JP7638902B2 - Method for Producing Continuously Cast, Hot Rolled, High Strength Steel Plate Products - Google Patents

Description

This application is a continuation-in-part of U.S. Patent Application No. 16/544,127, filed August 19, 2019, which is a continuation-in-part of U.S. Patent Application No. 15/591,344, filed May 10, 2017 (U.S. Patent No. 10,385,419), and claims priority to U.S. Provisional Application No. 62/334,189, filed May 10, 2016, and U.S. Provisional Application No. 62/396,602, filed September 19, 2016. This application also claims priority to U.S. Provisional Application No. 62/844,301, filed May 7, 2019. All of the above applications are incorporated herein by reference.

The present invention relates to a method for providing a continuously cast, hot rolled, high strength steel plate product, which method comprises quenching a finish rolled steel plate in a hot strip mill to produce a martensite dominated microstructure, followed by soaking at a temperature in the intercritical regime and holding at a low temperature to produce a ferrite and retained austenite dominated microstructure.

Over the past few years, the global steel industry has focused on developing Advanced High Strength Steels (AHSS) for the automotive market. These Generation 3 steels have a good balance of tensile strength and elongation, typically with a UTS-TE range of about 20,000 MPa% or more. However, the steel industry has had a difficult time commercializing the Generation 3 AHSS because many of the approaches require high alloying elements, typically greater than 4 wt.% manganese, which makes such steels difficult to manufacture with conventional steelmaking facilities. Additionally, the currently commercially available AHSS are difficult to weld by techniques such as spot welding, difficult to apply zinc-based plating coatings, and difficult to fabricate into the thin sheets required for a wide range of applications.

U.S. Patent No. 10,385,419 and U.S. Patent Application Publication No. 2020/0040422 disclose annealing processes for producing high strength steel products and are incorporated herein by reference.

The present invention provides a method for producing a continuously cast hot rolled steel sheet product, comprising the steps of continuously casting a steel slab, followed by hot rolling by finish rolling in a hot strip mill, quenching in the hot strip mill to produce a predominantly martensite microstructure, and a thermal cycling step including soaking at a temperature in the intercritical range followed by a low temperature hold. The thermal cycling step can be carried out in a continuous galvanize line or a continuous annealing line. The hot rolled steel sheet product thus obtained has a structure including ferrite and retained austenite. Steels processed according to the present invention exhibit good combined ultimate tensile strength and total elongation (UTS-TE) properties, e.g., greater than 25,000 MPa%. Steels with these properties are in the category of third generation advanced high strength steels and are highly desired by various industries, including automobile manufacturers.

One aspect of the present invention provides a method for producing a continuously cast hot-rolled high-strength steel plate product, which comprises continuously casting a steel slab containing 0.15 to 0.5% by weight of C, 1 to 3% by weight of Mn, and 0.8 to 3% by weight in total of Si and Al, and hot rolling the continuously cast steel slab in a hot strip mill at a finish rolling temperature of 820°C or higher to produce a hot-rolled steel plate product in the hot strip mill, and The method includes quenching in a strip mill to produce a martensite-dominated microstructure, subjecting the quenched hot rolled steel plate product to a thermal cycling process including soaking in an intercritical region of temperatures between 720 and 850°C and then holding at a temperature between 360 and 445°C, and quenching the thermally cycled steel product to room temperature, the steel plate product comprising ferrite and retained austenite grains and having a combined ultimate tensile strength and total elongation (UTS-TE) of greater than 25,000 MPa%.

Another aspect of the present invention is to provide a continuously cast, hot rolled, high strength steel plate product produced by the above method.

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

FIG. 1 illustrates the two-step annealing process and plots temperature versus time.

FIG. 2 shows the two-step annealing process and plots temperature versus time.

FIG. 3 shows a two-step annealing process combining two heating steps and an optional zinc-based hot dip coating in one manufacturing facility, plotting temperature versus time.

FIG. 4 is a plot of temperature versus time for the second stage of annealing, which comprises a soak zone and a hold zone in the thermal cycle.

