JP4888277B2 - Hot rolled steel bar or wire rod - Google Patents

Description

  The present invention is used as a material for gears, pulleys, shafts and the like, which are high-strength mechanical parts such as automobiles and industrial machines, and shear-cutability at 400 ° C. or lower, particularly 200 ° C. or lower, in the state of hot rolling. The present invention relates to a hot-rolled steel bar or a wire rod excellent in shear cutting performance.

  As a material for steel machine parts formed into a desired shape by hot forging or cold forging, generally, a long steel bar or wire cut into a certain length is used.

  As the above cutting method, there are methods such as shear cutting and saw cutting. From the viewpoint of production efficiency and cost, so-called “cold or warm” shear cutting is excellent.

  However, in the case of shear cutting, if the diameter of the steel bar or wire as the material to be cut is increased or the strength is increased, it is inevitable that the life of the shear cutting blade will be significantly shortened, and cracks will occur on the cut end face The problem is also likely to occur.

  In particular, in recent years, there has been a strong demand from the industry to reduce the weight, size and load of mechanical parts such as gears, pulleys, and shafts in order to improve automobile fuel efficiency and engine output. There is. In order to achieve weight reduction, downsizing, and high stress loading of these machine parts, it is necessary to increase the strength of parts subjected to carburizing and quenching or carbonitriding and quenching, and in particular, fatigue strength. In particular, it is necessary to increase the surface fatigue strength. Therefore, steel materials containing more alloy elements than conventional JIS standard SCr420 steel and SCM420 steel are often used as alloy steel for machine structure. Accordingly, the strength in the hot-rolled state is increased, and shear cutting at 400 ° C. or lower, particularly 200 ° C. or lower is difficult.

  For example, Patent Documents 1 to 3 propose a technique for improving shear cutting performance in warm and cold conditions.

  That is, in Patent Document 1, immediately after hot finish rolling, the surface is quenched by water cooling within a range of 5 to 15 seconds so that the surface does not become 200 ° C. or less, and self-maintained by internal retained heat. A “method for producing a steel bar material excellent in cold shear cutting ability” in which a tempering is performed to form a hardened layer on the cross-sectional surface layer portion is disclosed.

  Further, Patent Document 2 contains Ti in the range of 0.001 to 0.015% by weight, and the crystal grain size at the center measured by the crystal grain size test method of JIS standard is mainly No. 6 or more. “Steel” is disclosed.

  Patent Document 3 contains C: 0.15 to 1.50 mass%, and the average of the major axis / minor axis of the carbide is 2.50 or less, and occupies all the carbides of the carbide having the major axis / minor axis of 2.50 or less A “steel wire rod excellent in cold cutability” having a volume ratio of 70% or more is disclosed.

JP-A-2-209429 JP 2005-320610 A Japanese Patent Laid-Open No. 10-60595

  The technique proposed in Patent Document 1 prevents the occurrence of “burring” at the end of the cut surface in the cutting direction during cold shear cutting. And this technique was able to improve shear cutting property, when the steel kind with a small content of alloy elements and the diameter were not so large.

  However, if the steel type or diameter is high due to the high alloying element content and the diameter is large, the steel bar and wire are not necessarily stable in the hot-rolled state even if the burr is prevented. The shear could not be cut.

  The technique proposed in Patent Document 2 suppresses the occurrence of aging cracks or the like generated on the cut end face during shear cutting. And this technique was also able to improve shear cutting property in the case of a steel type with a small content of alloy elements.

  However, in the case of a steel type that has increased strength due to a high content of alloy elements, shear cutting is not necessarily stable in the state of hot-rolling steel bars and wires even if aging cracks that occur on the cut end face are suppressed. I couldn't.

  The technique proposed in Patent Document 3 suppresses the occurrence of cold shear cracking by spheroidizing carbide by performing spheroidizing annealing (hereinafter referred to as “spheroidizing annealing”) after hot rolling. It is. For this reason, it is a technique that enables stable shear cutting even when the steel type or the diameter is high because the alloy element content is large and the diameter is large.

  However, in order to perform spheroidizing annealing, in addition to the need to install an annealing furnace, the number of manufacturing processes will increase, and in addition, spheroidizing annealing itself consumes a lot of energy, and productivity is also increased. It is a process to reduce. For this reason, the manufacturing cost has increased, and it has been impossible to avoid a long manufacturing period.

  Accordingly, an object of the present invention is to provide a hot-rolled steel bar or wire rod that is excellent in shear-cutting properties, and is less likely to crack even when shear-cut at 400 ° C. or lower, particularly 200 ° C. or lower, in the state of hot rolling. Is to provide.

  The “hot rolled steel bar” in the present invention refers to a steel material that is hot rolled into a bar shape and cut to a predetermined length. The “wire” refers to steel that is hot-rolled into a rod shape and wound in a coil shape, and includes a so-called “burn-in coil”.

