JP5385899B2 - Thin cast strip with added microalloy and method for producing the same - Google Patents

Description

  This PCT application is based on US patent application No. 11 / 744,881 filed May 6, 2007, which is based on application number 11 / 255,604 filed October 20, 2005. This is a partial continuation application. This application is also based on US provisional patent application No. 60 / 943,781 filed Jun. 13, 2007.

  The present invention relates to the production of high strength thin cast strips and to a method for producing such cast strips in a twin roll caster.

In a twin roll casting machine, the molten metal is introduced between a pair of casting rolls that are cooled internally and rotated in the mutual direction, so that the metal shell solidifies on the surface of the moving roll, and in the roll gap between the rolls. A combined and solidified strip is produced and fed downward from the roll gap between the casting rolls. As used herein, the term “roll gap” (nip) refers to the entire region where the casting rolls are closest to each other. Molten metal is poured from a ladle through a metal feed system consisting of a tundish and a core nozzle positioned above the roll gap, and is supported by the roll casting surface above the roll gap and extends in the longitudinal direction of the roll gap. Form. This casting reservoir is usually defined between a refractory side plate or side weir that is slidably engaged and held on the end face of the roll in order to stop it from flowing out from both ends of the casting reservoir.

US Pat. No. 6,488,790

High strength, low carbon thin strips with a yield strength (YS) of 413 MPa (60 ksi) or more and a strip thickness of less than 3.0 mm have been conventionally produced by annealing of cold rolled strips. And cold rolling was necessary to achieve the desired thickness. Cold rolled strips are annealed to improve ductility without significantly reducing strength. However, the final ductility of the resulting strip is still relatively low, and the strip cannot achieve the total elongation level exceeding 6% required by some building codes for structural steel for structural components. Such annealed cold rolled low carbon steel is generally suitable only for simple forming operations such as rolling and bending, and is a steel with high ductility using cold rolling and annealing manufacturing techniques. It was not technically feasible to produce a strip with such a final strip thickness.

Traditionally, high strength steels have been made by microalloying with elements such as niobium, vanadium, titanium or molybdenum and have achieved the desired thickness and strength levels by hot rolling. Such microalloying requires expensive and high levels of niobium, vanadium, titanium or molybdenum, resulting in the formation of a bainite-ferrite microcrystalline structure typically containing 10-20% bainite. See Patent Document 1. Alternatively, the fine crystal structure may be ferrite containing 10 to 20% pearlite. These alloying elements will be partially precipitated during the hot rolling of the strip, so that the required strength level is achieved by providing sufficient age hardening to the microcrystalline structure that is mainly ferritized. This required relatively high alloy levels of niobium, vanadium, titanium or molybdenum elements. This high level of microalloying significantly increased the required hot rolling load and limited the range of hot rolled strip thicknesses that could be produced economically and practically. Such an alloyed high strength strip can be used for galvanization directly after pickling if the product has a thick end exceeding 3 mm thick.

  However, it is very difficult to make a high-strength steel strip with a thickness of less than 3 mm by adding niobium, vanadium, titanium or molybdenum to the basic steel component, especially when obtaining a wide strip by applying a high rolling load. Yes, it was not always commercially possible. Conventionally, adding these elements in large amounts is necessary to increase the strength of the steel, thereby reducing the elongation properties of the steel. Conventionally, high strength microalloyed hot rolled strips are relatively difficult to provide strength, are relatively expensive, and often have to be supplemented with other alloying elements.

  Although cold rolling is generally required to reduce the strip thickness, it is difficult to perform cold rolling on a hot-rolled high-strength strip because a high cold rolling load is required to reduce the strip thickness. there were. This high level of alloying also significantly increases the recrystallization annealing temperature required, and allows the construction and operation of an annealing line that provides the high annealing temperature necessary to fully recrystallize cold rolled strips. There was a need for high costs.

  In short, in the conventionally known microalloying method in which a high-strength thin strip is made using an element such as niobium, vanadium, titanium, or molybdenum, the alloying cost is high, the addition of the element is relatively inefficient, It was difficult to apply a high rolling load to the inter-rolling, and a high recrystallization annealing temperature was required.

The disclosed thin cast steel strip is less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, and less than 0.01% by weight. Aluminum , 0 . 01-0 . It contains 20% niobium, most of the fine crystal structure is bainite and acicular ferrite, and niobium in the solid solution exceeds 70%. Niobium may be less than 0.1%. The thin cast steel strip further comprises 0 . 05-0. 50% molybdenum , 0 . 01-0 . It may comprise at least one element selected from the group consisting of 20% vanadium and mixtures thereof.

The yield strength of the thin cast steel strip can be at least 340 MPa and the tensile strength (TS) can be at least 410 MPa. The yield strength of the thin cast steel strip may be at least 485 MPa and the tensile strength may be at least 520 MPa. The total elongation of the thin cast steel strip may be at least 6%, or at least 10%.

The steel article may be a thin cast steel strip. Optionally, the thin cast steel strip may have silicon and iron oxide particulates distributed through a steel crystallite structure having an average particle size of less than 50 nanometers.

The thin cast steel strip thickness may be less than 2.5 mm, or less than 2.0 mm. As yet another option, the thin cast steel strip thickness is 0 . It may be in the range of 5 to 2 mm.

Also disclosed is a hot rolled thin cast steel strip having a thickness of less than 3 millimeters, by weight of less than 0.25% carbon, 0.20 to 2.0% manganese, and 0.05 to 0.50%. and silicon, and less than 0.01% aluminum, 0. 01-0 . With 20% niobium, the majority of the microcrystalline structure is composed of bainite and acicular ferrite and can provide a yield strength of at least 410 MP under 20-40% reduction. The yield strength of the thin cast steel strip may be at least 485 MPa and the tensile strength may be at least 520 MPa. Alternatively, niobium may be less than 0.1%.

Optionally, hot rolled thin cast steel strip may have fine oxide particles of silicon and iron distributed through the steel micro fine crystal structure with an average particle size of less than 50 nanometers.

The hot rolled thin cast steel strip may have a total elongation of at least 6%, or at least 10%. The thickness of the hot rolled thin cast steel strip may be less than 2.5 mm, or less than 2.0 mm. In yet another option, the thickness of the hot rolled thin cast steel strip is 0 . It may be in the range of 5 to 2 mm.

A coiled thin cast steel strip is also disclosed, by weight, less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, and aluminum less than 0.01%, 0. 01-0 . 20% niobium , 0 . 01-0 . At least one element selected from the group consisting of 20% vanadium and a mixture thereof, and niobium and / or vanadium in the solid solution after winding and cooling exceeds 70%. Alternatively, niobium may be less than 0.1%.

The optional, thin cast steel strip wound may have fine oxide particles of silicon and iron with an average particle size is distributed over a steel micro fine crystal structure of less than 50 nanometers.

The yield strength of the wound thin cast steel strip may be at least 340 MPa and the tensile strength may be at least 410 MPa. Winding thickness of the thin cast steel strip taken is less than 3.0 mm, the yield strength is at least 485MPa of thin cast steel strip, the tensile strength may be at least 520 MPa.

Alternatively, the thickness of the wound thin cast steel strip may be less than 2.5 mm, or less than 2.0 mm. In yet another option, the rolled thin cast steel strip has a thickness of 0 . It is in the range of 5 to 2 mm. The total elongation of the wound thin cast steel strip may be at least 6%, or at least 10%.