FIG. 5 is an electron backscatter diffraction (EBSD) micrograph showing the microstructure of a high strength steel plate product. FIG. 6 is an electron backscatter diffraction (EBSD) micrograph showing the microstructure of a high strength steel plate product.

FIG. 7 is an optical micrograph of the heat treated steel sheet product shown in FIG. 1, showing dark ferrite grains and light austenite grains.

8 is a bar graph showing the aspect ratio of the austenite grains shown in FIG. 7.

FIG. 9 is a graph of a high strength steel plate product showing the austenite grain size distribution. FIG. 10 is a graph of a high strength steel product showing the ferrite grain size distribution.

2 is an EBSD micrograph showing the microstructure of a high strength steel plate product produced by the process shown in FIG. 1 .

FIG. 12 is an EBSD micrograph showing a steel sheet product produced by the process shown in FIG. FIG. 13 is an EBSD micrograph showing a steel sheet product produced by the process shown in FIG.

FIG. 14 is an EBSD micrograph of a steel plate product produced by the process shown in FIG.

1 is a graph showing the relationship between total elongation and ultimate tensile strength for a comparison of a high strength steel plate product with other steel plate products produced by processing conditions outside the scope of the present invention.

FIG. 16 is a graph showing the relationship between total elongation and ultimate tensile strength for high strength steel products produced in mill trials.

FIG. 17 is a plot of temperature versus time for cold rolled and hot rolled substrates subjected to thermal cycling.

FIG. 18 is an EBSD optical micrograph of the heat treated cold rolled steel substrate shown in FIG. 17, showing dark ferrite grains and lighter retained austenite grains.

FIG. 19 is an EBSD optical micrograph of the hot rolled steel substrate subjected to the heat treatment shown in FIG. 17, showing the dark ferrite grains and the lighter retained austenite grains.

FIG. 20 is a plot of temperature versus time in the first stage of a two-stage heat treatment process in which a hot rolled sheet is quenched on a runout table to produce martensite in accordance with one embodiment of the present invention.

FIG. 21 is a plot of temperature versus time in the second step of a two-step heat treatment process in accordance with one embodiment of the present invention, in which the quench annealed steel sheet of FIG. 17 is subjected to a thermal cycling process.

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

In addition to the amounts of C, Mn, Si, Al, Ti, and Nb listed in Table 1, the steel composition may contain small amounts or impurity levels of other elements, such as, for example, up to 0.015 S, up to 0.03 P, up to 0.2 Cu, up to 0.02 Ni, up to 0.2 Cr, up to 0.2 Mo, up to 0.1 Sn, up to 0.015 N, up to 0.1 V, and up to 0.004 B. As used herein with respect to the composition of steel plate products, the term "substantially free" means that the particular element or material is not intentionally added to the composition and is present only as an impurity or in trace amounts.

In the steel plate product of the present invention, C increases strength and promotes the formation of retained austenite. Mn contributes to hardening and acts as a solid solution strengthener. Si inhibits the precipitation of iron carbide during heat treatment and increases retained austenite. Al inhibits the precipitation of iron carbide during heat treatment and increases retained austenite. Ti and Nb can act as grain refiners that improve strength.

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, in some embodiments, Al may 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.

The steel plate products having the above composition are subjected to a two-stage annealing process, which is described in more detail below. The resulting steel plate products have been found to have favorable mechanical properties, including favorable ultimate tensile strength, high elongation, high lambda values, high bendability, and high yield ratio (YS/UTS).

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

In certain embodiments, the total elongation (TE) of the steel sheet product is typically 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%, such as 22-45%, or 25-40%.

The steel plate products typically have a lambda (λ) value measured by a standard hole expansion test that is greater than 20%, such as greater than 25%, or greater than 30%, or greater than 35%. The whole expansion ratio or lambda may be greater than 20%, such as between 22 and 80%, or between 25 and 60%.

In certain embodiments, high values of both total elongation (TE) and hole spread (λ) provide the steel sheet product with good global and local formability.

In the steel plate products of the present invention, a strength elongation balance (UTS-TE) of greater than 25,000 has been observed, which corresponds to the third generation steel grades highly sought after by industries such as the automotive industry. In certain embodiments, the UTS-TE value can be greater than 26,000, or greater than 27,000, or greater than 30,000.

