KR20080034981A - Martensitic stainless steel sheet for heat-resistant disc brake having excellent hardenability - Google Patents

Abstract

Disclosed is a martensitic stainless steel for heat-resistant disc brakes which is excellent in hardenability and can be easily quench hardened at relatively low temperatures while maintaining high heat resistance. This martensitic stainless steel for heat-resistant disc brakes is characterized by having a composition consisting of, in mass%, 0.05-0.10% of C, not less than 0.1% and not more than 1% of Si, 0.2-2.0% of Mn, not more than 0.04% of P, not more than 0.010% of S, 0.010-0.025% of N, 11-14% of Cr, 0.5-2% of Ni, 0.5-2% of Cu, 1-2% of Mo, 0.03-0.3% of Nb, not more than 0.01% of Al, not more than 0.1% of Ti and the balance of Fe and unavoidable impurities while satisfying that C + N is 0.06-0.1%. This martensitic stainless steel for heat-resistant disc brakes is also characterized in that Gp obtained by the formula (1) below is not less than 80. Gp = 420[%C] + 470[%N] + 23[%Ni] + 9[%Cu] + 7[%Mn]-11.5[%Cr]-11.5[%Si]-52[%Al]-12[%Mo]-47[%Nb] + 189 (1)

Description

Martensitic stainless steel sheet for heat-resistant disc brakes with excellent hardenability {MARTENSITIC STAINLESS STEEL SHEET FOR HEAT-RESISTANT DISC BRAKE HAVING EXCELLENT HARDENABILITY}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for disc brakes for motorcycles, and in particular, after processing with a brake, the hardness required as a brake is stably obtained as it is quenched at a relatively low temperature such as less than 1000 ° C., and the disc temperature at use is 650. The present invention relates to a martensitic stainless steel sheet having excellent heat resistance that is hard to soften even when it reaches a temperature.

The steel plate for disc brakes of motorcycles requires characteristics such as wear resistance, water resistance and toughness. Abrasion resistance generally increases with higher hardness. On the other hand, if the hardness is too high, a so-called brake sound is generated between the brake and the pad, so the hardness of the brake is required to be 32 to 38 HRC (Rockwell hardness C scale). From these required characteristics, martensitic stainless steel sheets are used for two-wheel disc brake materials.

Conventionally, SUS420J2 has been quenched and tempered to a desired hardness and used as a brake. However, in this case, there is a problem that two heat treatment processes, quenching and tempering, are required. In contrast, Japanese Laid-Open Patent Publication No. 57-198249 discloses a steel composition in which a desired hardness can be stably obtained at a wider quenching temperature range than conventional steel of SUS420J2, and can be used as it is. It is. This is supplemented by the addition of Mn, an austenite-forming element, to lower C, N, and to reduce the austenite temperature range, that is, the narrowing of the quenching temperature range. Further, Japanese Laid-Open Patent Publication No. Hei 8-60309 discloses a steel sheet for motorcycle disc brakes that can be used as it is quenched with low Mn steel. Instead of lowering Mn, this steel sheet is added with Ni and Cu having the same effect as the austenite forming element.

In recent years, even in a two-wheeled vehicle, the weight of the vehicle body is required, and the weight of the two-wheel brake disc has been examined. In this case, the problem is that the disk is deformed due to the softening of the disk material due to heat generation during braking. In order to solve this problem, it is necessary to improve the heat resistance of the disk material. One of these solutions is an improvement in temper softening resistance. Japanese Laid-Open Patent Publication No. 2001-220654 discloses a method for improving heat resistance by adding Nb and Mo. However, the heat resistance improvement effect was the limit of 530 degreeC. In addition, Japanese Laid-Open Patent Publication No. 2004-346425 discloses a steel sheet in which tempering softening hardly occurs even at a temperature exceeding 600 ° C. Or Japanese Unexamined Patent Publication No. 2005-133204 discloses a disk material having excellent heat resistance by quenching at a temperature over 1000 ° C.

