Micro-alloyed hot rolled strip with improved formability, up to 700 mpa

Abstract

The invention relates to a steel strip or sheet comprising 0.03 - 0.12 C, 0.3 - 2.2 Mn, 0.01 - 0.1 Al, 0.0001 - 0.08 V, 0.01 - 0.10 Nb, 0.001 - 0.2 Ti, ≤ 0.60 Si, ≤ 0.4 Cr, ≤ 0.25 Ni, ≤ 0.2 Mo, ≤ 0.4 Cu, balance Fe apart from impurities, the microstructure comprising (in vol. %): polygonal ferrite (15 - 65), bainite and quasi polygonal ferrite (35- 85), pearlite (< 3), cementite (< 3) and having a Yield strength of 250 - 700 MPa, a Hole expansion ratio (λ) of ≥ 70 %, and the strip or sheet having a thickness of 2.0-6.5 mm. The invention also relates to a method of producing the strip or sheet.

Description

TECHNICAL FIELD

[0001] The invention relates to a hot rolled steel strip or sheet and manufacturing method thereof.BACKGROUND ART

[0002] Conventional micro-alloyed hot-rolled strips having yield strength up to 700 MPa, such as SxxxMC according to EN10149-2, are typically produced on the hot wide strip mill (HWSM) at moderate cooling rates of ~20-30°C / s, from the hot rolling finish temperature to cooling temperatures of ~550-600°C.The microstructure of such steels typically contains ferrite, coarse cementite and perlite. The global ductility is good, e.g. elongation. However, the local ductility is worse, e.g. formability of punched edges: quantifiable by means of Hole Expansion Ratio HER.

[0003] With conventional micro-alloyed hot-rolled strip, complex forming on stamped edges often cannot be realised. There is a need for a micro-alloyed hot-rolled strips having yield strength up to 700 MPa which have an improvement in the formability of punched edges (HER).

[0004] EP3715491A1 disclose a hot rolled sheet and a manufacturing method thereof. The tensile strength is 440 MPa or higher. The microstructure comprises 30 - 70% of a first ferrite and another structure of at least one among bainite and a second ferrite such that the total sum is 95 % or more. The average grain size of the first ferrite is less than 5 µm and the average gran size of the other structure is less than 10 µm. The hot rolling finishing temperature is below Ae3.

[0005] EP3516085 B1 disclose a method of manufacturing a high-strength hot-rolled steel strip. The tensile strength is at least 570 MPa, preferably 780 MPa or higher. The method includes cooling the hot-rolled steel strip with a primary cooling rate between 50 to 150 °C / s to an intermediate temperature between 600 and 720 °C and coiling between 580 and 660 °C. The steel has a substantially single-phase ferritic microstructure.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Fig. 1 show the microstructure of an inventive sample, and Fig 2. Show the microstructure of a comparative sample. DISCLOSURE OF THE INVENTION

[0007] The invention is described in the claims.

[0008] In a preferred embodiment the strip or sheet has a composition consisting of the following alloying elements (in wt. %): C0.03 - 0.12Mn0.3 - 2.2Al0.01 - 0.1V0.0001 - 0.08Nb0.01 - 0.10Ti0.001 - 0.2Si≤ 0.60Cr≤ 0.4Ni≤ 0.25Mo≤ 0.2Cu≤ 0.4 balance Fe apart from impurities

[0009] The composition is excluding any coatings applied to the strip or sheet.

[0010] The steel balance strength and formability and is suitable for applications requiring improved edge stretch capability.

