Steel with high hardness and excellent toughness

Abstract

Steel of high hardness and excellent toughness, containing, in mass percent, 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 2.50% or less Ni and 1.00% or less Mo, wherein the amount of (C+V) is 0.60% or more in mass percent, the remainder consisting of Fe and unavoidable impurities, and wherein the steel has a microstructure which is a martensitic structure with finely dispersed Fe-based ε-carbides and a preceding austenite grain size of 20 µm or less. characterized in that 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, which are precipitated and distributed therein, and that an amount of the precipitated V-containing fine carbides is 0.10 - 0.90 volume percent of a total martensite volume, and that in the microstructure, an amount of precipitated cementite in the martensitic structure tempered at a low temperature of 130°C to 250°C constitutes 0.50 volume percent or less of a total martensite volume.

Description

Technical FieldThe present invention relates to steels having high hardness and excellent toughness, particularly superior in wear resistance and durability, which are used for machines including automobiles, aircrafts, ships and other transport machines, earthmoving machines, construction machines and industrial machines for which components including gears, shafts and other drive system components, speed reducer components, excavation mechanisms or their peripheral mechanism components and bearing components are used.Prior ArtSteels used for components of transportation and other machines, particularly steels used for components requiring excellent wear resistance and fatigue properties, are usually hardened by quenching before they are used. A steel material having primarily a martensitic structure as a result of quenching has a hardness determined by its C (carbon) content. An increased C content results in an increased hardness of the steel material. Increasing the hardness of a steel material, however, lowers the toughness thereof, making the steel material susceptible to cracking upon impact. The steel material thus requires a good balance between hardness and toughness.In this regard, JP 2013-104070 A discloses a high-strength steel and a high-strength bolt made of this high-strength steel. The high strength steel contains, by mass, C: 0.20 to 1.50%, Si: 0.20 to 5.00%, Mn: 0.10 to 3.00%, P: 0.0005 to 0.1000%, S: 0.0005 to 0.2000%, and N: 0.0020 to 0.0200%, and further contains one or two kinds of Cr: 0.01 to 5.00%, and Mo: 0.01 to 1.00%, if necessary. The remaining part of the steel is made of Fe and unavoidable impurities. The produced steel structure is a quenched martensite structure in which Fe-based ε carbide is finely dispersed. In addition, the size of the Fe-based ε carbide in the quenched martensite is 1 to 20 nm, and its area ratio is not less than 1.0%.Moreover, as a conventional technique, an invention of a rolling bearing component for high-temperature use that has excellent rolling fatigue life under foreign matter penetrated conditions and high-temperature environments has been proposed (see, for example, JP 2000-204444 A). The proposed invention does not require addition of V as an indispensable element as in the present invention; rather, it restricts the maximum diameter of carbides within the structure obtained after annealing to 8 μm or less. While the proposed invention ensures excellent rolling fatigue life even if carbides of 8 μm or almost 8 μm are contained in diameter, there is no statement as to whether high toughness can be achieved at the same time. Patent Literature 1 fails to propose any measures for achieving high toughness.On the other hand, an invention of a high-hardness and excellent-toughness steel used for components of transportation and other machines has been proposed (see, for example, JP 2017-57 479 A). According to the proposed invention, the steel is heated to a temperature range of a dual phase of austenite and cementite and then quenched to obtain a structure of martensite and spherical cementite. The size, shape, and distribution state of carbides are controlled, and in particular, carbides at grain boundaries are removed to significantly improve toughness. However, with the proposed invention, heating in the dual phase region and subsequent quenching are indispensable. In order to ensure appropriate conditions of the carbides, the holding times and temperatures must be controlled precisely, which leads to an increased burden on the practical process steps.SUMMARY OF THE INVENTIONTechnical TaskIt is an object of the present invention to provide a high hardness and excellent toughness steel containing carbon at least at an intermediate carbon level, i.e., a steel called an intermediate carbon steel or a high carbon steel, which can be subjected to a simple heat treatment such as a high temperature quenching from an austenite range of not less than a dissolution temperature of cementite.Solution of the ProblemThis object is achieved according to the invention by providing a steel having high hardness and excellent toughness, which has the features of independent claim 1.When a steel containing carbon at least at an intermediate carbon level as its chemical component is subjected to high temperature quenching from the austenite region, normally, cementite