JP6565656B2 - Hardness prediction method for high strength steel - Google Patents

Description

  The present invention relates to a method for predicting the hardness of high strength steel produced by quenching and tempering heat treatment.

  In recent years, high strength of steel materials has been demanded in various fields. For example, in the construction machinery field, with the increase in size of buildings, the size of construction machinery such as crane trucks has been increased, and in order to suppress the increase in the weight of structural members, the strength of steel materials has been demanded. Further, in the automobile field, from the viewpoint of environmental load, there is a demand for improvement in fuel efficiency by reducing the weight of the vehicle body, and in order to reduce the weight of structural members, the strength of steel materials is being increased.

  In order to increase the strength of the steel material, it is necessary to add an alloy. From the viewpoint of cost, it is desirable to quench the steel sheet to obtain a martensite structure in order to reduce the alloy content. However, an as-quenched martensite structure has low toughness and ductility, and thus needs to be tempered.

  In order to determine the heat treatment conditions, it is necessary to determine the temperature and time according to the component system, but various settings are possible. Conventionally, trial and error such as conducting a heat treatment and a property test (such as a tensile test) on a trial basis under some conditions have been required to find the optimum heat treatment conditions for obtaining desired properties.

  Therefore, a method for predicting a material without performing a physical characteristic test is desired. So far, in order to satisfy the required characteristics of the quenching material, a method of determining a heat treatment condition that gives a cooling curve during quenching has been proposed (see, for example, Patent Document 1 below).

  Moreover, in tempering heat treatment, after inserting a steel plate into a furnace, unsteady tempering, such as extracting in the middle of a steel plate reaching furnace temperature, may be performed. On the other hand, a tempering parameter has been proposed that can calculate unsteady tempering along various continuous temperature rise curves (for example, see Non-Patent Document 1 below).

  In addition, a method for predicting a material by calculating a change in a metal structure and a precipitation state using a mathematical model based on a steel plate component and manufacturing conditions has been proposed (for example, see Patent Document 2 and Patent Document 3 below). ).

Japanese Patent Laid-Open No. 01-198422 JP 05-072200 A JP 05-142126 A

Satoshi Inoue, “New tempering parameters and their application to the integration method of tempering effects along a continuous temperature rise curve”, Iron and Steel, September 1980, Vol. 66, No. 10, p. 1532-1541

  However, the method proposed in Patent Document 1 can be applied only when the material changes only by a cooling curve during quenching or the like, and cannot be applied to tempering heat treatment.

  Moreover, although the said nonpatent literature 1 shows the relationship between the tempering parameter and material of a specific steel material, there is no suggestion regarding the prediction of the hardness of the steel material of another component system.

  Furthermore, the method proposed in Patent Document 2 is intended for steel plates manufactured by the normalizing method, and cannot be applied to quenched-tempered steel. Further, the method proposed in Patent Document 3 is intended for steel plates having a tensile strength of 400 MPa to 900 MPa, and the application range is different from that of high strength steel plates having a tensile strength of 1000 MPa or more.

  Thus, in the conventional material prediction methods as proposed in Patent Document 1 to Patent Document 3 and Non-Patent Document 1 described above, the tensile strength produced by quenching and tempering heat treatment is as high as 1000 MPa or more. The hardness of high-strength steel cannot be predicted easily.

  Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to provide a steel material that is produced by quenching and tempering having a tensile strength of 1000 MPa to 1400 MPa. An object of the present invention is to provide a method for predicting the hardness of high-strength steel that can more easily predict the thickness.

  In order to ensure the tensile strength of 1000 MPa or more, the present inventors performed tempering heat treatment at a low temperature of 100 to 400 ° C., and observed the structural change of the steel material. As a result, it has been found that the change in the hardness of martensite due to tempering heat treatment is expressed only by solid solution strengthening by solid solution carbon and precipitation strengthening predictable from the amount and size of carbides.

  Furthermore, it has been found that there is a one-to-one relationship between the tempering parameter λ obtained from the thermal history during tempering, and the amount of solute carbon, carbide and carbide. And since the hardness of the martensite after hardening is decided only by the amount of carbon, it succeeded in predicting the hardness of a quenching-low temperature tempering material from the amount of added carbon and the heat history at the time of tempering.

  This invention is made | formed based on such knowledge, The summary is as follows.

