KR20220050935A - High-strength steel products and annealing processes for their manufacture - Google Patents

Abstract

The present invention provides a sheet product having a controlled composition that results in a sheet product having a desirable microstructure and advantageous mechanical properties (eg, high strength and ultra-high formability) through a two step annealing process. The steel sheet product may be cold rolled or hot rolled. The steel treated according to the present invention exhibits advantageous combined ultimate tensile strength and total elongation (UTS·TE) properties, and may fall into the category of third-generation advanced high-strength steels desirable in various industries including automobile manufacturers.

Description

High-strength steel products and annealing processes for their manufacture

The present invention relates to high-strength steel products having advantageous properties and to annealing processes for producing said products.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to U.S. Patent Application Serial No. 15/591,344, filed on May 10, 2017 (now U.S. Patent No. 10,385,419; U.S. Provisional Patent Application No. 62/334,189, filed on May 10, 2016, and 2016 It is a continuation-in-part of U.S. Provisional Application No. 62/396,602 filed on September 19, claiming priority. All of the aforementioned applications are incorporated herein by reference.

Over the past few years, the global steel industry has focused on developing third-generation advanced high-strength steels (AHSS) for the automotive market. These third-generation steels typically have a favorable balance of tensile strength and elongation in the UTS·TE range of about 20,000 MPa% or greater. However, the steel industry has struggled to commercialize third-generation AHSS, as most approaches require high alloy content (eg, typically 4 wt % or more manganese), which is usually It presents difficulties in manufacturing such steels using steel production equipment. Additionally, currently available AHSS was difficult to weld with techniques such as spot welding, difficult to coat with zinc-based galvanic coatings, and difficult to manufacture into thin gauge sheets required for a wide range of applications.

The present invention provides a steel sheet product having a controlled composition that has been subjected to a two-step annealing process to produce a sheet product having a desirable microstructure and advantageous mechanical properties (eg, high strength and ultra-high formability). The steel sheet product may be cold rolled or hot rolled. Steels treated in accordance with the present invention have advantageous combined ultimate tensile strength and total elongation (UTS·TE) properties, for example greater than 25,000 MPa%, when tested using standard subsize ASTM or full size JIS tensile testing procedures. indicates In addition, steels made according to the present invention exhibit an advantageous combination of TE and hole expansion (ie, both overall formability and local formability are good). Steel with these characteristics is classified as a third-generation advanced high-strength steel, and is highly required in various industrial fields including automobile manufacturers.

One aspect of the present invention is to provide a high strength rolled steel sheet product comprising 0.12 to 0.5 wt % C, 1 to 3 wt % Mn, and 0.8 to 3 wt % a combination of Si and Al, wherein The sheet product has been subjected to a two-step annealing process and contains substantially equiaxed retained austenite grains and ferrite with an average aspect ratio of less than 3:1 and a combination of ultimate tensile strength and total elongation greater than 25,000 MPa% ( UTS·TE).

Another aspect of the present invention is to provide a method for producing a high-strength rolled steel sheet product comprising 0.12 to 0.5 wt % C, 1 to 3 wt % manganese, and 0.8 to 3 wt % a combination of Si and Al. .

The method comprises subjecting the steel sheet product to a first step annealing process to achieve a predominant martensitic microstructure; and treating the steel sheet product in a second step process, wherein the steel sheet product is immersed in an intercritical regime of a temperature of 720 to 850° C. and then the steel sheet product is maintained at a temperature of 360 to 445° C. comprising the steps of

These and other aspects of the present invention will become more apparent from the following description.

