JP2019052856A - Deterioration degree determination method - Google Patents

Abstract

To provide a deterioration degree determination method capable of easily and accurately determining whether or not a polymer aqueous solution for quenching of steel materials can be used continuously.SOLUTION: A deterioration degree determination method includes: a step of acquiring the relationship between an average heat transfer coefficient and a maximum stress based on a steel material and a polymer aqueous solution, and a threshold value of a hardening crack stress; a step of performing a quenching test by using the polymer aqueous solution and the small test piece of the same type as the steel material to be determined in the operation process, and measuring the temperature change of each portion of the small test piece and the polymer aqueous solution with time; a step of calculating an average heat transfer coefficient Kin a predetermined temperature range including the transformation point of the steel material from the measurement results; a step of deriving the maximum stress σderived from the relationship between the average heat transfer coefficient and the maximum stress obtained in the acquisition step, on the basis of the average heat transfer coefficient K; and a step of determining whether the continuous use of the polymer aqueous solution is possible by the threshold value of the hardening crack stress and the maximum stress σobtained in the acquisition step.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a deterioration degree determination method. Specifically, the present invention relates to a method for determining the degree of deterioration of an aqueous polymer solution used for quenching steel.

  In a high strength steel material used for a crankshaft or the like of a large ship engine, it is common to secure a desired mechanical property by oil quenching a low alloy steel excellent in hardenability. However, in recent years, polymer quenching using an aqueous polymer solution has been used in place of oil quenching in order to reduce the burden on the environment and prevent the occurrence of a fire.

  Polymer quenching has advantages such as slow cooling in a wider temperature range than water quenching, and control of the cooling rate by changing the type and concentration of the polymer. For this reason, it is considered that polymer quenching tends to prevent quench cracking, which is often a problem in water quenching, even though it has a larger cooling rate than oil quenching.

  However, the polymer aqueous solution deteriorates with an increase in the number of times of quenching, the slow cooling performance is lowered, and quenching may occur in the quenched steel material. In that case, most of the steel material that has been cracked must be discarded, which may cause a great loss. In order to prevent this cracking, it is necessary to take measures such as replacing the polymer aqueous solution with a new one before the slow cooling performance deteriorates, or adding new liquid to the liquid in use to maintain the slow cooling performance. . On the other hand, when the frequency of this exchange or addition increases, the amount of the polymer aqueous solution used increases and the manufacturing cost of the steel material increases. Therefore, in order to minimize the frequency of replacement and addition of the aqueous polymer solution while preventing burning cracks, there is a demand for a method for determining the degree of deterioration that can easily and accurately determine whether the aqueous polymer solution is continuously used.

  In this regard, the time for which the test piece is cooled to a predetermined temperature by immersing the heated test piece in the quenching coolant to be managed is measured, and the reference data is obtained in advance from the measured cooling time. There has been proposed a cooling liquid management method for estimating the concentration of the cooling liquid to be managed using the relationship between the cooling time and the cooling liquid concentration (see Japanese Patent Application Laid-Open No. 2014-167487). In this cooling liquid management method, the concentration of the cooling liquid is used as a management index for determining whether or not the cooling liquid can be continuously used.

  However, the cooling liquid management method requires a complicated operation of immersing the heated test piece in the cooling liquid and measuring the cooling time until the temperature reaches a predetermined temperature.

  Further, it has been found by the inventors that when a polymer aqueous solution is used for quenching a steel material, the polymer has a low molecular weight due to thermal decomposition, oxidation, etc. with use. For this reason, for example, even if the aqueous polymer solution has the same concentration, the longer the use time, the lower the molecular weight of the polymer. Therefore, in the case of using a new polymer aqueous solution while adding it to the polymer aqueous solution in use, it is considered difficult to determine whether or not the coolant can be continuously used only by the concentration of the coolant as in the above-described coolant management method. .

  On the other hand, there has been proposed a deterioration degree determination method for determining the deterioration degree of a polymer aqueous solution not only based on the concentration of the polymer aqueous solution but also based on the kinematic viscosity of the polymer aqueous solution and the polymer concentration at the time of measuring the kinematic viscosity (see Japanese Patent Application Laid-Open No. 2017-37019). ). Since the kinematic viscosity has a correlation with the molecular weight of the polymer, it can be considered that the use of the aqueous polymer solution can be determined even when a new aqueous polymer solution is used by adding a new polymer aqueous solution.

  However, the deterioration degree determination method is not necessarily based on the theory of fire cracking. In addition, the occurrence of burn cracking of a metal material is indirectly predicted through the physical properties of an aqueous polymer solution, and a more accurate degradation degree determination method based directly on the crack crack generation mechanism is required.

JP 2014-167487 A JP 2017-37019 A

  The present invention has been made based on the above-described circumstances, and an object of the present invention is to provide a deterioration degree determination method capable of easily and accurately determining whether or not to continue using a polymer aqueous solution used for quenching steel.

