JP2009024213A - High carbon steel with excellent fracture separability, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high carbon steel which is reduced in component costs with no inclusion of alloy elements, such as Ti and V, and has satisfactory fracture separability even in the case of low hardness level. <P>SOLUTION: The high carbon steel with excellent fracture separability is characterized in having: a composition consisting of 0.70 to 1.20% C, ≤0.4% Si, 0.9% Mn, 0.10% P, ≤0.015% S, 0.5 to 2.0% Cr, ≤0.05% Al and the balance Fe with impurities; an old austenite grain size of ≥200 μm; and a Brinell hardness of ≤300. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high carbon steel excellent in break separation and a method for producing the same, and more particularly, break separation suitable as a raw material for a part to be used after being formed into a molded part by forging, cutting, or the like, for example, a cracking connecting rod. The present invention relates to a high carbon steel having excellent properties and a manufacturing method thereof. More specifically, it is suitable as a material for cracking products such as connecting rods having a low hardness (strength) level used in light cars and small passenger cars, etc. It relates to a manufacturing method.

  As a material steel for cracking connecting rods, especially products such as cracking connecting rods used in mini cars and small passenger cars, etc., it has excellent breaking separation properties in order to reduce component costs and reduce processing costs. There is a great demand for steel materials having the following.

  In the case of a connecting rod used in a light vehicle, a small passenger car, etc., a hardness (strength) level of Brinell hardness (hereinafter also referred to as “HB hardness”) of 300 or less may be used. .

  However, generally, when the hardness (strength) is low, ductility and toughness are increased. For this reason, it is difficult to manufacture a connecting rod having a low hardness (strength) level used for a light vehicle or a small passenger car as a “cracking connecting rod” by breaking and separating for the purpose of reducing processing costs.

  On the other hand, in order to reduce ductility and toughness and increase fracture separation, for example, when elements such as Ti and V are added to reduce toughness and ductility, the hardness (strength) increases. Therefore, workability such as machinability is remarkably deteriorated. In addition, in this case, the component cost also increases.

  Techniques relating to steel materials having excellent fracture separation properties have been proposed in Patent Document 1, Patent Document 2, and the like.

  Specifically, Patent Document 1 discloses a technique related to non-tempered steel that is low in ductility and toughness and can be separated by fracture by defining the chemical components of the steel.

  Patent Document 2 discloses a technique that achieves both machinability and fracture separation by defining the chemical components of steel.

JP-A-9-176785 JP 2003-27178 A

  The technique disclosed in the above-mentioned Patent Document 1 aims at a steel having a tensile strength of 800 MPa or more, and when viewed in the examples, it is substantially 1000 MPa, that is, a high hardness exceeding about 300 in terms of HB hardness ( Strength). For this reason, even when the hardness (strength) level is as low as 300 or less in terms of HB hardness, it cannot always be said that sufficient fracture separability can be ensured.

  In the case of the technique disclosed in Patent Document 2, since it is necessary to contain V and Ti, the component cost increases. In addition, as shown in the examples, the Charpy impact value as toughness and the elongation as ductility are high, and therefore it cannot be said that sufficient fracture separation is necessarily ensured.

  Accordingly, an object of the present invention is to provide sufficient fracture separation even if the component cost is low because it does not contain alloy elements such as Ti and V, and the hardness (strength) level is as low as 300 or less in HB hardness. It is providing the high carbon steel which can be provided with property, and its manufacturing method.

More specifically, the object of the present invention is low in component cost because it does not contain alloy elements such as Ti and V, and has a low hardness (strength) level of 300 or less in the HB hardness. However, when using a U-notch Charpy impact test piece defined by JIS Z 2202 (1998), the impact value at room temperature is 10 J / cm ² or less, the toughness is low, and the elongation in the tensile test at room temperature is 10%. The object of the present invention is to provide a high carbon steel excellent in fracture separation with low ductility and a method for producing the same.

  In order to solve the above-described problems, the present inventors have studied by changing various chemical components of steel, heating temperature, cooling rate, and the like. As a result, the following findings (a) to (d) were obtained.

