High-hardness, high-thermal-expansion steel

Abstract

To provide a steel having both high hardness and a high thermal expansion coefficient at low cost.SOLUTION: A high-hardness and high-thermal-expansion steel includes, in mass%, C: more than 0.10 to 0.60%, Si: 0.05 to 0.80%, Mn: 2.0 to 10.0%, P: 0.050% or less, S: 0.050% or less, Ni: 6.0 to 15.0%, Cr: 6.0 to 18.0%, Mo: 0.01 to 0.50%, Cu: 0.05 to 0.75%, Al: 0.001 to 0.100%, V: more than 0.50 to 3.00%, and N: 0.01-0.10%, with the balance being Fe and inevitable impurities, and satisfies Formula 1, Formula 2, and Formula 3. Formula 1: V / {4([C]+[N])}=0.5-1.5, Formula 2: 551-462([C]+[N]-0.07[V])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])-18.5[Mo]≤-40, and Formula 3: {(TA+273)×(20+log ta)} / 1000=17.0-22.0.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to an inexpensive steel material that possesses high hardness, high corrosion resistance, and a high coefficient of thermal expansion, making it suitable for tool holders that fix cutting tools by shrink-fitting. [Background technology]

[0002] In the manufacturing of various parts, machining processes such as drilling holes and cutting with inserts are frequently used. One method for accurately and stably machining parts to the desired shape is to use a tool in which the cutting tool and holder are joined by shrink fitting.

[0003] This method utilizes the thermal expansion of metal. For example, in shrink-fitting with a drill, a hole in a metal holder, which is machined to a size slightly smaller than the drill diameter, is heated and expanded to exceed the outer diameter of the drill. After inserting the drill, it is fixed in place by cooling and shrinking.

[0004] In production lines using these tools, increasing the speed of tool changes for cutting and drilling tools leads to increased production speed. Therefore, the metal material for shrink-fit holders must not only have the necessary strength to function as a holder, but also a high coefficient of thermal expansion that allows for easy expansion of the tool fixing space at the lowest possible temperature.

[0005] When considering steels suitable for shrink-fit applications, SUH660 heat-resistant steel, which possesses properties that balance hardness and thermal expansion, is cited as a suitable steel within the Japanese Industrial Standards (JIS). However, even if one were to repurpose SUH660, which is already a heat-resistant steel bar or wire, it would be a steel with high material costs due to its alloy composition containing a very high amount of Ni (24.0-27.0%). Furthermore, achieving the desired properties requires a heat treatment aging period of 15 hours or more, resulting in high manufacturing costs. Because SUH660 is such an expensive steel, it is not a material that can be easily repurposed.

[0006] Furthermore, a steel sheet for heat shrink bands has been proposed, characterized by containing C: 0.01% or less, Si: 1% or less, Mn: 0.1% to 2%, sol.Al: 0.01% or less, N: more than 0.005% to 0.1%, with the remainder being substantially Fe, and having a product μ × t of permeability μ and plate thickness t (mm) of 380 or more in a magnetic field of 0.3 Oe after shrink-fitting (see Patent Document 1). However, since it is a band that tightens around the panel portion of a color cathode tube, it is merely a material that can be shrink-fitted, and the strength required to hold cutting and drilling tools was not sufficient to begin with. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2001-032040 [Patent Document 2] Patent No. 6504870 [Overview of the project] [Problems that the invention aims to solve]

[0008] While common austenitic stainless steels exhibit high thermal expansion and good corrosion resistance, they have low hardness, and materials like SUH660 are considered too high-grade. Each material already on the market has its own challenges and drawbacks.

[0009] The problem that this invention aims to solve is to provide, at low cost, a steel material that possesses both high hardness and a high coefficient of thermal expansion, which is suitable from the viewpoint of productivity as a material for shrink-fit holders used in production lines that perform cutting and drilling processes.