[0011] The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients 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. 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 %. The amounts of the microstructural constituents are given in volume % (vol. %).C: 0.03 - 0.12 %

[0012] C (carbon) is an element having an action of improving the strength of the steel sheet. A certain amount is therefore required. Increasing the C content too much can lead to insufficient ductility, and that the strength becomes too high. Furthermore, iron-based carbides such as cementite (Fe3C) may form. These can be the starting point of cracking in punching operations and can cause deterioration of the stretch flangeability. A preferred range is 0.035 - 0.09 %.Mn: 0.3 - 2.2 %

[0013] Mn (manganese) improves the strength of the steel sheet through solid-solution strengthening and quench-hardening. Further, increasing Mn can expand the austenite temperature region at the lower temperature side. This improves hardenability and facilitates the formation of a low temperature transformation structure having an excellent burring property. The excellent burring property can be seen by improved Hole Expansion Ratio's. The lower limit may be set to 0.3, 0.35, or 0.4 %. The upper limit may be set to 2.2, 2.1, 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, or 1.0 %. A preferred range is 0.35 - 2.1 %, a more preferred range is 0.35 - 1.6 %.Al: 0.01 - 0.1 %

[0014] Al (aluminium) has an action of deoxidizing molten steel in a refining process of the steel to sound the steel. Al further has, similarly to Si, an action of suppressing the precipitation of the iron-based carbides such as cementite and thereby improving the strength and Hole expansion ratio. An upper limit of 0.1 % is set to suppress the generation of a non-metallic inclusion in the steel thereby improving the ductility and the low-temperature toughness. The lower limit may be set to 0.01 or 0.02 %. The upper limit may be set to 0.1, 0.08, or 0.06 %. A preferred range is 0.02 - 0.08 %.V: 0.0001- 0.08 %

[0015] V (vanadium) improves the strength of the steel sheet by precipitation strengthening or solid-solution strengthening. The upper limit may be set to 0.08, 0.07, 0.06, or 0.05 %. The lower limit may be set to 0.0001 or 0.001 %.Nb: 0.01 - 0.10 %

[0016] Nb(niobium) improves the strength of the steel sheet by precipitation strengthening or solid-solution strengthening. Furthermore, the addition of Nb can lead to a finer initial structure (the prior austenite grain size). A finer prior austenite grain size can increase the elongation at fracture and the Hole expansion ratio. The upper limit may be set to 0.10, 0.09, 0.08, 0.07, or 0.06 %. The lower limit may be set to 0.01 or 0.02 %. A preferred range is 0.02 - 0.06 %.Ti: 0.001- 0.2%

[0017] Ti (titanium) can improve the strength of the steel sheet by precipitation strengthening or solid-solution strengthening. The addition of Ti can lead to a finer initial structure (the prior austenite grain size). A finer prior austenite grain size can increase the elongation at fracture and the Hole expansion ratio. However, too much Ti can cause coarse titanium nitrides, which are bad for the Hole expansion ratio. The upper limit of Ti may be set to 0.2, 0.15, 0.1, 0.05, 0.01, or 0.005 %. The lowest amount of Ti may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %.Optional elementsSi: ≤ 0.6%

[0018] Si (silicon) is an element having an action of improving the strength of the steel sheet, but also functions as a deoxidizer in the steel melt. The upper limit may be 0.6, 0.5, 0.4,0.3, 0.25, 0.2, 0.15, 0.1, 0.08 %. A lower limit may be 0.01 %. A deliberate addition of Si is not necessary according to the present invention.Cr, Mo, Ni

[0019] Cr(chromium), Mo (molybdenum) and Ni (nickel) can improve the strength of the steel sheet by solid-solution strengthening or quench-hardening strengthening. Cr, Mo, and Ni affect the Ac3 temperature, and they also promote transformation behaviour, and facilitate obtaining a martensitic microstructure. Too much of Cr, Mo, and Ni can affect the Hole expansion ratio and the forming properties negatively.Cr ≤ 0.4%

[0020] The upper limit of Cr may be set to 0.4, 0.3, 0.2, 0.15, 0.1, or 0.05%. The lowest amount of Cr may be set to 0.001, 0.005, 0.01, 0.02 or 0.03 %. A deliberate addition of Cr is not necessary according to the present invention.Ni ≤ 0.25%

[0021] The upper limit of Ni may be set to 0.25, 0.2, 0.15, 0.1, or 0.05%. The lowest amount of Ni may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %. A deliberate addition of Ni is not necessary according to the present invention.Mo ≤ 0.2%

[0022] The upper limit of Mo may be set to 0.2, 0.15, 0.1, 0.05, 0.01 %. The lowest amount of Mo may be set to 0.001, or 0.002%. A deliberate addition of Mo is not necessary according to the present invention.Cu ≤ 0.4%