is completely dissolved at a high heating temperature, so that the steel loses the grain boundary pinning effect, resulting in coarsened austenite grains. The grain size, i.e., the prior austenite grain size, will remain coarse after quenching, in which case intergranular fracture, which is brittle fracture, would likely occur, resulting in reduced toughness.In view of the above, the present invention employs a steel containing carbon of at least an intermediate carbon content as its chemical component and having V added thereto. When V is contained as an indispensable additional element, V-containing fine carbides present in the austenite region and reaching a high processing temperature serve to fix the migration of austenite grain boundaries, so that the austenite grain size can be maintained fine. Accordingly, the grain size of martensite produced after quenching can be fine-grained and ductile fracture becomes predominant, thereby obtaining high toughness. More specifically, the inventors have found that the effects of the present invention can be obtained by the solutions of the present invention as follows:More specifically, the present invention according to independent claim 1 provides a high hardness and excellent toughness steel which contains, by mass percent, 0.40-1.00% of C, 0.10-2.00% of Si, 0.10-1.00% of Mn, 0.030% or less of P, 0.030% or less of S, 1.10 to 3.20% of Cr, 0.010 to 0.10% of Al and 0.15 to 0.50% of V and further contains at least one of 2.50% or less of Ni and 1.00% or less of Mo, wherein the amount of (C+V) is 0.60% or more by mass percent, and the balance is made of Fe and inevitable impurities. The steel has a microstructure which is a martensitic structure having finely divided Fe-based ε carbides and a preceding austenite grain size of 20 μm or less. The martensitic structure tempered at a low temperature of 130° C. to 250° C. includes V-containing fine carbides having a diameter of 0.50 μm or less precipitated therein, an amount of the precipitated V-containing fine carbides forms 0.10 - 0.90 volume percent of a total martensite volume, and in the microstructure, an amount of precipitated cementite in the martensitic structure tempered at a low temperature of 130° C. to 250° C. forms 0.50 volume percent or less of a total martensite volume.Effects of the InventionIn the present invention, high hardness that would not be obtained by high temperature annealing is obtained by using the martensite structure having Fe-based ε carbides finely dispersed as a result of low temperature annealing of 130° C. to 250° C. Moreover, with V contained as an indispensable additional element, V-containing fine carbides existing at the heating temperature for quenching serve to restrain the migration of austenite grain boundaries, whereby austenite grains having the grain size of not larger than 20 μm can be fine-held. Accordingly, the martensitic structure having the foregoing austenite grain size of 20 μm or less becomes fine after quenching, and thus the ductile fracture becomes predominant as a fracture mode, thereby achieving high toughness. These configurations produce useful effects in that components requiring high toughness, such as those for transportation and other machines, can be provided by making steel components using the high-hardness and high-toughness steel.Furthermore, in the martensitic structure, V-containing fine carbides having a diameter of 0.50 μm or less are dispersedly precipitated. When the precipitated amount is set to 0.10-0.90 volume percent of the total martensite volume, the grain refining effects can be obtained without causing a decrease in toughness due to the intrinsic brittleness of the V-containing fine carbides, and an coarsening of the foregoing austenite grain size is prevented, thereby achieving a high toughness together with a high hardness.Moreover, the amount of the precipitated cementite in the martensitic structure tempered at a low temperature of 130° C. to 250° C. is set to 0.50 volume percent or less of the total martensite volume. While cementite is usually expected to grow at grain boundaries and cause cracking along the boundaries after quenching and tempering, the amount of the precipitated cementite is quantitatively limited in the present invention, thereby preventing the decrease in toughness.DESCRIPTION OF AN EMBODIMENTBefore describing an embodiment of the present invention, the individual features of the invention according to the solutions of the present invention will be explained below in the following order: the reasons for restricting the chemical components of the steel except for Fe and inevitable 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 restricting the size and the precipitated amount of V-containing carbides in the martensitic structure, the reasons for restricting the ratio of the amount of precipitated cementite in the martensitic structure with respect to the entire martensite volume, and the reasons for restricting the foregoing austenite grain size. Note that %, which is used for chemical components, means mass percent.C: 0.40-1.00%C is an element that improves hardness, wear resistance, and fatigue life after quenching and tempering. However, if the carbon content is less than 0.40%, sufficient hardness cannot be obtained. On the other hand, if the carbon content is more than 