CN109563603B - High yield strength steel - Google Patents

Abstract

The present disclosure relates to high yield strength steels, wherein the yield strength can be increased without significantly affecting Ultimate Tensile Strength (UTS), and in some cases higher yield strengths can be obtained without significantly reducing ultimate tensile strength and total elongation.

Description

High yield strength steel

Cross Reference to Related Applications

The present application claims the benefit of U.S. provisional patent application serial No. 62/359,844 filed on 8/7/2016 and U.S. provisional patent application serial No. 62/482,954 filed on 7/4/2017, which are incorporated herein by reference in their entirety.

Technical Field

The present disclosure relates to high yield strength steels. Due to the unique structure and mechanism, yield strength can be increased without significantly affecting Ultimate Tensile Strength (UTS) and, in some cases, higher yield strength can be obtained without significantly reducing ultimate tensile strength and total elongation. These new steels may provide benefits for a number of applications (e.g., the passenger compartment of an automobile) where relatively high yield strength, as well as relatively high UTS and total elongation are desired.

Background

Third generation Advanced High Strength Steels (AHSS) are currently being developed for automotive applications, and in particular automotive body applications. Advanced High Strength Steel (AHSS) steels are classified by tensile strength greater than 700MPa and elongation from 4% to 30% and include types such as Martensitic Steels (MS), Dual Phase (DP) steels, transformation induced plasticity (TRIP) steels, and Complex Phase (CP) steels. An example target for generation 3 AHSS is provided in the banana-type chart of vehicle body Steel published by World Auto Steel (fig. 1).

Tensile properties such as Ultimate Tensile Strength (UTS) and total elongation are important benchmarks for establishing a combination of properties. AHSS materials, however, are not generally classified by Yield Strength (YS). The yield strength of the material is also of great importance to automotive designers because once a part is in service and if the part is stressed beyond yield, the part will permanently (plastically) deform. Materials with high yield strength resist permanent deformation to higher stress levels than those with lower yield strength. This resistance to deformation is useful by subjecting the structure made of the material to greater loads before the structure is permanently bent and deformed. Materials with higher yield strengths may thus enable automotive designers to reduce the associated part weight through gauge thinning while maintaining the same resistance to deformation in the part. Many types of emerging grades of third generation AHSS suffer from low initial yield strength despite various combinations of tensile strength and ductility.

Based on most design criteria, components in automobiles that undergo early yielding and undergo permanent plastic deformation during normal service would be unacceptable. However, a lower yield strength in the event of a collision, especially when combined with a high strain hardening coefficient, may be beneficial. This is particularly true in the front and rear ends of the passenger compartment, commonly referred to as the buffer zone. In these regions, due to the high initial ductility, a lower yield strength material with higher ductility may deform and strain harden to increase strength during a crash event resulting in a high level of energy absorption.

For other areas of the automobile, a low yield strength would be unacceptable. In particular, this will include the so-called passenger compartment of the car. In the passenger compartment, the material utilized must have a high yield strength as only very limited deformation/sinking into the passenger compartment is allowed. Once penetrating the passenger compartment, this can result in injury or death of the occupant(s). Therefore, materials with high yield strength are required for these regions.

The yield strength of the material can be increased in many ways on an industrial scale. The material can be cold rolled in small amounts (reduction < 2%) in a process called temper rolling. This process introduces a small amount of plastic strain in the material and slightly increases the yield strength of the material corresponding to the amount of strain the material undergoes during the flattening pass. Another method of increasing yield strength in materials is by reducing the crystal grain size of the material, which is known as Hall-Petch strengthening. Smaller crystal grains increase the shear stress required for initial dislocation movement in the material and delay initial deformation until higher applied loads. The grain size can be reduced by process modifications such as a modified annealing procedure to limit recrystallization that occurs during annealing after plastic deformation and grain growth during the growth process. Modification of the chemical composition of the alloy, such as addition of alloying elements present in solid solution, may also increase the yield strength of the material, however, the addition of these alloying elements must occur when the material is molten and may result in increased costs.

Developing a high yield strength in the passenger compartment from a low yield strength version of AHSS is a possible route. However, it is difficult to uniformly strain harden the finished part in many metalworking operations. This means that although the heavily cold worked areas of the part have much higher yield, there will still be areas of lower yield strength which can then deform and cause unacceptable subsidence into the passenger space.

Cold working steel from the fully annealed state is a known route to increase yield and tensile strength. Cold work can be applied uniformly across the sheet during processing by cold rolling to increase yield and tensile strength. However, this approach results in a reduction in total elongation and typically to a level much less than 20%. As elongation decreases, cold formability also decreases, reducing the ability to produce parts with complex geometries, resulting in reduced AHSS availability. Higher ductility with a minimum of 30% total elongation is generally required to form complex geometries by cold stamping processes. Although processes such as roll forming can be used to produce parts from lower elongation materials, the geometric complexity of the parts from these processes is limited. Cold rolling can also introduce anisotropy into the material, which will further reduce the ability of the material to be cold formed into parts.

Disclosure of Invention

A method of increasing yield strength in a metal alloy, comprising:

a. supplying a metal alloy comprising at least 70 atomic% iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy to 10%^-4K/s to 10³Cooling at a rate of K/s and solidifying to>A thickness of 5.0mm to 500 mm;

b. processing the alloy into a first sheet form having a thickness of from 0.5 to 5.0mm, wherein the first sheet has an X₁(total elongation, (%), Y₁Ultimate tensile strength and Z of (MPa)₁(MPa) yield strength;

c. permanently deforming the alloy at a temperature in the range of 150 ℃ to 400 ℃ into a second sheet form exhibiting one of the following tensile property combinations A or B:

a. (1) Total elongation X₂＝X₁±7.5％；

(2) Ultimate tensile strength Y₂＝Y₁+/-100 MPa; and

(3) yield strength Z₂≥Z₁+100MPa。

B. (1) ultimate tensile Strength Y₃＝Y₁+/-100 MPa; and

(2) yield strength Z₃≥Z₁+200MPa。

Optionally, the second sheet formed in step (c) indicating the above tensile property combination A or B may then be exposed to a permanent set at a temperature of ≦ 150 ℃.

Accordingly, the present invention also relates to a method of increasing yield strength in a metal alloy comprising:

a. supplying a metal alloy comprising at least 70 atomic% iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy to 10%^-4K/s to 10³Cooling at a rate of K/sec and solidifying to>A thickness of 5.0mm to 500 mm;

b. processing the alloy into a first sheet form having a thickness of from 5.0 to 0.5 mm;

c. permanently deforming the alloy into a second sheet form at a temperature in the range of 150 ℃ to 400 ℃;

d. permanently deforming the alloy at a temperature <150 ℃ into a second sheet form exhibiting the following combination of tensile properties:

(1) total elongation of 10.0 to 40.0%;

(2) ultimate tensile strength is 1150 to 2000 MPa;

(3) yield strength is 550 to 1600 MPa.

The metal alloys prepared herein provide particular utility in vehicles, rail tank cars/trailers, drill collars, drill pipes, casings, tool joints, wellheads, compressed gas storage tanks, or liquefied natural gas containers. More particularly, the alloy finds application in vehicle bodies in white, vehicle frames, chassis or panels.

Drawings

The following detailed description may be better understood with reference to the accompanying drawings, which are provided for illustrative purposes and are not to be construed as limiting any aspect of the invention.

FIG. 1 World Auto Steel "banana type plot" of the 3 rd generation AHSS target property.

FIG. 2 is an overview of method 1 for producing high yield strength in the alloys herein.

FIG. 3 is an overview of method 2 to produce a combination of high yield strength and targeted properties in the alloys herein.

FIG. 4 ultimate tensile strength in the alloys herein before and after cold rolling.

FIG. 5 tensile elongation in the alloys herein before and after cold rolling.

FIG. 6 yield strength in the alloys herein before and after cold rolling.

FIG. 7 magnetic phase volume percentages in the alloys herein before and after cold rolling.

FIG. 8 is a tensile stress-strain curve for alloy 2 after cold rolling with various reductions.

Figure 9 back-scattered SEM micrograph of the microstructure in the hot zone (hot band) from alloy 2: a) a low magnification image; b) high magnification images.

