JP2005213628A - Steel material having fine structure and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a high strength steel material in which fine crystal grains are produced even by simple heat treatment as well as a residual austenite structure is produced so as to realize not only high strength but high ductility, high toughness, resistance against brittle cracking, wear resistance, fatigue resistance and superior surface treatment property which are required for various usage in accordance with the usage such as cars, containers, tanks, buildings, ships, civil engineering, rails, electronic appliances, steel pipes and the like where a steel material is generally used. <P>SOLUTION: The composition is controlled to contain 0.05 to 1.5% C, ≤10.0% Mn, ≤3.0% Si, ≤3.0% Al, ≤1.0% Ni, and ≤5% Mo, and the material is heat treated at a temperature where the austenite phase is present by ≥70% by volume. Then, the material is cooled to control the grain size of the ferrite phase in the metal structure to ≤3.0 μm and to allow the austenite phase to remain. In order to obtain more significant effect of making a fine structure, the material is treated under predetermined conditions of thermal hysteresis including heating temperature/time, heating/cooling rate or the like as well as times of heat treatment, distortion added during heat treatment, or the like. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to all steel materials used as automobile members, building members, electrical equipment components, containers, tanks, shipbuilding, rails, original plates, steel bars, wire rods, steel pipes, etc. Steel materials that are excellent in strength, workability, weldability, toughness, wear resistance, fatigue properties, etc., and that can impart good adhesion such as plating and coating without impairing these properties, and It relates to its production method and its use.

  Conventionally, in steel materials used as members in various directions, the strength as a structural member and the workability to form the member, the weldability as the strength of the joint when joining with other members, and the toughness during use Various properties such as coating in the case of surface treatment for imparting wear resistance, fatigue strength and corrosion resistance, or adhesion of plating are required. When considering the improvement of these characteristics, it is often difficult to achieve both of them because some of these have conflicting actions. For example, with regard to strength and workability, in general, the workability of a material deteriorates as the strength increases. In particular, with regard to the increase in strength, against the background of increasing awareness of energy and environmental issues in recent years, there has been a trend toward reducing the weight and reducing the amount of materials used by applying high-strength materials. Dislocation strengthening, solid solution strengthening, structure strengthening, and the like are applied as methods for increasing the strength, but strengthening by refining the structure is attracting attention from the viewpoint of achieving compatibility with the various properties described above. Since this method does not require the addition of a large amount of special elements, it is expected to expand its application in a wide range from the viewpoint of recycling.

The refinement technology in steel materials mainly composed of ferrite phase, which is the subject of the present invention, is disclosed in Japan Iron and Steel Association, Iron and Steel, Vol. 85, p. 691, etc. The reachable crystal grain size was at most several μm.
In recent years, technological development to form a crystal grain size smaller than 1 μm in ferritic steel has been promoted by industry-academia cooperation. Japan Iron and Steel Institute, Materials and Processes, Vol. 14, p. 502, JP 2000-73034, JP 2000- The method is disclosed in Japanese Patent No. 96137, etc., but the method imparts very high strain by mechanical milling or the like, which is not only difficult for industrial practical use but also thermally unstable. There is a high possibility that problems will occur in applications involving welding. In addition, it has been reported that workability, particularly uniform elongation that is absolutely necessary for material processing, is significantly deteriorated with miniaturization, and the use is expected to be limited even if manufactured industrially. .

  As a high-strength steel with an excellent balance between strength and ductility, steel has been developed that utilizes a processing-induced transformation in which the austenite phase remaining in the steel mainly composed of the ferrite phase is transformed into hard martensite by processing. . This does not include expensive alloy elements, but uses 0.07 to 0.4% C, 0.3 to 2.0% Si, and 0.2 to 2.5% Mn as basic alloy elements, and generates austenite in a high-temperature two-phase region. This is a steel in which austenite remains in the metal structure even at room temperature by performing bainite transformation at about 400 ° C., and is generally called “residual austenitic steel” or “TRIP steel”. The technique is disclosed in, for example, Japanese Patent Laid-Open Nos. 1-2230715, 1-79345, 9-241788, and the like. However, these steels utilize their unique bainite transformation to retain austenite, so unless the heat treatment conditions (temperature, time) are strictly controlled, the intended metal structure cannot be obtained, and good strength and elongation are guaranteed. This is a factor that hinders yield improvement during manufacturing. Furthermore, since a large amount of Si content of 0.3 to 2.0% is essential, the adhesion of plating is poor in galvanizing and the like, and in hot dip plating, a favorable metal structure may be destroyed due to thermal history during plating. Industrial use is hindered. In addition, in steel materials utilizing these retained austenite structures, studies have not been made from the viewpoint of obtaining fine characteristics by controlling the complicated heat treatment as described above, and making it easy for those skilled in the art. No technology has been disclosed in detail to the extent that it can be implemented.

As a technique for solving these problems, the present inventor made Japanese steel No. 2003-27399 contain a large amount of N in the steel, which is unthinkable until now, based on ordinary steel (low Cr, Ni steel). An application has been filed regarding a technology related to steels that are mainly composed of a ferrite phase and have a retained austenite structure that has a crystal grain size of 1 μm or less only by simple heat treatment. However, this technology has a problem in terms of productivity because it is necessary to contain a high concentration of N that is difficult to manufacture in the molten steel-solidification currently performed in the normal steel manufacturing industry. there were.
Japan Iron and Steel Association, Materials and Processes, Volume 14, page 502 JP 2000-73034 A JP 2000-96137 A JP-A-1-230715 JP-A-1-79345 JP-A-9-241788 Japanese Patent Application No. 2003-27399 (prior application)

  An object of the present invention is to achieve both the formation of an ultrafine structure and a retained austenite structure in facilities and processes that are normally used at present, and to obtain more preferable characteristics.

