JP2020012173A - Steel material - Google Patents

Abstract

To provide a steel material having ultra-fine structure excellent in strength, ductility and toughness.SOLUTION: There is provided a steel material having a chemical composition of, by mass%, C:0.001 to 0.9%, Si:0.01 to 2.5%, Mn:0.01 to 6.0%, P:0.10% or less, S:0.030% or less, Al:0.001 to 2.5%, N:0.010% or less, Ni:0 to 10.0%, Cu:0 to 1.5%, Cr:0 to 5.0%, Ti:0 to 0.5%, Nb:0 to 0.5%, V:0 to 0.5%, Mo:0 to 1.5%, W:0 to 0.8%, B:0 to 0.005%, Co:0 to 0.5%, Ca:0 to 0.01%, Mg:0 to 0.01%, REM:0 to 0.01%, and the balance:Fe with impurities, and Ceq of 0.10 to 1.00, a metallic structure containing, by vol%, 95.0% or more of martensite, old austenite grain diameter of 5.0 μm or less and the number of packets in the old austenite grain of 3.0 or less.SELECTED DRAWING: None

Description

  The present invention relates to a steel material, and particularly to a steel material having an ultrafine structure.

  It is known that as the steel structure becomes finer, all of the strength, ductility and toughness are improved. Therefore, many studies have been made to obtain a steel having an ultrafine structure.

  For example, Patent Literature 1 discloses a mechanical structural component in which the prior austenite grains have an average grain size of 12 μm or less and the maximum grain size is four times or less the average grain size. Further, Patent Document 2 discloses a finely-processable fine structure composed mainly of martensite in which the average diameter of a packet of martensite or the average diameter of old austenite grains before forming martensite is 5 μm or less. A grain martensitic steel is disclosed. Further, Patent Document 3 discloses a Cu-containing steel containing Cu in an amount of 0.5 to 5% by mass and having an austenite structure of recrystallized grains having an average particle size of 0.5 to 5 μm.

JP 2006-52459 A JP-A-2-310338 JP 2006-9098 A

  In Patent Literature 1, high-frequency heating at a heating rate of 400 ° C./s or more and a reaching temperature of 1000 ° C. or less is performed using a steel material containing a fine bainite structure and / or a fine martensite structure as a raw material.

In addition, in Patent Document 2, a process of increasing the temperature to exceed the transformation point while performing plastic working on the low-temperature phase structure is performed. Further, in Patent Document 3, high-speed cumulative deformation with a cumulative strain of 2 or more and a strain rate of 1 / sec or more in a temperature range from the Ar ₃ transformation point to 980 ° C. for steel containing 0.5 to 5% by mass of Cu. Processing is to be performed.

  However, as a result of the subsequent research and development of the present inventors, it has been found that, even if an ultrafine structure is obtained, properties such as strength, ductility, and toughness may not be excellent in these conventional techniques. In the prior art, 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 has as its object to provide a steel material having an ultrafine structure having excellent strength, ductility, and toughness.

  The present invention has been made to solve the above-mentioned problems, and has the following steel materials.

(1) The chemical composition is expressed in mass%
C: 0.001 to 0.9%,
Si: 0.01 to 2.5%,
Mn: 0.01 to 6.0%,
P: 0.10% or less,
S: 0.030% or less,
Al: 0.001 to 2.5%,
N: 0.010% or less,
Ni: 0 to 10.0%,
Cu: 0 to 1.5%,
Cr: 0 to 5.0%,
Ti: 0 to 0.5%,
Nb: 0 to 0.5%,
V: 0 to 0.5%,
Mo: 0 to 1.5%,
W: 0 to 0.8%,
B: 0 to 0.005%,
Co: 0 to 0.5%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0-0.01%,
The balance: Fe and impurities,
Ceq defined by the following formula (i) is 0.10 to 1.00,
The metal structure comprises, by volume%, 95.0% or more of martensite;
The prior austenite particle size is 5.0 μm or less, and
The number of packets in the old austenite grains is 3.0 or less,
Steel.
Ceq = C + 1 / 24Si + ／ Mn + ／ 40Ni + ／ Cr + ／ Mo + ／ 14V (i)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel.