According to certain embodiments of the present invention, the final microstructure of the steel plate product is predominantly ferrite with a minor amount of retained austenite and a minor amount of fresh martensite, with the ferrite being, for example, at least 50%, up to 80% or more, the retained austenite being, for example, 5-25%, and the fresh martensite being, for example, 0-10% or 15%. The amounts of ferrite, austenite, and martensite can be determined by standard EBSD techniques. Alternatively, the retained austenite content can be determined by magnetic saturation techniques. Unless otherwise stated herein, the volume percentage of retained austenite is determined by EBSD techniques.

In certain embodiments, the retained austenite comprises 1-25 vol%, such as 5-20 vol%. The amount of fresh martensite may be less than 15 vol%, or less than 10 vol%, or less than 5 vol%. In certain embodiments, the steel sheet product is substantially free of fresh martensite. It has been found that amounts of fresh martensite greater than 15% significantly reduce hole expansion values, e.g., significantly reduce local formability.

At least a portion of the ferrite is formed by recrystallization and/or tempering of martensite during the soaking or heating section, or by decomposition of austenite during the cooling and holding section of the thermal cycle, as described below. A portion of the ferrite is considered bainitic ferrite. The ferrite, austenite and martensite phases are fine grained, with an average grain size of less than 10 microns, e.g., less than 5 microns, or less than 3 microns. For example, the grain size of the ferrite is in the range of less than 10 microns, e.g., less than 8 microns, or less than 6 microns. The average grain size of the austenite is in the range of less than 2 microns, e.g., less than 1 micron, or less than 0.5 microns. If martensite is present, the grain size of the martensite is in the range of less than 10 microns, e.g., less than 8 microns, or less than 6 microns.

The austenite grains are substantially equiaxed and have an average aspect ratio of less than 3:1 or less than 2:1, e.g., about 1:1. It has been found that when the amount of retained austenite is less than about 5%, the total elongation (TE) is significantly reduced. It has also been found that amounts of retained austenite greater than 25% are only obtained at very high carbon levels, which results in poor weldability.

In a particular embodiment of the present invention, a two-stage annealing process is used to produce advanced high strength steel products with the above-mentioned good mechanical properties. The first stage is performed in a hot strip mill after continuous casting of the steel slab and hot rolling, including a finish rolling step in the hot strip mill. The finish rolled steel sheet is quenched in the hot strip mill to produce a martensite-dominated microstructure. The second stage includes a soaking at a temperature in the intercritical range followed by a low temperature hold. The second stage can be performed in a continuous annealing line or a continuous galvanizing line. The final steel sheet product contains ferrite and retained austenite. That is, the final steel sheet product has a microstructure that is dominated by ferrite and retained austenite.

In some embodiments, the steel plate products can be produced in a combined continuous casting and hot rolling line, where molten steel is continuously cast to produce a slab of steel that is first subjected to a rough hot rolling to reduce the thickness, and then further reduced in thickness by a final hot rolling. The continuous casting/hot rolling line can include an endless strip production (ESP) process or the like. After the final hot rolling, the steel plate is sent to a run-out cooling table and subjected to a quenching process to a temperature below M _F , for example, using the thermo-mechanical schedule shown in FIG. 20. The quenched hot rolled plate can then be subjected to a second processing step, for example, by using the two-stage thermal cycle shown in FIG. 21.

In some embodiments, stage 1 of the two-stage process is carried out in a continuous casting and hot rolling facility using the processes disclosed in U.S. Patent Nos. 5,329,688, 5,497,821, 6,125,916, 7,343,961, 7,832,460, 7,967,056, 8,025,092, 8,162,032, 8,257,647, and 9,186,721 to Arvedi, which are incorporated herein by reference.