1 includes a plot of temperature versus time illustrating a two stage annealing process according to one embodiment of the present invention.
2 includes a plot of temperature versus time illustrating a two stage annealing process according to another embodiment of the present invention.
3 is a plot of temperature versus time showing a two stage annealing process combining a two stage thermal process and an optional zinc-based hot-dipped coating operation in a single production facility.
4 is a plot of temperature versus time for a second stage of an annealing process defining an immersion and hold zone in a thermal cycle according to one embodiment of the present invention.
5 and 6 are electron backscatter diffraction (EBSD) micrographs showing the microstructure of a high-strength steel sheet product according to an embodiment of the present invention.
FIG. 7 is an optical micrograph of a steel sheet product that has undergone the thermal process shown in FIG. 1 , and shows darker ferrite grains and brighter austenite grains.
FIG. 8 is a bar graph illustrating the aspect ratio of the austenite grains shown in FIG. 7 .
9 and 10 are graphs of a high-strength steel sheet product showing austenite and ferrite grain size distribution according to an embodiment of the present invention.
FIG. 11 is an EBSD micrograph showing the microstructure of a high-strength steel sheet product treated as shown in FIG. 1 .
12 and 13 are EBSD micrographs showing a steel sheet product treated as shown in FIG. 2 .
14 is an EBSD micrograph of a steel sheet product treated as shown in FIG. 3 .
15 is a graph of total elongation versus ultimate tensile strength for a high strength sheet product of the present invention compared to other sheet products treated outside the scope of the present disclosure;
16 is a graph of total elongation versus ultimate tensile strength for high strength steel products made in a mill trial in accordance with embodiments of the present invention.
17 is a plot of temperature versus time for cold rolled and hot rolled substrates subjected to thermal cycling in accordance with one embodiment of the present invention.
18 is an EBSD micrograph of a cold rolled steel sheet substrate subjected to the thermal process shown in FIG. 17, showing darker ferrite grains and lighter retained austenite grains.
19 is an EBSD micrograph of a hot-rolled steel sheet substrate subjected to the thermal process shown in FIG. 17, showing darker ferrite grains and brighter retained austenite grains.

The high strength steel sheet products of the present invention have a controlled composition that, in combination with a controlled annealing process, produces the desired microstructure and advantageous mechanical properties (eg, high strength and ultra-high formability). In certain embodiments, the steel composition may include carbon, manganese and silicon along with any other suitable alloying additives known to those skilled in the art. Examples of the steel compositions, including ranges of C, Mn, Si, Al, Ti, and Nb, are listed in Table 1 below.

Table 1. Steel composition (wt%)

In addition to the amounts of C, Mn, Si, Al, Ti and Nb listed in Table 1 above, the steel composition may contain traces 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. The term "substantially free" when referring to the composition of a steel sheet product herein means that a particular element or substance is not intentionally added to the composition and is only present as an impurity or in trace amounts.

In the steel sheet product of the present invention, C provides increased strength and promotes the formation of retained austenite. Mn provides hardening and acts as a solid solution strengthening agent. Si suppresses iron carbide precipitation during heat treatment and increases austenite retention. Al suppresses iron carbide precipitation during heat treatment and increases austenite retention. Ti and Nb can act as strength-enhancing grain refiners.

In certain embodiments, Al may be present in an amount of at least 0.1% by weight or at least 0.2% by weight. For example, Al can, in certain embodiments, be present in an amount of 0.5 to 1.2 weight percent, or 0.7 to 1.1 weight percent. Alternatively, the steel sheet product may be substantially free of Al.

A steel sheet product having the composition described above is subjected to a two-step annealing process as more fully described below. The resulting steel sheet product has been found to have advantageous mechanical properties, such as desirable ultimate tensile strength, high elongation, high lambda value, high bendability and high yield ratio (YS/UTS).

In certain embodiments, the ultimate tensile strength (UTS) of the steel sheet product ranges from 700 to 1,100 MPa or greater. In certain embodiments, the steel sheet product has an ultimate tensile strength greater than 700 MPa, for example from 720 to 1,100 MPa, or from 750 to 1,050 MPa.

In certain embodiments, the sheet product typically has a total elongation (TE) of greater than 22%, such as greater than 27%, or greater than 33%. For example, the steel sheet product may have a total elongation of at least 20%, or at least 25%, or at least 27%, for example, from 22 to 45%, or from 25 to 40%.

The sheet product may typically have a lambda (λ) value of greater than 20%, for example greater than 25%, or greater than 30%, or greater than 35%, as measured by the standard Hall expansion test. The overall expansion ratio or lambda may be greater than 20%, such as 22 to 80%, or 25 to 60%.