FIG. 10 bright field TEM micrograph of the microstructure in the hot zone from alloy 2: a) a low magnification image; b) high magnification images.

Figure 11 shows TEM micrographs of nanoscale precipitates in the hot band from alloy 2.

FIG. 12 Back-scattered SEM micrograph of the microstructure in cold rolled sheet from alloy 2: a) a low magnification image; b) high magnification images.

FIG. 13 TEM micrograph of the microstructure in a cold rolled sheet from alloy 2: a) a low magnification image; b) high magnification images.

Figure 14 shows a TEM micrograph of the nanoscale precipitates found in the alloy 2 sheet after cold deformation.

FIG. 15 is an engineered tensile stress-strain curve for alloy 2 after rolling with 20% reduction at different temperatures.

FIG. 16 change in volume percent magnetic phase (Fe%) in alloy 2 during tensile testing.

FIG. 17 is an engineering stress-strain curve for alloy 7 after rolling with 20% reduction at different temperatures.

FIG. 18 is an engineering stress-strain curve for alloy 18 after rolling with 20% reduction at different temperatures.

FIG. 19 is an engineering stress-strain curve of alloy 34 after rolling with 20% reduction at different temperatures.

FIG. 20 is an engineering stress-strain curve for alloy 37 after rolling with 20% reduction at different temperatures.

FIG. 21 is a representative engineering stress-strain curve of alloy 2 rolled at 200 ℃ to various rolling reductions.

FIG. 22 shows the yield and ultimate tensile strength of alloy 2 as a function of rolling reduction at 200 ℃. Note that the yield strength increases rapidly with increasing rolling reduction, while the ultimate tensile strength increases only slightly.

The yield strength and total elongation of alloy 2 of fig. 23 were varied with rolling reduction at 200 ℃. Note that the yield strength increases rapidly as the rolling reduction increases, while the total elongation decreases slowly up to 30% reduction and decreases rapidly at 40%.

FIG. 24200 ℃ Effect of rolling on deformation induced phase transformation in alloy 2 as a function of rolling reduction. Note that the transition measured in the as-rolled material increases slightly, while the transition after the tensile test decreases rapidly in the range of rolling reductions tested.

Figure 25 back-scattered SEM micrograph of the microstructure in the hot band from alloy 2: a) a low magnification image; b) high magnification images.

FIG. 26 is a back-scattered SEM micrograph of the microstructure in alloy 2 after rolling to 30% reduction at 200 ℃: a) a low magnification image; b) high magnification images.

FIG. 27 is a back-scattered SEM micrograph of the microstructure in alloy 2 after rolling to 70% reduction at 200 ℃: a) a low magnification image; b) high magnification images.

FIG. 28 is a bright field TEM micrograph of the microstructure in alloy 2 after rolling at 10% reduction at 200 ℃: a) low magnification image and b) high magnification image.

FIG. 29 is a bright field TEM micrograph of the microstructure in alloy 2 after rolling at 30% reduction at 200 ℃: a) low magnification image and b) high magnification image.

FIG. 30 is a bright field TEM micrograph of the microstructure in alloy 2 after rolling at 70% reduction at 200 ℃: a) low magnification image and b) high magnification image.

Fig. 31 is an engineering stress-strain curve of alloy 2 processed by a combination of rolling methods.

Note that specific processing condition variations are listed, including hot rolling temper conditions and single or multi-step rolling.

Fig. 32 engineering stress-strain curves for alloy 7 processed by a combination of rolling methods.

Note that specific processing condition variations are listed, including hot rolling temper conditions and single or multi-step rolling.

FIG. 33 is an engineering stress-strain curve for alloy 18 processed by a combination of rolling methods. Note that specific processing condition variations are listed, including hot rolling temper conditions and single or multi-step rolling.

FIG. 34 is an engineering stress-strain curve of alloy 34 processed by a combination of rolling methods. Note that specific processing condition variations are listed, including hot rolling temper conditions and single or multi-step rolling.

FIG. 35 compares the engineering stress-strain curves of alloy 2 sheets processed by different methods and combinations thereof. Note that specific processing condition variations are listed, including hot rolling temper conditions and single or multi-step rolling.

FIG. 36 tensile elongation and magnetic phase volume percent in tensile sample specifications after testing alloy 2 at different temperatures.

FIG. 37 volume percent magnetic phase as a function of rolling reduction at room temperature and at 200 ℃.

FIG. 38 is an example of engineering stress-strain curves for annealed sheets prepared by both cold rolling and rolling at 200 ℃.

FIG. 39 shows the rolling reduction limit and rolling temperature for alloy 2.

Detailed Description

Figure 2 represents an overview of a preferred method 1 of developing high yield strength from low yield strength materials by a route that results in either of two conditions as provided in condition 3a or 3 b. In step 1 of method 1, the starting conditions are to provide a metal alloy. Such metal alloys will comprise at least 70 atomic% iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C. Melting the alloy chemical composition and preferably at 10^-4K/s to 10³Cooling at a rate of K/s and solidifying to>A thickness of 5.0mm to 500 mm. The casting process may be accomplished using a variety of processes, including ingot casting, billet casting, continuous casting, thin slab casting, thick slab casting, thin strip casting, belt casting, and the like. The preferred method would be continuous casting in sheet form by thin slab casting, thick slab casting and thin strip casting. Preferred alloys will exhibit austenite (γ -Fe) fractions of at least 10 volume percent up to 100 volume percent and all increments therebetween over the temperature range of from 150 to 400 ℃.

In step 2 of method 1, the alloy is preferably processed into sheet form having a thickness of from 0.5 to 5.0 mm. This step 2 may comprise hot rolling or hot and cold rolling. If hot rolled, the preferred temperature range will be at a temperature of 700 ℃ and less than the Tm of the alloy. If cold rolling is used, it is understood that such temperatures are at room temperature. Note that after hot rolling or hot and cold rolling, the sheet may additionally be heat treated, preferably in the range from at a temperature of 650 ℃ to a temperature below the melting point (Tm) of the alloy.

The step of preparing the sheet from the cast product may thus vary depending on the particular manufacturing route and the particular objective. As an example, consider thick slab casting as one process route to obtain a sheet with this target thickness. The alloy will be cast, preferably by means of a water cooled mould, to a thickness typically in the range of 150 to 300mm in thickness. The cast ingot, preferably after cooling, is then prepared for hot rolling, which may include some surface treatment to remove surface defects (including oxides). The spindle will then pass through a roughing mill hot roll, which may include several passes, producing a slab of intermediate bars (transfer bar) typically from 15 to 100mm in thickness. This intermediate strip will then pass through successive finishing hot rolling stands to produce hot strip coils, typically from 1.5 to 5.0mm in thickness. Cold rolling can be accomplished in various reductions per pass, varying numbers of passes, and different rolling mills (including tandem, Z-mill, and reversing mill) if additional gauge reduction is desired. Typically the cold rolled thickness will be 0.5 to 2.5mm thick. Preferably, the cold rolled material is annealed in a temperature range from 650 ℃ to a temperature less than the melting point (Tm) of the alloy to restore ductility partially or completely lost from the cold rolling process.

Another example would be to work the cast material, preferably by a thin slab casting process. In this case, after the rolling, which is usually made 35 to 150mm thick by passing through a water-cooled die, the newly formed slab goes directly to hot rolling without directly raising the slab to the target temperature by cooling with an auxiliary tunnel furnace or applying induction heating. The slab is then hot rolled directly in a plurality of (multi-stand) finishing mills, preferably in a number of from 1 to 10. After hot rolling, the strip is rolled into a hot band coil having a typical thickness of from 1 to 5 mm. If further processing is required, cold rolling may be applied in a similar manner as above. Note that the billet casting will be similar to the above example but can be cast to greater thicknesses, typically from 200 to 500mm thick, and an initial breaker step is required to reduce the initial casting thickness to allow it to pass through a hot roughing mill.

Although the particular process from casting the material in step 1 to step 2, once the sheet is formed in the preferred range from 0.5mm to 5.0mm, the sheet will exhibit X₁(total elongation, (%), Y₁Ultimate tensile strength and Z of (MPa)₁(MPa) yield strength. Preferred properties for such an alloy would be an ultimate tensile strength value of from 900 to 2050MPa, a total elongation of from 10 to 70%, and a yield strength in the range of from 200 to 750 MPa.

