KR20170047254A - Cold rolled high strength low alloy steel - Google Patents

Abstract

The present invention relates to a high strength low alloy steel. According to the present invention, the high-strength low-alloy steel strip, sheet or blank coated with zinc or zinc alloy contains 0.03 to 0.07 wt% of C, 0.70 to 1.60 wt% of Mn, 0.01 to 0.2 wt% of Si, P: 0.03 wt.%, S: 0.025 wt.%, O: 0.01 wt.%, Ti: 0.02 wt.%, Cr: 0.1 wt.%, Cu: 0.2 wt.%, The steel strip, the sheet or the blank has a composition of 0.07 wt%, V: 0.04-0.15 wt%, Mo: 0.03 wt%, Nb: 0.03 wt%, Ca: And a yield strength (Rp0.2) of 420 MPa or more.

Description

{COLD ROLLED HIGH STRENGTH LOW ALLOY STEEL}

The present invention relates to a high strength low alloy steel strip, sheet or blank. The present invention also relates to a method of manufacturing such a high strength low alloy steel strip.

High strength low alloy steels (HSLA steels) are known in the art. HSLA steels are often used in the automotive industry. For example, HSLA steels are defined in the specifications of the German Automobile Manufacturers Association (VDA). Refer to VDA 239-100 Material Specification for August 2011. According to the VDA, cold-rolled HSLA steels are indicated by steel grade numbers, for example CR420LA where CR is cold rolling and numeral 420 is the lower limit of yield strength (Rp0.2) in the longitudinal direction , LA means low alloy. The VDA specification provides chemical compositions for HSLA steels, including Ti and Nb, in addition to the standard alloying elements C, Mn, Si and Al to provide high strength.

The thin HSLA steel strip, sheet or blank is typically coated with an aluminum coating or a zinc coating. If a zinc coating is used, the coating is often applied as a hot dip galvanized or a hot dip zinc galvanized coating.

HSLA steels cold-rolled at high strength levels have the disadvantage that due to their high strength, it is difficult to cold-roll the hot-rolled strip to a comparatively thin gauge at wide dimensions.

It is an object of the present invention to provide an HSLA steel strip which can be cold rolled to a relatively thin gauge in a wide dimension and which can be made of HSLA sheets and blank and which has the required strength.

A further object of the present invention is to provide an HSLA steel strip, sheet or blank having the required elongation.

It is a further object of the present invention to provide a method of making the HSLA steel strip.

According to the invention, the at least one purpose is achieved with a high strength, low alloy steel strip, sheet or blank coated with a zinc or zinc alloy having the following composition:

C: 0.03 - 0.07% by weight;

Mn: 0.70-1.60 wt%;

0.01 to 0.2% by weight of Si;

Al: 0.005-0.1 wt%;

Cr:? 0.1 wt%;

Cu:? 0.2 wt%;

N:? 0.008 wt%;

P:? 0.03% by weight;

S:? 0.025% by weight;

O:? 0.01 wt%;

Ti: 0.02-0.07 wt%;

V: 0.04 - 0.15 wt%;

Mo:? 0.03% by weight;

Nb:? 0.03% by weight;

Ca:? 0.05 wt%;

The balance iron and unavoidable impurities,

The steel strip, sheet or blank has a yield strength (Rp0.2) of 420 MPa or higher.

The inventors have discovered that when Ti and V are used as a combination of alloying elements instead of a combination of Ti and Nb as known in the VDA standard, a steel is produced that provides a lower mill load. The Ti and V levels should be used in combination with the specific levels of C, Mn and Si, as embodied in the present invention. Within the scope of the present invention, it is possible to achieve a yield strength (Rp0.2) of 420 MPa or more.

Preferably, the HSLA steel according to the present invention does not contain Cr, Cu, Mo and Nb. These elements are not required to provide HSLA steels with the required yield strength.

