WO2023148087A1

WO2023148087A1 - Method of manufacturing a low-carbon steel strip having improved formability

Info

Publication number: WO2023148087A1
Application number: PCT/EP2023/051964
Authority: WO
Inventors: Gerard August KRUSEMEIJER; An YUGUO; Edgar Matthijs TOOSE; Hendrik Bart Van Veldhuizen
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2022-02-03
Filing date: 2023-01-27
Publication date: 2023-08-10
Also published as: KR20240146658A; EP4473133A1

Abstract

The invention relates to a method of manufacturing a low-carbon steel strip having improved formability, the method comprising the steps of: providing a hot-rolled steel strip of a low- carbon steel composition comprising up to about 0.020 wt.% C; accelerated cooling of the hot- rolled steel strip from a hot-mill finishing temperature to below 630oC, and preferably to below 600°C; subjecting the hot-rolled steel strip to a batch heat-treatment by re-heating the hot-rolled strip to a temperature in a range of 630°C to 800°C to provide a hot-rolled and heat-treated steel strip; optionally pickling of the hot-rolled steel strip either prior to or after said heat treatment, or both before and after said heat treatment.

Description

METHOD OF MANUFACTURING A LOW-CARBON STEEL STRIP HAVING IMPROVED FORMABILITY

FIELD OF THE INVENTION

The invention relates to a method of manufacturing a low-carbon steel strip or sheet having improved formability. More specifically, the invention relates to a method of manufacturing an ultra-low carbon IF steel strip or steel sheet having improved formability in a cold forming operation. The invention relates also to a steel strip or steel sheet product obtained by said method and to the use of the steel strip or steel sheet obtained by said method.

BACKGROUND TO THE INVENTION

Ultra-low carbon (ULC) steel is widely used for various applications where good formability is required. The manufacturing route of ULC steel strip comprises the subsequent steps of steelmaking, continuous casting, hot-rolling in a hot strip tandem mill at a hot-mill finishing or exit-temperature above the Ar3 transformation point, accelerated water cooling to coiling temperature and coiling, cooling to ambient temperature, pickling, cold rolling in a cold strip tandem mill, annealing in a batch annealing furnace or a continuous annealing furnace, skin tempering, oiling and packing. Most often the cold rolled and annealed strip is also galvanized or galvannealed, but can also be supplied as an uncoated strip product. The final batch annealing of the cold rolled strip is usually performed at temperatures below about 725°C for ultra-low carbon steel. Conversely, the continuous annealing process of the cold rolled strip is usually performed at temperatures that are approximately 800°C, and the annealing time is set so that the cold rolled strip is fully recrystallised.

The term “Interstitial Free steel” or “IF steel” refers to the fact, that there are substantially no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. IF steels have interstitial free body centered cubic (bcc) ferrite matrix. These steels normally have low yield strength, high plastic strain ratio (r-value) and good formability, and have been used as automobile panels since the latter half of the 1980’s. In the production of IF steel the liquid steel is processed to reduce at least the N to levels low enough that the remainder can be stabilized by small additions of Ti alone or in combination with Nb. Sometimes also V is used for this purpose. The manufacturing route is otherwise analogue to ULC steel.

IF steel products for cold forming are for example standardised in EN 10346:2015 (E). In this standard the low carbon steels for cold forming are referenced in Table 1 and Table 7 and are identified as DX51 to DX57, wherein the higher numbers indicate a better formability. Within this standard the highest formability is provided by steel grade DX57. In patent document WO-2021/151896-A1 an interstitial-free low-carbon steel strip is disclosed of defined narrow compositional ranges. The disclosed steel strip is manufactured by a method comprising casting the required composition and after cutting into slabs hot-rolled with a hot rolling finishing temperature between 900°C and 950°C, most preferably between 900°C and 930°C, cooled in the run out table with a cooling rate between 25°C/s and 150°C/s, preferably between 60°C/s and 90°C/s. The coiling temperature is between 600°C and 750°C, preferably between 675°C and 725°C. After cooling down and pickling the coils are cold rolled with a reduction of between 78% and 88% and continuous annealed at a temperature of approximately 810°C. After standard hot-dip galvanising to provide a Gl coating the steel strips were skin passed with a reduction of between 0.4% and 0.7%.

