SE2051557A1 - Coiling temperature influenced cold rolled strip or steel - Google Patents

Abstract

The invention relates to a cold roll strip or sheet comprising in (wt%) C 0.08 - 0.28; Mn 1.4 -4.5; Cr 0.01 - 0.5; Si 0.01 - 2.5; A10.01 - 0.6; Si A1 > 0.1; Si A1 Cr > 0.4; Nh < 0.008; Ti < 0.02; Mo < 0.08; Ca < 0.005; V < 0.02; balance Fe apart from impurities. The steel being within the area defined by the coordinates A, B, C, D, where Ri/t (y-axle) is plotted vs TS(MPa)/YR (x-axle), and where A is [1200, 2), B is [2000, 4], C is [2000, 3], and D is [1200,

Description

COILING TEMPERATURE INFLUENCED COLD ROLLED STRIP OR STEEL TECHNICAL FIELDThe present invention relates to high strength steel strips and sheets suitable for applications in automobiles.

BACKGROUND ARTFor a great variety of applications increased strength levels are a pre-requisite for light-weightconstructions in particular in the automotive industry, since car body mass reduction results in reduced fuel consumption.

Automotive body parts are often stamped out of sheet steels, forming complex structural members ofthin sheet. However, such parts cannot be produced from conventional high strength steels, because ofa too low formability of the complex structural parts. For this reason, multi-phase TransformationInduced Plasticity aided steels (TRIP steels) have gained considerable interest in the last years, in particular for use in auto body structural parts and as seat frame materials.

TRIP steels possess a multi-phase microstructure, which includes a meta-stable retained austenitephase, which is capable of producing the TRIP effect. When the steel is deformed, the austenitetransforms into martensite, which results in remarkable work hardening. This hardening effect acts toresist necking in the material and postpones failure in sheet forming operations. The microstructure of a TRIP steel can greatly alter its mechanical properties.

TRIP steels have been known for long and attracted a lot of interest, mainly because the matrix allowsan excellent stretch flangability. Moreover, the TRIP effect ensured by the strain-inducedtransformation of metastable retained austenite islands into martensite, remarkably improves their drawability.

When producing cold rolled TRIP steel sheets a slab is initially provided. The slab is hot rolled inaustenitic temperature range to a hot rolled strip. The hot rolled strip is thereafter coiled. The coilingresistance is reduced with increasing temperature. Commonly a coiling temperature of 600 OC isemployed. The coiled strip is thereafter batch annealed, followed by cold rolling. The cold rolled strip is thereafter continuously annealed.

WO 2019/ 122963 Al and WO20l9l23043 Al both discloses a TRIP steel with improvedphosphatation coverage. A good phosphatation coverage is enabled. The improved phosphatation coverage was achieved by controlling the alloying elements and the process parameters of which one is to have a low coiling temperature. All inventive examples have a coiling temperature of 450 OC.Reference examples with higher coiling temperatures did not provide sufficient phosphatation coverage. A low coiling temperature increases cold rolling forces.

EP 2707514 Bl disclose a TRIP steel having a microstructure comprising of 5-20% polygonal ferrite,10-15% residual austenite, 5-15 % martensite and balance bainite. According to the document thepresence of polygonal ferrite between 5 and 20% makes it possible to exceed a V-bending angle of 90O without the occurrence of cracking.

WO2018116155 disclose a TRIP steel. The inventive examples disclose a lower coiling temperature of450 OC in combination with a higher batch annealing temperature of 620 OC respectively 650 OC, anda higher coiling temperature of 560 OC in combination with a lower batch annealing temperature of 460 OC.

Although these steels disclose several attractive properties there is demand for >950 MPa steel sheetor strip having an improved property profile with respect to advanced forn1ing operations, in particularbending properties. In particular bending property in relation to strength and toughness. Furtherdesirable properties are: reduced grain-boundary oxidation, reduced susceptibility to Liquid metal embrittlement, reduced susceptibility to hydrogen embrittlement, and a good phosphatabilißf.

DISCLOSURE OF THE INVENTION The present invention is directed to cold rolled steels having a tensile strength of at least 950 MPa andan excellent formability, wherein it should be possible to produce the steel sheets/strips on anindustrial scale in a Continuous Annealing Line (CAL) and in a llot Dip (šalvanizing Line (lflïšßís).The invention aims at providing a steel having a composition and microstructure that can be processed to complicated high strength structural members, where the bending properties are of importance.

