Double annealed steel sheet products with improved hole expansion ratios

Abstract

Steel sheet products are disclosed having controlled compositions that, in combination with controlled double annealing processes, produce desirable microstructures and favorable mechanical properties including high strengths and hole expansion ratios. The steel compositions may include carbon, manganese, chromium and molybdenum in controlled amounts, along with other alloying additions such as aluminum, silicon, niobium, boron and titanium.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 695,178 filed Sep. 16, 2024, which is incorporated herein by reference.FIELD OF THE INVENTION

[0002] The present invention relates to double annealed steel sheet products with improved hole expansion properties.BACKGROUND INFORMATION

[0003] Over the past several years, advanced high strength steel (AHSS) has been developed for automotive applications. These steels have favorable balances of tensile strength and elongation. However, improvements in manufacturability and mechanical properties such as hole expansion ratios are desired.SUMMARY OF THE INVENTION

[0004] The present invention provides steel sheet products having controlled compositions that, in combination with controlled double annealing processes, produce desirable microstructures and favorable mechanical properties including high strengths and hole expansion ratios. The steel compositions may include carbon, manganese, chromium and molybdenum in controlled amounts, along with other alloying additions such as aluminum, silicon, niobium, boron and titanium.

[0005] An aspect of the present invention is to provide a method of producing a rolled steel sheet product comprising from 0.02 to 0.3 weight percent C, from 1.5 to 2.7 weight percent Mn, from 0.01 to 1.5 weight percent Mo, and up to 0.5 weight percent Cr. The method comprises subjecting the steel sheet product to a first step annealing process at a temperature of from 800 to 980° C., and subjecting the steel sheet product to a second annealing process comprising soaking the sheet product at a temperature of from 750 to 900° C., followed by holding the sheet product a temperature of from 350 to 550° C., wherein the steel sheet product comprises ferrite, bainite, martensite, or a combination thereof, and has a hole expansion ratio of at least 30 percent.

[0006] Another aspect of the present invention is to provide a rolled steel sheet product made by the method described above.

[0007] A further aspect of the present invention is to provide a rolled double annealed steel sheet product comprising from 0.02 to 0.3 weight percent C, from 1.5 to 2.7 weight percent Mn, from 0.01 to 1.5 weight percent Mo, and up to 0.5 weight percent Cr, wherein the steel sheet product comprises ferrite, bainite, martensite, or a combination thereof, and has a hole expansion ratio of at least 30 percent.

[0008] These and other aspects of the present invention will be more apparent from the following description.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a partially schematic temperature versus time graph illustrating a double anneal cycle of the present invention.

[0010] FIG. 2 is a partially schematic temperature versus time graph illustrating a comparative single anneal cycle.

[0011] FIGS. 3-8 are HER graphs showing hole expansion ratios for double annealed steel products of the present invention in comparison with single annealed steel products.

[0012] FIGS. 9A and 9B are micrographs showing a double annealed steel sheet product in comparison with a single-annealed steel sheet product.

[0013] FIGS. 10A and 10B are micrographs showing a double annealed steel sheet product in comparison with a single annealed steel sheet product.

[0014] FIGS. 11 and 12 are temperature versus time graphs illustrating double anneal cycles of the present invention in comparison with single anneal cycles.

[0015] FIGS. 13 and 14 are micrographs showing microstructures of double annealed and single annealed steel products.

[0016] FIGS. 15 and 16 are micrographs showing microstructures of double annealed and single annealed steel products.

[0017] FIG. 17 is a temperature versus time graph illustrating a double anneal cycle of the present invention in comparison with a single anneal cycle.

[0018] FIGS. 18 and 19 are micrographs showing microstructures of double annealed and single annealed steel products.DETAILED DESCRIPTION

[0019] Steel sheet products of the present invention have controlled compositions that, in combination with controlled double annealing processes, produce desirable microstructures and favorable mechanical properties including high strengths and hole expansion ratios.

