EP-4627131-B1 - COLD ROLLED STEEL AND PRODUCTION PROCESS THEREOF

Inventors

• PLOBERGER, Mario
• HEBESBERGER, THOMAS

Dates

Publication Date

20260513

Application Date

20240117

Claims (3)

1. A cold rolled steel strip or sheet having: a) a composition consisting of the following elements in wt. %: C 0.02 - 0.10 Si 0.05 - 0.5 Mn 0.6 - 1.0 Al 0.015 - 1.5 Ti 0.03 - 0.12 Nb 0.01 - 0.06 Ti+ Nb 0.05 - 0.15 the balance consists of iron and impurities, wherein the impurities are limited to in wt. %: V ≤ 0.10 Cr ≤ 0.10 Mo ≤ 0.05 W ≤ 0.05 Ni ≤ 0.20 Cu ≤ 0.20 Sn ≤ 0.015 Zr ≤ 0.010 As ≤ 0.010 B ≤ 0.001 Ca ≤ 0.05 P ≤ 0.02 S ≤ 0.005 N ≤ 0.05 O ≤ 0.0003 H ≤ 0.0020 b) mechanical properties fulfilling the following conditions in accordance with the Industrial Standard DIN EN ISO 6892-1, wherein the samples are taken in the longitudinal direction of the strip: Yield strength R p0.2 800 - 950 MPa Tensile strength Rm 830 - 1000 MPa Total elongation A 80 ≥ 9% c) a hole expansion ratio λ determined by a hole expanding test according to ISO/WD 16630:2009 (E): hole expansion ratio λ ≥ 60 % d) a microstructure consisting of in vol%: ferrite ≥ 95 %, and perlite, cementite, carbides, nitrides, and/or carbonitrides < 5 % e) a bendability Ri/t in accordance with JIS Z2248 that fulfils: bendability Ri/t ≤ 1.0 wherein Ri is the limiting bending radius, which is defined as the minimum bending radius with no occurrence of cracks, and t is sheet thickness, and f) a carbon equivalent in wt. %: C Eq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5 ≤ 0.45
2. The strip or sheet according to any one of the preceding claims, wherein the carbon equivalent is in wt. %: C Eq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5 ≤ 0.28
3. A method of manufacturing of a cold rolled steel strip or sheet according to any one of the preceding claims, comprising the following steps: a) making steel slabs with a composition according to anyone of the preceding claims, b) reheating the slab to a temperature between 1200 °C and 1300 °C for a time in the range of 10 - 300 minutes, c) hot rolling the reheated slab completely in the austenitic range wherein the hot rolling finishing temperature is within the range 850 - 950 °C to obtain a hot rolled steel strip, d) cooling the hot rolled strips to a coiling temperature, e) coiling the hot rolled strip at a coiling temperature in the range of 470 - 670 °C, f) optionally subjecting the coiled strip to a scale removal process, such as pickling, g) cold rolling the steel strip with a reduction degree in the range of 20 - 40 %, h) final continuous annealing of the cold rolled strip at a temperature in the range of 700 - 800 °C for a time in the range of 25 - 175 s, i) cooling the final annealed strip at a cooling rate of 2 - 100 °C/s, j) optionally overaging the final annealed strip at 200 - 500 °C for a duration of 10 - 1000 s. k) optionally skin pass rolling of the strip at a rolling elongation of 0 - 2 %, l) optionally coating the strip with a zinc-based coating, and m) optionally cutting the strip to sheets.

