JP4590540B2 - Ultra-fine grain steel showing no up / down phenomenon - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The invention of this application relates to an ultrafine-grained steel that does not exhibit an up / down phenomenon. More specifically, the invention of this application relates to an ultrafine-grained steel not only showing no up / down yield phenomenon, but also showing a high strength up / down yield phenomenon.
[0002]
[Prior art]
An ultrafine-grained steel with a ferrite grain size of 3 μm or less has a simple composition, for example, Fe—C—Mn—Si, and therefore it is not necessary to use expensive trace elements for high strength. The present welding technique has an advantage that it can be used as it is. High strength is obtained as a result of the fine graining of ferrite, but in the ultrafine grain steel, the upper yield stress / tensile strength or the lower yield stress / tensile strength, that is, YR is high.
[0003]
In recent architectural design, there is a study that seismic energy is absorbed by a brace with a damper function, and columns and beams with high YR and wide elastic range are not plastically deformed, but give them the strength of the building. Yes. Ultra fine grain steel is useful for structural materials such as columns and beams in such buildings.
[0004]
However, in the current design method in which seismic energy is absorbed by plastic deformation of columns and beams, high YR is disadvantageous and it is difficult to apply ultra fine grain steel. In automobiles, ultrafine grained steel is effective for elastically designed parts, but high YR is disadvantageous when absorbing impact energy, and application of ultrafine grained steel is difficult.
[0005]
[Problems to be solved by the invention]
On the other hand, extremely low carbon IF steel (Interstitial Free steel) does not show up / down phenomenon. In this ultra-low carbon IF steel, C (carbon) is suppressed to 0.001 mass% to 0.002 mass%, Ti (titanium) and Nb (niobium) are added alone or in combination, Ti carbide, Nb carbide, Ti nitride, C and N (nitrogen) are fixed by Nb nitride. By fixing C and N, the decrease in up / down yield disappears, and solid solution strengthening disappears. Since it is extremely low carbon, the amount of carbide dispersion is extremely small, and dispersion strengthening by carbide can be kept small. Therefore, extremely low carbon IF steel has low strength and is excellent in workability, and is therefore used as an automobile steel plate (see, for example, Patent Document 1).
[0006]
However, as described above, the strength of the ultra-low carbon IF steel is low. No research has been conducted so far to increase the strength by refining.
[0007]
The invention of this application has been made in view of the circumstances as described above, and provides an ultrafine-grained steel not only showing no up / down yield phenomenon but also showing high / upset yield phenomena. It is a problem to be solved.
[0008]
[Patent Document 1]
Japanese Patent Publication No. 42-12348 [0009]
[Means for Solving the Problems]
The invention of this application, as to solve the above problem, containing 0.15 mass% or less of C (carbon), in ultra-fine grain steel building structures ferrite particle diameter of 3μm or less, Ti (titanium) At least one element selected from Nb (niobium), V (vanadium), and Mo (molybdenum) is added, and Ti carbide, Nb carbide, V carbide, Mo carbide, or two or more kinds of these carbides have a diameter. C (carbon) is fixed by precipitation at 150 nm or less, and all of N (nitrogen) is also obtained by Ti nitride, Nb nitride, V nitride, Mo nitride, or two or more of these nitrides. the fixed Rukoto, to provide a high strength building structures for ultra-fine grain steel which does not exhibit on and lower breakdown phenomenon, characterized by showing no breakdown phenomenon of the upper and lower.
[0011]
Hereinafter, the ultrafine grain steel which does not show the up / down yield phenomenon of the invention of this application will be described in more detail while showing examples.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the ultrafine-grained steel that does not show the upper / lower yield phenomenon of the invention of this application, in the ultrafine-grained steel having a ferrite grain size of 3 μm or less, Ti (titanium), Nb (niobium), V (vanadium), Mo ( Molybdenum) is added at least one element selected from within. C (carbon) in steel is all fixed by Ti carbide, Nb carbide, V carbide, Mo carbide or two or more of these carbides, and N (nitrogen) is Ti nitride, Nb nitride, V nitridation N (nitrogen) is all fixed by the metal, Mo nitride, or two or more types of these nitrides. As a result, all of C and N contained in the steel are fixed, and the upper / lowering phenomenon disappears.
[0013]
The carbon amount C ^* that can be fixed by carbides TiC, NbC, VC, and MoC having a stoichiometric composition is expressed by the following formula when the amount of each element bonded to carbon is Ti ^C , Nb ^C , V ^C , and Mo ^C Given in.