Conventionally, C has been used as an element to form retained austenite texture, but the transformation behavior from the austenite phase to the ferrite phase is complex, and pearlite, bainite, and martensite are representative for obtaining retained austenite texture. It was necessary to precisely control the transformation behavior. Therefore, it is extremely difficult to achieve both the formation of the ultrafine structure using the austenite phase by addition of C and the formation of the retained austenite structure, and even if realized, the productivity is extremely poor.
On the other hand, refinement of crystal grains has conventionally required precise temperature control and strain control in mechanical milling and hot rolling, addition of grain growth inhibiting elements, etc. , I could not escape the productivity disadvantage.
Under such circumstances, it was found that a steel containing a large amount of N can obtain a thermally stable and very fine structure containing residual austenite structure even with a very simple thermal history. Patent application 2003-27399 was filed for the grain refinement technology applying the principle.

While pursuing this mechanism, the present inventor obtained knowledge that there is a deep relationship between the refinement of this organization and the formation and suppression of nitrides.
Based on this knowledge, the present inventor applies the above knowledge to ordinary C-containing steel, that is, suppresses the formation of carbides, particularly cementite, in C-containing steel, and preferably controls heat treatment conditions such as heating rate. By combining the control of the cooling rate and the like, it has succeeded in obtaining the refinement of the organization and the formation of the retained austenite organization with ordinary equipment and processes.
In order to solve the above problems, the present invention is a component in which a large amount of austenite phase is formed during holding at high temperature in heat treatment and carbide formation during cooling is sufficiently suppressed, In the heat treatment, cooling is performed from a temperature range in which the austenite phase becomes a predetermined amount or more, and desirably, the heating rate and the cooling rate during the heat treatment are controlled within a predetermined range. The present invention eliminates the need for precise temperature control, strain control, grain growth inhibitory elements, etc. in hot rolling, etc., which have been conventionally performed in ultra-fine crystal structure, and is conventionally performed in the formation of residual austenite phase. Simple heating and cooling without the need to control the bainite transformation by concentrating the austenite phase-stabilizing element in the austenite phase by suppressing the formation of the austenite phase during high temperature holding, and by precise heat treatment during cooling By this heat treatment, an ultrafine structure containing an appropriate amount of retained austenite phase with the ferrite phase as the main phase is finally formed. This technology not only eliminates the need for special equipment for hot rolling under large pressure as in the prior art, but also increases the degree of freedom of thermal history during cooling, enabling highly efficient production. An adverse effect on surface treatment such as plating by heat treatment can be avoided.

That is, the gist of the present invention is that
1) The maximum temperature achieved in the heat treatment is controlled in consideration of the complete austenitizing temperature.
2) Thermal history so that the solid drag effect due to solute C during cooling, the pinning effect due to carbides formed at low temperatures compared to carbides, and the transformation nucleation from finely dispersed carbides formed at low temperatures can be fully utilized. To control.
3) To control the thermal history in consideration of the dispersion state of austenite as a composite structure, relaxation of strain accompanying volume change due to low-temperature transformation, and control of the morphology of the nitride that remains fine after the transformation is completed.
Specifically, the gist of the present invention is the following contents as described in the claims.
(1) By mass%, C: 0.05-1.5%, Si: 3.0% or less, Mn: 0.01-10.0%, P: 0.0001-0.3%, S: 0.0001-0.1%, Al: 3.0% or less, N: 0.0001 In the temperature range from room temperature to melting, there is a temperature range in which the austenite phase abundance ratio is 70% or more by volume ratio, and the crystal grain size mainly composed of ferrite phase is 3.0 μm or less on average A steel material having a fine structure characterized by having a structure of
(2) 3 * (0.5 * Mn + Ni) <8 + Cr + 1.5 * Sl + 1.5 * Al + 10 * P <4 * (0.5 * Mn + Ni + 2.5) Steel material.
(3) A steel material having a fine structure according to (1) or (2), characterized by containing, by mass%, Mo: 5.0% or less, Nb: 1.0% or less, and Ni: 10.0% or less.
(4) Further, the steel material having the fine structure according to any one of (1) to (3), further comprising, by mass%, Cr: 20% or less, Ti: 0.2% or less, and B: 0.02% or less. .
(5) The steel material having the fine structure according to any one of (1) to (4), wherein the volume fraction of the ferrite phase is substantially 50% or more and the volume fraction of the austenite phase is 20% or less.

(6) When manufacturing the steel materials according to (1) to (5), heat treatment is performed at a temperature at which the austenite phase is 70% or more in volume ratio, and then the crystal grain size is 3.0 μm by cooling. The manufacturing method of the steel materials which have the microstructure characterized by the following.
(7) The method for producing a steel material having a microstructure as described in (6), wherein the crystal grain size is 3.0 μm or less by performing heat treatment at Tmax−50 ° C. or more and then cooling.
Where Tmax: the temperature at which the austenite phase is austenitized when the steel material is completely austenitized, otherwise the temperature at which the austenite phase abundance is maximum (8) the temperature at which the austenite phase abundance is 70% or more by volume When the heat treatment is carried out, the heating rate is 2 ° C / second or more, the maximum temperature is the temperature at which the austenite phase is present at a maximum + 200 ° C or less, and the cooling rate is 2 ° C / second or more, so that the crystal grain size is 3.0. The method for producing a steel material having a fine structure according to (6) or (7), characterized in that the thickness is not more than μm.
(9) The method for producing a steel material having a microstructure according to any one of (6) to (8), wherein a heat treatment that causes ferrite-austenite transformation is performed a plurality of times.
(10) The method for producing a steel material having a fine structure according to any one of (6) to (9), wherein processing is performed in the middle of heat treatment.
(11) The fine structure according to (10), wherein the processing is performed in a temperature range of 200 ° C. or higher and Tmax + 200 ° C. or lower, and the strain in a specific direction to be applied is 0.1 or more in logarithmic strain. The manufacturing method of the steel materials which have this.
(12) After forming a structure having an average crystal grain size of 3.0 μm or less mainly composed of ferrite phase, stay in a temperature range of 50 to 550 ° C. for 10 seconds or more, and then do not hold it at a temperature exceeding 550 ° C. (6) thru | or the manufacturing method of the steel materials which have the fine structure as described in (11) characterized by the above-mentioned.
(13) After cooling from a temperature of 650 ° C. or higher to a temperature of 400 ° C. or lower at a cooling rate of 10 ° C./second or more to form a structure having an average crystal grain size of 3.0 μm or less mainly composed of a ferrite phase, The method for producing a steel material having a fine structure according to (12), wherein the steel material is allowed to stay for 10 seconds or more in a temperature range of 550 ° C., and thereafter is not maintained at a temperature exceeding 550 ° C.

