WO2015025746A1 - 被削性に優れた機械構造用鋼 - Google Patents
被削性に優れた機械構造用鋼 Download PDFInfo
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- WO2015025746A1 WO2015025746A1 PCT/JP2014/071081 JP2014071081W WO2015025746A1 WO 2015025746 A1 WO2015025746 A1 WO 2015025746A1 JP 2014071081 W JP2014071081 W JP 2014071081W WO 2015025746 A1 WO2015025746 A1 WO 2015025746A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D37/00—Tools as parts of machines covered by this subclass
- B21D37/01—Selection of materials
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
- B21J13/00—Details of machines for forging, pressing, or hammering
- B21J13/02—Dies or mountings therefor
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2261/00—Machining or cutting being involved
Definitions
- the present invention relates to a machine structural steel used for manufacturing various machine structural parts such as automobile parts and construction machine parts, and more particularly for machine structures having excellent machinability with reduced cutting finish surface roughness. It is about steel.
- various parts such as automobile parts and construction machine parts are finished into a final shape by performing a cutting process on a machine structural steel after performing a process such as forging.
- steel for machine structural use that exhibits excellent machinability in terms of component accuracy and manufacturing efficiency is required.
- the demand for cutting surface roughness is high, and machine structural steels that can obtain smaller cutting surface roughness are desired.
- the cutting surface roughness becomes large (rough), it is necessary to further finish the surface properties by grinding or the like, and there is a problem that the manufacturing process becomes complicated.
- Patent Document 1 defines a content of elements such as C, Mn, P, S, Pb, O, Si, and Al in a low-carbon sulfur free-cutting steel and includes MnS-based intervening materials. It has been shown that machinability can be improved by defining the average size of the product and the proportion of sulfide not bonded to the oxide. It is also shown that good finished surface roughness can be obtained.
- This technology includes lead (Pb) as a basic component as an element for improving machinability.
- Lead is a widely known element that improves machinability.
- Pb has been pointed out to be harmful to the human body and the environment. In recent years, Pb is required to exhibit good machinability without adding Pb.
- Patent Document 2 discloses that excellent machinability equivalent to that of Pb-added steel can be obtained by composite addition of S, Te and Ca.
- this technique discloses that machinability is further improved by adding Bi or rare earth elements (REM).
- REM rare earth elements
- machinability improving elements such as Te, Bi, and REM are expensive and there is a problem of cost increase in production.
- Patent Document 3 relates to a free-cutting steel for machine structures that exhibits both the mechanical properties and the chip breaking property of steel by containing a predetermined amount of Mg in the presence of sulfide inclusions. Proposed.
- Mg is added to control the sulfide inclusions in a predetermined shape and dispersion state.
- Mg has a low boiling point and easily evaporates, and is a strong deoxidizing element. Therefore, Mg is easily separated from molten steel as an oxide, so that the yield is low and an increase in cost is inevitable.
- Japanese Unexamined Patent Publication No. 62-23970 Japanese Unexamined Patent Publication No. 2004-292929 Japanese Unexamined Patent Publication No. 2002-69569
- the present invention has been made under such circumstances, and the object thereof is excellent machinability even with a normal chemical component composition without using Pb harmful to the human body or expensive free-cutting elements. It is to provide a steel for machine structural use that exhibits particularly good cutting surface roughness.
- the steel for machine structural use of the present invention capable of achieving the above object has a mixed structure consisting of a hard phase and a ferrite phase consisting of at least one selected from pearlite, bainite and martensite, and an average circle of ferrite grains It has a gist in that the equivalent diameter is 7 ⁇ m or less and the ferrite grain-ferrite grain coupling ratio X represented by the following formula (1) is 0.15 or less.
- [Ferrite grain-ferrite grain connection ratio X] [ferrite grain-ferrite grain interface number A] / [ferrite grain-hard phase interface number B] (1)
- the ferrite grain-ferrite grain interface number A is the intersection of the ferrite grain-ferrite grain interface and the straight line when a straight line of a predetermined length is drawn on the structure photograph taken using a scanning electron microscope.
- Number and The ferrite grain-hard phase interface number B indicates the number of intersections between the ferrite grain-hard phase interface and the straight line when a straight line having a predetermined length is drawn in the same manner as described above.
- the “average equivalent circle diameter” is an average value of diameters (equivalent circle diameters) when ferrite crystal grains are converted into circles having the same area.