1.00%, toughness will be impaired and, moreover, the hardness of the steel material will increase, which deteriorates workability such as chipping and workability. Therefore, the carbon content is set to 0.40-1.00%, preferably 0.50-1.00%, and more preferably 0.50-0.90%.Si: 0.10 - 2.00%Si is an element effective in deoxidation of the steel and serves to impart a required hardenability to the steel and improve its strength. In order to achieve these effects, the Si content must be 0.10% or more, or preferably 0.20% or more. On the other hand, if Si is contained in a large amount, it will increase the hardness of the material, which deteriorates the workability such as chipping and oscillability. Therefore, it is necessary to keep the Si content at 2.00% or less, and preferably 1.55% or less. Therefore, the Si content is set to 0.10-2.00%, and preferably 0.20-1.55%.Mn: 0.10 - 1.00%Mn is an element which is effective in deoxidation of the steel and is necessary for imparting the required hardenability to the steel and for improving its strength. For this purpose, the Mn content must be 0.10% or more, or preferably 0.15% or more. On the other hand, when Mn is contained in a large amount, it lowers the toughness. Further, it may combine with S to form MnS, which also reduces toughness or contributes to cracking during processing. It is therefore necessary to keep the Mn content at 1.00% or less, and preferably 0.70% or less. Accordingly, the Mn content is set to 0.10 to 1.00%, preferably 0.15 to 1.00%, and more preferably 0.15 to 0.70%.P: 0.030% or lessP is an impurity element inevitably contained in the steel. P precipitates in the grain boundary and deteriorates the toughness. Therefore, the P content is set to 0.030% or less, and preferably to 0.015% or less.S: 0.030% or lessS is an element that combines with Mn to form MnS, and deteriorates toughness. Therefore, the S content is set to 0.030% or less, and preferably to 0.010% or less.Cr: 1.10 - 3.20%Cr is an element that improves hardenability. In order to satisfactorily achieve the effect, the Cr content must be 1.10% or more, preferably 1.20% or more, and more preferably 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 after quenching, adversely affecting toughness. In order to avoid this, it is necessary to maintain the Cr content at 3.20% or less, preferably 2.50% or less, and more preferably 2.30% or less. Therefore, the Cr content is set to 1.10-3.20%, preferably 1.20-2.50%, and more preferably 1.35%-2.30%.Al: 0.010 - 0.10%Al is added because it is an indispensable element for deoxidation of the steel. Further, Al may combine with N to produce AlN, thereby preventing grain coarsening. In order to achieve these effects, the Al content must be 0.010% or more. On the other hand, if Al is added in a large amount, thermoformability will be impaired. Therefore, it is necessary to keep the Al content at 0.10% or less, and preferably 0.050% or less. Therefore, the Al content is set to 0.010 - 0.10%, and preferably 0.015 - 0.050%.V: 0.15-0.50%V is an element indispensable for achieving high toughness by refining grains because V forms fine carbides with C, and the carbides serve to define grain boundaries at the time of heating for quenching, thereby fine-keeping the grains. In order for the carbides to effectively define grain boundaries in the steel, the steel must be heated once to a temperature not lower than the carbide dissolution temperature in order to have the carbides dissolved, so that the carbides are finely precipitated at the time of heating to a quenching temperature. In this regard, however, it would not be possible to sufficiently dissolve the carbides even by heating the steel to 1250°C, which is considerably above the practical heating temperature of steel materials, if Nb, Ti or other carbide-forming element were added in terms of the C content in the components of the present invention. The pinning effect of the carbides would be insufficient and coarse carbides would likely remain, adversely affecting toughness. In contrast, V-containing carbides are dissolved at a lower temperature, so that they can be effectively used for fixing grain boundaries. In order to achieve this effect, V must be added in an amount of 0.15% or more, preferably 0.20% or more, and more preferably 0.25% or more. On the other hand, when V is contained in an amount of more than 0.50%, the effect of refining the grains is saturated and, moreover, coarse carbides containing V are formed, which carbides may impair hot workability or result in reduced toughness. Therefore, it is necessary to keep the V content at 0.5% or less, and preferably 0.45% or less. Therefore, the V content is set to 0.15-0.50%, preferably 0.20-0.50%, and more preferably 0.25-0.45%.Ni and Mo are elements, one or both of which are contained. They are limited for the following reasons:Ni: 2.50% or lessWhile Ni may be contained as an impurity (for example, in an amount of 0.07%) in the present invention, Ni is an element effective in improving hardenability and toughness, thus it may be added intentionally. On the other hand, Ni is an expensive element that increases the cost. When it is added, therefore, the Ni content is set to 2.50% or less, and preferably to 1.70% or less.Mo: 1.00% or lessWhile Mo may be contained as an impurity (for example, in an amount of 0.04%) in the present invention, Mo is an element effective in improving hardenability and toughness, thus it may be added intentionally. On the other hand, Mo is an expensive element that increases the cost. When it is added, therefore, the Mo content is set to 1.00% or less, and preferably 0.50% or less.C+V: 0.60% or moreIn order to achieve the grain refining function by distributing V-containing fine carbides, it is necessary to set a total amount of C and V to at least 0.60% or more.