JP2020059880A - Steel material and method for manufacturing the same - Google Patents

Abstract

To provide a steel material more excellent in ductility than that of conventional TRIP steel, and a method for manufacturing the same.SOLUTION: A steel material has a chemical composition containing, by mass%, C: 1.00% or less, Si: 0.80-3.00%, Mn:0.2-7.0%, Al:3.00% or less, Ni:0-10.0%, Cu:0-3.0%, Cr:0-10.0%, Ti:0-1.0%, Nb:0-1.0%, V:0-1.0%, Mo:0-2.0%, W:0-1.0%, B:0-0.01%, Co:0-1.0%, Ca:0-0.01%, Mg:0- 0.01%, REM:0-0.01%, and the balance Fe with inevitable impurities, Ceq: 0.10-1.00, has a metallic structure, by vol.%, composed of 40% or more of tempered martensite, 10% or more of bainite, 5% or more of retained austenite, and 5% or more of ferrite, and has an area ratio of crystal grains having an aspect ratio of less than 3.0 of 70% or more in the total amount of the tempered martensite, bainite and ferrite, and has a tensile strength of 1,150 MPa or more.SELECTED DRAWING: None

Description

  The present invention relates to a steel material and a method for manufacturing the steel material.

  A steel material containing retained austenite in the structure of steel is known. Work-induced transformation type 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 on the structure control for improving the properties of the TRIP steel have been made so far.

For example, in Patent Document 1, a steel sheet hot-rolled and cold-rolled 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 thin steel sheet composed of bainitic ferrite and residual γ is disclosed by cooling and holding at a cooling rate to a temperature of (Ms point −100 ° C.) to Bs point.

Further, Patent Document 2, a steel sheet subjected to hot rolling and cold rolling the material, the temperature range of annealing at (Ac ₃ point -50 ° C.) to Ac ₃ point heating at 2 ° C. / s or less. In the subsequent cooling process, it is cooled to a temperature range of Ms point −100 ° C. or lower at a cooling rate of 20 ° C./s or more and is reheated to a temperature range of 300 to 600 ° C., thereby tempering at 60 to 95%. A steel sheet of 1200 MPa or more having a structure containing martensite and residual γ is disclosed.

JP, 2006-207018, A International Publication No. 2009/0999079

  In the case of ordinary TRIP steels as disclosed in Patent Documents 1 and 2, sufficient consideration was given to the crystal grain sizes of tempered martensite, bainite, ferrite (and retained austenite) and their shapes that constitute the metal structure. There was no tissue control. Therefore, the conventional TRIP steel also has room for improvement from the viewpoint of improving ductility.

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

  The present invention has been made to solve the above problems, and has as its gist the following steel material and a method for manufacturing the same.

(1) The chemical composition is% by mass,
C: 1.00% or less,
Si: 0.80 to 3.00%,
Mn: 0.2 to 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 to 3.0%,
Cr: 0 to 10.0%,
Ti: 0 to 1.0%,
Nb: 0 to 1.0%,
V: 0 to 1.0%,
Mo: 0 to 2.0%,
W: 0 to 1.0%,
B: 0 to 0.01%,
Co: 0 to 1.0%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0 to 0.01%,
The balance: Fe and impurities,
Ceq defined by the following formula (i) is 0.10 to 1.00,
The metal structure contains 40% or more of tempered martensite, 10% or more of bainite, 5% or more of retained austenite, and 5% or more of ferrite in volume%.
Of the total amount of tempered martensite, bainite and ferrite, the area ratio of crystal grains having an aspect ratio of less than 3.0 is 70% or more,
Having a tensile strength of 1150 MPa or more,
Steel material.
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.

(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 is mass%,
Ni: 0.1 to 10.0%,
Cu: 0.3 to 3.0%, and Cr: 0.1 to 10.0%,
Containing one or more selected from,
The steel material according to (1) or (2) above.

(4) The chemical composition is 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 to 0.01%, and Co: 0.05 to 1.0%,
Containing one or more selected from,
The steel material according to any one of (1) to (3) above.

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

(6) The chemical composition according to any one of (1) and (3) to (5) above,
A steel material having a metal structure mainly composed of martensite is heated to a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C. at an average heating rate of 500 ° C./s or more, and then held for 5 to 30 seconds and then cooled. And 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, and then reheating to a temperature range of 300 to 500 ° C. After holding for 30 to 500 s, cool to room temperature,
Steel material manufacturing method.

