High-strength hot-rolled flat steel product with high local cold formability and method for manufacturing such flat steel product

Abstract

The object of this invention is to provide a high-strength hot-rolled flat steel product and a method for producing such a flat steel product, thus achieving a combination of high strength with high local cold formability and high economy in terms of steel. This is achieved through a high-strength hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8, and a porosity of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, at least 10%, preferably at least 16%, an elongation at break of at least 0.12, advantageously at least 0.17, and a ratio of local cold formability to overall cold formability of at least 5 and at most 13, and a microstructure consisting of more than 50% (by volume) bainite, no more than 10% (by volume), advantageously no more than 5% (by volume) carbon-rich microstructure such as martensite, retained austenite, pearlite, etc. The steel has a ferrite-based microstructure and the following chemical composition (wt%): C: 0.04–0.08; Si: 0.1–0.6; Mn: 1.0–2.0; P: maximum 0.06; S: ​​maximum 0.01; N: maximum 0.012; Al: not more than 0.06; Ti: not more than 0.18 and / or Nb: not more than 0.08; Mo: not more than 0.35; Ti+Nb greater than 0.06, wherein carbon and nitrogen exist in excess of stoichiometry according to the following formula: 1.0 < (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96), the remainder being iron, including unavoidable steel-related elements.

Description

Technical Field

[0001] This invention relates to a high-strength hot-rolled flat steel product with high local cold formability. Furthermore, this invention relates to a method for manufacturing such a flat steel product. Background Technology

[0002] Cold formability currently refers to the ability to be molded at temperatures ranging from 10°C to 700°C, preferably from 10°C to 200°C, particularly preferably from 10°C to 80°C, and especially preferably at room temperature ranging from 15°C to 40°C.

[0003] In particular, the present invention relates to a high-strength, microalloyed, predominantly bainitic hot-rolled strip having an optimized alloy composition and microstructure, for example, that is used as a chassis component in the automotive industry.

[0004] The present invention also relates to high-strength hot-rolled strip with a tensile strength of at least 760 MPa and high cold formability.

[0005] The description of cold formability is very complex and can only be fully quantified by combining the eigenvalues ​​using AND logic.

[0006] Therefore, in order to describe the cold formability of the flat steel product according to the present invention, the following characteristic values ​​are considered:

[0007] 1. Elongation at break (A)

[0008] 2. Pore Expansion Ratio (LA)

[0009] 3. Measurement of Cold Formability (FL)

[0010] 4. Local cold formability to overall cold formability ratio (LFR)

[0011] Characteristic values ​​such as elongation at break, uniform elongation, and porosity are established characteristic values ​​that describe cold formability.

[0012] To quantify high overall cold formability and high local cold formability, it is necessary to consider both local cold formability characteristic values ​​and overall cold formability characteristic values. Within the scope of this invention, the characteristic value of hole expansion ratio is selected as a representative of local formability, and the characteristic value of uniform elongation is selected as a representative of overall cold formability. True variables, rather than technical (percentage) variables, are given here, and their determination is explained in the description of the embodiments.

[0013] This invention particularly includes flat steel products made of steel with a multiphase microstructure, the multiphase microstructure mainly comprising bainite, i.e., bainite with a volume fraction greater than 50%, and the flat steel products having a yield strength ratio of at least 0.8. In addition to a high tensile strength of at least 760 MPa and an elongation at break of at least 10% A, the flat steel products also have high hole-expanding capacity, with a hole-expanding ratio LA of at least 30%, a cold formability measure FL of at least 0.12, and a local cold formability to overall cold formability ratio LFR in the range of at least 5 and at most 13.

[0014] As is well known, bainitic steel is characterized by relatively high yield strength and tensile strength, as well as sufficiently high ductility for cold forming processes. Due to its chemical composition, it exhibits good weldability. The microstructure typically consists of bainite as the main component and a certain proportion of ferrite. Other phases, such as martensite and retained austenite, may each contain small proportions within the microstructure.

[0015] Such steel is disclosed, for example, in publication DE 10 2012 002 079 A1. However, the disadvantage is that the hole-enlarging ability is still not high enough.

[0016] The highly competitive automotive market forces manufacturers to constantly seek solutions to reduce fleet fuel consumption and CO2 emissions while maintaining the highest possible levels of comfort and passenger protection. On the one hand, weight reduction of all vehicle components plays a decisive role, but on the other hand, the best possible performance of individual components under high static and dynamic loads, both during vehicle use and in the event of a collision, also plays a decisive role.

[0017] By providing high-strength steel or even ultra-high-strength steel with strengths up to 1050 MPa or higher, and by using these steels to reduce plate thickness, vehicle weight can be reduced, while the formability of the steel used is improved during manufacturing and operation.

[0018] Therefore, high-strength steel must meet relatively high requirements in terms of its strength, ductility and energy absorption, and must not exhibit the disadvantages compared to conventional steel during its processing, such as stamping, hot forming and cold forming, hot tempering (e.g., air hardening, pressure hardening), welding and / or surface treatments such as metal refinement, organic coating or painting.

[0019] Therefore, in addition to reducing weight by decreasing plate thickness, newly developed steels must also meet the increasing material requirements for yield strength, tensile strength, hardening behavior and elongation at break, while having good processing properties such as formability and weldability.

[0020] Therefore, in order to ensure the required reduction in plate thickness, high-strength steel with single-phase or multi-phase microstructure must be used to ensure sufficient strength of motor vehicle components and meet high forming requirements as well as high requirements for toughness, edge crack insensitivity, improved bending angle, bending radius, energy absorption, etc.

[0021] There is also an increasing need for improved joint applicability, in the form of better general weldability, manifested in a larger usable weld area in resistance spot welding and improved failure behavior (fracture mode) of the weld under mechanical stress, as well as sufficient resistance to delayed crack formation caused by hydrogen embrittlement.

[0022] Hole-expanding capacity is a material property that describes the material’s resistance to crack initiation and propagation in areas near the edges and previously sheared during forming operations, such as in circumferential conditions.

[0023] Hole enlargement testing is standardized, for example, in ISO 16630. Accordingly, a hole punched into a sheet metal is enlarged using a mandrel. The measured variable is the change in hole diameter relative to a reference initial diameter, at which the first crack penetrating the sheet metal appears at the edge of the hole.

[0024] Improved edge crack insensitivity implies increased formability at the sheet edges, described by increased hole expansion capacity. This is referred to by synonyms such as "Low Edge Crack" (LEC) or "High Hole Expansion" (HHE) and xpand. ® Known to the public.

[0025] A method for producing high-strength hot-rolled strip with a tensile strength of at least 570 MPa, preferably at least 780 MPa, is known from patent specification EP 3 516 085 B1. This method achieves good cold formability of the steel strip. The method includes the following steps:

[0026] - Cast the slab, and the next step is to reheat the solidified slab to a temperature between 1050 and 1260°C.

[0027] - Hot-rolled steel billets in the last final mill stand at a rolling temperature between 980 and 1100°C;

[0028] - Finish rolling is performed at a finishing temperature between 950 and 1080°C;

[0029] - Cool the hot-rolled strip to an intermediate temperature of 600 to 720°C on the ROT (outlet roller conveyor) at a primary cooling rate of 50 to 150°C / s, then:

[0030] - The latent heat generated by the phase transformation from austenite to ferrite allows for the gentle heating of steel between 0 and +10 °C / s; or

[0031] -Isothermal maintenance of steel; or

[0032] - Moderately cooling the steel results in a temperature change rate of -20 to 0°C / s for entering the second stage of ROT; - to achieve a winding temperature between 580 and 660°C.

