US3666572A - Process for the continuous heat treatment of a low alloy steel wire material - Google Patents

Abstract

A PROCESS FOR CONTINUOUSLY HEAT TREATING A WIRE, ROD, BAR AND STRIP MATERIAL MADE OF LOW ALLOY STEEL SUCH AS A CHROMIUM-VANADIUM, CHROMIUM-SILICON AND BORON STEEL, CHARACTERIZED BY HEATING THE MATERIAL TO A TEMPERATURE NOT LOWER THAN ITS A3 TRANSFORMATION POINT, COOLING THE HEATED MATERIAL AT A SUITABLE COOLING RATE NOT LOWER THAN ITS LOWER CRITICAL COOLING RATE, THE FORMER COOLING RATE BEING SUITABLY DETERMINED DEPENDING ON THE KIND OF STEEL AND THE DIAMETER OR THICKNESS OF THE MATERIAL, TO A TEMPERATURE RIGHT ABOVE ITS MS POINT, AND THEN REHEATING THE COOLED MATERIAL TO A TEMPERATURE NO HIGHER THAN ITS A1 TRANSFORMATION PONT, THEREBY RENDERING THE MATERIAL MAINLY FINE-PEARLITIC IN STRUCTURE; AND A LAOW ALLOY STEEL TREATED BY THE PROCESS.

Description

May 30, 1972 PROCESS FOR THE CONTINUOUS HEAT TREATIIENT AKIRA NAKAGAWA ETAL OF- A LOW ALLOY STEEL WIRE MATERIAL Filed Jan. 21. 1969 FORMATION POINT(MS) Y 400 MARTENSITE TRANS- I LL! I! 3 4 0: l-Ll '0. E ii --c0o| ms RATE CURVE ---REHEATING CURVE TIME (SECOND) Fig.l

United' States Patent-Oflice 3,666,572 Patented May 30, 1972 Int. 01. 621.1 1/18 US. Cl. 148-134 Claims ABSTRACT OF THE DISCLOSURE A process for continuously heat treating a wire, rod, bar and strip material made of low alloy steel such as a chromium-vanadium, chromium-silicon and boron steel, characterized by heating the material to a temperature not lower than its A transformation point, cooling the heated material at a suitable cooling rate not lower than its lower critical cooling rate, the former cooling rate being suitably determined depending on the kind of steel and the diameter or thickness of the material, to a temperature right above its Ms point, and then reheating the cooled material to a temperature not higher than its A; transformation point, thereby rendering the material mainly fine-pearlitic in structure; and a low alloy steel treated by the process.

This invention relates to a novel process for heat treating such materials as wires, rods, bars or strips made of a steel containing, on the basis of weight, 0.l50.65% carbon, at least one of up to 2.20% silicon, up to 2.05% manganese, 0.20-2.70% chromium, (US-3.80% nickel, 0.l52.10% molybdenum, 0.70l.20% aluminum, 050 150 tungsten, 0.10-0.50% vanadium, up to 0.006% boron, up to 0.05% niobium, up to 0.15% titanium and up to 0.15% zirconium, the balance being iron and incidental impurities; said process comprising cooling the materials such as the wires, rods, bars or strips to a temperature immediately above the Ms point thereof at a suitable cooling rate corresponding to the kind of steel the materials are made of and the diameter or thickness of the material and then reheating the thus-cooled materials. More particularly, this process is characterized by, while passing the materials continuously through a heat-treating apparatus, heating the materials at a temperature above their A trans-formation point for a suitably short time, this time being determined by calculating on the basis 0.10-2.0 minutes per mm. of diameter or thickness of the material, cooling the thus-heated materials to a temperature immediately above their Ms point at a cooling rate not lower than their lower critical cooling rate, this rate varying with the kind of steel the materials are made of and the diameter or thickness of the materials and then reheating the materials so cooled to a temperature below their A, transformation point, thereby allowing the materials to become mainly fine pearlite in structure.

Wires, rods, bars and strips made of such low alloy steel, when treated by the conventional heat treating processes, have a low strength because they are annealed in a bundle or coil and have a poor cold workability because they become spheroidal cementite (the carbide being spheroidized) or coarse pearlite in structure.

