High Strength Hot Dip Galvanised Steel Strip

Abstract

A high strength hot dip galvanised steel strip having, in mass percent, the following elements: 0.13-0.19% C; 1.70-2.50% Mn; max 0.15% Si; 0.40-1.00% Al; 0.05-0.25% Cr; 0.01-0.05% Nb; max 0.10% P; max 0.004% Ca; max 0.05% S; max 0.007% N; and optionally at least one of the following elements: max 0.50% Ti, max 0.40% V, max 0.50% Mo, max 0.50% Ni, max 0.50% Cu, max 0.005% B, the balance being Fe and inevitable impurities; wherein 0.40%<Al+Si<1.05% and Mn+Cr>1.90%. This steel offers improved formability at a high strength, has a good weldability, and surface quality together with a good producability and coatability.

Description

[0001] The invention relates to a high strength hot dip galvanised steel strip having improved formability, such as used in the automotive industry.

[0002] Such steel types are known and have been developed under the name of dual phase steel types. Such steel types do not provide the required formability as required in many applications for the automotive industry. For this reason, TRIP assisted dual phase steel types have been developed.

[0003] A document describing such steel types is EP 1 889 935 A1. This document describes a high strength hot dip galvanised steel sheet containing (in mass percent)

0.05-0.3% C

0.08-3% Mn

max 1.4% Si

0.1-2.5% Al

0.1-0.5% Cr

0.003-0.1% P

max 0.07% S

max 0.007% N

[0004] the balance being Fe and incidental impurities, and wherein Si+Al.gtoreq.0.5%. Optionally a number of other elements can be present. Thirty-one steel types have been tested at laboratory scale, of which nineteen are deemed inventive compositions. These examples show that a wide range of the amounts of especially Si and Al are fulfilling the inventiveness requirements according to EP 1 889 935 A1.

[0005] Formability, however, is not the only requirement for a TRIP assisted dual phase steel strip. The alloying elements should be low in amount to make the cost of the steel as low as possible, it should be as easy as possible to produce the steel strip and to coat it, the steel strip has to have high strength, good weldability and should also exhibit a good surface quality. These requirements are especially important for industrially produced TRIP assisted dual phase steel types, which have to be formed into for instance automotive parts that will be spot welded into a body in white.

[0006] It is thus an object of the invention to find a composition of a high strength hot dip galvanised steel strip that strikes a balance between the formability and the processability of the strip.

[0007] It is a further object of the invention to provide a high strength hot dip galvanised steel strip that has a good coatability during the hot dip galvanising process.

[0008] It is a still further object of the invention to provide a high strength hot dip galvanised steel strip that has a good weldability.

[0009] It is another object of the invention to provide a high strength hot dip galvanised steel strip that has a good surface quality.

[0010] It is still another object of the invention to provide a high strength hot dip galvanised steel strip having a cost price that is as low as possible.

[0011] One or more of these objects are met according to the invention by providing a high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements: [0012] 0.13-0.19% C [0013] 1.70-2.50% Mn [0014] max 0.15% Si [0015] 0.40-1.00% Al [0016] 0.05-0.25% Cr [0017] 0.01-0.05% Nb [0018] max 0.10% P [0019] max 0.004% Ca [0020] max 0.05% S [0021] max 0.007% N

[0022] and optionally at least one of the following elements: [0023] max 0.50% Ti [0024] max 0.40% V [0025] max 0.50% Mo [0026] max 0.50% Ni [0027] max 0.50% Cu [0028] max 0.005% B

[0029] the balance being Fe and inevitable impurities,

[0030] wherein 0.40%<Al+Si<1.05% and Mn+Cr>1.90%.

[0031] The inventors have found that by a careful selection of the amounts of the main constituting elements of the steel, being carbon, manganese, silicon, aluminium and chromium, a high strength hot dip galvanised steel strip can be produced that has the required formability, processability, strength and elongation, while at the same time providing a sufficient weldability, coatability and surface quality. It has been found by the inventors that none of the examples given in the state of the art provide all these requirements at the same time.