In step 3 of method 1, the alloy is permanently (i.e., plastically) deformed at a temperature in the range of from 150 ℃ to 400 ℃. Such permanent deformation may be provided by rolling and causing a reduction in thickness. This can be done, for example, in the final stages of steel coil development. It is now preferred to perform elevated temperature rolling, which is done in the target temperature range of 150 to 400 ℃, rather than performing traditional cold rolling with sheets starting at room temperature for final gauge reduction. One approach would be to heat the sheet to a target temperature range prior to passing through the cold mill. The sheet can be heated by various methods, including by tunnel rolling, radiant heaters, resistive heaters, or induction heaters. Another approach would be to directly heat the thinning rolls. A third example for illustration would be a low temperature batch annealed sheet and then sent to cold rolling mill(s) at the target temperature range. Alternatively, the sheet may be deformed into the part in an elevated temperature range using various processes (including roll forming, metal stamping, metal drawing, hydroforming, etc.) that provide permanent deformation during the manufacture of the part by various methods.

Although a particular process for permanently deforming an alloy in the temperature range of 150 to 400 ℃, two different conditions may be created, which are shown in fig. 2 as condition 3a and condition 3 b. In Condition 3a, the results in step 2 and after step 3 are comparedThe alloy, total elongation and ultimate tensile strength are relatively unaffected but yield strength is increased. In particular, the total elongation X₂Is equal to X₁7.5% and tensile strength Y₂Is equal to Y₁100MPa, and yield strength Z₂≥Z₁+100 MPa. The preferred property for such an alloy in condition 3a would be the ultimate tensile strength value (Y)₂) From 800 to 2150MPa, tensile elongation (X)₂) From 2.5% to 77.5%, and yield strength (Z)₂) Not less than 300 MPa. More preferably, the yield strength may fall within the range of 300 to 1000 MPa.

In condition 3b, the ultimate tensile strength is relatively unaffected but the yield strength is increased compared to the alloy in step 2 and after step 3. In particular, ultimate tensile strength Y₃Is equal to Y₁100MPa and yield strength Z₂≥Z₁+200 MPa. The preferred property for such an alloy in condition 3b would be the ultimate tensile strength value (Y)₃) From 800 to 2150MPa, and yield strength (Z)₃) Not less than 400 MPa. More preferably, the yield strength may fall within the range of 400 to 1200 MPa. In addition, unlike condition 3a, the total elongation decreased by more than 7.5%, i.e., the total elongation (X) in step B₃) Is defined as follows: x₃<X₁-7.5％。

As will be shown by the various example embodiments, with normal deformation, the metallic material will strain/work harden. This is for example achieved by between stress (σ) and strain (ε) σ ═ K εⁿThe strain hardening index (n) in the relationship is shown. The result is that the basic material properties change when the material is permanently deformed. Comparing the initial conditions with the final conditions will show typical and expected behavior, with an increase in yield strength and tensile strength and a corresponding decrease in overall ductility. Specific case examples are provided to illustrate this effect and then compare this effect with the new material behavior mentioned in this disclosure.

Fig. 3 confirms an overview of method 2 of the present disclosure. The first 3 steps in method 2 are the same as method 1, with step 4 being an additional step to method 2. Step 4 may be applied as shown to the alloy herein under condition 3a or condition 3 b.

As presented previously, in the description of fig. 2, a combination of properties (i.e., total elongation, ultimate tensile strength, and yield strength) is provided for each condition 3a or 3 b. As will be further explained in the detailed description and the subsequent case examples, the alloys under conditions 3a or 3b can also be characterized by their specific structure. This then also allows tailoring of the final properties by using a further optional step of permanently deforming the alloy at a temperature from ambient to ≦ 150 ℃, or more preferably in the temperature range of 0 ℃ to 150 ℃. This can be done, for example, by adding another step in the steel coil manufacturing process as illustrated in fig. 3. Step 4 in this case can be a temper rolling from 0.5 to 2.0% reduction (i.e. a small reduction rolling pass which is also sometimes used for surface quality improvement or levelling) or at a greater reduction from > 2% to 50% to develop a specific combination of properties. The alternating pattern may be performed, for example, in the manufacture of parts from sheet material processed by method 1. In optional step 4 of method 2, the sheet may be subsequently formed into a part using various deformation methods, including roll forming, metal stamping, metal drawing, hydroforming, and the like. While the exact method activates step 4 in method 2, the alloy can be used to develop final properties that are expected to exhibit properties with tensile elongation from 10 to 40%, ultimate tensile strength from 1150 to 2000MPa, and yield strength from 550 to 1600 MPa.

Alloy (I)

The organization and mechanism leading to the new process route for developing high yield strength in this application relies on the following chemical composition of the alloys provided in table 1.

TABLE 1

Chemical composition of alloy (atomic%)

As can be seen from table 1, the alloys herein are iron-based metal alloys having greater than 70 atomic% Fe. Additionally, it can be appreciated that the alloys herein are such that they comprise Fe and at least four or more, or five or more, or six elements selected from Si, Mn, Cr, Ni, Cu, or C. Thus, with respect to the presence of four or more, or five or more elements selected from Si, Mn, Cr, Ni, Cu or C, such elements are present in the atomic percentages indicated below: si (0 to 6.13 atomic%), Mn (0 to 15.17 atomic%), Cr (0 to 8.64 atomic%), Ni (0 to 9.94 atomic%), Cu (0 to 1.86 atomic%), and C (0 to 3.68 atomic%). Most preferably, the alloys herein are such that they comprise, consist essentially of, or consist of 70 at% or greater levels of Fe and Si, Mn, Cr, Ni, Cu and C, with impurity levels of all other elements in the range of from 0 to 5000ppm, at 70 at% or greater levels of Fe and Si, Mn, Cr, Ni, Cu and C.

Laboratory slab casting

The alloy was weighed to 3400 gram charge using commercially available iron additive (ferroaddive) powder and base steel feedstock with known chemical composition according to the atomic ratios in table 1. As mentioned above, impurities may be present at various levels depending on the starting materials used. The impurity elements will typically include the following elements: al, Co, Mo, N, Nb, P, Ti, V, W, and S, if present, will be in the range of from 0 to 5000ppm (parts per million), with a preferred range of from 0 to 500 ppm.

The charge was charged to a zirconia coated silica crucible placed in an Indutherm VTC800V vacuum tumble caster. The machine then evacuates the casting chamber and melting chamber prior to casting and fills to atmospheric pressure twice with argon to prevent oxidation of the melt. The melt was heated using a 14kHz RF induction coil until completely melted, approximately from 5 to 7 minutes, depending on the alloy composition and the charge mass. After the last solid was observed to melt it was heated for an additional 30 to 45 seconds to provide superheat and ensure melt homogeneity. The caster then evacuates the chamber and turns the crucible over and pours the melt into a 50mm thick, 75 to 80mm wide and 125mm deep channel in a water-cooled copper mold and will represent step 1 in fig. 2 and 3. The process can be adapted to preferred as-cast thicknesses ranging from >5.0 to 500 mm. The melt was allowed to cool under vacuum for 200 seconds before filling the chamber with argon to atmospheric pressure.

Laboratory hot rolling

The alloys herein are preferably processed into laboratory sheets. A laboratory alloy process was developed to simulate the hot strip preparation from slabs prepared by continuous casting and will represent step 2 in fig. 2 and 3. Industrial hot rolling is performed by heating the slab in a tunnel furnace to a target temperature and then passing the slab through either a reversing mill or a multi-stage mill or a combination of both in a preferred temperature range from 700 ℃ up to the melting point (Tm) of the alloy to reach the target gauge. The temperature of the slab steadily decreases during rolling on either mill type due to heat loss to the air and work rolls, so the final hot strip is at a much reduced temperature. This was simulated in the laboratory by heating to between 1100 ℃ and 1250 ℃ in a tunnel furnace and then hot rolling. Laboratory rolling mills are slower than industrial rolling mills, causing greater heat loss at each hot rolling pass, so the slab is reheated for 4 minutes between passes to reduce the temperature drop, the final temperature at the target specification when leaving the laboratory rolling mill is typically in the range from 1000 ℃ to 800 ℃, depending on the furnace temperature and final thickness.