In each of the first and second annealing or thermal cycle steps, several methods for performing the heat treatment can be used. Examples of the two-stage annealing process described in U.S. Pat. No. 10,385,419 and U.S. Patent Application Publication No. 2020/0040422 are shown in FIGS. 1-3 and described below. FIG. 1 shows the manufacturing route of a Continuous Annealing Line (CAL) followed by a Continuous Annealing Line (CAL). FIG. 2 shows the manufacturing route of a CAL + Continuous Galvanizing Line (CGL). FIG. 3 shows a line specially designed to be able to perform both steps of CAL + CAL or CAL + CGL in one facility. In FIG. 3, a Direct-fired Furnace (DFF) followed by a Radiant Tube (RT) furnace embodiment is shown, but other embodiments such as all radiant tube, electric radiant heating, etc. can be used to achieve the desired thermal cycle.

<Step 1>
The purpose of the first stage of the annealing process is to achieve a predominantly martensite microstructure in the continuously cast hot rolled steel sheet product. In the first stage, the hot rolled steel sheet has a finish rolling temperature above _A3 , e.g., a finish rolling temperature of 820°C or higher performed in a hot strip mill before quenching. In certain embodiments, the quench annealing temperature is typically in the range of 830-980°C, e.g., 830-940°C, or 840-930°C, or 860-925°C.

The continuously cast and finish rolled steel sheet is quenched to room temperature or to a controlled temperature above room temperature, as will be explained in more detail below. The quenching temperature does not necessarily have to be room temperature, but should be below the martensitic transformation start temperature (M _S ) and preferably below the martensitic transformation finish temperature (M _F ) to produce a predominantly martensite microstructure.

Quenching to a temperature below M _F (typically below 250° C.) can be accomplished by conventional techniques such as water quenching, submerged knife/nozzle water quenching, gas quenching, quenching using a combination of cold water, hot water, hot water, and gas, aqueous solution quenching, other liquid or gas fluid cooling, chilled roll quenching, water mist spray, wet flash cooling, non-oxidizing wet flash cooling, etc. For example, water quenching can be used in Stage 1, similar to that commonly used to cool strip after finish rolling on the run-out table of a conventional hot strip mill, CSP mill, or ESP mill.

Various types of cooling and quenching systems and processes known to those skilled in the art can be adapted for use in the process of the present invention. Suitable cooling/quenching systems and processes conventionally used on a commercial basis include water quenching, water mist cooling, dry flash, wet flash, oxidative cooling, non-oxidative cooling, alkane fluid to gas phase conversion cooling, hot water quenching, including two-stage water quenching, roll quenching, high percentage hydrogen or helium gas jet cooling, etc. For example, the dry flash and/or wet flash oxidative and non-oxidative cooling/quenching disclosed in International Publication No. WO 2015/083047 by Fives Stein can be used. Other Fives Stein patents describing cooling/quenching systems and processes that may be adapted for use in the process of the present invention include U.S. Pat. Nos. 6,464,808 B2, 6,547,898 B2, 8,918,199 B2, and U.S. Patent Application Publication Nos. 2009/0158975 A1, 2009/0315228 A1, and 2011/0266725 A1. Other examples of cooling/quenching systems and processes that may be adapted for use in the process of the present invention include U.S. Pat. Nos. 8,359,894 B2, 8,844,462 B2, 7,384,489 B2, and U.S. Patent Application Publication Nos. 2002/0017747 A1 and 2014/0083572 A1.

In certain embodiments, after the steel is quenched to produce martensite, the martensite is optionally tempered to soften the steel somewhat and make it easier to further process. Tempering is done by increasing the temperature of the steel from room temperature to about 500°C, e.g., 30-500°C, in a continuous annealing process and holding for up to 600 seconds, e.g., 1-600 seconds. Tempering can also be done in a batch annealing process, reaching the same temperature over a period of several days. For example, intermediate batch tempers can be done at temperatures of 125-500°C for up to 7 days. If tempering is used, the tempering temperature can be held constant, but can also be varied within this preferred range.

After tempering, the temperature can be ramped down to room temperature. The ramp-down rate is typically up to 40°C/sec, and can range, for example, from 0.1 to 20°C/sec.

<Step 2>
The second stage of the heat treatment process includes a first soaking step at a relatively high soaking temperature and a second holding step at a relatively low temperature. These steps are defined as the "soaking" zone and the "holding" zone, as depicted in Figures 4 and 21. The temperatures are controlled to promote the production of the desired microstructure in the final product.

