Steel with high hardness and excellent toughness

Abstract

A steel with high hardness and excellent toughness contains, in mass %, 0.40-1.00% C, 0.10-2.00% Si, 0.10-1.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-3.20% Cr, 0.010-0.10% Al, and 0.15-0.50% V, and further contains at least one or two of 2.50% or less Ni and 1.00% or less Mo, with an amount of (C+V) being 0.60% or more in mass %, with the balance consisting of Fe and unavoidable impurities. The steel has a microstructure which is a martensitic structure with finely dispersed Fe-based ε carbides, with its prior austenite grain size being 20 μm or less.

Claims

1. A steel with high hardness and excellent toughness, containing, in mass %, 0.40-1.00% C, 0.10-2.00% Si, 0.10-1.00% Mn, 0.030% or less P, 0.030% or less S, 1.10-3.20% Cr, 0.010-0.10% Al, and 0.15-0.50% V, and further containing at least one of 2.50% or less Ni and 1.00% or less Mo, with an amount of (C+V) being 0.60% or more in mass %, with the balance consisting of Fe and unavoidable impurities, the steel having a microstructure which is a martensitic structure with finely dispersed Fe-based ε carbides and with a prior austenite grain size of 20 μm or less, wherein the martensitic structure tempered at a low temperature of 130° C. to 250° C. has V-containing fine carbides with a diameter of 0.50 μm or less precipitated dispersively therein, an amount of the precipitated V-containing fine carbides constitutes 0.10-0.90 vol % of a total martensite volume, and an amount of precipitated cementite in the martensitic structure tempered at the low temperature of 130° C. to 250° C. constitutes 0.50 vol % or less of the total martensite volume.

Description

DESCRIPTION OF EMBODIMENT

(1) Prior to describing an embodiment of the present invention, the constituent features of the invention according to the solutions of the present invention will be explained below in order of: the reasons for limiting the chemical components of the steel, except for Fe and unavoidable impurities, the reasons for causing the microstructure of the inventive steel to be a martensitic structure tempered at a low temperature of 130° C. to 250° C., the reasons for limiting the size and precipitated amount of V-containing carbides in the martensitic structure, the reasons for limiting the proportion of the amount of precipitated cementite in the martensitic structure to the total martensite volume, and the reasons for limiting the prior austenite grain size. It should be noted that % used for chemical components is mass %.

(2) C: 0.40-1.00%

(3) C is an element which improves hardness, wear resistance, and fatigue life after quenching and tempering. However, if the C content is less than 0.40%, sufficient hardness cannot be obtained. On the other hand, if the C content is more than 1.00%, the toughness will be impaired, and further, the hardness of the steel material will increase, impairing the workability such as machinability and forgeability. Accordingly, the C content is set to 0.40-1.00%, desirably to 0.50-1.00%, and further desirably to 0.50-0.90%.

(4) Si: 0.10-2.00%

(5) 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. 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 keep the Si content to be 2.00% or less, and desirably 1.55% or less. Accordingly, the Si content is set to 0.10-2.00%, and desirably to 0.20-1.55%.

(6) Mn: 0.10-1.00%

(7) 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. Further, it may combine with S to form MnS, which will also decrease the toughness or contribute to cracking during processing. It is thus necessary to keep the Mn content to be 1.00% or less, and desirably 0.70% or less. Accordingly, the Mn content is set to 0.10-1.00%, desirably to 0.15-1.00%, and further desirably to 0.15-0.70%.

(8) P: 0.030% or less

(9) 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.

(10) S: 0.030% or less

(11) S is an element which 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.

(12) Cr: 1.10-3.20%

(13) Cr is an element which improves hardenability. To sufficiently obtain the effect, the Cr content needs to be 1.10% or more, desirably 1.20% or more, and further desirably 1.35% or more. On the other hand, if Cr is added in an excessively large amount, it will promote precipitation of carbides in grain boundaries during the cooling process following quenching, adversely affecting the toughness. To avoid this, it is necessary to keep the Cr content to be 3.20% or less, desirably 2.50% or less, and further desirably 2.30% or less. Accordingly, the Cr content is set to 1.10-3.20%, desirably to 1.20-2.50%, and further desirably to 1.35-2.30%.

(14) Al: 0.010-0.10%

(15) Al is added as it is an element indispensable to deoxidation of the steel. Further, Al may combine with N to generate AlN, thereby suppressing grain coarsening. For achieving these effects, the Al content needs to be 0.010% or more. On the other hand, if Al is added in a large amount, hot workability will be impaired. It is thus necessary to keep the Al content to be 0.10% or less, and desirably 0.050% or less. Accordingly, the Al content is set to 0.010-0.10%, and desirably to 0.015-0.050%.

