JP7680663B2 - Steel material and its manufacturing method - Google Patents

Description

The present invention relates to steel materials and manufacturing methods thereof.

It is known that the finer the steel structure, the greater its strength, ductility, and toughness. For example, in a multi-phase steel in which hard martensite is dispersed in a soft matrix, steel with many finely dispersed martensite particles has superior ductility and toughness compared to steel with the same martensite volume fraction in which the martensite is coarsened and connected.

In addition, if retained austenite is included in the structure, the austenite undergoes strain-induced martensitic transformation when strain is introduced. Therefore, steel containing retained austenite in the structure has excellent ductility and high strength, resulting in an excellent balance of strength and ductility.

DP steel or retained austenite steel is known as a material with an excellent balance of strength and ductility. For example, Patent Document 1 discloses a high-strength thin steel plate with excellent rigidity, having a tensile strength of 590 MPa or more, a yield ratio of 0.65 or more, and a Young's modulus of 225 GPa or more.

Patent Document 2 discloses a high-strength cold-rolled steel sheet used in automobiles, home appliances, machine structures, etc., which has high energy absorption in the low strain range and excellent crash resistance even without the introduction of strain by press working. Furthermore, Patent Document 3 discloses a high-strength cold-rolled steel sheet with excellent elongation and stretch flangeability and a high yield ratio.

JP 2007-92131 A JP 2008-231480 A JP 2015-34326 A

However, the steels described in Patent Documents 1 to 3 have the problem that the composition must be strictly restricted in order to control the structure. This limits the freedom in the chemical composition, and there is a concern that the manufacturing costs will increase.

The steels described in Patent Documents 1 to 3 contain soft ferrite to ensure ductility. Therefore, although the above steels have excellent formability, they have low deformation resistance in the low strain range, and when used as shock absorbing components for automobiles, for example, they have a problem of poor energy absorption ability at the initial stage of a collision (deformation). In addition, voids are likely to occur at the interface between the soft ferrite layer and the hard martensite layer, which also poses a problem in terms of local deformability.

The present invention was made to solve the above problems, and aims to provide a steel material with excellent ductility and a high yield ratio.

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: 0-10.0%,
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 to 0.01%,
REM: 0-0.01%,
The balance is Fe and impurities.
Ceq defined by the following formula (i) is 0.10 to 1.00,
The metal structure is, in volume percent,
Retained austenite: 5.0 to 30.0%; and
Total volume fraction of martensite and bainite: 2.5 to 50.0%;
The total volume fraction of retained austenite, martensite, tempered martensite, bainite and ferrite is 90.0% or more,
The average grain size of the retained austenite is 2.0 μm or less.
The average grain size of martensite and bainite is 3.5 μm or less,
The following formula (ii) is satisfied:
Steel material.
Ceq=C+1/24Si+1/6Mn+1/40Ni+1/5Cr+1/4Mo+1/14V...(i)
H _IT1 - _{H IT2} ≦3.5...(ii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero. Each element symbol in the above formula is defined as follows.
H _IT1 : Average nanohardness of martensite and bainite (GPa)
H _IT2 : Average nanohardness of tempered martensite and ferrite (GPa)

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

(3) The chemical composition is, in mass%,
Ti: 0.01 to 1.0%,
Nb: 0.01-1.0%,
V: 0.01-1.0%,
Mo: 0.05-2.0%,
W: 0.05-1.0%,
B: 0.0003 to 0.01%, and Co: 0.05 to 1.0%,
Contains one or more selected from
The steel material according to (1) or (2) above.

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

(5) The average nanohardness of tempered martensite and ferrite in the metal structure is 3.0 GPa or more.
A steel material according to any one of (1) to (4) above.

(6) Having a chemical composition according to any one of (1) to (4) above,
For steel materials having a metal structure mainly composed of cold worked martensite or cold worked tempered martensite,
A first heat treatment step and a second heat treatment step are carried out in sequence,
In the first heat treatment step, heating is performed to a temperature range of Ac ₃ or more and less than Ac ₃ +100° C. so that the average heating rate from the rapid heating start temperature T to Ac ₁ point is 150° C./s or more, and then cooling is started within 2.0 seconds to a temperature of 100° C. or less so that the average cooling rate from 800 to 500° C. is 150° C./s or more;
In the second heat treatment step, the steel is heated to a two-phase temperature range of Ac ₁ point or more and less than (Ac ₁ point + Ac ₃ point)/2, and then cooled within 2.0 seconds to a temperature of 100°C or less so that the average cooling rate from the two-phase temperature range to 300°C is 20 to 150°C/s.
Steel manufacturing method.
However, the rapid heating start temperature T is determined as follows.
If Ac ₁ ≧700° C., T=600° C.
When Ac ₁ <700° C., T=Ac ₁ −100° C.

