Martensitic stainless steel with improved softening resistance and manufacturing method thereof

Abstract

We provide martensitic stainless steel with improved softening resistance by inducing the formation of precipitates through the control of the alloy composition of Cu, Nb, etc. and the manufacturing method thereof. [Solution] The martensitic stainless steel with improved softening resistance of the present invention contains, by weight, C: 0.01% or more and 0.1% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.05% or more and 1.0% or less, Cr: 11.0% or more and 14.0% or less, Ni: 0.05% or more and 1.0% or less, Cu: 0.5% or more and 2.0% or less, N: 0.03% or more and 0.08% or less, Nb: 0.02% or more and 0.5% or less, Cu / Nb: 2 or more and 4 or less, with the remainder being Fe (iron) and other unavoidable impurities, and is characterized in that the difference in hardness before and after softening heat treatment is more than 0% and less than 10%.

Description

[Technical field]

[0001] The present invention relates to a martensitic stainless steel having improved softening resistance and a manufacturing method thereof, and more particularly to a martensitic stainless steel having improved softening resistance by inducing the formation of precipitates through an alloy composition and a manufacturing method thereof, and a manufacturing method thereof. [Background technology]

[0002] Generally, disk materials used in motorcycles are required to have high hardness to prevent disk wear, and therefore martensitic stainless steel, which has high hardness, is mainly used for disk materials.

[0003] When manufactured as a plate material, martensitic stainless steel is composed of ferrite and precipitates, and is punched into a disk shape and then hardened by heat treatment. The hardening heat treatment involves heating the material to a temperature at which the ferrite transforms into the austenite phase, holding the material for a certain period of time, and then quenching the material to form the martensite phase. When the martensite phase is formed, it achieves a high level of hardness that makes it resistant to wear due to friction.

[0004] Meanwhile, motorcycle discs generate frictional heat due to friction with the brake pads during braking, and are frequently exposed to high temperatures of 300 to 700°C. In this case, the hardness of the disc may decrease at high temperatures, causing a decrease in performance. Therefore, a material with improved softening resistance and less loss of hardness at high temperatures is required. Summary of the Invention [Problem to be solved by the invention]

[0005] In order to solve the above problems, an object of the present invention is to provide a martensitic stainless steel and a manufacturing method thereof in which the formation of precipitates is induced by controlling the alloy composition of Cu, Nb, etc. and the manufacturing method to improve softening resistance. [Means for solving the problem]

[0006] The martensitic stainless steel with improved softening resistance of the present invention contains, by weight, C: 0.01% or more and 0.1% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.05% or more and 1.0% or less, Cr: 11.0% or more and 14.0% or less, Ni: 0.05% or more and 1.0% or less, Cu: 0.5% or more and 2.0% or less, N: 0.03% or more and 0.08% or less, Nb: 0.02% or more and 0.5% or less, Cu / Nb: 2 or more and 4 or less, with the remainder being Fe (iron) and other unavoidable impurities, and is characterized in that the difference in hardness before and after softening heat treatment is more than 0% and less than 10%.

[0007] In addition, the martensitic stainless steel of the present invention with improved softening resistance has a number of precipitates with a major axis length exceeding 1 μm on a cross-sectional basis of 5 / 400 μm after softening heat treatment. 2 That's all.

[0008] In addition, the method for producing a martensitic stainless steel with improved softening resistance of the present invention is characterized by including the steps of producing a slab containing, by weight%, C: 0.01% or more and 0.1% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.05% or more and 1.0% or less, Cr: 11.0% or more and 14.0% or less, Ni: 0.05% or more and 1.0% or less, Cu: 0.5% or more and 2.0% or less, N: 0.03% or more and 0.08% or less, Nb: 0.02% or more and 0.5% or less, Cu / Nb: 2 or more and 4 or less, with the balance being Fe (iron) and other unavoidable impurities, hot rolling the slab to produce a hot-rolled material, and isothermally heat treating the hot-rolled material at 650 to 750°C.

