WO2004035850A1

WO2004035850A1 - Superfine grain steel having nitrided layer

Info

Publication number: WO2004035850A1
Application number: PCT/JP2003/013308
Authority: WO
Inventors: Saburo Matsuoka; Yoshiyuki Furuya; Hisashi Hirukawa; Shiro Torizuka; Hideyuki Kuwahara
Original assignee: National Institute For Materials Science
Priority date: 2002-10-17
Filing date: 2003-10-17
Publication date: 2004-04-29
Also published as: EP1580291A4; US20050241733A1; EP1580291A1; JP3931230B2; JP2004137561A; KR20050067184A; CN100359033C; CN1705762A

Abstract

A superfine grain steel having a nitrided layer, characterized in that it has a ferrite grain structure having an average grain diameter of 3 μm or less and has a nitrided layer formed on the surface thereof. In the superfine grain steel, a nitrided layer is formed without the addition of alloying elements which are expensive and are detrimental to the recycling of steel, such as Cr or Mo, and also provides a steel having high fatigue strength.

Description

Description Ultra-fine grained steel with nitrided layer Technical field

The invention of this application relates to an ultrafine grained steel having a nitrided layer. More specifically, the invention of this application relates to an ultrafine grain having a nitrided layer in which a nitrided layer is formed without adding an expensive and harmful alloying element such as Cr, Mo or the like during recycling, and has a high fatigue strength. It is about steel. BACKGROUND ART-Metal parts that are subjected to bending or torsional stress, such as rotating shafts, develop fatigue cracks from surfaces exposed to high stress, and eventually fracture. Therefore, hardening the surface and increasing the fatigue strength is effective in increasing the fatigue strength of the entire part. Hardening the surface is also effective from the viewpoints of wear resistance and corrosion resistance. The same is true for ultra-fine-grained steel having high strength and high toughness in which the ferrite crystal grain size is extremely small (for example, see Patent Document 1).

Conventionally, nitriding has been considered for surface hardening. For this purpose, alloying elements such as Cr, Mo, Ti, and Nb are added, and heated at 450 to 590 for 0.5 to 100 hours. It was necessary to generate a nitride of an alloy element (for example, see Non-Patent Document 1). Actually, when pure iron is nitrided using ammonia gas, a film-like iron nitride with a thickness of several to several tens of im is formed on its surface, and hardening is only about Hv250 at most. There is little or no precipitation of iron nitride inside and hardly contributes to hardening.

However, alloying elements such as Cr and Mo are expensive and harmful during recycling, so it is desirable to avoid their addition. The invention of this application was made in view of such circumstances, and a nitrided layer was formed without adding an expensive alloy element such as Cr or o, which would be harmful at the time of recycling. The task to be solved is to provide ultra-fine grained steel having a nitrided layer.

Patent document 1: Japanese Patent Application Laid-Open No. 2000-309850

Non-Patent Document 1: Hideyuki Kuwahara, Ph.D. Dissertation, "Study on Surface Modification of Iron Alloy by Plasma", November 1992, Kyoto University, Invention Disclosure

In order to solve the above-mentioned problems, the invention of this application has a nitride layer characterized by having a ferrite grain structure having an average particle diameter of 3 m or less and having a nitride layer formed on the surface. Provide ultra-fine grained steel (Claim 1). Further, the invention of this application is that the crystal grain growth during nitriding is suppressed by either precipitation of carbide or addition of a solid solution element (Claim 2 C content is 0.01 mass or more. (Claim 3) At least one element selected from the group consisting of Mn, Cr, Mo, Ti, Nb, V and P is added. (Claim 4 Mn content is 0.4 mass or more. Claim 5 P content is not less than 0.035 inass¾ (Claim 6 Carbon steel having a total content of Cr, Mo, Ti, Nb, and V of not more than 0.1 lmass% (Claim 7) The minimum limit is 1.6 times or more the Vickers hardness of the base material (Claim 8), and the molded product, part or member (Claim 9) formed from the ultrafine grained steel having the above nitrided layer Provided as one embodiment.

