Galvanized steel sheet products with high malleability and ductility

Abstract

Disclosed is a steel plate product having a controlled composition and high ductility, which, in combination with a controlled heating cycle, has a desired microstructure and produces favorable mechanical properties, including an ultimate tensile strength of at least 1180 MPa, high ductility, hole expandability, bendability, and formability. The steel composition includes controlled amounts of carbon, manganese, silicon, chromium, molybdenum, and aluminum. The rolled plate is subjected to a thermal cycle, which includes, after the heating step, quenching to a temperature below martensite start and aging.

Description

[Technical Field]

[0001] <Cross-reference of related applications> This application claims the benefit of U.S. Provisional Application No. 62 / 883,704, filed on 7 August 2019, all of which are incorporated herein by reference.

[0002] <Field of Invention> The present invention relates to a zinc-coated steel sheet product having high ductility, and more specifically, to a steel sheet product having controlled amounts of Si, Cr, Mo, and Al alloy additives, wherein the additives are subjected to a quench and partition process to obtain a steel sheet product having desirable mechanical properties including high ultimate tensile strength, high drawability, and high hole-expandability. [Background technology]

[0003] <Background information> Quench-and-partition steels typically contain a high amount of silicon to suppress carbide precipitation, retain austenite, and achieve high strength and ductility. The amount of silicon added is typically at least 1.5% by weight. However, such silicon addition can lead to the formation of a grain boundary oxide layer in hot-rolled steel, which is difficult to remove during pickling. Furthermore, silicon addition is associated with embrittlement of the molten metal during welding of galvanized steel sheets, reducing the strength of the weld. [Overview of the project]

[0004] <Summary of the Invention> The present invention provides a steel sheet product having a controlled composition and high ductility, which, in combination with a controlled heating cycle, produces a desired microstructure and favorable mechanical properties. The mechanical properties include an ultimate tensile strength of at least 1180 MPa, high ductility, hole-expandability, bendability, and formability. The steel composition contains controlled amounts of carbon, manganese, silicon, and chromium. Molybdenum and aluminum may also be included in controlled amounts. The rolled sheet is subjected to a thermal cycle. The thermal cycle includes a heating stage followed by quenching to a temperature below the martensite onset temperature and aging.

[0005] One aspect of the present invention provides a quench-and-partition steel sheet product containing 0.12 to 0.5 wt% C, 1 to 3 wt% Mn, 0.4 to 1.1 wt% Si, 0.2 to 0.9 wt% Cr, 0.5 wt% or less Mo, and 1 wt% or less Al. The steel sheet product contains martensite, ferrite, and retained austenite and has an ultimate tensile strength of at least 1180 MPa, a total elongation of at least 13%, and a hole-expanding capacity of at least 25%.

[0006] Another aspect of the present invention provides a method for manufacturing the above-mentioned quench-and-partition steel sheet product by heating the steel sheet product to a soaking temperature of 720°C or higher, quenching the heated steel sheet product to a quench temperature lower than the martensite onset temperature, and aging the quenched steel sheet product at a temperature above the quench temperature.

[0007] A further aspect of the present invention provides a method for producing a quench-and-partition steel sheet product containing 0.12 to 0.5 wt% C, 1 to 3 wt% Mn, 0.4 to 1.1 wt% Si, 0.2 to 0.9 wt% Cr, 0.5 wt% or less Mo, and 1 wt% or less Al. The method involves heating the steel sheet product to a soaking temperature of at least 720°C, quenching the heated steel sheet product to a quench temperature lower than the martensite onset temperature, and aging the quenched steel sheet product at an aging temperature higher than the quench temperature to produce a quench-and-partition steel sheet product. The steel sheet product contains martensite, ferrite, and retained austenite, and has an ultimate tensile strength of at least 1180 MPa, a total elongation of at least 13%, and a hole-expanding capacity of at least 25%.

[0008] These and other aspects of the present invention will become clearer from the following description. [Brief explanation of the drawing]

[0009] [Figure 1] Figure l plots temperature versus time for an arbitrary first annealing process and a subsequent quench-and-partition thermal cycle, the thermal cycle including quenching and aging according to one embodiment of the present invention.