  Conventionally, as proposed in Patent Document 3, it is well known that the shear cutting ability in cold can be improved by performing spheroidizing annealing to spheroidize and soften carbides. However, as already stated, this method cannot avoid an increase in manufacturing cost and a decrease in productivity.

  Therefore, the present inventors have found that the steel type that has increased strength due to the high content of alloy elements, and even when the diameter is large, the hot-rolled state remains at 400 ° C or lower, particularly 200 ° C or lower. In order to provide a hot-rolled steel bar or wire rod with excellent shear-cutability that does not generate cracks even when sheared with a shear, the surface layer portion and the entire cross-section hardness and microstructure used as the starting point during shear cutting are the above-mentioned temperatures. Investigation and research were repeated on the effect on shear cutting performance in the area. As a result, the following findings (a) to (d) were obtained.

  (A) The surface layer of the hot-rolled steel bar or wire serves as the starting point for shear cutting. For this reason, in order to improve shear cutting property, it is necessary to soften a surface layer part.

  (B) In order to improve shear cutting performance, not only the surface layer portion of the hot rolled steel bar or wire of (a) above, but also the average hardness needs to be a specific level or less.

  (C) In order to improve the shear cutting ability, it is necessary to control the microstructure in addition to the hardness control of the hot rolled steel bar or wire. Specifically, it is necessary to include a soft ferrite phase at a specific ratio or more by area ratio. This is presumed to be because the ferrite phase easily propagates cracks during shear cutting.

  (D) Hard pearlite phase and bainite phase are easily generated continuously in the rolling direction of hot-rolled steel bars or wires by microsegregation. However, in the rolling direction, the hard phase (structure) described above is continuously long. If present, shear cutting performance is reduced. This is presumed that the hard phase (structure) is an obstacle at the time of crack propagation during shear cutting.

  This invention is completed based on said knowledge, The summary exists in the hot-rolled steel bar or wire shown to following (1) and (2).

  (1) By mass%, C: 0.2 to 0.35%, Si: 0.3 to 1.5%, Mn: 0.4 to 1.5%, S: 0.003 to 0.05% , Cr: 0.8 to 3.0%, Al: 0.01 to 0.05%, N: 0.008 to 0.025%, and the balance is made of Fe and impurities, and O ( Oxygen): 0.002% or less and P: 0.025% or less in chemical composition, in the state of hot rolling, the average Vickers hardness is 240 or less, the Vickers hardness of the surface layer is 180 or less, In addition, the microstructure is composed of a ferrite pearlite structure or a ferrite pearlite bainite structure, and the ferrite phase area ratio is 30% or more, and the maximum length continuous in the rolling direction of the pearlite phase and / or bainite phase. Is less than 600μm Hot rolling steel bars or wire rods, characterized in that.

  (2) Instead of a part of Fe, by mass%, Mo: 0.8% or less, Nb: 0.08% or less, and V: 0.15% or less are contained. The hot-rolled steel bar or wire according to (1) above.

  “Average Vickers hardness” means a mirror-polished cross section perpendicular to the rolling direction (hereinafter also referred to as “C cross section”) using a Vickers hardness tester and a test force of 9.807 N from the center. It refers to the arithmetic average value of hardness obtained by measuring 10 points in total, each of 5 points each at a distance of 1/3 and 2/3 of the radius.

  The “surface layer Vickers hardness” means that the mirror-polished C cross section is arbitrarily placed at a position of 50 μm, 100 μm and 150 μm from the surface with a test force of 0.9807 N using a Vickers hardness tester. The arithmetic average value of hardness obtained by measuring 15 points in total by 5 points.

  Furthermore, the “microstructure” means that the cross section cut out in the rolling direction through the center line is mirror-polished and then corroded with nital, and the position at a distance of 1/3 and 2/3 of the radius from the center is multiplied. 50 times, and the size of the field of view is 1.6 mm × 1.2 mm, respectively, and refers to a case where 10 fields of observation are arbitrarily observed for each of 5 fields. And the area ratio of a ferrite phase is an arithmetic average value of each area ratio of the ferrite phase in 10 visual fields observed as mentioned above. Here, the ferrite phase is a portion that is brighter than other parts and is hardly corroded except for the grain boundary in nital. Each area ratio of the ferrite phase is obtained by multiplying the numerical value obtained by dividing the area of the ferrite phase in the visual field by a normal image analysis method and dividing the calculated area by the total area of the visual field.

  The “ferrite / pearlite structure” means a mixed structure of a ferrite phase and a pearlite phase. Similarly, “ferrite / pearlite / bainite structure” means a mixed structure composed of three phases of a ferrite phase, a pearlite phase, and a bainite phase.

