Field of the invention

This invention relates to invention relates to a hybrid high strength low alloy cold- rolled and annealed steel strip and a method for producing said steel strip.

Background of the invention

High strength low alloy steel (HSLA steel) is well known in the art. HSLA steels are often used in the automotive industry. HSLA steels are for instance defined in the specification of the Verband Der Automobilindustrie (VDA). Reference is made to the VDA 239-100 Material specification of August 2016. According to the VDA, cold-rolled HSLA steels are indicated with a steel grade number, for instance CR420LA, wherein CR stands for cold-rolled, the number 420 stands for the lower limit of the yield strength Rpo.2 (in short: Rp) in longitudinal direction, and LA stands for low alloy. The VDA specification gives a chemical composition for HSLA steels containing Ti and Nb, apart from the standard alloying elements C, Mn, Si and Al, to provide for the high strength. However, these ranges in the specification are still very wide. Another relevant international standard is EN 10002-l : 2001 relating to the tensile testing of metallic materials.

Thin HSLA steel strip, sheet or blank is usually coated with an aluminium coating or a zinc coating. If a zinc coating is used, the coating is often applied as a hot dip galvanised or hot dip galvannealed coating.

Cold-rolled HSLA steels at higher strength levels have the drawback that, due to their high strength, the hot rolled strip is difficult to cold-roll to a relatively thin gauge at wide dimensions.

In W02016030010-A1 a CR460LA is proposed based on an HSLA steel containing titanium and vanadium. However, although this grade meets the mechanical property requirements of the relevant standard for CR460LA, the cold-rolled and annealed product suffers from significant differences in mechanical properties in different directions (in-plane anisotropy). Moreover, vanadium is quite an expensive alloying element which moreover fluctuates significantly.

Objectives of the invention

It is an objective of the invention to provide a HSLA steel strip that can be cold- rolled to a relatively thin gauge at wide dimensions, and made into HSLA sheets and blanks, having the required strength.

It is a further objective of the invention to provide such a HSLA steel strip, sheet or blank having the required elongation. It is a further objective of the invention to provide such a HSLA steel strip, sheet or blank having a reduced in-plane anisotropy in mechanical properties.

It is another objective of the invention to provide a method for producing such a HSLA steel strip.

Description of the invention

One or more of the objectives are reached with the hybrid high strength low alloy (H-HSLA) cold-rolled and annealed steel strip consisting of, in wt.%:

C: 0.050 - 0.090 Nb: 0.030 - 0.060

Mn: 1.000 - 1.800 S: at most 0.015

Si: 0.050 - 0.300 P: at most 0.015

Al_sol : 0.020 - 0.080 N: 0.002 - 0.008

Ca+REM: at most 0.0050 optionally also comprising one or more of

B: 0.0001 - 0.0010 Ti: at most 0.050 the remainder being iron and unavoidable impurities, wherein the steel has a tensile strength Rm of 520 - 680 MPa and a yield strength Rp of 460 - 580 MPa, and wherein Rp/Rm is 0.70-0.80, and wherein the microstructure comprises a precipitation strengthened ferritic matrix comprising polygonal and/or acicular ferrite and a second phase comprising cementite, between 2 and 10 % of martensite optionally accompanied by one or more of pearlite and bainite.

A conventional dual phase steels has a microstructure of martensite dispersed in a clean and therefore ductile ferritic matrix and provides a good combination of ductility and high tensile strength. The specific favourable properties of the dual-phase steel result from the large differences in strength and ductility between the clean ferrite and the martensite.

A conventional HSLA steel has a carbon content between 0.05 and 0.25% to retain formability and weldability. Other alloying elements include manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare-earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes and are intended to alter the microstructure of carbon steels, which is usually a fine grained ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite matrix to create a precipitation hardened ferrite.

The inventor found that it is possible to combine the strong points of conventional dual-phase and conventional HSLA in a hybrid HSLA in a careful balancing act. This H- HSLA combines the formability of the dual phase steels resulting from the hard martensite phase in a ductile clean ferrite matrix with the strength of the precipitation hardened ferrite-pearlite aggregate. Since the specific favourable properties of the dual- phase steel result from the large differences in strength and ductility between the clean ferrite and the martensite it is highly counterintuitive to strengthen the ferrite by grain refinement and precipitation strengthening.

The ferritic matrix may comprise unrecrystallised ferrite if the annealing temperature and time is deliberately limited to prevent growth of the strengthening precipitates. The presence of un recrystallised ferrite may contribute to the overall strength, but this may come at the expense of some anisotropy. In order to limit the degree of anisotropy the recrystallised fraction is preferably at least 85%, preferably at least 87%. So by controlling the recrystallised fraction the degree of anisotropy and mechanical property values can be balanced accordingly.

