JPH0310046A - Fine-grained bainite steel - Google Patents

Abstract

PURPOSE:To make better the working treatability in the steel and to provide it with general usefulness by regulating the structure of the steel to the one essentially consisting of bainite in which the average size of the bucket of bainite and that of old austenite grains are regulated to the specified value or below of fine grains. CONSTITUTION:The structure of the steel is regulated to the one essentially consisting of bainite and having <=5mum average size of the bucket of bainite or that of old austenite grains before the formation of bainite. The fine-grained bainite steel can be manufactured by regulating the structural state of steel stock to the one in which at least a part is constituted of ferrite, raising its temp. to the range of the Ac1 point or above while plastic working is executed or, in succession to the temp.-raising, holding it to the temp. range of the Ae1, point or above for a certain time, inversely transforming a part or whole of the structure constituted of a ferrite phase into austenite for a time to bring ultra-fine austenite grains into existence and thereafter executing cooling.

Description

[Detailed description of the invention] <Industrial application field> This invention can be bent, cut, twisted and punched.

The present invention relates to a fine-grained bainite steel material that is highly versatile and has good processing properties such as rolling and wire drawing.

<Conventional technology and its issues] Traditionally, we have studied the properties of steel (strength, toughness, workability (plastic workability, cutting properties, bending properties, etc.), corrosion resistance, superplasticity, etc.)
It is widely known that "the finer the weave, the better the quality", and based on this recognition, various techniques are being used to refine the grains of steel and suppress grain growth. .And, for example, Fe-13~18wtχCr
-8J2wtχNi austenitic stainless steel is cold-worked at room temperature to induce strain-induced transformation of austenite into martensite, then heated and annealed to a stable austenite region, and the grain size is reversely transformed from martensite to austenite. : Report that austenitic steel material with 0.5m austenitic crystal structure can be obtained [Tetsu to Hagane, No. 74 (1988) No. 6, No. 1]
052-1057], or by strongly working low carbon steel in the austenite region above the transformation point to induce fine ferrite, and then immediately quenching it, the average grain size is reduced to less than 1 to 50%. Proposal to obtain a hot rolled steel material containing ferrite crystal grains of ~3 μm and having a hardened structure with the remainder being martensite or bainite [Special Publication No. 6]
No. 2-42021] was also made.

However, even with these known techniques, there is a limit to the refinement of the bainite structure itself in steel materials with a bainite-based structure, and it is difficult to easily determine trends in properties within the range of the microstructure achieved by these techniques based on conventional knowledge. It was not beyond the realm of possibility.

However, recently, the present inventors have discovered a means to achieve an ultra-fine structure that is far below the level known in the past for ferritic steel materials and pearlite A materials.
We succeeded in determining the fact that when the microstructure of these steels is refined to below a certain value, they begin to exhibit properties trends that exceed expectations. Furthermore, it has been confirmed that the properties of these ferrite-pearlite steel materials are almost controlled by the grain size of group va (ferrite grains or pearlite grains), which is the main component thereof.

Therefore, the present inventors are convinced that there is a method that can overcome the limit of microstructure refinement in conventional technology even in steel materials with bainite structure, and have developed a means for further microstructure refinement in steel materials with a bainitic structure as a main component. In order to elucidate the characteristic trends caused by

Means for Solving the Problems> Through the above research, the present inventors have obtained the following knowledge.

First, when a bainitic steel material is cooled from a hot state, a microstructure similar to "lamellae of pearlite grown in the same direction" is generated within the austenite grains. Each group of bainite leaves arranged in the same direction is called a ``pakeppi'', but in bainite steel, these packets are considered as ``units of structure'' similar to ferrite grains or pearlite grains in ferrite structure or pearlite structure. It has become clear that the carbon atoms play an extremely important role in determining the properties of steel materials.

By the way, since such a packet is generated from one austenite crystal grain, "the packet formed after bainite transformation" depends on the size of the original austenite grain, and changes according to the size. Therefore, in order to reduce the packet size of bainite, it is essential to refine the austenite grain size before transforming into bainite (original austenite grain size). However, although descriptions have been proposed to achieve considerable refinement of ferrite grains, it is difficult to refine austenite grains, and it is considered a dream to industrially realize an austenite grain structure of 5a or less, for example. was.

Therefore, it has been difficult to know the characteristics trends of fine bainite packet steel materials, which are considered to be difficult to form without such a fine-grained austenite structure as a pre-structure, and even to realize such steel materials.