(7) The steel material having a metallic structure mainly composed of martensite is previously cold-worked,
The method for producing a steel material according to (6) above.

  According to the present invention, it is possible to obtain a steel material having a ductility superior to that of the conventional TRIP steel by forming a metal structure containing a large amount of crystal grains having a low aspect ratio and containing retained austenite.

  The present inventors have earnestly studied a method for producing a steel having a ductility superior to that of the conventional TRIP steel, and as a result, have obtained the following findings.

  (A) In general, the substructure of martensite and bainite has a needle-like structure such as a packet or a block that develops, so that the aspect ratio of crystal grains in the structure tends to be large.

  (B) Crystal grains having a large aspect ratio are likely to break in the width direction and generate fine cracks when the steel material is processed and the crystal grains are deformed. It is considered that it is difficult to obtain a high ductility because a steel material that is susceptible to such cracks easily breaks.

  (C) On the other hand, the lumpy crystal grains are less likely to break when the steel material is processed. Therefore, it is considered that the steel having a metallographic structure composed of agglomerated crystal grains does not easily break even when subjected to high deformation and has excellent ductility.

  As a result of further studies on the method for producing steel having the above-described metal structure, the present inventors have obtained the following findings.

  (D) When a steel material having an initial structure mainly composed of martensite or cold-worked martensite is ultra-rapidly heated to austenite single phase region at once, austenite grains are generated.

  (E) At this time, the degree of fineness after heating largely changes due to the difference in the structure of the steel material before the ultra-rapid heating. When a steel having a large number of austenite nucleation sites in the metal structure is used as the steel material, a fine structure is easily obtained.

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

  (G) The generated ultrafine austenite grains are likely to grow into coarse grains in a high temperature state. Therefore, after the ultra-rapid heating, cooling is started within a fixed time to maintain the ultra-fine structure.

  (H) In order to prevent the growth and coarsening of ultrafine austenite grains, it is also effective to lower the transformation temperature. Since the movement of the grain boundaries is due to the diffusion of atoms, it is possible to maintain the fine grains as they are by lowering the temperature to reduce the diffusion rate.

  (I) The transformation temperature of the steel material can be lowered by adjusting the content of Mn and the like.

  (J) When ultrafine austenite grains are generated and then cooled, a phase transformation occurs, and ferrite, bainite, and martensite are generated. The shape of the ferrite grains and the blocks of bainite and martensite at that time largely depend on the grain size of the original austenite grains, and the finer the austenite grains, the more agglomerate structure is generated.

  (K) From the ultrafine austenite grains obtained by the production method of the present invention, the area ratio of the crystal grains having an aspect ratio of less than 3.0 is 70% or more in the total amount of tempered martensite, bainite and ferrite. The organization is obtained.

  The present invention is 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 addition, in the following description, "%" regarding the content means "mass%".

C: 1.00% or less C is an element that improves the strength of the steel material. The C content is selected according to the properties required for the steel material, but if it exceeds 1.00%, the Mf point will be too low, and some or all of the austenite generated during heating will transform during cooling. Otherwise, the required amount of martensite cannot be obtained and sufficient strength cannot be obtained. Therefore, the C content is 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: 0.80 to 3.00%
Si must be contained in order to suppress the precipitation of cementite and stabilize the retained austenite. However, if the content exceeds 3.00%, the hot workability is deteriorated and cracks easily occur during rolling. Therefore, the Si content is 0.80 to 3.00%. The Si content is preferably 1.00% or more, and preferably 2.50% or less.

Mn: 0.2-7.0%
Mn is an element effective in lowering the growth coarsening rate of the austenite phase by lowering the A ₁ transformation point and lowering the austenite formation temperature range. Further, Mn is an element that is distributed to the austenite phase. Furthermore, it is an effective element when it is desired to utilize retained austenite. In order to suppress the growth coarsening of the ultrafine austenite structure, it is necessary to contain 0.2% or more. On the other hand, when the Mn content exceeds 7.0%, the Mf point is too low, and a part or all of the austenite generated during heating does not transform during cooling, and the required amount of martensite is obtained. As a result, sufficient strength cannot be obtained. Therefore, the Mn content is set to 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 Although P is an element generally contained as an impurity, it 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 formability and toughness significantly decrease due to the segregation of grain boundaries. Therefore, the P content is 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, and further preferably 0.020% or less. The lower limit of the P content does not have to be specified in particular, but it is preferably 0.001% or more in order to obtain the above effects.