  In order to establish the degradation degree determination method, the present inventors conducted a quenching test of steel materials using a plurality of polymer aqueous solutions having different degradation levels, and measured, analyzed, etc. various physical property values of the steel materials and the polymer aqueous solutions. investigated. As a result, the present inventors can adopt a combination of the average heat transfer coefficient between the steel material and the polymer aqueous solution in the specific temperature range for each type of steel material and the maximum stress of the steel material as a management index of the deterioration degree. I found it. Specifically, the present inventors have carefully examined the occurrence mechanism of quench cracks, so that the maximum value (maximum stress) of the stress generated in the steel material due to quenching, the martensite transformation point and the bainite transformation point of the steel material or less. It has been found that there is a correlation between the average heat transfer coefficient between the steel material and the aqueous polymer solution in the temperature range. Accordingly, the present inventors have found that the combination of the maximum stress and the average heat transfer coefficient can be suitably used as a management index for the deterioration degree, and have completed the present invention based on this finding.

That is, the invention made in order to solve the above problems is a method for determining the degree of deterioration of a polymer aqueous solution used for quenching of steel materials, the relationship between the average heat transfer coefficient and the maximum stress based on the steel materials and the polymer aqueous solution, and the cracking stress. The quenching test is performed using the above-described polymer aqueous solution in the operation process to be determined and the same kind of small test piece as the steel material, and each part of the small test piece and the polymer aqueous solution for each time. Obtained in the step of measuring the temperature change, the step of calculating the average heat transfer coefficient K _t in a predetermined temperature range including the transformation point of the steel material, and the calculation step from the measurement results obtained in the measurement step. based on the average heat transfer coefficient K _t, the step of deriving the maximum stress sigma _t derived from the relationship between the obtained average heat transfer coefficient and the maximum stress in the acquisition step, baked obtained in the obtaining step Wherein the Re from the maximum stress sigma _t obtained in the threshold of the stress and the deriving step comprises a step of determining whether the continued use of the aqueous polymer solution.

In the degradation degree determination method, the process of obtaining the relationship between the average heat transfer coefficient and the maximum stress based on the steel material and the aqueous polymer solution and the threshold for the cracking stress is performed once for each combination of the steel material and the aqueous polymer solution and for each type of the steel material. Good. In each deterioration degree determination, a quenching test is performed using the above-described polymer aqueous solution in the operation process to be determined and a small test piece of the same type as the steel material, and each portion of the small test piece and the above-described aqueous polymer solution are timed. The average heat transfer coefficient K _t is calculated from the measured temperature change by software processing or the like and the maximum stress σ _t is derived, and the comparison between the σ _t and the threshold value of the cracking stress is performed. It is enough if you do. Therefore, according to the degradation degree determination method, it is possible to easily determine whether the polymer aqueous solution is continuously used without complicated measurement of the concentration and kinematic viscosity of the polymer aqueous solution. Also, the average heat transfer coefficient K _t between the steel material and the polymer solution directly affects the cooling rate in the quenching, the deterioration degree based on the threshold value of the maximum stress sigma _t and quenching crack stress of the steel material directly affect the occurrence of quench cracking Therefore, it is possible to accurately determine whether the polymer aqueous solution is continuously used. In addition, a quench test may be performed in a test room using a small test piece and a small amount of a polymer aqueous solution, and it is not necessary to perform a quench test in an operation facility. Therefore, it is not necessary to stop the operation for the quenching test or to prepare a large test piece, and it is possible to determine whether the continuous use of the aqueous polymer solution is inexpensive and safe.

The acquisition step performs a quenching test using the same kind of small test piece as the polymer aqueous solution and the steel material in the operation process to be judged, and changes the temperature of each part of the small test piece and the aqueous polymer solution over time. It is good to have the process of measuring, and the process of analyzing stress from the temperature data obtained at the above-mentioned measurement process. By having such a stress analysis step, the martensitic transformation region and the bainite transformation region of the steel material are more accurately identified, and the predetermined temperature range used for calculating the average heat transfer coefficient K _t is more appropriately determined. The degree of deterioration of the polymer aqueous solution can be determined with higher accuracy.

  It is preferable to use past performance data for obtaining the threshold value of the cracking stress in the obtaining step. Here, “past performance data” refers to measurement data or calculation data of the residual stress of the steel material in which quench cracking actually occurred when the steel material was quenched using a polymer aqueous solution. In this way, by obtaining the threshold value of the quench cracking stress based on the residual stress of the steel material that has undergone quench cracking, it is possible to determine the degree of deterioration more in line with the quenching in the operation equipment, and to obtain the threshold value of the cracking stress. Therefore, a quenching test, a residual stress measurement and the like are not required.

  For the acquisition of the threshold value of the cracking stress in the acquisition step, the measurement step and the analysis step may be performed using the polymer aqueous solution in a state where the cracking occurs. Here, the “burning crack occurrence state” refers to a deterioration state of the polymer aqueous solution in which quench cracking starts to occur when the steel material is quenched using the polymer aqueous solution. As described above, by performing the measurement step and the analysis step using the polymer aqueous solution in a state where the cracks are generated, the threshold value of the cracking stress can be obtained even when the past result data is not available.

  As described above, the degradation degree determination method of the present invention can easily and accurately determine whether the polymer aqueous solution used for quenching of the steel material is continuously used. In addition, since the deterioration degree determination method of the present invention can easily and accurately determine whether the polymer aqueous solution is used continuously, the frequency of replacement or addition of the polymer aqueous solution can be minimized, and the polymer aqueous solution Can be reduced.