However, in particular, [conventionally - steel material U used for boat fishing]
No matter how much the already existing austenite grains are processed by hot working, such as by loom refinement, as long as new austenite grains are generated by recrystallization during hot working, the austenite fineness remains at a high temperature. There is a limit to the size of the austenite grains, and the structure generated by transformation from the austenite grains is also constrained by the austenite grain size, so it is difficult to do anything about the fact that there is a natural limit to the size of the austenite grains. As a result of the research carried out, the present inventor has come to confirm the following facts. That is, when (al) steel is hot-worked, it is subjected to a thermal history or a working history similar to known hot working in the pre-processing stage, and then at least part of the steel structure changes to a low-temperature relaxation structure. After performing temperature control etc. to achieve the same result, as the final stage of processing, the temperature is raised to exceed the transformation point while adding plastic working to reverse transform the low temperature phase structure into austenite crystal grains, which is similar to conventional controlled rolling etc. It is possible to achieve an ultra-fine austenite structure that would be impossible to obtain with other methods.

(bl) Also, as mentioned above, the ultrafine austenite structure produced by reverse transformation is generated once a pre-structure (low-temperature relaxation structure) for reverse transformation is obtained during processing before hot working reaches the final stage. This can be achieved by placing the steel material under a certain temperature condition, and in the final stage of processing, plastic working is applied to this low-temperature structure while raising the temperature to exceed the transformation point. A subset VA (low-temperature relaxation structure) is prepared to form an austenitic structure through reverse transformation, and after first processing it in a cold temperature range or a warm temperature range, in the final stage of processing, This can also be achieved by increasing the temperature while applying plastic working to exceed the transformation point.

(C) As mentioned above, when applying plastic working to a low-temperature softened structure while raising the temperature to exceed the transformation point and reverse transform to an austenite structure, it is necessary to perform plastic working in order to fully complete the reverse transformation. It is also advantageous to employ means for maintaining the temperature at or above the A1 transformation point, ie, the Ae point, for a certain period of time in a complete equilibrium state after the temperature increase process carried out while adding is completed.

(d) The bainitic structure steel material obtained by cooling the ultrafine-grained austenite structure obtained in this way has an extremely fine packet bainite structure because the original austenite grains have been made ultrafine. It is possible.

(e) If the average diameter of the bainite packets in the "bainite-based steel" obtained by this process becomes less than 5, the strength of the steel material (workability
Strength, toughness, etc.) showed a significant improvement that was not expected based on conventional knowledge.

(f) By the way, in general, for bainite, especially bainite that has been tempered or aged, the state of the structure (
It is often easier to measure the size of prior austenite grains (austenite grains that are the precursor to bainite formation) based on the size of the prior austenite grains (rows of pro-eutectoid ferrite or network of columnar pro-eutectoid cementite); The austenite grain size and the average diameter of bainite packets are two sides of the same coin, and it is only when the former austenite grain size falls below 5 μm that the strength of steel materials mainly composed of bainite improves significantly. Because it is recognized, 3
The average packet diameter of bainite and the prior austenite grain size in the ftFfJ material can be considered to be the same index.

This invention was made based on the above findings, etc.
It has excellent workability that did not exist before.
The major feature is that it has realized an ultra-fine bainite steel material consisting of a bainite-based structure in which the average diameter of bainite packets or the average diameter of prior austenite grains before forming bainite is 5 μm or less. There is.

"Bainite packet" here refers to the above-mentioned ("Bainite packet")
It is defined as a region where the long planes of elongated bainite crystals are arranged almost parallel to each other, and the average packet diameter
refers to the average particle diameter when the above region is regarded as a particle. Furthermore, the "previous austenite grain average diameter" refers to the average austenite grain diameter in the austenite structure before bainite is transformed and generated, as described above.

In addition, the method of confirming the prior austenite grain boundaries in a steel material with a structure consisting mainly of bainite is that in hypo-eutectoid steels, the prior austenite grain boundaries are pro-eutectoid, which is generated by "ferrite transformation B at the austenite grain boundaries" that occurs prior to bainite transformation. In hypereutectoid steel, it is possible to confirm by checking the rows of ferrite, and in hypereutectoid steel, by checking the network of rows of pro-eutectoid cementite formed by "cementite precipitation at austenite grain boundaries" that occurs prior to bainite transformation.

