EP0338682A2

EP0338682A2 - Method of strengthening metal material and apparatus therefor

Info

Publication number: EP0338682A2
Application number: EP89302878A
Authority: EP
Inventors: Atsumi Ohno
Original assignee: O C C KK
Current assignee: O C C KK
Priority date: 1988-03-23
Filing date: 1989-03-22
Publication date: 1989-10-25
Also published as: JPH01242762A; EP0338682A3

Abstract

A method of strengthening a metal material of a cast product consisting of a unidirectionally solidified structure of crystals extending long in a direction of casting, and an apparatus for realizing this method. The cast product (12) is passed through a zigzag path formed by contact with circumferential surfaces of rotatable rolls (16, 18, 22, 26, 28, 31), so that repeatedly bending deformation is applied to the cast product to thereby harden the cast product. The repeatedly bending deformation grows dislocation in crystals of the cast product competitively, so that free movement in dislocation of crystals cannot be made any more. As a result of the growth competition of crystals, a strengthened metal material is obtained.

Description

The present invention relates to a metal material strengthening method for producing a tough metal material excellent in fatigue resistance, and relates to an apparatus therefor. More particularly, it relates to a metal material strengthening method for producing a metal material hardened by applying repeatedly bending deformation to a cast product having an unidirectionally solidified structure of crystals elongated in the direction of casting, and an apparatus for realizing this method.
In general, pure metal is soft and difficult to be harden, so that it cannot be used as a structural material. Therefore, metal has been used in the form of an alloy by addition of alloy elements to increase the strength of the metal. In the case where sufficient strength cannot be obtained by addition of alloy elements, the alloy has been hardened and strengthened by heat treatment such as quenching or by age hardening treatment so as to be used as a structural material.
For example, a large quantity of low specific gravity Al alloy has been used as a structural material of an airplane which requires both lightness and strength. In general, pure Al metal is soft and has not sufficient strength for use as a structural material of an airplane. Therefore, an Al alloy is used after strengthened through a process in that a deposition-hardening type alloy is prepared by addition of metals, such as Cu, Zn, Mg and the like, and the alloy is strengthened by the age-hardening treatment.
However, it is a matter of common knowledge that airplanes formed by use of this type alloy have brought about much accidental falls (crashes) and that the accident has been, in most cases, concluded to be caused by fatigue factors of the metal material. In general, a material, such as a structural material of an airplane, subjected to repeated stress such as vibration often brings about metal fatigue. It has been therefore necessary to constantly carry out a severe inspection for early detection of cracks caused by metal fatigue. It is, however, impossible to detect cracks occurring in the inside of the material before propagating to the surface of the material, though it may be possible to detect cracks occurring in the surface of the material. Such cracks occurring in the inside of the material may propagate to the surface of the material to cause destruction of the material at any time. Passengers carried by airplane have been in constant anxiety as to the happening of such an accident. Therefore, the advent of airplane structural material excellent in fatigue resistance has been desired eagerly.
The risk of happening of an accident caused by the destruction of the structural material due to metal fatigue is not only involved in airplane material but also always involved in machine parts subjected to repeated stress such as vibrations for a long time. Accordingly, it is very important to find a method of producing a structural material excellent in safety and reliability in which fatigue factors are removed.
On the other hand, it is a matter of common knowledge that, when a cast product is composed of columnar crystals arranged substantially vertically in its surface portions and equiaxed crystals, impurities are segregated easily in the inter-crystal grain boundary formed during solidification of the cast product to thereby make the grain boundary easily developed into a starting point of mechanical destruction. A conventional method which has been generally used as a casting method for producing a cast product comprises injecting a molten metal into a cooled mold to thereby cool the metal to thereby prepare a cast product. In the cast product prepared by the conventional method, as shown in Fig. 3a, a columnar crystal zone 2 in which crystals have grown side by side and perpendicularly to the surface of the cast product 1 is formed in the surface portions of the cast product 1, while an equiaxed crystal zone 3 is often formed in the inside of the cast product 1.
When the aforementioned cast product is subjected to cold treatment, cracks occurs easily at the grain boundary of the columnar crystals in the surface portions of the cast product. It is therefore necessary to soften the cast product by heating prior to the cold working to squeeze the columnar crystals in the surface layer to thereby prevent propagation of cracks from the surface portions to the inside of the cast product during the cold working.
Further, it is known that gases and impurities are easily segregated to thereby form fine holes in the grain boundary formed during solidification. The aforementioned solidified grain boundary is the most weak place in the structure of the cast product. When the solidified grain boundary exists in the surface of a machine part produced by working the cast product, concentration of stress occurs in the grain boundary so frequently that it may be developed into a starting point of destruction of the machine part. If such a grain boundary is once formed during casting, the grain boundary remains historically in a finished article though plastic treatment or heat treatment may be supplied to the cast product. In short, it is well known that such a solidified grain boundary in which impurities are segregated to make the inter-crystal binding imperfect is a weak place against destruction due to fatigue.