[0020] The steel compositions may include carbon, manganese, chromium and molybdenum in controlled amounts, along with other suitable alloying additions known to those skilled in the art. These alloying elements, among others, provide hardenability that allow for the production of dual phase and complex phase steels containing ferrite, martensite and / or bainite through continuous annealing or continuous galvanizing facilities. Examples of steel compositions including ranges of C, Mn, Cr, Mo, Al, Nb, B and Ti are listed in Table 1 below.TABLE 1Steel Compositions (weight %)ExampleCMnCrMoAlNbBTiA0.02-1.5-0-0-0-0-0-0-0.32.70.50.50.50.070.0040.06B0.05-1.8-0.01-0.01-0.01-0.01-0.001-0.01-0.22.60.40.30.20.060.00350.05C0.09-2.0-0.1-0.05-0.03-0.03-0.0014-0.015-0.182.50.30.250.070.050.00310.04D0.13-2.1-0.2-0.08-0.03-0.03-0.0014-0.015-0.172.50.30.210.060.050.00310.038E0.09-2.0-0.14-0.14-0.03-0.03-0.0014-0.02-0.112.30.260.20.070.040.00290.04

[0021] In addition to the amounts of C, Mn, Cr, Mo, Al, Nb, B and Ti listed in Table 1, the steel compositions may include minor or impurity amounts of other elements. For example, Si may comprise less than 0.4 weight percent, or less than 0.3 weight percent, or less than 0.2 weight percent, or less than 0.11 weight percent. Aluminum may comprise less than 0.5 or 0.4 weight percent, or less than 0.2 weight percent, or less than 0.1 weight percent. The total combined amount of Si and Al may be less than 0.5 weight percent, for example, less than 0.4 weight percent, or less than 0.3 weight percent, or less than 0.2 weight percent, or less than 0.1 weight percent. The alloys may contain 0.007 max S, 0.02 max P, 0.2 max Cu, 0.02 max Ni, 0.04 max Sn, 0.009 max N, and 0.01 max V (all values in weight percent). As used herein, the term “substantially free,” when referring to the composition of the steel sheet product, means that a particular element or material is not purposefully added to the composition, and is only present as an impurity or in trace amounts.

[0022] Steel sheet products having the compositions described above are subjected to a two-stage annealing process, as more fully described below. The resultant sheet products have been found to possess favorable mechanical properties including desirable ultimate tensile strengths, high hole expansion ratios, high clongations, high bendability and desirable yield ratios (YS / UTS).

[0023] In certain embodiments, the ultimate tensile strength (UTS) of the steel sheet products ranges from 590 to 1,200 MPa or more. In certain embodiments, the steel sheet product has an ultimate tensile strength of greater than 780 MPa, or greater than 980 MPa. For example, UTS may be in the 980 MPa range, or the 1180 MPa range. The steel sheet products may possess substantially isotropic UTS properties in the longitudinal and transverse orientations.

[0024] The steel sheet products may possess hole expansion ratios HER as measured by a standard hole expansion test typically greater than 30 percent, for example, greater than 35 percent, or greater than 40 percent, or greater than 50 percent, or greater than 60 percent, or greater than 70 percent, or greater than 80 percent. HER properties may be up to 110 percent, 120 percent, or more. For example, HER properties may be from 30 to 120 percent, or from 35 to 110 percent. HER is an important property in the global automotive industry as high HER values promote good edge cracking resistance during the stamping process and high stable energy absorption during a component crash event.

[0025] In certain embodiments, the steel sheet products have a total elongation (TE) typically greater than 8 percent, for example, greater than 10 percent, or greater than 15 percent, or higher.

[0026] In certain embodiments, increased values of both total elongation (TE) and HER (HER·TE) result in steel sheet products exhibiting good combinations of global formability and local formability.

[0027] The steel sheet products may possess favorable values of HER / (YS / UTS).

[0028] In certain embodiments, R / t bendability performance may be favorable, as measured by the standard 90° bend test with dies of different corner radii.

[0029] In accordance with certain embodiments of the invention, the final microstructure of the steel sheet products may primarily comprise bainite, ferrite or fresh martensite. The amounts of bainite, ferrite, martensite and other phases may be determined by standard EBSD techniques.

[0030] When bainite is the primary phase, it may comprise greater than 50 volume percent, for example, from 60 to 95 volume percent, or from 70 to 90 volume percent. Secondary phase(s) may include martensite in an amount of from 1 to 30 volume percent, or from 2 to 25 volume percent and / or ferrite in an amount of from 1 to 30 volume percent, or from 2 to 25 volume percent.

