JP2010280963A - Steel for cold working, method for producing steel for cold working, method for producing component for machine structure, and component for machine structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for cold working which has excellent cold workability and strength after cold working. <P>SOLUTION: The steel for cold working has a composition containing, by mass, 0.005 to 0.045% C, 0.005 to 0.050% Si, 0.4 to 1.0% Mn, ≤0.05% P, 0.005 to 0.050% S, 0.005 to 0.060% Al and 0.009 to 0.016% N, and the balance Fe with inevitable impurities, wherein the content of solid solution N is 0.008 to 0.015% and the structural fraction of a ferritic phase is ≥90%, and a difference between the maximum value and the minimum value in Vickers hardness (measured load of 9.8N) measured per 1 mm to a depth of 1/4 of the thickness of the steel from the surface of the steel is ≤15Hv. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a steel material for cold working excellent in cold workability and strength after cold working, a method for producing a steel material for cold working, a method for producing a machine structural component, and a machine structural component.

  In general, steel materials for cold working used for machine structural parts such as bolts, nuts, pinion gears, steering shafts, valve lifters, common rails, and the like are known to have excellent cold workability. For example, a technology that utilizes a cementite-free structure in order to obtain excellent cold workability (see Patent Document 1), or by controlling the solid solution C and the crystal grain size to suppress normal aging and cold forging A technique for age-hardening later (see Patent Document 2) is disclosed.

  That is, Patent Document 1 discloses a technique relating to a high-strength steel wire or steel bar having an average particle diameter of 500 nm or less and having a cementite-free ferrite structure and excellent deformability. In Patent Document 1, a steel material whose C amount is limited to a predetermined range is subjected to warm working within a range of 300 to 800 ° C., and then subjected to cold working, thereby ferrite main grains having an average crystal grain size of 500 nm or less. It has a phase structure that balances strength and deformability.

  Patent Document 2 discloses a technique that can suppress the progress of normal temperature aging and improve the strength of the component by aging treatment after cold forging. In this Patent Document 2, the steel amount is reduced as much as possible, and the ferrite material having 20 μm or more is made 90 area% or more. In this steel material, the ferrite grain size is made as large as possible, and room temperature aging is suppressed by increasing the distance at which solid solution C and solid N are fixed to dislocations at room temperature. That is, this steel material is configured such that normal temperature aging is less likely to occur as the ferrite grain size increases.

  In addition, the steel materials used for machine structural parts are cold-worked when manufactured into parts such as bolts. The cold processing (cold forging) performed here is a processing method in an atmosphere of 200 ° C. or lower. It is known that this cold working has higher productivity than hot working, and has good advantages in both dimensional accuracy and yield of steel.

Japanese Patent Laying-Open No. 2005-320630 JP-A-10-306345

However, the conventional cold working steel materials have the following problems.
In Patent Document 1, in order to make cementite free, the C content needs to be equal to or less than the solid solubility limit of the ferrite phase carbon at the point Ae1. The reason for this is that if C beyond the solid solubility limit exists, the solid solution C precipitates as cementite during warm working at 300 to 800 ° C., thereby degrading workability. That is, in the steel material of Patent Document 1, supersaturated solute C cannot be obtained, and N is treated as a harmful impurity, so that the inevitable mixing content must be limited.

  In Patent Document 2, strain aging is controlled by the amount of dissolved C, and it has been difficult to obtain a steel material having sufficient cold workability and sufficient strength after processing.

  In addition to the above, there is a method for increasing the amount of solute N in the steel structure in order to increase the strength of the parts after cold working, and N for increasing the deformation resistance of the steel material in order to improve the die life. There are also techniques to reduce other elements as much as possible. However, these methods are methods that utilize dynamic strain aging caused by the interaction between solid solution N in the structure and dislocations due to strain, so an increase in deformation resistance due to dynamic strain aging cannot be avoided. Therefore, there has been a need for a method for improving the component strength and die life after cold working without causing dynamic strain aging.

  In addition, under high-speed cold working, since the dislocation density can be increased as compared with working in a normal average strain rate region, dynamic strain aging is not required. Furthermore, under high-speed cold working, since adiabatic heat generation during cold working can be used, static strain aging can be promoted more than ordinary steel materials.

  The present invention has been made based on such a background, a steel material for cold work having both cold workability and strength after cold work, a method for producing a steel material for cold work, and a machine structural component. An object of the present invention is to provide a manufacturing method and a machine structural component.

  In order to solve the above problems, the steel for cold working according to claim 1 is: C: 0.005-0.045 mass%, Si: 0.005-0.050 mass%, Mn: 0.4-1 0.0 mass%, P: 0.05 mass% or less, S: 0.005-0.050 mass%, Al: 0.005-0.060 mass%, N: 0.009-0.016 mass% And the balance is a steel material for cold working having a composition composed of Fe and inevitable impurities, the solid solution N amount is 0.008 to 0.015 mass%, and the structure fraction of the ferrite phase is 90 The difference between the maximum value and the minimum value of Vickers hardness (measured load 9.8 N) measured every 1 mm from the steel surface to a depth of 1/4 of the thickness of the steel material is 15 Hv or less. And

  The steel material for cold work having such a configuration is because the amount of solute N is limited within a certain range and the structure fraction of the ferrite phase is 90% or more, whereby the solute N is fixed to dislocations. Dynamic strain aging can be suppressed. Moreover, by making the difference in Vickers hardness within a predetermined value, the strength distribution of the steel material before processing can be made uniform, and the effect of high-speed cold working can be maximized.

  Moreover, the steel material for cold work which concerns on Claim 2 is set as the structure in which the said composition contains at least 1 or more types among Cr: 2.0 mass% or less and Mo: 1.0 mass% or less further. Thus, the deformation resistance of the steel material can be reduced by adding a predetermined amount of either Cr or Mo.