By the way, it is advantageous in terms of energy cost and manufacturing cost to quench in the temperature range below 1000 degreeC lower than quenching at temperature over 1000 degreeC like the invention of Unexamined-Japanese-Patent No. 2005-133204. . However, on the premise of using it in a quenched state, for example, if the heat resistance is to be improved by adding an alloy or the like to express the tempering softening resistance effect even at a high temperature exceeding 600 ° C., at a relatively low temperature of about 900 ° C. to 1000 ° C. In hardening, there exists a problem that the phenomenon in which so-called hardenability falls easily tends to occur that hardness is hard to rise. With respect to such a problem, the invention described in JP-A-2004-346425 has no suggestion, and the invention described in JP-A-2005-133204 has been aimed at hardening at high temperatures from the beginning.

Accordingly, the present invention is based on the premise of use in the quenched state, and is easy to be quenched by quenching at a relatively low temperature of about 900 to 1000 ° C. while maintaining high heat resistance such as a tempering softening resistance effect even at a high temperature of more than 600 ° C. Another object is to provide a martensitic stainless steel sheet for heat-resistant disc brakes having excellent hardenability.

The present invention has been made to solve the above problems, the gist of which is as follows.

 (1) In mass%, C: 0.05% or more and 0.10% or less, Si: 0.1% or more and 1% or less, Mn: 0.2% or more and 2.0% or less, P: 0.04% or less, S: 0.010% or less, N: 0.010% 0.025% or more, Cr: 11% or more and 14% or less, Ni: 0.5% or more and 2% or less, Cu: 0.5% or more and 2% or less, Mo: 1% or more and 2% or less, Nb: 0.03% or more and 0.3% or less, Al: 0.01% or less, Ti: 0.1% or less, and C and N satisfy C + N: 0.06% or more and 0.1% or less, and are made of residual Fe and inevitable impurities, and are represented by the following formula (1) Martensitic stainless steel sheet for heat-resistant disc brakes having excellent hardenability, wherein γp satisfies 80 or more.

γp = 420 [% C] +470 [% N] +23 [% Ni] +9 [% Cu] +7 [% Mn] -11.5 [% Cr] -11.5 [% Si] -52 [% Al]- 12 [% Mo] -47 [% Nb] +189 ... (1)

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the result of the hardenability evaluation test about the high C low N steel and the low C high N steel which made the total amount of C and N the same.

It is a figure which shows the result of the evaluation test of the tempering softening resistance with respect to high C low N steel and low C high N steel which made the total amount of C and N the same.

Implement the invention  Best practice for

Best Mode for Carrying Out the Invention The present invention will be described in detail with the limitations.

The inventors have studied in detail the two-wheeled disc brake material that can be used as it is in the quenched state. Among them, the examination on the heat resistance has been conducted mainly on the examination of the improvement of the tempering softening resistance. In order to improve the temper softening resistance, an alloying element centered on Nb and Mo is added. Therefore, the so-called hardenability, which hardly rises in hardness even when quenched, has become a problem. The present inventors examined this point in detail and found that by combining C and Mo effectively, the improvement of temper softening resistance can be achieved, while securing hardenability. In other words, C and N, which are essential elements for obtaining a predetermined hardness after quenching, were noted that the effect on the deterioration of hardenability when the amount of alloy addition centering on Nb and Mo was increased was different.

In FIG. 1, the evaluation result of hardenability of the steel (A1 steel, A2 steel. For specific components see Table 1) in which the ratio of C and N differs with the same C + N amount (0.08%). From the figure, when N increases (A2 steel), hardenability will fall when alloy addition amount is increased, but when C increases (Al steel), hardenability hardly falls even if alloy addition amount is increased. This effect was also found to be remarkable when Mo was added.

In addition, regarding the heat resistance, it was also found that the tempering softening resistance is improved by restricting Ti when it contains a small amount of Nb.

First, limitation conditions regarding each component are demonstrated.

C is an essential element for obtaining a predetermined hardness after quenching, and is added in combination with N so as to have a predetermined hardness level. When the heat resistance is improved by adding an alloy centered on Nb and Mo as in the present invention, it is preferable to add a large amount of C to N. However, when the content exceeds 0.10%, the hardness becomes too high, causing problems such as brake noise and deterioration of toughness. Therefore, the upper limit is 0.10%. In addition, if it is less than 0.05%, N must be added excessively to obtain hardness, so the lower limit is 0.05%.