  (A) Ductility and toughness can be reduced by positively utilizing proeutectoid cementite that precipitates at grain boundaries when high-carbon steel is cooled from an austenite region at a slow cooling rate.

  (B) When the prior austenite grain size is 200 μm or more, the ductility and toughness are significantly reduced.

  (C) In order to make the prior austenite grain size 200 μm or more, in the case of a high carbon steel having a specific chemical composition, a heat treatment may be performed at 1150 ° C. or more for 30 minutes or more.

  (D) In the case of high carbon steel having the above specific chemical composition, in the cooling process after the heat treatment under the above conditions, by gradually cooling the temperature range of 730 to 700 ° C. at a rate of 20 ° C. or less per hour. The hardness becomes a low value of 300 or less in terms of HB hardness and has good machinability.

  The present invention has been completed based on the above findings, and the gist of the present invention is that of the high carbon steel excellent in break separation shown in (1) below and the high carbon steel excellent in break separation shown in (2). In the manufacturing method.

  (1) By mass%, C: 0.70 to 1.20%, Si: 0.4% or less, Mn: 0.9% or less, P: 0.10% or less, S: 0.015% or less, It contains Cr: 0.5 to 2.0% and Al: 0.05% or less, the balance is made of Fe and impurities, the prior austenite grain size is 200 μm or more, and the hardness is 300 or less in Brinell hardness. High carbon steel with excellent fracture separation characteristics.

  (2) By mass%, C: 0.70 to 1.20%, Si: 0.4% or less, Mn: 0.9% or less, P: 0.10% or less, S: 0.015% or less, It contains Cr: 0.5-2.0% and Al: 0.05% or less, and the remainder is processed into steel and made of Fe and impurities into a part shape, and then heated in a temperature range of 1150 ° C or higher for 30 minutes or more Then, further, a temperature range of 730 to 700 ° C. is cooled at a cooling rate of 20 ° C./hour or less, and a method for producing a high carbon steel excellent in fracture separability.

  Hereinafter, the invention related to the high carbon steel excellent in fracture separation of (1) and the invention related to the method of manufacturing high carbon steel excellent in fracture separation of (2) are referred to as “present invention (1)” and “ This is referred to as “the present invention (2)”. Also, it may be collectively referred to as “the present invention”.

  Since the steel of the present invention does not contain alloy elements such as Ti and V, the component cost is low, and it has sufficient fracture separation even at a low hardness (strength) level of 300 or less in HB hardness. In addition, since it has a low level of hardness (strength), it also has good machinability, so it can be cracked like a connecting rod with a low level of hardness (strength) used in light cars and small passenger cars. It can be used as a material for products. This steel can be produced by the method of the present invention.

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

(A) Chemical composition C: 0.70 to 1.20%
In order to reduce ductility and toughness by actively utilizing proeutectoid cementite that precipitates at grain boundaries, the C content needs to be 0.70% or more. If the C content is less than 0.70%, even if the prior austenite grain size described later is 200 μm or more, no proeutectoid cementite precipitates at the grain boundaries, so that the toughness does not fall even if the ductility is lowered. is there. On the other hand, if the C content exceeds 1.20%, the hardness increases and the machinability such as machinability is significantly reduced. Therefore, the content of C is set to 0.70 to 1.20%. The C content is preferably 0.90 to 1.10%.

Si: 0.4% or less Si is an element having a deoxidizing action. However, a large amount of Si causes a decrease in hot workability and increases the component cost. In particular, when the Si content exceeds 0.4%, the hot workability is significantly decreased and the component cost is increased. The rise will also increase. Therefore, the Si content is set to 0.4% or less. Note that the Si content is preferably set to 0.01% or more in order to reliably exhibit the deoxidizing action.

Mn: 0.9% or less Mn is an element having a deoxidizing action as in the case of Si. However, when the Mn content is increased, the hot workability is lowered and the component cost is increased. In particular, when the Mn content exceeds 0.9%, the hot workability is significantly reduced. As the cost increases, the cost of ingredients increases. Therefore, the Mn content is set to 0.9% or less. In addition, in order to fully exhibit a deoxidation effect | action, it is preferable that content of Mn shall be 0.1% or more.

