JP5747249B2 - High-strength steel material excellent in strength, ductility and energy absorption capacity and its manufacturing method - Google Patents

Description

  The present invention is a steel material used for structures such as buildings and bridges, undercarriage steel materials for automobiles, mechanical gears, etc., and is particularly a steel plate having high strength and high ductility and excellent energy absorption capability. It relates to non-heat treated steel such as steel bars and steel wires.

  In recent years, with the increase in the size of structures and the reduction in weight of automobile parts, stronger and higher performance steel materials are required than ever. In addition to this, it is an important issue to save resources and energy when manufacturing the steel material. And when manufacturing the said steel materials, it is desired that the target steel materials can be manufactured by a process saving rather than the conventional manufacturing process, without installing or newly installing an installation.

  Conventionally, many thin steel sheets having high strength and high ductility and excellent impact energy absorption ability have been developed mainly for automobile bodies. For example, Patent Document 1 discloses a technology related to a cold-rolled steel sheet for automobiles that has both high strength and high ductility and is excellent in press formability and impact energy absorption capability. This is a thin steel plate that suppresses the amount of expensive alloy elements added and increases the strength by refining ferrite crystal grains and has an excellent balance between strength and ductility, which is important for press formability. In the manufacturing process, after hot rolling, cold rolling is performed and appropriate annealing is performed. However, according to this technique, a small amount of expensive alloy elements such as Mo and Ni are essential elements to be added, and an annealing treatment is required after rolling into a thin steel plate.

  Further, Non-Patent Document 1 discloses a chemical composition steel according to a low carbon steel of 0.1% C-5% Mn-2% Si in which the contents of Mn and Si are increased without adding an expensive alloy element. Used, by increasing the fraction of retained austenite with a high content of Mn in low-temperature reheating treatment after annealing, and at the same time stabilizing the retained austenite with C discharged from ferrite into austenite with a high content of Si A steel sheet (called New TRIP steel) with an increased work hardening index is disclosed. However, this process requires an annealing process and a low-temperature reheating process, which are complicated processes after rolling into a thin steel sheet, and the problem from the viewpoint of energy saving has not been solved. And since the thin steel plate is made into steel material for manufacture, in addition to hot rolling, the cold rolling process is also essential.

On the other hand, many high-strength and high-tough steel materials used for structures and the like excluding thin steel plates have been developed as steels to be manufactured. For example, Patent Document 2 discloses a technique related to a high strength steel material having high strength, high ductility, excellent delayed fracture resistance, and drastically improved toughness. According to this technique, the tensile strength is 1660 to 1800 MPa, the elongation (total elongation) is 18.5 to 19.2%, and the impact absorption energy of the V-notch Charpy test at room temperature is 305 to 382 J / cm ² . The steel material which has is illustrated (refer Example 1 and Example 17 of Table 6 of patent document 2). However, even in this technique, about 1.0% of high-priced Mo is contained as a chemical component composition, and after manufacturing, annealing, tempering, or aging treatment is performed under conditions of a predetermined temperature and time. , A step of processing (warming) at a temperature of 350 ° C. or higher (A _C1 −20 ° C.) or lower is required.
As described above, the technologies disclosed so far have not solved the problem of resource saving and energy saving, and a processing apparatus is used in a normal production line to perform warm processing in a relatively low temperature region. For this reason, there is a problem in using it widely industrially.

JP 2007-321207 A International Publication WO2007 / 058364

H.Takechi, JOM.December 2008, p.22

  In view of the above points, the present invention is used for the above-mentioned various problems that cannot be solved by the prior art, that is, for structures such as buildings and bridges, undercarriage steel materials for automobiles, and mechanical gears. Steel material having high strength, high ductility, and energy absorption capacity (in this application, the product of tensile strength (TS) and elongation (El): TS × El is used as an index. This index is defined as (For example, it is described in the said nonpatent literature 1.) When manufacturing steel materials, such as a thick steel plate, a shaped steel, a deformed steel bar, a steel bar, and a steel wire, which is excellent in low carbon without adding an expensive alloy element Using steel with the chemical composition of steel to produce cheap and desired steel materials for many resources, high energy, and multiple processes in existing production lines without overloading production equipment To solve the problem It is intended to.