Hereinafter, the ultrafine grained steel having a nitrided layer according to the invention of the present application will be described in more detail with reference to examples. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a FE-SEM photograph showing the nitrided layer structure of Fe-C-Mn-based 0.002C coarse-grained steel plasma-nitrided at 550 "CX for 26 hours. Figure 2 shows the material of Fe-C-Mn-based 0.002C coarse grain steel and Fe-C-Mn-based 0.055C ultrafine grained steel plasma-nitrided under the conditions of 550 x 26 hours and the pick-up of the nitrided layer surface. 5 is a bar graph showing hardness.

Fig. 3 is a graph showing the hardness distribution after nitriding in a fatigue test specimen of an Fe-C-Mn-based 0.05C ultrafine-grained steel.

Figure 4 is a FE-SEM photograph showing the matrix structure after nitriding of a Fe-C-Mn-based 0.05C ultrafine-grained steel plasma-nitrided at 550 ^ X for 26 hours.

FIG. 5 is a graph showing the hardness distribution of Fe—C—Mn—Si-based 0.25C coarse-grained steel containing 0.37 mass% of Mn after nitriding under the condition of SOO X for 16 hours.

Figure 6 is a graph showing the hardness distribution of Fe-C-Mn-Si-based 0.45C coarse-grained steel containing 0.83 mass% of Mn after nitriding under the condition of 50 (TC X 16 hours). .

FIG. 7 is a graph showing a comparison between the results of a fatigue test of a Fe-C-Mn-based 0.05C ultrafine-grained steel material and a nitride material.

Fig. 8 is a graph showing the effect of Fe3C on the grain growth suppression of Fe-C-Mn-Si ultrafine grained steel.

Figure 9 shows (a) and (b) FE-SEM showing the microstructures of the Fe-C-Mn-Si-based 0.15C ultrafine-grained steel and 0.45C ultrafine-grained steel before nitriding, respectively. It is a photograph.

Figure 10 shows the effect of suppressing grain growth in a 0.15C-0.1P ultrafine grained steel with 0.1lmass% P added to an Fe-C-Mn-Si based 0.15C ultrafine grained steel. It is a graph.

FIG. 11 is a graph showing a hardness distribution after nitriding Fe-C-Mn-Si ultrafine grain steel 0.45C-0.1P. BEST MODE FOR CARRYING OUT THE INVENTION

Ultra fine-grained steel with a simple composition and a ferrite grain structure with an average grain size of 3 / m or less is maintained at a temperature of 450 to 590 in an ammonia gas atmosphere or an atmosphere containing ammonia gas for 0.5 to 100 hours. As a result, a nitride layer is formed on the surface, surface hardening occurs, and the fatigue strength is increased. Here, the ferrite grain structure means a structure mainly composed of ferrite grains. In this sense, the ferrite grain structure is

The two phases may include carbide, pearlite, martensite, austenite, and the like.

In the ultrafine grained steel having a nitrided layer according to the invention of the present application, the Mn content is preferably set to 0.4 mass% or more. In the case of Fe-C-Mn-based or Fe-C-Mn-Si-based ultrafine-grained steel with a Mn content of 0.37 mass%, nitriding at 500 for 16 hours causes hardening of the surface, If the Mn content is 0.83 mass%, the surface of the ultrafine grained steel hardens and a deep nitride layer is formed. However, in ordinary steel, at least 0.4 mass% of Mn is added as a measure against MnS. In view of the above, in the ultrafine grained steel having a nitrided layer of the invention of this application, the Mn content is preferably set to 0.4 nias s% or more.

In ultrafine grained steel with a C content of 0.05 mass to 0.15 mass%, if the temperature is maintained at about 550 for about a long time, grain growth occurs, and the ultrafine grain structure easily collapses. Therefore, in the method for increasing the fatigue strength of steel according to the invention of the present application, the amount of C is increased to precipitate carbides such as Fe3C, NbC, and TiC, or to form solid solution elements such as P (phosphorus) and V (vanadium). The grain growth can be inhibited or suppressed by adding or both. For example, nitriding for as long as 26 hours at temperatures around 50ITC is possible. This prolonged nitridation forms a nitrided layer that is deeper and effective for increasing fatigue strength. The preferred amount of C is at least 0.05 mass. Further, the amount of P is preferably set to 0.035 mass% or more.

