JP2021105197A - Steel - Google Patents

Abstract

To provide a steel that excels in strength and ductility and has an ultrafine structure.SOLUTION: A steel has a chemical composition consisting of, in mass%, C: 0.001-0.9%, Si: 0.01-2.5%, Mn: 0.01-6.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.001-2.5%, N: 0.010% or less, Ni: 0-10.0%, Cu: 0-1.5%, Cr: 0-5.0%, Ti: 0-0.5%, Nb: 0-0.5%, V: 0-0.5%, Mo: 0-1.5%, W: 0-0.8%, B: 0-0.005%, Co: 0-0.5%, Ca: 0-0.01%, Mg: 0-0.01%, REM: 0-0.01%, with the balance being Fe an impurities. The metallographic structure includes martensite of 90.0% or more in vol.%, the old γ grain size is 10.0 μm or less, the number fraction of cementite grains in martensite lath is 90% or more relative to the total number fraction of cementite grains in the steel, and an average minor axis length of the cementite grains in the martensite lath is 15 nm or less.SELECTED DRAWING: None

Description

The present invention relates to steel materials, and more particularly to steel materials having an ultrafine structure.

It is known that the finer the structure of steel, the better both strength and ductility. Therefore, many studies have been conducted to obtain steel having a hyperfine structure.

For example, Patent Document 1 discloses a mechanical structural component having an average particle size of 12 μm or less and a maximum particle size of 4 times or less of the average particle size of the former austenite grains. Further, Patent Document 2 discloses a spring steel and a spring having a tempered martensite structure having a former austenite crystal grain size of 1000 nm or less and an average block thickness of 200 nm or less. Further, Patent Document 3 describes a highly workable fine structure composed mainly of martensite in which the average diameter of martensite packets or the average diameter of former austenite grains before forming martensite is 5 μm or less. Granular martensite steel is disclosed.

Japanese Unexamined Patent Publication No. 2006-52459 Japanese Unexamined Patent Publication No. 2016-113671 Japanese Unexamined Patent Publication No. 2-310338

In Patent Document 1, a steel material containing a fine bainite structure and / or a fine martensite structure is used as a material, and high-frequency heating is performed at a heating rate of 400 ° C./s or higher and an ultimate temperature of 1000 ° C. or lower.

Further, in Patent Document 2, using a spring steel having a two-phase structure of equiaxed ferrite and spherical cementite as a material, _{it is heated to 3} points or more of Ac at a heating rate of 25 ° C./s or more, and reaches the heating temperature of 0 to 0. It is supposed to be rapidly cooled within 2 seconds. Further, in Patent Document 3, it is defined that the low temperature phase structure is subjected to a process of raising the temperature to exceed the transformation point while performing plastic working.

However, as a result of subsequent research and development by the present inventors, it has been found that in these conventional techniques, even if an ultrafine structure is obtained, properties such as strength and ductility may not be excellent. In the conventional technique, there is room for improvement in obtaining a steel having an ultrafine structure.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a steel material having an ultrafine structure having excellent strength and ductility.

The present invention has been made to solve the above problems, and the following steel materials are the gist of the present invention.

(1) It is a steel material
The chemical composition is mass%,
C: 0.001 to 0.9%,
Si: 0.01-2.5%,
Mn: 0.01-6.0%,
P: 0.10% or less,
S: 0.030% or less,
Al: 0.001-2.5%,
N: 0.010% or less,
Ni: 0-10.0%,
Cu: 0-1.5%,
Cr: 0-5.0%,
Ti: 0-0.5%,
Nb: 0-0.5%,
V: 0-0.5%,
Mo: 0-1.5%,
W: 0-0.8%,
B: 0 to 0.005%,
Co: 0-0.5%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
Remaining: Fe and impurities,
The metallic structure contains 90.0% or more martensite by volume and contains 90.0% or more of martensite.
The old austenite particle size is 10.0 μm or less,
The number fraction of cementite grains present in the martensite truss is 90% or more with respect to the total number fraction of cementite grains in the steel material, and
The average value of the minor axis lengths of the cementite grains present in the martensite truss is 15 nm or less.
Steel material.

(2) The chemical composition is mass%.
Ni: 0.01 to 10.0%,
Cu: 0.01-1.5%, and Cr: 0.01-5.0%,
Contains one or more selected from,
The steel material according to (1) above.

(3) The chemical composition is mass%.
Ti: 0.003 to 0.5%,
Nb: 0.003 to 0.5%,
V: 0.005-0.5%,
Mo: 0.01-1.5%,
W: 0.005-0.8%,
B: 0.0001 to 0.005%, and Co: 0.01 to 0.5%,
Contains one or more selected from,
The steel material according to (1) or (2) above.

