Method for manufacture of a metallic material having ultrafine grain structure

Abstract: A method for producing a metallic material having an ultrafine microstructure, the metallic material exhibiting a phase transformation of a low-temperature phase into a high-­ temperature phase is disclosed, the method comprising the steps of: preparing a metallic material which comprises at least a low-temperature phase; applying plastic deformation to the metallic material; and increasing the temperature of the metallic material to a point beyond a transformation point while applying the plastic deformation to effect reverse transformation of the low-­ temperature phase into a high-temperature phase.

Description: This invention relates to a method for manufacturing a metallic material having an ultra-fine microstructure according to the precharacterizing part of claim 1. The terms "high-temperature phase" and "low-temperature phase" are used to mean phases appearing at a temperature higher or lower, respectively, than a transformation temperature, and the term "metal" is used to include a variety of metals in which a low-temperature phase is transformed into a high-temperature phase, such as steel, Ti, Ti-base alloys, Zr, Zr-base alloys, Ni, and Ni-base alloys. In the case of steel, the high-temperature phase is austenite and the low-temperature phase is ferrite, or the high-temperature phase is δ -ferrite and the low-temperature phase is γ - austenite and in the case of titanium the former is β -phase and the latter is α -phase. For brevity, however, this invention will be described using steel and Ti-base alloys as examples, and the low-temperature phase is ferrite or α -phase and the high-temperature phase is austenite or β -phase. It is well known that refining the grain structure of a metal produces improvements in properties of the metal such as its low temperature toughness, ductility, yield strength, corrosion resistance, and superplasticity. Thus, many processes to prepare a fine metallic structure have been developed. However, prior art methods for refining the grain structure of a metal can attain a grain size of no smaller than 20 µm in diameter. An industrial manufacturing method to provide a grain structure having an average grain size of 10 µm or smaller in diameter, and generally 15 µm or smaller has not yet been developed. One industrial method for grain refining is the controlled rolling method. This is a method for preparing a fine grain structure for a hot-rolled steel material by controlling the hot rolling conditions, such as by lowering the finishing temperature to as low a level as possible. However, it is extremely difficult to obtain austenitic grains of the high-temperature phase which are 15 µm or smaller in diameter. Therefore, there is a limit to the grain size of a ferritic structure which is derived from the above-described austenitic grains, and it has been thought to be impossible from a practical viewpoint to obtain a uniform and ultra-fine ferritic grain structure comprising grains having an average diameter of 10 µm or smaller, especially 5 µm or smaller. The so-called accelerating cooling method has been developed for refining the grain size in a ferritic steel. In this method, the cooling rate is controlled after the completion of controlled rolling so as to increase the number of nuclei for the growth of ferritic crystal grains to further refine the crystal grains. However, according to this method, refinement of an austenitic structure before transformation occurs only during controlled rolling, and is not influenced by the subsequent cooling rate. Thus, there is still a limit to the grain size of an austenitic microstructure before transformation, and it is possible to obtain a uniform, ultra-fine grained austenitic structure. Since austenitic grains are rather large, the martensite derived therefrom does not have a fine-grained structure. Japanese Patent Publication No. 42021/1987 discloses a method of manufacturing hot rolled steel articles which comprises hot working a low-carbon steel with a high degree of deformation at a temperature higher than the transformation temperature to form a fine-grained ferritic structure so that recrystallization of austenitic grains can be prevented, and carrying out accelerated cooling to form bainite or martensite as well as to effect refinement of the thus-formed bainite or martensite. According to this method, a quenched structure which comprises ferritic grains having an average grain size of about 5 µm with the balance being bainite or martensite can be obtained. However, the resulting bainite or martensite has an average grain size of 20 - 30 µm. This is rather large. The Japanese journal "Iron and Steel" Vol.74 (1988) No. 6, pp. 1052-1057 discloses a method of manufacturing an ultra-fine austenitic grain structure by cold working an austenitic stainless steel (Fe-13/18wt%Cr-8/12wt%Ni) at room temperature to effect a strain-induced transformation of austenite into martensite, and annealing the resulting martensite by heating it at a temperature within a stable austenitic region to carry out reverse transformation of martensite into austenite, resulting in an ultra-fine austenitic grain structure. According to this method, a hot rolled stainless steel is subjected to cold rolling or a sub-zero treatment at a temperature lower than room temperature, and then is heated to a temperature in an austenitic region. This process corresponds to a conventional solution heat treatment of an austenitic steel. Such an ultra-fine microstructure can be obtained only for an austenitic high Cr-, high Ni stainless steel having a reverse transformation temperature of 500 - 600 °C . Therefore, as a general rule, it is impossible to obtain an austenitic microstructure having a grain size of 15 µm or smaller for a common steal by the above-described method. US-A-4,045,254 discloses a method to increase the toughness of a metallic material by extreme micro-refining of crystal grains. A load is applied while the metallic material is placed under a triangular-wave temperature cycle passing over the transformation point. US-A-3,278,345 discloses that an ultra-fine austenite grain size can be produced by rapidly heating a steel material to just about its transformation point and below its grain coarsening