(2) The chemical composition is represented by mass%
Ni: 0.01 to 10.0%,
Cu: 0.01 to 1.5%, and
Cr: 0.01 to 5.0%,
Containing at least one member selected from the group consisting of:
The steel material according to the above (1).

(3) the chemical composition is expressed in mass%;
Ti: 0.003 to 0.5%,
Nb: 0.003 to 0.5%,
V: 0.005 to 0.5%,
Mo: 0.01-1.5%,
W: 0.005 to 0.8%,
B: 0.0001 to 0.005%, and
Co: 0.01-0.5%,
Containing at least one member selected from the group consisting of:
The steel material according to the above (1) or (2).

(4) The chemical composition is represented by mass%,
Ca: 0.0001-0.01%,
Mg: 0.0001-0.01%, and REM: 0.0001-0.01%,
Containing at least one member selected from the group consisting of:
The steel material according to any one of the above (1) to (3).

  According to the present invention, it is possible to obtain a steel material having an ultrafine structure having an average crystal grain size of 5.0 μm or less and having excellent strength, ductility, and toughness.

  The present inventors have conducted intensive studies on a method for producing a steel material having an ultrafine structure and having excellent strength, ductility, and toughness, and as a result, have obtained the following knowledge.

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

  (B) The degree of fineness after heating changes greatly depending on the difference in the structure of the steel material before ultra-rapid heating. As the steel material, it is necessary to use steel in which a number of austenite nucleation sites exist in the metal structure. Specifically, steel made of processed martensite (cold-rolled martensite) is used as a material.

  (C) The generated ultrafine austenite grains tend to grow into coarse grains due to the movement of grain boundaries in a high temperature state. Therefore, immediately after the ultra-rapid heating, the material is rapidly cooled to a room temperature while maintaining the ultrafine structure.

  (D) By using martensite processed into a steel material, it becomes possible to reduce the number of packets in old austenite grains. When the austenite grain size is reduced, the yield stress increases, the yield ratio, which is the ratio to the tensile strength, increases, and the elongation tends to decrease.However, by reducing the number of packets, the yield stress decreases, and the elongation decreases. Can be improved.

  (E) 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 the grain boundary is caused by the diffusion of atoms, if the temperature is lowered and the diffusion speed is reduced, fine grains can be maintained.

  (F) By adjusting the content of Mn or the like, the transformation temperature of the steel material can be lowered.

  The present invention has been made based on 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 “% by mass”.

C: 0.001 to 0.9%
C is an element that improves the strength of the steel material. The C content is selected according to the required properties 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 0.01% or more. Further, the C content is preferably less than 0.3%, more preferably 0.2% or less.

Si: 0.01 to 2.5%
Si is an element distributed to the ferrite phase, and must be contained for deoxidation in order to suppress the coarse growth of the ultrafine austenite structure. On the other hand, when the Si content exceeds 2.5%, the hot workability is deteriorated, and the steel tends to crack during rolling. Therefore, the Si content is set to 0.01 to 2.5%. The Si content is preferably at least 0.05%, more preferably at least 0.10%. Further, the Si content is preferably 2.0% or less, more preferably 1.5% or less, and further preferably 1.0% or less.

Mn: 0.01-6.0%
Mn is effective to slow 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 use retained austenite. In order to suppress the growth coarseness of the ultrafine austenite structure, Mn must be contained at 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 at least 0.10%, more preferably at least 0.50%. Further, the Mn content is preferably at most 3.0%, more preferably at most 2.0%.

P: 0.10% or less P is an element generally contained as an impurity, but is also an element having an effect of increasing strength by solid solution strengthening. Therefore, P may be positively contained. However, P is an element that easily segregates, and if its content exceeds 0.10%, the reduction in formability and toughness due to grain boundary segregation becomes significant. 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 still more preferably 0.020% or less. The lower limit of the P content need not be particularly defined, but is preferably 0.001% or more when the above effects are desired.

S: 0.030% or less S is an element contained as an impurity, and forms sulfide-based inclusions in steel to lower the formability of the steel sheet. If the S content exceeds 0.030%, the moldability is significantly reduced. Therefore, the S content is set to 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 is not particularly limited, but is preferably 0.0001% or more from the viewpoint of suppressing an increase in refining cost.