P: 0.10% or less P is an element present as an impurity in steel. However, when the content of P, which is an impurity, is too large, the hot workability is deteriorated. In particular, when the content exceeds 0.10%, the hot workability is significantly reduced. Therefore, the content of P is set to 0.10% or less.

S: 0.015% or less S has a function of improving machinability by forming a sulfide together with Mn. However, if the S content increases, the amount of MnS also increases. In particular, if the S content exceeds 0.015%, the amount of MnS increases extremely. Even in the case where the slag treatment is performed, the degree of coarsening is reduced by the pinning action of MnS present in large quantities. And when the degree of coarse graining is small, the elongation as ductility becomes large, so that the break separation property is poor. Therefore, the S content is set to 0.015% or less. In order to exhibit the effect of improving the machinability of S, the S content is preferably 0.002% or more.

Cr: 0.5 to 2.0%
In order to stably precipitate network-like pro-eutectoid cementite at the prior austenite grain boundaries and reduce ductility and toughness, the Cr content needs to be 0.5% or more. However, if the Cr content exceeds 2.0%, the hardness increases and the machinability such as machinability is significantly reduced. Therefore, the Cr content is set to 0.5 to 2.0%. The Cr content is preferably 0.9 to 1.6%.

Al: 0.05% or less Al is an element having a deoxidizing action. However, even if Al is contained in excess of 0.05%, the above effect is saturated and the component cost is increased. Therefore, the Al content is set to 0.05% or less. In order to reliably exhibit the deoxidation effect, the Al content is preferably 0.001% or more.

  For the reasons described above, the high carbon steel excellent in fracture separation of the present invention (1) contains elements from C to Al in the above-mentioned range, and the remainder is defined as consisting of Fe and impurities.

  Moreover, in the manufacturing method of the high carbon steel excellent in fracture separation of the present invention (2), the element containing C to Al in the above range is used, and the balance is made of steel composed of Fe and impurities. did.

(B) Prior austenite grain size When the prior austenite grain size is 200 μm or more, the steel having the chemical composition described in the above section (A) has a significant decrease in ductility and toughness, and elongation in a tensile test at room temperature. 10% or less, and the impact value at room temperature when using a U-notch Charpy impact test piece defined by JIS Z 2202 (1998) is 10 J / cm ² or less, and has excellent fracture separation. It becomes.

  Therefore, the high carbon steel excellent in fracture separability of the present invention (1) is defined as having a prior austenite grain size of 200 μm or more. In order to secure even better break separation, the prior austenite particle size is preferably 400 μm or more.

  If the heat treatment is carried out at a high temperature for a long time, the prior austenite particle size becomes large, and the larger the prior austenite particle size, the better the break separation, but in addition to the increase in heat treatment cost, the productivity decreases, There are also problems in terms of equipment such as deterioration of the walls. For this reason, the practical maximum value of the prior austenite grain size is about 2000 μm.

  In the case of the steel having the chemical composition described in the section (A), network-like pro-eutectoid cementite is precipitated at the prior austenite grain boundaries. Therefore, the prior austenite grain size can be easily measured by observing the network structure of proeutectoid cementite, for example, by the method described in the following <1>. In addition, the prior austenite grain size can be measured by making an austenite grain boundary appear using a quenching treatment material as described in <2>.

  <1> After mirror-polishing the test piece embedded in the resin, the polished surface was corroded with an alkaline sodium picrate solution, the magnification was 50 times or 100 times, optical microscope photographs were taken for 4 fields, and image analysis was performed. The area surrounded by proeutectoid cementite is regarded as one prior austenite grain, and the grain size of prior austenite in each field of view is measured in accordance with the “cutting method” described in JIS G 0551 (2005). Then, the “old austenite particle size” is obtained by arithmetically averaging the prior austenite particle size of each visual field measured as described above.

  <2> After water-quenched or oil-quenched test specimens were subjected to a predetermined heat treatment and mirror-polished in a resin, the polished surface was corroded with a saturated aqueous solution of picric acid to which a surfactant was added, and the magnification was 50 times or At 100 times, optical micrographs are taken for 4 fields of view, image analysis is performed, and the grain size of prior austenite in each field of view is measured according to the “cutting method” described in JIS G 0551 (2005). Then, as in the case of <1>, the “old austenite particle size” is obtained by arithmetically averaging the prior austenite particle size of each field of view measured as described above.