The careful selection of alloying elements and process parameters reduces grain boundary oxidation.The reduced grain boundary oxidation improves bendability and reduces the risk of liquid metal embrittlement and susceptibility to hydrogen embrittlement. It further facilitates good phospahtabilityz BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a graph with the inventive samples within a within the dotted lines.Fig. 2a shows no an inventive sample with no grain boundary oxidation.

Fig, 2b shows the surface of the inventive sample of Fig. 2a.

Fig. Sa shows the grain boundary oxidation of a reference sample.

Fig. 3b is a zoom in on the grain boundary of Fig. Sa.Fig. 3c shows the surface of the reference sample of Fig. 3a-3b.Fig. 4 shows the phosphatation coverage of the inventive sample Fig 2a-2b.

Fig. 5 shows the phosphatation coverage of the reference sample of Fig 3a-3c.

DETAILED DESCRIPTION The invention is described in the claims.

The steel sheet has a composition consisting of the following alloying elements (in wt. %): C 0.08 - 0.128lVín 1.4 - 4.5Cr 0.01 - 0.5Si 0.01 - 2.5Al 0.01 - 0.6Si + A1 01Si-+-.Al+- Cr i: 0.4 Nb i' 0.1 'li 1.3.1 lVío 0.5 Ca í 0.05 V í 0.1 balance Fe apart from irripurities.

The importance of the separate elements and their interaction with each other as well as the limitationsof the chemical ingredients of the claimed alloy are briefly explained in the following. All percentagesfor the chemical composition of the steel are given in weight % (wt. %) throughout the description.Upper and lower limits of the individual elements can be freely combined within the limits set out inthe claims. The arithmetic precision of the numerical values can be increased by one or two digits forall 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 %. The amounts of the microstructural constituents are given in volume % (vol. %).

C: 0.08 - 0.28 %C stabilizes the austenite and is important for obtaining sufficient carbon within the retained austenitephase. C is also important for obtaining the desired strength level. Generally, an increase of the tensile strength in the order of 100 MPa per 0.1 % C can be expected. When C is lower than 0.08 % it is difficult to attain a tensile strength of 950 MPa. If C eXceeds 0.28 %, then the weldability is impaired.The upper limit may thus be 0.26, 0.24, 0.22, 0.20 or 0.18 %. The lower limit may be 0.10, 0.12, 0.14,or 0.16 %.

Mn: 1.4 - 4.5 % Manganese is a solid solution strengthening element, which stabilises the austenite by lowering the MStemperature and prevents ferrite and pearlite to be formed during Cooling. In addition, Mn lowers theA03 temperature and is important for the austenite stability. At a content of less than 1.5 % it might bedifficult to obtain the desired amount of retained austenite, a tensile strength of 950 MPa and theaustenitizing temperature might be too high for conventional industrial annealing lines. In addition, atlower contents it may be difficult to avoid the formation of polygonal ferrite. However, if the amountof Mn is higher than 4.5 %, problems with segregation may occur because Mn accumulates in theliquid phase and causes banding, resulting in a potentially deteriorated workability. The upper limitmay therefore be 4.2, 4.0, 3.8, 3.6, 3.4, 3.2, 3.0, 2.8, 2.6, or 2.4 %. The lower lin1it may be 1.5, 1.7,1.9, 2.1, 2.3, or 2.5%.

Cr: 0.01- 0.5 % Cr is effective in increasing the strength of the steel sheet. Cr is an element that forms ferrite andretards the formation of pearlite and bainite. The A03 temperature and the MS temperature are onlyslightly lowered with increasing Cr content. Cr results in an increased amount of stabilized retainedaustenite. When above 0.5% it may impair surface finish of the steel, and therefore the amount of Cr islimited to 0.5 %. The upper limit may be 0.45 or 0.40, 0.35, 0.30 or 0.25 %. The lower limit may be0.01, 0.03, 0.05, 0.07, 0.10, 0.15, 0,20 or 0.25 %. Preferably, a deliberate addition of Cr is not conducted according to the present invention.

Si: 0.01 - 2.5 % Si acts as a solid solution strengthening element and is important for securing the strength of the thinsteel strip. Si suppresses the cementite precipitation and is essential for austenite stabilization.However, if the content is too high, then too much silicon oXides will form on the strip surface, whichmay lead to cladding on the rolls in the CAL and, as a result there of, to surface defects onsubsequently produced steel sheets. The upper lin1it is therefore 2.5 % and may be restricted to 2.4,2.2, 2.0, 1.8 or 1.6 %. The lower limit may be 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.60, 0.80 or 1.0 %.