[0032] Especially, the inventors have found that a relatively high silicon content deteriorates the processability of the steel strip. Si is traditionally used to effectuate the TRIP effect, due to retardation of carbide formation in the presence of Si which leads to carbon enrichment and, hence, stabilisation of austenite at room temperature. The disadvantages of Si are that in very high quantities (above 0.4 wt. %) it interferes with the wettability of zinc, making galvanisation over tradition continuous annealing lines impossible. It has also been shown in EP 1 889 935 A1 that Si can be replaced by relatively high quantities of Al. The present invention shows that the addition of Si can be omitted and Al kept to a minimum by careful selection of the Cr content and with the addition of Nb. In this case a strip is produced which has low hot-rolling loads allowing a wider and thinner dimensional capability. However, silicon is usually present in trace quantities due to manganese addition. For this reason, the maximum silicon content is set on 0.15% Si. Above this amount of silicon, the rolling loads in the hot-strip mill are high. An amount of silicon below 0.15% makes it possible to produce wide and thin steel strip.

[0033] Moreover, the composition of the steel strip according to the invention is such that the formability of the steel is good and no necking occurs, and that the edge ductility of pressed parts is such that no cracking occurs.

[0034] The reason for the amounts of the main constituting elements is as follows.

[0035] C: 0.13-0.19 mass %. Carbon has to be present in an amount that is high enough to ensure hardenability and the formation of martensite at the cooling rates available in a conventional annealing/galvanising line. Martensite is required to deliver adequate strength. Free carbon also enables stabilisation of austenite which delivers improved work hardening potential and good formability for the resulting strength level. A lower limit of 0.13 mass % is desired for these reasons. A maximum level of 0.19 mass % has been found to be essential to ensure good weldability.

[0036] Mn: 1.70-2.50 mass %. Manganese is added to increase hardenability thus making the formation of martensite easier within the cooling rate capability of a conventional continuous annealing/galvanising line. Manganese also contributes to the solid solution strengthening which increases the tensile strength and strengthens the ferrite phase, thus helping to stabilise retained austenite. Manganese lowers the transformation temperature range of the dual phase steel, thus lowering the required annealing temperature to levels that can be readily attained in a conventional continuous annealing/galvanising line. A lower limit of 1.70 mass % is needed for the above reasons. A maximum level of 2.50 mass % is imposed to ensure acceptable rolling forces in the hot mill and to ensure acceptable rolling forces in the cold mill by ensuring sufficient transformation of the dual phase steel to soft transformation products (ferrite and pearlite). This maximum level is also given in view of the stronger segregation during casting and the forming of a band of martensite in the strip at higher values.

[0037] Al: 0.40-1.00 mass %. Aluminium is added to liquid steel for the purpose of de-oxidation. In the right quantity it also provides an acceleration of the bainite transformation, thus enabling bainite formation within the time constraints imposed by the annealing section of a conventional continuous annealing/galvanising line. Aluminium also retards the formation of carbides thus keeping carbon in solution thus causing partitioning to austenite during overaging, and promoting the stabilisation of austenite. A lower level of 0.40 mass % is required for the above reasons. A maximum level of 1.00 mass % is imposed for castability, since high aluminium contents lead to poisoning of the casting mould slag and consequently an increase in mould slag viscosity, leading to incorrect heat transfer and lubrication during casting.

[0038] Cr: 0.05-0.25 mass %. Chrome is added to increase hardenability. Chrome forms ferrite and suppresses the formation of carbides, thus enhancing the forming of retained austenite. A lower level of 0.05 mass % is required for the above reasons. A maximum level of 0.25 mass % is imposed to ensure satisfactory pickling of the steel strip, and to keep the cost of the strip sufficiently low.

[0039] Ca: max 0.004 mass %. The addition of calcium modifies the morphology of manganese sulphide inclusions. When calcium is added the inclusions get a globular rather than an elongated shape. Elongated inclusions, also called stringers, may act as planes of weakness along which lamellar tearing and delamination fracture can occur. The avoidance of stringers is beneficial for forming processes of steel sheets which entail the expansion of holes or the stretching of flanges and promotes isotropic forming behaviour. Calcium treatment also prevents the formation of hard, angular, abrasive alumina inclusions in aluminium deoxidised steel types, forming instead calcium aluminate inclusions which are softer and globular at rolling temperatures, thereby improving the material's processing characteristics. In continuous casting machines, some inclusions occurring in molten steel have a tendency to block the nozzle, resulting in lost output and increased costs. Calcium treatment reduces the propensity for blockage by promoting the formation of low melting point species which will not clog the caster nozzles.

[0040] P: max 0.10 mass %. Phosphorus interferes with the formation of carbides, and therefore some phosphorus in the steel is advantageous. However, phosphorus can make steel brittle upon welding, so the amount of phosphorus should be carefully controlled, especially in combination with other embrittling elements such as sulphur and nitrogen.

[0041] Sulphur and nitrogen are present in low amounts because these elements are detrimental for weldability.