[1] A step of _obtaining a relational expression between the tempering parameter λ obtained from the thermal history, the carbon content C _add of the steel material, and the solid solution carbon amount C _sol , the solid solution carbon amount C _sol , and solid solution strengthening and martensite hardness HV _s, a step of obtaining a relational expression, and precipitation strengthening the martensite hardness HV _P, and carbon content C _{the add} of the steel, and the solid solution carbon amount C _sol, and the tempering parameter λ , of a step of obtaining a relational expression, wherein the said the solid-solution strengthening martensite hardness HV _s, the and precipitation strengthening the martensite hardness HV _P, the hardness HV of the high strength steel which is the sum of the tempering A method for predicting the hardness of high-strength steel, which is calculated from a relational expression between the parameter λ and the carbon content C _add of the steel material.

Here, the tempering parameter λ is expressed by the following formula (1), and in the following formula (2), δ _τi is N from the tempering start time to the end time (N is an integer of 2 or more). Is the section time in the i (i = 1, 2,..., N) -th section when divided into sections, T _i is the temperature at the section time _δτi , and Q is iron Is the activation energy associated with self-diffusion, and R is a gas constant.

[2] A relational expression between the tempering parameter λ obtained from the thermal history, the carbon content C _{add of the} steel material, and the solid solution carbon amount C _sol is expressed by the following formula (3), and the solid solution: relationship between carbon content _{C sol} and the solid solution strengthening martensite hardness HV _s is represented by the following equation (4), wherein the precipitation and strengthening the martensite hardness HV _P, carbon content _{C the add} of the steel And the relational expression between the solid solution carbon amount C _sol and the tempering parameter λ is expressed by the following formula (5): the solid solution strengthened martensite hardness HV _s and the precipitation strengthened martensite hardness HV The relational expression of the hardness HV of the high-strength steel that is the sum of _P , the tempering parameter λ, and the carbon content C _add of the steel material is represented by the following formula (6), according to [1]. For predicting hardness of high-strength steel.
Here, in the following formulas (3) to (6), t, u, v, w, x, y, and z are constants.

  As described above, according to the present invention, the hardness of a steel sheet having a tensile strength of 1000 to 1400 MPa manufactured by quenching and tempering heat treatment can be predicted more easily.

  Further, according to the present invention, it is possible to determine appropriate tempering heat treatment conditions without actually producing a steel material and performing a characteristic test or reducing the load of the characteristic test. Therefore, according to the present invention, the industrial contribution of the present invention is extremely remarkable, such as making it possible to efficiently set the production conditions of a steel sheet having desired characteristics.

It is a graph explaining the relationship between the amount of solid solution carbon and solid solution strengthened martensite hardness. It is a graph explaining the relationship between tempering parameter (lambda) and cementite size (circle equivalent diameter). It is a graph figure which compares and demonstrates the tempering parameter (lambda) and the actual value of hardness of high-strength steel, and a predicted value.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

Below, the prediction method of the hardness of the high strength steel which concerns on embodiment of this invention is demonstrated in detail.
The high-strength steel according to the present embodiment is a steel material that has a tensile strength of 1000 MPa to 1400 MPa and is manufactured through a quenching-tempering heat treatment. Here, although it does not specifically limit about the chemical component of this steel material, For example, it is preferable to have the following chemical components. A steel material having the following chemical components can easily achieve a tensile strength of 1000 MPa to 1400 MPa through a known quenching-tempering heat treatment. That is, in order to realize high strength of 1000 MPa to 1400 MPa at a low alloy cost, it is important to form a martensite structure by quenching. The strength of the as-quenched martensite structure is determined only by the amount of carbon as described in detail below, but by setting the carbon content to 0.07 to 0.25% as in the following example, 1000 MPa A high strength of ˜1400 MPa can be achieved.

% By mass
C: 0.07 to 0.25%
Si: 0.5% or less Mn: 0.8 to 1.8%
P: 0.020% or less S: 0.005% or less Ni: 10% or less Cr: 5% or less Mo: 5% or less Al: 0.100% or less Nb: 0.050% or less Ti: 0.030% or less B: 0.030% or less N: 0.0080% or less Remainder: Fe and impurities

The tempering parameter λ noted below is a parameter indicating the degree of solid-phase reaction that occurs in the steel material in the tempering heat treatment. Such a tempering parameter λ can represent the thermal history of various tempering heat treatments. The tempering start time to the end time are divided into N sections (N is an integer of 2 or more), and the temperature T _i is set to each section time δτ _i (i = 1, 2,..., N). When it is assumed that the heat history curve is approximated as a rectangular section sum and is sequentially calculated, the tempering parameter λ can be obtained from the tempering heat history by the following equations (1) and (2).