[0023] Cu (copper) improves scale removal and thus has an influence on a better surface quality. If the steel is made from scrap, as is common when using an electric arc furnace, Cu can be up to 0.4 %. The upper limit of Cu may be set to 0.4, 0.3, 0.2, 0.15, 0.1, or 0.05 %. The lowest amount of Cu may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %. A deliberate addition of Cu is not necessary according to the present invention.Further preferred restrictionsV+ Nb + Ti: ≤ 0.2 %

[0024] The combined content of Ti, Nb and V is preferably restricted to ≤ 0.2%, more preferably ≤ 0.15%. Higher amounts of these elements are not necessary to achieve the desired properties of the steel.Impurities

[0025] The following impurities may optionally be limited as disclosed below.N: ≤ 0.015 %

[0026] N (nitrogen) is an element generally contained as an impurity. When the N content is too high, N causes cracking in the hot rolling, and deteriorates the aging resistance. Therefore, the N content is set to 0.015 % or less, preferably 0.10 % or less. A nominal amount may be in the range of 0.001 - 0.008 %.P: ≤ 0.10 %

[0027] P (phosphorus) is an element generally contained as an impurity. When the P content is more than 0.10 %, P causes cracking in the hot rolling, and is segregated at a grain boundary to decrease the low-temperature toughness and also decrease the workability and the weldability. Therefore, the P content is set to 0.10 % or less. The upper limit may be set to 0.10, 0.05, 0.04, 0.03 or 0.02 %.S: ≤ 0.010 %

[0028] S (sulphur) is an element generally contained as an impurity. When the S content is more than 0.010%, S causes cracking in the hot rolling, and can generate MnS inclusions in the steel which deteriorates the Hole expansion ratio. Therefore, the S content is set to 0.010 or less. The upper limit may be set to 0.010, 0.05, 0.03, 0.01, 0.005 or 0.001 % Other impurity elements may be comprised in the steel in normal occurring amounts. However, it is preferred to limit the amounts of As, Zr, Sn, Sb to the following optional maximum contents: As: ≤ 0.020, or ≤ 0.015% Zr: ≤ 0.010, or ≤ 0.006% Sn: ≤ 0.050, or ≤ 0.040% Sb: ≤ 0.020, or ≤ 0.010%

[0029] Oxygen and hydrogen can further be limited to O: ≤ 0.001, or ≤ 0.0003% H: ≤ 0.0050, or ≤ 0.0020%

[0030] Furthermore, the elements should preferably be balanced such that a parameter Q expressed by the relation below is less than 30, preferably less than 20, more preferably less than 10. Q = Ti / 48 / S / 32 wherein [Ti] indicates the Ti content (mass%) and [S] indicates the S content (mass%).Mechanical properties

[0031] The steel should fulfil the following conditions: YS, Yield strength (R p0.2 )250 - 700 MPa, preferably 300 - 650 MPa, more preferably 300 - 600 MPaHER, Hole expansion ratio (λ)≥ 70 %, preferably 80 - 200 %

[0032] And optionally one or more of the following mechanical properties: TS, Tensile strength (R m )300 - 900 MPa, preferably 400 - 850 MPa,TE, Total elongation≥ 15 %, preferably 20 - 40 %

[0033] TS, YS, TE are examples of properties related global ductility. HER is a property related to local ductility.

[0034] The R m , R p0.2 values are derived in accordance with the Industrial Standard DIN EN ISO 6892-1:2019, wherein the samples are taken in the longitudinal direction of the strip.

[0035] The Total Elongation is determined according to EN10149-2 2013 (D). For sheet thickness t <3mm, A 80mm used, and from t ≥3mm A 5 (proportional measuring length related to the sample cross-section) is used.

[0036] The Hole expansion ratio (λ) is determined by the Hole expansion test according to ISO 16630:2017. In this test a conical punch having an apex of 60 ° is forced into a 10 mm diameter punched hole made in a steel sheet having the size of 100 × 100 mm 2< . The test is stopped as soon as the first crack is determined, and the hole diameter is measured in two directions orthogonal to each other. The arithmetic mean value is used for the calculation.