(Causes for causing the microstructure to be a martensitic structure with Fe-based ε carbides finely dispersed)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 dispersed therein. Martensite with finely divided ε carbides is obtained by low temperature annealing at 130°C to 250°C. The steel of the present invention is capable of attaining the high toughness state upon quenching due to the chemical components and other limitations defined in the solutions of the present invention, and the excellent toughness is maintained at the low temperature annealing of 130°C to 250°C, so that alloying elements need not be added 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 annealing carried out at a temperature of 500° C. or higher instead of low-temperature annealing, the hardness will be lowered due to the small amount of alloying elements contributing to secondary hardening. Further, although the toughness may become higher in this case, high hardness cannot be obtained, making it difficult to acquire the required high hardness and high toughness. Therefore, the martensitic structure having Fe-based ε carbides finely dispersed therein as a result of low temperature annealing of 130°C to 250°C is employed.(Reasons for setting the maximum diameter of the V-containing carbides in martensite to 0.50 μm or less and the amount of precipitated V-containing carbides to 0.10-0.90 volume percent of the total martensite volume)When V-containing fine carbides having a diameter of 0.50 μm or less are dispersed in martensite, the foregoing austenite grain size is prevented from being coarsened and is limited to 20 μm or less, so that high toughness can be obtained simultaneously with high hardness. If the V-containing carbides that are dispersed have a diameter of 0.50 μm or more, the grain refining effect will become low and the toughness will decrease. If the amount of the precipitated V-containing carbides is less than 0.10 volume percent of the total martensite volume in terms of volume percent, the effect of refining the foregoing austenite grain size cannot be sufficiently obtained. For this reason, the amount of the precipitated V-containing fine carbides is preferably set to 0.15 volume percent or more. On the other hand, if the amount of the precipitated V-containing fine carbides exceeds 0.90 volume percent, the precipitated amount becomes too much, making the grains containing the V-containing carbides themselves brittle, resulting in reduced toughness. It is therefore set to 0.90 volume percent or less, and preferably 0.80 volume percent or less. Therefore, 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 is 0.40-0.90 volume percent, and preferably 0.15-0.80 volume percent of the total martensite volume.(Reasons for setting the ratio of the amount of precipitated cementite to the total martensite volume to 0.50 volume percent or less at maximum)Cmentite would be expected to grow at the austenite grain boundaries during heating, which may cause cracking along the grain boundaries after quenching and tempering, thereby degrading toughness. Therefore, the amount of the precipitated cementite is controlled to be 0.50 volume percent or less at the maximum of the total martensite volume.(Reasons for setting the foregoing austenite grain size to 20 μm or less, and preferably to 15 μm or less)If the foregoing austenite grain size in the quenched and tempered state is made fine, brittle fracture can be prevented, resulting in improved toughness. Moreover, if the foregoing austenite grain size is made small, the grain boundary area in the volume increases, and impurity elements such as P and S, which would cut off in the grain boundaries and deteriorate the toughness, are distributed over many grain boundaries, so that the amount of deposited impurities at the individual grain boundaries can be reduced, which also contributes to improved toughness. Therefore, the foregoing austenite grain size is set to 20 μm or less, and preferably to 15 μm or less.Hereinafter, an embodiment of the present invention will be described with reference to Examples and Tables.ExamplesSteels having the chemical compositions of Inventive Examples Nos. 1 to 9 and Comparative Examples Nos. 10 to 15 as shown in Table 1 below were produced in a 100 kilogram vacuum furnace. The obtained steels were each subjected to hot working at 1150°C to obtain a round steel of 26 mm in diameter. Note that Table 1 shows indispensable chemical components as well as P and S as impurities, with the remaining Fe and other unavoidable impurities omitted in Table 1. After forming the round steels described above, the round steels were subjected to normalization in which they were maintained at 1000° C. for 15 minutes, then gas-cooled to 600° C., and then air-cooled. In this heat treatment, most of the V is dissolved in the matrix, and the remainder precipitates as V-containing