  Here, in said formula (2), Q is the activation energy accompanying self-diffusion of iron, and the value is 67.2 kcal / mol. Moreover, in said formula (2), R is a gas constant and the value is 1.987cal / K * mol.

The number of sections N is not particularly limited, and may be set as appropriate according to the time from the temper start time to the end time and the length of each section time δτ _i . Although how to set the length of each section time δτ _i is not particularly limited, for example, 0.01 sec is preferable. Furthermore, also how to set the temperature T _i in each section, is not particularly limited, for example, it is preferable to 1 ° C..

Conventionally, there is no knowledge relating the tempering parameter λ and the hardness, and it has not been possible to predict the characteristics of the steel material from the thermal history of the tempering heat treatment. Therefore, in the present invention described in detail below, focusing on the strengthening mechanism in the low temperature tempering region of 100 to 400 ° C., a method for predicting the hardness by the tempering parameter λ and the carbon content C _add was studied.

  In the following description, “hardness” is not particularly limited, but for example, Vickers hardness measured according to JIS Z2244 is used. In the following description, it is assumed that tempering heat treatment is performed on the high-strength steel having the above chemical components in the low-temperature tempering region as described above. Here, the processing time of the tempering heat treatment is not particularly limited, and may be set as appropriate according to the required heat history.

The hardness of high-strength steel includes the hardness of martensite due to the amount of solute carbon (solid solution strengthened martensite hardness HV _s ), the hardness of martensite due to precipitation strengthening (precipitation strengthened martensite hardness HV _p ), and Is considered to be the sum of

[Solution strengthened martensite hardness HV _s ]
First, the solid solution carbon amount C _sol (unit: for example, mass%) can be expressed by the difference between the carbon content C _add (unit: for example, mass%) and the amount of carbon deposited by tempering heat treatment. This is because the amount of solid solution carbon immediately after quenching is equal to the carbon content, the solid solution carbon is reduced by the tempering heat treatment, and the carbon is precipitated in the steel as a carbide.

When the amount of carbon precipitated in the steel as carbide is expressed by an exponential function of the tempering parameter λ, the solute carbon amount C _sol is obtained by the following equation (3) using the carbon content C _add and the tempering parameter λ. be able to. It should be noted that the constant u and the index t in the following formula (3) are obtained by fitting through experiments. That is, for the steel material of _interest, after actually measuring the solute carbon amount C _sol and the carbon content C _add using a known method, the distribution of the measured values obtained is represented by the following exponential function (3) It is possible to specify specific values of the constant u and the index t by fitting with.

In addition, as a method of measuring solid solution carbon amount _Csol , the method of calculating _| requiring the amount of carbon which formed the deposit by the mapping by an electron beam microanalyzer or the electrolytic extraction residue analysis method, and deducting from carbon content is mentioned, for example. Can do. Examples of the method for measuring the carbon content C _add include a combustion infrared absorption method.

The value indicating the hardness of martensite increases in proportion to the square root of the solid solution carbon amount. Therefore, the solid solution strengthened martensite hardness HV _s can be expressed by the following equation (4) using the solid solution carbon amount C _sol . Here, in the following formula (4), v and w are certain constants. Specific values of the constants v and w are obtained by actually measuring the hardness of martensite whose solid solution carbon amount C _sol is known, and expressing the obtained measurement result by the following formula (4). It is possible to specify by fitting with a function form.

From the above formulas (3) and (4), the solid solution strengthened martensite hardness HV _s can be expressed as the following formula (7) using the carbon content C _add .

[Precipitation strengthened martensite hardness HV _p ]
Next, the martensite hardness HV _p due to precipitation strengthening, according to the Ashby-Olowan equation, between the martensite hardness HV _p due to precipitation strengthening, the volume fraction f of the precipitate, and the size d of the precipitate. The relationship expressed by the following equation (8) is established. Here, k in the following formula (8) is a certain constant.

The volume fraction f of the precipitate has a correlation with the difference between the carbon content C _add and the solid solution carbon amount C _sol, and can be expressed as in Expression (9) using a certain constant l.

  On the other hand, it has been found by the inventors that the size d of the precipitate can be expressed by a quadratic function of the tempering parameter λ as in the following formula (10) in the initial stage of the tempering heat treatment. Therefore, if an extracted replica sample is prepared and the size of carbides (for example, equivalent circle diameter) of various tempered materials is measured by observation with a transmission electron microscope (TEM), the equation (10) A constant m and a constant n can be obtained. That is, the tempering parameter λ corresponding to each extracted replica sample is specified, and the size (equivalent circle diameter, etc.) d of each extracted replica sample is measured by TEM observation, and the tempering parameter λ and carbide obtained The relationship with the size d is plotted on a plane. Thereafter, the following constant m and constant n can be obtained by fitting the obtained plot with a quadratic function.

