KR101527735B1 - A thin cast strip product with microalloy additions, and method for making the same - Google Patents

Abstract

The present invention relates to a process for the production of vanadium oxide comprising less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum, and 0.01-0.20% niobium and 0.01-0.20% , A steel product containing most of the microstructure consisting of bainite and acicular ferrite and containing molybdenum and / or vanadium in a solid state in excess of 70% The steel strip is started. The steel product increases in elongation and yield strength after age hardening. The age hardened steel article has niobium carbonitride particles having an average particle size of less than 10 nanometers and substantially no niobium carbonitride particles greater than 50 nanometers. The steel product may have a yield strength of at least 380 MPa or a tensile strength of at least 410 MPa, or both. The steel product or sheet steel strip has a total elongation of at least 6% or 10%.

Bainite, needle ferrite, microalloyed additives, laminated cast steel strip, twin roll casters, aged hardened steel products, yield strength, tensile strength, niobium, vanadium

Description

TECHNICAL FIELD [0001] The present invention relates to a thin sheet cast strip product having a micro alloy addition product and a method for manufacturing the same,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of high strength thin cast strips, and more particularly to a method for producing thin cast strips by means of a twin roll caster.

In a twin roll caster, the molten metal is poured between a pair of oppositely rotating internally cooled casting rolls to solidify on the roll surface on which the metal shells move, and the nip between the rolls are assembled together at the nip region to produce a solidified cast strip product that is fed down from the nip portion between the casting rolls. Here, the term "nip " is used to refer to the entire region where casting rolls are closest to each other. The molten metal is transferred from a ladle through a metal supply system comprising a core nozzle and a tundish located above the nip portion to be held on the casting surface of the rolls located above the nip portion, A casting pool of molten metal extending along the length of the casting pool is formed. Such a casting pool is typically formed between refractory side plates or dams, which are maintained in a slidable engagement configuration with the end surface of the roll, and both ends of the casting pool are connected to the outflow of molten metal to function as a dam to block outflow.

Previously, high strength low carbon thin strips having a yield strength of greater than or equal to 413 MPa (60 ksi) at strip thicknesses of less than 3.0 mm were prepared by recovery annealing of cold rolled strips. Cold rolling was required to produce the desired thickness. The cold rolled strip was then subjected to a recovery annealing to improve ductility without significantly reducing the strength. However, the final ductility of the resulting strip product was still relatively low and the strip could not achieve a total elongation level exceeding 6%, so that structural steel according to certain building regulations for the structures, . This recovered annealed cold rolled low carbon steel is generally suitable only for simple forming processes, such as roll forming and bending. The manufacture of such steel strips with higher ductility using cold rolling and recovery annealing manufacturing processes was technically unachievable at this final strip thickness.

Previously, high strength steels were prepared to achieve desired thickness and strength levels by microalloying and hot rolling the components with components such as niobium, vanadium, titanium or molybdenum. This microalloy treatment required an expensive high level of niobium, vanadium, titanium or molybdenum and resulted in the formation of a bainite-ferrite microstructure typically with 10 to 20% bainite. See U.S. Patent No. 6,488,790. Optionally, the microstructure may be a ferrite having 10-20% pearlite. Hot rolling the strips resulted in partial precipitation of these alloy components. As a result, it has been required to provide sufficient age hardening of the microstructure modified with ferrite dominant texture in Nb, V, Ti or Mo elements of relatively high alloy levels to achieve the desired strength level. This high microalloyle level not only significantly increased the required hot rolling load but also limited the thickness range of the hot rolled strip which could be produced economically and practically. High strength strips of such alloys can be used directly for galvanizing after pickling on the thicker end of the product range with a thickness greater than 3 mm.

However, it is very difficult to prepare high strength steel strips with a thickness of less than 3 mm by adding Nb, V, Ti or Mo to the base steel chemistry, especially for the wide strips due to the high rolling load , And it was not always feasible commercially. In the past, the addition of large amounts of these components was required to strengthen the steel, and additionally, it resulted in a reduction in the elongation properties of the steel. In the past, high strength microalloyed hot rolled strips were relatively inefficient in providing strength, were relatively expensive, and often required compensating addition of other alloy components.

In addition, although cold rolling is generally required for thinner strips, the high strength of hot rolled strips, however, makes such cold rolling processes more difficult due to the high cold rolling loads required to reduce the thickness of the strips. I make it. This high alloy level also requires the construction and operation of a high cost annealing process line which can significantly increase the required recrystallization annealing temperature to achieve the high annealing temperature required for complete recrystallization annealing of the cold rolled strip.

In summary, the application of conventional microalloying processes using Nb, V, Ti or Mo components to produce high strength thin strips requires high alloy cost, relative inefficiency of the additive components, high rolling load during hot and cold rolling due to difficulties due to rolling loads and the necessity of high recrystallization annealing.

The present invention relates to a process for the production of a composition comprising less than 0.25% carbon, from 0.20 to 2.0% manganese, from 0.05 to 0.50% silicon, less than 0.01% aluminum, and from about 0.01% to about 0.20% Most of the structures disclose steel products containing bainite and acicular ferrite structures and containing more than 70% solid solution niobium. Optionally, the niobium may be less than 0.1%. The steel product may further comprise at least one component selected from the group consisting of molybdenum in the range of about 0.05% to about 0.50%, vanadium in the range of about 0.01% to 0.20%, and mixtures thereof.

The steel product may have a yield strength of at least 340 MPa and a tensile strength of at least 410 MPa. The steel product may also have a yield strength of at least 485 MPa and a tensile strength of at least 520 MPa. The steel product has a total elongation of at least 6%. Optionally, the total elongation may be at least 10%.

The steel product may be a thin cast steel strip. Optionally, the thin sheet cast steel strip may comprise fine oxide particles of silicon and iron distributed through a steel microstructure having an average grain size of less than 50 nanometers.

The sheet steel strip has a thickness of less than 2.5 mm. Optionally, the sheet metal strip steel strip may have a thickness of less than 2.0 mm. As another option, the sheet steel strip may have a thickness ranging from about 0.5 mm to about 2 mm.

The present invention also provides a process for the production of silicon carbide containing less than 0.25% carbon, from 0.20 to 2.0% manganese, from 0.05 to 0.50% silicon, less than 0.01% aluminum, and from about 0.01% to about 0.20% niobium, The bulk of the structure is comprised of bainite and acicular ferrite structures and is capable of providing a yield strength of at least 410 MPa at a reduction of between 20% and 40% Discloses a hot rolled steel product. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least 520 MPa. Optionally, the niobium may be less than 0.1%.

Optionally, the hot rolled steel product may comprise fine oxide particles of iron and silicon distributed through a steel microstructure having an average particle size of less than 50 nanometers.

The hot rolled steel product has a total elongation of at least 6%. Optionally, the total elongation may be at least 10%. The hot rolled steel product has a thickness of less than 2.5 mm. Optionally, the hot rolled steel product may have a thickness of less than 2.0 mm. As another option, the hot rolled steel product may have a thickness ranging from about 0.5 mm to about 2 mm.

The present invention also provides a method of making a composite material comprising, on a weight basis, less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum and about 0.01% to about 0.20% niobium, About 0.20% vanadium, and mixtures thereof, and having a niobium and / or vanadium solid solution in excess of 70% after coiling and cooling. Optionally, the niobium may be less than 0.1%.

