Microtreatment of Iron-Based Alloy, Apparatus and Method Therefor, and Microstructure Resulting Therefrom

Abstract

This invention relates to a multi-phase transformation of an iron and carbon-containing alloy. While the phenomena for this cooling transformation are not fully understood, multiple theories are present. The first theory is that since the alloy is rapidly heated and carbon leveling has not occurred, carbon enriched areas transform to a first phase, perhaps martensite, while lesser carbon areas may transform to a second phase, perhaps bainite. Thus a dual phase alloy is produced.

Description

TECHNICAL FIELD

These inventions relate to treated iron-based alloys, and more particularly relate to processes and apparatuses for transforming and/or shaping the same and the various materials resulting therefrom obtained by treating low, medium, and high carbon steel and other iron-based alloys to a complex steel microstructure which may include portions of bainite, coalesced bainite, acicular ferrite, retained austenite and/or martensite and combinations thereof by micro-treating said iron based alloy.

BACKGROUND OF THE INVENTION

Traditionally, metallurgists have wanted to take low quality metals, such as low carbon steel, and turn them into high quality steels and more desirable products through inexpensive treatments, including annealing, quenching, and tempering to name a few. Previous attempts have met with limited success in that they did not always produce a desirable product. Other attempts have failed on a large scale due to high processing costs or the need to ultimately incorporate expensive alloying.

Processing of high strength steel generally takes intense capital equipment, high expenditures, expensive and dangerous heated fluids, such as quenching oils and quenching salts, and tempering/annealing processes which include the use of ovens, heated equipment, and residual heat from pouring molten steel. These quenching procedures are intended to raise the hardness of the steel to a desirable value. Bainite and martensite can be made by these processes and are very desirable materials for certain high strength applications as they generally have Rockwell C hardness of from about 30 and up. The increased hardness correlates to a comparable increase in tensile strength.

Typical Advanced High Strength Steels have generally included bainitic and/or martensitic phases. Bainite is generally acicular steel structured of a combination of ferrite and carbide that exhibits considerable toughness while combining high strength with high ductility. Usually formed by austempering, bainite is a very desirable product. One practical advantage of bainitic steels is that relatively high strength levels can be obtained together with adequate ductility without further heat treatment, after the bainite reaction has taken place. Such steels, when made as a low carbon alloy, are readily weldable, and bainite will form in the heat-affected zone adjacent to the weld metal, thereby reducing the incidence of cracking. Furthermore, these steels having a lower carbon content tend to improve the weldability and reduce stresses arising from transformation. When bainite is formed in medium and high carbon steels, weldability is reduced due to the higher carbon content.

The other conventional high strength steel, martensite, is another acicular steel made of a hard, supersaturated solid solution of carbon in a body-centered tetragonal lattice of iron. It is generally a metastable transitional structure formed during a phase transformation called a martensitic transformation or shear transformation in which larger workpieces of austenized steel may be quenched to a temperature within the martensite transformation range and held at that temperature to attain equalized temperature throughout before cooling to room temperature. Martensite in thinner sections is often quenched in water. Since chemical processes accelerate at higher temperatures, martensite is easily destroyed by the application of heat. In some alloys, this effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead. Since quenching can be difficult to control, most steels are quenched to produce an overabundance of martensite, and then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. Too much martensite leaves steel brittle, whereas too little martensite leaves it soft.

SUMMARY OF THE INVENTION

In accordance with the present invention, low grade ferrous alloys in strips, sheets, bars, plates, tubes, workpieces and the like are converted into multi-phase high strength steels with a minimum of cost, time and effort. Dual and multiple phase materials are achievable by practicing the present invention.

There are provided methods and apparatuses for extremely rapid micro-treating of low carbon iron-based alloys and articles made from and containing those alloys. The iron-based, or ferrous, alloys/articles start out having a first microstructure prior to the micro-treating, and are converted into a second microstructure by rapid heating and rapid cooling into high strength steels on at least a portion of the alloy/article.

A method for rapidly micro-treating an iron-based alloy is disclosed for forming at least one phase of a high strength alloy, where the method comprises the steps of providing an iron-based alloy having a first micro-structure with an austenite conversion temperature. This first microstructure is capable of being transformed to an iron-based alloy having a second micro-structure including the above mentioned phases by rapidly heating at an extremely high rate, such as 600° F./sec to 5000° F./sec. The traditional austenitic conversion temperatures are elevated for given alloys due to the short duration of the thermal cycle initiated by the rapid heating.

This heating step involves nearly immediate heating of the iron-based alloy to a selected temperature above its austenite conversion temperature. Then, the alloy is immediately quenched, also at an extremely fast rate, i.e. 600° F./sec to 10,000° F./sec on at least a portion of the iron-based alloy in a quenching unit adjacent the heating unit. This procedure forms at least one phase of a high strength alloy in a desired area, depending upon where the treatment was performed. Extremely rapid quenching will form at least one phase of a high strength alloy, as described more fully herein below.