  In addition, the “maximum length continuous in the rolling direction of the pearlite phase and / or bainite phase” means that the length of the continuous pearlite phase and the length of the continuous bainite phase in the rolling direction, as shown in the schematic diagram of FIG. It refers to the maximum length of the length and the length when the pearlite phase and the bainite phase are continuous.

  As already described, the “hot rolled steel bar” in the present invention refers to a steel material that is hot rolled into a bar shape and cut into a predetermined length. The “wire” refers to steel that is hot-rolled into a rod shape and wound in a coil shape, and includes a so-called “burn-in coil”.

  Hereinafter, the inventions related to the hot-rolled steel bars or wires of the above (1) and (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The hot-rolled steel bar or wire of the present invention is in a state of being hot-rolled, and is less likely to crack even when shear-cut at 400 ° C. or lower, particularly 200 ° C. or lower, and is excellent in shear-cutability. No increase in productivity or decrease in productivity. Moreover, since high fatigue strength can be obtained, it is suitable as a material for gears, pulleys, shafts and the like, which are high-strength machine parts such as automobiles and industrial machines.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” of the content of each element means “mass%”.

(A) Chemical composition of hot-rolled steel bar or wire C: 0.2 to 0.35%
C is an essential element for securing the strength of the parts when carburizing and quenching or carbonitriding and quenching. In order to satisfy the recent needs for improving fatigue strength, the content of less than 0.2% is insufficient, and it is necessary to contain 0.2% or more. However, if the content of C exceeds 0.35%, shear shearability becomes insufficient even if other items specified in the present invention are satisfied. Therefore, the content of C is set to 0.2 to 0.35%. In addition, it is preferable that content of C shall be 0.25 to 0.35%.

Si: 0.3 to 1.5%
Si has a large effect of increasing hardenability and temper softening resistance, and is an element effective in improving fatigue strength. In order to satisfy the recent needs for improving fatigue strength, the content is insufficient if less than 0.3%, it is necessary to contain 0.3% or more, when the content is 0.5% or more, High strength becomes remarkable. However, when the Si content exceeds 1.5%, not only the effect of increasing the strength is saturated, but also the shear cutting property is insufficient even if the other items defined in the present invention are satisfied. Therefore, the Si content is set to 0.3 to 1.5%. A more preferable Si content is 0.5 to 1.5%.

Mn: 0.4 to 1.5%
Mn has a large effect of enhancing hardenability and temper softening resistance, and is an element effective for improving fatigue strength. In order to satisfy the recent needs for improving fatigue strength, the content of less than 0.4% is insufficient, and it is necessary to contain 0.4% or more. However, when the content of Mn exceeds 1.5%, the effect of increasing the fatigue strength decreases. Therefore, the Mn content is set to 0.4 to 1.5%. In addition, it is preferable that content of Mn shall be 0.8 to 1.5%.

S: 0.003-0.05%
S combines with Mn to form MnS and has an effect of improving machinability. However, if the content is less than 0.003%, it is difficult to obtain the above effect. On the other hand, when the amount of S increases, coarse MnS tends to be generated and the fatigue strength tends to be reduced. When the content exceeds 0.05%, the fatigue strength is significantly reduced. Therefore, the content of S is set to 0.003 to 0.05%. In addition, it is preferable that content of S shall be 0.01-0.03%.

Cr: 0.8 to 3.0%
Cr is an element that has a large effect of enhancing hardenability and temper softening resistance and is effective in improving fatigue strength. In order to satisfy the recent needs for higher strength, the content of less than 0.8% is insufficient, it is necessary to contain 0.8% or more, when the content is 1.3% or more, The improvement in fatigue strength becomes significant. However, if the Cr content exceeds 3.0%, the hardenability becomes too high, and the shear cutting property becomes insufficient. Therefore, the Cr content is set to 0.8 to 3.0%. A more preferable Cr content is 1.3 to 3.0%.

Al: 0.01 to 0.05%
Al has a deoxidizing action, and at the same time, easily binds to N to form AlN, is effective in refining crystal grains in the quenched portion, and has an effect of increasing fatigue strength. However, if the Al content is less than 0.01%, the above effect is difficult to obtain. On the other hand, Al tends to form hard oxide inclusions, and when the Al content exceeds 0.05%, the fatigue strength is significantly reduced. Therefore, the Al content is set to 0.01 to 0.05%. The Al content is preferably 0.03 to 0.05%.

N: 0.008 to 0.025%
N is liable to form AlN, NbN and VN by combining with Al, Nb and V. The above AlN, NbN and VN are effective for refining crystal grains in the quenched portion and have the effect of increasing fatigue strength. However, if the N content is less than 0.008%, the above effect is difficult to obtain. On the other hand, when the N content exceeds 0.025%, the fatigue strength is significantly reduced. Therefore, the N content is set to 0.008 to 0.025%. The N content is preferably 0.015 to 0.025%.