The following Table 1 shows the results obtained from the test in which wire materials made of a chromiumvanadium (Cr-V) alloy steel and heat treated in the conventional manner, were drawn into finer wires which were then tested for tensile strength, reduction of area and elongation. The table shows that some of the finer wires so obtained, which are coarse pearlite in structure, remarkably increase in toughness and are unable of being further drawn when drawn to such an extent that a reduction of area of approximately 60% is reached, and that the remaining finer wires, which are spheroidal cementite in structure, also behave in this manner when drawn to an extent that a reduction of area of nearly 67% is obtained.

On the other hand, the process of this invention is different from conventional processes and comprises continuously heat treating wires, rods, bars and strips to provide them with superior subsequent workability although they can have excellent toughness and strength without any other treatments after the heat treatment.

The conventional processes for the continuous heat treatment of wires include a patenting process for the manufacture of cold-drawn wires such as music spring steel wires and other hard drawn wires, an oil quenching and tempering process for the manufacture of various oil tempered wires, and the like. Of those conventional processes, the patenting process can be applied only to carbon steel wires and it comprises heating a carbon steel wire to a predetermined temperature above the A transformation point thereof and then allowing the thus-heated steel to cool in the air or keeping it in a bath of molten lead to effect its semi-isothermal transformation. And the oil quenching and tempering process comprises heating a wire likewise to a temperature above the A transformation point thereof, quenching the heated wire in oil and then tempering the quenched Wire at a predetermined temperature. In these processes, wires in a coil being heat-treated are placed on the pay-off stand of a heat treating apparatus, thereafter pulled at their end to pass them through the heat treating unit of the apparatus and then wound on a take-up block.

Now, the process of this invention comprises heating low alloy steel wires, rods, bars and strips to a temperature above the A transformation point thereof, thereafter cooling the heated materials at a cooling rate not lower than the lower critical cooling rate thereof to a temperature immediately above the Ms point thereof by using suitable means and then reheating the cooled materials to a temperature not higher than the A transformation point thereof. This is accomplished, as in the case of the wires treated by the patenting and the continuous oil quenching and tempering process previously mentioned, by heating, cooling and reheating the starting wires in a continuous way, and then rewinding the thustreated wires on a take-up block, while introducing the starting wires on the pay-off stand to a heat treating apparatus. The process of this invention has advantages in that it requires a shorter time to heat treat materials to be treated and can treat several tons of materials such as wires simultaneously thereby rendering itself very economical and providing the materials with superior mechanical properties and cold workability.

The process of this invention is more particularly de scribed as follows:

It comprises austenitizing a starting low alloy steel material in a short time, continuously cooling at a suitable cooling rate corresponding to the kind and diameter or thickness of the material to a temperature immediately above the Ms point thereof by, for example, allowing the austenitized material to cool in the air or subjecting it to cooling with forcibly circulated air or with heated air so that the cooled material has a singleor multiphase structure such as supercooled austenite, supercooled austenite-bainite, supercooled austenite-bainite-ferrite, supercooled austenite-bainite-ferrite-pearlite or the like, and then reheating the cooled material to a temperature not higher than the transformation point A thereof to transform the supercooled austenite to fine pearlite or the coexisting bainite to secondary pearlite. The structure thus obtained is fine perlite and is dilferent from spheroidal cementite, coarse pearlite and the like, and the material having the fine pearlite structure is superior not only in toughness, strength and severe workability but also in capacity of being cold-drawn or cold-rolled, thereby allowing it to be drawn into a material having a smaller diameter or thickness with less frequent intermediate heat treatments.

Of alloy steels which may be treated by the process of this invention, Cr-V steel, Cr-Si steel, boron steel, Mi-Mn steel and Mn-Cr steel are shown with the range of composition thereof in Table 2.

This invention will be better understood by the following examples.

EXAMPLE 1 A Cr-V steel wire having a diameter of 9 mm., was continuously passed through a heat treating apparatus, and was simultaneously heated to a soaking temperature of 920 C. higher than its A transformation point and maintained at this temperature for 2.2 minutes, cooled in the air to a temperature of 455 C. immediately above its Ms point, reheated to a temperature of 550 C. below its A transformation point for a period of 2.8 minutes and then allowed to cool in the air. The wire so treated according to this invention was cold drawn to the extent that its original diameter 9.0 mm. was reduced finally to 2.9 mm., during which test pieces having the respective diameters of 7.4, 5.7, 4.1, 3.7 and 2.9 mm. were sampled. The samples were then tested for tensile strength, reduction of area, elongation and Vickers hardness; and the remainder of the treated wire, which was 9 mm. in. diameter and was not subjected to any further treatments such as drawing after the heat treatment, was then tested in the same manner as above. The particular chemical composition of the wire used, the operational conditions for heat treating it, and the relationship between the change in diameter and that in mechanical properties thereof. are respectively shown in Tables 3, 4 and 5.