S: 0.030% or less S is an element contained as an impurity and forms a sulfide-based inclusion in the steel to reduce the formability of the steel sheet. When the S content exceeds 0.030%, the moldability is significantly deteriorated. 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 further preferably 0.001% or less. The lower limit of the S content need not be specified in particular, but is preferably 0.0001% or more from the viewpoint of suppressing an increase in refining cost.

Al: 3.00% or less Al is an element that is distributed to the ferrite phase, and in order to suppress the growth coarsening of the ultrafine austenite structure, it is necessary to add more than the amount usually contained for deoxidation. is there. However, if the content exceeds 3.00%, the hot workability is deteriorated and cracks easily occur during rolling. Therefore, the Al content is 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 an effect of reducing the formability of the steel sheet. When the N content exceeds 0.010%, the formability is significantly reduced. Therefore, the N content is 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 have to be specified in particular, but considering the case where one or more kinds of Ti, Nb and V are contained to refine the steel structure as described later, the precipitation of carbonitride is promoted. Therefore, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more.

  In addition to the above elements, the steel used in 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 One or more elements selected from REM may be contained.

Ni: 0 to 10.0%
Ni is an element effective in lowering the growth coarsening rate of the austenite phase by lowering the A ₁ transformation point and lowering the austenite formation temperature range. Further, 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 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 to 3.0%
Cu is an element effective in lowering the growth coarsening rate of the austenite phase by lowering the A ₁ transformation point and lowering the austenite formation temperature range. Cu is an element that is distributed to the austenite phase. Therefore, Cu may be contained if necessary. However, if the Cu content exceeds 3.0%, the workability deteriorates and cracks easily occur during rolling. Therefore, the Cu content is 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 to 10.0%
Cr is an element that is distributed to the austenite phase, and is an element that is effective in suppressing the growth coarsening of the ultrafine austenite structure. Therefore, Cr may be contained if necessary. However, if the Cr content exceeds 10.0%, an imbalance between strength and ductility or strength and toughness occurs. Therefore, the Cr content is 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 to 1.0%
Ti is an element that is distributed to the ferrite phase and that diffuses slowly, and is an element that is effective in suppressing the growth coarsening of the ultrafine austenite structure. Therefore, Ti may be contained if necessary. However, if the Ti content exceeds 1.0%, the steel becomes brittle. Therefore, the Ti content is 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 to 1.0%
Nb is an element that is distributed into the ferrite phase and that diffuses slowly, and is an element that is effective in suppressing the growth coarsening of the ultrafine austenite structure. Therefore, Nb may be contained if necessary. However, if the Nb content exceeds 1.0%, the steel becomes brittle. Therefore, the Nb content is 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 to 1.0%
V is an element that is distributed to the ferrite phase, and is an element that is effective in suppressing the growth coarsening of the ultrafine austenite structure. Therefore, V may be contained if necessary. However, if the V content exceeds 1.0%, the steel becomes brittle. Therefore, the V content is 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 is distributed into the ferrite phase and that diffuses slowly, and is an element that is effective in suppressing the growth coarsening of the ultrafine austenite structure. Therefore, Mo may be contained if necessary. However, if the Mo content exceeds 2.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Mo content is 2.0% or less. The 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 to 1.0%
W is an element that is distributed to the ferrite phase and that diffuses slowly, and is an element that is effective in suppressing the growth coarsening of the ultrafine austenite structure. Therefore, W may be contained if necessary. However, if the W content exceeds 1.0%, the effect of suppressing grain growth becomes saturated. Therefore, the W content is 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 to 0.01%
B is an element that improves the 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 0.01%, the toughness deteriorates. Therefore, the B content is 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 to 1.0%
Co is an element that is distributed to the ferrite phase, and is an element that is effective in suppressing the growth coarsening of the ultrafine austenite structure. Therefore, Co may be contained if necessary. However, if the Co content exceeds 1.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Co content is 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 to 0.01%
Mg: 0-0.01%
REM: 0 to 0.01%
Ca, Mg and REM have a pinning effect of suppressing austenite grain growth and an effect of refining austenite grains. Therefore, one or more selected from these elements may be contained if necessary. However, if 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. Further, when two or more kinds are contained in combination, the total content thereof may be 0.03%. In order to obtain the above effects, it is preferable to contain 0.0001% or more of at least one 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 manufacturing method of the present invention, the balance is Fe and impurities.