Normally, when fatigue is considered, the hardened layer by nitriding, that is, the nitrided layer is about 0.5 im to 1.0 yard.When the surface is sufficiently hardened, the coarse grained steel starts from the base immediately below the nitrided layer. Fatigue rupture occurs. Generally, the fatigue strength of the entire nitrided material is determined by the strength governed by the stress at the starting point of fatigue rupture and the fatigue strength of the base metal. Fatigue strength much higher than expected from the above rules can be obtained. This is because the nitrided layer formed on the ultra-fine grained steel by nitriding is This is because the paste hardens. Therefore, the ultrafine-grained steel not only has high strength and high toughness but also has high fatigue strength by nitriding.

An increase in surface hardness leads to an improvement in wear resistance. In other words, a large compressive stress is applied to the nitride layer, and the remaining compressive stress cancels out the tensile stress due to sliding, and the tensile stress applied to the surface of the ultrafine-grained steel becomes relatively small. Further, in the ultrafine-grained steel having a nitrided layer according to the invention of the present application, the growth of crystal grains is prevented or suppressed at the nitriding temperature, so that the characteristics are hardly deteriorated even by heat generated by friction. Therefore, the ultrafine-grained steel having a nitrided layer according to the invention of this application shows good wear resistance.

The ultrafine-grained steel having a nitrided layer according to the invention of the present application can be in either a pulp or powder form.

When Fe-C-Mn-based or Fe-C-Mn-Si-based steel powders are nitrided, the strength of the powders is comparable to that of the nitrided layer formed in the ulcer-like ultrafine-grained steel. Therefore, the pulp obtained by sintering the nitrided powder can also be a high-strength material. In this case, the sintering is performed, for example, in an atmosphere in which nitrogen gas or ammonia gas is used alone or as a mixture, or a hydrogen gas is added to any of these gases, and the total pressure is 2 atm or less. It can be carried out by applying a compressive stress of 1,200 to the following temperature range. When sintering, not all the powders need to be nitrided. For example, pure iron powder can be blended as a sintering aid. Further, when a steel powder having a particle size of 20 im or less is used, a ceramic powder such as TiN or TiC having a particle size of 2 or less can be added in order to exhibit the above-described effect of suppressing grain growth.

As described above, the ultrafine-grained steel having a nitrided layer according to the invention of the present application has high fatigue strength and excellent wear resistance in addition to high strength and high toughness, and various molded products, parts, and members. Practical application is expected to expand the scope of application to Example

(a) Coarse grain steel

Table 2

(a) Coarse grain steel

Material Tensile strength (MPa) Total elongation (%) Vickers hardness

Fe-C-Mn 0.002C 114

Fe-C-Mn-Si 0.25C 51 1 34 155

0.45C 706 25 230

(b) Ultra fine grain steel

Material Tensile strength (MPa) Total elongation (%) Vickers hardness

Fe-C-Mn 0.05C 645 13 216

0.15C 842 17 286

0.45C 952 17 300

Fe-C-Mn-Si 0J5C 1 143 12 360

0.90C 364

0.15C-0.1 P 926 13 308

0.45C-0.1 P 1048 15 339 Table 1 shows the chemical composition and ferrite grain size of the materials used in the examples, and Table 2 shows the mechanical properties. The Fe-C-Mn coarse-grained steel 0.002C and the Fe-C-Mn-Si coarse-grained steel 0.25C have a ferrite grain size of about 20 m and a ferrite-pearlite structure. 0.45C Fe-C-Mn-Si coarse-grained steel has a tempered martensitic structure. Ultrafine-grained steel has a ferrite structure in which fine ferrite grains and carbides are dispersed. This ultrafine-grained steel was formed into a square bar of 18 mm X 18 mm by groove roll rolling.

As shown in Table 2, in the case of Fe-C-Mn ultrafine grained steel 0.055C and Fe-C-Mn-Si based coarse grained steel 0.15C, 0.45C, 0.75C, 0.90C However, as the amount of carbon increases, as shown in Fig. 9 (a) and (b), the amount of granular carbide increases, and the tensile strength and Vickers hardness increase due to the strengthening of precipitation of carbon. .

In 0.15C-0.1P and 0.45-0.1P of Fe-C-Mn-Si ultrafine-grained steel, the tensile strength and pitch The hardness is increasing. This is due to solid solution strengthening of P.