In the first soaking step of the second stage, the temperature of the soaking zone is in the intercritical region between _A1 and _A3 , for example, a soaking temperature of 720° C. or higher is used. In certain embodiments, the soaking temperature is typically in the temperature range of 720 to 850° C., for example, 760 to 825° C. In certain embodiments, the peak annealing temperature can be held for typically at least 15 seconds, for example, 20 to 300 seconds, or 30 to 150 seconds.

In the first step of the second stage, the temperature of the soaking zone can be achieved by heating the steel from a relatively low temperature, lower than Ms, e.g., room temperature, at an average rate of 0.5-50°C/s, e.g., about 2-20°C/s. In certain embodiments, the ramp-up takes 25-800 seconds, e.g., 100-500 seconds. The heating in the first step of the second stage can be achieved by any suitable heating system or process, such as radiant heating, induction heating, heating in a direct-fired furnace, etc.

After the soaking temperature is reached and held for a predetermined time, the steel is cooled to a holding zone at a controlled temperature above room temperature. In certain embodiments, the steel sheet product is maintained at a temperature above 300°C between the second soaking and the second holding. Cooling from the soaking zone to the holding zone can be accomplished by conventional techniques such as water cooling, gas cooling, etc. Average cooling rates of 5-400°C/sec can typically be used. Any suitable type of cooling and quenching system can be used to cool from the soaking temperature to the holding temperature.

In an embodiment of the invention, the holding zone stage is typically carried out at a temperature of 360-445°C, for example 370-440°C. The holding zone can be held for up to 800 seconds, for example 30 seconds to 600 seconds.

The temperature of the holding zone can be kept constant or can vary somewhat within the preferred temperature range. If the steel is to be hot-dip coated after the holding step, the steel can be reheated by induction heating or other heating methods and then placed in a hot-dip coating pot at the appropriate temperature to achieve good hot-dip coating results.

In certain embodiments, after the temperature of the holding zone is maintained for a desired period of time, the temperature can be ramped down to room temperature. Such ramping typically takes 10-1000 seconds, such as about 20-500 seconds. The rate of such ramping typically ranges from 1-1000°C/sec, such as about 2-20°C/sec.

Examples of heating systems suitable for use in the soaking and holding thermal cycle steps of the present invention are disclosed in U.S. Patent Nos. 5,798,007, 7,368,689, 8,425,225, and 8,845,324, all to Fives Stein, U.S. Patent Application Publication No. 2009/0158975, and International Publication WO/2015083047. Further examples of heating systems suitable for use in the soaking and holding thermal cycle steps of the present invention are U.S. Patent No. 7,384,489, all to Drever International, and U.S. Patent No. 9,096,918, all to Nippon Steel & Sumitomo Metal Corporation. However, other suitable known heating systems and processes may also be adapted for use in stages 1 and 2.

According to certain embodiments, the second stage thermal cycle treatment can be performed in a continuous annealing line (CAL). After the CAL process, the steel can 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. The galvanizing temperature is typically in the range of 440-480°C, for example 450-470°C. In certain embodiments, the galvanizing step can be performed as part of a second step annealing treatment in a continuous galvanizing line (CGL), for example as shown in FIG. 2. This CGL step can be used to produce hot-dip galvanized products based on zinc or zinc alloys, and can also produce iron-galvanized type coated products by reheating after coating. An optional nickel-based coating step can be performed before the CGL step to improve the zinc coating properties. The use of a continuous galvanizing line in the second stage can improve the production efficiency of GEN3 coated products.

The following examples are not intended to limit the scope of the invention.

Example 1
A cold rolled steel sheet having the composition of Sample No. 1 in Table 2 was subjected to the two-step annealing process shown in Figure 1. The microstructure of the resulting product is shown in Figures 5 and 6. Figure 5 shows the dark ferrite grains and the light austenite grains by EBSD technique using commercial EDAX orientation image microscope software.