EXAMPLES 2, 4, 5 AND 6 In these examples the same heat-treatment procedure was followed as in Example 1, but using Cr-Si steel. boron steel, Si-Mn steel and Mn-Cr steel wire samples the chemical compositions of which are respectively shown in Table 3 and under the operational conditions respectively shown in Table 4. The wire samples so heat treated were further treated in quite the same way as shose in Example 1 to obtain mechanical properties which are shown in Table 5.

Table 5 shows that in spite of the heat treatment for a shorter period of time, the heat-treated wire samples which had been drawn to the extent that the reduction of area thereof reached the values between approximately 87 and 93% at the end of the cold drawing, still indicated 4 the final reduction of area of 20 45% and elongation of 1.5-3.0% which represent excellent cold workability.

EXAMPLES 7-12 This invention can also be applied to the heat treatment of wires having a smaller diameter. This was confirmed and proved true by heat treating the wire samples, the chemical compositions of which are respectively shown in Table 3, under the conditions shown in Table 6, cold drawing the heat treated samples at ambient temperatures, testing the drawn samples for mechanical properties which are shown in Table 7 and then comparing the results of this table with those of Table 5.

The effects of the heat treatment of this invention will also be illustrated by the following examples.

EXAMPLES 13-18 In these examples, wire samples of Cr-Si steel having the same composition as the one shown in Table 3 were heat treated under the conditions indicated in Table 8. The mechanical properties of the wire samples thus treated are also shown in Table 8, from which it is seen that all the starting samples have each come to have excellent capacity of being drawn after the heat treatment at the different temperature not lower than the transformation point thereof during the different period of treating time. It is also seen from Table 8 that the soaking temperature and time which may practically be employed in this case, are the temperature from the A transformation point of Acm transformation point up to approximately 950 C. and the time of nearly two minutes per 1.0 mm. of diameter of the wire to be treated, respectively, that the use of temperatures and times exceeding the aforesaid ones will render the process less economical and tend to cause decarbonization, other surface defects and the like of the wire and that such treated wires as shown in this table may find their use in the industrial field because they even have superior mechanical properties without further treatment such as cold working.

FIG. 1 is a diagram showing the continuous cooling transformation of a Cr-Si steel wire, of a 4 mm diameter which contains 0.54% carbon, 1.43% silicon, 0.53% manganese, the balance consisting substantially of iron and which has been heat treated at a temperature of 900 C. for 5 minutes before cooling; and FIG. 2 is an enlarged photograph (magnification: 8000) showing the fine pearlite structure of a Cr-Si steel wire heat treated by the process according to this invention, which process comprises heating a starting Cr-Si steel wire of a 4 mm. diameter having the same composition as the Cr-Si steel in Table 3 at 880 C. for 1.6 minutes, air-cooling the heated wire to 375 C. and then reheating the cooled wire at a soaking temperature of 600 C. for 2 minutes to produce said heat-treated Cr-Si steel wire.

EXAMPLES l930 In the examples, test pieces of each of a Cr-Si steel wire and a Cr-V wire, the wires being 4.5 mm. in diam eter, were heat treated under the same conditions except that they were reheated at the respective soaking temperatures different from one another within the range of 650 C. to 400 C., as shown in Table 9, in order to find the variation of the thus-treated test pieces in mechanical properties.

The compositions of the steel wires used were as follows:

Cr-Si steel wire: 0.53% C, 1.58% Si, 0.78% Mn, 0.012% P, 0.012% S, 0.67% Cr, the balance consisting of Fe and impurities.

Cr-V steel wire: 0.51% C, 0.22% Si, 0.81% Mn, 0.011% P, 0.009% S, 0.93% Cr, 0.19% V, the balance con sisting of Fe and impurities.

TAB LE 9 [Variation of Cr-Si steel and Cr-V steel in mechanical properties with change in temperature at which the steel is reheated] Heating conditions Reheating conditions Mechanical properties Soaking Soaking Cooling Soaking Soaking Ex. temp. C.) time (min.) condition temp. 0.) time (min.) T.S. (kg/mm E. ILA/I.