[0009] In the method for producing a martensitic stainless steel having improved softening resistance according to the present invention, the hot-rolled material has a thickness of 2 to 5 mm.

[0010] In the method for producing a martensitic stainless steel having improved softening resistance according to the present invention, the isothermal heat treatment is performed for 18 to 22 hours. Effect of the Invention

[0011] According to the present invention, it is possible to provide a martensitic stainless steel and a manufacturing method thereof in which the formation of precipitates is induced by controlling the alloy composition of Cu, Nb, etc. and the manufacturing method, thereby improving softening resistance.

[0012] However, the effects of the present invention are not limited to those mentioned above, and other effects not mentioned will be clearly understood by those having ordinary skill in the art to which the present invention pertains from the following description. [Brief description of the drawings]

[0013] [Figure 1] 1 is a photograph of a cross section of the stainless steel according to Example 3 after softening heat treatment, taken with a scanning electron microscope (SEM). [Diagram 2] 1 is a photograph of a cross section of the stainless steel according to Comparative Example 2 after softening heat treatment, taken with a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The martensitic stainless steel with improved softening resistance of the present invention contains, by weight, C: 0.01% or more and 0.1% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.05% or more and 1.0% or less, Cr: 11.0% or more and 14.0% or less, Ni: 0.05% or more and 1.0% or less, Cu: 0.5% or more and 2.0% or less, N: 0.03% or more and 0.08% or less, Nb: 0.02% or more and 0.5% or less, Cu / Nb: 2 or more and 4 or less, with the remainder being Fe (iron) and other unavoidable impurities, and the difference in hardness before and after softening heat treatment is more than 0% and less than 10%.

[0015] Hereinafter, the present invention will be described in detail with reference to the drawings. The following embodiments are presented to fully convey the concept of the present invention to those skilled in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, parts that are not relevant to the description may be omitted in order to clarify the present invention, and the size of components may be somewhat exaggerated in order to facilitate understanding.

[0016] Throughout the specification, when a part is said to "comprise" certain elements, this does not mean to exclude other elements, but rather that it may further include other elements, unless specifically stated to the contrary.

[0017] The singular expression includes the plural expression unless the context clearly indicates otherwise.

[0018] The reasons for limiting the numerical values ​​of the alloy component contents in the examples of the present invention will be described below. Unless otherwise specified, the unit is % by weight.

[0019] The martensitic stainless steel with improved softening resistance of the present invention contains, by weight, C: 0.01% or more and 0.1% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.05% or more and 1.0% or less, Cr: 11.0% or more and 14.0% or less, Ni: 0.05% or more and 1.0% or less, Cu: 0.5% or more and 2.0% or less, N: 0.03% or more and 0.08% or less, Nb: 0.02% or more and 0.5% or less, Cu / Nb: 2 or more and 4 or less, with the remainder being Fe (iron) and other unavoidable impurities.

[0020] The C (carbon) content is 0.01% or more and 0.1% or less.

[0021] C is an element that is effective in increasing material hardness through solid solution strengthening. Taking this into consideration, C may be added in an amount of 0.01% or more. However, if the C content is excessive, it can easily combine with carbide-forming elements such as Cr, which can deteriorate corrosion resistance. Taking this into consideration, the upper limit of the C content is limited to 0.1%.

[0022] The content of Si (silicon) is 0.05% or more and 1.0% or less.

[0023] Silicon is an element added for its deoxidizing effect and is effective in improving corrosion resistance. In consideration of this, silicon may be added in an amount of 0.05% or more. However, if the silicon content is excessive, the hardness of the ferrite phase may increase rapidly, resulting in a decrease in elongation. In consideration of this, the upper limit of silicon content is limited to 1.0%.

[0024] The Mn (manganese) content is 0.05% or more and 1.0% or less.

