JP5618932B2 - Non-magnetic steel wire rod or bar, and method for producing the same - Google Patents

Description

  The present invention relates to a high-Mn non-magnetic steel wire rod and steel bar having excellent workability, that is, low deformation resistance during processing.

  Austenitic stainless steel is used for various applications as a non-magnetic structural member, but a rapid increase in deformation resistance due to processing often becomes a problem. That is, there is a problem that austenitic stainless steel undergoes processing-induced martensitic transformation with an increase in the amount of processing strain introduced during component processing. When the austenitic phase of the austenitic stainless steel undergoes a work-induced martensitic transformation, a ferromagnetic α ′ martensite phase is generated, so that the relative permeability is remarkably increased. Therefore, when processing austenitic stainless steel, in order to suppress the processing-induced α 'martensite transformation, it is either warm or hot worked, or solution reheat treatment is performed after cold working, and the structure is re- A method of restoring (decreasing) the relative permeability by using an austenite single phase has been used. However, these methods have a problem of deteriorating productivity.

  On the other hand, the high Mn nonmagnetic steel can be maintained with a very low relative magnetic permeability because the austenite phase is transformed into a nonmagnetic ε-martensite phase even when processing strain is applied. In addition, since high-Mn nonmagnetic steel is less expensive than austenitic stainless steel, its use as a nonmagnetic structural material has been expanding in recent years. However, high-Mn non-magnetic steel has high strength and a very high work hardening rate, so that deformation resistance increases rapidly during processing, and forging die cracks or steel material work cracks are likely to occur. There was a problem. Therefore, when using a high Mn non-magnetic steel for the application of austenitic stainless steel, it has been desired to have excellent workability, that is, low deformation resistance during processing.

  For example, Patent Documents 1 to 4 disclose techniques related to high Mn austenitic steels.

  Patent Document 1 discloses a technique related to a high Mn austenitic steel sheet having excellent anti-elasticity. In Patent Document 1, in addition to suppressing the deformation by increasing the strength of the steel sheet by having a multiphase structure composed of austenite and 10 to 60% by volume of processing-induced martensite, when the deformation proceeds. The kinetic energy is absorbed by the deformation, and the ballistic resistance is secured. In other words, in this technology, a high-strength martensite phase is present from the beginning, and because it is a steel material that is prone to work-induced martensite even during deformation, it is effective for shock absorption applications, but it can be used for complex structural parts. Processing is difficult.

  Patent Document 2 discloses a technique related to a nonmagnetic steel material having excellent hot workability and corrosion resistance. In this technique, hot workability and corrosion resistance are improved by casting austenitic stainless steel on the surface of high-Mn nonmagnetic steel, which has difficulties in corrosion resistance and hot workability. In addition, this technique aims to achieve high strength. The composition of the high-Mn nonmagnetic steel of the inner layer is adjusted not only for austenite stabilization but also for increasing the strength.

  Patent Document 3 discloses a method for producing a high Mn nonmagnetic steel excellent in local ductility. In this technique, notch sensitivity is reduced by controlling the shape of inclusions and the cleanliness of steel, and local ductility is improved. However, the steel component of Patent Document 3 is adjusted not only for austenite stabilization but also for increasing the strength.

  Patent Literature 4 discloses a method for producing a high-strength, high yield ratio, high-manganese nonmagnetic steel. In this technology, the components for securing the strength of the high manganese nonmagnetic steel and the rolling / cooling method are stipulated. In particular, in order to secure N effective for increasing the strength, it is necessary to increase the Mn and adjust the cooling rate. Has been done. Patent Document 4 is also a technique adjusted to increase the strength of steel, as in Patent Documents 2 and 3.

JP 2004-137579 A Japanese Patent Application Laid-Open No. 06-057379 Japanese Patent Laid-Open No. 06-088124 Japanese Patent Laid-Open No. 62-202023

In recent years, needs for processing methods are changing from hot processing to cold processing for the purpose of reducing the global environmental load. While cold working has the advantages of reducing CO ₂ in the manufacturing process and improving yield, there is a problem that the deformation resistance becomes high and the cost becomes high. All conventional high-Mn nonmagnetic steels are oriented to high strength (eg, Patent Documents 2 to 4 described above), and it has been difficult to process them into structural parts cold. In particular, a technique for reducing deformation resistance (particularly, deformation resistance during cold working) while maintaining a low relative permeability in high-Mn nonmagnetic steel has not been proposed.

  This invention is made | formed in view of the said situation, and it aims at providing the high Mn nonmagnetic steel which can make low relative permeability and cold workability (low deformation resistance) compatible.