Air-cooled Remarks: 'I.S., E. and R.A.'1. are as defined in Table 5.

EXAMPLES 31-33 Test pieces of a Cr-Si steel wire having a 4.5-mm. diameter and the same composition as the Cr-Si steel wire bainite-ferrite, when the wire samples heated are cooled :to a temperature immediately above the Ms point thereof.

TABLE 1 [Variation oi meehanical properties of Cr-V steel wire with change in diameter thereof made by drawing 1t--the Cr-V steel wire being heat treated by the conventional process] Diameter (min.)

shown in Table 9, were heat treated under the respective difierent conditions except that they were reheated at the same soaking temperature, as indicated in Table 10. It will be seen from the results shown in the table that such a low alloy steel can be provided with excellent mechanical properties by using a suitable soaking time in the heating and reheating steps even if a soaking temperature in the heating step is greatly varied.

TABLE 10 Remarks (l) The Cr-V steel wire used as the starting material contained 0.53 C, 0.30 Si, 0.74 Mn, 0.013 P, 0.014 S, 1.07 Cr, 0.20 V, the baalnoe consisting of Fe and incidental impurities.

(2) Some of the test pieces of the starting wire were [Efiects of heating conditions (soaking temperature and time) on mechanical properties of Cr-Si steel] Reheating conditions Heating conditions Soaking Soaking 1 Soaking Soaking Cooling temp. e '1.S. E. R.A.T. Ex. temp. C.) time (min.) condition 0.) (min.) (kgJmmfl) 31 880 1. 8 Air-co3%l5ed to 2. 2 112. 2 11.0 55.3

Remarks: T.S., E. and R.A.T. are as previously defined.

Referring to FIG. 1, curves A, B and C are the cooling rate curves which each show that the heat treated Cr-Si steel sample varies in structure with its diameter or thickness, kind of steel and cooling rate after heating. Cunve A indicates the formation of a supercooled austenite, B the formation of a supercooled austenitebainite and C the formation of a supercooled austenite- TABLE 2 [Chemical compositions of alloy steels which may be used in the practice of this invention (percentfl Kind of alloy steel Si MN P 8 Cr V B Cr-V steel 0. 45-0. 55 0. -0. 0. 65-0. 95 Up to 0.035 Up to 0.035 0. 80-1. 10 0.15-1.10 Cr-Si steel 0. -0. 1. 20-1. 50 0. 50-0. 80 Up to 0.040 Up to 0.040 0. 8 Bow nstee! 0. 50-0. 60 0. 150. 35 0.65-0.95 Up to 0.035 Up to 0.035 Si-Mn steei 0. 550. 1. 50-2. 20 0. -1. 00 Up to 0.035 Up to 0.035 Mn-Cr steel 0. 50-0. 60 0.15-0.35 0. 6H. 95 Up to 0.035 Up to 0.035

TABLE 3 [Chemical compositions of alioy steels used in the examples (percentfl Kind of alloy steel 0 Si Mn P S 01' V B Ms point C.)

Cr-V steel 0.50 0. 21 0.77 0. 014 011 318 Cr-Si steel 0. 55 1.37 0. 63 0. 009 .006 319 Boron steel 0. 57 0. 23 0. 87 0.015 009 298 Si-Mn steel 0.61 1. 71 0. 82 0. 018 314 Mn-Cr steel 0. 54 0. 22 0. 76 0. 014 320 TAB LE 4 [Heat treating conditions for wires having a larger diameter] Dtiami- Heating conditions Reheating conditions e er 0 wire Soaking Soaking Soaking Soaking Example Kind of steel (mm) temp. 0.) tune (m1n.) Mode of coohng temp. 0.) time (111111.)