The present invention makes it possible to obtain steel materials with excellent ductility and high yield ratios.

The inventors conducted extensive research into methods for obtaining steel materials with excellent ductility and a high yield ratio, and came to the following findings.

(a) In order to have excellent ductility and a high yield ratio, it is preferable for the metal structure to contain finely dispersed retained austenite, martensite, and bainite, with the remaining base material being primarily composed of relatively hard tempered martensite.

(b) To obtain a steel material having such a metal structure, the steel material is subjected to a first heat treatment process and a second heat treatment process in that order. In the first heat treatment process, the steel material is rapidly heated to the austenite single phase region, thereby generating austenite grains. Then, by immediately rapidly cooling from that state, fine martensite is obtained.

(c) The degree of fineness after heating varies greatly depending on the structure of the steel material. It is preferable to use a steel material that has many austenite nucleation sites in the metal structure. For this reason, it is preferable to use a steel material that has a metal structure mainly composed of cold-worked martensite or cold-worked tempered martensite.

(d) Many austenite nucleation sites are formed in the fine martensite obtained by the first heat treatment process. Therefore, by performing the second heat treatment process, it is possible to finely and abundantly disperse retained austenite, martensite, and bainite in the base material.

(e) In the second heat treatment step, the material is heated to the low temperature side of the ferrite/austenite two-phase region, which generates ultrafine austenite grains in part of the metal structure at high temperatures. Then, by immediately cooling from that state, finely dispersed retained austenite, martensite, and bainite are obtained.

(f) The untransformed areas after heating, i.e., the areas in which the martensite obtained in the first heat treatment process remains, are tempered and become tempered martensite.

(g) When heating is performed at a fast heating rate, the transformation completion temperature to austenite (Ac ₃ point) increases, so that the initial martensite can be tempered in a short period of time in the high temperature range.

(h) The ultrafine austenite grains that are formed tend to grow into coarse grains due to grain boundary migration at high temperatures. Therefore, cooling is started immediately after heating to cool the material to room temperature while maintaining the ultrafine structure.

(i) By slowing down the cooling rate to a certain extent, martensite and bainite are softened. Furthermore, carbon diffuses during the cooling process, concentrating carbon in the austenite, which also has the effect of stabilizing the retained austenite.

(j) To prevent the growth and coarsening of ultrafine austenite grains, it is also effective to lower the transformation temperature. Since the movement of grain boundaries occurs due to atomic diffusion, lowering the temperature and slowing down the diffusion rate makes it possible to maintain the grains at a fine size.

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

(l) The metal structure obtained by carrying out the first heat treatment process and the second heat treatment process can reduce the difference in hardness between the base material and the martensite and bainite. As a result, the generation of voids can be suppressed and ductility and local deformability can be improved.

The present invention was made based on the above findings. Each aspect of the present invention will be explained in detail below.

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

C: 1.00% or less C is an element that improves the strength of steel. The C content is selected according to the properties required for the steel, but if it exceeds 1.00%, the Mf point drops too much, and some or all of the austenite generated during heating does not transform during cooling, so the required amount of martensite is not 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, and more preferably 0.35% or less. In order to obtain the above effect, the C content is preferably 0.03% or more.

Si: 3.00% or less Si is an element distributed to the ferrite phase, and in order to suppress the growth and coarsening of the ultrafine austenite structure, it is necessary to contain more Si than the amount normally contained for deoxidation. However, if the content exceeds 3.00%, the hot workability deteriorates and the steel is easily cracked 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 effect, the Si content is preferably 0.01% or more, and more preferably 0.03% or more.

Mn: 0.2-7.0%
Mn is an effective element for lowering the growth and coarsening rate of the austenite phase by lowering the _A1 transformation point and lowering the austenite formation temperature range. Mn is also an element that is distributed to the austenite phase. Furthermore, it is an effective element when retained austenite is to be utilized. In order to suppress the growth and coarsening of the ultrafine austenite structure, it is necessary to contain 0.2% or more. On the other hand, if the Mn content exceeds 7.0%, the Mf point is lowered too much, and a part or all of the austenite generated during heating is not transformed during cooling, so that the required amount of martensite cannot be obtained and 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 P is an element that is generally contained as an impurity, but it is also an element that has the effect of increasing strength by solid solution strengthening. Therefore, P may be actively contained. However, P is an element that is easily segregated, and if its content exceeds 0.10%, the deterioration of formability and toughness due to grain boundary segregation becomes significant. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.05% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. There is no need to specify the lower limit of the P content, but if it is desired to obtain the above effect, it is preferable to set it to 0.001% or more.

S: 0.030% or less S is an element contained as an impurity, and forms sulfide-based inclusions in steel, thereby reducing the formability of the steel sheet. If the S content exceeds 0.030%, the formability is significantly reduced. Therefore, the S content is set to 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.001% or less. There is no need to specify a lower limit for the S content, but from the viewpoint of suppressing an increase in refining costs, it is preferably set to 0.0001% or more.

