Zinc or zinc alloy coated strip or steel with improved zinc adhesion

Abstract

Zinc or zinc alloy-coated rolled steel strip or sheet comprising: 0.08-0.28C, 1.4-4.5Mn, 0.01-0.5Cr, 0.01-2.5Si, 0.01-0.6Al. The steel has a tensile strength of 950-1550 MPa, a yield strength of 550-1400 MPa, a yield ratio ≥ 0.50, and Ri / t ≤ 4. The microstructure comprises: ≥ 70% tempered martensite + bainite, ≤ 10% fresh martensite, 2-20% retained austenite, and ≤ 20% polygonal ferrite. It has a decarburized zone in which the carbon content at a depth of 20 μm is no more than 75% of the carbon content in the middle of the steel strip and / or the microhardness at a depth of 20 μm is no more than 75% of the microhardness in the middle of the steel strip. The zinc or zinc alloy coating has an adhesion rating of 1 or 2 according to SEP 1931.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel strip or sheet coated with zinc or a zinc alloy (cold-rolled steel strip or sheet), and a method for producing the zinc- or zinc alloy-coated steel strip or sheet. The steel strip or sheet is suitable for use in automobiles. Background Art

[0002] Increased strength levels are a prerequisite for lightweight construction for a wide variety of applications, particularly in the automotive industry, as reduced vehicle body mass leads to reduced fuel consumption.

[0003] Automotive body parts are often stamped from sheet steel to form complex, thin-sheet structural components. However, such parts cannot be produced from conventional high-strength steels because the formability of these complex structural components is too low. For this reason, multiphase transformation-induced plasticity-assisted steels (TRIP steels) have garnered considerable attention in recent years, particularly for use in automotive body structural parts and as seat frame materials.

[0004] TRIP steel has a multiphase microstructure that includes a metastable retained austenite phase that produces the TRIP effect. When the steel is deformed, the austenite transforms into martensite, resulting in significant work hardening. This hardening effect acts to counteract necking in the material and delays failure during sheet forming operations. The microstructure of TRIP steel can significantly alter its mechanical properties.

[0005] TRIP-assisted steels have long been known and have attracted considerable attention. The TRIP effect, enabled by the strain-induced transformation of metastable islands of retained austenite to martensite, significantly improves their overall ductility. Depending on the steel matrix, this can also result in excellent stretch flanging properties or high uniform elongation.

[0006] Automotive parts are galvanized and galvannealed to improve corrosion resistance. However, high alloying contents such as Si and Mn can reduce Zn adhesion.

[0007] WO2020 / 162556 discloses hot-dip galvanized steel sheet. The composition of Steel A overlaps somewhat. Examples based on Steel A were soaked at 810-820°C and provided ferrite contents between 25 and 34% by volume. The high ferrite content likely stems from the low soaking temperature.

[0008] There is a need for steel sheets or strips >950 MPa with excellent surface quality, in particular a surface providing improved Zn adhesion. Further desired properties are improved bendability and reduced susceptibility to liquid (liquid) metal embrittlement. Summary of the Invention

[0009] The present invention relates to the production of zinc or zinc alloy coated steel strip or sheet cold rolled steel having a tensile strength of at least 950 MPa and excellent formability, wherein the steel sheet / strip is producible on an industrial scale in a continuous annealing line (CAL) and in a hot dip galvanizing line (HDGL).

[0010] The present invention aims to provide zinc or zinc alloy-coated steel strip or sheet with a composition and microstructure that can be processed into complex, high-strength structural components, and a method for its production, where zinc adhesion is important. Careful selection of alloying elements and process parameters, particularly regarding the atmosphere during soaking, introduces a soft decarburized zone at the steel surface. This decarburized zone improves zinc adhesion, bendability, and reduces the risk of liquid metal embrittlement.

