Steel with high hardness and excellent toughness

Abstract

A steel with high hardness and excellent toughness contains, in mass %, 0.55-1.10% C, 0.10-2.00% Si, 0.10-2.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-2.50% Cr, and 0.010-0.10% Al, with the balance consisting of Fe and unavoidable impurities. The structure of the steel after quenching is a dual phase structure of martensitic structure and spheroidized carbide. Spheroidized cementite particles with an aspect ratio of 1.5 or less constitute at least 90% of all cementite particles. The proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries to a total number of cementite particles is 20% or less.

Claims

1. A steel comprising, in mass %, 0.55-0.92% C, 0.10-2.00% Si, 0.10-2.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-2.50% Cr, and 0.010-0.10% Al, with the balance consisting of Fe and unavoidable impurities; a structure of the steel after quenching being a dual phase structure of martensitic structure and spheroidized carbide; spheroidized cementite particles with an aspect ratio of 1.5 or less constituting at least 90% of all cementite particles; regarding cementite on prior austenite grain boundaries, a proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries to a total number of cementite particles being 20% or less, wherein at least 90% of the spheroidized cementite particles on the prior austenite grain boundaries have a particle size of 1 μm or less.

2. The steel according to claim 1, further comprising, in mass %, one or two or more selected from among 0.10-1.50% Ni, 0.05-2.50% Mo, and 0.01-0.50% V.

3. The steel according to claim 1, wherein prior austenite grains have a grain size of 1-5 μm.

4. The steel according to claim 2, wherein prior austenite grains have a grain size of 1-5 μm.

5. The steel according to claim 1, wherein the steel has a HRC hardness of 58 HRC or more.

6. The steel according to claim 1, further comprising, in mass %, 0.01-0.50% V.

7. The steel according to claim 1, further comprising, in mass %, 0.01-0.08% Ni.

8. The steel according to claim 1, further comprising, in mass %, 0.30-2.50% Mo.

9. The steel according to claim 1, wherein prior austenite grains have a grain size of 1-2 μm.

10. The steel according to claim 1, wherein the proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries to the total number of cementite particles being 20% or less and 8% or more.

11. The steel according to claim 1, wherein the steel has a Charpy impact value of 51 J/cm.sup.2 or more.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram showing cracking occurring from a cementite particle having a large aspect ratio, circles and ellipses in the figure showing cementite particles, the deformation load being not limited to compression;

(2) FIG. 2 shows a pattern of pearlitization processing;

(3) FIG. 3 shows a pattern of spheroidizing annealing;

(4) FIG. 4 shows a pattern of quenching and tempering;

(5) FIG. 5 shows a shape of 10-RC notched Charpy impact test specimen; and

(6) FIG. 6 is a photograph, taken by a scanning electron microscope (SEM), showing the structure of a steel of Inventive Example No. 3 after quenching, which is a secondary electron image of 5000-fold magnification obtained using an accelerating voltage of 15 kV, the scale bar shown in the lower portion corresponding to 5 μm.

DESCRIPTION OF EMBODIMENT

(7) Prior to describing an embodiment of the present invention, a description will be made about the reasons for limiting the chemical components of the steel, the proportion of the number of spheroidized cementite particles having an aspect ratio of 1.5 or less, and the proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries, which are the constituent features of the invention recited in claim 1 of the present application, as well as the reasons for limiting the particle size of the spheroidized cementite particles on the prior austenite grain boundaries, and the grain size of the prior austenite grains. It should be noted that % used for chemical components is mass %.

(8) C: 0.55-1.10%

(9) C is an element which improves hardness, wear resistance, and fatigue life after quenching and tempering. If the C content is less than 0.55%, it will be difficult to obtain sufficient hardness. Desirably, the C content needs to be 0.60% or more. On the other hand, if the C content is more than 1.10%, the hardness of the steel material will increase, impairing the workability such as machinability and forgeability. In addition, the amount of carbides in the structure will increase more than necessary, and the alloy concentration in the matrix will decrease, leading to reduction in hardness and hardenability of the matrix. It is thus necessary to make the C content not more than 1.10%, and desirably not more than 1.05%. Accordingly, the C content is set to 0.55-1.10%, and desirably to 0.60-1.05%.

(10) Si: 0.10-2.00%

(11) Si is an element which is effective in deoxidation of the steel, and serves to impart required hardenability to the steel and enhance its strength. Si is dissolved in cementite in a solid state to increase the hardness of the cementite, thereby improving wear resistance. To achieve these effects, the Si content needs to be 0.10% or more, or desirably 0.20% or more. On the other hand, if Si is contained in a large amount, it will increase the hardness of the material, impairing the workability such as machinability and forgeability. It is thus necessary to make the Si content not more than 2.00%, and desirably not more than 1.55%. Accordingly, the Si content is set to 0.10-2.00%, and desirably to 0.20-1.55%.