  The "impurity" is a component that is mixed in during the industrial production of steel products due to various factors such as ores, raw materials such as scrap, and the manufacturing process, and is permitted within a range that does not adversely affect the present invention. Means something.

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

(B) Metallographic Structure The metallographic structure of the steel material according to the present invention includes, in volume%, 40% or more tempered martensite, 10% or more bainite, 5% or more retained austenite, 5% or more ferrite, and tempered martensite. The area ratio of crystal grains having an aspect ratio of less than 3.0 is 70% or more of the total amount of sites, bainite, and ferrite. The reasons for limiting each organization will be explained.

Tempered martensite: 40% or more Tempered martensite has a hard structure and improves the strength of steel. Therefore, the volume ratio of tempered martensite is set to 40% or more. When it is desired to emphasize the strength rather than the ductility, the volume ratio is preferably 50% or more, more preferably 60% or more. Since tempered martensite has an appropriate toughness, unlike martensite, which has a harder structure, the effect of inhibiting ductility is relatively small.

Bainite: 10% or more Bainite is a hard and tough structure. Therefore, the volume ratio of bainite is set to 10% or more. The upper limit is not particularly set, but is substantially less than 40% due to the relationship with other organizations. In addition, 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 ratio of retained austenite is set to 5% or more. The upper limit of the volume ratio of retained austenite is not particularly set, but it is substantially 45% or less due to the relationship with other structures. Further, if the volume ratio of retained austenite is excessive, the strength may decrease. Therefore, the volume ratio of retained austenite is preferably 40% or less, more preferably 30% or less, and further preferably 20% or less.

Ferrite: 5% or more Ferrite has a soft structure and improves the ductility of steel. Therefore, the volume ratio of ferrite is set to 5% or more. By setting the volume ratio of ferrite to 5% or more, the effect of improving the ductility of the steel material can be obtained. On the other hand, if the amount is excessive, the strength of the steel may decrease. Therefore, the volume ratio of ferrite is preferably 30% or less, more preferably 20% or less. Ferrite is formed by a phase transformation of part of austenite in the cooling process after rapid heating and holding. The average crystal grain size of ferrite is preferably 3.0 μm or less.

  By containing fine ferrite and retained austenite, it is effective in improving the elongation of the steel sheet, and the grain size is fine, so that bendability or stretch flangeability (hole expansion rate) is less likely to decrease. And a high yield stress is obtained. If coarse ferrite or retained austenite is contained, it tends to be the starting point of voids when the stretch flange is deformed, the hole expansion ratio is low, and the yield stress is low.

  The structure other than the above is not particularly limited, but one or more kinds selected from structures such as martensite and pearlite may be contained in addition to the above. However, the total volume ratio thereof is preferably 25% or less, more preferably 20% or less, and further preferably 15% or less.

  Further, in the metal structure, the area ratio of crystal grains having an aspect ratio of less than 3.0 is 70% or more of the total amount of tempered martensite, bainite, and ferrite. This further increases the ductility of the steel material. A crystal grain having an aspect ratio of less than 3.0 means that the crystal grain is nearly spherical. It is considered that the crystal grains that are close to spherical are less likely to break during deformation and have an effect of improving ductility. The area ratio of crystal grains having an aspect ratio of less than 3.0 is preferably 75% or more.

  In the present invention, the volume ratio of each structure, the average crystal grain size of ferrite grains, and the area percentage of crystal grains having an aspect ratio of less than 3.0 are measured by 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 becomes the observation surface. Then, the observation surface is mirror-polished and corroded with a Nital etchant, and then the structure is observed using a scanning electron microscope (SEM).

  At a depth of 1/4 of the thickness of the observation surface, an image of a region of 130 μm × 130 μm at 1000 times is photographed. The obtained microstructure photograph is subjected to black-and-white binarization processing and then image analysis is performed to identify bainite, ferrite, pearlite and other structures, and defined in JIS G 0551 (2013) standard, "Steel-grain size". The area ratio of each is determined using the method based on the "microscopic test method". Furthermore, 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 method described in Robert T. DeHoff and Frederik N. Rhines (ed., Quantitative Microscopy, 1968).