Optionally, the coiled steel product may comprise fine oxide particles of iron and silicon distributed through a steel microstructure having an average particle size of less than 50 nanometers.

The coiled steel product may have a yield strength of at least 340 MPa and a tensile strength of at least 410 MPa. The coiled steel product has a thickness of less than 3 mm. The steel product may also have a yield strength of at least 485 MPa and a tensile strength of at least 520 MPa.

Optionally, the coiled steel product has a thickness of less than 2.5 mm. Optionally, the coiled steel product may have a thickness of less than 2.0 mm. As a further option, the coiled steel article may have a thickness in the range of about 0.5 mm to about 2 mm. The coiled steel product has a total elongation of at least 6%. Optionally, the total elongation may be at least 10%.

The present invention also provides a method of making a composite material comprising, on a weight basis, less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum and about 0.01% to about 0.20% niobium, About 0.20% of vanadium, and a mixture thereof, wherein the majority of the microstructure comprises bainite and acicular ferrite structure, and further comprising at least one component selected from the group consisting of An increase in elongation and an increase in elongation. Optionally, the niobium may be less than 0.1%.

The age hardened steel product may include fine oxide particles of iron and silicon distributed through a steel microstructure having an average particle size of less than 50 nanometers as an additive.

The age hardened steel product may optionally have a yield strength of at least 340 MPa, or at least 380 MPa, or at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at least 550 MPa, or at least 600 MPa, or at least 650 MPa. The aged hardened steel article may optionally have a tensile strength of at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at least 550 MPa, or at least 600 MPa, or at least 650 MPa, or at least 700 MPa. The aged hardened steel product has a thickness of less than 3 mm. Optionally, the aged hardened steel product has a thickness of less than 2.5 mm. Optionally, the aged hardened steel product may have a thickness of less than 2.0 mm. As another option, the age hardened steel product may have a thickness ranging from about 0.5 mm to about 2 mm. The age hardened steel product has a total elongation of at least 6%. Optionally, the total elongation may be at least 10%.

The present invention also provides a method of making a composite material comprising, on a weight basis, less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum and about 0.01% to about 0.20% niobium, And about 0.20% vanadium, wherein the majority of the microstructure comprises bainite and acicular ferrite structures and has an average particle size of less than 50 nanometers Discloses a steel product comprising fine oxide particles of iron and silicon distributed through a steel microstructure. Optionally, the niobium may be less than 0.1%. Optionally, the steel product may comprise between about 0.05% and about 0.50% molybdenum.

The steel product may have a yield strength of at least 340 MPa and a tensile strength of at least 410 MPa. The steel product may also have a yield strength of at least 485 MPa and a tensile strength of at least 520 MPa. The steel product has a total elongation of at least 6%. Optionally, the total elongation may be at least 10%.

The present invention also relates to a microstructure comprising less than 0.25% carbon, from 0.20 to 2.0% manganese, from 0.05 to 0.50% silicon, less than 0.01% aluminum, from about 0.01% to about 0.20% niobium, Discloses an age hardened steel product comprising niobium carbonitride particles, most of which contain bainite and acicular ferrite structures and also have an average particle size of less than 10 nanometers. In the present specification and claims, the carbonitrides include carbides, nitrides, carbonitrides and combinations thereof. Optionally, the niobium may be less than 0.1%.

The age hardened steel product may be substantially free of any niobium carbonitride particles greater than 50 nanometers. The age hardened steel product may have a yield strength of at least 340 MPa and a tensile strength of at least 410 MPa. The age hardened steel product has a total elongation of at least 6%. Optionally, the total elongation may be at least 10%.

A method for manufacturing a coiled strip cast steel strip,

Assembling a cooling roll caster having casting rolls disposed sideways therebetween to form a nip portion therebetween and being supported on the casting rolls above the nip area and being supported on the casting rolls by a side dam, Forming a casting pool of molten iron confined near the end,

Rotating the casting rolls in opposite directions so that the metal shells solidify above the casting rolls as the casting rolls move through the casting pool,

Forming a steel strip from the metal shells downwardly through a nip portion between the casting rolls, and

A composition comprising less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum, and about 0.01% to about 0.20% niobium, about 0.01% to about 0.20% vanadium And mixtures thereof, wherein the majority of the microstructure comprises bainite and acicular ferrite structures, and further comprises at least one element selected from the group consisting of at least one element selected from the group consisting of A method for manufacturing a steel strip comprising cooling the steel strip at a rate of 10 占 폚 per second is disclosed.

The method may also provide fine oxide particles of iron and silicon distributed through a steel microstructure having an average particle size of less than 50 nanometers in a coiled steel product. The method may also include hot rolling the steel strip and making the hot rolled steel strip into a coil shape at a temperature between 450 [deg.] C and 700 [deg.] C. The method of manufacturing the coiled strip cast steel strip may further comprise hot rolling the steel strip and making the hot rolled steel strip into a coil shape at a temperature less than 600 < 0 > C . Optionally, cooling of the hot-rolled steel strip may be performed at a temperature less than 650 ° C.

The method may further comprise aging the steel strip to increase the tensile strength at a temperature of at least 550 < 0 > C. Optionally, the age hardening can occur at a temperature between 625 ° C and 800 ° C. As another option, the age hardening may occur at a temperature between 650 [deg.] C and 750 [deg.] C.

A method for manufacturing a thin cast steel strip,

Assembling a cooling roll caster having casting rolls disposed sideways therebetween to form a nip portion therebetween and being supported on the casting rolls above the nip area and being supported on the casting rolls by a side dam, Forming a casting pool of molten iron confined near the end,

Rotating the casting rolls in opposite directions so that the metal shells solidify above the casting rolls as the casting rolls move through the casting pool,

Forming a steel strip from the metal shells downwardly through a nip portion between the casting rolls, and

At least one component selected from the group consisting of less than 0.25% carbon, less than 0.01% aluminum, and from about 0.01% to about 0.20% niobium, from about 0.01% to about 0.20% vanadium, Cooling the steel strip at a rate of at least 10 ° C per second to provide a composition having greater than 70% niobium and / or vanadium solid solution, while the majority of the microstructure comprises bainite and needle ferrite;

And aging the steel strip at a temperature between 625 DEG C and 800 DEG C, thereby producing a thin sheet cast steel strip.

The method may further comprise aging the steel strip to increase the tensile strength. Optionally, the age hardening may occur at a temperature between 650 [deg.] C and 750 [deg.] C.

The process can provide a age hardened steel strip having niobium carbonitride particles having an average particle size of less than 10 nanometers. Optionally, the age hardened steel strip is substantially free of any niobium carbonitride particles greater than 50 nanometers.

The method may also provide fine oxide particles of iron and silicon distributed through a steel microstructure having an average particle size of less than 50 nanometers in a coiled steel product. The method may also include hot rolling the steel strip and coiling the hot rolled steel strip at a temperature less than 700 < 0 > C. Optionally, cooling of the hot rolled steel strip may occur at a temperature less than 650 < 0 > C.