The presence of some un recrystallised ferrite in the ferritic matrix is quite important because it allows reaching the required higher strength levels. However, it may not be too high in view of the required low anisotropy. It is preferred that the recrystallised fraction of the ferrite matrix microstructure is at least 85%, preferably at least 90%, more preferably at least 95% and even more preferably at least 98%. Preferably the average grain size of the ferritic matrix is at least 4 pm. Preferably the average grain size of the ferritic matrix is at most 9 pm. Preferably the average grain size of the ferritic matrix is between 4 and 8 pm. This average grainsize is measured at l/10 ^th of the thickness t, which is just below the surface of the strip.

The strip possesses the claimed properties in its uncoated, coated or temper-rolled state. This strip may have been temper rolled in an optional temper-rolling (aka skinpass rolling) step. The strip may also be coated with a metallic coating in an optional coating step. The strip also possesses the aforementioned mechanical properties after the optional temper rolling and/or the optional metalling coating. This is not necessarily the case after a following shaping operation like stamping, bending, deep drawing or via warm press forming or hot press forming.

The optional metallic coating serves primarily as protection against corrosion of the steel strip.

The hybrid HSLA according to invention is requires a niobium (Nb) addition and optionally also a titanium (Ti) addition as alloying elements. The presence of Nb is needed for grains refinement and precipitation strengthening. The optional Ti will also contribute to precipitation strengthening, but will also help the formation of martensite which allows the reduction of other elements such as manganese (Mn) and Nb.

The steel according to the invention also contains a higher concentration of silicon (Si) compared to common HSLA grades. Si acts as a solid solution strengthening element and allows reaching the target strengths. However, as Si is a ferrite promoting element and is known to cause problems with the surface of the hot rolled strip due to sticky oxide, its concentration should be limited to preserve the ability to form martensite in the steel's microstructure and to preserve good surface quality. This surface quality is not only relevant for the surface of the hot-rolled strip, but also for the surface of the cold-rolled strip and even of the galvanised strip surface. The presence of carbon (C) is needed for solid solution strengthening, carbide formation and the formation of 2 ^nd phases such as bainite and in particular martensite. At higher values than 0.090 wt.% the risk of too high a volume of hard 2 ^nd phases increases, and below 0.050 wt.% the risk of too low a volume of hard 2 ^nd phases increases. Preferably the steel comprises at most 0.085 wt.% of C, preferably at most 0.080 wt.% of C to reduce the risk of too high a volume fraction of hard 2 ^nd phases. Also the amount of pearlite and/or cementite is reduced if the carbon content is somewhat lower.

Normal hot-rolled HSLA steels rely on the grain refining character of the microalloying additions due to retardation of the recrystallisation during hot-rolling, and to a lesser extent on precipitation hardening. Cold-rolled HSLA steels rely on the precipitation hardening, and to a much lesser extent on grain refinement. Therefore, it has proven difficult to obtain high strength cold-rolled HSLA steels with sufficient formability. In W02016030010-A1 a CR460LA is proposed based on an HSLA steel containing titanium and vanadium. However, although this grade meets the mechanical property requirements of the relevant standard for CR460LA, the cold-rolled and annealed product suffers from significant differences in mechanical properties in different directions (in-plane anisotropy).

The presence of the elements C, Si, Nb and optionally Ti and the 2 ^nd phases in the right proportion allows the production of a steel grade which has a hybrid character of high yield stress, high elongation and low anisotropy. The steel according to the invention is therefore a hybrid between a HSLA steel and a dual-phase steel. This hybrid character is therefore due to the joint effect of the precipitation strengthening (the HSLA aspect) and the 2 ^nd phases, mainly martensite (the dual-phase aspect). The martensite volume fraction is therefore quite critical. If the volume fraction is too low (<2%) then the material behaviour is closed to a normal standard HSLA grade. If the volume fraction is too high (>10%), the material behaviour is closer to dual phase (DP) grades with too low a yield stress. For hybrid HSLA 0.70<Rp/Rm <0.80. When the ratio is below 0.70, the steel behaves like a DP-steel and the yield stress is too low. When the ratio is above 0.80, the steel behaves like standard HSLA and the grade has a low total elongation and high in-plane anisotropy. The presence of martensite between 2 and 10% allows to remove the in-place anisotropy of the mechanical properties to a very large extent.