  The present invention that has achieved the above-mentioned problems is as follows: C: 0.40 to 0.8% (meaning mass%; hereinafter, the chemical composition is the same), Si: 0.50% or less (not including 0%), Mn : 8 to 25%, P: 0.03% or less (not including 0%), S: 0.030% or less (not including 0%), Al: 0.010 to 0.10%, N: 0 .0010-0.020%, the balance is iron and inevitable impurities, the amount of N in a solid solution state is 0.001% or less (including 0%), and the structure is an austenite single phase structure, The number of austenite crystal grains having a crystal grain size of 30 to 80 μm is a non-magnetic steel wire rod or bar steel having 80% or more of all austenite crystal grains. The nonmagnetic steel wire or bar of the present invention preferably has a nitride compound amount of 0.0025% or more.

  The nonmagnetic steel wire rod or bar of the present invention may further comprise (a) Cr: 0.3% or less (not including 0%) and / or Mo: 1% or less (not including 0%), if necessary. b) Ti: 0.25% or less (not including 0%), Nb: 0.25% or less (not including 0%), V: 0.25% or less (not including 0%), and B: At least one selected from the group consisting of 0.01% or less (not including 0%), (c) Cu: 1% or less (not including 0%) and / or Ni: 1% or less (including 0%) It is also preferable to contain no).

  In the present invention, a steel having any one of the above chemical compositions is heated at 1050 to 1250 ° C., held at 800 to 1000 ° C. for 10 to 180 minutes, and subsequently cooled at a cooling rate of 0.5 ° C./second or more. It also includes a method for producing a non-magnetic steel wire rod or steel bar that is cooled to below C.

  According to the present invention, in particular, the amount of Mn is secured and the amount of dissolved N is suppressed, and the austenite crystal grains are sized, so that the relative permeability of the steel wire rod or the bar steel itself is low, A low relative magnetic permeability can be realized even after cold working, and deformation resistance during cold working can be suppressed.

  The non-magnetic steel wire and bar of the present invention (hereinafter, the steel wire and bar are collectively referred to as “steel wire”) suppresses the amount of solute N while securing the amount of Mn, and austenite crystal grains It is characterized in that the particle size is adjusted. By suppressing the amount of solute N after securing the amount of Mn, dynamic strain aging during cold working can be suppressed while austenite is stabilized. Further, by adjusting the austenite crystal grains, it is possible to suppress work-induced martensitic transformation and work hardening due to non-uniform deformation, and to reduce deformation resistance during cold working. This will be described in detail below.

  Usually, when Mn, which is an austenite stabilizing element, is increased, the solid solubility of N increases, so that the solid solution N easily increases. However, since N is also an austenite stabilizing element, it is effective to increase the solute N in order to obtain austenite single phase steel. However, solute N has the effect of generating dynamic strain aging during cold working and increasing deformation resistance. When dynamic strain aging occurs during cold working, a remarkable increase in deformation resistance and deterioration of deformability are likely to occur, making cold working difficult. However, simply reducing the solid solution N to reduce the deformation resistance makes the austenite less stable, so that the deformation-induced martensite transformation is likely to occur, so that the deformation resistance increases. That is, in order to reduce the deformation resistance, it is necessary to reduce the solid solution N while stabilizing the austenite (that is, maintaining the austenite single phase at room temperature and without causing the work-induced martensitic transformation during processing). is there. As a result of the study by the present inventors, it is effective to stabilize the austenite by increasing the amount of Mn and to reduce the amount of solid solution N by forming a nitride compound such as AlN. It has been found that the austenite crystal grain size can be made uniform (sized), and deformation resistance during processing can be reduced by this sized.

  The generation mechanism of the processing-induced martensitic transformation can be inferred as follows. In other words, the work-induced martensitic transformation is caused by locally increasing deformation stress due to nonuniform deformation. Larger crystal grains vary in the C and Mn concentrations in the grains, and smaller crystal grains are hardened by the refinement strengthening mechanism, so that larger crystal grains are likely to be locally deformed from smaller crystal grains. Therefore, with larger crystal grains, processing-induced martensitic transformation is likely to occur, and deformation resistance during processing increases. Moreover, since the work hardening rate increases when the distribution of crystal grain size is wide, the deformation resistance tends to increase.

  Therefore, in the present invention, an attempt was made to reduce the solid solution N by using a nitride compound-forming element such as Al and to adjust the grain size using a nitride compound such as AlN. As a result, it was found that when the steel was heated and held at an appropriate temperature, AlN was uniformly dispersed, and in addition to the effect of reducing solid solution N, abnormal growth of austenite grains could be suppressed. Since the austenite single phase steel in which the grain size is adjusted is deformed uniformly, the amount of strain necessary for the work-induced martensitic transformation can be increased, and work hardening accompanying cold work can be reduced. That is, in the present invention, austenite is stabilized by securing the amount of Mn, solid solution N is reduced by AlN or the like to suppress dynamic strain aging, and deformation resistance is reduced by regulating the austenite crystal grains by AlN or the like. A sufficiently reduced high-Mn nonmagnetic steel could be realized.