[0014]
C ^* = (12.01 / 47.86) Ti ^C + (12.01 / 92.90) Nb ^C + (12.01 / 50.94) V ^C
+ (12.01 / 94.94) Mo ^C
= (1 / 3.99) Ti ^C + (1 / 7.74) Nb ^C + (1 / 4.24) V ^C + (1 / 7.99) Mo ^C (1) where C ^* , Ti ^C , Nb ^C , V ^C , The unit of Mo ^C is mass%. The atomic weights of C, Ti, Nb, V, and Mo were set to 12.01, 47.86, 92.90, 50.94, and 94.94, respectively.
[0015]
Although the amount of N contained in the steel in the normal melting process is small, since it dissolves in the same manner as C, it must be fixed as a nitride. N ^* that can be fixed by nitrides TiN, NbN, VN, and MoN is the atomic weight of C in formula (1), where Ti ^N , Nb ^N , V ^N , and Mo ^N are the amounts of each element that binds to nitrogen. Substituting 12.01 with an atomic weight of N of 14.00 gives:
[0016]
N ^* = (1 / 3.42) Ti ^N + (1 / 6.63) Nb ^N + (1 / 3.64) V ^N + (1 / 6.85) Mo ^N (2)
Here, the unit of N ^* , Ti ^N , Nb ^N , V ^N , and Mo ^N is mass%.
[0017]
To fix all C and N contained in the steel,
C <C ^* , N <N ^* (3)
(C, N units are mass%).
[0018]
The amount of each element added is
Ti = Ti ^C + Ti ^N , Nb = Nb ^C + Nb ^N , V = V ^C + V ^N , Mo = Mo ^C + Mo ^N (4)
It becomes. Here, the unit of Ti, Nb, V, and Mo is mass%.
[0019]
In the ultrafine-grained steel that does not show the upper / lowering phenomenon of the invention of this application, ferrite is refined by the process of heat treatment, the grain size becomes 3 μm or less, strength is developed, and Ti, Nb, V, Mo As a result, all C and N in the steel are fixed and the up / down phenomenon disappears. Since ferrite is a fine grain with a grain size of 3 μm or less and does not show up / down yielding phenomenon, it has excellent Charpy impact characteristics and fatigue characteristics, and it does not show up / down yielding phenomenon of the invention of this application. Steel is effective as a structural material having high three major properties of strength, toughness, and fatigue properties.
[0020]
The ultrafine-grained steel that does not show the upper / lowering phenomenon of the invention of this application, except for the additive elements of Ti, Nb, V and Mo,
C: 0.001 to 0.90 mass%,
Mn: 0.8-3.0mass%,
Si: 0.80mass% or less,
Al: 0.10 mass% or less,
The balance can be exemplified by Fe and inevitable impurities.
[0021]
As other additive components, for example,
Cu: 0.05 to 2.5 mass%
Ni: 0.05-3 mass%,
Cr: 0.01-3 mass%,
W: 0.01-0.5mass%,
Ca: 0.001 to 0.01 mass%,
REM (rare earth element): 0.001 to 0.02 mass%,
B: 0.0001 ~ 0.006mass%
And one or more of the above can be added.
[0022]
In the ultra fine grain steel that does not show the upper / lower yield phenomenon of the invention of this application, Ti carbide, Nb carbide, V carbide, Mo carbide or two or more kinds of these carbides are used in the ferrite heat treatment process. It can be deposited and dispersed to a diameter of 150 nm or less, and the strength can be further increased.
[0023]
【Example】
[Example 1]
The chemical components of the test material used in Example 1 are as shown in Table 1. The balance is iron and inevitable impurities.
[0024]
[Table 1]
In Table 1, the unit is mass%. In Example 1, the C content was increased to 0.05 mass% as compared with the ultra-low carbon IF steel having C of 0.001 mass% to 0.002 mass%. Ti is not added to the A material, but Ti = 0.21 mass% is used to fix all C and N to the a material. When fixing C and N only with Ti, the minimum amount of Ti (mass%) obtained from the above formulas (1) to (4) is:
(Ti) min = 3.99C + 3.42N = 3.99 × 0.050 + 3.42 × 0.001 = 0.20292 (5)
Further, the addition amount of Ti is 0.21% larger than this value, and all of C and N can be fixed.
[0025]
Table 2 shows the processing conditions.
[0026]
[Table 2]
As a pretreatment, forging was performed after holding at 1100 ° C. for 1 hour, and air cooling was performed to room temperature. Next, after holding at 900 ° C. for 1 hour, it was air-cooled, and when it reached 550 ° C., it was 93% groove-rolled and water-cooled to produce an ultrafine-grained steel.
[0027]
FIGS. 1A and 1B are microscopic structure diagrams of the A material and the a material, respectively. In the A material shown in FIG. 1A, precipitates are coarse and segregated Fe ₃ C is dispersed. In the material a shown in FIG. 1B, fine TiC particles are uniformly dispersed. The ferrite grain size is about 0.5 μm for both the A material and the a material.
[0028]
2A and 2B are stress-strain curves of the A material and the a material obtained in the tensile test, respectively.