temperature, rapidly deforming the workpiece at such temperature to deform the steel grains throughout the workpiece and then rapidly cooling the workpiece to below the transformation point of the steel. The heating, working and cooling is repeated sufficient times to produce the fineness of grain desired. It is a general object of this invention to provide a metallic material comprising a high temperature phase of a uniform and ultra-fine grain structure. This is attained by a method as claimed in claim 1. It is a more specific object of this invention to provide a metallic material comprising a high-temperature phase of a uniform and ultra-fine grain structure, which in the case of steel is an austenite phase, the high-temperature structure having a grain size of 15 µm or smaller, preferably 10 µm or smaller and a method for producing the metallic material. It is another object of this invention to provide a metallic material comprising a uniform, ultra-fine grain structure, such as ferrite, martensite, bainite, or pearlite having an average grain size of 10 µm or smaller, preferably 5 µm or smaller, and a method of producing the metallic material from the before-mentioned uniform, ultra-fine austenitic structure. It is still another object of this invention to provide titanium or a titanium alloy having a uniform, ultra-fine grained microstructure, and a method for producing such a uniform, ultra-fine grained microstructure. The inventors of this invention made the following discoveries. (a) When steel which is phase-transformable between an austenitic phase and a ferritic phase is processed, i. e., when a metal which is phase-transformable between a high-temperature phase and a low-temperature phase is processed by hot working, as a pretreatment the metal is first subjected to a thermal treatment or deformation such as in conventional hot working so as to control of the microstructure such that at least part of the metallic structure comprises a low-temperature phase, and as a final step the temperature of the metal is increased to a point beyond the transformation temperature while plastic deformation is applied to the metal to effect a reverse transformation of the low-temperature phase into the high-temperature phase, resulting in an unexpectedly ultra-fine microstructure which cannot be obtained by conventional controlled rolling. (b) The above-described ultra-fine high-temperature microstructure can be obtained from a starting material which mainly comprises a low-temperature phase by first carrying out deformation in a low temperature region and a warm-temperature region, and then at the final stage of working by increasing the temperature beyond the phase transformation temperature while performing working to effect reverse transformation. (c) In order to complete the above-described reverse transformaton, it is preferable that the metallic material being processed be maintained at a prescribed temperature, e.g., at a temperature higher than the Ac₁ point in equilibrium conditions for a given length of time after the temperature rise caused by plastic deformation has ended. (d) The thus-obtained steel material having an ultra-fine, austenitic grain structure may be further subjected to a conventional treatment including air cooling, slow cooling, holding at high temperatures, accelerated cooling, cooling combined with deforming, quenching, or a combination of such treatments. The resulting steel product has a uniform and ultra-fine grain structure which has never been obtained in the prior art. In particular, when slow cooling is performed, a spheroidized or softened and annealed ultra-fine microstructure can be obtained. In addition, when the above-described austenitic steel is rapidly cooled only in a high temperature range without crossing a nose area of the CCT curve for the steel, a uniform, ultra-fine quenched microstructure can be obtained in a relatively easy manner. In the case of steel, the resulting metallurgical structure is austenite, ferrite, bainitem, martensite, or pearlite, which is determined depending on the heat treatment conditions employed. (e) Furthermore, according to this invention, in the case of a hot-worked steel product, since the steel product is subjected to the phase transformation "ferrite → austenite → ferrite", carbides and nitrides which have been precipitated during working and are effective to further strengthen steel are no longer coherent with the matrix with respect to their crystal lattice. The mechanism of strengthening steel is changed from "coherent precipitation strengthening" to "incoherent precipitation strengthening". Thus, it is possible to achieve precipitation strengthening without embrittlement. This is very advantageous from a practical viewpoint. This invention is based on the above findings. In a broad sense it resides in a metallic material and a method for producing the same in which the metallic material is phase-transformable between a low-temperature phase and a high-temperature phase, plastic deformation is applied when the material comprises at least a low-temperature phase, and the temperature of the material is raised beyond the transformation temperature to the temperature of the high-temperature phase while applying plastic deformation. The metallic material the temperature of which has been raised beyond the phase transformation point may be retained at such a high temperature. The resulting high-temperature structure has an ultra-fine grain structrue. The metallic material to which this invention can be applied is not restricted to any specific one so long as it has a phase transformation point from a low-temperature phase to a high-temperature phase. Examples of such metallic materials are steel, Ti, Ti-alloys, Zn, Zn-alloys, Ni, and Ni alloys. In the case of steel, the low-temperature is ferrite and the high-temperature phase is austenite, and it may be the case in which the low-temperature phase is γ-austenite and the high-temperature phase is δ-ferrite. In the former case, a steel comprising at least a ferritic phase can be used as a starting material for hot working. The term "steel" is used to include carbon steels, alloyed steels, and any other types having a structure comprising at least a ferritic phase, although it contains other additional elements. "Steel