Chisel and steel for chisel

Abstract

A steel constituting a chisel according to the present invention includes: 0.40-0.45% by mass of carbon, 0.50-0.80% by mass of silicon, 1.00-1.30% by mass of manganese, 0.001-0.005% by mass of sulfur, 2.90-3.80% by mass of chromium, and 0.20-0.40% by mass of molybdenum, with a balance consisting of iron and an unavoidable impurity, the steel has an ideal critical diameter DI defined by Equation (1) of 600 or more:
DI=7.Math.(% C).sup.1/2.Math.(1+0.64.Math.% Si).Math.(1+4.1.Math.% Mn).Math.(1+2.83.Math.% P).Math.(1−0.62.Math.% S).Math.(1+2.33.Math.% Cr).Math.(1+3.14.Math.% Mo)  (1).

Claims

1. A steel body constituting by a steel consisting of: 0.40% by mass or more and 0.45% by mass or less of carbon (C); 0.50% by mass or more and 0.80% by mass or less of silicon (Si); 1.00% by mass or more and 1.30% by mass or less of manganese (Mn); 0.001% by mass or more and 0.005% by mass or less of sulfur (S); 2.90% by mass or more and 3.80% by mass or less of chromium (Cr); 0.20% by mass or more and 0.40% by mass or less of molybdenum (Mo); and a balance consisting of iron and one or more unavoidable impurities with 0.020% by mass or less of phosphorous (P), wherein the steel has an ideal critical diameter DI defined by Equation (1) of 600 or more: $\mathrm{DI}=7.Math.{\left(\%C\right)}^{\frac{1}{2}}.Math.\left(1+0.64.Math.\%\mathrm{Si}\right).Math.\left(1+4.1.Math.\%\mathrm{Mn}\right).Math.\left(1+2.83.Math.\%P\right).Math.\left(1-0.62.Math.\%S\right).Math.\left(1+2.33.Math.\%\mathrm{Cr}\right).Math.\left(1+3.14.Math.\%\mathrm{Mo}\right)$ wherein % is by mass, and wherein the steel body is a rod having a diameter of at least 150 mm, and a hardness (HRC) of an entire cross section of the steel body is within the range of from 49 to 54 HRC.

2. The steel body according to claim 1, wherein a value of α defined by Equation (2) is 2.0 or more and 2.4 or less:
α=5.Math.% C+3.Math.% Si+% Mo−2.Math.% Mn−10.Math.% S  (2) where % is by mass.

3. The steel body according to claim 1, wherein sulfur (S) is present at 0.001% by mass or more and 0.004% by mass or less.

4. The steel body according to claim 1, wherein the steel has 3.01% by mass or more and 3.80% by mass or less of chromium (Cr).

5. The steel body according to claim 1, wherein the steel has 3.36% by mass or more and 3.80% by mass or less of chromium (Cr).

6. The steel body according to claim 1, wherein the steel has 0.27% by mass or more and 0.40% by mass or less of molybdenum (Mo).

7. The steel body according to claim 1, wherein the steel body is a rod having a diameter of at least 160 mm.

8. A chisel constituted by a steel consisting of: 0.40% by mass or more and 0.45% by mass or less of carbon (C); 0.50% by mass or more and 0.80% by mass or less of silicon (Si); 1.00% by mass or more and 1.30% by mass or less of manganese (Mn); 0.001% by mass or more and 0.005% by mass or less of sulfur (S); 2.90% by mass or more and 3.80% by mass or less of chromium (Cr); 0.20% by mass or more and 0.40% by mass or less of molybdenum (Mo); and a balance consisting of iron and one or more unavoidable impurities with 0.020% by mass or less of phosphorous (P), wherein the steel has an ideal critical diameter DI defined by Equation (1) of 600 or more: $\mathrm{DI}=7.Math.{\left(\%C\right)}^{\frac{1}{2}}.Math.\left(1+0.64.Math.\%\mathrm{Si}\right).Math.\left(1+4.1.Math.\%\mathrm{Mn}\right).Math.\left(1+2.83.Math.\%P\right).Math.\left(1-0.62.Math.\%S\right).Math.\left(1+2.33.Math.\%\mathrm{Cr}\right).Math.\left(1+3.14.Math.\%\mathrm{Mo}\right)$ wherein % is by mass, and wherein the chisel has a diameter of at least 150 mm, and a hardness (HRC) of an entire cross section of the chisel vertical to an axial direction of the chisel is within the range of from 49 to 54 HRC.