[0011] Cold-rolled steel strip or sheet, coated with zinc or zinc alloys,

[0012] a) having a composition (in wt. %) comprising:

[0013]

[0014] The balance Fe excluding impurities;

[0015] b) The following conditions are met:

[0016]

[0017]

[0018] c) having a multiphase microstructure comprising (in volume %)

[0019] Tempered martensite+

[0020]

[0021] d) having a decarburized zone in which the carbon content at a depth of 20 μm is not more than 75% of the carbon content in the middle of the steel strip and / or in which the microhardness at a depth of 20 μm is not more than 75% of the microhardness in the middle of the steel strip;

[0022] e) having a zinc or zinc alloy coating, wherein the zinc or zinc alloy coating has an adhesion of 1 or 2 when determined by a falling ball impact test according to SEP 1931.

[0023] The method for producing zinc or zinc alloy coated steel strip or sheet comprises the following steps:

[0024] i. Providing a cold-rolled steel sheet or strip having a nominal composition (in weight %) consisting of:

[0025]

[0026] The balance Fe excluding impurities;

[0027] ii. heating the plate or strip to a temperature in the range of 650-900° C. in a reducing atmosphere, optionally changing the atmosphere to an oxidizing atmosphere in the temperature range between 650 and 900° C.;

[0028] iii. soaking the plate or strip in a nitrogen atmosphere containing <5% by volume of hydrogen at a temperature in the range of 780-1000°C for a duration of 40 seconds to 180 seconds;

[0029] iv. Injecting steam during the soaking step iii) to obtain CO> 10000 ppm;

[0030] v. cooling the strip or sheet to a temperature between 200 and 500°C at a rate in the range of 10-400°C / second before coating, followed by isothermal holding for 50-10,000 seconds;

[0031] vi. coating the strip or sheet with a zinc or zinc alloy coating; and

[0032] vii. Optionally performing a galvanneal to alloy the coating into the steel strip. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Metallographic photographs of samples subjected to steam injection during soaking are shown.

[0034] Figure 2 Metallographic photographs of samples without steam injection during soaking are shown.

[0035] Figure 3 A graph showing the C content from the surface to the center according to one embodiment of the present invention is shown.

[0036] Figure 4 A graph showing microhardness from the surface to the center according to one embodiment of the present invention is shown. DETAILED DESCRIPTION

[0037] The invention is described in the claims.

[0038] composition

[0039] The steel sheet or strip has a composition consisting (in wt. %) of the following alloying elements:

[0040]

[0041] The balance Fe excluding impurities.

[0042] The following briefly explains the importance of the individual elements of the claimed alloys, their interactions with each other, and the limits of the chemical composition. All percentages for the chemical composition of the steel are given throughout the description in weight %. The upper and lower limits for the individual elements can be freely combined within the limits stated in the claims. For all values ​​given in this application, the numerical values ​​are rounded to one or two digits. Thus, a value given as, for example, 0.1% could also be expressed as 0.10 or 0.100%. The amounts of microstructural components are given in volume %.

[0043] C: 0.08-0.28%

[0044] Carbon stabilizes austenite and is important for obtaining sufficient carbon within the retained austenite phase. Carbon is also crucial for achieving the desired strength level. Generally, a tensile strength increase of the order of 100 MPa per 0.1% carbon can be expected. When carbon is below 0.08%, achieving a tensile strength of 950 MPa is difficult. If carbon exceeds 0.28%, weldability is impaired. Therefore, the upper limit may 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%.

[0045] Mn: 1.4-4.5%

[0046] Manganese is a solid solution strengthening element that reduces M s Temperature to stabilize austenite and prevent the formation of ferrite and pearlite during cooling. In addition, Mn reduces the c3 Temperature and is important for austenite stability. At a content of less than 1.5%, it may be difficult to obtain the desired amount of retained austenite, a tensile strength of 950 MPa, and the austenitizing temperature may be too high for conventional industrial annealing lines. In addition, at lower contents, it may be difficult to avoid the formation of polygonal ferrite. However, if the amount of Mn is higher than 4.5%, segregation problems may occur because Mn accumulates in the liquid phase and causes banding, resulting in potentially deteriorated workability. Therefore, the upper limit may be 4.2, 4.0, 3.8, 3.6, 3.4, 3.2, 3.0, 2.8, 2.6 or 2.4%. The lower limit may be 1.4, 1.5, 1.7, 1.9, 2.1, 2.3 or 2.5%.