[0031] When ferrite is the primary phase, it may comprise greater than 50 volume percent, for example, from 60 to 95 volume percent, or from 70 to 90 volume percent. In certain embodiments, when ferrite is the primary phase, fresh martensite may be a secondary phase, e.g., in an amount up to 30 volume percent or more, or from 1 to 30 volume percent, or from 2 to 25 volume percent. The martensite microstructure may exhibit reduced amounts of banding, which may result in improved hole expansion performance.

[0032] When fresh martensite is the primary phase, it may comprise greater than 50 volume percent, for example, from 60 to 95 volume percent, or from 70 to 90 volume percent. A secondary phase may include bainite in an amount of from 1 to 45 volume percent, or from 2 to 40 volume percent. The martensite microstructure may exhibit reduced amounts of banding, which may result in improved hole expansion performance.

[0033] In certain embodiments, the final microstructure may include tempered martensite, for example, at least 1 volume percent, or at least 5 volume percent, or at least 10 volume percent. Alternatively, the microstructure may comprise less than 1 volume percent tempered martensite, or may be substantially free of tempered martensite.

[0034] The microstructures may comprise less than 5 volume percent, or less than 2 volume percent, or less than 1 volume percent retained austenite. The microstructure may be substantially free of retained austenite.

[0035] The processing described herein may apply double annealing to produce complex phase (CP) and dual phase (DP) types of steel grades in order to improve the hole expansion ratio HER properties versus conventional single anneal variants. Double annealing line trials have shown extraordinary HER, for example, with materials showing HER from 40 to 110 percent. Double annealing has also shown considerable improvements in the ability for the annealing process line to control strip shape. Microstructure for this material depends largely on the traditional steel design it is applied to, though notably the double annealing process for the compositions of interest does not considerably contain a substantial amount of retained austenite.

[0036] As described below, a two-step annealing process may be used to produce steel products with favorable mechanical properties, such as those described above. Within each of the first and second annealing steps, multiple methodologies for undertaking the heat treatment may be used.

[0037] An example of a two-step annealing process is shown in FIG. 1 and described below. Exemplary production paths include a continuous annealing line (CAL) followed by a continuous annealing line (CAL) production route, or CAL plus a continuous galvanizing line (CGL) production route. A specially designed line may allow for both CAL+CAL or CAL+CGL steps to take place in a single facility. A direct-fired furnace (DFF) followed by a radiant tube (RT) furnace may be used. However, other embodiments such as all radiant tube, electric radiant heating, and the like may be used to achieve the desired thermal cycles.Step 1

[0038] The goal of the first step of the annealing process is to achieve a predominantly martensitic microstructure. In the first annealing stage of the first step (Soak 1 in FIG. 1), an annealing temperature above the A3 temperature may typically be used, for example, an annealing temperature of at least 830° C. may be used. In certain embodiments, the first stage annealing temperature may typically range from 830 to 980° C., for example, from 840 to 960° C., or from 850 to 950° C., or from 860 to 940° C., or from 880 to 920° C. In certain embodiments, the peak annealing temperature may be typically held for at least 20 seconds, for example, from 20 to 500 seconds, or from 30 to 200 seconds. Heating may be accomplished by conventional techniques such as a non-oxidizing or oxidizing direct-fired furnace (DFF), oxygen-enriched DFI, induction, gas radiant tube heating, electric radiant heating, and the like. Examples of heating systems that may be adapted for use in the processes of the present invention are disclosed in U.S. Pat. Nos. 5,798,007; 7,368,689; 8,425,225; and 8,845,324, U.S. Patent Application No. 2009 / 0158975, and Published PCT Application No. WO / 2015083047, assigned to Fives Stein. Additional examples of heating systems that may be adapted for use in the processes of the present invention include U.S. Pat. No. 7,384,489 assigned to Drever International, and U.S. Pat. No. 9,096,918 assigned to Nippon Steel and Sumitomo Metal Corporation. Any other suitable known types of heating systems and processes may be adapted for use in Step 1 and Step 2.

[0039] In the first stage, after the peak annealing temperature is reached and held for the desired period of time, the steel is quenched to room temperature, or to a controlled temperature above room temperature, as more fully described below. The quench temperature may not necessarily be room temperature but may be below the martensite start temperature (Ms), and may be below the martensite finish temperature (MF), to form a microstructure of predominantly martensite. In certain embodiments, between the first step process and the second step process, the steel sheet product may be cooled to a temperature below 300° C., for example, below 200° C., or below 150° C.

