.

[0017] Manganese is present in IF steel to impart strength, but at the same time, formability (or A80 and r-value) decreases with increasing Mn. Therefore, the maximum amount is preferably 0.20 wt.%, more preferably the maximum amount is 0.18 wt.% or 0.15 wt.%. The minimum amount is preferably 0.01 wt.%, more preferably 0.04 wt.%, depending on the amount of other elements in the IF steel.

[0018] Silicon can also be used to improve the strength of IF steel, but at the same time, formability (or A80 and r-value) decreases with increasing Si. For this reason, the maximum amount of silicon is preferably 0.10 wt%. Since silicon inhibits the ductility of the steel, the maximum amount of silicon is more preferably 0.05 wt%, and even more preferably 0.03 wt%. An even more preferred maximum amount of silicon is 0.015 wt% or 0.013 wt%. The preferred minimum amount of silicon is 0.001 wt% or 0.002 wt%, taking into account strength requirements.

[0019] Aluminum, like Ti, is used to kill the steel, i.e., to bond with N. For this reason, the amount of aluminum present in the steel is 0.01-0.20 wt.%. Preferably, the maximum amount of aluminum is 0.10 wt.%, more preferably 0.08 wt.%, so as not to interfere with the ductility of the IF steel.

[0020] Phosphorus can be used to improve the strength of IF steels, but at the same time, it reduces formability and is harmful to steelmaking. Phosphorus also leads to an increase in the ductile-brittle transition temperature (DBTT). Therefore, phosphorus should be present in a very small amount, preferably at most 0.02 wt%, more preferably at most 0.015 wt% or at most 0.013 wt%. The minimum amount, taking into account the required strength, is preferably 0.001 wt%, more preferably 0.002 wt%.

[0021] Sulfur and nitrogen are harmful to steelmaking and to the steel itself. Therefore, sulfur and nitrogen must be present in very small amounts, preferably at a maximum of 0.02% by weight and 0.01% by weight, respectively. More preferably, the maximum amount of S is 0.015% or 0.013% by weight, and the maximum amount of N is 0.008% or 0.006% by weight, and most preferably at a maximum of 0.004% by weight. In particular, nitrogen must be present in small amounts because it must be combined to form nitrides in IF steel. The preferred minimum amount of S is 0.001% or 0.003% by weight, and the preferred minimum amount of N is 0.001% by weight. These minimum amounts are due to steelmaking requirements.

[0022] Preferably, the sum of Mn+Si+(10×P) is at most 0.30 wt.%, more preferably at most 0.25 wt.%, most preferably at most 0.20 wt.% to achieve the required strength. The minimum value of this sum is preferably 0.06, more preferably 0.07 wt.%.

[0023] Titanium, niobium, and / or vanadium are added to bind carbon and nitrogen. Nitrogen can also be bound by aluminum or boron. In total, they must be present in sufficient amounts to ensure that neither carbon nor nitrogen remains in solid solution. However, considering the cost of these elements, they should also not be present in excess. Therefore, it is preferred that the total of Ti + Nb + V is at most 0.10 wt.%. More preferably, the amount of each of Ti and Nb and V is at most 0.09 wt.%, and the total of Ti + Nb + V is also at most 0.09 wt.%, and even more preferably, the amount of vanadium and / or niobium is at most 0.01 wt.%.

[0024] In a further preferred embodiment, vanadium, preferably neither vanadium nor niobium, is added to the IF steel, i.e., these elements are present only as unavoidable impurities. Titanium is preferably present in an amount of at least 0.01 wt.%, more preferably at least 0.03 wt.%, and most preferably at least 0.05 wt.%, especially when neither vanadium nor niobium is added to the IF steel.

[0025] Optional elements may be present in the amounts stated above. Preferably, the upper limits for these elements are even lower, for example, up to 60% of the amounts stated above for each added element. More preferably, these optional elements are not added to the IF steel at all, i.e., they are present only as unavoidable impurities.

[0026] The transverse yield strength Rp0.2 and the transverse tensile strength Rm preferably have minimum and maximum values ​​that are commercially used according to the norm.

[0027] An elongation in the transverse direction, A80, of at least 42% and an average plastic strain ratio, r, of at least 2.0 are among the most important factors determining the ductility of IF steel. In this case, the total elongation, designated A80, indicates a measure of the deformation that can be achieved when the steel is stretched to fracture. Thus, a high elongation, A80, is an indicator of the deformability of IF steel. The r-value is a measure of the resistance to wall thinning of steel used in deep drawing. The higher the r-value, the better the IF steel is for use in deep drawing. Thus, these parameters represent measures of achieving one or more of the objectives of the present invention. The steps for achieving these parameters are described below.