An age hardened thin cast steel strip is also disclosed, by weight less than 0.25% carbon, 0.20 to 2.0% manganese, 0.05 to 0.50% silicon, and aluminum less than 0.01%, 0. 01-0 . 20% niobium , 0 . 01-0 . At least one element selected from the group consisting of 20% vanadium and mixtures thereof, and the majority of the microcrystalline structure is composed of bainite and acicular ferrite to increase elongation and yield strength after age hardening . Alternatively, niobium may be less than 0.1%.

The age-hardened thin cast steel strip may further comprise silicon and iron oxide particulates distributed through a steel microcrystalline structure having an average grain size of less than 50 nanometers.

The yield strength of the thin cast steel strip may be 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, as desired. The tensile strength of the thin cast steel strip may be 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, as desired. The thickness of the age-hardened thin cast steel strip is less than 3.0 mm or less than 2.5 mm. Alternatively, the thickness of the age-hardened steel product is less than 2.0 mm. In yet another option, age hardened thin cast steel strip has a thickness of 0 . It may be in the range of 5 to 2 mm. The total elongation of the age-hardened thin cast steel strip may be at least 6%, or at least 10%.

The thin cast steel strip is by weight less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, and less than 0.01% aluminum. , 0 . 01-0 . Of 20% niobium及beauty 0. 01-0 . At least one element selected from the group consisting of 20% vanadium, distributed mostly through a steel microcrystalline structure composed of bainite and acicular ferrite and having an average grain size of less than 50 nanometers Contains oxide fine particles of silicon and iron. Alternatively, niobium may be less than 0.1%. Optionally, the thin cast steel strip is 0 . It may contain 0.5-0.50% molybdenum.

The yield strength of the thin cast steel strip may be at least 340 MP and the tensile strength may be at least 410 MPa. The yield strength of the thin cast steel strip may be at least 485 MPa and the tensile strength may be at least 520 MPa. The total elongation of the thin cast steel strip may be at least 6%, or at least 10%.

Age-hardened thin cast steel strips by weight are less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon, 0.01% Less than aluminum , 0 . 01-0 . It contains 20% niobium, most of the fine crystal structure is composed of bainite and acicular ferrite, and has niobium carbonitride particles having an average particle size of less than 10 nanometers. The carbonitride particles in the specification of the present invention and the appended claims include carbides, nitrides, carbonitrides, and mixtures thereof. Alternatively, niobium may be less than 0.1%.

Thin cast steel strip is age hardened in not have carbonitride niobium particles substantially exceeds 50 nm. The age-hardened thin cast steel strip may have a yield strength of at least 340 MPa and a tensile strength of at least 410 MPa. The total elongation of the age-hardened thin cast steel strip may be at least 6%, or at least 10%.

A method of making a wound thin cast steel strip is disclosed that assembles an internally cooled roll caster having laterally positioned casting rolls that form a roll gap therebetween, the casting roll above the roll gap. Forming a molten steel casting sump defined by a side weir supported above and adjacent to the end of the casting roll;
Rotating the casting roll in the mutual direction, solidifying the metal shell on the casting roll while the casting roll moves in the casting pool,
Form a steel strip downward from the metal shell through the roll gap between the casting rolls,
Cool the steel strip at a rate of at least 10 ° C./second and by weight less than 0.25% carbon, 0.20 to 2.0% manganese and 0.05 to 0.50% silicon. , and less than 0.01% aluminum, 0. 01-0 . 20% niobium , 0 . 01-0 . At least one element selected from the group consisting of 20% vanadium and mixtures thereof, most of the fine crystal structure is composed of bainite and acicular ferrite, and niobium and / or vanadium in the solid solution is 70% More than the steps of providing a composition.

The method may provide silicon and iron oxide particulates distributed in a steel microcrystalline structure having an average particle size of less than 50 nanometers in a wound thin cast steel strip. Further, the method may include hot rolling the thin cast steel strip and winding the hot rolled thin cast steel strip at a temperature of 450-700 ° C. Alternatively, the hot-rolled thin cast steel strip may be wound at a temperature below 650 ° C.

The method may further comprise age hardening the thin cast steel strip to increase the tensile strength to at least 550 ° C. Alternatively, age hardening may occur at a temperature of 625-800 ° C. In yet another option, age hardening may occur at a temperature of 650-750 ° C.

A thin cast steel strip manufacturing method is also disclosed,
Assemble an internally cooled roll casting machine having a casting roll positioned laterally and forming a roll gap therebetween, defined by a side dam supported by the casting roll above the roll gap and adjacent to the end of the casting roll Forming a molten steel casting reservoir,
Rotating the casting rolls in opposite directions, solidifying the metal shell on the casting rolls while the casting rolls move in the casting pool,
A steel strip is formed below the metal shell through the roll gap between the casting rolls, and the steel strip is cooled at a rate of at least 10 ° C./second, less than 0.25% carbon by weight, 0.01% Less than aluminum , 0 . 01-0 . 20% niobium , 0 . 01-0 . At least one element selected from the group consisting of 20% vanadium and mixtures thereof, and the majority of the fine crystal structure is composed of bainite and acicular ferrite, and niobium and / or vanadium in the solid solution is 70%. To provide a composition,
Including the steps of age hardening the thin cast steel strip at a temperature of 625-800 ° C.

The method may further comprise age hardening the thin cast steel strip to increase the tensile strength. Alternatively, age hardening may occur at a temperature of 650-750 ° C.

This method has an average particle size may provide a thin cast steel strip is age hardened with a carbonitride of niobium particles less than 10 nanometers. Or thin cast steel strip with age hardened essentially no free of carbonitride niobium particles exceeding 50 nm.

The method may provide silicon and iron oxide particulates distributed through a steel microcrystalline structure having an average particle size of less than 50 nanometers in a wound thin cast steel strip. Furthermore, the method, the thin cast steel strip hot rolling, the thin cast steel strip is hot rolled may include the step of winding at a temperature below 700 ° C.. Alternatively, the hot-rolled thin cast steel strip may be wound at a temperature below 650 ° C.

Thin cast steel strip manufacturing method
Assemble an internally cooled roll casting machine having a casting roll positioned laterally and forming a roll gap therebetween, defined by a side dam supported by the casting roll above the roll gap and adjacent to the end of the casting roll Forming a molten steel casting reservoir,
Rotating the casting rolls in opposite directions, solidifying the metal shell on the casting rolls while the casting rolls move in the casting pool,
Form a steel strip downward from the metal shell through the roll gap between the casting rolls,
Allow the steel strip to cool at a rate of at least 10 ° C./second and, by weight, less than 0.25% carbon, 0.20-2.0% manganese, 0.05-0.50% silicon , and less than 0.01% aluminum, 0. 01-0 . 20% niobium , 0 . 01-0 . At least one element selected from the group consisting of 20% vanadium and mixtures thereof, and the majority of the fine crystal structure is composed of bainite and acicular ferrite,
The thin cast steel strip may be age hardened at a temperature of 625-800 ° C. to include the steps of increasing elongation and increasing yield strength after age hardening.