[0033] The steel known therefrom comprises the following (by weight percentage): 0.015 to 0.15 C; up to 0.5 Si; between 1.0 and 2.0 Mn; up to 0.06 P; up to 0.008 S; up to 0.1 Aluminum sol; up to 0.02 N; between 0.02 and 0.45 V; and optionally one or more of the following: at least 0.05 and / or up to 0.7 Mo; at least 0.15 and / or up to 1.2 Cr; at least 0.01 and / or Up to 0.1 Nb; optionally, Ca consistent with the amount of calcium treatment used for inclusion control; the balance being Fe and unavoidable impurities; wherein the steel has a substantially single-phase ferrite microstructure comprising a mixture of polygonal ferrite (PF) and acicular / bainitic ferrite (AF / BF), and wherein the total volume percentage of the sum of the ferrite components is at least 95%, and wherein the ferrite components are hardened by precipitation of fine composite carbides and / or carbonitrides of V and optionally Mo and / or Nb.

[0034] However, it has been found that the cold forming capability of this flat steel product, especially its localized cold forming capability, is not high enough. In addition, the cost of the alloy solution is relatively high.

[0035] Furthermore, a method for producing hot-rolled steel is known from patent specification EP 3 492 611 B1, the hot-rolled steel having a tensile strength of at least 950 MPa and a microstructure comprising bainite with an area ratio of 70% or greater, wherein the difference is one or both of the following: martensite with an area ratio of 30% or less, and optionally ferrite with an area ratio of 20% or less, wherein the method comprises the following steps:

[0036] - Heating steel with the chemical composition to a temperature of at least 1250°C,

[0037] - Hot-rolled steel at a final rolling temperature of 850-930℃

[0038] - Quench the steel to a coiling temperature of 450-575℃.

[0039] - Coil the steel at this winding temperature.

[0040] - Cooling steel, and

[0041] - Roller rolling.

[0042] This steel has the following alloy composition (by weight):

[0043] The composition is as follows: C 0.07-0.10, Si 0.01-0.25, Mn 1.5-2.0, Cr 0.5-1.0, Ni 0.1-0.5, Cu 0.1-0.3, Mo 0.01-0.2, Al 0.01-0.05, Nb 0.015-0.04, V 0-0.1, Ti 0-0.1, with the difference being in Fe and unavoidable impurities. The steel used is relatively expensive due to the alloying additions of chromium, copper, and nickel, and still does not have sufficiently high cold formability. Local cold formability will not be discussed. Summary of the Invention

[0044] Based on this, the object of the present invention is to provide a high-strength hot-rolled flat steel product and a method for producing such a flat steel product, thus achieving a combination of high strength and high local cold formability and high economy in terms of steel.

[0045] This objective is achieved by a high-strength hot-rolled flat steel product having the features of claim 1 and by a method for producing the flat steel product having the features of claim 13. Advantageous designs of the invention are described in the dependent claims.

[0046] According to the present invention, a high-strength hot-rolled flat steel product with high local cold formability has a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8, a porosity of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, an elongation at break of at least 10%, preferably at least 16%, a cold formability measure of at least 0.12, advantageously at least 0.17, and a ratio of local cold formability to overall cold formability of at least 5 and at most 13, and a microstructure consisting of more than 50% (by volume) bainite, no more than 10% (by volume), advantageously no more than 5% (by volume) carbon-rich microstructure such as martensite, retained austenite, pearlite, retained austenite, and the remainder of precipitation-hardened ferrite. The chemical composition of the steel is as follows (wt%):

[0047] C: 0.04~0.08

[0048] Si: 0.1~0.6

[0049] Mn: 1.0~2.0

[0050] P: Maximum 0.06

[0051] S: Maximum 0.01

[0052] N: Maximum 0.012

[0053] Al: Not exceeding 0.06

[0054] Ti: not exceeding 0.18 and / or Nb: not exceeding 0.08

[0055] Mo: not exceeding 0.35

[0056] Ti+Nb is greater than 0.06% (by weight), wherein carbon and nitrogen exist in superstoichiometric proportions according to Equation 1:

[0057] 1.0<(C / 12+N / 14) / (Ti / 48+Nb / 93+Mo / 96),

[0058] The remainder is iron, including elements unavoidably associated with steel, and optionally alloys containing one or more of Cr, Ni, V, B, or Ca.

[0059] Specifically, the microstructure distribution of the flat steel product's thickness is characterized in three regions: near the surface, at 1 / 4 thickness, and at 1 / 2 thickness.

[0060] - In the near-surface region or 1 / 4 thickness region of the flat steel product, relative to the 1 / 2 thickness region of the flat steel product, the absolute deviation of the ferrite proportion is at most 12% (volume), advantageously at most 7% (volume), and / or

[0061] - In the three regions of the flat steel product, the deviation of the aspect ratio relative to the average value in the rolling direction in each of the three regions is less than 0.3, and / or

[0062] - In the three regions, the hardness difference HV0.1 compared to the average value over the entire thickness of the flat steel product is as large as 20 HV0.1, advantageously as large as 15 HV0.1, and even more advantageously as large as 10 HV0.1, representing a perfect combination of strength, ductility and formability.

[0063] For the measurement of microstructure and hardness on the thickness of flat steel products, it is irrelevant from which surface side these measurements are taken.

[0064] In alloys where one or more of Cr, Ni, V, B, and Ca are optionally added, it is specifically specified that the alloy contains up to 0.6% (by weight) Cr, up to 0.6% (by weight) Ni, up to 0.2% (by weight) V, up to 0.01% (by weight) B, and up to 0.01% (by weight) Ca. The microstructure preferably consists of more than 50% (by volume) bainite, with the remainder being precipitation-hardened ferrite.

[0065] In particular, the flat steel products are characterized by both high strength and excellent cold formability. Furthermore, the production of such flat steel products based on the alloying elements C, Si, Mn, Nb and / or Ti according to the present invention is relatively inexpensive.

[0066] The flat steel product according to the invention is characterized by a high elongation at break of at least 10%, a high porosity (LA) of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, a cold formability (FL) measure of at least 0.12, advantageously at least 0.17, and a ratio of local cold formability to overall cold formability (LFR) of at least 5 and at most 13, while having a tensile strength of at least 760 MPa.

[0067] In an advantageous improvement of the invention, to achieve an optimized performance combination, the steel alloy optionally further comprises one or more elements selected from Cr, Ni, V, or B, having the following content by weight percentage: Cr: greater than 0.1 to 0.6, Ni: greater than 0.1 to 0.6, V: greater than 0.01 to 0.2, and B: greater than 0.0005 to 0.01, wherein carbon and nitrogen are present in a superstoichiometric ratio according to Formula 2 below:

[0068] 1.0<(C / 12+N / 14) / (Ti / 48+Nb / 93+Mo / 96+V / 51).

[0069] In another advantageous improvement of the invention, Ca is added to the steel by alloying for inclusion control. Thus, MnS and Al2O3 inclusions, which are detrimental to the final properties, are replaced by Ca-containing inclusions, which are less harmful, especially in terms of morphology. The maximum amount added to the steel by alloying is 0.01% (by weight).

[0070] In a further advantageous improvement of the invention, in order to achieve a particularly advantageous combination of performance, the flat steel product comprises the following alloying components by weight percentage:

[0071] Ti: at least 0.02, advantageously at least 0.04, even more advantageously at least 0.06, Nb: at least 0.01, Mo: at least 0.05, and Ti+Nb: at most 0.2.