  In order to suppress the solid solution N amount, it is particularly important to appropriately adjust the N amount and the Al amount.

  When N forms a solid solution in steel, dynamic strain aging that prevents the movement of dislocations during deformation occurs, deformation resistance increases, and cracking occurs. On the other hand, when the amount of N is excessive, the amount of the nitride compound is excessively increased and the deformability is lowered. Therefore, the N amount is determined to be 0.020% or less. The upper limit of the N amount is preferably 0.018% or less, more preferably 0.016% or less (particularly 0.010% or less). From the viewpoint of workability, the smaller the amount of N, the better. However, it is difficult to reduce the amount of N to 0% in production. In the present invention, the amount of N is required to some extent in order to obtain austenite grain size reduction by a nitride compound such as AlN and to obtain a deformation resistance reduction effect during processing. Therefore, the N amount is set to 0.0010% or more. The lower limit of the N amount is preferably 0.0020% or more, more preferably more than 0.003% (more preferably 0.0035% or more, particularly 0.0040% or more).

  Al is not only effective as a deoxidizing element during melting, but can be precipitated as AlN at the time of cooling after hot rolling (or forging), and suppression of dynamic strain aging by reducing the solid solution N, It is an effective element for suppressing deformation resistance during cold working by adjusting the austenite grain size. In order to effectively exhibit such an effect and prevent the occurrence of gas defects due to lack of deoxidation during melting, the Al content is determined to be 0.010% or more. The lower limit of the Al content is preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, when the amount of Al is excessive, the steel material is easily embrittled and cold workability is deteriorated. Therefore, the Al amount is determined to be 0.10% or less. The upper limit of the Al amount is preferably 0.08% or less, more preferably 0.06% or less.

  In the present invention, the amount of solute N is 0.001% or less. Solid solution N has an effect of stabilizing austenite, but is an element that causes an increase in deformation resistance due to dynamic strain aging and causes cracking. In the present invention, since austenite can be stabilized by C and Mn, the amount of dissolved N is reduced as much as possible to improve cold workability. The amount of solute N is preferably 0.0005% or less, more preferably 0.0003% or less. The amount of solute N can be reduced to 0% by appropriately adjusting the amount of N, the amount of a nitride-forming element such as Al, and the production conditions described later.

  The structure of the steel wire rod of the present invention is an austenite single phase in order to achieve both cold workability and low relative magnetic permeability. Examples of phases other than the austenite phase include ferrite, martensite, bainite, and cementite. However, when the number of phases other than the austenite phase increases, not only the relative permeability is increased, but also the hardness and deformability are not uniform in the structure, so that an increase in deformation resistance and the occurrence of cracking are promoted. There are harmful effects. In order to avoid such harmful effects, the ratio of the austenite structure to the entire structure is usually 95% by area or more. The austenite phase is preferably 97 area% or more, more preferably 99 area% or more (particularly 100 area%).

  In addition to the austenite single phase structure, the structure of the steel wire rod of the present invention is also important that the crystal grain size is aligned throughout the entire structure. If the crystal grain size varies, strain tends to concentrate on the interface of relatively large grains, and processing-induced martensitic transformation tends to occur from the initial stage of deformation. By unifying the crystal grain size of the entire structure to approximately the same level, the entire structure is uniformly deformed, and the processing-induced martensitic transformation can be suppressed. Ideally, the austenite grains in the structure should be approximately the same size, but the number of austenite grains having a crystal grain size of 30 to 80 μm should be 80% or more of the total number of austenite grains. For example, the processing-induced martensitic transformation at the initial stage of processing can be suppressed. Even if the crystal grain size is uniform within the range of the crystal grain size of less than 30 μm, the grain size is too fine and the deformation resistance increases. Even if the crystal grain size is uniform in the range where the crystal grain size exceeds 80 μm, the deformation-induced martensitic transformation occurs due to the localization of deformation. Therefore, in the present invention, the size of the austenite grains is made uniform in the range of the crystal grain size of 30 to 80 μm. The number of austenite crystal grains having a crystal grain size of 30 to 80 μm is preferably 85% or more, and more preferably 90% or more. The crystal grain size means an equivalent circle diameter.

  The above-mentioned austenite grain sizing can be realized by securing an amount of a nitride compound such as AlN. In order to effectively exhibit the austenite grain-sizing effect, the amount of nitride compound is preferably 0.0025% or more, more preferably 0.0030% or more, and further preferably 0.0035% or more. On the other hand, when the amount of the nitride compound is excessive, the effect of grain size adjustment is increased, which is advantageous in that a uniform austenite single structure can be obtained. Acts as a starting point, and deformability deteriorates. Therefore, the amount of nitride compound is preferably 0.02% or less, more preferably 0.018% or less, and still more preferably 0.016% or less. It should be noted that the nitride compound includes AlN as well as nitrides of Ti, Nb, V, and B described later.