  In view of the above points, the present invention is used for the above-mentioned various problems that cannot be solved by the prior art, that is, for structures such as buildings and bridges, undercarriage steel materials for automobiles, and mechanical gears. Low C steel to which inexpensive Mn and Si are added to produce steel materials such as thick steel plates, shaped steels, deformed steel bars, steel bars and steel wires with high strength, high ductility and excellent energy absorption. By providing a steel material having a two-phase structure in which the formation ratio of ferrite and austenite is controlled without performing annealing while maintaining rolling under predetermined conditions, the steel material having a two-phase structure will be solved. It is what. Moreover, the effect which further improves a characteristic is shown by adding trace amount of Nb which is effective in structure | tissue refinement | miniaturization.

  And regarding the material property value of the steel material to be manufactured, as mechanical properties, the tensile strength (TS) is 1100 MPa or more, the elongation (El) is 25% or more, and the product of tensile strength and elongation ( TS × El) is to obtain a high-strength steel material excellent in strength, ductility and energy absorption capacity, characterized by being 30000 MPa ·% or more. In general, the ductility decreases as the tensile strength increases, whereas the present invention provides a high-strength steel material in which the total elongation is secured to a certain level or more.

  In order to solve the above-mentioned problems, the present inventor has eagerly studied the phase of a novel combination of microstructures of steel materials and the relationship between the composition ratio and material property values, and has also studied production conditions for obtaining such a structure. As a result, the present invention has been completed. The present invention has the following features.

  1stly, a chemical component composition is the mass%, C: 0.05-0.20%, Si: 1.0-3.5%, Mn: 4.5-5.5%, Al: 0.00. 001-0.080%, P: 0.030% or less, S: 0.020% or less, N: 0.010% or less, Nb: 0.01-0.045, the balance being Fe and inevitable impurities The microstructure is a two-phase structure composed of austenite in which the main phase is ferrite and the second phase accounts for 30% by volume or more. As mechanical properties, the tensile strength (TS) is 1100 MPa or more, the elongation (El) is 25% or more, and the product of the tensile strength and the elongation (TS × El) is 30000 MPa ·% or more. It is a high-strength steel material with excellent strength, ductility and energy absorption.

  Second, in the high-strength steel material excellent in strength, ductility, and energy absorption capability of the first invention, the microstructure of the high-strength steel material is an average crystal of ferrite of the main phase in a cross section parallel to the rolling direction. The grain size is 0.9 μm or less, and the average crystal grain size of the second phase austenite is 0.6 μm or less.

Third, the chemical composition is, in mass%, C: 0.05-0.20%, Si: 1.0-3.5%, Mn: 4.5-5.5%, Al: 0.00. 001-0.080%, P: 0.030% or less, S: 0.020% or less, N: 0.010% or less, Nb: 0.01-0.045, the balance being Fe and inevitable impurities A steel material consisting of
After uniformly heated at 1200 ° C., after processing or reduction rate of 84% by forging, and air-cooled to room temperature, in lath martensite main phase occupies more than 95 vol%, the residual of the second phase is less than 5 vol% A step of forming a two-phase structure composed of austenite (γR);
A step of immediately water-cooling the steel material of the above-mentioned two-phase structure to a structure of ferrite and fine cementite by recovery and recrystallization by imparting a strong work more than critical strain by groove rolling in a 650-700 ° C. warm region;
A method for producing a high-strength steel material excellent in strength, ductility and energy absorption capability, characterized by comprising a step of performing an annealing treatment in which the water-cooled steel material is heated for 2 minutes or more within a range of 650 to 700 ° C. is there.

  According to the conventional method for producing a high-strength steel material excellent in strength, ductility and energy absorption capacity using a low-carbon steel free from an expensive alloying element, characteristics close to the quality characteristics of the steel material of the present invention In order to obtain a steel material provided with a steel material, a heating time of about 30 minutes was required in the annealing treatment, but this was significantly shortened to achieve the purpose with a heating time of only about 2 minutes, and the steel material to be obtained Therefore, a steel material having excellent mechanical properties that cannot be obtained by the conventional steel material can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic explanatory drawing of the preparation process of the high strength steel materials which concern on this invention in Example 1. FIG. Isothermal transformation diagram when cooling from 1200 ° C. to room temperature at various cooling rates from 0.5 K / s slow cooling to 67 K / s rapid cooling of steels having chemical composition within the scope of the present invention shown in Table 1. (TTT diagram) A graph in which the martensitic transformation point (Ms) and Vickers hardness (HV) are written together. In Example 1, the SEM photograph which shows the form of the ferrite (FIG. 3- (1)) and the spheroidized cementite (FIG. 3- (2)) of the refined microstructure of the steel bar obtained after the warm groove roll rolling. . In Example 1, the ferrite (FIG. 4- (1)) and the austenite (FIG. 4- (2)) of the refined microstructure of the steel bar according to the present invention obtained after the annealing and overaging treatment steps are shown. SEM photograph shown. The stress-strain curve of the steel material which concerns on this invention obtained in Example 1. FIG. The schematic explanatory drawing of the preparation process of the steel materials outside the scope of the present invention in Comparative Example 1. In the comparative example 1, the SEM photograph which shows the form of the ferrite and residual austenite of the microstructure of the steel material outside the scope of the present invention obtained after the annealing and overaging treatment steps. The stress-strain curve of the steel material outside the scope of the present invention obtained in Comparative Example 1.