Like C, N is an element useful for obtaining a predetermined hardness after quenching, and is added in combination with C so as to have a predetermined hardness level. When heat resistance is improved by addition of alloys centering on Nb and Mo as in the present invention, a large amount of C may be added to N, and a small amount of N may be added. If the content is added in excess of 0.025%, the hardenability is lowered. Therefore, this is the upper limit. Moreover, when N is made into less than 0.010%, since steel-making cost increases, it will be 0.010% as a lower limit.

C + N is an amount directly related to the hardness after quenching, and the hardness increases as the amount added increases. 0.06% or more 0.1 in order to avoid the problem of hardenability deterioration caused by the addition of a large amount of alloying elements centered on Nb and Mo and to satisfy a predetermined hardness level of HRC 32 to 38 to improve heat resistance. It should be less than%.

Si is very strong as a ferrite-forming element and needs to be suppressed in this respect. However, it is useful as a deoxidizer. Therefore, the suitable range is made into 0.1% or more and 1% or less. If it exceeds 1%, it is not preferable because excessively adding Ni, Cu, and Mn, which are austenite forming elements, is not preferable because the deoxidation effect is weakened at less than 0.1%.

Mn is an important austenite forming element. In the present invention, it is necessary to add 0.2% or more together with Ni and Cu in order to secure an austenite phase at a high temperature and secure hardenability. If it exceeds 2.0%, deterioration of water resistance appears, so 2% is the upper limit. Unlike Ni and Cu, Mn has no effect of improving heat resistance, so when more heat resistance is required, 1% or less is preferable.

Although P is a component inevitably contained in steel, when it contains exceeding 0.04%, toughness will fall, and 0.04% was made into an upper limit.

Cr is a basic element necessary to secure the required water resistance as a two-wheel disc brake material, and if the content is less than 11%, sufficient water resistance cannot be obtained. In addition, since Cr is a ferrite-forming element, addition of more than 14% reduces the austenite phase formation temperature range, and produces a ferrite phase that does not change from the quenching temperature range to the martensite phase, thereby making it impossible to satisfy the hardness after quenching. . Therefore, Cr addition amount is made into 11% or more and 14% or less.

Ni is an austenite forming element like Mn, and is an effective element to secure the hardenability by securing the austenite phase at a high temperature. Moreover, since it contributes to the improvement of tempering softening resistance, it adds 0.5% or more. Adding more than 2% causes a decrease in toughness, so the upper limit is 2%.

Cu, like Mn and Ni, is an element effective for securing an austenite phase and securing hardenability. Since Cu is effective in suppressing softening of the disk due to brake braking heat generation, 0.5% or more is added when the braking heat generation of the disk increases in a large motorcycle use or the like. However, if it exceeds 2%, toughness deteriorates, so the upper limit is made 2%.

Al is very useful as a deoxidizer, but since it may deteriorate water resistance, it is suppressed as much as possible in the present invention. Therefore, let 0.01% be an upper limit.

Although S is a component inevitably contained in steel, in the present invention, since it is easy to produce CaS when it contains exceeding 0.010%, 0.01% is made into an upper limit. When S is less than 0.001%, the steelmaking cost is greatly increased. Therefore, the lower limit is 0.001%.

Mo is a very important element in the present invention. By adding an appropriate amount of Mo, the tempering softening resistance is remarkably improved and the hardenability is also improved. Although the mechanism for improving the tempering softening resistance is still unknown, when the amount of C is 0.05% or more, the improvement effect is remarkable, so that precipitation and coarsening of Cr carbide can be suppressed and dislocation motion can be suppressed even at high temperatures, thereby tempering. It is thought to improve the softening resistance. Moreover, also about the hardening improvement mechanism, it is presumed that the solid solution C is secured by suppressing precipitation of Cr carbide. On the other hand, in the prior art, since attention was not paid to the combination of C and Mo, it is considered that the improvement effect of hardenability was not found. However, since the effect cannot be acquired at less than 1%, it is necessary to add Mo 1% or more. In addition, since the addition of Mo exceeding 2% deteriorates toughness, the upper limit of Mo is made 2%.