A method for manufacturing a thin cast steel strip,

Assembling a cooling roll caster having casting rolls disposed sideways therebetween to form a nip portion therebetween and being supported on the casting rolls above the nip area and being supported on the casting rolls by a side dam, Forming a casting pool of molten iron confined near the end,

Rotating the casting rolls in opposite directions so that the metal shells solidify above the casting rolls as the casting rolls move through the casting pool,

Forming a steel strip from the metal shells downwardly through a nip portion between the casting rolls, and

From less than 0.25% carbon, from 0.20% to 2.0% manganese, from 0.05% to 0.50% silicon, less than 0.01% aluminum, and from about 0.01% to about 0.20% niobium, from about 0.01% to about 0.20% Vanadium, and mixtures thereof, wherein the majority of the microstructure is cooled at a rate of at least 10 ° C per second to provide a composition comprising a bainite and acicular ferrite structure, wherein the steel strip comprises at least one component selected from the group consisting of ,

And aging the steel strip at a temperature between 625 DEG C and 800 DEG C while providing an increase in elongation and an increase in yield strength after age hardening.

The method may also provide fine oxide particles of iron and silicon distributed through a steel microstructure having an average particle size of less than 50 nanometers in a coiled steel product. The method may also provide a age hardened steel strip having niobium carbonitride particles having an average particle size of less than 10 nanometers. Optionally, the age hardened steel strip is substantially free of any niobium carbonitride particles greater than 50 nanometers.

The method may include hot rolling the steel strip and coiling the hot rolled steel strip at a temperature less than 750 < 0 > C. Optionally, cooling of the hot rolled steel strip may occur at a temperature less than 700 < 0 > C.

BRIEF DESCRIPTION OF THE DRAWINGS For a more detailed description of the invention, reference will now be made, by way of example, to the accompanying drawings, in which: FIG.

Figure 1 shows a strip casting machine with a hot rolling mill and a coiler;

2 is a detailed view of a twin roll-type castor;

Figure 3 shows the effect of coiling temperature on strip yield strength in the presence or absence of niobium or vanadium additives;

4A is an optical micrograph of a niobium steel strip;

Figure 4b is an optical micrograph of a standard USS SS grade 380 steel strip;

5 is a graph showing the effect of post-coil age hardening on the yield strength of the steel strip of the present invention;

Figure 6 is a graph showing the effect of aging hardening after coiling on yield and tensile strength of steel strips of the present invention;

7 is a graph showing the effect of the hot rolling reduction on the yield strength;

8 is a graph showing the effect of yield strength on elongation;

9 is a graph showing the effect of the amount of niobium on yield strength in low level niobium;

Figure 10a shows a micrograph of the microstructure of a first sample of 0.065% niobium steel after hot rolling;

Figure 10b shows a micrograph of the microstructure of a second sample of 0.065% niobium steel after hot rolling;

11 is a graph showing the effect of the amount of niobium on the yield strength;

12 is a graph showing the effect of the coiling temperature on the yield strength;

13 is a graph showing the effect of the coiling temperature on the yield strength in low level niobium;

14 is a graph showing the effect of heat treatment conditions on yield strength;

15 is a graph showing the effect of the age hardening heat treatment temperature on the yield strength of 0.026% niobium steel;

16 is a graph showing the effect of maximum aging peak aging temperature on the yield strength of 0.065% niobium steel;

17 is a graph showing the effect of maximum age hardening temperature on yield strength of 0.065% niobium steel;

18 is a graph showing the effect of maximum age hardening temperature and hold time on the yield strength of 0.084% niobium steel;

19 is a graph showing the effect of yield strength on elongation before and after age hardening;

20 is a graph showing the results of a continuous annealing heat treatment;

21 is a graph showing the age hardening conditions;

22 is a graph showing the effect of temperature and time on hardness;

23 is a graph showing the effect of heat treatment on the yield strength of the vanadium steel of the present invention; And

24 is a graph showing the effect of the hot rolling reduction ratio on the yield strength of the vanadium steel of the present invention.

The following description of the embodiments is made in relation to high strength thin sheet cast strips with microalloyed additives produced by continuous cast steel strips using a twin roll caster.

Fig. 1 shows a part of a continuous process of a strip caster for continuously casting steel strips. Figures 1 and 2 show a twin roll caster 11 which continuously produces a cast steel strip 12 which passes through a guide table 13 and which is provided with a pinch roll 14 with pinch rolls 14A, And passes along the transit path 10 toward the stand 14. Immediately after exiting the pinch roll stand 14 the strip 12 passes through a hot rolling mill 16 having a pair of reduction rolls 16A and backing rolls 16B Where the cast strip is hot rolled and reduced to the desired thickness. The hot rolled strip passes through a run-out table 17 where the strip is fed by contact and convection with water supplied through a water injector 18 (or other suitable means) Can be cooled. The rolled and cooled strip is then fed to a coiler 19 through a pinch roll stand 20 having a pair of pinch rolls 20A. The final cooling of the cast strip takes place after coiling.

As shown in FIG. 2, the twin roll casters 11 include a main device frame 21, which supports a pair of laterally disposed casting rolls 22 having a casting surface 22A. The molten metal is fed from the ladle (not shown) through the tundish 23 to the distributor or mobile tundish 25 through the refractory lid 24 during the casting process, Is fed over the nip (27) region through the metal feed nozzle (26) between the rolls (22). The molten metal supplied between the casting rolls 22 forms one casting pool 30 on the nip area. The casting pool 30 is constrained at the ends of the casting rolls by a pair of side closure dams or plates 28 which are connected to hydraulic cylinder units (not shown) connected to the side plate support holders (Not shown) provided with a pair of thrusters (not shown) for pushing against the ends of the casting rolls. The upper surface of the casting pool 30 (commonly referred to as the "meniscus" level) is typically raised above the lower end of the feed nozzle so that the lower end of the feed nozzle is submerged in the casting pool 30. The casting rolls 22 are cooled internally in a water-cooled manner so that the shells solidify at the moving roller surface as they pass through the casting pool and are gathered together at the nip 27 between the casting rolls to form the cast strip 12, This is fed downwardly from the nip between the casting rolls.

The twin roll casters described above may be of the kind illustrated and described in detail in U.S. Patent Nos. 5,184,668 and 5,277,243 or 5,488,988. Reference should be made to the above-mentioned patents for the appropriate details of the twin roll casters suitable for use in one embodiment of the present invention.

High strength laminated cast strips can be manufactured using twin roll casters that overcome the drawbacks of conventional light gauge steel products and produce high strength lightweight steel strip products. The present invention utilizes components comprising niobium (Nb), vanadium (V), titanium (Ti) or molybdenum (Mo), or combinations thereof.

Microalloy components in steel are generally considered to refer to titanium, niobium, and vanadium components. These ingredients were previously added at levels typically less than 0.1%, but in some cases as high as 0.2%. These components can exert a powerful effect on steel microstructure and properties (such as the old carbonitride formers) through the combination of hardenability, grain refining and strengthening effect have. Although molybdenum is not normally regarded as a microalloyed component because it is a relatively weak carbonitride former in itself, it may be effective in the present case and may form complex carbonitride particles with niobium and vanadium have. As described below, in these hot rolled strips, carbonitride formation is inhibited by these components.

The high strength laminate cast strips combine several properties to achieve a light gauge cast strip product by forming microalloys with these components. The thickness of the strip may be less than 3 mm, less than 2.5 mm, or less than 2.0 mm, and may range from 0.5 mm to 2.0 mm. The cast strip is produced by hot rolling without the need for cold rolling to further reduce the strip to the desired thickness. Thus, the high strength thin sheet cast strip product overlaps both the light hot rolled thickness range and the cold rolled thickness range. The strips can be cooled at a rate of at least 10 ° C per second and most and typically form bainite and acicular ferrite dominated microstructures.