  Further, in the cold work steel material according to claim 3, the composition further includes at least one of Ti: 0.02 mass% or less, Nb: 0.02 mass% or less, and V: 0.02 mass% or less. It is set as the structure containing a seed | species or more. Thus, by adding a predetermined amount of any one of Ti, Nb, and V, an N compound portion can be formed to regulate the crystal grains, and variation in strength of parts after cold working can be suppressed.

  Moreover, the steel material for cold work which concerns on Claim 4 is set as the structure in which the said composition contains B: 0.005 mass% or less further. Thus, by adding a predetermined amount of B, it is possible to suppress a decrease in grain boundary strength due to P segregating at ferrite grain boundaries.

  Further, in the cold work steel according to claim 5, the composition further includes at least one of Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Co: 1.0% by mass or less. It is set as the structure containing a seed | species or more. Thus, by adding a predetermined amount of any one of Cu, Ni and Co, the static strain aging of the steel material can be promoted, and the strength of the parts after cold working can be improved.

  Further, in the steel for cold working according to claim 6, the composition further includes Ca: 0.01% by mass or less, REM: 0.01% by mass or less, Mg: 0.005% by mass or less, Li: 0.00%. It is set as the structure containing at least 1 or more types among 005 mass% or less, Pb: 0.5 mass% or less, Bi: 0.5 mass% or less. Thus, by adding a predetermined amount of any of Ca, REM, Mg, Li, Pb, and Bi, the sulfide compound inclusions such as MnS are spheroidized, and the cold workability of the steel material is improved. Machinability can be improved.

  Moreover, the manufacturing method of the steel material for cold work which concerns on Claim 7 is a manufacturing method of the steel material for cold work of any one of Claim 1 to 6, Comprising: A step of hot rolling or hot forging after heating to 1250 ° C, a step of heating and holding the steel material after hot rolling or hot forging at 400 to 700 ° C for 60 to 7200 sec, and the steel material after holding And a step of cooling to room temperature at a cooling rate of 0.1 ° C./s or more.

  The manufacturing method of the steel material for cold work provided with such a structure can control the solid solution N within a fixed range by setting the temperature at the time of hot rolling or hot forging within a predetermined range. Further, by performing heat treatment at a predetermined temperature for a predetermined time, the dislocations in the crystal grains can be arranged and the hardness of the steel material can be within an appropriate range without changing the shape of the ferrite phase.

  Moreover, the steel material for cold work which concerns on Claim 8 is set as the structure which further has the process of cooling a steel material to room temperature after the process of the said hot rolling or hot forging. By having this step, the grain size of the steel structure can be adjusted.

  Moreover, the manufacturing method of the machine structural component which concerns on Claim 9 is a manufacturing method of the machine structural component using the steel for cold work of any one of Claim 1-6, Comprising: The said composition Heating the steel material to 1050 to 1250 ° C. and then hot rolling or hot forging; and heating and holding the steel material after hot rolling or hot forging at 400 to 700 ° C. for 60 to 7200 seconds; The step of cooling the retained steel material to room temperature at a cooling rate of 0.1 ° C./s or more, and the cold-working of the cooled steel material at a starting temperature of less than 200 ° C. and an average strain rate of 50 / s or more. And a step of performing.

  In the method for manufacturing a machine structural component having such a configuration, the solid solution N can be controlled within a certain range by setting the temperature during hot rolling or hot forging within a predetermined range. Further, by performing heat treatment at a predetermined temperature for a predetermined time, the dislocations in the crystal grains can be arranged and the hardness of the steel material can be within an appropriate range without changing the shape of the ferrite phase. Furthermore, by cold working at an average strain rate of 50 / s or higher, the adiabatic temperature rise becomes 100 ° C. or higher, so that solid solution N easily moves in the structure after cold working. The strength of is improved.

  The mechanical structure manufacturing method according to claim 10 further includes a step of cooling the steel material to room temperature after the hot rolling or hot forging step. By having this step, the grain size of the steel structure can be adjusted.

  A mechanical structural component according to claim 11 is manufactured by cold working the steel for cold working according to any one of claims 1 to 6 at a start temperature of less than 200 ° C. And

  Machine structural parts with such a structure are used for mechanical structures with higher strength by making the steel material being cold-worked adiabatically deformed so that solid solution N can easily move through the structure after cold working. Parts can be manufactured.

  According to the steel material for cold working, the method for producing the steel material for cold working, the method for producing the component for machine structure, and the component for machine structure according to the present invention, the structure of the steel material is substantially uniform as a ferrite single phase. By applying a heat treatment in a temperature range in which the crystal grain size does not change after hot rolling or hot forging, the hardness in the steel can be adjusted appropriately to reduce hardness variation. Also, by setting the average strain rate during cold working to 50 / s or more, only static strain aging is generated by processing heat generation without generating dynamic strain aging, and the strength after cold working is increased. be able to. Solid solution N is an element that increases deformation resistance, but dynamic strain aging can be suppressed in the entire steel material by processing at high speed. Therefore, it is possible to provide a steel material for cold work and a machine structural component that are excellent in cold workability and strength after cold work.

  Hereinafter, the cold work steel material, the cold work steel production method, the machine structure component production method, and the machine structure component according to the present invention will be described in detail.

[Steel for cold working]
Hereinafter, the form for implementing the steel material for cold work which concerns on this invention is demonstrated.
The steel materials for cold working according to the present invention are: C: 0.005 to 0.045 mass%, Si: 0.005 to 0.050 mass%, Mn: 0.4 to 1.0 mass%, P: 0 0.05% by mass or less, S: 0.005 to 0.050% by mass, Al: 0.005 to 0.060% by mass, N: 0.009 to 0.016% by mass, the balance being Fe and inevitable A steel material for cold working having a composition composed of impurities, wherein the amount of solute N is 0.008 to 0.015 mass%, the structure fraction of the ferrite phase is 90% or more, and from the steel material surface The difference between the maximum value and the minimum value of the Vickers hardness (measured load 9.8 N) measured every 1 mm to a depth of 1/4 of the thickness of the steel material is 15 Hv or less.
Below, the numerical value range of the content of each component of the composition of the steel material for cold working according to the present invention, the reason for limiting the numerical range, and other conditions will be described in detail.