Al: 0.01- 0.6 %Al promotes ferrite formation and is also commonly used as a deoxidizer. Al, like Si, is not soluble inthe cementite and therefore it considerably delays the cementite formation during bainite formation. In addition, galvanization and reduced susceptibility to Liquid metal embrittlement can be improved.

Additions of A1 result in a remarkab1e increase in the carbon content in the retained austenite.HoWever, the MS temperature is also increased With increasing A1 content. A further drawback of A1 isthat it resu1ts in a drastic increase in the A03 temperature. HoWever, a main disadvantage of A1 is itssegregation behaviour during casting. During casting Mn is enriched in the midd1e of the s1abs and theA1-content is decreased. Therefore, in the midd1e of the s1ab a significant austenite stabi1ized region orband may be formed. This resu1ts at the end of the processing in martensite banding and that 1oW straininterna1 cracks are formed in the martensite band. On the other hand, Si and Cr are a1so enrichedduring casting. Hence, the propensity for martensite banding may be reduced by a11oying With Si andCr, since the austenite stabi1ization due to the Mn enrichment is counteracted by these e1ements. Forthese reasons the A1 content is preferab1y 1imited. The upper 1eve1 may be 0.6, 0.5, 0.4, 0.3, 0.2, 0.1%.The 1oWer 1imit may be set to 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1 %. If A1 is usedfor deoxidation on1y then the upper 1eve1 may then be 0.09, 0.08, 0.07 or 0.06 %. For securing acertain effect the 1oWer 1eve1 may set to 0.03 or 0.04 %.

Si + A1 2 0.1 %Si and A1 suppress the cementite precipitation during bainite formation. Their combined content is therefore preferab1y at 1east 0.1%. The upper 1imit may be 2 %.

Si+A1+Cr20.4%A certain amount of these e1ements is beneficia1 for the formation of austenite. Their combined content shou1d therefore be at 1east 2 0.4 %. The 1oWer 1imit can be 0.5, 0.6 or 0.7%.

Mn+Cr 1.7 - 5.0 %Manganese and Chromium affects the hardenabi1ity of the stee1. Their combined content shou1d therefore be Within the range of 1.7 - 5.0 %.

Optional e1ementsMo í 0.5% Mo1ybdenum is a poWerfu1hardenabi1ity agent. It may further enhance the benefits of NbCprecipitates by reducing the carbide coarsening kinetics. The stee1 may therefore contain Mo in anamount up to 0.5 %. The upper 1imit may be restricted to 0.4, 0.3, 0.2, or 0.1 %. A de1iberate additionof Mo is not necessary according to the present invention. The upper 1imit may therefore be restricted to S 0.01 %.

Nb: E 0.1%Nb is common1y used in 1oW a11oyed stee1s for improving strength and toughness, because of its influence on the grain size. Nb increases the strength e1ongation ba1ance by refining the matrix microstructure and the retained austenite phase due to precipitation of NbC. The steel may contain Nbin an amount of í 0.1%. The upper limit may be restricted to 0.09, 0.07, 0.05, 0.03, or 0.01 %. Adeliberate addition of Nb is not necessary according to the present invention. The upper limit may therefore be restricted to í 0.004 %.

V: E 0. 1 % The function of V is similar to that of Nb in that it contributes to precipitation hardening and grainrefinement. The steel may contain V in an amount of í 0.1 %. The upper limit may be restricted to0.09, 0.07, 0.05, 0.03, or 0.01 %. A deliberate addition of V is not necessary according to the present invention. The upper limit may therefore be restricted to í 0.01 %.

Ti: E 0. 1% Ti is commonly used in low alloyed steels for improving strength and toughness, because of itsinfluence on the grain size by forming carbides, nitrides or carbonitrides. In particular, Ti is a strongnitride former and can be used to bind the nitrogen in the steel. HoWever, the effect tends to besaturated above 0.1 %. The upper limit may be restricted to 0.09, 0.07, 0.05, 0.03, or 0.01 %. Adeliberate addition of Ti is not necessary according to the present invention. The upper limit may therefore be restricted to E 0.005%.

Ca E 0.05 Ca may be used for the modification of the non-metallic inclusions. The upper limit is 0.05% and maybe set to 0.04, 0.03, 0.01 %. A deliberate addition of Ca is not necessary according to the presentinvention. The upper limit may therefore be restricted to E 0.005%. mpurilCu: s 0.06 % Cu is an undesired impurity element that is restricted to í 0.06 % by careful selection of the scrap used.