  According to the present invention, an appropriate heat treatment is applied to a steel material whose components are controlled so that the amount of austenite phase generated at high temperature is favorable, and a crystal structure mainly composed of a ferrite phase is refined and a residual austenite structure is formed. In addition to increasing strength, steel products are generally used for automobiles, containers, tanks, buildings, shipbuilding, civil engineering, rails, electrical equipment, steel pipes, etc. It is possible to obtain a high-strength steel material that is compatible with high toughness, resistance to embrittlement cracking, wear resistance, high fatigue, and good surface treatment.

The reasons for limiting the steel components in the present invention will be described in detail below.
C, like ordinary TRIP steel, is an important element in the present invention. C is necessary to obtain the fine structure which is a feature of the present invention. That is, in combination with the influence of Mn described later, the ferrite-austenite transformation is cooled at a lower temperature in the cooling process from the temperature range where the austenite phase exists, and in the transformation process, a large amount of solute C is present in the austenite phase before the transformation. It is an indispensable element for exhibiting the effect of suppressing the grain growth of the ferrite phase after transformation and also suppressing the grain growth of the ferrite phase after transformation. Furthermore, depending on the content of Si, Mn, Mo, Nb, etc. and the thermal history, the carbide formation occurs at low temperature by controlling the amount of solid solution during cooling at a low temperature, so the carbide becomes finer and the pinning effect causes the ferrite phase to change. Suppress grain growth. In addition, the vicinity of the interface of fine carbides present in the austenite phase forms a C-depleted region and may become the nucleus of the ferrite transformation, which has the effect of contributing to the refinement of the ferrite structure after transformation. If the C content is less than 0.05%, the austenite structure becomes coarse during the high temperature holding required by the present invention, and the effect of ultrafine structure after the transformation of the present invention cannot be found, or a high concentration of alloy is added to obtain the effect. Or, since strict heat treatment is required, the lower limit is set to 0.05%. On the other hand, excessive C content tends to form a large amount of cementite in the steel and may impair ductility, so the upper limit is made 1.5%.
Regarding the lower limit, there is a tradeoff with other elements, particularly Mn, Si, Al, P, Cr, Mo, Nb, which are strongly related to transformation temperature and carbide formation, but preferably 0.085%, more preferably 0.10%, Preferably it is 0.15%, more preferably 0.20%, more preferably 0.25%, and still more preferably 0.30%. The upper limit is preferably 1.0%, more preferably 0.80%, still more preferably 0.60%, and still more preferably 0.50%.

  Mn is also an important element in the present invention and, like C, is an austenite stabilizing element. Therefore, Mn affects the above-described transformation behavior and contributes to the formation of ultrafine grains. In addition to C, increasing the amount of Mn makes it possible to effectively lower the transformation temperature while suppressing the formation of harmful excess carbides. If the amount of C is sufficiently high and the effects of other austenite stabilizing elements such as Ni can be used, the amount of Mn may not need to be increased. However, considering the alloy cost, Mn is an effective element. In some cases, it is inevitably contained from steel raw materials and the like, and it is not necessary to reduce the cost. The lower limit is 0.01%. Although depending on the amount of other elements, if the Mn concentration is less than 0.6%, the austenite structure becomes coarse during the high temperature holding required in the present invention, and the effect of ultrafine structure after transformation of the present invention is small or predetermined. In order to obtain an effect, a high concentration of alloy addition or strict heat treatment is required. For this reason, 0.8% or more is preferable, more preferably 1.0% or more, more preferably 1.2% or more, more preferably 1.5% or more, more preferably 1.9% or more, more preferably 2.2% or more, and further preferably 2.5% or more. To do. The upper limit does not need to be particularly limited, but excessive addition not only increases costs but also tends to cause casting problems, surface defects or surface treatment problems, and also stabilizes the austenite excessively and ultimately In some cases, a large amount of austenite phase remains at room temperature, and the effect of refining crystal grains mainly composed of ferrite phase may be impaired. More preferably, it is 6.0% or less, More preferably, it is 5.0% or less, More preferably, it is 4.0% or less, More preferably, it is 3.5% or less.

In general, Si is an element added to increase the strength by solid solution strengthening and to suppress the formation of cementite during cooling and increase the effect of refining the structure and the amount of retained austenite in the steel of the present invention. If the amount is too small, the austenite structure becomes coarse during the high temperature holding required in the present invention, and the effect of ultrafine structure after the transformation of the present invention may be reduced. On the other hand, if the amount added is increased, the transformation temperature rises and heat treatment at high temperature is required for refining the structure, and there is also a balance with the amount of C and Mn, but the austenite phase abundance at high temperature If it decreases, the effect of miniaturization due to transformation in the present invention cannot be obtained. Considering this, the content is set to 0.001 to 3.0%. Preferably it is 0.2 to 2.5%. The lower limit is more preferably 0.5% or more, more preferably 0.8% or more, further preferably 1.2% or more, more preferably 1.6% or more, and further preferably 2.0% or more. It is preferable that
Al is generally used as a deoxidizer, but the addition amount in the present invention is determined in the same manner as Si described above. In addition, molten steel containing a large amount of Al is liable to cause nozzle clogging or the like during casting, which hinders productivity. Furthermore, it is 3.0% or less because it may cause wrinkles on the steel surface. Preferably it is 0.2 to 2.5%. The lower limit is more preferably 0.5% or more, further preferably 0.8% or more, more preferably 1.2% or more, further preferably 1.6% or more, and further preferably 2.0% or more.

