JPH02310338A - Fine-grained martensitic steel - Google Patents

Abstract

PURPOSE:To obtain the fine-grained martensitic steel widely useful in general purposes by regulating its structure as the one having specified average size of the packet of martensite or that of old austenitic grains before the formation or martensite. CONSTITUTION:In the steel, the average size of the packet of martensite or that of old austenitic grains before the formation of martensite is regulated to <=5mum. The steel can be manufactured by regulating the structural state of steel stock into the one in which at least a part is constituted of ferrite, raising its temp. to the transformation point (Ac1 point) or above while plastic working is executed to reversely transform a part or whole of the structure into austenite for a time and to allow ultra-fine austenitic grains to appear and thereafter executing cooling or the like. The steel has various characteristics such as excellent workability in addition to the characteristics peculiar to a martensitic steel.

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 martensitic steel material that is highly versatile and has good processing properties such as wire drawing and rolling.

<Prior art and its challenges> It has been said that the mechanical properties of steel (strength, toughness, workability (plastic workability, cutting properties, bending properties, etc.), corrosion resistance, superplasticity, etc.) improve as the structure becomes finer. It is widely known that steel grains are made finer and grain growth is suppressed using various techniques based on this recognition. For example, Fe −13 to 18wtχC
r-8J2ivtχNi austenitic stainless steel is cold-worked at room temperature to cause strain-induced transformation of austenite into martensite, then heated to a stable austenite region and annealed, and martensite is reverse transformed into austenite. It is reported that an austenitic steel material having an austenite crystal structure with a diameter of 0.5- 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 ~300 mm and having a hardened structure with the remainder being martensite or bainite [Special Publication No. 62]
-42021] was also made.

However, even with these known techniques, there is a limit to the refinement of the martensitic structure itself, and the characteristics trends within the microstructure range achieved by these techniques are beyond the range that can be easily predicted based on conventional knowledge. It wasn't something that would come out.

However, recently, the inventors of the present invention have discovered a means of achieving ultra-fine structures that are far below the level known in the past for ferritic steel and pearlite steel, and have discovered a method for achieving ultra-fine structures that are far below the levels previously known. We succeeded in investigating the fact that when cattle are refined to a level below this value, they begin to exhibit characteristics that exceed expectations. Furthermore, these ferrites,
It has also been confirmed that the characteristics of pearlite steel are almost controlled by the grain size of its main structure (ferrite grains or pearlite grains).

Therefore, the present inventors believe that there is a method that can overcome the limit of structure refinement in the conventional technology even in martensitic structure steels, and 'Further group v6. We conducted research from various perspectives in order to elucidate the means of miniaturization and the characteristics trends resulting from them.

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

First, in martensitic steel materials, groups of martensite leaves (martensite packets) that are constrained by crystal orientation relationships within austenite grains due to rapid cooling from hot.
It is clear that in martensitic steel, these packets act as a weave unit similar to the ferrite grain size in ferritic steel, and play an extremely important role in determining the properties of the steel. It became.

By the way, since these packets are generated from a single austenite crystal grain, the size of the packet after martensitic transformation depends on the size of the original austenite grain. It is essential to refine the previous austenite grain size (original austenite grain size).However, although descriptions that achieve considerable refinement of ferrite grains have been proposed, For example, it was considered a dream to industrially realize an austenite grain structure with a grain size of 5 or less, so the characteristics of fine martensitic packet steel materials, which are thought to be difficult to form without such a fine austenite structure as a pre-structure. Not only was it difficult to know about trends, but even the realization of the steel material was in doubt.

However, in particular, no matter how much the already existing austenite grains are processed by hot working, such as with conventional steel structure refinement methods, new austenite grains cannot be regenerated by hot working. There is a limit to the refinement of austenite, which is a high-temperature alloy as long as it is produced by crystals, and therefore the transformation structure generated from the austenite grains is also constrained by the austenite grain size, so there is a natural limit to the refinement. As a result of further research based on the viewpoint that "it is difficult to do anything about it," the present inventors have come to confirm the following fact. That is, (a) When hot working steel, the steel undergoes a thermal history or processing history similar to that in known hot working prior to working, and then at least a portion of the steel structure exhibits a low-temperature relaxation structure. After performing temperature control, etc., 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 U weave into an austenite structure. An ultra-fine austenite structure that is impossible to obtain can be achieved.

(b) Furthermore, as mentioned above, the ultrafine austenite structure produced by reverse transformation is obtained 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 such temperature conditions, 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 pre-structure (low-temperature braid m) is prepared to obtain a monostenite structure through reverse transformation from the stage, and this is first processed in a cold temperature range or a warm temperature range, and then processed in the final stage of processing. This can also be achieved by performing a process called ``raising the temperature while adding 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 AI transformation point in a complete equilibrium state, that is, the temperature at Ae, for a certain period of time after the temperature raising process carried out while adding is completed.