Ni: E 0.08 %Ni is also an undesired impurity element that is restricted to í 0.08 % by careful selection of the scrap used.

B: E 0.0006%B is an undesired impurity element that is restricted to í 0.006 % by careful selection of the scrapused. B increases hardness but may come at a cost of reduced bendability and is therefore not desirable in the present suggested steel. B may further make scrap recycling more difficult and an addition of B may also deteriorate workability. A deliberate addition of B is therefore not desired according to the present invention.

Other impurity elements may be comprised in the steel in normal occurring amounts. However, it is preferred to limit the amounts of P, S, As, Zr, Sn to the following optional maximum contents: P: S 0.02 %S: S 0.005 %As S 0.010%Zr S 0.006%Sn S 0.015% It is also preferred to control the nitrogen content to the range:N: S 0.015 %, preferably 0.003 - 0.008 % In this range a stable fiXation of the nitrogen can be achieved.

OXygen and hydrogen can further be limited toO: S 00003H: S 0.0020 The microstructural constituents are in the following expressed in volume % (vol. %).

The cold rolled steel sheets of the present invention have a microstructure comprising at least 50%tempered martensite (TM) and bainite (B). The lower limit may restrict to at least 60, 70%, 75%, or 80%.

And further, at most 10 % fresh martensite (FM). The upper lin1it may be restricted 8 % or 5 %. Smallamounts of fresh martensite may improve edge flangeability and local ductility. The lower limit maybe restricted 1% or 2%. These un-tempered martensite particles are often in close contact with theretained austenite particles and they are therefore often referred to as martensite-austenite (MA) particles.

Retained austenite is a prerequisite for obtaining the desired TRIP effect. The amount of retainedaustenite should therefore be in the range of 2 - 20 %, preferably 5 - 15 %. The amount of retainedaustenite was measured by means of the saturation magnetization method described in detail in Proc.

Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61-64.

Polygonal ferrite (PF) is susceptible to Hydrogen Embrittlement and is therefore not a desiredmicrostructural constituent. Furthermore, the presence of ferrite may impart the steel with formabilityand elongation and also to a certain degree resistance to fatigue failure. It may also have negativeimpacts due to the fact that ferrite increases the gap in hardness with hard phases such as martensiteand bainite and reduces local ductility, resulting in lower hole expansion ratio. Polygonal ferrite (PF)is therefore limited to í 20 %, preferably í 10 %, í 5 %, í 3 % or í 1 %. Most preferably, the steel is free from PF.

The mechanical properties of the claimed steel are important, and the following requirements should be fulfilled: TS tensile strength (Rm) 950 - 1550YS yield strength (Rpog) 550 - 1400 MPa YR yield ratio (RPOQ/ Rm) 2 0.50, preferably 2 0.7bendability (Ri/t) í 5 MPa The Rm, Rpog values are derived according to the European norm EN 10002 Part 1, wherein thesamples are taken in the longitudinal direction of the strip. The total elongation (A50) is derived inaccordance with the Japanese Industrial Standard J IS Z 2241: 2011, wherein the samples are taken in the transversal direction of the strip.

The bendability is evaluated by the ratio of the limiting bending radius (Ri), which is defined as theminimum bending radius with no occurrence of cracks, and the sheet thickness, (t). For this purpose, a900 V-shaped block is used to bend the steel sheet in accordance with J IS Z2248. The value obtainedby dividing the limit bending radius with the thickness (Ri/t) should be less than 5, preferably less than4. Ri(t) may be further limited to 3, 2.5 or 2.

A yield ratio YR is defined by dividing the yield strength YS with the tensile strength TS. Lower limitfor YR can be 0.70, 0.75, 0.76, 0.77, or 0.78.

The steel should further be within the area defined by the coordinates A, B, C, D of Fig. 1, where Ri/T(y-aXle) is plotted vs TS/YR (X-aXle), and where A is [1200, 2), B is [2000, 4], C is [2000, 3], and D is[1200, 1]. The upper dotted line can be mathematically expressed as y= 0.Cf025*x v---1 and the low/erciotted line can be expressed as y=0.0025*x -l 'This provides a criteria 1§ 0.0íš25*ffS/YR - Ri/t í 2.Steels fulfilling the criteria has been found out to have a good balance between strength and bendability. 'The lower limit inay be 1.1, 1.2 or 1.3 and the ispper limit niay be 1.9 or 1.8.