[0042] Niobium is added in an amount between 0.01 and 0.05 mass % for grain refinement and formability. Niobium promotes transformation on the runout table and thus provides a softer and more homogeneous intermediate product. Niobium further suppresses formation of martensite at isothermal overaging temperatures, thereby promoting stabilisation of retained austenite.

[0043] The optional elements are mainly added to strengthen the steel.

[0044] In addition the reasons given above, the ranges for aluminium, chromium and manganese are chosen such that a correct balance is found to deliver complete transformation on the runout table to ensure a steel strip that can be cold rolled, and to provide a starting structure enabling rapid dissolution of carbon in the annealing line to promote hardenability and correct ferritic/bainitic transformation behaviour. Moreover, because aluminium accelerates and chromium decelerates the bainitic transformation, the right balance between aluminium and chromium has to be present to produce the right quantity of bainite within the timescales permitted by a conventional hot dip galvanising line with a restricted overage section.

[0045] Apart from the absolute contents of the elements as given above, also the relative amounts of certain elements are of importance.

[0046] Aluminium and silicon together should be maintained between 0.4 and 1.05 mass % to ensure suppression of carbides in the end product and stabilisation of a sufficient amount of austenite, with the correct composition, to provide a desirable extension of formability.

[0047] Manganese and chromium together should be above 1.90 mass % to ensure sufficient hardenability for formation of martensite and thus achievement of strength in a conventional continuous annealing line and hot dip galvanising line.

[0048] Preferably element C is present in an amount of 0.13-0.16%. In this range the hardenability of the steel is optimal while the weldability of the steel is enhanced.

[0049] According to a preferred embodiment element Mn is present in an amount of 1.95-2.40%, preferably in an amount of 1.95-2.30%, more preferably in an amount of 2.00-2.20%. A higher amount of manganese provides steel with a higher strength, so it is advantageous to raise the lower limit to 1.95 or even 2.00 mass % manganese. On the other hand, hot rolling and cold rolling of the steel is more difficult for higher amounts of manganese, so it is advantageous to lower the upper limit to 2.40, 2.30 or even 2.20 mass % manganese.

[0050] Preferably element Si is present in an amount of 0.05-0.15%. There is no objection to the presence of some Si in the steel, since Si ensures a better retardation of carbides during overaging which is advantageous for the formability of the steel.

[0051] According to a preferred embodiment element Al is present in an amount of 0.60-0.80%. A raised lower level of aluminium has the same effect as a higher amount of silicon, but also improves the bainite formation. A lower upper limit of aluminium improves the castability of the steel.

[0052] Preferably element Cr is present in an amount of 0.10-0.25%. A raised lower level increases the hardenability of the steel.

[0053] According to a preferred embodiment element Nb is present in an amount of 0.01-0.04%. As elucidated above, niobium improves the homogeneity of the intermediate product. The upper limit is mainly in consideration of the cost of niobium.

[0054] Preferably the steel has an ultimate tensile strength Rm of at least 700 MPa, more preferably an ultimate tensile strength Rm of at least 750 MPa. This strength can, due to the careful selection of the amounts of the elements present in the steel, be reached while the formability of a conventional 600 MPa dual phase steel is maintained.

[0055] According to a preferred embodiment the hot dip galvanised steel strip has an 0.2% proof strength Rp of at least 400 MPa, preferably an 0.2% proof strength Rp of at least 450 MPa. Also this strength can be reached due to the careful selection of the amounts of the elements present in the steel.

[0056] Preferably, the hot dip galvanised steel strip has a total elongation of at least 18%. This is a high elongation which is also reached by the chosen presence of the elements in the steel.

[0057] According to a preferred embodiment the hot dip galvanised steel strip has a hole expansion coefficient of at least 35% when Rm is 750 MPa and Rp is 450 MPa. This is a good hole expansion coefficient, as will be elucidated below. The hole expansion coefficient decreases with increasing strength.

[0058] Preferably the hot dip galvanised steel strip has an Erichsen cupping index of more than 10.5 mm when Rm is 750 MPa and Rp is 450 MPa. This is satisfactory for the usability of the steel. The Erichsen cupping index decreases with increasing strength.

[0059] According to a preferred embodiment the hot dip galvanised steel strip has a dual phase structure containing 8-12% retained austenite, 10-20% martensite, the remainder being a mixture of ferrite and bainite, preferably the hot dip galvanised steel strip containing not more than 10% bainite. With such microstructures, a high elongation and a high strength will be reached.