9. The chisel according to claim 8, wherein a value of α defined by Equation (2) is 2.0 or more and 2.4 or less:
α=5.Math.% C+3.Math.% Si+% Mo−2.Math.% Mn−10.Math.% S  (2) where % is by mass.

10. The chisel according to claim 8, wherein a hardness of a surface at room temperature after heating to 600° C. is 32 HRC or more, and a region including the surface has an impact value of 80 J/cm.sup.2 or more.

11. The chisel according to claim 8, wherein sulfur (S) is present at 0.001% by mass or more and 0.004% by mass or less.

12. The chisel according to claim 8, wherein the steel has 3.01% by mass or more and 3.80% by mass or less of chromium (Cr).

13. The chisel according to claim 8, wherein the steel has 3.36% by mass or more and 3.80% by mass or less of chromium (Cr).

14. The chisel according to claim 8, wherein the steel has 0.27% by mass or more and 0.40% by mass or less of molybdenum (Mo).

15. The chisel according to claim 8, having a diameter of at least 160 mm.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view schematically illustrating a configuration of a hydraulic breaker,

(2) FIG. 2 is a flowchart schematically showing a process of producing a chisel;

(3) FIG. 3 is a graph showing a relationship between a sample hardness and an impact value; and

(4) FIG. 4 is a graph showing a distribution of hardness of a sample in a radial direction.

DESCRIPTION OF EMBODIMENTS

(5) An embodiment of the present invention will now be described. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated.

(6) A steel for a chisel according to this embodiment can be used as a material constituting a chisel included in a hydraulic breaker, which will be described as an example. FIG. 1 is a cross-sectional view schematically illustrating a configuration of a hydraulic breaker. With reference to FIG. 1, a hydraulic breaker 1 according to this embodiment includes a chisel 10, a piston 20, and a frame 30.

(7) The chisel 10 has a rod shape. The chisel 10 includes a cylindrical base part 12 and a tapered part 11 which is connected to the base part 12 and whose cross sectional area taken vertically to the axial direction decreases toward the front end 11A. A proximal flat part 12A that is a flat part intersecting the axis axial direction is provided at a proximal end opposite to the front end 11A in the axial direction. An end of the chisel 10 close to the proximal flat part 12A in the axial direction is surrounded by the frame 30, and an end of the chisel 10 close to the front end 11A projects from the frame 30. A recess 12B is formed in a region of the chisel 10 surrounded by the frame 30. A stopper pin 50 is disposed in a region of an inner peripheral surface of the frame 30 corresponding to the recess 12B.

(8) The piston 20 has a rod shape. The piston 20 is disposed in a region surrounded by the frame 30. The piston 20 is disposed coaxially with the chisel 10. A distal flat part 21 that is a flat part intersecting the axial direction is formed at the distal end of the piston 20. The chisel 10 and the piston 20 are disposed in such a manner that the distal flat part 21 of the piston 20 faces the proximal flat part 12A of the chisel. The piston 20 is held to be axially movable relative to the frame 30.

(9) The piston 20 moves in the axial direction to strike the chisel 10 so that a striking force is transmitted to the chisel 10. In a hit chamber 31 defined at the inner periphery of the frame 30, contact of the distal flat part 21 of the piston 20 with the proximal flat part 12A of the chisel 10 causes a striking force to be transmitted from the piston 20 to the chisel 10. The chisel 10 breaks rocks or the like by the transmitted striking force.