The T S/YR value can further be liniited. such that TS/Ylš is vJithir11tÜ)00--2000 (MPa). The loyver' limitnray be 1 11,10, 1200, 1300, 14410, 1500, 1600, 1700, or 1800. The upper limit nray be 19110, 1800, 1700,1600, 1500, or 1400. A preferred range can be l2íl0-140t). Other ranges can e. g. be 1400-1600, orl6íltl-l800. or ißíltl-2íl00.

The hole expansion ratio (Ä) is preferably 2 20 %. The hole eXpanding ratio (Ä) is determined by thehole expanding test according to ISO/W D 166302009 (E). ln this test a conical punch having an apeXof 60 ° is forced into a 10 mm diameter punched hole made in a steel sheet having the size of 100 X100 mmz. The test is stopped as soon as the first crack is determined, and the hole diameter ismeasured in two directions orthogonal to each other. The arithmetic mean value is used for the calculation.

The hole expanding ratio (Ä) in % is calculated as follows: Ä = (Dh - Do)/Do X 100 wherein Do is the diameter of the hole at the beginning (10 mm) and Dh is the diameter of the hole after the test.

The cold rolled heat treated steel sheet of the present invention nlay raptionallyl be coated vaith zine or zinc alloys, or with aluminutrt or aluniinurrt alloys to improve its corrosion resistance.

The suggested steel can be produced by making steel slabs of the conventional metallurgy byconverter melting and secondary metallurgy with the composition suggested above. The slabs are hotrolled in austenitic range to a ltot rolled strip. Preferabiy hy reheating the slab to a temperaturebetween 1000 "C and 1280 °tf., rolling the slab completely in the austenitic range wherein the hotrolling finishing temperature is greater than or equal to 850 OC to obtain the hot rolled steel strip."Thereafter the hot roll ed strip is coileti at a ceiling ternperature in the range of 500 - 540 OC.Ûptionztlly subjectirag the coiled strip to a scale lrernotfal process, stich as picklirtg. The coiled strip isthereafter' batch annealed at a temperature in the range of 500 -650 "(1 preferably' 550-650 UC, for aduration of Såiífth. 'Iliereafter co1d rrilling the annealtïd steel strip witli a reduction rate between 35 andÜtilf/ïw, preferably around 40-609? reduction. liurtiler treating the cold rolled steel strip in a (lontinutiuslyAnnealing Tine (CAL) or in a liot Dip (lalvaniziiig Line (lflDGlo). in which the niiicrostructure is iinetuned. Both lines irtclticle subjecting the steel to a soaking ternperatttre (af 800 -l00fl ÛC, preferably åšílíl--90íl "(1 preferably' followed hy a rapid slow jet and rapid jet cooliiig to a holding ternperatuire of fff) - 450"C for a time of 150 to 1000 s, before cfaoling to room ternperature.

EXAihåÉPLJšJS ln lffig. 1 the limiting bending radiuses (Ri) divided by the cold rolling thickness has been plottedagainst the tensile strengths divided by the yield ratios, TS/ YR, for the steels in Example 1-4. Theinventive steels came within an area defined by the coordinates A, B, C, D when Ri/t (y-aXle) isplotted vs TS/YR (X-axle), Where A is [1200, 2), B is [2000, 4], C is [2000, 3], and D is [1200, 1].The upper dotted line can be mathematically expressed as y= íl0025>fx ---1 and the lovvcr dotted line can be expressed as yï0.íl025*x -2 Herice, the reference steels that were coiled at a higher temperature are all above the upper dotted linemathematically defined by:y = 0.0025*X -1, where y is Ri/t and X is TS (MPa)/YR.

The inventive steels of example 1-5 are all below the upper line.

The lower dotted line is defined byy = 0.0025*X -2, where y is Ri/t and X is TS (MPa)/YR.

The inventive steels of example 1-5 are all above the lower line.

Within these borders a good bending property in relation to strength and toughness is achieved.

EXAMPLE l Steels I1-I6, and reference steels R1 and R2 were produced by conventional metallurgy by convertermelting and secondary metallurgy. The compositions are shown in table 1, further elements werepresent only as impurities, and below the lowest levels specified in the present description. All steels having about the same composition.