In Japanese Patent Post-Exam Publication No. 55-4625 published November 21, 1980, the inventor of this application proposed a method of preparing a unidirectionally solidified cast product free from the solidified grain boundary formed by a columnar crystal zone and an equiaxed crystal zone, in which crystals has grown side by side and substantially perpendicularly to the surface of the cast product which is weak against destruction due to fatigue as described above. The invention disclosed in the above Japanese Patent Post-Exam Publication No. 55-46265 has been granted as a Japanese Patent No. 1049146. According to the proposed casting method, formation of a solidified shell along the side wall in the inside of a mold is prevented by keeping the temperature of the inside wall of the mold over the setting point of a cast metal and then solidification of the cast metal is started by quenching immediately after the cast metal is taken out of the mold. As shown in Fig. 3b, the cast product 4 prepared by this method has a unidirectionally solidified structure of columnar crystals longitudinally extending in the direction of casting. Continuously cast products of metals such as Al, Cu, Ni and the like can be prepared by the method.
It is considered however that the reason why the cast product thus prepared has not been used as a structural material exists in the two points as follows.
The first reason is in that the cast product has been considered to be unable to be produced as a structural material in mass-production with a low cost while it can be produced as a material of a special function, because a very low solidifying rate is required for preparing the unidirectionally solidified cast product so as to extremely lower the productivity thereof.
The second reason is in that it has been generally considered that a strong material must have a fine crystal structure, because large crystals are soft and difficult to be hardened by any treatment.
In general, when plastic working is applied to a cast product formed of polycrystals, the cast product is deformed by movement of dislocation generated in crystals. When the dislocation is moved to be converged into a crystal grain boundary, the movement stops there, so that crystals are hardened. As a result of hardening of crystals, destruction occurs from the grain boundary. Accordingly, a metal material having a smaller crystal grain size has been considered to be easy to harden. In other words, a metal material having a larger crystal grain size has been considered to be soft and difficult to be hardened.
In the case of single-crystals with no crystal grain boundary, dislocation generated by plastic working is moved out of crystal. Accordingly, there is no matter preventing movement of dislocation thus generated and hardening the material. The aforementioned fact suggests that it is necessary to find an effective means to prevent movement of dislocation in crystals for the purpose of hardening single-crystals.
It is therefore an object of the present invention to provide a novel method for strengthening a cast product having a unidirectionally solidified structure.
It is another object of the invention to provide an apparatus for strengthening a cast product having a unidirectionally solidified structure.
It is a further object of the invention to provide a strengthening method for producing a metal material strong in all directions by continuous treatment, and an apparatus for realizing this method.
The present invention provides a metal material strengthening method in which a continuously cast product having a unidirectionally solidified structure in which crystals are extended longitudinally in the direction of casting is passed through a bath drawing a zigzag locus to apply repeatedly bending deformation to the cast product to thereby strengthen the cast product.
Further, the invention provides a metal material strengthening apparatus which comprises a plurality of a rotatable rolls arranged at intervals so that the cast product is fed zigzag while being in contact with circumferential surfaces of the rolls so as to be repeatedly subject to bending deformation to thereby be hardened.
By applying repeated bending deformation treatment to a unidirectional solidified cast produced of, for example, single-crystals while merely changing the direction of bending, not by applying simple expanding deformation treatment such as rolling or drawing to the cast product or, in other words, not by applying plastic deformation causing a decrease of a section of the cast product, the growth of dislocation in crystals is accelerated competitively, so that free movement in dislocation of crystals cannot be made any more. As a result of the growth competition of crystals, a strengthened metal is obtained. It was thought that such a method as described above was effective. Based on the thought, it has been found that a hard and strong metal material excellent in fatigue resistance is obtained by the steps of: producing a cast product consisting of a single solid solution of metal having no solidification grain boundary causing occurrence of cracks, by a continuous casting method using a heat mold; and applying cold hardening treatment with repeated bending deformation to the cast product while changing the direction of bending. Hence, the prevent invention has been completed.
As a metal material having no inside defect and excellent in strength and fatigue resistance can be produced by applying repeated bending deformation treatment to a cast product as described above, it may be said that the method according to the invention is an epoch-making metal material strengthening method which can be applied to machines such as airplanes and cars in which destruction of the metal material caused by metal fatigue due to repeated stress affects people's life.
Other objects, features and effects of the present invention will become more apparent in the following description of preferred embodiments taken in connection with the accompanying drawing.