[0072] The microstructure is primarily composed of bainite and a smaller proportion of ferrite. Bainite is a mixture of components characterized by at least 50% (by volume) of major and minor components, wherein the major component consists of bainitic ferrite solidified by precipitation of (Ti,Nb,Mo)(C,N) or V (C,N), and the minor components consist of carbon-rich components such as martensite, retained austenite, lower bainite, and pearlite. Advantageously, the microstructure consists of more than 75% (by volume) bainite.

[0073] Furthermore, the microstructure may contain carbon-rich microstructure components. Particularly advantageous properties can only be achieved when the microstructure contains up to 10%, and advantageously up to 5%, of carbon-rich microstructure components (e.g., martensite, retained austenite, pearlite).

[0074] It has also proven advantageous that, measured below the surface of the flat steel product at a position at 1 / 2 thickness of the flat steel product, the grain elongation of all microstructures in the rolling direction is advantageously 1.6, characterized in that the aspect ratio of the area average of all microstructures in the rolling direction is at most 2.0, and / or the average of the three regions of the flat steel product near the surface, at 1 / 4 thickness, and at 1 / 2 thickness is at most 2.0.

[0075] Also indicating that high cold formability is the fact that half of the precipitates of solidified ferrite and the main components of bainitic ferrite (Ti,Nb,Mo)(C,N) or V (C,N) have an average diameter of less than 10 nm, and / or the average spacing of the precipitates is less than 750 nm.

[0076] It is also advantageous that the ratio of shear texture components to rolling texture components increases towards the surface and has the following values:

[0077] - Near the surface: at least 0.9;

[0078] - Half thickness of flat steel products: maximum 0.1.

[0079] The hot-rolled flat steel products according to the invention may be provided with a metallic or non-metallic coating. The metallic coating may be applied to the flat steel products electrolytically or by hot-dip immersion, and is advantageously zinc-based.

[0080] This hot-rolled flat steel product is advantageously used in the automotive industry for manufacturing components, particularly chassis components. The hot-rolled flat steel product according to the invention has a thickness of 1.6 to 6.0 mm. However, the invention also includes thicknesses less than 1.6 mm or greater than 6.0 mm.

[0081] Advantageously, the flat steel product according to the invention has a tensile strength Rm of at least 760 MPa along the rolling direction, a yield strength ratio of at least 0.8, an elongation at break of at least 10%, preferably at least 16%, and a hole expansion rate of at least 30%, advantageously at least 40%, or even at least 50%. The cold formability is at least 0.12, advantageously at least 0.17, and the ratio of local to overall cold formability is at least 5 and at most 13.

[0082] Alloying elements are typically added to steel to specifically influence particular properties. Therefore, alloying elements can affect different properties of different steels. The effects and interactions generally depend heavily on the quantity, state of presence, and dissolution state of other alloying elements in the material. The correlations are diverse and complex. The role of alloying elements in the alloys according to the present invention will be discussed in more detail below.

[0083] In the context of the numerical descriptions of alloying element content given below and in the claims, and in the context of all other numerical descriptions, these values ​​shall be included as limit values. The use of the term "to," for example, 0.01 to 1% (by weight), within a content range implies that limit values, in this case 0.01 and 1, are also included.

[0084] Carbon (C): Required to form carbides, particularly in combination with so-called microalloying elements Nb, V, and Ti, promoting the formation of martensite and bainite, stabilizing austenite, and generally increasing strength. Higher C content impairs weldability and leads to deterioration of elongation and toughness; therefore, the maximum content is set at no more than 0.08% (by weight). At least 0.04% (by weight) is needed to achieve sufficient strength in the material.

[0085] Manganese (Mn): Stabilizes austenite, increasing strength and toughness. Higher Mn content (>2.0% (by weight)) increases the risk of intermediate segregation, which significantly reduces ductility and thus product quality. Contents below 1.0% (by weight) do not allow the desired strength and toughness to be achieved at the desired moderate analytical cost. Therefore, the Mn content is set at 1.0 to 2.0% (by weight).

[0086] Aluminum (Al): Used for deoxidation in the steelmaking process. The amount of aluminum used depends on the process. Therefore, no minimum aluminum content is given. An Al content greater than 0.06% (by weight) will significantly impair casting performance in the continuous casting process. This will incur significant costs during casting. Therefore, the Al content is set at a maximum of 0.06% (by weight).

[0087] Silicon (Si): An element that allows for the inexpensive increase of steel strength through mixed-grain hardening. However, Si degrades the surface quality of hot-rolled strip by delivering a firmly adhered oxide scale onto the reheated slab. At high Si contents, removing the oxide scale requires considerable effort and may only result in insufficient removal. This is particularly detrimental in subsequent galvanizing. Therefore, the Si content is limited to a maximum of 0.6% (by weight). A lower limit of 0.1% (by weight) can be considered reasonable for the effectiveness of Si.

[0088] Calcium (Ca): Added to steel by alloying for inclusion control to prevent the formation of undesirable MnS and Al2O3 inclusions, and to form less morphologically harmful Ca-containing inclusions together with these elements. The maximum amount added to steel by alloying is 0.01% (by weight).

[0089] Typically, only very small amounts of microalloying elements (<0.2% (by weight) of each element) are added. Unlike alloying elements, they primarily function through precipitation formation, but can also affect properties in the dissolved state. Despite their small amounts, microalloying elements have a significant impact on target production conditions, as well as the processability and final properties of the product.

[0090] Typical microalloying elements include niobium and titanium. These elements are soluble in the iron lattice and form carbides, nitrides, and carbonitrides with carbon and nitrogen. Due to the relatively high cost of microalloying elements, their proportions in the alloy are kept as low as possible. On the other hand, in carbon-rich microstructures, carbon is superstoichiometric and therefore does not bind to precipitates of microalloying elements, contributing to cost-effectiveness and the necessary strength increase. Therefore, the superstoichiometric ratio of carbon and nitrogen, calculated according to Formula 1: (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96), is set to >1.

[0091] The effects of Nb and Ti depend particularly on how they are processed during hot rolling and subsequent cooling. The addition of microalloying elements aims to refine the grain size and produce precipitates in the nanoscale range. Therefore, an Nb+Ti content greater than 0.06% (by weight) is a prerequisite for achieving the desired strength and good ductility. Conversely, a total content exceeding 0.2% (by weight) no longer has any effect on improving the steel's properties because, when performing the aforementioned analysis using a conventional furnace, contents above the specified total content do not dissolve during slab reheating and therefore do not exhibit any positive effect.

[0092] Niobium (Nb): Niobium is added through alloying to refine the grain size, particularly by forming carbides during rolling, thereby simultaneously improving strength, toughness, and ductility. Furthermore, very fine niobium-containing precipitates can form after the phase transformation, which significantly contribute to the product's strength. Saturation behavior occurs when the content exceeds 0.08% (by weight), therefore a maximum content of less than or equal to 0.08% (by weight) is specified. For sufficient efficacy, a minimum content of 0.01% (by weight) is specified.

[0093] Titanium (Ti): Acts as a carbide forming agent by refining grain size, thereby simultaneously improving strength, toughness, and ductility. A Ti content exceeding 0.18% (by weight) weakens ductility and porosity by forming very coarse primary TiN precipitates; therefore, a maximum content of 0.18% (by weight) is set. For sufficient efficiency, minimum contents of 0.02%, advantageously 0.04%, and even more advantageously 0.06% (by weight) are specified.