[0025] Mn is an element effective in forming the austenite phase. Taking this into consideration, Mn may be added in an amount of 0.05% or more. However, if the Mn content is excessive, it will combine with S in the steel to form MnS, making it difficult to ensure corrosion resistance. Taking this into consideration, the upper limit of the Mn content is limited to 1.0%.

[0026] The Cr (chromium) content may be equal to or greater than 11.0% and equal to or less than 14.0%.

[0027] Cr is an element that is essentially added to improve corrosion resistance. Taking this into consideration, Cr may be added in an amount of 11.0% or more. However, if the Cr content is excessive, the size of precipitates may increase excessively. Taking this into consideration, the upper limit of the Cr content is limited to 14.0%.

[0028] The Ni (nickel) content is 0.05% or more and 1.0% or less.

[0029] Ni is an effective element for forming austenite phase during hardening heat treatment. Taking this into consideration, Ni is added in an amount of 0.05% or more. However, if the Ni content is excessive, it may lead to an increase in cost. Taking this into consideration, the upper limit of the Ni content is limited to 1.0%.

[0030] The Cu (copper) content is greater than or equal to 0.5% and less than or equal to 2.0%.

[0031] Cu is a key element in the present invention and may be added to form copper precipitates. In consideration of this, the Cu content is 0.5% or more. However, if the Cu content is excessive, it may promote cracks due to copper segregation and hinder productivity. In consideration of this, the upper limit of the Cu content is limited to 2.0%.

[0032] The N (nitrogen) content is greater than or equal to 0.03% and less than or equal to 0.08%.

[0033] N is an element that is effective in improving the hardness of steel. Taking this into consideration, the N content may be 0.03% or more. However, if the N content is excessive, surface defects may occur due to the generation of nitrogen pores. Taking this into consideration, the upper limit of the N content is limited to 0.08%.

[0034] The Nb (niobium) content is 0.02% or more and 0.5% or less.

[0035] Nb is a key element in the present invention, and may be added to form niobium precipitates that become the nuclei of the copper precipitates. In consideration of this, the Nb content may be 0.02% or more. However, if the Nb content is excessive, hot workability may be impaired. In consideration of this, the upper limit of the Nb content is limited to 0.5%.

[0036] Cu / Nb is 2 or more and 4 or less.

[0037] When Cu / Nb is less than 2, the amount of niobium precipitates increases too much and the amount of copper precipitates formed is relatively small, so that the desired improvement in softening resistance cannot be expected. On the other hand, when Cu / Nb exceeds 4, the amount of niobium precipitates that act as nuclei for copper precipitates formed is small, so that it is difficult to ensure sufficient softening resistance. Therefore, Cu / Nb may be 2 or more and 4 or less, and more preferably 3 or more and 4 or less.

[0038] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from the raw materials or the surrounding environment, and it is not possible to exclude them. Since any engineer of a normal manufacturing process would know about these impurities, the contents of all of them are not specifically mentioned in this specification.

[0039] The martensitic stainless steel of the present invention with improved softening resistance has a difference in hardness between before and after softening heat treatment of more than 0% and less than 10%.

[0040] The softening heat treatment is carried out by holding the material at 500°C for one hour. The smaller the difference in hardness before and after the softening heat treatment, the better the softening resistance can be evaluated. Therefore, the closer the difference in hardness before and after the softening heat treatment is to 0%, the better the softening resistance can be considered to be.

[0041] The martensitic stainless steel of the present invention with improved softening resistance has a cross-sectional basis of 5 precipitates / 400 μm in number with a major axis length exceeding 1 μm after softening heat treatment. 2 That's all.

[0042] The main reason for softening at high temperatures is the formation of chromium precipitates. Martensitic stainless steels have high hardness after hardening heat treatment due to the effects of C and N contained in them. However, when martensitic stainless steels are exposed to high temperatures for a long time, C and N may diffuse and combine with Cr to form chromium precipitates. Therefore, the C and N contents of the martensitic phase are reduced, which results in a lower hardness of the martensitic phase.