(C) Brinell hardness If the steel having the chemical composition described in the above section (A) has a low hardness of 300 or less in terms of HB hardness, it has good machinability. Therefore, the high carbon steel excellent in fracture separability of the present invention (1) is defined as having a hardness of 300 or less in terms of Brinell hardness. Note that if the hardness is too low, the machinability deteriorates, so the lower limit of the HB hardness is preferably about 150.

(D) Manufacturing method Next, the manufacturing method of the high carbon steel excellent in fracture separability of this invention (2) is described.

(D-1) Processing into a part shape In the method for producing high carbon steel excellent in fracture separability of the present invention (2), it contains elements from C to Al in the range described in the above section (A), The remainder uses steel made of Fe and impurities as a raw material, which is processed into a desired part shape. In addition, the method of processing into the component shape is not particularly limited, and a normal method such as hot forging or cutting may be used.

(D-2) Heat treatment after processing into part shape After processing into the part shape, it is necessary to perform a heating process in a temperature range of 1150 ° C or higher for 30 minutes or more.

When the heating temperature is lower than 1150 ° C., the old austenite grain size of 200 μm or more described in the above section (B) cannot be obtained. Therefore, desired low ductility and low toughness, that is, a tensile test at room temperature. The low ductility of 10% or less in elongation and the low toughness of 10 J / cm ² or less at room temperature when using a U-notch Charpy impact test piece defined by JIS Z 2202 (1998) cannot be obtained. For this reason, the break separation property is lowered.

Further, even when the heating temperature is 1150 ° C. or higher, when the heating holding time is less than 30 minutes, it becomes difficult to obtain the prior austenite grain size of 200 μm or more described in the above section (B). Ductility and low toughness, that is, low ductility of 10% or less in elongation at room temperature and an impact value at room temperature when using a U-notch Charpy impact test piece defined by JIS Z 2202 (1998) is 10 J / It becomes difficult to reliably obtain low toughness of cm ² or less.

  Therefore, in the manufacturing method of the high carbon steel excellent in break separability of the present invention (2), the above-described steel is processed into a part shape and then heated in a temperature range of 1150 ° C. or more for 30 minutes or more.

  In addition, from the viewpoint of the ability of a normal heat treatment furnace, productivity in actual production, and heat treatment cost, the upper limit of the heating temperature is desirably 1300 ° C., and the upper limit of the holding time in the heating temperature region is desirably 120 minutes.

(D-3) Cooling after heat treatment In the method for producing high carbon steel excellent in fracture separability according to the present invention (2), after performing the heat treatment of the item (D-2), in the vicinity of the temperature at which pearlite transformation occurs. More specifically, it is necessary to cool at a cooling rate of 20 ° C. or less per hour in a temperature range of 730 to 700 ° C.

  The steel having the chemical composition described in the section (A) is cooled under the above conditions after the heat treatment described in the section (D-2), so that the hardness becomes a low value of 300 or less in terms of HB hardness. Thus, it has good machinability.

  The cooling rate in the temperature range of 730 to 700 ° C. is preferably 15 ° C. or less per hour. However, if the cooling rate is too slow, productivity is lowered, resulting in an increase in cost. Therefore, the practical lower limit of the cooling rate in the temperature range is about 5 ° C. per hour.

  For the above reasons, the method for producing high carbon steel excellent in fracture separation of the present invention (2) has a temperature of 1150 ° C. or higher after processing the steel having the chemical composition described in the above section (A) into a part shape. Then, the temperature range of 730 to 700 ° C. was further cooled at a cooling rate of 20 ° C./hour or less.

  In the above, if the temperature range of 730 to 700 ° C. is cooled at a cooling rate of 20 ° C. or less per hour, a low hardness of 300 or less in HB hardness can be obtained, so the temperature exceeds 730 ° C. The cooling in the region and the temperature region below 700 ° C. need not be specified. For this reason, for example, means for increasing the cooling rate such as forced air cooling as cooling in a high temperature range up to 730 ° C. and a low temperature range lower than 700 ° C. after performing the heat treatment described in the section (D-2) By adopting, productivity can be increased.