Al: 0.001 to 2.5%
Al is an element distributed to the ferrite phase, and it is necessary to contain Al for deoxidation in order to suppress the coarse growth of the ultrafine austenite structure. On the other hand, if the Al content exceeds 2.5%, the hot workability is deteriorated, and cracking is likely to occur during rolling. Therefore, the Al content is set to 0.001 to 2.5%. The Al content is preferably at least 0.005%. Further, the Al content is preferably at most 0.50%, more preferably at most 0.10%, even more preferably at most 0.05%.

N: 0.010% or less N is an element contained as an impurity and has an effect of reducing the formability of a steel sheet. When the N content exceeds 0.010%, the moldability is significantly reduced. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.0080% or less, and more preferably 0.0070% or less. The lower limit of the N content does not need to be particularly defined. However, considering the case where one or more of Ti, Nb and V are contained as described below to achieve a finer steel structure, the precipitation of carbonitride is promoted. For this purpose, the N content is preferably at least 0.0010%, more preferably at least 0.0020%.

  In the steel subjected to the production method of the present invention, in addition to the above elements, the following amounts of Ni, Cu, Cr, Ti, Nb, V, Mo, W, B, Co, Ca, Mg, and One or more elements selected from REM may be contained.

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 distributed to the austenite phase. Therefore, Ni may be contained as 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 at most 5.0%, more preferably at most 2.0%. In order to obtain the above effects, the Ni content is preferably at least 0.01%, more preferably at least 0.1%.

Cu: 0 to 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 distributed to the austenite phase. Therefore, Cu may be contained as necessary. However, when the Cu content exceeds 1.5%, the workability is deteriorated and the material tends to crack during rolling. Therefore, the Cu content is set to 1.5% or less. Preferably, the Cu content is less than 0.5%. In order to obtain the above effects, the Cu content is preferably at least 0.01%, more preferably at least 0.1%.

Cr: 0 to 5.0%
Cr is an element distributed to the austenite phase, and is an effective element for suppressing the coarse growth of the ultrafine austenite structure. Therefore, Cr may be contained as 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 at least 0.01%.

Ti: 0 to 0.5%
Ti is an element that is distributed to the ferrite phase and has a slow diffusion, and is an effective element for suppressing the coarse growth of the ultrafine austenite structure. Therefore, you may make it contain Ti as needed. 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 at least 0.003%, more preferably at least 0.005%.

Nb: 0 to 0.5%
Nb is an element that is distributed to the ferrite phase and has a slow diffusion, and is an effective element for suppressing the coarse growth of the ultrafine austenite structure. Therefore, Nb may be contained as needed. However, if 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 at most 0.05%. 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 to 0.5%
V is an element distributed to the ferrite phase, and is an effective element for suppressing the coarse growth of the ultrafine austenite structure. Therefore, V may be contained as necessary. However, if the V content exceeds 0.5%, the steel becomes brittle. Therefore, the V content is set to 0.5% or less. 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 0.01% or more.

Mo: 0 to 1.5%
Mo is an element which is distributed to the ferrite phase and has a slow diffusion, and is an effective element for suppressing the coarse growth of the ultrafine austenite structure. Therefore, Mo may be contained as 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 at least 0.01%, more preferably at least 0.03%.

W: 0-0.8%
W is an element that is distributed to the ferrite phase and has a slow diffusion, and is an effective element for suppressing the coarse growth of the ultrafine austenite structure. Therefore, W may be contained as 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, more preferably 0.01% 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 as necessary. However, if the B content exceeds 0.005%, 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 coarse growth of the ultrafine austenite structure. Therefore, you may make it contain Co as needed. However, if 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 at least 0.01%, more preferably at least 0.1%.

Ca: 0 to 0.01%
Mg: 0 to 0.01%
REM: 0-0.01%
Ca, Mg, and REM have a pinning effect of suppressing austenite grain growth and have an effect of making austenite grains fine. Therefore, if necessary, one or more selected from these elements may be contained. However, when the content of each of these elements exceeds 0.01%, embrittlement occurs and workability deteriorates. Therefore, the content of each element is set to 0.01% or less. In the case where two or more kinds are combined, the total content may be 0.03%. In order to obtain the above effects, it is preferable that at least one selected from Ca, Mg, and REM be contained in an amount of 0.0001% or more.