[0028] Additionally, the strain hardening exponent n90 in the transverse direction of IF steel is at least 0.22. The n-value is a measure that defines the response of IF steel to cold working. The higher the n-value, the more ductile the steel. To assess the ductility of a steel, the elongation A80, r-value, and n90-value must all be examined.

[0029] According to a preferred embodiment, the elongation A80 in the transverse direction of the steel is at least 44%, preferably at least 46%, more preferably at least 48%, even more preferably at least 50% and most preferably at least 52%. The inventors have found that it is possible to achieve these high elongation A80 values ​​together with high r- and / or n-values.

[0030] It is also preferred if the strain hardening exponent n90 in the transverse direction of the steel is at least 0.23, preferably at least 0.24. Such high n values ​​are very important for providing highly ductile IF steels.

[0031] Furthermore, it is preferred that the steel has an average plastic strain ratio r of at least 2.15, preferably at least 2.20, more preferably at least 2.25, even more preferably at least 2.30 or at least 2.35, and most preferably at least 2.4. Higher average r values ​​also indicate improved ductility of the IF steel.

[0032] Furthermore, it is preferred that the plastic strain ratio r45 in the diagonal direction of the steel is at least 1.8 or at least 1.9, preferably at least 2.0, more preferably at least 2.1, even more preferably at least 2.2, and most preferably at least 2.3. Providing an IF steel with a high r-value r45 in the diagonal direction provides an additional measure of the steel's ductility. In fact, the r-values ​​in all three directions need to be high enough to provide an IF steel with sufficient ductility for deep drawing into complex automotive parts.

[0033] Preferably, one or more of the elements in the steel are in the following ranges: C: 0.0001 to 0.003 Mn: 0.01 to 0.20, preferably 0.04 to 0.18 Si: 0.001 to 0.05, preferably 0.002 to 0.015 Al: 0.01 to 0.10 Ti: 0.01 to 0.09 Nb: max 0.09, preferably max 0.01 V: maximum 0.09, preferably maximum 0.01 P: Maximum 0.015 S: Maximum 0.015 N: Maximum 0.008 Depending on the situation, Cr: Max 0.06 Ni: maximum 0.06 B: Maximum 0.0004 Ca: max. 0.005 Cu: max. 0.06 Mo: Max 0.06 Sn: Max 0.03 exists in There is provided an interstitial-free low carbon steel strip, sheet or blank having a sum of Ti+Nb+V of maximum 0.09.

[0034] These more limited ranges provide IF steels that are particularly suitable for the high formability that is often required.

[0035] According to a preferred embodiment, the composition is as follows, in weight percent: C: 0.0001 to 0.0022 Mn: 0.01 to 0.15 Si: 0.001 to 0.013 Al: 0.02 to 0.08 Ti: 0.03 to 0.09 P: 0.001 to 0.013 S: 0.001 to 0.013 N: 0.001 to 0.006 Depending on the situation, Nb: max. 0.003 V: max. 0.005 Cr: Max 0.05 Ni: max. 0.05 B: Maximum 0.0003 Ca: max. 0.002 Cu: max. 0.05 Mo: Max 0.04 Sn: Max 0.02 In accordance with the present invention, there is provided an ultra-low carbon interstitial-free steel strip, sheet or blank having

[0036] This preferred composition is well suited to achieving the mechanical properties described above.

[0037] According to a more preferred embodiment, the composition is as follows, in weight percent: C: 0.0010 to 0.0022 Mn: 0.04 to 0.13 Si: 0.002 to 0.013 Al: 0.03 to 0.07 Ti: 0.05 to 0.09 P: 0.002 to 0.013 S: 0.003 to 0.013 N: 0.001 to 0.004 Depending on the situation, Nb: Max 0.002 V: maximum 0.004 Cr: Max 0.04 Ni: maximum 0.04 B: Maximum 0.0002 Ca: maximum 0.001 Cu: maximum 0.04 Mo: Max 0.02 Sn: Max 0.01 In accordance with the present invention, there is provided an ultra-low carbon interstitial-free steel strip, sheet or blank having

[0038] This preferred composition can be used to achieve the highest ductility required by the automotive industry.

[0039] The upper and / or lower limits of each element in the above two preferred embodiments may also be used to qualify the respective element in the main embodiment of the present invention provided above.

[0040] As an indication of the ductility of the IF steel according to the invention, it is preferred if the (average plastic strain ratio r) x (strain hardening exponent in the transverse direction n90) is at least 0.44, preferably at least 0.48, even more preferably at least 0.52, most preferably 0.56. This combination of the average r-value and the n-value in the transverse direction, together with the elongation A80, gives a good indication of the ductility of the IF steel.

[0041] As another indicator of the ductility of the IF steel according to the invention, it is preferred if the (plastic strain ratio in the diagonal direction r45) x (strain hardening exponent in the transverse direction n90) is at least 0.40, preferably at least 0.44, more preferably at least 0.48, most preferably at least 0.52. This combination of the r-value in the diagonal direction and the n-value in the transverse direction together with the elongation A80 value gives a different indicator of the ductility of the IF steel.

