JP7218533B2 - Steel material and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material and its manufacturing method.

Steel materials containing retained austenite in the steel structure are known. A deformation-induced transformation steel (TRIP steel) having a mixed structure containing retained austenite has high strength and excellent ductility, and is therefore used in various fields. Therefore, many studies have been made so far to stabilize the austenite in the structure.

For example, in Patent Document 1, a hot-rolled and cold-rolled steel sheet is heated and held at a temperature of A ₃ point to (A ₃ point + 50 ° C.) for 10 to 1800 seconds, and then an average of 3 ° C./s or more A high-strength steel sheet comprising bainitic ferrite and retained γ is disclosed by cooling at a cooling rate from (Ms point −100° C.) to a temperature of Bs point and maintaining the temperature.

Further, in Patent Document 2, hot-rolled and cold-rolled steel sheets are used as materials, and are annealed at a temperature range of (Ac ₃ point-50° C.) to Ac ₃ point at 2° C./s or less. In the subsequent cooling process, it is cooled to a temperature range of Ms point −100 ° C. or less at a cooling rate of 20 ° C./s or more, and then reheated to a temperature range of 300 to 600 ° C., resulting in 60 to 95% tempering. A steel sheet of 1200 MPa or more having a structure containing martensite and retained γ is disclosed.

Japanese Unexamined Patent Application Publication No. 2006-207018 WO2009/099079

In the case of ordinary TRIP steels as disclosed in Patent Documents 1 and 2, retained austenite is often unevenly distributed at grain boundaries and the like. When retained austenite present at grain boundaries becomes martensite due to deformation-induced transformation, stress concentration occurs in the vicinity of grain boundaries, which can be the origin of fracture. Therefore, conventional TRIP steel still has room for improvement in terms of improving ductility.

An object of the present invention is to provide a steel material having a metallographic structure containing retained austenite and having superior ductility to conventional TRIP steel, and a method for producing the same.

The present invention has been made to solve the above problems, and the gist of the present invention is the following steel material and its manufacturing method.

(1) chemical composition, in mass %,
C: 1.00% or less,
Si: 3.00% or less,
Mn: 0.2-7.0%,
P: 0.10% or less,
S: 0.030% or less,
Al: 3.00% or less,
N: 0.010% or less,
Ni: 0 to 10.0%,
Cu: 0-3.0%,
Cr: 0 to 10.0%,
Ti: 0 to 1.0%,
Nb: 0 to 1.0%,
V: 0 to 1.0%,
Mo: 0-2.0%,
W: 0 to 1.0%,
B: 0 to 0.01%,
Co: 0 to 1.0%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
balance: Fe and impurities,
Ceq defined by the following formula (i) is 0.10 to 1.00,
the metallographic structure contains, by volume %, 40% or more tempered martensite, 10% or more bainite, 5% or more retained austenite, and 5% or more ferrite;
The area ratio of retained austenite existing inside the tempered martensite block or ferrite grain is 15% or more with respect to the total amount of retained austenite in the steel material,
Having a tensile strength of 1150 MPa or more,
steel.
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 (% by mass) of each element contained in the steel material.

(2) The average grain size of ferrite grains in the metal structure is 3.0 μm or less.
The steel material according to (1) above.

(3) the chemical composition, in mass %,
Ni: 0.1 to 10.0%,
Cu: 0.3-3.0%, and Cr: 0.1-10.0%,
containing one or more selected from
The steel material according to (1) or (2) above.

(4) the chemical composition, in mass %,
Ti: 0.01 to 1.0%,
Nb: 0.01 to 1.0%,
V: 0.01 to 1.0%,
Mo: 0.05-2.0%,
W: 0.05 to 1.0%,
B: 0.0003-0.01%, and Co: 0.05-1.0%,
containing one or more selected from
The steel material according to any one of (1) to (3) above.

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

(6) having the chemical composition described in any one of (1) and (3) to (5) above;
A tempering process and an annealing process are sequentially performed on a steel material having a metal structure mainly composed of martensite,
In the tempering step, a temperature range of 550 ° C. to Ac ₁ point is held for 60 minutes or more,
In the annealing step, after heating to a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C. at an average temperature increase rate of 500 ° C./s or more, cooling is started after holding for 5 to 30 s, and cooling is performed at 10 ° C./s or more. After cooling to a temperature range of Ms point -50 ° C. to Ms point -200 ° C. at an average cooling rate of less than 100 ° C./s, reheating to a temperature range of 300 to 500 ° C. and holding for 30 to 500 s, to room temperature. Cooling,
A method of manufacturing steel.