In certain embodiments, increased values of both total elongation (TE) and hole expansion (λ) provide a steel sheet product exhibiting good overall formability and local formability.

A strength elongation balance (UTS·TE) of greater than 25,000 is observed for the steel sheet product of the present invention, which puts the sheet product in the highly demanded third-generation steel category in industries such as the automotive industry. In certain embodiments, the UTS·TE value may be greater than 27,000, or greater than 30,000, or greater than 35,000.

According to certain embodiments of the present invention, the final microstructure of the steel sheet product may comprise predominantly (eg, at least 50% to 80% or more) ferrite, with a smaller amount (eg 5-25%) of residual austenite. nite, and small amounts (eg 0-10% or 15%) of fresh martensite. The amounts of ferrite, austenite and martensite can be determined by standard EBSD techniques. Alternatively, the retained austenite content can be determined by a magnetic saturation method. Unless otherwise specified herein, the volume percentage of retained austenite is determined by the EBSD technique.

In certain embodiments, the retained austenite comprises 1 to 25% by volume, such as 5 to 20% by volume. The amount of fresh martensite may account for less than 15% by volume, or less than 10% by volume, or less than 5% by volume. In certain embodiments, the sheet product is substantially free of fresh martensite. When the amount of fresh martensite is greater than 15%, it has been shown that the hole expansion value is significantly reduced (eg, local formability is greatly reduced).

At least a portion of the ferrite may be formed during the heating section by recrystallization and/or tempering of martensite, or formed during the cooling and holding section of the second annealing process by austenite decomposition, as described below. . Some ferrites can be considered bainitic ferrites. Ferritic, austenitic and martensitic phases are finely grained, for example having an average grain size of less than 10 μm, such as less than 5 μm, or less than 3 μm. For example, the ferrite grain size may range from less than 10 μm, such as less than 8 μm, or less than 6 μm. The average austenite grain size may range from less than 2 μm, such as less than 1 μm, or less than 0.5 μm. When present, the martensite grain size may range from less than 10 μm, such as less than 8 μm, or less than 6 μm.

The austenite grains may be substantially equiaxed (eg, having an average aspect ratio of less than 3:1 or less than 2:1, such as about 1:1). It has been found that the total elongation (TE) is significantly reduced when the amount of retained austenite is less than about 5%. In addition, amounts of retained austenite above 25% can only be obtained at very high carbon levels, which have been found to give poor weldability.

In certain embodiments of the present invention, a two-step annealing process is used to produce advanced high-strength steel products with advantageous mechanical properties (eg, those described above). Within each of the first and second annealing steps, several methodologies for performing the thermal treatment may be used. An example of a two-step annealing process is shown in Figures 1-3 and described below. 1 shows a continuous annealing line (CAL) and then a continuous annealing line (CAL) production path. 2 shows the CAL + continuous galvanizing line (CGL) production route. Figure 3 shows a line specially designed for both CAL + CAL or CAL + CGL steps to be performed in a single facility. A direct-fired furnace (DFF) and then a radiant tube (RT) furnace embodiment is shown in FIG. 3 , but other embodiments (e.g., all radiant tube, electric radiation heating, etc.) may be used.

Step 1

The goal of the first step of the annealing process is to achieve a martensitic microstructure in a cold rolled or hot rolled steel sheet product. In the first annealing step of the first step, an annealing temperature above the A ₃ temperature may be typically used, for example, an annealing temperature of 820° C. or higher may be used. In certain embodiments, the annealing temperature of the first step may typically range from 830 to 980 °C, such as from 830 to 940 °C, or from 840 to 930 °C, or from 860 to 925 °C. In certain embodiments, the peak annealing temperature can be typically maintained for at least 20 seconds, such as between 20 and 500 seconds, or between 30 and 200 seconds. Heating may be accomplished by conventional techniques such as non-oxidizing or oxidizing direct combustion furnaces (DFF), oxygen-enriched DFI, induction, gas radiant tube heating, electroradiative heating, and the like. Examples of heating systems that may be adapted for use in the method of the present invention are described in US Pat. Nos. 5,798,007; 7,368,689; 8,425,225; and 8,845,324, US Patent Application No. 2009/0158975, and International Patent Application Publication No. WO2015/083047 (assigned to Fives Stein). Additional examples of heating systems that may be suitable for use in the method of the present invention are described in US Pat. No. 7,384,489 (assigned to Drever International) and US Pat. No. 9,096,918 (Nippon Steel and Sumitomo). Metal Corporation (assigned to Sumitomo Metal Corporation). Any other suitable known type of heating system and process may be adapted for use in steps 1 and 2 above.