From the equations (8), (9), and (10) as described above, the hardness of martensite by precipitation strengthening (precipitation strengthened martensite hardness HV _p ) is expressed as in equation (5). Can do. Here, in the following formula (5), x, y, and z are certain constants. Furthermore, by using the relationship of the above formula (3), the formula (5) can be rewritten as the formula (5 ′).

[Hardness HV of high-strength steel]
As described above, the hardness after tempering is the total of the solid solution strengthened martensite hardness HV _s and the precipitation strengthened martensite hardness HV _p . Therefore, from the above formulas (4) and (5 ′), the prediction formula for the hardness HV after tempering is as shown in the following formula (6). In the following equation (6), constants t, u, v, w, x, y, and z are determined to be certain values by prior verification. Therefore, the variables in the following equation (6) are: The tempering parameter λ is determined from the carbon content C _add of the high strength steel of interest and the heat treatment conditions of the tempering heat treatment applied to the high strength steel of interest.

Therefore, for the high strength steel of _interest , the carbon content C _add is specified by a known method, and from the heat treatment conditions of the tempering heat treatment to be performed on the high strength steel, the above formulas (1) and (2) By calculating the tempering parameter λ based on the above and substituting the obtained carbon content C _add and the tempering parameter λ into the above equation (6), the hardness HV after tempering can be predicted.

Further, when it is desired to realize a desired tempered hardness HV in a high-strength steel, a high-strength steel in which the value of the hardness HV after tempering is set to a desired value and the carbon content C _add is focused on. The tempering parameter λ satisfying the above formula (6) may be calculated with the carbon content of This makes it possible to more easily specify the heat treatment conditions of the tempering heat treatment that can achieve the desired tempering hardness HV.

  Examples of the present invention will be described below. In addition, the Example shown below is only an example of the hardness prediction method of the high strength steel which concerns on this invention, Comprising: The hardness prediction method of the high strength steel which concerns on this invention is not limited to the following example. Absent.

  Tables 1 and 2 and FIGS. 1 to 3 shown below show the results of Examples according to the present invention. Table 1 shows the components of each production example. In the following examples, attention was paid to three types of steels whose components are shown in Table 1. All of the three types of steel shown in Table 1 are steel types used as high-strength steel that achieves a tensile strength of 1000 MPa to 1400 MPa.

Using the steel types shown in Table 1, tempering heat treatment was performed under the conditions shown in Table 2, a sample was taken, electrolyzed in a non-aqueous solvent, and separated into a solution and a residue with a filter. From the amount of carbon contained in the residue, the carbon content, and the tempering parameter λ, a relational expression (3) ′ between the solid solution carbon amount C _sol , the carbon content C _add and the tempering parameter λ was obtained by fitting. .

In each tempering conditions shown in Table 2, when calculating the tempering parameter λ is the section time .DELTA..tau _i and the temperature T _i in the formula (2) was set to 0.01 seconds and 1 ℃ respectively . The number of sections i was set according to the time from the temper start time to the end time and the length of each section time δτ _i . “Temperature” and “Time” of “Tempering” shown in Table 2 below are the heating temperature and holding time in the tempering heat treatment, and the tempering parameter λ is the time required for the temperature rise to reach the heating temperature, and It calculated including the time required for cooling after extraction from the heating furnace.

Next, the hardness (Vickers hardness) of martensite prepared separately (with the carbon content changed) (before tempering heat treatment) was measured according to JIS Z2244. If not subjected to tempering heat treatment, since the carbon content is considered equal to the solute carbon content C _sol, showing the relationship between solid solution carbon content C _sol the solid solution strengthening martensite hardness HV _s in FIG. From FIG. 1, the solid solution strengthened martensite hardness HV _s can be expressed as in equation (4) ′.

From the formulas (3) ′ and (4) ′, the solid solution strengthened martensite hardness HV _s can be expressed by the carbon content C _{add as} in the formula (7) ′.

When tempering heat treatment is performed under the conditions shown in Table 2, the precipitate is cementite, which is a carbide. At this time, the relationship between the precipitation strengthened martensite hardness HV _p , the volume fraction f and the size d of the carbide is expressed by the equation (8) ′. Further, the volume fraction f of the carbide can be expressed as in the formula (9) ′ by the difference between the carbon content C _add and the solute carbon amount C _sol .