[0047] Cr: 0.01-0.5%

[0048] Cr is effective in increasing the strength of steel sheets. Cr is an element that forms ferrite and delays the formation of pearlite and bainite. c3 Temperature and M sThe temperature decreases only slightly with increasing Cr content. Cr increases the amount of stabilized retained austenite. Above 0.5%, it can impair the surface finish of the steel, and therefore the amount of Cr is limited to 0.5%. The upper limit may be 0.45, 0.40, 0.35, 0.30, or 0.25%. The lower limit may be 0.01, 0.03, 0.05, 0.07, 0.10, 0.15, 0.20, or 0.25%. Preferably, no intentional addition of Cr is made according to the present invention.

[0049] Si: 0.01-2.5%

[0050] Si acts as a solid solution strengthening element and is important for ensuring the strength of thin steel strip. Si inhibits cementite precipitation and is essential for austenite stabilization. However, if the content is too high, excessive silicon oxides will form on the strip surface, potentially causing coatings on the rolls in the CAL process and, consequently, surface defects in the subsequently produced steel sheet. Therefore, the upper limit is 2.5%, and may be limited 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%.

[0051] Al: 0.01-0.6%

[0052] Al promotes ferrite formation and is also commonly used as a deoxidizer. Like Si, Al is insoluble in cementite, so it significantly delays cementite formation during bainite formation. Furthermore, it improves galvanizing and reduces susceptibility to liquid metal embrittlement. The addition of Al leads to a significant increase in the carbon content of retained austenite.

[0053] For some applications, limiting Al to 0.01-0.6% may also be appropriate. Here, the upper limit may be set at 0.5, 0.4, 0.3, 0.2, or 0.1%, and the lower limit may be set at 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1%. If Al is used solely for deoxidation, the upper limit may be 0.09, 0.08, 0.07, or 0.06%. To ensure a certain effect, the lower limit may be set at 0.03 or 0.04%.

[0054] For other applications, it may be appropriate to limit Al to 0.5-2.0%. Here, the upper limit may be further set to 2.0, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2 or 1.1%, and the lower limit may be set to 0.5, 0.6, 0.7, 0.8 or 0.9%.

[0055] Si+Al≥0.1%-2%

[0056] Si and Al inhibit cementite precipitation during bainite formation. Therefore, their combined content is preferably at least 0.1%. The lower limit can be set at 0.1, 0.2, 0.3, 0.4 or 0.5%. The upper limit can be set at 2%.

[0057] Si+Al+Cr≥0.4%-2.5%

[0058] A certain amount of these elements is beneficial to the formation of austenite. Therefore, their combined content should be at least ≥ 0.4%. The lower limit can be 0.5, 0.6 or 0.7%. The upper limit can be set at 2.5%.

[0059] Mn+Cr 1.7-5.0%

[0060] Manganese and chromium affect the hardenability of steel. Therefore, their combined content should be within the range of 1.7-5.0%.

[0061] Optional elements

[0062] Mo≤0.5%

[0063] Molybdenum is a strong hardenability enhancer. It can further enhance the benefits of NbC precipitation by reducing the kinetics of carbide coarsening. Therefore, the steel may contain up to 0.5% Mo. The upper limit may be 0.4, 0.3, 0.2, or 0.1%. According to the present invention, intentional Mo addition is not necessary. Therefore, the upper limit may be ≤ 0.01%.

[0064] Nb: ≤0.1%

[0065] Niobium (Nb) is commonly used in low-alloy steels to improve strength and toughness due to its effect on grain size. Niobium improves the strength-elongation balance by refining the matrix microstructure and the retained austenite phase through the precipitation of NbC. The steel may contain Nb in an amount of 0.1% or less. The upper limit may be 0.09%, 0.07%, 0.05%, 0.03%, or 0.01%. According to the present invention, intentional addition of Nb is not necessary. Therefore, the upper limit may be 0.004% or less.

[0066] V: ≤0.1%

[0067] V functions similarly to Nb, contributing to precipitation hardening and grain refinement. Steel may contain V in an amount of 0.1% or less. The upper limit may be 0.09%, 0.07%, 0.05%, 0.03%, or 0.01%. According to the present invention, intentional V addition is unnecessary. Therefore, the upper limit may be 0.01% or less.