It is a judgment figure showing typically the case where it is judged with the degradation degree judging method concerning one embodiment of the present invention that the continuous use of the polymer aqueous solution is possible. It is a judgment figure showing typically the case where it is judged with the degradation degree judging method concerning one embodiment of the present invention that the continuous use of polymer solution is impossible. It is a graph showing the approximate line which shows the relationship of the average heat transfer rate and the maximum stress based on the steel materials and polymer aqueous solution in the deterioration determination method which concerns on one Embodiment of this invention. FIG. 3 is a comparative diagram showing a comparison between a case where a strain of a steel material is actually measured at each temperature and a case where the strain is analyzed by a general-purpose finite element analysis software in a quenching test using a deterioration degree determination method according to an embodiment of the present invention. It is. It is typical sectional drawing of the small test piece used for the quenching test by the deterioration determination method which concerns on one Embodiment of this invention.

  Hereinafter, a degradation degree determination method according to an embodiment of the present invention will be described.

The said deterioration degree determination method is a deterioration degree determination method of the polymer aqueous solution used for hardening of steel materials. The deterioration degree determination method includes a step (acquisition step) of obtaining a relationship between an average heat transfer coefficient and a maximum stress based on the steel material and the polymer aqueous solution and a threshold value of a cracking stress, and the polymer aqueous solution in the operation process to be judged and A quenching test was performed using a small test piece of the same type as the steel material, and each step of the small test piece and the temperature change of the polymer aqueous solution for each time (measurement process) were obtained in the measurement process. measurement results, and step (calculating step) that calculates the average heat transfer coefficient K _t at a predetermined temperature range including the transformation of the steel material, based on the average heat transfer coefficient K _t obtained in the calculating step, the obtained Deriving the maximum stress σ _t derived from the relationship between the average heat transfer coefficient and the maximum stress obtained in the process (derivation process), the threshold value of the cracking stress obtained in the acquisition process, and the deriving process Was Characterized in that it comprises a large stress sigma _t from step of determining whether the continued use of the aqueous polymer solution (determination step).

(Steel)
There are no particular limitations on the type of steel processed by polymer quenching to which the degradation degree determination method is applied. For example, steel, nickel-chromium steel, nickel-chromium-molybdenum steel, chromium-molybdenum steel, chromium steel, manganese- Examples include chrome steel.

(Polymer aqueous solution)
The aqueous polymer solution in the degradation degree determination method is an aqueous polymer solution that quenches the steel material by immersing the heated steel material or spraying the heated steel material. The polymer used in the polymer aqueous solution is not particularly limited as long as it is easily dissolved in water and has a desired slow cooling performance. For example, polyethylene glycol (PEG), polypropylene glycol (PPG), polyalkylene glycol (PAG), Polyvinyl pyrrolidone (PVP) etc. are mentioned. You may use 2 or more types of these as needed.

  Although it does not specifically limit as a minimum of the number average molecular weight of the polymer used for the said polymer aqueous solution, For example, 4,000 are preferable and 4,500 are more preferable. If the number average molecular weight is less than the lower limit, the polymer concentration must be increased in order to obtain the desired slow cooling performance, which may increase the amount of polymer used and increase the production cost of steel.

  Although it does not specifically limit as a minimum of the polymer concentration of the said polymer aqueous solution, For example, 10 volume% is preferable and 15 volume% is more preferable. On the other hand, the upper limit of the polymer concentration is, for example, preferably 60% by volume, more preferably 50% by volume. If the polymer concentration is less than the lower limit, the desired slow cooling performance may not be obtained from the beginning. On the other hand, when the polymer concentration exceeds the upper limit, the amount of the polymer used may increase excessively and the production cost of the steel material may increase.

  Although it does not specifically limit as a minimum of the heat conductivity of the said polymer aqueous solution, For example, 0.40 W / (m * K) is preferable and 0.45 W / (m * K) is more preferable. On the other hand, the upper limit of the thermal conductivity of the aqueous polymer solution is preferably, for example, 0.65 W / (m · K), and more preferably 0.60 W / (m · K). When the said heat conductivity is less than the said minimum, there exists a possibility that the desired intensity | strength anticipated by quenching may not be obtained because the cooling rate of steel materials becomes too slow. On the other hand, when the thermal conductivity exceeds the upper limit, the slow cooling performance of the polymer is insufficient, and there is a risk that fire cracks are likely to occur.

<Acquisition process>
In the acquisition step, the relationship between the average heat transfer coefficient and the maximum stress based on the steel material and the polymer aqueous solution and the threshold value for the cracking stress are acquired. The acquisition step is a step of creating a determination standard used for determining the deterioration degree of the polymer aqueous solution in the operation process performed through the measurement step, the calculation step, the derivation step, and the determination step described later. Therefore, the acquisition process does not need to be performed for each deterioration degree determination, and may be performed once for each type of combination of the steel material and the aqueous polymer solution.