  Further, as described above, in the present invention, the amount of Mn is appropriately adjusted. Mn is an important austenite forming element. In particular, since Mn has a relatively small solid solution strengthening ability, even if it is added up to 25%, the deformation resistance is not increased so much. In the present invention in which the amount of N is reduced as much as possible, the amount of Mn added is determined by the balance with C described later, and the amount is 8-25%. If the amount of Mn is less than 8%, austenite tends to become unstable. However, if the amount of C is increased to stabilize austenite, cementite is likely to precipitate, resulting in an increase in relative permeability and deterioration of cold workability. Invite. On the other hand, if the amount of Mn exceeds 25%, the amount of C can be lowered, but the effect of solid solution strengthening due to Mn begins to become remarkable, deformation resistance tends to increase, and the deformability is deteriorated. The lower limit of the amount of Mn is preferably 9.0% or more, and more preferably 10.0% or more. The upper limit of the amount of Mn is preferably 22.5% or less, more preferably 20.0% or less.

  The steel wire rod of the present invention contains C, Si, P, and S as basic components in addition to the above-described N, Al, and Mn.

  C is an important element for stabilizing austenite at room temperature. In particular, in the present invention, N having an austenite stabilizing action is suppressed to an extremely low level, and the amount of C is determined to be 0.40% or more in order to stabilize austenite by C and Mn. If the amount of C is less than 0.40%, austenite becomes unstable even if Mn is added in a large amount, and the strength is likely to increase due to the formation of work-induced martensite. In order to stabilize austenite, the more C, the better. However, if it exceeds 0.8%, carbides are likely to be formed, making it difficult to obtain an austenite single-phase structure, and increasing the relative permeability. , Resulting in deterioration of cold workability. Therefore, the lower limit of the C amount, which is determined to be 0.8% or less, is preferably 0.45% or more, and more preferably 0.50% or more. The upper limit of the C amount is preferably 0.75% or less, more preferably 0.70% or less.

  Si is an element having an action of suppressing the formation of cementite during production, and is an essential additive element. In order to effectively exhibit such effects, the Si amount is preferably 0.05% or more, more preferably 0.08% or more, and further preferably 0.10% or more. On the other hand, Si not only strengthens austenite to increase the deformation resistance and deteriorates the deformability, but also increases the activity of the solid solution C and promotes the increase of the deformation resistance due to the solid solution C. The excessive addition of Si amount may be harmful from the viewpoint of workability. However, in the present invention, by suppressing variation in the austenite crystal grain size, it is possible to prevent deformation from concentrating locally and suppress work hardening of the steel material itself, so that the increase in deformation resistance due to Si can be mitigated, and the amount of Si is 0.50% or less. The upper limit of the Si amount is preferably 0.45% or less, and more preferably 0.40% or less.

  P is an element that tends to move and segregate to the austenite grain boundary during heating or hot working and needs to be kept low in order to promote cracking during working. In consideration of economy and effects, the P content is 0.03% or less. The amount of P is preferably 0.025% or less, more preferably 0.02% or less (particularly 0.015% or less). The smaller the amount of P, the better. The amount of P can be reduced as much as possible by devising the manufacturing conditions, but it is difficult to reduce the amount to 0%.

  S, like P, is an element that tends to segregate at austenite grain boundaries and needs to be kept low in order to promote cracking during processing. In consideration of economy and effects, the S amount is 0.030% or less. The amount of S is preferably 0.025% or less, more preferably 0.02% or less (particularly 0.015% or less). The smaller the amount of S, the better. The amount of S can be reduced as much as possible by devising the manufacturing conditions, but it is difficult to reduce the amount to 0%.

  The basic components of the steel wire rod of the present invention are as described above (C, Si, Mn, P, S, Al, N), and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Furthermore, the steel wire rod of the present invention preferably contains the following elements as necessary.

  Both Cr and Mo are elements having an austenite stabilizing effect. In particular, Cr is effective in improving the deformability of austenite in addition to the austenite stabilizing action, and Mo is effective in improving toughness by refining austenite grains. In order to effectively exhibit such action, the Cr content is preferably 0.01% or more, and the Mo content is preferably 0.05% or more. Cr and Mo may be used alone or in combination. The lower limit of the Cr amount is more preferably 0.03% or more, and further preferably 0.08% or more. The lower limit of the Mo amount is more preferably 0.08% or more, and further preferably 0.10% or more. On the other hand, when the amount of Cr and the amount of Mo become excessive, not only the austenite stabilizing action is saturated but also the steel material strength is increased, so that cold working becomes difficult. The upper limit of the Cr amount is preferably 0.3% or less, more preferably 0.25% or less, and still more preferably 0.20% or less. The upper limit of the Mo amount is preferably 1% or less, more preferably 0.80% or less, and still more preferably 0.60% or less.