Al: 3.00% or less Al is an element distributed to the ferrite phase, and in order to suppress the growth and coarsening of the ultrafine austenite structure, it is necessary to contain more Al than the amount normally contained for deoxidation. However, if the content exceeds 3.00%, the hot workability deteriorates and the steel is prone to cracking during rolling. Therefore, the Al content is set to 3.00% or less. The Al content is preferably 2.50% or less, and more preferably 2.00% or less. In order to obtain the above effect, the Al content is preferably 0.01% or more, and 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. If the N content exceeds 0.010%, the formability will be significantly reduced. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.008% or less, and more preferably 0.007% or less. There is no need to specify a lower limit for the N content, but in consideration of the case where one or more of Ti, Nb, and V are contained to refine the steel structure as described below, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more in order to promote the precipitation of carbonitrides.

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

Ni: 0-10.0%
Ni is an element that is 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. Ni is also an element that is distributed to the austenite phase. Therefore, Ni may be contained as necessary. However, when the Ni content exceeds 10.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Ni content is set to 10.0% or less. The Ni content is preferably set to 5.0% or less. In order to obtain the above effect, the Ni content is preferably set to 0.1% or more.

Cu: 0 to 3.0%
Cu is an element that is 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. Cu is also an element that is distributed to the austenite phase. Therefore, Cu may be contained as necessary. However, if the Cu content exceeds 3.0%, the workability deteriorates and the steel is prone to cracking during rolling. Therefore, the Cu content is set to 3.0% or less. The Cu content is preferably set to 2.5% or less. In order to obtain the above effect, the Cu content is preferably set to 0.3% or more.

Cr: 0-10.0%
Cr is an element that is distributed to the austenite phase and is effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, Cr may be contained as 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 set to 10.0% or less. The Cr content is preferably set to 8.0% or less. In order to obtain the above effect, the Cr content is preferably set to 0.1% or more.

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

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

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

Mo: 0 to 2.0%
Mo is an element that is distributed to the ferrite phase and diffuses slowly, and 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. The Mo content is preferably set to 1.0% or less. In order to obtain the above effect, the Mo content is preferably set to 0.05% or more.

W: 0 to 1.0%
W is an element that is distributed to the ferrite phase and diffuses slowly, and 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 is set to 1.0% or less. The W content is preferably set to 0.5% or less. In order to obtain the above effect, the W content is preferably set to 0.05% or more.

B: 0 to 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, if the B content exceeds 0.01%, toughness deteriorates. Therefore, the B content is set to 0.01% or less. The B content is preferably set to 0.005% or less. To obtain the above effects, the B content is preferably set to 0.0003% or more.

Co: 0 to 1.0%
Co is an element that is distributed to the ferrite phase and is 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 is set to 1.0% or less. The Co content is preferably set to 0.5% or less. In order to obtain the above effect, the Co content is preferably set to 0.05% or more.

Ca: 0-0.01%
Mg: 0-0.01%
REM: 0-0.01%
Ca, Mg and REM have a pinning effect to suppress austenite grain growth and have the effect of refining austenite grains. Therefore, one or more selected from these elements may be contained as 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. In addition, when two or more elements are contained in combination, 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: Sc, Y, and lanthanides, and the content of the REM refers to the total content of these elements.

In the chemical composition of the steel material according to the present invention, the balance is Fe and impurities. Note that "impurities" refer to components that are mixed in due to various factors in the manufacturing process and raw materials such as ores and scraps when industrially manufacturing steel material, and are acceptable within the range that does not adversely affect the present invention.

Ceq:0.10~1.00
Ceq means carbon equivalent and is defined by the following formula (i). If Ceq is less than 0.10, a martensite structure cannot be obtained even if quenching is performed. On the other hand, if Ceq exceeds 1.00, not only does toughness and ductility deteriorate, but also weldability and welded portion characteristics deteriorate when welding is performed. Therefore, Ceq is set to 0.10 to 1.00. Ceq is preferably 0.20 or more, and more preferably 0.30 or more.
Ceq=C+1/24Si+1/6Mn+1/40Ni+1/5Cr+1/4Mo+1/14V...(i)
In the formula, each element symbol represents the content (mass%) of each element contained in the steel, and when the element is not contained, it is set to zero.

(B) Metal structure of steel The metal structure of the steel according to the present invention is mainly a multi-phase structure of retained austenite, martensite, tempered martensite, bainite and ferrite. Specifically, the metal structure has a total volume ratio of retained austenite, martensite, tempered martensite, bainite and ferrite of 90.0% or more. The volume ratio of ferrite is preferably 5% or less. In addition, the steel may contain structures such as pearlite, but these structures are acceptable as long as their total volume ratio is less than 10.0%.