[0060] According to a preferred embodiment the hot dip galvanised steel strip has an average grain size of at most 5 .mu.m. This small grain size helps to achieve the above mentioned mechanical properties of the steel.

[0061] According to a second aspect of the invention there is provided a method for producing a high strength hot dip galvanised steel strip as defined above, wherein the cast steel is hot rolled and cold rolled to a strip having a desired thickness, after which the strip is reheated in an annealing line to a temperature above the Ac1 temperature and preferably between the Ac1 and the Ac3 temperature of the steel type and fast cooled at a cooling rate such as to avoid retransformation to ferrite, after which isothermal overaging is applied to form bainite, and the strip is hot dip galvanised.

[0062] In this method, the deformation schedule during hot rolling, the finish rolling temperature and the subsequent cooling pattern on a run-out table can be selected to achieve a microstructure in the hot rolled product which is conducive to further reduction of thickness in the cold mill. In particular attention can be paid to limiting the strength of the hot rolled strip so as to minimise the required cold rolling loads. The temperature in the annealing line can be chosen such that the steel strip comprises both ferrite and austenite. The cooling rate should be such that in principle no ferrite is formed, and the isothermal overaging is applied to promote the formation of bainite. Hot dip galvanising can be performed in the usual manner. During this method the temperature and duration of most steps is critical for the realisation of the desired balance between strength and ductility in the final product.

[0063] Preferably the annealing will be carried out a temperature between 750.degree. C. and 850.degree. C. and more preferably at a temperature between 780.degree. C. and 820.degree. C. At these temperatures the steel strip comprises both ferrite and austenite.

[0064] Preferably, the overaging is applied at a temperature between 360.degree. C. and 480.degree. C.

[0065] As known to the skilled person, the iron-carbon eutectoid system has a number of critical transformation temperatures as defined below. These temperatures are dependent on chemistry and processing conditions:

A1--temperature below which the microstructure is composed of a mixture of ferrite (alpha-Fe) and Fe3C/pearlite; A2--Curie temperature: temperature above which the material ceases to be magnetic; A3--temperature above which the microstructure is entirely composed of austenite. The suffixes c and r denote transformations in the heating and cooling cycle respectively.

[0066] The invention will be elucidated hereinafter; a number of compositions will be evaluated with regard to some well-known formability parameters that are elucidated first.

[0067] n-value: The work hardening coefficient or n-value is closely related to uniform elongation. In most sheet forming processes the limit of formability is determined by the resistance to local thinning or "necking" In uniaxial tensile testing necking commences at the extent of uniform elongation. n-value and uniform elongation derived from the tensile test can be taken as a measure of the formability of sheet steels. When aiming to improve formability of strip steels n-value and uniform elongation represent the most suitable optimisation parameters.

[0068] Hole expansion coefficient (HEC): To be successfully applied in industrial stamping operations, sheet metals must have a certain ability to withstand stretching of their sheared edges. This is tested in accordance with the international technical specification ISO/TS16630. A hole having a diameter of 10 mm is made in the centre of a test piece having the dimensions 90.times.90 mm. A cone punch of 40 mm diameter with a 60.degree. apex is forced into the hole while the piece is fixed with a die having an inner diameter of 55 mm. The diameter of the hole is measured when a crack had extended through the thickness of the test piece. The maximum HEC was determined by: Max HEC %=((Dh-Do)/Do).times.100, wherein Do is the original hole diameter and Dh is the diameter of the hole after cracking Stretch flangeability is evaluated on the basis of the maximum HEC and is deemed satisfactory when HEC>25%

[0069] Erichsen Index (EI): The Erichsen test describes the ability of metals to undergo plastic deformation in stretch forming and is tested in accordance with the international standard test ISO 20482:2003. A hemispherical punch is driven into a fully clamped sheet. As lubrication graphite grease is used on top of the punch. The punch travel is stopped when a through thickness crack is detected. Due to friction the fracture is not on top of the punch but to the side, so not in equi bi-axial strain but more towards plane strain. The depth of the punch penetration is measured. The value of the Erichsen cupping index (IE) is the average of a minimum of three individual measurements, expressed in millimetres and for the present invention is deemed satisfactory when EI>10 mm.