[0010] [Figure 2] Figure 2 is a micrograph of the steel sheet product subjected to the quench-and-partition process shown in Figure 1. [Modes for carrying out the invention]

[0011] <Detailed explanation> The highly ductile steel sheet products of the present invention, in combination with a controlled heating cycle, have a controlled composition that produces a desirable microstructure and desirable mechanical properties such as an ultimate tensile strength of at least 1180 MPa, high ductility, hole-expanding properties, bendability, and formability. The steel composition contains controlled amounts of carbon, manganese, silicon, chromium, and molybdenum, and may also contain aluminum, along with other suitable alloying additives known to those skilled in the art.

[0012] The steel composition of the present invention typically contains 0.12 to 0.5 wt% C, 1 to 3 wt% Mn, 0.4 to 1.1 wt% Si, 0.2 to 0.9 wt% Cr, and 0.5 wt% or less Mo. For example, the steel composition typically contains 0.15 to 0.4 wt% C, 2 to 2.8 wt% Mn, 0.5 to 1.0 wt% Si, 0.15 to 0.8 wt% Cr, and 0.1 or 0.15 to 0.4 wt% Mo. In certain embodiments, the steel composition may contain 0.2 to 0.25 wt% C, 2.1 to 2.5 wt% Mn, 0.6 to 0.9 wt% Si, 0.3 to 0.7 wt% Cr, and 0.2 to 0.3 wt% Mo. Aluminum may also be added to the steel composition in amounts up to 1% by weight, for example, 0.1-0.7% by weight, or 0.2-0.5% by weight.

[0013] We have found that a controlled combination of Mn, Si, Cr, Mo, and Al can yield highly ductile 1180 steel sheet products with excellent properties at relatively low Si content. Low Si content refers to less than 1.1% by weight, less than 1.0% by weight, less than 0.95% by weight, less than 0.90% by weight, less than 0.85% by weight, or 0.80% by weight. Low Si content results in excellent resistance to molten metal embrittlement during welding of zinc-clad sheets, making processing easier.

[0014] In the steel sheet products of the present invention, carbon (C) improves strength and promotes the formation of retained austenite. Manganese (Mn) contributes to hardening and has a solid solution strengthening effect. Si suppresses the precipitation of iron carbide during heat treatment and increases the amount of retained austenite. Cr, in combination with molybdenum (Mo), exhibits temper resistance and can suppress carbide precipitation, especially when used with Si, or when used with Si and Al. Al suppresses the precipitation of iron carbide during heat treatment and increases the amount of retained austenite. In addition, Ti and Nb can be optionally added as grain refiners to improve strength.

[0015] In addition to the above amounts of C, Mn, Si, Cr, Mo, and Al, the steel composition may contain other elements in trace amounts or as impurities, for example, Ti up to 0.05, Nb up to 0.05, S up to 0.015, P up to 0.03, Cu up to 0.2, Ni up to 0.2, Sn up to 0.1, N up to 0.015, V up to 0.1, and B up to 0.004. The term "substantially free" as used herein with respect to the composition of steel sheet products means that a particular element or material is not intentionally added to the composition but is present only as an impurity or in trace amounts.

[0016] The steel sheet product having the above composition is subjected to a quench-and-partition process, which will be explained in more detail below. The resulting steel sheet product was found to have good mechanical properties such as high elongation, desired ultimate tensile strength and yield strength, high bendability, and high hole expansion.

[0017] Steel sheet products can typically have high ductility, as measured by total elongation (TE) using the standard ASTM-L test, of at least 12%, for example, at least 13%, or at least 14%, or at least 15%. For example, steel sheet products can have a total elongation of 13 or 14% to 19% or higher.

[0018] The ultimate tensile strength (UTS) of the steel plate product is typically at least 1180 MPa, for example, 1180 - 1370 MPa. In some embodiments, the UTS may be less than 1370 MPa, or less than 1350 MPa, or less than 1320 MPa. The yield strength (YS) of the steel plate product is typically at least 700 MPa, for example, 700 - 1100 MPa.