'Table 1I1 0.105 0.0037 2.63 0.195 0.81 0.045I2 0.106 0.0038 2.67 0.197 0.84 0.048I3 0.106 0.0038 2.67 0.197 0.84 0.048I4 0.105 0.0037 2.63 0.195 0.81 0.045I5 0.118 0.0028 2.77 0.17 0.94 0.051I6 0.118 0.0028 2.77 0.17 0.94 0.051R1 0.112 0.0041 2.7 0.169 0.93 0.046R2 0.107 0.0051 2.63 0.199 0.85 0.041 The steels were continuously cast and cut into slabs. 11 The slabs Were reheated and hot rolled in austenitic range to a thickness of about 2.8 mm. The hot rolling finishing temperature Was about 900 OC.

The hot rolled steel strips *tvliere tliereafter coiled, steel 11-16 at a eoiling teniperature of Sfåí) OC and the reference steels R1 and R2 at about 630 OC.

The coiled hot rolled strips Were pickled and batch annealed at about 624 OC for 10 hours in order toreduce the tensile strength of the hot rolled strip and thereby reducing the cold rolling forces.

The strips Were thereafter cold rolled in a five stand cold rolling mill to a final thickness of about 1.41mm and finally subjected to continuous annealing in a Continuous Annealing Line (CAL). ln the CALthe strips Were heated to a soaking temp of about 850 OC and held there for about 120s. Afterannealing, the strips Were slow jet cooled to about 750 OC (SJ C), and then rapid jet cooled to a holdingtemperature of about 400 OC (RJ C). The strips Were hold at about 180 and then cooled to room ternperature.

The process pararneters are sliovari in table 2.

I1 2.8 530 623 1.41 50 850 750 393I2 2.8 530 623 1.41 40 850 750 39713 2.8 530 625 1.41 49 846 750 397I4 2.8 530 623 1.41 49 842 750 394I5 2.8 530 624 1.42 49 847 750 391I6 2.8 530 624 1.41 50 846 750 386R1 2.7 63_0 624 1.38 50 850 700 415R2 2.8 62_7 624 1.4 50 851 750 387 Yield strength YS and tensile strength TS Were derived according to the European norm EN 10002Part 1. The samples Were taken in the longitudinal direction of the strip.

Samples :Jf the produced strips yvtfre subjected to V bend test in accordaiïct? With J IS Z2248 to find outthe limiting bending radius (Ri). The samples Were examined both by eye and under optical microscope With 25 times magnification in order to investigate the occurrence of cracks. Ri/t Was 12 determined by dividing the limiting bending radius (Ri) with the thickness of the cold rolled strip (t).

Ri is the largest radius in which the material shows no cracks after three bending tests.

The limiting bending radius (Ri) of the steels 11 - 16 that were coiled at 539 OC were less than thoseRl, R2 that were coiled at 630 OC.

Steel 11 - 16 all fulfilled the condition l í G.()()25*'TS/'YR Ri/t í 2, valiereas Rl and RP. fell short.

The mechanical properties are shown in table 3.

I1 888 1088 0.81 8.1 18.4 2.0 1.4 1.8012 806 1018 0.79 8.1 18.2 2.5 1.8 1.4413 841 1088 0.81 8.2 14.2 2.5 1.8 1.48I4 817 1027 0.80 7.8 18.4 2.5 1.8 1.45IS 868 1084 0.80 8.2 18.5 2.5 1.8 1.6416 951 1114 0.85 6.8 11.8 2.0 1.4 1.85R1 958 1087 0.88 8.8 6.5 8.0 2.2 0.98R2 947 1092 0.87 8.4 5.9 8.5 2.5 0.65 Pig. Ba and 2153 shoxv an examination of inventive steel 16 coiled. at 530 OC and Fig. 3a-3c sliowv anexamination of a reference steel Rl coiled at 630 OC. The reference steel Rl showed grain boundary oxidation whereas the inventive steel 16 showed no grain boundary oxidation.

Fig. SC shows visible cracks on the sample surface of the reference steel Rl. These comes frombreakouts after pickling and cold rolling. Especially the grain boundary oxides lead to outbreaksaround the present grains, which could lead to full grain breakouts. The cracks/outbreaks are decremental for the bending ratio.

Fig. 2b show no visible cracks on the sample surface of the inventive steel. The lack of grain boundaryoxides and no visible cracks of the inventive steel improves the bending ratio and reduces the risk ofliquid metal embrittlement. 1t further facilitates griod phosphatalxility.

