Patent Grant
11268162
The present invention relates to high strength steel products having favorable properties, and to annealing processes for making such products.
Over past several years, the worldwide steel industry has focused on development of a third generation of advanced high strength steel (AHSS) for the automotive market. These Generation 3 steels have a favorable balance of tensile strength and elongation, typically in a UTS×TE range of about 20,000 MPa·% or greater. However, the steel industry has had a difficult time commercializing Generation 3 AHSS as the majority of approaches require high alloy content, e.g., typically greater than 4 weight percent manganese, which results in difficulties when manufacturing such steels with conventional steel production equipment. Additionally, currently available AHSS has been difficult to weld by techniques such as spot welding, have been difficult to coat with zinc-based galvanic coatings, have been difficult to manufacture into the thin gauge sheet needed for wide scale application.
The present invention provides steel sheet products having controlled compositions that are subjected to two-step annealing processes to produce sheet products having desirable microstructures and favorable mechanical properties such as high strength and ultra-high formability. Steels processed in accordance with the present invention exhibit combined ultimate tensile strength and total elongation (UTS·TE) properties of greater than 25,000 MPa-% when tested using standard subsize ASTM or full size JIS tensile testing procedures. In addition, steels produced in accordance with the present invention exhibit favorable combinations of TE and hole expansion, i.e., both global formability and local formability are good. Steels with these properties fall into the category of Generation 3 advanced high strength steels, and are highly desired by various industries including automobile manufacturers.
An aspect of the present invention is to provide a high strength cold rolled steel sheet product comprising from 0.12 to 0.5 weight percent C, from 1 to 3 weight percent Mn, and from 0.8 to 3 weight percent of a combination of Si and Al, wherein the steel sheet product has been subjected to a two-step annealing process, comprises ferrite and substantially equiaxed retained austenite grains having an average aspect ratio of less than 3:1, and has a combination of ultimate tensile strength and total elongation UTS·TE of greater than 25,000 MPa %.
Another aspect of the present invention is to provide a method of producing a high strength cold rolled steel sheet product comprising from 0.12 to 0.5 weight percent C, from 1 to 3 weight percent manganese, and from 0.8 to 3 weight percent of a combination of Si and Al. The method comprises subjecting the steel sheet product to a first step annealing process to achieve a predominantly martensitic microstructure, and subjecting the steel sheet product to a second step process comprising soaking the sheet product in an intercritical regime at a temperature of from 720 to 850° C., followed by holding the sheet product at a temperature of from 370 to 430° C.
These and other aspects of the present invention will be more apparent from the following description.
The high strength steel sheet products of the present invention have controlled compositions that, in combination with controlled annealing processes, produce desirable microstructures and favorable mechanical properties including high strengths and ultra-high formabilities. In certain embodiments, the steel composition may include carbon, manganese and silicon, along with any other suitable alloying additions known to those skilled in the art. Examples of steel compositions including ranges of C, Mn, Si, Al, Ti, and Nb are listed in Table 1 below.
0-1.5
In addition to the amounts of C, Mn, Si, Al, Ti and Nb listed in Table 1, the steel compositions may include minor or impurity amounts of other elements, such as 0.015 max S, 0.03 max P, 0.2 max Cu, 0.02 max Ni, 0.2 max Cr, 0.2 max Mo, 0.1 max Sn, 0.015 max N, 0.1 max V, and 0.004 max B. 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.
In the steel sheet products of the present invention, C provides increased strength and promotes the formation of retained austenite. Mn provides hardening and acts as a solid solution strengthener. Si inhibits iron carbide precipitation during heat treatment, and increases austenite retention. Al inhibits iron carbide precipitation during heat treatment, and increases austenite retention. Ti and Nb may act as a strength-enhancing grain refiners.
In certain embodiments, Al may be present in an amount of at least 0.1 weight percent or at least 0.2 weight percent. For example, Al may be present in an amount of from 0.5 to 1.2 weight percent, or from 0.7 to 1.1 weight percent, in certain embodiments. Alternatively the steel sheet product may be substantially free of Al.