Furthermore, "a structure mainly composed of bainite" refers to a structure containing 50% or more of bainite (herein referred to as bainite as it is transformed, tempered bainite, and aged bainite). When the ratio of bainite in the steel structure reaches 50%, the properties of the steel material are almost entirely controlled by the properties of bainite.

By the way, the composition of the steel material according to the present invention is not particularly limited as long as it has a structure mainly composed of haynite, and includes not only carbon steel but also low alloy steel, ferritic stainless steel, and precipitation hardened steel. Any material used in a bainite structure, such as type stainless steel or heat-resistant steel, may be used. Furthermore, B, V, Nb, T
t, Zr+ W, Co.

It may contain an appropriate amount of one or more alloying elements such as Ta, and depending on the purpose, it may contain rare earth elements such as La, Ce, (:a, Sl pb, Te, Bt and Se).
Also applicable are compositions containing free-cutting elements such as.

Next, the reason why the average diameter of bainite packets or the average diameter of prior austenite grains before bainite generation is set to 5 μm or less in the steel material of the present invention, and the means for producing the steel material of the present invention will be explained.

<Effect> The mechanical properties of bainite steel, especially elongation and reduction of area, improve as the bainite packet diameter or prior austenite grain size becomes finer, but when both of the above values become 5 μm or less, the improvement effect is greater than expected. Become recognized. In particular, when the packet diameter becomes 2 prn or less, the improvement effect becomes extremely remarkable. For this reason, the average diameter of the bainite packets, which account for more than 50% of the steel structure and control the properties of the steel of the present invention, or of the old austenite grains that are two sides of the same coin as mentioned above, was limited to 5 mm or less. However, it is desirable that it be 2 Em or less if possible.

By the way, the steel material according to the present invention is realized by the following manufacturing method. In other words, the material steel is made to have a structure in which at least a portion of it consists of ferrite (ferrite here refers to a structure consisting of a ferrite phase such as a ferrite structure, a pearlite structure, a bainite structure, and a martensitic structure), and this is given a plastic structure. While adding processing, the transformation point (
Ac, point) or above, or this temperature increase is followed by Ae.

This is a means of holding the material in a desired temperature range for a certain period of time to reversely transform part or all of the structure consisting of the ferrite phase into austenite so that ultrafine austenite grains appear, and then cooling.

Plastic working methods applied during the above-mentioned reverse transformation include known plate rolling mills, various rolling mills for seamless steel pipes, hole-type rolling mills for perforating two-strip steel, wire rods, etc., as well as well-known hammers,
Swager, stretch reducer, stretcher, twisting machine.

Any method can be used as long as it can be processed to the required degree of processing in the required temperature range by using an extruder, a drawing machine, etc., and is not particularly limited.

Note that the amount of strain during the plastic working is important in that it causes the following three effects. The first is that extremely fine austenite crystal grains are induced and generated from the work-hardened ferrite by processing ferrite, and the second is that the process of processing ferrite induces the production of extremely fine austenite crystal grains. This is an action that generates processing heat to raise the temperature of the workpiece, and the third action is to work harden the fine austenite crystals that are generated, and process even finer grains of the transformed structure during subsequent transformation. This is the action of producing induced transformation. From this point of view, the amount of strain in the plastic working is preferably 20% or more, preferably 50% or more.

It is essential that the temperature of the steel to be processed rises to a temperature at which ferrite reversely transforms into austenite, that is, the Act point or higher. Of course, even in the intended temperature range of Ac, if the temperature is below the Acx point, a two-phase mixed structure of ferrite and austenite will result, but if the temperature is increased and plastic working is added, Even in the temperature range of , the crystal grains are sufficiently refined by processing and recrystallization.

However, in order to fully exhibit the characteristic action and effect that "by processing ferrite, very fine austenite crystal grains are induced and generated from work-hardened ferrite", it is necessary to , it is desirable to raise the temperature to above the intended level. However, some products require a two-phase structure of ferrite and austenite, and for such products it is necessary to keep the heating temperature within the temperature range below the Ac= point. Needless to say.

As explained earlier, the reason why the temperature is raised while adding plastic working during reverse transformation from ferrite to austenite phase is "ferrite grain refinement due to working in the ferrite region".
, in order to achieve "work-induced generation of fine austenite grains from work-hardened ferrite grains,""refining austenite grains by working," and further "promote strain-induced transformation of fine bainite from work-hardened austenite grains." .

Next, the present invention will be explained more specifically based on Examples.