[0040] Quenching may be accomplished by conventional techniques such as water quenching, submerged knife / nozzle water quenching, gas cooling, rapid cooling using a combination of cold, warm or hot water and gas, water solution cooling, other liquid or gas fluid cooling, chilled roll quench, water mist spray, wet flash cooling, non-oxidizing wet flash cooling, and the like. A quench rate of 30 to 2,000° C. / sec may typically be used.

[0041] Various types of cooling and quenching systems and processes known to those skilled in the art may be adapted for use in the processes of the present invention. Suitable cooling / quenching systems and processes conventionally used on a commercial basis may include water quench, water mist cooling, dry flash and wet flash, oxidizing and non-oxidizing cooling, alkane fluid to gas phase change cooling, hot water quenching, including two-step water quenching, roll quenching, high percentage hydrogen or helium gas jet cooling, and the like. For example, dry flash and / or wet flash oxidizing and non-oxidizing cooling / quenching such as disclosed in published PCT Application No. WO2015 / 083047 to Fives Stein may be used. Other Fives Stein patent documents describing cooling / quenching systems and processes that may be adapted for use in the processes of the present invention include U.S. Pat. Nos. 6,464,808B2; 6,547,898B2; and 8,918,199B2, and U.S. Patent Application Publication Nos. US2009 / 0158975A1; US2009 / 0315228A1; and US2011 / 0266725A1. Other examples of cooling / quenching systems and processes that may be adapted for use in the processes of the present invention include those disclosed in U.S. Pat. Nos. 8,359,894B2; 8,844,462B2; and 7,384,489B2, and U.S. Patent Application Publication Nos. 2002 / 0017747A1 and 2014 / 0083572A1.

[0042] In certain embodiments, after the first-stage peak annealing temperature is reached and the steel is quenched to form martensite, the martensite can be optionally tempered to soften the steel somewhat to make further processing more feasible. Tempering may take place by raising the temperature of the steel in the range of from room temperature up to 500° C., or up to 450° C., or up to 400° C., or up to 500° C., and holding for up to 600 seconds. Tempering may take place at a temperature of at least 150° C., or at least 200° C. If tempering is utilized, the tempering temperature may be held constant, or may be varied within this preferred range. After tempering, the temperature may be ramped down to room temperature. The rate of such ramp-down may typically range from 1 to 40° C. / sec, for example, from 2 to 20° C. / sec. In the case of a single pass facility furnace, tempering may not be necessary.Step 2

[0043] The second step of the annealing process may include a first stage that is conducted at relatively high annealing temperature (Soak 2 in FIG. 1), and a second stage that is conducted at relatively low temperature (Hold in FIG. 1). The temperatures are controlled in order to promote the formation of the desired microstructure in the final product.

[0044] In the first annealing stage of the second step, a soaking zone temperature above A1, for example, in an intercritical range between A1 and A3, or a supercritical range above A3, may be used, for example, an annealing temperature of at least 800° C. may be used. In certain embodiments, the soaking zone temperature may typically range from 820 to 900° C., for example, from 840 to 880° C. In certain embodiments, the peak annealing temperature may be typically held for at least 15 seconds, for example, from 20 to 300 seconds, or from 30 to 150 seconds.

[0045] During the first stage of the second step, the soaking zone temperature may be achieved by heating the steel from a relatively low temperature below Ms, e.g., room temperature, at an average rate of from 0.5 to 50° C. / sec, for example, from about 2 to 20° C. / sec. In certain embodiments, the ramp-up may take from 25 to 800 seconds, for example, from 100 to 500 seconds. The first stage heating of the second step may be accomplished by any suitable heating system or process, such as using radiant heating, induction heating, direct fired furnace heating and the like.

[0046] After the soaking zone temperature is reached and held for the desired period of time, the steel may be cooled to a controlled temperature above room temperature to the holding zone. Cooling from the soaking to holding zone may be accomplished by conventional techniques such as water cooling, gas cooling and the like. An average cooling rate of 5 to 400° C. / sec may typically be used. Any suitable types of cooling and quenching systems may be adapted for use in cooling from the soaking temperature to the holding temperature, including those described above.