Steel sheet products having compositions as 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 elongations, high lambda values, high bendability and high yield ratios (YS/UTS).
In certain embodiments, the ultimate tensile strength (UTS) of the steel sheet products range from 700 to 1,100 MPa or more. In certain embodiments, the steel sheet product has an ultimate tensile strength of greater than 700 MPa, for example, from 720 to 1,100 MPa, or from 750 to 1,050 MPa.
In certain embodiments, the steel sheet products have a total elongation (TE) typically greater than 22 percent, for example, greater than 27 percent, or greater than 33 percent. For example, the steel sheet product may have a total elongation of at least 20%, e.g., from 22 to 45%, or from 25 to 40%.
The steel sheet products may possess lambda (λ) values as measured by a standard hole expansion test typically greater than 20 percent, for example, greater than 25 percent, or greater than 30 percent, or greater than 35 percent. The whole expansion ratio or lambda may be greater than 20%, for example, from 22 to 80%, or from 25 to 60%.
In certain embodiments, increased values of both total elongation (TE) and hole expansion (λ) result in steel sheet products exhibiting good global formability and local formability.
Strength elongation balances (UTS·TE) of greater than 25,000 are observed for the present steel sheet products, making them fall into the category of Generation 3 steels highly desired by industries such as the auto industry. In certain embodiments UTS·TE values may be greater than 30,000, or greater than 35,000.
In accordance with certain embodiments of the invention, the final microstructure of the steel sheet products may primarily comprise ferrite, e.g., at least 50% up to 80% or higher, with lesser amounts of retained austenite, e.g., from 5 to 25%, and minor amounts of fresh martensite, e.g., from 0 to 10% or 15%. The amounts of ferrite, austenite and martensite may be determined by standard EBSD techniques. Alternatively, retained austenite content may be determined by magnetic saturation methods, Unless otherwise specified herein, the volume percent of retained austenite is determined by the EBSD technique.
In certain embodiments, the retained austenite comprises from 1 to 25 volume percent, for example, from 5 to 20 volume percent. The amount of fresh martensite may comprise less than 15 volume percent, or less than 10 volume percent, or less than 5 volume percent. In certain embodiments, the steel sheet product is substantially free of fresh martensite. It has been found that when fresh martensite amounts are greater than 15%, hole expansion values decrease significantly, e.g., local formability is significantly decreased.
At least a portion of the ferrite may be formed during the heating section, as described below, by recrystallization and/or tempering of martensite, or during the cooling and holding section of the second annealing process by austenite decomposition. Some of the ferrite may be considered bainitic ferrite. The ferrite, austenite and martensite phases are fine grained, e.g., having average grain sizes of less than 10 microns, for example, less than 5 microns, or less than 3 microns. For example, ferrite grain size may range from less than 10 microns, for example, less than 8 microns, or less than 6 microns. Average austenite grain size may range from less than 2 microns, for example, less than 1 micron, or less than 0.5 micron. Martensite grain size, when present, may range from less than 10 microns, for example, less than 8 microns, or less than 6 microns.
The austenite grains may be substantially equiaxed, e.g., having average aspect ratios less than 3:1 or less than 2:1, for example, about 1:1. It has been found that amounts of retained austenite below about 5% results in significantly decreased total elongations (TE). It has further been found that amounts of retained austenite above 25% can only be obtained at very high carbon levels, which results in poor weldability.
In certain embodiments of the invention, a two-step annealing process is used to produce advanced high strength 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. Examples of two-step annealing processes are shown in
The goal of the first step of the annealing process is to achieve a martensitic microstructure. In the first annealing stage of the first step, an annealing temperature above the A3 temperature may typically be used, for example, an annealing temperature of at least 820° C. may be used. In certain embodiments, the first stage annealing temperature may typically range from 830 to 980° C., for example, from 830 to 940° C., or from 840 to 930° C., or from 860 to 925° 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.
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 should be below the martensite start temperature (Ms), and preferably 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.