It is not necessary to add P, but as with Si, if it is in an appropriate amount, it is an effective element to achieve high strength without deteriorating ductility. It can be used as necessary because it has the effect of refining and has the effect of supplementing the ultra-fine effect of the present invention and can compensate for the deterioration of ductility and increase the strength remarkably as much as possible. It is valid.
Although there is a trade-off with other elements, if the amount is too small, the austenite structure becomes coarse during the high temperature holding required in the present invention, and the ultrafine structure effect after transformation of the present invention may disappear. However, care must be taken to raise the transformation temperature in the same manner as Si, Al, and the like. Considering the cost of removing P and ductile deterioration due to excessive addition, it is 0.001 to 0.3%. Preferably it is 0.1% or less, more preferably 0.05% or less.
In the present invention, it is not necessary to add S as well, and it is preferable that it is low because MnS is formed and the effect of Mn required by the present invention is reduced. Further, if a large amount of coarse MnS is present, the ductility may be deteriorated, and the content is made 0.0001 to 0.1%. Preferably it is 0.05% or less, usually 0.02% or less.
N is usually 0.0002 to 0.04% without any problem as long as it can be produced by a production process of ordinary molten steel-steel making-continuous casting. If it is less than this, the steelmaking cost will increase, and if it is more than this, it will be difficult to obtain a good steel sheet. Preferably it is 0.0010 to 0.020%, More preferably, it is 0.0012 to 0.015%.

Mo, Nb and Ni have special meanings in the present invention.
Mo is known as an element that suppresses the formation of carbides and is effective for the effects of the present invention. Considering the addition cost, etc., make it 5.0% or less. Preferably 0.2% or more, more preferably 0.5% or more, more preferably 1.0% or more, more preferably 1.5% or more, still more preferably 2.0% or more, more preferably 3.0% or more.
Nb is also known to have the same inhibitory effect on carbide formation as Mo. It is also known that the structure is refined due to the segregation at the grain boundary and the solubilized drug effect. On the other hand, carbides may be formed and the solid solution C required in the present invention may be reduced. Therefore, excessive addition should be avoided and 1.0% or less should be avoided. Preferably it is 0.002-0.8%, More preferably, it is 0.02-0.6%, More preferably, it is 0.1-0.5%, More preferably, it is 0.2-0.4%.

Ni is an austenite stabilizing element and has a favorable effect on the effect of the present invention over Mn, but considering the addition cost, it is made 10% or less. Although there is a trade-off with other elements, if the amount is too small, the austenite structure becomes coarse during the high temperature holding required in the present invention, and the ultrafine structure effect after the transformation of the present invention may be lost. However, excessive addition may stabilize austenite excessively and finally leave a large amount of austenite phase to room temperature, which may impair the effect of refining crystal grains mainly composed of ferrite phase. It is preferably 0.2% or more, more preferably 0.5% or more, further preferably 1% or more, more preferably 1.5% or more, and further preferably 2% or more. It is much more advantageous in terms of cost to add a large amount of Mn, which has the same effect as Ni, for the ultrafine effect.
Note that Cr, Ti, and B may form carbides and reduce the amount of dissolved C required in the present invention.
Cr is a carbide forming element, and since it may reduce the amount of dissolved C during cooling, which has an important role in the present invention, excessive addition is not preferable. Considering the transformation temperature in order to avoid the effect on corrosion resistance and the addition cost, and to make it a completely non-transformed steel like Si, the preferable range is 20% or less. If it is preferably 10% or less, more preferably 5% or less, and 3% or less, the influence of carbide formation is greatly reduced.

Ti is also a strong nitriding element, and excessive addition is not preferable. However, when it is present in an appropriate amount, it has the effect of forming very fine nitrides and supplementing the effect of ultra-fine crystal grains, and it is possible to increase the strength significantly as it compensates for the deterioration of ductility. It is also effective to use as needed because the effect of delaying transformation is recognized. Ti is 0.2% or less, more preferably 0.1% or less.
B forms a carbon boride in steel with a high C content as in the present invention, and not only reduces the amount of solid solution C, but also excessive addition because it tends to cause embrittlement cracks starting from the carbon boride. Is not preferred. On the other hand, since it has an effect of stabilizing austenite, it can be added in an appropriate amount.
B is 0.02% or less, preferably 0.005% or less.
In addition, Sn, Sb, Bi, V, W, Ta, Se for the purpose of improving various usage characteristics not described in this specification, and for the purpose of improving manufacturing problems such as castability and rollability. Addition of appropriate amounts of various elements such as these does not impair the effects of the present invention. However, considering the effect on workability, etc., it is desirable to keep the elements at 0.5% or less and the total at 2% or less.