Patent document EP-1083237-A1 discloses a method of producing a ferritic Cr-containing steel sheet having excellent ductility, formability, and anti-ridging properties, and exhibiting excellent surface quality after forming, wherein a ferritic Cr-containing steel sheet contains, by mass%, 0.001 to 0.12% of C, 0.001 to 0.12% of N, and 9 to 32% of Cr, and has a crystal grain structure in which in a section of a hot-rolled annealed steel sheet in the thickness direction parallel to the rolling direction, an elongation index of crystal grains is 5 or less at any position, and in a section of a cold-rolled annealed steel sheet in the thickness direction parallel to the rolling direction, any colony of coarse grains oriented in the rolling direction has an aspect ratio of 5 or less. The production method includes hot rolling, pre-rolling by cold or warm rolling with a rolling reduction of about 2 to 15%, hot-rolled sheet annealing, cold rolling, and finish annealing; preferably the FDT of hot rolling is 850°C.

Patent document WO-2013/124264-A1 discloses a high-strength bake-hardenable ferritic steel strip and a method producing the same. The ferritic steel strip comprises, in wt.%, up to 0.01 % C_total; up to 0.5 % Si; up to 1.0% Mn; from 5 to up to 10% Al; up to 0.010% N; up to 0.019% Ti; up to 0.08% Nb; up to 0.1% Zr; up to 0.1% V; up to 0.01% S; up to 0.1% P; remainder iron and inevitable impurities, with defined C_solute, and defined controlled amounts of S, Ti, and N. The method comprises the steps of: providing a steel slab or thick strip by continuous casting, or by thin slab casting, or by belt casting, or by strip casting; optionally followed by reheating the steel slab or strip at a reheating temperature of at most 1250°C; hot rolling the slab or thick strip and finishing the hot-rolling process at a hot rolling finishing temperature of at least 850°C; coiling the hot-rolled strip at a coiling temperature of between 550°C-750°C. This hot-rolled strip can be subsequently further processed in a process comprising the steps of: cold-rolling the hot-rolled strip at a cold-rolling reduction of from 40%- 90% to produce a cold-rolled strip; annealing the cold-rolled strip in a continuous annealing process with a peak metal temperature of between 700°C-900°C; optionally galvanising the annealed strip in a hot-dip galvanising or electro-galvanising or a heat-to-coat process.

There is a demand for low-carbon steel strip products, in particular ULC IF steel strip products, having improved formability in a cold forming operation.

DESCRIPTION OF THE INVENTION

As will be appreciated herein, for any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.

As used herein, the term "about" when used to describe a compositional range or amount of an alloying addition means that the actual amount of the alloying addition may vary from the nominal intended amount due to factors such as standard processing variations as understood by those skilled in the art.

The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.10 wt.% Cu may include a steel composition having no Cu.

It is an object of the invention to provide a method of manufacturing a low-carbon steel strip having improved formability.

It is an object of the invention to provide a method of manufacturing a low-carbon steel strip, in particular an ultra-low carbon steel strip having improved formability in a cold forming operation.

These and other objects and further advantages are met by the present invention providing a method of manufacturing a low-carbon steel strip or sheet, preferably an ultra-low carbon steel, having improved formability, in particular having improved formability in a cold forming operation, the method comprising the process steps of, in that order, providing a hot-rolled steel strip of a low-carbon steel composition comprising up to about 0.020 wt.% C, and preferably up to about 0.010 wt.% C. In industrial practice this means hot rolling a continuous cast rolling ingot and whereby the hot-mill finishing temperature is above the Ar3 transformation point or transformation temperature, and preferably at a temperature in a range of about 890°C to 950°C. The hot-rolled steel strip has preferably a thickness of a range of about 1.5 mm to 6 mm, and more preferably of about 2.5 mm to 6 mm. accelerated cooling of the hot-rolled steel strip from the hot-mill finishing temperature to below 630°C, preferably to below 600°C, and more preferably to below 500°C. In industrial practice this means moving the hot-rolled strip over a run-out table and applying air cooling, laminar cooling or water jet cooling depending on the thickness of the steel strip. The cooling rate is at least 25°C/s, and preferably at least 50°C/s. A practical run-out table cooling rate is in a range of about 25-100°C/s, preferably of about 25-150°C/s, and more preferably of about 50 to 150°C/s. For practical reasons the run-out table cooling rate (ROT-CR) is defined as the average cooling rate of the surface of the steel strip. subjecting the hot-rolled steel strip to a purposive batch heat-treatment or batch annealing treatment by re-heating the hot-rolled strip to a temperature in a range of 630°C to 800°C, and preferably in a range of 630°C to 750°C, to provide a hot-rolled and heat-treated steel strip or annealed steel strip. Thus this purposive batch heat-treatment or batch annealing treatment is performed prior to any subsequent cold rolling operation, if any; cooling of the hot-rolled and batch heat-treated steel strip to below about 100°C, preferably cooling to ambient temperature. It has been found that the cooling rate is not critical and can be performed as is usual in the art. optionally pickling, typically at ambient temperature, of the hot-rolled steel strip either prior to or after said purposive heat treatment, or both before and after said heat treatment. The oxides (scale) on the hot-rolled steel strips are removed either by pickling in an acid solution (e.g. HCI) at warm temperatures (80-120°C) or by a combination of pickling and mechanical brushing of the strip surface. This step is necessary for rendering the steel strip surface suitable for direct use as uncoated hot-rolled steel or making it amenable to a coating process, when optionally needed for corrosion resistance.