  Moreover, since the retained austenite is difficult to distinguish from martensite by SEM, the volume ratio thereof is measured by the X-ray diffraction method. Then, the volume ratio of the remaining austenite is subtracted from the other volume ratios obtained by the above SEM observation to obtain the volume ratio of the balance (such as martensite).

  Further, the crystal orientation is measured and analyzed by an electron beam backscattering diffractometer (EBSD). In the measurement by EBSD, a crystal grain having a BCC structure surrounded by crystal grain boundaries having a crystal orientation difference of 15 ° or more is a ferrite grain. Then, the average crystal grain size of ferrite is obtained by calculating the average value of the circle equivalent 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 circle equivalent diameter of the i-th ferrite grain.

  In the above measurement, only crystal grains having a circle equivalent diameter of 0.3 μm or more are targeted.

  The aspect ratio of the crystal grains is determined by the following method. The structure is subjected to EBSD measurement, and in the regions of tempered martensite, bainite and ferrite, crystal grains surrounded by boundaries (large-angle grain boundaries) having a crystal orientation difference of 15 ° or more are specified. Program processing is performed using the obtained data to determine the aspect ratio of the specified crystal grains. By the program processing, the average coordinates (center of gravity) of the crystal grains, the straight line passing through the center of gravity, and the length of the section crossing the crystal grains are obtained. In the program processing, the average X coordinate of crystal grains, the average Y coordinate of crystal grains, and a straight line passing through the center of gravity are obtained by the following formula.

Average X coordinate of crystal grains: x (ave) = Σ _i (x _i / n)
Average Y coordinate of crystal grains: y (ave) = Σ _i (y _i / n)
A straight line passing through the center of gravity: y−x (ave) = a (xx−ave)
The inclination a of the straight line passing through the center of gravity is a = tan θ, and θ = 0 to 360 °.

  The ratio of the maximum length to the minimum length (= maximum length / minimum length) of the obtained lengths of the slices is defined as the aspect ratio of the crystal grain.

  The area ratio of the total area of the crystal grains having an aspect ratio of less than 3.0 obtained by the above method to the entire visual field is obtained.

(C) Mechanical Properties In the present invention, the tensile strength of the steel material is 1150 MPa or more. The tensile strength is preferably 1200 MPa or more. From the viewpoint of ensuring the balance between strength and ductility, the product of tensile strength and elongation is preferably 18000 MPa ·% or more.

(D) Manufacturing Method The steel material according to the present invention has the above-described chemical composition, and is rapidly heated to a temperature range where it becomes an austenite single phase, and then to the Ms point or lower. It can be manufactured by performing an annealing step of cooling 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 metal structure mainly composed of martensite is used.

  Here, the structure having “mainly martensite” means a metal structure having a volume ratio of 95.0% or more. There are cases where structures such as ferrite, pearlite, bainite, and retained austenite are mixed in the steel material, but these structures are acceptable if the total volume ratio is 5.0% or less.

  The method for producing the steel material is not particularly limited as long as the metallographic structure satisfies the above rules, and a general method may be used.

(D-2) Annealing step An annealing step is performed on the above steel material. The annealing step can be further subdivided into four steps of a temperature raising step, a holding step, a cooling step and a reheating step. The conditions in each step will be described in detail below.

<Temperature raising step>
The steel material having the above chemical composition and metal structure, at first, 500 ° C. / s or more an average heating rate for heating to a temperature range of Ac ₃ point to Ac ₃ point + 100 ° C.. A uniform structure can be obtained by heating to the austenite single-phase region of Ac ₃ or higher. On the other hand, if heating is performed at a temperature exceeding Ac ₃ point + 100 ° C., the grain growth rate increases and coarse austenite grains grow.

  A faster heating rate is preferable for making the austenite grains finer. The average heating rate up to the above temperature range is 500 ° C./s or more. The average heating rate is preferably 1000 ° C./s or more. The upper limit of the average heating rate is not particularly limited, but it is preferably 20,000 ° C./s or less as a practical range.

In the present invention, Ac ₃ point is determined by the following method. A plurality of test pieces having the same chemical composition and metal structure were prepared, and after heating to various temperatures at a predetermined heating rate, the holding time was set to 1 s or less, and the heating temperature was increased to 70 ° C. from 1000 ° C./s. Cool at average cooling rate. Then, the hardness of the test piece after that is the heating temperature applied to the test piece having the maximum quenching hardness is Ac ₃ point. Further, the Ac ₃ point gives the same result even if it is determined from the thermal expansion measurement during heating.