(7) Before the tempering step or between the tempering step and the annealing step, a further cold working step is performed.
A method for manufacturing a steel material according to (6) above.

According to the present invention, by forming a metal structure in which retained austenite is dispersed in grains, it is possible to obtain a steel material that is superior in ductility to conventional TRIP steel.

The present inventors have made intensive studies on a method for producing a steel having better ductility than conventional TRIP steel, and as a result, have obtained the following findings.

(a) As described above, when retained austenite is unevenly distributed at grain boundaries, hard martensite is generated at the grain boundaries due to working transformation, resulting in stress concentration near the grain boundaries.

(b) On the other hand, when retained austenite exists in crystal grains, martensite is generated in the grains due to deformation-induced transformation, resulting in stress concentration occurring in the crystal grains. The inside of the grain has a higher resistance to fracture due to stress concentration than the grain boundary.

(c) Therefore, a steel having a metallographic structure in which retained austenite is dispersed in crystal grains is considered to be superior in ductility to ordinary TRIP steel.

The inventors of the present invention have made further studies on a method for producing steel having the metal structure as described above, and as a result, have obtained the following findings.

(d) When a steel material having an initial structure mainly composed of martensite or cold-worked martensite is heat treated at a temperature of Ac ₁ point or less, cementite precipitates in the crystal grains, Elements such as Mn are concentrated in the cementite.

(e) When the above steel material is heated very rapidly to a single austenite phase region and held for a predetermined time, cementite is dissolved and austenite grains are generated. At this time, in the region where cementite existed in the initial structure, phase transformation occurs prior to the diffusion of the elements, so a region in which the austenite stabilizing element such as Mn is concentrated is formed in part of the austenite grains.

(f) After that, when rapidly cooled, the austenite is stabilized in the region where Mn or the like is concentrated, resulting in retained austenite, while the other region transforms into tempered martensite, ferrite, or the like.

(g) That is, retained austenite is formed in crystal grains such as tempered martensite or ferrite.

(h) Further, the degree of fineness after heating varies greatly depending on the difference in structure of the steel material before ultra-rapid heating. If a steel having many austenite nucleation sites in the metal structure is used as the steel material, a fine structure can be easily obtained.

(i) When it is desired to obtain a fine structure, it is preferable to cold work the steel material before performing the ultra-rapid heating.

(j) The produced ultrafine austenite grains tend to grow into coarse grains at high temperatures. Therefore, after ultra-rapid heating, the ultra-fine structure is maintained by starting cooling within a certain period of time.

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

(l) By adjusting the content of Mn, etc., it is possible to lower the transformation temperature of the steel material.

The present invention is made based on the above findings. Each requirement of the present invention will be described in detail below.

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

C: 1.00% or less C is an element that improves the strength of steel materials. The C content is selected according to the properties required of the steel material, but if it exceeds 1.00%, the Mf point is too low, and part or all of the austenite generated during heating is transformed during cooling. Otherwise, the required amount of martensite cannot be obtained, and sufficient strength cannot be obtained. Therefore, the C content is set to 1.00% or less. The C content is preferably 0.50% or less, more preferably 0.35% or less. In order to obtain the above effects, the C content is preferably 0.03% or more.

Si: 3.00% or less Si is contained in order to stabilize retained austenite. However, if the content exceeds 3.00%, the hot workability deteriorates and cracks easily during rolling. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less. In order to obtain the above effects, the Si content is preferably 0.10% or more, more preferably 0.30% or more.

Mn: 0.2-7.0%
Mn is an element effective in lowering the growth and coarsening rate of the austenite _phase by lowering the A1 transformation point and lowering the austenite formation temperature range. Also, Mn is an element distributed to the austenite phase. Furthermore, it becomes an effective element when it is desired to utilize retained austenite. In order to suppress the growth and coarsening of the ultrafine austenite structure, the content should be 0.2% or more. On the other hand, if the Mn content exceeds 7.0%, the Mf point is too low, and part or all of the austenite generated during heating does not transform during cooling, and the required amount of martensite is obtained. and sufficient strength cannot be obtained. Therefore, the Mn content should be 0.2 to 7.0%. The Mn content is preferably 5.0% or less, more preferably 3.0% or less.