The Fe-C-Mn-based 0.002C coarse-grained steel and the Fe-C-Mn-based 0.05C ultrafine-grained steel were plasma-nitrided at 550 ^ X for 26 hours. Figure 1 is a FE-SEM photograph showing the structure of the nitrided layer of the Fe-C-Mn-based 0.002C coarse-grained steel. From this FE-SEM photograph, it is confirmed that streak-like nitride is formed in the ferrite grains.

Figure 2 is a bar graph showing the Vickers hardness of the material and nitrided layer surfaces of the Fe-C-Μπ-based 0.002C coarse-grained steel and the Fe-C-Mn-based 0.05C ultrafine-grained steel. The picker hardness was measured by applying a load of 1 kg to the surface of a 1-mm-thick plasma-nitrided plate. It is confirmed that even a simple composition Fe-C-Mn-based steel is hardened by nitriding, and that ultrafine-grained steel is hardened more.

Fig. 3 is a graph showing the hardness distribution after nitriding in a fatigue test specimen of an Fe-C-Mn-based 0.05C ultrafine grained steel. The nitriding conditions were plasma nitriding at 550: x26 hours, the same as above, and the fatigue test piece was a sand clock type with a test part having a diameter Φ of 6. The Vickers hardness was measured under a load of 0.2 kg. From the hardness distribution shown in the graph of Fig. 3, it is estimated that the nitrided layer is about 1 place. Ma Also, it is confirmed that the hardness of the matrix after nitriding is lower than that of the material before nitriding. This is thought to be due to the coarsening of the matrix during nitriding.In fact, as can be seen from the FE-SEM photograph showing the matrix structure after nitriding in Fig. 4, the ferrite grain size was reduced. It was coarsened to a size of 5 m to 10 m. Fig. 5 and Fig. 6 show that Fe-C-Mn-Si-based 0.25C and 0.45C coarse particles with 0.37mass 0.83mass of Mil added (both round bars of 16 band diameter) 3 is a graph showing the hardness distribution after nitriding under the condition of X 16 hours. As with pure iron, the surface hardness of 0.25C coarse-grained steel with an Mn content of 0.37 mass% is high, but the hardness is not increased inside other than the surface. On the other hand, in 0.45C coarse-grained steel with an Mn content of 0.83 mass%, the internal hardness also increased, confirming that a deep nitride layer of about l mm was formed.

It is understood that nitriding is also possible in the Fe-C-Mn-Si system steel. In addition, as shown in Table 1, all other coarse-grained and fine-grained steels had Mn contents of 1.43 mass% or more, but all had a nitride layer of about 1 band. Therefore, in a Fe-C-Mn or Fe-C-Mii-Si based ultrafine-grained steel with a simple composition, the amount of Mn should be 0.37 mass to form a deep effective nitrided layer, that is, a hardened layer. It is understood that more than% is necessary.

Fig. 7 is a graph showing a comparison between the results of a fatigue test of a Fe-C-Mn-based 0.05C ultrafine-grained steel material and a nitride material. For the fatigue test, a Clause-type rotary bending fatigue tester and an hourglass type test piece with a test section diameter of 6 mm were used. The surface of the nitride material was removed by polishing to about 0.1 mm, and the defects introduced during nitridation were removed. As can be seen from Fig. 7, the nitrided material has significantly improved fatigue strength compared to the material, despite the fact that the matrix is coarse, and the fatigue limit of the material is 375MPa. The fatigue limit of the nitrided material was 640 MPa. The hardness of the base material of the nitride material is about HV160.

