Steel suitable for induction hardening

Abstract: A steel having such a good machinability that it can be directly cut without being annealed and a good induction hardenability. The steel consists essentially of C: 0.38 - 0.45 %, Si: up to 0.35 %, Mn: more than 1.0 % - up to 1.5 %, B: 0.0005 - 0.0035 %, Ti: 0.01 - 0.05 %, Al: 0.01 - 0.06 % and the balance of Fe, the content of N being up to 0.010 %, and has a fine structure of ferrite crystal grain size mumber 6 or more defined by JIS-G0552. In addition to the above basic composition, the alloy may further contain some optional alloying elements. The material is suitable for manufacturing machine structural parts such as drive shafts of automobiles.

Description: The present invention concerns a steel for induction hardening, more specifically, a steel which can be processed as rolled without being annealed by cutting or form rolling and is suitable for induction hardening. Taking a drive shaft with a homokinetic joint for automobiles as an example, it is manufactured in accordance with the steps of annealing or spheroidal annealing a steel for rolling such as SAE1541 to increase the machinability thereof, processing the steel by cutting or form rolling and strengthening the surface by induction hardening. The SAE1541 steel, however, has poor machinability as rolled, and therefore, it is difficult to process the steel without heat treatment. Thus, this steel is not a suitable material from an economical point of view. As the automobiles are getting more light-weighted and high-powered, the drive shafts should have higher strength. On the other hand, it is demanded that annealing be eliminated to enable direct cutting of the rolled steel at the request of cost reduction. To meet the request, there was proposed a steel having a composition in which manganese content of the SAE1541 steel is reduced to improve the machinability. The steel, however, has a drawback that the induction hardenability is low and the depth of surface hardened layer fluctuates. US-A-4537644 discloses a steel which consists essentially of 0.15 to 0.50% of C, up to 1.50% of Si, 0.20 to 1.50% of Mn, up to 2.00% of Cr, 0.0005 to 0.0030% of B, 0.005 to 0.10% of acid-soluble Al, up to 0.010% of P, up to 0.0020 % of N, 0.010 to 0.050% of Ti and the balance iron and inevitable impurities, and which has a tempered martensitic structure obtained by quenching and tempering. EP-A-0265402 discloses a calcium-treated steel alloy, the composition of which in percentages by weight is 0.01 - 0.5% carbon, 0.01 - 2.0% silicon, 0.4 - 1.6% manganese, 0.01 - 0.3% sulphur, 0 - 2.0% chromium, 0 - 0.8% molybdenum, 0.002 - 0.02% boron, 0 - 0.015% titanium, 0.005 - 0.014% aluminium, 0.0015 - 0.01% calcium and in which B = (0.5 - 2) x (N - Ti/3.4), wherein B is the boron content, N the nitrogen content and Ti the titanium content in percentages by weight. It would be desirable to provide a steel for induction hardening which has such a good machinability that it can be directly cut without being annealed as well as a good induction hardenability and thus, contributes to the improvement in strength of machine structural parts. The steel for induction hardening of the present invention comprises the following alloying elements: C: 0.37 to 0.45%, Si: 0 to 0.35%, Mn: more than 1.0% and up to 1.5%, B: 0.0005 to 0.0035%, Ti: 0.01 to 0.05%, Al: 0.01 to 0.06%, Cr: 0 to 1.0%, Mo: 0 to 0.20%, Ni: 0 to 1.0%, V: 0 to 0.30%, Nb: 0 to 0.10%, N: 0.005 to 0.022%, but not more than the equivalent to the total Al and Ti in the steel, the balance being Fe, the steel having been prepared by rolling at a temperature of not more than 1,100°C and finishing at a temperature of not more than 950°C, with a reduction in area of at least 70%, and having a fine structure of ferrite crystal grain size number 6 or higher as defined by JIS-G0552. The steel of the above basic alloy composition may further contain at the expense of the balance element Fe one or more of the following alloying elements: Pb: 0.01 - 0.20 %, S: 0.005 -0.30 %, Bi: 0.01 -0.10 %, Te: 0.0005 - 0.10 % and Ca: 0.0003 - 0.0050 %. DETAILED EXPLANATION The fine structure of ferrite crystal grain size of 6 or more can be attained by rolling the steel having one of the above alloy compositions at a relatively low temperature and under a high reduction of area. More specifically, the rolling is carried out at a heating temperature up to 1,100°C, with a finishing temperature of up to 