The feature of the steel of the present invention is that the crystal grain size is very fine. Since a normal high-strength steel material has a grain size of about several μm to 10 μm, the diameter of the crystal grain of the steel of the present invention is limited to 3.0 μm or less. Desirably 2.0 μm or less, more preferably 1.0 μm or less, and 0.5 μm or less by controlling the conditions for the refinement of the structure. Further, precise control of heat treatment conditions, multiple heat treatments, and the influence of strain due to processing Can be further refined to 0.2 μm or less and 0.1 μm or less. The finer the particle size, the clearer the characteristic features. Moreover, favorable effects can also be obtained with respect to characteristics that are expected to be improved from conventional knowledge, such as wear resistance and fatigue characteristics, due to the refinement of the structure.
Although the present invention basically has a ferrite phase as a main phase, it contains a relatively large amount of C, Mn, Ni, which are austenite stable elements, and relatively contains Si, Al, Mo, Nb, which suppresses the formation of cementite. The austenite structure remains in the structure because it is contained in a large amount and the refinement of the crystal structure is finally caused by the transformation from austenite to ferrite.
The retained austenite structure is effective in improving the strength-ductility balance. To obtain this effect, the volume ratio of the retained austenite structure is preferably 2% or more. Further, even if the volume ratio becomes a considerable amount, good characteristics due to the fine structure are not significantly inhibited. However, when extremely severe molding is performed on a material with a volume fraction of retained austenite exceeding 20%, the martensite phase generated by deformation induced by strain during processing causes stress concentration and decreases ductility. In some cases, and a large amount of martensite phase present in the press-molded state may cause a decrease in secondary workability and impact resistance, so the volume ratio of retained austenite is preferably 20% or less. Considering the existence of various phases such as martensite phase and bainite phase, which are present in small amounts in addition to the austenite phase, and various phases such as nitrides and carbides due to Fe or additive elements, the preferred range is the volume of the ferrite phase. The rate is 50% or more.

  Further, the size of the remaining austenite phase also affects the characteristics. For example, even if the main ferrite phase is as fine as 1 μm or less, if the austenite phase is about 5 μm, stress concentration may occur and the ductility may deteriorate as described above. For this reason, the size of the austenite phase, martensite phase, and bainite phase other than the ferrite phase needs to be approximately the same as that of the ferrite phase. Further, in the present invention, since a bainite transformation like a normal TRIP steel is not used as a main transformation mechanism, a large amount of bainite structure is not generated unlike a normal TRIP steel. In normal TRIP steel, the bainite transformation is used to form the retained austenite phase (volume ratio of bainite structure) / (volume ratio of austenite phase) is usually 1.0 or more. The ratio does not exceed 1.0, and is 0.5 or less, further .0.2 or less, 0.1 or less. However, it goes without saying that the presence of the bainite structure and the martensite phase is not completely zero. If the heat treatment disclosed in the present invention is followed, the above-mentioned state concerning the structure is almost satisfied.

  It is necessary to pay attention to the technology that forms the retained austenite structure by adjusting the amount of C, Si, Al, Mn, etc., and finely adjusting the thermal history when cooling from a high temperature range and controlling the transformation behavior. Therefore, enhancing the properties of steel materials such as strength and workability by itself is a technology with no novelty. However, a technique for refining the crystal structure to 3 μm and further to 1 μm or less only by heat treatment has not been put into practical use at present, and there is no technique aiming at coexistence with the formation of residual austenite structure. Further, in the present invention, the transformation from the austenite phase to the ferrite phase is mainly utilized during the heat treatment to express the effect of refining the structure and to form the retained austenite structure, whereas in the conventional TRIP steel, mainly from 300 ° C. A residual austenite structure is formed by utilizing the bainite transformation that occurs at 450 ° C., where no consideration is given to miniaturization, which is completely different from the technique intended by the present invention.

  In the present invention, the austenite phase needs to be generated in the steel by the heat treatment in order to achieve the refinement of the structure mainly composed of the ferrite phase in the final product by utilizing the ferrite-austenite transformation in the heat treatment. If the amount of formation is small, the structure of the portion that remains in the ferrite phase becomes coarse and exhibits a mixed grain structure and deteriorates the characteristics. This point is different from the conventional TRIP steel. In the conventional TRIP steel, C or Mn is concentrated in the austenite phase generated during high temperature holding, the austenite phase is stabilized, and remains after cooling. Therefore, the high temperature holding temperature is controlled so that the austenite phase does not increase so much.

The amount of austenite phase produced at high temperatures in normal TRIP steel is usually about 20-30%, at most about 50%. On the other hand, in the steel of the present invention, when the amount of austenite phase produced at high temperature is small as described above, a problem occurs, and the effect of ultra-fine structure of the structure of the present invention does not appear. For this reason, in the present invention, it is necessary to adjust the composition so that at least 70% by volume exists as an austenite phase in the range from room temperature to the melting temperature. Preferably it is 80% or more, more preferably 90% or more, more preferably 95% or more, and still more preferably 100% (complete austenite).
However, even if it is 100%, it is not 100% strictly if the precipitates are included, so it is substantially 100% in ordinary judgment. This transformation behavior can be easily known by those skilled in the art by using a general heat treatment-structural observation after quenching, automatic expansion measurement, a commonly used Formaster tester, etc. It can be estimated with high accuracy using empirical formulas based on vast knowledge of transformations up to the present and commercially available thermodynamic equilibrium calculation software. Based on the results, components and heat treatment temperatures described later can be easily determined. It is.

As described above, the steel component exhibiting the transformation behavior required in the present invention can be determined with high accuracy by using the current technology. However, in the present invention, a temporary standard is shown from the viewpoint of convenience. The basic concept is clear from the necessary requirements in the present invention. That is, the ferrite phase is mainly stable at room temperature, but the transformation occurs as the temperature rises, and the austenite phase needs to be determined to be the main structure in a specific high temperature range. For example, a component system that becomes a ferrite single phase in a wide temperature range, such as a magnetic steel sheet containing a large amount of Si or Al, or a ferritic stainless steel containing a large amount of Cr, or conversely, an austenitic stainless steel containing a large amount of Ni. In addition, a non-magnetic steel containing a large amount of Mn and a material that retains a large amount of austenite phase even at room temperature are not preferable. From this, the standard can be determined by the content ratio of the ferrite phase stabilizing element and the austenite phase stabilizing element. However, since the degree of stabilization of each phase by each element is different, it is necessary to multiply by some factor. Various factors affect this value. In the present invention,
3 * (0.5 * Mn + Ni) <8 + Cr + 1.5 * Si + 1.5 * Al + 10 * P <4 * (0.5 * Mn + Ni + 2.5)
Can be presented as a guide.
If the component deviates significantly from this range, it will be difficult to obtain the transformation behavior essentially required by the present invention. By the way, the above formula does not include C and N, which have a strong action as an austenite phase stabilizing element. This is because these elements greatly affect the phase stability above and below the eutectoid temperature. This is because it does not meet the purpose of being put in the formula to be presented in the invention.