<Holding process>
After heating under the above conditions and holding for 5 to 30 seconds, cooling is started. It is dissolved cementite in holding in a temperature range of Ac ₃ point to Ac ₃ point + 100 ° C.. If the holding time is less than 5 s, cementite may not be sufficiently dissolved and may remain in the final structure, so that a sufficient amount of retained austenite may not be obtained. On the other hand, if the holding time exceeds 30 s, the diffusion of elements during holding becomes remarkable.

<Cooling process>
In the cooling process, the cooling from the temperature range of the Ac ₃ point to Ac ₃ point + 100 ° C., at an average cooling rate of less than 10 ° C. / s or higher 100 ° C. / s to a temperature range of Ms point -50 ° C. Ms point -200 ° C. To do. During this cooling, a part of austenite is transformed into ferrite. Further, by reaching and maintaining the temperature range of Ms point −50 ° C. to Ms point −200 ° C., a part of the remaining austenite is transformed into martensite. The time for maintaining the temperature range of Ms point −50 ° C. to Ms point −200 ° C. is not particularly limited, but is preferably about 5 to 300 s.

  In this cooling step, ferrite, bainite, and martensite that are transformed from fine-grained austenite are fine grains, and a lumpy structure with a small aspect ratio is generated, so that the area of crystal grains with an aspect ratio of less than 3.0 is The ratio can be 70% or more. When the average cooling rate is lower than 10 ° C./s or the temperature range after cooling exceeds the Ms point −50 ° C., sufficient martensite does not appear in the structure and the strength decreases. Further, the temperature range after cooling is preferably Ms point −150 ° C. or higher from the viewpoint of ensuring sufficient ductility.

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

  By performing these steps, it becomes possible to obtain the steel material having the above-described metal structure.

(D-3) Cold Working Step When it is desired to obtain a finer structure, it is desirable to perform the cold working step before 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 in advance a large number of austenite nucleation sites in the metal structure. The nucleation site may be a grain boundary of the initial structure, an interface between a precipitate such as carbide and the grain of the base material, or the like. The tempered martensite structure has substructures such as packets, blocks and laths in the former austenite grains, and their boundaries can also be nucleation sites.

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

  In the case of heating martensite, C, which is a solid solution in martensite, precipitates as a carbide before the transformation of ferrite into austenite. Similar to austenite, carbide also preferentially precipitates at the crystal interface in the metal structure. As described above, since the interface between the precipitated carbide and the base structure is also an effective nucleation site, the cold-worked martensite structure is used as the starting structure, and a lot of fine carbides are formed during the heating process. More nucleation sites can be obtained by heating so that the austenite transformation starts from.

  The method of performing cold working is not particularly limited, and for example, when cold rolling is performed, it is desirable to set the condition such that the cold working degree is 20% or more.

  Hereinafter, the present invention will be described more specifically by way of 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 was hot forged into 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.

  Regarding test numbers 1 to 3, 5 to 11, 13, 15, 17 to 19, 21 to 24 and 29, the hot rolled steel sheet after hot rolling and tempering treatment was cooled by a cold rolling tester. Hot rolling was performed to obtain a cold rolled steel sheet having a thickness of 1 mm. As a result, cold-rolled steel sheets or hot-rolled steel sheets having the metal structure shown in Table 2 were produced and used as steel materials (test numbers 1 to 32).

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

  The metal structures of the test pieces before and after the heat treatment were 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 was the observation surface. Then, the observation surface was mirror-polished and corroded with a Nital etchant, and then the structure was observed using an SEM.

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

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

  Furthermore, the aspect ratio of the crystal grains (tempered martensite, bainite, and ferrite) having the BCC structure specified by the above EBSD was measured, and the total area of the crystal grains with an aspect ratio of less than 3.0 was measured for all fields of view. The area ratio was calculated.

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

  With reference to Tables 1 to 3, Test Nos. 1 to 18 satisfying 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 18000 MPa ·% or more, The result was an excellent balance of strength and ductility.

  On the other hand, in Test Nos. 19 to 29, although the chemical composition of steel satisfies the requirements of the present invention, the metallurgical structure satisfies the requirements of the present invention due to inappropriate manufacturing conditions. There wasn't.