[0019] The steel plate product can achieve a very high strength elongation balance (UTS·TE) of 15000 MPa, for example, greater than 17000 MPa%, or greater than 18000 MPa%, or greater than 20000 MPa%.

[0020] The steel plate product has high hole expansion properties, for example, at least 25%, or at least 30%, or at least 32%, or at least 34%.

[0021] The combination of UTS·TE·HE (MPa%) for the steel plate product , ,

[0023] is greater than 37.5x10 4 for example, greater than 42.5x10 4 or greater than 50x10 4 or greater than 54x10 4 or greater than 64x10 4 or greater than 68x10 4 or greater.

[0022] The steel plate product has high bendability (R / T), for example, at least 2R / T, or at least 2.5R / T.

[0023] In certain embodiments of the present invention, the final microstructure of the steel plate product mainly contains, for example, 50 to 80 volume% of martensite, with a small amount of ferrite, for example, 5 to 35 volume%, and a small amount of retained austenite, for example, 1 to 20 volume%. The retained austenite can typically contain more than 5 volume% or more than 8 volume%. In certain embodiments, the retained austenite may be 5 to 16 volume percent, or 8 to 15 volume percent, or 10 to 14 volume percent, or 11 to 12 volume. Bainite may be present in a small amount, for example, 0 to 5 volume%, or 10 volume%, or 15 volume%. The amounts of such phases can be determined by standard EBSD techniques.

[0024] The average crystal grain size of prior austenite can be 1 to 20 microns, for example, 5 to 10 microns. The average crystal grain size of ferrite can be 1 to 20 microns, for example, 3 to 5 microns. The average crystal grain size of retained austenite can be less than 2 microns, or less than 1 micron, or less than 0.5 microns. The retained austenite grains can be substantially equiaxed, and the average aspect ratio can be less than 3:1, or less than 2:1, or less than 1.9:1.

[0025] <Quenching and Partitioning Heat Cycle> In the Quenching and Partitioning heat cycle, after heating, it is quenched to a temperature lower than the martensite start temperature, and then direct aging treatment is carried out at the first quench temperature or a higher temperature. The precipitation of carbides is suppressed by appropriate alloying, and carbon is partitioned from the supersaturated martensite phase to the untransformed austenite phase, so the stability of the retained austenite is improved during subsequent cooling to room temperature. This treatment is called Quenching and Partitioning and is sometimes referred to as Q&P.

[0026] The first annealing or soaking stage can be performed at a relatively high annealing temperature, the second quenching or cooling stage is performed in which the temperature is reduced to a temperature lower than the martensite initiation temperature, and the third aging or holding stage is performed in which the steel sheet product is reheated to a relatively low holding temperature and held for a desired time. The temperature is controlled so as to promote the formation of the desired microstructure and mechanical properties in the final product.

[0027] After partial or complete austenitization during the soaking stage, the steel is quenched to a calculated temperature (QT) that produces a predetermined proportion of martensite and a balanced proportion of untransformed austenite. The steel is then heated to the partitioning temperature (PT), where carbon infiltrates the untransformed austenite, increasing its chemical stability. After partitioning and subsequent cooling to ambient temperature, the austenite remains. Because the untransformed austenite is enriched with carbon during partitioning, its effective Ms-Mf temperature range is suppressed. For chemical stabilization, Ms should be reduced to room temperature or lower.

[0028] In the first annealing stage, the temperature of the soaking zone can be between A1 and A3, for example, an annealing temperature of 720°C or higher can be used. In certain embodiments, the temperature of the soaking zone may typically be between 720 and 890°C, for example, in the range of 760 to 825°C. In certain embodiments, the peak annealing temperature may typically be 15 seconds or longer, for example, it may be held for 20 to 300 seconds or 30 to 150 seconds.

[0029] The temperature of the soaking zone can be achieved by heating the steel at a relatively low temperature lower than Ms, for example, room temperature, at an average rate of 0.5 to 50°C / second, for example, an average rate of about 2 to 20°C / second. In certain embodiments, the ramp-up may take 25 to 800 seconds, for example, 100 to 500 seconds. The first stage of heating in the second cycle can be carried out by any suitable heating system or process, such as radiation heating, induction heating, or direct-fired furnace heating.