The method in thin cast steel in strip wound, the average particle size may provide oxide particles of silicon and iron distributed through the steel micro fine crystal structure of less than 50 nanometers. Furthermore, the method may provide a thin cast steel strip is age hardened average particle size having a carbonitride of niobium particles less than 10 nanometers. Or thin cast steel strip with age hardened essentially no free of carbonitride niobium particles exceeding 50 nm.

The method a thin cast steel strip hot rolling, the thin cast steel strip is hot rolled may include the step of winding at a temperature below 750 ° C.. Alternatively, the hot-rolled thin cast steel strip may be wound at a temperature below 700 ° C.

  In order to describe the invention in greater detail, several embodiments will be described with reference to the accompanying drawings.

A strip casting facility incorporating an in-line hot rolling mill and a coiler is shown. The details of the twin roll strip casting machine are shown. The influence of the coiling temperature on the strip yield strength with and without addition of niobium or vanadium is shown. 2 is an optical micrograph of a niobium steel strip. 2 is an optical micrograph of a standard USC SS Grade 380 steel strip. It is a graph which shows the influence of the age hardening after winding with respect to the yield strength of the steel strip of this invention. It is a graph which shows the influence of the age hardening cycle simulated after winding with respect to the yield strength and tensile strength of the steel strip of this invention. It is a graph which shows the influence of the hot rolling reduction with respect to yield strength. It is a graph which shows the influence of the yield strength with respect to elongation. It is a graph which shows the influence of the amount of niobium on the yield strength when niobium is at a low level. It is a microscope picture of the fine crystal structure of the 1st sample of 0.065% niobium steel after hot rolling. It is a microscope picture of the fine crystal structure of the 2nd sample of 0.065% niobium steel after hot rolling. It is a graph which shows the influence of the amount of niobium with respect to yield strength. It is a graph which shows the influence of coiling temperature with respect to yield strength. It is a graph which shows the influence of coiling temperature with respect to the yield strength in case niobium is a low level. It is a graph which shows the influence of the heat processing conditions with respect to yield strength. It is a graph which shows the influence of age hardening heat treatment temperature with respect to the yield strength of 0.026% niobium steel. It is a graph which shows the influence of the peak aging temperature with respect to the yield strength of 0.065% niobium steel. It is a graph which shows the influence of the peak aging temperature and holding time with respect to the yield strength of 0.065% niobium steel. It is a graph which shows the influence of the peak aging temperature and holding time with respect to the yield strength of 0.084% niobium steel. It is a graph which shows the influence of the yield strength with respect to elongation before and after age hardening. It is a graph which shows a continuous annealing heat processing result. It is a graph which shows an age hardening state. It is a graph which shows the influence of the temperature and heat with respect to hardness. It is a graph which shows the influence of the heat processing with respect to the yield strength of the vanadium steel of this invention. It is a graph which shows the influence of the hot rolling reduction with respect to the yield strength of the vanadium steel of this invention.

    The following description of the examples refers to a high strength thin cast strip with added microalloy produced by continuous casting of a steel strip using a twin roll caster.

  FIG. 1 shows a series of parts of a strip casting machine for continuously casting a steel strip. The cast steel strip 12 continuously produced by the twin roll casting machine 11 shown in FIGS. 1 and 2 passes through the transition path 10, passes through the guide table 13, and reaches the pinch roll stand 14 having the pinch roll 14 </ b> A. Immediately after exiting the pinch roll stand 14, the strip is hot rolled through a hot rolling mill 16 having a pair of reduction rolls 16A and backup rolls 16B and reduced to the desired thickness. The hot-rolled strip reaches the run-out table 17 and can be cooled by contact and radiation between the convection and water supplied by water jets 18 (or other suitable means). The rolled and cooled strip is then passed through a pinch roll stand 20 comprising a pair of pinch rolls 20A and further through a coiler 19. Final cooling of the cast strip takes place after winding.

  As shown in FIG. 2, a main machine frame 21 constituting the twin roll casting machine 11 supports a pair of casting rolls 22 positioned in the lateral direction and having a casting surface 22A. During the casting operation, the molten metal flows from the ladle (not shown) to the tundish 23, to the distributor or movable tundish 25 via the refractory shroud 24, and from the distributor 25 to the roll gap via the metal feed nozzle 26. 27 between the upper casting rolls 22. Molten metal fed between the casting rolls 22 forms a casting pool 30 above the gap between the rolls. A pair of thrusters (not shown) in which a pair of side closing weirs or side closing plates 28 for closing both ends of the casting roll 30 of the casting reservoir 30 are connected to the side closing plate holder (Not shown) is pressed to both ends of the casting roll. Since the upper surface of the casting reservoir 30 (generally referred to as the “meniscus” level) is usually raised above the lower end of the feeding nozzle, the lower end of the feeding nozzle is immersed in the casting reservoir 30. Since the inside of the casting roll 22 is water-cooled, the shell is solidified on the surface of the moving roll passing through the casting pool, and the casting strip 12 is formed by joining the roll gap 27 between the rolls. It is sent downward.

  The twin roll caster may be of the type disclosed and described in some detail in US Pat. Nos. 5,184,668 and 5,277,243 or US Pat. No. 5,488,988. Reference may be made to these patents for appropriate structural details of twin roll casters suitable for use in embodiments of the present invention.

  A twin roll caster can be used to produce a high strength thin cast strip product that overcomes the disadvantages of conventional lightweight steel products, and a high strength, lightweight steel strip product is produced. In the present invention, an element including niobium (Nb), vanadium (V), titanium (Ti), molybdenum (Mo), or a combination thereof is utilized.

Usually, elements such as titanium, niobium and vanadium are mentioned as microalloying elements of steel. Traditionally, these elements have been added at levels typically below 0.1% and in some cases as high as 0.2%. These elements (conventionally as carbonitride formers) can have a strong influence on the fine crystal structure and properties of steel in combination of quench hardenability , grain refinement effect and strength improvement effect. Since molybdenum is a se relatively weak carbonitride former, but typically have not been regarded as a microalloying elements, it can become effective in environments of the present invention, niobium and complex carbonitride as with vanadium Particles can be formed. As will be explained below, carbonitride formation is suppressed in hot rolled strips containing these elements.

Several properties are combined in a high strength thin cast strip product, and a high strength light weight cast strip product is achieved by microalloying with these elements. The strip thickness may be less than 3 mm, less than 2.5 mm, less than 2.0 mm, or in the range of 0.5 to 2.0 mm. The cast strip is made by hot rolling without the need for cold rolling to reduce the strip to the desired thickness. Therefore, the high strength thin cast strip product overlaps the desired lightweight hot rolled thickness range and the cold rolled thickness range. The strip can be cooled at a rate of 10 ° C./second or more and still forms a fine crystal structure , most typically bainite and acicular ferrite.

Advantages obtained by manufacturing such a high-strength thin cast strip are that the production of conventional microalloyed steel requires relatively high alloy costs, the inefficiency of microalloying, hot rolling and cold rolling. This is in contrast to the difficulties in recrystallization annealing because conventional continuous galvanizing and annealing lines cannot provide the required high annealing temperatures. In addition, the problem of relatively low ductility found in strips produced by cold rolling and annealing is overcome.