Nb is also a very important element in the present invention. By adding an appropriate amount of Nb, the temper softening resistance is remarkably improved. Although this mechanism is still not clear, it is thought that Nb has a high correlation with N, suppresses precipitation and coarsening of Cr nitrides, and suppresses dislocation motion to improve temper softening resistance. Although N is suppressed in this invention, since it contains to some extent, the effect of improving the tempering resistance of Nb is also fully effective. However, Nb is easily bound to N and precipitates in the form of NbN, and precipitation in this form has no reinforcing function, reduces the solid-solution reinforcing effect of N, and also reduces hardenability. Need to be avoided. Therefore, in order to improve the tempering softening resistance, addition of 0.03% or more is required. On the other hand, if the addition exceeds 0.3%, hardenability deteriorates, so the upper limit is made 0.3%.

Ti is an element to be limited in the present invention. When Ti is added, it precipitates in the form of coarse TiN (which also employs C), so that there is no reinforcing function, and the solid solution N and the solid solution C are reduced to lower hardenability and heat resistance. Moreover, especially when tempering softening resistance is improved by the addition of trace amount of Nb like this invention, it has a remarkable influence. Therefore, Ti amount needs to be 0.1% or less. When the need for heat resistance is high, it is better to further reduce it to 0.05% or less.

In addition, since the above-mentioned elements can be quenched stably in the temperature range of 900-1100 degreeC among the component ranges, it is necessary to adjust each other so that gamma potential (gamma) p shown by following formula (1) may satisfy 80 or more. have. This is because if γp is less than 80, the ferrite phase may remain even after quenching, and the predetermined hardness level may not be reached.

γp = 420 [% C] +470 [% N] +23 [% Ni] +9 [% Cu] +7 [% Mn] -11.5 [% Cr] -11.5 [% Si] -52 [% Al]- 12 [% Mo] -47 [% Nb] +189 ... (1)

When the temperature is too high with respect to the quenching temperature, the production time is increased and the cost is increased. Therefore, the quenching temperature is preferably performed at 900 ° C or more and 1000 ° C or less. Satisfies the hardness range of.

γp is an index indicating the stability of austenite in the high temperature range, and the above formula (1) is the reference `` Etude des transformations isothermes dans les aciers inoxydabile semi-ferritiques a 17% de chrome '' (Memoires Scientifiques Rev. Metallurg., LXIII , N ° 7/8, 1966).

Next, a manufacturing method is demonstrated in detail.

A slab (or an ingot or the like) composed of the above-described component composition is formed into a hot rolled sheet having a sheet thickness of about 2 to 8 mm by hot rolling, followed by annealing to soften and then pickled to obtain a product steel sheet. You may finish with a short blast without pickling.

In the brake disc manufacturing process, these steel plates are quenched after being heated to 900 to 1000 ° C. after being processed into disc shapes, and both surfaces are polished to obtain a brake disc.

(Example 1)

The hot rolled sheet of thickness 6mm was obtained by hot-rolling the steel piece of thickness 200mm which has the chemical component shown in Table 1. Furthermore, the softening annealing which heated to 850 degreeC and cooled slowly was performed.

Hardenability evaluation test pieces were extract | collected from these steel plates, and the remainder was hold | maintained at 950 degreeC or 1000 degreeC for 10 minutes, and the hardening process performed by water cooling was performed.

 Hardenability evaluation was performed by measuring hardness by the Rockwell hardness test (HRC) based on JISZ2245, after hold | maintaining for 10 minutes at the temperature of 850 degreeC-1100 degreeC. 32-38 pass by HRC.

Various test pieces were extract | collected from the quenched steel plate, and the evaluation test was done. The tempering softening characteristics were evaluated by the Rockwell test as in the hardness test of the quenching material after tempering was performed at 550 ° C to 650 ° C for 1 hour. The pass criterion is that hardness after tempering does not fall below HRC 30. In the water resistance test, both surfaces of the test piece were polished to # 240, and a salt spray test (according to JIS Z 2371) was performed for 240 hours to examine the degree of water repellency. The non-water repellent thing passed and the water repellent thing failed.

The result of a hardenability evaluation test is shown in FIG. Al steel having the same C + N content and high C satisfies the predetermined range of HRC 32 to 38 in the temperature range of the quenching temperature of 900 ° C to 1100 ° C, and also exhibits almost constant hardness, but the high N A2 steel It can be seen that even at 950 ° C., only a predetermined range is reached, and even when the quenching temperature is raised, the reaching hardness is also lower than that of the Al steel, so that the Al steel has excellent hardenability.