  In the present invention, the contents of O (oxygen) and P in the impurities are limited as follows.

O (oxygen): 0.002% or less O is present as an impurity in the steel, and is liable to form a hard oxide-based inclusion by combining with Al, thereby reducing fatigue strength. In particular, when the O content exceeds 0.002%, the fatigue strength is significantly reduced. Therefore, the content of O in the impurities is set to 0.002% or less. In addition, although it is desirable to reduce the content of O in the impurities as much as possible, considering the cost in steelmaking, it is preferable to make it 0.001% or less.

P: 0.025% or less P is an element present in steel as an impurity, and segregates at the grain boundaries to embrittle the grain boundaries, resulting in a decrease in fatigue strength. In particular, its content is 0.025%. If it exceeds, the fatigue strength will be significantly reduced. Therefore, the content of P in the impurities is set to 0.025% or less. In addition, it is preferable that content of P in an impurity shall be 0.015% or less.

  For the above reasons, the hot-rolled steel bar or wire according to the present invention (1) contains C, Si, Mn, S, Cr, Al, and N in the above-mentioned range, and the balance consists of Fe and impurities. O (oxygen) in the medium was defined as having a chemical composition of 0.002% or less and P: 0.025% or less.

  The hot-rolled steel bar or wire according to the present invention (1) is replaced with a part of Fe in the present invention (1), and Mo: 0.8% or less, Nb: 0.08% or less, and V: One or more of 0.15% or less can be contained.

  That is, since Mo, Nb, and V have the effect | action which raises fatigue strength, in order to acquire this effect, you may add and contain said element.

  Hereinafter, the above elements will be described in detail.

Mo: 0.8% or less Mo is an element effective for increasing fatigue strength. That is, Mo has the effect of increasing the hardenability and temper softening resistance and thereby increasing the fatigue strength. Therefore, Mo may be added and contained in order to obtain this effect. However, the Mo content increases, and particularly when it exceeds 0.8%, the hardenability becomes too high and the shear cutting property becomes insufficient. Therefore, the Mo content when added is set to 0.8% or less.

  In addition, in order to acquire the above-mentioned effect of Mo reliably, it is preferable to make Mo content into 0.1% or more. For this reason, the more desirable Mo content in the case of adding is 0.1 to 0.8%, and more preferably 0.2 to 0.5%.

Nb: 0.08% or less Nb is an element effective for increasing fatigue strength. That is, Nb is easy to form NbC, NbN, and Nb (C, N) by combining with C and N, and is effective in supplementing the above-described grain refinement of the quenched portion by AlN, thereby improving the fatigue strength. In order to obtain this effect, it may be added and contained. However, if the Nb content increases, especially when it exceeds 0.08%, coarse Nb (C, N) is likely to be generated in the central segregation portion, and the fatigue strength is decreased. Therefore, the content of Nb when added is set to 0.08% or less.

  In order to surely obtain the effect of Nb described above, the Nb content is preferably set to 0.01% or more. For this reason, the more desirable Nb content in the case of adding is 0.01 to 0.08%, and more preferably 0.02 to 0.06%.

V: 0.15% or less V is also an element effective for increasing fatigue strength. That is, V is easy to form VN and VC by combining with C and N. Among these, VN is effective in supplementing the above-described grain refinement of the quenched portion by AlN and has the effect of increasing fatigue strength. Have. Moreover, when VN precipitates during carbonitriding, there is an effect of further increasing fatigue strength. For this reason, in order to acquire the above-mentioned effect, you may add and contain V. However, if the content of V increases, especially when it exceeds 0.15%, the strength of the hot-rolled steel bar or wire becomes too high to satisfy the Vickers hardness range defined in the present invention. , Causing a reduction in shear cutting ability. Therefore, when V is added, the content of V is set to 0.15% or less.

  In order to surely obtain the effect of V described above, the V content is preferably set to 0.02% or more. For this reason, the more desirable V content in the case of adding is 0.02 to 0.15%, more preferably 0.05 to 0.15%.

  Said Mo, Nb, and V can be contained only in one of them, or 2 or more types of composites.

  For the above reasons, the hot rolled steel bar or wire according to the present invention (2) is replaced with a part of Fe in the present invention (1), Mo: 0.8% or less, Nb: 0.08% or less, and V: It was defined as containing one or more of 0.15% or less.

  In addition, in this invention, in order to obtain the outstanding fatigue strength, it is preferable to restrict | limit content of Ti in an impurity as follows.