<Example> Steels A-E having the compositions shown in Table 1 were melted in a vacuum melting furnace, made into a 1-ton steel ingot, and then subjected to soaking and blooming rolling to make a square material with a cross section of 10100w x 50. .

Separately, a square piece of 45NX45 dragon cross section made of @B was prepared by hot forging, and then skewered.

Next, these steel materials were subjected to treatment under the conditions shown in Table 2, and steel materials having a structure mainly composed of bainite were fabricated. The Mi weave of each of the steel materials thus obtained was observed, and the results are also shown in Table 2.

Here, for test number 1 (comparative example), a skewered material with a cross section of 45n x 45n made of steel B was heated to 1000°C and then allowed to cool naturally, and the prior austenite grain size measured by the pro-eutectoid ferrite network was is 93.
Otrm, the final structure is 85% bainite and 15% ferrite, and the bainite packet diameter is 54.61rm.
Met. This structure is a general bainite structure obtained by normal heat treatment.

For test number 2 (comparative example), 45+ made of steel B
After heating the normalizing material with a cross section of nx45ms to 1000℃, 33
It was immersed in a salt bath at 0°C and held for 1 hour, then allowed to cool naturally outside the furnace. Although the prior austenite grain size is unclear due to the lack of pro-eutectoid ferrite, it is 90Q because the heating conditions are the same as test number 1. Estimated to be around. And the organization is 100%
With bainite, the packet diameter is slightly smaller than test number 1.
, it was 22.27+11.

Compared to such conventional materials, test numbers 3 to 11 relate to ultra-fine assembled ms materials obtained by a method of adding rolling work to the material steel in the process of heating it to austenite.

That is, in Test No. 3, a normalized material of Steel B with a 45wm x 45mm cross section was heated to 700°C and then high-speed rolled from that temperature to a thickness of 13.5mm using a 3-high continuous rolling mill. Since this rolling was high-speed, large-reduction rolling, the temperature of the rolled material rose to 905° C., which is higher than the transformation point. and,
The steel material obtained by this treatment has a bainite content of 90%
In the ferrite-bainite structure, the measurement result of the prior austenite grain size due to the pro-eutectoid ferrite network is 4.
Mark 03, the packet diameter of bainite was 3.67 mm, and the microstructure was an order of magnitude higher than that of conventional steel materials.

In Test No. 4, the same rolled material as in Test No. 3 was immediately immersed in a 330° C. salt bath for 1 hour after rolling, and then allowed to cool naturally. The thus obtained steel material had a prior austenite grain size of 3.88 μm, a bainite ratio of 95%, and a packet diameter of 1.591 rm.

In test number 5, a material with a thickness of 100 mm was heated to 900°C to austenite, and then allowed to cool to 750°C.
50% rolling was performed from this 750°C. In addition, the same rolling mill as in Test Nos. 1 to 4 was used for rolling, and the rolling speed was lowered so that the rolling end temperature was the same as the rolling start temperature. Immediately after rolling this 50 mm thick rolled material,
Insert and hold in electric furnace at 00℃ for 15 minutes
The mixed structure of pearlite and bainite was heated to 650° C. by high-frequency heating, and then rolled at a high speed of 89% from 650° C. to a thickness of 5.5 mm using a rolling mill of the same type as above. During this rolling, the temperature of the steel material increased to 900° C. due to processing heat, and the steel material exceeded the transformation point and changed to an austenite structure. Thereafter, this rolled material was immediately immersed in a salt bath furnace at 330° C. for 1 hour.

The thus obtained steel material had a prior austenite grain size of 1.79 μm, a bainite ratio of 95%, and a packet diameter refined to 1.04 mm.

In test number 6, a 100 mm thick material was heated to 650° C. in the ferrite range, and then rolled at high speed to a thickness of 50+n to give a reduction of 50% and a temperature rise to 850° C. At this stage, the rolled material that has become finely austenitized is
After ferrite-pearlite-bainite transformation in an electric furnace at 00°C, 5.5 mm as in test number 5.
It was heated and rolled to a thickness and then transformed into bainite in a salt bath.

Although the prior austenite grain size of the steel thus obtained was unclear, the bainite packet diameter was 0.43
It was 1.

In test number 7, after the first rolling to a thickness of 50 μl in test number 6, it was immediately quenched in water to obtain a quenched structure, which was then rapidly heated to 650°C in an induction heating furnace. The same processing as 6 was performed.

The prior austenite grain size of the steel thus obtained was barely discernible and was 0.31 μm. The bainite packet diameter was 0.19 μm, which was a fine structure that was completely unimaginable based on conventional knowledge.