[0030] The steel may be cooled to a controlled temperature higher than room temperature after reaching the temperature of the soaking zone and being held for a predetermined time. The steel is cooled to a temperature below the martensite onset temperature by water cooling, gas cooling, etc., and martensite is formed. Typical overall cooling rates are 5 to 200°C / second, and for example, 20 to 100°C / second or 30 to 80°C / second may be used. Quenching may reduce the temperature of the steel sheet product to a temperature typically of 150 to 350°C, for example, 220 to 300°C or 250 to 280°C. Cooling from the soaking temperature to the holding temperature can be adapted to any suitable type of cooling and quenching system, including the system described above.

[0031] In certain embodiments, multiple quenching speeds can be used, for example, a first relatively slow quenching speed followed by a second relatively fast quenching speed. For example, the first quenching speed may be 1 to 30°C / second until a first quenching temperature of 500 to 800°C is reached, and the second quenching speed may be 5 to 200°C / second until the final quenching temperature is reached. In certain embodiments, the first quenching speed may be 5 to 20°C / second until a first quenching temperature of 630 to 700°C is reached, and the second quenching speed may be 20 to 200°C / second until the final quenching temperature is reached.

[0032] After quenching, the steel is heated to a higher holding temperature for tempering and the partitioning process described above. In certain embodiments, the steel sheet product is maintained at a temperature higher than 300°C between the soaking and holding stages.

[0033] According to embodiments of the present invention, the aging or holding zone process is typically carried out at a temperature of 300 to 440°C, for example, at a temperature of 370 to 430°C. The holding zone can be held for up to 800 seconds, for example, 30 to 600 seconds. For example, aging can be carried out at a PT of 350 to 450°C for 30 to 300 seconds, or at 370 to 430°C for 60 to 180 seconds.

[0034] The temperature of the holding zone may be constant or may vary slightly within a selected temperature range. When hot-dip galvanizing steel, the steel may be reheated after holding to a temperature of, for example, about 470°C by a heating method such as induction heating, in order to charge the steel into the galvanizing pot at an appropriate temperature to obtain good results.

[0035] In certain embodiments, the temperature of the aging zone or holding zone can be maintained for a predetermined time, and then the temperature can be lowered to room temperature. Such a temperature drop (ramp-down) may typically take 10 to 1000 seconds, for example, about 20 to 500 seconds. The rate of this temperature drop is typically 1 to 1000°C / second, for example, in the range of 2 to 20°C / second.

[0036] In certain embodiments, the quench-and-partition steel sheet is hot-dip galvanized at the end of the retaining zone. The galvanizing temperature may typically be in the range of 440–480°C, for example, 450–470°C. Alternatively or additionally, Galvannealing The process is carried out at a typical temperature of 480°C to 530°C. So That's fine.

[0037] In certain embodiments, the zinc plating process can be carried out as part of the annealing process, which is the second step in a continuous galvanizing line (CGL). This CAL+CGL process is used to manufacture zinc-based or zinc alloy-based hot-dip galvanized products, or iron-zinc products. Galvanil type (iron-zinc galvanneal type) To manufacture coated products, the coating can be reheated after application. To improve the zinc coating properties, a nickel-based coating step can be optionally performed between the CAL and CGL steps in the process. By using a continuous zinc plating line in the second step, the production efficiency of coated products can be increased compared to using the CAL+CAL+EG route. Melting of zinc-plated products or zinc-based alloys. Covering The product can also be manufactured in a CGL (Continuously Integrated Laundry Facility) specially designed to allow for two annealing processes to be performed on a single line. In this case, Galvanic ring This may also be optional. Furthermore, it is possible to manufacture steel plate products by specially designing and constructing a single production facility and combining two heat treatment cycles.

[0038] <Initial thermal cycle> In certain embodiments of the present invention, two thermal cycling processes are used to produce highly ductile and high-strength steel products having the desirable mechanical properties described above. Multiple methods for heat treatment may be used in each of the first and second thermal cycles. An example of an annealing process for the first thermal cycle is described in U.S. Patent No. 10,385,419, which is incorporated herein by reference. A continuous annealing line (CAL) may be used in the first cycle, and a continuous galvanizing line (CGL) may be used in the subsequent second cycle.