comprising at least a ferritic phase" means steels comprising ferrite only as well as steels comprising a combined phase of ferrite with at least one of carbides, nitrides, and intermetallic compounds, steels comprising a combined phase of ferrite with austenite, and steels comprising a combined phase of ferrite with austenite and at least one of carbides, nitrides, and intermetallic compounds. According to this invention, not only carbon steel but also a variety of alloyed steels can be successfully treated to provide a hot-worked, high-strength steel having an ultra-fine microstructure without adverse effects which might be caused by alloying elements. The term "ferrite phase" or "ferrite structure" means a structure which comprises a ferritic phase distinguishable from an austenitic phase, including an equiaxed ferrite, acicular ferrite, and a ferrite-derived structure such as a bainite structure, martensite structure, or tempered martensite. Figure 1 is a schematic illustration of a hot rolling production line by which the method of this invention can be performed; and Figure 2 is a graph showing a CCT curve for steel. Figure 1 shows a hot rolling production line which can be used in this invention. In Figure 1, an induction heating furnace 1 covers a series of pair of rolls 2 and rolling is carried out within the furnace 1. In carrying out rolling, a steel rod 3 to be rolled is first heated by passing it through an infrared ray-heating furnace 4, and the heated rod is hot rolled within the induction-heating furnace 1 while further adjusting the temperature of rod by heating it with a series of induction heating coils 5 each of which is provided before each of the rolls. The rolled rod after leaving the final stage of rolling may be retained at a given temperature in a temperature-maintaining furnace 7 or it may be cooled slowly or it may be air-cooled or water-colled with water-spray nozzles 8. The thus heat-treated rolled rod is then coiled by a coiler 6. According to the method of this invention a starting microstructure for hot rolling is defined as a microstructure comprising at least a low-temperature phase, i.e., a single low-temperature phase microstructure or a microstructure mainly comprising the low-temperature phase, which is ferrite in the case of steel. While plastic deformation is applied, the ferrite is transformed into an austenitic phase so that an ultra-fine microstructure may be obtained. The resulting austenitic, ultra-fine grained struture, when subjected to further heat treatment, e.g. cooling, will have a uniform, ultra-fine transformed structure, such as an ultra-fine ferrite, martensite, bainite and pearlite. In this invention, the greater the amount of ferrite the better for the starting material. However, sometimes it is rather difficult to obtain 100% ferrite structure or 100% (ferrite + carbides or nitrides or other precipitates) structure during working. In addition, some steel products inevitably contain ferrite + austenite, or ferrite + austenite + carbides or nitrides or other precipitates. Therefore, it is desirable that the amount of ferrite be 20% by volume or more, and preferably 50% by volume or more. The amount of strain which is introduced during plastic deformation so as to effect reverse transformation of ferrite into austenite is preferably 20% or more for the purpose of this invention. The introduction of stain during plastic deformation is effective, firstly, to induce ultra-fine austenitic grains from the work-hardened ferrite. Secondly, it is effective to generate heat during plastic working so that the temperature of the work piece is increased beyond the transformation temperature at which ferrite is transformed into austenite. Thirdly, it is effective to produce work hardening in the resulting fine austenitic grains so that ultra-fine ferritic grains can be induced when followed by transformation into ferrite. However, when the amount of strain is less than 20%, the formation of ultra-fine austenitic grains induced by deformation during the reverse transformation is sometimes not enough to obtain a grain size of not larger than 15µm. Furthermore, when the strain is less than 20%, the amount of heat generated during working is so small that an auxiliary heating means should be provided in order to promote the reverse transformation of ferrite into austenite. This is disadvantageous from an economical viewpoint. In contrast, when the amount of strain is larger than 50%, there is no need for an additional heating means to effect the reverse transformation if the final shape of the steel product and the working speed are selected suitably. Therefore, the amount of strain is preferably 50% or higher. Means for providing strains to steel materials during working is not restricted to any specific one. It includes, for example, rolling mills such as strip rolling mills, pipe rolling mills, and rolling mills with grooved rolls, piercing machines, hammers, swagers, stretch-reducers, stretchers, and torsional working machines. Alternatively, such strains can be imparted solely by shot-blasting, which is a particularly easy and effective way to apply plastic deformation to wire. In carrying out shot-blasting, it is preferable to strike shot against the wire from four directions, i.e., from above and below and from right and left. The shot may be in the form of steel balls which are usually used to perform descaling under cold conditions. The diameter of the shot is preferably as small as possible. Needless to say, it is necessary to heat the steel being hot worked to a temperature higher than the point at which ferrite is transformed into austenite, i.e., the Ac₁ point in order to perform reverse transformation of ferrite into austenite. When the temperature is higher than the Ac₁ point but lower than the Ac₃ point, the resulting phase structure is a dual-phase structure comprising ferrite and austenite. According to this invention, however, since deformation is carried out while increasing the temperature, the size of crystal grains is thoroughly reduced due to plastic deformation and recrystallization even if the temperature does not increase to higher than the Ac₃ point. The rise in temperature is restricted to lower than the Ac₃ point when the production of a dual-phase structure comprising ferrite and austenite