KR101998496B1 - Hot-rolled steel and section - Google Patents

Abstract

A hot-rolled steel material according to one aspect of the present invention is a hot-rolled steel material having a predetermined chemical composition, the remainder including Fe and impurities, at least 90% by area of the metal structure being composed of ferrite and pearlite, The average number density of Mn sulfides having an aspect ratio of not less than 10 but not more than 30, which is measured along the rolling direction, is 50 to 200 / mm ² .

Description

Hot-rolled steel and section

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel part and a hot-rolled steel material thereof.

The present application claims priority based on Japanese Patent Application No. 2015-045855 filed on March 9, 2015, the contents of which are incorporated herein by reference.

Parts for engines and parts for suspensions of automobiles are obtained by forming the steel into hot forging and optionally subjecting the steel to a heat treatment such as quenching. The tempered part is referred to as a tempered part, and the uncoated part is referred to as a non-tempered part. In any case, the mechanical properties required for the part to be applied are secured. In recent years, from the viewpoint of economic efficiency in the manufacturing process, a part omitting the tempering, that is, non-tempered parts has been widely used.

An example of a component for an automobile engine is a connecting rod (hereinafter referred to as a connecting rod). This part is a part that transmits power when converting piston reciprocating motion into rotational motion by crankshaft in the engine. The connecting rod is constituted by a cap portion and a rod portion and is attached to a crankshaft by engaging an eccentric portion of a crankshaft called a pin portion between a cap portion and a rod portion so as to transmit power by a mechanism that rotates and slides with the pin portion .

In order to improve the consistency of the cap portion and the rod portion, it is necessary to make the mating surface of the cap portion and the rod portion smooth when manufacturing a normal connecting rod. Further, when the pin portion is to be fitted between the cap portion and the rod portion, it is necessary to align the cap portion and the rod portion. Therefore, when manufacturing a normal connecting rod, It needs to be formed. The machining process for smoothening the fit face and for forming the unevenness on the fit face increases the manufacturing time and manufacturing cost of the connecting rod. In order to omit the machining process for forming the concavities and convexities, breakage-breaking type connecting rods have been widely employed in recent years.

The fracture-detachable connecting rod is formed by forming the cap part and the rod part into a unitary shape by performing hot forging or the like on the steel, and then cutting the part corresponding to the boundary between the cap part and the rod part to break the connecting rod It is obtained by the method. The mating surface of the cap portion and the rod portion obtained by this method is a wave surface having irregularities obtained by fracture separation. By fitting the irregularities of the brittle fracture surface, it is possible to align the connecting rod when assembling the connecting rod to the crankshaft. Therefore, in the production of the fracture-disconnecting connecting rod, mechanical machining for enhancing the coherence of the mating face and machining for forming the positioning irregularities on the mating face can be omitted. Therefore, the fracture-breaking connecting rod can greatly reduce the machining process of the component, thereby greatly improving the economical efficiency at the time of manufacturing the component.

As a material of the breakaway type connecting rod, what is popular in Europe and America is C70S6 of the DIN standard. C70S6 is a high carbon non-refinement steel containing 0.7% by mass of C, and includes a pearlite structure whose metal structure is low in ductility and toughness in order to suppress dimensional change at the time of fracture separation. C70S6 is excellent in fracture tearability because the amount of plastic deformation near the fracture surface at fracture is small. The fracture toughness of a steel is an index for evaluating the perspiration of fractured surfaces of steel obtained by fracturing the steel. It was judged that a steel having a small amount of deformation in the vicinity of the wave-front, a large brittle fracture area ratio of the wave-front, and a small amount of defects at the time of fracture processing had good fracture toughness. However, C70S6 is applicable to high-strength connecting rods that have a low yield ratio (yield strength / tensile strength) and high buckling strength because the structure is coarser than that of a ferrite-pearlite structure of normal carbon steel for normal connecting rods There is a problem that it can not be done.

In order to increase the yield ratio of the steel, it is necessary to reduce the amount of carbon in the steel and increase the ferrite fraction of the steel. However, when the ferrite fraction of the steel is increased, the ductility of the steel is improved and the plastic deformation amount in the vicinity of the fracture surface becomes large at the time of fracture separation, so that the shape deformation of the connecting rod sliding portion fastened to the fin portion of the crankshaft is increased, There arises a problem in component performance that the roundness of the moving part is lowered.

In recent years, there has been a demand for prevention of misalignment between the cap portion and the rod portion of the connecting rod, that is, improvement of the perspiration and improvement of the fastening force, accompanied with an increase of the engine output due to the spread of the high output diesel engine or turbo engine. Of these, it is effective to control the texture of the steel so as to increase the unevenness of the surface that has been broken and separated.

Several non-tempered steels have been proposed as steels suitable for high-strength fracture-disconnecting connecting rods. Patent Documents 1 and 2 disclose techniques for improving the fracture toughness of a steel by adding a large amount of an embrittlement element such as Si or P to the steel to lower ductility and toughness of the steel. Patent Document 3 and Patent Document 4 describe a technique for improving fracture toughness of a steel by decreasing ferrite ductility and toughness of the steel using precipitation strengthening of second phase grains. Further, Patent Documents 5 to 7 disclose techniques for improving fracture tearability of steel by controlling the shape of Mn sulfide in the steel.

These techniques reduce the amount of deformation of the fractured portion and make the material fragile. Therefore, the steel obtained by these techniques causes defects at the time of fracture separation or when the fracture surfaces are fitted to each other. When the fracture of the fracture surface occurs, the positional displacement of the fitting portion occurs, and there arises a problem that it can not be fitted with high precision. In particular, increasing the concavity and convexity of the fracture surface increases the frequency of occurrence of defects and cracks at the time of fracture, so that a steel capable of attaining both concavity and convexity of the fracture surface and preventing both fracture and occurrence of gold at the time of fracture is required there was. As a solution for preventing the occurrence of defects and gold, there is a method of reducing the segregation of V as disclosed in Patent Document 8. V is a chemical component added for the purpose of increasing the strength.

However, in addition to the segregation of V, there is a cause of generating defects and gold. In practice, when the unevenness of the fracture surface is excessively large, the frequency of occurrence of defects and gold tends to increase. This is because cracks or depressions that are generated in the wavefront direction are also formed when the unevenness in the tensile direction of the fractured surface is formed. When stress is applied to the wave front in order to fit the wave fronts and fasten the wave front, it is considered that the crack or concaved portion that has propagated in the wave front direction becomes a stress concentration portion and fine breakage occurs here. On the other hand, it is necessary to increase the concavity and convexity of the fracture surface in order to improve the cohesion of the wavefronts. As described above, there is a relationship between improvement of perspiration due to the enlargement of the fractured concave and convex portions and prevention of defects and generation of gold, and both of them can not be achieved by the existing methods.

Japanese Patent No. 3637375 Japanese Patent No. 3756307 Japanese Patent No. 3355132 Japanese Patent No. 3988661 Japanese Patent No. 4314851 Japanese Patent No. 3671688 Japanese Patent No. 4268194 Japanese Patent No. 5522321

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a hot-rolled steel material and a steel member that are capable of reducing the deformation amount of the fracture surface at the time of fracture separation and increasing the concavity and convexity of the fracture surface, And to provide the above objects.

The gist of the present invention is as follows.