[0039] The initial annealing process can be used, for example, to obtain a martensite microstructure. In one embodiment of the present invention, in the first annealing stage of the first thermal cycle, an annealing temperature above the A3 temperature can be used, for example, an annealing temperature of 820 °C or higher can be used. In certain embodiments, the annealing temperature in the first stage may typically be in the range of 830 - 980 °C, for example 830 - 940 °C, or 840 - 930 °C, or 860 - 925 °C. In certain embodiments, the peak annealing temperature can typically be held for 20 seconds or more, and the holding time can be, for example, 20 - 500 seconds, or 30 - 200 seconds. Heating can be carried out by conventional techniques such as non-oxidizing or oxidizing direct combustion furnaces (DFFs), oxygen-enriched DFI, induction heating, gas radiant tube heating, electric radiant heating, etc. Examples of heating systems suitable for use in the process of the present invention are described in U.S. Patent Nos. 5,798,007, 7,368,689, 8,425,225, 8,845,324, assigned to Fives Stein, U.S. Patent Application No. 2009 / 0158975, and International Publication No. WO2015 / 083047. Other examples of heating systems suitable for use in the process of the present invention include U.S. Patent No. 7,384,489 assigned to Drever International and U.S. Patent No. 9,096,918 assigned to Nippon Steel & Sumitomo Metal Corporation. In addition to these, known suitable heating systems and processes can be used in the first thermal cycle and the second thermal cycle.

[0040] In the first stage, after reaching the peak annealing temperature and being held for a predetermined time, as described in detail below, the steel is quenched to room temperature or to a controlled temperature higher than room temperature. The quench temperature does not necessarily have to be room temperature, but should be lower than the martensite transformation start temperature (M S ), and in order to form a microstructure mainly composed of martensite, preferably, the martensite transformation end temperature (M FThe temperature is lower than 300°C. In certain embodiments, the steel sheet product may be cooled to a temperature below 300°C, for example, below 200°C, between the first and second processes.

[0041] Quenching can be performed by conventional techniques such as water quenching, submerged knife / nozzle water quenching, gas cooling, rapid cooling using a combination of cold water, hot water, hot water, and gas, aqueous solution cooling, other liquid or gas fluid cooling, chilled roll quenching, water mist spraying, wet flash cooling, and non-oxidative wet flash cooling. Quenching rates can typically range from 30 to 2000°C / second.

[0042] Various types of cooling and quenching systems and processes known to those skilled in the art can be adapted for use in the processes of the present invention. Suitable cooling / quenching systems and processes conventionally used on a commercial basis include water quenching, water mist cooling, dry flash, wet flash, oxidative cooling, non-oxidative cooling, alkane fluid-to-gas phase transition cooling, and hot water quenching, and include two-stage water quenching, roll quenching, and jet cooling with a high percentage of hydrogen or helium gas. For example, oxidative and non-oxidative cooling / quenching of dry flash and / or wet flash disclosed in Fives Stein International Publication No. WO2015 / 083047 can be used. Other Fives Stein patent documents describing cooling / quenching systems and processes configured for use in the processes of the present invention include U.S. Patent Nos. 6,464,808B2, 6,547,898B2, 8,918,199B2, and U.S. Patent Publication Nos. US2009 / 0158975A1, 2009 / 0315228A1, and 2011 / 0266725A1. Other examples of cooling / quenching systems and processes configured for use in the processes of the present invention include U.S. Patent Nos. 8,359,894B2, 8,844,462B2, 7,384,489B2, and U.S. Patent Publication Nos. 2002 / 0017747A1 and 2014 / 0083572A1.

[0043] In certain embodiments, after reaching the first-stage peak annealing temperature and quenching the steel to generate martensite, the martensite can be optionally tempered to soften the steel somewhat, making further processing easier. Tempering is performed by raising the temperature of the steel from room temperature to approximately 500°C and holding it for 600 seconds. When tempering is used, the tempering temperature may be kept constant, or it may be varied within this preferred range.