[0047] In accordance with embodiments of the invention, the holding zone step is carried out at a typical temperature of from 350 to 550° C., for example, 370 to 520° C., or from 390 to 500° C. The holding zone may be held for up to 800 seconds, for example, from 20 to 600 seconds.

[0048] The holding zone temperature may be held constant or may be varied somewhat within the preferred temperature range. After holding, the steel may be reheated, such as by induction or other heating method, to enter a hot-dip coating pot at the proper temperature for good coating results, if the steel is to be hot-dip coated.

[0049] In certain embodiments, after the holding zone temperature has been maintained for a desired period of time, the temperature may be ramped down to room temperature. Such a ramp-down may typically take from 10 to 1,000 seconds, for example, from about 20 to 500 seconds. The rate of such ramp-down may typically range from 1 to 1,000° C. / sec, for example, from 2 to 20° C. / sec.

[0050] In accordance with certain embodiments, one or both of the first-step and second-step annealing processes may be performed on a continuous annealing line (CAL). After going through a CAL+CAL process, the steel may be electrogalvanized to produce a zinc based coated product.

[0051] In certain embodiments, the annealed steel sheet is hot-dip galvanized at the end of the holding zone. Galvanizing temperatures may typically range from 440 to 480° C., for example, from 450 to 470° C. In certain embodiments, the galvanizing step may be performed as part of the second-step annealing process on a continuous galvanizing line (CGL). This CAL+CGL process can be used to produce both a zinc-based or zinc alloy-based hot-dip galvanized product or reheated after coating to produce an iron-zinc galvanneal type coated product. An optional nickel-based coating step can take place between the CAL and CGL steps in the process to improve zinc coating properties. The use of a continuous galvanizing line in the second step increases the production efficiency of producing a coated product versus using a CAL+CAL+EG route.

[0052] A galvanized product or zinc-based alloy hot-dip coated product can also be made on a specially designed CGL in which the two-step annealing can take place in a single line. Galvannealing can also be an option in this case. Thus, the Step 1 and Step 2 heat treatment may be performed in a single pass on an appropriately configured galvanize line. Furthermore, a single production facility can also be specially designed and built to combine the two-step thermal process to produce uncoated steels.

[0053] The following examples are intended to illustrate various aspects of the present invention and are not intended to limit the scope of the invention.Example 1

