High-strength cold-rolled steel with reduced hydrogen embrittlement sensitivity and its manufacturing method

Abstract

The high-strength cold-rolled flat steel articles with reduced hydrogen embrittlement sensitivity according to the present invention comprise a steel substrate consisting of, by weight percent: C: 0.20% to 0.40%, Mn: 1.50% to 3.00%, Si: 0.90% to 1.50%, Al: 0.005% to 1.00%, V: 0.01% to 0.30%, optionally Cr: 0.01% to 1.00%, optionally Mo: 0.005% to 0.20%, optionally B: 0.00001% to 0.002%, optionally Nb and Ti: the total content of Nb and Ti is 0.005% to 0.2%, P The steel substrate comprises, by area %: up to 0.020% Fe, up to 0.005% S, up to 0.008% N, with the balance being Fe and unavoidable impurities, the total proportion of which is ≤0.8%. The microstructure of the steel substrate comprises, by area %, 65% to 92% primary (tempered) martensite and at least 8% retained austenite (RA), with the remainder consisting of up to 27% secondary (untempered) martensite, up to 10% bainite or bainitic ferrite, and / or up to ≤5% polygonal ferrite, the total proportion of which is ≤27%. The present invention also provides a method for reliably manufacturing the flat steel article.

Description

[0001] This invention relates to a high-strength cold-rolled flat steel product with reduced hydrogen embrittlement sensitivity, and a method for manufacturing such a flat steel product.

[0002] In this article, "flat steel products" refers to rolled products whose length and width are significantly greater than their thickness. Therefore, flat steel products specifically include steel strips, steel plates, and billets made from them.

[0003] Unless otherwise expressly stated, all information regarding the composition of steel alloys in this application is in mass. Therefore, all percentage data relating to the composition of steel alloys or other alloys mentioned herein, unless otherwise specified, should be understood as "mass percentage" ("mass%).

[0004] To achieve emissions reduction targets in the automotive and transportation industries through lightweighting while improving passenger safety, flat steel products that combine higher strength with better ductility are needed. These two conflicting goals of strength and ductility have been incorporated into the advanced third-generation high-strength steel concept (the so-called "AHSS"), which includes steels known as "quenched and tempered steel" ("Q&P steel").

[0005] Q&P steel utilizes retained austenite (“RA”) as a component of its microstructure to enhance strain hardening and tensile strength, while also improving elongation through the transformation-induced plasticity (“TRIP”) effect, which is well-known. The retained austenite is embedded within a quenched and tempered martensite (primary martensite) matrix. The microstructure of Q&P steel may also contain small amounts of bainite (bainitic ferrite), polygonal ferrite, and newly formed martensite (secondary martensite).

[0006] As tensile strength increases, the risk of sensitivity to hydrogen embrittlement also increases, as hydrogen embrittlement can lead to an unexpected decrease in ductility and strength.

[0007] It is known that the addition of microalloying elements such as Ti, Nb, or V can minimize the susceptibility of steel to hydrogen embrittlement. Microalloying elements form fine carbide or carbonitride precipitates that are coherent or semi-coherent and have a size of approximately <10 nm, preferably <5 nm.

[0008] In professional literature, precipitates formed by microalloying elements are often referred to as "diffusing hydrogen traps." This is because hydrogen atoms have a relatively strong binding energy to these fine precipitates. Therefore, hydrogen atoms that penetrate into the steel containing these fine precipitates in its microstructure will bind to the precipitate interface or to the misalignments created by the mismatch between the precipitate and the surrounding matrix.

[0009] The abundance of such traps in the matrix slows down the diffusion of hydrogen atoms in the microstructure, thereby reducing their negative impact on the deformation and fracture processes.

[0010] However, combining microalloying elements with Q&P steel is not an easy task. Because microalloying elements can form carbides with the carbon present in the steel, they consume some of the carbon that needs to be used to stabilize the residual austenite during the distribution process.

[0011] Against this backdrop, the present invention will specifically describe a flat steel product that is high-strength, easy to form, and has reduced hydrogen embrittlement tendency.

[0012] This problem is solved by the following cold-rolled flat steel products, which comprise a steel substrate, the steel substrate being composed of the following by weight percent:

[0013] C: 0.20% to 0.40%,

[0014] Mn: 1.50% to 3.00%,

[0015] Si: 0.90% to 1.50%,

[0016] Al: 0.005% to 1.00%,

[0017] V: 0.01% to 0.30%,

[0018] Optional Cr: 0.01% to 1.00%,

[0019] Optional Mo: 0.005% to 0.20%,

[0020] Optional B: 0.00001% to 0.002%,

[0021] Optional Nb and Ti: The total content of Nb and Ti is 0.005% to 0.2%.

[0022] P: at most 0.020%,

[0023] S: at most 0.005%,

[0024] N: at most 0.008%,

[0025] The balance consists of Fe and unavoidable impurities, with the total proportion of impurities ≤ 0.8%.

[0026] The steel substrate exhibits a microstructure, which, as determined by ISO 9042, comprises, in area % as follows:

[0027] 65% to 92% of primary martensite, and

[0028] At least 8% retained austenite (RA),

[0029] The remaining parts are filled with the following:

[0030] Up to 27% secondary martensite,

[0031] Up to 10% bainite or bainitic ferrite,

[0032] And / or at most ≤5% polygonal ferrite, the total proportion of secondary martensite, bainite, or bainitic ferrite and polygonal ferrite is ≤27%.

[0033] Among them, the density of V-based precipitates with a diameter of less than 10 nm in the microstructure of the steel substrate of the flat steel product is ≥1000 precipitates / μm. 2 .

[0034] Furthermore, the present invention will specifically describe a method that allows for the reliable manufacture of such flat steel products.

