JP2011058045A - Dispersed-particle hardening steel and method for producing the same - Google Patents

Abstract

Disclosed is a particle dispersion strengthened steel capable of making maximum use of the strengthening ability of precipitates and a method for producing the same.
SOLUTION: The present invention has a precipitate having a NaCl-type crystal structure that satisfies the orientation relationship of bcc iron or bct iron and Baker-Nutting as a parent phase, and the outer diameter d and thickness t of the precipitate are expressed by the following equations: Particle dispersion strengthened steel characterized by satisfying; a ₀ ≤ d <b / e, a ₀ ≤ t ≤ 2a ₀ , where a ₀ : lattice constant of precipitates, b: Burgers of bcc iron or bct iron vector, e: a lattice misfit between the precipitate and a bcc iron or bct iron, with a lattice constant a ₁ of bcc iron or bct iron expressed by ^{_{(a 0/2 1/2 -a 1}} ) / a 1 .
[Selection] Figure 2

Description

  The present invention relates to a particle dispersion strengthened steel that makes the most of particle dispersion strengthening and a method for producing the same.

  In general, precipitation strengthening and particle dispersion strengthening are known as methods for strengthening metallic materials using precipitates.

  Precipitation strengthening is a strengthening method that uses the strain existing between the precipitate and the matrix, and the greater the size of the precipitate, the greater the strain that occurs, resulting in higher strength. However, since the strain relaxes when the size of the precipitate exceeds a certain limit, in the case of the precipitation strengthening method, the size of the precipitate that is most effective for strengthening exists. As an example mainly using this precipitation strengthening, Patent Document 1 describes the ratio of the lattice constant of the parent phase of the metal material to the lattice constant of the second phase that matches and precipitates in the metal material (the lattice mismatch ratio, that is, the lattice constant). A curve diagram showing the relationship between (misfit) and the number of crystal lattices of the individual precipitates in which the second phase precipitated in the metal material can exist stably is created. A method of strengthening a metal material for designing the size of the second phase is disclosed.

  On the other hand, particle dispersion strengthening is a strengthening method using precipitates as strong obstacles to dislocation motion, and the smaller the size of the precipitates, the higher the strength. As an example mainly using this particle dispersion strengthening, Patent Document 2 describes that any one of Ti, Nb, V, W, Mo, and Cr in a high-strength steel sheet mainly composed of a ferrite structure having a tensile strength of 500 MPa or more. One or two or more types of carbide forming elements are contained in a total amount of 0.7 to 1.3 times the C content, and the carbide forming elements are 20 to 80% of the steel in mass ratio to the total amount. There is disclosed a high-strength steel sheet excellent in stretch flangeability that is solid-solved therein and the remainder exists as a carbide having a particle size of 3 nm or less. Patent Document 3 discloses a method for designing a precipitation-strengthening-type high-strength steel sheet obtained by precipitating carbide in a steel structure, and has an electronegativity of less than 1.8 and a MC-type carbide as a metal element constituting the carbide. 1 type or 2 or more types of first metal element M1 and 1 type or 2 or more types of second metal element M2 having an electronegativity of 1.8 or more, the first metal element M1 and the above The first step of selecting in combination such that the difference in atomic radius with the second metal element M2 is less than 10%, and a carbide containing the first metal element M1 and the second metal element M2 is generated. Thus, a design method of a precipitation strengthening type high strength steel sheet has been proposed, which has a second step of determining the amount of addition of the first metal element M1, the second metal element M2, and C.

  However, precipitation strengthening and particle dispersion strengthening are not necessarily clearly separated, and these two strengthening methods are used more or less in the strengthening method using precipitates. In general, it is said that the precipitation strengthening is dominant when the size of the precipitate is less than several tens of nm, and the particle dispersion strengthening is dominant when the size is larger. Precipitation strengthening is strengthening due to the resistance of the host phase to strain dislocation movement caused by the precipitate, and the precipitate itself has no effect of stopping the dislocation, and the precipitate is cut by the dislocation. On the other hand, grain dispersion strengthening has the effect of stopping dislocations in the precipitates themselves, and dislocations cannot move by cutting the precipitates (Orowan mechanism). FIG. 1 shows a general relationship between the size of precipitates and the amount of strengthening in precipitation strengthening and particle dispersion strengthening. The amount of strengthening becomes maximum when the size of the precipitates is some. Below this size, precipitates are cut by dislocations and the strengthening mechanism transitions to precipitation strengthening mechanisms, but above this size, precipitates are not cut by dislocations and are strengthened by particle dispersion strengthening mechanisms. Further, the amount of strengthening depends not only on the size of the precipitates but also on the amount of precipitates. Therefore, the amount of strengthening can be easily increased by adding a large amount of precipitation elements.