[0010] Furthermore, for tools that are exposed to cutting fluid (including water) and used for extended periods, these steel materials must also possess corrosion resistance.

[0011] The present invention aims to provide, at low cost, a steel material that, in addition to corrosion resistance, is particularly suitable for use on the concave side of components and tools that utilize shrink fitting, and that achieves both high hardness and a high coefficient of thermal expansion. [Means for solving the problem]

[0012] The inventors of this invention have diligently investigated how to actively utilize Mn as an element to stabilize the γ phase, which has a high coefficient of thermal expansion. This will reduce the amount of expensive Ni added, thereby suppressing cost increases, while simultaneously achieving high hardness with V(C,N) precipitates, which precipitate and grow faster than intermetallic compounds, and further reducing costs by shortening the required heat treatment time.

[0013] Therefore, in order to provide a material that is inexpensive and possesses both high hardness and a high coefficient of thermal expansion, we came up with the idea of ​​using a steel that satisfies the following three points.

[0014] First, the range of alloy components for the steel was defined, and then, by further defining it to satisfy equation 1: V / {4([C]+[N])}=0.5~1.5, it was decided to obtain a low-cost, high-hardness, and high-coefficient-of-thermal-expansion steel.

[0015] Next, by specifying that equation 2:551-462([C]+[N]-0.07[V])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])-18.5[Mo] satisfies -40 or less, it was determined that the γ phase, which has a high coefficient of thermal expansion, is stable even at room temperature.

[0016] Furthermore, Equation 3: Equation 3: {(T A (+273) × (20 + log t) a By specifying the range that satisfies )} / 1000 = 17.0 to 22.0, it is possible to perform aging heat treatment that increases the coefficient of thermal expansion while ensuring high hardness.

[0017] Incidentally, the present inventor has also developed a non-magnetic corrosion-resistant steel material with high strength and excellent hydrogen embrittlement resistance (see Patent Invention 2). Since this patent invention is for a steel with a completely different application purpose for use in an environment where contact with hydrogen is impossible, there is no mention or suggestion whatsoever of the term "expansion", and in fact, the invention does not consider at all things such as a high coefficient of thermal expansion. Although it is a steel for a completely different application, the present inventor considered that in view of the high coefficient of thermal expansion of the γ phase, if the steel for hydrogen embrittlement resistance in Patent Document 2 containing an austenite structure excellent in hydrogen embrittlement resistance was further advanced, there might be room to expect the effects of the present invention. Therefore, considering that the rationality of the three ideas in the present invention would not be denied, the present inventor decided to proceed with the study of the present invention.

[0018] Therefore, the first means for solving the problems of the present invention is a high-hardness and high-thermal-expansion steel which, in mass%, contains C: over 0.10% to 0.60%, Si: 0.05% to 0.80%, Mn: 2.0% to 10.0%, P: 0.050% or less, S: 0.050% or less, Ni: 6.0% to 15.0%, Cr: 6.0% to 18.0%, Mo: 0.01% to 0.50%, Cu: 0.05% to 0.75%, Al: 0.001% to 0.100%, V: over 0.50% to 3.00%, N: 0.01% to 0.10%, with the balance being Fe and inevitable impurities, and satisfies Formula 1, Formula 2, and Formula 3. However, Formula 1 is Formula 1: V / {4([C]+[N])}=0.5 to 1.5, Formula 2 is Formula 2: 551 - 462([C]+[N] - 0.07[V]) - 9.2[Si] - 8.1[Mn] - 13.7[Cr] - 29([Ni]+[Cu]) - 18.5[Mo] ≤ -40, and Formula 3 is Formula 3: { (T A +273)×(20 + log t a )} / 1000 = 17.0 to 22.0. For each element symbol in the formulas, the value of the mass% of its component is substituted. Also, T A is the aging heat treatment temperature (°C), and t a is the aging holding time (hr).

