JP5896458B2 - Ultra fine martensite high hardness steel and its manufacturing method - Google Patents

Description

  The present invention relates to steel used for parts such as structures such as buildings and bridges, undercarriage steel for automobiles, and mechanical gears, and particularly to high-hardness steel.

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

  Conventionally, many high strength steel plates have been developed. 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 sheet that suppresses the addition amount of expensive alloy elements to increase the fineness of ferrite crystal grains and is excellent in balance with ductility when it 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 for manufacture, in addition to hot rolling, the cold rolling process is also essential.

On the other hand, a large number of high-strength and high-tough steels that are used as structures to be manufactured for structures other than thin steel sheets have been developed. For example, Patent Document 2 discloses a technique related to high strength steel having high strength, high ductility, excellent delayed fracture resistance, and dramatically 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 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 for automobiles, and mechanical gears. As a steel, high strength (in this application, tensile strength (TS)) is used as an index. This steel is described in Non-Patent Document 1, for example. When manufacturing steels such as thick steel plates, shaped steels, deformed steel bars, steel bars and steel wires that are excellent in), they are manufactured using steels with chemical composition of low carbon steel without adding expensive alloying elements. The present invention is intended to solve the problem that it is impossible to produce desired steel at a low cost due to multiple resources, high energy, and multiple processes in an existing production line without imposing an excessive load on the equipment.

  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 for automobiles, and mechanical gears. In order to produce steels such as thick steel plates, shaped steels, deformed steel bars, steel bars and steel wires with excellent strength, steel based on low C steel added with inexpensive Mn and Si, Is intended to solve the problem by providing a steel having an ultrafine martensite structure in which martensite is controlled without performing annealing treatment under rolling under predetermined conditions.

  Regarding the material property value of the steel to be manufactured, the mechanical property is to obtain a high hardness steel having an excellent Vickers hardness of 490 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 and the relationship between the composition ratio and material property values, and has also studied the 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 to 0.080%, P: 0.030% or less, S: 0.020% or less, N: 0.010% or less , the balance is made of Fe and inevitable impurities, and the microstructure is a martensite structure In addition, as a mechanical property, it is a high-hardness steel excellent in hardness, characterized by having a Vickers hardness of 490 or more.

  Secondly, in the high hardness steel excellent in hardness of the first invention, the microstructure of the high hardness steel has an average crystal grain size of the martensite of 2.6 μm or less in a cross section parallel to the rolling direction. Provide some hard steel.

  Thirdly, the high hardness steel having the chemical composition of the first invention is uniformly heated at 1200 ° C., then cooled to a temperature range of 600 ° C. to 400 ° C., and the surface reduction rate is 88 by performing forging once in that temperature range. Provided is a method for producing a high-hardness steel that is air-cooled to room temperature after processing at least%.

  According to the method for producing a high hardness steel having excellent hardness according to the present invention, in order to obtain a steel having a level of characteristics close to the quality characteristics of the steel according to the present invention, it has been conventionally necessary to add an expensive alloy element. However, by using a structure called ultrafine martensite that could not be expressed by conventional methods, the objective was achieved with a normal steel composition that did not contain expensive alloy elements, and the quality characteristics of the steel obtained were In this way, a steel with excellent mechanical properties that cannot be obtained is obtained.

Pre-organization photomicrograph (HV421). Sample and anvil configuration in a 25 ton hot working simulator. Experimental conditions: Strain rate 10 / s 50% compression processing temperature 900, 600, 400 ° C. Time-load recording in the heat processing state of the sample. Distribution state of plastic equivalent strain in processing state of sample (by FEM calculation). Hv hardness of tissue. Relationship between average particle size and hardness of sample. Comparison of structure between Example and Comparative Example: A line of a large angle grain boundary (15 degrees or more) by EBSD measurement is displayed.

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

<Chemical composition of steel>
The range of the chemical component composition in the high hardness steel 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%, there is a possibility that the tensile strength of the steel according to the present invention may not be sufficiently satisfied. On the other hand, if it exceeds 0.20%, the steel tends to have a lower ductility and lower weldability, 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, is an element effective for increasing the hardness of steel, and is preferably 1.0% or more. However, if the Si content is excessively high, a large amount of Si scale is generated during heating during hot working, and there is a problem that extra cost is required for removing the scale and surface flaws are likely to occur due to the scale. Therefore, the upper limit is set to 3.5%.

Mn: 4.5 to 5.5%.
In order to generate martensite by low temperature processing at 400 ° C or higher, which is a characteristic of this material, it is necessary to highly stabilize austenite. To ensure this, a high Mn content exhibits an effective action. To do.