is required. According to this invention, as already mentioned, the reverse transformation is carried out by applying palstic deformation and by simultaneously increasing the temperature. The purposes of carrying out the reverse transformation are to refine the ferrite grains by working in a ferrite-forming temperature range, to promote the work-induced formation of fine austenitic grains from work-hardened ferrite grains, to refine the austenite grains by working, and to promote the strain-induced transformation of work-hardened austenite grains into fine ferritic grains. When the starting structure for the reverse transformation contains carbides, the carbides are mechanically crushed into fragments which are then uniformly dispersed throughout the matrix during the above-mentioned plastic deformation. Furthermore, such fine carbides constitute nuclei for transformation of ferrite into austenite to promote the formation of finer grains of austenite. Working is effective for accelerating the decomposition of carbides and their incorporation into a solid-solution, and the decomposition of carbides also accelerates the reverse transformation into austenite. When carrying out hot working and heating of steel so as to effect the reverse transformation into austenite in accordance with this invention, there is a tendency for the rate of deformation to be high and therefore for the temperature to rise rapidly. In fact, sometimes there is not enough time to complete the reverse transformation into austenite before cooling. In such a situation the hot-worked steel might be cooled before deformed ferrite is thoroughly transformed into austenite, and large grains of ferrite will remain without being transformed. Therefore, after hot working is completed and the temperature is increased to a point higher than the transformation point, it is preferable that the resulting hot-worked steel material be kept at a temperature higher than the Ae₁ point so as to allow sufficient time for the ferrite grains containing strains to transform into austenite. For this purpose the rolled material can be held at a temperature higher than the Ae₁ point. If it is held at a temperature lower than the Ae₁ point, the reverse transformation will no longer take place for the reasons of thermodynamic principles. A necessary period of time for hot-worked metallic material to be maintained at a temperature higher than the Ae₁ point is preferably determined based on the working conditions and the kind of metallic material. A period of as little as 1/100 seconds is enough for highly-pure iron metal, while some types of high-alloy steel require several tens of minutes to complete the reverse transformation. In general, one hour at the longest is enough for high-alloyed steels which are widely used today in industry. Therefore, it is desirable to employ a retaining time which is long enough to complete transformation and is reasonable from the viewpoint of economy to ensure proper operating efficiency. Thus, according to this invention the upper and lower limits are not restricted to specific ones. After finishing the reverse transformation of this invention, direct annealing may be applied to the hot-rolled product by controlling the cooling rate. Such a heat treatment is already known in the art. When applying annealing, the suitable cooling rate is rather slow and it depends on the desired product as well as the intended transformed structure which includes, for example, a well-recovered, soft ferrite having an ultra-fine grain structure, an ultra-fine grain structure comprising an ultra-fine ferrite and spherical carbides, and an annealed, ultra-fine structure comprising ferrite and spherical carbides or soft pearlite, which is free from a quenched structure such as martensite and bainite. The cooling rate is not restricted to a specific one, and a suitable one can be chosen based on the above factors and practical considerations. According to this invention, a quenched structure can be obtained. Namely, the resulting austenitic structure, i.e., the structure of a high-temperature phase comprising ultra-fine grains can be quenched to provide an ultra-fine martensite structure. However, as is well known, the finer the austenitic grains the worse is the hardenability. Since the transformation temperature from austenite to ferrite shifts to a higher position for an austenite having a finner microstructure, more coarse ferritic grains are easily formed for an austenite having finner grains even if the same cooling rate is employed. This is contrary to the purpose of providing a steel product having an ultra-fine microstructure by refining an austenitic structure. In addition, the nose area of a CCT curve moves towards the short-time side as shown by a white arrow in Figure 2 when the austenite comprises finer grains, and it is rather difficult to obtain a quenched structure, but ferrite/pearlite are easily formed. In this case the bainite-forming region also moves towards the short-time side. Therefore, in order to obtain an ultra-fine, quenched microstructure in spite of these problems it is necessary to carry out rapid cooling at a rate higher than the critical cooling rate so as not to cross the nose area of the CCT curve. Such rapid cooling can be performed using a large amount of a cooling medium such as water, oil, or air, or it can be performed by spraying such a cooling medium against an object to be cooled at a high pressure and at high speed. However, the cooling rate is usually higher in a high-temperature region than in a low-temperature region. Therefore, in order to avoid passing through the nose area of the CCT curve, rapid cooling is carried out only in a high temperature region, i.e., in a temperature region from the Ae₁ point to the Ms point. This is advantageous from the industrial point of view. In a preferred embodiment of this invention, after quenching in the above-manner, a quenched structure may be slowly cooled. Such slow cooling may be accomplished by air cooling or natural cooling, too. Thus, according to this invention, a high-temerature phase with an ultra-fine microstructure of the high-temperatrue phase can be obtained, and the resulting ultra-fine high-temperature phase can be further heat treated to produce the following various steel materials. (1) Ultra-fine