Fig. 1 is a front view showing partly in section an embodiment of the apparatus for realizing the strengthening method according to the present invention;
Fig. 2 is a schematic perspective view showing another embodiment of the apparatus for realizing the strengthening method according to the present invention.
Fig. 3a is a schematic view showing a macro structure of a cast product prepared by a conventional casting method; and
Fig. 3b is a schematic view showing a macro structure of a cast product prepared by a casting method used in the present invention.

An embodiment of the apparatus for realizing the metal material stengthening method in accordance with the present invention will be described hereunder. Referring now to Fig. 1, two feed pinch roll pairs 13 and 14 are arranged side by side in the entrance side of a strengthening apparatus 11. A continuously cast product 12 having a unidirectionally solidified structure is fed into the apparatus 11 by the feed pinch rolls 13 and 14 while the cast product 12 is nipped by the feed pinch rolls 13 and 14. Each of the feed pinch roll pairs 13 and 14 is constituted by a pair or rolls separated verti cally in Fig. 1 so that the cast product 12 can be nipped therebetween. The second pinch roll pair 14 is arranged at a distance from the first pinch roll pair 13 at the same level as the first pinch roll pair 13. A first guide 15 and a first bending roll 16 are arranged in the exit side of the second pinch roll pair 14 at a suitable distance from the second pinch roll pair 14. The first bending roll 16 and the first guide 15 are arranged so that the circumferential surface of the first bending roll 16 faces a guide surface 17 of the first guide 15 through the cast product 11. The guide surface 17 of the first guide 15 is arranged so that the guide surface 17 horizontally receives the cast product 11 substantially horizontally fed out of the second pinch roll pair 14 and then guides the cast product 11 so as to make the cast product 11 go down along a circular arc traced by the circumferential surface of the first bending roll 16. A second bending roll 18 and a second guide 19 are arranged in the exit side of the first guide 15 and the first bending roll 16. In Fig. 1, the axis of the second bending roll 18 is located at a position higher than the axis of the first bending roll 16. Hereinafter, all the positional relations are described with reference to Fig. 1. The second guide 19 has a guide surface shaped like a circular arc substantially equal to that of the circumferential surface of the second bending roll 18. A third bending roll 22 and a third guide 23 are arranged in the exit side of the second bending roll 18 and the second guide 19. The axis of the third bending roll 22 is located substantially in the same level as the axis of the first bending roll 16. The third guide 23 is formed in the same manner as the second guide 19. A fourth bending roll 24 and a fourth guide 25 are arranged in the exit side of the third bending roll 22 and the second guide 23. The axis of the fourth bending roll 24 is located substantially in the same level as the axis of the second bending roll 18. The fourth guide 24 is formed in the same manner as the second guide 19. A fifth bending roll 26 and a fifth guide 27 are arranged in the exit side of the fourth bending roll 24 and the fourth guide 25. A sixth bending roll 28 and a fifth guide 29 are arranged in the exit side of the fifth bending roll 26 and the fifth guide 27. The axis of the fifth bending roll 26 is located substantially in the same level as the axis of the first bending roll 16, whereas the axis of the sixth bending roll 28 is located substantially in the same level as the axis of the second bending roll 18. The guide surfaces of the fifth and sixth guides 27 and 29 are respectively formed in the same manner as in the second guide 19. A seventh bending roll 31 and a seventh guide 32 are arranged in the exit side of the sixth bending roll 28 and the sixth guide 29. The axis of the sixth bending roll 28 is located substantially in the same level as the axis of the second bending roll 18. The axis of the seventh bending roll 31 is located substantially in the same level as the axis of the first bending roll 16. The seventh guide 32 has a guide surface 33 composed of a surface ascending along a circular arc traced by the circumferential surface of the seventh bending roll 31, and another surface extending substantially horizontally to the exit thereof. Accordingly, the cast product 12 is substantially horizontally fed out of the seventh bending roll 31 and the seventh guide 32. These bending rolls (from the second bending roll to the seventh bending roll) 18, 22, 24, 26, 28, 31 are supported rotatably in the same manner as the first bending roll 16. The bending rolls (from the first bending roll to the seventh bending roll) 16, 18, 22, 24, 26, 28 and 31 and the guides (from the first guide to the seventh guide) 15, 19, 23, 25, 27, 29 and 32 are respectively formed of metal.