[0043] In order to prevent such a decrease in hardness, it is necessary to induce the formation of separate precipitates at the high temperature. In the present invention, when softening occurs due to the formation of chromium precipitates at high temperatures, copper precipitates are simultaneously formed, thereby improving softening resistance.

[0044] On the other hand, copper precipitates are more likely to form when there is another precipitate to act as a nucleus. In the present invention, the formation of copper precipitates is induced using niobium precipitates as nuclei. However, as described above, in order to ensure sufficient softening resistance, the ratio of Cu and Nb contents can play an important role.

[0045] The method for producing a martensitic stainless steel with improved softening resistance of the present invention includes the steps of producing a slab containing, by weight, C: 0.01% to 0.1%, Si: 0.05% to 1.0%, Mn: 0.05% to 1.0%, Cr: 11.0% to 14.0%, Ni: 0.05% to 1.0%, Cu: 0.5% to 2.0%, N: 0.03% to 0.08%, Nb: 0.02% to 0.5%, Cu / Nb: 2 to 4, with the balance being Fe (iron) and other unavoidable impurities, hot rolling the slab to produce a hot-rolled material, and isothermally heat treating the hot-rolled material at 650 to 750°C.

[0046] The reasons for limiting the numerical values ​​of the ranges of the components of each alloy composition are as described above, and each manufacturing step will be described in more detail below.

[0047] First, a slab satisfying the above-mentioned alloy composition is manufactured, and then undergoes a series of steps including hot rolling and isothermal heat treatment.

[0048] The hot rolling may be performed in a temperature range that is usually used, and is preferably performed at 950 to 1150° C. to ensure an appropriate hardness.

[0049] The hot-rolled material produced through the hot rolling may have a thickness of 2 to 5 mm, although the thickness of the hot-rolled material may vary depending on the application.

[0050] Next, the hot-rolled material is subjected to isothermal heat treatment at 650 to 750° C. The isothermal heat treatment is maintained for 18 to 22 hours.

[0051] If the isothermal heat treatment temperature is low or the holding time is short, it is difficult to sufficiently transform the austenite phase formed during hot rolling into the ferrite phase. In addition, it may be difficult to effectively form niobium precipitates as nuclei for copper precipitates. However, if the isothermal heat treatment temperature is too high or the holding time is too long, the crystal grains may become coarse, resulting in poor hardness.

[0052] The present invention will be described in more detail with reference to the following examples. However, the description of the examples is for illustrating the implementation of the present invention, and the present invention is not limited by the description of the examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from the claims.

[0053] {Example} Slabs were manufactured for various alloy composition ranges shown in Table 1 below. The manufactured slabs were hot-rolled to 4 mm at a temperature of 1100°C to manufacture hot-rolled materials. Next, the hot-rolled materials were subjected to isothermal heat treatment at 700°C for 20 hours. After that, a hardening heat treatment step was performed, in which the temperature was raised to 1000°C and the material was water-cooled immediately after reaching the target temperature. The heating temperature range of the hardening heat treatment step may be selected from the range of 950 to 1100°C depending on the characteristics of the present invention composition, and methods such as mold and oil may be selected in addition to water cooling.

[0054] [Table 1]

[0055] The number of precipitates and softening resistance are shown in Table 2 below. The precipitates may include copper precipitates or niobium precipitates, which are useful for improving softening resistance, and it can be determined that the more precipitates exceeding 1 μm in size, the more excellent the softening resistance.

[0056] The number of precipitates was measured by softening the stainless steel by holding it at 500°C for 1 hour, then photographing the cross section with a JSM-7900F Scanning Electron Microscope (SEM). 2 The number of particles with a major axis length exceeding 1 μm was counted within the area.

[0057] The softening resistance was shown by measuring the difference in hardness before and after the softening heat treatment.