Specifically, in Test No. 19, since the holding temperature in the annealing step exceeded Ac ₃ + 100 ° C., the austenite during annealing was coarsened, and the area ratio of the crystal grains having an aspect ratio of less than 3.0 was 70% or more. It got smaller.

  In Test No. 20, since the holding time in the annealing step exceeded 5 s, the austenite during annealing was coarsened and the area ratio of the crystal grains having an aspect ratio of less than 3.0 was smaller than 70%.

  In Test No. 21, since the heating rate in the annealing step was less than 500 ° C./s, the austenite during annealing was coarsened, and the area ratio of the crystal grains having an aspect ratio of less than 3.0 was smaller than 70%.

  Further, in Test Nos. 22 and 29, since the metal structure of the steel material before heat treatment was mainly ferrite / pearlite, the austenite during annealing was coarsened, and the area of the crystal grains with an aspect ratio of less than 3.0 The ratio became smaller than 70%.

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

  In Test No. 24, since the reheating temperature in the annealing step exceeded 500 ° C., austenite was transformed into cementite at this temperature, and residual austenite was not obtained in the final structure.

  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 was reduced. At the same time, the ferrite grain size exceeded 5.0 μm.

  Test No. 26 has a heating rate of less than 500 ° C./s and a cooling rate of less than 10 ° C./s in the annealing step, so that the area ratio of crystal grains having an aspect ratio of less than 3.0 is more than 70%. It got smaller.

  In Test No. 27, the reheating temperature in the annealing step was lower than 300 ° C., so that all the austenite was transformed into martensite, and retained austenite could not be obtained in the final structure.

  In Test No. 28, the cooling stop temperature in the annealing step 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 transformed at the reheating temperature. However, the area ratio of tempered martensite was not sufficiently obtained in the final structure.

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

  Test numbers 30 and 31 are examples in which the Mn or C content is excessive and the Ceq value is high. As a result, in Test Nos. 30 and 31, the volume ratio of ferrite in the structure was reduced, and the balance between strength and elongation was low.

  Test No. 32 had a low Ceq value and low hardenability, and therefore had a structure composed of only ferrite. Therefore, the strength became low.

  According to the present invention, it is possible to obtain a steel material having a ductility superior to that of the conventional TRIP steel by forming a metal structure containing a large amount of crystal grains having a low aspect ratio and containing retained austenite.

Claims (7)

The chemical composition is% by mass,
C: 1.00% or less,
Si: 0.80 to 3.00%,
Mn: 0.2 to 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 to 3.0%,
Cr: 0 to 10.0%,
Ti: 0 to 1.0%,
Nb: 0 to 1.0%,
V: 0 to 1.0%,
Mo: 0 to 2.0%,
W: 0 to 1.0%,
B: 0 to 0.01%,
Co: 0 to 1.0%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0 to 0.01%,
The balance: Fe and impurities,
Ceq defined by the following formula (i) is 0.10 to 1.00,
The metal structure contains 40% or more of tempered martensite, 10% or more of bainite, 5% or more of retained austenite, and 5% or more of ferrite in volume%.
Of the total amount of tempered martensite, bainite and ferrite, the area ratio of crystal grains having an aspect ratio of less than 3.0 is 70% or more,
Having a tensile strength of 1150 MPa or more,
Steel material.
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. The average grain size of ferrite grains in the metal structure is 3.0 μm or less,
The steel material according to claim 1. The chemical composition is% by mass,
Ni: 0.1 to 10.0%,
Cu: 0.3 to 3.0%, and Cr: 0.1 to 10.0%,
Containing one or more selected from,
The steel material according to claim 1 or 2. The chemical composition is% by 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 to 0.01%, and Co: 0.05 to 1.0%,
Containing one or more selected from,
The steel material according to claim 1. The chemical composition is% by mass,
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.01%,
Containing one or more selected from,
The steel material according to any one of claims 1 to 4. Having the chemical composition according to any one of claims 1 and 3 to 5,
A steel material having a metal structure mainly composed of martensite is heated to a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C. at an average heating rate of 500 ° C./s or more, and then held for 5 to 30 seconds and then cooled. And 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, and then reheating to a temperature range of 300 to 500 ° C. After holding for 30 to 500 s, cool to room temperature,
Steel material manufacturing method. Steel material having a metal structure mainly composed of martensite, cold working in advance,
The method for manufacturing the steel material according to claim 6.