In test number 8, a 100fi thick normalized material of steel A was heated to 650°C in the same way as test number 7, and the temperature was increased from 650°C to 850°C.
After rolling at 50% temperature to ℃ and quenching in water, further 6
Rolling at 89% temperature from 50℃ to 900℃, 33
Bainite transformation was carried out in a 0°C salt bath.

The steel thus obtained had a prior austenite grain size of 1.06 grains and a packet diameter of 0.621 km.

Test No. 9 is an example in which Steel C was subjected to the same treatment as Test No. 8, but the steel material obtained in this example had a prior austenite grain size of 0.75 μm, a packet diameter of 0.155 μI, and 80% bainite. The structure was a mixture of 15% retained austenite and 5% martensite.

Test No. 10 is an example of a tempered bainitic steel material obtained by subjecting steel to the same treatment as Test No. 8 and then tempering it at 500° C. for 1 hour.

This steel material has a prior austenite grain size of 1.62 μm,
Haynight's packet diameter was 0.98 mm.

In test number 11, 100 Long Atsushi semi-hardened material made of Steel E was
After heating to 00°C, the steel material temperature was raised to 850°C by high-speed rolling to a thickness of 50 m, and then allowed to cool naturally to room temperature. At this stage, the steel structure was a mixed structure of bainite, one-flutenside, and residual austenite, but this was further rapidly heated to 700°C by induction heating to create a structure of fine ferrite and spherical carbides.
mm JAW with 89% high speed and large reduction rolling
The temperature was increased to °C. After rolling, it was allowed to cool naturally.

The final structure of the steel thus obtained is 80% bainite, 15% martensite, and 5% retained austenite, which is micro! The prior austenite grain size estimated from the J1 weave was 0.35 μm, and the bainite packet diameter was 0.16 μm.

Next, a round bar micro tensile test piece with a parallel part diameter of 3.5 mm and a 10nx5
Cut out a 21 mV nanometer specimen (according to JIS d) of the weight, and machined a round bar with a diameter of 51 mm and a length of 45011 mm, and each was subjected to a tensile test at room temperature and a Charpy impact test. and subjected to a 4-point bending fatigue test.

These results are also shown in Table 2.

Note that the scoring distance for elongation measurement in the tensile test was 17.5 mm, which is five times the diameter. Furthermore, regarding the fatigue test, since the durability limit ratio (fatigue limit/tensile strength) is an important evaluation for such high-strength materials, Table 2 also shows the durability limit ratio.

As is clear from the results shown in Table 2, it can be seen that the ultrafine bainite steel material according to the present invention not only has improved strength, but also remarkable improvement in elongation and reduction of area compared to the conventional material (comparative material). Furthermore, the ultra-fine bainite steel material of the present invention has a significantly improved impact value, and it is clear that it is a steel material with unprecedented properties of toughness and high ductility.

Furthermore, regarding fatigue strength, if we focus on the durability limit ratio, conventional materials have a low strength of 0.45 to 100 kg.
Approximately 0.55, 0.3 to 0 when the strength reaches 150 kg class
．． 4, and 0.2 to 0.3 in the 200 kg class, whereas in the steel of the present invention, it is 0.3 to 0 in the 200 kg class.
．． It can be seen that it shows a surprisingly high value of 5.

By the way, for reference, in Table 2, test numbers 2 and 3~
The results of the investigation (corrosion resistance) of the corrosion progression rate by artificial seawater spray test for the bainitic steel material obtained in Test No. 7 are also shown, but this corrosion resistance investigation shows that the corrosion progression rate of the conventional material according to Test No. 2 was 0.04911/ In contrast, the steel materials of the present invention according to test numbers 3 to 7 have the same composition, so it clearly shows that the corrosion resistance is significantly improved to 0.011 to 0.002 m/year.

(Summary of Effects) As explained above, according to the present invention, it is possible to provide an ultra-fine bainite steel material that could not be practically realized with the conventional technology, and in addition to the characteristics unique to bainite steel material, it is possible to provide an ultra-fine bainite steel material that has extremely It has become possible to stably supply steel materials with unprecedented properties such as excellent workability, etc.
Industrially, extremely useful effects are brought about.

Claims (1)

[Claims] A fine-grained bainite steel material with excellent workability, consisting of a bainite-based structure in which the average diameter of bainite packets or the average diameter of prior austenite grains before bainite generation is 5 μm or less.