(1) A hot-rolled steel material according to one embodiment of the present invention has a chemical composition of 0.35 to 0.45 mass% of C, 0.6 to 1.0 mass% of Si, 0.60 to 0.90 mass% of Mn, 0.010 to 0.035 mass% of P, From 0.002 to 0.10 mass% of S, from 0.02 to 0.25 mass% of Cr, from 0.20 to 0.40 mass% of V, from 0.0001 to 0.0050 mass% of Z, from 0.0060 to 0.0150 mass% of N, from 0 to 0.050 mass% 0 to 0.030% by mass of Nb, 0 to 0.0050% by mass of Mg, and 0 to 0.0010% by mass of REM, the remainder including Fe and impurities, and 90% or more of the metal structure is composed of ferrite and pearlite And the average number density of Mn sulfides whose aspect ratio is 10 or more and 30 or less along the rolling direction, which is measured in a cross section parallel to the rolling direction, is 50 to 200 / mm ² .

(2) The hot-rolled steel material according to (1), wherein the chemical component is 0.005 to 0.050 mass% of Ti, 0.005 to 0.030 mass% of Nb, 0.0005 to 0.0050 mass% of Mg and 0.0003 to 0.0010 mass of REM %, And the like.

(3) A steel part according to another aspect of the present invention is characterized by comprising a steel having a chemical composition of 0.35 to 0.45 mass% of C, 0.6 to 1.0 mass% of Si, 0.60 to 0.90 mass% of Mn, 0.010 to 0.035 mass% of P, : 0.06 to 0.10 mass%, Cr: 0.02 to 0.25 mass%, V: 0.20 to 0.40 mass%, Zr: 0.0001 to 0.0050 mass%, N: 0.0060 to 0.0150 mass%, Ti: 0 to 0.050 mass% : 0 to 0.030 mass%, Mg: 0 to 0.0050 mass%, and REM: 0 to 0.0010 mass%, the balance including Fe and impurities, and 90% or more of the metal structure is composed of ferrite and pearlite , And the average number density of Mn sulfides whose aspect ratio is more than 10 and not more than 30, which is measured in a cross section parallel to the rolling direction, is 50 to 200 pieces / mm ² .

(4) The steel component according to (3), wherein when the steel component is subjected to tensile stress by tensile stress parallel to the rolling direction to form a wave front, A step having a height difference of 80 占 퐉 or more toward a direction parallel to the tensile stress and having an angle of 45 degrees or less with respect to the direction parallel to the tensile stress is formed on the wave surface at an average number density of 2 or more per 10 mm, Direction, the angle with respect to said direction parallel to said tensile stress being greater than 45 degrees and extending over a length of 80 占 퐉, and a part of said crack or concave The average number density of the portions is limited to less than 3 points per 10 mm on the wave front, and the brittle fracture wave front on the wave front may be 98% or more area percent.

(5) The steel component according to (3) or (4), wherein the chemical component contains 0.005 to 0.050 mass% of Ti, 0.005 to 0.030 mass% of Nb, 0.0005 to 0.0050 mass% of Mg, To 0.0010% by mass, and at least one of them may be contained.

The hot-rolled steel material and the steel part according to one embodiment of the present invention have a small plastic deformation amount in the vicinity of the fracture surface when fractured and separated, and the occurrence of fracture of the fracture surface is reduced. Therefore, when the fractured end faces are fitted, positional deviation does not occur, and it is possible to fit with high precision, thereby improving the precision and yield of the steel part. Further, by using the steel material and the steel part of the present invention, it is possible to omit the step of removing defects, thereby reducing the manufacturing cost, thereby greatly improving the economic efficiency of the industry. Therefore, the steel material of the present invention is suitable for use in steel parts obtained by hot forging, and the steel material and steel parts of the present invention are suitable for use in which they are used after being fractured and then joined again to each other.

1A is a plan view of a test piece for evaluation of fracture toughness.
1B is a side view of a test piece for evaluating fracture toughness.
Fig. 2 is a cross-sectional photograph of a fractured surface observed as a rugged state of a fractured surface.
3 is a schematic view of crack propagation of a steel part according to the present embodiment.

Hereinafter, the hot-rolled steel and the steel part according to the embodiment of the present invention will be described.

The inventor of the present invention has found that by controlling the shape of the Mn sulfide present in the steel, the size of the irregularities of the wave front obtained by fracture separation in the vertical direction of the wavefront can be preferably controlled to suppress the amount of defects.

According to the findings of the present inventors, the concavo-convex shape of the wave front is affected by the degree of elongation and the distribution frequency of the Mn sulfide. When the elongation of the Mn sulfide is excessive (that is, the aspect ratio of the Mn sulfide is large), the size of the irregularities in the vertical direction of the wave front becomes remarkably large, so that defects or gold are generated at the wavefront during fracture separation or wavefront fitting, Voids are generated at the time of fitting, and the perspiration is lowered. On the other hand, if the distribution frequency of the elongated Mn sulfide is increased, the number of irregularities of the wave front increases, and the synthesis is improved.

The present inventors speculate that the above-described phenomenon is caused by the following mechanism. The Mn sulfide of the steel component is stretched in the rolling direction at the time of hot rolling the hot rolled steel which is the material of the steel part. 3, when the steel member 10 is fractured and separated in a direction substantially perpendicular to the rolling direction, the crack 12 is first developed perpendicularly from the rupture starting point 13 in the rolling direction. However, when the crack 12 reaches the stretched Mn sulfide 11 in the rolling direction, the advancing direction of the crack 12 largely changes, and the crack 12 causes the Mn sulfide, It is considered to be approximately parallel to the rolling direction. When the crack 12 reaches the end of the Mn sulfide 12, the direction of advance of the crack 12 changes in the direction of stress, and the crack 12 again advances substantially perpendicular to the rolling direction. It is considered that the fracture 12 progresses perpendicular to the rolling direction and advances parallel to the rolling direction while fracture separation progresses to form irregularities on the wave front. Based on the above reasons, the inventors of the present invention have estimated that the number of irregularities increases when the number of Mn sulfides 11 increases, and that the size of the irregularities increases along the rolling direction when the aspect ratio of the Mn sulfides 11 is large.

The present invention is obtained by the knowledge of the inventors of the present invention described above. The chemical components of the hot-rolled steel and the steel part according to the embodiment of the present invention, the shape of the Mn sulfide and the shape of the fracture surface obtained by fracture splitting will be described below.

In addition, the chemical composition of the hot-rolled steel does not change with hot working. Further, since the size of the Mn sulfide is very small compared to the size of the strain imparted by hot working, the shape of the Mn sulfide of the hot-rolled steel is hardly changed by the hot working. Therefore, the chemical composition of the steel part and the shape of the Mn sulfide obtained by hot working the hot-rolled steel according to the present embodiment are the same as those of the hot-rolled steel according to the present embodiment described below. Since the shape of the wave front obtained by the fracture splitting is determined according to the chemical composition and the shape of the Mn sulfide, the shape of the wave front obtained by the fracture splitting is the same as that of the hot rolled steel according to the present embodiment, The parts are the same.

The hot-rolled steel material of the present embodiment is a steel material containing C, Si, Mn, P, S, Cr, V, Zr and N at a predetermined content as a chemical component. The hot-rolled steel of the present embodiment contains the chemical components described below, so that the ductility is preferably controlled to improve the ratio of the brittle fracture wavefront at the wave front (tensile wavefront) obtained by tensile stress, It is possible to increase the size of the unevenness of the wavefront in the vertical direction of the wavefront. Thereby, the hot-rolled steel material of the present embodiment has a high frictional resistance obtained by fracture-splitting. The hot-rolled steel of the present embodiment may optionally contain at least one of Ti, Nb, Mg, and REM as a chemical component.

The reason for limiting the chemical composition of the hot-rolled steel and the steel part of the present embodiment will be described below. Hereinafter, unless otherwise specified, the reason for limiting the chemical composition of the hot-rolled steel is equivalent to the reason for limiting the chemical composition of the steel part.