(16) V: 0.15-0.50%

(17) V is an element indispensable for achieving high toughness by refining of grains, as V combines with C to form fine carbides, and the carbides serve to pin the grain boundaries at the time of heating for quenching, thereby keeping the grains fine. In order for the carbides to effectively pin the grain boundaries in the steel, the steel needs to be once heated to a temperature not lower than the dissolution temperature of the carbides to let the carbides dissolved, so that the carbides are precipitated finely at the time of heating to a quenching temperature. In this regard, however, if Nb, Ti, or other carbide-forming element were added with respect to the C content in the components of the present invention, it would not be possible to let the carbides dissolved sufficiently even by heating the steel to 1250° C. which is considerably higher than the practical heating temperature of steel materials. The pinning effect of the carbides would be insufficient, and coarse carbides would likely remain, adversely affecting the toughness. In contrast, V-containing carbides are dissolved at a lower temperature, so they can be effectively utilized for pinning the grain boundaries. To obtain this effect, V needs to be added in an amount of 0.15% or more, desirably 0.20% or more, and further desirably 0.25% or more. On the other hand, if V is contained in an amount of more than 0.50%, the effect of refining the grains will become saturated, and further, coarse carbides containing V will be formed, which carbides may impair hot workability or lead to reduced toughness. It is therefore necessary to keep the V content to be 0.5% or less, and desirably 0.45% or less. Accordingly, the V content is set to 0.15-0.50%, desirably to 0.20-0.50%, and further desirably to 0.25-0.45%.

(18) Ni and Mo are elements from which one or both are contained. They are limited for the following reasons.

(19) Ni: 2.50% or less

(20) While Ni may be contained as an impurity in the present invention (in an amount of 0.07%, for example), Ni is an element effective in improving the hardenability and toughness, so it may be added intentionally. On the other hand, Ni is an expensive element, increasing the cost. Accordingly, the Ni content, when added, is set to 2.50% or less, and desirably to 1.70% or less.

(21) Mo: 1.00% or less

(22) While Mo may be contained as an impurity in the present invention (in an amount of 0.04%, for example), Mo is an element effective in improving the hardenability and toughness, so it may be added intentionally. On the other hand, Mo is an expensive element, increasing the cost. Accordingly, the Mo content, when added, is set to 1.00% or less, and desirably to 0.50% or less.

(23) C+V: 0.60% or more

(24) In order to achieve the grain refining function by dispersion of V-containing fine carbides, it is necessary to set a total amount of C and V to be at least 0.60% or more.

(25) (Reasons for Causing the Microstructure to be a Martensitic Structure with Finely Dispersed Fe-Based ε Carbides)

(26) In order to impart high hardness to the steel of the present invention, the microstructure is made to be martensite having Fe-based ε carbides finely dispersed therein. The martensite with finely dispersed Fe-based ε carbides is obtained through low-temperature tempering at 130° C. to 250° C. The steel of the present invention, by virtue of the chemical components and other restrictions defined in the solutions of the present invention, is capable of attaining the state of high toughness as quenched, and the excellent toughness is maintained in the low-temperature tempering at 130° C. to 250° C., eliminating the need to add alloy elements more than necessary. On the other hand, if the steel having the components within the scope of the present invention is subjected to high-temperature tempering conducted at a temperature of 500° C. or higher, instead of the low-temperature tempering, the hardness will be decreased due to the small amount of alloy elements contributing to secondary hardening. In such a case, although toughness may become still higher, high hardness cannot be achieved, hindering acquisition of required high hardness and high toughness. Accordingly, the martensitic structure having Fe-based ε carbides finely dispersed therein as a result of low-temperature tempering at 130° C. to 250° C. is adopted.

(27) (Reasons for Setting the Maximum Diameter of V-Containing Carbides in Martensite to be 0.50 μm or Less and the Amount of Precipitated V-Containing Carbides to be 0.10-0.90 Vol % of the Total Martensite Volume)

(28) When V-containing fine carbides having a diameter of 0.50 μm or less are dispersed in the martensite, the prior austenite grain size is prevented from coarsening and it is restricted to 20 μm or less, so that high toughness can be achieved simultaneously with high hardness. If the V-containing carbides being dispersed have a diameter of 0.50 pun or more, the grain refining effect will become small and toughness will decrease. If the amount of precipitated V-containing carbides in terms of volume % is less than 0.10 vol % of the total martensite volume, the effect of refining the prior austenite grain size cannot be obtained sufficiently. Therefore, the amount of precipitated V-containing carbides is set to 0.10 vol % or more, and the amount of precipitated V-containing fine carbides is desirably set to 0.15 vol % or more. On the other hand, if the amount of precipitated V-containing fine carbides exceeds 0.90 vol/%, the precipitated amount becomes too much, making the grains themselves including the V-containing carbides brittle, leading to decreased toughness. It is therefore set to 0.90 vol % or less, and desirably to 0.80 vol % or less. Accordingly, the maximum diameter of the V-containing carbides is controlled to be 0.50 μm or less and the amount of the precipitated V-containing carbides to be 0.10-0.90 vol %, and desirably 0.15-0.80 vol %, of the total martensite volume.