(4) The chemical composition is mass%.
Ca: 0.0001-0.01%,
Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.01%,
Contains one or more selected from,
The steel material according to any one of (1) to (3) above.

(5) The steel material according to any one of (1) to (4) above, wherein the Ceq defined by the following formula (i) is 0.10 to 1.00.
Ceq = C + 1 / 24Si + 1 / 6Mn + 1 / 40Ni + 1 / 5Cr + 1 / 4Mo + 1 / 14V ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel material, and if it is not contained, 0 is substituted.

According to the present invention, it is possible to obtain a steel material having an ultrafine structure having an old austenite particle size of 10.0 μm or less and having excellent strength and ductility.

As a result of diligent studies on a method for obtaining a steel material having an ultrafine structure and excellent strength and ductility, the present inventors have obtained the following findings.

(A) Ultra-fine austenite grains can be obtained by ultra-rapidly heating the steel material and heating it to the austenite single-phase region at once.

(B) The degree of fineness after heating changes greatly depending on the structure of the steel material before ultra-rapid heating. As the steel material, it is necessary to use steel in which a large number of austenite nucleation sites are present in the metallographic structure. Specifically, it is desirable to use steel composed of martensite, bainite, cold-worked martensite, and cold-worked bainite as a material.

(C) The produced ultrafine austenite grains tend to grow into coarse grains due to the movement of grain boundaries in a high temperature state. Therefore, it is desirable to quickly quench after ultra-rapid heating.

(D) On the other hand, spherical cementite grains produced in the manufacturing process are precipitated at the boundary of the structure of the steel material. Therefore, when the steel material is rapidly cooled immediately after ultra-rapid heating, the cementite grains cannot be sufficiently solid-solved and remain in the steel material. The cementite grains remaining at the boundary of the structure of the steel material in this way serve as the starting point of fracture of the steel material.

(E) Therefore, after ultra-rapid heating, the cementite granules precipitated at the boundary of the tissue are sufficiently solid-solved and then rapidly cooled while being held for an appropriate time and maintaining the hyperfine structure. As a result, the residual cementite grains can be suppressed at the boundary of the tissue or the like. In the cooling step, martensite undergoes self-tempering to become tempered martensite, and cementite grains may precipitate in the tempered martensite. However, if the cementite particles precipitated at this time can be finely dispersed in the lath, it will not be the starting point of fracture. This makes it possible to improve ductility while maintaining the strength of the steel material.

Here, the tissue boundary and the like mean the boundary of the former austenite grain boundary of the martensite structure and the boundary of packets, blocks, laths and the like which are the substructures of the former austenite grain.

(F) It is also effective to lower the transformation temperature in order to prevent the growth and coarsening of the ultrafine austenite grains. Since the movement of grain boundaries is due to the diffusion of atoms, it is possible to maintain fine grains by lowering the temperature and reducing the diffusion rate.

(G) By adjusting the content of Mn and the like, it becomes possible to lower the transformation temperature of the steel material.

The present invention has been made on the basis of the above findings. Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.001 to 0.9%
C is an element that improves the strength of steel materials. The C content is selected according to the required characteristics of the steel material and needs to be 0.001% or more. On the other hand, when the C content exceeds 0.9%, an imbalance between strength and ductility or strength and toughness occurs. Therefore, the C content is set to 0.001 to 0.9%. The C content is preferably 0.010% or more. The C content is preferably less than 0.3%, more preferably 0.2% or less.

Si: 0.01-2.5%
Si is an element distributed to the ferrite phase and needs to be contained for deoxidation in order to suppress the growth coarsening of the hyperfine austenite structure. On the other hand, if the Si content exceeds 2.5%, the hot workability deteriorates and it becomes easy to crack during rolling. Therefore, the Si content is set to 0.01 to 2.5%. The Si content is preferably 0.05% or more, more preferably 0.10% or more. The Si content is preferably 2.0% or less, more preferably 1.5% or less, and even more preferably 1.0% or less.

Mn: 0.01-6.0%
Mn is effective to reduce the growth coarsening rate of the austenite phase to lower the austenite-forming temperature range by lowering the A ₁ transformation point, and Mn is an element which is distributed to the austenite phase. In addition, it is an effective element when it is desired to utilize retained austenite. In order to suppress the growth coarsening of the hyperfine austenite structure, it is necessary to contain Mn in an amount of 0.01% or more. On the other hand, when the Mn content exceeds 6.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Mn content is set to 0.01 to 6.0%. The Mn content is preferably 0.10% or more, and more preferably 0.50% or more. The Mn content is preferably 3.0% or less, more preferably 2.0% or less.