[0037] The Hole expansion ratio (λ) in % is calculated as follows: λ = Dh − Do / Do × 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.

[0038] The lower limit of the Hole expansion ratio (λ) can be 70, 80, 90 or 100 %. The upper limit may be 150 or 200 %.

[0039] The strip or sheet thickness of the final product is 2.0 - 6.5 mm, preferably 2.5 - 5 mm, more preferably 3-4 mm. The strip or sheet width may be 500 - 2000 mm, preferably 700 - 1750 mm in non-slit condition.Microstructure

[0040] The microstructure of the steel contains the main phases ferrite and bainite. High elongation is associated with ferrite, and bainite is associated with good edge stretchability expressed as HER. The steel balance strength and formability and is suitable for applications requiring improved edge stretch capability.

[0041] The microstructural constituents are in the following expressed in volume % (vol. %). a) polygonal ferrite15 - 65, preferably 20 - 60, most preferably 20 - 55 b) bainite and quasi polygonal ferrite35 - 85, preferably 40 - 80, most preferably 45 - 80 c) pearlite< 3, preferably < 2, most preferably <1cementite< 3, preferably < 2, most preferably <1retained austenite< 1

[0042] The phases a) or b) in the list above may balance the microstructure.

[0043] Bainite represents a carbon-rich second phase of the present steel. Bainite can be upper bainite and / or lower bainite and / or granular bainite. Quasi polygonal ferrite looks similar to granular bainite (irregular, undulating grain boundaries and a dislocation substructure) and is therefore included in the range.

[0044] Pearlite and / or cementite may be present in lesser amounts. The content of pearlite and / or cementite can be 0%.

[0045] Retained austenite should preferably not be present but may be present at smaller amount. The content may be 0%.

[0046] The microstructure, including the amount of each phase, can be identified in scanning electron microscope (SEM) using 20000 times magnification. Preferably by cutting out a sample from a steel plate and polishing a cross section of a plate parallel to the rolling direction. The microstructure should be taken from ¼ of the thickness. The surface can be etched to make the phases easier to identify.

[0047] The amount of retained austenite can e.g. be determined by means of the saturation magnetization method such as described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61 - 64.Production of the coiled strip.

[0048] The coiled strip can be produced by the following steps: a) making steel slabs with the composition defined above. The steel slabs may e.g. be produced by converter melting or in an electric steel plant and secondary metallurgy to finalise the composition b) Reheating the slabs to a reheating temperature TRH between 1150 °C and 1300 °C at a heating rate of 0.05 - 10 °C / s. c) Hot rolling the slabs in austenitic range including finishing rolling to a final thickness t (2 - 6.5 mm), wherein the hot rolling finishing temperature FRT is within the range of 880 to 930 °C, preferably 900 to 930 °C. Furthermore, the hot rolling finishing temperature FRT is preferably above Ae3 as defined by the formula: Ae 3 ° C = 919 − 266 × C + 38 × Si − 28 × Mn − 27 × Ni + 12 × Mo . The amount of the elements in brackets are in weight %. By having the hot rolling finishing temperature above Ae3, it can be ensured to accomplish rolling in the austenite region, including conditioning of the austenitic grain. d) Cooling the hot rolled strip wherein the cooling rate (CR) is 40 - 60 °C / s in the temperature range from 800 to 600 °C. Cooling from 600 °C to the coiling temperature may be done at any rate. e) Coiling the cooled strip at a coiling temperature between Bs -200 °C and Bs -100 °C, where Bs (°C)= 637-58 × [C] - 20 × [Mn] - 15 × [Ni] - 34 × [Cr] - 41 × [Mo] + 67 × [V + Nb +Ti]. The amount of the elements in brackets are in weight %. f) optionally batch annealing of the coiled strip at 400 - 600 °C for 1 - 48 h, or sheet annealing at 400 - 650 °C for 1-60 minutes, or hot dip galvanising of the strip