In the first step, as described in more detail below, the cold rolled or hot rolled steel sheet has reached a peak annealing temperature and held for a desired period of time, followed by quenching to room temperature or a controlled temperature above room temperature. The quench temperature does not necessarily have to be room temperature, but it must be below the martensite starting temperature (M _s ), preferably below the martensitic end temperature (M _F ) to form a predominantly martensitic microstructure. In certain embodiments, between the first stage process and the second stage process, the steel sheet product may be cooled to a temperature of less than 300 °C, for example less than 200 °C.

The quenching may be accomplished by conventional techniques, such as water quenching, immersed knife/nozzle water quenching, gas cooling, rapid cooling using cold water, lukewarm water or a combination of hot water and gas, aqueous solution cooling, cooling of other liquid or gaseous fluids, cooling rolls This may be achieved by quenching, water mist spraying, wet flash cooling, non-oxidizing wet flash cooling, and the like. A quench rate of 30 to 2,000° C./sec can 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 methods of the present invention. Suitable cooling/quenching systems and processes commonly used on a commercial basis include water quenching, water mist cooling, dry flash and wet flash, oxidative and non-oxidative cooling, alkane fluid to gas phase change cooling, hot water quenching, such as 2 Step water quenching, roll quenching, high proportion hydrogen or helium gas jet cooling, and the like. For example, dry flash and/or wet flash oxidation and non-oxidative cooling/quenching may be used, such as disclosed in WO2015/083047 (Fives Stain). Other Fine Stein patents that describe cooling/quenching systems and processes that may be adapted for use in the methods of the present invention include US Pat. Nos. 6,464,808B2; 6,547,898B2; and 8,918,199B2, and US Patent Application Publication Nos. 2009/0158975A1; 2009/0315228A1; and 2011/0266725A1. Other examples of cooling/quenching systems and processes that may be suitable for use in the methods of the present invention are described in US Pat. Nos. 8,359,894B2; 8,844,462B2; and 7,384,489B2, and US Patent Application Publication Nos. 2002/0017747A1 and 2014/0083572A1.

In a specific embodiment, after the peak annealing temperature of the first step is reached and the steel is quenched to form martensite, the martensite is optionally tempered to slightly soften the steel, further realizing further processing can make it possible The tempering is performed by raising the temperature of the steel in the room temperature range to about 500° C. and holding it for 600 seconds or less. If tempering is used, the tempering temperature may be kept constant or may vary within the above 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. For single pass plant furnaces, tempering may not be necessary as in FIG. 3 .

Step 2

The second step of the annealing process may include a first step performed at a relatively high annealing temperature and a second step performed at a relatively low temperature. These steps are defined as the "absorption" and "hold" zones of the second annealing, as shown in FIG. 4 . The temperature is controlled to promote the formation of the desired microstructure in the final product.

In the first annealing step of the second step, the immersion zone temperature of A ₁ to A ₃ may be used, for example, an annealing temperature of 720° C. or higher may be used. In certain embodiments, the immersion zone temperature may typically range from 720 to 850°C, for example from 760 to 825°C. In certain embodiments, the peak annealing temperature can be typically maintained for at least 15 seconds, such as between 20 and 300 seconds, or between 30 and 150 seconds.