P: 0.10% or less P is an element generally contained as an impurity, but it is also an element having an effect of increasing the strength by strengthening the solid solution. Therefore, P may be positively contained. However, P is an element that is easily segregated, and when its content exceeds 0.10%, the moldability and toughness are significantly reduced due to grain boundary segregation. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. The lower limit of the P content does not need to be specified in particular, but when the above effect is desired, it is preferably 0.001% or more.

S: 0.030% or less S is an element contained as an impurity and forms sulfide-based inclusions in the steel to reduce the formability of the steel sheet. When the S content exceeds 0.030%, the moldability is significantly reduced. Therefore, the S content is 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.001% or less. The lower limit of the S content does not need to be specified, but it is preferably 0.0001% or more from the viewpoint of suppressing an increase in the refining cost.

Al: 0.001-2.5%
Al is an element distributed to the ferrite phase and needs to be contained for deoxidation in order to suppress the growth coarsening of the hyperfine austenite structure. On the other hand, if the Al content exceeds 2.5%, the hot workability deteriorates and it becomes easy to crack during rolling. Therefore, the Al content is set to 0.001 to 2.5%. The Al content is preferably 0.005% or more. The Al content is preferably 0.50% or less, more preferably 0.10% or less, and even more preferably 0.05% or less.

N: 0.010% or less N is an element contained as an impurity and has an action of lowering the moldability of the steel sheet. When the N content exceeds 0.010%, the moldability is significantly reduced. Therefore, the N content is 0.010% or less. The N content is preferably 0.0080% or less, more preferably 0.0070% or less. The lower limit of the N content does not need to be specified in particular, but considering the case where one or more of Ti, Nb and V are contained to miniaturize the steel structure as described later, precipitation of carbonitride is promoted. The N content is preferably 0.0010% or more, and more preferably 0.0020% or more.

In addition to the above elements, the steel according to the present invention is further selected from the following amounts of Ni, Cu, Cr, Ti, Nb, V, Mo, W, B, Co, Ca, Mg and REM. It may contain one or more elements.

Ni: 0 to 10.0%
Ni is by lowering the austenite formation temperature range by lowering the A ₁ transformation point is an element effective in reducing the growth coarsening rate of the austenite phase. Ni is an element that is distributed to the austenite phase. Therefore, Ni may be contained if necessary. However, when the Ni content exceeds 10.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Ni content is set to 10.0% or less. The Ni content is preferably 5.0% or less, more preferably 2.0% or less. In order to obtain the above effects, the Ni content is preferably 0.01% or more, more preferably 0.10% or more.

Cu: 0-1.5%
Cu is by lowering the austenite formation temperature range by lowering the A ₁ transformation point is an element effective in reducing the growth coarsening rate of the austenite phase. Cu is an element that is distributed to the austenite phase. Therefore, Cu may be contained if necessary. However, if the Cu content exceeds 1.5%, the workability deteriorates and it becomes easy to crack during rolling. Therefore, the Cu content is set to 1.5% or less. The Cu content is preferably less than 0.5%. In order to obtain the above effects, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more.

Cr: 0-5.0%
Cr is an element that is distributed to the austenite phase and is an effective element for suppressing the growth coarsening of the hyperfine austenite structure. Therefore, Cr may be contained if necessary. However, if the Cr content exceeds 5.0%, an imbalance between strength and ductility or strength and toughness occurs. Therefore, the Cr content is set to 5.0% or less. The Cr content is preferably 3.0% or less. In order to obtain the above effects, the Cr content is preferably 0.01% or more.

Ti: 0-0.5%
Ti is an element that is distributed to the ferrite phase and diffuses slowly, and is an effective element for suppressing the growth coarsening of the hyperfine austenite structure. Therefore, Ti may be contained if necessary. However, if the Ti content exceeds 0.5%, the steel becomes brittle. Therefore, the Ti content is set to 0.5% or less. The Ti content is preferably 0.05% or less. In order to obtain the above effects, the Ti content is preferably 0.003% or more, and more preferably 0.005% or more.

Nb: 0-0.5%
Nb is an element that is distributed to the ferrite phase and diffuses slowly, and is an effective element for suppressing the growth coarsening of the hyperfine austenite structure. Therefore, Nb may be contained if necessary. However, when the Nb content exceeds 0.5%, the steel becomes brittle. Therefore, the Nb content is set to 0.5% or less. The Nb content is preferably 0.05% or less. In order to obtain the above effects, the Nb content is preferably 0.003% or more, and more preferably 0.005% or more.