The optional addition of Ti helps to form martensite and thereby allows to reduce Mn while Nb is used for grains refinement. The presence of Ti also helps recrystallisation during the annealing of cold-rolled material at low temperatures (<850°C). At a similar annealing temperature, HSLA without Ti will be less recrystallised and contain a lower martensite fraction. Therefore, it will have a higher anisotropy and lower elongation values. Without Ti annealing at a higher temperature is needed, which will lower the strengths due to coarsening of Nb(C, N) precipitates. The steel optionally comprises at least 0.001 wt.% titanium, preferably at least 0.005 wt.% titanium and/or at most 0.035 wt.%, preferably at most 0.020 wt.% titanium.

In order to make the steel according to the invention more suitable for hot- stamping and direct quenching applications small amounts of boron (B) may optionally be added to the steel. Preferably the steel comprises at least 0.0002 (= 2 ppm) and/or at most 0.0010 wt.% (= 10 ppm) boron, preferably at most 0.0008 wt.% B and more preferably at most 0.0006 wt.% boron. Too much boron leads to copious amounts of martensite and bainite, and increased the strength too much and reduces the formability of the steel so that it no longer satisfies the CR460LA requirements.

Sulphur (S) and phosphorus (P) are in these steels almost inevitable impurities of which the amounts should be limited to as low a value as is technically and economically feasible. Preferably the steel comprises at most 0.010 wt.% of S and preferably at most. Also, the steel preferably comprises at most 0.010 wt.% of P and at most 0.005 wt.% of P.

Manganese is an austenite forming element, and too high a content increases the risk of martensite formation, particularly if the silicon content is as low as is the case in the steel according to the invention. Too low a Mn content result in too low strength values. Preferably the steel according to the invention comprises at least 1.200 wt.% of Mn, more preferably at least 1.350 wt.% of Mn and/or preferably at most 1.600 wt.% of Mn, more preferably at most 1.550 wt.% of Mn.

To improve edge stretching or bending in some applications, high strength low alloy steel grades can be optionally specified with sulphide inclusion control. A special steel making practice is used to control the shape and content of predominantly manganese sulphide inclusions. The producer may first limit the sulphide inclusion content via ultra-low sulphur steelmaking practices. To control the inclusion shape, the producer may also utilize a calcium (CA) and/or rare-earth element (REM, e.g. Ce) addition to the steel melt. Small globular particles are preferred. For this reason, the optional Ca+REM indicates the sum of the amounts of calcium and rare earth elements like Ce in the steel for sulphide inclusion control. If present, a suitable minimum amount of any of these elements is 0.0005 wt.%.

The cold-rolled and annealed strip is optionally provided with a metallic coating, preferably applied by means of hot-dip coating, although cold application techniques like PVD, CVD or electrodeposition are also feasible.

In an embodiment of the present invention the metallic coating can be also a hot- dip galvanized (GI) coating applied in a continuous process by passing the sheet steel through a molten bath with a zinc content of at least 99 % (all coating percentages are in wt.% unless otherwise indicated). The metallic coating is a zinc alloy coating. The metallic coating can be an electrogalvanized (EG) coating with a zinc content of at least 99.9 % electrolytically applied in a continuous coating process on a suitable prepared steel surface.

The metallic coating can be zinc-iron alloy coating that is generated by immersing the prepared strip in a molten bath containing a zinc content of at least 99 % and a subsequent annealing as a result of which iron diffuses into the zinc layer. The resulting zinc-iron coating has an iron content of normally to 13 % by mass and is referred to as a galvannealed (GA) coating. The metallic coating can also be an aluminium-silicon (AlSi) coating by passing the prepared strip through a molten aluminium bath with a silicon content of 8 to 11 %.

The metallic coating can also be a zinc-magnesium (MZ) coating by passing the prepared strip through a molten zinc bath alloyed with magnesium and aluminium. The zinc alloy coating (including the Fe2Als barrier layer) preferably comprises 0.3-4.0% Mg and 0.3-6.0% Al; optionally at most 0.2% of one or more additional elements; unavoidable impurities; the remainder being zinc. More preferably the alloying element contents in the coating shall be 1,0 - 2,0 % Magnesium and 1,0 - 3,0 %. Aluminium, optionally at most 0.2% of one or more additional elements, unavoidable impurities and the remainder being zinc. In an even more preferred embodiment, the zinc alloy coating comprises at most 1.6% Mg and between 1.6 and 2.5% Al, optionally at most 0.2% of one or more additional elements, unavoidable impurities and the remainder being zinc.