  Hereinafter, the chemical composition of the steel material according to the present invention, the characteristics of the microstructure and mechanical properties, and the characteristics of the method for producing the steel material will be described in detail.

<Chemical composition of steel>
The range of the chemical component composition in the high-strength steel material according to the present invention is as follows (hereinafter, “% of components” indicates “% by mass”).

 C: Set to 0.05 to 0.20%. C is necessary for securing the tensile strength, but if it is less than 0.05%, the tensile strength of the steel material according to the present invention may not be sufficiently satisfied, so it is defined as 0.05% or more. On the other hand, if it exceeds 0.20%, a tendency to lower the ductility of steel and a tendency to lower weldability are exhibited, so the upper limit is defined as 0.20%.

Si: 1.0 to 3.5%. Si is a substitutional solid solution strengthening element that greatly hardens the material, and is an effective element for increasing the strength of the steel material, and is also a solid solution C in ferrite during heating in the annealing process of the manufacturing process of the present invention. Is discharged and concentrated in austenite to stabilize austenite. In order to exhibit the latter effect more fully, 1.0% or more is desirable. However, if the Si content is excessively high, a large amount of Si scale is generated during heating during hot working, and an extra cost is required for scale removal, or surface flaws due to the scale tend to occur. Therefore, the upper limit is 3.5%

Mn: 4.5 to 5.5%.
“In the manufacturing process of the first manufacturing method of the product of the present invention, in the annealing process by heating for the very short time (about 2 minutes), which is the most characteristic condition, the temperature of the two-phase structure formation of ferrite and austenite By heating at 650 to 700 ° C. belonging to the region and diffusing C in the fine spheroidized cementite into the fine-grained ferrite at a remarkably high rate, the fine-grained austenite is stably generated, and a predetermined fraction or more In order to secure the austenite, a high Mn content exhibits an effective action.

 In order to fully exhibit these effects, it is desirable that the Mn content is 4.5% or more. On the other hand, if the Mn concentration is high, the low temperature toughness of the steel material is deteriorated, and if the Mn concentration is excessively high, segregation of Mn in the steel at the time of solidification becomes excessive and the uniformity inside the material is impaired. Further, surface cracks are likely to occur in the hot working step in the raw material preparation step. Therefore, the upper limit is set to 5.5%.

  Al: 0.001 to 0.080%. Al is added for deoxidation of molten steel, but even when a vacuum melting furnace is used, the effect is insufficient if it is less than 0.001%. In the case of converter refining, 0.010% or more is usually desirable for sufficient deoxidation. On the other hand, if it exceeds 0.080%, the problem of embrittlement may occur due to the formation of AlN, and oxide inclusions may increase and impair toughness. %. In addition, in the present invention, as a steel melting step, a converter steelmaking method or an electric furnace steelmaking method, which is a normal industrial mass production method, is a precondition, and there is no need for vacuum refining, The lower limit is specified assuming small production using a vacuum melting furnace.

P: 0.030% or less. P is an impurity element inevitably mixed in the steel and lowers the toughness, so the upper limit of its content is limited to 0.030%. A more desirable upper limit of the P content is 0.015% or less. The lower limit value is not particularly limited, but may be appropriately determined in consideration of cost.

 S: Set to 0.020% or less. S is an impurity element that is inevitably mixed in the steel as in the case of P, and since workability and toughness are impaired, the upper limit of the content is limited to 0.020%. A more desirable upper limit of S is 0.005%. The lower limit value is not particularly limited, but may be appropriately determined in consideration of cost.

 N: 0.010% or less. N is an element inevitably contained in the steel, and degassing refining or the like is required to actively reduce it, resulting in high manufacturing costs. Further, since N depends on the N content in the raw material particularly when the electric furnace steelmaking method is used, no lower limit is particularly defined. On the other hand, if the N content exceeds 0.0080%, nitrides increase and the toughness is impaired, so the upper limit is made 0.0100%.