The “average heat transfer coefficient based on the steel material and the aqueous polymer solution” is the heat between the steel material and the aqueous polymer solution at each time point calculated by the following formula (1) and the like in a predetermined temperature range including the transformation point of the steel material. It is a value [W / (m ² · K)] obtained by averaging the transmission rate [W / (m ² · K)]. Here, the “transformation point of the steel material” refers to the martensitic transformation point and the bainite transformation point of the steel material, and the “predetermined temperature range including the transformation point of the steel material” refers to the martensitic transformation point of the steel material and A predetermined temperature range including the bainite transformation point. The reason why these transformation points are included in the predetermined temperature range is that burning cracks are likely to occur particularly in the temperature range below these transformation points. The average heat transfer coefficient can be calculated from the measurement results obtained in the measurement process described later, for example, by software processing using spreadsheet software. As this spreadsheet software, for example, “Microsoft Excel” manufactured by Microsoft Japan is preferably used.

As described above, the predetermined temperature range including the transformation point of the steel material needs to be determined in advance. In this determination, the martensitic transformation region and the bainite transformation region of the steel material are first identified. This identification can be performed by software processing using, for example, general-purpose finite element analysis software. As this general-purpose finite element analysis software, for example, “FORGE” manufactured by TRANSVALOR is preferably used. Specifically, first, the heat transfer coefficient [W / (m ² · K)] between the steel material and the polymer aqueous solution at each time point calculated by the formula (1) described later is calculated, and then this heat transfer is calculated. The steel material is subjected to stress analysis for each temperature using the rate data, and the martensitic transformation region and bainite transformation region of the steel material are identified from the heat transfer coefficient data and the result of the stress analysis. Then, the predetermined temperature range is determined so as to include these transformation regions. At that time, it is preferable that the upper limit of the predetermined temperature range is not too high and the lower limit is not too low so that the average heat transfer coefficient can be easily calculated. As this upper limit, 550 degreeC is preferable, for example, 500 degreeC is more preferable, and 450 degreeC is further more preferable. On the other hand, as this lower limit, for example, 100 ° C is preferable, 120 ° C is more preferable, and 140 ° C is more preferable. The predetermined temperature range may be determined once for each type of steel material.

  In the stress analysis, coupled analysis of temperature, phase transformation, stress, strain, etc. of each part of the small test piece in the quenching test is performed. The stress-strain curve of the steel material used for this coupled analysis can be created using stress and elongation or strain measured by a tensile test based on JIS-Z2241 (2011) of the steel material. In addition, measured values can be used as thermophysical values such as thermal conductivity and specific heat of the steel used for the coupled analysis. Furthermore, the isothermal transformation diagram (TTT diagram) used for estimating the transformation start temperature and transformation fraction in the coupled analysis can be the same or similar component as the steel material. Note that, as shown in FIG. 4, the analysis value of the strain at each temperature by the above-described general-purpose finite element analysis software processing and the measured value of this strain agree very well regardless of the cooling rate (CR), The reliability of the stress analysis was confirmed.

  The “maximum stress based on the steel material and polymer aqueous solution” is the maximum value [MPa] of stress generated in the steel material in the predetermined temperature range when the steel material is quenched using the polymer aqueous solution. This maximum stress may be actually measured, or the maximum value of the stress in the predetermined temperature range may be obtained by the stress analysis, and this maximum value may be used.

  “Relationship between average heat transfer coefficient and maximum stress based on the steel material and polymer aqueous solution” means that the steel material is quenched using the polymer aqueous solution having a specific average heat transfer coefficient with the steel material. The relationship between a specific average heat transfer coefficient and the maximum stress of the steel material. Specifically, it refers to the relationship between the average heat transfer coefficient and the maximum stress indicated by the approximate line described later. This relationship and the approximate line can be obtained by using a plurality of polymer aqueous solutions having different degrees of deterioration and repeating a series of steps from a measurement process to a derivation process, which will be described later, as many times as necessary to create the approximate line.

  The “threshold for cracking stress” refers to the maximum stress [MPa] at which when cracking of steel begins to occur when the steel is quenched using an aqueous polymer solution. This threshold value may be obtained by measuring or calculating the residual stress of the steel material when the steel material is actually quenched using the polymer aqueous solution, and performing the stress analysis. Thus, the maximum stress in the predetermined temperature range may be obtained, and the maximum stress may be set as the threshold value.

<Measurement process>
In the measurement process, a quenching test is performed using the same kind of polymer aqueous solution in the operation process to be judged and the same kind of small specimen as the steel material, and the temperature change of each part of the small specimen and the aqueous polymer solution is measured over time. To do.

  In order to accurately determine the degree of deterioration of the aqueous polymer solution, the quenching test performed in the measurement process is preferably performed in an aspect as close as possible to the actual quenching performed in the operation equipment using the steel material and the aqueous polymer solution. Therefore, the quenching test is not limited to a mode in which the heated small test piece is immersed in the polymer aqueous solution, but may be a mode in which the aqueous polymer solution is uniformly sprayed onto the heated small test piece. The embodiment may be cooled with an aqueous solution or other embodiments.

  The determination object of the present invention is an aqueous polymer solution in an operation process that is actually used for quenching and is feared to deteriorate. When performing the quenching test in a mode in which a heated small test piece is immersed in a polymer aqueous solution, the amount of the polymer aqueous solution used for one quenching test depends on the size of the small test piece, for example, 2 L Can be about.

  The “small test piece” of the present invention refers to a test piece having such a size that it can be used for a quenching test in a test room. The shape of the small test piece is not particularly limited as long as it does not hinder the temperature measurement of each part of the small test piece, and examples thereof include a spherical shape, an oblate shape, a cubic shape, a regular polygonal column shape, and a cylindrical shape. Among these, a cylindrical shape that is widely used in the quenching test and allows easy stress analysis is preferable.