V: 0-0.5%
V is an element distributed to the ferrite phase and is an effective element for suppressing the growth coarsening of the hyperfine austenite structure. Therefore, V may be contained if necessary. However, when the V content exceeds 0.5%, the steel becomes brittle. Therefore, the V content is set to 0.5% or less. The V content is preferably 0.1% or less. In order to obtain the above effects, the V content is preferably 0.005% or more, and more preferably 0.010% or more.

Mo: 0-1.5%
Mo is an element that is distributed to the ferrite phase and diffuses slowly, and is an effective element for suppressing the growth coarsening of the hyperfine austenite structure. Therefore, Mo may be contained if necessary. However, when the Mo content exceeds 1.5%, the effect of suppressing grain growth becomes saturated. Therefore, the Mo content is set to 1.5% or less. The Mo content is preferably 1.0% or less. In order to obtain the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.

W: 0-0.8%
W is an element that is distributed to the ferrite phase and diffuses slowly, and is an element that is effective in suppressing the growth coarsening of the hyperfine austenite structure. Therefore, W may be contained if necessary. However, when the W content exceeds 0.8%, the effect of suppressing grain growth becomes saturated. Therefore, the W content is set to 0.8% or less. The W content is preferably 0.5% or less. In order to obtain the above effects, the W content is preferably 0.005% or more, and more preferably 0.010% or more.

B: 0 to 0.005%
B is an element that improves hardenability and is an effective element for obtaining a structure containing martensite. Therefore, B may be contained if necessary. However, if the B content exceeds 0.005%, the toughness deteriorates. Therefore, the B content is set to 0.005% or less. The B content is preferably 0.003% or less. In order to obtain the above effects, the B content is preferably 0.0001% or more.

Co: 0-0.5%
Co is an element distributed to the ferrite phase and is an effective element for suppressing the growth coarsening of the hyperfine austenite structure. Therefore, Co may be contained if necessary. However, when the Co content exceeds 0.5%, the effect of suppressing grain growth becomes saturated. Therefore, the Co content is set to 0.5% or less. The Co content is preferably 0.3% or less. In order to obtain the above effects, the Co content is preferably 0.01% or more, and more preferably 0.10% or more.

Ca: 0-0.01%
Mg: 0-0.01%
REM: 0-0.01%
Ca, Mg and REM have a pinning effect of suppressing the growth of austenite grains and have an effect of refining austenite grains. Therefore, one or more selected from these elements may be contained as needed. However, if the content of each of these elements exceeds 0.01%, it becomes brittle and the workability deteriorates. Therefore, the content of each element is set to 0.01% or less. Further, when two or more kinds are contained in a complex manner, the total content may be 0.03%. In order to obtain the above effects, it is preferable to contain 0.0001% or more of one or more selected from Ca, Mg and REM.

Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements.

In the chemical composition of the steel used in the production method of the present invention, the balance is Fe and impurities.

The "impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when the steel material is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means things.

Ceq: 0.10 to 1.00
In order to obtain the structure specified in the present invention, it is preferable that the carbon equivalent Ceq defined by the following formula (i) is within an appropriate range. If Ceq is less than 0.10, a sufficient martensite structure may not be obtained even after quenching. On the other hand, when Ceq exceeds 1.00, not only ductility and toughness deteriorate, but also weldability and welded portion characteristics may deteriorate when welding is performed. Therefore, Ceq is preferably 0.10 to 1.00. Ceq is more preferably 0.20 or more, and even more preferably 0.30 or more. Further, Ceq is more preferably 0.90 or less, and further preferably 0.80 or less, or 0.70 or less.
Ceq = C + 1 / 24Si + 1 / 6Mn + 1 / 40Ni + 1 / 5Cr + 1 / 4Mo + 1 / 14V ... (i)
However, each element symbol in the formula represents the content (mass%) of each element contained in the steel material, and if it is not contained, 0 is substituted.

(B) Metallic structure of steel material The metal structure of the steel material according to the present invention is mainly martensite. Specifically, it has a metal structure in which the volume fraction of martensite is 90.0% or more. If the volume fraction of martensite is less than 90.0%, the strength will be insufficient. Martensite shall also include martensite that has been self-tempered in the process of cooling from the heating temperature. The volume fraction of martensite is preferably 95.0% or more. Structures such as bainite, ferrite, pearlite, retained austenite, and tempered martensite may coexist in the steel material, but these structures are allowed if the total volume fraction is 10.0% or less.