Advantages that can be gained from the manufacture of high-strength thin-sheet cast strip products include the relatively high cost of alloying, the inefficiency in microalloys, the difficulty of hot and cold rolling, the difficulty of recrystallization annealing, As compared to conventional microalloy steels because conventional continuous galvanizing and annealing process lines can not provide the required high annealing temperatures. Also, the relatively low ductility found in the strips produced by cold rolling and repair annealing processing manufacturing processes is overcome.

In microalloyed steels made in conventional manner, components such as niobium and vanadium could not remain solid solution through coagulation, hot rolling, coiling and cooling. Niobium and vanadium diffuse through microstructures that form carbonitride particles at various stages of the high temperature coil manufacturing process. In the present specification and claims, the carbonitride particles include carbides, nitrides, carbonitrides and combinations thereof. The formation and growth of carbon and nitrogen particles in high temperature post-slab and subsequent coiling processes of microalloyed steels produced in the conventional manner will result in smaller grain sizes of austenite in hot plates , Whereby the hardenability of the steel is further reduced. In these conventional steels, the effect of particles of hot plates (slabs) needs to be overcome by increasing the amount of microalloy components, reheating the casting plate to a higher temperature, and lowering the carbon content.

In contrast to steel made in conventional manner, the present invention comprises less than 0.25% carbon, from 0.20% to 2.00% manganese, from 0.05% to 0.50% silicon, less than 0.06% aluminum, and At least one component selected from the group consisting of from about 0.01% to about 0.20% titanium, from about 0.01% to about 0.20% niobium, from about 0.05% to about 0.50% molybdenum, from about 0.01% to about 0.20% vanadium And a high strength thin sheet cast steel strip product having a majority of bainite was produced. The steel product may further comprise a fine oxide structure of silicon and iron distributed through a steel microstructure having an average particle size of less than 50 nanometers. The steel product may further comprise a more uniform distribution of microalloys through microstructures than previously produced with conventional steel plate casting products.

Alternatively, the high strength thin sheet cast steel strip may comprise less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum, and from about 0.01% to about 0.20% % Of niobium, and most of the microstructure includes bainite and acicular ferrite, and may contain more than 70% soluble niobium.

Alternatively, the coiled steel article may comprise less than 0.25% carbon, from 0.20% to 2.0% manganese, from 0.05% to 0.50% silicon, less than 0.01% aluminum, and from about 0.01% 0.20% niobium, about 0.01% to about 0.20% vanadium, and mixtures thereof, and may include at least one component selected from the group consisting of soluble niobium and vanadium exceeding 70% after coiling and cooling You have it according to your choice. The coiled high strength thin plate steel strip product may optionally have more than 70% soluble niobium and vanadium, depending on the hot rolled reduction and subsequent coiling and after age hardening, optionally. The microstructure may be a mixture of bainite and needle-like ferrite. Alternatively, the microstructure of the hot-rolled and subsequent coiling-treated cooled steel described above may comprise bainite and needle-like ferrite in which more than 80% of the niobium and / or vanadium remains in the solid solution, It may exceed 90%.

Optionally or additionally, the steel product may have a total elongation of more than 6% or more than 10%. The steel product may have a yield strength of at least 340 MPa (about 49 ksi), or a tensile strength of at least 410 MPa, or both and exhibit satisfactory ductility. 8 shows the relationship between the yield strength and the total elongation of a hot rolled product.

The hot rolled steel strip after hot rolling may be a coil at a temperature in the range of about 500-700 < 0 > C. Thin sheet cast steel strips can also be age hardened to increase the tensile strength at a temperature of at least 550 < 0 > C. The above-mentioned age hardening may occur at a temperature in the range of 550 ° C to 800 ° C or 625 ° C to 750 ° C, or 675 ° C to 750 ° C. Thus, conventional furnaces of continuous galvanizing or annealing lines can provide the age hardening temperature needed to strengthen the cast alloy strip product of microalloyed.

For example, one steel composition was prepared by making a steel composition of 0.026% niobium, 0.04% carbon, 0.85% manganese, 0.25% silicon on a weight basis, cast by a sheet metal strip strip process. The above strips were cast to a thickness of 1.7 mm using a twin roll caster as shown in Figs. 1 and 2 and hot-rolled in series in the range of 1.5 mm to 1.1 mm strip thickness. The strip was coiled at a coiling temperature of 590 캜 to 620 캜 (1094 - 1148.).

As shown in Figure 3, the yield and tensile strength levels achieved in the cast strip of the present invention are the yield and tensile strength levels achievable in a cast strip steel composition of base non-microalloyed over a range of coiling temperatures And tensile strength. It can be seen that the niobium steel strip achieved a yield strength in the range of 420-440 MPa (about 61-64 ksi) and a tensile strength of about 510 MPa (about 74 ksi). The cast strip product of the present invention is compared to a C-Mn-Si base steel composition treated with a coiling temperature such as microalloyed steel, wherein the niobium steel produces a substantially higher level of strength. The comparative base steel strips needed to be coiled at very low temperatures to approach the strength level comparable to cast niobium steel products. Cast niobium steel products did not require coiling at low coiling temperatures to achieve its strengthening capability with hot rolling. In addition, the yield and tensile strength levels for cast niobium steel were not significantly affected by the degree of series hot rolling with a reduction of at least 19% to 37% as shown in FIG.

The hardenability of the steel of the present invention is shown in Fig. As shown in FIG. 9, as little as 0.007% of the level of niobium was effective in increasing the strength of the final strip and a yield strength level exceeding 380 MPa was achieved with a niobium level of greater than about 0.01%. It should be noted that a niobium level of less than about 0.005% may be considered to remain. Therefore, the addition of very small amounts of microalloy components can be effective in substantial strengthening.

High strength has been achieved by using niobium microalloyed additives to increase the hardenability of steel through the suppression of the formation of proeutectic ferrite. Figure 4b shows that proutectic ferrite was formed along the austenite allotriomorphic ferrite in base steel, but it did not exist in the niobium steel shown in Figure 4a. The hardenability effect of the niobium additive suppresses the ferrite conversion, thus making it possible to produce stronger bainite and needle-like ferrite microstructures while using the usual cooling rate and higher coiling temperature during cooling. The final microstructure of the niobium steel of the present invention includes most of the combinations of bainite and needle-like ferrite. The base steel shown in Figure 4b was cooled to a relatively low coiling temperature, i.e., a cooling condition known to inhibit ferrite formation at austenite grain boundaries, of less than 500 ° C.

The effect of high temperature reduction on the yield strength is deteriorated in the niobium steel of the present invention. In prior C-Mn products there is typically a decrease in strength as the hot reduction rate is increased. In contrast, as shown in Fig. 7, the effect of hot pressing on the yield strength is significantly reduced in the steel product of the present invention. In this experiment, the coiling temperature was kept constant and the strip thickness ranged from 1.0 mm to 1.5 mm when the range of hot rolling reduction was applied to at least 40%. Unlike non-microalloyed base steels, the strength level of the niobium microalloyed steels disclosed herein in hot rolled cast strip products is relatively insensitive to the degree of hot rolling reduction for shrinkage up to at least 40%. This high strength level was also achieved using conventional coiling temperatures in the range of 550 [deg.] C to 650 [deg.] C, as shown in Fig.