- the chemical component composition of the steel for machine structural use of the present invention is not particularly limited as long as it is steel for machine structural use.
- C 0.2 to 1.2% (“mass%”)
- the chemical composition is the same.)
- Si 0.05 to 0.5%
- Mn 0.2 to 1.8%
- P 0.03% or less (excluding 0%)
- S 0.03% or less (excluding 0%) is included, and the balance is iron and inevitable impurities.
- Cr 0.5% or less (not including 0%)
- Cu 0.5% or less (not including 0%)
- Ni 0.5% or less (if necessary)
- Mo 0.5% or less (not including 0%)
- Mo depending on the type of element to be included.
- the present invention includes a method for improving the cutting surface properties, and a steel product with improved cutting surface properties can be obtained by cutting the steel for machine structure as described above.
- a molding die having a good surface property can be efficiently produced without performing finishing such as grinding.
- the present invention has a mixed structure composed of at least one hard phase selected from pearlite, bainite and martensite and a ferrite phase, the average equivalent circle diameter of ferrite is 7 ⁇ m or less, and a predetermined relational expression
- [Ferrite Grain-Ferrite Grain Connection Ratio X] expressed, it is possible to realize a steel for machine structural use that exhibits excellent machinability and has particularly good cutting surface roughness.
- FIG. 4 is a drawing-substituting photograph showing a procedure for obtaining a ferrite grain-ferrite grain coupling ratio X, and is a diagram showing a case where X ⁇ 0.15.
- FIG. 6 is a drawing-substituting photograph showing a procedure for obtaining a ferrite grain-ferrite grain coupling ratio X, and showing a case where X> 0.15.
- the inventors of the present invention have studied particularly the relationship with the metal structure in order to realize a steel for machine structural use that can obtain a good machined surface roughness even with a normal chemical composition.
- the idea that the roughness of the machined surface during machining is one of the causes is that the phases with different hardness are mixed in the structure.
- FIGS. 1A to 1C are schematic diagrams for explaining the relationship between the structure of steel material and the roughness of the cut surface.
- 1 indicates a ferrite phase
- 2 indicates a hard phase (a hard phase composed of at least one selected from pearlite, bainite, and martensite), and has a mixed structure structure in which these are mixed.
- the upper side of the steel material indicates the steel material surface to be cut.
- the soft ferrite phase 1 in the mixed structure is deformed so as to be pushed out by the hard phase 2. Subsequently, as shown in FIG.
- the ferrite phase 1 is removed by a cutting edge (blade edge) of the tool in a state where the ferrite phase 1 is extruded.
- a cutting edge blade edge
- FIG. 1C since the cutting edge of the tool passes through the cutting portion (processed surface), the ferrite phase 1 on the processed surface is deformed so that the ferrite phase 1 is drawn by the elastic recovery of the hard phase 2.
- a recess 3 is generated in the surface, and the presence of this recess deteriorates the surface roughness of the steel material.
- the present inventors have further investigated the requirements for obtaining good cutting surface roughness. As a result, it has been found that if the average equivalent circle diameter of ferrite grains is set within a predetermined range and the number of portions where the ferrite grains are connected is reduced, it is possible to realize a machine structure that can obtain good finished surface roughness. Was completed.
- the size of ferrite grains In order to make the soft ferrite phase finely dispersed, it is necessary to make the size (particle size) of the ferrite grains as small as possible.
- the size of ferrite grains In the steel for machine structure of the present invention, in order to ensure the desired dispersion state of ferrite, the size of ferrite grains needs to be 7 ⁇ m or less in terms of average equivalent circle diameter.
- the average equivalent circle diameter of the ferrite is preferably 6 ⁇ m or less, and more preferably 5 ⁇ m or less.
- the minimum with a preferable average equivalent circle diameter of a ferrite is 2 micrometers or more.
- the pearlite refers to a structure having a structure in which ferrite and plate-like cementite are alternately arranged in layers. Such a structure is collectively referred to as “pearlite”.
- the ferrite targeted in the present invention does not take into account the layered ferrite in pearlite, but refers to a phase that looks white with a scanning electron microscope when it is subjected to nital etching.