Vanadium provides precipitation strengthening and some grain refinement. At a V concentration of less than 0.04% by weight, the volume of the vanadium carbide precipitate is insufficient to provide enough additional precipitation hardening to reach a strength of 420 MPa for Rp0.2. At concentrations greater than 0.15 wt% V, recrystallization during annealing is suppressed. This limits elongation.

Titanium also provides precipitation strengthening and some grain refinement. At a Ti concentration greater than 0.07% by weight, the work hardening during cold rolling will increase significantly and limit high cold reduction. On the other hand, the inventors have found that a Ti concentration of less than 0.02% by weight will reduce the overall elongation of the steel strip, sheet or blank. The combination of the appropriate amounts of Ti and V appears to generate certain microstructures that provide both high strength and elongation.

Carbon is useful for obtaining greater strength by increasing solution strengthening. Therefore, 0.03 wt% or more of C should be added. However, too high concentrations will limit cold rolling and reduce elongation. For this reason, the amount of carbon is limited to 0.07% by weight.

Manganese is also used for strengthening employment and has an effect similar to C, but with lower strength. Therefore, in order to cope with the increase in strength, the minimum amount required is 0.7 wt% Mn. In addition, a large amount of addition will affect the surface quality and increase the cost. Therefore, the upper limit used is 1.60 wt% Mn.

Nitrogen has an effect similar to C. This element will be preferentially combined with Al and Ti to form AlN and TiN precipitates. TiN precipitates are already formed in the reheating oven as well as at high temperatures during hot rolling and coiling. The precipitate is a large precipitate (several microns) and does not increase the strength. AlN can also be formed at high temperatures. Despite rapid cooling and a coiling temperature of less than 650 ° C., the precipitation can be partially stopped and the sources of Al and N, which can deposit during the continuous annealing and contribute to precipitation strengthening, are retained in the solid solution. If a large amount of N (> 0.008 wt.%) Is added, the elongation will decrease and cracks in the slab will occur.

Silicon is used for solid solution strengthening, but at high concentration (> 0.2 wt% Si), silicon will degrade the surface quality. If the concentration is less than 0.01% by weight Si, the smelting cost for Si removal will be very large.

 Phosphorus is used to strengthen employment, but high concentrations will reduce the ductility. Therefore, the concentration should be less than 0.03 wt.% P.

Aluminum is used as a deoxidizer in steel and the minimum amount of aluminum to ensure deoxidation should be 0.005 wt% Al. At Al concentrations greater than 0.1 wt.%, The generation of surface defects resulting from alumina clusters increases.

Niobium is kept as low as possible and even avoided because it greatly increases the work hardening and limits the cold pressing of the wide strip. Also, at Nb concentrations above 0.03 wt.%, Niobium has a large effect on the recrystallization temperature utilizing a high annealing temperature (higher than 800 DEG C) required to obtain a properly recrystallized HSLA.

Cr, Cu, S, O, Mo and Ca should all be low. For example, a high level of S, as is known in the art, will degrade the ductility of the steel.

According to one preferred embodiment, the one or more alloying elements may be present in a limited amount as follows:

C: 0.04 - 0.06% by weight; And / or

Mn: 0.80-1.40% by weight and preferably Mn: 0.80-1.30% by weight; And / or

0.01 to 0.1% by weight of Si and 0.01 to 0.05% by weight of Si; And / or

Al: 0.015 - 0.055 wt%; And / or

Cr:? 0.05 wt%; And / or

Cu:? 0.05 wt%; And / or

N: 0.002 to 0.008% by weight; And / or

O:? 0.005 wt%; And / or

Ti: 0.02-0.06 wt%; And / or

V: 0.05 - 0.15 wt%; And / or

Mo:? 0.01 wt% and preferably 0? Mo? 0.01 wt%; And / or

Nb: 0.02% by weight and preferably Nb: 0.01% by weight; And / or

Ca:? 0.01 wt%;

The object of the present invention is to maximize elongation at a given strength level and to roll as wide as possible for a given gauge at a given strength level.