(C: 0.35 to 0.45% by mass)

C has the effect of ensuring the tensile strength of the hot-rolled steel and the steel part of the present embodiment, and the effect of realizing good fracture separation by reducing the amount of plastic deformation near the fracture surface at the time of fracture. As C increases, the volume fraction of the pearlite structure rises, resulting in an increase in tensile strength and a decrease in ductility and toughness. In order to maximize these effects, the C content in the steel is set to 0.35 to 0.45 mass%. If the C content exceeds the upper limit, the pearlite fraction of the hot-rolled steel becomes excessively large, and the frequency of occurrence of defects at breakage increases. Further, when the C content is less than the lower limit, the amount of plastic deformation near the fracture surface of the hot-rolled steel material increases, and the reduction of the fracture surface deteriorates. The lower limit of the C content is preferably 0.36 mass% or 0.37 mass%. The upper limit value of the C content is preferably 0.44 mass%, 0.42 mass%, or 0.40 mass%.

(Si: 0.6 to 1.0% by mass)

Si strengthens ferrite by solid solution strengthening, thereby lowering ductility and toughness of hot-rolled steel and steel parts. The plastic deformation amount in the vicinity of the fracture surface at the time of fracture separation becomes small due to the deterioration of ductility and toughness and the fracture detachability of the hot-rolled steel and the steel part is improved. In order to obtain this effect, it is necessary to set the lower limit of the Si content to 0.6 mass%. On the other hand, if Si is contained excessively, the frequency of occurrence of the fracture of the fracture surface increases, so that the upper limit of the Si content is set to 1.0% by mass. The lower limit of the Si content is preferably 0.7% by mass. The upper limit value of the Si content is preferably 0.9 mass%.

(Mn: 0.60 to 0.90% by mass)

Mn strengthens ferrite by solid solution strengthening, thereby lowering ductility and toughness of hot-rolled steel and steel parts. The plastic deformation amount in the vicinity of the fracture surface at the time of fracture separation becomes small due to the deterioration of ductility and toughness and the fracture detachability of the hot-rolled steel and the steel part is improved. Further, Mn combines with S to form a Mn sulfide. When a steel component obtained from the hot-rolled steel of the present embodiment is fractured and divided, the crack propagates along the Mn sulfide stretched in the rolling direction, so that the size of the irregularities of the fracture surface in the rolling direction is increased. Therefore, Mn has an effect of preventing positional deviation when fitting a fracture surface. On the other hand, when Mn is excessively contained, the ferrite becomes too hard and the frequency of occurrence of defects at break increases. Taking these into consideration, the Mn content is 0.60 to 0.90 mass%. The lower limit value of the Mn content is preferably 0.65 mass%, 0.70 mass%, or 0.75 mass%. The upper limit value of the Mn content is preferably 0.85% by mass, 0.83% by mass, or 0.80% by mass.

(P: 0.010 to 0.035% by mass)

P lowers the ductility and toughness of ferrite and pearlite, thereby lowering ductility and toughness of hot-rolled steel and steel parts. The plastic deformation amount in the vicinity of the fracture surface at the time of fracture separation becomes small due to the deterioration of ductility and toughness and the fracture detachability of the hot-rolled steel and the steel part is improved. However, P causes embrittlement of the grain boundaries, and the fracture of the fracture surface is liable to occur. Therefore, it is not preferable to lower the ductility and toughness by using P from the viewpoint of preventing defects. In consideration of the above, the range of the P content is 0.010 to 0.035 mass%. The lower limit value of the P content is preferably 0.012 mass%, 0.013 mass%, or 0.015 mass%. The upper limit value of the P content is preferably 0.030 mass%, 0.028 mass%, or 0.025 mass%.

(S: 0.06 to 0.10% by mass)

S combines with Mn to form Mn sulfide. The crack propagates along the Mn sulfide stretched in the rolling direction when the steel component obtained from the hot-rolled steel material according to the present embodiment is fractured and divided so that the size of the Mn sulfide in the vertical direction of the wave- There is an effect of preventing the positional deviation. In order to obtain the effect, it is necessary to set the lower limit of the S content to 0.06 mass%. On the other hand, if S is contained excessively, the plastic deformation amount in the vicinity of the fracture surface at the time of fracture splitting is increased, and the fracture detachability is lowered. Also, the excessive amount of S may promote the defect of the fracture surface. For the reasons described above, the suitable range of S is set to 0.06 to 0.10 mass%. The lower limit value of the S content is preferably 0.07 mass%. The upper limit value of the S content is preferably 0.09 mass%.

(Cr: 0.02 to 0.25% by mass)

Cr, like Mn, strengthens ferrite by solid solution strengthening and lowers ductility and toughness of hot-rolled steel and steel parts. The decrease in ductility and toughness reduces the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture, thereby improving the fracture tearability of the hot-rolled steel and the steel part. In order to obtain the effect, it is necessary to set the lower limit of the Cr content to 0.02 mass%. However, if Cr is excessively contained, the lamellar spacing of the pearlite becomes small, and the softness and toughness of the pearlite become high. Therefore, the plastic deformation amount in the vicinity of the fracture surface at the time of fracture becomes large, and the fracture detachability is deteriorated. In addition, if Cr is contained excessively, bainite structure is easily produced, and breakability can be greatly deteriorated. Therefore, when Cr is contained, the content thereof is set to 0.25 mass% or less. The lower limit value of the Cr content is preferably 0.05% by mass, 0.08% by mass, or 0.10% by mass. The upper limit value of the Cr content is preferably 0.23 mass%, 0.20 mass%, or 0.18 mass%.

(V: 0.20 to 0.40% by mass)

V forms carbides or carbonitrides mainly at the time of cooling after hot forging to strengthen ferrite and lower ductility and toughness of hot-rolled steel and steel parts. The lowering of the ductility and toughness of the hot-rolled steel and the steel part reduces the plastic deformation amount of the hot-rolled steel and the steel part in the vicinity of the fracture surface at the time of fracture, thereby improving the fracture tearability of the steel part including the hot-rolled steel . Further, V has the effect of increasing the yield ratio of the hot-rolled steel by precipitation strengthening of carbide or carbonitride. In order to obtain these effects, it is necessary to set the lower limit of the V content to 0.20 mass%. On the other hand, even if V is excessively contained, the effect is saturated, so the upper limit of the V content is 0.40 mass%. The lower limit value of the V content is preferably 0.23 mass% or 0.25 mass%. The upper limit value of the V content is preferably 0.38 mass% or 0.35 mass%.

(Zr: 0.0001 to 0.0050% by mass)

Zr forms an oxide, and this Zr oxide becomes a nucleation or precipitation nucleus of the Mn sulfide, and uniformly and finely disperses the Mn sulfide. This finely dispersed Mn sulfide serves as a propagation path for cracking during breakage and has an effect of reducing the amount of plastic deformation in the vicinity of the fracture surface of the hot-rolled steel and the steel part, thereby improving fracture tearability. However, even if Zr is contained excessively, the effect is saturated, so the upper limit of the Zr content is set to 0.0050 mass%. In order to fully exhibit this effect, the lower limit of the Zr content is set to 0.0001 mass%. The lower limit value of the Zr content is preferably 0.0005 mass% or 0.0010 mass%. The upper limit value of the Zr content is preferably 0.0045 mass%, 0.0040 mass%, or 0.0030 mass%.