[0054] Cold rolled steel sheets having compositions as listed in Table 2 are subjected to two-step annealing processes as schematically illustrated in FIG. 1, with details of the soaking and holding temperatures indicated in Table 3. The same steels were comparatively heat treated in a single anneal manner, i.e., without the pre-martensitic step, as shown schematically in FIG. 2, also with details listed in Table 3. Mechanical properties are also listed in Tables 3. The double annealed materials have combinations of high UTS, favorable YS, adequate TE, and high hole expansion ratio HER. Box and whisker plots of hole expansion ratios (HER) comparing double and single annealing are shown in FIGS. 3 through 8 for the Steels A, B, C, D, E, and F annealed under production conditions 1 through 53 in Table 3. The enhancement in hole expansion properties with double annealing is evident. Representative microstructures comparing double annealed versus single annealed products for composition B (production conditions 22 and 24) are shown in FIGS. 9A and 9B, and F (production conditions 48 and 52) are shown in FIGS. 10A and 10B. With the martensite appearing white in these LePera etched optical images, it is clear is that the degree of martensite banding is significant in the single annealed samples whereas it is eliminated or greatly minimized in the double annealed variant. As martensite bands are known to be detrimental to hole expansion properties, the more homogenous microstructure in the double annealed variants results in enhanced HER.TABLE 2SampleComposition (wt %)IDCMnCrMoAlNbBTiA0.0682.120.0370.0050.0460.0370.00010.021B0.0862.090.250.290.0350.0010.00010.001C0.1051.840.050.100.0420.0210.00180.022D0.1182.440.270.080.0340.0020.00010.056E0.1582.250.240.160.0570.0390.00250.023F0.1532.310.260.150.0480.0230.00190.023G0.1022.140.200.200.0450.0350.00220.03H0.1082.230.260.200.0340.00250.043TABLE 3SoakSoakHoldHoldYield cleTemp 1Temp 2TemptimeStressUTSTEHER ype(C.)(C.)(C.)(seconds)(MPa)(MPa)(%)(%) uble8998294713039759130.675 uble8998714713038258732.783 ngleNone8294713045762430.466 ngleNone8714713042261329.959 uble89982947112039759131.387 uble89987147112038958933.068 uble89982949312044856132.492 ngleNone82947112046862428.956 ngleNone87147112041861629.872 uble89982936012037059330.889 ngleNone82936012046260027.258 uble8998294713061186312.670 uble8998714713061395312.266 ngleNone8294713059389914.239 ngleNone8714713070999010.958 uble89982947112064790512.749 uble89987147112069594711.279 uble89982949312052280520.456 uble89987149312059084811.745 ngleNone82947112057590115.444 ngleNone871471120719100611.265 uble8998294713046575822.852 uble89987147130468745Bad55break ngleNone8294713046277421.440SoakSoakHoldHoldYieldGradeCycleTemp 1Temp 2TemptimeStressUTSTEHERCodeType(C.)(C.)(C.)(seconds)(MPa)(MPa)(%)(%)DDouble8998294713052592219.025DDouble8998714713052790618.928DSingleNone8294713055198515.919DSingleNone8714713055196116.521DDouble89982947112051493021.624DDouble89987147112052489814.825DDouble89982949312055194716.125DDouble89987149312055692910.525DSingleNone82947112055298317.420DSingleNone87147112054695316.621EDouble8998294713099513669.227EDouble8998714713099913538.144ESingleNone82947130100314029.332ESingleNone87147130103014018.741EDouble899829471120100313609.563EDouble899871471120101313609.366EDouble89982949312097513559.249EDouble89987149312099513599.759ESingleNone829471120101114158.949ESingleNone871471120997138810.244FDouble89982941612095112879.741FDouble89987141612098613259.140FDouble899829360120103012438.575SoakHoldHoldYieldTemp 2TemptimeStressUTSTEHER(C.)(C.)(seconds)(MPa)(MPa)(%)(%)87136012098512119.879829416120875123210.33087141612096213219.83282936012085912209.73387136012094513068.936 indicates data missing or illegible when filedExample 2Cold rolled steel sheets having the composition of Steel G as listed in Table 2 were subjected to a two-step annealing process, and a comparative single-step annealing process, as illustrated in FIGS. 11 and 12. FIG. 11 corresponds to a 980CP version, and FIG. 12 corresponds to a 980DP version. Microstructures of the resultant 980CP double-annealed and single-annealed products are shown in FIGS. 13 and 14, respectively. Mechanical properties are as follows: 980CP double annealing processing microstructure using Steel G grade material yields OYS=766 Mpa, UTS=1045 Mpa, TE=9.2% and HER=64%; 980CP single annealing processing microstructure using Steel G material yields OYS=814 Mpa, UTS=1064 Mpa, TE=7.9% and HER=37%. Microstructures of the double-annealed and single annealed Steel G DP products are shown in FIGS. 15 and 16, respectively. Mechanical properties are as follows: 980DP double annealing processing microstructure using Steel G material yields OYS=654 Mpa, UTS=1025 Mpa, TE=10.8% and HER=39%; 980DP single annealing processing microstructure using Steel G material yields OYS=728 Mpa, UTS=1115 Mpa, TE=14.1% and HER=25%. The results show a substantial improvement of HER when going from single to double annealing for both 980 variants.Example 3

[0056] A 980CP sheet product having the Steel H chemistry shown in Table 2 was subjected to a mill production trial under the double annealing processing conditions shown in Table 4. The results of tensile tests for the production material are shown in Table 4 from the center location of the coil at 4 separate locations: Location No. 1-head; Location No. 2-Body 1; Location No. 3-Body 2; and Location No. 4-tail. Each longitudinal measurement thus has a corresponding transverse measurement. The comparative YS and UTS measurements demonstrate isotropic strength properties. This mill trial double-annealed Steel H material has a combination of high YS and UTS, modest TE, excellent HER at above 75%, and isotropic YS and UTS in longitudinal and transverse orientations.TABLE 4 ldHoldYield mptimeTensileTensileStressUTSTEHER )(seconds)LocationOrientation(MPa)(MPa)(%)(%) 5135Location 1Longitudinal89810099.483 5135Location 1Transverse88710119.783 5135Location 2Longitudinal90110079.689 5135Location 2Transverse90410069.189 5135Location 3Longitudinal90010069.376 5135Location 3Transverse91510159.076 5135Location 4Longitudinal886100510.285 5135Location 4Transverse88810109.585 indicates data missing or illegible when filedExample 4