[0094] Molybdenum (Mo): Improves hardenability or reduces the critical cooling rate, thereby promoting the formation of fine bainite. Furthermore, the use of small amounts of Mo has delayed the coarsening of fine precipitates, which should be as fine as possible to improve the strength of the microalloyed structure. For sufficient efficiency, a minimum content of 0.05% (by weight) is specified, and for cost reasons, it is limited to a maximum of 0.35% (by weight).

[0095] Phosphorus (P): A trace element in iron ore, dissolved in the iron lattice as a substitutional atom. Phosphorus increases hardness and improves hardenability through mixed-crystal hardening. However, it is generally desirable to minimize phosphorus content as much as possible because it exhibits a particularly strong tendency to segregate and significantly reduces toughness. Phosphorus adheres to grain boundaries, causing cracks along these boundaries during hot rolling. Furthermore, phosphorus increases the transition temperature from tough to brittle properties by up to 300°C. However, with targeted measures and precise control during processing, it is possible to inexpensively increase strength using small amounts of P. For these reasons, phosphorus content is limited to a maximum of 0.06% (by weight).

[0096] Sulfur (S): Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel because it leads to unwanted MnS inclusions, thus deteriorating elongation and toughness. Therefore, efforts are made to achieve the lowest possible sulfur content in the melt, and elongated inclusions can be transformed into more favorable geometries through a process known as Ca treatment. For the reasons mentioned above, sulfur content is limited to a maximum of 0.01% (by weight).

[0097] Nitrogen (N): Also a byproduct of steel production. Steel containing free nitrogen tends to exhibit strong aging effects. Nitrogen diffuses into misalignments and blocks them even at low temperatures. Therefore, it leads to an increase in strength, but a rapid loss of toughness. It is possible to combine nitrogen in the form of nitrides, for example, through alloying with aluminum, niobium, or titanium. However, these alloying elements are subsequently no longer used to form small precipitates that are highly effective in terms of strength in subsequent processes. For these reasons, nitrogen content is limited to a maximum of 0.012% (by weight).

[0098] Chromium (Cr): As a selectively alloying element, Cr improves strength, reduces corrosion rate, and slows the formation of ferrite and pearlite. The maximum content is set at 0.6% (by weight), as higher contents lead to impaired ductility. For sufficient effectiveness, contents greater than 0.1% (by weight) are specified.

[0099] Nickel (Ni): Selective use of very small amounts of Ni can improve ductility while maintaining strength. Due to its relatively high cost, the Ni content is limited to a maximum of 0.6% (by weight). For sufficient effectiveness, a content greater than 0.1% (by weight) is specified.

[0100] Vanadium (V): For current alloying schemes, the addition of vanadium is not absolutely necessary. For cost reasons, the vanadium content is limited to a maximum of 0.2% (by weight). Nevertheless, if V is specified, according to Formula 2: (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96 + V / 51), the superstoichiometric ratio of carbon and nitrogen is set to >1. Then, for sufficient efficiency, a V content greater than 0.01% (by weight) is also specified.

[0101] Boron (B): Boron is an effective element for improving hardenability, and it is effective even in small amounts. In this case, the martensitic initiation temperature is unaffected. For it to be effective, boron must be present in a solid solution. Due to its high affinity for nitrogen, nitrogen must first be removed, preferably by stoichiometric amounts of titanium. Due to its low solubility in iron, dissolved boron preferably adheres to the austenite grain boundaries. There, it partially forms Fe-B carbides, which are coherent and lower the grain boundary energy. Both of these effects work by delaying the formation of ferrite and pearlite, and thus increasing the hardenability of the steel. However, excessive boron content is detrimental because it forms iron boride, which negatively impacts the hardenability, formability, and toughness of the material.

[0102] For the reasons stated above, the boron content in the alloy scheme according to the present invention is limited to a maximum value of 0.01% (by weight). For sufficient efficiency, a content greater than 0.0005% (by weight) is specified.

[0103] A method according to the invention for producing hot-rolled flat steel products with high local cold formability, the flat steel products having a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8, a hole expansion ratio of more than 30%, advantageously at least 40%, particularly advantageously at least 50%, a cold formability measure of at least 0.12, advantageously at least 0.17, and a ratio of local cold formability to overall cold formability of at least 5 and at most 13, the method comprising the steps of:

[0104] - Melt a steel melt containing the following (by weight percentage):

[0105] C: 0.04~0.08

[0106] Si: 0.1~0.6

[0107] Mn: 1.0~2.0

[0108] P: Maximum 0.06

[0109] S: Maximum 0.01

[0110] N: Maximum 0.012

[0111] Al: Not exceeding 0.06

[0112] Ti: not exceeding 0.18 and / or

[0113] Nb: Not exceeding 0.08

[0114] Mo: not exceeding 0.35

[0115] Ti+Nb greater than 0.06, containing carbon and nitrogen in superstoichiometric proportions according to Equation 1: 1.0 < (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96), selectively added by alloying with one or more elements from Cr, Ni, V, B, or Ca, with the remainder being iron, including unavoidable steel-related elements.

[0116] - Steel molten material is cast into slabs or thin slabs using horizontal or vertical slab casting processes.

[0117] - Reheat the slab or sheet to 1100°C to 1270°C, and then hot-roll the slab or sheet by the following directly following steps:

[0118] - At the final rolling temperature EWT, the hot-rolled strip is rolled to the desired final thickness in the final rolling pass, wherein the following conditions apply:

[0119] EWT≥EWTmin=682℃+464C+6445Nb–644×Nb0.5+732V–230V0.5+890Ti+363Al–36Si (Formula 3)

[0120] - Cooling is performed at an average cooling rate of 30 K / s to 150 K / s.

[0121] - The hot-rolled strip is wound into coils at a sufficiently low winding temperature (HT) to set a favorable microstructure.

[0122] HT≤HTmax=761℃–217×C–77×Mn+97×Si–47×Mo–53×Cr–34×Ni–21×V (Equation 4), on the other hand, it is suitable to provide sufficient precipitation-hardening in the subsequent time-varying cooling process T(t), as specified by (Equation 5)17000≤HP≤18800, and HP(T,t)=T(t)×(In(t)+20), where the temperature T is expressed in K and the duration t is expressed in h.

[0123] - In the cooling process T(t), the coiling temperature is cooled at an average cooling rate of 5 K / h to 50 K / h between 100 °C and the coiling temperature, and then cooled to room temperature in still air.

[0124] In alloys in which one or more of the elements of Cr, Ni, V, B and Ca are optionally added, it is specifically specified that the alloys contain up to 0.6% (by weight) Cr, up to 0.6% (by weight) Ni, up to 0.2% (by weight) V, up to 0.01% (by weight) B and up to 0.01% (by weight) Ca.

[0125] The concepts upon which this invention is based will be explained below and described in more detail with examples.

[0126] To produce high-strength, microalloyed hot-rolled strip, thermomechanical rolling is now commonly used. In this process, finishing rolling is carried out at temperatures below EWTmin, where austenite no longer recrystallizes. Therefore, as phase transformation begins, the accumulated dislocations lead to a higher nucleation density, resulting in a finer microstructure in the hot-rolled strip. A fundamental goal of thermomechanical rolling is to improve strength and ductility through the small grain size of the hot-rolled strip microstructure.

[0127] In the case of alloys where carbon and nitrogen are present in supra - stoichiometric ratios relative to the micro - alloying elements, in contrast to nitrogen, carbon does not precipitate completely in the form of micro - alloy precipitates that enhance strength. The carbon that does not precipitate in the micro - alloy precipitates leads to the formation of different carbon - rich constituents of the matrix and of the bainite. For cold formability, it is crucial that the carbon - rich constituents of the matrix and of the bainite are present in a favorable manner in terms of size and distribution. "Favorable" means small size and as uniform a distribution as possible.