[0029]
As shown in FIG. 2A, the up / down yield phenomenon appears in the A material to which no Ti is added. On the other hand, in the “a” material to which Ti is added, as shown in FIG.
[0030]
Table 3 shows various characteristics obtained from the stress-strain curves shown in FIGS.
[0031]
[Table 3]
Since the material a did not show the up / down yielding phenomenon, 0.2% proof stress was shown instead of the yield stress.
[0032]
As can be seen from Table 3, material A is Ti carbide (TiC) and N is Ti nitride (TiN) compared to material A, so there is no solid solution strengthening of C and N, and 0.2% yield strength And the tensile strength is reduced. However, (0.2YS / TS) calculated in place of the upper yield stress / tensile strength (UYS / TS) and the lower yield stress / tensile strength (LYS / TS) is from either UYS / TS or LYS / TS of the A material. Is also low. That is, YS of material a is low.
[Example 2]
The chemical composition of the test material used in Example 2 is shown in Table 4 (the balance is iron and inevitable impurities), and the heat treatment conditions are shown in Table 5.
[0033]
[Table 4]
[0034]
[Table 5]
The C content of the test material was 0.05, 0.15, and about 0.45 mass%. Ti was added to all specimens, but for B, D and E materials, 3.99C + 3.42N> Ti was added to show the up / down phenomenon, and for b, d and e materials, 3.99C + 3.42N <Ti was added so as not to show the up / down phenomenon. The aim was to increase the strength by increasing the TiC content and strengthening the precipitation. Regarding the thermomechanical treatment conditions, for the purpose of increasing the amount of dissolved Ti before groove roll rolling, a pretreatment was carried out for air cooling to room temperature after holding at 1200 ° C. for 1 hour. In the groove roll rolling, for the purpose of precipitating fine TiC, the step of holding at 900 ° C. for 1 hour from the processing heat treatment condition I shown in Table 2 was removed.
[0035]
The mechanical properties obtained for each material are shown in Table 6.
[0036]
[Table 6]
In B, D, and E materials, the up / down yield phenomenon appeared and YR increased. In the b, d and e materials, the up / down yield phenomenon disappeared, and the YR became low.
[0037]
When the materials b, d and e were compared, the 0.2% proof stress and the tensile strength were lower with the d material of C = 0.15 mass% than the b material of C = 0.05 mass%. This is because when the amount of C increases, the amount of Ti that can be dissolved before groove roll rolling decreases, so that there is a large TiC before groove roll rolling, and fine TiC precipitates during groove roll rolling. This is because the dispersion is not sufficiently strengthened and the strength characteristics are deteriorated. Compared with the d material of C = 0.15 mass%, the e material of C = 0.43 mass% increased the strength characteristics because of the larger TiC before groove roll rolling.
[Example 3]
Table 7 shows the chemical composition of the test material used in Example 3 (the balance is iron and inevitable impurities), and Table 8 shows the conditions for the heat treatment.
[0038]
[Table 7]
[0039]
[Table 8]
The C amount of the test material was about 0.05 and 0.075 mass%, and the Ti amount was 3.99C + 3.42N <Ti for both materials. Regarding the heat treatment conditions, as a pretreatment, forging was carried out after holding at 1200 ° C. for 1 hour, and then air-cooled to room temperature. Three types of conditions were adopted for the groove roll rolling, the III condition was substantially the same as the I condition of Example 1, and the IV condition was the same as the II condition of Example 2. V condition is new, and in order to precipitate a lot of fine TiC, air-cooled after holding at 1200 ° C for 1 hour, not air-cooled to room temperature, and groove roll rolling with 91% area reduction at 650 ° C Went.
[0040]
Table 9 shows the mechanical properties obtained for each material.
[0041]
[Table 9]
In both the f and g materials, the upper / lower yield phenomenon disappeared under any processing conditions, and the YR was as low as the a, b, d, and e materials.
[0042]
In the ultrafine-grained steel that does not show the upper and lower yield phenomenon in Example 1-3, the combination of the specimen with the highest strength and the thermomechanical treatment condition is the specimen g with C content = 0.075 mass%, It was a combination of thermomechanical treatment conditions V.
[0043]
FIG. 3 is a microstructure diagram obtained by observation with a transmission electron microscope of a thin film of the g material prepared under the thermomechanical treatment condition V. Although the particle size is about the same as that of the material a shown in FIG. 1B (about 0.5 μm), fine carbides having a diameter of 10 nm or less are observed. Furthermore, as a result of observing the extracted replica with a transmission electron microscope, it was confirmed that the diameter of the carbide was 10 nm or less. As a result of observing the thin film of the f material having the lowest strength in Table 9 and its extracted replica with a transmission electron microscope, the ferrite particle diameter was about 0.5 μm and the carbide diameter was 10 nm to 150 nm. The material “a” having a microscopic structure shown in FIG. 1B has substantially the same chemical composition and processing heat treatment conditions as the material “f”, but the carbide diameter is approximately 50 nm.