Ti: 0.003% or less Ti is an element that is easily bonded to N to form TiN. Since TiN is very hard and has a square shape, it tends to be a starting point for fatigue failure, leading to a decrease in fatigue strength. In particular, when the content of Ti in the impurities exceeds 0.003%, TiN is easily formed, and the fatigue strength may be significantly reduced. Therefore, in the hot rolled steel bar or wire of the present invention, the content of Ti in the impurities is preferably 0.003% or less.

(B) Hardness and structure in the state as hot-rolled According to the study by the present inventors, the hot-rolled steel bar or wire according to the present invention has an average Vickers hardness of 240 or less and a surface layer in the state as hot-rolled. The Vickers hardness of the portion is 180 or less, and the microstructure is composed of a ferrite pearlite structure or a ferrite pearlite bainite structure, and the area ratio of the ferrite phase is 30% or more, and the pearlite phase and / or It became clear that the maximum continuous length in the rolling direction of the bainite phase should be 600 μm or less. Hereinafter, the above items will be described in detail.

  First, the inventors adjusted the components of steel α and steel β satisfying the chemical composition described in the section (A) shown in Table 1 in a 70-ton converter, and then sufficiently conducted electromagnetic stirring of the molten steel, Casting was performed to obtain a 400 mm × 300 mm square bloom.

  The Ti contents of steel α and steel β were 0.001% and 0.002%, respectively.

Next, the above bloom was subjected to a homogenization treatment under the following three conditions.
(I) None,
(B) 1280 ° C x 5 hours,
(C) 1280 ° C. × 12 hours.

  After the homogenization treatment, the billet was split and rolled under normal conditions to produce a 180 mm × 180 mm square billet. In addition, the bloom which has not been subjected to the homogenization treatment (b) is subjected to heat treatment at 1250 ° C. for 1 hour in a heating furnace and then subjected to block rolling, while the homogenization of (b) and (c) above. The treated bloom was taken out of the soaking furnace and rolled as it was.

  Next, the above 180 mm × 180 mm square billet was cared only at the corner portion and hot-rolled under the conditions shown in Table 2 to obtain a steel bar having a diameter of 70 mm. The reduction ratio in the hot rolling, that is, “the cross-sectional area of the billet before rolling / the cross-sectional area of the rolled product (rolled steel bar)” is 8.4.

  For each steel bar having a diameter of 70 mm obtained as described above, the average Vickers hardness in the hot-rolled state (hereinafter, Vickers hardness is referred to as “Hv hardness”), Hv hardness of the surface layer portion, micro Tissue and shear cutting properties were investigated by the following method.

  First, a cross section (C cross section) perpendicular to the rolling direction of a steel bar having a diameter of 70 mm is mirror-polished, and then an Hv hardness tester is used in accordance with “Vickers hardness test-test method” in JIS Z 2244 (2003). Then, with a test force of 9.807 N, the position at a distance of 1/3 and 2/3 of the radius from the center was measured arbitrarily for each of 5 points in total, 10 points in total, and from the arithmetic average value, “average Hv hardness " In addition, a position of 50 μm, 100 μm, and 150 μm from the surface was measured at a test force of 0.9807 N, each arbitrarily measuring 5 points at a total of 15 points, and “Hv hardness of the surface layer portion” was determined from the arithmetic average value. .

  Next, the cross-section cut in the rolling direction through the center line of a steel bar having a diameter of 70 mm is mirror-polished and then corroded with nital, and the position at a distance of 1/3 and 2/3 of the radius from the center is multiplied by 50. The phase was specified by observing 10 fields with a total of 5 fields, each with a magnification of 1.6 mm × 1.2 mm.

  In addition, image analysis by a normal method was performed, the area ratio of the ferrite phase in each visual field was obtained, and the average value of the area ratios of the ferrite phase in a total of 10 visual fields was calculated. Here, the ferrite phase is a portion that is brighter than other parts and is hardly corroded except for the grain boundary in nital. The area ratio of the ferrite phase in each visual field is a value obtained by dividing the numerical value obtained by dividing the area of the ferrite phase in the visual field by the total area of the visual field by 100.

  Moreover, the maximum continuous length in the rolling direction of the pearlite phase and / or the bainite phase was measured by the observation method described above.

  Further, using a general shear cutting device, the shear cutting property was investigated by cutting the steel bar having a diameter of 70 mm so as to have a length of 150 mm in a direction perpendicular to the rolling direction at room temperature. In addition, the case where there is a crack of 2 mm or more on the cut end face is rejected, and when each of the steel bars is cut by 50 pieces having a length of 150 mm, the shear cutting ability is 1 or less. It was good.