- ferrite grain-ferrite grain coupling ratio X [ferrite grain-ferrite grain interface number A] / [ferrite grain-hard phase interface number B] (1)
- the ferrite grain-ferrite grain interface number A is the intersection of the ferrite grain-ferrite grain interface and the straight line when a straight line of a predetermined length is drawn on the structure photograph taken using a scanning electron microscope.
- Number and The ferrite grain-hard phase interface number B indicates the number of intersections between the ferrite grain-hard phase interface and the straight line when a straight line having a predetermined length is drawn in the same manner as described above.
- “ferrite grain-ferrite grain coupling ratio X” is calculated based on the formula (1).
- the observation area when observing is preferably 40000 ⁇ m 2 or more from the viewpoint of improving the accuracy as much as possible.
- the total length (total length) of the line segments drawn horizontally at equal intervals is 1000 ⁇ m or more for the same reason as described above.
- ferrite grain-ferrite grain coupling ratio X defined as described above means that there are few regions where ferrite grains and ferrite grains are continuous, that is, ferrite grains are not continuous and hard. It is surrounded by phases and shows an isolated and dispersed state. Conversely, a large value of “ferrite grain-ferrite grain coupling ratio X” indicates that there are many regions where the ferrite grains are continuous, that is, the ferrite grains tend to form a large phase.
- FIG. 2A shows an example in which the ferrite grain-ferrite grain coupling ratio X is 0.15 or less
- FIG. 2B shows an example in which the ferrite grain-ferrite grain coupling ratio X exceeds 0.15.
- the ferrite grain-ferrite grain coupling ratio X needs to be 0.15 or less. Preferably it is 0.13 or less, More preferably, it is 0.10 or less.
- the purpose can be achieved by satisfying the above requirements, and the ferrite area ratio (the area fraction of ferrite in the entire structure) is not limited at all.
- the ferrite area ratio (the area fraction of ferrite in the entire structure) is not limited at all.
- it is about 30 to 80 area%.
- machine structural steel is assumed, and its steel type may be any chemical composition as long as it is a normal chemical structural steel, but C, Si, Mn, P and S It is better to adjust to the appropriate range. From these viewpoints, the appropriate ranges of these chemical components and the reasons for setting the ranges are as follows.
- C (C: 0.2-1.2%) C is an element effective for securing the strength of steel parts manufactured from machine structural steel. If the C content is too low, it is difficult to adjust the material within the specified range of the ferrite grain-ferrite grain coupling ratio X. If the C content is excessive, the hardness becomes too high and the material is cut. (For example, tool life) decreases. Therefore, the C content is preferably 0.2% or more (more preferably 0.25% or more) and 1.2% or less (more preferably 1.1% or less).
- Si 0.05-0.5%)
- Si is contained as a deoxidizing element and for the purpose of increasing the strength of the steel part by solid solution hardening. However, if it is less than 0.05%, such an effect is not exhibited effectively, and exceeds 0.5%. When it contains excessively, hardness will rise too much and machinability (for example, tool life) will fall.
- the more preferable minimum of Si content is 0.1% or more, and a more preferable upper limit is 0.4% or less.
- Mn is an effective element for deoxidizing and desulfurizing steel during melting, and is an effective element for improving the hardenability and increasing the strength of steel parts.
- Mn content is less than 0.2%, these effects are not exhibited.
- Mn content exceeds 1.8%, the hardness is excessively increased and the cold workability is deteriorated.
- the more preferable minimum of Mn content is 0.3% or more, and a more preferable upper limit is 1.5% or less.
- P 0.03% or less (excluding 0%)
- P is an element inevitably contained in the steel, but segregates at the ferrite grain boundary and deteriorates cold workability. Therefore, it is preferable to reduce P as much as possible, but an extreme reduction leads to an increase in steelmaking cost, and since it is difficult to make it 0%, it is 0.03% or less (not including 0%). It is preferable.
- the upper limit with more preferable P content is 0.025% or less.
- S is an element inevitably contained in steel like P, but it is present as MnS in steel and is a harmful element that deteriorates cold workability, so it needs to be reduced as much as possible.
- the S content is preferably 0.03% or less (more preferably 0.025% or less).
- S is an unavoidable impurity, it is industrially difficult to reduce the amount to 0%.
- the basic component composition of the steel for machine structure of the present invention is as described above, and the balance is substantially iron.
- substantially iron can accept trace components (eg, Sb, Zn, etc.) that do not impair the properties of the steel material of the present invention in addition to iron, and inevitable impurities other than P and S ( For example, Al, N, O, H, etc.) may also be included.