The purposive batch heat-treatment or batch annealing treatment in accordance with the invention is performed at a temperature of at least 630°C, and more preferably of at least 640°C, and most preferably of at least 645°C. The temperature is maximum about 800°C, and more preferably does not exceed 750°C, to avoid excessive grain growth of ferrite grains.

In accordance with the invention it has been found that the batch heat-treatment or batch annealing treatment of the hot-rolled steel strip prior to any substantial cold rolling reduction provides an increase in the r-value while maintaining a sufficiently high average strain hardening component n. In particular the r-value in the 45°-direction is increased. The important increase of this critical parameter improves the cold forming capability of the steel sheet, in particular in a deep drawing forming operation. Lowering an r-value would suggest an increased probability of undesired fracturing of the steel sheet in a subsequent forming operation. It has been found also that the planar anisotropy in the r-value is reduced. The final strip product also has a low yield strength. This enables the production of complex geometry three-dimensional formed parts, in particular when formed in a cold forming operation comprising at least one deep- drawing step.

As the low-carbon steel strip preferably also comprises Ti in a range of 0.005-0.12 wt.%, and more preferably 0.01-0.10 wt.% Ti, during the batch heat-treatment and subsequent cooling the steel strip exhibits TiC-based particles greater than 10 nm in mean radius, and preferably of at least 6 nm in mean radius, contributing to the improved formability characteristics of the steel strip.

The heat-treatment in accordance with the invention is performed as a batch process. In an industrial environment this means hot rolled strip leaves the hot-rolling mill, typically at a temperature in a range of about 890°C to 950°C, is accelerated cooled on a run-out table with a cooling rate of more than about 25°C/s, preferably between 25 to 150°C/s, and coiled into individual coils at a temperature in the range of up to 730°C, preferably in a range of 400°C to 730°C, and preferably 450°C to 650°C. The individual coils are allowed to cool to below 630°C, preferably to below 600°C, and more preferably to below 500°C, and typically to ambient temperature or room temperature. Next the cooled coils are re-heated and batch heat-treated or batch annealed according to the invention (for the purpose of this invention these are equivalent expressions). In industrial scale batch annealing, tightly wound cylindrical coils of rolled steel are stacked two, three, four or five high on bases with convector spacers between the coils. A cover is lowered onto the stack and is sealed at the base. The atmosphere surrounding the coils is purged and replaced with slightly reducing atmosphere, commonly used in the steel industry are a mixed nitrogen hydrogen gas, pure hydrogen, pure nitrogen, or pure argon. Both singlestack and multi-stack furnaces can be used for this purpose. The stack of coils is heat-treated via a temperature controlled cycle including a heat-up cycle, a soaking cycle and a cooling cycle. The batch annealing in accordance with the invention is preferably performed at a temperature in the range of 630°C to 720°C. In an embodiment the temperature is at least 640°C, and preferably at least 645°C. In an embodiment the temperature does not exceed about 700°C, and preferably does not exceed 695°C. As is well known to the skilled person during batch annealing the coils of steel sheet undergo a set temperature-time cycle. There is a ramp-up cycle and when a pre-defined soak temperature is reached, a soak timer is started and on expiry the steel cools naturally until an accelerated cooling temperature (nominally about 550°C or about 580°C) is reached, at which time a cooling fan is pulse started to accelerate cooling. At approximately 100°C to 150°C the cooling fan is switched of and the furnace can be removed. Most often a soak timer can be set at 0 s., such that when the pre-defined soak temperature (e.g. 650°C or 670°C) is reached, the cooling cycle of the steel coils can start. A typical batch anneal temperature-time cycle for a soaking temperature of 650°C is shown in Fig. 1 herein.