During the first stage of the second stage, the immersion zone temperature is applied at an average rate of 0.5-50 °C/sec (eg about 2-20 °C/sec) from a relatively low temperature (eg room temperature) below M _s to the steel. This can be achieved by heating with a furnace. In certain embodiments, the ramp-up may take between 25 and 800 seconds, for example between 100 and 500 seconds. The first stage heating of the second stage may be accomplished by any suitable heating system or process, such as using radiant heating, induction heating, direct combustion furnace heating, and the like.

After the dip zone temperature has been reached and held for the desired time, the steel may be cooled to a controlled temperature above room temperature up to the hold zone. In certain embodiments, the sheet product is maintained at a temperature greater than 300° C. between immersion in the second stage process and maintenance of the second stage process. Cooling from the immersion zone to the 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 can typically be used. Any suitable type of cooling and quenching system (eg, those described above) may be configured for use in cooling from an immersion temperature to a holding temperature.

According to an embodiment of the present invention, said holding zone step is carried out at a typical temperature of 360 to 445 °C, for example 370 to 440 °C. The holding zone may be maintained for 800 seconds or less, for example, 30 to 600 seconds.

The holding zone temperature may be held constant or may vary somewhat within a desired temperature range. When the steel is hot dip coated, the steel can be reheated after holding, for example by induction or other heating methods, and introduced into the hot dip coating pot at an appropriate temperature for good coating results.

In certain embodiments, after the holding zone temperature is maintained for a desired period of time, the temperature may be ramped down to room temperature. The 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.

According to certain embodiments, one or both of the first and second stage annealing processes may be performed in a continuous annealing line (CAL). After going through the CAL+CAL process, the steel can be electrogalvanized to produce a zinc-based coating product.

In certain embodiments, the annealed steel sheet is hot-dip galvanized at the end of the holding zone. The galvanizing temperature 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 a second stage annealing process for a continuous galvanizing line (CGL), for example as shown in FIG. 2 . The CAL + CGL process can be used to produce zinc-based or zinc alloy-based hot dip galvanized products, or can be reheated after coating to produce iron-zinc galvanyl based type coated products. To improve the zinc coating properties, an optional nickel-based coating step may be performed between the CAL and CGL steps in the process. When the continuous galvanizing line is used in the second step, the production efficiency of producing the coated GEN3 product is increased compared to the case of using the CAL + CAL + EG route.

Galvanized products or zinc-based alloy hot dip coated products can also be produced in a specially designed CGL where two-step annealing can be performed in a single line, as shown in FIG. 3 . In this case, galvannealing may be an option. In addition, a single production facility may be specially designed and constructed to combine a two-step thermal process to produce uncoated third-generation steel as defined herein.

The following examples are intended to illustrate various aspects of the invention and are not intended to limit the scope of the invention.

Example 1

A cold rolled steel sheet (Sample No. 1) having a composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 1 . The microstructure of the resulting product is shown in FIGS. 5 and 6 . The EBSD technique using commercial EDAX oriented imaging microscopy software shows dark ferrite grains and bright austenite grains in FIG. 5 .

Example 2

A cold rolled steel sheet (Sample 2) having a composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 1 . The microstructure of the resulting product is shown in FIG. 11 . The mechanical properties of Sample 2 are listed in Table 2 below. The grain size distributions of austenite and ferrite are shown in FIGS. 9 and 10, respectively. The average grain size of austenite is less than 1 μm, and the average grain size of ferrite is less than 10 μm.

The microstructure comprises about 80% by volume ferrite having an average grain size of about 5 μm, about 10% by volume retained austenite having substantially equiaxed grains and an average grain size of about 0.5 μm, and an average of about 5 μm. and about 10% by volume of fresh martensite having a grain size. The mechanical properties of Sample 1 are listed in Table 2 below.

Example 3

A cold-rolled steel sheet (Sample 3) having a composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 2 . The microstructure of the resulting product is shown in FIGS. 12 and 13 . In Fig. 13, austenite is a light color and ferrite is a dark color. The mechanical properties of Sample 3 are listed in Table 2 below.

Example 4

A cold rolled steel sheet (Sample 4) having a composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 3 . The microstructure of the resulting product is shown in FIG. 14 . In Fig. 14, austenite is a light color and ferrite is a dark color. The mechanical properties of Sample 4 are listed in Table 2 below.