- the steel for machine structure of this invention may contain the following selective components as needed. The reasons for limiting the component range when these components are contained are as follows.
- Cr 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Ni: 0.5% or less (not including 0%), and Mo: 0 .1 or more selected from the group consisting of 5% or less (excluding 0%)) Cr, Cu, Ni and Mo are all effective elements for increasing the strength of the final product by improving the hardenability of the steel material, and are contained alone or in combination of two or more as required.
- the respective preferable upper limits are set as described above. More preferably, both are 0.45% or less (more preferably 0.40% or less).
- the preferable minimum for exhibiting those effects effectively is 0.015% or more (more preferably 0.020% or more). It is.
- a steel that satisfies the above component composition is hot-rolled under normal conditions to obtain a hot-rolled material. Heating to a temperature of 800 to 950 ° C., holding at that temperature for about 10 to 25 minutes (holding time), and then cooling to 500 ° C. or less at an average cooling rate of 2 ° C./second or more. In addition, as long as it exists in the range of these manufacturing conditions, you may change the conditions in the middle. These manufacturing conditions will be described.
- Heating temperature 800-950 ° C
- the heating temperature heating temperature after hot rolling
- the austenite grain size at the time of heating becomes coarse, so that the total area of the austenite grain boundary per unit volume decreases, and the ferrite grains precipitated from the austenite grain boundary approach. This makes it difficult to isolate the ferrite grains.
- the heating temperature is less than 800 ° C., the ferrite grains existing in the material before the heat treatment do not completely change to the austenite phase, and it is difficult to control the ferrite grain-ferrite grain coupling ratio X to 0.15 or less. It becomes.
- the more preferable lower limit of this heating temperature is 820 ° C. or higher (more preferably 850 ° C. or higher), and the more preferable upper limit is 930 ° C. or lower (more preferably 900 ° C. or lower).
- the holding time in the heating temperature range is a factor affecting the ferrite grain-ferrite grain coupling ratio X. If the holding time at this time is less than 10 minutes, the change from the ferrite phase before the heat treatment to the austenite phase does not proceed sufficiently, and the ferrite phase remains in the structure. Also, if the holding time exceeds 25 minutes, the austenite grains become coarse and the total area of the austenite grain boundaries per unit volume decreases, so that the ferrite grains precipitated from the austenite grain boundaries approach, so that the ferrite grains It becomes difficult to isolate. A more preferable lower limit of the holding time is 15 minutes or more, and a more preferable upper limit is 20 minutes or less.
- the average cooling rate up to 500 ° C. or less (cooling stop temperature) is less than 2 ° C./second, the ferrite average particle size cannot be made 7 ⁇ m or less. From such a viewpoint, the average cooling rate needs to be 2 ° C./second or more.
- the average cooling rate is more preferably 5 ° C./second or more, and further preferably 7 ° C./second or more.
- the cooling form which changes a cooling rate may be sufficient.
- the cooling stop temperature is preferably 500 ° C. or lower. If this temperature is higher than 500 ° C., the ferrite particle size tends to be coarse, and it becomes difficult to make the ferrite average particle size 7 ⁇ m or less. On the other hand, as the cooling stop temperature is lowered, there is no influence on the material structure. Therefore, after cooling to 500 ° C. or lower, normal cooling (cooling) is performed, Just do it.
- the steel for machine structure of the present invention has excellent machinability, and a steel product with improved cutting surface properties (cut surface roughness) can be obtained by cutting such steel for machine structure. Further, if the steel for machine structural use of the present invention is cut, a good cutting surface property can be obtained, so that it can be applied as it is as a molding die without performing a finishing process such as grinding.
- steel types A to D having the chemical composition shown in Table 1 below, hot rolled material (thick plate material: plate thickness: 30 mm) was used under normal hot rolling conditions.
- Steel type A shown in Table 1 is S55C equivalent steel (JISG4051)
- Steel type B is S60C equivalent steel (JISG4051)
- Steel type C is S50C equivalent steel (JISG4051)
- Steel type D is S45C equivalent steel (JISG4051). It is.
- the obtained hot rolled material was used as various test materials under the production conditions (heating temperature, holding time, average cooling rate after heating, cooling method) shown in Table 2 below (Test Nos. 1 to 7).