In an embodiment of the method according to the invention the method further comprises one or more of the following subsequent processing steps:

(i) cold rolling of the hot-rolled and heat-treated steel strip to further reduce the thickness. Preferably the steel strip is cold rolled to a cold rolling finishing thickness in the range of about 0.3 to 3 mm, and preferably of about 0.5 to 2.3 mm. The cold rolling reduction is preferably in the range of about 50% to about 95%. In an embodiment the cold rolled rolling reduction is at least 70%, and preferably at least 78%, and more preferably at least 80%. In an embodiment the cold rolling reduction is at most 92%, and more preferably at most 90%. By means of a cold rolling reduction in these ranges the r-value of the steel strip is further improved. As is known in the art, preferably the steel strip is pickled prior to the cold rolling operation.

(ii) annealing of the cold rolled steel strip. The annealing of the cold rolled strip can be done, as known in the art, by batch annealing or continuous annealing. The continuous annealing is typically performed at a peak metal temperature in the range of 780°C to 860°C, e.g. at 805°C, and for a time in a range of 10 to 180 s., e.g. for about 20 s., and cooling the annealed steel strip from that temperature to ambient temperature at a cooling rate of 3°C/s or more.

(iii) the annealed steel strip product can be a bare product or it can be provided on one or both of the main surfaces with a thin metallic coating as is known in the art, typically up to about 150 g/m² per side of the sheet, and preferably up to about 100 g/m² per side, and with the metallic coating preferably selected from the group comprising an aluminium alloy coating (e.g., an Al-Si alloy, or Al-Zn alloy), a zinc coating, and a zinc alloy coating (e.g., a Zn-AI alloy, Zn-Mg alloy, Zn-Fe alloy, Zn-AI-Mg alloy, or Zn-Mg-AI alloy).

(iv) optionally applying a temper rolling reduction or a skin pass reduction of less than about 3%, preferably less than about 1%, more preferably less than about 0.8% (e.g. about 0.5% or about 0.6%), and more than about 0.25%, next coiled and stored until for further processing, for example further processing in a forming operation into a three-dimensional formed product. Preferably such a forming operation is a cold forming operation, and more preferably the cold forming operation includes at least a deep drawing step. In accordance with the invention the steel strip is a low-carbon steel strip with a carbon content of up to 0.020%, and preferably is an ultra-low carbon (ULC) steel with a carbon content up to 0.010%, and is more preferably an ultra-low carbon (ULC) interstitial free (IF) steel with a carbon content up to 0.010%.

In an embodiment the steel strip has a composition comprising of, in wt.%.,

C up to about 0.020%, preferably up to about 0.010%, and more preferably up to about 0.005%;

Mn up to about 0.70%, preferably up to about 0.50%, and more preferably up to about 0.30%, and most preferably about 0.03-0.30%;

Si up to about 0.50%, preferably up to about 0.30%, and more preferably up to about 0.15%, and most preferably about 0.001-0.10%;

Al up to 0.20%, preferably 0.005-0.20%, more preferably 0.005-0.20%, and most preferably 0.005-0.10%;

Ti 0.005-0.12%, preferably 0.01-0.10%;

Nb up to about 0.09%, preferably up to about 0.05%, and more preferably up to about 0.03%;

V up to about 0.09%, preferably up to about 0.05%, and more preferably up to about 0.02%;

P up to about 0.1%, preferably up to about 0.05%, more preferably up to about 0.03%;

S up to about 0.05%, preferably up to about 0.03%;

N up to about 0.01%, preferably up to about 0.008%, and more preferably 0.001-0.006%; and optionally one or more elements selected from the group of:

(Cr up to about 0.10%, preferably up to about 0.06%; Ni up to about 0.10%, preferably up to about 0.06%; B up to about 0.0050% (50 ppm), preferably up to about 0.0030% (30 ppm); Ca up to about 0.01 %, preferably up to about 0.005%; Cu up to about 0.20%, preferably up to about 0.10%, and more preferably up to about 0.05%; Mo up to about 0.10%, preferably up to about 0.06%, and more preferably up to about 0.04%; Sn up to about 0.05%, preferably up to about 0.03%, and more preferably up to about 0.02%); and wherein the balance is being made by Fe and inevitable impurities resulting from the ironmaking and steelmaking process.