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

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

  Note that “impurities” are components that are mixed due to various factors in the ore, scrap, and other raw materials and the manufacturing process when steel products are industrially manufactured, and are acceptable as long as they do not adversely affect the present invention. Means things.

Ceq: 0.10-1.00
Ceq means carbon equivalent and is defined by the following formula (i). When Ceq is less than 0.10, a martensite structure cannot be obtained even when quenching is performed. On the other hand, when Ceq exceeds 1.00, not only the toughness and ductility are deteriorated, but also when welding is performed, the weldability and the properties of the welded portion are deteriorated. Therefore, Ceq is set to 0.10 to 1.00. Ceq is preferably 0.20 or more, and more preferably 0.30 or more.
Ceq = C + 1 / 24Si + ／ Mn + ／ 40Ni + ／ Cr + ／ Mo + ／ 14V (i)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel.

(B) Metal Structure of Steel Material The metal structure of the steel material according to the present invention is mainly composed of martensite. Specifically, it has a metal structure in which the volume fraction of martensite is 95.0% or more. In the steel material, structures such as bainite, ferrite, pearlite, retained austenite, and tempered martensite may be mixed, but these structures are permissible if the total volume ratio is 5.0% or less.

  If the volume fraction of martensite is less than 95.0%, the strength becomes insufficient. In particular, when a ferrite structure is mixed, fine ferrite is generated, austenite grain boundaries disappear, and fine austenite grains cannot be obtained.

  Further, the prior austenite grain size of the martensite structure contained in the steel material is 5.0 μm or less. The mechanical properties of the steel material improve with the refinement of the crystal grain size. In particular, when the prior austenite grain size is 5.0 μm or less, the balance between strength, ductility and toughness is improved. Here, the “old austenite grain size” means the average grain size of the old austenite grains.

  As described above, when the structure is made ultrafine, the yield stress tends to increase, but an extreme increase in the yield ratio is not preferable. However, it is possible to reduce the yield stress by reducing the number of packets in the prior austenite grains. Therefore, the number of packets in the old austenite grains is set to 3.0 or less.

  The volume fraction of martensite, the prior austenite grain size, the number of packets in the prior austenite grains and the packet diameter are measured by the following methods.

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

  An image of 300 μm × 300 μm is taken at a magnification of 1000 times at a depth of 1/4 of the thickness of the observation surface. The obtained microstructure photograph was subjected to a black-and-white binarization process and then subjected to image analysis to specify pearlite, bainite and ferrite, and to determine the “microstructure test method of steel-grain size” specified in JIS G 0551 (2013) standard. Is used to determine the total amount of those area ratios.

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

  Although it is difficult for SEM to distinguish retained austenite from martensite, the volume fraction of retained austenite can be measured by X-ray diffraction. Then, the volume ratio of martensite is determined by subtracting the total volume ratio 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 corroded test piece. Thereby, the old austenite grain size, the number of packets in the old austenite grains, and the packet diameter are measured. Here, the packet is a group of laths having a specific arrangement in the martensite organization, and the inside of the packet is a group of parallel laths.

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

<Steel material>
As a steel material before heat treatment, a steel material having a metal structure mainly composed of cold-worked martensite is used. 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 the metal structure in advance. The nucleation sites can be at the grain boundaries of the initial structure, at the interface between the precipitates such as carbides and the base crystal grains. The martensite structure has substructures such as packets, blocks, and laths in the prior austenite grains, and their boundaries can also be nucleation sites. Therefore, the martensite structure has more austenite nucleation sites than the ferrite / pearlite structure and the like.

  Further, when cold working is performed on the martensite structure, the crystal grains become finer, so that the number of nucleation sites further increases, and the martensite structure is finely dispersed in the metal structure. Therefore, a fine structure can be obtained by using cold-worked martensite as a steel material.

  When martensite is heated, C dissolved in martensite precipitates as carbide before the ferrite is transformed into austenite. Similarly to austenite, carbides are preferentially precipitated at a crystal interface or the like in a metal structure. Since the interface between the precipitated carbide and the base structure is also an effective nucleation site as described above, the cold-processed martensite structure is used as the starting structure, and through the process of forming many fine carbides during the heating process. By heating so that austenite transformation starts from, more nucleation sites can be obtained.