[0044] After tempering, the temperature can be lowered to room temperature. The rate of this decrease is typically in the range of 1 to 40°C / second, for example, 2 to 20°C / second. Tempering is not necessary for single-pass furnaces.

[0045] In certain embodiments, one or both of the initial thermal cycling process and the quench-and-partition thermal cycling process may be carried out on a continuous annealing line (CAL). After the CAL+CAL process, the steel can be electro-galvanized to produce zinc-based coated products, and optionally, Galvanized It is also possible.

[0046] The following examples are intended to illustrate various aspects of the present invention and are not intended to limit the scope of the invention. [Examples]

[0047] Cold-rolled steel sheets having a composition containing 0.22 wt% C, 2.3 wt% Mn, 1.0 wt% Si, 0.5 wt% Cr, 0.25 wt% Mo, and 0.4 wt% Al were subjected to the two-cycle heating process shown in Figure 1. As shown in Figure 1, in the first cycle, the steel sheet was heated to 890°C to austenitize the steel, and then rapidly cooled. In the second quench-and-partition cycle, after reaching a peak temperature of 823°C, it was slowly cooled to 660°C, rapidly cooled to 230°C, overaged at 400°C, and coated with zinc from approximately 470°C. The processing parameters (°C) used were 930 RTS1, 660 SJC1, 30 RJC1, 800 RTS2, 660 SJC2, 231 RJC2, 400 OA1, 400 OA2, and 470 GI. The resulting steel product had a retained austenite (RA) content of 12.7%, and its mechanical properties were 41% HE, 10¹⁶ MPa YS, 12²⁰ MPa UTS, 12.3% UE, 16.6% TE, and 20²⁸⁵ MPa·% UTS·TE. The microstructure of the resulting product is shown in Figure 2. This structure consists mainly of tempered martensite, with 12.7% of retained austenite consisting of elongated interlath grains and small equiaxed grains. Small amounts of equiaxed ferrite may also be present. Additionally, small amounts of non-carbide bainite may be present. [Examples]

[0048] Cold-rolled steel sheets with the compositions shown in Table 1 were subjected to the quench-and-partition process shown in Table 2. The mechanical properties of the resulting steel sheets are shown in Table 3. [Table 1] [Table 2] [Table 3]

[0049] In several embodiments of the present invention, the amount of Si is reduced while adding a relatively small amount of Al. On the other hand, replacing a portion of the Si with a significant amount of aluminum reduces strength and also reduces the strength-ductility balance. The properties of a comparative steel with 0.24 wt% carbon, 2.4 wt% manganese, 0.6 wt% Si, and 0.8 wt% Al were 1018 MPa YS, 1100 MPa UTS, 8.6% UE, and 14.2% TE. The processing parameters (°C) used for this sample were 930 RTS1, 800 SJC1, 30 RJC1, 900 RTS2, 730 SJC2, 270 RJC2, 360 OA1, 3600 A2, 470 G, and 510 GA. With these annealing parameters, the desired ultimate tensile strength of 1180 MPa was not achieved.

[0050] With 1 wt% Si and a relatively small amount of Al, adding 0.4-0.8 wt% Cr results in a total elongation of 12-14%, but the strength may be at its lowest (see samples No. 4-6, 10-13, and 38-39). Adding aluminum may decrease the strength, but the total elongation increases somewhat. Table 4 compares Al-containing samples No. 12 and 13 with Al-free samples No. 3 and 9. [Table 4]

[0051] When Mn was increased (see samples No. 14-19), both strength and elongation increased. While the strength was relatively high at approximately 1300 MPa, the hole-expanding ability decreased. Furthermore, when aluminum was added, as shown in Table 5, the strength decreased and the elongation increased, but the hole-expanding ability remained low.