fine carbides. Thereafter, the steels were roughly formed into 10R-C Charpy impact bending test specimens, and those of Invention Examples Nos. 1 to 9 and Comparative Examples Nos. 10, 12, 13, 14 and 15 were maintained at 950°C in the austenite range not lower than the dissolution temperature of cementite for 60 minutes, and then oil-quenched.In the above-described heat treatment, in the steels of Inventive Examples Nos. 1 to 9, the V-containing carbides contained therein, which precipitate finely while the steels are heated and held during quenching, serve to fix the grains. Note that the quenching heating temperature conditions for the steels of Inventive Examples Nos. 1 to 9 were selected to be 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 within the scope of the present invention was subjected to normalization and then spheroidizing annealing at a heating temperature of 810°C, and roughly molded into a 10R-C Charpy impact bending test specimen. It was then subjected to a process of holding at a temperature of 810°C in the dual phase region of cementite and austenite for 30 minutes and oil quenching, which process was repeated twice. The heating conditions for quenching 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.Thereafter, all the roughly-shaped specimens were subjected to quenching and tempering, where they were maintained at a temperature range of 130° C. to 250° C. for low-temperature tempering for 180 minutes, and then were air-cooled. Moreover, the roughly-shaped specimens were subjected to post-processing to obtain 10R-C Charpy impact test specimens.Although not carried out in the above-described processing, the steels of Inventive Examples Nos. 1 to 9 and Comparative Examples Nos. 10, 12, 13, 14 and 15 as the heat treatment may be additionally subjected to spheroidizing annealing after the normalization processing to improve the material processability. In such a case, spheroidizing annealing conditions can be adjusted appropriately in accordance with the types of steels, and are not limited to the temperature upper limits described in the present examples.Table 2 shows the hardness with respect to HRC, the maximum diameter of the V-containing carbides, the amount of the precipitated V-containing carbides with respect to the total martensite volume, the amount of the precipitated cementite, the foregoing austenite grain size, and the Charpy impact value for the steels of the invention examples and the comparative examples under the conditions of the embodiment of the invention. The steels of Inventive Examples Nos. 1 to 9 all have excellent toughness, wherein the 10R-C Charpy impact value exceeds 100 J / cm 2 while showing a high hardness of 57 HRC or more. Such high toughness is achieved because with the steels of the present invention having V incorporated inevitably, the test specimens do not suffer brittle fracture when struck by a Charpy impact tester, but exhibit ductile deformation to some extent before they are broken. 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 therein and its chemical components are within the scope of the present invention, the results of the heat treatment are outside the scope of the present invention. The steels of the comparative examples all have a low notch impact value compared with the steels of the invention examples.More specifically, the results of No. 11 show that it is useful to appropriately control the microstructure, let alone the chemical components, to simultaneously achieve satisfactory hardness and toughness. From the results of Nos. 14 and 15, it is also apparent that although V and Nb are in the same group in the periodic table, they cannot be easily substituted because both satisfactory hardness and toughness can be obtained with V, while Nb-containing carbides cannot be effectively used for fixing grain boundaries, which at the same time hinders the achievement of good hardness and toughness. It has thus been found to be useful to add V as an additional element.It is to be understood that the embodiment and examples disclosed herein are illustrative and not restrictive in all respects. The scope of the present invention is defined by the terms of the claims rather than the above description, and is intended to include all modifications within the scope and meaning corresponding to the terms of the claims.

Claims

A steel having high hardness and excellent toughness, comprising, by mass percent, 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 comprising at least one of 2.50% or less Ni and 1.00% or less Mo, wherein the amount of (C+V) is 0.60% or more by mass percent, the balance being Fe and unavoidable impurities, and wherein the steel has a microstructure which is a martensitic structure having finely divided Fe-based ε carbides and a preceding austenite grain size of 20 μm or less, characterized in that, wherein the martensitic structure tempered at a low temperature of 130° C. to 250° C. includes V-containing fine carbides having a diameter of 0.50 μm or less precipitated therein, an amount of the precipitated V-containing fine carbides forms 0.10 - 0.90 volume percent of a total martensite volume, and an amount of precipitated cementite in the martensitic structure tempered at a low temperature of 130° C. to 250° C. forms 0.50 volume percent or less of a total martensite volume.