[0044]
In this way, in addition to fine grain strengthening of ferrite, fine precipitation of Ti carbide can cause precipitation dispersion strengthening and increase the strength of ultrafine grain steel.
Moreover, higher strength can be obtained by increasing the amount of fine Ti carbide.
[0045]
The Charpy impact characteristics of all the ultrafine-grained steels obtained in Example 1-3 are shown in FIGS. 4-6.
[0046]
The test piece was a No. 4 test piece (width 10 mm, thickness 10 mm, V notch depth 2 mm) of JIS Z2202 “Metallic material impact test piece”. In the JIS-SM50B rolled steel for welded structures, which is also widely used as the current building steel, the Charpy impact energy at a use temperature of 0 ° C is 27.5 J. However, the use temperature of recent building steel is -80. The development goal is a Charpy impact energy of 100 J at ℃. For materials a, b, d, e, f, and g that did not show the up / down phenomenon, the ductile / brittle transition temperature was −100 ° C. or less, except for the e material with a C content of 0.43 mass%, and Charpy impact energy. Is over 100J, exceeding the above development target. The e material shows a value about twice that of the current steel. A, B, D, E materials showing up / down phenomenon are high strength, but impact energy is slightly higher than materials a, b, d, e, f, g which do not show up / down materials. Inferior.
[0047]
FIG. 7 shows the high cycle fatigue characteristics of the specimens A and a obtained in Example 1, that is, the relationship between the stress amplitude and the fracture life.
[0048]
Using an hourglass test piece having a test part diameter of 6 mm, a rotating bending fatigue test was conducted. The fatigue limit σ _w determined by the number of repetitions of 10 ⁷ is known to have a good correlation with the tensile strength TS. For example, Fatigue Data Sheet (Saburo Matsuoka, National Institute for Materials Science) According to Nobuo Nagashima and Satoshi Nishijima, Metal Material Strength Data Sheet Material 17 “Indicator Characteristics on Fatigue of Metal Materials for Mechanical Structures”, Institute for Materials Research (currently National Institute for Materials Science), 1999) (TS = 450~750MPa) with σ _w = 0.395TS, empirical formula σ _w = 0.522TS in tempered martensite steel (TS = 700~1200MPa) is obtained.
[0049]
From FIG. 7, for materials A and a, σ _w = 480 MPa and 420 MPa are obtained, and σ _w /TS=0.57 and 0.58. These figures are much higher than ferrite-pearlite steel, which is a relative of ultrafine-grained ferritic steel, and slightly higher than tempered martensitic steel. In other words, regardless of the presence or absence of the up / down phenomenon, ultrafine-grained steel exhibits excellent fatigue properties.
[0050]
Of course, the invention of this application is not limited to the above embodiments and embodiments. It goes without saying that various aspects are possible for the details.
[0051]
【The invention's effect】
As described above in detail, according to the invention of this application, an ultrafine grain steel not only showing no up / down phenomenon but also having high strength and no up / down phenomenon is provided.
[Brief description of the drawings]
FIGS. 1A and 1B are microscopic structure diagrams of a material A and a material obtained in Example 1, respectively.
FIGS. 2A and 2B are stress-strain curves of material A and material a obtained in a tensile test in Example 1, respectively.
FIG. 3 is a microstructure diagram obtained by observation with a transmission electron microscope of a thin film of material g produced under a thermomechanical treatment condition V.
4 is a graph showing the Charpy impact characteristics of the ultrafine-grained steel obtained in Example 1. FIG.
5 is a graph showing the Charpy impact characteristics of the ultrafine-grained steel obtained in Example 2. FIG.
6A and 6B are diagrams showing Charpy impact characteristics of the ultrafine-grained steel obtained in Example 3, respectively.
7 is a graph showing the high cycle fatigue characteristics of specimens A and a obtained in Example 1. FIG.

Claims (1)

Ti (titanium), Nb (niobium), V (vanadium), Mo (molybdenum) in ultrafine-grain steel for building structures containing 0.15 mass% or less of C (carbon) and having a ferrite grain size of 3 μm or less At least one element selected from the group consisting of Ti carbide, Nb carbide, V carbide, Mo carbide, or two or more of these carbides precipitates with a diameter of 150 nm or less, so that C (carbon) is all fixed. while being, Ti nitrides, Nb nitrides, V nitrides, by Rukoto fixed all N (nitrogen) also by Mo nitride or two or more nitrides of these show no upper and lower breakdown phenomenon, A high-strength steel for high-strength building structures that does not show up / down phenomenon, characterized by Charpy impact absorption energy at room temperature of 100 J or more.