In the above equation (8) ′, the coefficient 9.3 is the specific coefficient value of the Ashby-Olowan equation calculated theoretically. In addition, regarding the above formula (9) ′, the volume ratio f is obtained by multiplying the carbon concentration used for precipitation (C _prep = C _add −C _sol ) by a coefficient l. Here, in calculating the coefficient l, it is assumed that the carbide is all cementite (Fe ₃ C), and cementite has a specific gravity almost equal to that of iron. Therefore, the coefficient l is stoichiometrically as follows: Is calculated.

l = ((mass number of Fe) × 3 + (mass number of C)) / (mass number of C) × (specific gravity of Fe) / (specific gravity of Fe ₃ C) / 100
= (56 × 3 + 12) /12×1/100=0.15

  Further, an extracted replica sample was prepared, and the equivalent circle diameter of the carbide was measured by TEM to obtain FIG. As shown in FIG. 2, the relationship between the carbide size d and the tempering parameter λ is expressed as in Expression (10) ′.

From formula (8) ′, formula (9) ′ and formula (10) ′, the precipitation strengthened martensite hardness HV _p is expressed as in formula (5) ″. Further, the high strength steel after tempering heat treatment The hardness is the sum of the solid solution martensite hardness HV _s of the formula (7) ′ and the precipitation strengthened martensite hardness HV _p of the formula (5) ″, and the hardness prediction formula is the following formula ( 6) '.

  Table 2 shows a comparison between the measured value of hardness of the manufactured high-strength steel and the result of calculation prediction by the method of the present invention. As shown in Table 2, the measured value of hardness is in good agreement with the predicted value. Moreover, as shown in FIG. 3, the plot of the measured value of the hardness of the high-strength steel against the tempering parameter λ is in good agreement with the prediction formula curve. In FIG. 3, ◯ indicates the calculated hardness related to steel type A, Δ indicates the calculated hardness related to steel type B, and ◇ indicates the calculated hardness related to steel type C.

  As is apparent from the results, even if the steel material is produced by quenching-tempering having a tensile strength of 1000 MPa to 1400 MPa by using the hardness prediction method for high-strength steel according to the present invention, the hardness of the steel material. Can be predicted more easily.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

The present invention is a method for predicting the hardness of a high-strength steel sheet, and can be used to determine heat treatment conditions for obtaining a desired strength. And according to this invention, it becomes possible to determine the heat processing conditions for manufacturing a high strength steel plate, without performing a characteristic test or reducing the load of a characteristic test.

Claims (2)

A step of _obtaining a relational expression between the tempering parameter λ obtained from the thermal history, the carbon content C _add of the steel material, and the solute carbon amount C _sol ;
_Obtaining a relational expression between the solid solution carbon amount C _sol and the solid solution strengthened martensite hardness HV _s ;
And precipitation strengthening the martensite hardness HV _P, and carbon content C _{the add} of the steel, a step of determining said solid solution carbon amount C _sol, and the tempering parameter lambda, the relational expression,
Including
Wherein the solid-solution strengthening martensite hardness HV _s, the and precipitation strengthening the martensite hardness HV _P, the hardness HV of the high strength steel which is the sum of the tempering parameter λ and the carbon content C _{the add} of the steel A method for predicting the hardness of high-strength steel, calculated from the relational expression

Here, the tempering parameter λ is expressed by the following equation (1):
In the following formula (2),
_δτi is the i (i = 1, 2,..., N) -th section when dividing the tempering start time to the end time into N sections (N is an integer of 2 or more). Is the interval time at
T _i is the temperature at the interval time _δτi ,
Q is the activation energy associated with self-diffusion of iron,
R is a gas constant.

A relational expression between the tempering parameter λ obtained from the thermal history, the carbon content C _{add of the} steel material, and the solute carbon amount C _sol is represented by the following formula (3):
A relational expression between the solid solution carbon amount C _sol and the solid solution strengthened martensite hardness HV _s is represented by the following formula (4):
Wherein the precipitation strengthening the martensite hardness HV _P, and carbon content C _{the add} of the steel, and the solid solution carbon amount C _sol, the relationship between the tempering parameter lambda, expressed by the following equation (5),
And hardness HV of the high strength steel wherein the sum of the solid solution strengthening martensite hardness HV _s and the precipitation strengthening the martensite hardness HV _P, and the tempering parameter lambda, the carbon content C _{the add} of the steel The hardness prediction method for high-strength steel according to claim 1, wherein the relational expression:
Here, in the following formulas (3) to (6), t, u, v, w, x, y, and z are constants.

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