Prior to hot rolling, the laboratory slabs were preheated in a Lucifer EHS3GT-B18 furnace. Depending on the alloy melting point and the point in the hot rolling process, the furnace set point is varied between 1100 ℃ to 1250 ℃, with the initial temperature set higher to promote greater thinning and the latter temperature set lower to minimize surface oxidation on the hot strip. The slabs were soaked for 40 minutes prior to hot rolling to ensure they reached the target temperature and then pushed out of the tunnel oven into a Fenn Model 0612 automatic mill (high rolling mill). The 50mm castings were hot rolled by rolling mills for 5 to 10 passes before being allowed to air cool. The final thickness after hot rolling preferably ranges from 1.8mm to 4.0mm, with a variable reduction per pass ranging from 20% to 50%.

After hot rolling, the slab thickness has been reduced to a hot band final thickness of from 1.8mm to 2.3 mm. The process conditions can be adjusted by varying the amount of hot rolling and/or adding a cold rolling step to produce a preferred thickness range from 0.5 to 5.0 mm. Tensile specimens were cut from the laboratory hot strip using wire EDM. Tensile properties were measured on an Instron mechanical test frame (model 3369) using the Instron Bluehill control and analysis software. The tensile properties of the alloy, which has been processed to a thickness of from 1.8 to 2.3mm, under hot rolled conditions are listed in table 2.

The ultimate tensile strength values may vary from 913 to 2000MPa, while the tensile elongation varies from 13.8 to 68.5%. The yield strength is in the range from 250 to 711 MPa. The mechanical properties of the hot band from the steel alloys herein depend on the alloy chemistry, the processing conditions, and the material mechanical response to the processing conditions.

TABLE 2

Tensile properties of alloys under hot rolling conditions

Example embodiments

Control example #1 conventional response to ambient temperature rolling

For comparison purposes, the hot strip from the alloys listed herein in table 1 was cold rolled to a final target gauge thickness of 1.2mm by multiple cold rolling passes. Tensile coupons were cut from each cold rolled sheet using wire EDM. Tensile properties were measured on an Instron mechanical test frame (model 3369) using the Instron Bluehill control and analysis software. All tests were run in displacement control at ambient temperature.

The tensile properties of the alloys herein after cold rolling are listed in table 3. As can be seen, the yield strength increased significantly in the tropical range, with a maximum of 711MPa (table 2). The yield strength after cold rolling varied from 1037 to 2000 MPa. The ultimate tensile strength value after cold rolling is in the range of from 1431 to 2222 MPa. However, a decrease in tensile elongation was recorded for each of the alloys herein after cold rolling, varying from 4.2 to 31.1%. The general trend of the effect of cold rolling on the tensile properties of the alloys herein is illustrated in fig. 4-6.

TABLE 3

Tensile properties of the alloy at final gauge after cold rolling

The relative magnetic phase content was measured by Feritscope for each of the alloys herein listed in Table 4 both in the hot band and after cold rolling and the selected alloys are illustrated in FIG. 7. The volume percentage of the magnetic phase in the hot band of 0.1 to 56.4 Fe% increases to the range from 1.6 to 84.9 Fe% after cold rolling, confirming the phase transformation during deformation.

TABLE 4

Volume percent magnetic phase in alloy after cold rolling (Fe%)

This control example demonstrates that the yield strength in the alloys herein can be improved by cold rolling (i.e., at ambient temperature). Ultimate tensile strength is also increased but cold rolling results in a significant reduction in alloy ductility indicated by the reduction in tensile elongation, which may be a limiting factor in certain applications. Strengthening as shown by the increase in ultimate tensile strength involves a phase transformation of austenite to ferrite as described by measurements of magnetic phase volume percentages before and after cold rolling.

Control example # 2: effect of Cold Rolling reduction on yield Strength in alloy 2

Alloy 2 was processed into a hot band having a thickness of 4.4 mm. The hot strip is then cold rolled in multiple cold rolling (i.e., at ambient temperature) passes with different reductions. After cold rolling, the samples were heat treated with an intermediate anneal at 850 ℃ for 10 minutes. This represents the starting condition for each alloy, which represents the fully annealed condition that removed the prior cold work. From this starting condition, subsequent cold rolling at different percentages (i.e., 0%, 4.4%, 9.0%, 15.1%, 20.1%, 25.1%, and 29.7%) as provided in table 5 was applied so that the final gauge for the tensile test would be a target constant thickness of 1.2 mm. The corresponding increase in material yield strength is indicated by the tensile stress-strain curve in fig. 8 with increasing cold rolling reduction as the final step after annealing. The tensile properties from the test are listed in table 5. The yield strength of alloy 2 increased to a range from 666 to 1140MPa, depending on the level of thinning compared to the initial value in the annealed state (table 5). Also, as shown in table 5, the volume percent of magnetic phase measured by fetiscope increased up to 12.9 Fe% compared to the initial value of 1.0 Fe% in the annealed state. It should be noted that the increase in yield strength is achieved at the expense of the alloy ductility and the reduction in tensile elongation after cold rolling.

TABLE 5

Tensile properties and magnetic phase volume percent in alloy 2 after cold rolling

This control case example #2 demonstrates that the yield strength of the alloys herein can be varied by cold rolling reduction to achieve relatively higher yield strength values with increased tensile strength but decreased ductility. Higher cold rolling reduction was applied, higher yield strength was achieved and lower tensile elongation was recorded.

Control example #3 texture transformation during cold rolling of the hot strip from alloy 2

The hot strip from alloy 2 with a thickness of 4mm was cold rolled to a final thickness of 1.2mm by multiple cold rolling passes with intermediate annealing at 850 ℃ for 10 minutes. The microstructure of the hot and cold rolled sheets was investigated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).

To prepare SEM samples, the work piece was cut by EDM and embedded in epoxy resin and polished stepwise with 9, 6 and 1 μm diamond suspension solutions and finally with 0.02 μm silica. To prepare TEM coupons, samples were cut from the sheet with EDM and then thinned by grinding each time with a pad of reduced particle size. Further thinning to a thickness of 60 to 70 μm was accomplished by polishing with 9 μm, 3 μm and 1 μm diamond suspension solutions, respectively. Discs of 3mm diameter were punched out of the foil and final polishing was performed using electropolishing using a dual jet polisher. The chemical solution used was 30% nitric acid mixed in a methanol substrate. In the case of insufficiently thin regions for TEM observation, a TEM sample may be ion-ground using a Gatan Precision Ion Polishing System (PIPS). Ion milling is typically done at 4.5keV and the tilt angle is reduced from 4 ° to 2 ° to open up thin regions. TEM studies were done using a JEOL 2100 high resolution microscope operating at 200 kV.

SEM analysis of tropical tissues revealed relatively large austenite grains with straight boundaries (fig. 9). Bright field TEM images show that the tropical structure contains very few dislocations and the grain boundaries are straight and sharp (fig. 10), which is typical for a recrystallized structure. TEM studies also showed the presence of nanosized precipitates in the microstructure (fig. 11).

When the hot band is subjected to cold rolling, the austenite phase transforms to a refined ferrite phase in selected regions of the hot band structure under stress. Back-scattered SEM images of cold rolled sheets show the presence of transformed and refined texture and deformed twins (fig. 12). As shown in the TEM image in fig. 13, a high dislocation density was generated in the residual austenite grains, and refined ferrite grains having a size of 200 to 300nm were formed. Deformation twins are also observed in the residual austenite grains. Additional nanoprecipitation was also observed during cold rolling as part of the phase transformation process (fig. 14).

This example demonstrates the microstructure evolution from the initial hot band austenite during cold rolling, resulting in alloy strengthening (increase in ultimate tensile strength) by grain refinement through phase transformation to ferrite with nano-precipitates, and increased dislocation density and deformation twins.

Example #4 Effect of Rolling temperature on yield Strength of alloy 2

The starting material was hot strip from alloy 2 having a thickness of approximately 2.5mm, prepared by hot rolling a 50mm thick laboratory cast slab, mimicking the processing in commercial hot strip preparation. The starting material had an average ultimate tensile strength of 1166MPa, an average tensile elongation of 53.0% and an average yield strength of 304 MPa. The starting material also had a magnetic phase volume percent of 0.9 Fe%.