[0035] This problem was solved using the following method, which includes the following steps:

[0036] a) Provide a steel melt comprising, by mass %: C: 0.2% to 0.4%, Mn: 1.5% to 3.0%, Si: 0.9% to 1.5%, Al: 0.005% to 1.0%, V: 0.01% to 0.3%, optional Cr: 0.01% to 1.00%, optional Mo: 0.005% to 0.20%, optional B: 0.00001% to 0.002%, optional Nb and Ti: the total content of Nb and Ti is 0.005% to 0.2%, P: up to 0.020%, S: up to 0.005%, N: up to 0.008%, and the balance being Fe and unavoidable impurities, the total proportion of which is ≤0.8%;

[0037] b) Cast the molten steel into slabs;

[0038] c) Reheat the slab to a reheating temperature of 1000°C to 1300°C;

[0039] d) The reheated slab is hot-rolled into a hot-rolled strip, wherein the hot rolling is completed at a hot rolling finishing temperature of 850°C to 980°C;

[0040] e) Cool the hot-rolled strip to a coiling temperature of 400°C to 600°C, with the cooling completed within a maximum of 25 seconds after hot rolling, and then coil the hot-rolled strip into a roll.

[0041] f) Optionally, the hot-rolled strip is pickled;

[0042] g) Cold rolling hot-rolled strip into cold-rolled strip, with a cold rolling reduction rate of 20% to 80%.

[0043] h) Perform final annealing on the cold-rolled strip using the following method:

[0044] - The cold-rolled strip is heated to a homogenization temperature TS at a heating rate ΘS of 2°C / s to 10°C / s, which is at least 50°C higher than the Ac3 temperature of the corresponding steel and at most 950°C, wherein the Ac3 temperature of the corresponding steel is determined according to SEP.

[0045] The measurements were taken using the dilatation method from 1681 to 1998-06.

[0046] -Then the cold-rolled strip is kept at a homogenization temperature TS for a homogenization time tS of more than 40 seconds and less than 200 seconds;

[0047] -The cold-rolled strip is then quenched at a quenching rate ΘQ of 20°C / s to 100°C / s to a quenching stop temperature TQ, which is below the martensitic initiation temperature T_MS of the steel and at least equal to the temperature TQ_min, wherein at temperature TQ_min, the microstructure of the cold-rolled strip contains 65% to 92% primary martensite by area, wherein the T_MS temperature of the steel is determined using the dilatation method according to SEP1681-1998-06; and

[0048] - Hold the annealed cold-rolled strip at the quenching stop temperature TQ for 4 to 20 seconds;

[0049] i) The cold-rolled strip after final annealing undergoes an over-aging treatment, which includes the following steps:

[0050] -Heating the cold-rolled strip to an over-aging temperature TP of 380°C to 460°C,

[0051] - Hold the cold-rolled strip at the over-aging temperature for 50 to 200 seconds.

[0052] as well as

[0053] - Cool the cold-rolled strip to below 100°C at a cooling rate of 0.5°C / s to 20°C / s.

[0054] Advantageous embodiments of the invention and the overall concept of the invention are described in detail below.

[0055] Thus, according to the present invention, a high-strength cold-rolled flat steel product with reduced hydrogen embrittlement sensitivity comprises, by weight percent, the following:

[0056] C: 0.20% to 0.40%,

[0057] Mn: 1.50% to 3.00%,

[0058] Si: 0.90% to 1.50%,

[0059] Al: 0.005% to 1.00%,

[0060] V: 0.01% to 0.30%,

[0061] Optional Cr: 0.01% to 1%,

[0062] Optional Mo: 0.005% to 0.2%,

[0063] Optional B: 0.00001% to 0.002%,

[0064] Optional Nb and Ti: The total content of Nb and Ti is 0.005% to 0.2%.

[0065] P: at most 0.020%,

[0066] S: at most 0.005%,

[0067] N: at most 0.008%,

[0068] Furthermore, the balance is Fe and unavoidable impurities, the total proportion of which is ≤0.8%, and

[0069] The flat steel product exhibits a microstructure, by area percentage, consisting of the following:

[0070] 65% to 92% primary (tempered) martensite, and

[0071] At least 8% retained austenite (RA),

[0072] The remaining parts are filled with the following:

[0073] Up to 27% secondary (untempered) martensite,

[0074] Up to 10% bainite or bainitic ferrite,

[0075] and / or at most ≤5% polygonal ferrite,

[0076] The total proportion of the secondary (untempered) martensite, the bainite or bainitic ferrite, and the polygonal ferrite is ≤27%.

[0077] The flat steel articles according to the invention have a tensile strength of at least 1300 MPa, typically in the range of 1300 MPa to 1600 MPa, a yield strength of at least 1000 MPa, and a total elongation of at least 10% A80, wherein the tensile strength, the yield strength and the elongation are determined according to the currently valid DIN ENISO 6892 (sample form 2).

[0078] The flat steel products according to the present invention not only have high strength, but also have good formability, which is reflected in the expansion ratio HER exceeding 20%, which is determined according to the currently valid ISO 16630.

[0079] Furthermore, numerous tests have demonstrated, as shown by the quantitative results of slow strain rate tests (with and without hydrogen-filled media) conducted according to the currently valid DIN ENISO 7539-7, that the flat steel products according to the present invention have improved resistance to hydrogen embrittlement.

[0080] Therefore, this invention offers significant advantages in structural and collision-related components in the automotive and transportation industries, including battery casings for electric vehicles. These applications require both high yield strength and a large capacity to absorb plastic deformation. High yield strength ensures minimal intrusion in the event of a collision, while high elongation enables large energy absorption.

[0081] The method for manufacturing flat steel products according to the present invention comprises the following steps:

[0082] a) Provide a steel melt comprising, by mass %: C: 0.2% to 0.4%, Mn: 1.5% to 3.0%, Si: 0.9% to 1.5%, Al: 0.005% to 1.0%, V: 0.01% to 0.3%, optional Cr: 0.01% to 1%, optional Mo: 0.005% to 0.2%, optional B: 0.00001% to 0.002%, optional Nb and Ti: the total content of Nb and Ti is 0.005% to 0.2%, P: up to 0.020%, S: up to 0.005%, N: up to 0.008%, and the balance being Fe and unavoidable impurities, the total proportion of said impurities being ≤0.8%;

[0083] b) Cast the molten steel into slabs;

[0084] c) Reheat the slab to a reheating temperature of 1000°C to 1300°C;

[0085] d) The reheated slab is hot-rolled into a hot-rolled strip, wherein the hot rolling is completed at a hot rolling finishing temperature of 850°C to 980°C;

[0086] e) Cool the hot-rolled strip to a coiling temperature of 400°C to 600°C, the cooling being completed within a maximum of 25 seconds after the hot rolling is finished, and then coil the hot-rolled strip into a roll;

[0087] f) Optionally, the hot-rolled strip is pickled;

[0088] g) The hot-rolled strip is cold-rolled into a cold-rolled strip, with a cold-rolling reduction rate of 20% to 80%.