1 Cr-V steel 9. 0 920 2. 2 Air-cooled to 455 C. 550 2. 8 2 Cr-Si steel 8. 0 875 2.0 Air-cooled to 420 C. 570 2. 5 4 Boron steel 7. 5 870 2. 0 Air-cooled to 405 C. 610 2. 5 5 S1-Mn steel 9. 5 920 2. 2 Fureibly cooled to 315 C. 570 2. 8 6 Mn-Cr steel 9. 5 900 2. 5 Air-cooled to 390 C. 570 3, 1

TABLE 5 {Variation of mechanical properties of wires with change in diameter thereof by drawing them-the Wires being heat treated according to this invention] D.W. (111111.) 9. 0 7. 4 5. 7 4. 1 3. 7 2. 0 R.A.D 0%) 0 33. 0 60. 4 79. 3 83. 1 89. 0

(not drawn) Cr-V T.S. (kgJmm. 109.0 128.0 146.0 165. 0 169.0 173. 0 1 steel R.A.T. (%J) 46. 5 41.0 38.5 35. 0 34.5 36 0 E. 4. 2 3. 7 3.5 3. 5 2.5 2.2 V.H. 335 425 445 505 515 530 D.W. (111111.) 8.0 6.4 5.0 4. 2 3.4 2. 7 R.A.D- 0 36. 0 61. 0 72. 3 82. 0 88. 6

(not 111 wn) E 2 Cr-Si T.S. (kg. mm. 109. 5 123.6 139.6 147. 0 157.5 170. 1 steel 11.1111. 58.1 53. 5 es. 2 so. 0 43. 2 s9. 2 E. 0%) 15. 0 5. 5 5.3 4. 5 3. 0 2. 0

V.H. 299 379 412 424 439 467 D.W. (111111.) 7. 5 6.0 4. 7 3. 9 3. 2 2. 5 RA. 0 35. 1 60. 7 73.0 81.8 88. 9

(not drawn) E 4 Boron T.S. (kgJmm. 107.5 138.8 145. 2 154. 1 163.3 170. 5 steel R.A.T. 51. 5 48. 5 48. 7 42. 7 40. 0 E. 13. 0 6. 0 4.3 3. 5 2.3 2.0 17.11. 320 409 430 448 470 493 D.W. (mm.) 9 5 7. 5 6.0 4. 5 3.9 3. 4 R.A.D. (75) 0 37. 7 60. 2 77. 5 83. 2 87. 2

(not drawn) E 5 Si-Mn 'I.S. (kgJmm. 105. 133. 2 142. 7 155.0 162. 5 169. 0 51.661 11.111. 46 5 48.5 45.2 45.0 40.8 41.8 E. 15 0 5.0 4.5 4. 3.1 2.0 V.H. 317 394 425 455 473 497 DW.( 9.5 75 6.0 4.5 3.9 3.4 R A.D. 0 37. 7 60. 2 77. 5 83. 2 87. 2

(not drawn) E 6 Mil-Cr T S. (kgJmmJ) 99. 2 127.3 7 5 150. 5 160.4 167.4 steel R.A.T. 57. 3 50. 0 55. 0 55. 0 45. 0 41. 5 E 14. 5 5. 5 3.8 2.3 2. 0 V.H. 288 374 401 433 456 479 Remarks: D.W.:Diameter of wire obtained.

R.A.D.=Reduction of area reached at the end of drawing. T.S.=Te11sile strength. R.A.T.=Reduct1on of area obtained in tension test. 10.: Elongation. V.H.:Vickers hardness.

TABLE 6 [Heat treating conditions for wires having a smaller diameter] Heating conditions Reheating conditions Diameter of Soaking Soaking Soaking Soaking Example Kind of steel wire (min.) temp. 0.) time (min.) Mode of cooling temp. 0.) time (min.)

7 Cr-V steel 4. 5 900 2. 6 Air-cooled to 405 C. 550 3. 3 8 (Jr-Si steel 2. 900 0. 8 Air-cooled to 370 C. 600 1. 0 10 Boron steel 5. 6 900 1. 4 Air-cooled to 355 C. 580 1. 8 11 Si-Mn steel 3. 2 880 1. 2 Air-coo1ed to 365 C. 570 1. 12 Mn-Cr steel 5. 6 880 1. 4 Air-cooled to 380 C. 600 1. 8

TAB LE 7 [Variation of mechanical properties of wires with change in diameter thereof by drawing them-the wires being heat treated according to this invention] D.W. (111111.) 4. 5 3. 5 3. 0 2. 85 l. 5 1. 2 R.A.D. 0 36.0 55. 0 60. 0 88. 9 92. 8 Ex. 7 Or-V (not drawn) Steel T.S. (kgJmmJ) 105.0 188. 0 147.0 150. 0 198. 2 215. 2 R.A.'1. 53. 5 46. 5 43. 5 43. 0 38.5 20. 0 V.H. 340 400 410 415 565 575 D.W. (min.) 2. 0 1. 6 1. 3 1. 2 1.1 1. 0 R.A.D. 0 36. 0 57. 7 64. 0 69. 7 75. 0