P: 0.10% or less P is an element that is generally contained as an impurity, but it is also an element that has the effect of increasing the strength by solid-solution strengthening. Therefore, P may be positively contained. However, P is an element that easily segregates, and when its content exceeds 0.10%, the formability and toughness due to grain boundary segregation are remarkably lowered. Therefore, the P content should be 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, even more preferably 0.020% or less. Although the lower limit of the P content does not need to be specified, it is preferably 0.001% or more if the above effects are to be obtained.

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 remarkably lowered. Therefore, the S content should be 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less, even more preferably 0.001% or less. Although the lower limit of the S content does not need to be specified, it is preferably 0.0001% or more from the viewpoint of suppressing the increase in refining cost.

Al: 3.00% or less Al is an element distributed in the ferrite phase, and in order to suppress the growth and coarsening of the ultrafine austenite structure, it is necessary to contain more than the amount normally contained for deoxidation. be. However, if the content exceeds 3.00%, the hot workability deteriorates and cracks easily during rolling. Therefore, the Al content is set to 3.00% or less. The Al content is preferably 2.50% or less, more preferably 2.00% or less. In order to obtain the above effects, the Al content is preferably 0.01% or more, more preferably 0.03% or more.

N: 0.010% or less N is an element contained as an impurity and has the effect of reducing the formability of the steel sheet. When the N content exceeds 0.010%, the moldability is significantly deteriorated. Therefore, the N content should be 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 of containing one or more of Ti, Nb and V to refine the steel structure as described later, it promotes the precipitation of carbonitrides. The N content is preferably 0.0010% or more, more preferably 0.0020% or more, in order to

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

Ni: 0-10.0%
Ni is an element effective in lowering the growth and coarsening rate of the austenite _phase by lowering the A1 transformation point and lowering the austenite formation temperature range. Also, 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 should be 10.0% or less. The Ni content is preferably 5.0% or less. In order to obtain the above effects, the Ni content is preferably 0.1% or more.

Cu: 0-3.0%
Cu is an element effective in lowering the growth and coarsening rate of the austenite _phase by lowering the A1 transformation point and lowering the austenite formation temperature range. Also, Cu is an element distributed to the austenite phase. Therefore, Cu may be contained as necessary. However, if the Cu content exceeds 3.0%, workability deteriorates and the steel tends to crack during rolling. Therefore, the Cu content should be 3.0% or less. The Cu content is preferably 2.5% or less. In order to obtain the above effects, the Cu content is preferably 0.3% or more.

Cr: 0-10.0%
Cr is an element distributed to the austenite phase, and is an element effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, Cr may be contained as necessary. However, when the Cr content exceeds 10.0%, an imbalance occurs between strength and ductility or between strength and toughness. Therefore, the Cr content should be 10.0% or less. The Cr content is preferably 8.0% or less. In order to obtain the above effects, the Cr content is preferably 0.1% or more.

Ti: 0-1.0%
Ti is an element that distributes to the ferrite phase and diffuses slowly, and is an element that is effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, Ti may be contained as necessary. However, when the Ti content exceeds 1.0%, the steel becomes embrittled. Therefore, the Ti content should be 1.0% or less. The Ti content is preferably 0.5% or less. In order to obtain the above effects, the Ti content is preferably 0.01% or more.

Nb: 0-1.0%
Nb is an element that distributes to the ferrite phase and diffuses slowly, and is an element that is effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, Nb may be contained as necessary. However, when the Nb content exceeds 1.0%, the steel becomes embrittled. Therefore, the Nb content should be 1.0% or less. The Nb content is preferably 0.5% or less. In order to obtain the above effects, the Nb content is preferably 0.01% or more.

V: 0-1.0%
V is an element distributed in the ferrite phase, and is an element effective in suppressing the growth and coarsening of the ultrafine austenitic structure. Therefore, V may be contained as necessary. However, when the V content exceeds 1.0%, the steel becomes embrittled. Therefore, the V content should be 1.0% or less. The V content is preferably 0.5% or less. In order to obtain the above effects, the V content is preferably 0.01% or more.

Mo: 0-2.0%
Mo is an element that distributes in the ferrite phase and diffuses slowly, and is an element that is effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, Mo may be contained as necessary. However, when the Mo content exceeds 2.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Mo content is set to 2.0% or less. Mo content is preferably 1.0% or less. In order to obtain the above effects, the Mo content is preferably 0.05% or more.

W: 0-1.0%
W is an element that distributes in the ferrite phase and diffuses slowly, and is an element that is effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, W may be contained as necessary. However, when the W content exceeds 1.0%, the effect of suppressing grain growth becomes saturated. Therefore, the W content should be 1.0% or less. The W content is preferably 0.5% or less. In order to obtain the above effects, the W content is preferably 0.05% or more.

