JP4604251B2 - S-containing high strength fine grain steel precursor steel - Google Patents

Description

The invention of this application relates to a precursor steel that is optimal for producing ultrafine-grained steel suitable for building steel, automobile steel, and the like that require high strength and high toughness. More specifically, the processing temperature is from 550 ° C. to Ae1 + 50. The present invention relates to a high strength fine grain steel precursor steel for further refinement of grain size by strain processing in a temperature range of ° C.

There is known a method for producing a steel material having characteristics comparable to a high-strength alloy steel such as S45CQT material by finely pulverizing ferrite of Fe-C-Mn-Si ferrite / pearlite steel which is ordinary steel. (Patent Document 1). Such a ferrite fine graining method is expected to be a steel material having high strength characteristics that can be applied to automotive parts by adjusting the C (carbon) concentration. It is expected to be a high-strength steel material that does not need to be used and has the advantage that the welding / joining technology currently used can be used as it is, and is excellent in recyclability.
JP 2000-309850 A

However, there are two problems in increasing the strength in such ferrite refinement. One problem is a decrease in toughness, particularly a decrease in the upper shelf energy in the Charpy impact test, and another problem is a decrease in the machinability associated with the increase in strength. The problem of lowering the upper shelf energy and lowering of the machinability is a large practical problem for parts processing. As a method of coping with the lowering of the upper shelf energy, a method of developing a lamellar structure can be considered. The method of developing this lamellar structure is that the crack propagation energy is absorbed during the crack splitting process when the layer structure that has developed vertically in the notch is present, and the crack propagation energy is absorbed. Based on the perception that energy will increase.

As another problem, it is effective to disperse the Pb (lead) particles in the steel material, which have hard and easily breakable physical properties, which can finely cut the cutting waste into the material. It is considered to be. However, Pb is toxic to the human body, and the use of such harmful substances is restricted.

Therefore, in view of the circumstances as described above, the invention of this application is a high-strength ultrafine grain that can suppress a decrease in toughness and a decrease in machinability, which is a problem when increasing the strength by refining ferrite. It is an object to provide a steel precursor steel.

In order to solve the above-mentioned problems, the invention of this application is as follows. First, by performing strain processing at a processing temperature range of 550 ° C. to Ae1 + 50 ° C., a high grain structure having an average particle size of 3 μm or less is obtained. It is a precursor steel used for producing a high- strength fine-grained steel, and has a structure composed of at least one of ferrite and austenite, pearlite, cementite, and martensite having an average particle diameter of 10 μm or less, and C (carbon ) 0.3 to 0.5 mass% and S (sulfur) 0.1 to 0.3 mass%, Si: 0.8 mass% or less, Mn: 0.05 to 3.0 mass%, the balance Fe and providing a precursor steel characterized by Rukoto such unavoidable impurities.

According to the first invention of this application, it was possible to obtain a steel material having excellent strength and machinability and having high strength characteristics applicable to building materials and automotive parts.

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

As described above, the invention of this application suppresses a decrease in toughness and machinability of a high-strength ultrafine-grained steel using a ferrite fine graining method, but suppresses a decrease in toughness and machinability. Is based on the finding that the addition of S (sulfur) is effective.

However, in order to produce ultrafine-grained steel by the ferrite fine graining method, it is required to add a plastic equivalent strain of about 2.0 in the temperature range of the α phase region corresponding to the components of the steel material. ) It is extremely strict to apply this method to a steel material with a high concentration, and there arises a problem that destruction occurs because MnS (manganese sulfide) is formed along the layer in the manufacturing process.

Therefore, in the invention of this application, in the ferrite fine granulation method, high strength ultrafine particles having excellent toughness and machinability while avoiding material destruction during the manufacturing process, which is a problem when adding S (sulfur). In order to produce a grain steel, the amount of S (sulfur) added and the average grain size of the ferrite fine grain steel are specified within the optimum range of the heat treatment conditions. That is, S (sulfur) is added in a range of 0.01 to 0.3 mass%, and strain processing is performed in a temperature range of 550 ° C. to Ae1 + 50 ° C.

In strain processing in this case, it is preferable that the strain is 0.7 or more.

In the chemical composition, it is preferable in the invention of this application that the C (carbon) content is in the range of 0.1 mass% to 0.75 mass%. More preferably, it is in the range of 0.3% to 0.5%.

The elemental composition other than S (sulfur) and 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.

Hereinafter, the present invention will be described in more detail with reference to examples.

Table 1 shows the chemical components of the test materials used in the examples. P1 and S1 contain 0.15 mass% and 0.45 mass% of C (carbon), respectively, but do not contain S (sulfur). F2, H5, H6, H7, H8, S2, and S2X contain C (carbon) in a range of 0.15 mass% or 0.45 mass% and S (sulfur) in a range of 0.01 to 0.1 mass%. The S (sulfur) -added ultrafine-grained steel contained in is shown.