[0070] Weldability: Resistance spot welding is the major joining technique used in the automotive industry, with an average car containing around 2000-3000 spot welds. Traditionally spot welds have always been a very cheap and reliable joint type, however since the introduction of AHSS, this reliability has been compromised. The weldability is measured by the ability of the material to be spot-welded. Welding conditions were taken from BS1140: 1993 which are standard for industry, although not necessarily optimised for AHSS. Spot-weldability is measured by the failure mode of the resultant spot-weld (plug). When a material cannot be welded then the plug will split along the interface between the two joining surfaces. In a fully welded material the failure will be in the parent metal, outside of the plug and preferably also outside the heat-affected zone. This is known as full-plug failure, that is the full plug is pulled out of the parent metal. Spot-weldability can be expressed on the scale between full-interface failure and full-plug failure with the former being deemed un-weldable.

[0071] One of the aims of the present invention is to provide a high strength hot dip galvanised steel strip that has a formability in the range of a 600 MPa AHSS hot dip galvanised steel strip, but having a strength level of at least 700 MPa. This is achieved by realising a suitable increase in the uniform elongation and n-value.

[0072] During the development of the high strength hot dip galvanised steel strip according to the invention a number of coils of strip have been produced along with comparative examples. The chemistry of the different alloys is given in Table 1 (in 10.sup.-3%). The processing conditions along with the resultant mechanical properties are given in Table 2. In Table 2 RA indicates retained austenite, M indicates martensite, and F indicates ferrite plus bainite.

TABLE-US-00001 TABLE 1 Chemical composition of invented steel and some comparative examples Alloy C Mn Al Cr Nb Si Notes A 154 1660 570 400 0 120 Example B 143 1944 680 103 21 72 Invention C 156 2054 729 105 21 60 Invention D 151 1730 580 110 0 410 Example E 150 1880 610 519 22 430 Example F 155 2027 707 92 20 57 Invention

TABLE-US-00002 TABLE 2 Processing conditions and resultant mechanical properties of invented steel and some comparative examples Iso- Anneal thermal temp holding % Rp Rm n- EI, Alloy .degree. C. temp .degree. C. % F % M RA MPa MPa Ag % A80 % value HEC % mm A 800 420 71 7 12 345 701 18.8 23.4 0.210 B 780 360 79 12 9 457 727 17.6 25.5 0.178 B 780 380 78 11 11 487 738 16.8 22.5 0.174 B 790 360 78 11 11 490 751 17.4 26.1 0.174 35 10.8 B 790 370 79 11 10 502 750 14.8 19.0 0.173 42 10.6 B 800 420 76 12 12 523 751 15.4 18.5 0.180 B 810 380 82 10 8 489 701 16.4 21.3 0.187 B 830 480 78 12 10 492 754 15.6 21.4 0.159 38 10.7 B 830 490 81 15 4 532 804 13.0 17.3 0.145 C 815 385 81 10 9 534 784 15.3 19.0 0.17 37 10.5 C 820 440 80 11 9 524 791 14.7 18.1 0.17 33 10.1 D 800 420 90 5 5 442 702 21.3 26.6 0.239 E 830 420 80 10 10 427 772 14.7 20.8 0.158 25 9.5 F 780 400 82 9 9 471 787 15.0 20.7 0.15 F 780 410 82 10 8 454 781 14.6 19.2 0.17 32 10.4 F 790 410 79 12 9 496 795 15.3 20.1 0.16

[0073] Table 1 shows six different alloys A to F, of which alloys B, C and F have a composition according to the invention and alloys A, D and E are comparative examples. For alloys A, D and E processing conditions according to the invention have been applied as shown in Table 2. The mechanical properties of the alloys A and E are clearly outside the values as desired for according to the invention, because for alloy A the Rp value is below 400 MPa, and for alloy E the hole expansion coefficient is only 25% and the Erichsen cupping index is less than 10. Alloy D has a composition according to the invention, but the Si level is too high. Table 2 shows that the mechanical properties as measured fall within the values as desired for according to the invention, though the Rm value is only just above the lower limit of 700 MPa, and clearly lower than the preferred value of at least 750 MPa. However, the structure of alloy D is not in accordance with the desired structure, because the amount of martensite and retained austenite is too low.

[0074] Alloys D and E both have Si contents which are well outside of the range of the invention which leads to high rolling loads thereby reducing the available dimensions for the final product.

[0075] Alloy B is an alloy according to the invention. For this alloy, different processing conditions have been applied as shown in Table 2. Different annealing temperatures and different isothermal holding temperatures result in mechanical properties that fulfil the desired values according to the invention as regards the ultimate tensile strength and the 0.2% proof strength, though desired total elongation is not always met. This occurs when the overaging temperature is above the optimum bainite holding temperature (i.e. above 480.degree. C.), thus reducing the amount of retained austenite available.