The media was grit blasted to remove oxides and fed into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to bring the panels to temperature. The hot strip was rolled on a Fenn Model 061 mill with a steadily decreasing roll gap and was charged into the furnace for at least 10 minutes between passes to ensure a constant starting temperature (i.e., 50, 100, 150, 200, 250 ℃, 300 ℃, 350 ℃, and 400 ℃) for each subsequent rolling pass for a target 20% total reduction. Samples were EDM cut according to ASTM E8 standard geometry. Tensile properties were measured on an Instron mechanical test frame (model 5984) using the Instron Bluehill control and analysis software. All tensile tests were run in displacement control at ambient temperature with the bottom clamp held stationary and the top clamp moved; a pressure sensor is attached to the top clamp.

The tensile properties of alloy 2 after rolling at the identified temperatures are listed in table 6. Depending on the rolling temperature, the yield strength increases to a range from 589 to 945MPa compared to values of 250 to 711MPa in the hot band (table 2). Alloy 2 had an ultimate tensile strength varying from 1132 to 1485MPa and a tensile elongation varying from 21.2 to 60.5%. Example stress-strain curves are shown in fig. 15. As can be seen, rolling the hot strip from alloy 2 at a temperature of 200 ℃ shows the possibility of increasing the yield strength with minimal change in ductility and ultimate strength consistent with step 3a in fig. 3.

The volume percent magnetic phase (Fe%) was measured after rolling and at least 10mm from break is reported in table 7 in tensile specifications. As can be seen, the volume percent of magnetic phase after rolling at temperatures above 100 ℃ is significantly lower, ranging from 0.3 to 9.7 Fe%, compared to the volume percent of magnetic phase after cold rolling alloy 2 at ambient temperature (18.0 Fe%, table 4). A significant increase in the volume percent of magnetic phase was measured in alloy 2 after rolling and tensile testing at temperature (table 7, fig. 16). After the tensile test, the volume percent of magnetic phase in the tensile specification of the sample varied from 25.2 to 52.1 Fe%, depending on the rolling temperature.

TABLE 6

Tensile Properties of alloy 2 after-20% Rolling reduction at different temperatures

TABLE 7

Magnetic phase volume percent (Fe%) as a function of rolling temperature before and after tensile testing of alloy 2

This example demonstrates that the yield strength in the alloys herein can be increased by rolling at elevated temperatures thereby reducing the austenite to ferrite phase transformation. A significant drop in Fe% occurs when the rolling temperature is greater than 100 ℃. Moreover, rolling a hot strip from the alloys herein at a temperature of 150 ℃ to 400 ℃ indicates the ability to increase yield strength (e.g., increase yield strength to a value at least 100MPa or greater than the initial value) without significantly changing ductility (i.e., the change is limited to plus or minus seven percent five (+ -7.5% tensile elongation)) and maintaining the ultimate tensile strength at about the same level (i.e., ± 100MPa compared to the initial value).

Example #5 Effect of Rolling temperature on the yield Strength of alloy 7, alloy 18, alloy 34 and alloy 37

The starting material was hot strip from each of alloy 7, alloy 18, alloy 34 and alloy 37 with an initial thickness of approximately 2.5mm, prepared by hot rolling a 50mm thick laboratory cast slab, to simulate commercial processing. Alloys 7, 18, 34 and 37 were processed to a hot band with a thickness of approximately 2.5mm by hot rolling at a temperature between 1100 ℃ and 1250 ℃ and subsequent media blasting to remove oxides. The tensile properties of the hot strip material are previously listed in table 2. The media was grit blasted to remove oxides and fed into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to bring the plate to the desired temperature. The resulting cleaned hot strip was rolled on a Fenn Model 061 mill with a steadily decreasing roll gap and was charged into the furnace for at least 10 minutes between passes to ensure a constant temperature. The hot strip was rolled to 20% target reduction and the samples were EDM cut according to ASTM E8 standard geometry. Tensile properties were measured on an Instron mechanical test frame (model 5984) using the Instron Bluehill control and analysis software. All tensile tests were run in displacement control at ambient temperature with the bottom clamp held stationary and the top clamp moved; a pressure sensor is attached to the top clamp.

The response of each alloy, in particular their elongation, yield strength and ultimate tensile strength, was monitored over the entire range of temperatures investigated. Each alloy was tested after rolling at a temperature ranging from a minimum of 100 ℃ to a maximum of 400 ℃. For alloy 7, the tensile elongation ranged from 14.7% to 35.5% in the temperature range investigated, the ultimate tensile strength ranged from 1218MPa to 1601MPa, and the yield strength ranged from 557MPa to 678MPa (table 8), while the Fe% number ranged from 29.9 to 41.7 before the tensile test, and 57.7 to 65.4 after the test (table 9). For alloy 18, the tensile elongation ranged from 43.0% to 51.9% from 150 to 400 ℃, the ultimate tensile strength ranged from 1083MPa to 1263MPa, and the yield strength ranged from 772MPa to 924MPa (table 10), while the Fe% number ranged from 6.8 to 12.3 before the tensile test and 31.5 to 39.6 after the test (table 11) in the range of 150 to 400 ℃. For alloy 34, the tensile elongation ranged from 21.1% to 31.1%, the ultimate tensile strength ranged from 1080MPa to 1140MPa, and the yield strength ranged from 869MPa to 966MPa (table 12) in the range of 150 to 400 ℃, while the Fe% number ranged from 0.4 to 1.0 before the tensile test and 0.8 to 2.1 after the test (table 13). For alloy 37, the tensile elongation ranged from 1.5% to 9.0%, the ultimate tensile strength ranged from 1537MPa to 1750MPa, and the yield strength ranged from 1384MPa to 1708MPa (table 14) in the range of 150 to 400 ℃, while the Fe% number ranged from 74.5 to 84.3 before tensile testing and 71.1 to 85.6 after testing (table 15).

TABLE 8

Tensile Properties of alloy 7 after-20% Rolling reduction at different temperatures

TABLE 9

Fe% before and after testing alloy 7 at different temperatures

Watch 10

Tensile properties of alloy 18 after-20% rolling reduction at different temperatures

TABLE 11

Fe% before and after testing alloy 18 at different temperatures

TABLE 12

Tensile properties of alloy 34 after-20% rolling reduction at different temperatures

Watch 13

Fe% before and after testing alloy 34 at different temperatures

TABLE 14

Tensile Properties of alloy 37 after-20% Rolling reduction at different temperatures

Watch 15

Fe% before and after testing alloy 37 at different temperatures

Representative curves for each of the alloys herein are shown in fig. 17-20, where reference curves from the tested hot band and after cold rolling to the same approximately 20% reduction are used for parallel comparison.

This example demonstrates that the yield strength in the alloys herein can be increased despite the reduced austenite to ferrite phase transformation when rolled at temperatures of 100 ℃ or greater up to 400 ℃. Examples of changes in yield strength, ultimate tensile strength and tensile elongation are provided for both steps 3a and 3b in fig. 2.

Example #6 Effect of reduction on yield Strength of alloy 2 rolled at 200 ℃

Alloy 2 was machined from a laboratory casting into a hot band having a thickness of approximately 2.5 mm. After hot rolling, alloy 2 was rolled at 200 ℃ to varying rolling reductions ranging from about 10% to 40%. Between rolling passes, the alloy 2 sheet material was placed in a convection oven at 200 ℃ for 10 minutes to maintain the temperature. When the desired rolling reduction was achieved, ASTM E8 tensile samples were cut by wire EDM and tested.

The tensile properties of alloy 2 after rolling with different rolling reductions (0.0 to 70.0%) at 200 ℃ are listed in table 16, which also includes the data prior to any rolling experiments. Fig. 21 shows a representative tensile curve for alloy 2 as a function of rolling reduction at 200 ℃. It was observed that the yield strength of the material increased rapidly with increasing reduction, while the ultimate tensile strength did not change up to 30% reduction (i.e., a change of plus or minus 100 MPa). Fig. 22 provides a comparison of the trends for yield strength and ultimate tensile strength as a function of rolling reduction at 200 ℃, showing that while the yield strength increases relatively rapidly, the ultimate tensile strength change is consistent with the step 3a property change in fig. 2 up to 30.4% rolling reduction and the step 3b property change at 39.0% rolling reduction.