Next, manufacturing conditions will be described.
As the heat treatment conditions required in the present invention, the austenite amount during the heat treatment needs to be increased to 70% by volume as described in the above component control. Preferably it is 80% or more, more preferably 90% or more, more preferably 95% or more, and still more preferably 100% (complete austenite). However, as described above, 100% is not a strict meaning, but is based on ordinary judgment. If there is a site where transformation does not occur, the structure of this site may become coarse, resulting in a mixed grain structure in the final product, which may deteriorate characteristics. The transformation behavior is affected by the steel composition and the heat history up to that point, and also by the heating rate and holding time, etc., but it cannot be said unconditionally. However, if it is fully austenitized, it will be completely austenitized, otherwise it will be austenite. The temperature at which the abundance of the phase is maximized is defined as Tmax, and it is desirable to reach Tmax−50 ° C. or higher. Preferably, Tmax-20 ° C. or higher, Tmax or higher, there is no problem in obtaining the effects of the present invention. A holding time of several seconds is sufficient, but it may be held for several minutes or several hours if necessary. The upper limit of the maximum temperature achieved in this heat treatment is not particularly limited, but when the temperature rises and there is a portion that remains in the ferrite phase, the structure of this portion becomes coarse and the mixed grain structure after cooling becomes It becomes remarkable and is not preferable. In addition, even if it is completely austenitic, coarsening of the austenite structure occurs, and it is not preferable for refinement of the structure by ferrite transformation in the subsequent cooling process, and it is wasted in terms of energy. Inadvertently raising the temperature should be avoided. The upper limit is about Tmax + 200 ° C., preferably Tmax + 100 ° C., and more preferably about Tmax + 50 ° C.

  An important point in this heat treatment is the heating and cooling rate. In general, in the transformation of C-Mn steel, it is known that the higher the heating rate or the cooling rate, the finer the microstructure after transformation. This is also effective in the present invention. Even if the heating or cooling rate is low, the effect of the present invention is not lost and it is possible to obtain a sufficiently fine structure as compared with conventional steel, but it is meaningless to lower these rates intentionally. It is not preferable. In particular, the cooling rate should be high enough not to cause a problem in the present invention. Depending on the size of the steel material, these speeds are preferably 2 ° C./second or more. If it has a certain size (thickness, thickness), it is important to increase the cooling rate of the surface layer part and refine the structure of the surface layer part depending on the purpose even though the cooling rate of the whole steel material cannot be increased. Has meaning. When it is thin or thin like a thin plate or wire, the effect of the present invention becomes remarkable by heating and cooling preferably at 10 ° C./second, more preferably at 30 ° C./second, more preferably at 100 ° C./second. .

The important point in the present invention is the refinement of the structure by transformation, but it is generally known that the structure of the C-Mn steel is refined by repeating such a ferrite-austenite transformation a plurality of times. However, this also has an effect in the present invention. Rather, the steel according to the present invention is more effective than the conventional C-Mn steel. As described above, in the steel of the present invention as described above, due to the drag effect due to solute C at the time of transformation, the pinning effect due to fine carbides precipitated at low temperature, and further the increase in the number density of transformation nuclei with fine carbides as nuclei, This is because the growth of austenite grains during temperature rise and the growth of ferrite grains during temperature fall are remarkably suppressed. The number of repetitions is not limited, but is 5 times or less, preferably 2 or 3 times in consideration of saturation of the refinement effect and industrial productivity.
In addition, it is well known that the structure of a normal C-Mn steel is refined by processing during transformation, but it is effective to refine the structure in the steel of the present invention. . Of course, if the same processing is performed, the steel of the present invention will always have a finer structure than ordinary C-Mn steel. It should be noted that the effect of processing appears more markedly in the steel of the present invention even when the transformation amount is the same at the same temperature as compared with the normal C-Mn steel. This is probably because the steel of the present invention has a markedly high effect of suppressing grain growth and nucleation efficiency as described above. Also, in the steel of the present invention, the carbide formation behavior differs from the carbide formation behavior of normal C-Mn steel, and it is difficult to form a transformation structure such as pearlite and bainite, so austenite exists up to a relatively low temperature and the effect of machining The temperature range in which sapphire appears spreads to a temperature range that is particularly lower than that of ordinary C-Mn steel. The effective temperature range is 200 ° C or higher and Tmax + 200 ° C or lower. Preferably they are 350 degreeC or more and Tmax + 50 degrees C or less, More preferably, they are 500 degreeC or more and Tmax or less.

  Moreover, it is possible to further improve the balance of strength ductility by maintaining at an intermediate temperature in the final step of the structure refinement heat treatment. This is because the solute C and Fe carbide forms in the steel are preferably changed, and the strain remaining in the steel is removed by transformation. The holding temperature is 50 to 550 ° C. Even in this range, the holding in the high temperature range excessively promotes the formation of Fe carbide and the ductility may be extremely deteriorated, so the upper limit is preferably 500 ° C., more preferably 450 ° C. The lower limit is preferably 80 ° C. or higher, more preferably 100 ° C. or higher, because it takes a long time to change the preferred carbide morphology in the low temperature range. Fe carbide produced near 100 to 150 ° C has a high Fe ratio, whereas Fe carbide produced at high temperature around 350 to 450 ° C has a lower Fe ratio than that produced at low temperature, and the composition of Fe carbide depends on the temperature. Since the characteristics to be improved are expected to be different from each other due to the different forms, it is important to select a temperature range according to the application. It goes without saying that the residence time in this temperature range needs to be long at low temperatures, but 10 seconds or more is desirable to obtain a clear effect.