Example 2
A cold rolled steel sheet having the composition of sample No. 2 in Table 2 was subjected to the two-stage annealing process shown in Figure 1. The microstructure of the resulting product is shown in Figure 11. The mechanical properties of sample No. 2 are listed in Table 2. The grain size distribution of austenite is shown in Figure 9 and the grain size distribution of ferrite is shown in Figure 10. The average grain size of austenite is less than 1 micron and the average grain size of ferrite is less than 10 microns.

The microstructure includes about 80 volume percent ferrite with an average grain size of about 5 microns, about 10 volume percent retained austenite with substantially equiaxed grains with an average grain size of about 0.5 microns, and about 10 volume percent fresh martensite with an average grain size of about 5 microns. The mechanical properties of Sample No. 1 are listed in Table 2.

Example 3
A cold rolled steel sheet having the composition of sample No. 3 in Table 2 was subjected to a two-step annealing process as shown in Figure 2. The microstructure of the resulting product is shown in Figures 12 and 13. In Figure 13, austenite is light in color and ferrite is dark in color. The mechanical properties of sample No. 3 are listed in Table 2.

Example 4
A cold rolled steel sheet having the composition of sample No. 4 in Table 2 was subjected to a two-step annealing process as shown in Figure 3. The microstructure of the resulting product is shown in Figure 14, where austenite is light in color and ferrite is dark in color. The mechanical properties of sample No. 4 are listed in Table 2.

Example 5
A cold-rolled steel sheet having the composition of Sample No. 5 in Table 2 was subjected to the two-stage annealing process shown in Figure 1. The mechanical properties of Sample No. 5 are listed in Table 2.

Example 6
A cold rolled steel sheet having the composition of Sample No. 6 in Table 2 was subjected to the two-step annealing process shown in Figure 1. The mechanical properties of Sample No. 6 are listed in Table 2. Figure 7 is an optical image showing the microstructure of a steel having the composition of Sample No. 6 in Table 2, which was subjected to the two-step annealing process shown in Figure 1. In Figure 7, the dark areas in the photograph are ferrite grains and the light areas are austenite grains. Figure 8 is a graph showing the aspect ratio of the austenite grains shown in Figure 7. The optical image of Figure 7 was used to determine the aspect ratio of the austenite grains using image analysis by commercially available software. Figure 7 shows that the average aspect ratio of the austenite grains is less than 3:1.

Example 7
A cold-rolled steel sheet having the composition of Sample No. 7 in Table 2 was subjected to the two-stage annealing process shown in Figure 2. The mechanical properties of Sample No. 7 are listed in Table 2.

Example 8
A cold-rolled steel sheet having the composition of Sample No. 8 in Table 2 was subjected to the two-stage annealing process shown in Figure 3. The mechanical properties of Sample No. 8 are listed in Table 2.

The UTS values of the steels in Examples 1 to 8 were in the range of 700 to 1,100 MPa.

<Comparative Examples 1 to 4>
Cold-rolled steel sheets having the compositions of Sample Nos. C1 to C4 in Table 2 were subjected to the two-step annealing process shown in Figure 1. The mechanical properties of Sample Nos. C1 to C4 are listed in Table 2. The UTS values of the steels of Comparative Examples 1 to 4 were less than 700 MPa.

<Comparative Examples 5 to 8>
Cold-rolled steel sheets having the compositions of Sample Nos. C5 to C8 in Table 2 were subjected to the two-step annealing process shown in Figure 1. The mechanical properties of Sample Nos. C5 to C8 are listed in Table 2. The UTS values of the steels of Comparative Examples 5 to 8 were above 1,100 MPa.

<Comparative Examples 9 to 11>
Cold rolled steel sheets having compositions of Sample Nos. C9 to C11 in Table 2 were subjected to the two-step annealing process shown in Figure 1, except that the soaking or holding temperature of the second annealing was outside the preferred range of the present invention. The mechanical properties of Sample Nos. C9 to C11 are listed in Table 2.

<Comparative Example 12>
A cold rolled steel sheet having the composition of Sample No. C12 in Table 2 was subjected to a two-step annealing process similar to that shown in Figure 2, except that the holding zone of the second anneal was outside the preferred range of the present invention. The mechanical properties of Sample No. C12 are listed in Table 2.