Narrowing the carbon range provides maximum elongation at a given strength level. Increasing the minimum C level increases the proof stress of the material. Reducing the upper C level of the upper limit minimizes the cold rolling load and achieves the highest combination of maximum width and elongation at these higher strength levels.

Manganese not only provide employment intensification, but also assist in recrystallization. By increasing the minimum level of Mn, a better bond of strength and ductility is achieved. Excessive Mn increases the likelihood of MnS stringers which are not good for surface conditions and can be detrimental to ductility. It is therefore also advantageous to reduce the maximum Mn level. Reduced silicon levels have an advantage over surface quality.

Narrowing the aluminum range improves deoxidation and limits the risk of surface defects.

Titanium delays recrystallization. Minimizing the maximum Ti level can help to optimize the elongation for a given intensity level.

Vanadium delays recrystallization. Minimizing the maximum V level can help to optimize elongation for a given intensity level.

Minimizing the niobium level further aids in broad rolling at a given strength level of cold rolled and annealed products.

Minimizing the residual elements further aids in improving elongation at a given strength level.

Preferably, the steel strip, sheet or blank has a yield strength (Rp0.2) of at least 460 MPa in the longitudinal direction, and more preferably a yield strength (Rp0.2) of at most 580 MPa. The automotive industry prefers to use HSLA steels having the above yield strength according to the VDA standard.

According to one preferred embodiment, the steel strip, sheet or blank has an elongation (A 80 mm) of at least 15% in the longitudinal direction. This is the elongation required for the CR460LA grade according to the VDA standard.

Preferably, the steel strip, sheet or blank has a tensile strength (Rm) of at least 480 MPa in the longitudinal direction, more preferably a tensile strength (Rm) of at least 520 MPa, more preferably a tensile strength at Rm ). The tensile strength is preferred in the automotive industry according to the VDA standard.

According to one preferred embodiment, the zinc or zinc alloy coating is a hot dip galvanized or hot dip zinc galvanized coating. These coatings are zinc coatings commonly used in the automotive industry.

In another preferred embodiment, the zinc alloy coating comprises from 0.5 to 4 wt% Al and from 0.5 to 3.2 wt% Mg, the balance zinc and minor amounts of other elements. The coating preferably has a thickness of 5 to 15 占 퐉 per side, more preferably 6 to 13 占 퐉 per side. This is a so-called AlMgZn coating that provides corrosion protection compared to conventional zinc coatings. Other elements that may be present are Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn, Bi and Sb are usually added to form a sequin. The elements may be expressed in small amounts, each less than 0.5 wt.%, Typically less than 0.2 wt.% Each, often together less than 0.2 wt.%.

According to a second aspect of the present invention there is provided a method of making a high strength low alloy steel strip comprising the steps of:

According to a first aspect of the present invention, there is provided a method for producing a molten steel,

Casting a molten steel in a casting apparatus,

Hot rolling the casting to a strip at a terminal temperature of at least 880 DEG C,

Coiling the hot rolled strip at a coiling temperature of 500 ° C to 650 ° C,

Cold-rolling the strip at a total reduction of 50-75%;

Continuously annealing the strip at an annealing temperature of 750 ° C to 820 ° C.

Due to the coiling temperature, the reduction rate and the annealing temperature of the strip according to the method of the second aspect of the present invention, an HSLA strip having the composition according to the first aspect of the present invention with a yield strength (Rp0.2) of 420 MPa or more is provided It is possible to do.

The coiling temperature affects the deposition of V and mainly VC. At 550 캜, there is a small amount of VC to assist in cold rolling (less work hardening). At higher coiling temperatures, the volume of VC precipitate will increase and will increase work hardening, making cold rolling more difficult and eventually limiting the width of the cold-rolled strip under defined cooling downs. Above 650 캜, the VC precipitate will begin to coalesce, and then the advantage of precipitation strengthening in the cold rolled annealed end material will be reduced. At a coiling temperature of less than 500 占 폚, there is a possibility of bainite formation in the hot rolled coils. Bainite will increase the cold rolling load. Preference is given to avoiding bainite, and temperatures below 500 ° C are not recommended.