The total elongation of alloy 2 is plotted in fig. 23 as a function of the rolling reduction at 200 ℃. It shows that although the yield strength of alloy 2 increases with additional reduction during rolling at 200 ℃, the ductility available up to > 30% reduction does not decrease rapidly. Note that this was simulated using laboratory rolling, and commercial rolling methods (including tandem rolling, Z-mill rolling, and reversing mill rolling) would additionally apply strip tension during rolling, so the exact amount of reduction can be changed and thus the ductility reduction can be changed.

The magnetic phase volume percent (Fe%) was measured using Fischer ferriscope FMP30 on samples in tensile gauge (i.e., the portion of reduced gauge present in the tensile specimen) after rolling at 200 ℃ and again after tensile testing. These measurements, shown in table 17, indicate the amount of deformation-induced phase transformation that occurred in the alloy during the rolling process and during subsequent tensile testing. The amount of deformation-induced phase transformation in alloy 2 after rolling and tensile testing is shown in fig. 24. It can be seen that the deformation-induced phase transformation is mostly suppressed at 200 ℃, while the magnetic phase volume percentage increases only slightly with increasing rolling reduction. It is shown that rolling at 200 ℃ also has an effect on deformation induced phase transformation during tensile testing, wherein increased rolling reduction suppresses the amount of transformation in the material.

TABLE 16

Average tensile properties of alloy 2 after rolling to various reduction levels at 200 deg.C

Different processes were applied: alloy 2 was processed at 1250 ℃ into a hot band with a thickness of approximately 9.3mm, followed by media blasting to remove oxides and then rolled to 4.6mm (-50% reduction) at 200 ℃. The material was then annealed at 850 ℃ for 10 minutes and rolled at 200 ℃ to approximately 50.4, 60.1 and 70% reduction.

TABLE 17

Volume percent magnetic phase (Fe%) changes with rolling reduction

Rolling reduction (%)	Fe% after rolling	Fe% in the tensile specification
			0.0	0.9	42.6
10.7	3.0	46.7
			20.1	4.2	37.9
30.4	5.8	26.7
			39.0	5.1	16.2
50.4	2.5	15.3
			60.1	2.4	13.5
70.0	2.3	16.1

This example demonstrates that the yield strength of the alloys described herein can be tailored by varying the rolling reduction at temperatures greater than ambient (by rolling at 200 ℃ as shown herein for alloy 2). In the broad context of the present disclosure, the temperature range is expected to be between 150 ℃ to 400 ℃, as provided in the previous case embodiment of table 7. During such rolling, the deformation path is modified such that relatively limited deformation-induced phase transformation occurs, which results in the ability to maintain a large amount of ductility and maintain ultimate tensile strength while increasing yield strength in the cold rolled state. Thus, the rolling parameters can be optimized to improve the yield strength of the material without losing ductility or ultimate tensile strength.

Example #7 microstructure in alloy 2 after rolling at 200 deg.C

Alloy 2 was processed from a laboratory casting into hot bands with a thickness of 9mm to mimic the processing in a commercial hot band preparation. The hot strip was cold rolled with 50% reduction and annealed at 850 ℃ for 10 minutes with air cooling to simulate the cold rolling process in commercial sheet preparation. Dielectric blasting is used to remove oxides formed during annealing. The alloy is then cold rolled again until failure or rolling limit reduction. Samples were heated in a convection oven to 200 ℃ for at least 30 minutes to ensure they were at a uniform temperature prior to cold rolling and were reheated between passes for 10 minutes to ensure a constant temperature. Alloy 2 sheet was first cold rolled with a reduction of 30% and then to a maximum reduction of 70%. The initial structure and the microstructure after rolling were studied by Scanning Electron Microscopy (SEM). To prepare SEM samples, the work piece was cut by EDM and embedded in epoxy resin and polished stepwise with 9, 6 and 1 μm diamond suspension solutions and finally with 0.02 μm silica.

Figure 25 shows a back-scattered SEM image of the microstructure prior to cold rolling, which is mainly austenite with annealing twins inside the micro-sized grains. After cold rolling with 30% reduction as shown in fig. 26, the band structure can be seen in different regions with different orientations. It is presumed that the bands having similar orientations are deformed twins in one austenite grain and the bands of different directions are twins in the other crystal orientation grain. Some grain refinement may be observed in the selected areas.

After the rolling reduction increased to 70%, the strip was no longer visible and a thinning of the texture within the volume was visible (fig. 27). As shown in the high magnification image in fig. 27b, fine islands with a size much smaller than 10 μm can be discerned. In view of the high deformation imposed in the austenite stabilised during the rolling process, the austenite can be significantly refined, typically in the range of 100 to 500 nm. The Feritscope measurements indicate that austenite is stable at 200 ℃ and retains close to 100% austenite after rolling.

This example demonstrates the stability of austenite (i.e., resistance to transformation to ferrite) in the alloys herein and the refinement of the austenitic microstructure when compared to cold rolling when refinement occurs through austenite transformation to ferrite at 200 ℃ even during rolling at high rolling reductions of 70%.

Example #8 Effect of Rolling reduction at 200 ℃ on microstructure in alloy 2

Rolling at temperature results in a significant increase in the yield strength of alloy 2 while maintaining high tensile elongation. A TEM study was performed on alloy 2 rolled at 200 ℃ to analyze the change in structure with rolling strain during rolling at 200 ℃. In this example, a 50mm thick laboratory cast slab was first hot rolled and the resulting hot band was then rolled at 200 ℃ to different strains. To show the tissue evolution, the microstructure of the rolled sheet was studied by Transmission Electron Microscopy (TEM). To prepare TEM coupons, samples were cut from the sheet using wire EDM and then thinned by grinding each time with a pad of reduced grain size. Further thinning to 60 to 70 μm thick samples was done by polishing with 9 μm, 3 μm and 1 μm diamond suspension solutions, respectively. A disk of 3mm in diameter was punched out of the foil and final polishing was performed by electropolishing using a double jet polisher. The chemical solution used was 30% nitric acid mixed in a methanol substrate. In the case of insufficiently thin regions for TEM observation, TEM samples were ion-ground using a Gatan Precision Ion Polishing System (PIPS). Ion milling is typically done at 4.5keV and the tilt angle is reduced from 4 ° to 2 ° to open up thin regions. TEM studies were done using a JEOL 2100 high resolution microscope operating at 200 kV.

FIG. 28 shows a bright field TEM image of the microstructure in alloy 2 rolled at 10% reduction at 200 ℃. It can be seen that the entangled dislocations fill the austenite grains and exhibit a dislocation cell structure. However, due to the relatively low rolling strain, the initial austenite grain boundaries are still visible. It is noted that austenite is stable during rolling at 200 ℃. Electron diffraction indicates that austenite is the predominant phase, which is also consistent with the Feritscope measurements. Rolling at 200 ℃ at 10% reduction increased the average yield strength from 303MPa to 529MPa in the hot band (see table 16). When the sheet was rolled to 30%, TEM qualitatively showed a higher dislocation density in the grains (as shown in fig. 29) and showed a clear dislocation cell structure. In addition, some deformation twins were seen within the austenite grains. Similar to the 10% rolled samples, the austenite phase was maintained as confirmed by electron diffraction. However, the initial grain boundaries of the austenite are no longer visible. Rolling at 30% reduction at 200 ℃ resulted in an average yield strength of 968MPa (table 16). After rolling at 70% reduction (fig. 30), it can be seen from the TEM that the qualitative higher dislocation density continues, and the dislocation cells are similar to those in the 30% rolled sample (fig. 29). In addition, dislocation twins were also present in the sample. Similar to the 30% rolled sample, the austenite remains stable during rolling as evidenced by electron diffraction.

This example demonstrates that the alloys herein maintain an austenitic structure during rolling at 200 ℃ with up to 70% reduction. The structural change including dislocation cell formation and twinning resulted in an increase in yield strength after rolling at 200 ℃.

Example #9 Process route by Rolling Process combination

Alloy 2, alloy 7, alloy 18, and alloy 34 were processed into hot bands having a thickness of-2.7 mm, media blasted to remove oxides and rolled at 200 ℃ to 20% reduction. The material was split and then rolled at ambient temperature in a series of passes. ASTM E8 tensile samples were cut by wire EDM and tested on the Instron 5984 frame using the Bluehill software of the Instron.