(12) Mn: 0.10-2.00%

(13) Mn is an element which is effective in deoxidation of the steel and necessary for imparting required hardenability to the steel and enhancing its strength. To this end, the Mn content needs to be 0.10% or more, or desirably 0.15% or more. On the other hand, if Mn is contained in a large amount, it will decrease the toughness. It is thus necessary to make the Mn content not more than 2.00%, and desirably not more than 1.00%. Accordingly, the Mn content is set to 0.10-2.00%, and desirably to 0.15-1.00%.

(14) P: 0.030% or less

(15) P is an impurity element which is contained unavoidably in the steel. P segregates in the grain boundary and deteriorates the toughness. Accordingly, the P content is set to 0.030% or less, and desirably to 0.015% or less.

(16) S: 0.030% or less

(17) S is an impurity element which is contained unavoidably in the steel. S combines with Mn to form MnS, and deteriorates the toughness. Accordingly, the S content is set to 0.030% or less, and desirably to 0.010% or less.

(18) Cr: 1.10-2.50%

(19) Cr is an element which improves hardenability and also facilitates spheroidization of carbides by spheroidizing annealing. To obtain such effects, the Cr content needs to be 1.10% or more, or desirably 1.20% or more. On the other hand, if Cr is added in an excessively large amount, cementite will become brittle, leading to deterioration in toughness. It is thus necessary to make the Cr content not more than 2.50%, and desirably not more than 2.15%. Accordingly, the Cr content is set to 1.10-2.50%, and desirably to 1.20-2.10%.

(20) Al: 0.010-0.10%

(21) Al is an element effective in deoxidation of the steel. Further, Al is an element effective in suppressing grain coarsening, as it combines with N to generate AlN. For achieving the effect of suppressing grain coarsening, the Al content needs to be 0.010% or more. On the other hand, if Al is added in a large amount, it will generate nonmetallic inclusions, which will become origins of cracking. Accordingly, the Al content is set to 0.10% or less, and desirably to 0.050% or less.

(22) Ni, Mo, and V are elements from which any one or two or more elements are contained selectively. They are contained under this condition and limited for the following reasons.

(23) Ni: 0.10-1.50%

(24) Ni is an element which is contained under the above-described condition of being contained selectively. Although Ni needs to be contained in an amount of 0.10% or more for dissolution and it is an element effective in improving the hardenability and toughness, Ni is an expensive element, increasing the cost. Accordingly, the Ni content is set to 0.10-1.50%, and desirably to 0.15-1.00%.

(25) Mo: 0.05-2.50%

(26) Mo is an element which is contained under the above-described condition of being contained selectively. Although Mo needs to be contained in an amount of 0.05% or more for dissolution and it is an element effective in improving the hardenability and toughness, Mo is an expensive element, increasing the cost. Accordingly, the Mo content is set to 0.05-2.50%, and desirably to 0.05-2.00%.

(27) V: 0.01-0.50%

(28) V is an element which is contained under the above-described condition of being contained selectively. V needs to be contained in an amount of 0.01% or more for dissolution. Further, V forms carbides, and it is an element effective in refining the grains. However, if V is contained in an amount of more than 0.50%, the effect of refining the grains will become saturated, and the cost will increase. Further, V is an element which may form carbonitrides in a large amount, deteriorating processing property. Accordingly, the V content is set to 0.01-0.50%, and desirably to 0.01-0.35%.

(29) That the spheroidized cementite particles with an aspect ratio of 1.5 or less constitute at least 90% of all cementite particles.

(30) An aspect ratio defining the ratio of major axis to minor axis of spheroidized carbide provides an indication of spheroidization. Cementite particles having a large aspect ratio, such as those having plate-like shape or nearly columnar shape, would likely become origins of cracking as stress would focus on the ends of such cementite particles during deformation. In contrast, cementite particles of nearly spherical shape would have no portion on which stress concentrates, so they have a lower risk of causing cracking. FIG. 1 is a schematic diagram showing that a cementite particle having a large aspect ratio becomes an origin of cracking. Thus, as compared to a structure in which a large number of cementite particles having a large aspect ratio are distributed, a structure in which a large number of cementite particles having an aspect ratio close to 1, i.e. cementite particles of nearly spherical shape, are distributed has a lower risk of causing cracking from the cementite particles when a load is applied, and has improved toughness. When a cementite particle has an aspect ratio of 1.5 or less, its harmful effect of becoming an origin of cracking can be lowered, and it is more preferable that the proportion of the number of such cementite particles to the total number of cementite particles takes a larger value. Accordingly, it is configured such that the spheroidized cementite particles with an aspect ratio of 1.5 or less constitute at least 90%, and preferably at least 95% (including 100%), of all the cementite particles. It should be noted that the deformation load shown by arrows in FIG. 1 is not limited to compression.