(N: 0.0060 to 0.0150 mass%)

The N promotes the ferrite transformation by forming V nitride or V carbonitride mainly at the time of cooling after hot forging and acting as a transformation nucleus of ferrite. Thereby, N has the effect of suppressing the formation of bainite structure which greatly deteriorates the fracture tearability of the steel component obtained from the hot-rolled steel. In order to obtain this effect, the lower limit of the N content is set to 0.0060 mass%. If N is excessively contained, the hot-rolled steel of the hot-rolled steel and the steel part may deteriorate, and cracks or scratches may easily occur at the time of hot working. Therefore, the upper limit of the N content is set to 0.0150 mass%. The lower limit of the N content is preferably 0.0065 mass%, 0.0070 mass%, or 0.0080 mass%. The upper limit value of the N content is preferably 0.0140 mass%, 0.0130 mass%, or 0.0120 mass%.

The hot rolled steel according to the present embodiment may further contain 0.050 mass% or less of Ti, 0.030 mass% or less of Nb, 0.0050 mass% or less of Mg, or 0.0010 mass% or less of REM And at least one kind selected from the group consisting of However, even when these elements are not contained, the lower limit value of Ti, Nb, Mg, and REM is 0 mass% because the hot rolled steel and the steel according to this embodiment can solve the problems.

(Ti: 0 to 0.050 mass%)

Ti forms carbides or carbonitrides mainly at the time of cooling after hot forging and strengthens ferrite by precipitation strengthening, thereby lowering ductility and toughness of hot-rolled steel and steel parts. The decrease in ductility and toughness reduces the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture, thereby improving fracture tearability of the hot-rolled steel and the steel component. However, if Ti is contained excessively, the effect becomes saturated. When Ti is contained in order to obtain the above-mentioned effect, the upper limit of the Ti content is set to 0.050 mass%. In order to sufficiently exhibit the effect of Ti, it is preferable to set the lower limit of the Ti content to 0.005 mass%. The lower limit of the Ti content is more preferably 0.015 mass%, 0.018 mass%, or 0.020 mass%. A more preferable upper limit value of the Ti content is 0.040 mass%, 0.035 mass%, or 0.030 mass%.

(Nb: 0 to 0.030% by mass)

Nb forms carbides or carbonitrides mainly at the time of cooling after hot forging and strengthens ferrite by precipitation strengthening, thereby lowering ductility and toughness of hot-rolled steel and steel parts. The decrease in ductility and toughness reduces the amount of plastic deformation in the vicinity of the fracture surface at the time of fracture, thereby improving fracture tearability of the hot-rolled steel and the steel component. However, if Nb is contained excessively, the effect becomes saturated. When Nb is contained in order to obtain the above-mentioned effect, the upper limit of the Nb content is set to 0.030 mass%. In order to sufficiently exhibit the effect of Nb, it is preferable to set the lower limit of the Nb content to 0.005 mass%. The lower limit value of the more suitable Nb content is 0.010 mass%. The upper limit value of the more suitable Nb content is 0.030 mass%, 0.028 mass%, or 0.025 mass%.

(Mg: 0 to 0.0050 mass%)

Mg forms oxides and becomes nucleation or precipitation nuclei of Mn sulfides, thereby uniformly and finely dispersing Mn sulfides. This Mn sulfide serves as a propagation path of cracks at the time of fracture, thereby reducing the amount of plastic deformation in the vicinity of the fracture surface and enhancing fracture tearability of the hot-rolled steel and the steel part. However, even if Mg is contained in excess, the effect is saturated, so the upper limit of the Mg content is set to 0.0050 mass%. In order to fully exhibit this effect, the lower limit of the Mg content is preferably 0.0005 mass%. The lower limit value of the more preferable Mg content is 0.0006 mass%. The upper limit value of the more preferable Mg content is 0.0045 mass%, 0.0040 mass%, 0.0035 mass%, 0.0030 mass%, or 0.0025 mass%.

(REM: 0 to 0.0010% by mass)

The REM forms an oxysulfide to become a nucleation or precipitation nucleus of the Mn sulfide, thereby uniformly and finely dispersing the Mn sulfide. This Mn sulfide serves as a propagation path of cracks at the time of fracture, thereby reducing the amount of plastic deformation in the vicinity of the fracture surface and enhancing fracture tearability of the hot-rolled steel and the steel part. However, if REM is contained excessively, problems such as clogging of the nozzle in the casting step occur in the steel material manufacturing step. Therefore, the upper limit of the REM content is set to 0.0010 mass%. In order to sufficiently exhibit this effect, the lower limit of the REM content is preferably 0.0003 mass%. A lower limit value of a more suitable REM content is 0.0004 mass%, or 0.0005 mass%. A more preferable upper limit value of the REM content is 0.0009 mass%, 0.0008 mass%, or 0.0007 mass%. The term " REM " refers to 17 elements including Sc, Y and lanthanoid, and the " content of REM " means the total content of these 17 elements. When lanthanoids are used as REMs, industrially, REMs are added in the form of mischmetals.

The balance of the chemical components of the hot-rolled steel and the steel part according to the present embodiment is Fe and impurities. Impurities are those which are mixed in raw materials such as ores and scrap and in a manufacturing environment and do not affect the properties of hot-rolled steel and steel parts according to the present embodiment. In addition to the above components, the hot-rolled steel and the steel part according to the present embodiment may contain 0 to 0.01% of Te, 0 to 0.01% of Zn, and 0 to 0.01% of Sn ≪ / RTI >

(Metal structure: at least 90% by area is composed of ferrite and pearlite)

The hot rolled steel material and the metal structure of the steel part of the present embodiment have a so-called ferrite-pearlite structure. In some cases, bainite or the like is contained in the metal structure. Bainite is undesirable because it damages fracture splittability. Therefore, the inventors of the present invention have defined that the hot rolled steel of the present embodiment and the metal structure of steel parts contain ferrite and pearlite in a total area of 90% or more by area. With this provision, the amount of bainite is limited to not more than 10% by area, and the fracture splitting property of the hot-rolled steel and the steel part is satisfactorily maintained. The hot rolled steel of the present embodiment and the steel structure may contain ferrite and pearlite in a total of 92 area%, 95 area%, or 98 area% or more.

The ratio of the ferrite and the pearlite is not particularly limited as far as the total amount of the ferrite and the pearlite is within the above-mentioned range. For example, even if ferrite or pearlite is 0% by area, good fracture splittability is maintained. The constitution of the remainder of the metal structure is not particularly limited as long as the total amount of the ferrite and the pearlite is within the above-mentioned range. The amount of ferrite and pearlite contained in the metal structure is obtained by photographing an optical microscope picture of the polished and etched cross section and analyzing the picture by image analysis.

(Average number density of Mn sulfides having an aspect ratio of not less than 10 but not more than 30 stretched along the rolling direction: 50 to 200 / mm ² )

Mn sulfides are formed in the hot rolled steel material and steel parts of the present embodiment. The Mn sulfide is elongated along the rolling direction of the hot rolled steel. The elongated Mn sulfide is an indispensable inclusion for forming a concave-convex shape on the wave front obtained by tensile fracturing the hot-rolled steel and the steel part. In order to elongate the Mn sulfide, it is necessary to set the reduction ratio from the billet to the steel bar at least 80% when the steel is produced by hot rolling.

In the hot-rolled steel material and the steel part according to the present embodiment, the elongated Mn sulfide having an aspect ratio of not less than 10 and not more than 30 with the rolling direction as the long axis side is distributed in the range of 50 to 200 per 1 mm ² . The elongated Mn sulfide forms irregularities in the tensile direction on the fracture surface formed by the tensile fracture in the rolling direction, thereby enhancing the sensibility of the fracture surfaces. The Mn sulfide having an aspect ratio of more than 10 and not more than 30 can appropriately size the unevenness in the tensile direction. When the number of Mn sulfides having an aspect ratio of 10 to 30 or less is 50 to 200 / mm ² , the number of irregularities can be optimized.