 Nb: 0.045% or less. Nb has the effect of finely dispersing the carbide in the steel to refine the structure. This is because Nb reacts with C in the steel to produce NbC, and this fine precipitate suppresses the growth of γ grains in the high temperature γ region by grain boundary pinning. If 0.045% or more is added, carbon in the steel is consumed, and there is a risk of lowering the volume fraction of austenite and degrading the properties of the steel material.

<Microstructure and mechanical properties>
Next, the microstructure of the high-strength steel material according to the present invention will be described.
The microstructure of the high-strength steel material according to the present invention is a two-phase structure in which the main phase is ferrite and the second phase is composed of austenite (γ _R ). At that time, the fraction of austenite (γ _R ) is 30 volumes. Occupy more than 50%. In addition to austenite (γ _R ), the second phase substantially does not contain any of polygonal ferrite, quasi-polygonal ferrite, bainite, bainitic ferrite, tempered martensite, pearlite and cementite. It is an organization. The phrase “substantially free” means that the presence is not confirmed even by observation with a SEM and TEM at a magnification of 10,000 times. Having such a microstructure is one of the necessary conditions for satisfying the required mechanical property values. For this purpose, it is premised on satisfying the chemical composition of the steel described above.

The high-strength steel material according to the present invention has the following mechanical properties (1) to (3):
It satisfies all of the above.

A steel material having the above-described chemical component composition and having such a mechanical characteristic value has not been found so far.

In order to satisfy the mechanical characteristic values of the above formulas (1) to (3), in addition to the chemical composition and microstructure described above, the microstructure of the high-strength steel material has a main phase of ferrite and a second phase. Is a two-phase structure of 30 volume% or more, and in a cross section parallel to the rolling direction, the average crystal grain size of ferrite is 0.9 μm or less, and the average crystal grain size of austenite is 0.6 μm or less. Is desirable.

<Manufacturing method>
Next, the preferable manufacturing method for obtaining the steel material of this invention is demonstrated.
(1) About the hot plastic working conditions of the raw material (0.1% C-2% Si-5% Mn steel) As the hot plastic working method of the raw material obtained above, it is industrially performed. Any of flat roll rolling in a thick steel plate production line, forging in a very thick steel plate production line, groove roll rolling in a bar or steel wire production line, and shape roll rolling in a steel bar or shape steel production line may be used. Any one of these processing methods gives a desired plastic equivalent strain to the material.

Depending on the above processing method, the way of entering the compressive strain and shear strain introduced into the material is different. Therefore, there is a finite element method (FEM) as a method for theoretically calculating the plastic strain with respect to the amount and distribution of the total stress component and the total strain component. The calculation of plastic strain is described in detail in the reference (Kasaburo Harumi, et al. “Introduction to Finite Element Method” (Kyoritsu Shuppan Co., Ltd .: March 15, 1990). The plastic equivalent strain that can be used in the calculation may be used, but it is more desirable to use the plastic strain obtained by the finite element method calculation, but here the plastic equivalent strain defined by the following formula (4), which is industrially simple, is used. Let (e) be an index of plastic strain.
However, R is a surface reduction rate (%), and when the C direction sectional area of the material is S ₀ and the C direction sectional area after hot working is S, it is expressed by the following formula (5).

In the test of Example 1 and Example 2 to be described later, after heating a steel ingot (raw material) of 0.1% C-2% Si-5% Mn of 95% square within the chemical component composition range at 1200 ° C, The microstructure obtained when forging to 38 mm square is lath martensite in which the main phase accounts for 95% by volume or more, the major axis is 7.0 μm or less, the minor axis is 1.0 μm or less, and the second phase is 5 volumes. Less than% retained austenite (γ _R ), the L direction cross section was 5.0 μm, and the C direction cross section was 0.2 μm. Such a fine grain structure is obtained when the plastic equivalent strain (e) is somewhat large. From the results in Example 1 and Example 2, it is desirable that e ≧ 1.8.

(2) Production method corresponding to claim 3 This production method comprises a main phase of lath martensite occupying 95% by volume or more obtained in the above item (1) and a second phase of residual austenite of less than 5% by volume. By introducing a strain larger than the critical strain into the steel material comprising the following, dynamic recovery or recrystallization occurs simultaneously with processing, and the crystal grains are refined regardless of the phase transformation. Thus, a dual phase steel having a fine ferrite of 1.0 μm or less in the cross section in the C direction and a fine spheroidized cementite having a particle size of 0.2 μm or less is prepared. The processing temperature in that case is industrially desirable in the range of 650 to 700 ° C. When the temperature is lower than 650 ° C., the load on the processing equipment gradually increases. On the other hand, as the temperature becomes higher than 700 ° C., the grain growth gradually becomes disadvantageous for miniaturization.
The steel composed of the fine ferrite and fine cementite thus obtained is subjected to an annealing treatment by heating for a short time in a temperature range of 650 to 700 ° C. The heating time may be only 2 minutes or more. This is because since both ferrite and spheroidized cementite are fine, C in the cementite diffuses into the ferrite at a high speed to generate austenite. Moreover, the austenite to be produced is fine and is produced in an amount of 30% by volume or more, and the ferrite is also fine, so that desired TS and El can be obtained, and TS × El is also at a high level.

  Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the present invention is not limited by the following examples, and can be carried out by making appropriate modifications within a range that can be adapted to the above and the gist of the following, all of which are within the technical scope of the present invention. Contained within.

<Example 1>
FIG. 1 shows a schematic preparation process for explaining a method for preparing a high-strength steel material according to the present invention in Example 1. Details will be described below with reference to FIG.

(1) First step of Example 1: Using hot forged electrolytic iron, electrolytic Mn and metal Si as main raw materials for melting and melting using a high frequency vacuum induction melting furnace, length 95 mm × width 95 mm X A steel ingot having a height of 450 mm was cast as a raw material. Table 1 shows the chemical composition of the material.

After heating and heating the 95 mm square material (steel ingot) for 1 hour at 1200 ° C., as shown in Table 2 for the forging schedule, Then, six sets of press forging were alternately performed one by one in the vertical and horizontal directions without reheating on the way, and a square cross section of 38 mm long x 38 mm wide (referred to as 38 mm square. And finally the whole material was straightened to obtain a 38 mm square bar. In this hot forging, the area reduction ratio (R) from 95 mm square to 38 mm square is R = 84.0%, the plastic equivalent strain (e) is e = 1.83, and the forging end temperature is It was 680 ° C. Immediately after that, it was air-cooled, cooled to room temperature, and used as a bar.
Here, the area reduction ratio (R) and the plastic equivalent strain (e) were calculated by the following formulas (6) and (7). S ₀ is a cross-sectional area in the direction perpendicular to the rolling of the material (C direction), and S is a cross-sectional area in the direction perpendicular to the rolling after hot forging (C direction).
The microstructure of the 38 mm square bar material obtained by this hot forging is a lath martensite in which the main phase occupies 95% by volume or more, and the second phase consists of residual austenite (γ _R ) of less than 5% by volume. Phase organization,
The average crystal grain size of lath martensite is 7.0 μm or less in major axis and 1.0 μm or less in minor axis, and the average crystal grain size of retained austenite (γ _R ) is 5. It was 0 μm and was 0.2 μm in the longitudinal cross section (C cross section).

In addition, even when air-cooled after completion of forging, the reason why martensite is formed by 95% by volume or more as described above is mainly due to high Mn as in the component composition of Table 1. FIG. 2 shows the phase transformation when cooling from 1200 ° C. to room temperature at various cooling rates from 0.5 K / s slow cooling to 67 K / s rapid cooling using the test pieces having the components shown in Table 1. The continuous cooling transformation diagram (CCT diagram) created by thermal expansion measurement in the master test is shown. It can be seen from FIG. 2 that the Ms point is about 340 ° C. regardless of the cooling rate. This is probably because only the martensite phase was generated because the temperature dropped to the martensitic transformation point (Ms) without reaching the nose of the isothermal transformation diagram (TTT diagram).
In FIG. 2, the measured value of the average hardness of the entire two-phase structure is also shown by the Vickers test with a test load of 10 kg.

(2) Second step of Example 1: A 38 mm square bar material obtained by hot forging of a 38 mm square bar material was formed into a 14.3 mm square steel bar by warm groove roll rolling. . As shown in Table 3, after rolling and holding the material at 650 ° C. for 1 hour as shown in Table 3, the rolling temperature was set to 14.3 mm square by groove roll rolling for a total of 11 passes in the warm temperature range of 650-680 ° C. A straightening roll for straightening the entire material was performed to finish a 14.3 mm square steel bar. In the warm groove roll rolling step, heating at 650 ° C. for 2 minutes is performed three times as shown in Table 3 in the middle. The cumulative plastic equivalent strain (e) here is 2.06.
In the normal line of actual operation, when rolling by the above pass schedule, if the material is first heated to 650 ° C. and then entered into a continuous rolling process, the exothermic action due to rolling is added, and 650-680 ° C. without reheating. It is thought that it can be rolled at a degree. The steel bar finished to 14.3 mm square as described above was immediately cooled with water and cooled to room temperature. The temperature at the end of rolling was 675 ° C.