Example 5

A cold rolled steel sheet (Sample 5) having a composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 1 . The mechanical properties of Sample 5 are listed in Table 2 below.

Example 6

A cold-rolled steel sheet (Sample 6) having a composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 1 . The mechanical properties of Sample 6 are listed in Table 2 below. 7 is an optical image showing the microstructure of the steel sheet shown in FIG. 2 (Sample 6 that has undergone the two-step annealing process shown in FIG. 1). In FIG. 7 , dark areas in the micrograph are ferrite grains, and bright areas are austenite grains. FIG. 8 is a graph showing the aspect ratio of the austenite grains shown in FIG. 7 . Using the optical images of FIG. 7 , the aspect ratio of the austenite grains was determined using image analysis using commercially available software. 7 shows that the average aspect ratio of the austenite grains is less than 3:1.

Example 7

A cold-rolled steel sheet (Sample 7) having the composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 2 . The mechanical properties of Sample 7 are listed in Table 2 below.

Example 8

A cold-rolled steel sheet (Sample 8) having the composition listed in Table 2 below was subjected to a two-step annealing process as shown in FIG. 3 . The mechanical properties of Sample 8 are listed in Table 2 below.

The steels of Examples 1-8 exhibited UTS levels ranging from 700 to 1,100 MPa.

Comparative Examples 1 to 4

Cold rolled steel sheets (Samples C1 to C4) having the compositions listed in Table 2 below were subjected to a two-step annealing process as shown in FIG. 1 . The mechanical properties of samples C1 to C4 are listed in Table 2 below. The steels of Comparative Examples 1-4 exhibited UTS levels of less than 700 MPa.

Comparative Examples 5 to 8

Cold rolled steel sheets (Samples C5 to C8) having the compositions listed in Table 2 below were subjected to a two-step annealing process as shown in FIG. 1 . The mechanical properties of samples C5 to C8 are listed in Table 2 below. The steels of Comparative Examples 5-8 exhibited UTS levels above 1,100 MPa.

Comparative Examples 9 to 11

Cold rolled steel sheets (Samples C9 to C11) having the compositions listed in Table 2 below were subjected to two steps similar to those shown in FIG. 1, except that the immersion or holding temperature in the second annealing was outside the preferred range of the present invention. It was treated by an annealing process. The mechanical properties of samples C9 to C11 are listed in Table 2 below.

Comparative Example 12

A cold rolled steel sheet (Sample C12) having the composition listed in Table 2 below was subjected to a two-step annealing process similar to that shown in FIG. 2, except that the holding zone temperature of the second annealing was outside the preferred range of the present invention. did The mechanical properties of sample C12 are listed in Table 2 below.

Table 2

Table 2 (continued)

15 plots Total Elongation (TE) and Ultimate Tensile Strength (UTS) of Samples C1 - C12 of Comparative Examples C1 - C12 as well as Samples 1 - 8 of Examples 1 - 8 . A line corresponding to UTS·TE of 25,000 is roughly drawn in FIG. 15 . As can be seen, the high strength steel sheet sample prepared according to the present invention has an excellent combination of strength and elongation compared to the comparative sample (i.e., for the inventive examples, high total elongation properties are observed at high UTS levels) . The steels of Samples 1-8 belong to the third generation advanced high-strength steel category, which is highly desirable for automotive and other industries.

Example 9

Using the CAL+CAL or CAL+CGL process, a mill test was performed on the samples labeled M1 to M5 in Table 3 below. For samples M1, M2 and M5, the CAL+CAL treatment time and temperature shown in FIG. 1 were used. For samples M3 and M4, the CAL+CGL treatment time and temperature shown in FIG. 2 were used.

Table 3. Wheat test results

16 depicts the strength-elongation balance of the wheat test material that all meet the minimum UTS·TE of 25,000. The test substance exhibited a lambda value of greater than 20%.