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

  A slab obtained by subjecting steels 1 to 7 having the chemical composition shown in Table 1 to 70-ton converter melting-continuous casting was subjected to block rolling to obtain a square member having a side of 180 mm.

  A round bar having a diameter of 25 mm was produced by hot forging with heating at 1200 ° C. and finishing at 1000 ° C. using a square material having a side of 180 mm obtained as described above.

  Next, the above-mentioned round bar having a diameter of 25 mm was held at 1100 ° C. or 1200 ° C. for 30 minutes, then allowed to cool in the atmosphere, and once charged in the low temperature furnace, it was once charged in the low temperature furnace, and 730 to 700 in the low temperature furnace. The cooling rate in the temperature range of ° C was controlled. The cooling in the temperature range below 700 ° C. was again allowed to cool in the atmosphere.

  Table 2 shows details of heat treatment and subsequent cooling.

  Various test pieces were collected from the round bar having a diameter of 25 mm obtained as described above, and HB hardness, prior austenite particle size, tensile properties, and impact properties were investigated.

  The HB hardness was determined by cutting out three test pieces from each round bar, each having a plane perpendicular to the forging axis (hereinafter referred to as “cross section”) as a test surface. Represents the radius of a round bar.) Measured at a test force of 29.42 kN using an indenter with a diameter of 10 mm for one point and a total of three points by the method described in JIS Z 2243 (1998). Was obtained by arithmetic averaging.

  The prior austenite grain size was determined as follows. That is, a micro test piece is cut out from each round bar so that the cross section becomes a test surface, embedded in a resin and mirror-polished, and then the polished surface is corroded with an alkaline sodium picrate solution, and the magnification is 50 times or 100 times. As shown, optical micrographs were taken for four fields of view at the R / 2 site. Then, image analysis was performed, and the area surrounded by pro-eutectoid cementite was regarded as one old austenite grain, and the grain size of the prior austenite in each field of view was measured according to the “cutting method” described in JIS G 0551 (2005). And it calculated | required by carrying out the arithmetic average of the prior austenite particle size of each visual field.

  In addition, with respect to Test No. 12 in which the heat treatment was performed on the steel 5, cementite at the grain boundaries could not be produced by the above method. Therefore, for test number 12, a round bar test piece was separately prepared from a hot forged material having a diameter of 25 mm of steel 5 and held at the heating temperature in Table 2, that is, 1200 ° C. for 30 minutes, followed by water quenching. A micro test piece was cut out from the round bar test piece so that the cross section was the test surface, embedded in a resin and mirror-polished, and then the polished surface was corroded with a saturated aqueous solution of picric acid to which a surfactant was added. Was taken 50 times or 100 times, and optical micrographs were taken for 4 fields of view at the R / 2 site. Then, the image was analyzed, and the particle size of the prior austenite in each field of view was measured according to the “cutting method” described in JIS G 0551 (2005), and the old austenite particle size in each field of view was arithmetically averaged. The austenite particle size was determined.

  Tensile properties were obtained by cutting out the 14A test piece described in JIS Z 2201 (1998) from the center of each round bar (however, the parallel part had a diameter and length of 6 mm and 42 mm, respectively), and the gauge distance was The tensile test was performed at room temperature with 30 mm, and the elongation was measured. The target for elongation at room temperature was 10% or less.

For impact characteristics, a U-notch Charpy impact test piece defined by JIS Z 2202 (1998) was cut out from the central portion of each round bar, and a Charpy impact test was performed at room temperature to measure the impact value. The target for the Charpy impact value at room temperature was 10 J / cm ² or less.

  Table 2 also shows the results of the austenite grain size, HB hardness, elongation, and impact value of the Charpy impact test. FIG. 1 shows the relationship between the prior austenite grain size and elongation.