In the following explanation, newly precipitated martensite is sometimes called "new martensite" to distinguish it from tempered martensite, which is initial martensite that has been tempered. In addition, the soft tempered martensite and ferrite are sometimes collectively called the "base structure," and the newly precipitated martensite and bainite are sometimes collectively called the "hard structure."

The volume fraction of the retained austenite in the metal structure is 5.0-30.0%, and the average crystal grain size is 2.0 μm or less. The total volume fraction of the newly formed martensite and bainite is 2.5-50.0%, and the average crystal grain size is 3.5 μm or less. By dispersing a specified amount of fine retained austenite, newly formed martensite, and bainite in the metal structure, it is possible to suppress the decrease in elongation while maintaining high strength.

The mechanical properties of steel improve as the grain size becomes finer, and the improvement is particularly noticeable when the average grain size of the newly formed martensite and bainite is 3.5 μm or less. It is desirable for the average grain size to be 3.0 μm or less. Note that the grain size of the newly formed martensite and bainite here refers to either the previous austenite grain size or the packet grain size of the martensite and bainite that can be specified.

In the present invention, the volume fraction and average crystal grain size of each structure are measured by the following method.

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

At a depth position of 1/4 of the plate thickness of the above observation surface, an area of 300 μm x 300 μm is photographed at 1000x magnification. The microstructure photograph obtained is then binarized to black and white, and image analysis is performed to identify areas judged to be newly formed martensite, bainite, or retained austenite. The total area ratio of these areas is calculated using a method based on the "Microscopic test method for steel grain size" stipulated in the JIS G 0551 (2013) standard.

Next, in the same field of view (photograph), from the areas determined to be other than areas determined to be new martensite, bainite, or retained austenite, the areas that are tempered martensite or ferrite are identified. Ferrite, pearlite, and tempered martensite are distinguished by the presence or absence of carbide precipitation. In addition, pearlite and tempered martensite are distinguished by the shape and position of the carbides. Then, in image processing, the total area ratio of the identified areas that are tempered martensite or ferrite is calculated.

Next, crystal orientation is measured and analyzed using an electron backscatter diffraction (EBSD) device in the same field of view as that in which the SEM observation of the same sample was performed. The measurement is performed in an area of 30 μm x 30 μm or more, in steps of 0.05 μm or less. The FCC phase is then identified from the analysis results, and its area ratio is calculated based on JIS G 0551 (2013) in the same manner as above, and this is taken as the area ratio of retained austenite.

The area ratio of each structure obtained by the above method is converted to a volume ratio by the line segment method. The line segment method is based on the method described in, for example, Quantitative Microscopy, co-edited by Robert T. DeHoff and Frederik N. Rhines, 1968.

Then, the total volume fraction of the newly formed martensite and bainite is calculated by subtracting the volume fraction of the retained austenite from the total volume fraction of the newly formed martensite, bainite and retained austenite. Furthermore, the total volume fraction of the newly formed martensite, bainite and retained austenite is added to the total volume fraction of the tempered martensite and ferrite to calculate the total volume fraction of the retained austenite, newly formed martensite, bainite, tempered martensite and ferrite.

In addition, the crystal grains identified as FCC phase are approximated as spheroids to determine their diameter. Furthermore, the average diameter of all crystal grains within the field of view is calculated to calculate the average crystal grain size of the retained austenite.

Furthermore, from the photograph taken using the above-mentioned SEM, the parts in the same field of view that were identified as retained austenite by EBSD measurement are removed and the image is processed to identify the structure that is determined to be newly formed martensite or bainite. The identified structure is then approximated as a spheroid by image analysis to determine its diameter. The average diameters of all crystal grains in the field of view are then averaged to calculate the average crystal grain size of the newly formed martensite and bainite.

In addition, in the steel material of the present invention, the relationship between the average nano-hardness of the newly formed martensite and bainite and the average nano-hardness of the matrix structure satisfies the following formula (ii).
H _IT1 - _{H IT2} ≦3.5...(ii)
Each symbol in the above formula is defined as follows.
H _IT1 : Average nanohardness of martensite and bainite (GPa)
H _IT2 : Average nanohardness of tempered martensite and ferrite (GPa)

Tempered martensite, which is the initial martensite that has been tempered, is softer than newly formed martensite and bainite. In addition, the ferrite that is produced when austenite that appears during heating is transformed into ferrite during cooling is also soft. However, if there is a large difference in hardness between the newly formed martensite and bainite and the base structure, voids will occur at the boundary between the hard layer and the soft layer during processing, causing a deterioration in ductility and local deformability.