[0019] The second means is a high-hardness and high-thermal-expansion steel that contains, in addition to the chemical components described in the first means, one or more of B: 0.010% or less, Ca: 0.050% or less, and Mg: 0.050% or less by mass%, with the balance being composed of Fe and unavoidable impurities and satisfying Formula 1, Formula 2, and Formula 3. However, Formula 1 is Formula 1: V / {4([C]+[N])}=0.5~1.5, Formula 2 is Formula 2: 551-462([C]+[N]-0.07[V])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])-18.5[Mo]≦-40, and Formula 3 is Formula 3: {(T A +273)×(20+log t a )} / 1000=17.0~22.0. In each formula, the value of the mass% of the component is substituted into the element symbol. Also, T A is the aging heat treatment temperature (°C), and t a is the aging holding time (hr).

Advantages of the Invention

[0020] The steel of the present invention that satisfies the chemical components defined in the present invention and Formulas 1 to 3 is an inexpensive material, yet has a high hardness with an aging hardness of 31 HRC or more, and also has a high thermal expansion coefficient of 17.0×10 -6 / °C or more from room temperature to 200°C. Therefore, as a high-hardness and high-thermal-expansion steel in which high hardness and high thermal expansion coefficient are compatible, the steel of the present invention is suitable as a material for shrink-fit fixtures.

[0021] In addition, the steel of the present invention can stably perform hot working such as forging without frequent cracking and becoming impossible to process. Furthermore, it exhibits excellent corrosion resistance such that the number of pitting rusts in a salt spray test is 20 or less.

Embodiments for Carrying Out the Invention

[0022] Prior to the description of the embodiments for carrying out the present invention, the reasons for defining the chemical components of the steel of the present invention and the reasons for defining Formulas 1, 2, and 3 will be described. In the chemical components, % is mass%.

[0023] C: more than 0.10~0.60%, Carbon (C) is a necessary component because it strengthens the precipitation by forming V(C,N) and increases the coefficient of thermal expansion through solid solution. From this perspective, the amount of C should be greater than 0.10%. On the other hand, if there is too much C, the corrosion resistance will be reduced due to the formation of coarse carbonitrides. Therefore, the amount of C should be 0.60% or less.

[0024] Si: 0.05~0.80%, Si is a component that acts as a deoxidizing agent during the steelmaking process. Therefore, the Si content should be 0.05% or more. On the other hand, if there is too much Si, the ductility of the material will decrease, and the thermal expansion coefficient will decrease due to ferrite formation. From these perspectives, the Si content should be 0.80% or less.

[0025] Mn: 2.0~10.0%, Mn is a relatively inexpensive austenite-stabilizing element and a useful component for ensuring a high coefficient of thermal expansion. From this perspective, the amount of Mn should be 2.0% or more. However, if Mn is added in excess, the coefficient of thermal expansion will decrease. Therefore, the amount of Mn should be 10.0% or less.

[0026] P: 0.050% or less, P is an unavoidable impurity. If the P content exceeds 0.050%, the ductility, toughness, and hot workability will be inferior. Therefore, the P content should be kept below 0.050%.

[0027] S: 0.050% or less, S is an unavoidable impurity. If the S content exceeds 0.050%, the ductility, toughness, and hot workability will be inferior. Therefore, the S content should be kept below 0.050%.

[0028] Ni: 6.0~15.0%, Ni is a useful component for γ-stabilization and ensuring a high thermal expansion coefficient. Therefore, the amount of Ni should be 6.0% or more. On the other hand, if the amount of Ni is excessive, the effect saturates and costs increase. Therefore, the amount of Ni should be 15.0% or less.

[0029] Cr: 6.0~18.0%, Cr is a useful component for improving corrosion resistance. From this perspective, the Cr content should be 6.0% or more. On the other hand, if the Cr content is excessive, the effect will saturate, and the thermal expansion coefficient will decrease due to ferrite formation. Therefore, the Cr content should be 18.0% or less.