  Ti, Nb, V, and B are all nitride compound forming elements, elements that act to reduce solid solution N and to austenite grain size regulation, and contain one or two or more kinds selected arbitrarily. Such an effect is exhibited. In order to effectively exhibit such an action, the Ti amount, the Nb amount, and the V amount are each preferably 0.005% or more, more preferably 0.010% or more. The amount of B is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, the effect increases as the content of these elements increases. However, when the content is excessive, a large amount of hard carbides are generated and the cold workability is deteriorated. Therefore, the Ti, Nb, and V amounts are all preferably 0.25% or less, more preferably 0.20% or less, and still more preferably 0.15% or less. The amount of B is preferably 0.01% or less, more preferably 0.0080% or less, and still more preferably 0.0060% or less.

  Cu and Ni are both elements that have the effect of stabilizing austenite at room temperature and the effect of improving toughness by increasing the stacking fault energy of the austenite phase. In order to exert such an effect, Cu and Ni may be used alone or in combination, and both the Cu content and the Ni content are preferably 0.08% or more, more preferably 0.10. % Or more. On the other hand, when these elements become excessive, the strength of the steel material increases and cold working becomes difficult. Therefore, the amount of Cu and the amount of Ni are both preferably 1% or less, more preferably 0.80% or less, and still more preferably 0.60% or less.

  In order to realize the steel wire rod of the present invention, it is effective to appropriately adjust the heating temperature, the holding temperature and the holding time, the cooling stop temperature, and the cooling rate up to the cooling stop temperature in the manufacturing process of the steel wire rod. The heating temperature is 1050 to 1250 ° C., which is an austenitizing temperature and a temperature at which a nitride compound such as AlN decomposes. The holding temperature is 800 to 1000 ° C., and the holding time is 10 to 180 minutes. These are the temperature and time at which a nitride compound such as AlN can precipitate while being uniformly dispersed. The cooling stop temperature is 400 ° C. or less, and the cooling rate to the cooling stop temperature is 0.5 ° C./second or more. These are conditions under which cementite does not precipitate. The heat treatment described above may be achieved by hot working the steel during heating at 1050 to 1250 ° C., and then holding and cooling under the above conditions, or the above heating is applied to the hot / cold worked steel. It may be achieved by holding, cooling. In addition, the said hot processing is a processing process with the heating above recrystallization temperature, and can illustrate plastic processing, such as hot rolling and hot forging. Further, even if the temperature of the steel wire enters the holding temperature range described above during plastic processing such as hot rolling, the time during the processing is not included in the holding time. The steel wire of the present invention can be realized by holding and cooling under the above-described conditions after the processing is completed. Each of the above conditions will be described in detail below.

  By setting the heating temperature to 1050 to 1250 ° C., the carbonized compound and the nitride compound can be decomposed. When the temperature is lower than 1050 ° C., the decomposition does not occur sufficiently, resulting in problems such as a uniform austenite single structure not being obtained after cooling. Further, when hot working during heating, if the heating temperature is lower than 1050 ° C., the deformation resistance of the steel is not sufficiently lowered, so that desired processing becomes difficult. From the viewpoint of decomposition of carbonized compounds and nitride compounds, and from the viewpoint of reducing the deformation resistance of steel, the higher the heating temperature, the better. The upper limit is not particularly limited, but if the heating temperature is too high, dripping occurs at the steel end. This causes problems such as deterioration of the handleability of the steel due to this, or excessive processing due to the deformation resistance becoming too low. Therefore, the upper limit of the heating temperature is 1250 ° C. The upper limit of the heating temperature is preferably 1225 ° C. or less, more preferably 1200 ° C. or less. The minimum of heating temperature becomes like this. Preferably it is 1075 degreeC or more, More preferably, it is 1100 degreeC or more.

  By holding at 800 to 1000 ° C. for 10 to 180 minutes after the above heating, a nitride compound such as AlN can be uniformly dispersed and precipitated, so that solid solution N can be reduced and austenite crystal grains can be adjusted. Can be granulated. When the holding temperature exceeds 1000 ° C., the nitride compound cannot be sufficiently precipitated, so that the effect of reducing the solid solution N and austenite sizing cannot be obtained, and the deformation resistance is likely to increase. On the other hand, if the holding temperature is less than 800 ° C., it takes too much time for the precipitation of the nitride compound, and the solid solution N cannot be sufficiently reduced in the above-described time of 10 to 180 minutes. In addition, austenite may grow before the nitride compound precipitates, making it difficult to adjust the austenite crystal grains. The lower limit of the holding temperature is preferably 825 ° C or higher, more preferably 850 ° C or higher, and the upper limit is preferably 975 ° C or lower, more preferably 950 ° C or lower.

