JP2005194547A - High carbon extra-fine grain steel and its producing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new steel material having such merit so as not to use expensive trace elements, and excellent recycling characteristic by which compared with the conventional S45C.QT (quenching and tempering) steel having about 0.4% C content and 800-1,200 MPa assumed strength used for car parts, such as axle, gear, crank shaft; in this new material, C content is the same level and regarding other alloy elements, added elements are held to the degree as the plain steel and the present 400 MPa class Fe-C-Mn-Si base ferrite-pearite steel is changed to the high strength by making fine grain of the ferrite. <P>SOLUTION: The steel containing 0.35-1.0 wt% C and having the structure of at least one or more kinds of ferrite, austenite, pearite, cementite and martensite, of ≤10μm average grain diameter, is processed to ≥0.7 strain in the temperature range from 550°C to Ae1 point +50°C to produce the high carbon extra-fine grain steel having fine structure of ≤ 3μm average grain diameter. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The invention of this application relates to a high C (carbon) added ultra fine grain steel and a method for producing the same. More specifically, the invention of this application relates to a high-strength, high-toughness, high C (carbon) -added ultrafine-grained steel useful as a structural steel material, automotive steel material, and the like, and a method for producing the same.

  Conventionally, S45CQT steel having a characteristic of a specified strength of about 800 MPa to 1200 MPa is used in automobile parts such as an axle, a gear, and a crankshaft. In this S45CQT steel, C (carbon) is added at a high content of about 0.4%.

  However, in the case of this S45CQT steel, its chemical composition is special, and expensive trace elements are added, so there are difficulties in recycling compared to ordinary steel, and QT treatment is also necessary. As a result, the cost is high and the manufacturing burden has become a problem.

  In such a situation, as a measure for improving recyclability without adding expensive trace elements in the composition and reducing the burden on cost and manufacturing, increasing the strength by making the structure finer Attention has been paid.

  Actually, the strength of the 400 MPa class Fe—C—Mn—Si ferritic pearlite steel has already been increased by refining ferrite grains, and the inventors of this application have already achieved an average ferrite grain size of 2 μm or less. Have proposed a high-strength, high-toughness steel material and a method for producing the same (Patent Document 1).

However, at present, steel materials based on a normal steel composition having characteristics equal to or higher than those of S45CQT steel have not been realized due to the refinement of the structure.
JP 2000-309850 A

  The invention of this application was made on the basis of the background as described above, and it is not necessary to add expensive trace elements as in S45CQT steel, nor to make QT treatment essential in its production. Based on the composition, the object is to provide a new fine-grained steel that is excellent in recyclability, is inexpensive, and has the same or better characteristics as S45CQT steel, and a method for producing the same.

  The invention of this application solves the above-mentioned problems. First, a ferrite having a chemical composition containing 0.35 to 1.0% by weight of C (carbon) and an average particle size of 10 μm or less. And steel having at least one structure of austenite, pearlite, cementite, and martensite at a temperature range of 550 ° C. to Ael point + 50 ° C., and processing with a strain of 0.7 or more, an average particle size of 3 μm Provided is a method for producing a high-C (carbon) -added ultrafine-grained steel characterized in that the steel has the following microstructure.

  Further, the invention of this application has a chemical composition containing 0.35 to 1.0% by weight of C (carbon), a ferrite grain structure having an average particle size of 3 μm or less, and a tensile strength of 900 MPa or more. Provided is a high C (carbon) -added ultrafine grained steel characterized by having a total elongation of 20% or more.

  According to the first invention of this application as described above, it is possible to recycle based on the composition of ordinary steel without adding expensive trace elements as in S45CQT steel and making QT treatment essential in the production. It is possible to produce a new fine-grained steel that is excellent in properties, is inexpensive, and has the same or better properties as S45CQT steel.

  In addition, according to the second invention of this application, a ferrite fine grain steel having both excellent strength and toughness is provided.

  The invention of this application has the features as described above, and an embodiment thereof will be described below.

  In the invention of this application, the amount of C is equal to or higher than the S45CQT steel of about 0.4% C with an assumed strength of 800 MPa to 1200 MPa used for automobile parts such as conventional axles, gears, and crankshafts. Other alloy elements are limited to the additive elements of ordinary steel, and the current 400 MPa class Fe-C-Mn-Si ferritic pearlite steel is strengthened by making the ferrite finer, thereby reducing expensive trace elements. The present invention provides a new steel material that has the advantage of not being used and has excellent recyclability.

  That is, in the invention of this application, as described above, in the chemical composition, 0.35 to 1.0% by weight of C (carbon) and an average particle size of 10 μm or less of ferrite and austenite, pearlite, cementite, and A steel having a microstructure of at least one kind of martensite is processed at a strain of 0.7 or more in a temperature range from 550 ° C. to an Ael point + 50 ° C., and has a fine structure with an average grain size of 3 μm or less. (Carbon) added ultra fine grain steel is manufactured.