In conventionally produced microalloyed steels, elements such as niobium and vanadium cannot remain in the solid solution through solidification, hot rolling, winding and cooling, and are heated by niobium and vanadium diffused through the fine crystal structure. Carbonitride particles are formed at various stages of the interwinding manufacturing process. As used herein and in the appended claims, carbonitride particles include carbides, nitrides, carbonitrides , and combinations thereof. The micro-alloying steel, built conventionally, further reduces the austenite grain size during high temperature slab by the carbon particles and nitrogen particles are taken up by and after forming and growth during high temperature slab reduced hardenability of steel Was. In these conventional steels, the effects of particles in the high temperature slab had to be overcome by increasing the amount of microalloying elements, raising the cast slab and reheating to temperature, and lowering the carbon content.

In contrast to conventionally produced steel, the high strength thin cast steel strip product of the present invention is less than 0.25% carbon, 0.20-2.00% manganese, 0.05-0 by weight. 50% silicon, less than 0.06% aluminum, about 0.01 to about 0.20% titanium, about 0.01 to about 0.20% niobium, about 0.05 to about 0.0. and at least one element selected from the group consisting of 50% molybdenum and about 0.01 to about 0.20% vanadium is largely made has a fine crystal structure is bainite. The steel article may further be composed of oxide fine particles of silicon and iron distributed through a steel fine crystal structure having an average particle size of less than 50 nanometers. In the steel product, the microalloy may be distributed more uniformly through the fine crystal structure than the conventional slab cast product.

Alternatively, the high strength thin cast steel strip product is by weight less than 0.25% carbon, 0.20 to 2.0% manganese, 0.05 to 0.50% silicon, and 0.01 % Of aluminum and from about 0.01 to about 0.20% niobium, with the majority of the microcrystalline structure containing bainite and acicular ferrite , and soluble niobium may exceed 70%.

In another option, the wound steel product is less than 0.25% carbon, 0.20 to 2.0% manganese, 0.05 to 0.50% silicon by weight, Less than 0.01% aluminum and at least one element selected from the group consisting of about 0.01 to about 0.20% niobium, about 0.01 to about 0.20% vanadium and combinations thereof; And more than 70% soluble niobium and vanadium as selected after winding and cooling. The wound high strength thin cast steel strip product may have more than 70% soluble niobium and vanadium as selected, especially after hot rolling reduction and subsequent winding and before age hardening. . The fine crystal structure may be a mixture of bainite and acicular ferrite. Alternatively, the microcrystalline structure of the steel that has been hot rolled and later wound and cooled may be bainite and acicular ferrite with more than 80% niobium and / or vanadium remaining in the solid solution, or 90% An excess amount may remain in the solid solution.

  Alternatively or additionally, the steel article may have a total elongation of greater than 6% or greater than 10%. The steel article 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 sufficient ductility. FIG. 8 shows the relationship between yield strength and total elongation in a hot-rolled product.

  After hot rolling, the hot-rolled steel strip can be wound at a temperature of about 500-700 ° C. Thin cast steel strips can also be increased in tensile strength at temperatures of at least 550 ° C. by further age hardening the steel strip. Age hardening can occur at temperatures of 550-800 ° C, 625-750 ° C, or 675-750 ° C. Thus, conventional furnaces with continuous galvanizing lines or annealing lines can provide the age-hardening temperature necessary to cure microalloyed cast strip articles.

  For example, making a steel composition with 0.026% niobium, 0.04% carbon, 0.85% manganese and 0.25% silicon cast in a thin cast strip process. A steel composition was produced. A twin roll caster as shown in FIGS. 1 and 2 was used to cast the strip to a thickness of 1.7 mm and in-line hot rolled to a strip thickness in the range of 1.5 to 1.1 mm. The strip was wound at a winding temperature of 590-620 ° C (1094-1148 ° F).

  As shown in FIG. 3, the yield strength and tensile strength levels obtained with the cast strip of the present invention, and the yield strength and tensile strength levels obtainable with a basic cast strip steel composition that is not microalloyed, Were compared over a specific coiling temperature range. It can be seen that the niobium steel strip gained a yield strength of 420-440 MPa (about 61-64 ksi) and a tensile strength of about 510 MPa (about 74 ksi). Compared to the C-Mn-Si base steel composition in which the cast strip product of the present invention was treated at the same coiling temperature as the microalloyed steel, the niobium steel produced an essentially high strength level. The base steel strips that were compared had to be wound at very low temperatures to achieve a strength level comparable to cast niobium. The cast niobium steel product did not need to be wound at a low coiling temperature in order to obtain strength improvement characteristics by hot rolling. Furthermore, the yield strength and tensile strength levels of cast niobium steel were not significantly affected by the degree of in-line hot rolling at a reduction of at least 19-37% as shown in FIG.

The hardenability of the steel of the present invention shown in FIG. As shown in FIG. 9, a niobium level as low as 0.007% is effective in increasing the strength of the final strip, yield strength levels exceeding 380 MPa were obtained at niobium levels above about 0.01%. . It should be noted that niobium levels below about 0.005% can be considered residual. Therefore, the addition of a very small amount of microalloying elements can be effective for improving the essential strength.

High strength was obtained using the microalloying addition of niobium, and the quench hardenability of the steel increased through suppressing the formation of proeutectoid ferrite. FIG. 4b shows that in the base steel, pro-eutectoid ferrite is formed along the preceding austenite grain boundary (other-type ferrite), which is not present in the niobium steel shown in FIG. 4a. Because the quenching and hardening action of niobium suppresses ferrite conversion, stronger bainite and acicular ferrite fine crystal structures can be created using conventional cooling rates and higher coiling temperatures in cooling. The final fine crystal structure of the niobium steel of the present invention mostly contain a combination of bainite and acicular ferrite. The base steel shown in FIG. 4b was cooled to a relatively low coiling temperature of less than 500 ° C., which is a cooling condition known to suppress ferrite formation at the austenite grain boundaries.

  The effect of hot rolling on yield strength is reduced in the niobium steel of the present invention. In conventional C-Mn products, strength typically decreases with increasing hot rolling. In contrast, as shown in FIG. 7, the effect of hot reduction on the yield strength is sufficiently small in the steel product of the present invention. In this experiment, the coiling temperature was kept constant and the hot rolling reduction of at least 40% with a strip thickness of 1.0-1.5 mm was covered. Unlike basic steels that are not microalloyed, the strength levels of the niobium microalloyed steels of the present invention in hot-rolled cast strips are relatively high under hot rolling pressures of at least 40%. In addition, these high strength levels were achieved using conventional winding temperatures of 550-650 ° C. as shown in FIG.

To further investigate this effect, the austenite grain size was measured by changing the thickness of 0.026 % niobium steel. The base steel showed a tendency to complete recrystallization under hot pressure of about 25% or more, whereas the 0.026 % niobium steel showed only a part of recrystallization even under 40% reduction. It can be seen from this that niobium in the solid solution suppresses the static recrystallization of deformed austenite after hot rolling, thereby reducing the influence of hot reduction on strength properties. FIG. 10 shows this, and it can be seen that there is no recrystallization into fine particles even when austenite grains are elongated by hot rolling. Microparticles are reducing the steel hardenability by increasing the austenite grain boundary area. However, even if recrystallization to austenite fine grain size is suppressed, it is known that the ferrite deformation start temperature increases due to such high hot rolling pressure. Furthermore, the high hot rolling pressure can cause a local strain region , usually called a shear band, in the austenite grains, which can act as an intraparticle nucleation site for ferrite nucleation. The steel of the present invention, the ferrite formation in the deformed austenite grains hardenability effect of the niobium becomes sufficiently is suppressed, exhibits very affected hardly intensity level to the degree of hot rolling.