(10) An oil chamber 32 that receives hydraulic oil for driving the piston 20 is defined between the piston 20 and the frame 30. A control valve mechanism 40 is disposed on a side surface of the frame 30. Supply of hydraulic oil from the control valve mechanism 40 to the oil chamber 32 causes the piston 20 to be driven in the axial direction and hit the chisel 10. The chisel 10 breaks rocks or the like by the striking force transmitted from the piston 20.

(11) In a case where the thus-configured chisel 10 is used under a severe environment, the temperature near the front end 11A thereof increases to about 600° C. In the chisel 10 to be used in such an environment, the hardness and the impact value after tempering at a high temperature (600° C.) are increased, and the hardness of the core portion after tempering (after tempering at 210° C.) performed for removing strains in quenching is increased. In this manner, abrasion resistance and cracking resistance can be increased, and thereby, high durability can be obtained. The chisel 10 according to this embodiment is constituted by a steel for a chisel including 0.40% by mass or more and 0.45% by mass or less of carbon, 0.50% by mass or more and 0.80% by mass or less of silicon, 1.00% by mass or more and 1.30% by mass or less of manganese, 0.001% by mass or more and 0.005% by mass or less of sulfur, 2.90% by mass or more and 3.80% by mass or less of chromium, and 0.20% by mass or more and 0.40% by mass or less of molybdenum, with a balance consisting of iron and an unavoidable impurity, and an ideal critical diameter DI defined by Equation (1) is 600 or more.

(12) The chisel 10 according to this embodiment constituted by the steel described above has a hardness of 32 HRC or more in the surface at room temperature after heating to 600° C. and an impact value of 80 J/cm.sup.2 or more in a region including the surface. In the chisel 10, the hardness of the core portion (hardness after tempering for reducing strains after quenching) is 45 HRC or more. Thus, the chisel 10 according to this embodiment has high durability under severe environments.

(13) In the steel for a chisel constituting the chisel 10, the value of α defined by Equation (2) may be 2.0 or more and 2.4 or less. In this case, high levels of the hardness and the impact value after high-temperature tempering can be obtained, and durability of the chisel 10 can be further enhanced.

(14) In the steel for a chisel constituting the chisel 10, the content of phosphorus included as an impurity is preferably 0.020% by mass or less. In this case, the influence of phosphorus on toughness can be reduced. The content of phosphorus is more preferably 0.015% by mass or less. This can increase the impact value after high-temperature tempering, and further increase cracking resistance of the steel for a chisel.

(15) An example method for producing the chisel 10 will now be described with reference to FIG. 2. FIG. 2 is a flowchart schematically showing a process of producing a chisel. In the method for producing the chisel 10 according to this embodiment, a steel material preparation step is performed as step (S10). In this step (S10), a solid cylindrical steel material having the composition of the steel for a chisel described above is prepared, for example.

(16) A processing step is performed as step (S20). In this step (S20), processing such as cutting is performed on the steel material prepared in step (S10). In this manner, the material is processed into a general shape of the chisel 10 according to this embodiment.

(17) Next, a quenching step is performed as step (S30). In this step (S30), the formed body obtained in step (S20) is subjected to quenching. The quenching is performed in such a manner that the formed body heated to a temperature of about 870° C. in an atmospheric furnace is subjected to oil cooling or water cooling, for example.

(18) Thereafter, a tempering step is performed as step (S40). In this step (S40), tempering is performed on the formed body subjected to quenching in step (S30). The tempering is performed in such a manner that the formed body heated to 210° C. in a heating furnace is subjected to air cooling.

(19) A finishing step is performed as step (S50). In this step (S50), a finishing process such as cutting, grinding, shot blasting, or coating is performed on the formed body subjected to tempering in step (S40) as necessary. Through the foregoing procedure, the chisel 10 according to this embodiment can be produced.