There were basically two types of thermomechanical treatment conditions. The first condition (A condition) is the method used for P1, S1, and S2. As a pretreatment, forging is performed after holding at 1100 ° C. for 1 hour, air cooling to room temperature, and further heating to 1100 ° C. to 30 After holding the minute, WQ is performed. Next, after holding at 650 ° C. for 1 hour, groove roll rolling with an area reduction rate of 85% is performed and water-cooled to produce ultrafine-grained steel. The second condition (Condition B) is a method used for F2, H5, H6, H7, and H8. As a pretreatment, after holding at 1100 ° C. for 1 hour, forging is performed and air cooling is performed to room temperature. Next, after holding at 900 ° C. for 1 hour, air cooling is performed, and when the temperature reaches 550 ° C., groove roll rolling with a reduction in area of 93% is performed and water cooling is performed to produce an ultrafine grained steel. In addition, F2, H6, and H8 were cracked during the groove roll rolling, and a product could not be produced. FIG. 1 shows the shape after processing of H7 processed without breakage and H8 which was broken vertically by a 40 mm opening during groove roll processing. Moreover, in order to determine the conditions under which cracking during processing does not occur in the 550 ° C. groove roll processing, which is a manufacturing condition of H8, specimen materials SX2-1, SX2-2, and SX2-3 were manufactured under the conditions shown in Table 2.

Although SX2-1 was successful, SX2-2 and SX2-3 had vertical cracks during processing. The shape after processing of SX2-2 is shown in FIG. 2, and the shape after processing of SX2-3 is shown in FIG. In both cases, vertical cracks occurred from the center of the rod, and it was observed that the sample was completely torn into two.

FIG. 4 shows a microstructure after processing of S1 (comparative material) and FIG. 5 shows S2 (material of the present invention). S1 and S2 have the same C (carbon) concentration of 0.45%, but S2 contains 0.1% of S (sulfur) and is often fine Fe3 C particles. It is observed that the particle size is uniformly dispersed.

6 is S1 (comparative material), FIG. 7 is S2 (material of the present invention), FIG. 8 is H5 (reference material), and FIG. 9 is stress-strain obtained by tensile test using H7 (reference material). A curve is shown. Table 3 shows the mechanical properties of the steel materials.

P1 and S1 are comparative materials containing no S (sulfur) and 0.15 mass% and 0.45 mass% of C (carbon), but S2 (material of the present invention) performed under the A condition is S (sulfur). ) Is contained in an amount of 0.1 mass%, YS (yield stress), TS (tensile strength), T. EI. (Total elongation), U.I. EI. The physical property of (uniform elongation) is maintained at the same level as that of 0.45% S (sulfur) -free (S1) having the same C (carbon) content. In H5 and H7 (reference materials) performed under the B condition, UYS increased from 572 MPa to 886 MPa and from 740 MPa to 986 MPa compared to P1 and S1. There is a slight decrease in ductility, but this is thought to be due to the processing temperature decreasing from 650 ° C to 550 ° C.

FIG. 10 shows a Charpy impact test curve prepared under the condition A. However, since the comparative material (S1) has a high C (carbon) content concentration of 0.45%, the upper shelf energy tends to be low.

However, it is recognized that S2 (the material of the present invention) dramatically increases the upper shelf energy from the 100J level to 200J despite the same composition and the same manufacturing conditions except for containing S (sulfur). FIG. 11 is a Charpy impact test curve produced under the B condition, and it is recognized that the comparative materials (P1, S1) tend to have lower upper shelf energy. On the other hand, in the case of the present invention material (H5, H7), the upper shelf energy is remarkably increased in the same composition and the same production conditions except for containing S (sulfur).

As described above in detail, in the high strength ultrafine grain steel of the invention of this application containing S (sulfur), the strength is expressed by trickling and is comparable to an alloy steel that requires QT treatment such as S45C. The steel material which has the intensity | strength characteristic to have and has a high upper shelf energy is implement | achieved. Furthermore, this ultrafine-grained steel is excellent in toughness and fatigue properties in addition to strength properties, and has the characteristics of having the three major basic properties necessary for structural materials. As described above, the invention of this application is to solve the problems of toughness and machinability, which are problems when increasing the strength by refining ferrite, in order to solve the deterioration of toughness and machinability. In this case, a method for avoiding material breakage during the manufacturing process, which is a problem, is provided.

Of course, the invention of this application is not limited to the above examples, and various modes are possible for details.

The shape after processing of H7 steel and H8 steel is compared. The shapes before and after the processing of S2X-2 are compared. The shapes before and after the processing of S2X-3 are compared. It shows the microscopic tissue of S1. It shows the microscopic tissue of S2. It is a stress-strain curve of S1. It is a stress-strain curve of S2. It is a stress-strain curve of H5. It is a stress-strain curve of H7. It is the Charpy impact test curve produced on H conditions. It is a Charpy impact test curve produced on L conditions.

Claims (1)

Precursor steel used for producing high-strength fine-grained steel having a fine grain structure with an average grain size of 3 μm or less by performing strain processing in a temperature range of 550 ° C. to Ae1 + 50 ° C. Has a structure composed of at least one of ferrite and austenite, pearlite, cementite, and martensite having a size of 10 μm or less, C (carbon) is 0.3 to 0.5 mass%, and S (sulfur) is 0.1 to 0.1%. 0.3mass%, Si: 0.8mass% or less, Mn: contains 0.05~3.0mass%, the precursor steel characterized by Rukoto a balance of Fe and unavoidable impurities.