Fig. 4 slifaw' the phosplizatzatiori cfawferage for 16 "llhe microstructure of16 was cleterrriirietl to: 13 Eainite -4- Ternpered híztrterisite > 855/'03 lïlreslï niartensite about 5949;retained allstcaiite about 5 9%.EXAMPLE 2Steel 17 and reference steel R3 were produced by conventional metallurgy by converter melting andsecondary metallurgy. The compositions are shown in table 4, further elements were present only asimpurities, and below the lowest levels specified in the present description. All steels having about thesame composition. The steels 17 and R3 having higher Cr and C contents and lower Si and Mn contents than the steels of example 1. This provides a steel having a higher yield strength and a highertensile strength. 'able 4 I7 0.223R3 0.223 0.0052 1.49 0.380.0052 1.49 0.38 0.1450.145 0.0440.044 The steels were treated in the same process as Example 1, in which steel 17 was coiled at a coiling ternperature of 532 °C and the reference steel R3 at 626 °C. 1n the CAL the strips were heated to a soaking temp of about 850 °C and held there for about 120s.After annealing, the strips were slow jet cooled to about 700 °C (SJ C), and then rapid jet cooled to aholding temperature of about 250 °C (RJ C). The strips were hold at about 180 s and then coriled to room teniperature. All other process parameters were about the same as those of Example 1. "llhe process paranieters are shoyvn in table 5.

Table 5 17 2.3 532 625 1.39Rs 2.3 g 625 1.39 0.50 850 700 2500.50 850 700 250 Samples :Jf the produced strips yvere the subjected to the same tests as thrise crf Exaniple 1.The limiting bending radius (Ri) of the steel 17 that was coiled at 532 °C was less than that of the steelR3 that was coiled at 625 °C. 14 Steel 17 fulfilled the condition 1 í 0.()025*"1"S/'YR Ri/'t í 2. whereas 1313 fe11 short.

The mechanical properties are shown in table 6.

Table 6 17 1181 1522 0.78 3.8 6.5 .0 3.6 1.31 R3 1221 1489 0.82 3.4 .9 6.0 4.3 0.22 The nricrostructure of 17 was determined to: Bainite+ 'Tlernpererl niarterisite retaiaieri austenittf íålXAh/FPLE 3 Steel 18 and reference steel R4 were produced by conventional metallurgy by converter melting andsecondary metallurgy. The compositions are shown in table 7, further elements were present only asimpurities, and below the lowest levels specified in the present description. All steels having about thesame composition. The steels 18 and R4 having higher Si and C contents and lower Cr content than the steels of example 1. This results in a steel having a slightly higher tensile strength than that of example 1. "able 7 about 9 %; about 5 %. 18 0.198 0.0037 2.51 0.029 1.49 0.0 R4 0.202 0.0053 2.53 0.027 1.45 0.0 The steels were treated in the same process as Example 1, in which steel 18 was coiled at a coiling terriperattrre of 535 °C and the reference steel R4 at 633 °C. All other process parameters were about the same as those of EXample 1.

The process pararneters are shovvn in table 8. 'able 8 18 3.2 535 630 1.19 0.37 850 700 350 R4 3.5 63_3 630 1.47 0.42 850 700 350 Samples riff the produced strips yvere the subjected to the same tests as thrise riff Examiple 1.

The lirniting bending radius (Ri) of the steel 18 that was coiled at 535 °C was less than that of the steelR4 that was coiled at 633 °C.

Steel 18 fulfilled the condition 1 í í).01)25>*:'1'S/YR låi/t í 2, yvhertïas R4 fell short.

The mechanical properties are shown in table 9. 'r br» 9 _ ..« ma: _ IS 933 1198 0.78 10.0 15.8 3.0 2.5 1.32R4 864 1212 0.71 10.5 16.5 5.0 3.4 0.85EXl/-XÄÃPIÄÉ 4 Steel 19 and reference steel R5 were produced by conventional metallurgy by converter melting andsecondary metallurgy. The compositions are shown in table 10, further elements were present only asimpurities, and below the lowest levels specified in the present description. All steels having about thesame composition. The steels 19 and R5 having slightly higher C content and slightly lower Mn and Si content than the steels of example 1.

Table 10 19 0.155 0.0061 2.33 0.0061 2.33 0.24 0.441 0.053 R5 0.155 0.24 0.441 0.053 1n example 4 the CAL line weas replaced by a Hot Dip Galvanizing Line. Prior to the Hot DipGalvanizing Line the steels were treated in a similar process as EXample 1, in which steel 19 wascoiled at a coiling ternperaturcë of 520 °C and the reference steel R5 at 630 °C. The batch annealing temperature was 570 °C.

The process pararneters are sliovvri in table 11.