  In order to obtain the effect of reducing the solute N by precipitation of the nitride compound and austenite sizing, the holding time is 10 minutes or more. The longer the holding time, the more effective the reduction of the solid solution N. The holding time is preferably 20 minutes or more, more preferably 30 minutes or more. On the other hand, if the holding time is too long, it is effective for reducing the solid solution N, but the effect of regulating the crystal grains cannot be sufficiently obtained due to the aggregation of the nitride compounds. Therefore, the holding time is 180 minutes or less. The holding time is preferably 150 minutes or less, more preferably 120 minutes or less.

  Here, “holding” includes not only the case where the temperature of the steel material is maintained at a constant temperature, but also the case where the steel material passes through the range of 800 to 1000 ° C. over 10 to 180 minutes even if the temperature of the steel material decreases. Specifically, if a cover, a heater, or the like is used, the temperature of the steel material can be prevented from naturally decreasing due to air cooling, and the temperature and time can be maintained.

  After the heating and holding described above, an austenite single phase structure can be obtained at room temperature by cooling to 400 ° C. or lower at a cooling rate of 0.5 ° C./second or higher. When the cooling rate is less than 0.5 ° C./second, or when the cooling stop temperature exceeds 400 ° C., carbides are formed on the austenite grain boundaries, so that cold workability and magnetic properties are deteriorated. If the cooling rate is 0.5 ° C./second or more, the precipitation of carbides is suppressed, and the austenite structure is not particularly changed depending on the cooling rate. The cooling rate is preferably 0.8 ° C./second or more, more preferably 1.0 ° C./second or more. Considering productivity and the like, the upper limit of the cooling rate is about 100 ° C./second. The cooling stop temperature is preferably 375 ° C. or lower, and more preferably 350 ° C. or lower. The lower limit of the cooling stop temperature is not particularly limited, and is, for example, room temperature (about 20 ° C.). In addition, when the above-mentioned cooling stop temperature is higher than room temperature, the cooling from a cooling stop temperature to room temperature is not specifically limited, For example, what is necessary is just to cool by air.

The steel wire rod of the present invention has a stable austenite structure and is excellent in cold workability. In addition, the relative permeability is low, and the relative permeability after cold working can be kept low. For example, the deformation resistance when the steel wire material of the present invention is 60% compressed (the compressibility is 60%) by cold working in the axial direction can be 1500 MPa or less (preferably 1450 MPa or less), and cracking occurs after the compression. Absent. Further, the relative permeability of the steel wire rod of the present invention is, for example, 1.3 or less (preferably 1.2 or less), and the relative permeability after cold working is also 1.3 or less (preferably 1.2 or less). ) And can. Therefore, the steel wire rod of the present invention includes a lifting magnet, various electric motors / generators, a transformer, a gas circuit breaker, a linear motor car track facility, a fusion reactor member, a solenoid valve, which have been manufactured with austenitic stainless steel. It can be suitably used for non-magnetic structural parts such as, can be manufactured at a lower cost than before, and can improve productivity. Further, high Mn non-magnetic steel parts that have been processed by hot working so far with high deformation resistance can be produced by cold working, which can contribute to reduction of CO ₂ emissions and improvement of productivity in the parts manufacturing process. Furthermore, the conventional austenitic stainless steel could not be solid-phase bonded as represented by friction welding due to the formation of a passive film, but the steel wire of the present invention is capable of solid-phase bonding, so it is magnetic, for example. By friction welding with steel, hybrid parts of magnetic and non-magnetic materials can be manufactured and used for a wide range of applications.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

  150 kg of test steels having the chemical compositions shown in Tables 1 and 2 were melted in a vacuum induction furnace and cast into an ingot having a top surface of 245 mm, a bottom surface of 210 mm, and a height of 480 mm. Subsequently, a steel bar was obtained by the following production method (1) or (2). In the table, steel types 1A to 2L were produced by production method (1), and steel types 2M to 2V were produced by production method (2).

Manufacturing method (1)
1. Heated to 1200 ° C., hot forged into 155 mm square billet from the above ingot and cooled.
2. The end of the billet is cut and welded to a dummy billet (155 mm square x length (9-10) m).
3. The billet after welding is heated to 950 to 1200 ° C., and hot rolled to a round bar of φ45 mm at 750 to 1050 ° C. After hot rolling, hold at 750 to 1050 ° C. for 1 to 500 minutes. Then, it is cooled to 20 to 500 ° C. at 0.3 to 12.0 ° C./second.