The Mn sulfide having an aspect ratio of 10 or less can not sufficiently increase the size of the unevenness of the fracture surface in the tensile direction and does not contribute to improvement of the seam composition of the fracture surfaces. The Mn sulfide having an aspect ratio exceeding 30 remarkably makes irregularities on the fracture surface, but increases the frequency of fracture and deficiency, thereby impairing the sensation of fracture surfaces. Therefore, it is preferable that the number density of Mn sulfides having an aspect ratio of 10 or less or 30 or more is small. However, when the number density of Mn sulfides having an aspect ratio of more than 10 and equal to or less than 30 is within the above-mentioned range and the contents of Mn and S which are sources of Mn sulfide are within the above-mentioned range, Mn and S in the chemical components become Mn sulfides having an aspect ratio of 10 or less or 30 or more are sufficiently suppressed. Therefore, the number density of Mn sulfides having an aspect ratio of 10 or less or 30 or more is not particularly limited.

When the average number density of Mn sulfides having an aspect ratio of more than 10 and not more than 30 does not reach the lower limit value, the number of irregularities of the fracture surface becomes small, and insufficient synthesis of the fracture surface after fracture separation becomes insufficient. When the average number density of Mn sulfides having an aspect ratio of more than 10 and less than 30 does not reach the lower limit value, the number density of Mn sulfides having an aspect ratio of less than 10 or more than 30 is increased, which may deteriorate fracture tearability. On the other hand, when the average number density of Mn sulfides having an aspect ratio of more than 10 and not more than 30 exceeds the upper limit value, breakage or breakage occurs in the fracture plane, and in this case also, collapse of the fracture plane is damaged.

A method of measuring the average number density of Mn sulfides having an aspect ratio of not less than 10 but not more than 30 in the hot rolled steel and steel parts along the rolling direction is as follows.

First, the hot rolled steel and the steel part are cut parallel to the rolling direction, and the cut surface is polished. Mn sulphide is stretched along the rolling direction. Therefore, when cutting hot-rolled steel and steel parts, the stretching direction of the Mn sulfide can be regarded as the rolling direction of the hot-rolled steel and the steel part.

Subsequently, an enlarged photograph of the cut surface is photographed by an optical microscope or an electron microscope. The magnification at this time is not particularly limited, but is preferably about 100 times. Since the Mn sulfide is distributed substantially uniformly, the area for photographing is not particularly limited.

By analyzing the photographs, the number density of Mn sulfides having an aspect ratio of more than 10 and not more than 30 in the area where the photograph was taken can be obtained. Further, some of the elongated Mn sulfides are divided and distributed in the form of heat in the rolling direction. However, two Mn sulphides with a spacing of 10 탆 or less are considered as one elongated Mn sulphide. Two Mn sulfides arranged in the elongation direction and having an interval of not more than 10 mu m have the same effect as one Mn sulfide in that they propagate in the tensile direction the cracks generated when the hot rolled steel or steel part is subjected to tensile fracture I think I have it.

The average number density of the Mn sulfides having an aspect ratio of not less than 10 but not more than 30 in the rolling direction of the hot-rolled steel and the steel part is not less than 30% by the number density obtained by repeating the photographing and the analysis at least ten times, Is obtained.

(Hot-rolled steel and method for manufacturing steel parts)

Next, a method of manufacturing a hot-rolled steel material and a steel part according to the present embodiment will be described. The method of manufacturing a hot-rolled steel material according to the present embodiment includes a step of obtaining blooms by solvent and continuous casting of a steel having the same chemical composition as that of the hot-rolled steel material according to the present embodiment, a step of hot- And a step of hot rolling the billet to obtain a round bar, wherein Zr is added at an early stage of secondary refining in the solvent so that the total reduction ratio in the hot rolling is 80% or more, and in the hot rolling, Of not less than 50% at a temperature of 1000 占 폚 or less. The method for manufacturing a steel part according to the present embodiment includes the steps of heating the hot-rolled steel material according to the present embodiment to 1150 to 1280 캜 for hot forging and a step of cooling the hot-rolled hot-rolled steel material to room temperature by air cooling or air- Cold forging the hot-rolled steel material according to the present embodiment, and cutting the cooled hot-rolled steel material to obtain a steel component having a predetermined shape.

Details of the method of manufacturing the hot-rolled steel material according to the present embodiment are as follows. First, a steel having the same chemical composition as that of the hot-rolled steel according to the present embodiment is melted in a converter, and is continuously cast to produce a bloom. Zr is added to the molten steel during the secondary refining or the secondary refining at the converter solvent. (For example, RH (Ruhrstahl-Heraeus)) or the like is used in order to float the crude Zr oxide from the molten steel sufficiently and to finely disperse the Mn sulfide that generates the Zr oxide as nuclei in the molten steel Zr must be added before the degassing treatment is performed on the molten steel, or during the degassing treatment by RH and within 15 minutes after the start of the treatment). When the addition of Zr is performed after 15 minutes from the start of the degassing treatment using RH or the like, the time for making the Mn sulfide using the Zr oxide becomes insufficient, so that the Mn sulfide in the bloom is coarsened. When the Mn sulfide in the bloom is coarsened, the Mn sulfide is excessively stretched in the subsequent Bloom rolling step, the Mn sulfide having an aspect ratio of more than 30 is increased, and the number density of Mn sulfides having an aspect ratio of more than 10 and less than 30 is insufficient.

The obtained bloom is further subjected to a crushing and rolling step to form a billet. The resulting billet is also round bar by hot rolling. Thus, the hot-rolled steel of the present embodiment is produced. In order to elongate the Mn sulfide, it is preferable that the rolling reduction ratio when the billet is formed into a round bar shape is 80% or more. Further, in order to remarkably elongate the Mn sulfide, it is necessary to perform hot rolling in a temperature region where the high temperature hardness of the Mn sulfide is relatively low relative to the steel material, that is, the temperature region where the Mn sulfide is easily elongated. Concretely, it is necessary to set the rolling reduction ratio at 1000 ° C or lower to 50% or more. Thereby, the Mn sulfide in the steel can be elongated. If these rolling conditions are not satisfied, the Mn sulfide is not sufficiently stretched. Further, the hot rolled steel after hot rolling may be cooled to room temperature, or may be provided for hot forging before cooling.

Details of the manufacturing method of the steel member according to the present embodiment are as follows. The hot-rolled steel material obtained by the above-described method is heated to, for example, 1150 to 1280 占 폚 for hot forging and cooled to room temperature by air cooling (air cooling in the atmosphere) or air blast cooling (cooling the steel by wind). The forged member after cooling is cut to obtain a steel member having a predetermined shape. When hot-rolled steel is forged, it may be subjected to cold forging instead of hot forging.

The hot-rolled steel material and the steel part of the present embodiment exhibit a tensile stress of 80 占 퐉 toward a direction parallel to the tensile stress observed in a cross section parallel to the rolling direction when the fractured surface is formed by tensile stress caused by tensile stress parallel to the rolling direction And a step having an angle of 45 degrees or less with respect to a direction parallel to the tensile stress is formed on the wavefront at an average number density of at least 2 per 10 mm and a tensile stress in a direction parallel to the tensile stress And the average number density of the cracks or concavities in which a part of the cracks or recesses are developed inside the steel member is limited to less than 3 points per 10 mm in the wave front, The brittle fracture wave front at the wave front is 98% or more area%.

The reasons for defining the characteristics of the wavefront will be described below. When the wavefronts formed by the tensile fracture are fitted to each other and stress is applied to the wavefront in the horizontal direction, the stress is applied to the wavefront in the horizontal direction and in two normal directions (inclination directions of 90 deg. And vertical direction). In this case, the applied stress is dispersed as the size of the wave-front unevenness in the tensile direction is larger. The inventors have found that when the step formed by the unevenness has an angle of 45 degrees or less with respect to the direction parallel to the tensile stress and a height difference of 80 mu m or more toward the direction parallel to the tensile stress, . Further, as long as the size of the step in the tensile direction of the step of the wave front is larger, the positional deviation at the time of application of the stress can be more reliably prevented as long as the wave front defect does not occur.