0t drawn) Ex. 8 Cr-Si T.S. (kg/mm 114. O 140. 6 148. 2 150. 6 156. 5 164. 5 steel R.A.T. 32. 0 39.0 28. 0 26. 5 23. 4 21. 1

D.W. 5. 6 3. 6 2. 9 2. 35 1. 9 1. 7 R.A.D 0 58. 6 73. 0 82.0 88. 5 90. 8 Ex. 10 Boron ot drawn) steel T.S. (kgJmmfi) 121. 5 158. 0 168. 0 179. 5 196. 5 206. 0 3.11.1. 55. 3 50. o 50. o 57. 9 45. s 45. o V.H. 0 476 498 500 508 515 D.W. (111111.) 3. 2 2. 75 2.05 1. 65 1. 35 1. 0 R.A.D. 0 26. 0 58.9 73. 3 82. 7 90. 8 Ex. 11 Si-Mn ot drawn) Steel T.S. (kg/mm 108. 2 136. 0 149. 0 159. 0 168. 4 187. 0 R.A.'I. 51. 5 40. 2 47. 8 55. l 47. 5 46. 0 V.H. 312 425 465 470 480 486 D.W. (111.111.) 5.6 4.5 3.6 2.9 2. 35 1.7 R.A.D. 0 25. 5 58. 6 73. 0 82. 0 90. 8 Ex. 12 Mn-Cr (not drawn) steel T.S. (kgJmm 99. 3 124. 0 136. 0 146. 5 157. 5 175. 5 R.A.1. 56. o 43. 0 45.8 53. s 55. 1 50. o V.H. 297 382 408 424 432 459 Remarks: D.W., R.A.D., T.S., R.A.'1., and V.H. are as defined in Table 5.

TABLE 8 [Effects of heat treating conditions] Wireshaving 8-mm. diameter Wire so heat treated and then heat treated according to this drawn from 8-mm. to 2.7 mm

Heating conditions Reheating conditions invention in diameter (R.A.D.: 88.6%)

Soaking Soaking Soaking I Soaking T.S. R.A.T. E. T.S. R.A.T. E Ex temp. 0.) time (min.) Cooling temp. 0.) time (min.) (kg/mini) (kgJmmJ) 13 800 4 Aitggoslsdc 570 5. 0 113. 3 52. 0 14. 2 179. 2 38. 3 1. 8

o 14 800 6 Air-cooled 570 7. 5 122. 8 49. 2 11. 1 190. 0 81. 4 1. 5

to 360 C 15 875 2 mtr-caogt l ed3 570 2. 5 109. 5 58. 1 15.0 170. 1 39. 2 2. 0

o 16 875 4 Aig-crgg ed 570 5.0 122. 1 50. 3 11. 0 189. 7 31. 7 1. 5

o 17 950 1 Aihcggialgta 570 1. 110. 4 57. 7 16. 2 172. 2 30. 0 2. 2

o 18 950 2 Air-cooled 570 2. 5 117. 8 54. 3 12. 8 184. 0 30. 2 2. 0

Remarks: T.S., R.A.T., E., and R.A.D. are as defined in Table 5.

What is claimed is:

1. A process for heat treating a low alloy steel wire, rod, bar or strip material wherein the material to be treated is made of a Cr-V-steel consisting essentially of 0.45 to 0.55% carbon, 0.15 to 0.35% silicon, 0.65 to 0.95% manganese, 0.80 to 1.10% chromium, 0.15 to 0.25% vanadium, the balance consisting of iron and incidental impurities, characterized by firstly heating the material at a temperature not lower than the A transformation point there of and keeping it at that temperature for a suitably short time, determined on the basis of 0.12.0 minutes per mm. of the diameter or thickness of the material to be treated; cooling the heated material at a specific cooling rate not specific cooling rate being determined according to the kind of steel and the diameter or thickness of the material, to a temperature immediately above the Ms point thereof; then reheating the cooled material at a tempera ture not higher than the A transformation point thereby rendering the material mainly fine-pearlitic in structure, the temperature not lower than the A point is 820 C. to 950 C., the temperature above the Ms point is more than 318 C. to 418 C. and the temperature not higher than the A point is 500 to 650 C.