[0052] [Table 5]

[0053] When 0.25 wt% of Mo was added (see samples No. 20-25, 40-50), the strength increased, as did the elongation, and UTS·TE increased. As shown in sample No. 21 in Table 6, the strength increased to approximately 1300 MPa, which is almost the maximum value of the desired strength range. However, when Al was added along with Mo, as shown in sample No. 41 in Table 6, the total elongation and hole-expanding properties improved, but the strength remained similar to that of the case without Mo. [Table 6]

[0054] Furthermore, regarding the optimal combination of properties obtained by alloying Si, Cr, Al, and Mo, it was confirmed that reducing the Si content further improved pickling and welding behavior (see samples No. 45 and 50). This favorable property was maintained until the Si content decreased to 0.6-0.7 wt%, as shown in Table 7. Additionally, while the addition of Nb increased strength and ductility, it reduced hole expandability, and the hole expandability did not reach the desired range (see sample No. 51 in Table 7). [Table 7]

[0055] As used herein, terms such as “including,” “comprising,” and “containing” are open-ended and are understood not to exclude the existence of additional elements, materials, phases, or method steps not described in this application. As used herein, the term “consisting of” is understood to exclude the existence of any elements, materials, phases, or method steps not specified. As used herein, the term “consisting essentially of” is understood to include, where applicable, the specified elements, materials, phases, or method steps, and also to include any unspecified elements, materials, phases, or method steps that do not significantly affect the fundamental or novel features of the invention.

[0056] While the numerical ranges and parameters representing the broad scope of this invention are approximations, the numerical values ​​shown in specific examples are described as accurately as possible. However, every numerical value inherently contains some degree of error that inevitably arises from the standard deviation observed in each test measurement.

[0057] Furthermore, it should be understood that all numerical ranges described herein are intended to include all subranges contained within them. For example, the range "1" to "10" is intended to include subranges between a minimum value of 1 and a maximum value of 10, where the minimum value is 1 or greater than 1 and the maximum value is 10 or less than 10.

[0058] In this application, unless otherwise specified, the use of the singular form includes the plural form, and the use of the plural form includes the singular form. Also, in this application, unless otherwise specified, the use of "or" means "and / or," even when the words "and / or" are explicitly used in a particular embodiment. In this specification and claims, the articles "a," "an," and "the" include multiple referents unless they are explicitly and obviously limited to a single referent.

[0059] While specific embodiments of the present invention have been described above for illustrative purposes, it will be apparent to those skilled in the art that many modifications to the details of the present invention can be made without departing from the present invention.

Claims

1. Quench and partition steel sheet product comprising steel containing 0.12 to 0.5 wt% C, 1 to 3 wt% Mn, 0.4 to 1.1 wt% Si, 0.15 to 0.9 wt% Cr, 0.1 to 0.5 wt% Mo, and up to 1 wt% Al, optionally containing up to 0.05 wt% Ti, optionally up to 0.05 wt% Nb, optionally other elements, with the remainder being Fe and impurities, wherein the other elements are at least one element selected from the group consisting of up to 0.015 wt% S, up to 0.03 wt% P, up to 0.2 wt% Cu, up to 0.2 wt% Ni, up to 0.1 wt% Sn, up to 0.015 wt% N, up to 0.1 wt% V, and up to 0.004 wt% B, Quench-and-partition steel sheet products comprising martensite, ferrite, and retained austenite, having an ultimate tensile strength of at least 1180 MPa, a total elongation of at least 13%, and a hole-expanding property of at least 25%.

2. The quench and partition steel plate product according to claim 1, wherein the steel plate product comprises 0.15 to 0.4 wt% C, 2 to 2.8 wt% Mn, 0.5 to 1.0 wt% Si, 0.15 to 0.8 wt% Cr, 0.15 to 0.4 wt% Mo, and 0.1 to 0.7 wt% Al.

3. The quench-and-partition steel sheet product according to claim 1, wherein the steel sheet product comprises 0.2 to 0.25 wt% C, 2.1 to 2.5 wt% Mn, 0.6 to 0.9 wt% Si, 0.3 to 0.7 wt% Cr, 0.2 to 0.3 wt% Mo, and 0.2 to 0.5 wt% Al.