In an embodiment the steel strip has a composition consisting of (in wt.%), C up to 0.020%, and preferably up to 0.010%; Mn up to 0.70%; Si up to 0.50%; Al up to 0.20%; Ti 0.005- 0.12%; Nb up to 0.09%; V up to 0.09%; P up to 0.1%; S up to 0.05%; N up to 0.01%; and optionally one or more elements from the group consisting of (Cr up to 0.10%; Ni up to 0.10%; B up to 0.005%; Ca up to 0.01 %; Cu up to 0.20%; Mo up to 0.10%; Sn up to 0.05%); and wherein the balance is made by Fe and production related unavoidable impurities, and with preferred narrower compositional ranges as herein described and claimed.

Carbon provides strength to the steel sheet, but a too high content may adversely effect ductility. Carbon is present in an amount of up to about 0.020%, preferably up to about 0.010%, and more preferably up to 0.005%.

Manganese is present in an IF steel to provide strength and can be present up to about 0.70%, preferably up to about 0.50%, and more preferably 0.30%. A too high Mn adversely affects formability, in particular A50, A80 and the r-value decrease to undesirable levels. To provide a desirable balance in strength and formability, the Mn-content is preferably at least 0.03%, and more preferably at least 0.04%.

Silicon is also present in an IF steel to provide strength and should not exceed 0.50%, preferably is present up to 0.30%, and more preferably up to 0.15%. A too high Si adversely affects formability. To provide a desirable balance in strength and formability, the Si-content is preferably at least 0.001%, and more preferably at least 0.002%.

Aluminium is an element required for killing the steel and should be present in an amount of up to 0.20%. A too high Al content adversely affects the ductility of the steel strip and for that reason preferably the Al-content should not exceed 0.15%, and preferably is maximum 0.10%. The in embodiment the Al content is at least 0.005%, and preferably of at least 0.01%.

Titanium is added to bind the carbon and nitrogen in the steelmaking process and should be at least 0.005%, and is preferably at least 0.01%, and more preferably at least 0.015%. In the heat-treatment method according to the invention the Ti forms also Ti-C based precipitates positively contributing to the observed improvement in deep drawing formability and the r-value and anisotropy of the r-value in particular. The Ti-content should not exceed 0.12%, and preferably does not exceed 0.10%.

Nb and/or V can be added in conjunction with Ti, and each of Nb and V should not exceed 0.09%, preferably each does not exceed 0.05%, and more preferably does not exceed 0.03% for Nb and 0.02% for V.

Each of the optional elements selected from the group comprising Cr, Ni, B, Ca, Cu, Mo, and Sn can be present in the range, and narrower preferred ranges, as herein disclosed. The balance is made by Fe and production related unavoidable impurities.

In an aspect of the invention it relates to a hot-rolled steel strip of a composition as herein described and claimed having been hot-rolled, accelerated cooled at a cooling rate of at least 25°C/s, and preferably in a range of 25 to 150°C/s, of the hot-rolled steel strip from the hot-mill finishing, preferably coiled, and cooled to below 630°C, and preferably to below 600°C, and next batch heat-treated according to this invention and after subsequently being cooled to below 500°C, and preferably to ambient temperature, exhibits TiC-based particles greater than 10 nm in mean radius, and preferably of at least 6 nm in mean radius.

In accordance with the invention it has been found that this microstructure prior to any substantial cold deformation (e.g., a cold rolling operation in a cold strip tandem mill resulting in a thickness reduction of the strip of more than 50% and subsequent annealing) contributes to the observed improved formability of the strip product.

In an embodiment of such a hot-rolled steel strip product it is provided in the form of a coiled strip product without any substantial cold rolling deformation. The thickness of the hot- rolled steel strip is in a range of about 1.5 to 6 mm, and preferably 2.5 to 6 mm. Already in this form the steel strip according to the invention has technical and commercial relevance and can be formed in a forming operation into shaped three-dimensional product. The strip product can be a bare product or it can be provided with a thin metallic coating as is known in the art, typically up to about 150 g/m² per side of the sheet, and preferably up to about 100 g/m², and preferably selected from the group comprising an aluminium alloy coating, a zinc coating, and a zinc alloy coating, as disclosed herein.