- Test No. 1 a hot-rolled material of S55C equivalent steel (steel type A) was heated to 850 ° C., held at that temperature for 20 minutes, and then air-cooled without blowing with a blower (average cooling rate: 3 ° C./second) ).
- Test No. In No. 2 the S60C equivalent steel (steel type B) hot-rolled material was heated to 850 ° C., held at that temperature for 20 minutes, and then cooled in the furnace (average cooling rate: 0.8 ° C./second). It is the obtained test material.
- Test No. 3 a hot-rolled material of S50C equivalent steel (steel type C) was heated to 900 ° C., held at that temperature for 20 minutes, and then air-cooled while blowing with a blower (average cooling rate: 6 ° C./second) It is a test material obtained by doing. Test No. In No. 3, a hot-rolled material of S50C equivalent steel (steel type C) was heated to 900 ° C., held at that temperature for 20 minutes, and then air-cooled while blowing with a blower (average cooling rate: 6 ° C./second) It is a test material obtained by doing. Test No. In No.
- a hot-rolled material of S55C equivalent steel (steel type A) was not heated.
- Test No. 7 a hot-rolled material of S45C equivalent steel (steel type D) was heated to 700 ° C. and held at that temperature for 30 minutes, and then air-cooled without blowing with a blower (average cooling rate: 3 ° C./second) ). In all cases, the cooling stop temperature was set to 500 ° C. or lower.
- the average equivalent-circle diameter of ferrite and the ferrite grain-ferrite grain coupling ratio X were measured by the following method.
- the cutting experiment was performed on the conditions shown in following Table 3, and the cutting surface roughness of each test material after cutting was evaluated.
- the cutting experiment at this time was performed by two-dimensional cutting (planing) using a machining center.
- the cutting surface roughness which is a criterion for machinability, was measured by moving the stylus parallel to the cutting direction using a stylus type roughness meter.
- the evaluation standard of the cutting surface roughness was evaluated as good surface properties when the cutting surface roughness was less than 0.10 ⁇ m in the calculated average roughness Ra.
- the cutting experiment is performed by two-dimensional cutting, and it is expected that a more complicated three-dimensional cutting will be obtained by normal cutting. However, since the three-dimensional cutting can be regarded as an accumulation of two-dimensional cutting with a narrow width, the effect in the three-dimensional cutting can be predicted from the data in the two-dimensional cutting.
- Test No. Examples 1, 3, and 4 are examples that satisfy all of the requirements defined in the present invention (ferrite grain-ferrite grain connection ratio X, average circle equivalent diameter of ferrite), and have a good cutting surface roughness (Ra). It can be seen that the values are shown. Of these, in particular, test no. No. 1 is an example in which the heating temperature and the holding time are set to more preferable values, and the ferrite grain-ferrite grain connection ratio X is a more preferable value, and it is understood that the most excellent surface properties are obtained.
- test no. Nos. 2, 5 to 7 are examples that do not satisfy any of the requirements defined in the present invention, and all have a large cut surface roughness. That is, test no. 2 and test no. No. 5 is an example in which the average cooling rate is 0.8 ° C./second, the average equivalent circle diameter of ferrite is large, and the cutting surface roughness (Ra) is large.
- Test No. No. 6 is an example in which the heat treatment was not performed after the hot rolling, and the ferrite grain-ferrite grain connection ratio X is large, and the cut surface roughness (Ra) is large.
- Test No. No. 7 is an example in which the heating temperature is low, the ferrite grain-ferrite grain coupling ratio X is large, and the cutting surface roughness (Ra) is large.
- the steel for machine structure of the present invention is useful for various machine structure parts such as automobile parts and construction machine parts, and is excellent in machinability, so that it is particularly suitable for application to a molding die.