<Composition>
(C: 0.005-0.045% by mass)
C needs to be reduced as much as possible in order to make the steel for cold working a ferrite single phase. However, if the amount of C is extremely small, deoxidation during melting becomes difficult. On the other hand, if the amount of C in the structure does not exceed 0.045% by mass, the ferrite single phase is substantially formed although fine cementite is slightly present.

  When the amount of C exceeds 0.045% by mass, cementite forms pearlite and forms a multiphase structure of ferrite and pearlite, so that the fraction of the ferrite phase decreases. Further, since pearlite is a hard phase, when pearlite is formed in the structure, the hardness of the steel material is increased, the cold workability is deteriorated, and cracks occur in the parts after the cold work. On the other hand, when C is less than 0.005% by mass, deoxidation is insufficient, gas defects are likely to occur during melting, and the yield decreases. In addition, cracks occur in the parts after cold working. The amount of C is preferably 0.010% by mass or more, more preferably 0.015% by mass or more. Moreover, Preferably it is 0.043 mass% or less, More preferably, it is 0.040 mass% or less.

(Si: 0.005 to 0.050 mass%)
Si is an effective element as a deoxidizing element during melting. Si strengthens the steel structure ferrite phase in solid solution. However, when the amount of Si exceeds 0.05% by mass, deformation resistance increases, cold workability deteriorates, and cracks occur. On the other hand, if the amount of Si is less than 0.005% by mass, the effect of deoxidation is not exhibited, gas defects are likely to occur during melting, and cracks occur in parts after cold working. The amount of Si is preferably 0.007% by mass or more, more preferably 0.010% by mass or more. Moreover, Preferably it is 0.04 mass% or less, More preferably, it is 0.03 mass% or less.

(Mn: 0.4 to 1.0% by mass)
Mn is an effective element as a deoxidizing / desulfurizing element during melting. Moreover, Mn can improve the deformability of steel materials by combining with S. However, if the amount of Mn exceeds 1.0% by mass, deformation resistance is remarkably increased by solid solution strengthening, cold workability is deteriorated, and cracks are generated. On the other hand, if the amount of Mn is less than 0.4% by mass, the effect of deoxidation / desulfurization cannot be sufficiently exhibited, and the deformability of the steel material is lowered. In addition, cracks occur in the parts after cold working. The amount of Mn is preferably 0.42% by mass or more, more preferably 0.45% by mass or more. Moreover, Preferably it is 0.98 mass% or less, More preferably, it is 0.95 mass% or less.

(P: 0.05% by mass or less)
P is an element inevitably contained as an impurity in the structure. However, if the amount of P exceeds 0.05% by mass, it segregates at the grain boundaries of the ferrite phase to lower the cold workability, and strengthens the ferrite phase by solid solution strengthening to increase the deformation resistance of the steel material. In addition, cracks occur in the parts after cold working. Although it is desirable to reduce the amount of P as much as possible, an extreme reduction leads to an increase in the steelmaking cost, and it is technically difficult to reduce it to 0% by mass. The amount of P is preferably 0.04% by mass or less, more preferably 0.03% by mass or less.

(S: 0.005 to 0.050 mass%)
S is an element inevitably contained as an impurity in the structure. S combines with Fe to precipitate as a film on the grain boundary as FeS, thereby degrading cold workability. Therefore, it is necessary to combine S with Mn and precipitate MnS. However, if the amount of S exceeds 0.050% by mass, the amount of MnS deposited increases, the cold workability deteriorates, and cracks occur in the parts after cold working. On the other hand, when the amount of S is less than 0.005% by mass, problems such as a decrease in the machinability of the steel material and an increase in the steelmaking cost are caused. The amount of S is preferably 0.007% by mass or more, more preferably 0.010% by mass or more. Moreover, Preferably it is 0.04 mass% or less, More preferably, it is 0.03 mass% or less.

(Al: 0.005-0.060 mass%)
Al is an effective element as a deoxidizing element during melting. Further, by precipitating Al as AlN during the heat treatment after hot rolling or hot forging, the crystal grains are easily sized. However, when the Al content exceeds 0.060 mass%, the solid solution N is easily bonded and the solid solution N amount is decreased, so that the strength of the component after cold working is lowered. On the other hand, if the Al content is less than 0.005% by mass, the deoxidation effect is not exhibited, gas defects are likely to occur during melting, and the yield decreases. In addition, cracks occur in the parts after cold working. The amount of Al is preferably 0.007% by mass or more, more preferably 0.010% by mass or more. Moreover, Preferably it is 0.050 mass% or less, More preferably, it is 0.045 mass% or less.

(N: 0.009 to 0.016 mass%)
N is an important element for improving the strength of parts after high-speed cold working by static strain aging. However, if the N amount exceeds 0.016% by mass, dynamic strain aging cannot be sufficiently suppressed even in high-speed cold working, and an increase in deformation resistance, deterioration of cold workability, variation in steel strength, It causes cracking of parts after cold working. On the other hand, if the N content is less than 0.009% by mass, a sufficient improvement in the component strength cannot be obtained even when cold-working the steel material at high speed. Therefore, cracks occur in the parts after cold working. The N amount is preferably 0.0095% by mass or more, more preferably 0.0100% by mass or more. Moreover, Preferably it is 0.0140 mass% or less, More preferably, it is 0.0135 mass% or less.

(Solution N amount: 0.008 to 0.015 mass%)
N dissolved in steel (solid solution N) fixes many dislocations generated during high-speed cold working, thereby strengthening static strain aging and increasing the strength beyond work hardening. Has an effect. However, when the amount of solute N exceeds 0.015 mass%, cold workability deteriorates. On the other hand, if the amount of dissolved N is less than 0.008% by mass, the strength of the parts after cold working cannot be improved. The amount of solute N is preferably 0.0085% by mass or more, more preferably 0.0090% by mass or more. Moreover, Preferably it is 0.0130 mass% or less, More preferably, it is 0.0125 mass% or less.

