JPS58174544A - Super fine grain ferrite steel - Google Patents

Abstract

PURPOSE:To obtain the super fine grain ferrite steel consisting of metallographical structure including many equiaxed ferrite grain surrounded with great inclination grain boundary, by working, in a hot state, low alloy steel by regulating C content without alloying any special element. CONSTITUTION:The steel slab containing not more than 0.3% C, alloy content excepting C of not more than 3%, and not practically containing Nb, Ta, Mo and W, is rolled within a range of temperature practically consisting of austenite range near the transformation point Ar3 with continuous rolling, etc. When processed in this hot rolling, the transformation and recrystallization of austenite are promoted and the grain size is reduced. The super fine grain ferrite steel of metallographical structure including not less than 70% of equiaxed ferrite crystal surrounded with great inclination grain boundary of not more than 4mum in average particle size can be provided as rolled in a hot state thereby. This steel member is excellent in characteristics such as tensile strength, yield stress, etc. and has sufficient ductility and workability as practical steel.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an ultra-fine-grained 7-eight steel, especially a heat-resistant compressor,
Moreover, it relates to a next-layer fine-grained ferritic steel that is mainly composed of hypo-eutectoid steel that does not contain special alloying elements such as Nb.

Various attempts have been made to refine the grains of sipherite steel. In other words, refining the grains of the 7-ethite copper material is the only way to increase yield stress (also tensile strength) and improve toughness (fracture transition temperature &) at the same time.
Rumor has it that special heat treatment methods have been used, such as adding special alloying elements such as Nb and MO, or combining the two.

However, a method of providing ultra-fine grain steel of 4JI or less by a method that is industrially viable for practical hypoeutectoid steel has not yet been achieved.

The present invention relates to such a revolutionary Hiko ultra-fine grain steel that can be obtained industrially, and is characterized by O:0.
3X or less, a hot rolled steel material with an alloy content other than C of 3X or less, which does not subtly contain Nb, Ta, Mo, or Wt gold, and is equiaxed with a large angle grain boundary K11l with an average grain size of 4μ or less. The ultra-fine grains consisting of a metal structure containing ferrite crystal grains t-70X or more will be explained in more detail.

The copper of the present invention is ultra-fine ferrite copper as described above,
In the present invention, the composite fibers referred to as fine-grained 7E2ite have a grain shape that is elongated and isotropic.

In addition, as a general rule, it refers to a structure consisting of crystal grains surrounded by so-called high-angle grain boundaries, and is not considered to be a subgrain boundary (small tendency grain boundary).

This is the main reason for determining the composition range of the steel of the present invention.

In other words, although the carbon content was specified to be 0.3X or less, in general, as the carbon content increases, the 7E2ite amount inevitably decreases and the Par2ite f increases, which is the normal phase diagram for the steel of the present invention. 7 elites are generated more than expected. Carbon is 0.3
If the amount exceeds 70X, the amount of other structures such as per2ite increases, making it difficult to obtain a ferrite amount of 70X or more, so the above component range is limited.

In principle, the steel of the present invention can be obtained with or without O alloying elements other than carbon, but the optimum fM degree of hot working is 700 to 900.
℃, and Ar5 transformation point, Ar1+
It is often desirable to adjust the O''e and Ar3 transformation points with alloying elements, but the total amount of one alloying element is
If it exceeds $, Ar5 will be too low and it will be difficult to obtain fine grains, so the content of alloys other than carbon is set to 3X or less.

In addition, these alloying elements include asls Mn, □r
A large amount of 1illII, etc., is usually contained in steel: 8% of 4%, and 9% of steel usually contributes to improving ductility and toughness.

t*, Nb, TI, MO, W, etc. are all known as elements that retard the recrystallization and transformation of austenite.In the steel of the present invention, the transformation and recrystallization of austenite are promoted during hot working to make the grains finer. Because it is a thing%
Nb, Ta, Mo, W, etc. are elements that inhibit this, so they must not be included in the steel of the present invention.

Steel with the above composition is manufactured by the following manufacturing method.

The above steel is subjected to IA pass
The total reduction rate of 2 passes or more is 8o・- or more, and the - is 1
For a short time of less than 1 second, rolling is performed at a total reduction rate of 50% or more at l/(s) or 2 nos or more, and -m is caused by rolling to generate fine ferrite crystal grains.