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 from 30 to 2,000° C./sec may typically be used.
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.
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 takes place by raising the temperature of the steel in the range of room temperature to about 500° C. and holding for up to 600 seconds. If tempering is utilized, the tempering temperature may be held constant, or may be varied within this preferred range.
After tempering, the temperature is 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, as in
The second step of the annealing process may include a first stage that is conducted at relatively high annealing temperature, and a second stage that is conducted at relatively low temperature. These stages are defined as the “soaking” and “holding” zones of the second anneal, as described in
In the first annealing stage of the second step, a soaking zone temperature between A1 and A3 may be used, for example, an annealing temperature of at least 720° C. may be used. In certain embodiments, the soaking zone temperature may typically range from 720 to 850° C., for example, from 760 to 825° 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.
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.
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. In certain embodiments, the steel sheet product is maintained at a temperature above 300° C. between the second step soaking process and the second step holding process. 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 from 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.
In accordance with embodiments of the invention, the holding zone step is carried out at a typical temperature of from 360 to 440° C., for example, from 370 to 430° C. The holding zone may be held for up to 800 seconds, for example, from 30 to 600 seconds.
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.
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.
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.
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), e.g., as shown in
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, as shown in
The following examples are intended to illustrate various aspects of the present invention, and are not intended to limit the scope of the invention.
A cold rolled steel sheet having a composition as listed in Table 2, Sample No. 1, was subjected to a two-step annealing process as illustrated in
A cold rolled steel sheet having a composition as listed in Table 2, Sample No, 2, was subjected to a two-step annealing process as illustrated in
The microstructure includes about 80 volume percent ferrite with an average grain size of about 5 microns, about 10 volume percent retained austenite having substantially equiaxed grains and an average grain size of about 0.5 micron, and about 10 volume percent fresh martensite with an average grain size of about 5 microns. Mechanical properties of Sample No. 1 are listed in Table 2 below.
A cold rolled steel sheet having a composition as listed in Table 2, Sample No. 3, was subjected to a two-step annealing process as illustrated in
A cold rolled steel sheet having a composition as listed in Table 2, Sample No. 4, was subjected to a two-step annealing process as illustrated in
A cold rolled steel sheet having a composition as listed in Table 2, Sample No. 5, was subjected to a two-step annealing process as illustrated in
A cold rolled steel sheet having a composition as listed in Table 2, Sample No. 6, was subjected to a two-step annealing process as illustrated in
A cold rolled steel sheet having a composition as listed in Table 2, Sample No. 7, was subjected to a two-step annealing process as illustrated in
A cold rolled steel sheet having a composition as listed in Table 2, Sample No. 8, was subjected to a two-step annealing process as illustrated in
The steels in Examples 1-8 exhibited UTS levels in the range of 700 to 1,100 MPa.
Cold rolled steel sheets having compositions as listed in Table 2, Sample Nos. C1-C4, were subjected to a two-step annealing process as illustrated in
Cold rolled steel sheets having compositions as listed in Table 2, Sample Nos. C5-C8, were subjected to a two-step annealing process as illustrated in
Cold rolled steel sheets having compositions as listed in Table 2, Sample Nos. C9-C11, were subjected to a two-step annealing process similar to that illustrated in
A cold rolled sheet having a composition listed in Table 2, Sample No. C12, was subjected to a two-step annealing process similar to that illustrated in
Mill trials were conducted for samples labeled M1-M5 in Table 3 below using either the CAL+CAL or CAL+CGL process. For Samples M1, M2 and M5, the CAL+CAL processing times and temperatures shown in
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.
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.
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.
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.
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.
This application is a divisional of U.S. patent application Ser. No. 15/591,344 filed May 10, 2017, now 1,i, Patent No, 10,385,419, which claims the benefit of U.S. Provisional Application No. 62/396,602 filed Sep. 19, 2016, and U.S. Provisional Application No. 62/334,189 filed May 10.201.6. All of these applications are incorporated herein by reference.
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| Child | 16119742 | US |