[0089] h) The cold-rolled strip is finally annealed in the following manner:

[0090] - The cold-rolled strip is heated to a homogenization temperature TS at a heating rate ΘS of 2°C / s to 10°C / s, which is at least 50°C higher than the Ac3 temperature of the corresponding steel and at most 950°C.

[0091] -Then the cold-rolled strip is kept at a homogenization temperature TS for a homogenization time of more than 40 seconds and less than 200 seconds;

[0092] -The cold-rolled strip is then quenched at a quenching rate ΘQ of 20°C / s to 100°C / s to a quenching stop temperature TQ, which is lower than the martensite initiation temperature T_MS of the steel and at least equal to the temperature TQ_min, wherein at the temperature TQ_min, the microstructure of the cold-rolled strip contains 65% to 92% primary martensite in terms of area %.

[0093] - Hold the annealed cold-rolled strip at the quenching stop temperature TQ for 4 to 20 seconds;

[0094] i) The cold-rolled strip undergoes an aging treatment (also known as a "split treatment") after final annealing, which includes:

[0095] -Heating the cold-rolled strip to an over-aging temperature TP of 380°C to 460°C,

[0096] - Hold the cold-rolled strip at the over-aging temperature for 50 to 200 seconds, and

[0097] - The cold-rolled strip is cooled to below 100°C at a cooling rate of ΘC from 0.5°C / s to 20°C / s.

[0098] To protect the flat steel articles according to the invention from corrosion, an anti-corrosion coating may be provided on at least one of their surfaces in a conventional manner. Such a coating may be applied by electroplating, hot-dip galvanizing, or zinc annealing. The coating is typically composed of an alloy, the main components of which are zinc (“Zn”) or aluminum (“Al”), and other alloying elements such as silicon (“Si”), magnesium (“Mg”), and iron (“Fe”) may be added in well-known manner to optimize the properties of the coating.

[0099] The steel composition of the steel substrate of the flat steel product according to the present invention is determined as follows:

[0100] In the steel of the flat steel products according to the invention, carbon (“C”) accounts for 0.20% to 0.40% by mass. C is a fundamental element for improving the strength of the steel sheet and reliably obtaining the required amount of stable retained austenite in the microstructure. During the quenching process in the final annealing (working step h of the method according to the invention) and the subsequent over-aging treatment step (working step i of the method according to the invention), the retained austenite is stabilized by the diffusion and distribution of C until room temperature is reached. For the stability of the retained austenite phase, the C content must reach at least 0.20% by mass. Furthermore, the required amount and strength of martensite formed during the first quenching (working step h of the method according to the invention) or the final quenching (working step i of the method according to the invention) up to a temperature below 100°C (particularly room temperature) is determined by the C content. Steel sheets with a C content below 0.2% will not exhibit a combination of sufficiently high strength and good formability. However, if the C content exceeds 0.4% by mass, the martensite initiation temperature is significantly reduced, which will result in only a small amount of martensite being produced, thereby reducing the strength level. Furthermore, a carbon content exceeding 0.4% reduces the weldability of the steel plate according to the invention. When the carbon content is at least 0.22% by mass, the positive effect of carbon on the properties of the flat steel products according to the invention can be obtained particularly reliably. The presence of carbon is especially effective when the carbon content in the flat steel products according to the invention is at most 0.3% by mass.

[0101] The flat steel products according to the present invention contain 1.50% to 3.00% manganese (“Mn”) added to the steel. Mn effectively increases the hardness of the steel, thereby increasing its strength. Furthermore, the presence of Mn within the range specified in the present invention suppresses the formation of ferrite and pearlite during quenching. A suitable microstructure containing martensite and retained austenite is obtained after the first quenching (working step i) by using a quenching rate of <100°C / s. Mn is also a solid solution strengthening element, stabilizing austenite by lowering the Ms temperature. To reliably obtain these positive effects of Mn, the Mn content in the steel of the flat steel products according to the present invention needs to reach at least 1.5% by mass, particularly at least 1.9% by mass. However, a high Mn content exceeding 3.00% by mass deteriorates weldability and leads to segregation, thus also deteriorating the mechanical properties of the flat steel products. To reliably avoid these negative effects of Mn, the Mn content in the steel of the flat steel products according to the present invention can be limited to 2.8% by mass.

[0102] In the steel of the flat steel products according to the invention, silicon (“Si”) accounts for 0.90% to 1.50% by mass. Si contributes to the strength of the steel through solid solution strengthening. Furthermore, silicon is insoluble in cementite, thus inhibiting the formation of iron carbide during the distribution process (working step i) and supporting the stability of retained austenite, thereby improving ductility. To achieve these effects, at least 0.90% by mass of Si is required in the steel of the flat steel products according to the invention. In addition, the combination of Al and 0.9% Si also shows a similar effect on the stability and amount of retained austenite, thereby improving ductility. However, a high silicon content exceeding 1.50% by mass can adversely affect the coatability of the steel sheet.

[0103] The flat steel according to the invention contains 0.005% to 1.00% by mass aluminum (“Al”). Al is commonly used as a deoxidizer in steelmaking and forms aluminum nitride, thereby contributing to the strength of the steel. Like Si, Al is insoluble in cementite, thus inhibiting the formation of iron carbide during decomposition (working step i). However, with increasing aluminum content, the Ac3 temperature of the steel used to manufacture the flat steel products of the invention rises sharply to values ​​that are excessively high for typical industrial annealing lines. If Al is used only as a deoxidizer, the Al content can be limited to a maximum of 0.1% by mass to avoid the formation of AlN. If Al is to be used for deoxidation, the Al content must reach at least 0.005% by mass. Typically, in the steel alloy according to the invention, the Al content for the above purposes ranges from 0.005 to 0.100% by mass, particularly 0.005 to 0.070% by mass or 0.005 to 0.060% by mass. However, if it is necessary to use Al to promote the retention of austenite, a content of up to 1.0% is permitted. In this case, an Al content of at least 0.060% by mass, particularly at least 0.1% by mass or at least 0.3% by mass, is suitable.