Quenching may be accomplished nearly instantaneously by various methods and apparatuses, including water baths, water sprays, chilled forming dies, air knives, open air convection, final operation chilled progressive dies, final stage chilled line dies, chilled roll forming dies, and quenching hydroforms among others.

Rapid quenching closely following rapid austenization has been shown to develop a dual hardness microstructure as illustrated in the attached drawings, herein incorporated by reference. Experimentation has shown that flash processing of AISI 4130 yields multiple hardness peaks of approximately 525 and 625 Vickers hardness. The combination of hardnesses has been verified by single sensor differential thermal analysis showing that two temperature ranges have transformation occurring during the single quenching operation. In AISI 4130, transformation occurs from 650C to 550C and again from 470C to 360C during water cooling.

While the phenomena for this double cooling transformation is not fully understood, multiple theories are present. The first is that since the steel is rapidly heated and carbon leveling has not occurred that carbon enriched areas transform to martensite while lesser carbon areas transform to bainite.

Another possible theory is that the upper transformation temperature occurs when austenite transforms to nano-scale platelets. The second transformation occurrence during cooling is the coalescing of the platelets into larger plates. This leads us to another embodiment of this invention. Since a double transformation is occurring, one could allow the first transformation to occur but halt the second. For example, rapidly heat the iron based alloy, a few seconds later, quench the iron alloy in a medium that is below the first transformation finish temperature but above the second transformation start temperature. The material would complete the first transformation but never get to the second transformation.

This may cause for example, the first stage of Flash Bainite to form, for example, just the nano coalesced bainite phase, but then leave a significant amount of another phase, possibly retained austenite, or some other austenite daughter product. The material could then be brought down from the temperature between the first transformation end temperature (i.e. 550C) and the second transformation start temperature (470C).

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and advantages of the expected scope and various embodiments of the present invention, reference shall be made to the following detailed description, and when taken in conjunction with the accompanying drawings, in which like parts are given the same reference numerals, and wherein;

FIG. 1A is a FEGSEM micrograph of bainite processed in accordance with the present invention;

FIG. 1B is a FEGSEM micrograph of bainite processed in accordance with the present invention;

FIG. 2A is a graph of typical temperature measurements at the inside wall of the processed tube;

FIG. 2B is a cooling cycle time/temperature graph of the process in accordance with the present invention;

FIG. 3 is a transform analysis graph of temperature versus differential of temperature;

FIG. 4A is a mechanical heterogeneity analysis of a raw material;

FIG. 4B is a similar analysis of the flash processed material;

FIG. 5 is a graph of elongation versus temperature; and

FIG. 6 is a stress versus strain graph of various examples of material.

DETAILED DESCRIPTION OF THE DRAWINGS

It is a first aspect of the present invention to provide an inexpensive, quick and easy way to produce a low, medium, or high carbon iron-based alloy containing an appreciable percentage of nano-sized platelet bainite while having some of the desirable mechanical properties of nano-sized laths of martensite. While other thermo-mechanical processing techniques require lengthy thermal processing to obtain a complex bainitic-martensitic microstructure, flash bainite processing does so with a single, rapid quenching operation which takes less than 10 seconds from above the lower austenitic temperature to below the lower martensitic temperature.

It is a second aspect of the present invention to provide a method and apparatus for micro-treating low, medium, or high carbon iron-based alloys to contain a desirable quantity of flash bainite processed complex microstructural material with bainite and martensite interspersed within the same prior austenitic grains. The micro-treated low, medium, or high carbon iron-based alloy may have varying thicknesses for different applications and may be readily weldable while having high tensile strength, along with the ability to save material and reduce weight. One aspect of the present invention for the elevated interruption temperature is to use a quench medium at this temperature that could be molten salts, among others. This aspect causes the first iron based alloy transformation that is stopped by molten salt. All other iron alloy transformations are intentionally occurring in molten salt through either continuous cooling transformation or time temperature transformation. From this temp, 550-470° C., the steel could be cooled in a manner in which the remaining austenite is brought down to ambient temperature with either no further transformation occurring or transformation to some other desirable austenite daughter phases.

Another aspect of this invention has to do with the heating and quenching apparatus. Other previously filed patent applications for apparatus employ single or multiple heating and quenching heads to cool the material. The present method employs a single heating unit to heat multiple pieces of material. For example, a rectangular induction coil could have material passing by it and heated both inside and outside of the coil. If the coil were appropriately sized, a rectangular tube could be heated inside the coil while other pieces, such as pieces of bar stock could be heated on the outside of the coil simultaneously.

Another aspect of this invention has to do with heating interrupted pieces of material. For example, a strip could have multiple cutouts removed from its shape. These pieces could be manufacture in the soft state and then Flash Bainite Processed in their final hardness state. Sometimes, when such a strip is heated, the edges near the cutouts will concentrate heat and melt the corners. The present aspect of this invention will allow plugs of similar material to be held in place of the interruptions to absorb the heat. This will thus prevent the melted corners.