[0068] Ti: ≤0.1%

[0069] Ti is commonly used in low-alloy steels to improve strength and toughness due to its effect on grain size by forming carbides, nitrides, or carbonitrides. Specifically, Ti is a strong nitride former and can be used to bind nitrogen in the steel. However, this effect saturates above 0.1%. The upper limit can be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. According to the present invention, intentional addition of Ti is not necessary. Therefore, the upper limit can be limited to ≤ 0.005%.

[0070] Ca≤0.05%

[0071] Ca can be used to modify non-metallic inclusions. The upper limit is 0.05%, and can be set to 0.04, 0.03, or 0.01%. According to the present invention, intentional addition of Ca is not necessary. Therefore, the upper limit can be limited to ≤ 0.005%.

[0072] impurities

[0073] Cu: ≤0.06%

[0074] Cu is an undesirable impurity element and is limited to ≤ 0.06% by careful selection of the scrap used.

[0075] Ni: ≤0.08%

[0076] Ni is also an undesirable impurity element and is limited to ≤ 0.08% by careful selection of the scrap used.

[0077] B: ≤0.0006%

[0078] Boron is an undesirable impurity element, and careful selection of the scrap used limits its content to 0.0006% or less. Boron increases hardness, but may come at the expense of reduced bendability, and is therefore undesirable in the steels proposed in this invention. B may further complicate scrap recycling, and its addition may also degrade workability. Therefore, intentional addition of B is undesirable according to the present invention.

[0079] Other impurity elements may be contained in the steel in the amounts normally present. However, it is preferred to limit the amounts of P, S, As, Zr and Sn to the following optional maximum contents:

[0080] P: ≤0.02%

[0081] S: ≤0.005%

[0082] As≤0.010%

[0083] Zr≤0.006%

[0084] Sn≤0.015%

[0085] It is also preferred to control the nitrogen content to the following range:

[0086] N: ≤0.015%, preferably 0.003-0.008%

[0087] Within this range, stable nitrogen fixation can be achieved.

[0088] Oxygen and hydrogen can be further restricted to

[0089] O: ≤0.0003

[0090] H: ≤0.0020

[0091] microstructure

[0092] The microstructural components are expressed below in volume % (vol. %).

[0093] The cold rolled steel sheet of the present invention has a microstructure comprising at least 70% tempered martensite (TM) and bainite (B), and further, at most 10% fresh martensite (FM) and at most 20% polygonal ferrite (PF).

[0094] Retained austenite is a prerequisite for achieving the desired TRIP effect. Therefore, the amount of retained austenite should be in the range of 2-20%, preferably 5-15%. The amount of retained austenite is measured using the saturation magnetization method described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, pp. 61-64.

[0095] Depending on the Al content, tempered martensite (TM) and bainite (B), fresh martensite (FM) and polygonal ferrite (PF) can be further limited as described below.

[0096] The microstructure of the steel can be further limited with Al in the range of 0.01-0.6.

[0097] The microstructure comprises at least 70% tempered martensite (TM) and bainite (B). The lower limit may be limited to at least 70%, 75% or 80%.

[0098] And further, up to 10% fresh martensite (FM). The upper limit can be limited to 8% or 5%. A small amount of fresh martensite can improve edge flanging and local ductility. The lower limit can be limited to 1% or 2%. These untempered martensite particles are often in close contact with retained austenite particles, and therefore they are often referred to as martensite-austenite (MA) particles.

[0099] Polygonal ferrite (PF) will be further limited to ≤ 20%, preferably ≤ 10%, ≤ 5%, ≤ 3% or ≤ 1%.Most preferably, the low Al steel contains no PF.

[0100] Retained austenite as described above.

[0101] Mechanical properties

[0102] The declared mechanical properties of the steel are important and should meet the following requirements:

[0103]

[0104] R m 、R p0.2 The values ​​are obtained according to European Standard EN 10002 Part 1, where samples are taken in the longitudinal direction of the strip.

[0105] Bendability is evaluated by the ratio of the ultimate bending radius (Ri), defined as the minimum bending radius without cracking, to the sheet thickness (t). For this purpose, the steel sheet is bent using a 90° V-block according to JIS Z2248. The value (Ri / t), obtained by dividing the ultimate bending radius by the thickness, should be less than 4, preferably less than 3. Increasing CO to above 10,000 ppm during soaking using steam injection can improve bendability by 10-30%, compared to the same level without steam injection.