To further investigate this effect, the austenite grain size was measured at each thickness of 0.026 Nb steel. Base steel tends to be completely recrystallized beyond about 25% hot reduction, while 0.026 Nb steel shows only limited recrystallization at 40% reduction. This indicates that the niobium in the solid solution decreased the effect of hot rolling on the strength characteristics by suppressing the static recrystallization of the austenite after the hot rolling. This is shown in FIG. 10, where the austenite grains are elongated by hot rolling without recrystallization to finer grains. The finer particles increase the austenite grain boundary area, thereby reducing the hardenability of the steel. However, it is known that recrystallization to finer austenite grain size is inhibited while increasing the ferrite denaturation starting temperature at such hot rolling. In addition, it is possible to cause a local high strain region, usually referred to as shear bands, in the above-mentioned austenite grains subjected to hot rolling at high temperature, Which can act as intragranular nucleation sites. The hardenability effect of niobium in the steel of the present invention was sufficient to inhibit the formation of ferrite within the austenite grains modified, which resulted in a level of strength insensitive to the extent of hot rolling.

The sheet cast striped niobium steel article described above had a constant yield and tensile strength level for the applied hot rolling range and was able to provide a yield strength of at least 410 MPa with a reduction of 20% to 40%. The prior austenite grain size was determined for each strip thickness. According to the austenite grain size measurements, only very limited recrystallization occurred under hot hot rolling, whereas in comparable base steel strips, microstructures were almost completely recrystallized under hot rolling over about 25%. The addition of niobium to the cast steel strips inhibited the recrystallization of coarse as-cast austenite grain size during the hot rolling process and resulted in the hardenability of the steel being retained after hot rolling and molten niobium retention It was concluded.

The higher strength of the steel strip after hot rolling is mostly due to the formation of microstructures. As shown in FIG. 4A, the microstructure of cast niobium steel includes many, if not most, bainites for all strip thicknesses. However, as shown in FIG. 4B, the compared non-microalloyed steels showed a similar strength by coiling at low coiling temperatures and had a microstructure comprising mostly needle-shaped ferrites with some grain bound ferrites. The addition of niobium to the steel strip increases the hardenability of the steel, inhibits the formation of grain bound ferrites, and also promotes the bainite microstructure even at significantly higher coiling temperatures.

The yield and tensile strength results obtained from the test steels shown in Table 2 below at as-hot rolled conditions are summarized in FIG. The strength level is a yield strength of at least 340 MPa, which increases with increasing niobium content to a level of up to about 500 MPa immediately after the hot rolling. The tensile strength will be at least 410 MPa. The initial rapid increase in strength is due to the inhibition of pro-eutectic ferrite formation and the acceleration of bainite and needle ferrite, while the subsequent curing is due to the continuous microstructure purification and solid solution hardening possible from niobium maintained in solid solution can do.

Further, according to the electron microscope (TEM) test, no substantial niobium precipitate appeared in the cast strip immediately after hot rolling. This indicates that niobium was retained in the solid solution and the hardenability provided was mainly due to the enhanced hardenability effect of niobium, which resulted in the formation of microstructures predominantly of bainite and possibly bainite. The hardenability of the cast steel strip also appears to be enhanced by the rough austenite grains produced during the formation of the cast strip. It is believed that denaturation to bainite rather than ferrite is the main factor in suppressing precipitation of the microalloy addition of niobium in the thin strip cast strip during cooling of the coils from the coiling temperature.

Electron microscopy (TEM) tests can be used to determine the size, identity, and volume fraction of niobium carbonitride particles present in steel. The absence of any niobium carbonitride particles in the electron microscope test supported the view that the observed intensity was mainly due to microstructure consisting mainly of bainite rather than ferrite. The subsequent observed increase in strength resulting from the age hardening heat treatment thus reaches the conclusion that the niobium has been substantially melted in the hot rolled strip. The volume fraction of the carbonitride particles of the microstructure can be determined using electron microscopy analysis, and then the amount of the microalloyed component of the solid solution can be determined.

Thin foils or carbon replicates may be evaluated by TEM in determining the amount of the carbonitride particles of the present invention. In the analysis of the present inventor, JEOL 2010 electron microscope was used. However, according to my experience with these devices, Nb particles below 4 nanometers may not be analyzed in heavily dislocated ferrites.

A foil is prepared for the analysis of foil foil. The foil is cut and polished to a thickness of 0.1 mm. The sample was then electro-polished in a Tenupole-2 electro-polishing unit using 5% perchloric acid, 95% acetic acid electrolyte, Thin. The sample can then be moved directly to the TEM.

For carbon replication, the polished sample is etched in Nital (a solution of alcohol and nitric acid) after etching, the sample is coated with carbon, and then the carbon coating is exposed to a suitable size (e.g., The desired sample can be prepared by scoring with a sample (e.g., mm). After marking, the carbon replicas can be liberated from the sample by melting the ferrite matrix in 3% Nital solution. The carbon copy samples are collected in a 3 mm diameter support grid and subsequently washed in an ethanol / water solution. The carbon extracts having the support grid can then be transferred under an electron microscope.

An additional factor judged to account for the lack of niobium carbonitride particles in the hot-rolled cast strip is the nature of the dispersion of niobium with the rapid coagulation of the strip during the formation of the strip by the process of continuously making the cast strip described above Lt; / RTI > Relatively long time intervals in conventional microalloy high strength strips have been accompanied by solidification processes with slab cooling, slab reheating, and thermomechanical treatment processes, which can be accomplished with carbonitride (Nb, V, Ti, Mo) Enabling kinetics for subsequent precipitation through various stages of the manufacturing process while allowing for the opportunity for pre-clustering and / or solid state precipitates of the particles. In the process of the present invention described above, the cast strip is continuously formed in a casting pool between the casting rolls, wherein an excessively rapid initial solidification in forming the cast strip is effected by solid state precipitates of carbonitride particles and / or pre-clustering ), And in turn, slows down and reduces kinetic energy for precipitation of microalloys in subsequent processing, including rolling and coiling operations. This means that the microalloys of Nb, V, Ti, Mo are relatively more uniformly distributed on austenite and ferrite than on thin steel strips previously produced by conventional slab casting and manufacturing processes.

Atom probe analysis for niobium cast strips formed from casting pools between casting rolls as described above resulted in a more uniform distribution of microalloys in both as-cast and hot rolled strips when made into coils below 650 ° C Reduced pre-clustering and / or solid state solidification). This more uniform distribution of ingredients is believed to be due to the fine coherent precipitates of such components inhibiting the formation of carbonitrides in the coiling process under conditions previously encountered in micro alloy slab cast steels that have been conventionally prepared and treated . The lack of solid state formation and / or pre-clustering of the carbonitride in microalloyed cast strips produced by twin roll casting also slows down the provision of kinetic energy for the formation of carbonitrides during subsequent thermomechanical processes such as annealing do. This then provides an opportunity for age hardening at higher temperatures than those where the particles in previously traditionally treated strips lose their strengthening ability through a coarsening mechanism (Ostwald ripening) .