4. The quench and partition steel plate product according to claim 1, wherein Si contains less than 1.0% by weight.

5. The quench and partition steel plate product according to claim 1, wherein Si contains less than 0.95% by weight.

6. The quench and partition steel plate product according to claim 1, wherein Si contains 0.6 to 0.8% by weight.

7. The quench and partition steel plate product according to claim 6, wherein Al contains less than 0.5% by weight.

8. The quench and partition steel plate product according to claim 1, wherein Al contains less than 0.5% by weight.

9. The quench and partition steel plate product according to claim 1, wherein Mo contains 0.1 to 0.4% by weight.

10. The quench and partition steel plate product according to claim 1, wherein Mo contains 0.2 to 0.3% by weight.

11. The quench-and-partition steel sheet product according to claim 1, wherein the retained austenite contains 5 to 16 volume percent.

12. The quench-and-partition steel plate product according to claim 1, wherein the ultimate tensile strength is less than 1370 MPa.

13. The quench and partition steel plate product according to claim 1, wherein the total elongation is at least 14%.

14. The quench and partition steel plate product according to claim 1, wherein the steel plate product has a UTS-TE, which is a combination of ultimate tensile strength and total elongation, greater than 17,000 MPa%.

15. The quench-and-partition steel plate product according to claim 1, wherein the hole-expanding ability is at least 30%.

16. The quench and partition steel plate product according to claim 1, wherein the steel plate product has a UTS-TE-HE, which is a combination of ultimate tensile strength, total elongation, and hole expandability, greater than 37.5 × 10⁴ MPa².

17. The quench and partition steel plate product according to claim 16, wherein UTS-TE-HE is greater than 50 x 104 MPa%².

18. The quench and partition steel plate product according to claim 1, wherein the quench and partition steel plate product includes a galvanized coating.

19. The quench and partition steel sheet product according to claim 1, wherein the quench and partition steel sheet product includes a galvanyl coating.

20. A method for manufacturing the quench and partition steel plate product described in claim 1, Heating steel plate products to a uniform temperature of 720°C or higher, Quenching a heated steel plate product to a quench temperature lower than the martensite onset temperature, A method for producing the quenched and partitioned steel product according to claim 1, comprising aging the quenched steel product at the quench temperature or a temperature higher than said quench temperature.

21. A method for manufacturing quench and partition steel plate products, The steel plate product includes the steel described in claim 1, and the method is The steel plate product is heated to a uniform temperature of at least 720°C, Quenching a heated steel plate product to a quench temperature lower than the martensite onset temperature, This includes aging the quenched steel plate product at the quench temperature or a temperature higher than said quench temperature, The steel sheet product comprises martensite, ferrite, and retained austenite, and has an ultimate tensile strength of at least 1180 MPa, a total elongation of at least 13%, and a hole-expanding capacity of at least 25%, in a method.

22. The method according to claim 21, wherein the soaking temperature is 760 to 825°C, the quench temperature is 150 to 350°C, and the aging temperature is 330 to 440°C.

23. The method according to claim 21, further comprising zinc plating the quench-and-partition steel plate product at a temperature of 440 to 480°C.

24. The method according to claim 23, further comprising galvanizing a quench-and-partition steel sheet product at a temperature of 480 to 530°C.

25. The method according to claim 21, wherein the steel plate product comprises 0.15 to 0.4 wt% C, 2 to 2.8 wt% Mn, 0.5 to 1.0 wt% Si, 0.15 to 0.8 wt% Cr, 0.15 to 0.4 wt% Mo, and 0.1 to 0.7 wt% Al.

26. The method according to claim 21, wherein the steel plate product comprises 0.2 to 0.25 wt% C, 2.1 to 2.5 wt% Mn, 0.6 to 0.9 wt% Si, 0.3 to 0.7 wt% Cr, 0.2 to 0.3 wt% Mo, and 0.2 to 0.5 wt% Al.

27. The method according to claim 21, wherein the total elongation is at least 14%, the steel plate product has a UTS-TE, which is a combination of ultimate tensile strength and total elongation, greater than 17,000 MPa%, the hole expandability is at least 30%, and the steel plate product has a UTS-TE-HE, which is a combination of ultimate tensile strength, total elongation, and hole expandability, greater than 50 × 104 MPa%².

28. The method according to claim 21, further comprising, before the step of heating the steel plate product to a soaking temperature, annealing the steel plate product at an annealing temperature of 820°C or higher, and then quenching it to a temperature lower than the martensite onset temperature.

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