[0106] The bendability may be ≤4, ≤3.5, ≤3, ≤2.5, or ≤2. The lower limit may be 1, 1.5, or 2.

[0107] The yield ratio YR is defined by dividing the yield strength YS by the tensile strength TS.

[0108] Depending on the Al content, the mechanical properties can be further restricted.

[0109] The mechanical properties of steels with Al in the range of 0.01-0.6 can be further limited to:

[0110]

[0111] The lower limit of YR of the steel having Al in the range of 0.01-0.6 may be further set to 0.70, 0.75, 0.76, 0.77 or 0.78.

[0112] Decarburized area and microhardness

[0113] The steel has a decarburized zone in which the carbon content at a depth of 20 μm is not more than 75% by weight of the carbon content in the middle of the steel strip. The carbon content at a depth of 20 μm can be further set to not more than 50%, 40% or 30% of the carbon content in the middle of the steel strip.

[0114] The microhardness at a depth of 20 μm is not higher than 75% of the microhardness of the middle portion of the steel strip. The microhardness at a depth of 20 μm can be further set to not higher than 70%, 60% or 50% of the microhardness of the middle portion of the steel strip.

[0115] The decarburized zone improves the steel's zinc adhesion, bendability, and reduces the risk of liquid metal embrittlement.

[0116] zinc coating

[0117] The steel sheet or strip comprises a zinc or zinc alloy coating. The coating may be applied via hot-dip galvanizing (GI), galvannealing (GA) or by electrolytic galvanizing (EG).

[0118] The zinc alloy coating may contain, in weight %,:

[0119] At least one of the following:

[0120] Mg 0.1-10

[0121] Al 0.1-10

[0122] Sn 1-10

[0123] The balance is Zn and impurities.

[0124] The galvannealed coating may contain 5-20 wt. % diffused Fe.

[0125] Zinc adhesion

[0126] The decarburized areas improve the zinc adhesion of the steel. Thus, the steel has a Zn adhesion of 1 or 2 when determined by a falling ball impact test according to SEP 1931: Prüfung der Haftung von Zinküberzügen auf feuerverzinktem Feinblech, Kugelschlagprüfung, 1991.

[0127] Production of cold rolled strip

[0128] The steel can be produced by producing conventional steel slabs having the above-recommended composition through converter melting and secondary metallurgy. The slabs are hot rolled in the austenitic range to form hot-rolled strip. Preferably, the slabs are rolled entirely in the austenitic range by reheating to a temperature between 1000°C and 1280°C, with the hot rolling finish temperature being greater than or equal to 850°C. The hot-rolled strip is then coiled at a coiling temperature in the range of 500-700°C. The coiled strip is optionally subjected to a descaling process, such as pickling. The coiled strip is then batch annealed at a temperature in the range of 500-650°C, preferably in the range of 550-650°C, for a period of 5-30 hours. The batch-annealed strip is then cold rolled at a reduction of between 35% and 90%, preferably approximately 40-60%.

[0129] Annealing and coating of cold rolled strip

[0130] The cold-rolled strip can be processed, for example, in a continuous annealing line (CAL) and a subsequent continuous galvanizing line (CEL) or in a hot-dip galvanizing line (HDGL).

[0131] Annealing and coating include the following steps:

[0132] i)

[0133] Cold rolled steel sheets or strips are provided having nominal compositions as described in the Components or Exemplary Compositions and Mechanical Properties sections.

[0134] ii)

[0135] The plate or strip is heated in a reducing atmosphere to a temperature in the range of 650-900° C., optionally changing the atmosphere to an oxidizing atmosphere in the temperature range between 650 and 900° C. Heating can be carried out, for example, in a furnace such as a direct heated furnace (DFF) or a non-oxidizing furnace (NOF).

[0136] iii)

[0137] The plate or strip is soaked in a nitrogen atmosphere containing <5% by volume of hydrogen at a temperature in the range of 780-1000° C. for a duration of 40 seconds to 180 seconds. The soaking furnace may be, for example, a radiant tube furnace.