[0104] In the steel used to manufacture the flat steel articles according to the invention, the vanadium (“V”) content ranges from 0.01% by mass to 0.3% by mass. V is a microalloying element with a dissolution temperature much lower than that of Ti or Nb. This allows V-based precipitates to partially or completely dissolve during the final annealing (working step h) and precipitate during subsequent cooling, or most preferably precipitate by carbon diffusion and partitioning during the partitioning stage of the final annealing cycle (working step i). This results in particularly fine V-based precipitates, which are particularly effective at capturing diffusing hydrogen. The positive effect of V becomes apparent when the concentration exceeds 0.01% by mass and continues to increase to a maximum of 0.15% by mass. This effect is observed to saturate when the concentration exceeds 0.3% by mass. The presence of V has proven to be most effective in the range of up to 0.25% by mass, and particularly up to 0.20% by mass. The effect is particularly pronounced if the V content is at least 0.07% by mass.

[0105] Chromium (“Cr”) may optionally be added to the steel alloy of the steel base material used in the manufacture of the flat steel articles according to the invention to effectively inhibit the formation of pearlite and bainite and improve strength. This effect is achieved by adding at least 0.01% by mass of Cr. However, to avoid grain boundary oxidation, the Cr content is limited to 1% by mass. This is particularly effective when Cr is present in the steel of the flat steel articles according to the invention at a Cr content of at least 0.1% by mass. To avoid the negative effects of Cr presence, the Cr content may be limited to 0.5% by mass.

[0106] As another optional element, 0.005% to 0.2% by mass of molybdenum (“Mo”) may be added to the steel alloy used in the manufacture of the flat steel articles according to the invention. Mo increases the strength of the steel sheet and inhibits the formation of pearlite. This is particularly effective when the Mo content is at least 0.02% by mass, and the positive effect of Mo is particularly significant when the Mo content is at most 0.15% by mass.

[0107] Niobium (“Nb”) and titanium (“Ti”) are microalloying elements that can be optionally added, either in combination or individually, to the steel alloy of the flat steel articles according to the invention, effectively increasing the strength of the steel sheet through precipitation hardening and refining the microstructure. However, the dissolution temperatures of Nb and Ti are much higher than that of V. This means that, for example, if Ti is present in combination with V, mixed carbides will form. These mixed carbides tend not to dissolve during the final annealing process, and therefore remain coarse or grow to ineffective sizes during the final annealing. Therefore, the total content of Nb and Ti is limited to a maximum of 0.2% by mass. On the other hand, the beneficial effects of the presence of Nb and / or Ti on the strength and formability of the flat steel articles can be reliably utilized when the total content of Nb and / or Ti is at least 0.005% by mass. The strengthening of steel is particularly effective when the total content of Nb and / or Ti ranges from 0.02% to 0.15% by mass.

[0108] Optionally, 0.00001% to 0.002% by mass of boron (“B”) can be added to the steel of the flat steel articles according to the invention to suppress the formation of ferrite and segregation along grain boundaries, thereby preventing ferrite migration. This results in a fine-grained microstructure, which contributes to improved mechanical properties of the steel. In this respect, a B content in the range of 0.0001% to 0.001% by mass is particularly effective.

[0109] The term “impurity” includes all elements that enter the steel during the steelmaking process or cannot be completely removed from the steel. In the steel of the flat steel articles according to the invention, impurities may be detectable by measurement but are not listed herein as necessary or optional components, or, for optional added elements, their content is very low, below the effective limits specified herein. To avoid the overall detrimental effects of impurities, the total impurity content is limited to a maximum of 0.8% by mass. Specifically, phosphorus (“P”), sulfur (“S”), and nitrogen (“N”) are unavoidable impurities. However, it has been demonstrated that a P content of up to 0.020% by mass, a S content of up to 0.005% by mass, and a N content of up to 0.008% by mass do not degrade the properties of the steel of the flat steel articles according to the invention. Typically, steel contains 0.001% to 0.020% by mass of P, 0.0001% to 0.005% by mass of S, and 0.0001% to 0.008% by mass of N. The content of copper (“Cu”) is at most 0.5% by mass, the content of nickel (“Ni”) is at most 0.5% by mass, and the content of oxygen (“O”) is at most 0.0080% by mass; these are also considered unavoidable impurities. Of course, other elements such as W, Co, Sn, Ca, Mg, REM, Zr, Te, As, Bi, etc. are also considered impurities.

[0110] The flat steel articles according to the invention exhibit a microstructure comprising 65 to 92 area % primary (tempered) martensite and at least 8 area % retained austenite (RA). The retained austenite fills the portion not occupied by the primary martensite and optionally other microstructure components permitted according to the invention. Thus, if no other components are present in the microstructure of the steel, the retained austenite accounts for 8 to 35 area % of the microstructure. However, if only a minimum amount of primary (tempered) martensite (65 area %) and retained austenite (8 area %) are present in the microstructure, the total amount of components optionally present according to the invention is 27 area %. Theoretically, the corresponding remaining portion of the microstructure can be filled solely by secondary (untempered) martensite, by bainite or bainitic ferrite, and / or by polygonal ferrite, wherein these optional components typically appear in combination.

[0111] The microstructure of the flat steel product according to the present invention further includes V-based precipitates with a diameter of less than 10 nm and a density of ≥1000 precipitates / μm. 2 Therefore, the flat steel articles according to the present invention exhibit improved resistance to hydrogen embrittlement.

[0112] The density of the vanadium-based precipitates was determined using carbon-extracted replicas via transmission electron microscopy (TEM) combined with X-ray microscopy (EDX). The carbon-extracted replicas were obtained from longitudinal sections. Measurement magnifications ranged from 10,000x to 200,000x. Based on these images, the diameter of the precipitates within the measurement regions was calculated using computer-aided image analysis. For this purpose, measurements were taken in five regions in each case. The results from the five regions were then averaged. The size of the measurement regions depended on the selected magnification and ranged from 18.5 μm x 14.5 μm at 10,000x magnification to 0.925 μm x 0.725 μm at 200,000x magnification. Simultaneously, the properties of the precipitates were determined using EDX (energy-dispersive X-ray spectroscopy). In this process, the electron beam emitted from the TEM excites the atoms in the precipitates. The elemental distribution in the sample can be determined by the emitted X-rays, thus enabling the identification of the vanadium precipitates.