To explain the history in detail, the above-mentioned steel has a normal Ars & B point (refers to the temperature at which the 7-elite transformation starts during gradual cooling from a temperature in the austenite region of the steel, hereinafter simply referred to as Ars) and A
1 between (r1+50℃) and (Ar5+100℃) based on r1 [point 11, which similarly refers to the temperature at which pearlite transformation begins during slow cooling, and hereinafter simply referred to as Arl).
A total rolling reduction of 80X or higher at two or more points or a rolling reduction of 5ON or higher in a short period of less than 1 second is carried out.6゜The grain refining method using fjAK is a ferrite process due to the strain applied to the diaustenite phase during rolling. Based on the principle of inducing or promoting transformation, rolling is carried out at a temperature f( Ill more than 16) people! Yoshishi started, Ar
The transformation starts at a temperature that is no lower than the point s. In normal transformation, the next driving force for the transformation to proceed is supercooling, so the transformation start temperature λr5 varies depending on the thickness of the steel plate and the cooling rate. do. In this kind of transformation, the number of 7-item grains is determined by the grain boundary area of the austenite phase before transformation and the dense cape of the dead center with high strain, which becomes the transformation nucleus.
Ferrite particle size company 8 when granulation countermeasures are not taken
~10 voices mFK is normal. Working strain also becomes a driving force for the initiation and progression of transformation, and this is well known from the fact that the Ar15 point of austenite, where working strain remains, also increases (in the case where it does not remain). However, it has been known that under conditions where ferrite transformation occurs at the moment or after a very short time when austenite is subjected to rough rolling, the transformed ferrite grains are extremely fine. The present invention is novel in that it thoroughly utilizes machining strain as a driving force for transformation. Immediately after processing, fine ferrite grains are formed at grain boundaries of -hi-stenite immediately above the Ars point.
KIT precipitates after a short time of 7'ta, and when further processing strain is applied, the interface between ferrite grains and untransformed austenite F
L, repeating the process of precipitation of ferrite grains,
If the processing strain is sufficiently large, it is desirable to cool the material at a cooling rate of 20° C./sec or higher to reach a temperature of 600° C. or lower in order to produce new ferrite grains on the entire surface at the end of rolling.

After that, various thermal histories can be taken depending on the required characteristics.

For example, in order to improve the strength by 1, it is sufficient to rapidly cool it to around room temperature, and in order to improve the workability, K can be used.

After rapidly cooling to around 400° C., carbon may be precipitated in the same bath by slowly cooling from that temperature.

Examples of the steel of the present invention will be described below.

Converter melted steel containing the ingredients shown in Table 1 with a thickness of 200mm
This slab was heated to 1100° C. and hot rolled in a Horasis strip mill. Rough rolling was performed in 5 passes to a thickness of 200 mm and 50 mm, and finish rolling was performed to a thickness of 5 mm in 6 nonces. Finish rolling to make the final 2 noses 1
An example in which a pressure of 58X was applied within seconds is referred to as A (the present invention), and 1
．． A pressure of 27 N was applied to the final armpit within 2 seconds of □1'1° to give 9 examples ¥t'B (comparative example).

The desired finishing temperature of the present invention is from the transformation point of 2m steel to 680-870°C for steel, and 660-870°C for B.
The temperature is 890°C. The water cooling started approximately lf$ after the end of rolling.The finishing pass schedule for each is shown in Table 1, Section 2! ! ! Table 3 shows the microstructure 1 mechanical properties of the nine-gauge steel produced by the same method.

In this table, test A! -4.8 to 10 are the steels of the present invention, among which test JEI, 2.4, 8. iΩ is finish rolling
Test M5 of p1 comparative example, which is an example of fabrication with a cooling rate of 20C/see or higher to soo℃ or below, has a high rolling temperature (
(normal conditions), bainite and martensitic) O quenched structure has a large amount and the amount of ferrite is low at 40X, and the strength is high but the ductility is low. Therefore, the relatively coarse ferrite grains are formed into a processed structure in which the ferrite grains are stretched and stretched, but the average ferrite grain size is large.

Therefore, the ductility is somewhat insufficient and the strength is also low. In Tests A7 and 11, the rolling reduction was insufficient, so the ferrite grains were transformed during cooling, the ferrite grains were not thin enough, and the second phase of pearlite and bainite occupied 40XS degrees, and although the core strength increased, the ductility decreased. is not enough.

Details 1llKII! @do.

Figure 112 shows the optical microscopic structure of Test A4, and it can be seen that most of the structure consists of ultra-fine glazed ferrite grains. This organization! The electron microscope structure shown in Figure 3 is enlarged to reveal that it is composed of fine equiaxed crystal grains (ferrite) with an incandescent contrast. This composite fiber contains 70X or more of ferrite grains with a zero grain size of 4μ or less, and furthermore, the zero grain boundaries have a large tilt angle, that is, the crystal orientations of adjacent crystal grains are greatly different. When the zero grain size at grain boundaries becomes smaller.

Strength and toughness are improved, as shown in Figure 1.

When the average grain size becomes 4 or less, the yield stress and toughness improve rapidly compared to 900 million, which is predicted by the 0petch relational expression. 11 IWI of ultra-fine grain steel such as superplasticity
P-ness also appears in 4 tones or less.

Such a phenomenon was first obtained in a hot-rolled ultrafine ferritic steel such as the steel of the present invention. In other words, it appears only when the average grain size of the steel structure is 4μ or less and the grain boundaries are large-angle grain boundaries), for example, 7
In the case of a subgrain structure, which occurs when steel with a light structure is hot-processed, the grain boundaries have a slight tendency to differ slightly in orientation between the grains, so the fine grains have a negative effect on mechanical properties. The effects of change are not necessarily fully manifested.

However, the steel of the present invention has a structure after rolling.

It is a matter of course that within the grains surrounded by such large tendency grain ascension, there is further dislocation [0 uplift and subgrain formation] as a substructure.