[0138] The soaking temperature is preferably in the range of 830-890°C.

[0139] The soaking temperature is preferably defined as follows: c3 Above: A c3 =910-203*C 1 / 2-15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W. The soaking temperature can be at least A c3 +20℃ or at least A c3 +30℃.

[0140] The upper limit of hydrogen may be 5.0, 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.7 or 1.5%. The lower limit of hydrogen may be 0.1, 0.5, 1.0, 1.2 or 1.3%.

[0141] iv)

[0142] During the soaking step iii) steam is injected to bring CO to >1 vol% and create a decarbonisation zone. The CO content can be controlled, for example, by measuring the CO level in the off-gas from the soaking furnace.

[0143] The upper limit of CO may be 2% or 1.5%.

[0144] v)

[0145] Before coating the strip or sheet is cooled to a temperature between 200 and 500°C at a rate in the range of 10-400°C / sec and then held isothermally for 50-10000 sec. Cooling of the strip can be carried out, for example, by slow spray cooling followed by rapid spray cooling.

[0146] The final cooling temperature and the holding temperature can be set at M S Above or below. M S It can be defined by the following formula: S =692-502*(C+0.68N) 0.5 -37*Mn-14*Si+20*Al-11*Cr.

[0147] The lower limit of the isothermal holding time can be set to 50 or 100 seconds. The upper limit of the time can be 10,000, 5,000, 1,000, or 500 seconds. The lower limit of the isothermal holding temperature can be 200, 250, 300, or 330°C. The upper limit of the temperature can be 500, 450, or 400°C.

[0148] vi)

[0149] The strip or sheet is coated with a zinc or zinc alloy coating. The coating may be applied, for example, by hot-dip galvanizing (GI), galvannealing (GA) or electrolytic galvanizing (EG).

[0150] vii)

[0151] Optionally, a galvannealing (GA) is performed to alloy the coating into the steel strip.If the coating is applied using hot dip galvanizing, the strip or sheet may be annealed to alloy the coating into the steel strip or sheet.

[0152] Galvannealing can be performed at a temperature in the range of 450-600°C.

[0153] Exemplary Composition and Mechanical Properties

[0154] The microstructure and mechanical properties of Examples 1-5 may be limited according to the aforementioned disclosure of limitations on steels having Al in the range of 0.01-0.6, while the microstructure and mechanical properties of Examples 6 and 7 may be limited according to the aforementioned disclosure of limitations on steels having Al in the range of 0.5-2.0.

[0155] Zinc adhesion and decarburization zones are as described above.

[0156] According to a first example, steel:

[0157] a) having a composition (in wt. %) comprising:

[0158]

[0159] The balance Fe excluding impurities;

[0160] b) At least one of the following conditions is met:

[0161] TS tensile strength (R m ) 950-1550MPa

[0162] YS yield strength (R p0.2 ) 550-1400MPa

[0163] YR yield ratio (R p0.2 / R m ) ≥0.50

[0164] Flexibility (Ri / t) ≤4; and

[0165] According to the second example, steel:

[0166] a) having a composition (in wt. %) comprising:

[0167]

[0168] The balance of Fe excluding impurities; and

[0169] b) At least one of the following conditions is met:

[0170]

[0171] According to the third example, steel:

[0172] a) having a composition (in wt. %) comprising:

[0173]

[0174]

[0175] The balance of Fe excluding impurities; and

[0176] b) At least one of the following conditions is met:

[0177] According to the fourth example, steel:

[0178] a) having a composition (in wt. %) comprising:

[0179] The balance of Fe excluding impurities; and

[0180] b) At least one of the following conditions is met:

[0181] According to the fifth example, steel:

[0182] a) having a composition (in wt. %) comprising:

[0183]

[0184] The balance of Fe excluding impurities; and

[0185] b) At least one of the following conditions is met:

[0186]

[0187] Example

[0188] The five steels AE were produced by conventional metallurgy with converter melting and secondary metallurgy. The compositions are shown in Table 1, with further elements present only as impurities and below the minimum levels specified in this specification. The compositions are shown in Table 1.