[0057] Cold rolled steel sheet having the composition of Steel G were subjected to a two-step annealing process, and a comparative single-step annealing process, as illustrated in FIG. 17. FIGS. 18 and 19 show microstructures of the double annealed and single annealed 980MP Steel G material, respectively. Mechanical properties are as follows: 980MP double annealing processing microstructure using Steel G grade material yields OYS=718 Mpa, UTS=1065 Mpa, TE=11.5% and HER=58%; 980MP single annealing processing microstructure using Steel G material yields OYS=803 Mpa, UTS=1136 Mpa, TE=9.0% and HER=43%. Using double annealing processing conditions resulted in a significant improvement in HER properties, while maintaining sufficient OYS, UTS and TE properties. Bands of martensite are shown in the single-anneal material of FIG. 19, but are absent in the double-anneal material of FIG. 18.

[0058] As used herein, “including,”“containing” and like terms are understood in the context of this application to be synonymous with “comprising” and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases or method steps. As used herein, “consisting of” is understood in the context of this application to exclude the presence of any unspecified element, material, phase or method step. As used herein, “consisting essentially of” is understood in the context of this application to include the specified elements, materials, phases, or method steps, where applicable, and to also include any unspecified elements, materials, phases, or method steps that do not materially affect the basic or novel characteristics of the invention.

[0059] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0060] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0061] In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and / or” unless specifically stated otherwise, even though “and / or” may be explicitly used in certain instances. In this application and the appended claims, the articles “a,”“an,” and “the” include plural referents unless expressly and unequivocally limited to one referent.

[0062] Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

Claims

1. A method of producing a rolled steel sheet product comprising from 0.02 to 0.3 weight percent C, from 1.5 to 2.7 weight percent Mn, from 0.01 to 1.5 weight percent Mo, and up to 0.5 weight percent Cr, the method comprising:subjecting the steel sheet product to a first step annealing process at a temperature of from 800 to 980° C.; andsubjecting the steel sheet product to a second annealing process comprising soaking the sheet product at a temperature of from 750 to 900° C., followed by holding the sheet product a temperature of from 350 to 550° C., wherein the steel sheet product comprises ferrite, bainite, martensite, or a combination thereof, and has a hole expansion ratio of at least 30 percent.

2. The method of claim 1, wherein the first annealing process is performed at a temperature above 820° C.

3. The method of claim 1, wherein the first annealing process is performed at a temperature of from 830 to 940° C.

4. The method of claim 1, wherein the soaking step of the second annealing process is performed at a temperature of from 780 to 880° C., and the holding step of the second annealing process is performed at a temperature of from 370 to 520° C.

5. The method of claim 1, wherein the soaking step of the second annealing process is performed at a temperature of from 790 to 860° C., and the holding step of the second annealing process is performed at a temperature of from 370 to 520° C.

6. The method of claim 1, wherein the steel sheet product is cooled to a temperature below 300° C. between the first annealing process and the second annealing process.

7. The method of claim 1, wherein the Cr is at least 0.01 weight percent.

8. The method of claim 1, wherein the C is from 0.09 to 0.17 weight percent, the Mn is from 2.0 to 2.5 weight percent, the Cr is from 0.15 to 0.3 weight percent, and the Mo is from 0.08 to 0.25 weight percent.

9. The method of claim 1, wherein the sheet product comprises less than 0.4 weight percent Si and less than 0.4 weight percent Al.

10. The method of claim 9, wherein the total combined amount of Si and Al is less than 0.5 weight percent.

11. The method of claim 1, wherein the first annealing process is performed on a continuous annealing line, and the second annealing process is performed on a continuous annealing line.

12. The method of claim 11, wherein the same continuous annealing line is used for both the first annealing process and the second annealing process.

13. The method of claim 11, wherein separate continuous annealing lines are used for the first annealing process and the second annealing process.

14. The method of claim 1, wherein the first annealing process is performed on a continuous annealing line, and the second annealing process is performed on a continuous galvanizing line.