In a preferred embodiment the steel strip after the batch heat-treatment is an intermediate product and in accordance with the invention it can be further processed using the process steps of (i) cold rolling using a cold rolling reduction of at least 50%, (ii) batch or continuous annealing, (iii) optionally providing a metallic coating, and (iv) optionally receiving a skin pass reduction of less than 3%, next (v) coiled and stored until for further processed in a forming operation into a formed product.

The invention is also embodied in a cold-rolled steel strip as herein disclosed and claimed, manufactured using the method according to this invention and having been further processed by substantial cold rolling (e.g., in the range of 50% to 95%) and preferably also batch or continuous annealed, optionally provided with a metallic coating, and optionally having receiving a skin pass reduction, all steps as disclosed herein, and is characterised by one or more of the following mechanical properties (being the statistical average of at least three measurements), and preferably by three or more of the following mechanical properties:

(a) yield strength (Rp) in the T-direction (T is transverse) in the range of 60 to 160 MPa, preferably of 75 to 160 MPa, and more preferably of 80 to 160 MPa. (b) tensile strength (Rm) in the T-direction in the range of 250 to 330 MPa.

(c) elongation A50mm in the T-direction of at least 43%, and preferably of at least 44%, for sheet thickness of more than 0.7 mm; and of at least 42% for sheet thickness of less than 0.7 mm.

(d) an average plastic strain ratio r of at least 1.90, and preferably of at least 2.00, and most preferably of at least 2.10. The average plastic strain ratio r or average r-value is defined as:

(rO + 2xr45 + r90)/4, and wherein: rO is the plastic strain ratio in the rolling or longitudinal direction, r45 is the plastic strain ratio in diagonal direction, and r90 is the plastic strain ration in transverse direction; and wherein the rO, r45, and r90 have been measured on A50 mm test specimen.

(e) a plastic strain ratio r in the diagonal or 45°-direction (r45) of at least 2.00, and preferably at least 2.05.

(f) an average strain hardening component n of at least 0.23, and preferably of at least 0.25. It is important that the average strain hardening component remains at a sufficiently high level to maintain good formability characteristics of the steel sheet. The average strain hardening component n or average n-value is defined as: (nO + 2xn45 + n90)/4, and wherein: nO is the strain hardening component in the rolling or longitudinal direction, n45 is the strain hardening component in diagonal direction, and n90 is the strain hardening component in transverse direction; and wherein the nO, n45, and n90 have been measured on A50 mm test specimen.

The 0.2% offset proof strength or yield strength (Rp), ultimate tensile strength (Rm), uniform elongation (Ag) and tensile elongation (A50), r-values, and n-values were determined from quasistatic (strain rate 3 x 10^-4 s^-1) tensile tests at room temperature with A50 specimen geometry according to EN 10002-1/150 6892-1. The geometry of the tensile specimens consisted in 50 mm gauge length in the rolling direction, 12.5 mm in width and a thickness depending on the final gauge.

The invention is also related to the use or method of use of the steel strip according to this invention or obtained by the method according to this invention in the subsequent manufacture of a flat steel strip into a complex three-dimensional shaped product or formed product. In particular products shaped in a cold press forming operation comprising a deep- drawing step take benefit from the advantages of this invention. Shaped products include in particular automotive parts requiring improved formability, in particular in a cold deep drawing forming operation, and include body sides, door liners, tailgates, etc.

The invention is also embodied in a formed or shaped automotive component manufactured from a low-carbon steel strip obtained by the method according to this invention and formed into a three-dimensional shaped product in a cold press forming operation comprising a deep-drawing step.

BRIEF DESCRIPTION OF THE FIGURE

The invention is also explained by means of the following, non-limiting figure:

Fig. 1 shows the temperature-time cycle for the batch annealing in accordance with the invention for samples 3A and 3B being batch annealed of the hot-rolled strip at a temperature of 650°C.

The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention.

EXAMPLE

On an industrial scale of manufacturing an ULC-IF steel grade has been produced having the composition listed in Table 1.

Table 1. Steel composition, in mwt.%, except for C and N which in ppm, balance impurities and Fe.

The steel has been hot rolled to a thickness of 3.17 mm, the hot rolled steel was actively cooled from the austenitic phase field with a mixture of water and air to an end temperature in the ferritic phase field at the run-out table in the range of 565 to 615°C with an average cooling rate of about 120°C/s, and coiled at a temperature of about 580°C and air cooled to ambient temperature. From this industrial produced hot-rolled strip material samples were taken for subsequent investigation at laboratory scale.