In short, the second, fourth and sixth bending rolls 18, 24 and 28 are located above the first, third, fifth and seventh bending rolls, so that the path for feeding the cast product 12 is formed zigzag.
Two take-out pinch roll pairs 34 and 35 are arranged in the exit side of the seventh bending roll 31 and the seventh guide 32. The take-out pinch roll pairs 34 and 35 are constructed in the same manner as the feed pinch roll pairs 13 and 14. The take-out pinch roll pairs 34 and 35 serve to take the fed cast product 12 out of the apparatus. A cooler 36 such as a cooling water spray is arranged above each of the second, fourth and sixth bending rolls 18, 24 and 28. The cooler 36 serves to prevent the cast product 12 from softening due to recrystallization during repeated deformation.
Accordingly, the cast product 12 fed from the first and second feed pinch roll pairs 13 and 14 is passed successively by the bending rolls (from the first bending roll to the seventh bending roll) 16, 18, 22, 24, 26, 28 and 31, by which the cast product 12 is repeatedly bent and hardened. Then the cast product 12 thus hardened is continuously moved in the direction of the arrow by the first and second take-out pinch roll pairs 34 and 35 to thereby attain a final product. When repeated bending deformation is given to the cast product 12, the temperature of the cast product 12 rises. Therefore, in order to prevent the cast product from softening due to recrystallization during repeated deformation, the cast product 12 is cooled by the coolers 36 at the respective positions of the second, fourth and sixth bending rolls 18, 24 and 28. In this embodiment, the stengthening apparatus applies bending deformation vertically to the cast product 12. If the cast product 12 taken out of the second take-out pinch roll pair 35 is rotated clockwise or counterclockwise by about 90° about its longitudinal axis and then is put in the first feed pinch roll pair 13 again so that bending is applied to the cast product 12 by the bending rolls (from the first bending roll to the seventh bending roll) 16, 18, 22, 24, 26, 28 and 31, repeated bending deformation can be applied to the cast product 12 both vertically and horizontally with respect to a section of the cast product.
Fig. 2 shows a second embodiment of the present invention. The apparatus 11 as shown in Fig. 1 is constructed so that bending is continuously unidirectionally applied to the cast product 12. Accordingly, when bending should be applied to the cast product 12 in various directions, it is necessary to rotate the cast product 12 by a predetermined angle about the longitudinal axial of the cast product 12 as described above for the purpose of applying bending to the cast product again by the bending rolls (the first bending roll to the seventh bending roll) 16, 18, 22, 24, 26, 28 and 31. However, a strengthening apparatus 41 of the second embodiment shown in Fig. 2 is constructed so that bending can be applied to the cast product 12 in various directions. In short, the apparatus 41 has first bending roll means 42 which are constructed to apply bending vertically to the cast product 12 in the same manner as in the bending rolls of the first embodiment, and second bending roll means 43 which are arranged in the exit side of the first bending roll means 42 and constructed to apply bending horizontally to the cast product 12 which has been subject to the bending in the vertical direction. The first bending roll means 42 are constituted by four vertically bending rolls 44, 45, 46 and 47. The axis of the first and third vertically bending rolls 44 and 46 are located below the axes of the second and fourth vertically bending rolls 45 and 47. The second bending roll means 43 are constituted by four horizontally bending rolls 48, 49, 51 and 52. The axes of these rolls are arranged perpendicularly to the axes of the vertically bending rolls 44, 45, 46 and 47. Also in the four horizontally bending rolls, the axis of the first and third horizontal bending rolls 48 and 51 are located in positions shifted horizontally relative to the axes of the second and fourth horizontal bending rolls 45 and 47. In short, a vertically zigzag path for the cast product 12 is formed by the first bending roll means 42 and, further, a horizontally zigzag path is formed by the second bending roll means 43. Roll shafts 53 of the bending rolls are provided with motors 54 for driving the rolls. Those motors 54 are controlled synchronously by a control means (not shown).