[0030] Mo: 0.01~0.50%, Mo is a useful component for improving corrosion resistance. From this perspective, the Mo content should be 0.01% or higher. On the other hand, excessive Mo content increases costs. Also, the formation of ferrite reduces the coefficient of thermal expansion. Therefore, the Mo content should be 0.50% or less.

[0031] Cu: 0.05~0.75%, Cu is a γ-stabilizing element and is a useful component for ensuring a high coefficient of thermal expansion. From this perspective, the amount of Cu should be 0.05% or more. On the other hand, if the amount of Cu is excessive, the hot workability will be poor. Therefore, the amount of Cu should be 0.75% or less.

[0032] Al: 0.001~0.100%, Al is a useful component for deoxidation. Therefore, the amount of Al should be 0.001% or more. Preferably, the amount of Al should be 0.015% or more. On the other hand, if the amount of Al is excessive, the ductility decreases and the coefficient of thermal expansion decreases due to the formation of ferrite. From these viewpoints, the amount of Al should be 0.100% or less.

[0033] V: more than 0.50~3.00%, V is a useful component for strengthening precipitation through the formation of V(C,N). From this perspective, V should be greater than 0.50%. On the other hand, if V is excessive, the corrosion resistance will be poor due to the formation of coarse carbonitrides. It will also increase costs. Therefore, V should be 3.00% or less.

[0034] N: 0.01~0.10%, N is a useful component for precipitation strengthening through the formation of V(C,N), matrix strengthening through solid solution, and improvement of corrosion resistance. From these perspectives, the N content should be 0.01% or more. On the other hand, if the N content is excessive, corrosion resistance will be reduced due to the formation of coarse carbonitrides. Furthermore, the formation of nitrides will reduce ductility. In addition, manufacturing costs will increase. Therefore, the N content should be 0.10% or less.

[0035] The steel of the present invention may contain one or more of B, Ca, and Mg as selective additive components within the following ranges.

[0036] B: 0.010% or less, Component B improves hot workability, resulting in good manufacturability. However, if the amount is too high, its effect will saturate, and the hot workability will actually worsen. From this perspective, the amount of B should be 0.010% or less.

[0037] Ca: 0.050% or less, Ca is an ingredient that improves hot workability, resulting in good manufacturability. However, if it is present in too much, its effect will saturate, and the hot workability will actually deteriorate. From this perspective, the amount of Ca should be kept below 0.050%.

[0038] Mg: 0.050% or less, Mg is an ingredient that improves hot workability, resulting in good manufacturability. However, if the amount is too high, its effect will saturate, and the hot workability will actually deteriorate. From this perspective, the amount of Mg should be 0.050% or less.

[0039] Next, we will explain the reasons for specifying that equations 1, 2, and 3 must be satisfied.

[0040] Equation 1: V / {4([C]+[N])}=0.5~1.5. Substitute the mass percentage value of the element for the elemental symbol in Equation 1. In Equation 1, V / {4([C]+[N])} is an indicator that shows how effectively the components V, C, and N can be utilized for precipitation hardening. If V / {4([C]+[N])} is less than 0.5, corrosion resistance deteriorates due to excess C and N. On the other hand, if V / {4([C]+[N])} exceeds 1.5, the cost increases due to excess V. Therefore, the value of V / {4([C]+[N])} is assumed to be within the range of 0.5 to 1.5.

[0041] Formula 2:551-462([C]+[N]-0.07[V])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])-18.5[Mo]≦-40, Note that the elemental symbols in Equation 1 should be replaced with the mass percentage values ​​of those components. Equation 2 is an index representing the stability of the γ phase, which has a large coefficient of thermal expansion. If 551-462([C]+[N]-0.07[V])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])-18.5[Mo] shows a value of -40 or less, the lower the value, the more stable the γ phase is, and therefore the higher its coefficient of thermal expansion.