  In the chemical composition, it is essential in the invention of this application that the C (carbon) content is in the range of 0.35 to 1.0%. If it is less than 0.35%, sufficient improvement effect on strength and toughness is not necessarily obtained as compared with S45CQT steel, and if it exceeds 1.0%, problems such as a decrease in toughness occur. It will be.

The elemental composition other than C (carbon) can be based on the composition of ordinary steel. For example, as weight percent,
Si: 0.8% or less Mn: 0.05 to 3.0%
Al: 0.1% or less Cu: 2.5% or less Ni: 3.0% or less Ti: 0.1% or less Cr: 3.0% or less Mo: 1.0% or less W: 0.5% or less Nb : 0.1% or less V: 0.1% or less can be considered.

  Various means for processing with a strain of 0.7 or more may be considered. For example, groove roll processing, rolling, forging, etc. are considered.

Therefore, an example will be shown below and will be described in more detail. Of course, the invention is not limited by the following examples.

  The following two types (a) and (b) as chemical compositions (% by weight);

Each steel was processed by the following two methods.

  The method A and method B in this case are generally characterized as follows. That is, the method A has a final finish at a temperature as low as 550 ° C. as compared with the method B, and the particle size becomes finer.

In addition, there is a large amount of residual strain, which increases the tensile strength. On the other hand, the ductility and the upper shelf energy of the Charpy test are lowered.

  By the processing of the method A and the method B, two kinds of invention materials of this application: Ab and Bb and two kinds of comparative materials: Aa and Ba were produced.

  1 and 2 attached herewith are micrographs showing the microstructures of the inventive material B-b and the comparative material Ba, respectively. Compared to the comparative material Ba (FIG. 2) having a C (carbon) concentration of 0.15%, the inventive material B-b (FIG. 1) increased to a C (carbon) concentration of 0.45%. It can be seen that more fine Fe 3 C particles are uniformly dispersed and the ferrite particle diameter is 1 μm or less.

Note that the initial structure before processing of the inventive material Bb is a martensitic structure that is WQ (water quench) from 1100 ° C.

FIG. 3 and FIG. 4 show the stress-strain curves for the inventive material B-b and the comparative material Ba.

  Moreover, what showed the stress-strain curve about each of invention material Ab and comparative material Aa is FIG. 5 and FIG.

  Therefore, Table 3 shows various characteristics obtained from the above stress-strain curve.

  Although Aa and Ba show the case of a comparative material containing 0.15% C, in the case of method A, the present invention material (Ab) has a TS of 834.9 MPa to 970.8 MPa. 131.4 MPa, and the total elongation is maintained at 23.8%. In the case of Method B, it can be seen that the TS of the present invention material (B-b) is increased from 571.9 MPa to 734 MPa and 162 MPa, and the total elongation is maintained at 32.4%.

  For example, from the above results, a high C (carbon) -added ultrafine grain steel having a tensile strength of 900 MPa or more and a total elongation of 20% or more is provided.

  FIG. 7 shows the Charpy impact test characteristics. Since this invention material (Ab, Bb) has high C density | concentration, the tendency for an upper shelf energy to be low is recognized. However, when the strength is increased by 130 MPa or more and the application as an undercarriage member of an automobile is considered, a comparison of the characteristics of the total balance with a conventional high alloy steel such as S45C that requires QT treatment, as shown in FIG. It can be seen that the material of the present invention occupies a higher position in the overall characteristics of the QT material. This is a very important feature of the invention of this application.

  As explained in detail above, according to the invention of this application, the amount of C is the same as that of S45CQT steel of about 0.4% C with an assumed strength of 800 MPa to 1200 MPa used in conventional automotive parts such as axles, gears, and crankshafts. Although it is a level, other alloy elements are limited to the additive elements of ordinary steel, and the current 400 MPa class Fe—C—Mn—Si based ferrite pearlite steel is increased in strength by refining ferrite, Provided is a new steel material that has the advantage that it is not necessary to use expensive trace elements and that is excellent in recyclability.

It is the microscope picture which showed the micro structure of invention material (Bb). It is the microscope picture which showed the micro structure of the comparative material (Ba). It is the figure which showed the stress-strain curve of invention material (Bb). It is the figure which showed the stress-strain curve of the comparative material (Ba). It is the figure which showed the stress-strain curve of invention material (Ab). It is the figure which showed the stress-strain curve of the comparative material (Aa). It is the figure which showed the result of the Charpy impact test. It is the figure which showed the comparison of total balance.

Claims (2)

  In the chemical composition, it contains 0.35 to 1.0% by weight of C (carbon), and has an average particle size of at least one structure of ferrite, austenite, pearlite, cementite, and martensite having a particle size of 10 μm or less. Addition of high C (carbon), characterized in that the steel is processed into a steel having a microstructure with an average grain size of 3 μm or less by processing at a strain of 0.7 or more in a temperature range from 550 ° C. to Ael point + 50 ° C. Manufacturing method of ultra fine grain steel.   In the chemical composition, it contains 0.35 to 1.0% by weight of C (carbon), has a ferrite grain structure with an average particle size of 3 μm or less, has a tensile strength of 900 MPa or more and a total elongation of 20% or more. A high-C (carbon) -added ultrafine-grained steel.

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