Example 10

Cold-rolled and hot-rolled steel sheets having a composition of 0.23 wt% C, 2.3 wt% Mn, 0.6 wt% Si and 0.8 wt% Al, corresponding to Sample Nos. 9A to 12B in Table 4 below, are shown in FIG. As shown, a two-step annealing process was performed. In Table 4 below, cold rolled samples are listed as "CR" substrate type and hot rolled samples are listed as "HR" substrate type. The mechanical properties of Samples 9A-12B are listed in Table 4 below. The hot rolled substrate samples exhibited excellent YS, UTS, TE and hole expansion properties comparable to the cold rolled samples, indicating that hot rolled substrates directly treated with a two-step annealing process can produce 3rd generation AHSS properties shows Also, as shown in the EBSD phase maps shown in FIGS. 18 and 19 (where the retained austenite grains are brighter than the ferrite grains), similar austenite content, distribution and shape is observed. Fig. 18 shows the austenite content of the cold-rolled sample 11A, and Fig. 19 shows the austenite content of the hot-rolled sample 12A. A fine and predominantly equiaxed distribution of austenite is observed in both of these microstructures.

Table 4

As used herein, “including,” “comprising,” and similar terms, in the context of this application, are to be understood as synonymous with “comprising” and are thus open-ended and additional unrecited or unrecited elements, materials, phases or methods. It does not rule out the existence of steps. As used herein, “consisting of,” in the context of this application, is understood to exclude the presence of any non-specified element, material, phase or method step. As used herein, "consisting essentially of" in the context of the present application includes, where applicable, specified elements, materials, phases or method steps, and also does not materially affect the basic or novel properties of the invention. is understood to include non-specified elements, materials, phases or method steps of

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 deviation found in its respective testing measurements.

It should also be understood that any numerical range recited herein is intended to include all subranges subsumed therein. For example, a range from “1 to 10” is intended to include all subranges between the recited minimum value of 1 and the recited maximum value of 10 (ie, having a minimum value of 1 or greater and a maximum value of 10 or less).

In this application, unless explicitly stated otherwise, the use of the singular includes the plural and the plural includes the singular. Also, use of “or” herein means “and/or” unless explicitly stated otherwise, although in certain instances “and/or” may be used explicitly. In this application and in the appended claims, the singular includes the plural referent unless explicitly and explicitly limited to one referent.

While specific embodiments of the invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that numerous modifications to the details of the invention may be made without departing from the invention.

Claims (33)