(20) As described above, a steel material constituted by a steel for a chisel having the composition described above is processed to obtain a formed body, and the formed body is subjected to the heat treatment and then to the finishing treatment as necessary, thereby obtaining the chisel 10 according to this embodiment. Even if this chisel 10 is used under such a severe environment that the chisel is tempered by heating to have its distal temperature increase to about 600° C., the chisel 10 can obtain high abrasion resistance and high cracking resistance.

Examples

(21) Experiments were performed to observe compositions suitable for a steel for a chisel to be used under severe environments. The experiments were conducted in the following procedure.

(22) First, steel materials having compositions shown in Table 1 below were prepared. The steel materials were quenched by rapidly cooling from 870° C., and then heated to 200° C. to be subjected to tempering, thereby producing samples. In anticipation of use environments of chisels, the samples were heated to 600° C. to be subjected to tempering. The hardnesses and impact values of the resulting samples were measured. The hardnesses were measured with a Rockwell hardness tester. The impact values were measured with a 2-mm V-notch Charpy impact test (sample shape: a length of 55 mm; a square cross section of 10 mm at each side; a notch depth of 2 mm; a notch angle of 45°; and a notch bottom radius of 0.25 mm).

(23) Table 1 provides a listing of values of carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), molybdenum (Mo), niobium (Nb), vanadium (V), titanium (Ti), and boron (B) of each steel in units of % by mass. The balance consists of iron and one or more unavoidable impurities. Although phosphorus is an unavoidable impurity, but is included in the table in consideration of a large influence on the impact value. Table 1 also shows hardnesses (HRC) and impact values (unit: J/cm.sup.2) obtained through the examples described above. Table 1 also shows values of the ideal critical diameter DI defined by Equation (1). Table 1 also shows values of α defined by Equation (2).

(24) TABLE-US-00001 TABLE 1 Impact Hardness value DI α C Si Mn P S Cr Mo Nb V Ti B (HRC) (J/cm.sup.2) value value A 0.44 0.71 1.11 0.014 0.003 3.51 0.30 — — — — 34 112 693 2.38 B 0.40 0.69 1.18 0.014 0.002 3.73 0.35 — — — — 33 129 787 2.04 C 0.42 0.74 1.08 0.013 0.005 3.45 0.27 — — — — 33 108 626 2.38 D 0.43 0.78 1.24 0.013 0.003 3.01 0.28 — — — — 34 122 652 2.26 E 0.45 0.67 1.10 0.012 0.002 3.36 0.31 — — — — 35 91 665 2.35 F 0.41 0.49 1.09 0.015 0.003 1.01 0.40 0.03 — 0.04 0.002 33 97 253 1.71 G 0.47 0.92 1.01 0.015 0.009 3.85 0.26 — — — — 35 45 736 3.26 H 0.41 1.01 1.29 0.015 0.003 1.51 0.24 — — 0.04 0.002 35 38 383 2.71 I 0.41 1.00 2.00 0.015 0.003 1.50 0.02 — — 0.04 0.002 33 11 336 1.04 J 0.41 1.01 1.30 0.015 0.003 2.80 0.02 — — 0.04 0.002 34 11 389 2.47 K 0.29 0.20 1.80 0.015 0.010 1.36 0.44 — — — — 29 199 367 −1.21 L 0.44 0.26 0.35 0.008 0.008 1.98 1.02 0.03 0.11 — 0.003 45 47 317 3.22 M 0.37 0.30 1.33 0.015 0.013 0.62 0.13 — — 0.04 0.002 30 122 117 0.09 N 0.42 0.25 0.82 0.008 0.009 0.94 0.15 — — — — 30 137 110 1.27

(25) Materials A through E in Table 1 are steels for chisels of the present invention (examples), and materials F through N are steels falling outside the scope of the present invention (comparative examples). FIG. 3 shows relationships between the hardness and the impact value of samples obtained from the steels. In FIG. 3, the abscissa represents the hardness at room temperature after tempering at 600° C., and the ordinate represents the impact value at room temperature after tempering at 600° C. In FIG. 3, data points of the samples of the examples are plotted as circles, and data points of the samples of the comparative examples are plotted as diamonds.