According to a second aspect the invention is also embodied in a method for producing a hybrid dual phase steel strip according to the invention as described herein above and in any one of claims 1 to 11, comprising the following steps:

• Continuously casting a steel slab or strip and hot-rolling said slab or strip to a hot- rolled strip, wherein the hot-rolled strip has a composition consisting of, in wt.%: C: 0.050 - 0.090 Nb: 0.030 - 0.060

Mn: 1.000 - 1.800 S: at most 0.015

Si: 0.050 - 0.300 P: at most 0.015

Al_sol: 0.020 - 0.080 N: 0.002 - 0.008

Ca: at most 0.0050 optionally also comprising one or more of B: 0.0002 - 0.0006 Ti : 0.001 - 0.050 the remainder being iron and unavoidable impurities, wherein the hot-rolled strip has a thickness of 2.0 - 4.5 mm and wherein finishrolling is performed while the strip has an austenitic microstructure; • Cooling the hot-rolled strip after finish-rolling, preferably with a cooling rate of at least 30 °C/s;

• Coiling the cooled strip at a coiling temperature CT in the range of 500 to 660 °C and allowing the coiled strip to cool to ambient temperatures;

• Uncoiling the coiled hot-rolled strip, followed by pickling and cold-rolling with a reduction of 40 - 80%;

• Continuous annealing of the cold-rolled strip by i. Heating the strip; ii. intercritically annealing the strip; ill. cooling the intercritically annealed strip to an intermediate temperature iv. optionally holding the strip at this intermediate temperature for a time t_oa in the range of 5 to 100 s v. optionally hot dip coating the strip vi. further cooling of the strip to the post-annealing coiling temperature at a cooling rate sufficient to induce martensite formation in the annealed strip, i.e. higher than the critical cooling rate; vii. coiling the strip.

• wherein the strip is optionally provided with a metallic coating by a) hot-dip coating in step v. or b). with a metallic coating by a cold application technique after step vii.;

• Optionally temper rolling the coated steel strip with a reduction of between 0.05 and 3.00 %;

• Wherein the optionally coated and optionally temper-rolled steel strip has a tensile strength Rm of 520 - 680 MPa and a yield strength Rp of 460 - 580 MPa, and wherein Rp/Rm is 0.70-0.80;

• Coiling the coated steel strip or cutting the coated steel strip into sheets or blanks;

• Optionally followed by shaping the coated steel strip, sheet or blank via cold-forming operations like stamping, bending, deep drawing or via warm press forming or hot press forming.

The importance of the composition of the steel was discussed already. The different processing steps all contribute to reaching the delicate hybrid balance between the properties of a standard HSLA and those of a standard DP. For instance, the high cooling rate is needed to form sufficient amounts (but not too high amounts) of 2 ^nd phases such as bainite and in particular martensite.

The coiling temperature is also an important parameter because it determines the starting condition of the hot-rolled strip for the subsequent processing steps. Preferably the coiling temperature is at most 650 °C. It is preferred that the cold-rolling reduction is at most 75%, more preferably at most 70%. A lower cold-rolling reduction limits the rolling forces during cold-rolling and thereby reduces the risk of shape defects in the strip.

The continuous annealing after the first cold-rolling step is an intercritical annealing treatment which involves heating to, and holding at, a temperature between the Acl and Ac3 temperatures to obtain partial austenisation of the cold-rolled steel strip. After the continuous annealing the microstructure comprises a precipitation strengthened ferritic matrix comprising polygonal and/or acicular ferrite and a second phase comprising cementite, between 2 and 10 % of martensite optionally accompanied by one or more of pearlite and bainite, and wherein the recrystallised fraction of the ferrite matrix is at least 85%. The cooling rate after annealing must therefore be higher than the so-called critical cooling rate to induce the transformation of austenite into martensite in the resulting continuously annealed cold-rolled strip. This critical cooling rate can be defined as the slowest cooling rate at which unstable austenite can be transformed into stable martensite and can be easily determined by means of routine experiments. Martensite is stable if the post-annealing coiling temperature is below the Mf-temperature.

In an embodiment the temper rolling reduction is at most 2.50%, preferably at most 1.30%, more preferably at most 0.80%. Preferably the temper rolling reduction is at least 0.10%, more preferably at least 0.20 %. The temper rolling reduction is important to obtain the desired final properties, strip shape and surface texture.