Therefore, the value on the left side of equation (ii) is 3.5 GPa or less. The value on the left side of equation (ii) is preferably 3.0 GPa or less. The smaller the value on the left side of equation (ii), the more improved the ductility, but for steel materials produced by the manufacturing method of the present invention, the value on the left side of equation (ii) is 0 GPa or more. In addition, the average nanohardness of the tempered martensite and ferrite (base structure) is preferably 3.0 GPa or more, and more preferably more than 3.2 GPa.

The average nanohardness of the base structure is measured using the nanoindentation method. Specifically, the measurement is performed under the following conditions. A diamond cube-corner type indenter is used as the indenter, and the continuous stiffness method is used as the indentation method. The load is 500 μN, and measurements are performed at 10 locations (with intervals of 5 μm or more) at room temperature. The average value of these measurements is then taken as the average nanohardness. The average nanohardness of the hard structure is measured in the same way as the average nanohardness of the base structure.

(C) Manufacturing Method The steel material according to the present invention can be manufactured by sequentially carrying out a first heat treatment and a second heat treatment on a steel material having the above-mentioned chemical composition and a predetermined metal structure. Each condition will be described in detail below.

(C-1) Steel Material The steel material to be used before the heat treatment has a metal structure mainly composed of cold worked martensite or cold worked tempered martensite. The reason is as follows.

In order to disperse a large number of fine austenite grains during heating, it is necessary to obtain a large number of austenite nucleation sites in advance in the metal structure. Possible nucleation sites include the grain boundaries of the initial structure and the interfaces between precipitates such as carbides and the grains of the base material. Martensite structures have substructures such as packets, blocks, and laths within prior austenite grains, and the boundaries between these structures can also become nucleation sites. Therefore, compared to ferrite/pearlite structures, martensite structures have more austenite nucleation sites, and a fine structure is obtained when heated.

In addition, when martensite is cold worked, the crystal grains become finer, which further increases the number of nucleation sites and makes them finely dispersed in the metal structure. Therefore, a fine structure can be obtained by using cold worked martensite as a steel material.

When martensite is heated, the C dissolved in the martensite precipitates as carbides before the ferrite transforms into austenite. Like austenite, carbides also precipitate preferentially at crystal interfaces within the metal structure. As mentioned above, the interfaces between the precipitated carbides and the base structure are also effective nucleation sites, so by using cold-worked martensite as the starting structure and heating it so that austenite transformation begins after a process in which many fine carbides are formed during the heating process, more nucleation sites can be obtained.

Note that, because martensite is hard and has poor ductility, cold working may not be possible depending on the composition. In such cases, tempering may be performed before cold working to produce tempered martensite. Even when using a cold-worked tempered martensite structure as the starting structure, it is possible to obtain many nucleation sites.

Here, "mainly cold-worked martensite or cold-worked tempered martensite" means a metal structure in which the total volume fraction of cold-worked martensite and cold-worked tempered martensite is 95.0% or more. Although structures such as ferrite, pearlite, bainite, and retained austenite may be mixed in the steel material, these structures are acceptable as long as their total volume fraction is less than 5.0%.

The manufacturing method of the steel material is not particularly limited as long as it is possible to control the metal structure to the above structure, and a general method may be used. There is also no particular limit to the method of cold working the steel material. For example, when cold rolling is performed, it is desirable to use conditions that result in a cold working degree of 20% or more. Furthermore, there is also no particular limit to the tempering treatment conditions, and it is sufficient to use conditions that reduce the hardness to a level that allows cold working.

(C-2) First heat treatment step The above steel material is subjected to a first heat treatment step. The conditions of the first heat treatment step are described in detail below.

<Heating process>
A steel material having the above-mentioned chemical composition and metal structure is heated to a temperature range of Ac ₃ point or more and less than Ac ₃ +100°C so that the average heating rate from the rapid heating start temperature T to Ac ₁ point is 150°C/s or more. By heating to the austenite single phase region of Ac ₃ point or more, a uniform structure can be obtained. On the other hand, if the heating temperature is Ac ₃ +100°C or more, the grain growth rate increases and the residence time in the austenite single phase temperature region, including during cooling, becomes long, resulting in the growth of coarse austenite grains. Therefore, the maximum temperature is set to Ac ₃ point or more and less than Ac ₃ +100°C.

In addition, the average heating rate at _one point from the rapid heating start temperature T to Ac is 150° C./s or more. By setting the average heating rate _{at one} point from the rapid heating start temperature T to Ac to 150° C./s or more, the temperature range in which recrystallization progresses can be rapidly heated, and the disappearance of the nucleation site can be suppressed. The average heating rate is preferably 500° C./s or more, and more preferably 1000° C./s or more. There is no particular limit to the upper limit of the average heating rate, but it is preferable that it is 20000° C./s or less as a practical range.