Manufacturing method (2)
1. Heated to 1200 ° C., hot forged into 155 mm square billet from the above ingot and cooled.
2. The billet was heated to 1150 ° C and hot forged into a 45mm round bar at 900 ° C. After hot forging, hold at 900 ° C for 60 minutes. Cool to 20 ° C at 3.0 ° C / sec.

  In Tables 3 and 4, the experiment No. Uses the same symbols as the steel type used. For example, 1B-1 to 1B-17 all use steel type 1B, and these are examples in which the production conditions are changed.

Evaluation of cold workability The steel bar was cut to produce a compression test piece having a diameter of 25 mm and a length of 37.5 mm. Using a 1600 ton press machine, with the end face of the test piece constrained, cold forging of machine structural parts was performed by cold forging at room temperature and a strain rate of 10 / sec. A material was prepared. The processing strain rate is an average value of strain rates during processing (plastic deformation). The compression ratio, H ₀ the compression direction length of the test piece, when the compressed length of the cold forging material after compression was H, calculated in _{100 × (H 0 -H) /} H 0 Value.

  Cold workability was judged by the maximum deformation resistance at 60% compression and the presence or absence of cracks when the surface state was observed with a stereomicroscope (magnification 20 times) after the compression test.

Magnetic evaluation method A test piece for measuring relative permeability of 5 mm square was cut out from the D / 4 position (D is the diameter of the steel bar) of the steel bar obtained by the above manufacturing method. Similarly, a test piece for measuring relative permeability of 5 mm square was cut out from the D / 4 position of the test piece after the cold workability evaluation test. Using a vibrating sample magnetometer, the relative permeability before and after cold working was examined.

Evaluation method of solid solution N amount and nitride compound amount The value of the solid solution N amount was measured in accordance with JIS G1228, and was calculated by subtracting the total nitride compound amount from the total N amount in the steel.
1. The total N amount in the steel uses an inert gas melting method-thermal conductivity method. A sample was cut out from the test steel material, put into a crucible, melted in an inert gas stream, extracted N, transported to a thermal conductivity cell, and the change in thermal conductivity was measured.
2. For the total amount of nitrided compounds in steel, ammonia distillation separation indophenol blue absorptiometry is used. A sample is cut out from the test steel material, 10% AA electrolyte (non-aqueous solvent electrolyte that does not produce a passive film on the steel surface, specifically 10% acetylacetone, 10% tetramethylammonium chloride, Constant current electrolysis is performed in the remainder: methanol). About 0.5 g of the sample is dissolved, and the insoluble residue (N compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. The insoluble residue is decomposed by heating in sulfuric acid, potassium sulfate and pure Cu chips and combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed by dilute sulfuric acid. Phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) were added to form a blue complex, and the absorbance was measured using a photometer. The solute N amount in the steel was calculated by subtracting the total N compound amount from the total N amount in the steel determined by the above method.

Austenite Area Ratio and Grain Size Measurement The austenite area ratio and crystal grain size distribution were measured as follows.
1. The steel bar was cut in a cross section perpendicular to the longitudinal direction.
2. The cut steel bar was embedded in resin, polished in order by emery paper, diamond buffing, and electrolytic polishing, and the sample surface was mirror finished.
3. Images were taken using a field emission scanning electron microscope (FE-SEM), observed at an acceleration voltage of 20 kV and an observation magnification of 400 times.
4). Image analysis was performed using a crystal orientation analyzer (EBSP) to determine the austenite area ratio and crystal grain size distribution. The crystal grain size distribution is such that the horizontal axis is the austenite crystal grain size and the vertical axis is the frequency based on the number of crystal grains, and the austenite crystal grains having a crystal grain size of 30 to 80 μm by the crystal grain size distribution The ratio (number basis) to austenite crystal grains was determined.

  The results are shown in Tables 5 and 6.

  Experiment No. 1A, 1B-2, 1B-3, 1B-5, 1B-8, 1B-9, 1B-12 to 1B-16, 1C to 1Z, 2M to 2V, the chemical composition and production conditions satisfy the requirements of the present invention. Both the cold workability and the relative magnetic permeability (before and after cold work) are satisfactory. On the other hand, other experiment No. Then, since at least one of the chemical composition and the manufacturing conditions does not satisfy the requirements of the present invention, at least one of the cold workability and the relative magnetic permeability is deteriorated.

  1B-1 is an example in which the heating temperature was low, the nitrided compound could not be decomposed sufficiently, and the sizing effect was insufficient, so that work hardening during cold working became remarkable, deformation resistance was high, and cracking occurred. There has occurred. 1B-4 has a low holding temperature and a low driving force for precipitation of the nitride compound, so that the nitride compound does not sufficiently precipitate in 60 minutes, the amount of solute N increases, and the deformation resistance during cold working is high. And cracks occurred.