[0113] For example, the density of V precipitate can be determined by the following steps:

[0114] 1. Provide carbon extraction replicas of longitudinal slices of flat steel products;

[0115] 2. Using TEM and computer-aided image analysis at 100,000x magnification, the diameters of all precipitates in five different measurement regions measuring 1.85μm x 1.45μm were determined;

[0116] 3. Vanadium precipitates are identified based on detected X-ray quantum mechanics;

[0117] 4. Calculate the number of vanadium precipitates with a diameter less than 10 nm in each of the five measurement regions, and determine the density of vanadium precipitates in each of the five measurement regions;

[0118] 5. The average density of the five measurement areas is determined as the density of vanadium precipitates in the flat steel product.

[0119] If the austenite grains are too large, the number of martensite nucleation sites will decrease, potentially leading to excessively large martensite packets, which in turn reduces the local formability of the final product. The width of a single martensite lath is a function of its length. Thinner laths are advantageous for supporting the microstructural processes that occur during over-aging treatment (working step i of the method according to the invention). According to the invention, the precipitation of fine V-based precipitates along the phase boundary between martensite and retained austenite grains yields lath thicknesses up to 1000 nm, with lath lengths often reaching a maximum of 500 nm. The microstructure can be determined using transverse sections located at 1 / 3 t of the layer, i.e., sections taken at one-third of the thickness of the steel substrate. These sections are prepared for scanning electron microscopy (SEM) and treated with 3% Nital etchant. Due to the fineness of the microstructure, it is observed using a 5000x magnification scanning electron microscope. The measured lath thickness corresponds to the average of five measurements.

[0120] Furthermore, precipitates located at phase boundaries can also act as hydrogen traps, thereby improving resistance to hydrogen embrittlement. Controlling the microstructure by precipitating fine V-based precipitates along the phase boundaries of martensite and / or retained austenite grains leads to superior resistance to hydrogen embrittlement and mechanical properties. Due to the use of the alloy specified in this invention and the method for manufacturing flat steel products according to this invention, fine V(C,N) precipitates are formed, resulting in finer original austenite grains and a thinner martensite / retained austenite lath structure. V(C,N) nanoparticles are positioned along the lath interfaces, where they trap hydrogen penetrating the steel substrate, thereby improving resistance to hydrogen embrittlement.

[0121] The properties of the flat steel product according to the present invention described above can be reliably obtained by the method for manufacturing such a flat steel product proposed in the present invention.

[0122] To manufacture the flat steel product according to the invention, a steel melt with a composition corresponding to that specified in the invention is prepared using conventional methods. The steel melt is then cast using conventional methods to form at least one slab (steps a) and b) according to the method of the invention.

[0123] In preparation for subsequent hot rolling, the slab is reheated to a temperature in the range of up to 1000°C to 1300°C, or, if the temperature of the slab after casting is high enough, it is maintained in that temperature range to ensure that the temperature of the entire volume of the slab is uniform (operating step c of the method according to the invention).

[0124] In step d) of the method according to the invention, the slab is hot-rolled into a hot-rolled strip at a sufficiently high temperature, such that the final rolling temperature is in the range of 850°C to 980°C. If the temperature during hot rolling is too low, sufficient static recrystallization will not occur between rolling steps. Therefore, the resulting microstructure retains a robust texture and a high dislocation density due to dynamic recrystallization. Thus, the final rolling temperature must not be lower than 850°C. A final rolling temperature higher than 980°C is technically not feasible.

[0125] According to step e) of the method of the invention, the obtained hot-rolled strip should be cooled to the coiling temperature within a maximum of 25 seconds after hot rolling to prevent the hot-rolled strip from precipitating or forming polygonal ferrite on the run-out table it passes through before coiling. Cooling is preferably completed within a maximum of 18 seconds, and even more preferably within a maximum of 15 seconds, to avoid such an effect.

[0126] In step e), cooling to the winding temperature can be performed using any method known in the art, with cooling rates typically ranging from 20°C / s to 1000°C / s. High cooling rates can be achieved, for example, by water quenching.

[0127] The coiling temperature should be a maximum of 600°C to avoid pearlite formation. Furthermore, at higher coiling temperatures, oxidizing elements (such as Si, Cr, or Mn) diffuse to the grain boundaries and form oxides, which degrades the surface quality of the hot-rolled material, thus limiting the surface quality of hot-rolled and optionally coated flat steel products. Limiting the coiling temperature to a maximum of 600°C also prevents the formation of unwanted polygonal ferrite. Coiling temperatures below or at most 580°C increase the bainite content in the microstructure of the hot-rolled material. Setting the coiling temperature at a lower range of 400°C to 500°C effectively avoids grain boundary oxidation. However, low coiling temperatures also promote the formation of a large amount of martensite, resulting in high hardness in the hot-rolled strip; therefore, batch annealing is required during cold rolling; typically, annealing for more than 6 hours but less than 24 hours in the temperature range of 550°C to 600°C is performed to decompose bainite / martensite. Therefore, by winding the hot-rolled strip at a winding temperature of 400°C to 600°C, preferably 400°C to 580°C, and especially usefully 500°C to 580°C, a uniform microstructure can be reliably obtained without generating a large amount of martensite, thereby enabling smaller thickness and width tolerances to be achieved in the subsequent cold rolling process.

[0128] After the hot-rolled strip is wound into a coil and cooled to room temperature, it can be descaled using conventional methods, and if necessary, pickling can be performed to remove dirt from the strip surface or improve surface quality (optional working step f according to the method of the invention).

[0129] Cold rolling is typically performed in one or more rolling steps on a conventional tandem mill, with intermediate batch annealing as described above if necessary. For cold rolling, reversible mill stands can be used to achieve higher reductions. In this case, it is advantageous to achieve a cold rolling reduction of up to 80%. The minimum reduction obtained during cold rolling should typically be 20% to ensure sufficient recrystallization in the final annealing step (step h of the method according to the invention). However, higher reductions contribute to obtaining a finer-grained microstructure, thereby achieving the aforementioned article properties. Therefore, in the method according to the invention, a cold rolling reduction of 20% to 80% is appropriate when cold rolling a hot-rolled strip into a cold-rolled strip (working step g of the method according to the invention).