Fatigue limit = 1.6 X Vickers hardness

Is used, the fatigue limit of the base metal is estimated to be 1.6 X l60 = 256 [MPa]. Rotating bending test of a specimen with a diameter of Φ 6 mm (radius of 3 mm) with the thickness of the nitride layer as l mm Considering the stress gradient at, the stress amplitude σ a 'acting immediately below the nitride layer is aa' / σ a = (3-1) / 3 = 0.67 with respect to the nominal stress amplitude of the surface. Therefore, the fatigue limit of the nitrided material estimated from the matrix hardness and the stress gradient is about 256 / 0.67 = 382 [MPa], but the actual fatigue limit of the nitrided material is 640 MPa as described above. It is confirmed that the fatigue strength is much higher than the fatigue limit expected from the hardness of the mother ground, and the fatigue strength has been increased. When the same estimation as above is performed using the fatigue limit of 376 MPa measured for the material, the nitriding depth that shows the same hardness as the material is about 0.6ππιι from the results shown in Fig. 3, and oa '/ σ a = (3−0.6) /3=0.8. Therefore, the predicted fatigue limit of the nitrided material is 375 / 0.8 = 469 [MPa]. It is understood that the actual fatigue limit of nitrided material, 640MPa, is larger than this expected value.

As described above, even in the case of ultra-fine-grained steel, even if it is coarsened somewhat with nitriding, a large nitriding effect can be obtained in terms of fatigue strength. However, in consideration of actual parts and members, it is desirable that the base material maintain the ultrafine grain structure and maintain its strength even by nitriding.

Therefore, Fig. 8 shows the results of examining the grain growth effect of Fe3C. Specifically, using a fatigue test specimen with a test part with a diameter of 6 band made of Fe-C-Mn-Si-based ultrafine-grained steel, simulating high temperature and long-time holding during nitriding, The hardness change after holding at 500 for up to 30 hours using an oven was measured. As shown in the graph of Fig. 8, the hardness of 0.05C (0.05Cnmss¾) ultra-fine grained steel and 0.15C (0.15Cmass%) Decreased to about Hv200, and coarsening was confirmed. On the other hand, in the case of 0.45 (0.45Cmass%) ultrafine grained steel with a high C content, the hardness is slightly reduced even after being maintained for 30 hours, and there is no sign of coarsening. Similar results were obtained for 0.75C ultrafine grained steel and 0.90C ultrafine grained steel.

Figures 9 (a) and (b) are FE-SEM photographs showing the base metal structures of the Fe-C-Mn-Si series 0.15C ultrafine grained steel and 0.45C ultrafine grained steel before nitriding, respectively. It is. As described above, a large number of Fe3Cs (white spots) are precipitated in 0.45C ultrafine-grained steel. This It is presumed that the 0.45C ultrafine-grained steel did not coarsen due to the effect of Fe3C precipitates on grain growth suppression. In addition, it is confirmed that the ferrite grain size is 11 or less in both ultrafine-grained steels.

Fig. 10 shows the effect of suppressing grain growth in a 0.15C-0.1P ultrafine grained steel with 0.1lmass% P added to an Fe-C-Mn-Si based 0.15C ultrafine grained steel. It is a graph. As can be seen from Fig. 10, the hardness of the 0.15C-0.1P ultrafine-grained steel is smaller than that of the 0.15C ultrafine-grained steel, and coarsening is suppressed. . This is presumed to be due to the effect of solid solution of P on grain growth suppression. From the above results, it is concluded that coarsening can be prevented or suppressed by the grain growth effect of carbides or solid solution elements to maintain an ultrafine grain structure, and that long-term nitriding is possible while maintaining high strength. .

In order to further confirm the effectiveness of nitriding of ultrafine grained steel using the grain growth suppression effect, the solidification of Fe-C-Mn-Si ultrafine grained steel 0.45C and P using Fe3C precipitates was investigated. The 0.15C-0.1P using melting and the 0.45C-0.1P using both were actually nitrided and subjected to a fatigue test. Nitriding was plasma nitriding under the condition of 500 ^ X 16 hours. In the fatigue test, an hourglass type test piece with a diameter of 6 mm at the test part and a Clause type rotary bending tester were used, and a stepwise test was carried out on a single test piece in units of 107 times to obtain only the fatigue limit. Was. Fig. 11 is a graph showing the hardness distribution after nitriding of 0.45C-0.1P Fe-C-Mn-Si ultrafine grained steel. As can be seen from the graph shown in Fig. 11, the 0.45C-0.1P nitrided material shows a Hv of about 300 even in the matrix, and the ultrafine grained structure is maintained. Table 3 shows the fatigue test results for each nitrided material.