The grain size of the old austenite of the martensite structure contained in the steel material shall be 10.0 μm or less. The mechanical properties of the steel material improve as the crystal grain size becomes finer, and in particular, when the old austenite grain size is 10.0 μm or less, the balance between strength and ductility becomes good. In addition, toughness is also improved. The particle size of the old austenite is preferably 5.0 μm or less, and more preferably 3.0 μm or less. The "former austenite particle size" here means the average particle size of the former austenite grains.

As described above, the ductility of the steel material can be improved by sufficiently dissolving the cementite grains produced in the manufacturing process of the steel material in the steel material. If a large amount of coarse cementite particles remain undissolved at the boundary of the structure or the like, not only the ductility deteriorates but also the hydrogen embrittlement resistance deteriorates.

In addition, when martensite is self-tempered, cementite grains are precipitated in the tempered martensite. Most of the cementite grains precipitated at this time can be precipitated in the martensite truss by appropriately controlling the heat treatment conditions, as will be described later. However, it is not possible to completely prevent the precipitation of cementite grains at the boundary of the tissue or the like.

As described above, a part of cementite grains is inevitably present at the boundary of the structure or the like due to undissolved residue or precipitation by self-tempering, but it is permissible if the number fraction is 10% or less. Therefore, the number fraction of cementite grains existing in the martensite truss is 90% or more with respect to the total number fraction of cementite grains in the steel material. As a result, it is possible to prevent the cementite grains from agglomerating at the boundary of the structure and improve the ductility of the steel material.

In addition, the cementite grains precipitated in the martensite truss need to be needle-shaped crystals. If the minor axis length of the cementite grains precipitated in the martensite truss is more than 15 nm, it tends to be a starting point of fracture, which deteriorates the ductility of the steel material. Therefore, the cementite grains precipitated in the martensitic truss have a minor axis length of 15 nm or less when approximated by an ellipse. The major axis length of the cementite grains precipitated in the martensite truss varies depending on the grains, and is not particularly limited.

In the present invention, the volume fraction of martensite, the particle size of the former austenite, the number fraction of cementite grains, and the minor axis length can be measured by, for example, the following methods.

First, a sample is taken so that the cross section parallel to the rolling direction and the plate thickness direction of the steel material is the observation surface. Then, the observation surface is mirror-polished, corroded with a nital corrosive solution, and then the structure is observed using a scanning electron microscope (SEM).

At a depth of 1/4 of the plate thickness of the observation surface, a range of 300 μm × 300 μm is photographed at 1000 times. The obtained microstructure photograph is subjected to black-and-white binarization and then image analysis is performed to identify pearlite, bainite and ferrite, and the "steel-crystal grain size microscopic test method" specified in JIS G 0551 (2013) standard. The total amount of those area ratios is calculated by using the method based on.

Further, the conversion from the area ratio of the structure to the volume ratio is performed by the line segment method. The line segment method is based on, for example, the method described in Robert T. DeHoff and Frederik N. Rhines co-edited (Quantitative Microscopy, 1968).

Although it is difficult to distinguish retained austenite from martensite by SEM, the volume fraction of retained austenite can be measured by X-ray diffraction. Then, the volume fraction of martensite is obtained by subtracting the total volume fraction of pearlite, bainite, ferrite and retained austenite obtained by the above method from 100%.

Then, the crystal orientation is measured and analyzed by EBSD using the above-mentioned test piece after corrosion. Thereby, the old austenite particle size is measured.

The number fraction of cementite grains is measured by SEM or transmission electron microscope (TEM). When measuring with SEM as an observation sample, the above-mentioned test piece after corrosion is used. When measuring by TEM, a thin film having a thickness of 100 nm is used, which is taken from a depth position of 1/4 of the plate thickness in a cross section parallel to the rolling direction and the plate thickness direction of the steel material.

Then, in each case, a field of view of 5 μm × 5 μm is observed at 10 places at 10000 times. The number of cementite grains present at the boundary of the tissue and the number of cementite grains present in the martensite truss are counted. Then, the average value of the number fraction of the cementite grains existing in the martensite truss is calculated.

The minor axis length of cementite grains is measured by TEM. Using the above test piece, observe 10 places of a 5 μm × 5 μm field of view at 10000 times. Then, the cementite grains are approximated by an ellipse to measure the minor axis, and the average value is taken as the minor axis length of the cementite grains.

(C) Manufacturing Method The steel material according to the present invention is manufactured by subjecting a steel material having the above-mentioned chemical composition and a predetermined metal structure to a heat treatment that immediately starts cooling after ultra-rapid heating. It is possible. Each condition will be described in detail below.