Accordingly, the cast product 12 fed from the first vertically bending roll 44 of the first bending roll means 42 is subjected to bending continuously vertically by the first bending roll means, is fed to the second bending roll means 43 from the first horizontally bonding roll 44 and is subjected to bending continuously horizontally by the second bending means 43. The cast product 12 taken out of the fourth horizontal bending roll 52 has been subjected to bending both vertically and horizontally with respect to a section of the cast product 12. As a result of the bending deformation, a strengthened metal material can be obtained.
For simplification of the drawing, the feed pinch roll pairs, the take-out pinch roll pairs, the guides and the cooler as shown in Fig. 1 are not shown in Fig. 2. However, those parts can be provided easily in the same manner as in the first embodiment. In this case, the feed pinch roll pairs and the take-out pinch roll pairs are driven by motors synchronized with the motors of the bending rolls. Although the second embodiment has shown the case where each of the first and second bending roll means are constituted by four rolls, it is a matter of course that the number of rolls can be changed if necessary. Though not shown, third and fourth bending roll means following the second bending roll means 43 may be added. In this case, for example, those bending roll means may be arranged so that the axes of the rolls of the third bending rolls means are inclined clockwise by an angle of 45° relative to the axes of the rolls of the second bending roll means 43, and, at the same time, the axes of the rolls of the fourth bending roll means are inclined counterclockwise by an angle of 45° relative to the axes of the rolls of the second bending roll means 43, respectively. By such a configuration, continuous bending is applied to the cast product 12 in the four directions (vertically, horizontally, rightward obliquely and leftward obliquely) with respect to a section of the cast product 12, so that the cast product can be hardened due to competition of dislocation in the inside of the cast product. Consequently, the cast product 12 having hardness distributed uniformly over all the length can be taken out constantly. Because the crystal structure of a cast product prepared by continuous casting with a heat mold has a unidirectionally solidified structure of crystals, not always restricted to single-crystals, extended in a casting direction, the cast product has no solidified grain boundary perpendicular to the casting direction which may be a source of growth of cracks due to bending deformation. Accordingly, a strengthened metal material can be obtained without occurrence of cracks by repeatedly applying such bending deformation to the cast product.
To obtain a structural material more strengthened by such a cold bending deformation process, pure metal is too soft. It is therefore necessary to use an alloy containing other metals in the amount free from crystallization of a second phase. In this case, when a weak plate-like second layer is crystallized out of the solidified structure by the addition of ally elements, the second layer is developed into a starting point of a crack generated in the repeated deformation process. However, the fact that the second phase is not crystallized as a solid out of a liquid phase but the cast product temporarily forms a supersaturated single solid solution, is rather suitable for hardening the cast product. Such a supersaturated solid solution alloy cast product can be obtained by quenching the alloy in a continuous casting method using a heat mold.
All the conventional structural materials are prepared as alloys. The conventional materials generally have a surface defect developed into a starting point of destruction due to bending stress or fatigue. In other words, the conventional materials generally have solidified grain boundaries, second-phase crystallized matters such as chemical compounds, and the like, in the surfaces of the materials. The present invention can provide a metal material excellent in reliability, in which there is no solidified grain boundary and no second-phase crystallized matter developed into a starting point of occurrence of lateral cracks in the inside of the material as well as there is no surface defect.