[0076] Alloy C is also an alloy according to the invention. For this alloy, two different processing conditions have been applied as shown in Table 2. For both processing conditions, the ultimate tensile strength Rm is more than 750 MPa and the total elongation is more than 18%. The Erichsen Index is more than 10, the n-value is 0.17 and the HEC is above 30.

[0077] Alloy F is another alloy according to the invention. Three different processing conditions have been applied, as shown in Table 2, and in all three cases the ultimate tensile strength is more than 750 MPa, the total elongation is even more than 19%, and the n-value is 0.15 or more. Where measured, the Erichsen Index is more than 10 and the HEC is above 30.

[0078] Hole expansion coefficient HEC and Erichsen cupping index EI are not always measured but where measured the value is satisfactory. Also the work hardening coefficient or n-value is good in all cases.

[0079] The TRIP assisted dual phase steel according to the invention with low amounts of silicon thus shows to be a steel type that gives satisfactory mechanical properties, contrary to the expectation by the person skilled in the art that a certain amount of added silicon is always needed.

Claims

1. A high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements: 0.13-0.19% C 1.70-2.50% Mn max 0.15% Si 0.40-1.00% Al 0.05-0.25% Cr 0.01-0.05% Nb max 0.10% P max 0.004% Ca max 0.05% S max 0.007% N and optionally at least one of the following elements: max 0.50% Ti max 0.40% V max 0.50% Mo max 0.50% Ni max 0.50% Cu max 0.005% B the balance being Fe and inevitable impurities, wherein 0.40%<Al+Si<1.05% and Mn+Cr>1.90%, wherein the hot dip galvanised steel strip has a microstructure containing 8-12% retained austenite, 10-20% martensite, the remainder being a mixture of ferrite and bainite, the hot dip galvanised steel strip containing not more than 10% bainite, and wherein the hot dip galvanised steel strip has an ultimate tensile strength Rm of at least 700 MPa, a 0.2% proof strength Rp of at least 400 MPa and a total elongation of at least 18%.

2. The steel strip according to claim 1, wherein element C is present in an amount of 0.13-0.16%.

3. The steel strip according to claim 1, wherein element Mn is present in an amount of 1.95-2.40%.

4. The steel strip according to claim 1, wherein element Si is present in an amount of 0.05-0.15%.

5. The steel strip according to claim 1, wherein element Al is present in an amount of 0.60-0.80%.

6. The steel strip according to claim 1, wherein element Cr is present in an amount of 0.10-0.25%.

7. The steel strip according to claim 1, wherein the element Nb is present in an amount of 0.01-0.04%.

8. The steel strip according to claim 1, wherein the hot dip galvanised steel strip has an ultimate tensile strength Rm of at least 750 MPa.

9. The steel strip according to claim 1, wherein the hot dip galvanised steel strip has an 0.2% proof strength Rp of at least 450 MPa.

10. The steel strip according to claim 1, wherein the hot dip galvanised steel strip has a hole expansion coefficient of at least 35% when Rm is 750 MPa and Rp is 450 MPa.

11. The steel strip according to claim 1, wherein the hot dip galvanised steel strip has an Erichsen cupping index of more than 10.5 mm when Rm is 750 MPa and Rp is 450 MPa.

12. The steel strip according to claim 1, wherein the hot dip galvanised steel strip has an average grain size of at most 5 .mu.m.

13. A method for producing a high strength hot dip galvanised steel strip according to claim 1, comprising: hot rolling and cold rolling the cast steel to a strip having a desired thickness, after said rolling to the desired thickness reheating the strip in an annealing line to a temperature above the Ac1 temperature of the steel, and fast cooling the reheated strip at a cooling rate to avoid retransformation to ferrite, after which isothermal overaging is applied to form bainite, and the strip is hot dip galvanised.

14. The method according to claim 13, wherein the annealing is applied at a temperature between 750.degree. C. and 850.degree. C.

15. The method according to claim 13, wherein the overaging is applied at a temperature between 360.degree. C. and 480.degree. C.

16. The steel strip according to claim 1, wherein element Mn is present in an amount of 1.95-2.30%.

17. The steel strip according to claim 1, wherein element Mn is present in an amount of 2.00-2.20%.

18. The method according to claim 14, wherein the annealing is applied at a temperature between 780.degree. C. and 820.degree. C.

19. The method according to claim 14, wherein the strip is reheated in the annealing line to a temperature between the Ac1 and the Ac3 temperature of the steel type.