ferritic steels: When the above-described ultra-fine austenite is cooled from its high-temperature state under usual ferrite-forming conditions, according to this invention, a steel mainly comprising a ferritic structure of equiaxed ferritic grains is obtained. The steel exhibits excellent properties when the grain size is 5 µm or less. The equiaxed ferrite is distinguishable from non-equiaxed ferrite which is included in pearlite, bainite and martensite. (2) Ultra-fine bainitic steels: When the above-described ultra-fine austenite is cooled from its high-temperature state under usual bainite-forming conditions, according to this invention, a steel mainly comprising a bainitic structure of ultra-fine bainitic packet is obtained. The steel exhibits excellent properties including good workability, strength, and toughness when the packet size is 5 µm or less. The bainite packet is a region in which the longitudinal axes of the bainitic grains are aligned. (3) Ultra-fine martensitic steels: When the above-described ultra-fine austenite is cooled from its high-temperature state under the before-mentioned martensite-forming conditions, according to this invention, a steel mainly comprising a martensitic structure of ultra-fine martensitic packet is obtained. The steel exhibits excellent properties including good workability, strength, and toughness when the packet size is 5 µm or less. The martensitic packet is a region in which the longitudinal axes of the martensitic grains are aligned. In the case of the above ultra-fine, martensitic carbon steel or alloyed steel having a carbon content of 0.6% by weight or less, when tempering is carried out at a temperature lower than the Ac₁ point, a highly-ductile PC steel can be obtained which has a relaxation value of 1.5% at room temperature, a relaxation value of 10% or less at warm temperatures, a tensile strength of 95 kgf/mm² or higher, and uniform elongation of 3.0% or more. During tempering, deformation with a total of plastic strains of 3 -90% may be applied. (4) Ultra-fine pearlitic steels: When the above-described ultra-fine austenite of high carbon steel is cooled from its high-temperature state under usual pearlite-forming conditions, according to this invention, a steel mainly comprising a pearlite structure of ultra-fine pearlite grains is obtained. The steel exhibits excellent workability when the average pearlite colony size is 5 µm or less. A pearlite colony is a region of pearlite structure in which ferrite lamellae and cementite lamellae are aligned in the same direction. When a steel having a carbon content of 0.70 - 0.90% is used for the above described ultra-fined, pearlitic steel and controlled cooling such as lead patenting or air-blasting is applied to the ultra-fine austenitic structure after completion of the reverse transformation, a filament which can be successfully used as cord for automobile tires is obtained. A conventional wire has a strength of at most 320 kgf/mm². In contrast, according to this invention a wire having a tensile strength of 380 kgf/mm², 20 twists or more, and a probability of fracture by bending of 5% or less and which is suitable for wire drawing can be obtained. The types and compositions of the above-described steels are not restricted to any specific ones so long as an intended ultra-fine microstructure can be attained. Furthermore, if necessary, at least one alloying element such as B, V, Nb, Ti, Zr, W, Co, and Ta can be added. Depending on the purpose of the steel, a rare earth metal such as La and Ce and an element which promotes free-cutting properties such as Ca, S, Pb, Te, Bi, and Te can be added. This invention can be applied to any metallic materials which exhibit a phase transformation from a low-temperature phase to a high-temperature phase and vice versa, such as titanium and titanium alloys. In the case of titanium and titanium alloys, the high-temperature phase corresponds to β-phase and the low-temperature phase corresponds to α-phase. According to one embodiment of this invention, titanium material comprising at least an α-phase is hot-worked to increase its temperature to a point higher than the transformation point while carrying out plastic deformation with plastic strains of 20% or more. It is then kept at this temperature for not longer than 100 seconds to perform the reverse transformation of at least part of the α-phase into β-phase. It is then cooled to obtain titanium or a titanium alloy with an ultra-fine microstructure. In the case of titanium or a titanium alloy, it is preferable that the particle size of the resulting β-phase grains, i.e., the particle size of the β-phase grains before cooling be 100 µm or smaller. It is well known in the art that the particle size of β-phase grains can be easily and accurately determined on the basis of the arrangement of α-phase grains, the etched surface appearance, and the like. The structure "comprising at least an α-phase" means not only a structure comprising α-phase only, but also a structure comprising a combined phase of α-phase with precipitated phases of rare earth metals and/or oxides of rate earth metals, a structure comprising a combined phase of α-phase of β-phase, and a structure comprising a combined phase of α-phase with β-phase and precipitated phases of rare earth metals and/or oxides of rare earth metals. After finishing the reverse transformation into β-phase, the titanium or titanium alloy is cooled. Rapid or slow cooling can be performed. This invention will be further described in conjunction with the following working examples which are presented merely for illustrative purposes. Example 1 The steel compositions shown in Table 1 were melted in air using an induction heating furnace and were poured into 3-ton ingots. After soaking, the ingots were hot-rolled to form square bars each measuring 130 X 130 mm in section. The bars were divided into 100 kg pieces which were then hot-forged to form billet measuring 50 x 30 mm in section. For Steel A through Steel H the resulting billets were heated to 950 °C to give normalized structures. For Steel I and Steel J the resulting billets were heated to 1150°C and furnace-cooled. The resulting heat-treated billets were then rolled to form billets measuring 9 mm, 10 mm, 12 mm, 15 mm, 20 mm, or 25 mm in thickness and 30 mm in