In the present invention, the Ac ₁ point and Ac ₃ point are determined by the following method. A plurality of test pieces having the same chemical composition and metal structure are prepared, heated to various temperatures at a predetermined heating rate, and then cooled from the heating temperature to 70°C at an average cooling rate of 1000°C/s with a holding time of 1 s or less. Then, each test piece is observed under a microscope, and the lowest heating temperature at which newly formed martensite is observed in the metal structure is defined as the Ac ₁ point. Furthermore, the hardness of each test piece is measured, and the heating temperature applied to the test piece at which the hardness is the maximum quenched hardness is defined as the Ac ₃ point. The Ac ₁ point and Ac ₃ point can also be obtained by measuring the thermal expansion during heating to obtain the same results. The rapid heating start temperature T is determined as follows.
If Ac ₁ ≧700° C., T=600° C.
When Ac ₁ <700° C., T=Ac ₁ −100° C.

<Holding process>
After heating under the above conditions, cooling is started within 2.0 seconds. If the holding time in the above temperature range exceeds 2.0 seconds, austenite grows during holding, making it difficult to obtain a fine martensite structure. The holding time is preferably 1.0 seconds or less.

<Cooling process>
In the cooling step, the above-mentioned heating temperature is cooled to a temperature of 100°C or less so that the average cooling rate from 800 to 500°C is 150°C/s or more. If the average cooling rate is less than 150°C/s, ferrite transformation, pearlite transformation, and bainite transformation occur during cooling, and some of the austenite grains become structures other than martensite, making it difficult to make the structure after the first heat treatment step fine martensite. In addition, ferrite remains until after the second heat treatment step, making it difficult to make a hard base structure mainly composed of tempered martensite, so the average cooling rate is 150°C/s or more. The average cooling rate is preferably 300°C/s or more, and more preferably 500°C/s or more. There is no particular limit to the upper limit of the average cooling rate, but it is preferable that it is 20,000°C/s or less as a practical range.

By subjecting the steel material to the first heat treatment process under the above conditions, fine martensite can be obtained. Specifically, it is preferable that the average crystal grain size of the prior austenite grains in the fine martensite is 15 μm or less, and the area ratio of the martensite is 90% or more. Since many austenite nucleation sites are formed in the fine martensite, by subjecting the steel material to the second heat treatment process described below, it is possible to finely and abundantly disperse the retained austenite, martensite, and bainite in the base material.

In addition, the fine martensite obtained by the first heat treatment process is tempered by the second heat treatment process to become fine tempered martensite, resulting in a hard matrix structure.

(C-3) Second Heat Treatment Step The steel material after the first heat treatment is subjected to a second heat treatment step. The conditions of the second heat treatment step are described in detail below.

<Heating process>
A steel material having the above-mentioned chemical composition and metal structure is first heated to a temperature range of Ac ₁ or more and less than (Ac ₁ point + Ac ₃ point)/2. By heating to the above-mentioned temperature range, fine austenite can be generated in a part of the metal structure. In addition, the region where the martensite obtained in the first heat treatment step remains is tempered at a high temperature for a short time, and becomes tempered martensite with fine carbides dispersed therein.

There are no particular restrictions on the heating rate. However, in order to prevent excessive softening of the base structure as the tempering progresses, and to prevent coarsening of newly formed martensite and bainite in the final structure due to the growth of austenite formed during the heating process, the average heating rate up to the above temperature range is preferably 150°C/s or more, and more preferably 500°C/s or more. There are no particular restrictions on the upper limit of the average heating rate, but a practical range of 20,000°C/s or less is desirable.

In the present invention, the Ac ₁ point and Ac ₃ point are determined by the following method. A plurality of test pieces having the same chemical composition and metal structure are prepared, heated to various temperatures at a predetermined heating rate, and then cooled from the heating temperature to 70°C at an average cooling rate of 1000°C/s with a holding time of 1 s or less. Then, each test piece is observed under a microscope, and the lowest heating temperature at which newly formed martensite is observed in the metal structure is defined as the Ac ₁ point. Furthermore, the hardness of each test piece is measured, and the heating temperature applied to the test piece at which the hardness is the maximum quenched hardness is defined as the Ac ₃ point. The Ac ₁ point and Ac ₃ point can also be obtained by measuring the thermal expansion during heating, and similar results can be obtained.

<Holding process>
After heating under the above conditions, cooling is started within 2.0 seconds. If the holding time in the above temperature range exceeds 2.0 seconds, austenite grows during holding, making it difficult to obtain fine newly formed martensite, bainite, and retained austenite. In addition, in the process in which austenite grows during holding, the carbon concentration in the austenite decreases with an increase in the volume fraction of austenite, so that the stability of the austenite is impaired, and the amount of retained austenite obtained after cooling is reduced. The holding time is preferably 1.0 seconds or less.