[0130] The heat treatment steps h) and i) according to the method of the present invention are preferably carried out on a heat treatment production line, wherein the corresponding flat steel products pass through the production line continuously and uninterruptedly.

[0131] During the final annealing of the cold-rolled strip in step h) of the method according to the invention, most of the properties of the flat steel products according to the invention are adjusted. When heating the cold-rolled strip to the homogenization temperature TS, the average heating rate “ΘS” should be between 2°C / s and 10°C / s. Heating rates above 10°C / s jeopardize normal recrystallization prior to austenitization, while heating rates below 2°C / s are uneconomical in continuous annealing production lines. The homogenization temperature TS should be at least 50°C higher than the Ac3 temperature of the corresponding steel composition to ensure proper homogenization of carbon in the fully austenitic microstructure.

[0132] For a given composition, the Ac3 temperature can be experimentally determined using the expansion measurement method in a known manner, or estimated according to the following formula (1):

[0133] Ac3[℃]=910℃+(-203√(%C)–15.2%Ni+

[0134] 44.7% Si + 31.5% Mo – 21.1% Mn) ℃ / mass%

[0135] Where %C = the corresponding C content in the steel alloy (in mass%)

[0136] %Ni = the corresponding Ni content in the steel alloy (in mass%).

[0137] %Si = The corresponding Si content in the steel alloy (in mass%).

[0138] %Mo = the corresponding Mo content in the steel alloy (in mass%).

[0139] %Mn = The corresponding Mn content in the steel alloy (in mass %).

[0140] For the steel alloys covered by the alloy parameters of the present invention, the temperature range of Ac3 is typically from about 750 °C to about 920 °C. The Ac3 temperature of the steel alloys covered by the alloy parameters of the present invention is experimentally determined by dilatometry according to SEP 1681-1998-06. When measuring, a dilatometer is used to compare the average thermal expansion coefficient of the steel sample with the thermal expansion coefficient of the quartz tube. The quartz tube expands linearly with temperature over a wide temperature range (far exceeding 1000 °C), while the steel sample undergoes phase changes during heating and cooling. Record the change of the linear expansion rate during uniform heating or cooling, so as to show the phase change temperature. For this purpose, commercially available dilatometers can be used, such as 805 dilatometer.

[0141] The upper limit of the soaking temperature TS is 950 °C in order to fully dissolve any V-carbides in the semi-finished product. The soaking time tS should be long enough to achieve chemical homogenization and carbide dissolution. However, at the same time, the soaking time must be limited to prevent excessive growth of austenite grains.

[0142] In order to promote the precipitation of V(C,N) precipitates, the soaking time tS should be at least 40 seconds, preferably more than 40 seconds, and at most 200 seconds (40 s ≤ tS ≤ 200 s), preferably shorter than 200 seconds (40 s < tS < 200 s). A soaking time tS exceeding 60 seconds and shorter than 120 seconds (60 s < tS < 120 s) is particularly useful in the present invention.

[0143] After the soaking stage, the steel strip enters the primary cooling step. The average cooling rate "ΘQ" of the flat steel product according to the present invention in the primary cooling step must be high enough to minimize the formation of polygonal ferrite, bainite, bainitic ferrite and any precipitated carbides. Therefore, the lower limit of the cooling rate ΘQ is set to 20 °C / s. The upper limit of ΘQ depends on process stability and the cooling capacity of the primary cooling step. The present invention does not require increasing the cooling capacity of the primary cooling step above 100 °C / s. On the contrary, a cooling rate exceeding 100 °C / s during the primary cooling process will increase production costs and result in insufficient cooling, which is not conducive to the mechanical properties of the final product. Therefore, the cooling rate ΘQ is set to 20 °C / s to 100 °C / s, preferably 30 °C / s to 70 °C / s.

[0144] The quenching stop temperature TQ at the end of the primary cooling step during cooling carried out in this way must be lower than the martensite start temperature T_MS.

[0145] The martensite start temperature T_MS can be experimentally determined by dilatometry in a known manner or estimated according to the following formula (2):

[0146] T_MS[℃])=539℃+(-423%C–30.4%Mn–7.5%Si+30%Al)℃ / mass%

[0147] Where %C = the corresponding C content in the steel alloy (in mass%)

[0148] %Mn = the corresponding Mn content in the steel alloy (in mass%).

[0149] %Si = The corresponding Si content in the steel alloy (in mass%).

[0150] %Al = The corresponding Al content in the steel alloy (in mass %).

[0151] For the steel alloys covered by the alloy parameters of this invention, the T_MS temperature range is typically from about 265°C to about 435°C. The T_MS temperatures of the steel alloys covered by the alloy parameters of this invention were experimentally determined by the dilatation measurement method according to SEP 1681-1998-06. This method is well known to those skilled in the art and is performed using a dilatometer, as described above in the determination of the Ac3 temperature.

[0152] The quenching stop temperature shall not be lower than temperature TQ_min, where at this temperature TQ_min, the microstructure of the cold-rolled strip contains 65% to 92% area of ​​primary martensite (T_MS≥TQ≥TQ_min).

[0153] To determine the proportion of primary martensite after initial quenching, and thus obtain the temperature TQ_min, the Koistinnen-Marburger equation can be used. This equation is typically expressed as follows:

[0154] f m =1–exp(-0.011(T_MS–TQ))

[0155] Among them, f m It is the austenitic portion that transforms into martensite when quenched to a temperature TQ below T_MS (Speer et al., MS&T 2003, pp. 505-522, 2003). According to the present invention, the amount of primary martensite should not exceed 92% by area and should not be less than 65% by area.

[0156] Following a cooling process, the flat steel product according to the invention is then held at the corresponding quenching temperature TQ for at least 4 seconds and no more than 20 seconds. This holding is necessary to homogenize the temperature across the entire thickness of the sheet and to allow the isothermal phase transformation in the final stage to occur.