width. For Steel A through Steel H the resulting billets were again heated to 950 °C to give normalized structures. For Steel I and Steel J the resulting billets were heated to 1150°C and furnace-cooled to prepare stock for rolling. Experiment i The thus-prepared rolling billets of Steel A through Steel K measuring 20 mm X 30 mm were heated in an induction heating furnace to the temperatures indicated in Table 2 and were hot rolled to plates measuring 7.5 mm in thickness in a single pass using a planetary mill. As shown in Table 2, the structure prior to hot rolling was a single phase of ferrite, a combined structure of ferrite with austenite or a combined structure of ferrite with austenite further containing carbides, or intermetallic compounds. The temperature of the rolled plates at the outlet of the rolling mill was increased by the heat generated during severe working with the planetary mill to the temperatures indicated as "finishing temperatures" in Table 2. It was confirmed that the temperature to be attained can be controlled by varying the rolling speed. After hot-rolling the structures of eight steel samples including Steel A through Steel H were determined. The ferritic grain size was measured for the samples which had been air-cooled after hot rolling. The original austenitic grain size was measured by preferentially etching original austenitic grain boundaries for samples which had been water-quenched after rolling. For comparison, stock of Steel A and Steel E measuring 20 mm X 30 mm in section was heated to 950 °C and was then hot rolled at temperatures of 850 - 825 °C with three passes using an experimental mill for rolling plates. This process was referred to as "controlled rolling". For further comparison, after controlled rolling, some of the samples were cooled rapidly to 650°C by water-spraying and then air-cooled. This process was referred to as "controlled rolling + rapid cooling". The austenitic grain size was measured on a structure which after controlled rolling had been brine-quenched and then tempered. The results of measurements are also shown in Table 2. Experiments ii Steel G was used as stock for rolling. Six types of billets of Steel G measuring 9 mm, 10 mm, 12 mm, 15 mm, 20 mm, or 25 mm in thickness were hot rolled with various degrees of working. For the billets having a thickness of 9 mm and 10 mm, hot rolling was carried out using the above-mentioned planetary mill to a thickness of 7.5 mm with one pass as in Experiment (i). Since in these cases the temperature of the rolled plates just after rolling increased to only 765°C and 790°C, respectively, the temperature was increased rapidly by heating the plates to 905°C with an induction heating coil disposed at the outlet of the mill. Some of the hot-rolled plates were retained at 905 °C for 5 seconds and then water cooled. The other plates were directly air-cooled without being held at 950°C. On the other hand, for the billets having a thickness of 12 mm - 20 mm, hot rolling was carried out using the planetary mill as in Experiment (i). However, this time the temperature of the plates just after rolling increased to 905 °C. Some of the hot rolled plates were air-cooled immediately after finishing hot rolling, and the others were held at the outlet temperature for 5 seconds within the induction furnace disposed at the outlet of the mill and then water cooled. Furthermore, the billet measuring 25 mm thick was subjected to four continuous passes of rolling with a reduction in 5 mm for each pass using an experimental mill for rolling plates and an induction heating furnace to obtain hot-rolled steel plates. Between each pass, heating with the induction heating furnace was performed to increase the temperature of the rolled plates by 50°C. The test results are summarized in Table 3 together with processing conditions. Experiment iii Steel A and Steel G were used as stock for rolling. Plates of these steels measuring 20 mm thick were hot rolled in the same manner as in Experiment (i). The temperature of the rolled plates was increased at the outlet of the mill due to the heat generated during rolling, since the degree of deformation was large. The temperature which was reached depended on the rolling speed of the planetary mill. Therefore, the temperature of the plate after finishing rolling was adjusted by varying the rolling speed. Immediately after rolling some plates were water-cooled directly, and the others were held at the final rolling temperature for one minute by means of induction heating and then were water-cooled. The test results are shown in Table 4 together with processing conditions. Experiment iv Steel D was used as stock for rolling. Billets of this steel with a thickness of 20 mm were first heated to 740°C, 780°C, or 850°C in order to change the ratio of the area of austenite to the area of ferrite. The resulting plates were then hot rolled in the same manner as in Experiment (i). The finishing temperature was adjusted to be about 810°C by controlling the rolling speed. In addition, the microstructure prior to hot rolling was examined on a material which, after heating, was quenched instead of being hot rolled. Immediately after rolling, the hot-rolled plates were water-cooled or air-cooled. The test materials designated as Run 4-7 and Run 4-8 were held at 810 °C for one minute after rolling. The test resutls are shown in Table 5. Experiment v Billets of Steel G of Table 1 with a thickness of 20 mm were used as stock for rolling. The billets were heated to 875°C in an infrared heating furnace and were then air-cooled to 675°C, 650 °C, 625 °C, or 600°C prior to hot rolling. At the indicated temperatures the billets were hot rolled with the planetary mill in the same manner as in Experiment (i). The finishing temperature was adjusted to be about 850°C by controlling the rolling speed. In addition, the same billet was heated to 875°C and then was air-cooled to 675 - 600 °C. After quenching and tempering, without hot rolling, the grain size of the billet was observed. On the basis of observations, the microstructure prior to hot rolling was estimated. Furthermore, plates of Steel G measuring 20 mm thick were prepared. Some of the plates were subjected to a patenting treatment in a salt bath to form bainite structure. The others were oil-quenched and then