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Abstract
Description
[フェライト粒-フェライト粒連結率X]=[フェライト粒-フェライト粒界面数A]/[フェライト粒-硬質相界面数B] …(1)
式中、フェライト粒-フェライト粒界面数Aは、走査型電子顕微鏡を用いて撮影された組織写真に所定長さの直線を引いた時の、フェライト粒-フェライト粒界面と前記直線との交点の数を示し、
フェライト粒-硬質相界面数Bは、前記と同様にして所定長さの直線を引いた時の、フェライト粒-硬質相界面と前記直線との交点の数を示す。
[フェライト粒-フェライト粒連結率X]=[フェライト粒-フェライト粒界面数A]/[フェライト粒-硬質相界面数B] …(1)
式中、フェライト粒-フェライト粒界面数Aは、走査型電子顕微鏡を用いて撮影された組織写真に所定長さの直線を引いた時の、フェライト粒-フェライト粒界面と前記直線との交点の数を示し、
フェライト粒-硬質相界面数Bは、前記と同様にして所定長さの直線を引いた時の、フェライト粒-硬質相界面と前記直線との交点の数を示す。
Cは、機械構造用鋼から製造される鋼部品の強度を確保するために有効な元素である。C含有量が低すぎると、フェライト粒-フェライト粒連結率Xの規定範囲内に材料を調整するのは困難であり、またC含有量が過剰になると、硬さが高くなりすぎて、被削性(例えば、工具寿命)が低下する。そのため、C含有量は0.2%以上(より好ましくは0.25%以上)、1.2%以下(より好ましくは1.1%以下)とするのが良い。
Siは、脱酸元素として、および固溶体硬化による鋼部品の強度を高くすることを目的として含有させるが、0.05%未満ではこうした効果が有効に発揮されず、また0.5%を超えて過剰に含有されると硬度が過度に上昇して被削性(例えば、工具寿命)が低下する。尚、Si含有量のより好ましい下限は0.1%以上であり、より好ましい上限は0.4%以下である。
Mnは、溶製中の鋼の脱酸、脱硫元素として有効であり、また焼入れ性を向上させて鋼部品の強度を高めるのに有効な元素である。Mn含有量が、0.2%未満ではこれらの効果が発揮されず、1.8%を超えて過剰に含有されると、硬度が上昇しすぎて冷間加工性を劣化させる。尚、Mn含有量のより好ましい下限は0.3%以上であり、より好ましい上限は1.5%以下である。
Pは、鋼中に不可避的に含まれる元素であるが、フェライト粒界に偏析し、冷間加工性を劣化させる。従って、Pは極力低減することが好ましいが、極端な低減は製鋼コストの増大を招き、0%とすることは製造上困難であるので、0.03%以下(0%を含まない)とすることが好ましい。P含有量のより好ましい上限は0.025%以下である。
SもPと同様に鋼中に不可避的に含まれる元素であるが、鋼中でMnSとして存在し、冷間加工性を劣化させる有害な元素であるので、なるべく低減する必要がある。こうした観点から、S含有量は0.03%以下(より好ましくは0.025%以下)とすることが好ましい。しかしSは、不可避的に含まれる不純物であるので、その量を0%とすることは工業的に困難である。
Cr,Cu,NiおよびMoは、いずれも鋼材の焼入れ性を向上させることによって最終製品の強度を増加させるのに有効な元素であり、必要によって単独でまたは2種以上で含有される。しかしながら、これらの元素の含有量が過剰になると、強度が高くなり過ぎ、冷間加工性を劣化させるので、上記のように夫々の好ましい上限を定めた。より好ましくは、いずれも0.45%以下(更に好ましくは0.40%以下)である。尚、これらの元素による効果はその含有量が増加するにつれて大きくなるが、それらの効果を有効に発揮させるための好ましい下限は、いずれも0.015%以上(より好ましくは0.020%以上)である。
フェライト粒-フェライト粒連結率Xを0.15以下に制御するためには、加熱温度(熱間圧延後の加熱温度)を800~950℃に制御する必要がある。このときの加熱温度が950℃を超えると、加熱時のオーステナイト粒径が粗大化することで、単位体積当たりのオーステナイト粒界の総面積が減少し、オーステナイト粒界から析出するフェライト粒が接近することとなって、フェライト粒同士を孤立化させることが困難となる。また、加熱温度が800℃未満であると、熱処理前の材料に存在するフェライト粒が完全にオーステナイト相に変化せず、フェライト粒-フェライト粒連結率Xを0.15以下に制御することが困難となる。この加熱温度のより好ましい下限は820℃以上(更に好ましくは850℃以上)であり、より好ましい上限は930℃以下(更に好ましくは900℃以下)である。