(31) That the proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries to a total number of cementite particles is 20% or less.

(32) The steel as recited in claim 1 of the present application falls within the range of hypereutectoid steel in view of the content of C in the chemical components. In a hypereutectoid steel, the mode of brittle fracture deteriorating the shock resistance property is primarily intergranular fracture along the prior austenite grain boundaries. This is caused by cementite on the prior austenite grain boundaries (particularly, reticular carbides along the grain boundaries). Cementite that precipitates and exists at the grain boundaries is easier to become an origin of fracture and more harmful as compared to cementite in the grains. Thus, it is not preferable that such cementite exists at the grain boundaries. Accordingly, it is configured such that the proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries to the total number of cementite particles is 20% or less, desirably 10% or less, and further desirably 5% or less (including 0%).

(33) That at least 90% of the spheroidized cementite particles on the prior austenite grain boundaries have a particle size of 1 μm or less.

(34) As explained in the above paragraph, it is not preferable that cementite particles exist on the prior austenite grain boundaries. Particularly, reticular carbides or similarly coarse carbides along the grain boundaries have increased risks of becoming origins of intergranular fracture. Therefore, it is configured such that at least 90%, and preferably at least 95% (including 100%), of the spheroidized cementite particles have a particle size of 1 μm or less, which is low in harmfulness.

(35) It should be noted that % here is the proportion when the total number of carbides observable by a scanning electron microscope with a magnification of about 5000 times is set to be 100%. Very fine carbides which cannot be observed with that magnification power are not taken into account, as they will hardly influence the toughness.

(36) That the prior austenite grains have a grain size of 1-5 μm.

(37) Refining prior austenite grains can reduce the unit of fracture of intergranular fracture or cleavage fracture, and can increase the energy required for fracture, leading to improved toughness. Further, finer prior austenite grains can reduce segregation of impurity elements such as P and S, which would segregate at the grain boundaries and deteriorate toughness. As such, refining the grains is a very effective way of enhancing the toughness without decreasing the hardness. The reasons for setting the grain size of the prior austenite grains to 1-5 μm are as follows. Producing products having prior austenite grains with a grain size of less than 1 μm in an industrially stable manner is difficult and increases the cost, so the lower limit of the grain size of the prior austenite grains is set to 1 μm. When the upper limit of the grain size of the prior austenite grains is set to 5 μm, the above effects become noticeable, making it possible to obtain a steel material having balanced hardness and toughness. Accordingly, it is configured such that the prior austenite grains have a grain size of 1-5 μm.

(38) An embodiment of the present invention will be described below with reference to Examples and Tables.

Examples

(39) Steels having the chemical compositions of Inventive Examples Nos. 1 to 7 and Comparative Examples Nos. 8 to 11 shown in Table 1 below were produced in a 100-kg vacuum melting furnace. The obtained steels were each subjected to hot forging at 1150° C. to obtain a round bar having a diameter of 26 mm, which was then cut into 250 mm in length to form a test sample. Next, heat treatment was carried out, as pearlitization processing as shown in FIG. 2, in which each round bar steel was held at 1000° C. for 15 minutes and then gas-cooled to 600° C. It was held at 600° C. for three hours and then air-cooled. Thereafter, spheroidizing annealing was carried out, as shown in FIG. 3, in which heat treatment of furnace-cooling the bar steel from 780° C. to 650° C. was repeated twice. The resultant bar steels were then each shaped roughly into a 10-RC notched Charpy impact test specimen, which was then subjected to processing as shown in FIG. 4. Specifically, each test specimen was held at a temperature range of 780-840° C. for 30 minutes for oil quenching, which was performed at least twice. Then, for preventing season cracking, it was subjected to temporary tempering processing in which it was held at 150° C. for 40 minutes before being air-cooled. It was then subjected to tempering processing in which it was held at a temperature range of 180-220° C. for 90 minutes before being air-cooled. Further, the resultant rough-shaped specimens were subjected to finishing work, whereby the 10-RC notched Charpy impact test specimens as shown in FIG. 5 were obtained.