(Measurement method of solid solution N)
The value of the solid solution N can be calculated by measuring the total N amount and the total N compound amount in the steel material according to the following procedure and subtracting them in accordance with JIS G 1228, for example.

  For the total amount of N in the steel material, an inert gas melting method-thermal conductivity method is used. A sample is cut out from the test steel material, placed in a crucible, melted in an inert gas stream, extracted N, transported to a thermal conductivity cell, and the change in thermal conductivity is measured. The total amount of N compounds in the steel can be measured by ammonia distillation separation indophenol blue absorptiometry. This method is as follows.

  First, about 0.5 g of a sample cut out from the test steel material was converted into a 10% AA electrolyte solution (a non-aqueous solvent electrolyte solution that does not generate a passive film on the surface of the steel material. % Acetylacetone, 10% tetramethylammonium chloride, balance: methanol). The dissolved sample (and electrolyte solution) is filtered through a polycarbonate filter having a mesh size of 0.1 μm to separate into an insoluble residue (nitrogen compound) and the filtrate. The insoluble residue is heated and decomposed in sulfuric acid, potassium sulfate, and pure Cu chips, and then mixed with the filtrate. The mixed solution is alkalized with sodium hydroxide and then steam distilled to absorb the distilled ammonium in dilute sulfuric acid. To the solution is added phenol, sodium hypochlorite, and sodium pentacyanonitrosyl iron (III) to form a blue complex. The absorbance of this blue complex is measured using a photometer, and the amount of N in the N compound is determined from this absorbance.

The steel for cold working according to the present invention preferably includes the following optional components.
(Cr: 2.0 mass% or less, Mo: at least one of 1.0 mass% or less (excluding 0%))
Cr and Mo are effective elements for improving cold workability and strength after cold working, and can be selectively added only in a predetermined amount. When the Cr amount is 2.0 mass% and the Mo amount exceeds 1.0 mass%, deformation resistance increases and cold workability deteriorates. In order to appropriately obtain the effects of addition of Cr and Mo, it is preferable to add Cr in an amount of 0.10% by mass or more and Mo in an amount of 0.04% by mass or more.

  The amount of Cr is preferably 0.2% by mass or more, more preferably 0.3% by mass or more. Moreover, Preferably it is 1.5 mass% or less, More preferably, it is 1.0 mass% or less. The amount of Mo is preferably 0.04% by mass or more, more preferably 0.12% by mass or more. Moreover, Preferably it is 0.8 mass% or less, More preferably, it is 0.4 mass% or less.

(Ti: 0.02 mass% or less, Nb: 0.02 mass% or less, V: at least one of 0.02 mass% or less (excluding 0%))
Ti, Nb, and V are elements effective for suppressing variation in the strength of parts after cold working because they form an N compound by bonding with N to regulate crystal grains. However, since Ti, Nb, and V have strong affinity with N, if they are added in excess of 0.02% by mass, N compounds are excessively formed and the amount of solute N is reduced. In order to appropriately obtain the effects of adding Ti, Nb and V, it is preferable to add 0.001% by mass or more of Ti, Nb and V, respectively. The amount of Ti, Nb, and V is preferably 0.002% by mass or more, more preferably 0.003% by mass or more. Moreover, Preferably it is 0.015 mass% or less, More preferably, it is 0.010 mass% or less.

(B: 0.005 mass% or less (excluding 0%))
Since B tends to gather at the ferrite grain boundary, B is an effective element for suppressing a decrease in grain boundary strength due to segregation of P at the ferrite grain boundary. However, since B has a strong affinity with N, if it is added in an amount exceeding 0.005% by mass, BN is formed and the amount of solute N is reduced, and the BN is excessively segregated at the ferrite grain boundaries. The field strength is reduced. The amount of B is preferably 0.0004% by mass or more, and more preferably 0.0006% by mass or more. Moreover, Preferably it is 0.0035 mass% or less, More preferably, it is 0.0020 mass% or less.

(Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Co: at least one of 1.0 mass% or less (excluding 0%))
Cu, Ni, and Co are all effective elements for accelerating the static strain aging of steel and improving the strength after cold working. However, if the amount of Cu, Ni, Co exceeds 1.0% by mass, the effect is saturated and cracking of the steel material is promoted. In order to appropriately obtain the effect of adding Cu, Ni and Co, it is preferable to add 0.01% by mass or more of Cu, Ni and Co, respectively. The amount of Cu, Ni, Co is preferably 0.02% by mass or more, more preferably 0.03% by mass or more. Moreover, Preferably it is 0.8 mass% or less, More preferably, it is 0.5 mass% or less.

(Ca: 0.01% by mass or less, REM (rare earth element): 0.01% by mass or less, Mg: 0.005% by mass or less, Li: 0.005% by mass or less, Pb: 0.5% by mass or less, Bi: at least one of 0.5% by mass or less (excluding 0%))
Ca, REM, Mg, and Li are elements that spheroidize sulfide compound inclusions such as MnS and contribute to improving the cold workability and machinability of steel materials. When Ca and REM exceed 0.01% by mass and Mg and Li exceed 0.005% by mass, the effect is saturated and an effect commensurate with the amount added cannot be expected, which is not economical. In order to appropriately obtain the effects of adding Ca, REM, Mg, and Li, it is preferable to add 0.005% by mass or more of Ca and REM and 0.0001% by mass or more of Mg and Li.

  The amount of Ca and REM is preferably 0.0010% by mass or more, more preferably 0.0015% by mass or more. Moreover, Preferably it is 0.008 mass% or less, More preferably, it is 0.005 mass% or less. The amount of Mg and Li is preferably 0.0003 mass% or more, more preferably 0.0005 mass% or more. Moreover, Preferably it is 0.003 mass% or less, More preferably, it is 0.001 mass% or less.