JP-A-6-65628 JP 2007-262487 JP 2005-120430 A

Y.Funakawa et al: ISIJ Int., Vol.44 (2004), No.11, 1945

  However, the amount of strengthening becomes maximum when the precipitate has a certain size as described above, but it is not easy to determine the size of the precipitate at that time. The method described in Patent Document 1 is intended to make maximum use of precipitation strengthening, and it is not obvious whether the maximum effect can be obtained including the enhancement of particle dispersion. In the steel described in Patent Document 2, the upper limit of the size of the precipitate is determined so as to satisfy a predetermined strength, and the particle dispersion strengthening is not necessarily utilized to the maximum extent. Similarly, the steel described in Patent Document 3 does not make maximum use of particle dispersion strengthening by precisely controlling the size of precipitates. Therefore, in such a conventional technique, the amount of strengthening using precipitates cannot always be maximized.

  An object of this invention is to provide the particle dispersion strengthened steel which can utilize the strengthening ability of a precipitate to the maximum, and its manufacturing method.

  As a result of intensive studies on the relationship between the type and size of the precipitates and the strength of the steel material, the present inventors have found that a precipitate having a NaCl-type crystal structure is a Baker- In the case of precipitation with a Nutting orientation relationship, it has been found that even precipitates of a size that would conventionally contribute to strengthening as precipitation strengthening contribute to strengthening as particle dispersion strengthening. In other words, when the size of the precipitate is reduced in Fig. 1, in the case of the precipitate having the NaCl type crystal structure, the strengthening mechanism does not transition from the particle dispersion strengthening to the precipitation strengthening, and the particle dispersion strengthening is effectively exhibited. It was found that the amount of strengthening of the steel material using the precipitate can be maximized by controlling the size of the precipitate based on the lattice constant of the precipitate.

  Here, the orientation relationship of Baker-Nutting is, as shown in FIG. 2, when the parent phase is α of bcc and the precipitate is MX (M is an alloy element, X is carbon or nitrogen or both) 001] α // [110] MX, {200} α // {002} MX. In addition, when the precipitate satisfies the above orientation relation with the matrix, the outer diameter d of the precipitate is [[011] direction of the matrix when observed from the [011] direction with respect to the matrix by a transmission electron microscope (TEM) [ 011] is defined as the length of the precipitate, and the thickness t of the precipitate is defined as the length of the precipitate in the [100] direction of the parent phase when observed from the [001] direction with respect to the parent phase. Is done.

The present invention has been made on the basis of such knowledge, and has a precipitate having a NaCl-type crystal structure that satisfies the orientation relationship of Baker-Nutting with bcc iron or bct iron of the parent phase, Provided is a particle dispersion strengthened steel characterized in that an outer diameter d and a thickness t satisfy the following formula.
a ₀ ≤ d <b / e
a ₀ ≤ t ≤ 2a ₀
here,
a ₀ : lattice constant of the precipitate,
b: Burgers vector of bcc iron or bct iron,
e: a lattice misfit between the precipitate and a bcc iron or bct iron, with a lattice constant a ₁ of bcc iron or bct iron expressed by ^{_{(a 0/2 1/2 -a 1}} ) / a 1.

  Examples of precipitates in the particle dispersion strengthened steel of the present invention include TiC, (Ti, Mo) C, and (Ti, V) C.

The particle dispersion strengthened steel of the present invention is, for example, in mass%, C: 0.015-0.025%, Si: 0.3% or less, Mn: 0.8-1.5%, P: 0.03% or less, S: 0.003% or less, Ti: 0.02 Steel slab containing ~ 0.04%, the balance consisting of Fe and inevitable impurities, heated to 1000 ° C or higher, after hot rolling at the Ar ₃ transformation point or higher, cooled at a cooling rate of 300 ° C / sec or higher, It can be manufactured by a winding method at a winding temperature of 590 to 630 ° C.

  In the method for producing a particle dispersion strengthened steel of the present invention, a steel slab containing at least one of Mo: 0.02 to 0.05% and V: 0.02 to 0.05% by mass% can be used.

  According to the present invention, it is possible to produce a particle dispersion strengthened steel that can utilize the strengthening ability of precipitates to the maximum. The particle dispersion strengthened steel of the present invention is superior in workability compared to work strengthening, which is a strengthening method that increases dislocation density because it uses particle dispersion strengthening, and is used to increase the strength of thin steel plates and thick steel plates. it can. In addition, since the strengthening ability of the precipitate can be utilized to the maximum, it is possible to further increase the strength if the amount of additive elements is the same as in the conventional case, and to further reduce the amount of alloy if the strength level is the same.