B: 0-0.01%
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, when the B content exceeds 0.01%, the toughness deteriorates. Therefore, the B content should be 0.01% or less. The B content is preferably 0.005% or less. In order to obtain the above effects, the B content is preferably 0.0003% or more.

Co: 0-1.0%
Co is an element distributed in the ferrite phase, and is an element effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, Co may be contained as necessary. However, when the Co content exceeds 1.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Co content should be 1.0% or less. The Co content is preferably 0.5% or less. In order to obtain the above effects, the Co content is preferably 0.05% or more.

Ca: 0-0.01%
Mg: 0-0.01%
REM: 0-0.01%
Ca, Mg and REM have a pinning effect of suppressing austenite grain growth and have an effect of refining austenite grains. Therefore, one or more selected from these elements may be contained as necessary. However, when the content of each of these elements exceeds 0.01%, the steel becomes embrittled and the workability deteriorates. Therefore, the content of each element should be 0.01% or less. Moreover, when 2 or more types are contained compositely, the total content may be 0.03%. In order to obtain the above effect, 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 lanthanoids, and the REM content means the total content of these elements.

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

Note that "impurities" are components that are mixed in by various factors in the manufacturing process, such as raw materials such as ores and scraps, when manufacturing steel materials industrially, and are allowed within a range that does not adversely affect the present invention. means something

Ceq: 0.10-1.00
Ceq means carbon equivalent and is defined by the following formula (i). If the Ceq is less than 0.10, sufficient strength of the steel material cannot be obtained. On the other hand, if the Ceq exceeds 1.00, not only the toughness and ductility deteriorate, but also the weldability and weld properties deteriorate when welding is performed. Therefore, Ceq is set to 0.10 to 1.00. Ceq is preferably 0.20 or more, more preferably 0.30 or more. Also, Ceq is preferably 0.90 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 (% by mass) of each element contained in the steel material.

(B) Metallographic structure The metallographic structure of the steel according to the present invention contains 40% or more by volume of tempered martensite, 10% or more of bainite, 5% or more of retained austenite, and 5% or more of ferrite. The reason for the limitation of each organization will be explained.

Tempered martensite: 40% or more Tempered martensite is a hard structure and improves the strength of steel. Therefore, the volume fraction of tempered martensite is set to 40% or more. When the strength is to be considered more important than the ductility, the volume fraction is preferably 50% or more, more preferably 60% or more. Since tempered martensite has moderate toughness, it has a relatively small effect of inhibiting ductility unlike martensite, which is a harder structure.

Bainite: 10% or more Bainite is a hard and tough structure. Therefore, the volume fraction of bainite is set to 10% or more. Although there is no particular upper limit, it is substantially less than 40% in relation to other organizations. Incidentally, bainite is a mixed structure of bainitic ferrite having a body-centered cubic structure of iron and iron carbide (cementite having a chemical component of Fe ₃ C). Of these, bainitic ferrite is distinguished from ferrite described later.

Retained Austenite: 5% or More Retained austenite is a structure that improves the ductility of steel due to the TRIP effect. Therefore, the volume fraction of retained austenite is set to 5% or more. Although no particular upper limit is set for the volume fraction of retained austenite, it is substantially 45% or less in relation to other structures. Moreover, when the volume fraction of retained austenite is excessive, the strength may be lowered. Therefore, the volume fraction of retained austenite is preferably 40% or less, more preferably 30% or less, and even more preferably 20% or less.

Ferrite: 5% or more Ferrite is a soft structure and improves the ductility of steel. Therefore, the volume fraction of ferrite is set to 5% or more. Since ferrite is a soft structure, its volume fraction is preferably 10% or more when ductility is more important than strength. On the other hand, if the amount is excessive, the strength of the steel may decrease, so the volume fraction of ferrite is preferably 30% or less, more preferably 20% or less. Note that ferrite is generated by phase transformation of a part of austenite in the cooling process after rapid heating and holding. The average grain size of ferrite is preferably 3.0 μm or less.

The inclusion of fine ferrite and retained austenite is effective in improving the elongation of steel sheets, and the fine grain size makes it difficult to reduce bendability or stretch-flangeability (hole expansion ratio). and a high yield stress is obtained. If coarse ferrite or retained austenite is contained, it tends to become the starting point of voids during stretch flanging deformation, resulting in a low hole expansion ratio and a low yield stress.