The amount of defect occurrence correlates with the presence of cracks in the wavefront direction of the fractured surface or presence of concave portions in the wavefront direction. That is, as the cracks in the wavefront direction or the concave portions in the wavefront direction are larger than a certain size, the amount of defects is increased. It is considered that cracks or concave portions in the wavefront direction act as stress concentration regions and break finely when the fracture surfaces are fitted, resulting in defects. The inventors of the present invention have found that it is necessary to minimize cracks or depressions in the wavefront direction in order to suppress the amount of defect generation at the fracture surface. Specifically, in order to sufficiently suppress the amount of defect generation, an angle of more than 45 degrees with respect to a direction parallel to the tensile stress, which is observed on a cross section parallel to the rolling direction, is formed over a length of 80 占 퐉 or more, The inventors have found that the average number density of cracks or recesses developed inside the parts should be limited to less than 3 points per 10 mm.

Particularly, since the shape and dispersion state of Mn sulfide greatly affect the shape of the wave front, it is important to control the morphology and dispersion state of the Mn sulfide in order to maximize the unevenness of the wave front in a range not causing defects. More specifically, the Mn sulphide, which is a path of crack propagation, is dispersed in a proper range while being stretched in a suitable range, contributing to increasing the size of the unevenness in the tensile direction of the fracture surface. Thus, in the present embodiment, the concave-convex shape that can be experimentally realized within a range that does not cause the fracture of the fracture surface at the time of fracture is defined as above.

The hot rolled steel and the steel part according to the present embodiment are preferably controlled in chemical composition so that at least 90% by area of the metal structure becomes ferrite and pearlite, and the Mn sulfide having a predetermined shape is dispersed therein Therefore, at least 98% by area of the wave front obtained by dividing the hot-rolled steel and the steel according to the present embodiment by tensile stress parallel to the rolling direction becomes a brittle wave front. Since deformation occurs in the ductile wavefront, the ductile wavefront impairs the convolution of the wavefront. When 98% by area or more of the wave front is a brittle wave front, the wave front synthesis is preferably maintained.

The evaluation method of the wavefront shape is as follows.

The area ratio of the brittle fracture surface occupying in the fracture plane is determined by analyzing the photograph according to a conventional wavefront analysis method to define a region in which a brittle fracture surface composed of cleavage fracture, cleavage fracture or grain boundary cracking occurs, And the ratio of the area of the wave-front area to the area of the entire wave-front end is calculated.

The amount of deformation due to fracture is determined by measuring the difference between the inner diameter in the fracture direction and the inner circumference in the direction perpendicular to the fracture direction by tightening the hot-rolled steel material or steel part after rupture with a bolt, . ≪ / RTI >

The amount of defects generated at the fracture surface is determined by loosening the bolt and releasing the fracture surface by repeating the operation ten times until the total weight of the detached fragments is measured , And this total weight is determined as the amount of defect occurrence of the fracture surface.

(Tensile step) having an elevation difference of 80 占 퐉 or more toward a direction parallel to the tensile stress and having an angle of 45 degrees or less with respect to a direction parallel to the tensile stress, which is observed in a cross section parallel to the rolling direction, Direction and a part of which is formed over a length of 80 占 퐉 or more and has a crack or a concave portion in the inside of the steel member Cracks) is evaluated by the following method. First, the hot-rolled steel material or the steel part having the wave-front is cut in parallel to the tensile direction so that the wave-front shape can be observed from the direction perpendicular to the tensile direction. The wave front shape may be maintained at the time of cutting by embedding the wave front into the resin before cutting. By observing the wave-front shape on the above-mentioned cut surface, the unevenness in the tensile direction and the unevenness in the wave-surface direction can be observed. Further, the cut surface can be formed at any place of the test piece parallel to the tensile direction. For convenience, it is preferable to form the cut surface so that the wave surface at the cut surface becomes as large as possible. Observation is carried out at an arbitrary five visual field or longer on the cut surface, and at the time of observation, the tensile step in each field of view and the number density per 10 mm of the crack in the wavefront direction are measured and their average values are obtained. Thereby, the tensile step and the number density of cracks in the wavefront direction are obtained.

The method of breaking the hot rolled steel material and the steel part according to the present embodiment is not particularly limited, but it is preferable that the hot rolled steel material is broken by using a tensile stress parallel to the rolling direction. Since the hot rolled steel according to the present embodiment and the Mn sulfide of the steel part are stretched in parallel to the rolling direction, tensile stress parallel to the rolling direction is applied to form a wave front substantially perpendicular to the rolling direction, The effect is maximized. In addition, in order to improve the fracture tearability, it is preferable to apply a notch process to a portion forming the wave front before applying tensile stress. The method of notch machining is not particularly limited, and for example, notch machining may be performed by broaching or laser machining.

Example

The present invention will be described in detail below with reference to Examples. These examples are for the purpose of illustrating the technical significance and effect of the present invention and do not limit the scope of the present invention.

Example  One

The blooms were produced by continuous casting of the steel 1 to 28 and the steel 101 to 115 which had been melted in the converter having the chemical compositions shown in Tables 1-1 and 1-2. The blooms were subjected to a crushing rolling process, And a round bar having a diameter of 56 mm was formed by hot rolling. Zr was added to the molten steel before the degassing treatment was performed on the molten steel using the RH or within 15 minutes after the start of the degassing treatment when the steel 1 to 28, the steel 101 to 112, the steel 114 and the steel 115 were dissolved. Zr was not added to the steel 113. When the billets were rolled into hot rolled bars, the total reduction ratio was 90%, and the reduction ratio in the temperature range of 1000 占 폚 or less was 80%. The symbol " - " in the table indicates that the content of the element with respect to the place where the symbol is written is an impurity level. The heating temperature and the heating time of the blooms before the block rolling were 1270 캜 and 140 min respectively, and the heating temperature and heating time of the billets before hot rolling were 1240 캜 and 90 min, respectively. Values underlined in Tables 1-1 and 1-2 are figures outside the scope of the present invention.

The average number density of Mn sulfides having an aspect ratio of not less than 10 but not more than 30, which were measured in a cross section parallel to the rolling direction and which were included in Examples 1 to 28 and Comparative Examples 101 to 115 obtained by the above- Mn sulfide number density) was calculated by the following method. First, Examples 1 to 28 and Comparative Examples 101 to 115 were cut parallel to the rolling direction, and the cut surfaces were polished. Subsequently, enlarged photographs of the cut surfaces of Examples 1 to 28 and Comparative Examples 101 to 115 were photographed by an optical microscope or an electron microscope. The magnification at this time was 100 times. Then, by analyzing the photographs, the number density of Mn sulfides having an aspect ratio of not less than 10 and not more than 30 in the area where the photograph was photographed was obtained. Further, the two Mn sulfides having an interval of 10 탆 or less were regarded as one elongated Mn sulfide. The photographing and the analysis were repeated ten times, and the number density obtained thereby was averaged to obtain the Mn sulfides of Examples 1 to 28 and Comparative Examples 101 to 115 having an aspect ratio of not less than 10 but not more than 30 stretched along the rolling direction The average number density was obtained. The " Mn sulfide number density " in Tables 1-1 and 1-2 is an average number density of Mn sulfides whose aspect ratio is 10 or more and 30 or less, which is drawn along the rolling direction and is measured in a cross section parallel to the rolling direction.