In order to make the holding effect in the intermediate temperature range more prominent, cooling at a rate of 10 ° C./second or more from a temperature of 650 ° C. or more to a temperature of 400 ° C. or less in the heat treatment that has reached 650 ° C. or more was performed immediately before that. It is effective to cool at a speed. Preferably, it is 50 ° C./second or more.
However, although excessively rapid cooling depends on the steel composition and the cooling end temperature, it is necessary to be careful because it may generate an excessive martensite phase in the steel and deteriorate the ductility. It should be noted that the heat treatment at this intermediate temperature is completely different from the heat treatment performed at 300 to 450 ° C. in ordinary TRIP steel. That is, in ordinary TRIP steel, the bainite transformation is controlled during heat treatment in this temperature range, and C and Mn are concentrated in the austenite phase that is going to transform in the cooling process from high temperature. In contrast, the heat treatment at an intermediate temperature in the present invention is not related to the remaining austenite phase. That is, the heat treatment at an intermediate temperature in the present invention is performed on a steel material in which an ultrafine structure and a retained austenite structure are formed regardless of the heat treatment at the intermediate temperature. This is because the heat treatment exhibits a preferable effect by controlling the form of solid solution C and carbide by performing after the formation of the retained austenite structure.

After the steel structure is preferably controlled by heat treatment in the intermediate temperature range described above, heating to a temperature exceeding 550 ° C. should be avoided in order to maintain this structure. And a considerable part of the characteristic improvement effect by holding | maintenance at said intermediate temperature lose | disappears.
Above 580 ° C, the effect of improving characteristics by holding at an intermediate temperature is hardly observed.
The microstructure that is a feature of the present invention does not require that all parts of the steel material be fine depending on the application, and in order to improve wear resistance and fatigue properties, a significant effect can be obtained if only the surface layer is miniaturized. Can do. In addition, since the structures are partially different, it is possible to provide a composite function in which fine grains such as strength and toughness are advantageous, and coarse grains such as ductility are advantageous. As a method of partially changing the structure, for example, it is conceivable to make the components non-uniform or to make the heat treatment conditions non-uniform.

  The use of the steel of the present invention is not limited at all by its shape and the like, and steel materials such as automobiles, containers, tanks, buildings, shipbuilding, civil engineering, rails, electrical equipment, steel pipes are generally used as steel materials The effect of the present invention can be obtained by applying to the above. Moreover, the effect of the invention is not lost even if strength adjustment and shape adjustment are performed by performing some processing after forming fine particles.

First, a common characteristic evaluation method in this embodiment will be described.
The evaluation of the crystal grain size is performed regardless of the manufactured product, and the cross-sectional area per crystal grain is obtained from the number of crystal grains observed within a specific area in the usual cross-sectional structure observation, and the cross section of this crystal grain is further determined. The diameter was obtained when the shape was a circle.
In the present invention, cooling from a structure containing at least 70% of the austenite phase is necessary to form a microstructure, but the volume of the austenite phase at a temperature just before the start of transformation for the formation of a refined structure. The rate was measured by observing the cross-sectional structure of the sample cooled with water from the cooling start temperature.
The volume fraction of retained austenite was measured by the 5-peak method of X-ray diffraction using MoKα rays.
When the product partially had a fine structure, the site was measured.
In the present embodiment, there is a considerable difference in the desired properties in view of the preferred component range and production conditions among the inventive steels. For this reason, some characteristics in the inventive steel were evaluated by ranking the characteristics as follows.
A: Highest level
B: Remarkably good
C: Good (over conventional steel)
D: Conventional steel level

特性の評価結果を表2に示す。
本発明のように成分調整しオーステナイト相が70％以上存在する温度域からの熱処理により結晶組織を微細化した本発明鋼は従来、強度一延性バランスが優れていると評価されているTRIP鋼よりも優れた特性を示す。また、比較鋼では合金成分のためめっき性が劣るが本発明鋼ではめっきにおいて何ら問題を生じなかった。

(Example 1)
Steel strips containing the components in Table 1 were hot-rolled to 4.5 mm at a heating temperature of 1200 ° C and a coiling temperature of 650 ° C, pickled, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.6 mm. After various heat treatments shown in Fig. 1, temper rolling was performed at 0.6%, and the workability and plating properties were investigated. The workability was evaluated on a thin steel plate having a thickness of 0.6 mm, and was evaluated by a normal temperature tensile test using a JIS No. 5 tensile test piece with a gauge length of 50 mm and a tensile speed of 10 mm / min. Plating property is evaluated on a thin steel plate with a thickness of 0.6 mm, non-plating occurrence and plating adhesion are performed on a steel plate that has been alloyed hot-dip galvanized under practical conditions. As for the plating adhesion, a tape test was conducted after a 60 degree V-bending test of the plated steel sheet, and if the tape test blackening degree was less than 20%, it was considered acceptable.

Table 2 shows the evaluation results of the characteristics.
The steel of the present invention in which the component structure is adjusted and the crystal structure is refined by heat treatment from a temperature range in which an austenite phase is present at 70% or more as in the present invention is conventionally compared to TRIP steel, which has been evaluated as having an excellent balance of strength and ductility. Also exhibits excellent properties. The comparative steel is inferior in plating property due to the alloy component, but the steel of the present invention has no problem in plating.