<Steel material>
The steel material before heat treatment is mainly a martensite structure, a bainite structure, or a mixed structure thereof, or a cold-worked martensite structure, a cold-worked bainite structure, or a mixed structure thereof. Use one with a metallographic structure. The reason is as follows.

In order to disperse a large number of fine austenite crystal grains during heating, it is necessary to obtain a large number of austenite nucleation sites in advance in the metal structure. Possible nucleation sites are the grain boundaries of the initial structure, the interface between precipitates such as carbides, and the crystal grains of the substrate. The martensite structure and bainite structure have substructures such as packets, blocks, and laths in the old austenite grains, and their boundaries can also be nucleation sites. Therefore, the martensite structure and the bainite structure have more austenite nucleation sites than the ferrite / pearlite structure and the like, and a fine structure can be obtained when heated.

Further, when the martensite structure and the bainite structure are cold-processed, the crystal grains become finer, so that the number of nucleation sites is further increased, and the martensite structure and the bainite structure are finely dispersed in the metal structure. Therefore, if cold-worked martensite and cold-worked bainite are used as the steel material, a finer structure can be obtained.

When martensite and bainite are heated, C dissolved in martensite is precipitated as carbide prior to transformation into austenite. Carbides, like austenite, are preferentially deposited at the boundaries of the structure within the metal structure. On the other hand, fine carbides are originally precipitated in bainite. Since the interface between the carbide and the substrate structure is also an effective nucleation site, the austenite transformation occurs after a process in which a large number of fine carbides are formed in the heating process, starting from the cold-processed martensite structure and bainite. More nucleation sites can be obtained by heating to initiate.

Here, the above-mentioned "martensite structure, bainite structure, or a mixed structure thereof, or a cold-processed martensite structure, a bainite structure, or a metal structure mainly composed of a mixed structure thereof" refers to martensite, aus. It means a metallographic structure in which the total volume ratio of foamed martensite, bainite, cold-worked martensite and cold-worked bainite is 95.0% or more. Structures such as ferrite, pearlite, retained austenite, and tempered martensite may coexist in the steel material, but these structures are allowed if the total volume fraction is 5.0% or less.

The method for producing the steel material is not particularly limited as long as the metal structure satisfies the above-mentioned regulations, and a general method may be used. Further, there is no particular limitation on the method of cold-working a steel material mainly composed of martensite structure and / or bainite structure. For example, in the case of cold rolling, the cold workability is 20% or more. It is desirable to do.

<heating process>
The steel material having the above-mentioned chemical composition and metallographic structure is first heated to a temperature range _{of Acc 3 points or more at an average temperature rise rate of 500 ° C./s or more.} A uniform structure can be obtained by heating to the austenite single-phase region of _{Ac 3 points or more.} The upper limit of the heating temperature is not particularly limited, but _{if the heating exceeds the Ac 3} point, the rate of grain growth increases and the grains grow into coarse austenite grains. Therefore, it is desirable that the heating temperature is Ac ₃ points + 50 ° C. or less.

Further, according to a preferred embodiment of the present invention, it is important to perform ultra-rapid heating so that the austenite grains produced during the transformation into austenite do not grow and coarsen. In addition, when the heating rate is slow, cementite particles are precipitated at the boundary of the structure and in the martensite truss during heating, and coarsen into a spherical shape. Therefore, the average heating rate up to the temperature range of _{3 points or more of Ac is set to 500 ° C./s or more.} The average heating rate is preferably 1000 ° C./s or higher. The upper limit of the average temperature rise rate is not particularly limited, but it is desirable that it is 20000 ° C./s or less as a practical range.

In the present invention, _three Ac points are obtained by the following method. Prepare a plurality of test pieces having the same chemical composition and metal structure, heat them to various temperatures at a predetermined heating rate, and then set the holding time within 1 s, and average 1000 ° C./s from the heating temperature to 70 ° C. Cool at the cooling rate. Then, the heating temperature applied to the test piece in which the hardness of the subsequent test piece becomes the maximum quenching hardness is defined as Ac ₃ points. Further, _{the same result can be obtained even if the three} points of Ac are obtained from the measurement of thermal expansion during heating.

<Holding process>
After heating under the above conditions, cooling is started within more than 2 s and 500 s. If _{the holding time in the temperature range of 3} points or more is 2 s or less, the cementite grains contained in the steel material cannot be sufficiently solid-solved. Therefore, coarse cementite grains remain at the boundary of the structure of the steel material, and the ductility of the steel material deteriorates. Even if the holding time is more than 2 s, the austenite grains do not become coarse immediately, and the influence on the strength of the steel material is small. However, if the holding time exceeds 500 s, grain growth of austenite occurs during holding, and fine grains cannot be obtained.