2. A process for heat treating a low alloy steel wire rod, bar or strip material wherein the material to be treated is made of a Cr-Si steel consitsing essentially of 0.50 to lower than the lower critical cooling rate thereof, the 0.60% carbon, 1.20 to 1.60% silicon, 0.50 to 0.80% manganese, 0.50 to 0.80% chromium, the balance consisting of iron and incidental impurities, characterized by firstly heating the material at a temperature not lower than the A transformation point thereof and keeping it at that temperature for a suitably short time, determined on the basis of 0. 12.0 minutes per mm. of the diameter or thickness of the material to be treated; cooling the heated material at a specific cooling rate not lower than the: lower critical cooling rate thereof, the specific cooling rate being determined according to the kind of steel and the diameter or thickness of the material, to a temperature immediately above the Ms point thereof; then reheating the cooled material at a temperature not higher than the A transformation point thereby rendering the material mainly fine-pearlitic in structure, the temperature not lower than the A point is 820 C. to 950 C., the temperature above the Ms point is more than 319 C. to 419 C. and the temperature not higher than the A point is 500 to 650 C.

3. A process for heat treating a low alloy steel wire rod, bar or strip material wherein the material to be treated is made of a boron steel consisting essentially of 0.50 to 0.60% carbon, 0.15 to 0.35% silicon, 0.65 to 0.95% manganese, 0.65 to 0.95% chromium, not less than 0.0005% boron and the balance consisting of iron and incidental impurities, characterized by firstly heating the material at a temperature not lower than the A transformation point thereof and keeping it at that temperature for a suitably short time, determined on the basis of 0.1-2.0 minutes per mm. of the diameter or thickness of the material to be treated; cooling the heated material at a specific cooling rate not lower than the lower critical cooling rate thereof, the specific cooling rate being determined according to the kind of steel and the diameter or thickness of the material, to a temperature immediately above the Ms point thereof; then reheating the cooled material at a temperature not higher than the A transformation point thereby rendering the material mainly fine-pearlitic in structure, the temperature not lower than the A point is 820 C. to 950 C., the temperature above the Ms point is more than 298 C. to 398 C. and the temperature not higher than the A; point is 500 to 650 C.

4. A process for heat treating a low alloy steel wire rod, bar or strip material wherein the material to be treated is made of a Si-Mn steel consisting essentially of 0.55 to 0.65% carbon, 1.50 to 2.20% silicon, 0.70 to 1.0% manganese, the balance consisting of iron and incidental impurities, characterized by firstly heating the material at a temperature not lower than the A transformation point thereof and keeping it at that temperature for a suitably short time, determined on the basis of 0.12.0 minutes per mm. of the diameter or thickness of the material to be treated; cooling the heated material at a specific cooling rate not lower than the lower critical cooling rate thereof, the specific cooling rate being determined according to the kind of steel and the diameter or thickness of the material, to a temperature immediately above the Ms point thereof; then reheating the cooled material at a tern perature not higher than the A transformation point thereby rendering the material mainly fiue-pearlitic in structure, the temperature not lower than the A point is 820 C. to 950 C., the temperature above the Ms point is more than 314 C. to 414 C. and the temperature not higher than the A point is 500 to 650 C.

5. A process for heat treating a low alloy steel 'Wire, rod, bar or strip material wherein the material to be treated is made of a Mn-Cr steel consisting essentially of 0.50 to 0.60% carbon, 0.15 to 0.35% silicon, 0.65 to 0.95% manganese, 0.65 to 0.95 chromium, the balance consisting of iron and incidental impurities, characterized by firstly heating the material at a temperature not lower than the A transformation point thereof and keeping it at that temperature for a suitably short time, determined on the basis of 0.1-2.0 minutes per mm. of the diameter or thickness of the material to be treated; cooling the heated material at a specific cooling rate not lower than the lower critical cooling rate thereof, the specific cooling rate being determined according to the kind of steel and the diameter or thickness of the material, to a temperature immediately above the Ms point thereof; then reheating the cooled material at a temperature not higher than the A transformation point thereby rendering the material mainly fine-pearlitic in structure, the temperature not lower than the A point is 820 C. to 950 C., the temperature above the Ms point is more than 320 to 420 C. and the tem perature not higher than the A; point is 500 to 650 C.

References Cited UNITED STATES PATENTS 3,458,365 7/1969 Nickola et al. 148-143 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

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