上記加熱温度範囲での保持時間は、フェライト粒-フェライト粒連結率Xに影響を及ぼす要因である。このときの保持時間が10分未満であると、熱処理前のフェライト相からオーステナイト相への変化が十分に進行せず、組織内にフェライト相が残った状態となる。また、保持時間が25分を超えると、オーステナイト粒が粗大化し、単位体積当たりのオーステナイト粒界の総面積が減少するため、オーステナイト粒界から析出するフェライト粒が接近することで、フェライト粒同士を孤立化させることが困難となる。この保持時間のより好ましい下限は15分以上であり、より好ましい上限は20分以下である。
500℃以下(冷却停止温度)までの平均冷却速度が2℃/秒未満であると、フェライト平均粒径を7μm以下とすることができなくなる。こうした観点から、平均冷却速度は2℃/秒以上とする必要がある。この平均冷却速度は、より好ましくは5℃/秒以上であり、更に好ましくは7℃/秒以上である。尚、このときの冷却については、2℃/秒以上となる平均冷却速度の範囲内であれば、冷却速度を変えるような冷却形態であっても良い。
冷却停止温度は、500℃以下とすることが好ましい。この温度が500℃よりも高くなると、フェライト粒径が粗大化しやすく、フェライト平均粒径を7μm以下にすることが困難となる。一方、冷却停止温度が低くなる分には、材料組織に対して何ら影響が無いため、500℃以下まで冷却を終えた後は、通常の冷却(放冷)を行って、室温までの温度とすれば良い。
各供試材を鏡面に研磨し、3%ナイタール液で腐蝕させて金属組織を現出させた後、概略170μm×230μmの領域の5視野について、倍率:400倍の走査型電子顕微鏡(SEM)にて組織観察し、撮影した。それらの写真を元に、画像のコントラストから白い部分をフェライト粒子と判別してマーキングし、画像解析によって、フェライト粒子の円相当直径を求め、5視野の平均値を求めた。
各供試材を鏡面に研磨し、3%ナイタール液で腐蝕させて金属組織を現出させた後、面積40000μm2の領域において、倍率:400倍の走査型電子顕微鏡(SEM)にて組織観察を行い、撮影した。そして、組織写真中に等間隔で平行な線を総長さ1000μm以上となるように引き、前述した手順に従ってフェライト粒-フェライト粒連結率Xを求めた。
本出願は、2013年8月22日出願の日本特許出願(特願2013-172546)に基づくものであり、その内容はここに参照として取り込まれる。
2 硬質相
3 凹部
Claims (5)
- パーライト、ベイナイトおよびマルテンサイトから選ばれる少なくとも1種からなる硬質相とフェライト相とからなる混合組織を有し、フェライト粒の平均円相当直径が7μm以下であり、下記(1)式で表されるフェライト粒-フェライト粒連結率Xが0.15以下であることを特徴とする被削性に優れた機械構造用鋼。
[フェライト粒-フェライト粒連結率X]=[フェライト粒-フェライト粒界面数A]/[フェライト粒-硬質相界面数B] …(1)
式中、フェライト粒-フェライト粒界面数Aは、走査型電子顕微鏡を用いて撮影された組織写真に所定長さの直線を引いた時の、フェライト粒-フェライト粒界面と前記直線との交点の数を示し、
フェライト粒-硬質相界面数Bは、前記と同様にして所定長さの直線を引いた時の、フェライト粒-硬質相界面と前記直線との交点の数を示す。 - C:0.2~1.2%(「質量%」の意味。以下、化学成分組成について同じ。)、
Si:0.05~0.5%、
Mn:0.2~1.8%、
P :0.03%以下(0%を含まない)、
S :0.03%以下(0%を含まない)、
を夫々含有し、残部が鉄および不可避的不純物である請求項1に記載の機械構造用鋼。 - 更に他の元素として、
Cr:0.5%以下(0%を含まない)、
Cu:0.5%以下(0%を含まない)、
Ni:0.5%以下(0%を含まない)、および
Mo:0.5%以下(0%を含まない)よりなる群から選択される1種以上を含有するものである請求項2に記載の機械構造用鋼。 - 請求項1~3のいずれかに記載の機械構造用鋼を切削する、切削面性状の改善方法。
- 請求項1~3のいずれかに記載の機械構造用鋼を切削する、成型用金型の製造方法。
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US14/910,888 US20160194741A1 (en) | 2013-08-22 | 2014-08-08 | Steel for use in machine-construction excellent in machinability |
CN201480045836.1A CN105473750A (zh) | 2013-08-22 | 2014-08-08 | 可切削性优异的机械结构用钢 |
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