[0049] The cooling rate of 40 - 60 °C / s in the temperature range from 800 to 600 °C and the coiling temperature between Bs -200 °C and Bs -100 °C facilitates a ferritic matrix without coarse cementite and without or at most small amounts of pearlite. If the cooling rate is too high and / or the coiling temperature is too low, martensite may form. Preferably the coiling temperature is between Bs - 175 °C and B S - 125 °C.EXAMPLE

[0050] Steel slabs were produced by conventional metallurgy by converter melting and secondary metallurgy. The compositions of the produced steels 1-11 are shown in Table. 1. Table 1 SteelCSiMnPSAlCrNiMoCuVNbTiV+Nb+TiN10.0390.0240.3840.00890.00150.0440.0250.0120.0030.0110.0030.0240.0010.0280.003420.0590.0130.4210.00810.00190.0480.0230.0110.0030.0090.0030.030.0010.0340.004430.0650.0170.5210.00860.0110.0540.0320.0210.0050.0340.0020.0360.0030.0410.003140.0710.0160.80.0110.00190.0430.0320.0130.0050.0140.0030.0390.0010.0430.002750.0780.0180.810.00640.00420.0530.0450.030.0120.020.0020.0430.0020.0470.002960.0810.0181.030.00790.00420.040.0250.0120.0040.010.0490.050.0010.1000.004770.0730.071.370.00810.00110.0450.0250.0180.0050.0280.0480.0520.0020.1020.004280.0780.0181.370.0080.00430.0520.0440.0340.0050.020.0490.0530.0010.1030.005190.0750.051.560.00830.00080.0480.0360.0150.0060.0120.0480.0580.0010.1070.0061100.0790.0211.440.00790.00370.0420.0350.0110.0070.0110.0290.0440.0640.1370.0051110.080.0341.440.00840.00130.0510.0350.0120.0070.010.0070.0480.0660.1210.0047 Table 2 Steelt [mm]FRT [°C]CR 800-600°C [°C / s]CT [°C]YS [Mpa]UTS [Mpa]A5 [%]HER [%]Remarks12.891153B S - 16035142632145inventive23.091749B S - 16439447031105inventive33.190526B S - 84324963266comparative43.092959B S - 15644452528101inventive53.090630B S - 424765392966comparative63.090624B S + 65335973035comparative73.091564B S - 1545005922289inventive83.190725B S - 65156342441comparative93.089949B S - 1265506332088inventive103.188739B S - 696226972041comparative113.191757B S - 1485956781981inventive Underlined values are outside the scope of the invention.

[0051] The steel strips were produced from the steels. Process values and mechanical parameters are shown in Table 2.

[0052] The steels #1-11 were reheated to a reheating temperature TRH at a heating rate of about 0.1 °C / s. The reheated samples where thereafter hot rolled in austenitic range to a hot rolled strip. The hot rolling finishing temperature FRT ranged from 887 °C to 929 °C and was above Ae3.

[0053] The hot rolled strip were thereafter cooled and coiled at a coiling temperature CT. The inventive steels were cooled at a temperature in the range 40 to 60 °C / s between 600 and 800 °C whereas the comparative steels were cooled at lower cooling rates. By cooling the strip faster in the range 600 to 800 °C, the formation of coarse cementite and perlite can be avoided or supressed. The inventive steels were all coiled at a coiling temperature within Bs -200 °C and Bs -100 °C, whereas the comparative examples were coiled at temperatures closer to Bs. The coiling temperatures CT for the inventive steels were chosen to avoid or supress formation of coarse cementite and perlite and to achieve a high fraction of bainite in the microstructure. After coiling phase transformation is finished for these steels.

[0054] The mechanical properties of the produced strips #1-11 were determined using the measurement methods defined in the description. Mechanical properties are shown in Table 2. As can be seen, the inventive steels #1, 2, 4, 7, 9, and 11 met all mechanical criterions, particular in having a Tensile strength YS of 250 - 700 MPa while having a Hole expansion ratio HER ≥ 70 %. However, the comparative examples failed to meet the requirements of the Hole expansion ratio HER.