<Cooling process>
In the cooling step, _{the average cooling rate from the temperature range of 3} points or more to the temperature range of less than 600 ° C. is set to 100 ° C./s or more. When the cooling shutdown temperature is 600 ° C. or higher, the structure of bainite or the like increases, so that sufficient martensite cannot be obtained. Further, the solid-dissolved C is precipitated in martensite as cementite grains and coarsened into a spherical shape. Therefore, the cooling shutdown temperature is set to less than 600 ° C. Cooling from a temperature range of less than 600 ° C. to room temperature may be water cooling or free cooling. If the cooling rate is less than 100 ° C./s, many structures other than martensite are formed in the structure of the steel material after cooling, and the structure is not mainly martensite. Therefore, the cooling rate is set to 100 ° C./s or more.

Steel materials containing martensite and bainite are characterized by a high dislocation density. Generally, dislocations in steel disappear by heating. However, if the steel material is rapidly cooled rapidly after ultra-rapid heating, the steel material is cooled without a margin for dislocations to disappear due to heating. Therefore, it is cooled while maintaining a high dislocation density.

In the cooling process, martensite inevitably undergoes self-tempering to become tempered martensite. At that time, cementite grains may precipitate in the tempered martensite. However, in the present invention, cooling can be performed while maintaining a high dislocation density. Therefore, the cementite grains can be finely dispersed in the lath. This makes it possible to improve the ductility of the steel material.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and hot forged to obtain a steel piece having a thickness of 30 mm. The obtained slab was hot-rolled by a hot-rolling tester to obtain a hot-rolled steel sheet having a thickness of 2 mm. Then, various heat treatments were performed so that the main metal structure became the structure shown in Table 2. For test numbers 9, 20, and 21 shown in Table 2, the hot-rolled steel sheet after the above heat treatment was used as the steel material. The other hot steel sheets were further cold-rolled with a cold rolling tester to obtain a 1 mm-thick cold-rolled steel sheet, which was used as a steel material.

From the obtained steel material, test pieces having a width of 10 mm, a length of 50 mm, and a thickness of 1 mm were collected. Each of the collected test pieces was heat-treated according to the conditions shown in Table 2. The heating was carried out by energization heating, and after the power supply of the energization heating was cut off, the mixture was cooled to the temperature shown in Table 2 and then allowed to cool to room temperature. Table 2 also shows the _{three Ac points of each material.} Each test piece before and after the heat treatment was subjected to microstructure observation.

The metallographic structure of the test piece before and after the heat treatment was measured by the following method.

First, an observation sample was taken from the test piece before and after the heat treatment so that the cross section parallel to the rolling direction and the plate thickness direction became the observation surface. Then, the observation surface was mirror-polished, corroded with a nital corrosive solution, and then the structure was observed using SEM.

At a depth of 1/4 of the plate thickness of the observation surface, a range of 130 μm × 130 μm was photographed at 1000 times. The obtained microstructure photograph is subjected to black-and-white binarization processing and then image analysis is performed to identify pearlite, bainite and ferrite, and the area fraction of each is obtained based on JIS G 0551 (2013), and the area fraction of each is obtained by the linear method. Converted to each volume fraction.

The volume fraction of retained austenite was measured by an X-ray diffraction method. Then, the volume fraction of martensite was determined by subtracting the total volume fraction of pearlite, bainite, ferrite and retained austenite obtained by the above method from 100%.

The old austenite grain size was determined by measuring and analyzing the crystal orientation by EBSD at a depth of 1/4 of the plate thickness from the surface of the steel plate after being corroded with nital.

The number fraction and minor axis length of cementite grains were measured by TEM. First, in a cross section parallel to the rolling direction and the plate thickness direction of the steel plate, a thin film having a thickness of 100 nm was prepared from a depth position of 1/4 of the plate thickness, and 10 visual fields of 5 μm × 5 μm were observed at 10000 times. Then, the number of cementite grains existing at the boundary of the tissue and the number of cementite grains existing in the martensite truss were counted. Then, the average value of the number fraction of cementite grains existing in the martensite truss was calculated. In addition, each grain was approximately elliptical and the minor axis was measured, and the average value was taken as the minor axis length of the cementite grains.