[0157] Following the holding step, the flat steel product enters the overaging step i) according to the method of the invention, also known in technical terms as the "separation step". In step i), carbon (C) is partitioned from primary martensite to adjacent retained austenite. However, the separation of C is a thermally activated process that competes with the precipitation of nanoparticles. This means that if the partitioning occurs at excessively high temperatures or for excessively long times, iron carbide may form, consuming a large amount of carbon required to stabilize the retained austenite and reducing both the tensile strength and elongation of the flat steel product. To avoid this effect, the invention sets an upper limit of the overaging temperature TP at 460°C. The lower limit of the overaging temperature TP is set at 380°C to ensure the precipitation of a sufficient amount of V(C,N) nanoparticles, which would not occur at lower temperatures. In this respect, limiting the overaging temperature TP to a maximum of 460°C has also proven particularly advantageous, as it also prevents the coarsening of V(C,N) nanoparticles, which would reduce the mechanical properties of the flat steel product. By setting the over-aging temperature to 380°C to 460°C (380°C TP 460°C) and limiting the time tP for holding the corresponding flat steel products at temperature TP to 50 seconds to 200 seconds, the precipitate density of V-based precipitates with a diameter of less than 10 nm in the microstructure of the products according to the present invention can reliably reach ≥1000 precipitates / μm. 2 .

[0158] After the corresponding time period tP ends, the annealed steel strip is cooled to below 100°C at a cooling rate ΘC of 0.5°C / s to 20°C / s. This immediately stops the dispensing process and avoids any carbide coarsening or other thermally induced effects that could reduce the mechanical properties of the flat steel product according to the invention. After the flat steel product is cooled to below 100°C, the cooling process to room temperature is not critical and can be carried out in any suitable manner.

[0159] Tests conducted according to DIN ENISO 7539-7 have demonstrated that the hydrogen embrittlement resistance of the flat steel products of the present invention is improved. For this purpose, samples of the flat steel products according to the present invention were exposed to natural air, and other samples of the same flat steel products were exposed to an aqueous solution containing 5% NH4SCN as a hydrogen-filling medium. Subsequently, the maximum tensile stress Rm_max (air) of the sample exposed to the air and the maximum tensile stress Rm_max (NH4SCN) of the sample exposed to the hydrogen-filling medium were determined according to standard procedures. Based on the tensile stresses thus determined, the "hydrogen embrittlement sensitivity coefficient" ("H_sf") was calculated as follows:

[0160] H_sf = 1 - Rm_max(NH4SCN) / Rm_max(air)

[0161] The results show that the hydrogen embrittlement sensitivity coefficient H_sf of the flat steel products according to the invention is reliably at most 0.35 (H_sf ≤ 0.35). Therefore, the tests demonstrate that the hydrogen embrittlement sensitivity of the flat steel products according to the invention is significantly lower than that of the comparative steel plate samples, which were subjected to the same test conditions and evaluated in the same manner, but which were composed of conventional steel plates in which the steel was not alloyed according to the invention, i.e., particularly lacking the V content specified in the invention. In contrast to the samples of the invention, the comparative samples generally show a hydrogen embrittlement sensitivity coefficient H_sf of at least 0.35.

[0162] In each of the tables 1 to 5 shown below, values ​​that do not conform to the specifications of this invention are underlined.

[0163] To conduct practical tests and demonstrate the effects of this invention, we melted steel melt AM, the composition of which is shown in Table 1. Values ​​and examples outside the scope of this invention are underlined in Table 1.

[0164] Table 2 shows the Ac3 temperature and T_MS temperature measured by the expansion measurement method according to SEP 1681-1998-06 for each melt AM.

[0165] The melt AM is cast into a slab, which is then reheated to a temperature of 1150°C to 1300°C for subsequent hot rolling.

[0166] The slabs reheated in this manner are then hot-rolled into hot-rolled strips on a conventional hot-rolling production line, with a final hot-rolling temperature (FT) ranging from 850°C to 980°C. Table 2 also shows the final hot-rolling temperature (FT) maintained during the hot-rolling process for steels A to M.

[0167] After leaving the last hot rolling stand of the hot rolling production line, the corresponding hot-rolled strip is cooled to its respective coiling temperature CT within a time period tC of up to 25 seconds, with CT ranging from 400°C to 600°C. Table 2 also lists the respective time periods tC and the respective coiling temperatures CT.

[0168] After the hot-rolled strip is wound into coils and cooled to room temperature, it is pickled in a manner known to those skilled in the art to remove dirt from its surface.

[0169] The hot-rolled strip is then rolled into cold-rolled strip, using the same rolling method as ordinary cold rolling. The total cold-rolled density (CRD) achieved during the cold rolling process ranges from 20% to 80%, and the calculation formulas for each CRD are as follows:

[0170] CRD = (D1 – D2) / D1 * 100%

[0171] Where D1: the thickness of each hot-rolled strip before cold rolling.

[0172] D2: Thickness of each cold-rolled strip after cold rolling

[0173] Table 2 also lists their respective cold rolling degree (CRD).

[0174] The cold-rolled strip samples were heated to a homogenization temperature TS at a heating rate ΘS for final annealing, followed by holding each sample at the homogenization temperature TS for a homogenization time tS. Immediately after homogenization, the samples were quenched at a quenching rate ΘQ to a quenching stop temperature TQ, such that the proportion of primary martensite (PM%) in the microstructure of the samples annealed in this manner was between 65% and 92% area. After quenching, the annealed cold-rolled strip samples were then held at the quenching stop temperature TQ for a period of time tQ ranging from 4s to 20s. Table 3 lists the homogenization temperature TS, heating rate ΘS, homogenization time tS, quenching rate ΘQ, quenching stop temperature TQ, proportion of primary martensite PM% (corresponding to tM in Table 5), and time period tQ.

[0175] After final annealing, the cold-rolled strip samples undergo an aging treatment, also known in technical terms as a "partitioning treatment".

[0176] For this purpose, the sample was first heated to temperature TP and then held at that temperature for a time period tP. Finally, the sample was cooled to a temperature below 100°C at a cooling rate ΘC. Table 3 also lists the temperature TP and the time period tP.

[0177] For each sample treated in this manner, Table 4 lists the yield strength YS, tensile strength TS and elongation A80, YS / TS ratio, porosity HER as determined according to ISO 16630, and the product of TS x A80 for each sample as determined according to DIN EN ISO 6892 (sample form 2).

[0178] In addition, the microstructure of each sample was analyzed according to ISO 9042 for optical microscopy. Table 5 lists the residual austenite content RA, primary (tempered) martensite content tM, secondary (fresh) martensite content fM, total bainite content, bainitic ferrite content bF, and polygonal ferrite content pF determined by this method.