Table 11 16 850 850 Saïnphes of the produced strips tvere the suhjeeted to the sahle tests as those of Exampíe 1.

The limiting bending radius (Ri) of the steel 19 that Was coiled at 520 OC Was less than that of the steelR5 that Was coiled at 630 OC.

Steel 19 fulfilled the condition 1 í 0.()025*TS/"YR - Rí/t í 2, Whereas RS 1e11 short.

The mechanical properties are shown in table 12.

Tabhà 12w19 727 1000 0.73 9 15.1 3 2.1 1.30R5 739 1012 0.73 9 15.3 4 2.9 0.61 The microstrueture of 19 »Vas determíned to: Bainite+ Temperett Pw/íartensíttë Ffresh ïnartensite retahaed austenite about 5 %: about 10 about 85 %;

Claims (2)

1. A cold rohed steel strip or shieet a) b) å) having a cciinprisiticiii consisting of (in vit. 90): C 0.08 - (128Mn 1.4- - 4.5(lr (101 - 0.5Si (101 - 2.5Ai 0.01 - 0.6Si + Al 0.Si + fäí + (lr Ii 0.4- Nh íTi i' 0.Mo i' 0.Ca í 0.V í 0.balance Fe apart frøni iinpuritiiäs, iiiiíiflirig the iiisílnwviiig conditions: TS tensile strength (Rm) 950 - 1550 MPa YS yield strength (Rpog) 550 - 1400 MPaYR yield ratio (Rpoa/ Rm) 2 0.50bendability (Ri/t) íbeing Within the area defined by the coordinates A, B, C, D, Where Ri/t (y-axle) isplotted vs TS(MPa)/YR (X-axle), and Where A is [1200,

2. ), B is [2000, 4], C is[2000, 3], and D is [1200, 1]; having a niuitiphase iriicrostructure cuniprisiiag (in t/oïilë) tenipiäiïed inarteiisiste +ibainite 2 50fresh rnarterisite i' 10retainefii austenitepoifyfgon al. íerrittfSJ)The cold roll strip or sheet according to claim 1, Wherein the composition comprising (in Wt%): Cl 0.1 - 0.25lvír: lfl - 3(Ir 0.01 - 0.5Si 0.1 - lAl 0.01 - 0.1Si + Al E *(3,Si -e- Al -+- Cr i: 0.Nb i' 0.008'Ti íl02Mo 0.Ca í 0.005V í 0.balance Fe apart from irripcurities. The cold roll strip or sheet according to claim 1 or 2 Wherein the hole expansion ratio (k) 220%.The cold roll strip or sheet according to any one of the preceding claims Wherein the composition fulfilling at least one of the following requirements: Si 2 0.4Si+Al 2 0.8Al í 0.1lVír: -+- Cr l.7 - 5.The cold roll strip or sheet according to claim 4 fulfilling all of the requirements of claimThe cold roll strip or sheet according to any one of the preceding claims Wherein the microstructure fulfils at least one of the following requirements: ternperetl rnarterasite +-bairiite 2 60fresh niartensite l~retained austenite 5-l /\I-A polygtanal ferrite A niethod an' nianuíactiiriiig of a fcold rolltäd steel strip or sheet according to any onie of claiins ti), cornprising the ibllowing steps: a)h) providing a steel slab having a eornpositioii according to ariyone oi the preceding claimshot rolling the slah in the austenitirf range to a hot rolled strip; eoiling the hot rolled strip at a coiling teinperature in the range of540 WC;optionallyf performing scale reinoval process on the coiltäd steel strip; batch annealing the coiled strip at a temperature in the range of 500 -650 °C for a tiluratitinot' fš-Éàtlh; cold rolling the annealetl steel strip With a reduction rate betvifeen 35 and 90%; fuiftheëi* treating the cold rolled steel strip in a (Üoiitiiitiouslyf Annealisn g Line or in a. ifiotDip Galvanizing Line; and further coolirig the steel strip dorvri to room ternperatuie. The nietiiriti according to clairn it), fulfilling at least one of the following conditions: - in step b) reheating the slab to a temperature between 1000 “C and 1280 °C, rolling the slab completely in the austenitic range Wherein the hot rolling finishing temperature is greater than or equal to 850 “C to obtain the hot rolled steel strip; - in step e) batch annealing in the range of 550-650 OC; - in step g) a sraaking teniperatttre is Stift -ltltl0 ÛC. preferabßf šštl0-9ílll OC; arid -- in step g) a holding temperature is 350 -- 450°C for a tiine of l50 to 1000 s,