tempered at 200 °C. The resulting plates were also used as stock for rolling. After hot rolling and the above-described post-treatment the resulting microstructure was observed. The test results together with experimental conditions are summarized in Table 6. Experiment vi Rectangular bars of Steel I of Table 1 measuring 50 mm X 30 mm in section were heated to 200 °C, and then were hot forged into rectangular bars measuring 20 mm X 30 mm in a temperature range of 1050 - 700 °C by means of an air hammer. Following the hot-forging, the bars were held at 700 °C for from 5 minutes to 2 hours to form a combined structure comprising austenite, spherical carbides and nitrides, ferrite, and pearlite. After being removed from the furnace at 700°C, the hot-forged bars were hot rolled in the same manner as in Experiment (i), and then were air-cooled. The hot-rolled bars were colled to room temperature and immediately tempered. The tempered bars were observed to determine the original grain size of austenite. The test results together with experimental conditions are summarized in Table 7. Example 2 Experiment vii In this experiment, the procedure of Experiment (i) was repeated except that the hot-rolled plates were retained at the finishing temperature for various periods of time of up to 1 hour. The grain size of ferritic grains of the as-quenched structure was measured and determined as grain size before cooling. The grain size of austenitic grains before cooling was determined by measuring the grain size of a structure which had been subjected to tempering after quenching. The test results are summarized in Table 8. Experiment viii In this experiment, the procedure of Experiment (ii) was repeated except that some of the processing conditions were changed as shown in Table 9. The test conditions and results are summarized in Table 9. Experiment ix In this experiment, the procedure of Experiment (iii) was repeated using Steel A, Steel G, and Steel H except that some of the processing conditions were changed as shown in Table 10. The test conditions and results are summarized in Table 10. Experiment x In this experiment, the procedure of Experiment (iv) was repeated except that some of the processing conditions were changed as shown in Table 11. The test conditions and results are summarized in Table 11. Experiment xi In this experiment, the procedure of Experiment (v) was repeated except that some of the processing conditions were changed as shown in Table 12. The test conditions and results are summarized in Table 12. Experiment xii In this experiment, the procedure of Experiment (vi) was repeated except that some of the processing conditions were changed as shown in Table 13. The test conditions and results are summarized in Table 13. In the preceding examples, plastic deformation was carried out by hot rolling in order to carry out reverse transformation. In another embodiment of this invention, the reverse transformation may be carried out by shot-blasting in place of hot rolling. It was confirmed that when shot-blasting was performed on steel wire with an initial surface temperature of 710 °C, it was possible to increase the surface temperature to 920°C. Example 3 In this example, the method of the present invention was used for the manufacture of titanium and titanium alloys. Pure titanium and the titanium alloys shown in Table 14 were melted using a vacuum arc melting furnace and were poured into alloy ingots. These ingots were hot-forged with a heating temperature of 1500°C and a finishing temperature of 1300°C to provide rods measuring 60 mm X 40 mm in section. Test pieces measuring 50 mm X 30 mm in section were cut from the rods after annealing. Experiment xiii Pure titanium and titanium alloys (Sample A through Sample E) shown in Table 14 were prepared and were heated to the temperatures indicated in Table 15. After heating, they were hot-rolled to a thickness of 7.5 mm using a planetary mill or a conventional mill for rolling plate. When a conventional plate-rolling mill was used, rolling was carried out in three passes. When rolling was carried using the planetary mill, the temperature of the plates at the outlet of the mill was increased due to the heat generated during rolling with a high degree of reduction. The temperature attained during rolling can be controlled by varying the rolling speed. In this experiment every sample could be heated to a temperature higher than its transformation temperature. Immediately after the hot-rolling or after the plates were maintained at the finishing temperature for a period of time of up to 1 hour the resulting plates were water-cooled and then their microstructure was observed. The grain size of β -grains before water-cooling was determined by observing the microstructure of the stock for rolling. The test results and processing conditions are summarized in Table 15. Experiment xiv Titanium Alloy C in Table 14 was used as stock for rolling. It was hot-rolled with a planetary mill. Heat generation was controlled by changing the degree of reduction in order to effect reverse transformation. After finishing rolling, the rolled plates were kept at the finishing temperature for 10 seconds, and then were water-cooled. The microstructure of the resulting titanium alloys was then observed. The degree of reduction with the planetary mill, i.e., the amount of strain was adjusted to be 20%, 30%, 40% or 50%. This amount of reduction was not enough to increase the temperature thoroughly high over the transformation temperature of the alloy, an induction coil was disposed at the outlet of the mill and performed supplemental heating to heat the alloy to a temperature higher than the transformation temperature, e. g., 1050°C. The observed grain sizes are summarized in Table 16. Example 4 In this example steel materials comprising mainly ferrite were prepared using the steel samples of Table 17 by controlling the cooling rate from austenite. The mechanical properties of these materials were determined and are shown in Table 18. Example 5 Steel materials comprising mainly bainite were prepared using Steel A through Steel E shown in Table 19 by controlling the cooling rate from austenite. The mechanical properties of these materials were determined and are shown in Table 20. Example 6 Steel materials comprising mainly martensite