Then, in order to investigate the fracture detachability, test pieces 1 to 28 and 101 to 115 corresponding to the forged connecting rod were produced by hot forging. Concretely, the steel 1 to 28 and the steel 101 to 115 made of material bars having a diameter of 56 mm and a length of 100 mm were heated to 1150 to 1280 占 폚 and then forged perpendicular to the longitudinal direction of the steel bar by the above- Respectively. The steels 1 to 27 and the steels 101 to 115 were cooled to room temperature by air cooling (air cooling in the atmosphere), and the steel 28 was cooled to room temperature by air-blast cooling (cooling the specimen by cooling). From the cooled forged material, a tensile test specimen of JIS No. 4 and a test piece for evaluating the fracture tearability of the connecting rod large end portion were cut. The JIS No. 4 tensile test specimen was taken along the longitudinal direction of the forging material at a position 30 mm from the side surface of the forging material. As shown in Fig. 1, a test piece for evaluation of fracture tearability was prepared by punching a hole with a diameter of 50 mm in a central portion of a plate shape having a size of 80 mm x 80 mm and a thickness of 18 mm. On the inner surface of a hole having a diameter of 50 mm, V notch processing was performed at 45 degrees at a position of 1 mm in depth and a tip curvature of 0.5 mm in two positions at ± 90 degrees with respect to the longitudinal direction of the bar. A through hole having a diameter of 8 mm was drilled as a bolt hole so that the center line thereof was located at a position of 8 mm from the side of the notched side.

The test apparatus for the evaluation of fracture toughness consists of a split mold and a descent tester. The split mold is formed by dividing a cylindrical cylinder having a diameter of 46.5 mm formed on a rectangular steel into two along the center line, one of which is fixed and one of which moves on the rail. Wedges are machined in the mating faces of the two semicircular columns. In the fracture test, a hole having a diameter of 50 mm of the test piece is inserted into a cylinder having a diameter of 46.5 mm of the split mold, and a wedge is inserted into the cylinder. The mass of the parachute is 200kg, and it falls down along the guide. When dropping down, the wedge is inserted, and the specimen undergoes tensile fracture in two. The tensile stress applied to the test piece at the time of tensile fracture becomes parallel to the hot rolling direction. Further, the periphery of the test piece is fixed so as to be pressed against the split mold so that the test piece is not released from the split mold at the time of breakage.

The method of measuring the area ratio (brittle fracture area ratio) of the brittle fracture surface in the fracture plane was as follows. First, the specimen was broken at a fall height of 100 mm, and an optical microscope photograph of the fracture surface was taken. By analyzing photographs in accordance with a conventional wavefront analysis method, an area in which a brittle wavefront composed of cleavage breakage, cleavage breakage or grain boundary cracking is generated is defined, and the area of this brittle wavefront area is divided by the area of the entire wavefront .

The method of measuring the amount of deformation during fracture separation was as follows. The test piece after the fracture was tightened with a bolt, and the difference between the inner diameter in the fracture direction and the inner curve in the direction perpendicular to the fracture direction was measured. This difference was regarded as a deformation amount due to fracture splitting.

The method of measuring the amount of defect occurrence on the fracture surface was as follows. After the above-described deformation amount measurement was carried out, the fracture section was bolted together with a torque of 20 N · m to assemble it, and then the bolt was loosened and the fracture section was released ten times. The gross weight of the removed fragments was measured, and the gross weight was defined as the amount of defects generated at the fracture surface.

A steel having good fracture tearability is a steel which is brittle and has a small amount of deformation in the vicinity of the fracture surface due to fracture separation. The present inventors regarded a specimen having a brittle fracture surface area ratio of 98% or more, a deformation amount in the vicinity of the wave-front surface of 100 m or less and a defect generation amount of 1.0 mg or less as a sample having good fracture tearability. Further, it is preferable that an angle of 45 degrees or more with respect to a direction parallel to the tensile stress, which is observed in a cross section parallel to the rolling direction, is formed over a length of 80 占 퐉 or more, (Wave-front direction cracks) having a wave number limited to less than 3 points per 10 mm was considered as a sample having good fracture tearability.

It is necessary that the size of the concavo-convex in the tensile direction of the fracture surface (that is, the size of the step formed by the concavo-convex) is large and the concavities and convexities are present at a high frequency. The present inventors have found that the number of steps (tensile step) having an elevation difference of 80 占 퐉 or more toward a direction parallel to a tensile stress observed at a cross section parallel to the rolling direction and having an angle of 45 degrees or less with respect to a direction parallel to the tensile stress A specimen having a wave front having a density of 2 or more per 10 mm was regarded as a specimen with high sensitivity.

The tensile direction step of the fracture surface and the number density of the fracture direction cracks were measured by the following methods. First, the test piece was cut parallel to the tensile direction, and the wave form was observed in a direction perpendicular to the tensile direction. The irregularities in the tensile direction and the irregularities in the wavefront direction were observed by observing the wave-front shape on the above-mentioned cut surface. The cut surface was formed to include the center of the wave front. Observations were made in any 5 fields of view at the section. At the time of observation, tensile step differences in each field of view and the number density per 10 mm of cracks in the wavefront direction were measured and their average values were determined. Fig. 2 shows an example of a cross-sectional observation photograph used for evaluating the unevenness situation of the fractured surface shown above.

The results are shown in Tables 2-1 and 2-2.

In Examples 1 to 28, the average number density of Mn sulfides having an aspect ratio of not less than 10 but not more than 30 elongated along the rolling direction was not less than 50 per 1 mm ² . In addition, the chemical compositions of Examples 1 to 28 were within the range of the present invention. Thus, all of Examples 1 to 28 were excellent in fracture tearability, and at the same time, they were good in persimmon synthesis. In other words, in Examples 1 to 28, when subjected to breakage after cold forging or air cooling after hot forging, the plastic deformation amount in the vicinity of the fracture plane was small and the fracture fracture was less likely to occur, . In Examples 1 to 28, since the plastic deformation amount of the fracture surface was small and the occurrence of defects was small, Examples 1 to 28 did not cause positional deviation at the time of fitting of the fracture surfaces, Thereby improving the yield. Further, with these features, in Examples 1 to 28, the step of removing defects can be omitted, leading to a reduction in manufacturing cost, which is very effective in industry.

On the other hand, in Comparative Examples 101 to 115, the content of any one of C, Si, Mn, P, S, Cr, V, Zr and N is out of the scope of the present invention. These do not satisfy the requirements of the present invention for the following reasons.

In Comparative Examples 101, 103, 107, 112 and 115, the content of C, Si, P, V and N was less than the lower limit of the range of the present invention. As a result, it was judged that Comparative Examples 101, 103, 107, 112 and 115 did not have good fracture tearability.

In Comparative Examples 102, 104, 106, and 108, the content of C, Si, Mn, and P exceeded the upper limit of the range of the present invention. As a result, it was judged that Comparative Examples 102, 104, 106 and 108 did not have good fracture toughness.

In Comparative Example 105, since the content of Mn is less than the lower limit of the range of the present invention, the average number density of Mn sulfides having an aspect ratio of 10 to 30 or less elongated along the rolling direction is less than 50 per 1 mm ² , And the degree of elongation was insufficient. As a result, in Comparative Example 105, it was judged that the number of concavities and convexities on the wave front was insufficient, and that there was no good convolution.

In Comparative Example 109, since the content of S exceeded the upper limit of the range of the present invention, the average number density of Mn sulfides having an aspect ratio elongated along the rolling direction of 10 to 30 or more exceeded 200 per 1 mm ² . As a result, in Comparative Example 109, it was judged that the occurrence of defects at fracture exceeded 1.0 mg, and the plastic deformation amount at fracture separation exceeded 100 탆 and did not have good fracture toughness.