[0128] To achieve a balanced ratio of local and overall cold formability, in addition to different alloy compositions, different processes are used. Basically, three process routes can be distinguished, in which the type and distribution of the carbon - rich constituents of the matrix and the precipitation state of the micro - alloying elements are set. The type and distribution of the carbon - rich constituents of the matrix influence cold formability, and the precipitation state of the micro - alloying elements influences strength.

[0129] The process routes are:

[0130] 1. A low coiling temperature, e.g., 450 < HT < 550 °C, results in the formation of low - temperature bainite, which has a very finely distributed carbon - rich constituent, e.g., lower bainite. The product produced exhibits high cold formability and a significant proportion of local cold formability ("high local cold formability"). However, the strength is relatively low because only a small fraction of the micro - alloying elements precipitate at the low coiling temperature, thus contributing less to precipitation hardening.

[0131] 2. A high coiling temperature, e.g., HT > 650 °C, is used to produce a ferritic microstructure. The carbon is present in hard constituents, such as carbides, pearlite or martensite. The product produced has high cold formability, accompanied by a lower proportion of local cold formability. The strength is greater because a higher proportion of the micro - alloying elements are precipitated.

[0132] 3. An average coiling temperature, e.g., 550 < HT < 650 °C, has not achieved the desired goal so far. It is used to produce a mixed microstructure consisting of high - temperature bainite (e.g., upper bainite and granular bainite) and ferrite, which has both high local cold formability and high strength due to a high precipitation proportion. Due to the high precipitation proportion, either only high cold formability or only high strength has been achieved.

[0133] Importantly, current tests have shown that when the alloy composition and superstoichiometry ratio 1.0 < (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96) are combined, the bainite-dominant microalloyed hot-rolled strip steel exhibits both high strength and high local cold formability. The flat steel product is finished rolled at a final rolling temperature of at least EWTmin according to Formula 3, followed by coiling and cooling at a certain temperature for a certain time. It is characterized by a maximum coiling temperature HTmax according to Formula 4 and 17000 ≤ HP ≤ 18800, where HP is calculated according to Formula 5.

[0134] Experiments have shown that when hot-rolled flat steel products are coiled and cooled within a temperature time window characterized by 17000≤HP≤18800 (compared to a temperature time window characterized by HP≤15990), the contribution of precipitation deformation to tensile strength is at least 80 MPa or more. This strength contribution is necessary for achieving high tensile strength and high yield strength ratio at low cost. Furthermore, if HTmax is maintained within the stated temperature time window, a microstructure favorable to local formability and strength is formed.

[0135] While maintaining the temperature-time window, the flat steel product according to the invention is characterized in that the half-precipitates of (Ti, Nb, Mo)(C, N) and / or V(C, N) that harden the main components of ferrite and bainitic ferrite have a diameter of less than 10 nm, and / or these precipitates have an average spacing of less than 750 nm.

[0136] Surprisingly, it has been found that, while maintaining the temperature-time window, combined with a final rolling temperature of at least EWTmin, local cold formability is high, while, while maintaining the temperature-time window, combined with a final rolling temperature below EWTmin, local cold formability is low.

[0137] In addition to cost-effective alloy solutions, the flat steel products produced according to the present invention also possess high strength and high local cold formability. Furthermore, the production method according to the present invention is characterized by high process stability.

[0138] Unlike ferrite, bainite is typically composed of a variety of compositions. These compositions arise from the austenitic phase as the temperature decreases after final rolling during the production of hot-rolled strip. Compared to ferrite, bainite forms at lower temperatures and has a higher average dislocation density.

[0139] Only a predominantly bainitic microstructure can achieve both high strength and high local cold formability. This is because bainitic microstructure has a high dislocation density and small grain size. A ferrite-dominant microstructure cannot achieve high local cold formability. This is because ferrite has a relatively large grain size, and superstoichiometric carbon precipitates at phase boundaries as very hard and coarse carbides. Due to localized stress concentration, these carbides can lead to premature material failure during cold forming.

[0140] Using previously known solutions, hot-rolled strips with a combination of properties that are either relatively low strength and relatively high local cold formability or a combination of properties that are relatively high strength and relatively low local cold formability can be obtained.

[0141] Conversely, this invention allows for a combination of high strength and high local cold formability. This is because localized damage accumulated during the forming process, especially when there are significant differences in elongation, can only be adequately accounted for when considering the actual dimensions.

[0142] - Definition of true uniform elongation: , where Ag is the technical uniform elongation.

[0143] - Definition of true porosity: , where LA is the technical aperture expansion rate.

[0144] The measure of cold formability is described by the geometric mean of local and overall formability:

[0145] - Definition of cold formability measure (formability grade "FL"): (True uniform elongation × True porosity) 0.5 .

[0146] The ratio of local to overall cold formability is defined as: Local formability ratio (LFR) = (True porosity / True uniform elongation).

[0147] For applications requiring high local cold formability, particularly in the flat steel products according to the present invention, the following standards are required:

[0148] -A≥10%

[0149] -LA≥30%

[0150] - Cold formability measure ≥ 0.12

[0151] -5≤Ratio of partial to overall cold forming≤13.

[0152] The test results shown in the appendix cover examples with a tensile strength of at least 760 MPa. For these examples, and particularly for the aforementioned application areas, high local cold formability requires the following criteria:

[0153] -≥16%

[0154] -LA≥50%

[0155] - Cold formability measure ≥ 0.17

[0156] -5≤Ratio of local cold formability to overall cold formability≤13.

[0157] Within the testing scope, the mechanical properties and microstructure of the resulting hot-rolled flat steel products were tested. In addition to the tensile test according to ISO 6892-1 for determining tensile strength Rm, yield strength Rp0.2, elongation at break A, and uniform elongation Ag, a hole expansion test was also performed according to ISO 16630.

[0158] The following abbreviations are used in the tables listed in the appendix and in the following instructions:

[0159] EWT = Final rolling temperature

[0160] HT = winding temperature

[0161] MW = Average

[0162] Leg. = Alloy

[0163] GOS = Grain Orientation Distribution

[0164] KAM = Kernel Average Misorientation

[0165] IQ = Image Quality

[0166] AR = Aspect Ratio

[0167] SGV = Yield-to-tensile strength ratio

[0168] S P =Intensity contribution of precipitate formation

[0169] S M = The strength of a bainite-dominant microstructure, which has no precipitates due to the low value of the parameter HP.

[0170] For hot-rolled strip steel with a thickness > 3 mm, a sample form proportional to the elongation at break A is used. Conversely, for hot-rolled strip steel with a thickness ≤ 3 mm, a non-proportional sample form with an initial measurement length of 80 mm is used. For better comparability, when using a non-proportional sample form, the elongation at break value is derived from the uniform elongation using A = AG × b, where b is predetermined to be 2.254 using a reference sample. In the expansion test, the average of at least three individual tests is always indicated.

[0171] For the metallographic evaluation of the microstructure of hot-rolled strip steel, the following regions were defined within different thickness ranges of the samples:

[0172] - Near-surface: The measurement area is 100µm × 100µm, with a distance of 0.1mm from the sample surface.

[0173] -1 / 4 thickness: The measurement area is 100µm × 100µm, located between the surface and the center of the sample.

[0174] -1 / 2 thickness: The measurement area is 100µm × 100µm, with a distance of 0.1mm from the center of the sample.

[0175] from Figure 2 The location of the measurement area can be seen in the sketch.

[0176] Metallographic testing is performed on the sample longitudinally relative to the rolling direction.