  Here, the “structure mainly composed of cold-worked martensite” refers to ausformed martensite or cold-worked martensite, or a metal having a total volume ratio of 95.0% or more. Means organization. In the steel material, structures such as bainite, ferrite, pearlite, and retained austenite may be mixed, but these structures are permissible as long as the total volume ratio is 5.0% or less.

  The method of producing the steel material is not particularly limited as long as the metal structure is mainly composed of cold-worked martensite, and a general method may be used. Also, there is no particular limitation on a method of performing cold working on a martensite-based steel material. For example, in the case of performing cold rolling, it is desirable that the cold working degree be 20% or more.

<Heating process>
First, a steel material having the above-described chemical composition and metal structure is heated to a temperature range of _three or more Ac at an average heating rate of 500 ° C./s or more. By heating to an austenite single phase region of _three or more Ac, a uniform structure can be obtained. The upper limit of the heating temperature is not particularly limited. However, if the heating temperature greatly exceeds the Ac ₃ point, the rate of grain growth increases, and coarse austenite grains grow. Therefore, the heating temperature is desirably set to _three points of Ac + 150 ° C.

Further, according to a preferred embodiment of the present invention, it is important to perform ultra-rapid heating so that austenite grains generated when ferrite is transformed into austenite are not grown and coarsened. Therefore, the average heating rate up to the temperature range of _three or more Ac is 500 ° C./s or more. The average heating rate is desirably 1000 ° C./s or more. The upper limit of the average heating rate is not particularly limited, but is preferably 20,000 ° C./s or less as a practical range.

In the present invention, Ac ₃ points are determined by the following method. A plurality of test pieces having the same chemical composition and metal structure are prepared, and after heating to various temperatures at a predetermined heating rate, the holding time is set to within 1 s, and the average of 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 highest quenching hardness is defined as _three points Ac. The same result can be obtained when the Ac ₃ points are obtained from the measurement of thermal expansion during heating.

<Holding process>
After heating under the above conditions, cooling is started within 2 seconds. If the holding time in the temperature range of Ac ₃ points or more exceeds 2 s, austenite grain growth occurs during holding, and fine grains cannot be obtained. The holding time is desirably 1 s or less.

<Cooling process>
In the cooling step, cooling is performed to a temperature of 300 ° C. or less so that the average cooling rate from the temperature range of _three or more Ac to 300 ° C. is 100 ° C./s or more. If the cooling rate is less than 100 ° C./s, recrystallization starts, a lot of structures other than martensite are formed in the structure of the steel material after cooling, and the structure does not become mainly martensite.

  In order to suppress recrystallization during cooling as much as possible, it is desirable that the time during which the steel material is kept in a temperature range of 500 ° C. or more between the above-mentioned heating step and the above-mentioned cooling step be within 5 s. .

  Hereinafter, the present invention will be described more specifically 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 into a slab having a thickness of 30 mm. The obtained slab was subjected to hot rolling by a hot rolling tester to obtain a hot-rolled steel having a thickness of 2 mm. Thereafter, Test Nos. 1 to 12 and 14 to 23 shown in Table 2 were heated to 1000 ° C. at a heating rate of 1 ° C./s, held for 1 hour, and then rapidly cooled to room temperature to obtain a martensitic structure. Further, for test numbers 1 to 18, 21, 23 and 24, the hot-rolled steel was cold-rolled by a cold-rolling test machine to obtain a cold-rolled steel having a thickness of 1 mm. Thus, a cold-rolled steel or a hot-rolled steel having a metal structure shown in Table 2 was produced, and was used as a steel material (test numbers 1 to 24).

Test pieces having a width of 10 mm, a length of 50 mm, and a thickness of 1 mm were collected from the obtained steel material. A heat treatment was performed on each of the collected test pieces in accordance with the conditions shown in Table 2. Heating was carried out by electric heating, and immediately after the power supply of the electric heating was cut off, cooling water was injected to cool to room temperature. Table 2 also shows _three Ac points of each material. Each test piece before and after the heat treatment was subjected to structure observation.