  Pb and Bi are elements that contribute to the improvement of machinability. However, if the amount of Pb and Bi exceeds 0.5% by mass, a problem in production such as a rolling mill occurs. In order to appropriately obtain the effect of adding Pb and Bi, it is preferable to add 0.01% by mass or more of Pb and Bi, respectively. The amount of Pb and Bi is preferably 0.03% by mass or more, more preferably 0.05% by mass or more. Moreover, Preferably it is 0.3 mass% or less, More preferably, it is 0.1 mass% or less.

<Ferrite phase structure fraction (area ratio)>
The steel material for cold work according to the present invention has a soft ferrite phase as a main structure in order to impart cold workability. By making the ferrite single phase in this way, the entire structure is simultaneously and uniformly deformed and hardened when cold-working the steel material for cold-working to produce a machine structural component. Accordingly, an increase in deformation resistance is suppressed as a whole, and cold workability does not deteriorate. Further, it is not always necessary to have a complete ferrite single-phase structure, and it is sufficient if the area ratio (structure fraction) of the ferrite phase to the entire structure is 90% or more. This is because even if cementite precipitates at some grain boundaries, it does not deteriorate cold workability if it is spheroidized.

  However, when the ferrite fraction has a structure fraction of less than 90%, the interface between ferrite and cementite tends to be the starting point of cracking, and cold workability is deteriorated. Further, the structure fraction of the ferrite phase is preferably 93% or more, and more preferably 95% or more.

(Measurement method of ferrite phase structure fraction)
An example of a method for measuring the structure fraction of the ferrite phase is observation with an optical microscope. Moreover, as a position for observing the structure, a position at a depth of 1/4 of the thickness of the steel material from the steel surface is preferable, and by observing a plurality of visual fields in the vicinity (for example, five visual fields), Can be determined by average. Specifically, the steel material for cold working is cut out so that the observation position is included in the cut surface, the cut surface is polished to a mirror surface, and then corroded with a nital solution (3% ethanol solution of nitric acid). When observed with an optical microscope at a magnification of about 100, the white area is the ferrite phase. In order to obtain the structure fraction, for example, a plurality of points (for example, 100 points) are randomly selected from the optical micrograph, the structure of each point is discriminated, and the percentage of the total number of points of the ferrite phase may be calculated. . Or you may process an optical micrograph with commercially available image analysis software, and may obtain | require the area ratio of a white area | region.

<Vickers hardness difference>
In order to maximize the effects of high-speed cold working, it is necessary to make the strength distribution of the steel material before working uniform. When a steel material is cold worked at high speed, deformation may occur locally. In other words, if the hardness of the steel material is high or low, even if it appears to be uniformly deformed macroscopically, it is deformed unevenly microscopically, resulting in a large variation in component strength. . Accordingly, the difference between the maximum value and the minimum value of Vickers hardness (measured load 9.8 N) measured every 1 mm from the steel surface to a depth of 1/4 of the thickness of the steel material is 15 Hv or less. The difference in the Vickers hardness can be controlled by adjusting the temperature and holding time of the heat treatment step in the method for producing a steel material for cold working described later. In addition, the difference in the Vickers hardness is preferably 12 Hv, and more preferably 10 Hv or less.

(Measurement method of Vickers hardness)
As a measuring method of Vickers hardness, Vickers hardness measurement is performed every 1 mm from the surface of the steel material for cold work to the depth of 1/4 of the thickness of the steel material, and the difference between the maximum hardness and the minimum hardness is calculated. The measurement conditions are a micro Vickers hardness tester and the load is 1000 g (9.8 N). Then, this measurement is performed for three lines, and the average value of the differences is calculated.

[Method of manufacturing steel for cold working]
Next, the manufacturing method of the steel material for cold work is demonstrated in detail. The manufacturing method of the steel material for cold work which concerns on this invention is the process of hot rolling or hot forging after heating what melted the steel material of the said composition by a well-known method to 1050-1250 degreeC, 400 A step of heating and holding at ˜700 ° C. for 60 to 7200 seconds and a step of cooling to room temperature at a cooling rate of 0.1 ° C./s or more are performed.
Hereinafter, each element in the manufacturing method of the steel material for cold work is demonstrated.

<Hot rolling or hot forging at 1050 to 1250 ° C>
In the steel material according to the present invention, it is necessary to ensure the amount of solute N required by decomposing or dissolving AlN by hot rolling or hot forging. When the heating temperature is less than 1050 ° C., AlN is not sufficiently decomposed, and it becomes difficult to secure the required amount of dissolved N, and the strength after cold working is insufficient. On the other hand, the decomposition of AlN tends to proceed as the temperature increases. However, when the temperature exceeds 1250 ° C., not only the effect on the decomposition of AlN is saturated, but also the end of the billet is deformed and hot rolling is difficult. Become. The heating temperature in this step is preferably 1075 ° C or higher, more preferably 1100 ° C or higher, preferably 1225 ° C or lower, more preferably 1200 ° C or lower.

<Heat holding at 400 to 700 ° C. for 60 to 7200 sec>
It is a heat treatment necessary for organizing dislocations in the crystal grains, and the steel material hot-rolled or hot-forged is heated and held at 400 to 700 ° C. for 60 to 7200 seconds without changing the shape of the ferrite phase. It is possible to improve the cold workability by arranging the dislocations in the crystal grains. That is, by performing the heat treatment in this step, the difference between the maximum value and the minimum value of Vickers hardness (measured load 9.8 N) measured every 1 mm from the steel surface to a depth of 1/4 of the thickness of the steel material is 15 Hv or less. Can be controlled. Details will be described later in an example comparing an example satisfying the requirements of the present invention with a comparative example not satisfying the requirements of the present invention. On the other hand, if the heat treatment in this step is not performed, the hardness of the steel material increases and the cold workability deteriorates, and cracks occur in the parts after the cold work. In addition, this process is performed after cooling the steel materials hot-rolled or hot forged in the previous process to 400 to 700 ° C., which is the heating temperature of this process.