[0177] To characterize the microstructure, electron backscattered diffraction (EBSD) images were captured in the measurement field defined above. For this purpose, a longitudinal section was fabricated and mechanically ground and polished to 1 µm. Subsequently, the sample was polished with OP-S for approximately 10 minutes to produce a surface that was as undeformable as possible. For measurement, [the following was used]. The camera has a 10×10 binning ratio and a 140Hz shooting rate, with an accelerating voltage of 15kV. In each case, the step size between measurement points is 0.1µm. The parameters relevant to this invention are determined as follows:

[0178] GOS (Grain Orientation Spread): The average orientation error of all measurement points within a grain relative to the average orientation of the grain. A 15° cleavage angle was used to determine the grain.

[0179] KAM (Kernel Average Orientation Difference) and GKAM: To calculate the KAM value, the average orientation difference of an EBSD measurement point relative to its next-next neighbor is determined. The maximum permissible orientation difference is 4°. For GKAM, the KAM values ​​of all measurement points for a grain are averaged, with a 15° cutoff angle used to determine the grain.

[0180] The types of tissues that appear are defined according to metallography as follows:

[0181] - Ferrite is composed of polygonal and quasi-polygonal ferrite, and the grains are determined by orientation difference angles ( The grain boundaries are >15°. Ferrite grains do not have small-angle grain boundaries <15°, the grain orientation distribution (GOS) value is <2°, and the grain average orientation difference (GKAM) value is typically <0.4°. TEM images show high density of (Ti,Nb,Mo)(C,N) precipitates within the grains. In particular, (Fe,Mn) carbides can exist in the triple-linked regions of the grains.

[0182] Granular bainitic grains are defined by grain boundaries >15°. Due to the displacement phase transformation from austenite to bainite, small-angle grain boundaries appear within the grains, with GOS values ​​≥2° and GKAM values ​​typically ≥0.4°. "Lancers" in different directions within the grains are usually visible in the EBSD IPF (inverse pole figure). "Lancers" not showing a second phase in the EBSD image quality map are referred to as "bainitic ferrite" below. Between the bainitic ferrite grains, a carbon-rich second phase exists in the form of martensite, MA phase, lower bainite, or pearlite. In particular, (Fe, Mn) carbides can be present in the triplet regions of the grains. The surface proportion of the second phase decreases with increasing coiling temperature, ranging from 0-10%.

[0183] Aspect Ratio: EBSD measurements of the sample were performed using an electron microscope, aligning the rolling direction with the Y-axis of the measurement field. Using the MTEX ​​toolbox in Matlab, ellipses were fitted to the shape of each grain (with a 15° slit angle) and parameterized by their major and minor semi-axes, as well as the orientation of the major axis. Ellipses were calculated based on these parameters for each grain, and the intersections of these ellipses with the X and Y axes of the coordinate system were then determined. The ratio of the intersection of the grain ellipse with the X-axis to the intersection of the grain ellipse with the Y-axis corresponds to the aspect ratio of the grain to the plate normal in the rolling direction. This calculation method ensures that only the extension of some grains is determined in the rolling or plate normal direction, where the major axis of these grains does not perfectly point in the rolling direction.

[0184] Hardness testing (HV0.1) was performed on the polished sample at points with varying spacing relative to the surface. Measurements were not taken at distances of 0.1 mm from the surface and center. Additionally, the following applies:

[0185] The hardness value is expressed as the average of 6 individual measurements.

[0186] In each case, the three hardness indentations near the surface are located between 0% and 10% of the distance between the surface and the sheet thickness, and between 90% and 100%.

[0187] In each case, the three hardness indentations at the 1 / 4 position are located between 20% and 30% and between 70% and 80% of the distance between the surface and the sheet thickness.

[0188] In each case, the three hardness indentations at the 1 / 2 thickness location are located between 40% and 50% of the distance between the surface and the sheet thickness, and between 50% and 60%.

[0189] from Figure 2 The location of the hardness imprint can be seen in the sketch.

[0190] The alloy compositions of the two examples are summarized in Table 1. Alloys A and B are single castings, therefore all examples A1-A14 and B1-B20 have the same composition. Similarly, Table 1 shows the calculated superstoichiometric ratios of carbon and nitrogen relative to the microalloying elements (Formula 2), i.e.

[0191] 1.0<(C / 12+N / 14) / (Ti / 48+Nb / 93+Mo / 96+V / 51).

[0192] Table 1 illustrates the alloy composition of the two embodiments.

[0193] Tables 2 and 3 illustrate the results of different embodiments. Evaluations of the results for achieving the desired characteristic values ​​are also described, where J (achieved) and N (not achieved). If a value does not conform to the specifications according to the invention, it is underlined in the second row of the tables. Listed values ​​are rounded according to commercial practice.

[0194] For alloys A and B, Table 2 lists the mechanical property values ​​under different processing conditions. Underlined values ​​exceed the required mechanical properties or the favorable processing conditions.

[0195] Regarding the final rolling temperature (EWT), complete recrystallization must be ensured across the strip thickness within each thickness range. This is achieved when EWT – EWTmin ≥ 0, where EWTmin = 682℃ + 464℃ + 6445Nb - 644 × Nb 0.5 +732V-230V 0.5 +890Ti+363Al–36Si (Formula 3). All element specifications are given as a percentage (by weight).

[0196] During the subsequent strip coiling process, it is essential to ensure the formation of a microstructure consisting of more than 50% (by volume) bainite. This is achieved when HT-HTmax ≤ 0, where HTmax = 761℃ – 217×C – 77×Mn + 97×Si – 47×Mo – 53×Cr – 34×Ni – 21×V (Equation 4). All elemental specifications are given in % (by weight). The formability characteristics specified in Table 2 can be achieved if the conditions of EWT and HT are met. The results show that only during the subsequent cooling process, after coiling, can harden S through precipitation. PA sufficient contribution to the tensile strength Rm is required to achieve a cost-effective increase in strength. For this purpose, it is necessary to maintain a suitable temperature T for a suitable duration t during the cooling process T(t). This is given by 17000 ≤ HP ≤ 18800 and HP(T,t) = T(t) × (In(t) + 20) (Formula 5), where T is always expressed in K and t is always expressed in h when calculating HP.

[0197] To calculate the parameter HP, the following procedure is performed:

[0198] 1. The cooling process T(t) is divided into n equal time intervals ti, accompanied by corresponding temperatures Ti, where n is chosen to be large enough such that when divided into significantly more time intervals, the result remains almost the same.

[0199] 2. Calculate each parameter .

[0200] 3. Calculate the time interval , where T* represents any temperature, such as the coiling temperature HT.

[0201] 4. Calculate the parameter HP,

[0202] The strength contribution S caused by precipitation formation P is determined by the following steps:

[0203] 1. Determine the strength S in a microstructure mainly consisting of bainite M , which has no precipitates due to the low value of the parameter HP. With the help of TEM testing, it is determined that this state exists regardless of the alloy when HP = 15990. As a first step, for all application examples, plot the data Rm versus HP in the range 16080 < HP < 18000, and then perform a linear regression. In the second step, determine the strength at HP = 15990 with the help of the regression line. In the current case, this is the strength contribution S M,A (15990) = 804 MPa for alloy A and the strength contribution S M,B (15990) = 762 MPa for alloy B.

[0204] 2. Calculate the theoretical strength of the precipitate-free microstructure depending on HP by S M (HP) = S M (15990) - 0.0495(HP - 15990).

[0205] 3. Calculate the strength contribution S P (HP) by S M (HP) = Rm(HP) - S P (HP).