Higher tensile strengths were found to be achievable with age hardening heat treatments. For example, with 0.026% niobium addition, an increase in yield strength from 410 to 450 MPa (about 60-65 psi) was observed with at least 35 MPa (about 5 ksi). When 0.05% of niobium is added, it is judged that an increase of at least 10 ksi is expected by age hardening, and it is judged that at the addition of 0.1% of niobium, an increase of at least 20 ksi is expected by age hardening. The microstructure of the age hardened steel product according to the present invention may have niobium carbonitride particles having an average particle size of less than 10 nanometers. The microstructure of the aged hardened steel article may not substantially contain any niobium carbonitride particles in excess of 50 nanometers.

Experimental aging heat treatment was performed on samples of 0.026% niobium steel at various temperatures and times to attract the niobium activity that is considered to remain a solid solution in the hot rolled strip. As shown in Fig. 5, the aging heat treatment shows a significant increase in strength with a yield strength of about 480 MPa (about 70 ksi). This means that niobium is retained as a solid solution and can provide aging hardening in subsequent aging, for example, through the use of an annealing furnace on a continuous galvanizing line or by using a continuous annealing line . Thus, short-term aging hardening is performed to stimulate the aging potential by treating the niobium microalloy cast steel product through an annealing furnace attached to a continuous galvanizing line or through a conventional continuous annealing line. In the latter case, the high strength strip product that has been aged hardened may subsequently be galvanized, painted, or otherwise uncoated.

As shown in FIG. 6, the results clearly show that significant enhancement has been realized to have a strength level close to that performed for a longer time at lower temperatures for a maximum processing temperature of 700 DEG C (1292 DEG F). The tensile properties of the niobium sheet cast steel products after brief aging treatment at a maximum temperature of 700 DEG C (1292 DEG F) are provided in Table 1. In addition to the high strength of the cast strip products, ductility and formability are satisfactory for high quality structural products. The produced cast strip products are thin sheet high strength strip products suitable for structural applications through the use of niobium microalloys. It is believed that higher micro alloy levels potentially realize higher yield strengths in excess of 550 MPa (about 80 ksi).

Strip thickness, mm Yield strength, MPa Tensile strength, MPa Total elongation,%
YS / TS 'n' value 'r' value 1.1 477 563 18 0.85 0.12 0.90

In recent years, steel containing 0.014 wt.% And 0.065 wt.% Niobium addition in addition to producing 0.026 wt.% Niobium steel has been successfully produced through this process. The heat treated compositions are shown in Table 2 below.

steel C (wt%) Mn (wt%) Si (wt%) Nb (wt%) V (wt%) N (wt%) A 0.032 0.72 0.18 0.014 <0.003 0.0078 B 0.029 0.73 0.18 0.024 <0.003 0.0063 C 0.038 0.87 0.24 0.026 <0.003 0.0076 D 0.032 0.85 0.21 0.041 <0.003 0.0065 E 0.031 0.74 0.16 0.059 <0.003 0.0085 F 0.030 0.86 0.26 0.065 <0.003 0.0072 G 0.028 0.82 0.19 0.084 <0.003 0.0085 H 0.026 0.90 0.21 <0.003 0.042 0.0070 Base steel 0.035 0.85 0.27 <0.003 <0.003 0.0060

The yield strength achieved for Steel C and Steel F is shown in Figure 12 and the yield strength results for 0.014% Nb heat treated steel A produced with lower Mn content are shown in FIG. The addition of niobium increased the yield strength at all coiling temperatures in relation to the base steel composition. The yield strength was increased by about 70 to 100 MPa (10 to 15 ksi) for 0.014% Nb and 0.026% Nb additives and increased by about 140 to 175 MPa (20 to 25 ksi) for 0.065% Nb additive . From Figure 12 it is understood that 0.026% Nb steel has a higher yield strength than 0.8 Mn base steel for similar coil ring temperatures and that the 0.8 Mn base steel achieves a comparable yield strength when coiled at lower temperatures do. Alternatively, the strength achieved in the above-mentioned 0.8 Mn base steel at low coil ring temperatures (about 500 DEG C) can be achieved at higher coiling temperatures (about 600 DEG C) with these Nb additives.

In addition, it has been found that the microalloyed additive of the present invention as compared to microalloyed steels produced in the conventional manner, inhibits the formation of carbonitride particles in hot rolled and subsequently coiled and cooled steels. Instead, the microstructure of the hot rolled and subsequently coiled and cooled steel includes bainite and needle-like ferrite with niobium and / or vanadium in excess of 70% remaining in the solid solution. Alternatively, the microstructure of the hot-rolled and subsequently coiled and cooled steel described above may comprise bainite and needle-like ferrite having niobium and / or vanadium in excess of 80% remaining in the solid solution, Optionally, it may comprise bainite and needle-like ferrite with niobium and / or vanadium in excess of 90% remaining in the solid solution.

Thus, it can be seen that the niobium cast strip results in a light gauge, high strength steel product. The niobium additive can first inhibit austenite recrystallization during hot rolling, which improves the hardenability of the steel by maintaining a relatively rough as-cast austenite size. Niobium, which is retained as a solid solution in austenite after hot rolling, directly increases the hardenability of the steel, which, even at relatively high coiling temperatures, is responsible for converting the austenite to the final microstructure, It is helpful. Formation of bainite microstructure promoted the preservation of the niobium content in the solid state in the hot rolled strip.

The properties of the steel of the present invention can be further improved by aging the steel. In previous microalloyed and non-microalloyed steels, an increase in strength could be obtained by age hardening, but in such conventional steels a decrease in elongation occurs with increasing strength. The present inventors have found that both the elongation and the increase in strength can be obtained by age-hardening the steel of the present invention.

Preservation of microalloyed components such as niobium and vanadium in solid solution state by conventional processing conditions provided significant curing ability for subsequent age hardening cycles. Such age hardening cycles may be made using a suitable continuous galvanizing line or continuous annealing equipment. Therefore, microalloyed steel strips produced using a sheet strip casting process in combination with an age hardening heat treatment process provided by an appropriate galvanizing line or an annealing facility are uniquely designed to provide a unique reinforcing approach to this type of steel product .

Isothermal aging treatment of hot rolled 0.026% Nb cast strip material was performed at 600 and 650 ° C (1110 ° F and 650 ° C, respectively) to induce the formation of niobium carbonitride or Nb (C, N), as confirmed by electron microscopy 1200 &lt; 0 &gt; F) for 20 minutes. This resulted in an increase in the yield strength of the material as shown in Fig. Also as shown in FIGS. 6 and 14, the thermal cycle of the strip through the annealing section of the galvanizing line resulted in a significant increase in strength, approaching that achieved by isothermal aging at lower temperatures.

The increase in hardenability provided by the addition of microalloys through the suppression of ferrite denaturation significantly lowers the austenite decomposition temperature to the bainite / needle ferrite temperature range. This lower denaturation starting temperature provides the potential to maintain the majority of the microalloys in the solid solution by applying a conventional runout table cooling rate and an appropriate coiling temperature.

Microalloyed components present in solid solution state such as niobium and vanadium can utilize aging hardening to increase strength during subsequent heat treatment. An age hardening study in the laboratory confirms that substantial hardening can be achieved with relatively short annealing cycles, such as using continuous annealing lines and galvanizing lines. As shown in Figs. 15-18, the results in the laboratory assumed continuous annealing cycles applied to sample steel C (0.026% Nb), steel F (0.065% Nb), steel G (0.084% Nb).