Figure 15 is a plot of total elongation (TE) versus ultimate tensile strength (UTS) for Samples No. 1-8 of Examples 1-8 and Samples No. C1-C12 of Comparative Examples. A line corresponding to a UTS-TE of 25,000 is plotted generally in Figure 15. As shown, the high strength steel plate samples produced in accordance with the present invention have a superior combination of strength and elongation compared to the comparative examples. That is, the examples of the present invention exhibit high total elongation at high UTS values. The steels of Samples 1-8 fall within the category of third generation advanced high strength steels that are highly preferred for the automotive and other industries.

<Example 9>
Factory trials were conducted on samples designated M1-M5 in Table 3 below using either the CAL+CAL process or the CAL+CGL process. For samples M1, M2 and M5, the CAL+CAL process time and temperature shown in Figure 1 was used. For samples M3 and M4, the CAL+CGL process time and temperature shown in Figure 2 was used.

Figure 16 shows the strength-elongation balance of the factory experimental materials, all of which met the minimum UTS-TE of 25,000. The lambda values of the test materials were greater than 20%.

The cold-rolled and hot-rolled steel sheets having compositions corresponding to Samples No. 9A to 12B in Table 4, and containing 0.23 wt% C, 2.3 wt% Mn, 0.6 wt% Si, and 0.8 wt% Al, were subjected to the two-step annealing process shown in FIG. 17. In Table 4, the cold-rolled samples are indicated as CR in the column for Substrate Type, and the hot-rolled samples are indicated as HR. The mechanical properties of Samples No. 9A to 12B are shown in Table 4. The hot-rolled substrate samples showed excellent YS, UTS, TE, and hole expansion properties equivalent to those of the cold-rolled samples. This indicates that the hot-rolled substrates directly processed in the two-step annealing process can achieve third generation AHSS properties. Furthermore, the hot rolled material exhibits a similar austenite content, distribution and morphology to the cold rolled material, as shown in the EBSD phase maps of Figures 18 and 19, where the retained austenite grains appear brighter than the ferrite grains. Figure 18 shows the austenite content of cold rolled sample 11A, and Figure 19 shows the austenite content of hot rolled sample 12A. In both microstructures, the austenite is finely divided and mainly equiaxed.

According to one embodiment of the present invention, a hot rolled steel sheet with the composition C=0.29%, Mn=1.85%, Si=1.55%, A1=0.04%, and the balance typical of steel, was subjected to a two-stage process shown in Figures 20 and 21. In stage 1, martensite is produced in a hot strip mill by finish rolling using the thermomechanical schedule shown in Figure 20 followed by water quenching. At the end of this thermal cycle, an optional overaging step (not shown) can be added to soften and toughen the hot band for further processing. In stage 2, the thermal cycle shown in Figure 21 is performed. The resulting mechanical properties were YS=864 MPa, UTS=864 MPa, total elongation=31.2%, UTS x TE=26957 MPa%.

As used herein, the terms "including, comprising," "containing," and the like are open-ended terms and are understood to not exclude the presence of additional elements, materials, phases, or process steps not described in this application. As used herein, the term "consisting of" is understood to exclude the presence of any unspecified elements, materials, phases, or process steps. As used herein, the term "consisting essentially of" is understood to include the specified elements, materials, phases, or process steps, if applicable, and to include any unspecified elements, materials, phases, or process 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. However, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

It should also be understood that all numerical ranges described herein are intended to include all subranges subsumed therein. For example, a range of "1" to "10" is intended to include the subranges between a minimum value of 1 and a maximum value of 10, where the minimum value is 1 or greater than 1 and the maximum value is 10 or less than 10.

In this application, the use of the singular includes the plural and the plural includes the singular, unless expressly stated otherwise. Also, in this application, the use of "or" means "and/or," even if the word "and/or" is explicitly used in a particular embodiment, unless expressly stated otherwise. In this specification and claims, the articles "a," "an," and "the" include plural referents unless expressly and unambiguously limited to one referent.

While particular embodiments of the invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that many variations in the details of the invention can be made without departing from the invention.