Samples of the hot-rolled strip have been pickled and subsequently heat-treated or annealed using different annealing temperatures (610°C, 650°C, and 670°C; wherein 650°C and 670°C are according to this invention) and also a sample has been produced having not been heat-treated to represent standard production material. The heat-treatment has been performed as a batch annealing using an HNx-gas to minimise surface oxidation as in known in the art. A representative temperature-time batch annealing cycle is shown in Fig. 1. Fig. 1 is for the annealing temperature of 650°C, the other batch annealing cycles followed a similar T-t cycle except having a different target temperatures (610°C and 670°C). The samples have been cold rolled using various cold rolling degrees (80% and 83%) to cold rolling finishing thickness (0.63 mm and 0.54 mm respectively). Next the cold rolled strip materials have been annealed on laboratory scale mimicking an industrial continuous annealing cycle with a soaking temperature of about 800°C and a soaking time of about 50 seconds. A summary of the relevant processing steps is given in Table 2.

Various mechanical properties have been measured (average over three samples) in the 0°, 45° and 90° rolling direction in accordance with ISO6892-1. The results of the various tests are listed in Table 3.

Table 2. Summary of the sample numbering and the process applied.

From the results of Table 3 it can be seen that regular manufacturing (Sample 1A) provides a defined set of mechanical and formability properties. By batch annealing at 610°C (Sample 2A and 2B) of the hot-rolled strip the balance of properties in the final steel sheet are not improved. Compared to Sample 1A in Sample 2A and 2B the yield strength is increased and the elongation A50 is reduced, which indicates towards a reduced formability. Table 3. Test results.

However, by batch annealing in accordance with this invention at 650°C and 670°C (Samples 3A to 4B) the balance in mechanical and formability properties is improved in the final sheet products. Compared to sample 1A, 2A and 2B, the yield strength in the samples according to this invention (Samples 3A to 4B) is slightly reduced while still at sufficient high level, which is good for the formability of the steel sheet. The average n-value remains favourably high for all samples manufactured. In accordance with the invention it has been found that the average r- value is increased. More in particular, there is an important improvement in the plastic strain ratio r in the diagonal or 45°-direction (r45), which direction is commonly perceived as the more critical direction in the steel sheet in a cold forming operation, in particular of deep-drawing is involved. An increase in the range of about 0.10 to 0.20, and in the best examples even more, has been found against regular manufacturing represented by sample 1A. This important increase in r-value provides a significant improvement of the balance in mechanical and formability properties. An increase in r-value means an increased resistance against thinning of the sheet material in a cold forming operation, i.e. a deep-drawing operation, and thus in a reduced risk of crack initiation and subsequent cracking of the steel sheet. In the steel sheets manufactured according to this invention the average n-value is not adversely affected and remains at a high level. Also the elongation A50 remains very high and in some examples is even increased. The yield strength and tensile strength remain at the same level or are slightly reduced. This slight reduction of yield strength and tensile strength means that the steel sheet is somewhat softer which is favourable for the formability.

The improved balance in mechanical and formability properties obtained by the method according to this invention renders the steel strip ideally suitable for subsequent manufacture of a flat steel strip into a complex three-dimensional shaped product or formed product. In particular products shaped in a cold press forming operation comprising a deep-drawing step take benefit from the advantages of this invention. Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as herein described.

Claims

1. Method of manufacturing a low-carbon steel strip having improved formability, the method comprising the steps of: providing a hot-rolled steel strip of a low-carbon steel composition comprising up to 0.020 wt.% C, and preferably up to 0.010 wt.% C, and more preferably an ultra-low interstitial free (IF) steel having up to 0.010 wt.% C; accelerated cooling of the hot-rolled steel strip from a hot-mill finishing temperature to below 630°C, and preferably to below 600°C, at a cooling rate of at least 25°C/s, and preferably in a range of 25 to 150°C/s; subjecting the hot-rolled steel strip to a batch heat-treatment by re-heating the hot- rolled strip to a temperature in a range of 630°C to 800°C, preferably 630°C to 750°C, to provide a hot-rolled and heat-treated steel strip; optionally pickling of the hot-rolled steel strip either prior to or after said heat treatment, or both before and after said heat treatment.