<Cooling process>
In the cooling step, the material is cooled to a temperature of 100°C or less so that the average cooling rate from the heating temperature to 300°C is 20 to 150°C/s. By cooling under such conditions, a part of the austenite is transformed into martensite, and then carbon is diffused during cooling, and the newly formed martensite is tempered by auto-tempering, softening it. In addition, a part of the austenite is transformed into bainite, and soft bainite is obtained.

If the cooling rate is less than 20°C/s, most of the austenite will transform into ferrite and pearlite structures during cooling, and ultrafine martensite and bainite will not be obtained. On the other hand, if the cooling rate exceeds 150°C/s, most of the austenite will transform into martensite during cooling, reducing the amount of retained austenite, and the newly formed martensite and bainite will not be softened sufficiently, making it difficult to achieve a hardness difference of 3.5 GPa or less. Furthermore, since the austenite that existed before the start of cooling will transform into martensite with almost the same shape, the average crystal grain size of the newly formed martensite and bainite in the final structure may become coarse.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

A 180 kg steel ingot with the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and hot forged into a 40 mm thick steel slab. The resulting slab was hot rolled using a hot rolling test machine to produce a 2 mm thick hot-rolled steel plate.

Then, the decarburized layer on the surface of the hot-rolled steel sheet was mechanically ground to produce a steel sheet 2 mm thick, which was then subjected to various heat treatments so that the main metal structure would become the structure shown in Table 2. Furthermore, the sheet was cold-rolled using a cold-rolling test machine to produce a cold-rolled steel sheet 1 mm thick, which was used as the steel material.

From the obtained steel material, a test piece having a width of 30 mm, a length of 200 mm, and a thickness of 1 mm was taken. Each of the taken test pieces was subjected to heat treatment according to the conditions shown in Tables 2 and 3. Heating was performed by electrical heating, and after the target temperature was reached, the power source for electrical heating was cut off, and cooling water or cooling gas was immediately sprayed to cool to room temperature. Tables 2 and 3 also show Ac ₁ point and Ac ₃ point for each material. In addition, the heating rate in Table 2 means the average heating rate from the rapid heating start temperature T to Ac ₁ point, and the cooling rate means the average cooling rate from 800 to 500 ° C. The cooling rate in Table 3 means the average cooling rate from the heating temperature to 300 ° C.

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

First, observation samples were taken from the test pieces before and after heat treatment so that the cross sections parallel to the rolling direction and plate thickness direction would be the observation surfaces. The observation surfaces were then mirror-polished and etched with a nital etching solution, after which the structure was observed using a SEM.

A 300 μm x 300 μm area was photographed at 1000x magnification at a depth of 1/4 the plate thickness of the above observation surface. The microstructure photographs obtained were then binarized to black and white and then image analyzed to identify areas deemed to be newly formed martensite, bainite, or retained austenite. The total area ratio of these areas was calculated based on JIS G 0551 (2013) and converted to a volume ratio using the line segment method.

Next, in the same field of view (photograph), from the areas determined to be other than areas determined to be newly formed martensite, bainite, or retained austenite, the areas that were tempered martensite or ferrite were identified. Ferrite, pearlite, and tempered martensite were identified based on the presence or absence of carbide precipitation, while pearlite and tempered martensite were identified based on the shape and position of the carbides. Then, in image processing, the total area ratio of the identified tempered martensite and ferrite (base structure) was calculated based on JIS G 0551 (2013), and converted to a volume ratio using the line segment method.

Next, the crystal orientation was measured and analyzed by EBSD in the same field of view as that in which the SEM observation of the same sample was performed. The measurement was performed in an area of 30 μm x 30 μm or more in steps of 0.05 μm or less. The FCC phase was identified from the analysis results, and its area ratio was calculated based on JIS G 0551 (2013), and converted to the volume ratio of retained austenite using the line segment method. In addition, the crystal grains identified as FCC phase were approximated as spheroids to determine their diameter, and the average diameter of all crystal grains in the field of view was averaged to calculate the average crystal grain size of retained austenite.

The total volume fraction of the newly formed martensite and bainite was then calculated by subtracting the volume fraction of the retained austenite from the total volume fraction of the newly formed martensite, bainite and retained austenite. Furthermore, the total volume fraction of the retained austenite, newly formed martensite, bainite, tempered martensite and ferrite (complex phase structure) was calculated by adding the total volume fraction of the newly formed martensite, bainite and retained austenite to the total volume fraction of the tempered martensite and ferrite.

Furthermore, newly formed martensite or bainite was identified by excluding the parts of the photograph taken using the above-mentioned SEM that were identified as retained austenite in the same field of view by EBSD measurement and processing the images. The identified newly formed martensite or bainite was then approximated as a spheroid by image analysis to determine its diameter, and the average grain size of the newly formed martensite and bainite was calculated by averaging the diameters of all crystal grains in the field of view.