[0042] Equation 3: {(T A (+273) × (20 + log t) a )} / 1000=, Note that the elemental symbols in Equation 3 should be replaced with the mass percentage values ​​of those components. Also T A The aging heat treatment temperature (°C), t a This refers to the statute of limitations period (hr). Equation 3 is an index based on the temperature and holding time of the aging heat treatment. By satisfying the value of 17.0 to 22.0 in Equation 3, high hardness can be ensured by precipitation hardening, and a decrease in the coefficient of thermal expansion due to excessive precipitation (decrease in solid solution alloy elements) can also be avoided. In other words, Equation 3 is an index for maximizing hardness and the coefficient of thermal expansion. {(T A (+273) × (20 + log t) a Equation 3 is considered satisfied when the value of )} / 1000 is between 17.0 and 22.0.

[0043] <Examples> The manufacturing method for high-hardness, high-thermal-expansion steel that achieves both high hardness and high thermal expansion coefficient according to the present invention will be described below through a series of examples. First, steels having the chemical compositions of Invention Examples No. 1 to 17, which are embodiments of the present invention shown in Table 1, and Comparative Examples No. 18 to 39, shown in Table 2, were melted and refined under vacuum using a 100 kg vacuum induction melting furnace (VIM) to obtain steel ingots. Next, this steel ingot was heated to 1150°C and forged to a diameter of 15 mm. Furthermore, these materials undergo a solution heat treatment by heating and holding them at 1250°C or below (1000~1250°C) for 10 minutes or more, followed by water quenching under the temperature conditions T listed in Tables 1 and 2. A and retention time t a As described, an aging treatment was performed by heating and holding, followed by air cooling. Subsequently, test specimens were appropriately prepared using the obtained steel for each of the following tests.

[0044] [Table 1]

[0045] [Table 2]

[0046] <Hot workability> For parts that were successfully forged to a diameter of 15 mm in the above process, they were marked with a circle (○) in the φ15 forging column of Tables 3 and 4, indicating excellent hot workability. On the other hand, parts that had poor hot workability and were unable to continue processing due to frequent cracking were marked with a cross (×).

[0047] <Aging hardness (HRC hardness)> Age hardness was measured using Rockwell hardness, with a rating of ○ for 34 HRC or higher (excellent), △ for 31-33 HRC (acceptable), and × for less than 31 HRC (poor). The results are shown in Tables 3 and 4.

[0048] <Coefficient of thermal expansion> The average linear thermal expansion coefficient was measured from room temperature to 200°C. 17.0 × 10⁻⁶ -6 A value of 17.0 × 10°C or higher indicates excellent thermal expansion, and 17.0 × 10°C is marked with a circle (○). -6 Values ​​below / ℃ were marked with an asterisk (×) indicating poor thermal expansion, and are shown in Tables 3 and 4.

[0049] <Corrosion resistance evaluation> First, the test pieces were processed into rod-shaped corrosion test specimens measuring φ12 × 21 mm L. Then, a salt spray test was conducted on each of these specimens by spraying them with a 50 ppm dilute salt solution at 35°C for 16 hours. After the salt spray test, the surface of the test pieces was observed, and those with 20 or fewer point rust spots of 1 mm or larger in length were marked with ○ for excellent corrosion resistance, and those with more than 21 spots were marked with × for poor corrosion resistance, as shown in Tables 3 and 4.

[0050] [Table 3]

[0051] [Table 4]

[0052] As shown in Table 3, it was confirmed that all of the invention examples No. 1 to 17 are steels that exhibit excellent hot workability, have an age hardness of 31 HRC or higher, have excellent thermal expansion resistance, and also possess corrosion resistance.