[0058] [Table 2]

[0059] As shown in Table 2, Examples 1 to 9 satisfied the alloy composition, component range, and manufacturing process presented in the present invention, so after softening heat treatment, the cross section of the stainless steel was 400 μm 2 In each of the comparative examples 1 to 5, the number of precipitates with a major axis exceeding 1 μm in length was 5 or more, and the difference in hardness before and after the softening heat treatment was less than 10%. However, in the comparative examples 1 to 5, the Cu / Nb value exceeded 4, and therefore few niobium precipitates were formed to act as nuclei for copper precipitates. Therefore, in the comparative examples 1 to 5, the number of precipitates with a major axis exceeding 1 μm in length in the stainless steel cross section was 5 or more in each of the comparative examples 1 to 5 after the softening heat treatment. 2 In the above area, the number of precipitates having a major axis length exceeding 1 μm did not satisfy the requirement that the number be 5 or more. In addition, in Comparative Examples 1 to 5, the difference in hardness before and after the softening heat treatment did not satisfy the requirement that the difference be less than 10%.

[0060] In Comparative Examples 6 and 7, the Cu / Nb value was less than 2, and therefore niobium precipitates were formed in excess. 2 In Comparative Examples 6 and 7, the number of precipitates having a major axis length exceeding 1 μm in the area was not 5 or more. In addition, Comparative Examples 6 and 7 did not satisfy the requirement that the difference in hardness before and after the softening heat treatment be less than 10%.

[0061] FIG. 1 is a photograph taken with a scanning electron microscope (SEM) of a cross section of the stainless steel of Example 3 after softening heat treatment, and FIG. 2 is a photograph taken with a scanning electron microscope (SEM) of a cross section of the stainless steel of Comparative Example 2 after softening heat treatment.

[0062] As shown in FIG. 1 and FIG. 2, it can be seen that the stainless steel according to the example of the present invention has a greater number of precipitates whose major axis length exceeds 1 μm. [Industrial Applicability]

[0063] INDUSTRIAL APPLICABILITY The present invention provides a martensitic stainless steel and a manufacturing method thereof in which the formation of precipitates is induced and softening resistance is improved, and therefore the present invention is found to be industrially applicable.

Claims

1. In weight percent, it contains C: 0.01% or more and 0.1% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.05% or more and 1.0% or less, Cr: 11.0% or more and 14.0% or less, Ni: 0.05% or more and 1.0% or less, Cu: 0.5% or more and 2.0% or less, N: 0.03% or more and 0.08% or less, Nb: 0.02% or more and 0.5% or less, Cu / Nb: 2 or more and 4 or less, and the balance is Fe (iron) and other unavoidable impurities, A martensitic stainless steel having improved softening resistance, characterized in that the difference in hardness between before and after softening heat treatment is more than 0% and less than 10%.

2. After softening heat treatment, the number of precipitates with a major axis length exceeding 1 μm on a cross-sectional basis was 5 / 400 μm. ２ 2. The martensitic stainless steel having improved softening resistance according to claim 1, characterized in that:

3. A step of producing a slab containing, by weight, C: 0.01% to 0.1%, Si: 0.05% to 1.0%, Mn: 0.05% to 1.0%, Cr: 11.0% to 14.0%, Ni: 0.05% to 1.0%, Cu: 0.5% to 2.0%, N: 0.03% to 0.08%, Nb: 0.02% to 0.5%, Cu / Nb: 2 to 4, with the remainder being Fe (iron) and other unavoidable impurities; hot rolling the slab to produce a hot rolled material; and isothermally heat treating the hot-rolled material at 650 to 750°C.

4. The method for producing martensitic stainless steel with improved softening resistance according to claim 3, wherein the hot-rolled material has a thickness of 2 to 5 mm.

5. 4. The method of claim 3, wherein the isothermal heat treatment is performed for 18 to 22 hours.

6. 4. The method of claim 3, further comprising a hardening heat treatment step of heating at 950 to 1100° C. and then quenching after the isothermal heat treatment step.

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