Although the method according to the present invention makes it a rule that the bending deformation working applied to the continuously cast product does not cause reduction in section of the material, it is to be understood that the invention is not limited to the rule and that the invention is applicable to the case where the bending deformation causes somewhat reduction in the section as long as the effect of the invention can be attained.
Further, the metal material strengthening method according to the present invention can be applied not only to materials merely using Al, Cu and Ni but also to any metal materials such as Fe, Co, Mg and the like so long as they are capable of being treated by solution casting and work hardening.
Although the aforementioned embodiments have shown the case where the axes of bending rolls are arranged so as to be shifted respectively relative to the axes of adjacent bending rolls, it is a matter of course that the axes of all the bending rolls may be arranged in one and the same level.
In the following, specific example of the invention and a comparative example are described.
An Al-4.5%Cu alloy molten was cast by a continuous casting method using a heat mold in accordance with the above-mentioned Japanese Patent No. 1049146 to prepare a unidirectionally solidified cast product of linear single-crystals having a 6 mm diameter. On the other hand, a linear cast product consisting of a polycrystal structure of the same Al alloy prepared by a general metal casting method was used as a comparative material.
The tensile strength of the linear single-crystal cast product was 19.4 kg/mm², the Vickers hardness thereof was 64, and the repetition number up to complete breaking thereof, measured by a canti-lever type rotary bending fatigue test machine was 6 X 10⁴ under a load of 6 kg/mm². On the other hand, the tensile strength of the comparative material was 20 kg/mm², the Vickers hardness thereof was 60, and the repeti tion number up to breaking thereof was 3 X 10⁴.
Then, the linear single-crystal cast product and the comparative material were treated as follows by use of an apparatus having seven 12mm-diameter bending rolls arranged as shown in Fig. 1. While the cast product drawn out of the take-out pinch roll pairs was rotated by 90° about its longitudinal axis, the cast product was passed through the bending rolls from the feed pinch roll pairs again. Then, the cast product drawn out of the take-out pinch roll pairs was further rotated by 45° abut its longitudinal axis and then similar bending deformation was applied to the cast product. Lastly, the cast product was further rotated by 90° and then similar bending deformation was applied to the cast product. In short, repeated bending deformation test in the four directions (vertically, horizontally, rightward obliquely and leftward obliquely) was conducted on the cast product.
In the case of the comparative material, cracks occurred in the course of the first-time passage by the rolls, so that the comparative material was broken. The Vickers hardness of the broken portion of the comparative material was 70.
On the contrary, the Vickers hardness of the single-crystal cast product reached 130 after four-times passage by the rolls, and the tensile strength thereof increased to 35 kg/mm².
Further, the repeated bending fatigue test was conducted on the hardened single-crystal cast product. As a result of the test, the repetition number up to breaking reached 1.8 X 10⁶ under a load of 6 kg/mm². The hardened single-crystal product showed remarkable prolongation of fatigue life, compared with the comparative material. Then, the single-crystal material reached a limit of fatigue under a load of 5 kg/mm².
It will be apparent from the result of the test that the strength of an alloy which could not be used as a strength material or a structural material can be increased by the metal material strengthening method according to the present invention which comprises hardening an unidirectionally solidified cast product having no solidified grain boundary by repeated bending treatment, and that the unidirectionally solidified cast product having no solidified grain boundary weak against work can be formed easily from the alloy merely by changing the conventional casting method.
While the present invention has been described with reference to preferred embodiments, it is to be understood that the description thereof is illustrative and not restrictive and that various changes and modifications may be made in the invention without departing from the scope of the appended claims.