Table 3 Fatigue limit (MPa)

0.45C 0.15C-0.1 P 0.45C-0.1 P

Material 500 520 580

Fe-C-Mn-Si ¾

Nitride 700 700 780 The fatigue limit after nitriding was 700MPa for 0.45C ultrafine grained steel, 780MPa for 0.15C-0. IP ultrafine grained steel, and 700MPa for 0.45C-0. IP ultrafine grained steel. As shown in Table 2, the Vickers hardness of each base is 300, 308, and 339, so the ratio of fatigue limit / Pickers hardness of the base is 2.33, 2.53, 2.06, respectively. All are 1.6 or higher.

By the way, ultrafine grained steel 0.45C and 0.15C-0.1P were fatigue fractures originating from the surface, whereas 0.45C-0.1P ultrafine grained steel It was a fatigue rupture. From this, the original fatigue limit of the nitrided structure was obtained in 0.45C and 0.15C-0.1P ultrafine-grained steel, but 0.45C-0.1P ultrafine-grained steel It can be understood that the fatigue limit has been reduced due to the inclusion-initiated fatigue fracture as shown in the figure. For example, if the size of inclusions is reduced by using high-purification technology, etc. to prevent inclusion-originated fatigue fracture, the 0.45C-0.1P ultrafine-grained steel has high hardness Therefore, it is expected that the fatigue limit exceeding 780MPa of 0.15C-0.1P ultrafine grained steel will be obtained.

The higher the hardness, the better the wear resistance. As shown in Fig. 2, the difference in hardness between the material and the nitrided layer is more than twice as large for ultrafine-grained steel as for coarse-grained steel. This means, in other words, that the nitriding of the ultrafine-grained steel results in a higher hardness than expected for coarse-grained steel, and that the ultrafine-grained steel has excellent wear resistance. From the comparison between Fig. 2 and Fig. 11, the precipitation of carbides such as Fe3C and the addition of solid solution elements such as P increase the hardness of the nitrided layer by precipitation strengthening and solid solution strengthening, respectively. It is expected that the wear resistance of ultrafine-grained steel will be further improved.

Moreover, as shown in FIGS. 8, 10, and 11, no grain growth occurs even at the nitriding temperature. From this, even if heat is generated by friction, it is possible to maintain an ultra-fine grain structure with respect to the temperature rise of the friction surface up to the nitriding temperature, and there is no or small strength reduction and good wear resistance. It is thought that the property can be obtained.

Of course, the invention of this application is limited by the above-described embodiments and examples. It is not specified. It goes without saying that various aspects are possible for details such as the chemical composition of the steel, the nitriding conditions, and the nitriding method. Industrial applicability

As described in detail above, according to the invention of this application, a nitrided layer is formed without adding an alloy element that is harmful to recycling at a high price such as Cr, Mo, etc. Grain steel is provided.

Claims

The scope of the claims

1. Ultra-fine grained steel having a nitrided layer characterized by having a ferrite grain structure with an average grain size of 3 m or less and a nitrided layer formed on the surface.

2. The ultrafine-grained steel having a nitrided layer according to claim 1, wherein crystal grain growth during nitriding is suppressed by either or both of precipitation of a carbide and addition of a solid solution element.

3. The ultrafine-grained steel having a nitrided layer according to claim 1, wherein the C content is 0.01 iiiass¾ or more.

4. The ultrafine grained steel having a nitrided layer according to any one of claims 1, 2 and 3, wherein at least one element selected from the group consisting of Mn, Cr, Mo, Ti, Nb, V and P is added. .

5. The ultrafine-grained steel having a nitrided layer according to claim 4, wherein the amount of Mil is 0.4 mass% or more.

6. The ultrafine grained steel having a nitrided layer according to claim 4 or 5, wherein the P content is 0.035 mass% or more.

7. The ultrafine-grained steel having a nitrided layer according to any one of claims 4, 5 and 6, wherein the carbon steel has a total content of Cr, Mo, Ti, Nb and V of 0.1 lmass or less.

8. The ultrafine grained steel having a nitrided layer according to any one of claims 1, 2, 3, 4, 5, 6, and 7, wherein the fatigue limit is 1.6 times or more of the Vickers hardness of the base material.

9. A molded product, component or member formed from the ultrafine grained steel having a nitrided layer according to any one of claims 1, 2, 3, 4, 5, 6, 7 and 8.