The structure other than the above is not particularly limited, but in addition, one or more selected from structures such as martensite and pearlite may be included. However, their total volume fraction is preferably 25% or less, more preferably 20% or less, and even more preferably 15% or less.

Moreover, as described above, the state of dispersion of retained austenite is important in order to further improve the ductility of the steel material compared to the conventional TRIP steel. Specifically, the area ratio of retained austenite present in tempered martensite blocks or inside ferrite grains (in the following description, also simply referred to as “the inside of grains”) is defined as the total amount of retained austenite in the steel material. should be 15% or more.

In the present invention, the volume fraction of each structure, the dispersed state of retained austenite, and the average grain size of ferrite grains are measured by the following methods.

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

A range of 130 μm×130 μm is photographed at a magnification of 1000 at a depth of 1/4 of the plate thickness of the observation surface. After subjecting the obtained microstructure photograph to black and white binarization processing, image analysis is performed to identify tempered martensite, ferrite, bainite, and other structures, and defined in JIS G 0551 (2013) standard, "Steel - Obtain the area ratio of each using the method based on "Microscopic test method for grain size". Furthermore, the conversion from the area ratio to the volume ratio is performed by the line segment method. The line segment method is based on, for example, the technique described in Robert T. DeHoff and Frederik N. Rhines (Quantitative Microscopy, 1968).

Moreover, since it is difficult to distinguish retained austenite from martensite by SEM, the volume ratio is measured by X-ray diffraction. Then, by subtracting the volume ratio of retained austenite from the other volume ratios obtained by the SEM observation, the volume ratio of the remainder (corresponding to martensite, etc.) is obtained.

Furthermore, in order to obtain the dispersed state of retained austenite, the crystal orientation is measured and analyzed by an electron beam backscatter diffraction device (EBSD). Specifically, the EBSD measurement counts the number of blocks of tempered martensite or ferrite grains adjacent to retained austenite grains. Here, in the present invention, grains having a BCC structure surrounded by grain boundaries with a crystal orientation difference of 5° or more are defined as tempered martensite grains and ferrite grains.

Adjacent tempered martensite blocks or retained austenite grains in which the number of ferrite grains is one are defined as grains present inside the grains. On the other hand, a retained austenite grain having two or more adjacent tempered martensite blocks or ferrite grains is defined as a grain existing at a grain boundary. Then, by dividing the total area ratio of the retained austenite grains existing inside the crystal grains by the total area ratio of all the retained austenite grains, the crystal grains existing inside the crystal grains occupying the total crystal grains of the retained austenite Calculate the area ratio of

In the above EBSD measurement, a ferrite grain is defined as a grain having a BCC structure surrounded by grain boundaries with a crystal orientation difference of 15° or more. Then, the average crystal grain size of ferrite is obtained by calculating the average value of the equivalent circle diameters of the specified ferrite grains based on the following formula. However, Ai in the following formula represents the area of the i-th ferrite grain, and di represents the equivalent circle diameter of the i-th ferrite grain.

In addition, in the above measurement, only retained austenite having an equivalent circle diameter of 0.3 μm or more is targeted.

(C) Mechanical Properties In the present invention, the tensile strength of the steel material shall be 1150 MPa or more. Preferably, the tensile strength is 1200 MPa or more. From the viewpoint of ensuring a balance between strength and ductility, the product of tensile strength and elongation is preferably 20000 MPa·% or more.

(D) Manufacturing method The steel material according to the present invention has the chemical composition described above and a steel material having a predetermined metal structure. It can be manufactured by sequential annealing steps of cooling below the point and then reheating. Each condition will be described in detail below.

(D-1) Steel Material As the steel material before heat treatment, one having a metallographic structure mainly composed of martensite is used.

Here, the structure "mainly composed of martensite" means a metal structure having a volume fraction of 95.0% or more. Structures such as ferrite, pearlite, bainite, and retained austenite may be mixed in the steel material, but these structures are allowed if the total volume fraction is 5.0% or less.

The method of manufacturing the steel material is not particularly limited as long as the metal structure satisfies the above regulations, and a general method may be used.

(D-2) Tempering Step The above steel material is held in a temperature range of 550° C. to Ac ₁ point for 60 minutes or longer. By performing the heat treatment under such conditions, cementite precipitates in the crystal grains of each structure described above, and elements such as Mn are concentrated in the cementite. If the holding temperature in this step is less than 550° C. or the holding time is less than 60 minutes, precipitation of cementite and concentration of elements in cementite will be insufficient. On the other hand, when the holding temperature exceeds the Ac ₁ point, austenite transformation occurs and cementite is less likely to precipitate.