It is a figure which shows the relationship between the magnitude | size of a precipitate, and the amount of reinforcement | strengthening. It is a figure which shows the orientation relationship of Baker-Nutting between the precipitate of outer diameter d and thickness t, and bcc iron.

  The point of the present invention is that the type and size (outer diameter and thickness) of the precipitates are controlled to maximize the use of particle dispersion strengthening, and the strength of the steel material using the precipitates is increased. is there. The details will be described below.

1) Types of precipitates: In the NaCl-type strengthening mechanism that satisfies the orientation relationship between iron and Baker-Nutting, the effect of particle dispersion strengthening or precipitation strengthening generally depends on the type and size of the precipitates. As a result of our study, when the precipitate is a carbide, nitride or carbonitride having a NaCl-type crystal structure, the precipitate is not cut by dislocation regardless of its constituent elements, and acts as a particle dispersion strengthening factor. It was. Particle dispersion strengthening, in which the strength increases as the precipitates become smaller, can reduce the size of the precipitates compared to the precipitation strengthening that has the optimum size, so it is possible to increase the number density of precipitates without increasing the amount of alloy elements. High strength can be achieved. The reason why precipitates with a NaCl-type crystal structure are not cut by dislocations is not necessarily clear, but the hardness of NaCl-type precipitates such as TiC deposited in steel is sufficiently hard compared to the parent phase iron. This is probably one of the reasons.

  In addition, the lattice misfit between precipitates with NaCl-type crystal structure and bcc iron or bct iron is generally less than 10%, and precipitates with NaCl-type crystal structure are consistent with bcc iron or bct iron. Because of its good properties, it tends to precipitate due to the Baker-Nutting orientation relationship. When in such an azimuth relationship, the size of the precipitate can be set within the following range by an appropriate heat treatment described later.

  A typical example of such a precipitate having a NaCl-type crystal structure is Ti carbide such as TiC. Further, composite carbide (Ti, Mo) C or (Ti, V) C composed of Ti and Mo or V described in Patent Document 3 can be mentioned.

2) Outer diameter d of precipitate: a ₀ ≤ d <b / e
When the outer diameter d of the precipitate is less than the lattice constant a _{0 of the} precipitate phase, the precipitate cannot form a complete NaCl-type structure and is cut by dislocation, and does not function as a particle dispersion strengthening factor. When the outer diameter d of the precipitate is greater than or equal to b / e, the distortion of the matrix phase due to precipitation increases and the precipitation strengthening amount increases, but the particle dispersion strengthening amount decreases. When the precipitate has an NaCl type structure, the effect of precipitation strengthening is smaller than that of particle dispersion strengthening, so the outer diameter d of the precipitate is set to be less than b / e from the viewpoint of utilizing particle dispersion strengthening.

3) Precipitate thickness t: a ₀ ≤ t ≤ 2a ₀
When the thickness t of the precipitate is less than a _{0 of the} precipitate phase, the precipitate cannot form a complete NaCl-type structure and is cut by dislocation, and does not function as a particle dispersion strengthening factor. When the thickness t of the precipitate becomes 2a ₀ or more, the number density of the precipitate is remarkably reduced, and the particle dispersion strengthening cannot be utilized to the maximum.

Here, according to Non-Patent Document 1, the lattice constant a ₀ of TiC and (Ti, Mo) C is 0.433 nm. The lattice constant a ₀ of (Ti, V) C was 0.423 nm as a result of X-ray diffraction. On the other hand, since the lattice constant a _{1 of} iron is 0.287 nm, in the case of TiC and (Ti, Mo) C, the lattice misfit is e = (0.433 / 2 ^1/2 -0.287) /0.287=0.067, Since the Burgers vector b of iron is 0.25 nm, b / e is 3.7 nm. Similarly, in the case of (Ti, V) C, b / e is 6.0 nm.

4) Example of manufacturing method As described above, in order to make the most of particle dispersion strengthening, precipitates such as TiC, (Ti, Mo) C, (Ti, V) C having NaCl-type crystal structure are used. It is necessary to form and to control the outer diameter and thickness appropriately.

For example, in mass%, C: 0.015-0.025%, Si: 0.3% or less, Mn: 0.8-1.5%, P: 0.03% or less, S: 0.003%, Ti: 0.02-0.04%, further necessary Accordingly, a steel slab containing at least one of Mo: 0.02 to 0.05% and V: 0.02 to 0.05%, with the balance being Fe and inevitable impurities, is heated to 1000 ° C or higher and heated above the Ar ₃ transformation point. After the hot rolling is completed, the steel sheet is cooled at a cooling rate of 300 ° C./sec or more and wound at a winding temperature of 590 to 630 ° C. to obtain a hot rolled steel sheet.