As far as cold rolling is concerned, in principle there is no limiting factor as long as it has a strong mill to cold-roll up to 90%. Further, the higher the cold reduction ratio, the easier the recrystallization of the steel species becomes. It will allow the use of low annealing temperatures that are high cooling pressures.

Therefore there is a duality between the cooling-down percentage and the annealing temperature. As described above, it will allow a lower annealing temperature, which results in a higher cooling pressure. The upper limit of the annealing temperature is controlled by the coarsening / dissolution of the VC precipitate. The upper limit value should be lower than the solubility temperature of the VC precipitate by 20 ° C or more. The solubility of the VC precipitate depends on the V (and C) concentration. In the corresponding part, the volume of VC precipitate will affect recrystallization of the steel; The larger the VC volume, the higher the recrystallization temperature.

In each variation of the concentration of V in the steel composition, a balance must be made between the concentrations of C, Mn, N and Ti and the cold rolling reduction to define the annealing temperature.

Preferably, the annealed strip is dip-dipped zinc coated with a zinc or zinc alloy coating. Continuous annealing usually follows immediately after hot dip zinc coating with zinc or zinc alloys.

According to a preferred embodiment, the coated strip is cold-rolled in a temper mill at a reduction rate of 0.1-3.0%, preferably 0.2-2.0%. Temper rolling provides improved surface quality to the strip. At higher levels of temper rolling, the removal of the Luders line as well as increased yield strength is also seen.

Preferably, the strip is cold rolled to a gauge of 0.7 to 2.0 mm at a width of at least 1400 mm, preferably at least 1600 mm wide, more preferably at least 1800 mm wide. This is possible because HSLA with Ti and V has improved ductility compared to HSLA with Ti and Nb or Nb and V. [

According to a preferred embodiment, the coiling temperature of the hot-rolled strip is between 550 ° C and 600 ° C and / or the overall cold rolling reduction is between 60-70% and / or the annealing temperature is between 760 ° C and 800 ° C. To achieve optimal ductility, utilizing one or more of the limited ranges in the required steps provides optimal properties after cold rolling and galvanizing. This makes it easier to cold-roll with the required gage and width.

Preferably, the steel used in the process has the composition provided by the preferred embodiment of the composition according to the first aspect of the present invention.

According to a preferred embodiment, the produced steel strip has a yield strength (Rp0.2) of at least 420 MPa, preferably a yield strength (Rp0.2) of at least 460 MPa, more preferably a yield strength (Rp0.2) of at most 580 MPa, .

Preferably, the steel strip produced has an elongation (A 80 mm) of at least 15%.

The present invention will be described with reference to the following examples.

A plurality of strips are manufactured as complete production materials. Samples of the strip are referred to as 1, 2, 3 and 4. In each of the samples, Modifications A and B are tested, and as shown in Table 1, Modifications A and B each have the same composition, but different coiling temperatures and different temper rolling pressures are used for Modifications A and B do. The information on the coiling temperature and the temper rolling is given in Table 2 together with the cold reduction percentage and the annealing temperature.

Table 2 shows that for the composition according to the invention it is possible to reach a yield strength (Rp0.2) of 420 MPa or more for a cold reduction of 60%, and as shown in samples 2, 3 and 4, It is possible to reach a yield strength (Rp0.2) of 460 MPa by correct selection of ring temperature and annealing temperature. The rough rolling reduction for these samples is at most 1%.

Table 2 also shows that the elongation (A80 mm) for the samples tested is typically at least 15%. Only sample 4A with the highest yield strength (Rp0.2), elongation (A80mm) is slightly lower than 15%.

It should be noted here that elongation in sample 1A-2B (A80 mm) is measured in the rolling direction of the strip, but elongation in samples 3A-4B is measured in the transverse direction of the strip. This explains to some extent the reason why the elongation (A80 mm) is lower in samples 3A-4B, although higher yield strengths (Rp0.2) typically represent lower elongation (A80 mm).