[0042] In a further indication of the ductility of an IF steel according to the invention, the (plastic strain ratio in the transverse direction r90) x (strain hardening exponent in the transverse direction n90) is at least 0.50, preferably at least 0.55, more preferably at least 0.58 or at least 0.60, even more preferably at least 0.62, and most preferably at least 0.64 or at least 0.66. Any combination of r- and n-values ​​in the transverse direction indicates ductility in the transverse direction, which is usually higher than the values ​​in the oblique directions.

[0043] Another indicator of the ductility of the IF steel according to the invention is given by the combination of the plastic strain ratio in the diagonal direction r45 and the strain hardening exponent in the diagonal direction n45, where (plastic strain ratio in the diagonal direction r45) x (strain hardening exponent in the diagonal direction n45) is at least 0.35, preferably at least 0.40, more preferably at least 0.45, even more preferably at least 0.48, and most preferably at least 0.50. Since both the r-value and n-value in the diagonal direction are usually lower than those in the transverse and rolling directions, this combination represents a valuable measure of the ductility of the steel.

[0044] According to a particularly preferred embodiment of the IF steel according to the invention, the yield strength Rp0.2 in the transverse direction is 110 to 155 MPa. The lower the yield strength of the IF steel, the better the formability.

[0045] The IF steel according to the invention is usually coated with a metal coating, preferably an aluminum alloy coating or a zinc alloy coating. Such coatings are frequently required in the automotive industry. These coatings are known to those skilled in the art and are usually applied to steel strips by hot-dip coating.

[0046] The method steps that can be used to obtain the IF steel of the present invention are described below.

[0047] The inventors have found that the ductility of the IF steel according to the present invention is greatly affected by the cold rolling reduction during cold rolling of the steel. It has been found that a cold rolling reduction of 80% to 85% is necessary to obtain a high r-value.

[0048] Furthermore, the skin-pass rolling of IF steel strip, which is applied after hot-dip galvanizing, also has a significant impact on the strength and ductility of the strip. The skin-pass reduction must be limited to less than 1.0%, preferably 0.4% to 0.7%.

[0049] Furthermore, the inventors have found that the temperature of the steel strip in the final stand of the hot-rolling finishing mill should preferably be 900-950°C, and the cooling rate in the run-out table should preferably be about 60°C / s to about 90°C / s, and therefore preferably be 25°C / s to 150°C / s. The coiling temperature of the hot-rolled strip is preferably about 700°C, and therefore preferably be 600°C to 750°C. All of these values ​​are valid for the center of the coil, and the values ​​at the head and tail ends may be slightly different. These parameters have been found to be particularly advantageous for IF steels containing titanium in an amount greater than 0.05% by weight.

[0050] Thus, in the method for producing interstitial-free low-carbon steel strip as described above, molten steel having a given composition is cast and cut into slabs, followed by hot rolling at a finishing temperature of 900-950°C, preferably 900-940°C, more preferably 900-930°C, and then cooled on a run-out table at a cooling rate of 25°C / s to 150°C / s, preferably 60°C / s to 90°C / s. The coiling temperature is 600-750°C, preferably 675-725°C. After cooling and pickling, the coil is cold rolled at a reduction of 78% to 88%, preferably 80% to 85%, and then continuously annealed at a temperature of about 810°C (800-820°C). After standard hot-dip galvanizing to form a GI coating, the strip is skin-passed at a reduction of 0.4% to 0.7%, preferably about 0.6%. Further method steps are known to those skilled in the art and are generally standard. [Example]

[0051] The invention will now be illustrated with reference to the following examples.

[0052] Table 1 shows the compositions of 16 examples of cast, hot rolled, cold rolled, hot dip galvanized, and skin-passed coils. Table 1a shows the elements that are significant, intentionally added, or present in large amounts. Table 1b shows the elements that are present in small amounts or as unavoidable impurities. All elements in Tables 1a and 1b are listed in milliwt%. Alzo stands for acid soluble aluminum.

[0053] All examples are examples of the present invention, as can be seen from Table 2. The yield strength Rp0.2, tensile strength Rm, elongation A80, plastic strain ratio r (r value), and strain hardening exponent n (n value) are shown in three directions: longitudinal direction (rolling direction of the coil), oblique direction (direction at 45 degrees to the rolling direction), and transverse direction (direction at 90 degrees to the rolling direction).

[0054] Also shown at the end of Table 2 is the average r-value (r_AVG), calculated as (r0 + (2 × r45) + r90) / 4, where r0 is the plastic strain ratio in the rolling or longitudinal direction, r45 is the plastic strain ratio in the oblique direction, and r90 is the plastic strain ratio in the transverse direction.