  Here, when the heating temperature is less than 400 ° C., dislocations in the crystal grains cannot be sufficiently arranged, the Vickers hardness in the steel becomes uneven, the cold workability deteriorates, and the parts after the cold work Cracks occur. On the other hand, when the heating temperature exceeds 700 ° C., AlN starts to be generated, so that the amount of dissolved N decreases and the strength after cold working decreases.

  Also, if the holding time is less than 60 seconds, dislocations in the crystal grains cannot be sufficiently arranged, the Vickers hardness in the steel material becomes uneven, cold workability deteriorates, and the parts after cold work are cracked. Will occur. On the other hand, when the holding time exceeds 7200 sec, the effect is saturated.

  The heating temperature in this step is preferably 400 to 675 ° C, more preferably 400 to 650 ° C. The holding time is preferably 120 sec or more, more preferably 300 sec or more, 6600 sec or less is preferable, and 6000 sec or less is more preferable.

<Cooling to room temperature at a cooling rate of 0.1 ° C./s or higher>
If the cooling after hot working or hot forging is moderate, AlN precipitates in the steel material, so cooling is performed at a cooling rate of 0.1 ° C./s or more. The cooling rate is preferably 0.3 ° C./s or more, more preferably 0.5 ° C./s or more. However, the cooling rate can be appropriately changed according to the facility capacity and the shape of the hot-worked material.

  In addition, in the manufacturing method of the steel material for cold work which concerns on this invention, it is preferable to cool to room temperature and to heat-process the next process after the said hot rolling or hot forging. By having this step, the grain size of the steel structure can be adjusted. The cooling rate at that time is preferably 1 ° C./s or more because it is necessary to prevent AlN from being precipitated again.

[Manufacturing method of machine structural parts]
Next, a method for manufacturing a machine structural component will be described in detail.
In the present invention, paying attention to the fact that dynamic strain aging, which greatly affects deformation resistance (cold workability), is governed by temperature, the amount of solid solution elements, and the average strain rate, if the strain rate is increased, It has been found that even when solute N is contained in steel, dynamic strain aging can be suppressed. Also, paying attention to the fact that static strain aging, which greatly affects the strength of parts after cold working, is governed by temperature, the amount of solid solution elements, and the density of movable dislocations. It was found that static strain aging can be promoted.

  Further, when high-speed cold working is performed on a steel material, the balance between the diffusion rate of the solid solution element and the moving speed of the dislocation changes, so that dynamic strain aging hardly occurs. Further, the higher the processing speed, the more adiabatically occurs, so that the part is kept at a high temperature even after processing. In addition, depending on the form of the structure, dislocations grow at the same time, so that the mobile dislocation density tends to increase. Therefore, the steel material was deformed at high speed, and the structural form of the steel material suitable for coexistence of mold life and part strength was searched, and the following points were found.

(1) In the case of high-speed deformation, even if there is a difference in hardness between the structures, the structure is uniformly deformed. However, if the structure is made uniform, the component strength is more easily improved. It is also effective in reducing deformation resistance (cold workability).
(2) When the strain rate is 50 / s or more, the adiabatic temperature rise is 100 ° C. or more, and the solid solution N easily moves after the parts are processed, so that the component strength is easily improved.
(3) In order to sufficiently suppress dynamic strain aging due to the amount of solute N, in the case of the steel according to the present invention, an average strain rate region of 50 / s or more is most effective, and the deformation resistance is sufficiently reduced. Can be made.

The method of manufacturing a machine structural component according to the present invention includes a step of hot rolling or hot forging after heating a steel material having the above composition to 1050 to 1250 ° C., and a steel material after the hot rolling or hot forging. , The step of heating and holding at 400 to 700 ° C. for 60 to 7200 seconds, the step of cooling the steel material after the holding to room temperature at a cooling rate of 0.1 ° C./s or more, and the steel material after cooling, starting temperature And cold working at an average strain rate of 50 / s or higher at a temperature lower than 200 ° C.
Hereinafter, each element in the manufacturing method of the machine structural component will be described. However, with respect to the elements already described, the step of hot rolling or hot forging at 1050 to 1250 ° C., the step of heating and holding at 400 to 700 ° C. for 60 to 7200 sec, and the step of cooling at 0.1 ° C./s or more. The description is omitted.

<Cold working at an initial temperature of less than 200 ° C. and an average strain rate of 50 / s or more>
By cold-working the steel material at the start temperature and the average strain rate, the adiabatic temperature rise becomes 100 ° C. or more, and the solid solution N easily moves after parts processing, so that the strength of the parts is easily improved. On the other hand, when the starting temperature exceeds 200 ° C. or the average strain rate is less than 50 / s, dynamic strain aging occurs, leading to an increase in deformation resistance and deterioration of deformability.

[Mechanical structural parts]
Next, the machine structural component manufactured by the above-described method will be described. The machine structural component according to the present invention is H1 having a Vickers hardness (Hv) at a measurement load of 9.8 N when cold-worked at an average strain rate of 0.001 / s, and is cooled at an average strain rate of 100 / s. It is characterized in that 20 ≦ H2−H1 is satisfied when the Vickers hardness (Hv) at a measurement load of 9.8 N in the case of hot working is H2. By increasing the average strain rate, the mechanical structural component having such characteristics makes it easier for solute N to move through the structure after cold working, and it is possible to manufacture a mechanical structural component with higher strength. . Examples of the mechanical structure parts include bolts / nuts, pinion gears, steering shafts, valve lifters, and common rails.

  Next, an example that satisfies the requirements of the present invention and a comparative example that does not satisfy the requirements of the present invention will be described in detail. The steel materials and parts after cold working used in this example were produced by the following two methods.