[0053] On the other hand, as shown in Table 4, the comparative examples were evaluated in the following order: hot workability, age hardness, thermal expansion, and corrosion resistance. However, if any of these evaluations were poor, subsequent evaluations were omitted. Of Comparative Examples No. 18-39, Comparative Examples No. 22, 23, 27, and 32-34 had poor hot workability and could not be properly forged. Therefore, these lacked the prerequisites for confirming hardness and thermal expansion properties, and their practicality could not be recognized without even evaluating them. Comparative Example No. 18 had an excessive amount of carbon, resulting in poor corrosion resistance. Comparative Example No. 19 had insufficient carbon (C), and therefore did not achieve age hardness. Comparative Example No. 20 had an excessive amount of Si, resulting in poor thermal expansion properties. Comparative Example No. 21 had an excessive amount of Mn, resulting in poor thermal expansion properties. Comparative Example No. 24 had insufficient Ni content, resulting in poor thermal expansion properties. Comparative Example No. 25 had insufficient Cr content, resulting in poor corrosion resistance. Comparative Example No. 26 had an excessive amount of Cr, resulting in poor thermal expansion properties. Comparative Example No. 28 had an excessive amount of Al, resulting in poor thermal expansion properties. Comparative Example No. 29 had an insufficient V value, resulting in inferior hardness. Comparative Example No. 30 had an excessive amount of V, resulting in poor corrosion resistance. Comparative Example No. 31 had an excessive amount of nitrogen, resulting in poor corrosion resistance. Comparative Example No. 35 did not satisfy the value in Equation 1 because C+N was in excess, resulting in inferior corrosion resistance. Comparative Example No. 36 did not satisfy Equation 2 and exhibited inferior thermal expansion properties. Comparative Example No. 37 did not satisfy Equation 3 and was inferior in hardness. Comparative Example No. 38 did not satisfy Equation 3 and was inferior in hardness.

[0054] Comparative Example No. 39 is a steel equivalent to JIS SUH660 steel, and is a steel that is excellent in both age hardness and thermal expansion. This comparative example is presented as an example to confirm that the steel of the present invention is as good as the heat-resistant steel SUH660 in terms of age hardness and thermal expansion. In addition, since SUH660 has an excessive amount of Ni, the material itself is expensive, and there is a need for a material with better alternatives, and the present invention is superior in terms of cost.

Claims

1. In mass percent, C: more than 0.10 to 0.60%, Si: 0.05-0.80%, Mn: 2.0 to 10.0%, P: 0.050% or less, S: 0.050% or less, Ni: 6.0 to 15.0%, Cr: 6.0-18.0%, Mo: 0.01-0.50%, Cu: 0.05-0.75%, Al: 0.001-0.100%, V: more than 0.50 to 3.00%, N: 0.01-0.10%, A high-hardness, high-thermal-expansion steel having a remainder of Fe and unavoidable impurities, satisfying equations 1 and 2, having a hardness of 31 HRC or higher, and an average thermal expansion coefficient of 17.0 × 10⁻⁶ / °C or higher from room temperature to 200°C. However, equation 1: V / {4([C]+[N])} = 0.5 to 1.5, Formula 2: 551-462([C]+[N]-0.07[V])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])-18.5[Mo]≦-40. Note that the elemental symbols in each formula should be replaced with the mass percentage values ​​of the corresponding components.

2. In addition to the chemical components described in claim 1, by mass%, B: 0.010% or less, Ca: 0.050% or less, Mg: Contains one or more of the following in an amount of 0.050% or less. A high-hardness, high-thermal-expansion steel having a remainder of Fe and unavoidable impurities, satisfying equations 1 and 2, having a hardness of 31 HRC or higher, and an average thermal expansion coefficient of 17.0 × 10⁻⁶ / °C or higher from room temperature to 200°C. However, equation 1: V / {4([C]+[N])} = 0.5 to 1.5, Formula 2: 551-462([C]+[N]-0.07[V])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])-18.5[Mo]≦-40. Note that the elemental symbols in each formula should be replaced with the mass percentage values ​​of the corresponding components.

Images