In addition, the mechanical properties do not exhibit sufficient ** unless the O''ejii fine grain structure, which generally appears as an average of the properties of minute parts, occupies the majority, but the steel of the present invention has 70
Depending on the production conditions, fine-grained 7-elite of approximately 100X can be obtained in a volume of J&, which also satisfies these conditions.

In addition, in the above-mentioned IIIi fine grain structure of the steel of the present invention, %7 elite grains are overwhelmingly large regardless of the carbon content, and secondary grains such as pearlite are present.
A special feature is the small number of 1111.

For such the steel of the present invention, as shown in the optical microscopic structure shown in Comparative Example Test A50#I Hiro No. 411, the ferrite amount was about 40Xt, Kana (remaining #) Fi bainite and martensite structure, This causes poor ductility. In addition, test A6 (D steel has an amount of ferrite of about 85X, as is clear from the optical microscope structure in Figure 5, but it has a processed elongated structure, which causes poor ductility and insufficient strength. There is.

In the above description, the case where hot strip is manufactured as a product has been described, but it is clear that the steel material of the present invention can also be obtained by plate rolling, wire rod rolling, hot extrusion, etc.

As described in detail above, the steel of the present invention has a tensile strength of at least 5°/- or more, a yield stress of 40 kp/- or more, and sufficient ductility and workability as a practical steel. within a specific temperature range (600m 800℃)
It exhibits a superplastic phenomenon and has significantly improved ductility and good friction bonding properties.

:.

Moreover, since steel materials having such properties and properties can be obtained by hot rolling using steel materials mainly made of hypo-eutectoid steel pipes, the industrial effects thereof are enormous.

[Brief explanation of drawings]

Figure 1 Relationship between grain size, yield stress, and toughness wJ% No. 211~
In the micrographs of the 51I Hiro metal structure, 1wE281 is an optical micrograph of the steel of the invention, FIG. 3 is an electron micrograph of the steel of the invention, and No. 4m and 5wi are optical micrographs of the comparative steel. Agent: Masamitsu Aki Sawa et al. 24 265 20 Volume 3 yen / θ folding crystal 4 diagrams 1 hole 5 diagrams, :) Q, g (gold yen) Showa,
□year. Month. 2B Director General of the Japan Patent Office 1. Indication of the case Patent Application No. 55648/1983 3. Person making the amendment Relationship to the case Applicant Address (Residence) 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (Name) ) (665) Nippon Steel Corporation 4.
Address: Taiyo Building 4, 12-1 Nihonbashi Kabutocho, Chuo-ku, Tokyo
Date of 18' Showa Month: Day (Delivery) 5゛ Notification of reasons for refusal,,. 6. Number of inventions increased by amendment The description of contents of the amendment shall be amended as follows. (1) On the last line of page 2, ``proposed grain boundary)'' is corrected to ``r-angle grain boundary)''. (2) In the 4th purchase first line, ``In the element ru,'' is corrected to ``It refers to the element ru, but this.'' (3) Page 4, line 6, “Transformation and recrystallization of tenite
"..." is corrected to "transformation of tenite to ferrite and/or recrystallization of this and ferrite...". (4) On the last line of the fourth page, "(Hagane...") was replaced with "(Hagane...")
” he corrected. (5) Page 5, lines 10 to 12, “Therefore, rolling...
・Correct "The following is the same and" to "Therefore, rolling is effective for grain refinement." (6) In the 7th line from the bottom of page 5, "Starts from the temperature" is corrected to "From the temperature." (7) In the 6th line from the bottom of page 5, “Terminate at temperature.”
Within the temperature range. ” he corrected. (8) On page 5, lines 5 to 3 from the bottom, "It changes because it's supercooled." is corrected to "It's dark because of supercooling." (9) On the 5th page, 3rd line from the bottom, "The number of... is different" is changed to "・
The number of... is mainly strange.'' 0 [On the second to last line from the bottom of page 5, "grain boundary area, density, etc." is corrected to 1 grain size." 0υ Page 6, line 2, ``about 8 to 10 μ J° to ``8 to 1
"About 0μ or more," he corrected. (121, page 6, lines 3 to 4, "This means that machining distortion remains" is corrected to "therefore". 0'5 page 6, lines 4 to 6, rAr 3 points teeth···
Are known. ”, the rArg point rises. ” he corrected. 04) On page 6, lines 8 and 9, correct ``as it happens...'' to ``at this time.'' 09 In Table 3 on page 10, the amount of ferrite in Test 45 (
Correct "30" in h) to "40". aQ Page 14, lines 4-3 from the bottom, “Steel...
・Correct "Mainly made of steel" to "Steel made of hypo-eutectoid steel with few alloying elements."-26't

Claims (1)

[Claims] (1) O: 0.3X or less % Alloy content other than 0 3X
%substantially Nb,'raw MO,w
Hot-rolled steel material that does not contain T'', with large angle grain boundaries KWi rare 9 equiaxed 7E2ite crystal grains t-70X with an average grain size of 4# or less
Ultra-fine-grained 7-elite steel having a metal structure containing the above.