  The microstructure of the steel bar obtained by groove rolling in the warm temperature range is composed of ferrite and cementite, and the average crystal grain size of ferrite is 0.64 μm in a cross section perpendicular to the rolling direction, The average particle diameter of cementite was spheroidized cementite of 0.11 μm or less in a cross section perpendicular to the rolling direction. SEM photographs of these microstructures in the cross section perpendicular to the rolling direction (C direction) are illustrated in FIGS.

In this figure, the outline of each particle of ferrite or cementite measured on the cross section and the area surrounded by it (unit: μm ² ) are entered, the square root of the area is regarded as the particle size, The average value of the 10 grain sizes was defined as the average grain size of ferrite or the diameter of spheroidized cementite.
The reason why a steel bar having such a microstructure is obtained by warm groove roll rolling is considered as follows (for example, see paragraph number 0103 of JP-A-2005-194550).

  At the above rolling temperature, a strain larger than the critical strain is introduced into the material. As a result, the microscopic local orientation difference of the crystal grain due to this strain becomes the starting point of the fine crystal grain, and in the recovery process that occurs during or after the rolling process. This is because the grain boundary is formed at the same time as the dislocation density in the grains is lowered, and a fine grain structure is formed. That is, dynamic recovery or recrystallization occurs simultaneously with the processing, and the crystal grains are refined regardless of the phase transformation. However, if the processing temperature is too high, the crystal grains become coarse due to discontinuous recrystallization or normal grain growth. Conversely, if the processing temperature is too low, recovery occurs even if a strain greater than the predetermined critical strain is applied. Therefore, a processed structure with a high dislocation density remains.

(3) Third step of Example 1: Annealing treatment + over-aging treatment of a warm groove roll rolled material The present invention is applied to a 14.3 mm square steel bar obtained by the warm groove roll rolling of the above production step 2. An annealing process by short-time heating, which is the greatest feature in the manufacturing method according to the above, was performed, specifically, a short-time heat treatment of heating at 675 ° C. for 2 minutes, and then cooled to room temperature with He gas. Next, this was further heated at 400 ° C. for 5 minutes, and then cooled to room temperature with He gas.

  The steel bar thus obtained was examined for microstructure observation and mechanical properties. The test results are as follows.

(A) Microstructure observation result According to the SEM observation result of the longitudinal cross section of the steel bar (= the cross section in the rolling direction at the time of the warm groove roll rolling, the cross section in the L direction), the main phase is ferrite and the retained austenite (γ _R ) Is the second phase. Moreover, the fraction of the retained austenite (γ _R ) was found to occupy 30% by volume or more by the X-ray diffraction test. The average grain size of the main phase ferrite was 0.85 μm fine grains, and the average grain size of retained austenite (γ _R ) was 0.56 μm fine grains.
The SEM photograph in the L direction cross section of these micro structures is illustrated to FIG. 4- (1) and-(2). However, in FIG. 4, the outline of each particle of ferrite or retained austenite (γ _R ) measured on the cross section and the area surrounded by it (unit: μm ² ) are entered, and the square root of the area is defined as the grain. The average value of the 10 grain sizes was regarded as the average crystal grain size of ferrite or retained austenite (γ _R ).

In addition, as a result of further testing, the present inventor conducted the annealing process that was heated at 675 ° C. for 2 minutes, and then performed the above heat treatment at 400 ° C. for 5 minutes and not. By comparing the cases, it was confirmed that substantially no difference was observed in the morphology of the microstructure between the two.
Further, the overaging treatment under the above conditions is not an essential requirement of the invention of the manufacturing method according to claim 3 of the present application, but the machine over time due to the movement of interstitial elements (hydrogen etc.) at or near normal temperature. From the viewpoint of preventing changes in mechanical properties and the occurrence of delayed fracture, it is even more desirable.

The mechanism by which the above characteristic microstructure is formed is as follows. In this annealing treatment, since the steel bar was previously controlled to have a fine grained ferrite and the cementite was also finely spheroidized as described above, 675 ° C. belonging to the range of Ac ₁ to Ac _3. The austenite was able to be generated in a temperature range where the two-phase structure of ferrite and austenite was formed and the diffusion rate of C in the cementite into the ferrite was remarkably large. Moreover, the produced austenite can be stabilized in a fine grain state. As a result, it became possible to secure an austenite amount of 30% by volume or more by an extremely short annealing process with a heating and holding time of 2 minutes. At the same time, it was possible to secure fine grains having an average austenite grain size of 0.56 μm and ferrite having an average grain diameter of 0.85 μm.