  When the small test piece is cylindrical, the lower limit of the diameter of the bottom surface of the cylinder is preferably, for example, 10 mm, and more preferably 15 mm. On the other hand, the upper limit of the diameter is, for example, preferably 40 mm, and more preferably 30 mm. When the said diameter is less than the said minimum, there exists a possibility that the reliability of the average heat transfer coefficient calculated by the temperature difference of the center and side surface of a small test piece becoming small may fall. On the other hand, when the diameter exceeds the upper limit, the scale of the quenching test becomes unnecessarily large, and there is a possibility that the deterioration degree of the aqueous polymer solution cannot be determined at a low cost.

  When the small test piece is cylindrical, the lower limit of the height of the cylinder is preferably, for example, 30 mm, and more preferably 40 mm. On the other hand, the upper limit of the height is preferably 120 mm, for example, and more preferably 100 mm. When the height is less than the lower limit, the temperature difference between the center of the small test piece, the top surface, and the bottom surface becomes too small, and the reliability of the average heat transfer coefficient calculated may decrease. On the other hand, when the height exceeds the upper limit, the scale of the quenching test becomes unnecessarily large, and there is a possibility that the deterioration degree of the aqueous polymer solution cannot be determined at a low cost.

  The temperature at which the small test piece is heated is not particularly limited, but in order to accurately determine the degree of deterioration of the polymer aqueous solution, the same temperature as the heating temperature when actually quenching the steel material is preferable. It can be set to a predetermined temperature of not more than ° C. The heating method is not particularly limited, and a known method such as high-frequency heating can be appropriately used.

  The instrument for measuring the temperature change of each part of the small test piece and the aqueous polymer solution with respect to time is not particularly limited as long as it can be measured quickly and accurately, and includes, for example, a thermocouple thermometer.

  Each site | part of the said small test piece is not specifically limited as long as it is a site | part which can represent the temperature change for every time of the said small test piece. In the case where the shape of the small test piece is a columnar shape, each of the parts includes, for example, the center of gravity of the upper cylindrical region having a predetermined thickness from the top surface, the center of gravity of the bottom cylindrical region having the predetermined thickness from the bottom surface, and the above-mentioned from the side surface. A position at a half of the predetermined thickness from the side surface in the side cylindrical area of a predetermined thickness and a position at a half of the height of the small test piece, and the upper cylindrical area, the bottom cylindrical area, and the side The center of gravity of the central columnar region surrounded by the partial cylindrical region can be used, and the temperature change can be measured by installing thermocouples at these positions. On the other hand, the polymer aqueous solution can measure the said temperature change by installing a thermocouple in the arbitrary positions which can represent the temperature change of the polymer aqueous solution for every time.

  The initial temperature of the aqueous polymer solution when the heated small test piece is immersed is not particularly limited, but in order to accurately determine the degree of deterioration of the aqueous polymer solution, the initial temperature is the same as the initial temperature when actually quenching the steel material. Preferably, for example, a predetermined temperature of 30 ° C. or higher and 80 ° C. or lower can be set.

  The temperature change for each time can be measured by measuring the temperature of each part of the small test piece and the aqueous polymer solution at regular intervals. This fixed time is not particularly limited as long as it does not affect the reliability of the average heat transfer coefficient, and is, for example, 0.1 seconds, 0.5 seconds, 1.0 seconds, 2.0 seconds, 5.0 seconds, or the like. be able to.

<Calculation process>
In the calculation step, an average heat transfer coefficient K _t in a predetermined temperature range including the transformation point of the steel material is calculated from the measurement result obtained in the measurement step. Here, the “measurement result obtained in the measurement step” refers to a measurement result of temperature change of each part of the small test piece and the aqueous polymer solution over time.

From the measurement results obtained in the measurement step, an average heat transfer coefficient K _t between the steel and polymer aqueous solution at the predetermined temperature range. When the small test piece is cylindrical, the K _t is calculated by substituting the measurement result into the following formula (1), for example, to calculate the heat transfer coefficient h _i between the steel material and the polymer aqueous solution at each time point. The average value of h _{i in} the predetermined temperature range is calculated as K _t .

Where t is the center of gravity of the upper cylindrical region having a predetermined thickness from the top surface of the small test piece, b is the center of gravity of the bottom cylindrical region having the predetermined thickness from the bottom surface, and s is the predetermined thickness from the side surface. The position of half the height of the small test piece at the position of 1/2 of the predetermined thickness from the side surface in the side cylindrical region of the height, c is the upper cylindrical region, the bottom cylindrical region and the side portion This is the center of gravity of the central columnar region surrounded by the cylindrical region. m _i (i is t, s, b) is the mass [kg] of each of the upper cylindrical region, the side cylindrical region, and the bottom cylindrical region. A _i is the contact area [m ² ] between the upper columnar region, the side cylindrical region, the bottom columnar region, and the central columnar region. C _px (x is c, t, s, b) is the specific heat [J / (kg · K)] of each of the central columnar region, the upper columnar region, the side cylindrical region, and the bottom columnar region. . l _i (i is t, s, b) is a distance [m] from c to each of the top, side, and bottom surfaces. T _x (x is c, t, s, b) is the temperature [K] at each point of c, t, s, and b, and T _bulk is the temperature [K] of the aqueous polymer solution. the upper surface of the h _i the test piece, a heat transfer coefficient at the side and bottom surfaces ^{[W / (m 2 · K} )].