[0189] Table 1

[0190] steel C N Mn Cr Si Al A 0.108 0.0035 2.66 0.185 0.85 0.044 B 0.221 0.005 1.47 0.378 0.13 0.042 C 0.196 0.0035 2.49 0.027 1.47 0.052 D 0.153 0.0059 2.31 0.238 0.42 0.051 E* 0.168 0.0031 2.31 0.228 0.44 0.958

[0191] *E-based steel has a composition outside the scope of the claims of the present invention,

[0192] The steel is continuously cast and cut into slabs. The slabs are reheated and hot-rolled in the austenitic range to a thickness of approximately 2.8 mm. The hot rolling finish temperature is approximately 900°C. The hot-rolled strip is then coiled at a coiling temperature of 630°C. The coiled hot-rolled strip is pickled and batch annealed at approximately 624°C for 10 hours to reduce the tensile strength of the hot-rolled strip and thus the cold rolling force. The strip is then cold-rolled in a five-stand cold rolling mill to a final thickness of approximately 1.4 mm.

[0193] The cold-rolled steel strip is conveyed to a hot-dip galvanizing line. The strip is heated to a temperature of 800° C. in a non-oxidizing heating furnace in a reducing atmosphere. Thereafter, the strip is conveyed to a soaking furnace and soaked at the temperature and conditions according to Table 2. Each steel A, ..., E is treated with steam injection for the present invention and without steam injection as a reference. The steels according to the present invention are denoted by A1, ..., D1, E1*, and the reference steels are denoted by A2, ..., E2. The basic atmosphere is N2 + 1.9% by volume H2. The CO content is determined by measuring the exhaust gas. For the steels A2, ..., E2 soaked without steam injection, the CO content is below 2000 ppm. For the steels A1, ..., E1 with steam injection, the amount of injected steam is adjusted so that the CO content reaches above 10000 ppm.

[0194] After soaking, the steel was cooled by slow spray cooling (SJC) followed by rapid spray cooling (RJC), with the final temperatures for SJC and RJC shown in Table 2. The strip was isothermally held at the final temperature for rapid spray cooling for approximately 180 seconds, after which it was hot-dip galvanized to apply the zinc coating. The process parameters are shown in Table 2.

[0195] Table 2

[0196]

[0197] The zinc adhesion was determined by a falling ball impact test according to SEP 1931: Prüfung der Haftung von Zinküberzügen auffeuerverzinktem Feinblech, Kugelschlagprüfung, 1991.

[0198] The decarburized area of ​​steel C1 (>10000 ppm CO) was examined and compared with that of C2 (<2000 ppm CO). Figure 1 and shows the lack of decarburization in C2 Figure 2 Seen in. Figure 3 A plot showing the C content from the surface to the center of C1 compared to C2 is shown. And Figure 4 Shown is the microhardness map from the surface to the center of C1 compared with C2.

[0199] The mechanical properties are shown in Table 3. It can be seen that the zinc adhesion and bending properties of steels A1 , ..., E1 soaked at >10000 ppm CO are much improved compared to those of A2, ..., E2 soaked at <2000 ppm CO.

[0200] Table 3

[0201]

[0202] Yield strength YS and tensile strength TS are obtained according to European standard EN 10002 Part 1. Samples are taken in the longitudinal direction of the strip. Total elongation (A 50 ) was obtained according to Japanese Industrial Standard JIS Z 2241:2011, where the sample was taken in the transverse direction of the belt.

[0203] Samples of the produced strip were subjected to a V-bend test according to JIS Z2248 to determine the ultimate bending radius (Ri). The samples were examined visually and under an optical microscope at 25x magnification to investigate the occurrence of cracks. Ri is the maximum radius at which the material exhibits no cracks after three bend tests. Ri / t is determined by dividing the ultimate bending radius (Ri) by the thickness (t) of the cold-rolled strip.

[0204] A c3 Determined by the following formula:

[0205] A c3 =910-203*C 1 / 2 -15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W.

[0206] The microstructures of A1, B1, D1, and E1 were determined and are shown in Table 4.