were prepared using steel samples shown in Table 21. The mechanical properties of these materials were determined and are shown in Table 22. Example 7 Steel materials comprising mainly pearlite were prepared using steel samples shown in Table 23. The mechanical properties of these materials were determined and are shown in Table 24. Example 8 Carbon steel (0.80%C-0.22%Si-0.51%Mn) was hot rolled using a heating temperature of 650°C, a finishing temperature of 900°C, a rate of temperature increase of 100 °C/s, and a reduction of 70% to form steel wire with a diameter of 5.2 mm. Following the hot rolling, water-cooling to 800 °C was performed, and then controlled cooling was carried out so as to complete the transformation into pearlite. The resulting pearlite steel wire was then subjected to conventional cold wire drawing to form a filament which was used as cord wire for the manufacture of automobile tires. The resulting filament had a maximum tensile strength of 408 kgf/mm², a torsion strength of 25 cycles, and a bending fracture probability of 4.0%. Example 9 Steel bars of carbon steel (0.53%C-0.28%Si-0.79%Mn) were heated to 950 °C and hot rolled to a diameter of 22.5 mm at a temperature of 780°C using an 8 stand tandem mill. After hot-rolling the resulting wire was air-cooled to 500°C, and then rapidly heated to 700°C by high-frequency heating. After heating to 700 °C the steel wire was hot-rolled to a diameter of 15.0 mm using the tandem mill with a reduction of 56%. The temperature of the wire at the outlet of the mill was 890 °C. After rolling, the wire was quenched in 0.6 seconds. The wire was then reheated to 690°C by high-frequency heating, and then high speed rolling with the tandem mill was carried out to roll the wire to a diameter of 7.4 mm with a reduction of 76%. The roll finishing temperature was 880°C, and after water-cooling a PC steel bar with a diameter of 7.4 mm was obtained. The resulting PC steel bar had a tensile strength of 155.0 kgf/mm², a yield strength of 142.7 kgf/mm², an elongation of 14.6%, a uniform elongation of 10.3%, a relaxation value at 180°C of 6.0%, and an impact fracture energy of 7.26 kgf-mm/mm³.

Claims: A method for producing a metallic material having an ultra-fine microstructure, the metallic material exhibiting a phase transformation of a low-temperature phase into a high-temperature phase, the method comprising the steps of: preparing a metallic material which comprises at least a low-temperature phase; increasing the temperature of the metallic material to a point beyond a transformation point; and effecting reverse transformation of the low-temperature phase into the high-temperature phase, characterized in that the reverse transformation is carried out by increasing the temperature of the metallic material to a point beyond a transformation point while performing plastic deformation of the metallic material by applying strains in an amount of 20% or more to the metallic material. A method as set forth in Claim 1 wherein the metallic material is selected from the group consisting of steel, titanium, titanium alloys, zirconium, zirconium alloys, nickel, and nickel alloys. A method as set forth in Claim 1 or 2, further compriisng the step of cooling the high-temperature phase to room temperature. A method as set forth in Claim 2 wherein the metallic material is steel, the low-temperature phase is ferrite, and the high-temperature phase is austenite. A method as set forth in Claim 2 wherein the metallic material is steel, the low-temperature phase is γ-austenite, and the high-temperature phase is δ-ferrite. A method as set forth in any one of Claims 1 to 5, further comprising the step of retaining the metallic material at an attained temperature after having increased the temperature to a point higher than the phase transformaton point to promote the reverse transformation of the low-temperature phase into the high-temperature phase. A method as set forth in any one of the preceeding claims, wherein the metallic material is a steel material, the low-temperature phase is ferrite and the high-temperature phase is austenite, the transformation point is the Ac₁ point, and cooling the steel material to effect reverse transformation of at least part of the ferrite into austenite. A method as set forth in Claim 7, further comprising the step of retaining the steel material at a temperature higher than the Ae₁ point after having increased the temperature to a point higher than the Ac₁ point to promote the reverse transformation of ferrite into austenite. A method as set forth in Claim 3 or 7 wherein the step of cooling is carried out in a manner selected from air-cooling, slow cooling, and rapid cooling. A method as set forth in any one of Claims 1 to 9 wherein the plastic deformation is carried out by shot blasting. A method as set forth in any one of claims 1 to 3, wherein the metallic material is a titanium or titanium alloy material, the low-temperature phase is the α-phase and the high-temperature phase is the β-phase, after applying the plastic deformation retaining the material at the attained temperature for no longer than 100 seconds to transform at least a portion of the α-phase into β-phase, and cooling the material to room temperature. A method as set forth in Claim 7, wherein the steel is a high carbon steel wire for use in wire drawing and after transformation into austenite controlled cooling is performed to promote the transformation of the austenite into pearlite. A method as set forth in Claim 7, wherein the steel is a highly-ductile PC steel and the step of carrying out transformation into austenite is performed at least one time, immediately after the transformation step the material is cooled at a cooling rate higher than the critical cooling rate to form a structure comprising martensite in which the average size of a martensitic packet or an original austenitic grain is 5 µm or less, and after the cooling, tempering is carried out at a temperature of Ac₁ or lower.

Inventors: AIHARA KENJI, HAGITA HYOJI, HAYASHI CHIHIRO, MURAI NOBUHIRO, TSUKAMOTO TAKASHI, AIHARA, KENJI, HAGITA, HYOJI, HAYASHI, CHIHIRO, MURAI, NOBUHIRO, TSUKAMOTO, TAKASHI
Applicants: SUMITOMO METAL IND, SUMITOMO METAL INDUSTRIES, LTD.
Application year: 1989
Main classification: C21D8
IPC codes: B21B1, C21D8, C22F1, B21B3, B21B45