<Steel material production method 1>
(1) Melting / Casting 150 kg of steel was melted in a vacuum induction furnace to cast an ingot having an upper surface φ245 mm × lower surface φ210 mm × length 480 mm.
(2) Hot forging This ingot was hot forged into billets (155 mm square) at 1150 to 1250 ° C. Specimen No. No. 3 was hot forged at 1000 ° C.
(3) Cutting and welding The end of the billet was cut and welded to a dummy billet (155 mm square × 9 to 10 m long).
(4) Hot rolling The billet after welding was hot rolled into a round bar of φ80.
(5) Heat treatment A round rod of φ80 was heated at 400 to 700 ° C. for 60 to 7200 sec and cooled to room temperature at 0.5 ° C./s or more. The test piece No. No. 11 did not perform cooling after hot forging and hot rolling. In addition, test piece No. No. 23 was not heat-treated. In addition, test piece No. No. 24 has a heating temperature of 300 ° C. 29 was 800 degreeC. In addition, test piece No. 30 has a holding time of 10 sec.
(6) Cutting / processing A φ6 × 9 mm compression test piece was cut out from a position at a depth of 1/4 of the thickness and used for evaluation. The steel material produced by this method was indicated as “hot rolling” in the “others” column of Table 1.

<Steel material production method 2>
Since (1) and (2) are the same as the steel material manufacturing method 1 described above, description thereof is omitted.
(3) Hot forging The billet was hot forged at 1050 to 1250 ° C. on a round bar of φ80.
(4) Heat treatment A round rod of φ80 was heated at 400 to 700 ° C. for 60 to 7200 sec and cooled to room temperature at 0.5 ° C./s or more.
(5) Cutting / processing A φ6 × 9 mm compression test piece was cut out from a position at a depth of 1/4 of the thickness and used for evaluation. The steel material produced by this method was indicated as “hot forging” in the “others” column of Table 1.

  Table 1 shows the composition of the steel material and the steel material production conditions. In Table 1, those not satisfying the scope of the present invention are shown underlined.

  The properties (cold workability) of the steel materials manufactured under the conditions shown in Table 1 were evaluated. Moreover, the characteristics (the strength of the cold-worked part) of the machine structural parts (hereinafter, parts) produced by cold-working each steel material were also evaluated. The method for producing the component is as follows.

<Parts manufacturing method>
Using a hot working reproducibility test device (Cermec Master X, manufactured by Fuji Radio Engineering Co., Ltd.) The steel was compressed and evaluated for deformation resistance and cracking. A part obtained by compression processing was cut in a longitudinal section, embedded in resin, and the surface was mirror-polished with emery paper or diamond buff.

  Table 2 shows the results of measuring the properties of steel materials and parts manufactured under the conditions in Table 1. In Table 2, those not satisfying the scope of the present invention are shown underlined.

About the characteristic of steel materials, the following items were measured.
(Solution N amount)
About each test piece, the amount of solid solution N was measured with the above-mentioned method, and the thing in the range whose solid solution N amount is 0.008-0.015 mass% was made favorable, and the thing outside the said range was made bad.

(Ferrite area ratio)
About each test piece, the ferrite area ratio was measured by the above-described method, and those having a ferrite area ratio of 90% or more were determined to be good and those having a ferrite area ratio of less than 90% were determined to be defective.

(Difference in Vickers hardness of steel)
About each test piece, the average value of the difference of the maximum value of Vickers hardness and the minimum value was measured by the above-mentioned method, and the thing whose said average value was 15 Hv or less was made favorable, and the thing exceeding 15 Hv was made bad. In the items in Table 2, “Vickers hardness” is simply described under the “characteristics of steel” column.

In addition, the following items were measured for parts manufactured by cold working steel materials.
(Difference in Vickers hardness of parts)
About each test piece (part), Vickers hardness (measuring load 9.8N) was measured in the position of the depth of 1/4 of thickness. A micro Vickers hardness tester was used for the measurement, and the average value of 5 points was regarded as the Vickers hardness of the part. Moreover, the average hardness by the average strain rate of the same steel material is calculated, and the difference between the hardness (H1) with an average strain rate of 0.001 / s and the Vickers hardness (H1) with an average strain rate of 100 / s is calculated. It was also confirmed whether or not 20 ≦ H2−H1 was satisfied. And the difference of Vickers hardness of 20Hv or more satisfying the above formula was determined to be good, and the difference of less than 20Hv not satisfying the above formula was determined to be defective. In the items in Table 2, “Vickers hardness” is simply written under the “part properties” column.

(With or without cracks)
About each test piece, the presence or absence of the crack was measured. And the thing in which the crack did not arise in the component after cold processing was made into favorable "(circle)", and the thing in which the crack produced was made into defect "x".

(Comprehensive evaluation)
Regarding the above items, the overall evaluation was “good” if it was all good, and the overall evaluation “x” if there was any defect.

  As shown in Table 2, the test piece No. whose steel composition and steel production conditions satisfy the requirements of the present invention. 1, 2, 4-22, 25-28, 31-34, 36-57 (Examples) are within the range specified by the present invention for the properties of steel materials and parts, and cold workability and strength of parts. It is understood that is superior. On the other hand, test piece No. in which either the steel structure or the steel production condition does not satisfy the necessary condition of the present invention. 3, 23, 24, 29, 30, 35, 58 to 69 (comparative example), any of the characteristics of the steel material and the component is outside the range defined by the present invention, and the cold workability and the strength of the component are inferior. You can see that Hereinafter, the comparative example will be specifically described.

  Specimen No. In No. 3, since the heating temperature in hot rolling was less than 1050 ° C., the amount of solute N was less than 0.008% by mass. The Vickers hardness of the parts was also less than 20 Hv.

  Specimen No. No. 23 was not heat-treated in the manufacturing process of the steel material, so the Vickers hardness of the steel material exceeded 15 Hv, and cracks occurred in the parts.

  Specimen No. In No. 24, since the heating temperature in the heat treatment was set to less than 400 ° C., the Vickers hardness of the steel material exceeded 15 Hv, and cracks occurred in the parts.

  Specimen No. In No. 29, since the heating temperature in the heat treatment was over 700 ° C., the amount of solute N was less than 0.008% by mass. The Vickers hardness of the parts was also less than 20 Hv.