The average nanohardness of the matrix structure was measured using the nanoindentation method under the following conditions. A diamond cube-corner type indenter was used as the indenter, and the continuous stiffness method was adopted as the indentation method. The load was 500 μN, and measurements were taken at 10 locations (with intervals of 5 μm or more) at room temperature. The average value of these measurements was taken as the average nanohardness. The average nanohardness of the hard structure was also measured in the same manner as the average nanohardness of the matrix structure.

These results are shown in Table 4.

In addition, some of these test pieces were subjected to tensile tests to measure the mechanical properties.

The tensile test was carried out using an Instron tensile tester in accordance with the provisions of ASTM standard E8. From the above test pieces, an Instron-type tensile test piece (parallel section length: 30 mm, parallel section plate width: 6.0 mm) was taken so that the test direction was parallel to the rolling direction. Note that with the electrical heating and cooling equipment used in this example, the number of uniformly heated areas that can be obtained from a sample with a length of about 200 mm is limited, so we decided to use a half-size plate test piece of ASTM standard E8.

The results are shown in Table 5. In this example, it was determined that a steel has excellent ductility and a high yield ratio when TS×EL is 17,000 MPa·% or more and the yield ratio is 0.60 or more.

As shown in Table 5, test numbers 1, 2, and 29 to 31, which satisfy the provisions of the present invention, were excellent in ductility and had a high yield ratio of 0.60 or more. On the other hand, test numbers 15 and 32 to 40, which are comparative examples that do not satisfy the provisions of the present invention, showed poor results in at least one of TS×EL and the yield ratio.

The present invention makes it possible to obtain steel materials with excellent ductility and high yield ratios.

Claims (6)

The chemical composition, in mass%, is
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: 0-10.0%,
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 to 0.01%,
REM: 0-0.01%,
The balance is Fe and impurities.
Ceq defined by the following formula (i) is 0.10 to 1.00,
The metal structure is, in volume percent,
Retained austenite: 5.0 to 30.0%; and
Total volume fraction of martensite and bainite: 2.5 to 50.0%;
The total volume fraction of retained austenite, martensite, tempered martensite, bainite and ferrite is 90.0% or more,
The average grain size of the retained austenite is 2.0 μm or less.
The average grain size of martensite and bainite is 3.5 μm or less,
The following formula (ii) is satisfied:
Steel material.
Ceq=C+1/24Si+1/6Mn+1/40Ni+1/5Cr+1/4Mo+1/14V...(i)
H _IT1 - _{H IT2} ≦3.5...(ii)
In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero. Each element symbol in the above formula is defined as follows.
H _IT1 : Average nanohardness of martensite and bainite (GPa)
H _IT2 : Average nanohardness of tempered martensite and ferrite (GPa) The chemical composition, in mass%,
Ni: 0.1 to 10.0%,
Cu: 0.3-3.0%, and Cr: 0.1-10.0%,
Contains one or more selected from
The steel material according to claim 1. The chemical composition, in mass%,
Ti: 0.01 to 1.0%,
Nb: 0.01-1.0%,
V: 0.01-1.0%,
Mo: 0.05-2.0%,
W: 0.05-1.0%,
B: 0.0003 to 0.01%, and Co: 0.05 to 1.0%,
Contains one or more selected from
The steel material according to claim 1 or 2. The chemical composition, in mass%,
Ca: 0.0001-0.01%,
Mg: 0.0001-0.01%, and REM: 0.0001-0.01%,
Contains one or more selected from
The steel material according to any one of claims 1 to 3. The average nanohardness of tempered martensite and ferrite in the metal structure is 3.0 GPa or more.
The steel material according to any one of claims 1 to 4. A method for producing a steel material according to any one of claims 1 to 5,
The chemical composition according to any one of claims 1 to 4,
For steel materials having a metal structure mainly composed of cold worked martensite or cold worked tempered martensite,
A first heat treatment step and a second heat treatment step are carried out in sequence,
In the first heat treatment step, heating is performed to a temperature range of Ac ₃ or more and less than Ac ₃ +100° C. so that the average heating rate from the rapid heating start temperature T to Ac ₁ point is 150° C./s or more, and then cooling is started within 2.0 seconds to a temperature of 100° C. or less so that the average cooling rate from 800 to 500° C. is 150° C./s or more;
In the second heat treatment step, the steel is heated to a two-phase temperature range of Ac ₁ point or more and less than (Ac ₁ point + Ac ₃ point)/2, and then cooled within 2.0 seconds to a temperature of 100°C or less so that the average cooling rate from the two-phase temperature range to 300°C is 20 to 150°C/s.
Steel manufacturing method.
However, the rapid heating start temperature T is determined as follows.
If Ac ₁ ≧700° C., T=600° C.
When Ac ₁ <700° C., T=Ac ₁ −100° C.