(D-3) Annealing step Annealing step is performed on the above steel material. The annealing process can be further subdivided into four steps, namely, a heating process, a holding process, a cooling process and a reheating process. Conditions in each step will be described in detail below.

<Temperature rising process>
A steel material having the chemical composition and metallographic structure described above is first heated to a temperature range of Ac ₃ point to Ac ₃ point+100° C. at an average heating rate of 500° C./s or more. A uniform structure can be obtained by heating to an austenitic single-phase region of Ac ₃ or higher. On the other hand, when heated above Ac ₃ point+100° C., the speed of grain growth increases and coarse austenite grains grow.

Also, it is important to heat very rapidly so that the elements concentrated in the cementite do not diffuse. Therefore, the average heating rate to the above temperature range is set to 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 it is preferably 20000° C./s or less as a practical range.

In addition, in the present invention, Ac ₃ points are obtained by the following method. Prepare a plurality of test pieces having the same chemical composition and metallographic structure, heat to various temperatures at a predetermined heating rate, hold the temperature within 1 s, and heat from the above heating temperature to 70 ° C. at 1000 ° C./s. Cool at the average cooling rate. Then, the heating temperature applied to the test piece having the maximum hardness after quenching is set to Ac ₃ points. Similar results can be obtained even if Ac ₃ points are obtained from thermal expansion measurement during heating.

<Holding process>
After heating under the above conditions and holding for 5 to 30 seconds, cooling is started. Cementite is dissolved by holding in the temperature range from Ac ₃ point to Ac ₃ point +100°C. If the holding time is less than 5 seconds, the cementite will not dissolve sufficiently and will remain in the final structure, making it impossible to obtain a sufficient amount of retained austenite. On the other hand, if the holding time exceeds 30 s, diffusion of the elements during holding becomes significant.

<Cooling process>
In the cooling step, from the temperature range of Ac ₃ points to Ac ₃ points + 100 ° C., cooling to a temperature range of Ms point -50 ° C. to Ms point -200 ° C. at an average cooling rate of 10 ° C./s or more and less than 100 ° C./s. do. Part of the austenite transforms into ferrite during this cooling. Furthermore, by reaching and maintaining the temperature range of Ms point −50° C. to Ms point −200° C., part of the remaining austenite transforms into martensite. The time for holding in the temperature range of Ms point -50°C to Ms point -200°C is not particularly limited, but it is preferably about 5 to 300 seconds.

At this time, the Mn-enriched region where cementite existed remains untransformed as retained austenite. If the average cooling rate is less than 10°C/s, or if the temperature range after cooling exceeds -50°C at the Ms point, sufficient martensite will not appear in the structure and the strength will decrease. Further, the temperature range after cooling is preferably -150° C. or more at the Ms point from the viewpoint of ensuring sufficient ductility.

<Reheating process>
After that, the steel material is reheated to 300 to 500° C. and held for 30 to 500 seconds. At this time, since martensite is tempered, a structure having high strength and excellent toughness can be obtained. Furthermore, during holding at 300-500° C., carbon diffuses into the untransformed austenite, increasing the stability of the austenite. Due to this phenomenon, 5% or more of retained austenite is obtained in the final structure, and the elongation of the steel can be increased. The steel plate after reheating is cooled to room temperature at an arbitrary cooling rate.

By carrying out these steps, it is possible to obtain a steel material having a metal structure in which retained austenite is dispersed inside crystal grains.

(D-4) Cold Working Step To obtain a finer structure, it is desirable to perform a cold working step before the tempering step or between the tempering step and the annealing step. 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 metallographic structure. The nucleation sites can be grain boundaries of the initial structure, interfaces between precipitates such as carbides and grains of the matrix, and the like. The tempered martensite structure has substructures such as packets, blocks, laths, etc. in prior austenite grains, and their boundaries can also be nucleation sites.

When the martensite structure or tempered martensite structure is subjected to cold working, the crystal grains become finer, and the number of nucleation sites further increases, resulting in finely dispersed states in the metal structure. Therefore, if cold-worked tempered martensite is used as a steel material, a fine structure can be obtained.

When martensite is heated, C dissolved in martensite precipitates as carbide before ferrite transforms into austenite. Like austenite, carbides are preferentially precipitated at crystal interfaces and the like within the metal structure. As described above, the interface between the precipitated carbides and the base structure is also an effective nucleation site. More nucleation sites can be obtained by heating so that the austenite transformation starts from .