2. Method according to claim 1 , wherein the batch heat-treatment is performed as a batch process at a temperature in the range of 630°C to 720°C, preferably in the range of 640°C to 720°C, and more preferably in the range of 645°C to 700°C.

3. Method according to claim 1 or 2, wherein the hot-mill finishing temperature is above the Ar3 transformation temperature of the steel, and is preferably a temperature in a range of 890°C to 950°C.

4. Method according to any one of claims 1 to 3, wherein the hot-rolled strip is accelerated cooled at a cooling rate of at least 25°C/s, and preferably in a range of 25 to 150°C/s, and next coiled at a temperature in the range of 400°C to 730°C, and preferably coiled at a temperature in the range of 450°C to 650°C, into coiled hot-rolled strip, and the coiled hot-rolled strip is cooled to a temperature to below 630°C, and preferably to below 600°C.

5. Method according to any one of claims 1 to 4, wherein the hot-rolled and batch heat- treated steel strip is further reduced in thickness by cold rolling to a cold-rolled steel sheet, preferably to a thickness in the range of 0.3 to 3 mm, and preferably with a cold rolling degree of 50% to 95%, and more preferably with a cold rolling degree of 70% to

6. Method according to claim 5, wherein the cold rolled steel strip is annealed, preferably the cold rolled strip is continuous annealed, and more preferably continuous annealed at a temperature in a range of 780°C to 860°C, and preferably for 10 to 180 s.

7. Method according to claim 5 or 6, wherein the heat-treated strip on at least one of its faces is provided with a metallic coating, preferably an aluminium alloy coating, or a zinc coating or a zinc alloy coating.

8. Method according to any one of claims 1 to 7, wherein the steel strip has a composition comprising of, in wt.%,

C up to 0.020%, preferably up to 0.010%, and more preferably up to 0.005%;

Mn up to 0.70%, preferably up to 0.50%;

Si up to 0.50%, preferably up to 0.30%;

Al up to 0.20%;

Ti 0.005-0.12%, preferably 0.01-0.10%;

Nb up to 0.09%;

V up to 0.09%;

P up to 0.1%;

S up to 0.05%;

N up to 0.01%; and optionally:

Cr up to 0.10%;

Ni up to 0.10%;

B up to 0.005%;

Ca up to 0.01 %;

Cu up to 0.20%;

Mo up to 0.10%;

Sn up to 0.05%; the balance is made by Fe and unavoidable impurities.

9. Method according to any one of claims 5 to 8, wherein the cold-rolled steel strip has a yield strength (Rp) in the transverse direction in the range of 60 to 160 MPa, and preferably of 75 to 160 MPa. Method according to any one of claims 5 to 9, wherein the cold-rolled steel strip has a tensile strength (Rm) in the transverse direction in the range of 250 to 330 MPa. Method according to any one of claims 5 to 10, wherein the cold-rolled steel strip has an average plastic strain ratio (r) of at least 1.90, and preferably of at least 2.00. Method according to any one of claims 5 to 11 , wherein the cold-rolled steel strip has an plastic strain ratio (r) in the 45°-direction of at least 2.00, and preferably of at least 2.05. A hot-rolled low-carbon steel strip having a composition according to claim 8 and being manufactured according to any one of claims 1 to 3, and comprising TiC precipitates larger than 10 nm in mean radius, and preferably larger than 6 nm in mean radius. Low-carbon steel strip comprising up to 0.020 wt.% C obtainable by the method according to any one of claims 1 to 12, and having three or more of the following mechanical properties: yield strength (Rp) in the T-direction in the range of 60 to 160 MPa, preferably of 75 to 160 MPa; tensile strength (Rm) in the T-direction in the range of 250 to 330 MPa; elongation A50mm in the T-direction of at least 43%, and preferably of at least 44%, for sheet thickness of more than 0.7 mm; an average plastic strain ratio r (based on A50 mm test specimen) of at least 1.90, and preferably of at least 2.00; , and most preferably of at least 2.10; a plastic strain ratio r in the diagonal or 45°-direction (r45) of at least 2.00, and preferably at least 2.05; an average strain hardening component n (based on A50 mm test specimen) of at least 0.23, and preferably of at least 0.25. A formed automotive component manufactured from a low-carbon steel strip obtained by the method according to any one of claims 1 to 12, and preferably formed in a forming process comprising deep drawing.