(29) (Reasons for Setting the Proportion of the Amount of Precipitated Cementite to the Total Martensite Volume to be at Most 0.50 Vol % or Less)

(30) Cementite would likely grow on the austenite grain boundaries during heating, which may cause cracking along the grain boundaries after quenching and tempering, thereby degrading the toughness. Accordingly, the amount of precipitated cementite is controlled to be at most 0.50 vol % or less of the total martensite volume.

(31) (Reasons for Setting the Prior Austenite Grain Size to 20 μm or Less, and Desirably to 15 μm or Less)

(32) When the prior austenite grain size in the quenched and tempered state is made fine, brittle fracture can be suppressed, leading to improved toughness. Further, when the prior austenite grain size is made small, the grain boundary area in the volume increases, and impurity elements such as P and S that would segregate in the grain boundaries and deteriorate toughness are dispersed over many grain boundaries, so that the amount of segregated impurities on individual grain boundaries can be decreased, which also contributes to improved toughness. Accordingly, the prior austenite grain size is set to 20 μm or less, and desirably to 15 μm or less.

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

EXAMPLES

(34) Steels having the chemical compositions of Inventive Examples Nos. 1 to 9 and Comparative Examples Nos. 10 to 15 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 steel of 26 mm in diameter. It should be noted that Table 1 shows indispensable chemical components as well as P and S as impurities, with the remaining Fe and other unavoidable impurities being omitted in Table 1.

(35) TABLE-US-00001 TABLE 1 (Unit: mass %) No. C Si Mn P S Ni Cr Mo Al V Nb C + V Steel of Inventive Example 1 0.81 0.48 0.20 0.011 0.005 0.07 2.02 0.04 0.010 0.30 — 1.11 2 0.80 0.26 0.21 0.010 0.005 0.07 2.03 0.04 0.012 0.30 — 1.10 3 0.80 0.26 0.20 0.010 0.005 0.07 2.32 0.04 0.021 0.34 — 1.14 4 0.70 0.25 0.20 0.009 0.005 0.07 2.01 0.04 0.012 0.31 — 1.01 5 0.61 0.26 0.20 0.010 0.005 0.07 2.00 0.04 0.022 0.30 — 0.91 6 0.48 0.40 0.44 0.006 0.005 0.07 1.96 0.05 0.015 0.23 — 0.71 7 0.45 0.70 0.44 0.006 0.005 0.07 1.96 0.04 0.015 0.31 — 0.76 8 0.60 1.01 0.41 0.009 0.005 0.07 2.01 0.04 0.012 0.31 — 0.91 9 0.62 0.99 0.39 0.011 0.005 0.07 1.98 0.30 0.015 0.30 — 0.92 Steel of Comparative 10 1.00 0.26 0.40 0.005 0.005 0.08 1.35 0.04 0.018 — — 1.00 Example 11 0.80 0.26 0.21 0.010 0.005 0.07 1.36 0.04 0.014 0.30 — 1.10 12 0.80 0.26 0.20 0.010 0.005 0.07 2.02 0.04 0.013 — — 0.80 13 0.60 0.26 0.20 0.010 0.005 0.08 2.00 0.04 0.017 — — 0.60 14 0.70 0.26 0.20 0.009 0.005 0.07 2.00 0.04 0.021 — 0.05 0.70 15 0.49 0.49 0.50 0.006 0.006 0.07 1.92 0.04 0.018 — 0.05 0.49 *Shaded values are outside the scope of the claims.

(36) Following forming the round bar steels described above, the round bar steels were subjected to normalizing, where they were held at 1000° C. for 15 minutes, then gas-cooled to 600° C., and then air-cooled. In this heat treatment, most part of V is dissolved in the matrix, with the rest being precipitated as V-containing fine carbides. Thereafter, the steels were roughly shaped into 10R C-notched Charpy impact test specimens, and those of Inventive Examples Nos. 1 to 9 and Comparative Examples Nos. 10, 12, 13, 14, and 15 were held at 950° C., in the austenite region of not lower than the dissolution temperature of cementite, for 60 minutes and then oil-quenched.