(Example 2)
A steel slab containing the components shown in Table 1 was hot-rolled to 2.0 mm at a heating temperature of 1200 ° C and a winding temperature of 650 ° C. These materials were heated at 800 ° C. to 1100 ° C. and then hot pressed. Thermal history was performed in the intermediate temperature range after pressing. A test piece was cut out from the member thus manufactured and examined for hardness, toughness and delayed fracture resistance. The toughness was a method conforming to JIS, and the delayed fracture resistance was applied at 0.9 times the breaking weight, the cathode hydrogen was continuously charged by electrolysis in an ammonium thiocyanate solution, and the time until breaking was evaluated. Regarding the cooling rate, the temperature drop of the material due to the contact with the mold by the press was excluded, the contact with the mold immediately after the material was press-molded, and the subsequent thermal history was controlled. Table 3 shows the evaluation results of the characteristics.
The steel of the present invention in which the component structure is adjusted and the crystal structure is refined by heat treatment from a temperature range in which 70% or more of the austenite phase is present as shown in the present invention exhibits excellent properties and is improved by heat treatment at an intermediate temperature.

(Example 3)
A steel slab containing the components shown in Table 1 was formed into a steel wire having a thickness of 1.0 mm in a normal manufacturing process. Next, tempering heat treatment was performed under the conditions shown in Table 4, and some materials were subsequently heat treated at 900 ° C. for 10 seconds. After that, all the materials were reheated to 300 ° C. and held for 3 minutes, and then air-cooled. The characteristics were evaluated based on the temperature at which embrittlement cracking occurred when the bending test was performed while changing the breaking stress and deformation temperature in tensile deformation. The evaluation results are shown in Table 4.
The steel of the present invention in which the component structure is adjusted and the crystal structure is refined by heat treatment from a temperature range in which an austenite phase is 70% or more as shown in the present invention exhibits high strength and good brittle resistance at low temperatures, and is within the scope of the invention. Properties are improved by multiple heat treatments.

(Example 4)
Rail steel was produced from continuous cast steel pieces having the components shown in Table 1 by ordinary continuous hot working. For the last two stages of machining, the machining temperature and the machining amount were changed. The cooling conditions and the like are applied to general rail steel, and are all constant in this example. Characteristic evaluation is based on the change in weight per unit length of the test material when sliding on the rail head surface while pressing the standard steel material formed into a cylindrical shape against the test material with a certain amount of weight, and sliding it 1000000 times. The degree of occurrence of fatigue defects on the surface was evaluated. The results are shown in Table 5. The structure was observed at a depth of 5 mm from the rail head surface.
Abrasion resistance and fatigue fracture resistance are improved by refining the crystal structure by heat treatment and processing as in the present invention.

Claims (13)

  In mass%, C: 0.05 to 1.5%, Si: 3.0% or less, Mn: 0.01 to 10.0%, P: 0.0001 to 0.3%, S: 0.0001 to 0.1%, Al: 3.0% or less, N: 0.0001% to 0.04 In the temperature range from room temperature to melting, the austenite phase existing ratio is 70% or more by volume, and the grain size mainly composed of ferrite phase is 3.0 μm or less on average A steel material having a fine structure characterized by having   2. The steel material having a fine structure according to claim 1, wherein 3 * (0.5 * Mn + Ni) <8 + Cr + 1.5 * Sl + 1.5 * Al + 10 * P <4 * (0.5 * Mn + Ni + 2.5).   3. The steel material having a fine structure according to claim 1 or 2, characterized by containing, by mass%, Mo: 5.0% or less, Nb: 1.0% or less, and Ni: 10.0% or less.   4. The steel material having a fine structure according to claim 1, further comprising, by mass%, Cr: 20% or less, Ti: 0.2% or less, and B: 0.02% or less.   5. The steel material having a fine structure according to claim 1, wherein the volume fraction of the ferrite phase is substantially 50% or more and the volume fraction of the austenite phase is 20% or less.   When producing the steel material according to claim 1 to claim 5, heat treatment is performed at a temperature at which the abundance of the austenite phase is 70% or more by volume ratio, and then the crystal grain size is set to 3.0 μm or less by cooling. A method for producing a steel material having a fine structure. 7. The method for producing a steel material having a microstructure according to claim 6, wherein the crystal grain size is set to 3.0 μm or less by performing a heat treatment at Tmax−50 ° C. or more and then cooling.
Where Tmax: the temperature at which the austenite phase is maximized when the steel material is completely austenitized, otherwise the austenite phase is present   When heat treatment is performed at a temperature at which the austenite phase abundance is 70% or more, the heating rate is 2 ° C / second or more, the maximum temperature is the temperature at which the austenite phase abundance is maximum + 200 ° C or less, the cooling rate 8. The method for producing a steel material having a microstructure according to claim 6 or 7, wherein the crystal grain size is 3.0 μm or less by setting the temperature to 2 ° C./second or more.   9. The method for producing a steel material having a fine structure according to claim 6, wherein the heat treatment causing the ferrite-austenite transformation is performed a plurality of times.   10. The method for producing a steel material having a fine structure according to claim 6, wherein processing is performed in the middle of heat treatment.   11. The steel material having a fine structure according to claim 10, wherein the processing is performed in a temperature range of 200 ° C. or more and Tmax + 200 ° C. or less, and a strain in a specific direction to be applied is 0.1 or more in logarithmic strain. Manufacturing method.   After forming a structure with an average grain size of 3.0 μm or less mainly composed of ferrite phase, it is allowed to stay for 10 seconds or more in a temperature range of 50 to 550 ° C., and then not maintained at a temperature exceeding 550 ° C. 12. A method for producing a steel material having a fine structure according to claim 6. After cooling from a temperature of 650 ° C. or higher to a temperature of 400 ° C. or lower at a cooling rate of 10 ° C./second or more, a structure having an average crystal grain size of 3.0 μm or less mainly composed of a ferrite phase is formed. 13. The method for producing a steel material having a fine structure according to claim 12, wherein the steel material is allowed to stay in a temperature range for 10 seconds or longer and is not thereafter maintained at a temperature exceeding 550 ° C.