(B) Test results of mechanical properties According to the tensile test results, tensile strength (TS) = 1142 MPa, elongation (El) = 27.2%, tensile strength (TS) × elongation (El) = 31062 MPa・ It was excellent at%. The aperture (RA) was excellent at 48.7%.
In FIG. 5, the stress-strain curve of the steel material of Example 1 is shown. The tensile test was performed with a round bar tensile test piece in the L direction (the diameter of the parallel part of the test part was 3.5 mmφ and the length was 24.5 mm).

  Table 4 summarizes the accuracy test results obtained in Example 1 above and the material reliability test results in the intermediate process until it is obtained.

Next, Comparative Example 1 that is outside the scope of the present invention will be described. In Comparative Example 1, short-time annealing and overaging treatment were performed under the same conditions as in Example 1 without refining the structure by warm groove roll rolling after hot forging performed in Example 1. This is the case.
<Comparative Example 1>
The schematic preparation process of the steel material in Comparative Example 1 is shown in FIG.

(1) First step of Comparative Example 1: Hot forging of raw materials Using electrolytic iron, electrolytic Mn and metal Si as main raw materials for melting, using a high-frequency vacuum induction melting furnace, the chemical composition composition shown in Table 5 Molten steel (unit: mass%) was melted. This chemical composition is almost the same as that of Example 1. This was cast into a steel ingot having a length of 95 mm × width of 95 mm × height of 450 mm, and this was used as a material.

  The material was forged into a 38 mm square as in Example 1 under the same conditions as the hot forging conditions in Example 1. That is, a 95 mm square material was heated and held at 1200 ° C. for 1 hour according to the forging schedule shown in Table 2, and then six sets of press forging were performed alternately in the vertical and horizontal directions, and forged to 38 mm square. During this time, forging was performed without reheating in the middle. Finally, the shape of the entire material was straightened to obtain a 38 mm square bar. In the forging, the area reduction ratio (R) from 95 mm square to 38 mm square is R = 84.0%, the plastic equivalent strain (e) is e = 1.83, and the forging end temperature is 688 ° C. Met. The forging end temperature was almost the same as 680 ° C. in Example 1. Immediately after that, it was air-cooled, cooled to room temperature, and used as a bar.

The microstructure of the 38 mm square bar obtained by this hot forging was substantially the same as the 38 mm square bar obtained by the hot forging of Example 1. That is, the main phase is a lath martensite occupying 95% by volume or more, and the second phase is a two-phase structure composed of residual austenite (γ _R ) of less than 5% by volume, and the average crystal grain size of the lath martensite is The major axis is 7.0 μm or less and the minor axis is 1.0 μm or less, and the average crystal grain size of retained austenite (γ _R ) is 5.0 μm in the longitudinal section (L section). (Cross section) was 0.2 μm.

(2) Second step of comparative example 1: annealing for 38 mm square hot forged material + overaging treatment Next, for the 38 mm square bar material obtained by the above hot forging, the same as in Example 1 Conditioned annealing treatment and overaging treatment were performed. Specifically, after a short heat treatment of heating at 675 ° C. for 2 minutes, the mixture was cooled to room temperature with He gas. Next, this was over-aged by heat treatment at 400 ° C. for 5 minutes, and then cooled to room temperature with He gas.
The steel bar thus obtained was examined for microstructure observation and mechanical properties. The test results are as follows.

(A) Microstructure observation results According to SEM observation results of the longitudinal cross section of the steel bar (= the cross section in the rolling direction at the time of the warm groove roll rolling, the cross section in the L direction), the retained austenite is mainly composed of ferrite (α). It is a two-phase structure having (γ _R ) as the second phase.
The fraction of retained austenite (γ _R ) was found to occupy 20% by volume or more by an X-ray diffraction test. The average particle diameter of the main phase ferrite (α) was 5.0 μm, and the average particle diameter of retained austenite (γ _R ) was 0.2 μm.
The SEM photograph in the L direction cross section of these micro structures is illustrated in FIG.

(B) Test results of mechanical properties On the other hand, according to the tensile test results,
Tensile strength (TS) = 1160 MPa, Elongation (El) = 23.7%, Tensile strength (TS) × Elongation (El) = 27492 MPa ·%
Met.
In FIG. 8, the stress-strain curve of the steel material of the comparative example 1 is shown. The tensile test was performed with a round bar tensile test piece in the L direction (the diameter of the parallel part of the test part is 3.5 mmΦ and the length is 24.5 mm).