(37) In the above-described heat treatment, in the steels of Inventive Examples Nos. 1 to 9, the V-containing carbides contained therein, which are finely precipitated while the steels are heated and held during the quenching, serve to pin the grains. It should be noted that, for the steels of Inventive Examples Nos. 1 to 9, the heating temperature conditions for quenching were selected so as to fall within the scope claimed in the present invention, while the steels of Comparative Examples Nos. 10, 12, 13, 14, and 15 having no V added thereto were heated according to the heating conditions of the steels of Inventive Examples. The steel of Comparative Example No. 11, containing V and having the chemical components falling within the scope of the present invention, was subjected to normalizing and then spheroidizing annealing with the heating temperature of 810° C., and roughly shaped into a 10R C-notched Charpy impact test specimen. It was then subjected to processing of holding at a temperature of 810° C., in the dual phase range of cementite and austenite, for 30 minutes and oil-quenching, which processing was repeated twice. The heating conditions for quenching of this steel of Comparative Example No. 11 are conditions for measuring the Charpy impact value when V-added steel is heated within the dual phase range of cementite and austenite. This test was carried out for comparison with the steels of Inventive Examples Nos. 1 to 9 of the present application.

(38) Thereafter, all the roughly shaped test specimens were subjected to quenching and tempering, where they were held at a temperature range of 130° C. to 250° C. for low-temperature tempering for 180 minutes and then air-cooled. Further, the roughly shaped specimens were subjected to finishing work to obtain 10R C-notched Charpy impact test specimens.

(39) As for the heat treatment, although not performed in the above-described processing, the steels of Inventive Examples Nos. 1 to 9 and Comparative Examples Nos. 10, 12, 13, 14, and 15 may be additionally subjected to spheroidizing annealing after the normalizing processing for the purposes of improving the material workability. In such a case, the spheroidizing annealing conditions may be adjusted as appropriate in accordance with the steel types, not limited to the upper-limit temperatures described in the present examples.

(40) Table 2 shows hardness in terms of HRC, maximum diameter of V-containing carbides, amount of precipitated V-containing carbides with respect to total martensite volume, amount of precipitated cementite, prior austenite grain size, and Charpy impact value for the steels of Inventive Examples and Comparative Examples under the conditions of the embodiment of the invention.

(41) TABLE-US-00002 TABLE 2 Maximum Amount of diameter of precipitated Amount of Prior Charpy V-containing V-containing precipitated austenite impact Hardness carbides carbides cementite grain size value No. (HRC) (μm) (vol %) (vol %) (μm) (J/cm.sup.2) Steel of Inventive Example 1 60 0.43 0.36 0 9 112 2 60 0.46 0.34 0 8 112 3 59 0.36 0.34 0 11 151 4 59 0.47 0.30 0 7.5 239 5 59 0.37 0.25 0 4.5 239 6 58 0.35 0.25 0 11 140 7 57 0.32 0.16 0 5.7 234 8 60 0.36 0.35 0 6 148 9 60 0.38 0.46 0 4.8 180 Steel of Comparative Example 10 61 includes no 0 0 27 7 V-based carbide 11 61 0.65 0.67 0.71 9 41 12 60 includes no 0 0 28 14 V-based carbide 13 60 includes no 0 0 30 92 V-based carbide 14 60 includes no 0 0 12 60 V-based carbide 15 60 includes no 0 0 30 63 V-based carbide *Shaded values are outside the scope of the claims.

(42) The steels of Inventive Examples Nos. 1 to 9 are all excellent in toughness with the 10R C-notched Charpy impact value exceeding 100 J/cm.sup.2, while exhibiting high hardness of 57 HRC or more. Such high toughness is achieved because, with the steels of the present invention having indispensably added V, the test specimens do not suffer brittle fracture when hit by a Charpy impact tester, but experience ductile deformation to some extent before being fractured. The steels of Comparative Examples Nos. 10, 12, 13, 14, and 15 have no V added thereto. While the steel of Comparative Example No. 11 has V added thereto and its chemical components are within the scope of the present invention, the results of heat treatment fall outside the scope of the present invention. The steels of Comparative Examples all have a low impact value as compared to the steels of Inventive Examples.

(43) In particular, the results of No. 11 show that it is useful to control the microstructure appropriately, let alone the chemical components, to achieve satisfactory hardness and toughness simultaneously. It is also clear from the results of Nos. 14 and 15 that, while V and Nb are in the same group on the periodic table, they cannot be easily substituted because with V, satisfactory hardness and toughness can both be achieved whereas with Nb, Nb-containing carbides cannot be utilized effectively for pinning the grain boundaries, hindering achievement of good hardness and toughness at the same time. It has thus become apparent that adding V as an additive element is useful.

(44) It should be understood that the embodiment and the 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.