(26) With reference to Table 1 and FIG. 3, materials A through E as steels for chisels of the present invention obtained hardnesses of 32 HRC or more and impact values of 80 J/cm.sup.2 or more, which are target values after tempering at 600° C. The materials of the comparative examples having a values outside the range from 2.0 to 2.4, both inclusive, showed hardnesses and impact values less than the target values, except for material F. On the other hand, the materials of the examples having values of a within the range from 2.0 to 2.4, both inclusive, obtained target values of both the hardness and the impact value. Material F had a DI value less than a target value of 600. Material F showed insufficient hardenability.

(27) In addition, an experiment for confirming a hardness in the core portion in the case of producing chisels was carried out. First, solid cylindrical steel materials having a diameter of 160 mm and compositions shown in Table 2 below were prepared. The steel materials were quenched and then heated to 210° C. to be subjected to tempering, thereby producing samples. For Example A, quenching was carried out by performing oil cooling from 880° C. For Example B, quenching was carried out by performing water cooling from 880° C. For Comparative Examples A and B, quenching was carried out by performing water cooling from 870° C. Comparative Examples A and B have compositions similar to those of materials N and M in Table 1. The compositions of materials N and M correspond to compositions of steels currently used as steels for chisels.

(28) TABLE-US-00002 TABLE 2 DI α C Si Mn P S Cr Mo B value value Example A 0.42 0.74 1.10 0.013 0.003 3.45 0.31 — 680 2.40 Example B 0.42 0.73 1.08 0.014 0.002 3.48 0.28 — 642 2.39 Comparative 0.39 0.23 0.77 0.012 0.016 1.09 0.20 — 123 1.14 Example A Comparative 0.37 0.30 1.31 0.015 0.012 0.61 0.12 0.002 112 0.13 Example B

(29) Then, a hardness distribution in a cross section vertical to the axial direction of each sample was measured. The hardness measurement was carried out with a Rockwell hardness tester. FIG. 4 shows results of the measurement.

(30) In FIG. 4, the abscissa represents the distance from the surface, and the ordinate represents the hardness. With reference to FIG. 4, in steels of the comparative examples that are currently used steels and have DI values less than 600, only surface portions are sufficiently hardened by quenching, but core portions are insufficiently hardened by quenching. The hardnesses in the core portions are below 45 HRC. On the other hand, in the steels of the examples having DI values of 600 or more, regions from surface portions to core portions are sufficiently hardened by quenching. Although Example A was subjected to oil quenching, Example A shows a hardness distribution substantially equivalent to that of Example B subjected to water quenching. The hardnesses in core portions of Examples A and B are 45 HRC or more. In the entire region of each cross section, the hardness is within the range from 49 to 54 HRC. Examples A and B show uniform hardness distributions.

(31) From the foregoing results of the experiments, it was confirmed that steels for chisels according to the present invention can obtain high abrasion resistance and cracking resistance even when used in a severe environment, and thus, show high durability. With reference to FIG. 1, the steel for a chisel can also be used as a steel constituting the stopper pin 50.

(32) It should be understood that the embodiment and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

(33) A chisel and a steel for a chisel according to the present invention are applicable particularly advantageously as a chisel to be used in severe environments and a material for such a chisel.

DESCRIPTION OF REFERENCE NUMERALS

(34) 1: hydraulic breaker, 10: chisel, 11: tapered part, 11A: front end, 12: base part, 12A: proximal flat part, 12B: recess, 20: piston, 21: distal flat part, 30: frame, 31: hit chamber, 32: oil chamber, 40: control valve mechanism, and 50: stopper pin.