The concept of rapid heating, quenching, reheating, and quenching was discussed in my previous patent applications filed on Oct. 2, 2006, which is incorporated herein by reference, referring to an iron based alloy component. The method could be applied as well to a rolling strip of metal. A similar thermal technique known as quench and partitioning has been used. Quench and partitioning technology austenizes steel over many minutes, quenches to below the martensite start temperature, either holds temp or reheats below martensitic start temperature and then quenches to ambient. Another aspect of Quench and partitioning technology austenizes steel over many minutes, quenches to below the martensite start temperature, reheats above martensitic start temperature and bainite finish temperature and then quenches to ambient after a desired amount of transformation has occurred. The present innovation is a new technique of Quench/Partition technology. As before with an iron based alloy part, the heater will rapidly austenize the steel strip, quench the material to enact a transformation, hold or reheat with the second heater to a subaustenitic temperature to stabilize or transform the existing microstructure, and then quench to room temp with the second quench.

The resulting high strength steel may include at least one portion of the resulting high strength material made of coalesced bainite, bainite, martensite, ferrite, austenite, pearlite, and/or dual or complex phase combinations thereof, depending on the placement of the treatments described and claimed herein below.

Complex phase materials can be made, such as martensite and bainite located next to ferrite. These highly desired complex phase materials are achievable in the same workpiece by quenching only in various patterns so that a pattern of high strength steel can be manufactured in desired areas across the surface and/or cross section of an article after it has been heated. By only quenching certain areas, various material phases are possible in various locations where desired.

Looking first with combined reference to FIGS. 1A and 1B, there can be seen that the flash bainite includes a bi-modal distribution of platelets or plates which exhibit highly desirable combinations of strength, ductility and toughness. The flash processing of the present invention creates almost distortion free flat sheets, bars, plates and straight tubing. As can be seen in these figures, the microstructure produces a fine grain structure within the bi-modal distribution of microstructures which yields the surprising strength and ductility.

With reference to FIG. 2A, a graph is shown charting temperature in degrees C. versus time in seconds to illustrate the cooling cycle as processed at the inside wall of one of the test tubes. Typical temperature measurements of this inside wall are showing that there is a very low temperature-time history ratio. In this particular example, utilizing AIS 414130 sheet metal tubing has a lower temperature to time history ratio.

Looking now to FIG. 2B, there is shown a graph of temperature versus time showing the flash processing temperature to time history ratio in addition to the conventional continuous annealing temperature to time history. Clearly, the temperature to time history ratio for the continuous annealing is much greater than that ratio for the flash processing.

FIG. 3 illustrates an analysis of temperature in degrees centigrade versus the change in temperature also in degrees centigrade. This analysis shows transformations at between 550 and 649 degrees C. and 360 to 459 degrees C. This analysis suggests that we have two different transformation conditions leading to very localized microstructural heterogeneity, although experiencing homogeneity on a macro scale.

Looking now to FIGS. 4A and 4B collectively, there can be seen two mechanical heterogeneity analyses showing that there are two distinct regions of microstructure between the raw material and the flash processed material in accordance with the present invention. These findings are consistent with the previous analyses showing two separate transformations during the flash processing procedure. Both FIG. A and FIG. B are graphs of normalized frequencies versus hardness, which illustrates the distribution of hardness. FIG. 4A shows the base metal hardness distribution as very slight, while the material that has proceeded through flash processing illustrates both a high hardness zone as well as a low hardness zone over a broader distribution of hardness.

Looking now to FIG. 5, yet another aspect of the invention is illustrated with fully strengthened with AISI 1010 material that has been flash processed. This graph shows elongation versus peak flash temperatures, to show that the highest flash temperature, 1180 degrees C. having an elongation of 7.9%. At a peak flash temperature of 1010 degrees C., the elongation percentage is 5.6. Optimum elongation is found at larger grain sizes. The chemistry of this material in percent by weight is 0.10 C, 0.31 Mn, less than 0.01 Si, sulfur, phosphorus, and 99.41 iron.

Last, we look at FIG. 6, which is a graph of tensile strength in KSI versus tensile strain in percentage. With an example of AISI 1020 after heating to various temperatures in a range from 400 to 700 degrees C., 8 examples are shown with varying widths in inches. This experiment shows that even after 300 seconds of tempering at 500 degrees C., flash processed AISI 1020 will retain 79% of its “as quenched” tensile strength. Furthermore, the elongation does not drop to + or −5% with less than 5 seconds of tempering.

In summary, numerous benefits have been described which result from employing any or all of the concepts and the features of the various specific embodiments of the present invention, or those that are within the scope of the invention. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings with regards to the specific embodiments. The embodiment was chosen and described in order to best illustrate the principles of the invention and its practical applications to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims which are appended hereto.

Claims

1. A metal treatment process for transforming an iron-containing alloy into a high strength steel, comprising: rapidly heating the alloy to at least about 1650 degrees C., immediately quenching the alloy at a rate of from about 10,000 degrees C., whereby two temperature ranges have transformations occurring during the single quenching operation, first transformation occurs from 650C to 550C and second transformation occurring from 470C to 360C during cooling.