  In order to fully exhibit this effect, it is desirable to make Mn content 4.5% or more. On the other hand, if the Mn concentration is high, the low temperature toughness of the steel is deteriorated. 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%. Further, a more desirable upper limit of S is 0.05%. 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 0.010%.

  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. When 0.045% or more is added, carbon in the steel is consumed, and there is a risk that the driving force of the martensitic transformation is lowered and the properties of the steel are deteriorated.

<Microstructure and mechanical properties>
Next, the microstructure of the high steel according to the present invention will be described.
The microstructure of the high steel according to the present invention is characterized in that the main phase is martensite and the particle size is 2.6 μm or less, which is much smaller than normal martensite (> 100 μm). Particularly, the one processed at 600 ° C. is 2.55 μm and the one processed at 400 ° C. is 1.60 μm, and this fine structure is characterized by being considerably higher in the state of martensite itself but higher. Compared with the Vickers hardness, the HV = 448 in the microstructure with a diameter of 4.30 μm, whereas HV = 495 in the 2.55 μm and HV = 504 in the 1.60 μm, which cannot be achieved without adding an expensive alloy element. It is characterized in that high hardness can be achieved with a normal composition.
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 hardness steel according to the present invention has the following formula (1) as its mechanical characteristic value:
It satisfies.

  A steel having the above chemical composition and having such mechanical characteristic values has not been found so far.

<Production method of specimen>
Next, the manufacturing method of the test material for obtaining the steel 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, Any of flat roll rolling, forging in a heavy 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 (2) is industrially simple. Let (e) be an index of plastic strain.
However, R is the area reduction ratio (%), and S ₀ is the cross-sectional area of the surface perpendicular to the longitudinal direction of the material, and S is the cross-sectional area of the surface perpendicular to the hot-worked direction. 3).

  In the test of Example 1 described later, a 15 mm square steel ingot (material) of 0.1% C-2% Si-5% Mn within the chemical composition range was heated at 1200 ° C. for 3 minutes, and then 5K / After cooling at s, holding at 600 ° C. or 400 ° C. for 5 s, and compressing to 7.5 mm thickness, the microstructure at the location of the plastic deformation equivalent to 1.1 in the central portion has a main phase of almost 100 volumes. The average particle size consisting of% martensite was 2.55 μm (processed at 600 ° C.) and 1.60 μm (processed at 400 ° C.).

  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.

<Examples 1 and 2>
FIG. 1 shows a schematic preparation process for explaining the method for preparing high steel according to the present invention in Examples 1 and 2. Details will be described below with reference to FIG.

(1) Preparation of sample materials of Examples 1 and 2: Hot forged electrolytic iron, electrolytic Mn and metal Si were used as main raw materials for melting, and melted using a high-frequency vacuum induction melting furnace, length 95 mm X Casting into a steel ingot with a width of 95 mm and a height of 450 mm, which was used as a material. Table 1 shows the chemical composition of the material.

The above 95 mm square material (steel ingot) is heated and heated and held at 1200 ° C. for 1 hour, and then the above vertical 95 mm × 95 mm square material is vertically reheated without reheating. 6 press forgings, alternately and side by side, and forging to a 38 mm x 38 mm square cross section (referred to as 38 mm square, hereinafter referred to as this) Finally, the entire 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 obtained by this hot forging was lath martensite in which the main phase accounted for 95 volume% or more. An SEM photograph of this structure is shown in FIG. In this structural state, the hardness was HV421. Using this steel bar as a base material, a 15 mm square bar was cut out and used as a test material for the process of manufacturing the inventive material.

(2) Process of Examples 1 and 2: Structure control of 15mm square bar material by 25t simulator Structure control by 25t simulator using 15mm square bar material cut out from 38mm square bar material obtained by hot forging went. The 25t simulator used is a device capable of rapid heating, rapid large strain processing, and rapid cooling of a sample, and is a device that enables tissue control under extreme conditions. An outline of the actual organization control is shown in FIG. Here, the shape, size, and positional relationship between the sample and the anvil during the 25t simulator experiment are shown. The sample is a 15 mm prism, and has a positional relationship in which an anvil having a width of 25 mm, a depth of 30 mm, and a sample contact portion width of 15 mm is sandwiched above and below the sample.