Claims

1. A metal material strengthening method in which a continuously cast product consisting of a unidirectionally solidified structure in which crystals are extended long in a direction of casting is passed through a path drawing a zigzag locus to apply repeatedly bending deformation to said cast product to thereby strengthen said cast product.

2. A metal material strengthening method according to Claim 1, in which said path is composed of a first path portion drawing a horizontally zigzag locus and a second path portion drawing a vertically zigzag locus.

3. A metal material strengthening method according to Claim 1, in which said path is arranged zigzag and formed of rotatable rolls having circumferential surfaces being in contact with said cast product.

4. A metal material strengthening method according to Claim 1, in which at least one part of said path is cooled.

5. A metal material strengthening method according to Claim 1, in which said path is arranged so that bending deformation is applied horizontally, vertically and obliquely to a section of said cast product.

6. A metal material strengthening method according to Claim 1, in which said cast product is of single-crystals.

7. A metal material strengthening method according to Claim 1, in which said cast product is of a single solid solution alloy.

8. A metal material strengthening method according to Claim 2, in which said first and second path portions are arranged in different axial directions and formed a plurality of rotatable rolls having circumferential surfaces being in contact with said cast product.

9. An apparatus for strengthening a metal material of a continuously cast product (12) having a unidirectionally solidified structure of crystals extended long in a direction of casting, and apparatus comprising a plurality of rotatable rolls (16, 18, 22, 24, 26, 28, 31; 44, 45, 46, 47, 48, 49, 51, 52) 3arranged at intervals so that said cast product is fed zigzag while being in contact with circumferential surfaces of said rolls so as to be subject repeatedly to bending deformation.

10. A metal material strengthening apparatus according to Claim 9, further comprising a driving source (54) for rotating said rolls synchronously with each other.

11. A metal material strengthening apparatus according to Claim 9, further comprising guide members (15, 19, 23, 25, 27, 29, 32) having guide surfaces and arranged respectively by said rolls at a distance from said circumferential surfaces of said rolls to permit passing of said cast product.

12. A metal material strengthening apparatus according to Claim 9, further comprising a cooler means (36) for cooling said cast product passing by said rolls.

13. A metal material strengthening apparatus according to Claim 9, further comprising at least a pair of feed pinch rolls (13, 14) for feeding said cast product and at least a pair of take-out pinch rolls (34, 35) for taking out said cast product passed by said rolls, said pair of feed pinch rolls being arranged in the entrance side of said rolls, said pair of take-out pinch rolls being arranged in the exit side of said rolls.

14. A metal material strengthening apparatus according to Claim 9, in which said rolls are constituted by at least first and second roll means (42, 43), each of said first and second roll means (42, 43) being constituted by a plurality of rolls (44, 45, 46, 47; 48, 49, 51, 52), said rolls constituting said first roll means being arranged in one axial direction (53), said rolls constituting of said second roll means being arranged in another axial direction (53) different from said one axial direction.