Table 3 shows experimental samples 5 and 6 of the same production material as used in samples 1A and 1B and samples 5 and 6 are treated with annealing temperatures near or beyond the limits given in accordance with the present invention.

Sample 5 shows that Rp0.2 will be very low at very high annealing temperatures. Sample 6 shows that the elongation (A80 mm) will be lower than desired when the annealing temperature is very low. Samples 5 and 6 therefore show that the annealing temperature is very important for reaching the desired properties.

As a comparative example, an experimental sample containing more Ti but less (almost) V than is required according to the present invention is tested. The composition is shown in Table 4. The amounts of P and S were not measured, but the elements were not added and therefore fall within the given limits of the present invention.

Processing conditions are given in Table 5. Although the selected annealing temperature exceeds the upper limit according to the invention, the comparative example shows very low yield strength (Rp0.2) and the use of Ti without V will not result in the required yield strength.

A further plurality of strips are also produced as complete manufacturing materials. A sample of the strips is referred to as numeral 1 when having the same composition as Sample 1 above. Other examples are referred to as Samples 7-14. In Sample 1, Modifications C, D, and E are tested, and as shown in Table 6, these deformations each have the same composition, but different coiling temperatures and different temper rolling depressions are used for these modifications. Modifications A and B are tested for Samples 7, 8, 9 and 10, which have the same composition each time, but in most cases different coiling temperatures, annealing temperatures or temper rolling are used. In Samples 11-14 only one variant is tested each time. The information on the coiling temperature and the temper rolling is given in Table 7, together with the cold pressing percentage and the annealing temperature.

Table 7 shows that it is possible to reach a yield strength (Rp0.2) of 420 MPa or higher at a 60 or 65% cold reduction rate for the composition according to the present invention, and with appropriate selection of composition, coiling temperature and annealing temperature, , 1E and samples 7A, 7B, 8A, 8B, 11, 12, 13 and 14, it is possible to reach a yield strength (Rp0.2) of 460 MPa or more. The rough rolling reduction for most of the samples is at most 1%; Only in samples 8A, 8B and 14 the temper rolling reduction is 1.4%.

Table 7 also shows that the elongation (A80 mm) is typically at least 15% for the samples tested. Only for Samples 13 and 14 with very high yield strength (Rp0.2), the elongation (A80 mm) is slightly less than 15%.

For Samples 1C-1E and 7A-10B, the elongation (A80 mm) was measured in the rolling direction of the strip, but for Samples 11-14 the elongation was measured in the transverse direction of the strip. This explains to some extent the reason why the elongation (A80 mm) is lower for Samples 11-14, although higher yield strengths (Rp0.2) typically represent lower elongation (A80mm).

In the compositions of Tables 1, 4 and 6, no molybdenum was added. The amount of molybdenum described in the tables above is the amount of molybdenum present as a residual element, and thus an unavoidable impurity. Chromium and copper are not listed in Tables 1, 4 and 6, and the elements are also not added to the steel, but the elements are present as residual elements and therefore inevitable impurities.

From the discussion of the steel composition, it is clear that Mo, Cr and / or Cu can be added to the steel.

Claims (15)