Description

TECHNICAL FIELD The present invention relates to a cold rolled strip or sheet having a high strength combined with good local formability. The invention also relates to a continuous annealing process for producing the strip or sheet. BACKGROUND The development of materials that combines high strength and formability has contributed to the reduction of the weight of components in automotive applications. The development involves improving formability without reducing the strength of cold-rolled steel sheets for general purposes and increasing the strength while maintaining the formability of various high strength steel sheets. For some component a good local formability is particularly desirable, such as hole expansion ratio and bending. An example of a component that requires high yield strength and good bendability is a seat rail. For other components, hole expansion may be of greater importance. Cost is also an important factor. Typically seat have been formed from high strength micro alloyed steel strips that are produced in a production line using reversible cold rolling mills and batch annealing of the cold rolled strips. The dimensions of such steel strips are smaller than those in continuous cold rolling mill and a continuous annealing line. The strips are typically uncoated. For wider strips, batch annealing is also a common annealing process, but generally for steel grades with lower strength classes. For higher strength classes, continuous annealing lines are usually employed. WO 2022/1845780 A1 disclose a cold rolled steel product having a ferritic microstructure. The steel is alloyed with at least one element from the group Ti, Nb, V and at least one element from the group Cr, Mo, W. The use of expensive alloying elements can lead to higher cost of the product. It is cold worked to a reduction degree of 5 - 70 % and continuously annealed at a temperature in the range of 700 - 900 °C. CN112680655A disclose a cold rolled steel product having a yield strength (Rp0.2) of 700 - 850 MPa. The steel is micro alloyed with Ti and Nb. The amount of Mn is 1.0 - 1.5 %, preferably 1.2 - 1.4 %. It is cold worked to a reduction degree of 60-70 % and continuously annealed at a temperature in the range of 770 - 800 °C. After the annealing, the product is slowly cooled to a temperature of 630 - 670 °C, followed by rapidly cooling to 400 - 450 °C where it is overaged. The microstructure comprises austenite. EP1399598 B2 disclose a cold rolled steel product. The steel is alloyed with at least one element from the group Ti, Nb, V. It is cold worked to a reduction degree of 5 - 20 %, particularly 5 - 10% and annealed at a temperature in the range of 450 -700 °C, preferably 550 - 650 °C. All examples are high in Mn and Cr. CN109207842A disclose a cold rolled steel product. The steel is micro alloyed with Ti and Nb. The amount of Mn is 1.1 - 1.5 %, preferably 1.2 -1.5% to ensure the strength of the steel. It is cold worked to a reduction degree of 40 - 60 %, preferably 45 - 50 %. It would be advantageous to provide an improved wide steel strip and a process for producing a wide steel strip that can be used for components requiring high yield strength and good local formability, such as hole expansion ratio and bendability. The steel strip should preferably be able to be produced in continuous annealing line, at low cost and high volumes. Furthermore, process stability is a desirable outcome to minimise losses due to discarding products not meeting the requirements. It should preferably be easy to weld. DETAILED DESCRIPTION The invention is described in the claims. In a preferred embodiment the cold rolled steel strip or sheet has a composition consisting of the following alloying elements (in wt. %): C0.02 - 0.10Si0.05 - 0.5Mn0.6 - 1.0Al0.015 - 1.5Ti0.03 - 0.12Nb0.01 - 0.06Ti+ Nb0.05 - 0.15 the balance consists of iron and impurities, and wherein the impurity elements are limited to V≤ 0.10Cr≤ 0.10Mo≤ 0.05W≤ 0.05Ni≤ 0.20Cu≤ 0.20Sn≤ 0.015Zr≤ 0.010As≤ 0.010B≤ 0.001Ca≤ 0.05P≤ 0.02S≤ 0.005N≤ 0.05O≤ 0.0003H≤ 0.0020 The importance of the separate elements and their interaction with each other as well as the limitations of the chemical composition of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. The microstructural constituents are expressed in volume % (vol. %) throughout the description. Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims. The arithmetic precision of the numerical values can be increased by one or two digits for all values given in the present application. Hence, a value of given as e.g. 0.1 % can also be expressed as 0.10 or 0.100 %. C: 0.02 - 0.10 % C is important for obtaining the desired strength level. When C is lower than 0.02 % it is difficult to attain a tensile strength of 830 MPa. If the C content is too high, n