The evaluation result of a tempering softening resistance is shown in FIG. Even if A1 steel which is an example of this invention is 650 degreeC and 1 h tempering, hardness is HRC30 or more and shows the outstanding heat resistance. Although A2 intensity | strength which is a comparative example shows the outstanding heat resistance very much, initial hardness may be low and it is below HRC30 at 600 degreeC or more.

The results of the water resistance test were both steel passes.

As mentioned above, it is clear that this invention steel is excellent in heat resistance and excellent in hardenability.

 (Example 2)

After melt | dissolving the steel piece of thickness 200mm which consists of chemical components shown in Table 2, the hot rolled sheet of thickness 6mm was obtained by hot rolling. Furthermore, the softening annealing which heated to 850 degreeC and cooled slowly was performed.

After holding these steel plates for 10 minutes at 900 degreeC, 950 degreeC, and 1000 degreeC, the hardening process performed by water cooling was performed.

Various test pieces were extract | collected from the quenched steel plate, and the evaluation test was done. The tempering softening property was tempered at 600 ° C. and 650 ° C. for 1 hour, followed by Rockwell hardness test (HRC) in accordance with JIS Z 2245. 32-38 pass by HRC. The pass criterion is that hardness after tempering does not fall below HRC 30. In the water resistance test, both surfaces of the test piece were polished with # 240, and after performing a salt spray test (according to JIS Z 2371) for 240 hours, the degree of water repellency was examined. What did not repel water was passed and what was repelled was rejected.

These results are shown in Table 3.

The B1 steel to the B5 steel are obtained by changing Mo to 12.5 Cr-1.2 Ni-1Cu. B1 steel has insufficient hardenability and low heat resistance. This is presumably because Mo is low and insufficient suppression of precipitation of carbides is obtained. B2 steel and B4 steel are steels of the present invention, and both hardenability and heat resistance are sufficient. Although B5 steel has many Mo, hardenability and heat resistance are sufficient, but since it is low in toughness and there exists a danger of cracking during use, it is not preferable.

The C1 steel to the C5 steel are components other than Mo, and are C1 steel and C2 strength steel of the present invention, and both hardenability and heat resistance are sufficient. C3 steels with low C + N have poor hardness even when fully quenched. In addition, C4 strength hardenability is low because γp is low, and hardness after quenching is low. In addition, the Ti-rich C5 steel has an HRC of 30 or less after tempering and fails in heat resistance.

In addition, the water resistance of this publication was all passed. It is estimated that it is because a large amount of elements which improve water resistance, such as Ni, Cu, Mo, is contained.

From the above, it is clear that the steel of the present invention is excellent in heat resistance and hardenability, and is very suitable as a disc brake material.

According to the present invention, the steel sheet for a two-wheeled disc brake used in the quenched state, even if quenched in a relatively low quenching temperature range of 900 to 1000 ℃, it is possible to obtain a hardness of 32 to 38 HRC, excellent in hardenability, 600 Since martensitic stainless steel sheets for heat-resistant two-wheeled disc brakes having a temper softening resistance effect can be provided even at a high temperature exceeding ℃, not only manufacturers but also users of this steel sheet can obtain great benefits.

Claims (1)

In mass%, C: 0.05% or more and 0.10% or less, Si: 0.1% or more and 1% or less, Mn: 0.2% or more and 2.0% or less, P: 0.04% or less, S: 0.010% or less, N: 0.010% or more and 0.025% or less, Cr: 11% or more and 14% or less, Ni: 0.5% or more and 2% or less, Cu: 0.5% or more and 2% or less, Mo: 1% or more and 2% or less, Nb: 0.03% or more and 0.3% or less, Al: 0.01% or less, Ti: 0.1% or less Containing and C and N are C + N: 0.06% or more and 0.1% or less The martensitic stainless steel sheet for heat-resistant disc brakes having excellent hardenability, which satisfies the above, and is composed of the balance Fe and unavoidable impurities, and γp represented by Equation (1) below 80 is satisfied. γp = 420 [% C] +470 [% N] +23 [% Ni] +9 [% Cu] +7 [% Mn] -11.5 [% Cr] -11.5 [% Si] -52 [% Al]- 12 [% Mo] -47 [% Nb] +189 ... (1)

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