950°C, and under a reduction of area of 70 % or more. The depth of decarburization of the rolled material (defined by JIS-G0588) is preferably up to DM-T:0.20 mm. If the depth of decarburization is too large, effect of the induction hardening will be slight and formation of the surface hardened layer is dissatisfactory, and further, deeper cutting will be necessary. The following is the reasons for choosing the alloy compositions of the present invention as noted above: C: 0.3⁷ - 0.45 % Carbon content of 0.37 % or more is necessary for maintaining the strength required for the structural parts. As the carbon content of the steel increases machinability and processability in form rolling decrease, and sensitivity of quenching crack and hardness due to rolling increase. Thus, a suitable content should be chosen in a range up to 0.45 %. Si: up to 0.35 % Silicon is used in a certain amount as a deoxidizer. In order to suppress increase in the hardness caused by rolling low, the amount of addition must be in the above limit. Mn: 1.0 - 1.5 % To attain the induction hardenability manganese of 1.0 % or more is necessary. On the other hand, increase of manganese content lowers machinability and processability in form rolling and heightens sensitivity of quench cracking. 1.5 % is thus the upper limit. B: 0.0005 - 0.0035 % Boron is important as the component improving hardenability without significant increase of hardness as rolled condition. The effect is appreciable at such a low content as 0.0005 % or so, and saturates as the content increases. With a higher content of boron, hot workability becomes low, and therefore,the addition is limited to 0.0035 % or less. Ti: 0.01 - 0.05 %, Al: 0.01 - 0.06 % Both of the elements have a function of fixing nitrogen and oxygen contained in the material. Solid-dissolved nitrogen forms BN to decrease the hardenability-improving effect of boron. If, however, titanium or aluminum is contained, formation of TiN or AlN occurs preferentially and the effect of boron is thus maintained. For this purpose, addition of at least 0.01 % of either element is necessary. On the other hand, too much addition is meaningless, and from consideration of the cleanliness of the steel, the upper limits of 0.05 % for Ti and 0.06 % for Al are set. N: 0.005 to 0.022%. As noted above, nitrogen forms BN to prevent improvement in the hardenability; it is important to limit the content of nitrogen to such an amount as not exceeding the equivalents to titanium and aluminum. It is not preferable to fix a large amount of nitrogen with a large amount of titanium, because this results in increase of TiN-based non-metallic inclusions. The groups of alloying elements optionally added as mentioned above have the following effects, and limitations of the amounts of the elements are as follows: One or more of Cr: up to 1.0 %, Mo: up to 0.2 % and Ni: up to 1.0 % Any of these elements may be used in the above noted limits when a further improvement of hardenability is desired. Too much addition not only results in no furhter increase of the effect, but also lowers the machinability and processability in form rolling. One or two of V: up to 0.3 % and Nb: up to 0.1 % These elements are added in case where further reduction of the crystal grain size is contemplated. Addition in the amounts exceeding the above limits will give no increase of the effect. One or more of Pb: 0.01 - 0.20 %, S: 0.005 - 0.30 %, Bi: 0.01 - 0.10 %, Te: 0.0005 - 0.10 % and Ca: 0.0003 - 0.050 % Needless to say, these are the elements added in case where particularly high machinability is required. Addition in the amounts more than the lower limits will give the effect. The upper limits are set in view of deterioration of the mechanical properties of the steel. The reason why the structure of the steel of the present invention must be of such fine grains as ferrite crystal grain size 6 or more is to ensure required toughness of the products. EXAMPLES Low temperature rolling was carried out on the alloy steels having the compositions shown in Table 1 under the condition of heating temperature 1,050°C, finishing temperature 850°C and reduction of area 97 % to prepare round rods of diameter 32 mm. (Only Control Run 5 was operated under the condition of heating temperature 1,250 °C, and finishing temperature 1,050°C.) The samples were subjected to the tests of the conditions shown below: Ferrite Crystal Grain Size Number JIS-G0552 Machinability: Drilling. When the