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

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

(fl) By the way, in general, martensite, especially tempered martensite, is determined by etching methods and structure conditions rather than by measuring the size of martensite packets. It is often easier to measure the size of prior austenite grains, and as mentioned above, the prior austenite grain size and the average diameter of martensite packets are two sides of the same coin. A significant improvement in the properties of a steel material is only recognized when the prior austenite grain size is below 5. Therefore, the average packet diameter of martensite and the prior austenite grain size in the steel material are similar indicators. It's okay to think about it.

This invention was made based on the above findings, etc.
It has excellent workability that did not exist before.
The average diameter of martensite packets or the average diameter of old austenite grains before forming martensite is 5 gm
The major feature is that it has realized an ultra-fine martensite M material consisting of the following martensite-based structure.

A "martensite packet" here is defined as a region in which the longitudinal directions of elongated martensite crystals are arranged almost parallel to each other, and the "packet average diameter" refers to the region considered to be a grain. It refers to the average grain diameter when Moreover, the "prior austenite grain average diameter" refers to the average austenite grain diameter in the austenite structure before martensite is transformed and generated, as described above.

Various methods are known for confirming the prior austenite grain boundaries in steel materials with a martensitic structure, but in practice, martensite is tempered and then a corrosive solution containing a surfactant is used to reveal the prior austenite structure.・It is preferable to adopt a method of confirmation.

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

By the way, the composition of the steel material according to the present invention is not particularly limited as long as it can obtain a structure mainly composed of martensite, and carbon steel may be used.

Low alloy steel as well as ferritic stainless steel.

Any material used in a martensitic structure, such as PH stainless steel or marage steel, may be used. Moreover, +3. V, Nb+ Ti, Zr, W
+It may contain an appropriate amount of one or more alloying elements such as Co, Ta, etc., and depending on the purpose, it may contain rare earth elements such as La, Cc, or free-cutting materials such as Ca, S, Pb, Te+old and Se. Component compositions with added elements are also covered.

Next, the reason why the average diameter of martensite packets or the average diameter of prior austenite grains before martensite generation is set to 5p 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> Mechanical properties of martensitic steel, especially strength.

The elongation and the aperture value improve as the martensite packet diameter or prior austenite grain size becomes finer, but when both of the above values become 51 rrn or less, a more significant improvement effect than expected is observed.

In particular, when the packet diameter becomes 24 m or less, the improvement effect becomes extremely remarkable. For this reason, the average diameter of the martensite packets, which account for 50% or more of the steel structure and control the properties of the steel of the present invention, or the prior austenite grains that are two sides of the same coin as mentioned above, was limited to 5 rrm or less. However, if possible, it is desirable that the number is 2 or less.

By the way, the steel material according to the present invention is realized by the following manufacturing method. That is, the material network 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 martensite structure), and this is given a plastic structure. While adding processing, the transformation point (
Ae.

This is a means of maintaining the temperature in the temperature range on the point plate for a certain period of time to reversely transform a 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 induce further fine ferrite grains during subsequent ferrite formation. This is the effect of producing metamorphosis. 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 material to be processed rises to a temperature at which ferrite reversely transforms into austenite, that is, above Ac, a point plate. Of course, even if the temperature is in the temperature range on the Ac point plate, if the temperature is less than the Ac3 point, a two-phase mixed structure of ferrite and austenite will result, but if the temperature is increased and plastic working is added, the Ac point Even in the temperature range below 100 mL, 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 point. 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 martensite from work-hardened austenite grains." .

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

<Example> Steels having the composition shown in Table 1 were melted in a vacuum melting furnace, and steels A, B, and C were made into 1-ton steel ingots, and then subjected to soaking and blooming rolling. It was made into a piece of rope with a cross section of 20m x 120mm.

In addition, for the rope and rope, the vacuum melted material was remelted in an ESR melting furnace and then made into a steel ingot.

Next, these steel slabs and steel ingots were heated to 980°C and hot forged, and then externally milled to form a 35m steel bar.
A steel material with a structure mainly composed of martensite was produced by processing the conditions shown in the table.

Regarding test number 1 (comparative example), a 35 m long steel bar made of w4A was heated to 980°C, rolled to a diameter of 17 mm in an 8-stand tandem mill, quenched in water in a rolling line, and then directly quenched. The input material was tempered at 650°C.

In test number 2, a 35Dφ steel bar made of steel A was heated to 700°C.
After heating, the same treatment as Test No. 1 was performed. At this time,
The rolling completion temperature of the rolled material had risen to 915°C, which exceeds the transformation point.