The results of a full scale experiment using steels B and F using the heat treatment conditions set forth in the laboratory study are shown in Figures 20 and 21, respectively. A substantial increase in strength was achieved with steels B and F. Yield strength levels exceeding 450 MPa were recorded with steel B (0.024% Nb steel) and yield strengths exceeding 550 MPa were recorded with steel F (0.065% Nb steel). The increase in strength due to age hardening was about 70 MPa for steel B (0.024% Nb steel) and about 100 MPa for steel F (0.065% Nb steel). The above-mentioned 0.065% Nb steel may achieve a yield strength exceeding 600 MPa under aged hardening treatment conditions.

Steel F Thickness mm Yield strength, MPa Tensile strength, MPa Elongation,% High temperature band 0.996 512 599 11.47 Galvanizing 0.991 581 645 14.16

Samples of Steel F were age hardened using age hardening conditions found in the galvanizing line. As shown in Table 3, the aged hardened steel has a strength of almost 70 MPa and an elongation increased from 11.47% to 14.16%. The relationship between the yield strength and the total elongation for the niobium steel that is currently started in the as-hot rolled condition and in the age hardening and galvanizing conditions (longitudinal test direction) is shown in Fig.

As shown in Figure 16, the inventors have found that a 10 second hold cycle can be used between 675 캜 and 725 캜 to prevent overaging. However, the temperature range is a function of the holding time. Increasing the hold time to 20 seconds lowers the temperature range somewhat, but for zero hold time the temperature range increases somewhat as shown in FIG. The age hardening temperature range may be between about 625 ° C and 800 ° C, depending on the overall heat treatment cycle time, i.e., heating rate, holding time, and cooling rate.

Lower temperatures in the range of 500 [deg.] C to 650 [deg.] C may be used in the case of longer time heat treatment. In FIG. 6, it can be seen that a heat treatment at 600 占 폚 for 20 minutes produces a similar strength level of as much as 10 seconds in a continuous annealing cycle at 700 占 폚. 22 shows the results of a heat treatment in a laboratory conducted for 20 minutes and 120 minutes. The results indicate that substantial curing was achieved for a heat treatment at 550 캜 for 120 minutes, but aging at a temperature above about 650 캜 for 120 minutes lowered the hardness of the steel. Other post-coiling cooling for hot-rolled coils designed to deposit the preserved niobium by batch annealing in a temperature range of 500 [deg.] C to 650 [deg.] C, or controlled cooling through a temperature range of 500 [ A longer annealing time, such as the method, can be used with the overall coil annealing process.

Electron microscopic (TEM) irradiation was carried out on the samples of steel C and F, and they were subjected to heat treatment at 650 ° C for 60 minutes. Fine particles ranging in size from 4 to 15 nanometers were found. These microparticles are known to contain niobium carbonitride, which indicates that such curing can be due to age hardening by the micro-niobium carbonitride particles.

The microstructure of the age hardened microalloyed steel product may have niobium carbonitride particles having an average particle size of less than 10 nanometers. The microstructure of the aged hardened steel product described above may be substantially free of any niobium carbonitride particles greater than 50 nanometers. Samples of the niobium steel of the present invention were examined using an electron microscope and portions of such microstructures did not contain any measurable amounts of carbonitride particles.

The present inventors have found that in the age hardened steel of the present invention, portions of microstructures where the enhanced strength / elongation relationship is substantially free of particles of a size exceeding 5 nanometers, or "precipitate free zone" It is believed to be due to the nano-cluster. Development of the non-precipitate zone near the grain boundaries can affect the strength and tensile elongation relationship by causing a decrease in hardness in the regions adjacent to the grain boundaries. It has been reported that the relaxation of stress concentration in the no-sediment area improves strength and elongation. The beneficial effect of the no-sediment zone on elongation and strength can be seen when the above-mentioned no-sediment zones are narrow and the size of the particle boundary sediments is small.

The addition of components in the steels of the present invention can provide increased elongation with increased strength after age hardening by producing smaller un-precipitated zone widths and smaller hardness changes than in niobium steels typically produced. Owing to the more uniform distribution of the components in the rapidly solidified steel, the kinetics of age hardening are delayed to effectively expand the time-temperature window in which the formation of nano-clusters can be safely controlled. The nano-clusters of the above components can provide enhancement in the early stages of age hardening. Cluster strengthening may be due to the extra energy required to dislocate the diffusion boundary of a cluster of solute species. The clusters described above can provide substantial enhancement without reducing ductility because their resiliently flexible boundaries do not strictly inhibit dislocation movement or cause pile-ups in the way normal normal second phase particles do.

The more uniformly distributed components of the steel of the present invention are retained in solid solution during rapid solidification of the steel. In contrast to the previously traditionally produced niobium and vanadium steels, the hot rolling of the present invention and the microstructures of the subsequently coiling and cooled steels, with the niobium and / or vanadium additions in excess of 70% remaining in the solid solution Bainite, and needle-like ferrite, and has substantially no niobium and / or vanadium particles greater than 50 nanometers. Alternatively, the microstructure of the hot-rolled and subsequently coiling and cooled steel described above may comprise bainite and needle-like ferrite having a niobium and / or vanadium addition in excess of 80% present in solid solution, , It may have a proportion exceeding 90% which is present in the solid solution.

The components are present in the hot rolled coil in a molten state trapped and do not precipitate if the coil ring temperature is less than 650 ° C. Formation is effectively delayed because the association of atoms, which usually occurs in conventional slab casting and reheating for hot strip rolling (such as in the form of particles), is prevented in the present process. The observed increase in strength in the hot rolling coils will therefore be due largely to the hardenability and solid solution hardening effect.

The formation of the carbonitride particles can be activated during the heat treatment. Additionally, pre-precipitation clusters and finer particles during age hardening are stable over an extended range of time and temperature due to the significant amount of niobium and / or vanadium in solid state prior to age hardening. Precipitate zones formed near particle boundaries, such as the normal precipitation phenomenon, typically contain nano-clusters and finer precipitates that are narrower and more uniformly distributed than for steel produced. Thus, the change in hardness in the no-sediment region with respect to the interior of the particle is relatively small for the present steel. The present inventors have found that in a no-deposit region that reduces microcracks from differential stress preferential deformation in the no-deposit region due to a smaller hardness variation across the narrower no-deposit region and the no-deposit region, I believe concentration is declining. The inventors believe that cluster strengthening is characterized by an increase in strength without degradation of ductility because dislocation pile-up does not occur in the clusters. It is believed that the combination of a narrow no-deposit area and a cluster strengthening action results in no-deposit areas of current steel. This results in improved elongation because the cracks are more difficult to start and less constrained to the grain boundary non-sediment region. In addition, the above-described nano-clusters may coexist with distinct particles within the intra-particle regions for certain annealing temperature / time combinations.

An annealing furnace can be used to perform age hardening, which is not a solution for current enhancement to process such products. The annealing conditions may be one continuous annealing cycle having a peak temperature of at least 650 캜 and better than 675 캜 to 750 캜 at less than 800 캜. Alternatively, such enhancement can be achieved in a manufacturing environment using a very short age-hardening cycle as is possible with conventional annealing furnaces incorporated in continuous galvanizing lines. The final strength levels recorded in a full scale plant test were similar to those produced as a heat treatment in the laboratory of each steel.

Similar results were observed with niobium between about 0.01% and about 0.20% titanium, between about 0.05% and about 0.50% molybdenum, and between about 0.01% and about 0.20% vanadium as well as between 0.01% and about 0.20% .