[0206] With alloy compositions A and B, high strength Rm is achieved in a particularly cost-effective manner because the cooling process with a specified HP range allows for strength contribution S through precipitation formation. P ≥80MPa.

[0207] Furthermore, high local cold formability with high strength can only be observed when 17000≤HP≤18800, but not when HP>18800 or HP<17000 (Table 2).

[0208] By utilizing the microstructure and characteristics in the longitudinal section, the reasons for the different local cold forming properties at high strength were analyzed from a materials science perspective.

[0209] The bainitic microstructure of the hot-rolled strip produced according to the present invention consists of ≥50% major and minor components, wherein the major component is formed by bainitic ferrite hardened by precipitates (Ti,Nb,Mo)(C,N). Transmission electron microscopy (TEM) tests on the precipitates of various representative samples show that half of the precipitates (Ti,Nb,Mo)(C,N) hardened by the major component consisting of bainitic ferrite have a diameter of less than 10 nm, and / or these precipitates have an average spacing of less than 750 nm. The minor components consist of carbon-rich components, such as martensite, MA phase, lower bainite, and pearlite. Since the major component has higher formability than the minor component, a minimum proportion of ≥50% for the major component is advantageous.

[0210] Table 3 presents the microstructure test results of Alloy A for different final rolling temperatures according to Formula 3, different coiling temperatures according to Formula 4, and HP values ​​according to Formula 5.

[0211] The following applies to samples not treated according to the present invention: the microstructure of the hot-rolled strip samples is non-uniform and anisotropic across the strip thickness. The non-uniformity and anisotropy of two samples, A2 and A6, with HP values ​​of 17232 and 18380, can be described as follows:

[0212] a. The sample consists of a ferrite-bainite microstructure. The ferrite proportions are 48% and 66%.

[0213] b. The ferrite proportions at the near-surface and 1 / 4 thickness locations deviate the most from those at the 1 / 2 thickness location, by 59% and 17%, respectively.

[0214] c. The tissue has high extensibility. This is especially suitable for the 1 / 2 thickness position; here, the aspect ratio is 2.9 and 2.5.

[0215] d. Hardness varies considerably with thickness. Especially on the surface, the hardness is low, differing from the average hardness of the sample thickness by -24HV0.1 and -26HV0.1.

[0216] e. The shear texture components vary considerably, with values ​​of 0.92 and 0.96 near the surface, and 0.01 and 0.01 at half the thickness.

[0217] It is well known that local material behavior during cold forming and the high requirements for local cold forming capability are negatively affected by microstructure inhomogeneity, as damage is located early and leads to material failure. In the present case, the features listed in a.-d. directly or indirectly contribute to elongated regions of reduced formability, such as regions with an increased proportion of carbon-rich granular bainite second phase. However, the effect of the features listed in e. on local cold formability remains unclear.

[0218] The cause of the microstructure inhomogeneity across the thickness of hot-rolled strip was identified as the distinct and complete recrystallization of austenite across the strip thickness immediately after the final rolling step and before cooling. With the finishing rolling temperature range increasing beyond EWTmin but at a constant average coiling temperature, complete recrystallization across the strip thickness can be achieved within the scope of this invention, resulting in a uniform ferrite-bainite microstructure across the strip thickness.

[0219] The results were surprising, because the absence of recrystallization in the final pass of the rolling process resulted in the microstructure of the superstoichiometric bainitic hot-rolled strip steel becoming coarser as expected, but contrary to expectations, local cold formability was positively affected.

[0220] In order to achieve complete recrystallization on the strip thickness within each thickness range, according to Formula 3, it is necessary that EWT be at least EWTmin.

[0221] The reasons for high local cold formability were analyzed from a materials science perspective by utilizing the microstructure and characteristics in the longitudinal section. The microstructure analysis results of samples A7 and A9 processed according to Table 3 based on this invention are as follows:

[0222] - The microstructure of the hot-rolled strip steel samples is relatively uniform and isotropic across the strip thickness. The uniformity and isotropy of the two samples with HP values ​​of 18380 (sample A7) and 17232 (sample A9) can be described as follows:

[0223] a. The sample is mainly composed of bainitic microstructure. The ferrite content is 22% and 49%.

[0224] b. The ferrite proportions at the near-surface and 1 / 4 thickness locations deviate by a maximum of 7% and -2% respectively from those at the 1 / 2 thickness location.

[0225] c. The tissue has relatively weak stretch. This is especially true at the 1 / 2 thickness position; here, the aspect ratio is 1.5 and 1.5.

[0226] d. The hardness variation across the thickness is relatively small. This is especially true near the surface. The surface hardness deviates from the average value by -10HV0.1 and 0HV0.1.

[0227] e. The shear texture components are 0.98 and 0.98 near the surface, and 0.01 and 0.03 at half the thickness.

[0228] Further testing revealed the following characteristics of the desired tissue:

[0229] At least half of the precipitates (Ti,Nb,Mo)(C,N) hardened by the main component consisting of bainitic ferrite have a diameter of <10 nm, and / or these precipitates have an average spacing of less than 750 nm.

[0230] Figure 1 To reiterate, the scope of cold formability claimed according to the invention and limited by FL and LFR specifications is shown again.

[0231] Figure 2 The locations of the hardness imprints are shown, with a distance from the surface (0% and 100%) of 0.1 mm and a distance from the center (50%) of 0.1 mm; and the locations of the EBSD measurement fields are shown, with a distance from the surface (0%) of 0.1 mm and a distance from the center (50%) of 0.1 mm.

[0232]

[0233] Table 1

[0234]

[0235]

[0236] Table 2

[0237]

[0238] Table 3

Claims

1. A high-strength hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8, a porosity of at least 30%, an elongation at break of at least 10%, a cold formability measure of at least 0.12, a ratio of local cold formability to overall cold formability of at least 5 and at most 13, and a microstructure consisting of more than 50% by volume bainite, no more than 10% by volume carbon-rich microstructure (i.e., martensite, retained austenite, pearlite), and the remainder ferrite, wherein the chemical composition of the steel is as follows by weight percentage: C: 0.04 to 0.08 Si: 0.1 to 0.6 Mn: 1.0 to 2.0 P: Maximum 0.06 S: Maximum 0.01 N: Maximum 0.012 Al: Not exceeding 0.06 Ti: not exceeding 0.18 and / or Nb: not exceeding 0.08 Mo: not exceeding 0.35 Ti+Nb is greater than 0.06, where, According to the following formula, there exists carbon and nitrogen in superstoichiometric proportions: 1.0<(C / 12+N / 14) / (Ti / 48+Nb / 93+Mo / 96), The remainder is iron, including elements inevitably associated with steel, and optionally one or more of Cr, Ni, V, B, or Ca are added through alloying. Among them, the microstructure distribution in the three regions of near-surface, 1 / 4 thickness, and 1 / 2 thickness of the flat steel product is characterized by the following: - In the near-surface region or 1 / 4 thickness region of the flat steel product, relative to the 1 / 2 thickness region of the flat steel product, the absolute deviation of the ferrite proportion is a maximum of 12% by volume, and / or - In the three regions of the flat steel product, the deviation of the grain aspect ratio relative to the average value in the rolling direction is less than 0.3 in each of the three regions, and / or - In the three regions, the hardness difference HV0.1 was the largest at 20HV0.1 compared to the average across the entire thickness of the flat steel product.

2. The flat steel product according to claim 1, characterized in that, Add up to 0.01% by weight of Ca by alloying.

3. The flat steel product according to claim 1 or 2, characterized in that, The steel contains, by weight percentage: Ti+Nb: Maximum 0.