drill abrades to cut no longer, the tool is regarded to come to the end of life. Tool: SKH51, diameter 5 mm, 118° Feed Rate: 0.1 mm/rev Depth of Holes: 20 mm (blind hole) Induction Hardenability Test Piece: diameter 30 mm, length 100 mm Frequency: 8 KHz Out Put: 200 KW Transfer Rate: 6 mm/sec Effective Depth of Hardened Layer: Hv 400 Twist Strength Test Piece: diameter 30 mm, length 450 mm Toughness JIS-Z2242 The test results are shown in Table 2. The data of Example runs in Table 2 show that the invented steels have such a good machinability that they can be directly cut or processed by form rolling without being annealed, and such a good hardenability that they may obtain a satisfactory hardness by induction hardening. On the other hand, the data of control runs show that the control steels are inferior to the invented steels. (The underlines in Table 2 indicate the inferior properties.) In detail, Control No.1 has a hardened depth shallower than those of the invented steels owing to the lower Mn-content. Control No.2 exhibits a deep hardened depth and a high twist strength, however, the machinability is extremely low. Control No.3, which contains corbon in the amount smaller than the lower limit of the invention, has a very low twist strength. The increased nitrogen content of Control No.4, which is higher than those of the invented steels, results in a shallower hardened depth. Toughness of Control No.5 is lower than those of the invented steels because of the large ferrite grain size. The steel, therefore, enables enjoying high productivity and low cost in production of various products inclusive of the above mentioned homokinetic joint. No. C Si Mn S B Ti Al N Others Example Runs 1 .42 .25 1.05 .016 .0013 .017 .022 .005 2 .39 .13 1.30 .021 .0015 .040 .021 .022 Ni .10, Cr .05 3 .38 .09 1.45 .035 .0018 .038 .017 .011 4 .42 .23 1.03 .025 .0012 .034 .035 .016 Ni .07, Cr .50 5 .44 .31 1.18 .022 .0025 .030 .031 .013 Cr .12, Mo .09 6 .41 .25 1.05 .013 .0015 .025 .020 .006 Ni .85 7 .39 .23 1.04 .028 .0015 .040 .021 .022 Ni .10, Cr .05 V .15 8 .37 .25 1.22 .034 .0018 .038 .017 .010 Nb .02 9 .41 .20 1.08 .024 .0011 .038 .013 .015 Ni .04, Cr .11 Bi .06 10 .42 .04 1.45 .028 .0014 .030 .023 .019 Te .02 11 .38 .28 1.15 .035 .0015 .028 .011 .008 Mo .17, Ca .003 12 .37 .22 1.11 .080 .0020. .042 .019 .018 Ni .05, Cr .40 Mo .05, Pb .07 Control Runs 1 .40 .24 .60 .018 .0012 .035 .027 .010 Ni .01, Cr .10 Mo .02 2 .41 .23 1.80 .018 .0015 .040 .021 .012 Ni .02, Cr .09 Mo .01 3 .33 .21 1.23 .018 .0017 .040 .018 .009 Ni .02, Cr .07 Mo .01 4 .42 .25 1.13 .025 .0018 .019 .021 .017 Ni .02, Cr .05 Mo .01 5 the same as Example Run No.1 (high temperature rolling) No. Ferrite Crystal Grain Number Effective Depth of Hardened Layer (mm) Tool Life (relative value) Twist Strength (kgf/mm²) Toughness (kgf-m/cm²) Example Runs 1 9.1 6.2 100 156 7.7 2 8.6 7.3 80 170 6.4 3 7.8 9.1 60 185 6.6 4 6.6 7.8 70 173 6.0 5 8.5 7.7 90 170 6.5 6 7.8 9.1 60 190 8.2 7 9.3 8.0 80 180 8.0 8 9.5 7.9 80 180 8.2 9 7.9 7.8 150 172 6.6 10 9.1 9.0 90 182 7.1 11 8.3 7.2 100 165 6.8 12 7.6 7.1 200 161 6.9 Control Runs 1 8.1 4.5 200 121 8.0 2 8.3 9.5 20 187 5.0 3 8.1 7.4 180 115 8.2 4 8.1 5.1 90 120 7.0 5 5.1 6.1 120 131 4.5

Claims: A steel for induction hardening, characterised in that it comprises the following alloying elements: C: 0.37 to 0.45%, Si: 0 to 0.35%, Mn: more than 1.0% and up to 1.5%, B: 0.0005 to 0.0035%, Ti: 0.01 to 0.05%, Al: 0.01 to 0.06%, Cr: 0 to 1.0%, Mo: 0 to 0.20%, Ni: 0 to 1.0%, V: 0 to 0.30%, Nb: 0 to 0.10%, N: 0.005 to 0.022%, but not more than the equivalent to the total Ti and Al in the steel, the balance being Fe, the steel having been prepared by rolling at a temperature of not more than 1,100°C and finishing at a temperature of not more than 950°C, with a reduction in area of at least 70%, and having such fine structure as No 6 or higher of the ferrite crystal grain size number as defined by JIS-G0552. A steel according to claim 1, wherein the steel, in addition to the alloying elements set forth in claims 1, 2 and 3, further contains at the expense of the balance element Fe one or more of Pb: 0.01 - 0.20%, S: 0.005 - 0.30%, Bi: 0.01 - 0.10%, Te: 0.0005 - 0.10% and Ca: 0.0003 - 0.005%.

Inventors: NAMIKI KUNIO, URITA TATSUMI, NAMIKI, KUNIO, URITA, TATSUMI
Applicants: DAIDO STEEL CO LTD, NTN TOYO BEARING CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA, NTN CORPORATION
Application year: 1991
Main classification: C22C38
IPC codes: C22C38