In Comparative Example 110, since the content of S was less than the lower limit of the range of the present invention, the average number density of Mn sulfides having an aspect ratio of not less than 10 but not more than 30 drawn along the rolling direction was less than 50 per 1 mm ² , The number and elongation were insufficient. As a result, in Comparative Example 110, it was judged that the number of concavities and convexities on the wave front was insufficient, and that there was no good convolution.

In Comparative Example 111, it was judged that the content of Cr exceeded the upper limit of the range of the present invention, and therefore, the plastic deformation amount at the time of fracture separation exceeded 100 탆 and did not have good fracture toughness.

In Comparative Example 113, the number of irregularities on the wave front was not more than 50, and the distribution of the Mn sulfide was roughly dispersed in the MnS steel containing no Zr and having an aspect ratio of not less than 10 but not more than 30 per 1 mm ² . And the plastic deformation amount at the time of fracture separation exceeded 100 탆 which is a condition of good fracture separation.

In Comparative Example 114, the N content exceeded the upper limit of the range of the present invention, and scratches were caused in the steel material manufacturing step, that is, the casting and hot rolling step. Therefore, Comparative Example 114 was judged to be an inappropriate example of the fracture splitting material, and the fracture splittability was not evaluated.

In Comparative Example 115, it was judged that the ferrite transformation was not promoted and the plastic deformation amount at the time of fracture separation exceeded 100 탆, because the content of N was less than the lower limit of the range of the present invention, and thus the ferrite was not satisfactorily detached.

[Table 1-1]

[Table 1-2]

[Table 2-1]

[Table 2-2]

Example  2

Strengths 1-2 to 1-4 having the same chemical composition as Steel 1 described in Table 1-1 were produced under the conditions described in Table 3 and the number density of Mn sulfides having an aspect ratio exceeding 10 and not more than 30 contained in these steels Respectively. The "time until Zr injection" in Table 3 is the time (minutes) from the start of degassing treatment to the input of Zr, the "total reduction ratio" is the total reduction ratio (%) in hot rolling, Quot; is the total number of reduction ratios (%) in a period in which the steel temperature is not higher than 1000 캜 in hot rolling, and the " Mn sulfide number density " is the number density (number / mm ² ) of Mn sulfides having an aspect ratio of not less than 10 but not more than 30 . The production conditions not shown in Table 3 were the same as those of Examples 1 to 28 and Comparative Examples 101 to 115.

[Table 3]

As shown in Table 1-1, since the chemical composition and the manufacturing conditions were both within the scope of the present invention, the number density (Mn sulfide number density) of the Mn sulfides exceeding the aspect ratio of 10 to 30 was within the scope of the present invention . On the other hand, as shown in Table 3, steels 1-2 to 1-4 had chemical compositions within the range of the present invention, but the production conditions were out of the range of the present invention, so that the number density of Mn sulfides having an aspect ratio of more than 10 and 30 or less But fell below the scope of the present invention.

Strength 1-2 is an example in which Zr is added after lapse of 15 minutes from the start of the degassing treatment. It is presumed that the number density of Mn sulfides exceeding the aspect ratio of 10 to 30 in the steel 1-2 is insufficient because the time for the Zr oxide to sufficiently refine the Mn sulfide is not ensured.

Steels 1-3 are examples in which the total reduction ratio during hot rolling is less than 80%, and steels 1-4 are examples in which the reduction ratio in a temperature region of 1000 占 폚 or less, which is a temperature region where Mn sulfide is liable to be elongated, is less than 50%. Since the Mn sulfides were not sufficiently stretched at the time of hot rolling, it is assumed that the number density of the Mn sulfides exceeding the aspect ratio of 10 to 30 in the steels 1-3 and 1-4 was insufficient.

The hot-rolled steel material and the steel part according to the present embodiment have a small plastic deformation amount in the vicinity of the fracture surface when fractured and separated, and the occurrence of fracture of the fracture surface is reduced. Therefore, when the fracture surfaces are fitted, positional deviation does not occur, so that it is possible to fit with high precision, and it is possible to simultaneously realize the improvement of the precision of the steel part and the improvement of the yield. Further, by using the hot-rolled steel material and the steel part of the present embodiment, it is possible to omit the step of removing defects, thereby reducing the manufacturing cost, thereby greatly improving the economic efficiency of the industry.

1: Specimen
2: hole
3: V notch
4: Through hole
10: Steel parts
11: Mn sulfide
12: Crack
21: Crack in wavefront direction
22: tensile direction step

Claims (5)

Chemical composition
C: 0.35 to 0.45% by mass,
0.6 to 1.0% by mass of Si,
Mn: 0.60 to 0.90 mass%
0.010 to 0.035 mass% of P,
S: 0.06 to 0.10% by mass,
0.02 to 0.25 mass% or less of Cr,
V: 0.23 to 0.40% by mass,
Zr: 0.0001 to 0.0050 mass% or less,
N: 0.0060 to 0.0150% by mass,
Ti: 0 to 0.050 mass%
0 to 0.030 mass% of Nb,
Mg: 0 to 0.0050 mass%, and
REM: 0 to 0.0010 mass%
, The remainder including Fe and impurities,
At least 90% by area of the metal structure is composed of ferrite and pearlite,
Wherein an average number density of Mn sulfides having an aspect ratio of not less than 10 but not more than 30, which is elongated along the rolling direction, measured in a cross section parallel to the rolling direction is 50 to 200 pieces / mm ² . The method according to claim 1,
Wherein the chemical component comprises
0.005 to 0.050 mass% of Ti,
0.005 to 0.030 mass% of Nb,
Mg: 0.0005 to 0.0050 mass%, and
REM: 0.0003 to 0.0010 mass%
Wherein the hot-rolled steel sheet contains at least one member selected from the group consisting of iron, steel, and iron. Chemical composition
C: 0.35 to 0.45% by mass,
0.6 to 1.0% by mass of Si,
Mn: 0.60 to 0.90 mass%
0.010 to 0.035 mass% of P,
S: 0.06 to 0.10% by mass,
0.02 to 0.25 mass% or less of Cr,
V: 0.23 to 0.40% by mass,
Zr: 0.0001 to 0.0050 mass% or less,
N: 0.0060 to 0.0150% by mass,
Ti: 0 to 0.050 mass%
0 to 0.030 mass% of Nb,
Mg: 0 to 0.0050 mass%, and
REM: 0 to 0.0010 mass%
, The remainder including Fe and impurities,
At least 90% by area of the metal structure is composed of ferrite and pearlite,
And an average number density of Mn sulfides having an aspect ratio of not less than 10 and not more than 30, measured in a cross section parallel to the rolling direction, is 50 to 200 pieces / mm ² . The method of claim 3,
When the steel component is subjected to tensile stress by tensile stress parallel to the rolling direction to form a wave front,
A step having a height difference of 80 占 퐉 or more toward a direction parallel to the tensile stress observed at the cross section parallel to the rolling direction and having an angle with respect to the direction parallel to the tensile stress of 45 占 or less, Is formed at an average number density of two or more per side,
Wherein an angle with respect to said direction parallel to said tensile stress, which is observed in said cross-section parallel to said rolling direction, is greater than 45 degrees and is formed over a length of 80 占 퐉, Or the average number density of the recesses is limited to less than 3 points per 10 mm in the wave front,
And the brittle fracture wave front at the wave front is 98% or more by area. The method according to claim 3 or 4,
Wherein the chemical component comprises
0.005 to 0.050 mass% of Ti,
0.005 to 0.030 mass% of Nb,
Mg: 0.0005 to 0.0050 mass%, and
REM: 0.0003 to 0.0010 mass%
And at least one member selected from the group consisting of iron and iron.

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