In test numbers 3 to 5, first, a 35nφ steel bar made of mA was heated to 980°C and then milled for 17m using an 8-stand tandem mill.
The material was rolled in a 10-stand tandem mill and allowed to cool to 500° C., then heated to 700° C. in a high-frequency heating furnace, and then rolled with a 90% reduction to a diameter of 5.5 cranes in a 10-stand tandem mill. After this rolling, test numbers 3 and 5 were quenched in water, and test number 4 was heated to room temperature. Furthermore, in test number 3, the rolled material was tempered at 650°C.

In addition, in test number 4, the rolled and cooled material was heated to 70% by high-frequency heating.
After reheating to 0°C, rolling was carried out to 87% reduction to 2 Omm, and quenching was performed by spray cooling after rolling, but the rolling end temperature at this time had risen to 925°C. Then, the rolled material after quenching was tempered at 650°C.

In test number 5, the water-quenched material with a diameter of 5.51 m was
After heating to 0°C by high frequency heating, 2. Rolling is performed with a 10-stand tandem mill at a reduction of 87% to Owmφ.
After rolling, it was spray cooled and quenched.

Although the 2.0iΦ rolling completion temperature at this time was 925°C, this rolled material was tempered at 650°C.

In test number 6, a 351m steel bar consisting of rope A was tested at 980m.
After heating to .degree. C., it was rolled to a thickness of 17 m using an 8-stand mill at 780.degree. C., then cooled with shower water to 450.degree. C., and then allowed to cool naturally. Then, this material is heated to 70% by high frequency heating.
After heating to 00℃ and rolling to 5.5mm diameter, rolling finish temperature:
940° C.) and immediately cooled with spray ice. Furthermore, this rolled material of 5.5 crane φ was again high-frequency heated to 700°C, and rolled with a reduction of 86.8% using a 10-stand mill.
．． After making Ofiφ and quenching with spray water cooling, 350
Tempering treatment was carried out at ℃.

In test number 7, a 351m long steel bar made of w4C was rolled under the same conditions as test number 6, but the shower water cooling after rolling 17 φ was set to 500°C, and 17n+φ-5.5Wm
The heating temperature was 750° C. during rolling into the medium and 5.5Hφ→2, Omm. And the final 2. The On+φ rolled and directly quenched material was aged at 550°C.

Test number 8 was conducted under the same conditions as test number 6.
A steel bar of Sflφ was processed, but in this case, the steel bar was subjected to 2.0 mm rolling and direct quenching, and no subsequent tempering treatment was performed.

Test No. 9 is an example in which the 2.0 nφ rolled and directly hardened material obtained in Test No. 8 was further subjected to aging treatment at 530°C.

In test number 10, a 35 mm diameter steel bar made of steel E was tested at 98 mm.
After heating to 0°C, it was rolled at 830°C to a thickness of 17.0 mm, then allowed to cool naturally, and then heated to 60 mm by high-frequency heating.
It was heated to 0°C, rolled to a diameter of 5.5 cranes, and then allowed to cool. In addition,
At this time, the rolling end temperature had risen to 800°C. Then, the 5.5 crane medium-rolled material was further reheated by high-frequency heating and then rolled to 2.0 crane diameter (rolling end temperature 2800℃).
), the material was rolled and then allowed to cool, resulting in a completely martensitic structure), and this material was aged at 500°C.

Table 2 shows the results of observing the structure and investigating the mechanical properties of each of the steel materials thus obtained. The prior austenite grain size was measured by a method of exposing the prior austenite structure by etching.

As is clear from the results shown in Table 2, the martensitic M material according to the present invention has higher strength and superior ductility than the conventional material (comparative material), and moreover, test number 2 As seen in Figures 5 to 5, it can be seen that the improvement in characteristics accompanying the miniaturization of the group wi is extremely remarkable.

By the way, Table 2 also shows the investigation results (corrosion resistance) of the rate of corrosion progression (corrosion resistance) of the obtained martensitic steel materials by artificial seawater spray tests, and this corrosion resistance investigation also showed that the materials of the present invention have low alloys! Significantly superior results compared to 0.093 m/year of conventional structured steel material (average austenite grain size: 14.7 dust controlled rolling-direct quenching material, i.e. test number 1 material) based on l14 (Steel A). is obtained.

(Summary of Effects) As explained above, according to the present invention, it is possible to provide an ultra-fine martensitic steel material that could not be practically realized with the conventional technology, and in addition to the characteristics unique to martensitic steel materials, This brings about extremely useful effects industrially, such as making it possible to stably supply steel materials with unprecedentedly excellent mechanical properties such as extremely excellent workability.

Claims (1)

[Claims] The average diameter of martensite packets or the average diameter of prior austenite grains before forming martensite is 5 μm
Consisting of the following martensite-based tissues:
Fine-grained martensitic steel with excellent workability.