Thin cast strip niobium steel articles have consistent yield strength and tensile strength levels throughout the range of hot rolling applied and can provide a yield strength of at least 410 MPa at 20-40% reduction. Conventionally, the austenite grain size is determined by the strip thickness. By measuring the austenite grain size, only a very limited recrystallization occurs under high hot rolling pressure, and the comparative basic steel strip has almost complete microcrystalline structure under hot rolling pressure of over 25%. It was found to be recrystallized. Recrystallization coarse as-cast austenite grain size during the hot rolling process is suppressed by the addition of niobium to the cast steel strip, the niobium is held in hardenability of the steel is retained after hot rolling in solution Became.

The high strength of the steel strip of the present invention after hot rolling was mostly attributed to the fine crystal structure formed. As shown in FIG. 4a, the fine crystal structure of the cast niobium steel was composed mostly of bainite, if not most, at any strip thickness. In contrast, as shown in FIG. 4b, the comparative non-alloyed steel achieves similar strength by coiling at low coiling temperatures, with some containing intergranular ferrite, mostly acicular ferrite . It had a fine crystal structure. The addition of niobium to the steel strip increased the hardenability of the steel even at fairly high coiling temperatures, suppressed the formation of grain boundary ferrite, and promoted the bainite fine crystal structure.

The results of the yield strength and tensile strength of the hot-rolled test steel shown in Table 2 below are summarized in FIG. The strength level increases as the niobium content increases, with the yield strength at a level of at least 340 MPa and a maximum of about 500 MPa in the hot rolled state. The tensile strength can be at least 410 MPa. The initial strength surge is attributed to the suppression of proeutectoid ferrite formation and the promotion of bainite and acicular ferrite, but later strength enhancement is due to continued fine crystal structure refinement and possibly the retention of niobium in solid solution. It can be attributed to solid solution hardening.

Further, a transmission electron microscope (TEM) test was conducted, and there was substantially no niobium precipitation in the hot-rolled cast strip. This is mainly attributed to the fact that niobium is held in solid solution and the resulting strength improvement has resulted in the formation of a majority and similarly dominant bainite microcrystalline structure with increased quenching and hardening properties of niobium. It shows that you are doing. It is believed that the quench hardenability of the cast steel strip is also enhanced by the retention of coarse austenite grains in the formation of the cast strip. Deformation to bainite, not ferrite, seems to be the main factor that suppresses the precipitation of niobium added with the microalloy from the strip when the wound thin cast strip is cooled from the coiling temperature.

A transmission electron microscope (TEM) test can be used to determine the size, identity and volume fraction of niobium carbonitride particles present in the steel. The absence of niobium carbonitride particles in the TEM test supported the view that the observed strength was largely attributed to the fine crystal structure being mostly bainite rather than ferrite. Thus, the subsequent increase in strength caused by age hardening heat treatment leads to the conclusion that niobium is essentially in solid solution in the hot rolled strip. After determining the volume fraction of carbonitride particles in the fine crystal structure using TEM analysis, the amount of the microalloy element in the solid solution can be determined.

The thin foil or carbon replica can be evaluated by knowing the amount of the carbonitride particles of the present invention by TEM. In our analysis, a JEOL 2010 transmission electron microscope was used. However, from our experience with this device, niobium particles below 4 nanometers are not soluble in ferrites with high dislocation density .

  A foil is prepared for thin foil analysis, and the foil is cut and polished to a thickness of 0.1 mm. The sample is then thinned for electron transmission by electropolishing using a 5% perchloric acid, 95% acetic acid electrolyte in a Tenupole-2 electropolishing apparatus. The sample can then be transferred directly to the TEM.

The desired sample for the carbon replica is obtained by etching the polished sample with Nital (Nital: a solution of alcohol and nitric acid), coating the sample with carbon after etching, and then applying the carbon coating to an appropriate size for TEM analysis ( For example, it can be prepared by giving a notch of 2 mm square. After scoring, the carbon replica can be released from the sample by dissolving the ferrite matrix in 3% nital. Carbon replica samples are collected on a 3 mm diameter support grid and then washed repeatedly with an ethanol / water solution. The carbon extraction replica can then be transferred to the TEM along with the support grid.

An additional factor believed to be the absence of niobium carbonitride particles in the hot-rolled cast strip relates to the nature of the niobium dispersion that rapidly solidifies the strip during strip formation in the cast strip continuous manufacturing process described above. Conventionally produced microalloyed high-strength strips involve relatively long time intervals in slab cooling solidification, slab reheating and thermomechanical processing, and carbonitride (CN) particles such as niobium, vanadium, titanium or molybdenum It gives the opportunity for pre-clustering and solid state precipitation, thus enabling kinetics of post-precipitation through various stages of the manufacturing process. In the method described above, if the cast strip is formed continuously from the casting pool between the casting rolls, very rapid solidification (about 160 microseconds) in the formation of the cast strip results in pre-clustering and solid precipitation of carbonitride particles. It is considered that the reaction rate mechanism of microalloy precipitation in post-treatment including rolling and winding operations is delayed or reduced. This means that the niobium, vanadium, titanium and molybdenum microalloys are relatively more evenly distributed in the austenite and ferrite phases than in the case of thin steel strips made by conventional slab casting and processing. That is.

By performing atom probe analysis on the niobium cast strip formed from the casting pool between the casting rolls as described above, the microalloy is relatively uniform in both an as-cast strip and a hot-rolled strip when wound at about 650 ° C. or less. It has been demonstrated that it is distributed (indicating reduced pre-clusters and solid precipitation). By relatively uniformly distributing the elements in this way, it is possible to perform winding operations under conditions where such elements are consistently and considerably precipitated in the case of microalloyed slab cast steel that has been manufactured and processed in the past. It is considered that the formation of carbonitride is hindered. Microalloyed cast strips produced by twin-roll casting also reduce the rate of carbonitride formation kinetics during thermomechanical processing, such as subsequent annealing, by reducing or lacking carbonitride preclusters and solids formation . For this reason, age-hardening opportunities are given at temperatures higher than those at which the strength of the particles is lost due to the roughening (Ostwald growth ) mechanism in the case of conventional strips.

It has been found that high tensile strength can be achieved with age hardening heat treatment. For example, with 0.026% niobium addition, an increase of at least 35 MPa (about 5 ksi) was observed with a yield strength of 410-450 MPa (about 60-65 ksi). Addition of 0.05% niobium is expected to increase at least 10 ksi by age hardening, and addition of 0.1% niobium is expected to increase by at least 20 ksi by age hardening. The fine crystal structure of the age-hardened steel product of the present invention may have niobium carbonitride particles having an average particle size of 10 nanometers or less. The microcrystalline structure of age hardened steel articles can essentially have no niobium carbonitride particles greater than 50 nanometers.

Laboratory aging heat treatments were performed on 0.026% niobium steel samples at various temperatures and times to elicit the action of niobium that is believed to be retained in solid solution in the hot rolled strip. As shown in FIG. 5, the aging heat treatment resulted in a significant strength improvement, yield strength of about 480 MPa (about 70 ksi). This confirms that niobium is retained in solid solution and can provide age hardening with subsequent aging, for example, by using an annealing furnace on a continuous galvanizing line, or by using a continuous annealing line. Therefore, a short time age hardening was performed to simulate the aging expected in the treatment of niobium microalloyed cast steel products in an annealing furnace attached to a continuous galvanizing line or by a conventional continuous annealing line. In the latter case, the age-hardened high-strength strip can be later galvanized or painted, or it can be used uncoated.