  Tables 3 and 4 summarize the results of the above investigations. In Tables 3 and 4, the symbols (i) to (c) in the homogenization treatment column are “no homogenization treatment”, “perform homogenization treatment for 1280 ° C. × 5 hours” and “1280 ° C. × 12”, respectively. It means “perform time homogenization”. Further, “F”, “P” and “B” in the microstructure column indicate the ferrite phase, pearlite phase and bainite phase, respectively, and “P and B maximum length” indicate the pearlite phase and / or bainite phase, respectively. It means the maximum continuous length in the rolling direction.

From Table 3 and Table 4, in the state of hot rolling,
-Average Hv hardness is 240 or less,
-Hv hardness of the surface layer portion is 180 or less,
-The microstructure is composed of ferrite-pearlite structure or ferrite-pearlite-bainite structure, and the area ratio of ferrite phase is 30% or more,
The maximum continuous length in the rolling direction of the pearlite phase and / or bainite phase is 600 μm or less,
When all the conditions are satisfied, it is clear that the number of rejects is 1 or less and the shear cutting property is good.

  Therefore, the hot-rolled steel bar or wire according to the present invention has an average Hv hardness of 240 or less, a Hv hardness of the surface layer portion of 180 or less, and a microstructure of ferrite pearlite in the hot-rolled state. It is composed of a structure or a ferrite / pearlite / bainite structure, and the ferrite phase area ratio is 30% or more, and the maximum continuous length in the rolling direction of the pearlite phase and / or bainite phase is 600 μm or less. did.

  In addition, since C content in this invention is 0.2% or more, the upper limit of the area ratio of a ferrite phase will be about 75%.

  In addition, since the cost for increasing the rolling finishing temperature to 750 ° C. or lower is increased, the lower limit of the maximum length that continues in the rolling direction of the pearlite phase and / or the bainite phase is about 50 μm.

  From Tables 3 and 4, when the average Hv hardness is 210 or less and the Hv hardness of the surface layer portion is 150 or less, the number of rejects is 0, that is, the cut end face has a length of 2 mm or more. It did not occur, and the shear cutting property was particularly good.

  Therefore, it is preferable that the average Hv hardness and the Hv hardness of the surface layer portion in the hot-rolled state of the hot-rolled steel bar or wire according to the present invention are 210 or less and 150 or less, respectively.

  However, since the C content in the present invention is 0.2% or more, the lower limit of the average Hv hardness is about 130.

  Moreover, the lower limit of the Hv hardness of the surface layer portion is about 100.

  In addition, the chemical composition of steel, the segregation state of components, the hot rolling conditions, the cooling rate after rolling, and the like affect the Hv hardness and structure in the state of hot rolling.

  Therefore, a steel containing C: 0.2-0.25%, Si: 0.3-0.8%, Mn: 0.5-0.8% and Cr: 1.3-1.8% For example, the Hv hardness and structure as hot-rolled can be obtained by satisfying the conditions shown in the following <1> to <6>, for example.

<1> Perform sufficient magnetic stirring of the molten steel.
<2> The bloom is subjected to a homogenization heat treatment at 1250 to 1300 ° C. for 5 hours or more.
<3> Care for billets is not performed except for corners.
<4> The heating of the billet is performed at a heating temperature of 1150 to 1230 ° C. and a heating time of 1 hour or more.
<5> Rolling finish temperature is set to 850 to 950 ° C., and after finish rolling is cooled to 600 ° C. or less at a cooling rate of cooling or less.
<6> Rolling is performed with a reduction ratio from billet to bar or wire, that is, “cross-sectional area of billet before rolling / cross-sectional area of rolled product (rolled bar or wire)” being 6 or more.

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

  Components of the steel a having the chemical composition shown in Table 5 were adjusted in a 70-ton converter, and then the molten steel was sufficiently agitated and continuously cast to obtain a 400 mm × 300 mm square bloom.

  Next, the bloom was subjected to a homogenization treatment at 1280 ° C. for 12 hours, and then taken out from the soaking furnace and subjected to split rolling under normal conditions as it was to produce a billet of 160 mm × 160 mm square.

  Further, steels b to n having chemical compositions shown in Table 6 were melted in a 30 kg vacuum furnace and then cast into an ingot.

  Steels cf in Table 6 are steels that satisfy the chemical composition defined in the present invention. On the other hand, the steel b and the steels g to n are steels that deviate from the chemical composition conditions defined in the present invention, and the steel b is steel corresponding to JIS standard SCr420.

  Each ingot of the steels b to n is held at 1250 ° C. for 10 hours, once cooled to room temperature, then heated again at 1200 ° C. for 1 hour, hot forged with a finishing temperature of 900 ° C. or higher, and a diameter of 30 mm A steel bar was obtained.

  Next, each steel bar having a diameter of 30 mm was heated at 925 ° C. for 1 hour and then allowed to cool to room temperature, and then machined to form a small roller for roller pitching test as shown in FIG. Was made.