The optional temper rolling reduction may be important to obtain the right balance of final properties and surface quality (aspects like flatness, waviness, bow, ..). That balance may not always be the same either. Sometimes the importance of strip flatness prevails over mechanical properties (provided requirements imposed by the relevant standard are met), and sometimes it is the properties that prevail. By playing with the chemistry and the processing the balance of the various requirements can be shifted somewhat, but always within the requirements of the relevant standard. Some temper rolling is required up to a maximum of 3.00% temper rolling reduction (TRR). However, since a higher TRR eats away from the formability potential of the steel the TRR has to be as low as possible to balance the surface and shape requirements of the steel strip with the strength and formability requirements. Preferably the temper rolling reduction is at most 2.50%, preferably at most 1.30%, more preferably at most 1.00%. Preferably the temper rolling reduction is at least 0.10%, preferably at least 0.20 %, more preferably at least 0.50 or even at least 0.65%.

The method to produce the hybrid dual phase cold-rolled and annealed strip optionally comprises the application of a metallic coating, preferably applied by means of hot-dip coating, although cold application techniques like PVD, CVD or electrodeposition are also feasible.

According to another aspect the invention is also embodied in the use of the steel produced according to the invention in automotive applications. Automotive applications of the steel according to the invention may consist of, but is not limited to, application in the body structure, inner and outer panels, doors and trunk closures, application in the chassis or suspension, or in wheels, fuel tank, steering and braking systems. The automotive applications may be in passenger cars or in other vehicles such as trailers, trucks, trains, yellow goods and the like.

Examples and drawings

The invention will now be explained by means of the following, non-limiting examples and drawings.

Cold-rolled and annealed product

Results shown in Table 1 in Figure 4, with the considered annealing cycles, indicate the effect as well of alloying elements as the annealing cycle to produce a steel grade with the "hybrid" character. A too high Nb, Ti or C content give steel grade with Rp/Rm >0.8 and which contain a high fraction of non-recrystalised areas. A decrease in Si concentration makes it difficult to reach the VDA strengths specifications. The comparison between the Nb and NbTi alloy indicates that the sample containing Ti has higher strengths and is less sensitive to variations in annealing temperature, due to the formation of higher martensite percentages and a higher degree of recrystallisation.

Figure la-b shows Klemm etching micrographs for la) Nb and lb) NbTi: Martensite appears as bright spots on the micrographs. Samples were annealed at 800°C; Figure Ic-d shows Nital etched micrographs for figure 1c) Nb and Id) NbTi showing presence of polygonal ferrite (recrystallised + unrecrystallised) acicular ferrite as well as of pearlite and cementite precipitates for samples annealed at 800°C.

As shown in Figure 1 martensite appears as white spots in the micrographs in an etched sample. The etching was performed by means of a standard Klemm etching (polishing the surface of the sample to optical grade, etching the surface a few seconds with Klemm etchant (which is a solution of saturated aqueous sodium thiosulphate and 1 g potassium metabisulphites) and subsequently washed with ethanol). These spots have been analysed using the open-source software for processing and analysing scientific images Image! (https://imagej.nih.gov) to determine the percentage of martensite precipitates and their mean size. The etched image is converted into a black and white image, and the software determines the distribution of martensite in the steel (see figure 2a) micrograph of Klemm etched Nb sample (annealing temperature = 820°), 2b) After processing with Image! software, 2c) Martensite size distribution). Optical assessment of the microstructures of the steels according to the invention reveals that the high degree of recrystallisation of the microstructure is an important contribution to the realisation of the balanced properties. EBSD measurements have confirmed the correctness of the values of the optical measurements. The SEM conditions were 15 kV, 120 pm aperture with high current on. The EBSD conditions were 16 mm working distance and 100 fps scanning speed. Scans of 300x1000 pm2 (step size 0.5 pm) and 200x200 pm2 (step size 0.2 pm) gave similar results.

Table 3: Grains size at different sample thickness depth - Measurements done using ImageJ software

Table 2: Recrystallisation percentage estimated from optical micrographs after Nital etching of samples annealed at 800°C.

Hot stamping

The steel according to the invention is also suitable for hot-stamping applications. This is demonstrated by subjecting the Nb and NbTi composition to a direct hot stamping process. A typical t-T-schedule is as follows: the samples were heated in a reheating furnace to 900°C for 5 to 7 minutes. The samples were then removed from the furnace and cooled in air for 5 to 10 seconds during the transfer between furnace and hot press before being cooled. A typical experimental heating and cooling curve is shown in figure 3. In such a curve the hot pressing would occur around 800°C.

After hot pressing the NbTi grade has mechanical properties that are comparable to various commercially available grades (see Table 4 and 5). These results underline the versatility of the steel grade according to the invention which is the result of the balanced composition and microstructure. Table 4: Experimental processing parameters and strengths after lab hot press simulation for NbTi and Nb grades (note that for hot-pressed material the CR460LA requirements are not applicable). Table 5: Comparison of Nb and NbTi with comparative commercial grades.