  FIG. 3 shows the process in the organization control. There are three types of conditions. Among these three types, the basic steps are the same, and the only processing temperatures are different from 400, 600, and 900 ° C. Here, a sample processed at 400 ° C. is referred to as Example 1, a sample processed at 600 ° C. as Example 2, a sample processed at 900 ° C. as Comparative Example 1, and a sample processed as non-processed as Comparative Example 2. First, the sample was heated at 5 K / s, and after reaching 1200 ° C., held at 1200 ° C. for 3 minutes, cooled to any of 400, 600, and 900 ° C. at 5 K / s, and held at that temperature for 5 s. Thereafter, it is compressed by 50% at a strain rate of 10 / s and air-cooled (AC). A graph in which the reaction force during the compression processing is recorded as a function of time is compared and plotted in each case of 400 ° C., 600 ° C., and 900 ° C. processing is shown in FIG. From this, it can be seen that the reaction force is higher as the processing temperature is lower. Considering that the high processing reaction force is comparable to the low dislocation disappearance, it is expected that the crystal grain size becomes finer at lower temperatures. Incidentally, it was 141 kN at 400 ° C., 100 N at 600 ° C., and 56 N at 900 ° C.

  FIG. 5 shows the result of FIM (finite element method) calculation of the plastic equivalent strain distribution in the sample cross section at 50% working (iron and steel Vol. 86 (2000) No. 12). From this it can be seen that at 50% compression, the plastic equivalent strain at the center of the sample is 1.1. As a result, a plastic equivalent strain of 1.1 is derived as the amount of processing strain to be added, which is a necessary condition for manufacturing the material of the present invention.

  The Vickers hardness distributions of the samples after the processes of Examples 1 and 2 are shown in FIG. As a result, the HV at the center is 508 when processed at 400 ° C. (Example 1), 495 when processed at 600 ° C. (Example 2), and 488 when processed at 900 ° C. (Comparative Example 1). You can see that Considering that HV = 423 is a sample (Comparative Example 2) that remains γ-modified at 1200 ° C. and does not undergo processing, the effect of processing is clear.

FIG. 7 shows the relationship between the average particle diameter and hardness of the sample. Here, it can be seen that the −1/2 power of the particle size and the hardness are in a linear relationship, and the hardness increases as the particle size decreases. FIG. 8 shows large-angle grains (azimuth difference> 15 degrees) from the EBSD measurement in which the particle diameter was determined. From this, it can be seen that the particle sizes of Examples 1 and 2 (400 ° C. and 600 ° C. processing) are much finer than those of Comparative Example 1 (600 ° C. processing).
The reason why an ultrafine-grained martensite structure that has not been obtained in the present invention is obtained will be described below. Although the composition used in the present invention is basically 0.1% C-2% Si-5% Mn, it has been clarified from our experiments that this composition greatly enhances the stability of austenite (γ). . That is, the structure is 100% γ even when the entire sample is γ-modified at 1200 ° C. and the temperature is lowered to 400 ° C. by cooling. Here, 50% reduction is applied, but in the case of ordinary steel, the γ → α transformation progresses by this processing, but this steel has a very high γ stability, so the phase transformation Does not happen. Thereafter, it is cooled to around 340 ° C. by air cooling, reaches the MS point (martensitic transformation start point) for the first time, and becomes martensite. Assuming that the refining of γ grains becomes finer as the processing temperature is lower, the steel of the present invention martensite from the ultrafine γ structure at around 340 ° C. It is considered that a martensite structure was obtained while maintaining an ultrafine structure.

A summary of the experimental results is shown in Table 2. Thus, it is clearly recognized that the steel of the present invention has a smaller particle size and higher hardness than the comparative steel.

The present invention is steel used for parts such as structures such as buildings and bridges, undercarriage steel for automobiles, gears for machinery, etc., and is particularly suitable for non-tempered steel plates, steel bars, steel wires, etc. It is about steel.

Claims (2)

Chemical composition is mass%, C: 0.05-0.20%, Si: 1.0-3.5%, Mn: 4.5-5.5%, Al: 0.001-0. 080%, P: 0.030% or less, S: 0.020% or less, N: 0.010% or less, and the balance is a high-hardness steel composed of Fe and inevitable impurities, and the microstructure is an average crystal particle size 2.6μm is less ultrafine martensitic, high hardness steel Vickers hardness of the martensite structure is characterized in that 490 or more. It is a manufacturing method of the high hardness steel of Claim 1, Comprising: After heating the raw material of the said chemical component uniformly at 1200 degreeC, it cools to the temperature range of 600 to 400 degreeC, and is equivalent to plasticity by forging once in the temperature range. A process for producing high-hardness steel, characterized by air cooling to room temperature after processing at a strain of 1.1 or more .