  Before hot rolling, it is necessary to heat the slab to 1000 ° C. or higher so that the elements constituting the precipitate are sufficiently dissolved. In addition, in order to precipitate TiC, (Ti, Mo) C, (Ti, V) C, etc. with a NaCl-type crystal structure and control their outer diameter and thickness appropriately, it is synchronized with the transformation from austenite to ferrite. It is optimal to use the phase interface precipitation phenomenon that occurs as a result. In order to take advantage of this phase interface precipitation phenomenon, after hot rolling, cooling is performed at a cooling rate of 300 ° C / sec or more to prevent ferrite transformation and precipitation during cooling, and winding at a winding temperature of 630 ° C or less. Therefore, it is necessary to synchronize the ferrite transformation and the precipitation of precipitates. However, if the coiling temperature is less than 590 ° C, cementite precipitates and precipitates having a NaCl-type crystal structure are not generated, so the lower limit of the coiling temperature is 590 ° C.

  Steels Nos. A to D having the chemical compositions shown in Table 1 were melted, and these steels were heated at 1250 ° C. and then hot-rolled at a finishing temperature of 950 ° C. Then, it cooled at the cooling rate shown in Table 2, and wound up on the conditions of the temperature and time shown in Table 2, and produced hot-rolled steel plates No. 1-9. From these steel sheets, JIS No. 5 tensile test specimens were collected in the vertical direction of rolling, and the tensile strength was determined by a method based on JIS Z 2241. Regarding the amount of particle dispersion strengthening, the difference from the tensile strength of the base steel plate No. 1 containing no precipitated elements was used as the amount of strengthening. In addition, the average outer diameter d and average thickness t of the precipitates in the sample were measured using a TEM, the outer diameter of the precipitates was observed in the [011] direction, and the thickness was 250,000 times as observed from the [001] direction of the parent phase. Obtained from the statue. The same operation was performed for a total of 20 or more precipitates with 3 or more fields of view, and the arithmetic average value was calculated. Furthermore, the structure was observed using a TEM, and it was observed whether or not the precipitate was a row-like precipitation that is characteristic of phase interface precipitation.

  The tensile strength of the base steel plate No. 1 is 293 MPa. In steel sheet Nos. 2, 5, 7, and 9 within the scope of the present invention where the coiling temperature is 590 ° C. to 630 ° C., according to TEM observation, precipitates are finely deposited at the phase interface in the steel sheet. High strength is obtained by dispersion strengthening. When the winding temperature exceeds 630 ° C. as in the case of steel plates No. 3, 6, and 8, the outer diameter and thickness of the precipitate are coarsened and the strength is lowered. In the case of steel sheet No. 4 with a cooling rate of less than 300 ° C / sec until the winding process, precipitation starts during cooling, and the precipitates become coarse, or some of them cause pearlite transformation and cementite precipitates. The amount of reinforcement decreases. Further, comparing Nos. 7 to 9, it can be seen that even when the size of the precipitate is 3 nm or less, the particle dispersion strengthening amount is increased and the strength is further increased as the size of the precipitate is reduced.

Claims (4)

It has a precipitate having a NaCl-type crystal structure that satisfies the orientation relationship of bcc iron or bct iron and Baker-Nutting in the parent phase, and that the outer diameter d and thickness t of the precipitate satisfy the following formula: Characterized particle dispersion strengthened steel;
a ₀ ≤ d <b / e
a ₀ ≤ t ≤ 2a ₀
here,
a ₀ : lattice constant of the precipitate,
b: Burgers vector of bcc iron or bct iron,
e: a lattice misfit between the precipitate and a bcc iron or bct iron, with a lattice constant a ₁ of bcc iron or bct iron expressed by ^{_{(a 0/2 1/2 -a 1}} ) / a 1.   2. The particle dispersion strengthened steel according to claim 1, wherein the precipitate is any one of TiC, (Ti, Mo) C, and (Ti, V) C. In mass%, C: 0.015-0.025%, Si: 0.3% or less, Mn: 0.8-1.5%, P: 0.03% or less, S: 0.003% or less, Ti: 0.02-0.04%, the balance being Fe and inevitable A steel slab made of natural impurities is heated to 1000 ° C or higher, cooled at a cooling rate of 300 ° C / sec or higher after the hot rolling at the Ar ₃ transformation point or higher, and wound at a winding temperature of 590 to 630 ° C. A method for producing a particle dispersion strengthened steel.   4. The method for producing a particle dispersion strengthened steel according to claim 3, wherein a steel slab containing at least one of Mo: 0.02 to 0.05% and V: 0.02 to 0.05% in mass% is used.