The tensile properties of the selected alloys after the combination rolling are listed in tables 18 to 21. A significant increase in yield strength after combination of rolling methods was observed in all three alloys compared to the tropical state or after rolling with a rolling thickness-20% reduction only at 200 ℃ and subsequent rolling reduction at room temperature. Yield strengths up to 1216MPa (309 MPa in hot band and 803MPa after rolling at 200 ℃), up to 1571MPa in alloy 7 (333 MPa in hot band and 575MPa after rolling at 200 ℃), up to 1080MPa in alloy 18 (390 MPa in hot band and 834MPa after rolling at 200 ℃), and up to 1248MPa in alloy 34 (970 MPa in hot band and 1120MPa after rolling at 200 ℃) were recorded for alloy 2. Fig. 31 to 34 show the corresponding tensile curves for alloys 2, 7, 18 and 34, respectively. An increase in ultimate tensile strength and a decrease in tensile elongation after cold rolling were also observed in all alloys herein (see tables 18 to 21). The analysis of the volume percent magnetic phase of the alloys selected herein under each of the inspection conditions (before and after the tensile test) is set forth in tables 22-25. Cold rolling results in higher Fe% in the worked sheet from the alloys herein, and then the Fe% is further increased due to the transformation that occurs during the tensile test.

TABLE 18 tensile Properties of alloy 2 after combination of Rolling methods

TABLE 19 tensile Properties of alloy 7 after combination of Rolling methods

TABLE 20 tensile properties of alloy 18 after combination of rolling methods

TABLE 21 tensile Properties of alloy 34 after combination of Rolling methods

TABLE 22 magnetic phase volume percent (Fe%) in alloy 2 after rolling process combination

TABLE 23 magnetic phase volume percent (Fe%) in alloy 7 after rolling process combination

TABLE 24 magnetic phase volume percent (Fe%) in alloy 18 after rolling process combination

TABLE 25 magnetic phase volume percent (Fe%) in alloy 34 after rolling process combination

This example demonstrates a path that produces a different third set of property combinations that can be achieved by processing the alloy into a sheet of 0.5mm to 5.0mm thickness, followed by deformation (rolling) and thickness reduction in one pass at a temperature in the range of 150 ℃ to 400 ℃, and then subsequent thickness reduction at a temperature of ≦ 150 ℃. This was observed to provide a relatively high yield strength compared to cold rolling alone, and a higher tensile strength compared to rolling at temperature alone.

Example method for customizing property combinations for case embodiment #10

The hot band from alloy 2 was processed into sheets by different methods herein towards higher yield strength and property combinations according to the steps provided in fig. 2 and 3. Alloy 2 was first cast and then processed by hot rolling into a sheet, which was from 2.5 to 2.7mm thick. For tensile comparison, the reference hot strip was hot rolled to-1.8 mm to thin gauge prior to testing. For the example of fig. 2 (i.e., rolling 20% at 200 ℃), the hot band was rolled at 200 ℃ at 20% reduction. Before rolling, it was heated up to 200 ℃ for 30 minutes before rolling 20% at 200 ℃ and reheated between rolling passes for 10 minutes to maintain the temperature. For the fig. 3 example (i.e., rolling 20% at 200 ℃ and then 10% cold rolling at ambient temperature), the process steps are repeated, including the additional steps of 20% reduction at 200 ℃ and applying a 10% ambient temperature rolling reduction. Tensile specimens were cut from the sheets processed by each method using wire EDM. Tensile properties were measured on an Instron mechanical test frame (model 5984) using the Instron Bluehill control and analysis software. All tests were run in displacement control at ambient temperature.

A near-optimal representative stress-strain curve with the combination of properties achieved in each processing method is shown in fig. 35. As can be seen, the yield strength can be significantly increased (i.e. 469MPa increase) by rolling at 200 ℃, with minimal change in the alloy ultimate tensile strength (i.e. 34MPa increase) and elongation (i.e. 1.8% decrease). This is provided by example condition 3a in fig. 2. For the sample rolled additionally at 10% at ambient temperature from the start condition of step 3, then this would satisfy step 4 in fig. 3. As can be seen, in this case, this is a path of higher yield strength (i.e. 688MPa increase) and tensile strength (i.e. 224MPa increase) but with a decrease in total elongation (i.e. 25.1% decrease). Note that satisfying step 4 in fig. 3 can also be accomplished by cold stamping the part, for example, by various processes, whereby the regions in the stamped part will experience higher yield and tensile strengths and proportionately lower ductility that is partially depleted when the part is formed.

This example demonstrates the achievement of high yield strength in the alloys herein by various methods or combinations thereof that provide various strength/elongation combinations in sheets obtained from the alloys herein.

EXAMPLES example #11 test of the Effect of temperature on the tensile Properties of alloy 2

Alloy 2 was prepared from a slab in sheet form having a thickness of 1.4mm by hot and cold rolling to a target thickness followed by annealing. Tensile specimens were cut from alloy 2 sheet using wire EDM. Tensile properties were measured at different temperatures ranging from-40 ℃ to 200 ℃.

The tensile properties of the alloy 2 sheet at different temperatures are listed in table 26. The volume percent of magnetic phase measured in the tensile sample specification after testing at each temperature using Feritscope is also listed in Table 26. As can be seen, as the test temperature is increased, the yield and ultimate tensile strength decrease and the tensile elongation increases. The tensile elongation and magnetic phase volume percent (Fe%) changes with test temperature are plotted in fig. 36, showing that despite the higher elongation at elevated temperatures, the magnetic phase volume percent in the tensile sample specification dropped significantly after the test and approached zero after the test at 200 ℃. The reduction in the volume percent of magnetic phase in the tensile sample specification after testing indicates that the higher austenite stability at elevated temperatures inhibits the transformation of austenite to ferrite under stress.

TABLE 26 tensile Properties of alloy 2 tested at different temperatures

This example demonstrates that multi-component alloying of the alloys herein results in a significant improvement in austenite stability and shows inhibition of transformation to ferrite during rolling at elevated temperatures compared to cold rolling as clearly provided in the last column in table 26. Which provides inherently higher ductility during rolling and higher formability in subsequent sheet forming operations (e.g., stamping, drawing, etc.).

Example #12 thinning to target specification in processing step

Alloy 2 was processed into a hot band having a thickness of 4.4 mm. Then two sections of hot band were rolled, one at room temperature and one at 200 ℃. The plate was heated in a mechanical convection oven at 200 ℃ for 30 minutes before rolling and 10 minutes between passes to ensure a constant temperature.

In the case of rolling at ambient temperature, failure occurred at approximately 42% reduction, whereas when the limit of the mill was reached, more than 70% reduction was applied without failure during rolling at 200 ℃. The mill limit occurs when a Fenn model 061 mill can no longer produce significant reduction per pass during cold rolling while the material still has the ability to roll further down.

The magnetic phase volume percentage (Fe%) was measured by fetiscope at different levels of reduction during cold rolling and rolling at 200 ℃. The data are shown in fig. 37. As can be seen, the magnetic phase volume percent (Fe%) increases rapidly with thinning at ambient temperature (resulting in a material rolling limit at-42%). In the case of rolling at 200 ℃, the magnetic phase volume percentage (Fe%) remains below 3 Fe% even at > 70% of maximum rolling reduction.

Sheets from alloy 2 having a final thickness of 1.2mm were prepared by using both cold rolling and rolling at 200 ℃. In the case of cold rolling, the alloy is rolled with an intermediate annealing cycle to recover the ductility of the alloy and achieve the target thickness at 29% reduction in the final rolling step. Tensile samples were EDM cut from sheets having a thickness of 1.2mm prepared both by the rolling method and annealed at 1000 ℃ for 135 seconds. Tensile properties were measured on an Instron mechanical test frame (model 3369) using the Instron Bluehill control and analysis software. All tests were run at ambient temperature in a displacement control, with the bottom clamp held stationary and the top clamp moved; a pressure sensor is attached to the top clamp.

An example of the engineering stress-strain curves of annealed sheets prepared by both cold rolling and rolling at 200 ℃ is shown in fig. 38. As can be seen, the final properties of the sheet after annealing are similar, although the different rolling methods are towards the target thickness.