  Although the minimum of the said predetermined thickness is not specifically limited, For example, 1 mm is preferable and 2 mm is more preferable. On the other hand, the upper limit of the predetermined thickness is not particularly limited, but is preferably 5 mm, for example, and more preferably 4 mm. When the predetermined thickness is less than the lower limit, it is difficult to install the thermocouple at an accurate position, and the reliability of the average heat transfer rate may be reduced. On the other hand, when the predetermined thickness exceeds the upper limit, it is difficult to accurately measure the temperature in the vicinity of the surface of the small test piece, and the reliability of the average heat transfer coefficient may be reduced.

<Derivation process>
In the derivation step, the maximum stress σ _t is derived from the relationship between the average heat transfer coefficient obtained in the acquisition step and the maximum stress based on the average heat transfer coefficient K _t obtained in the calculation step. Specifically, σ _t is read using an approximate line indicating the relationship between the average heat transfer coefficient and the maximum stress and K _t . Details of this reading will be described in an embodiment described later.

<Judgment process>
In the determination step, whether or not the polymer aqueous solution is continuously used is determined from the threshold value of the cracking stress obtained in the acquisition step (hereinafter also referred to as “σ _th ”) and the maximum stress σ _t obtained in the derivation step. Specifically, sigma tolerance of maximum stress sigma _t based on _th (hereinafter also referred to as "sigma _ac") is determined and the maximum stress sigma _t Toko obtained using the polymer solution to be determined with sigma _ac Whether the continuous use of the aqueous polymer solution is determined based on the magnitude relationship.

For example, as shown in FIG. 1, when the maximum stress σ _t is less than σ _ac , that is, when the following equation (2) is satisfied, it is determined that no burning crack occurs and the continuous use of the polymer aqueous solution is possible. In FIGS. 1 and 2, the average heat transfer coefficient K _t and the maximum stress σ _t are indicated as K _test and σ _test , respectively.
σ _t <σ _ac (2)

On the other hand, as shown in FIG. 2, when the maximum stress σ _t is equal to or greater than the allowable value σ _ac of the maximum stress σ _t , that is, when the following formula (3) is satisfied, there is a risk of burning cracking. It is determined that continuous use is not possible. In this case, the entire amount of the aqueous polymer solution may be replaced suddenly, but instead of replacing the entire amount, a new liquid is added to the determined aqueous polymer solution, the deterioration degree is again determined for the added liquid, and the above formula ( It is preferable to satisfy 2). By doing in this way, the usage-amount of polymer aqueous solution can be saved. Here, although the amount of liquid to be added depends on the degree of deterioration, for example, it is conceivable that 20 to 30 parts by mass of a new liquid is added to 100 parts by mass of the determined liquid.
σ _t ≧ σ _ac (3)

In the determination step, the threshold value σ _th of the cracking stress may be set as the allowable value σ _ac of the maximum stress σ _t as it is, but in order to prevent the cracking more reliably, a value obtained by dividing σ _th by a safety factor exceeding 1 is used. σ _ac is preferable. The safety factor is not particularly limited, but the lower limit of the safety factor is preferably 1.1, more preferably 1.2, and still more preferably 1.3, for example. On the other hand, as the upper limit of this safety factor, for example, 2.2 is preferable, 2.1 is more preferable, and 2.0 is more preferable. If the safety factor is less than the lower limit, _σac is too close to _σth, and there is a possibility that firing cracks cannot be sufficiently prevented. On the other hand, when the safety factor exceeds the upper limit, σ _ac becomes too small, the amount of the aqueous polymer solution used increases, and the manufacturing cost of the steel material may increase.

  The deterioration degree determination of the aqueous polymer solution may be performed every time the steel material is quenched, and when it can be expected that the quenching can be performed a predetermined number of times or for a predetermined time without occurrence of quench cracking, the predetermined number of times or the predetermined time You may go every time.

<Advantages>
The method for determining the degree of deterioration involves cooling the heated small test piece with an aqueous polymer solution, and measuring the temperature change of each portion of the small test piece and the aqueous solution of the polymer with time. Whether the aqueous solution is continuously used can be easily determined. In addition, the degradation degree determination method uses the average heat transfer coefficient between the steel material and the polymer aqueous solution that directly affects the cooling rate, and the threshold value and maximum stress of the steel material that directly affect the cracking as the management index. Therefore, it is possible to accurately determine whether the polymer aqueous solution is continuously used. Furthermore, the deterioration degree determination method may be performed by performing a quenching test in a test room using a small amount of the polymer aqueous solution, and can determine whether the continuous use of the polymer aqueous solution is inexpensive and safe.

[Other Embodiments]
The said embodiment does not limit the structure of this invention. Therefore, in the above-described embodiment, the components of each part of the above-described embodiment can be omitted, replaced, or added based on the description and common general knowledge of the present specification, and they are all interpreted as belonging to the scope of the present invention. Should.