As a result, as shown in FIG. 6, a significant strength improvement is realized at a peak processing temperature of 700 ° C. (1292 ° F.), and the strength level approaches that achieved at longer and lower temperatures. Is clearly shown. Table 1 shows the tensile properties of the niobium thin cast steel product after a short time aging treatment using a peak temperature of 700 ° C. (1292 ° F.). In addition to the high strength of the cast strip product, the ductility and formability of the structural product are satisfactory. The manufactured cast strip product is a thin and high-strength strip product for architectural use using niobium microalloying. With higher microalloying levels, it may also be contemplated to achieve higher yield strengths, possibly well above 550 MPa (about 80 ksi).

Recently, in addition to the production of 0.026 wt% niobium steel, 0.014 wt% and 0.065 wt% niobium added steel have been successfully produced by this method. The hot steel components are as shown in Table 2 below.

The yield strength obtained with steel C and steel F is shown in FIG. 12, and the yield strength of 0.014% niobium thermal steel A made with a low manganese content is shown in FIG. Compared with the basic steel components , the yield strength increased at any winding temperature by adding niobium. With 0.014% niobium addition and 0.026% niobium addition, the yield strength increase was about 70-100 MPa (10-15 ksi), and with 0.065% niobium addition, it was about 140-175 MPa (20-25 ksi). . From FIG. 12, 0.026% niobium steel achieves higher yield strength than 0.8 % manganese base steel at the same winding temperature and is comparable to 0.8 % manganese base steel wound at lower temperatures. It can be seen that the yield strength is achieved. Alternatively, the strength obtained at a low coiling temperature (about 500 ° C.) in 0.8 % manganese basic steel can be achieved at a high coiling temperature (about 600 ° C.) by adding this niobium.

In addition, in contrast to conventionally produced microalloyed steels, we have shown that the addition of microalloys suppresses the formation of carbonitride particles in steel that is hot rolled and subsequently wound and cooled. I found it. Instead, the fine crystal structure of steel that is wound and cooled after hot rolling contains more than 70% niobium and / or vanadium containing bainite and acicular ferrite that remain in the solid solution. Alternatively, the fine crystal structure of steel that is hot rolled and later wound and cooled may include bainite and acicular ferrite in which more than 80% or more than 90% of niobium and / or vanadium remains in the solid solution.

Thus, niobium cast strips have been shown to be lightweight high strength steel products. The addition of niobium can first suppress austenite recrystallization during hot rolling , and also enhances the hardenability of the steel by maintaining a relatively coarse as-cast austenite grain size. It hardenability of the steel is increased directly with niobium in solid solution is held in the austenite after hot rolling, it is mostly the final composed bainite in austenite relatively high coiling temperature It helps to deform into a fine crystal structure . The formation of the bainite fine crystal structure promoted the addition of niobium in the solid solution in the hot-rolled strip.

  Further improvement in properties can be obtained by age hardening the steel of the present invention. In conventional microalloyed steel and non-microalloyed steel, strength improvement was obtained by age hardening, but in such conventional steel, elongation is reduced due to strength improvement. We have found that both the elongation improvement and the strength improvement can be obtained by age hardening the steel of the present invention.

Microalloying elements niobium and vanadium in solid solution by being retained, that a significant hardenable by age hardening cycle later is provided has been found by prior treatment state. Such an age hardening cycle can be made using a suitable continuous galvanizing line or continuous annealing facility. Therefore, microalloyed steel strips produced using a thin strip casting process are combined with an age hardening heat treatment provided by a suitable galvanizing line or annealing line to provide a unique for this type of steel product. It is a unique manufacturing method that provides a means to improve strength.

Isothermal aging treatment of hot-rolled 0.026% niobium cast strip material was carried out at 600 ° C. and 650 ° C. (1110 ° F. and 1200 ° F.) for 20 minutes to produce niobium carbonitride or niobium (C, N). Caused formation, which was confirmed by TEM testing. This improved the yield strength of the material as shown in FIG. Also, as shown in FIGS. 6 and 14, the thermal cycle of the strip due to the annealed portion of the galvanizing line also causes a significant strength improvement that is close to that achieved by isothermal aging at low temperatures.

By hardenable by microalloying additive is improved by the suppression of ferrite deformation, the bainite / acicular ferrite temperature range austenite decomposition temperature is reduced significantly. This lower deformation start temperature offers the possibility of maintaining most of the microalloy addition in the solid solution by applying a conventional run-out table cooling rate and an appropriate coiling temperature.

  By the presence of microalloying elements such as niobium and vanadium in the solid solution, age hardening is obtained by subsequent heat treatment and the strength is improved. Laboratory age hardening studies have demonstrated that significant strength improvements can be achieved even with relatively short heat treatment cycles such as those obtained with continuous annealing and galvanizing lines. Results of laboratory simulated continuous annealing cycles applied to test steel C (0.026% niobium), steel F (0.065% niobium) and steel G (0.084% niobium) are shown in FIGS. Show.

  The results of full scale plant tests on Steel B and Steel F using heat treatment conditions established from laboratory studies are shown in FIGS. 20 and 21, respectively. Significant strength improvements were achieved with Steel B and Steel F. A yield strength level exceeding 450 MPa was recorded for 0.024% niobium steel (steel B), and a yield strength exceeding 550 MPa was recorded for 0.065% niobium steel (steel F). The strength improvement by age hardening reached about 70 MPa (10 ksi) for 0.024% niobium steel (steel B) and about 100 MPa (15 ksi) for 0.065% niobium steel (steel F). It is believed that 0.065% niobium steel can achieve a yield strength in excess of 600 MPa when age hardened.

  Steel F samples were age hardened using age hardening conditions found in galvanizing lines. As shown in Table 3, the age-hardened steel had a strength of approximately 70 MPa, and the elongation increased from 11.47% to 14.16%. FIG. 19 shows the relationship between the yield strength (in the longitudinal test direction) and the total elongation of the niobium steel disclosed in the present invention in the hot-rolled state and the age-hardened and galvanized state.

  As shown in FIG. 16, we have found that a 10 second hold cycle can be used at about 675-725 ° C. to prevent overaging. However, since the temperature range is a function of the retention time, increasing the retention time to 20 seconds slightly decreased the temperature range as shown in FIG. 17, while the zero retention time slightly increased the temperature range. Depending on the total heat treatment cycle time, i.e. for all heating rates, holding times and cooling rates, the age hardening temperature can be in the range of about 625-800C.

  In the case of long-time heat treatment, a low temperature of 500 to 650 ° C. can be used. It can be seen from FIG. 6 that a heat treatment at 600 ° C. for 20 minutes produces the same strength level as 10 seconds in a 700 ° C. continuous annealing cycle. FIG. 22 shows the results of laboratory heat treatments performed for 20 minutes and 120 minutes. As the results show, intrinsic hardening was achieved by heat treatment at 550 ° C. for 120 minutes, but aging for 120 minutes at temperatures above about 650 ° C. reduced the hardness of the steel. A long heat treatment time can be used in the temperature range of 500 to 650 ° C. in all coil annealing processes such as batch annealing, and cooling is controlled to a temperature range of 500 to 650 ° C. in other cooling methods after winding the hot-rolled coil. By doing so, the retained niobium is deposited.