  Specimen No. No. 30, because the holding time in heat treatment was less than 60 seconds, the Vickers hardness of the steel material exceeded 15 Hv, and cracks occurred in the parts.

  Specimen No. No. 35 had a cooling rate in heat treatment of less than 0.1 ° C./s, so the amount of solute N was less than 0.008% by mass. The Vickers hardness of the parts was also less than 20 Hv.

  Specimen No. In No. 58, since the amount of C contained in the steel material was less than the lower limit, cracking occurred in the parts. Specimen No. In No. 59, since the amount of C contained in the steel material exceeded the upper limit, the ferrite area ratio was 90% or less, and the Vickers hardness of the steel material was 15 Hv or more. In addition, cracks occurred in the parts.

  Specimen No. In No. 60, since the amount of Si contained in the steel material was less than the lower limit, cracking occurred in the part. Specimen No. In No. 61, since the amount of Si contained in the steel material exceeded the upper limit, cracking occurred in the part.

  Specimen No. In No. 62, since the amount of Mn contained in the steel material was less than the lower limit, cracking occurred in the part. Specimen No. In No. 63, since the amount of Mn contained in the steel material exceeded the upper limit value, cracks occurred in the parts.

  Specimen No. In No. 64, since the amount of P contained in the steel material exceeded the upper limit, cracks occurred in the parts. Specimen No. In No. 65, since the amount of S contained in the steel material exceeded the upper limit value, cracks occurred in the parts.

  Specimen No. In No. 66, since the amount of Al contained in the steel material was less than the lower limit, cracking occurred in the part. Specimen No. In No. 67, since the amount of Al contained in the steel material exceeded the upper limit, the amount of dissolved N was less than the lower limit, and the Vickers hardness of the component was less than 20 Hv.

  Specimen No. In No. 68, since the amount of N and the amount of solute N contained in the steel material were less than the lower limit values, the Vickers hardness of the component was less than 20 Hv, and the component was cracked. Specimen No. In No. 69, since the amount of N contained in the steel material exceeds the upper limit value, the Vickers hardness of the steel material exceeds the upper limit value, and cracks occurred in the parts.

  The steel material for cold working according to the present invention, the method for producing the steel material for cold working, the method for producing the component for machine structure, and the component for machine structure have a uniform strength throughout the structure, with the structure of the steel material being substantially a ferrite single phase. In addition, by performing heat treatment in a temperature region in which the crystal grain size does not change after hot rolling or hot forging, the hardness variation in the steel material is appropriately adjusted to reduce the hardness variation. Also, by setting the average strain rate during cold working to 50 / s or higher, dynamic strain aging is not generated, but only static strain aging is generated by processing heat generation. Even if a large amount of solute N is present, dynamic strain aging can be suppressed in the entire steel material. Therefore, it is possible to provide a steel material for cold work and a machine structural part that are excellent in cold workability and strength of the parts after cold work.

Claims (11)

C: 0.005-0.045 mass%, Si: 0.005-0.050 mass%, Mn: 0.4-1.0 mass%, P: 0.05 mass% or less, S: 0.005 -0.050 mass%, Al: 0.005-0.060 mass%, N: 0.009-0.016 mass%, the steel material for cold work which has a composition which remainder consists of Fe and an unavoidable impurity Because
The amount of solute N is 0.008 to 0.015 mass%,
The structure fraction of the ferrite phase is 90% or more,
The difference between the maximum value and the minimum value of Vickers hardness (measured load 9.8 N) measured every 1 mm from the steel surface to ¼ depth of the steel thickness is 15 Hv or less,
A steel material for cold working, characterized by   The steel composition for cold work according to claim 1, wherein the composition further contains at least one of Cr: 2.0 mass% or less and Mo: 1.0 mass% or less.   The composition further comprises at least one of Ti: 0.02 mass% or less, Nb: 0.02 mass% or less, and V: 0.02 mass% or less. The steel material for cold work as described in 2.   The said composition further contains B: 0.005 mass% or less, The steel materials for cold work of any one of Claim 1 to 3 characterized by the above-mentioned.   The composition further comprises at least one of Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, and Co: 1.0% by mass or less. The steel material for cold work as described in any one of 4.   The composition is further Ca: 0.01% by mass or less, REM: 0.01% by mass or less, Mg: 0.005% by mass or less, Li: 0.005% by mass or less, Pb: 0.5% by mass or less, The steel for cold working according to any one of claims 1 to 5, wherein at least one of Bi: 0.5% by mass or less is contained. It is a manufacturing method of the steel material for cold work of any one of Claim 1 to 6,
A step of hot rolling or hot forging after heating the steel having the above composition to 1050 to 1250 ° C .;
A step of heating and holding the steel material after the hot rolling or hot forging at 400 to 700 ° C. for 60 to 7200 sec;
Cooling the steel after the holding to room temperature at a cooling rate of 0.1 ° C./s or more;
The manufacturing method of the steel material for cold work characterized by having.   The method for manufacturing a steel material for cold working according to claim 7, further comprising a step of cooling the steel material to room temperature after the step of hot rolling or hot forging. A method for manufacturing a machine structural part using the cold work steel according to any one of claims 1 to 6,
A step of hot rolling or hot forging after heating the steel having the above composition to 1050 to 1250 ° C .;
A step of heating and holding the steel material after the hot rolling or hot forging at 400 to 700 ° C. for 60 to 7200 sec;
Cooling the steel after the holding to room temperature at a cooling rate of 0.1 ° C./s or more;
Cold-working the steel after cooling at a starting temperature of less than 200 ° C. and an average strain rate of 50 / s or more;
A method for manufacturing a machine structural component comprising:   The method for manufacturing a machine structural component material according to claim 9, further comprising a step of cooling the steel material to room temperature after the step of hot rolling or hot forging.   A machine structural component manufactured by cold working the steel material for cold working according to any one of claims 1 to 6 at a starting temperature of less than 200 ° C.