In a high strength low alloy steel strip, sheet or blank having the following composition and coated with zinc or zinc alloy,
C: 0.03 - 0.07% by weight;
Mn: 0.70-1.60 wt%;
0.01 to 0.2% by weight of Si;
Al: 0.005-0.1 wt%;
Cr:? 0.1 wt%;
Cu:? 0.2 wt%;
N:? 0.008 wt%;
P:? 0.03% by weight;
S:? 0.025% by weight;
O:? 0.01 wt%;
Ti: 0.02-0.07 wt%;
V: 0.04 - 0.15 wt%;
Mo:? 0.03% by weight;
Nb:? 0.03% by weight;
Ca:? 0.05 wt%;
The balance iron and unavoidable impurities,
The steel strip, sheet or blank has a yield strength (Rp0.2) of 420 MPa or higher. The method according to claim 1,
C: 0.04 - 0.06% by weight; And / or
Mn: 0.80-1.40% by weight and preferably Mn: 0.80-1.30% by weight; And / or
0.01 to 0.1% by weight of Si and 0.01 to 0.05% by weight of Si; And / or
Al: 0.015 - 0.055 wt%; And / or
Cr:? 0.05 wt%; And / or
Cu:? 0.05 wt%; And / or
N: 0.002 to 0.008% by weight; And / or
O:? 0.005 wt%; And / or
Ti: 0.02-0.06 wt%; And / or
V: 0.05 - 0.15 wt%; And / or
Mo:? 0.01 wt% and preferably 0? Mo? 0.01 wt%; And / or
Nb: 0.02% by weight and preferably Nb: 0.01% by weight; And / or
Ca:? 0.01 wt%;
In, steel strip, sheet or blank. 3. The method according to claim 1 or 2,
The steel strip, sheet or blank has a yield strength (Rp0.2) of at least 460 MPa in the longitudinal direction, preferably a yield strength (Rp0.2) of at most 580 MPa. 4. The method according to any one of claims 1 to 3,
Wherein the steel strip, sheet or blank has an elongation (A 80 mm) of at least 15% in the longitudinal direction. 4. The method according to any one of claims 1 to 3,
The steel strip, sheet or blank has a tensile strength (Rm) of at least 480 MPa in the longitudinal direction, preferably a tensile strength (Rm) of at least 520 MPa, more preferably a tensile strength (Rm) of at most 680 MPa, Sheet or blank. 5. The method according to any one of claims 1 to 4,
Wherein the zinc or zinc alloy coating is a hot dip galvanized or hot dip galvanized coated steel strip, sheet or blank. 5. The method according to any one of claims 1 to 4,
The zinc alloy coating comprises from 0.5 to 4 wt% Al and from 0.5 to 3.2 wt% Mg, the balance zinc and minor amounts of other elements, preferably the coating has a thickness of from 5 to 15 μm per side, A steel strip, sheet or blank having a thickness of 6 to 13 microns per side. A method of manufacturing a high strength low alloy steel strip comprising the steps of:
Producing a molten steel having the composition of claim 1,
Casting the molten steel in a casting apparatus,
Hot rolling the casting to a strip at an end temperature of at least 880 DEG C,
Coiling the hot rolled strip at a coiling temperature of 500 ° C to 650 ° C,
Cold-rolling the strip at a total reduction of 50-75%, and
And continuously annealing the strip at an annealing temperature of 750 ° C to 820 ° C. 9. The method of claim 8,
Wherein the annealed strip is melt immersion coated with a zinc or zinc alloy coating. 10. The method of claim 9,
Wherein the coated strip is cold-rolled in a temper mill at a reduction of 0.1-3.0%, preferably a reduction of 0.2-2.0%. 11. The method according to any one of claims 8 to 10,
Wherein said strip is cold-rolled to a gauge of 0.7-2.0 mm at a width of at least 1400 mm, preferably at least 1600 mm wide, more preferably at least 1800 mm wide. The method according to any one of claims 8 to 11,
Wherein the coiling temperature of the hot-rolled strip is from 550 占 폚 to 600 占 폚 and / or the overall cold rolling reduction is 60-70% and / or the annealing temperature is 760 占 폚 to 800 占 폚. 13. The method according to any one of claims 8 to 12,
Wherein the steel has the composition provided by claim 2. 14. The method according to any one of claims 8 to 13,
The produced steel strip has a yield strength (Rp0.2) of 420 MPa or higher, preferably a yield strength (Rp0.2) of at least 460 MPa, more preferably a yield strength (Rp0.2) of at most 580 MPa, Gt; 15. The method according to any one of claims 8 to 14,
Wherein the produced steel strip has an elongation (A80 mm) of 15% or more.