In the above-described embodiment, the measurement process, the calculation process, the derivation process, and the determination process are independent processes, and are performed in this order. However, these processes may be combined. For example, the derivation process and the determination process may be combined to determine whether continuous use is performed while deriving the maximum stress σ _t . By doing in this way, it can be judged more rapidly whether the polymer aqueous solution can be used.

In the said embodiment, although the quenching test was done in the aspect which immerses the heated small test piece in polymer aqueous solution, you may carry out in another aspect. For example, it is good also as an aspect sprayed uniformly on the test piece which heated polymer aqueous solution with the spray apparatus provided with a some nozzle. In this case, the ejection speed of each nozzle is not particularly limited, and can be appropriately adjusted within a range of, for example, 100 to 1200 L / (m ² · min). In this case, the temperature change of the aqueous polymer solution can be measured, for example, by measuring the temperature of the aqueous polymer solution near the nozzle at regular intervals.

  In the above embodiment, the temperature of the portion (t, s, b) in the vicinity of the surface of the small test piece that is most easily cooled and the center portion (c) of the small test piece that is most difficult to be cooled are measured. If there is no hindrance in calculating the rate, for example, only the temperature at the intermediate position between the surface of the small test piece and the central portion may be measured. By doing in this way, it can be determined more easily whether continuous use of the polymer aqueous solution.

  EXAMPLES Hereinafter, although this invention is further demonstrated based on an Example, this invention is not interpreted limitedly based on description of this Example.

(Acquisition of average heat transfer coefficient and maximum stress)
As a polymer aqueous solution for performing a quenching test, about 2 L of samples that reached a state where a crack occurred due to continuous use was collected. Moreover, as shown in the schematic cross-sectional view of the small test piece in FIG. 5, a cylindrical small test piece having a diameter of 20 mm and a height of 60 mm made of a low alloy steel of the same type as the steel to be quenched was prepared.

The small test piece was divided into four virtual regions with a thickness of 3 mm from the surface, and a thermocouple was installed in each divided region. Specifically, in the schematic cross-sectional view of FIG. 5, a cylindrical region having a thickness of 3 mm from the top surface is an upper portion R _t , a cylindrical region having a thickness of 3 mm from the bottom surface is a bottom portion R _b , and a cylindrical shape having a thickness of 3 mm from the side surface. The columnar region surrounded by the side portion R _s , the top portion R _t , the bottom portion R _b, and the side portion R _s was defined as a central portion R _c . The center of gravity of the upper part R _t is t, the center of gravity of the bottom part R _b is b, the center of gravity of the center part R _c is c, the position of 1.5 mm in thickness from the side surface in the side part R _s is s, and the position of 30 mm in height is s. , B, s and c were provided with thermocouples. In the case of a polymer aqueous solution, the position of the polymer aqueous solution at a depth of 1/2 in the middle of the side surface of the small test piece when the small test piece is completely immersed in the polymer aqueous solution and the inner side surface of the container containing the polymer aqueous solution. Thermocouples were installed at three positions, an intermediate position between the upper surface of the small test piece and the liquid surface of the aqueous polymer solution, and an intermediate position between the bottom surface of the small test piece and the inner bottom surface of the container.

  Immediately after heating the small test piece provided with the thermocouple to 900 ° C., it is immersed in the polymer aqueous solution kept at 50 ° C. by hot water using a traverser, and t, b, s every 0.5 seconds from the start of immersion. The temperature at 4 points of c and c and the above three temperatures of the polymer aqueous solution were measured. This measurement was performed until the temperature at the point c was cooled to 70 ° C.

The change in the amount of heat at the top portion R _t , the side portion R _s, and the bottom portion R _b is determined by the amount of heat released to the polymer aqueous solution and the amount of heat input from the central portion R _c. By substituting the temperature at each time point of points t, s, b, and c and the average value of the temperature at each of the above three points of the aqueous polymer solution, the aqueous polymer solution at each time point, the upper part R _t , and the side part R _s and it was calculated heat transfer coefficient h _i between the bottom R _b. This calculation was performed by the above-mentioned “Microsoft Excel”.

In the formula (1), t is the center of gravity of the upper part _{R t} of thickness 3mm from the upper surface of the small specimen, b is the center of gravity of the bottom _{R b} of the thickness 3mm from the bottom, s a side portion of the thickness of 3mm from the side A position at a thickness of 1.5 mm from the side surface at R _s and a height of 30 mm, c is the center of gravity of the central portion R _c surrounded by the upper portion R _t , the bottom portion R _b and the side portion R _s . m _i (i is t, s, b) is the mass [kg] of each of the upper part R _t , the side part R _s and the bottom part R _b . A _i is the contact area [m ² ] between the top portion R _t , the side portion R _{s, the} bottom portion R _b, and the central portion R _c . C _px (x is c, t, s, b) is the specific heat [J / (kg · K)] of each of the central portion R _c , the upper portion R _t , the side portion R _s and the bottom portion R _b . l _i (i is t, s, b) is a distance [m] from c to each of the top, side, and bottom surfaces. T _x (x is c, t, s, b) is the temperature [K] at each point of c, t, s, and b, and T _bulk is the average temperature at each of the above three points of the aqueous polymer solution. Value [K]. h _i is the upper surface of the small test strip, the heat transfer rate in the side and bottom surfaces ^{[W / (m 2 · K} )].