  Since 1B-6 had a high holding temperature, 1B-7 had a short holding time, so that a nitride compound was not sufficiently formed in all cases, so that the solid solution N was high and the sizing effect was insufficient. Work hardening and work-induced martensitic transformation during processing became remarkable, deformation resistance was high, and cracking occurred.

  1B-10 has a long holding time, so that the nitrided compound aggregates and the sizing effect is insufficient. Therefore, work hardening and work-induced martensitic transformation during cold working become remarkable, deformation resistance is high, and cracks are not generated. Occurred. In 1B-11, the cooling rate after holding was slow, so carbides precipitated during cooling and did not become an austenite single phase structure. In addition, martensitic transformation during cold working became prominent, deformation resistance increased, cracks occurred, and the relative permeability after cold working increased. 1B-17 was an example in which the cooling stop temperature was high, and carbides precipitated after cooling, and did not become an austenite single phase structure. In addition, martensitic transformation during cold working became prominent, deformation resistance increased, cracks occurred, and the relative permeability after cold working increased.

  Since 2A had a small amount of C, 2D had a small amount of Mn. Therefore, in both cases, austenite became unstable at room temperature, a part of the structure became martensite and carbide, and the austenite fraction became insufficient, and the relative permeability was low. It became high. In addition, martensitic transformation during cold working became prominent, the deformation resistance increased, and the relative permeability after cold working also increased. Since 2B had a large amount of C, a part of the structure became carbide, the austenite fraction became insufficient, and the relative permeability increased. In addition, martensitic transformation during cold working became prominent, the deformation resistance increased, and the relative permeability after cold working also increased.

  Since 2C had a large amount of Si and 2E had a large amount of Mn, both had high deformation resistance due to solid solution strengthening, and cracks occurred during cold working.

  Since 2F had a large amount of P, the workability deteriorated when the P concentration at the crystal grain boundary increased, and cracks occurred during cold working.

  Since 2G had a large amount of S, the amount of MnS having a large strength difference from the parent phase was increased, and the fracture starting point was increased, thereby causing cracks during cold working.

  Since 2H had a small amount of Al and 2J had a small amount of N, neither nitride compound was sufficiently formed, the austenite sizing effect was insufficient, work hardening during cold working became remarkable, and cold working As the deformation resistance increased, cracks occurred. Since 2I had a large amount of Al, the amount of solid solution Al in the steel increased, and solid solution strengthening (embrittlement) resulted in high deformation resistance during cold working, and workability deteriorated and cracking occurred. did.

  Since 2K had a large amount of N, the amount of dissolved N was high, work hardening due to dynamic strain aging became remarkable, deformation resistance during cold working was high, and cracking occurred. 2L has a large amount of N and a low amount of solid solution N, but since a nitride compound is excessively generated and coarsens, it becomes a starting point of fracture, and cracking occurs during cold working.

Claims (6)

C: 0.40 to 0.8% (meaning mass%, hereinafter the same for chemical composition),
Si: 0.50% or less (excluding 0%),
Mn: 8-25%,
P: 0.03% or less (excluding 0%),
S: 0.030% or less (excluding 0%),
Al: 0.010 to 0.10%,
N: 0.0010 to 0.020% included, the balance being iron and inevitable impurities,
The N amount in a solid solution state is 0.001% or less (including 0%),
A nonmagnetic steel wire rod or steel bar, wherein the structure is an austenite single phase structure and the number of austenite crystal grains having a crystal grain size of 30 to 80 μm is 80% or more based on all austenite crystal grains.   The nonmagnetic steel wire or bar according to claim 1, wherein the amount of nitride compound is 0.0025% or more.   Furthermore, Cr: 0.3% or less (0% is not included) and / or Mo: 1% or less (0% is not included), The nonmagnetic steel wire rod or steel bar of Claim 1 or 2 containing. Furthermore,
Ti: 0.25% or less (excluding 0%),
Nb: 0.25% or less (excluding 0%),
Any one of claims 1 to 3 containing at least one selected from the group consisting of V: 0.25% or less (not including 0%) and B: 0.01% or less (not including 0%). A nonmagnetic steel wire or steel bar described in 1.   Furthermore, Cu: 1% or less (not including 0%) and / or Ni: 1% or less (not including 0%), the nonmagnetic steel wire or bar according to any one of claims 1 to 4. Steel having the chemical composition according to any one of claims 1 to 5,
After heating at 1050 to 1250 ° C., holding at 800 to 1000 ° C. for 10 to 180 minutes, and subsequently cooling to 400 ° C. or less at a cooling rate of 0.5 ° C./second or more or Steel bar manufacturing method.