This example demonstrates that rolling at 200 ℃ (where austenite is stable and does not transform to ferrite as shown here) for alloy 2 significantly improves the rolling ability of the alloys herein, which will allow for thinning to the target sheet gauge in the processing step. Thus, such elevated temperature rolling can be used to reach an approximate final target specification with high cold rolling reduction (e.g., > 70% as provided in this example). This near-final gauge material may then be annealed to restore the initial properties (i.e., initial conditions). The final target specification can then be obtained by rolling in the temperature range from 150 to 400 ℃ provided in the present application and then the steps and procedures in fig. 2 or fig. 3.

Example #13 Change in ultimate Rolling reduction

The hot band was prepared from alloy 2 having a thickness of about 9 mm. It was heated to 200 to 250 ℃ for 60 minutes and rolled to approximately 4.5mm, with an additional 10 minutes between rolling passes to ensure a constant temperature. Once at 4.5mm, it was sectioned and annealed at 850 ℃ for 10 minutes and allowed to air cool. The material was media blasted to remove oxides and heated to the desired temperature for at least 30 minutes prior to rolling and 10 minutes more between passes to ensure a constant temperature. The material was rolled until failure (visible cracking), characterized by such visible cracking propagating at least 2 inches from the end of the sheet. It is difficult for a rolling mill at about 70% reduction to achieve the necessary load to thin the material and the rolling stops, which is a plant limit and not a material limit. The control material used for room temperature rolling was a 4.4mm thick hot band, which was rolled at room temperature until failure. The results of the maximum rolling reduction as a function of rolling temperature are provided in table 27 and fig. 39.

This example demonstrates that the ultimate rolling reduction increases with increasing temperature for the alloys herein. It can thus be seen that the alloys herein are expected to allow for a permanent set of greater than 20% thickness reduction before failure when heated to a temperature falling within the range of 150 ℃ to 400 ℃. More preferably, the alloys herein are such that they are expected to have a permanent set of greater than 40% thickness reduction before failure when heated in such a temperature range. This provides much greater potential for deformation for rolling operations, including processing industrial materials to target specifications. Greater thinning prior to cracking means that fewer steps (i.e., cold rolling and recrystallization annealing) may be required to achieve a particular target gauge in the steel making process. In addition, greater formability indicated at elevated temperatures would be beneficial for the manufacture of parts from various forming operations (including stamping, roll forming, drawing, hydroforming, etc.).

TABLE 27 Rolling reduction Limit and Rolling temperature for alloy 2

Temperature (. degree.C.)	Rolling reduction limit
		23	41.4％
100	53.8％
		150	68.6％
200	>70％
		250	>70％

Claims (19)

1. A method of increasing yield strength in a metal alloy, comprising:

a. supplying a metal alloy consisting of at least 70 atomic% of iron and at least four elements selected from Si, Mn, Cr, Ni, Cu or C and unavoidable impurities, melting said alloy to 10%^-4K/s to 10³Cooling at a rate of K/sec and solidifying to>A thickness of 5.0mm to 500mm, wherein when Si is present in the alloy, it is present in an amount of greater than 0 to 6.13 atomic%; when Mn is present in the alloy, it is present in an amount of greater than 0 to 15.17 atomic%; when Cr is present in the alloy, it is present in an amount of greater than 0 to 8.64 atomic percentAt least one of the following steps; when Ni is present in the alloy, it is present in an amount of greater than 0 to 9.94 at%; when Cu is present in the alloy, it is present in an amount greater than 0 to 1.86 atomic%; and when C is present in the alloy, it is present in an amount greater than 0 to 3.68 atomic%;

b. processing said alloy into a first sheet having a thickness of from 0.5 to 5.0mm, wherein the first sheet has X in%₁Total elongation of (2), Y in MPa₁Ultimate tensile strength and Z in MPa₁The yield strength of (d);

c. permanently deforming said first sheet at a temperature in the range of 150 ℃ to 400 ℃ to produce a second sheet having one of the following combinations of tensile properties A or B:

a. (1) Total elongation X₂＝X₁±7.5％；

(2) Ultimate tensile strength Y₂＝Y₁+/-100 MPa; and

(3) yield strength Z₂≥Z₁+100MPa

B. (1) ultimate tensile Strength Y₃＝Y₁+/-100 MPa; and

(2) yield strength Z₃≥Z₁+200MPa。

2. The method of claim 1, wherein said alloy consists of at least 70 atomic percent iron and at least five elements selected from the group consisting of Si, Mn, Cr, Ni, Cu, or C.

3. The method of claim 1, wherein said alloy consists of at least 70 atomic% of iron and Si, Mn, Cr, Ni, Cu and C and unavoidable impurities.

4. The method of claim 1, wherein X₁In the range of 10.0 to 70.0%, Y₁In the range of 900MPa to 2050MPa, and Z₁In the range of 200MPa to 750 MPa.

5. The method of claim 1, wherein said second sheet has said combination of tensile properties a, X₂2.5 to 77.5%, Y₂800MPa to 2150MPa, and Z₂≥300MPa。

6. The method of claim 1, wherein said second sheet has said combination of tensile properties B, Y₃800MPa to 2150MPa, and Z₃≥300MPa。

7. The method of claim 1, wherein in step (c) said first sheet is permanently deformed into said second sheet by reducing the thickness of said first sheet.

8. The method of claim 1, wherein step (b) is performed at a temperature of 700 ℃ to less than the melting point Tm of the alloy.

9. The method of claim 1, wherein the alloy is heat treated after step (b) at a temperature of 650 ℃ to a temperature less than the melting point Tm of the alloy.

10. The method of claim 1 wherein the first sheet is permanently deformed in step (c) by reducing its thickness by more than 20% prior to failure.

11. The method of claim 1, wherein the first sheet is permanently deformed into said second sheet in step (c) by roll forming, metal stamping, metal drawing, or hydroforming.

12. The method of claim 1, wherein said alloy formed to a thickness of >5.0mm to 500mm contains more than 10 volume percent austenite.

13. The method of claim 1, further comprising permanently deforming the second sheet at a temperature of 150 ℃.

14. A method of increasing yield strength in a metal alloy, comprising:

a. supplying a metal alloy consisting of at least 70 atomic% iron andat least four elements selected from Si, Mn, Cr, Ni, Cu or C and inevitable impurities, melting the alloy to 10 deg.C^-4K/s to 10³Cooling at a rate of K/sec and solidifying to>A thickness of 5.0mm to 500mm, wherein when Si is present in the alloy, it is present in an amount of greater than 0 to 6.13 atomic%; when Mn is present in the alloy, it is present in an amount of greater than 0 to 15.17 atomic%; when Cr is present in the alloy, it is present in an amount greater than 0 to 8.64 atomic%; when Ni is present in the alloy, it is present in an amount of greater than 0 to 9.94 at%; when Cu is present in the alloy, it is present in an amount greater than 0 to 1.86 atomic%; and when C is present in the alloy, it is present in an amount greater than 0 to 3.68 atomic%;

b. processing the alloy into a first sheet having a thickness of from 5.0 to 0.5 mm;

c. permanently deforming the first sheet at a temperature in the range of 150 ℃ to 400 ℃ to produce a second sheet;

d. permanently deforming the second sheet at a temperature <150 ℃ into a permanently deformed part having the following properties:

(1) total elongation of 10.0 to 40.0%;

(2) ultimate tensile strength is 1150 to 2000 MPa;

(3) yield strength is 550 to 1600 MPa.

15. The method of claim 14, wherein step (b) is performed at a temperature of 700 ℃ to less than the melting point Tm of the alloy.

16. The method of claim 14, wherein the alloy is heat treated after step (b) at a temperature of 650 ℃ to a temperature less than the melting point Tm of the alloy.

17. The method of claim 14, wherein the first sheet is permanently deformed into said second sheet in step (c) by roll forming, metal stamping, metal drawing, or hydroforming.

18. The method of claim 14, further comprising placing said permanently deformed part formed in step (d) in a vehicle frame, a vehicle chassis, or a vehicle panel.

19. The method of claim 14, further comprising placing the permanently deformed part of step (d) in one of a drill collar, drill pipe, casing, tool joint, wellhead, compressed gas storage tank, railroad tank car/tank trailer, or liquefied natural gas container.

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