The method of cold working is not particularly limited. For example, in the case of cold rolling, it is desirable to set the conditions such that the degree of cold working is 20% or more.

EXAMPLES The present invention will be described in more detail with reference to examples below, 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 steel slab 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.

Thereafter, as shown in Table 2, the samples other than Test No. 28 were subjected to tempering heat treatment. In addition, for test numbers 1 to 5, 8 to 11, 13 to 16, 18, 19, 21 to 24, 26, 29 and 31, cold rolling tests were performed on hot rolled steel sheets after hot rolling and after tempering. It was cold-rolled by a machine to obtain a cold-rolled steel sheet with a thickness of 1 mm. As a result, cold-rolled steel sheets or hot-rolled steel sheets having metal structures shown in Table 2 were produced and used as steel materials (test numbers 1 to 31).

A test piece having a width of 50 mm, a length of 70 mm, and a thickness of 0.5 mm was taken from the obtained steel material. Heat treatment was performed according to the conditions shown in Table 2 for each sampled test piece. Heating was performed by electric heating, and cooling was performed by injecting nitrogen gas. Table 2 also shows Ac ₃ points of each material. Each test piece before and after heat treatment was subjected to structure observation and tensile test.

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

First, samples for observation were collected from the test pieces before and after the heat treatment so that the cross section parallel to the rolling direction and the plate thickness direction was the observation surface. Then, the observed surface was mirror-polished, corroded with a nital corrosive solution, and then subjected to structural observation using an SEM.

An image of a range of 130 μm×130 μm was photographed at a magnification of 1000 at a depth position of 1/4 of the plate thickness of the observation surface. After subjecting the obtained microstructure photograph to black and white binarization processing, image analysis is performed to identify bainite, ferrite, pearlite and other structures, and the area ratio of each is determined based on JIS G 0551 (2013), Each volume ratio was converted by the line segment method. Also, the volume fraction of retained austenite was measured by an X-ray diffraction method.

Furthermore, in order to determine the dispersed state of retained austenite, the crystal orientation was measured and analyzed by EBSD. Specifically, the number of tempered martensite blocks or ferrite grains adjacent to retained austenite grains was measured by EBSD.

Then, the number of adjacent tempered martensite blocks or ferrite crystal grains is one, and the retained austenite grains are the crystal grains existing inside the crystal grains. It was considered as existing crystal grains. Then, by dividing the total area ratio of the retained austenite grains present inside the tempered martensite block or ferrite crystal grains by the total area ratio of all the retained austenite grains, the crystal grains occupying the total crystal grains of the retained austenite The area ratio of crystal grains existing inside was calculated.

Also, the average grain size of ferrite was obtained by calculating the average value of the circle-equivalent diameters of the ferrite grains specified in the above EBSD measurement.

In the above measurement, only retained austenite having an equivalent circle diameter of 0.3 μm or more was targeted.

With reference to Tables 1 to 3, test numbers 1 to 18 that satisfy all the conditions specified in the present invention have a tensile strength of 1150 MPa or more and a product of tensile strength and elongation of 20000 MPa ·% or more. The result was an excellent balance between strength and ductility.

On the other hand, in Test Nos. 19 to 28, although the chemical composition of the steel satisfies the provisions of the present invention, the metallographic structure does not satisfy the provisions of the present invention due to inappropriate manufacturing conditions. I didn't.

Specifically, in Test No. 19, since the holding temperature in the annealing step exceeded Ac ₃ +100° C., a locally concentrated region of Mn was not formed, and the ratio of retained austenite grains inside the crystal grains after the annealing step was small. rice field.

In Test No. 20, since the holding time of the annealing process exceeded 5 s, the local enrichment region was lost due to the diffusion of Mn, and the ratio of retained austenite grains inside the grains was small after the annealing process.

In Test No. 21, since the heating rate in the annealing process was less than 500° C./s, the local enrichment region was lost due to the diffusion of Mn, and the proportion of retained austenite grains inside the grains was small after the annealing process.

In Test Nos. 22 and 28, since the metallographic structure of the steel material before heat treatment was mainly composed of ferrite/pearlite, a locally concentrated region of Mn was not formed. The proportion of austenite grains was small.

In Test No. 23, the cooling stop temperature in the annealing process was lower than the Ms point of −200° C., so all the austenite was transformed into martensite, and retained austenite was not obtained in the final structure.