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

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

  An image of 130 μm × 130 μm was photographed at a magnification of 1000 at a depth of １ ／ of the thickness of the observation surface. The obtained microstructure photograph is subjected to black and white binarization processing and then image analysis is performed to identify pearlite, bainite, and ferrite, determine the area ratio of each based on JIS G 0551 (2013), and use the line segmentation method. Each volume ratio was converted.

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

  Further, the crystal orientation was measured and analyzed by EBSD using the corroded test piece. Thus, the prior austenite grain size, the number of packets in the old austenite grains, and the packet diameter were measured. Table 3 shows the results.

  Referring to Tables 1 to 3, Test Nos. 1 to 12 manufactured by a method satisfying all the conditions specified in the present invention are mainly composed of martensite, have an ultrafine structure, and have a packet number of 3 0.0 or less.

On the other hand, in Test No. 13, since the metal structure of the steel material before the heat treatment was mainly composed of ferrite / pearlite, the average crystal grain size exceeded 5.0 μm, and an ultrafine structure could not be obtained. The number of packets also exceeded 3.0. In Test No. 14, since the heating temperature was lower than the Ac ₃ point, a metal structure mainly composed of martensite could not be obtained. In Test No. 15, the average crystal grain size was coarse because the temperature rising rate was low.

  In Test No. 16, the average crystal grain size became coarse because the retention time was too long. In Test No. 17, since the cooling rate was low, a metal structure mainly composed of martensite could not be obtained. Further, in Test No. 18, the metal structure mainly composed of martensite could not be obtained because the cooling stop temperature was too high. In Test No. 19, since the steel material was hot-rolled steel, the average crystal grain size was 5.0 μm or less, but the result was that the number of 3.0 packets exceeded 3.0.

  Test Nos. 20 to 23 are examples in which the content of Mn or C is excessive and the value of Ceq is high. As a result, in Test Nos. 20 to 22, the retained austenite in the final structure was increased, and the structure was not martensite-based. Further, in Test No. 23, since the C content was too high, cracks occurred during cold rolling, and the material could not be produced. Further, Test No. 24 had a structure mainly composed of ferrite and pearlite because of low Ceq value and low hardenability.

  According to the present invention, it is possible to obtain a steel material having an ultrafine structure having an average crystal grain size of 5.0 μm or less and having excellent strength, ductility, and toughness.

Claims (4)

Chemical composition in mass%
C: 0.001 to 0.9%,
Si: 0.01 to 2.5%,
Mn: 0.01 to 6.0%,
P: 0.10% or less,
S: 0.030% or less,
Al: 0.001 to 2.5%,
N: 0.010% or less,
Ni: 0 to 10.0%,
Cu: 0 to 1.5%,
Cr: 0 to 5.0%,
Ti: 0 to 0.5%,
Nb: 0 to 0.5%,
V: 0 to 0.5%,
Mo: 0 to 1.5%,
W: 0 to 0.8%,
B: 0 to 0.005%,
Co: 0 to 0.5%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0-0.01%,
The balance: Fe and impurities,
Ceq defined by the following formula (i) is 0.10 to 1.00,
The metal structure comprises, by volume%, 95.0% or more of martensite;
The prior austenite particle size is 5.0 μm or less, and
The number of packets in the old austenite grains is 3.0 or less,
Steel.
Ceq = C + 1 / 24Si + ／ Mn + ／ 40Ni + ／ Cr + ／ Mo + ／ 14V (i)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel. The chemical composition is expressed in mass%;
Ni: 0.01 to 10.0%,
Cu: 0.01 to 1.5%, and
Cr: 0.01 to 5.0%,
Containing at least one member selected from the group consisting of:
The steel material according to claim 1. The chemical composition is expressed in mass%;
Ti: 0.003 to 0.5%,
Nb: 0.003 to 0.5%,
V: 0.005 to 0.5%,
Mo: 0.01-1.5%,
W: 0.005 to 0.8%,
B: 0.0001 to 0.005%, and
Co: 0.01-0.5%,
Containing at least one member selected from the group consisting of:
The steel material according to claim 1 or 2. The chemical composition is expressed in mass%;
Ca: 0.0001-0.01%,
Mg: 0.0001-0.01%, and REM: 0.0001-0.01%,
Containing at least one member selected from the group consisting of:
The steel material according to any one of claims 1 to 3.