A high-strength rolled steel sheet product comprising 0.12 to 0.5% by weight of C, 1-3% by weight of Mn, and 0.8 to 3% by weight of a combination of Si and Al;
The steel sheet product has been subjected to a two-step annealing process and has a substantially equiaxed retained austenite grain, and ferrite, having an average aspect ratio of less than 3:1. wherein the combination of ultimate tensile strength and total elongation (UTS·TE) is greater than 25,000 MPa%. According to claim 1,
Si accounts for not more than 2% by weight,
Al accounts for not more than 2% by weight,
The high-strength rolled steel sheet product, wherein the rolled steel sheet product further comprises 0.05 wt% or less of Ti and 0.05 wt% or less of Nb. 3. The method of claim 2,
C accounts for 0.15 to 0.4% by weight,
Mn accounts for 1.3 to 2.5% by weight,
Si accounts for 0.2 to 1.8% by weight,
Al accounts for not more than 1.5% by weight,
Ti accounts for 0.03% by weight or less,
A high-strength rolled steel sheet product, wherein Nb accounts for 0.03% by weight or less. According to claim 1,
The ferrite occupies 50% by volume or more,
The retained austenite accounts for 5 to 25% by volume,
The average aspect ratio of the retained austenite grains is less than 2: 1, high-strength rolled steel sheet product. According to claim 1,
wherein the retained austenite has an average grain size of less than 10 μm. 6. The method of claim 5,
wherein the retained austenite has an average grain size of less than 1 μm. 6. The method of claim 5,
The high-strength rolled steel sheet product, wherein the rolled steel sheet product contains less than 15% by volume of fresh martensite. According to claim 1,
The high-strength rolled steel sheet product, wherein the rolled steel sheet product has an ultimate tensile strength of 720 to 1,100 MPa and a total elongation of 20% or more. According to claim 1,
wherein the rolled steel sheet product has a hole expansion ratio greater than 20%. According to claim 1,
The UTS · TE is 27,000 MPa% or more, high-strength rolled steel sheet product. According to claim 1,
A high-strength rolled steel sheet product further comprising a zinc-based coating on the rolled sheet product. According to claim 1,
The rolled steel sheet product is a hot rolled (hot rolled), high-strength rolled steel sheet product. A method for producing a high-strength rolled steel sheet product comprising 0.12 to 0.5% by weight of C, 1-3% by weight of manganese, and 0.8 to 3% by weight of a combination of Si and Al;
The method is
subjecting the sheet product to a first step annealing process to achieve a predominantly martensitic microstructure; and
treating the steel sheet product in a second step process, immersing the sheet product in an intercritical regime at a temperature of 720 to 850° C. and then maintaining the steel sheet product at a temperature of 360 to 445° C. step including
including,
wherein the sheet product comprises substantially equiaxed retained austenite grains and ferrite 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%; , manufacturing method. 14. The method of claim 13,
The manufacturing method, wherein the first step annealing process is performed at a temperature of 820 °C or higher. 14. The method of claim 13,
The manufacturing method, wherein the first step annealing process is performed at a temperature of 830 to 940 °C. 14. The method of claim 13,
The immersion of the second step process is carried out at a temperature of 720 to 850 °C,
The manufacturing method, wherein the maintenance of the second step process is performed at a temperature of 370 to 440 °C. 14. The method of claim 13,
wherein the steel sheet product is cooled to a temperature of less than 300° C. between the first step process and the second step process. 14. The method of claim 13,
The method of claim 1, wherein the steel sheet product is maintained at a temperature of 300° C. or higher between immersion in the second step process and maintenance of the second step process. 14. The method of claim 13,
The first step annealing process is performed in a continuous annealing line,
wherein the second step process is performed in a continuous annealing line. 20. The method of claim 19,
and the same continuous annealing line is used for both the first stage annealing process and the second stage process. 20. The method of claim 19,
wherein separate continuous annealing lines are used for the first step annealing process and the second step process. 14. The method of claim 13,
The first step annealing process is performed in a continuous annealing line,
wherein the second step process is performed in a continuous galvanizing line. 14. The method of claim 13,
and electrolytically coating the rolled steel sheet product with a zinc-based coating. 14. The method of claim 13,
Si accounts for not more than 2% by weight,
Al accounts for not more than 2% by weight,
The method of claim 1, wherein the rolled steel sheet product further comprises 0.05 wt% or less of Ti and 0.05 wt% or less of Nb. 26. The method of claim 25,
C accounts for 0.15 to 0.4% by weight,
Mn accounts for 1.3 to 2.5% by weight,
Si accounts for 0.2 to 1.8% by weight,
Al accounts for not more than 1.5% by weight,
Ti accounts for 0.03% by weight or less,
Nb accounts for 0.03% by weight or less. 14. The method of claim 13,
wherein the rolled steel sheet product is hot rolled prior to the first step annealing process. 14. The method of claim 13,
The ferrite occupies 50% by volume or more,
The retained austenite comprises 5 to 25% by volume,
wherein the average aspect ratio of the retained austenite grains is less than 2:1. 14. The method of claim 13,
wherein the retained austenite has an average grain size of less than 10 μm. 30. The method of claim 29,
wherein the retained austenite has an average grain size of less than 1 μm. 14. The method of claim 13,
The method of claim 1, wherein the rolled steel sheet product contains less than 15% by volume of fresh martensite. 14. The method of claim 13,
wherein the rolled steel sheet product has an ultimate tensile strength of 720 to 1,100 MPa and a total elongation of at least 20%. 14. The method of claim 13,
wherein the rolled steel sheet product has a hole expansion ratio greater than 20%. 14. The method of claim 13,
and applying a zinc-based coating to the rolled steel product.

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