From Table 2, the test number “test example”, that is, test number 1, test number 6, test number 8, and test number 10 satisfying the conditions defined in the present invention (1) are 300 or less in HB hardness. In spite of having a low hardness level, the elongation at room temperature and the impact value at room temperature using a U-notch Charpy impact test piece specified in JIS Z 2202 (1998) are 10% or less and 10 J / It is clear that the goal of cm ² or less has been achieved and that it has low ductility and low toughness.

On the other hand, the test number of “Comparative Example” with an HB hardness exceeding 300 is considered to be inferior in machinability, and has an elongation at room temperature of 10% or less and JIS Z 2202 (1998). If the impact value at room temperature using the U-notch Charpy impact test specimen specified in (1) is at least one of the targets of 10 J / cm ² or less, the HB hardness is less than 300 or less. It is inferior to break separation.

Specifically, in Test No. 2, Test No. 3 and Test No. 5, the elongation at room temperature and the impact value at room temperature using a U-notch Charpy impact test piece are 10% or less and 10 J / cm ² or less, respectively. The HB hardness is 309, 314, and 307, respectively, exceeding the upper limit value of 300 HB hardness defined in the present invention (1). Therefore, it is thought that it is inferior to machinability.

  Test No. 4, Test No. 7, Test No. 9 and Test No. 11 have old austenite particle sizes of 83.05 μm, 141.60 μm, 168.41 μm and 146.12 μm, respectively, and are the old specified in the present invention (1). It is below the lower limit of 200 μm of the austenite grain size. Therefore, the elongation at room temperature exceeds the target value. For this reason, the HB hardness specified in the present invention (1) is inferior in fracture separability under a hardness level as low as 300 or less.

  In Test No. 12, since the C content of Steel 5 is lower than the lower limit of 0.70% C content defined in the present invention, although the prior austenite grain size is 200 μm or more, proeutectoid cementite is present at the grain boundaries. No precipitation occurs, and the impact value at room temperature using a U-notch Charpy impact test piece exceeds the target value. For this reason, the HB hardness specified in the present invention (1) is inferior in fracture separability under a hardness level as low as 300 or less.

  Test No. 13 is that the C content of steel 6 exceeds the upper limit of 1.20% C content specified in the present invention, so that the elongation at room temperature and the impact at room temperature using a U-notch Charpy impact test piece Although both values achieved the target, the HB hardness was 314, exceeding the upper limit value of 300 HB hardness defined in the present invention (1). Therefore, it is thought that it is inferior to machinability.

  Test No. 14 is intended for the impact value at room temperature using a U-notch Charpy impact test piece because the Cr content of steel 7 is below the lower limit of 0.50% Cr content defined in the present invention. The value is exceeded. For this reason, the HB hardness specified in the present invention (1) is inferior in fracture separability under a hardness level as low as 300 or less.

  Since the steel of the present invention does not contain alloy elements such as Ti and V, the component cost is low, and it has sufficient fracture separation even at a low hardness (strength) level of 300 or less in HB hardness. In addition, since it has a low level of hardness (strength), it also has good machinability, so it can be cracked like a connecting rod with a low level of hardness (strength) used in light cars and small passenger cars. It can be used as a material for products. This steel can be produced by the method of the present invention.

It is a figure which arrange | positions and shows the relationship between the prior austenite particle size and elongation in test numbers 1-14 of an Example.

Claims (2)

  In mass%, C: 0.70 to 1.20%, Si: 0.4% or less, Mn: 0.9% or less, P: 0.10% or less, S: 0.015% or less, Cr: 0 0.5 to 2.0% and Al: 0.05% or less, the balance is Fe and impurities, the prior austenite grain size is 200 μm or more, and the hardness is 300 or less in terms of Brinell hardness High carbon steel with excellent fracture separation.   In mass%, C: 0.70 to 1.20%, Si: 0.4% or less, Mn: 0.9% or less, P: 0.10% or less, S: 0.015% or less, Cr: 0 0.5 to 2.0% and Al: 0.05% or less, and the balance is processed into a part shape of steel composed of Fe and impurities, and then heated in a temperature range of 1150 ° C. or higher for 30 minutes or more. A method for producing a high carbon steel excellent in break separability, wherein the temperature range of 730 to 700 ° C. is cooled at a cooling rate of 20 ° C./hour or less.

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