  The small roller for roller pitching test is carburized and quenched under the conditions shown in FIG. 3 using a gas carburizing furnace, and then tempered at 170 ° C. for 2 hours, and for the purpose of removing heat treatment strain, The grip portion was finished and the test portion was ground by 20 μm. Note that “CP” in FIG. 3 means a carbon potential.

  The roller pitting test small roller obtained as described above and steel a shown in Table 1 are used as a raw material, and the shape shown in FIG. 2B, that is, the diameter is 130 mm, and the R shape of the contact portion is 150 mmR. A roller pitching test was conducted under the conditions shown in Table 7 using a large roller for a roller pitching test.

  The large roller pitching test roller is produced by a general manufacturing process, that is, “normalizing, specimen processing, eutectoid carburizing with a gas carburizing furnace, low temperature tempering and polishing”.

For each of the steels b to n, the number of tests in the roller pitching test was set to 5, an SN diagram was prepared with the surface pressure on the vertical axis and the number of repetitions until the occurrence of pitching on the horizontal axis. The surface pressure at × 10 ⁷ times was defined as surface fatigue strength.

In addition, when the area of the largest thing became 1 mm < ² > or more among the places where the surface of the test part of a small roller was damaged, it was set as pitching generation | occurrence | production.

  The surface fatigue strength of steel b, which is a steel corresponding to JIS standard SCr420, was standardized to 1.00, and the goal was to have a surface fatigue strength of 1.15 times or more. As described above, in order to achieve weight reduction, downsizing, and high stress loading of machine parts, the strength of parts subjected to carburizing and quenching or carbonitriding and quenching should be increased. This is because, in particular, it is necessary to increase the surface fatigue strength.

  Table 8 summarizes the results of the above roller pitching test.

  As is apparent from Table 8, steel g with a C content of less than 0.2%, steel h with a Mn content of less than 0.4%, steel i with a S content of more than 0.05%, Steel k with Cr content lower than 0.8%, Steel l with Al content higher than 0.05%, Steel m with N content higher than 0.025%, O content 0.002 In the case of steel j in which the content of steel n and P in excess of% exceeds 0.025%, the target surface fatigue strength is not obtained.

  Therefore, next, it was decided to investigate the shear cutting ability of the component system that was able to obtain the target surface fatigue strength.

  That is, the contents of C, Mn, and Cr are 0.2% or more, 0.4% or more, and 0.8% or more, respectively, and the contents of S, Al, N, O, and P are each 0%. .05% or less, 0.05% or less, 0.025% or less, 0.002% or less, and 0.025% or less, and the steel shown in Table 9 having a Si content of 0.3% or more Components of ox were adjusted in a 70-ton converter, and then the molten steel was sufficiently agitated and continuously cast to obtain a 400 mm × 300 mm square bloom.

Next, the above bloom was subjected to a homogenization treatment under the following three conditions.
(I) None,
(B) 1280 ° C x 5 hours,
(C) 1280 ° C. × 12 hours.

  After the homogenization treatment, the billet was split and rolled under normal conditions to produce a 180 mm × 180 mm square billet. In addition, the bloom which has not been subjected to the homogenization treatment (b) is subjected to heat treatment at 1250 ° C. for 1 hour in a heating furnace and then subjected to block rolling, while the homogenization of (b) and (c) above. The treated bloom was taken out of the soaking furnace and rolled as it was.

  Next, the above 180 mm × 180 mm square billet was cared only at its corners and hot-rolled under the conditions shown in Table 2 to obtain a steel bar having a diameter of 70 mm. The reduction ratio in the hot rolling, that is, “the cross-sectional area of the billet before rolling / the cross-sectional area of the rolled product (rolled steel bar)” is 8.4.

  About each steel bar 70 mm in diameter obtained as mentioned above, the average Hv hardness in the state as hot-rolled, the Hv hardness of a surface layer part, a microstructure, and shear cutting ability were investigated by the following method.

  First, a cross section (C cross section) perpendicular to the rolling direction of a steel bar having a diameter of 70 mm is mirror-polished, and then an Hv hardness tester is used in accordance with “Vickers hardness test-test method” in JIS Z 2244 (2003). Then, with a test force of 9.807 N, the position at a distance of 1/3 and 2/3 of the radius from the center was measured arbitrarily for each of 5 points in total, 10 points in total, and from the arithmetic average value, “average Hv hardness " In addition, a position of 50 μm, 100 μm, and 150 μm from the surface was measured at a test force of 0.9807 N, each arbitrarily measuring 5 points at a total of 15 points, and “Hv hardness of the surface layer portion” was determined from the arithmetic average value. .

  Next, the cross-section cut in the rolling direction through the center line of a steel bar having a diameter of 70 mm is mirror-polished and then corroded with nital, and the position at a distance of 1/3 and 2/3 of the radius from the center is multiplied by 50. The phase was specified by observing 10 fields with a total of 5 fields, each with a magnification of 1.6 mm × 1.2 mm.