Next, a stress analysis of the small specimens each temperature by the "FORGE" using h _i calculated above. Furthermore, the results of data and the stress analysis of h _i calculated above, to identify the martensitic transformation range and the bainite transformation region of the steel, the predetermined temperature range used for calculating the average heat transfer coefficient K _t, of It was determined to be 140 ° C. or higher and 440 ° C. or lower including the transformation region. And the average heat transfer coefficient _Kk [W / (m _< ² > K)] between the said steel materials and polymer aqueous solution in this temperature range was computed. On the other hand, the maximum stress σ _k [MPa] of the steel material in this temperature range was extracted from the result of the stress analysis.

The average heat transfer coefficient K _k and the maximum stress σ _k were plotted in a biaxial orthogonal coordinate system with the average heat transfer coefficient as the horizontal axis and the maximum stress as the vertical axis.

  Using a plurality of polymer aqueous solutions having the same composition and different degrees of deterioration, the above average heat transfer is repeated by repeating a series of steps from the heating of the small test piece corresponding to the measurement step, calculation step and derivation step to the plot. A sufficient number of data was plotted to obtain an approximate line showing the relationship between rate and maximum stress. An approximate line TL shown in FIG. 3 was obtained by applying a least square method to the plotted data and performing a first-order approximation.

In addition, when applying the least square method when acquiring the approximate line, higher-order approximation higher than quadratic approximation may be performed. By doing so, a faithful approximate line can be obtained from the plotted data of the average heat transfer coefficient K _k and the maximum stress σ _k , and the maximum stress σ _t can be derived more accurately. An approximation method other than the least square method may be applied.

(Acquisition of threshold value of burning crack stress)
On the other hand, the maximum stress of the steel material in the temperature range of 140 ° C. or higher and 440 ° C. or lower was obtained from the result of the stress analysis, and this maximum stress was defined as the threshold value σ _{th of the} cracking stress. Next, a safety factor was set to 1.25, and a value obtained by dividing σ _th by 1.25 was defined as a maximum stress allowable value σ _ac .

(Derivation of maximum stress σ _t and determination of deterioration)
In the same manner as the method used for obtaining the approximate line TL, the average heat transfer coefficient K _t [W / (m ² · K)] of the aqueous polymer solution to be determined is calculated, and the biaxial orthogonal coordinate system shown in FIG. Are plotted on the average heat transfer coefficient axis (hereinafter referred to as “K-axis”). Next, a straight line passing through this point (K _t , 0) and parallel to the maximum stress axis (hereinafter referred to as “σ axis”) is drawn, and an intersection (hereinafter referred to as “first intersection”) between this straight line and the approximate line TL is obtained. Asked. Then, a straight line passing through the first intersection point and parallel to the K axis is drawn, and an intersection point between the straight line and the σ axis (hereinafter referred to as “second intersection point”) is obtained, and the maximum stress value corresponding to the second intersection point is determined as the maximum stress. Read as σ _t [MPa]. This reading was performed using the above-mentioned “Microsoft Excel”. Then, as shown in FIG. 1, since this σ _t does not satisfy the maximum stress allowable value σ _ac and satisfies the above equation (2), it was determined that the polymer aqueous solution can be used continuously.

  As described above, the degradation degree determination method of the present invention can be easily and accurately determined whether or not the aqueous polymer solution used for quenching of the steel material is used, so that it is suitably used for the manufacture of steel materials that use the aqueous polymer solution for quenching. be able to.

The center of gravity of the TL approximate line R _b bottom R _t top R _s side R _c central b R _b of the center of gravity t R _t of the center of gravity s place in the R _s c R _c

Claims (4)

A method for determining the degree of deterioration of an aqueous polymer solution used for quenching steel,
A step of obtaining a relationship between an average heat transfer coefficient and a maximum stress based on the steel material and the polymer aqueous solution, and a threshold value of a cracking stress;
A step of performing a quenching test using the same kind of small test piece as the polymer aqueous solution and the steel material in the operation process to be determined, and measuring a temperature change of each part of the small test piece and the aqueous polymer solution over time,
A step of calculating an average heat transfer coefficient K _t in a predetermined temperature range including the transformation point of the steel material from the measurement result obtained in the measurement step;
Deriving the maximum stress σ _t derived from the relationship between the average heat transfer coefficient obtained in the acquisition step and the maximum stress based on the average heat transfer coefficient K _t obtained in the calculation step;
And a step of determining whether the polymer aqueous solution is continuously used from the threshold value of the burning crack stress obtained in the acquisition step and the maximum stress σ _t obtained in the derivation step. The acquisition process is
A step of performing a quenching test using the same kind of small test piece as the polymer aqueous solution and the steel material in the operation process to be determined, and measuring a temperature change of each part of the small test piece and the aqueous polymer solution over time,
The degradation degree determination method according to claim 1, further comprising a step of analyzing stress from the temperature data obtained in the measurement step.   The deterioration determination method according to claim 2, wherein past performance data is used for acquiring the threshold value of the cracking stress in the acquisition step. The deterioration determination method according to claim 2, wherein the measurement step and the analysis step are performed using the polymer aqueous solution in a state where the cracks are generated in acquiring the threshold value of the cracking stress in the acquisition step.

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