2.

4. The flat steel product according to claim 1 or 2, characterized in that, The steel contains, by weight percentage: Minimum Ti: 0.02 Minimum Nb: 0.01 Mo minimum: 0.

05.

5. The flat steel product according to claim 1 or 2, characterized in that, The steel contains, by weight percentage: Cr: Not exceeding 0.6 Ni: Not exceeding 0.6 V: Not exceeding 0.2 B: Not exceeding 0.01 Among them, carbon and nitrogen exist in superstoichiometric proportions according to the following formula: 1.0<(C / 12+N / 14) / (Ti / 48+Nb / 93+Mo / 96+V / 51).

6. The flat steel product according to claim 5, characterized in that, The steel contains, by weight percentage: Cr: greater than 0.1 Ni: greater than 0.1 V: greater than 0.01 B: Greater than 0.0005.

7. The flat steel product according to claim 1 or 2, characterized in that, The bainite is a mixture of multiple components, characterized by at least 50% by volume of a major component and a minor component, wherein the major component consists of bainitic ferrite hardened by precipitation of (Ti,Nb,Mo)(C,N) and / or V(C;N), and the minor component consists of carbon-rich components, namely martensite, retained austenite, lower bainite and pearlite.

8. The flat steel product according to claim 1 or 2, characterized in that, The microstructure consists of more than 50% by volume bainite and residual ferrite.

9. The flat steel product according to claim 8, characterized in that, The microstructure consists of more than 75% by volume bainite and residual ferrite.

10. The flat steel product according to claim 1 or 2, characterized in that, The grain elongation of all microstructures at the 1 / 2 thickness position of the flat steel product is characterized by the fact that the aspect ratio of all microstructures in the rolling direction is at most 2.0, and / or the average value of the three regions of the flat steel product near the surface, at 1 / 4 thickness and at 1 / 2 thickness is at most 2.

0.

11. The flat steel product according to claim 1 or 2, characterized in that, Half of the (Ti,Nb,Mo)(C,N) and / or V(C,N) precipitates, which are hardened by ferrite and bainitic ferrite as the main components, have a diameter of less than 10 nm, and / or the precipitates have an average spacing of less than 750 nm.

12. The flat steel product according to claim 1 or 2, characterized in that, The ratio of the shear texture component to the rolling texture component increases toward the surface and has the following values: - Near surface: at least 0.9; -1 / 2 thickness: maximum 0.

1.

13. The flat steel product according to claim 1, characterized in that, The flat steel product has a hole expansion rate of at least 40%.

14. The flat steel product according to claim 1, characterized in that, The flat steel product has a hole expansion rate of at least 50%.

15. The flat steel product according to claim 1, characterized in that, The flat steel product has an elongation at break of at least 16%.

16. The flat steel product according to claim 1, characterized in that, The flat steel product has a cold formability measure of at least 0.

17.

17. The flat steel product according to claim 1, characterized in that, The flat steel product has a microstructure consisting of more than 50% by volume bainite, no more than 5% by volume carbon-rich microstructure (i.e., martensite, retained austenite, and pearlite), and the remainder ferrite.

18. The flat steel product according to claim 1, characterized in that, The microstructure distribution across the thickness of flat steel products is characterized in three regions: near the surface, at 1 / 4 thickness, and at 1 / 2 thickness. In the near-surface region or 1 / 4 thickness region of the flat steel product, relative to the 1 / 2 thickness region of the flat steel product, the absolute deviation of the ferrite proportion is 7% by volume.

19. The flat steel product according to claim 1, characterized in that, The microstructure distribution across the thickness of flat steel products is characterized in three regions: near the surface, at 1 / 4 thickness, and at 1 / 2 thickness. - In the three regions, the difference in hardness HV0.1 was the largest at 15HV0.1 compared to the average across the entire thickness of the flat steel product.

20. The flat steel product according to claim 1, characterized in that, The microstructure distribution across the thickness of flat steel products is characterized in three regions: near the surface, at 1 / 4 thickness, and at 1 / 2 thickness. - In the three regions, the hardness difference HV0.1 was the largest of 10HV0.1 compared to the average value over the entire thickness of the flat steel product.

21. The flat steel product according to claim 4, characterized in that, The steel contains, by weight percentage: The steel contains, by weight percentage: Minimum Ti: 0.

04.

22. The flat steel product according to claim 4, characterized in that, The steel contains, by weight percentage: Minimum Ti: 0.

06.

23. The flat steel product according to claim 10, characterized in that, The average value of the three regions of flat steel products—near the surface, 1 / 4 thickness, and 1 / 2 thickness—is at most 1.

6.

24. A method for producing hot-rolled flat steel products with high local cold formability, the flat steel products having a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8, a hole expansion ratio of more than 30%, a cold formability measure of at least 0.12, and a local cold formability to overall cold formability ratio of at least 5 and at most 13, the method comprising the steps of: - Melt a steel melt containing the following by weight percentages: C: 0.04 to 0.08 Si: 0.1 to 0.6 Mn: 1.0 to 2.0 P: Maximum 0.06 S: Maximum 0.01 N: Maximum 0.012 Al: Not exceeding 0.06 Ti: not exceeding 0.18 and / or Nb: not exceeding 0.08 Mo: not exceeding 0.35 Where Ti+Nb is greater than 0.06, and the superstoichiometric ratio of carbon and nitrogen exists according to the following formula: setting 1.0 < (C / 12 + N / 14) / (Ti / 48 + Nb / 93 + Mo / 96), one or more elements selected from Cr, Ni, V, B or Ca are selectively added by alloying, with the remainder being iron, including unavoidable steel-related elements. - Using horizontal or vertical slab casting processes, molten steel is poured into slabs. - Reheat the slab to 1100°C to 1270°C, and then hot-roll the slab using the following directly following steps: - In the final rolling pass, the hot-rolled strip is rolled to the desired final thickness at the final rolling temperature EWT, where the following conditions apply: EWT≥EWTmin=682℃+464C+6445Nb–644×Nb 0.5 +732V–230V 0.5 +890Ti+363Al–36Si - Cooling is performed at an average cooling rate of 30 K / s to 150 K / s. - The hot-rolled strip is wound into coils at a sufficiently low winding temperature (HT) to set a favorable microstructure. ， On the other hand, it is suitable to provide sufficient precipitation hardening in the subsequent cooling process T(t), as specified by 17000≤HP≤18800, and HP(T,t)=T(t)×(In(t)+20), where T is represented by K and t by h. - During the cooling process T(t), the coiling temperature is cooled at an average cooling rate of 5 K / h to 50 K / h between 100 °C and the coiling temperature, followed by cooling to room temperature in still air.

25. The method according to claim 24, characterized in that, The slab is hot-rolled into flat steel products with a thickness of 1.6 mm to 6.0 mm.

26. The method according to claim 24, characterized in that, Flat steel products are coated with a metal coating by electrolysis or hot-dip galvanizing.

27. The method according to claim 26, characterized in that, The metal coating is zinc-based.

28. The method according to any one of claims 24 to 27, used for producing flat steel products according to any one of claims 1 to 23.

29. The method according to claim 24, characterized in that, This flat steel product has a hole expansion rate of at least 40%.

30. The method according to claim 24, characterized in that, This flat steel product has a hole expansion rate of at least 50%.

31. The method according to claim 24, characterized in that, The flat steel product has a cold formability measure of at least 0.

17.

32. Use of a flat steel product according to any one of claims 1 to 23 in the production of components in the automotive industry.

33. The flat steel product according to claim 32 is used in the production of chassis components.

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