[0055] [Table 1]

[0056] [Table 2]

[0057] All coils shown in Tables 1 and 2 were cast with the composition shown in Table 1 and hot rolled to a finishing temperature of 920°C to 930°C. The cooling rate on the run-out table was approximately 60°C / sec, and the coiling temperature was approximately 710°C. All values ​​are valid for the center of the coil; values ​​at the head and tail ends may be slightly different.

[0058] After cooling and pickling, the coils were cold rolled with a reduction of 82% and continuously annealed at a temperature of about 810°C. After standard hot dip galvanizing to form the GI coating, the strips were skin passed with a reduction of 0.6%.

[0059] [Table 3]

[0060] The relatively low Rp0.2 and high levels of A80, r-values, and n-values, or a combination thereof, can be attributed to the relatively high amounts of Ti and Mn combined with the relatively low amounts of C and Si. Naturally, the process steps shown are preferred for achieving such values.

[0061] It will be apparent to one skilled in the art that process steps can be modified to arrive at the values ​​set forth in the claims.

[0062] The protection sought is not limited by the examples, in this respect only the limitations of the claims are of importance.

Claims

1. The following elemental composition in weight percent: C: 0.0010-0.0022 Mn: 0.04-0.13 Si:0.002~0.013 Al: 0.03~0.07 Ti: 0.05-0.09 Nb: maximum 0.002 V: maximum 0.004 P:0.002-0.013 S:0.003~0.013 N: 0.001-0.004 Depending on the situation, Cr: maximum 0.04 Ni: maximum 0.04 B: Maximum 0.0002 Ca: maximum 0.001 Cu: maximum 0.04 Mo: Maximum 0.02 Sn: maximum 0.01 Fe and inevitable impurities: balance and the sum of Ti+Nb+V is a maximum of 0.10, The interstitial-free low-carbon steel Yield strength in the transverse direction: 110-170 MPa, Tensile strength in the transverse direction: 250 to 330 MPa, Elongation in the transverse direction A80: at least 42%; Average plastic strain ratio r: at least 2.00; Strain hardening exponent n90 in the transverse direction: at least 0.22, and Plastic strain ratio r45 in the diagonal direction: at least 1.8 and wherein the values ​​of A80, r and n90 are, in the case of interstitial-free low carbon steel strip, the average values ​​of at least three measurements performed on at least three different strips from one casting; in the case of interstitial-free low carbon steel sheet, the average values ​​of at least three measurements performed on at least three different sheets from one casting; and in the case of interstitial-free low carbon steel blanks, the average values ​​of at least three measurements performed on at least three different blanks from one casting; the interstitial-free low carbon steel is coated with a zinc alloy coating; 1. A strip, sheet or blank of interstitial-free low carbon steel, wherein the interstitial-free low carbon steel is a steel standardized in EN 10346:2015.

2. 2. The interstitial-free low carbon steel strip, sheet or blank according to claim 1, wherein the interstitial-free low carbon steel has an elongation in the transverse direction A80 of at least 44%.

3. 3. Interstitial-free low carbon steel strip, sheet or blank according to claim 1 or 2, wherein the interstitial-free low carbon steel has a strain hardening exponent n90 in the transverse direction of at least 0.

23.

4. 4. Interstitial-free low carbon steel strip, sheet or blank according to any one of claims 1 to 3, wherein the interstitial-free low carbon steel has an average plastic strain ratio r of at least 2.

15.

5. 5. Interstitial-free low carbon steel strip, sheet or blank according to any one of claims 1 to 4, wherein the interstitial-free low carbon steel has a plastic strain ratio r45 in a diagonal direction of at least 1.

9.

6. 6. Interstitial-free low carbon steel strip, sheet or blank according to any one of claims 1 to 5, wherein (average plastic strain ratio r) x (strain hardening exponent in transverse direction n90) is at least 0.

44.

7. 7. Interstitial-free low carbon steel strip, sheet or blank according to any one of claims 1 to 6, wherein (plastic strain ratio in diagonal direction r45) x (strain hardening exponent in transverse direction n90) is at least 0.

40.

8. 8. Interstitial-free low carbon steel strip, sheet or blank according to any one of claims 1 to 7, wherein (plastic strain ratio in transverse direction r90) x (strain hardening exponent in transverse direction n90) is at least 0.

50.

9. 9. An interstitial-free low carbon steel strip, sheet or blank according to any one of claims 1 to 8, wherein (plastic strain ratio in diagonal direction r45) x (strain hardening exponent in diagonal direction n45) is at least 0.

35.

10. An interstitial-free low carbon steel strip, sheet or blank according to any one of the preceding claims, having a yield strength in the transverse direction Rp0.2 of 110 to 155 MPa.

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