[0055] The microstructures of the inventive steels all fulfilled in vol. %: polygonal ferrite20 - 55bainite and quasi polygonal ferrite45 - 80pearlite< 1cementite< 3retained austenite< 1

[0056] Specifically inventive steel 11 had the following microstructure in vol. %: polygonal ferrite23bainite and quasi polygonal ferrite76pearlite0cementite1

[0057] Fig. 1 is metallographic image of steel 11.

[0058] Whereas the comparative steel 10 had the following microstructure in vol. %: polygonal ferrite90bainite and quasi polygonal ferrite0pearlite9cementite1

[0059] Fig. 2 is metallographic image of steel 10.

Claims

1. A hot rolled steel strip or sheet having a composition consisting of the following alloying elements (in wt. %): C0.03 - 0.12Mn0.3 - 2.2Al0.01 - 0.1V0.0001 - 0.08Nb0.01 - 0.10Ti0.001 - 0.2Si≤ 0.60Cr≤ 0.4Ni≤ 0.25Mo≤ 0.2Cu≤ 0.4 balance Fe apart from impurities, and having microstructure comprising (in vol. %): polygonal ferrite15 - 65bainite and quasi polygonal ferrite35- 85pearlite< 3Cementite< 3 determined by use of scanning electron microscope (SEM) at 20000 times magnification, and having the following mechanical properties: YS, Yield strength250 - 700 MPaHole expansion ratio (λ)≥ 70 % the yield strength is derived in accordance with the Industrial Standard DIN EN ISO 6892-1:2019 in which samples are taken in the longitudinal direction of the strip or sheet and the Hole expansion ratio (λ) determined by the Hole expansion test according to ISO 16630:2017, and the strip or sheet having a thickness of 2.0-6.5 mm.

2. The hot rolled steel strip or sheet according to claim 1, having a composition consisting of the following alloying elements (in wt. %): C0.035 - 0.09Mn0.35 - 2.1Al0.02 - 0.08V0.001 - 0.06Nb0.02 - 0.06Ti0.005 - 0.15Si≤ 0.08Cr≤ 0.15Ni≤ 0.2Mo≤ 0.15Cu≤ 0.15 balance Fe apart from impurities.

3. The hot rolled steel strip or sheet according to claim 1 or 2, wherein the Hole expansion ratio (λ) is at least 80 %.

4. The hot rolled steel strip or sheet according to any one of claims 1 - 3, wherein the microstructure comprising (in vol. %): polygonal ferrite20 - 60bainite and quasi polygonal ferrite40 - 80pearlite< 2cementite< 35. The hot rolled steel strip or sheet according to claim 4, wherein the microstructure comprising (in vol. %): polygonal ferrite20 - 55bainite and quasi polygonal ferrite45 - 80pearlite< 1cementite< 36. The hot rolled steel strip or sheet according to any one of claims 1 - 5, wherein the microstructure is free of pearlite.

7. A method for producing the hot rolled steel strip or sheet according to any one of claim 1 - 6: the method comprising the steps of: a) making steel slabs according to the composition; b) reheating the slabs to a reheating temperature (TRH) between 1150 °C and 1300 °C at a heating rate of 0.05 - 10 °C / s; c) hot rolling the slabs in austenitic range to a hot rolled strip having a final thickness (t) of 2 - 6.5 mm, wherein the hot rolling finishing temperature (FRT) is within the range of 880 to 930 °C; d) cooling the hot rolled strip wherein the cooling rate (CR) is 40 - 60 °C / s in the temperature range from 800 to 600 °C, e) coiling the cooled strip at a coiling temperature between Bs -200 °C and Bs -100 °C, where BS = 637-58 × [C] - 20 × [Mn] - 15 × [Ni] - 34 × [Cr] - 41 × [Mo] + 67 × [V + Nb +Ti], and the content of the element is in weight %, f) optionally batch annealing the coiled strip at 400 - 600 °C for 1 - 48 h, or sheet annealing at 400 - 650 °C for 1-60 minutes, or hot dip galvanising of the strip.

8. The method according to claim 7, wherein the coiling temperature is between Bs - 175 °C and BS - 125 °C.

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