(40) In Table 1, “*” added to 0.06-0.08% Ni, “*” added to 0.04% Mo, and the hyphens for V mean that they are unavoidable impurities. Therefore, the steels of Inventive Examples No. 1 and No. 2 correspond to the steel recited in claim 1, and the steels of Inventive Examples Nos. 3 to 7 correspond to the steel recited in claim 2.

(41) TABLE-US-00001 TABLE 1 (Unit: mass %) No. C Si Mn P S Ni Cr Mo Al V Steel of 1 1.00 0.26 0.40 0.015 0.005 0.08* 1.35 0.04* 0.018 — Inventive 2 0.89 0.27 2.00 0.013 0.006 0.08* 1.99 0.04* 0.023 — Example 3 0.92 0.26 0.20 0.012 0.005 0.07* 2.03 0.15 0.020 — 4 0.91 0.26 0.21 0.012 0.005 0.07* 1.34 1.99 0.030 0.15 5 0.90 1.50 1.00 0.011 0.005 0.07* 1.34 0.04* 0.014 0.14 6 0.90 1.53 0.41 0.012 0.005 0.06* 1.35 0.50 0.017 0.15 7 0.97 0.25 0.99 0.014 0.006 0.99  1.35 0.30 0.018 — Steel of 8 0.99 0.25 2.03 0.013 0.005 0.08* 1.36 0.04* 0.016 — Comparative 9 1.00 0.25 0.40 0.014 0.005 1.99  1.34 0.04* 0.016 — Example 10 1.01 0.25 0.99 0.015 0.006 1.99  1.36 0.30 0.500 — 11 1.00 1.01 0.42 0.012 0.005 1.00  1.36 0.15 0.525 0.15 1) The underlined values are outside the scope of the present invention. 2) “*” means that they are unavoidable impurities.

(42) These 10-RC notched Charpy impact test specimens were subjected to a Charpy impact test at room temperature. Further, these test specimens were subjected to hardness measurement, and also to scanning electron microscopy to obtain the size of prior austenite grains.

(43) Table 2 below shows the prior austenite grain size (μm), the HRC hardness, and the Charpy impact value (J/cm.sup.2) as the results of the above-described Charpy impact test, hardness measurement, and scanning electron microscopy. Table 2 also shows, as the features of the structure after quenching, the proportion of the number of spheroidized cementite particles having an aspect ratio of 1.5 or less, the proportion of the number of spheroidized cementite particles on the prior austenite grain boundaries, and the particle size of the spheroidized cementite particles on the prior austenite grain boundaries.

(44) TABLE-US-00002 TABLE 2 Proportion of cementite Proportion of the number of Proportion of cementite particles with aspect cementite particles on prior particles with particle size Prior Charpy ratio of 1.5 or less to austenite grain boundaries of 1 μm or less among the austenite impact the total number of to the total number of cementite particles on prior grain HRC value No. cementite particles (%) cementite particles (%) austenite grain boundaries size (μm) hardness (J/cm.sup.2) Steel of 1 92 18 96 5 61 55 Inventive 2 97 10 98 4 60 52 Example 3 95 16 94 3 58 78 4 97 10 95 2 59 51 5 98  8 96 2 60 60 6 95 14 92 1 61 56 7 95  9 92 4 62 45 Steel of 8 85 18 85 6 61 29 Comparative 9 93 27 93 6 60 37 Example 10 83 16 91 4 60 28 11 91 23 84 3 61 33 1) The underlined values for the steels of Comparative Examples are outside the scope of the present invention.

(45) In Table 2, the underlined values for the steels of Comparative Examples Nos. 8 to 11 are outside the claimed invention. These steels of Comparative Examples falling outside the claimed invention each had a Charpy impact value of less than 40 J/cm.sup.2, and it was not possible to obtain enough hardness and toughness at the same time with these steels. In contrast, the steels of Inventive Examples fulfilling all the requirements of the claims each have a hardness of 58 HRC or more and a Charpy impact value of 40 J/cm.sup.2 or more, showing that they support both enough hardness and enough toughness. FIG. 6 shows, as an exemplary structure, the structure of the steel of Inventive Example No. 3 after quenching. It is a dual phase structure of martensitic structure and cementite. Regarding the cementite in the structure, the amount of cementite particles having an aspect ratio of 1.5 or more is small, and the amount of cementite particles on the prior austenite grain boundaries is small. Of the cementite particles on the prior austenite grain boundaries, the amount of cementite particles having a size of greater than 1 μm is small, and the prior austenite grains have a grain size of 3 μm. It is thus recognized that the structure obtained falls within the scope of the claimed invention.

(46) It should be understood that the embodiment and the inventive 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.