15. The method of claim 1, wherein the first annealing process and the second annealing process are performed in a single pass on a galvanizing line.

16. The method of claim 1, further comprising electrolytically coating the rolled steel sheet product with a zinc-based coating.

17. The method of claim 1, wherein the rolled steel sheet product is hot rolled prior to the first annealing process.

18. The method of claim 1, wherein the rolled steel sheet product is cold rolled prior to the first annealing process.

19. The method of claim 1, wherein the steel sheet product comprises less than 5 volume percent austenite.

20. The method of claim 1, wherein the steel sheet product comprises less than 2 volume percent austenite.

21. The method of claim 1, wherein the steel sheet product is substantially free of austenite.

22. The method of claim 1, wherein the steel sheet product comprises at least 50 volume percent bainite.

23. The method of claim 22, wherein the bainite comprises from 60 to 95 volume percent.

24. The method of claim 22, wherein the steel sheet product further comprises from 1 to 30 volume percent martensite, from 1 to 30 volume percent ferrite, or a combination thereof.

25. The method of claim 1, wherein the steel sheet product comprises at least 50 volume percent ferrite.

26. The method of claim 25, wherein the ferrite comprises from 60 to 95 volume percent.

27. The method of claim 25, wherein the steel sheet product further comprises from 1 to 30 volume percent fresh martensite.

28. The method of claim 1, wherein the steel sheet product comprises at least 50 volume percent fresh martensite.

29. The method of claim 28, wherein the fresh martensite comprises from 60 to 95 volume percent.

30. The method of claim 28, wherein the steel sheet product further comprises from 1 to 45 volume percent bainite.

31. A rolled steel sheet product made by the method of claim 1.

32. A rolled double annealed steel sheet product comprising from 0.02 to 0.3 weight percent C, from 1.5 to 2.7 weight percent Mn, from 0.01 to 1.5 weight percent Mo, and up to 0.5 weight percent Cr, wherein the steel sheet product comprises ferrite, bainite, martensite, or a combination thereof, and has a hole expansion ratio of at least 30 percent.

33. The rolled double annealed steel sheet product of claim 32, wherein the Cr is at least 0.01 weight percent.

34. The rolled double annealed steel sheet product of claim 32, wherein the C is from 0.09 to 0.17 weight percent, the Mn is from 2.0 to 2.5 weight percent, the Cr is from 0.15 to 0.3 weight percent, and the Mo is from 0.08 to 0.25 weight percent.

35. The rolled double annealed steel sheet product of claim 32, wherein the sheet product comprises less than 0.4 weight percent Si and less than 0.4 weight percent Al.

36. The rolled double annealed steel sheet product of claim 35, wherein the total combined amount of Si and Al is less than 0.5 weight percent.

37. The rolled double annealed steel sheet product of claim 32, wherein the steel sheet product comprises less than 5 volume percent austenite.

38. The rolled double annealed steel sheet product of claim 32, wherein the steel sheet product comprises less than 2 volume percent austenite.

39. The rolled double annealed steel sheet product of claim 32, wherein the steel sheet product is substantially free of austenite.

40. The rolled double annealed steel sheet product of claim 32, wherein the steel sheet product comprises at least 50 volume percent bainite.

41. The rolled double annealed steel sheet product of claim 40, wherein the bainite comprises from 60 to 95 volume percent.

42. The rolled double annealed steel sheet product of claim 40, wherein the steel sheet product further comprises from 1 to 30 volume percent martensite, from 1 to 30 volume percent ferrite, or a combination thereof.

43. The rolled double annealed steel sheet product of claim 32, wherein the steel sheet product comprises at least 50 volume percent ferrite.

44. The rolled double annealed steel sheet product of claim 43, wherein the ferrite comprises from 60 to 95 volume percent.

45. The rolled double annealed steel sheet product of claim 43, wherein the steel sheet product further comprises from 1 to 30 volume percent fresh martensite.

46. The rolled double annealed steel sheet product of claim 32, wherein the steel sheet product comprises at least 50 volume percent fresh martensite.

47. The rolled double annealed steel sheet product of claim 46, wherein the fresh martensite comprises from 60 to 95 volume percent.

48. The rolled double annealed steel sheet product of claim 46, wherein the steel sheet product further comprises from 1 to 45 volume percent bainite.

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