[0179] In addition, the areal density of V(C,N) precipitates in the microstructure of the samples was determined by preparing carbon replicas and using conventional transmission electron microscopy.

[0180] Finally, the hydrogen embrittlement sensitivity coefficient H_sf was determined according to the method described above. The results show that the samples according to the invention, which meet all the requirements of the invention in terms of composition, production, and heat treatment, have significantly lower hydrogen embrittlement sensitivity than those samples that were not alloyed according to the invention and / or not manufactured and heat-treated according to the invention.

[0181]

[0182]

[0183]

[0184]

[0185]

Claims

1. A high-strength cold-rolled flat steel product with reduced hydrogen embrittlement sensitivity, the flat steel product comprising a steel substrate, said steel substrate comprising, by weight percent: C： 0.20% to 0.40%, Mn: 1.50% to 3.00%, Si: 0.90% to 1.50%, Al: 0.005% to 1.00%, V: 0.01% to 0.30%, Optional Cr: 0.01% to 1.00%, Optional Mo: 0.005% to 0.20%, Optional B: 0.00001% to 0.002%, Optional Nb and Ti: The total content of Nb and Ti is 0.005% to 0.2%. P: at most 0.020%, S: at most 0.005%, N: at most 0.008%, The balance consists of Fe and unavoidable impurities, the total proportion of which is ≤0.8%. The steel substrate exhibits a microstructure, which, as determined by ISO 9042, comprises, in area % as follows: 65% to 92% of primary martensite, and At least 8% retained austenite (RA), The remaining parts are filled with the following: Up to 27% secondary martensite, Up to 10% bainite or bainitic ferrite, And / or at most ≤5% polygonal ferrite, wherein the total proportion of the secondary martensite, the bainite or bainitic ferrite and the polygonal ferrite is ≤27%. The density of V-based precipitates with a diameter of less than 10 nm in the microstructure of the steel substrate of the flat steel product is ≥1000 precipitates / μm. 2 .

2. The flat steel product according to claim 1, characterized in that, The primary martensite contained in the microstructure of the steel substrate of the flat steel product has a fine lath structure, and the maximum length of the lath is 1000 nm.

3. The flat steel product according to claim 2, characterized in that, The maximum length of the slats is 500 nm.

4. The flat steel product according to any one of the preceding claims, characterized in that, The carbon content in the steel substrate of the flat steel product is 0.22% to 0.3% by mass.

5. The flat steel product according to claim 1 or 2, Its features are, The Mn content in the steel substrate of the flat steel product is 1.9% to 2.8% by mass.

6. The flat steel product according to claim 1 or 2, characterized in that, The V content in the steel substrate of the flat steel product is 0.07% to 0.20% by mass.

7. The flat steel product according to claim 1 or 2, Its features are, The flat steel product is tested in accordance with DIN EN ISO 6892 in sample form 2, and has a yield strength of at least 1000 MPa, a tensile strength of at least 1300 MPa, and an elongation A80 of at least 10%.

8. The flat steel product according to claim 1 or 2, characterized in that, According to ISO 16630, the porosity of the flat steel product is at least 20%.

9. The flat steel product according to claim 1 or 2, Its features are, At least one surface of the flat steel product is provided with an anti-corrosion coating.

10. The flat steel product according to claim 9, characterized in that, The coating is applied by electroplating, hot-dip galvanizing, or galvanizing annealing.

11. A method for manufacturing a flat steel article according to any one of the preceding claims, comprising the following steps: a) Provide a steel melt, which comprises, by mass percent, the following: C： 0.2% to 0.4%, Mn: 1.5% to 3.0%, Si: 0.9% to 1.5%, Al: 0.005% to 1.0%, V: 0.01% to 0.3%, optional Cr: 0.01% to 1.00%, optional Mo: 0.005% to 0.20%, optional B: 0.00001% to 0.002%, optional Nb and Ti: the total content of Nb and Ti is 0.005% to 0.2%, P: up to 0.020%, S: up to 0.005%, N: up to 0.008%, and the balance being Fe and unavoidable impurities, the total proportion of which is ≤0.8%; b) Cast the molten steel into slabs; c) Reheat the slab to a reheating temperature of 1000°C to 1300°C; d) The reheated slab is hot-rolled into a hot-rolled strip, wherein the hot rolling is completed at a hot rolling finishing temperature of 850°C to 980°C; e) Cool the hot-rolled strip to a coiling temperature of 400°C to 600°C, the cooling being completed within a maximum of 25 seconds after the hot rolling is finished, and then coil the hot-rolled strip into a roll; f) Optionally, the hot-rolled strip is pickled; g) The hot-rolled strip is cold-rolled into a cold-rolled strip, with a cold rolling reduction rate of 20% to 80%. h) The cold-rolled strip is finally annealed in the following manner: - The cold-rolled strip is heated to a homogenization temperature TS at a heating rate ΘS of 2°C / s to 10°C / s, which is at least 50°C higher than the Ac3 temperature of the corresponding steel and at most 950°C, wherein the Ac3 temperature of the corresponding steel is determined using the expansion measurement method according to SEP1681-1998-06. -Then the cold-rolled strip is held at the homogenization temperature TS for a homogenization time of more than 40 seconds and less than 200 seconds; -The cold-rolled strip is then quenched at a quenching rate ΘQ of 20°C / s to 100°C / s to a quenching stop temperature TQ, which is lower than the martensite initiation temperature T_MS of the steel and at least equal to the temperature TQ_min, wherein at the temperature TQ_min, the microstructure of the cold-rolled strip contains 65% to 92% primary martensite in terms of area %, wherein the T_MS temperature of the steel is determined using the expansion measurement method according to SEP 1681-1998-06; and - Hold the annealed cold-rolled strip at the quenching stop temperature TQ for 4 to 20 seconds; i) The cold-rolled strip after final annealing undergoes an over-aging treatment, which includes the following steps: -Heating the cold-rolled strip to an over-aging temperature TP of 380°C to 460°C, - Hold the cold-rolled strip at the over-aging temperature for 50 to 200 seconds, and - The cold-rolled strip is cooled to below 100°C at a cooling rate of ΘC from 0.5°C / s to 20°C / s.

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