Fig. 8 Stress-strain curve of the steel material of Comparative Example 1
Table 6 summarizes the accuracy test results obtained in the above Comparative Example 1 and the material reliability test results in the intermediate process until it is obtained.

<Summary of comparison between Example 1 and Comparative Example 1>
From the above results, the material properties obtained after annealing and overaging in each of Example 1 and Comparative Example 1 are compared as follows.
(1). According to Example 1, a microstructure steel having a two-phase structure composed of retained austenite in which the main phase is ferrite and the second phase occupies 30% by volume or more, tensile strength (TS) ≧ 1100 MPa, elongation (El) A high-strength steel material having a high strength and high ductility of ≧ 25% and a high absorption energy of TS × E1 ≧ 30000 MPa ·% was obtained.
On the other hand, in Comparative Example 1, such an excellent high strength steel material could not be obtained in all of tensile strength (TS), elongation (El), and TS × El.
(2). The reason why the above-described excellent mechanical properties were obtained in Example 1 is that the microstructure of the high-strength steel material is reduced in the average crystal grain size of the main phase ferrite in the cross section in the direction perpendicular to the rolling direction. This is because the average crystal grain size of the retained austenite of the second phase is 0.6 μm or less. On the other hand, in the comparative example 1, the microstructural refinement to the extent seen in the example 1 is not achieved.
(3). Regarding the manufacturing method, the reason why the characteristics of the above items (1) and (2) are achieved in Example 1 is that the microstructure of the material subjected to the short-time annealing is in a cross section perpendicular to the rolling direction. This is because a microstructure steel made of ferrite having an average particle diameter of 1.0 μm or less and spheroidized cementite having a diameter of 2 μm or less was provided.
On the other hand, in Comparative Example 1, the material subjected to the short-time annealing is not provided with the material as fine as that in Example 1.
Further, as Example 2, the same components as in Example 1 were added, but the material added with 0.045% Nb was also subjected to experiments, and the structure and characteristics almost equivalent to those in Example 1 were obtained. As a slight difference, the material of Example 2 has a slightly fine structure due to the effect of Nb addition, and therefore the ductility was about 5% higher.

The present invention is a steel material used for structures such as buildings and bridges, undercarriage steel materials for automobiles, mechanical gears, etc., and is particularly a steel plate having high strength and high ductility and excellent energy absorption capability. It relates to non-heat treated steel such as steel bars and steel wires.

Claims (3)

The chemical composition is mass%,
C: 0.05 to 0.20%,
Si: 1.0 to 3.5%
Mn: 4.5 to 5.5%,
Al: 0.001 to 0.080% ,
P: 0.030% or less,
S: 0.020% or less,
N: 0.010% or less ,
Nb: 0.01~ 0.045%,
And the balance consists of Fe and inevitable impurities,
As the microstructure, the main phase is ferrite, the second phase is a two-phase structure consisting of austenite occupying 30% by volume or more,
As the mechanical properties, the tensile strength (TS) is 1100 MPa or more, the elongation (El) is 25% or more, and the product of the tensile strength and the elongation (TS × El) is 30000 MPa ·% or more. High strength steel with excellent strength, ductility and energy absorption.   In the microstructure of the high-strength steel material, the average crystal grain size of the cross section parallel to the rolling direction of the main phase ferrite is 0.9 μm or less, and the average crystal of the cross section parallel to the rolling direction of the second phase austenite The high-strength steel material excellent in strength, ductility, and energy absorption capacity according to claim 1, wherein the particle size is 0.6 μm or less. The chemical composition is mass%,
C: 0.05 to 0.20%,
Si: 1.0 to 3.5%
Mn: 4.5 to 5.5%,
Al: 0.001 to 0.080% ,
P: 0.030% or less,
S: 0.020% or less,
N: 0.010% or less ,
Nb: 0.01~ 0.045%,
And the balance is a steel made of Fe and inevitable impurities,
After uniformly heated at 1200 ° C., after processing or reduction rate of 84% by forging, and air-cooled to room temperature, in lath martensite main phase occupies more than 95 vol%, the residual of the second phase is less than 5 vol% A step of forming a two-phase structure composed of austenite (γR);
A step of immediately water-cooling the steel material of the above-mentioned two-phase structure to a structure of ferrite and fine cementite by recovery and recrystallization by imparting a strong work more than critical strain by groove rolling in a 650-700 ° C. warm region;
A method for producing a high-strength steel material excellent in strength, ductility, and energy absorption capability, comprising a step of subjecting the water-cooled steel material to an annealing treatment in which heating is performed for 2 minutes or more within a range of 650 to 700 ° C.