A transmission electron microscope (TEM) was used for samples of Steel C and Steel F that were heat-treated at 650 ° C. for 60 minutes, and fine particles having a diameter of 4 to 15 nanometers were found. These fine particles were found to contain niobium carbonitride , which indicates that the strength enhancement can be attributed to age hardening by the niobium carbonitride fine particles.

The fine crystal structure of age hardened and microalloyed steel articles can have niobium carbonitride particles with an average particle size of 10 nanometers or less. The fine crystal structure of age hardened steel products can essentially have no niobium carbonitride particles exceeding 50 nanometers. As a result of inspecting the niobium steel sample of the present invention using TEM evaluation, the portion of the fine crystal structure did not have a measurable amount of niobium carbonitride particles.

The improvement in strength / elongation relationship in the age-hardened steels of the present invention appears to be attributed to fine crystallographic parts, ie “precipitated zones” and nanoclusters, essentially free of particles above 5 nanometers. . The development of a precipitation-free zone near the grain boundary can affect the strength and tensile elongation relationship by reducing the hardness of the region near the grain boundary. It has been reported that relaxation of stress concentration in the precipitation-free zone increases strength and elongation. When the precipitation-free zone is narrow and the size of the grain boundary precipitates is small, the beneficial effect of the precipitation-free zone on elongation and strength can appear.

  In the steels of the present invention, element addition can result in increased elongation with increased strength after age hardening by producing smaller precipitation-free band widths and smaller hardness changes than conventionally produced niobium steel. Since the elements are relatively uniformly dispersed in the rapidly solidified steel, the time-temperature frame in which the formation of nanoclusters can be stably controlled can be effectively expanded by delaying the reaction rate mechanism of age hardening. Elemental nanoclusters can provide strength enhancement in the early stages of age hardening. The increase in cluster strength can be due to the extra energy required for dislocations that cut the cluster diffusion boundary of solute species. The clusters can provide substantial strength enhancement without reducing ductility because their elastic soft boundaries do not strictly control dislocation motion, ie normal second phase particles do This is because such a pile-up is not caused.

In the steel of the present invention, the elements remain relatively uniformly dispersed in the solid solution during rapid solidification of the steel. In contrast to the conventionally produced niobium and vanadium steels, the microcrystalline structure of the hot rolled, later wound and cooled steel contains bainite and acicular ferrite , with over 70% niobium and / or vanadium addition Remain in the solid solution and essentially no niobium carbonitride particles greater than 50 nanometers. Alternatively, the microcrystalline structure of the steel that has been hot rolled and later wound and cooled can include bainite and acicular ferrite, with more than 80% niobium and / or vanadium addition remaining in the solid solution, Alternatively, it can remain in the solid solution in excess of 90%.

If the coiling temperature is below about 650 ° C., the element remains dissolved and caught in the hot rolled coil and does not precipitate. The formation is effectively delayed because the process prevents the prior chaining of atoms (such as in the form of particles) that normally occurs during reheating for conventional slab casting and hot strip rolling. The strength enhancement observed to occur in hot rolled coils can therefore be largely attributed to quench hardening and solid solution hardening effects.

The formation of carbonitride particles can be initiated during heat treatment. In addition, since a significant amount of niobium and / or vanadium is present in the solid solution prior to age hardening, the predeposited clusters and particulates are stable over a considerable time and temperature range during age hardening. As a normal precipitation phenomenon, the non-precipitation zone formed in the vicinity of the grain boundary is narrower, and includes nanoclusters and fine precipitates that are relatively uniformly dispersed as compared with conventionally produced steel. Therefore, the hardness change of the precipitation-free zone with respect to the grain interior is relatively small in the steel of the present invention. It is considered that the narrow precipitation-free zone and the small hardness change in the precipitation-free zone reduce the stress concentration in the precipitation-free zone and reduce microcracks from preferential deformation in the precipitation-free zone. The cluster strength improvement is characterized by strength improvement without ductile deterioration because dislocation pileup does not occur in the cluster. It seems that the precipitation-free zone of the steel of the present invention is a combination of the narrow precipitation-free zone and the cluster strength improvement mechanism. This results in improved elongation because cracking is relatively difficult to initiate and is not constrained by grain boundary free precipitation zones. In addition, the nanoclusters can coexist with distinct particles over a specific annealing temperature / time combination in the interior region of the grains.

  An annealing furnace that is not currently used as a strength improvement measure for age hardening can be used to perform such treatment. The annealing conditions can be a continuous annealing cycle having a peak temperature of at least 650 ° C. below 800 ° C., more preferably 675-750 ° C. Alternatively, strength enhancement can be achieved in a manufacturing environment that uses a very short age hardening cycle obtained in a conventional annealing furnace incorporated in a continuous galvanizing line. The final strength levels recorded in full scale plant tests were similar to those obtained with laboratory heat treatment of each steel.

  Similar results were obtained from about 0.01 to about 0.20% niobium and from about 0.01 to about 0.20% titanium, from about 0.05 to about 0.50% molybdenum and from about 0.01 to about 0. Expected at 20% vanadium.

The composition of the steel of the present invention using vanadium is shown in Table 2 as steel H. The yield strength of steel H is shown in FIG. Vanadium steel was made at two different coiling temperatures and later aged at 650 ° C. and 700 ° C. for 20 minutes to cause strength enhancement by vanadium in the solid solution. The results show that a significant improvement in strength can be obtained under these heat treatment conditions. The increase in strength was slightly more for materials made at higher coiling temperatures, which may be due to the effects of processes that resist precipitation hardening and microcrystalline structural softening. The strength improvement achieved with materials made at low coiling temperatures was comparable to that achieved with 0.026% niobium steel.

  The yield strength of steel H in the hot-rolled and galvanized state is shown in FIG. FIGS. 23 and 24 show that vanadium steel achieved higher strength levels than ordinary carbon base steel despite being made using high coiling temperatures. In the sample shown in FIG. 24, the winding temperature of steel H was 570 ° C., and the basic steel winding temperature was below 500 ° C.

  As shown in FIG. 24, the strength improvement was realized with vanadium steel by age hardening using an annealing furnace in a continuous galvanizing line, but the strength improvement was smaller than in the case of an equivalent niobium content. The yield strength of the sample of FIG. 24 at the galvanizing line is about 450 MPa in the galvanized state, which is the extent obtained by the long-term laboratory heat treatment shown in FIG. The strength of vanadium steel can be more independent of coiling temperature than niobium steel.

This thin cast strip makes it possible to produce a new type of steel product as follows.
1. A galvanized strip that is age-hardened during the galvanization process, utilizing a high-strength, lightweight, fine-grained bainite-based component. Annealed portions of the galvanizing line can be used to cause age hardening of niobium and / or vanadium in hot rolled thin cast strips.
2. Uncoated strip by utilizing a microcrystalline structure that is high-strength and lightweight, mostly bainite and age-hardened by processing in a continuous annealing line. Conventional continuous annealing high temperature furnaces can be used to cause activation of niobium and vanadium elements retained in solid solution by bainite microcrystalline structure after hot rolling of thin cast strips.
3. Hot-rolled cast strip product that is high-strength and lightweight, and the strength level is independent of the degree of hot rolling reduction applied. The bainite microcrystalline structure produces a relatively high strength product (yield strength of 380 MPa or more (˜55 ksi)). By suppressing austenite recrystallization during hot rolling or after hot rolling, a final strength level can be provided regardless of the degree of hot rolling reduction. The final strength level is consistent regardless of the thickness that can be produced in the thin cast strip process.