The composition of the current steel utilizing vanadium is shown as steel H in Table 2. The yield strength of steel H is shown in Fig. Vanadium steel is produced with two different coiling temperatures and subsequently subjected to aging at 650 ° C and 700 ° C for 20 minutes to induce curing by vanadium in the solid state. The results show that a significant enhancement was achieved from this heat treatment condition. The enhancement enhancement was slightly higher for the material produced with the higher coiling temperature, which may be due to the effects of conflicting processes of precipitation hardening and microstructure softening. The enhancement increment implemented with the material produced at the lower coiling temperature was about the same as achieved with steel containing 0.026% Nb.

The yield strength of steel H in as-hot rolled and galvanizing conditions is provided in Fig. 23 and 24 show that vanadium steel was produced using higher coiling temperatures but achieved a higher level of strength than ordinary carbon-based steels. In the samples shown in Fig. 24, the coiling temperature of steel H was 570 占 폚 and the base steel coiling temperature was less than 500 占 폚.

Also, as shown in Fig. 24, an increase in strength from age hardening using annealing furnaces in a continuous galvanizing line was realized in vanadium steel, but the increase in strength was lower than realized from an equivalent niobium component. The yield strength of the sample in Fig. 24 in the galvanizing line was about 450 MPa in the galvanized condition, which is the degree achieved by the longer time laboratory heat treatment shown in Fig. The strength of vanadium steel may be more sensitive to the coiling temperature than niobium steel.

These thin strip cast strips enable the production of new kinds of steel products with:

1. Galvanized strips of high strength and thin thickness by utilizing age hardening during galvanizing and microstructure including bainite as main component. The annealing section of the galvanizing line can be used to induce age hardening of the niobium and / or vanadium in the hot rolled sheet metal cast strip.

2. High strength, thin thickness uncoated strip utilizing bainite and age hardened microstructure during processing in successive annealing lines. The hot furnace of conventional continuous annealing can be used to induce activation of the niobium and vanadium components remaining in the solid solution state by the bainite microstructure after hot rolling of the sheet metal strip.

3. High strength, thin-walled hot-rolled cast strip product whose strength level is insensitive to the degree of applied hot rolling reduction. The bainite microstructure produces relatively high strength products (YS ≥ 380 MPa (~ 55 ksi)). The inhibition of austenite recrystallization during or after hot rolling can provide a final strength level that is insensitive to the degree of hot rolling reduction. The final strength level will be consistent over a range of thicknesses that can be produced by the sheet metal strip process.

While the invention has been illustrated and described in detail in the foregoing drawings and description, it should be understood that they are to be considered illustrative rather than limiting in nature, and that only exemplary embodiments thereof have been shown and described. And all changes and modifications that fall within the scope of the invention as described by the following claims shall be protected. Additional features of the invention will be apparent to those of ordinary skill in the art in view of the detailed description. Such variations will be made without departing from the scope and scope of the invention.

The present invention is used in the manufacture of high strength thin cast strips.

Claims (28)

By weight, less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum, and 0.01-0.20% niobium, 0.01-0.20% vanadium, And comprising niobium and / or vanadium exceeding 70% of the solid solution state after coiling and cooling. The coil steel article of claim 1 wherein said niobium is less than 0.1%. The coil steel product of claim 1, further comprising fine silicon oxide and iron oxide particles distributed through a steel microstructure having an average particle size of less than 50 nanometers. 2. The coil steel product of claim 1, wherein the steel product has a yield strength of at least 340 MPa. The product of claim 1, wherein the steel product has a tensile strength of at least 410 MPa. The coil steel product of claim 1, wherein the steel product has a thickness of less than 3.0 mm. The product of claim 1, wherein the steel product has a thickness of less than 2.5 mm. The product of claim 1, wherein the steel product has a thickness of less than 2.0 mm. 2. The coil steel product of claim 1, wherein the steel product has a thickness in the range of 0.5 mm to 2 mm. The product of claim 1, wherein the steel product has a total elongation of at least 6%. The product of claim 1, wherein the steel product has a total elongation of at least 10%. From a group consisting of less than 0.25% carbon, from 0.20% to 2.0% manganese, from 0.05% to 0.50% silicon, less than 0.01% aluminum, and from 0.01% to 0.20% niobium and from 0.01% to 0.20% Wherein the microstructure comprises bainite and acicular ferrite structures and comprises at least one element selected from the group consisting of silicon and iron distributed through a steel microstructure having an average particle size of less than 50 nanometers Steel products containing fine oxide particles. 13. The steel product according to claim 12, wherein the niobium is less than 0.1%. 13. The steel product according to claim 12, wherein the steel product has a yield strength of at least 340 MPa. 13. A steel product according to claim 12, wherein the steel product has a tensile strength of at least 410 MPa. 13. A steel product according to claim 12, wherein the steel product has a total elongation of at least 6%. 13. A steel product according to claim 12, characterized in that the steel product has a total elongation of at least 10%. From less than 0.25% carbon, from 0.20 to 2.0% manganese, from 0.05 to 0.50% silicon, less than 0.01% aluminum, and from 0.01% to 0.20% niobium, from 0.05% to 0.50% molybdenum, and from 0.01% And 0.20% vanadium, wherein the microstructure comprises bainite and acicular ferrite structure, wherein the microstructure comprises a steel microstructure having an average particle size of less than 50 nanometers &Lt; RTI ID = 0.0 &gt; and / or &lt; / RTI &gt; 19. The steel product according to claim 18, wherein the niobium is less than 0.1%. 19. A steel product according to claim 18, wherein the steel product has a yield strength of at least 340 MPa. 19. The steel product of claim 18, wherein the steel product has a tensile strength of at least 410 MPa. 19. A steel product according to claim 18, wherein the steel product has a total elongation of at least 6%. 19. A steel product according to claim 18, wherein the steel product has a total elongation of at least 10%. A method of making a thin cast steel coil steel strip, Assembling a cooling roll caster having casting rolls disposed sideways therebetween to form a nip portion therebetween and being supported on the casting rolls above the nip portion and being supported by side dams in the vicinity of the ends of the casting rolls Forming a casting pool of limited molten iron, Rotating the casting rolls in opposite directions so that the metal shells solidify above the casting rolls as the casting rolls move through the casting pool, Forming a steel strip from the metal shells downwardly through a nip portion between the casting rolls, and By weight of carbon, less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, less than 0.01% aluminum, and 0.01-0.20% niobium, 0.01-0.20% vanadium, At least one component selected from the group consisting of bainite and needle-like ferrite, and further comprising at least a ratio of 10 DEG C / sec to provide a composition having niobium and / or vanadium in solid solution state of more than 70% &Lt; / RTI &gt; and cooling the steel strip to a predetermined temperature. 25. The method of claim 24, wherein the coil steel strip has fine oxide particles of iron and silicon distributed through a steel microstructure having an average particle size of less than 50 nanometers. . 25. The method of claim 24, Hot rolling the steel strip, and Further comprising the step of forming said hot rolled steel strip into a coil shape at a temperature between 450 [deg.] C and 700 [deg.] C. 25. The method of claim 24, Hot rolling the steel strip, and Further comprising the step of forming said hot rolled steel strip into a coil shape at a temperature of less than 600 &lt; 0 &gt; C. 25. The method of claim 24, further comprising aging hardening the steel strip at a temperature of at least 550 DEG C to increase tensile strength.

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