In Test No. 24, since the tempering temperature after hot rolling was less than 550° C., no locally concentrated region of Mn was formed, and the proportion of retained austenite grains inside the grains was small after the annealing process.

In Test No. 25, since the cooling rate in the annealing step was lower than 10°C/s, a large amount of ferrite was generated during cooling, and the area ratio of martensite in the final structure decreased. At the same time, the ferrite grain size exceeded 5.0 μm.

In Test No. 26, the reheating temperature in the annealing process was lower than the Ms point, so all the austenite was transformed into martensite, and retained austenite was not obtained in the final structure.

In Test No. 27, the cooling stop temperature in the annealing process was higher than the Ms point -50 ° C., so the transformation from austenite to martensite did not occur sufficiently, and instead, a large amount of bainite was transformed at the reheating temperature. , a sufficient area ratio of tempered martensite was not obtained in the final structure.

As a result, these steels have either low strength or low ductility, resulting in a poor balance of strength and ductility.

Test numbers 29 and 30 are examples of excessive Mn or C content and high Ceq values. As a result, in Test Nos. 29 and 30, the volume ratio of ferrite in the structure was low, and the balance between strength and elongation was low.

Test No. 31 had a low Ceq value and low hardenability, so that the structure was composed only of ferrite. Therefore, the strength was lowered.

According to the present invention, by forming a metal structure in which retained austenite is dispersed in grains, it is possible to obtain a steel material that is superior in ductility to conventional TRIP steel.

Claims (6)

The chemical composition, in mass %,
C: 1.00% or less,
Si: 3.00% or less,
Mn: 0.2-7.0%,
P: 0.10% or less,
S: 0.030% or less,
Al: 3.00% or less,
N: 0.010% or less,
Ni: 0 to 10.0%,
Cu: 0-3.0%,
Cr: 0 to 10.0%,
Ti: 0 to 1.0%,
Nb: 0 to 1.0%,
V: 0 to 1.0%,
Mo: 0-2.0%,
W: 0 to 1.0%,
B: 0 to 0.01%,
Co: 0 to 1.0%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
balance: Fe and impurities,
Ceq defined by the following formula (i) is 0.10 to 1.00,
The metal structure contains, by volume %, 40% or more tempered martensite, 10% or more bainite, 5% or more retained austenite, 5% or more ferrite, and the balance is 1% or more,
The area ratio of retained austenite existing inside the tempered martensite block or ferrite grain is 15% or more with respect to the total amount of retained austenite in the steel material,
The average grain size of ferrite grains in the metal structure is 3.0 μm or less,
Having a tensile strength of 1150 MPa or more,
steel.
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 (% by mass) of each element contained in the steel material. The chemical composition, in mass %,
Ni: 0.1 to 10.0%,
Cu: 0.3-3.0%, and Cr: 0.1-10.0%,
containing one or more selected from
The steel material according to claim 1 . The chemical composition, in mass %,
Ti: 0.01 to 1.0%,
Nb: 0.01 to 1.0%,
V: 0.01 to 1.0%,
Mo: 0.05-2.0%,
W: 0.05 to 1.0%,
B: 0.0003-0.01%, and Co: 0.05-1.0%,
containing one or more selected from
The steel material according to claim 1 or 2 . The chemical composition, in mass %,
Ca: 0.0001 to 0.01%,
Mg: 0.0001-0.01%, and REM: 0.0001-0.01%,
containing one or more selected from
The steel material according to any one of claims 1 to 3 . A method for manufacturing the steel material according to any one of claims 1 to 4,
Having a chemical composition according to any one of claims 1 to 4 ,
A tempering step and an annealing step are sequentially performed on a steel material having a metal structure with a martensite volume fraction of 95.0% or more ,
In the tempering step, a temperature range of 550 ° C. to Ac ₁ point is held for 60 minutes or more,
In the annealing step, after heating to a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C. at an average temperature increase rate of 500 ° C./s or more, cooling is started after holding for 5 to 30 s, and cooling is performed at 10 ° C./s or more. After cooling to a temperature range of Ms point -50 ° C. to Ms point -200 ° C. at an average cooling rate of less than 100 ° C./s, reheating to a temperature range of 300 to 500 ° C. and holding for 30 to 500 s, to room temperature. Cooling,
A method of manufacturing steel. Before the tempering step or between the tempering step and the annealing step, a further cold working step is performed.
The method for manufacturing the steel material according to claim 5 .