[0207] Table 4

[0208]

[0209]

Claims

1. A zinc or zinc alloy coated cold rolled steel strip or sheet, a) comprising, in wt. %: composition: The balance Fe excluding impurities; b) The following conditions are met: c) having a multiphase microstructure comprising, in volume %, d) having a decarburized zone in which the carbon content at a depth of 20 μm is not more than 75% of the carbon content in the middle of the steel strip or plate and / or in which the microhardness at a depth of 20 μm is not more than 75% of the microhardness in the middle of the steel strip or plate; e) having a zinc or zinc alloy coating, wherein the adhesion of the zinc or zinc alloy coating is 1 or 2 when determined by the falling ball impact test according to SEP 1931; f) wherein the amount of retained austenite is 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, pages 61-64; g) wherein the bendability is determined according to JIS Z2248; and h) wherein the yield strength and tensile strength are determined according to European Standard EN 10002 Part 1.

2. The zinc or zinc alloy coated cold rolled steel strip or plate according to claim 1 , wherein the steel strip has a decarburized zone in which the carbon content at a depth of 20 μm is not more than 50% of the carbon content in the middle of the steel strip and / or in which the microhardness at a depth of 20 μm is not more than 60% of the microhardness in the middle of the steel strip.

3. The zinc or zinc alloy coated cold rolled steel strip or sheet according to claim 1 or 2, wherein the composition satisfies at least one of the following conditions, in weight %: The balance Fe excluding impurities.

4. The zinc or zinc alloy coated cold rolled steel strip or sheet according to claim 1 or 2, a) having a composition comprising, in weight %,: The balance of Fe excluding impurities; and b) At least one of the following conditions is met:

5. The zinc or zinc alloy coated cold rolled steel strip or sheet according to claim 1 or 2, a) having a composition comprising, in weight %,: The balance of Fe excluding impurities; and b) At least one of the following conditions is met: TS tensile strength (R m )1300-1550MPa YS yield strength (R p0.2 )1000-1300MPa YR yield ratio (R p0.2 / R m )≥0.

70.

6. The zinc or zinc alloy coated cold rolled steel strip or sheet according to claim 1 or 2, a) having a composition comprising, in weight %,: The balance of Fe excluding impurities; and b) At least one of the following conditions is met:

7. The zinc or zinc alloy coated cold rolled steel strip or sheet according to claim 1 or 2, a) having a composition comprising the following in wt. %: The balance of Fe excluding impurities; and b) At least one of the following conditions is met:

8. The zinc or zinc alloy coated cold rolled steel strip or sheet according to any one of claims 1 to 2, wherein Al≤0.1。 9. A method for producing a zinc or zinc alloy coated cold rolled steel strip or sheet according to any one of claims 1 to 8, comprising the steps of: i. providing a cold rolled steel sheet or strip having a nominal composition as defined by clause a) of any preceding claim, the cold rolled strip being produced by: manufacturing a conventional metallurgical steel slab via converter melting and secondary metallurgy; rolling the slab entirely in the austenitic range with a hot rolling finish temperature greater than or equal to 850° C.; coiling the hot rolled strip in the range of 500-700° C.; batch annealing the coiled strip in the range of 500-650° C. for a duration of 5-30 hours; and cold rolling the batch annealed steel strip at a reduction between 35 and 90%. ii. heating the plate or strip in a reducing atmosphere to a temperature in the range of 650-900 ° C, optionally changing the atmosphere to an oxidizing atmosphere in the temperature range between 650 and 900 ° C; iii. soaking the plate or strip in a nitrogen atmosphere containing <5% by volume of hydrogen at a temperature in the range of 780-1000° C. for a duration of 40 seconds to 180 seconds, wherein the soaking temperature is defined by A as follows. c3 Above: A c3 =910-203*C 1 / 2 -15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W; iv. Injecting steam during the soaking step iii) to obtain CO> 10000 ppm; v. cooling the strip or sheet to a temperature between 200 and 500°C at a rate in the range of 10-400°C / second before coating, followed by isothermal holding for 50-10,000 seconds; vi. coating the strip or sheet with a zinc or zinc alloy coating; and vii. Optionally performing a galvanneal to alloy the coating into the steel strip.

10. The method according to claim 9, wherein the soaking temperature in step iii) is in the range of 830-890°C.

11. The method according to claim 10, wherein the soaking temperature in step iii) is c3 +20℃ or above.

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