Claims (19)

1. A method for producing a hot rolled steel sheet product having high strength , comprising the steps of:
(a) continuously casting a steel slab consisting essentially of 0.15-0.5 wt.% C, 1-3 wt.% Mn, 0.8-3 wt.% of a combination of Si and Al, with the balance being Fe and impurities ;
(b) a hot rolling step of hot rolling the continuously cast steel slab, the hot rolling step including a finish rolling step of finish rolling the slab at a finish rolling temperature of 820°C or more in a hot strip mill, and forming a hot rolled steel plate in the hot strip mill;
(c) a hot-rolled steel sheet quenching step of quenching the hot-rolled steel sheet in the hot strip mill to generate a microstructure mainly composed of martensite;
(d) subjecting the quenched hot-rolled steel sheet to a thermal cycle process, the thermal cycle process comprising soaking the hot-rolled steel sheet in a critical region at a temperature of 720 to 850°C, and then holding the hot-rolled steel sheet at a temperature of 360 to 445°C;
(e) quenching the hot-rolled steel sheet subjected to the thermal cycle step to room temperature;
A hot-rolled steel sheet product is manufactured by the steps (a) to (e),
The method of claim 1, wherein the hot rolled steel sheet product comprises ferrite grains and retained austenite grains and has a combined ultimate tensile strength and total elongation, UTS·TE, greater than 25,000 MPa%. 2. The method of claim 1, wherein the hot rolling step (b) occurs immediately after the continuous casting step (a) . 2. The method of claim 1, wherein the hot rolled steel sheet quenching step (c) in the hot strip mill is performed on a run-out table of the hot strip mill. 2. The method of claim 1, wherein the step (c) of quenching the hot rolled steel sheet in the hot strip mill comprises water cooling. 2. The method according to claim 1, wherein the hot rolled steel sheet quenching step (c) in the hot strip mill is carried out at a cooling rate of 30 to 1000 ° C./s. 2. The method of claim 1, wherein the hot rolling step (b) further comprises rough rolling the continuously cast steel slab prior to the finish rolling step. 2. The method according to claim 1, wherein the finish rolling temperature of the finish rolling step of the hot rolling step (b) is 830 to 940°C. 2. The method of claim 1, further comprising subjecting the quenched hot rolled steel sheet to an intermediate tempering step prior to the thermal cycling step (d) . The method according to claim 8, wherein the intermediate tempering step is carried out at a temperature between 30 and 500° C. for up to 7 days. The method according to claim 8, wherein the intermediate tempering step is carried out at a temperature of from 125 to 500°C. The method of claim 1, wherein C is greater than 0.2% by weight. 2. The method of claim 1, comprising 0.2-0.4 wt.% C , 1.3-2.5 wt.% Mn , 0.2-1.8 wt.% Si, and up to 1.5 wt.% Al . 2. The method of claim 1, wherein the ferrite comprises at least 50% by volume of the hot rolled steel product and the retained austenite comprises 5-25% by volume of the hot rolled steel product. 2. The method of claim 1, wherein the hot rolled steel product comprises less than 15 volume percent fresh martensite. 2. The method of claim 1, wherein the hot rolled steel sheet product has an ultimate tensile strength of 720-1100 MPa and a total elongation of 20% or more. 2. The method of claim 1, wherein the hot rolled steel sheet product has a hole expansion ratio of greater than 20%. 10. The method of claim 1 further comprising the step of applying a zinc-based coating to said hot rolled steel sheet after holding said hot rolled steel sheet in said thermal cycling step (d) . The steel slab is
Optionally, Ti is 0 to 0.05 wt. %; and
10. The method of claim 1 , optionally comprising 0-0.05 wt.% Nb . 2. The method of claim 1, wherein the steel slab is allowed to contain at least one other element selected from the group consisting of: maximum 0.015 wt.% S, maximum 0.03 wt.% P, maximum 0.2 wt.% Cu, maximum 0.02 wt.% Ni, maximum 0.2 wt.% Cr, maximum 0.2 wt.% Mo, maximum 0.1 wt.% Sn, maximum 0.015 wt.% N, maximum 0.1 wt.% V, and maximum 0.004 wt.% B.

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