Furthermore, these test pieces were subjected to a tensile test to measure their mechanical properties. The tensile test was carried out with an Instron tensile tester in accordance with the standards of ASTM Standard E8. From the above test piece, an Instron type tensile test piece (parallel part length: 30 mm, parallel part plate width: 6.0 mm) was collected so that the test direction was parallel to the rolling direction. In the energization heating device cooling device used in this example, since the soaking portion obtained from a sample having a length of about 200 mm is limited, it was decided to use an ASTM standard E8 half-size plate-shaped test piece. Then, it was judged that the one having a tensile strength (TS) of 1000 MPa or more was excellent in strength, and the one having an elongation (EL) of 15.0% or more was judged to be excellent in ductility. These results are shown in Table 3.

With reference to Tables 1 to 3, test numbers 1 to 13 produced by a method satisfying all the conditions specified in the present invention are made of steel containing bainite and martensite and have an ultrafine structure. The particle size was 10.0 μm or less. In addition, the result was excellent in mechanical properties.

On the other hand, in Test No. 14, since ferrite pearlite was used as the steel material, the old austenite grains became coarse and the strength was inferior. In Test No. 15, since the heating temperature was _{less than 3} points of Ac, the volume fraction of martensite was less than 90.0%, resulting in inferior strength. In Test No. 16, since the heating rate was less than 500 ° C./s, the old austenite grains became coarse, and coarse cementite grains were precipitated in the martensite truss, making it difficult to dissolve in solid solution, resulting in poor ductility. ..

In Test No. 17, since the holding time was 2 s or less, the cementite grains in the steel material could not be sufficiently solid-solved, resulting in inferior ductility. In Test No. 18, since the cooling rate was less than 100 ° C./s, the volume fraction of martensite was less than 90.0%, resulting in inferior strength. In Test No. 19, since the cooling shutdown temperature was 600 ° C. or higher, sufficient martensite could not be obtained, and the cementite grains became coarse, resulting in inferior strength and ductility.

In Test No. 20, since the Mn content was more than 6.0%, quenching was insufficient, the volume fraction of martensite was low, and the strength was deteriorated. In Test No. 21, since the C content was more than 0.9%, quenching was insufficient and the volume fraction of martensite was low. Moreover, since Ceq was more than 1.00, ductility deteriorated. In test number 22, since Ceq was less than 0.10, the volume fraction of martensite was low, and since ferrite pearlite was used as the steel material, the old austenite grains became coarse and the strength was inferior.

According to the present invention, it is possible to obtain a steel material having an ultrafine structure having an old austenite particle size of 10.0 μm or less and having excellent strength and ductility.

Claims (5)

It is a steel material
The chemical composition is mass%,
C: 0.001 to 0.9%,
Si: 0.01-2.5%,
Mn: 0.01-6.0%,
P: 0.10% or less,
S: 0.030% or less,
Al: 0.001-2.5%,
N: 0.010% or less,
Ni: 0-10.0%,
Cu: 0-1.5%,
Cr: 0-5.0%,
Ti: 0-0.5%,
Nb: 0-0.5%,
V: 0-0.5%,
Mo: 0-1.5%,
W: 0-0.8%,
B: 0 to 0.005%,
Co: 0-0.5%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
Remaining: Fe and impurities,
The metallic structure contains 90.0% or more martensite by volume and contains 90.0% or more of martensite.
The old austenite particle size is 10.0 μm or less,
The number fraction of cementite grains present in the martensite truss is 90% or more with respect to the total number fraction of cementite grains in the steel material, and
The average value of the minor axis lengths of the cementite grains present in the martensite truss is 15 nm or less.
Steel material. When the chemical composition is mass%,
Ni: 0.01 to 10.0%,
Cu: 0.01-1.5%, and Cr: 0.01-5.0%,
Contains one or more selected from,
The steel material according to claim 1. When the chemical composition is mass%,
Ti: 0.003 to 0.5%,
Nb: 0.003 to 0.5%,
V: 0.005-0.5%,
Mo: 0.01-1.5%,
W: 0.005-0.8%,
B: 0.0001 to 0.005%, and Co: 0.01 to 0.5%,
Contains one or more selected from,
The steel material according to claim 1 or 2. When the chemical composition is mass%,
Ca: 0.0001-0.01%,
Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.01%,
Contains one or more selected from,
The steel material according to any one of claims 1 to 3. The steel material according to any one of claims 1 to 4, wherein the Ceq defined by the following formula (i) is 0.10 to 1.00.
Ceq = C + 1 / 24Si + 1 / 6Mn + 1 / 40Ni + 1 / 5Cr + 1 / 4Mo + 1 / 14V ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel material, and if it is not contained, 0 is substituted.