  In addition, image analysis by a normal method was performed, the area ratio of the ferrite phase in each visual field was obtained, and the average value of the area ratios of the ferrite phase in a total of 10 visual fields was calculated. Here, the ferrite phase is a portion that is brighter than other parts and is hardly corroded except for the grain boundary in nital. The area ratio of the ferrite phase in each visual field is a value obtained by dividing the numerical value obtained by dividing the area of the ferrite phase in the visual field by the total area of the visual field by 100.

  Moreover, the maximum continuous length in the rolling direction of the pearlite phase and / or the bainite phase was measured by the observation method described above.

  Further, using a general shear cutting device, the shear cutting property was investigated by cutting the steel bar having a diameter of 70 mm so as to have a length of 150 mm in a direction perpendicular to the rolling direction at room temperature. In addition, the case where there is a crack of 2 mm or more on the cut end face is rejected, and when each of the steel bars is cut by 50 pieces having a length of 150 mm, the shear cutting ability is 1 or less. This was judged to be good, and this was the target of the present invention.

  Tables 10 and 11 collectively show the results of the above investigations. In Tables 10 and 11, the symbols (i) to (c) in the homogenization column are “No homogenization”, “Implement homogenization for 1280 ° C. × 5 hours”, and “1280 ° C. × 12, respectively. It means “perform time homogenization”. Further, “F”, “P” and “B” in the microstructure column indicate the ferrite phase, pearlite phase and bainite phase, respectively, and “P and B maximum length” indicate the pearlite phase and / or bainite phase, respectively. It means the maximum continuous length in the rolling direction.

  From Table 10 and Table 11, the test numbers of comparative examples deviating from the conditions defined in the present invention, specifically, test numbers 43-45, test numbers 48-50, test numbers 53-55, test numbers 58-60 In the case of test numbers 63 to 65 and test numbers 68 to 77, the number of rejects when cutting 50 steel bars having a length of 150 mm exceeded 1 and the target shear cutting performance It is clear that is not obtained.

  Compared to the above comparative example, the test number of the present invention example that satisfies the conditions specified in the present invention, specifically, test number 46, test number 47, test number 51, test number 52, test number 56, test number 57 In the case of Test No. 61, Test No. 62, Test No. 66 and Test No. 67, each steel bar has reached the target of 1 or less rejects when 50 steel pieces having a length of 150 mm are cut. It is clear that it has excellent shear cutting ability.

  Further, among the inventive examples, in the case of test number 46 and test number 47 in which the average Hv hardness is 210 or less and the Hv hardness of the surface layer portion is 150 or less, the number of rejects is 0, that is, cutting It is clear that no cracks having a length of 2 mm or more are formed on the end face, and the shear cutting property is particularly good.

  The hot-rolled steel bar or wire of the present invention is in a state of being hot-rolled, and is less likely to crack even when shear-cut at 400 ° C. or lower, particularly 200 ° C. or lower, and is excellent in shear-cutability. No increase in productivity or decrease in productivity. Moreover, since high fatigue strength can be obtained, it is suitable as a material for gears, pulleys, shafts and the like, which are high-strength machine parts such as automobiles and industrial machines.

It is a figure which illustrates typically the maximum length which continues in the rolling direction of a pearlite phase and / or a bainite phase. It is a figure explaining the shape of the roller for a roller pitching test, (a) is a figure which shows the shape of a small roller, (b) is the figure which shows the shape of a large roller. It is a figure which shows the conditions of the carburizing quenching given to the small roller for a roller pitching test using the gas carburizing furnace.

Claims (2)

  In mass%, C: 0.2 to 0.35%, Si: 0.3 to 1.5%, Mn: 0.4 to 1.5%, S: 0.003 to 0.05%, Cr: 0.8 to 3.0%, Al: 0.01 to 0.05%, N: 0.008 to 0.025%, the balance is made of Fe and impurities, O (oxygen) in the impurities: It has a chemical composition of 0.002% or less and P: 0.025% or less, and in the state of hot rolling, the average Vickers hardness is 240 or less, the surface layer part has a Vickers hardness of 180 or less, and is micro The structure is composed of ferrite pearlite structure or ferrite pearlite bainite structure, and the area ratio of the ferrite phase is 30% or more, and the maximum continuous length in the rolling direction of the pearlite phase and / or bainite phase is 600 μm. Must be Hot rolling steel bars or wire rods, characterized.   Instead of a part of Fe, it contains at least one of Mo: 0.8% or less, Nb: 0.08% or less, and V: 0.15% or less in mass%. Item 2. A hot-rolled steel bar or wire according to item 1.

Images