  While the invention has been shown and described in detail in the drawings and description above, it is illustrative and not of a limiting nature, and merely exemplary embodiments of the invention have been presented and disclosed, It should be understood that protection of all changes and modifications within the scope of the present invention as set forth in the following claims is desired. Additional features of the invention will be apparent to those skilled in the art from consideration of the specification. Changes and modifications can be made without departing from the spirit of the invention.

Claims (44)

By weight, and less than 0.25% carbon, 0.20 to 2.0% manganese, and from .05 to 0.50% of silicon, and less than 0.01% aluminum, 0. 01-0 . A thin cast steel strip comprising 20% niobium, the majority of the fine crystal structure being composed of bainite and acicular ferrite, with more than 70% niobium in solid solution.   The thin cast steel strip of claim 1 wherein niobium is less than 0.1%. 0 . 05% to 0 . 50% molybdenum , 0 . 01% to 0 . The thin cast steel strip of claim 1 further comprising at least one element selected from the group consisting of 20% vanadium and mixtures thereof.   The thin cast steel strip according to claim 1, wherein the yield strength is at least 340 MPa.   The thin cast steel strip of claim 1 having a tensile strength of at least 410 MPa.   The thin cast steel strip according to claim 1, wherein the yield strength is at least 410 MPa.   The thin cast steel strip of claim 1 having a tensile strength of at least 485 MPa.   The thin cast steel strip according to claim 1, wherein the yield strength is at least 450 MPa.   The thin cast steel strip of claim 1 having a tensile strength of at least 520 MPa.   The thin cast steel strip of claim 1 having a total elongation of at least 6%.   The thin cast steel strip of claim 1 having a total elongation of at least 10%. By weight, less than 0.25% carbon, 0.20 to 2.0% manganese, 0.05 to 0.50% of silicon, aluminum及Beauty 0 less than 0.01%. 01-0 . A thin cast steel strip having a thickness of less than 3 millimeters, containing 20% a niobium, the majority of the microcrystalline structure comprising bainite and acicular ferrite, and having more than 70% niobium in solid solution.   The thin cast steel strip of claim 12 wherein niobium is less than 0.1%. In addition , 0 . 05-0. 50% molybdenum , 0 . 01-0 . 13. The thin cast steel strip of claim 12, comprising at least one element selected from the group consisting of 20% vanadium and mixtures thereof.   13. The thin cast steel strip of claim 12, further comprising silicon and iron oxide particulates distributed through a steel fine crystal structure having an average particle size of less than 50 nanometers.   13. A thin cast steel strip according to claim 12, wherein the yield strength is at least 340 MPa.   13. A thin cast steel strip according to claim 12, wherein the tensile strength is at least 410 MPa.   13. A thin cast steel strip according to claim 12, wherein the yield strength is at least 410 MPa.   The thin cast steel strip of claim 12 having a tensile strength of at least 485 MPa.   13. The thin cast steel strip according to claim 12, wherein the yield strength is at least 450 MPa.   13. A thin cast steel strip according to claim 12, wherein the tensile strength is at least 520 MPa.   13. A thin cast steel strip according to claim 12, wherein the thickness is less than 2.5 mm.   13. A thin cast steel strip according to claim 12, wherein the thickness is less than 2.0 mm. The thickness is 0 . 13. A thin cast steel strip according to claim 12, which is in the range of 5 to 2 mm.   13. A thin cast steel strip according to claim 12, wherein the total elongation is at least 6%.   13. A thin cast steel strip according to claim 12, wherein the total elongation is at least 10%. By weight, and less than 0.25% carbon, 0.20 to 2.0% manganese, and from .05 to 0.50% of silicon, and less than 0.01% aluminum, 0. 01-0 . 20% niobium, most of the microcrystalline structure contains bainite and acicular ferrite, and more than 70% niobium in solid solution, providing yield strength of at least 410MP under 20-40% pressure A hot rolled thin cast steel strip with a thickness of less than 3 millimeters.   28. The hot rolled thin cast steel strip of claim 27, wherein niobium is less than 0.1%. In addition , 0 . 05-0. 50% molybdenum , 0 . 01-0 . 28. The hot rolled thin cast steel strip of claim 27, comprising at least one element selected from the group consisting of 20% vanadium and mixtures thereof.   28. The hot-rolled thin cast steel strip of claim 27, further comprising silicon and iron oxide particulates distributed through a steel fine crystal structure having an average particle size of less than 50 nanometers.   28. The hot rolled thin cast steel strip of claim 27, wherein the yield strength of the thin cast steel strip is at least 340 MPa.   28. A hot-rolled thin cast steel strip according to claim 27, having a tensile strength of at least 410 MPa.   28. A hot rolled thin cast steel strip according to claim 27 having a thickness of less than 2.5 mm.   28. A hot rolled thin cast steel strip according to claim 27 having a thickness of less than 2.0 mm. The thickness is 0 . 28. Hot-rolled thin cast steel strip according to claim 27, in the range of 5 to 2 mm.   28. A hot rolled thin cast steel strip according to claim 27, wherein the total elongation is at least 6%.   28. A hot rolled thin cast steel strip according to claim 27, wherein the total elongation is at least 10%. Assemble a roll casting machine that has a laterally positioned casting roll that forms a roll gap between them and is internally cooled, and is supported on the casting roll above the roll gap by side dams adjacent to both ends of the casting roll. A molten steel casting pool is formed, and the casting rolls are rotated in the mutual direction to solidify the metal shell on the casting roll while passing through the casting pool, and downward from the roll gap between the metal shell and the casting roll. The steel strip is cooled at a rate of at least 10 ° C./second, and by weight is less than 0.25% carbon, 0.20-2.0% manganese, 0 0.05 to 0.50% silicon, less than 0.01% aluminum, and 0 . 01-0 . A process for producing a wound thin cast steel strip comprising 2% niobium and providing a composition in which the fine crystal structure is predominantly composed of bainite and acicular ferrite and more than 70% niobium is in solid solution .   40. The wound thin cast steel strip of claim 38, wherein the wound steel strip has silicon and iron oxide particulates distributed through a steel microcrystalline structure and having an average particle size of less than 50 nanometers. Manufacturing method. 39. The method of producing a wound thin cast steel strip according to claim 38, further comprising hot rolling the steel strip and winding the hot rolled steel strip at 450-700C.   40. The method of producing a wound thin cast steel strip according to claim 38, further comprising the steps of hot rolling the steel strip and winding the hot rolled steel strip at a temperature below 600C.   40. The method of making a wound thin cast steel strip according to claim 38, further comprising age hardening the steel strip to increase the tensile strength at least at 550C.   40. The method of making a wound thin cast steel strip according to claim 38, further comprising the step of age hardening can occur at a temperature of 625-800C.   40. The method of producing a wound thin cast steel strip according to claim 38, further comprising the step of age hardening can occur at a temperature of 650-750C.