JP5739689B2 - Mechanical structural member - Google Patents

Description

  The present invention relates to a steel plate of carburized boron steel that has both good workability and hardenability and is suitable for various machine parts including automobile parts.

  Since carbon steel (boron steel) to which a small amount of B is added has excellent hardenability, it is low in various parts applications that require sufficient quenching to the center of the plate thickness (becomes a martensite structure). Used as a cost material. An annealed steel sheet of boron steel is used after being processed into a predetermined part shape by press forming such as punching and bending, forging and cutting, and then subjected to heat treatment such as carburizing and quenching. Therefore, the boron steel sheet as a material is required to have workability, hardenability, and characteristics after heat treatment according to the application.

  Parts that require wear resistance are carburized to increase the surface hardness. However, when a boron steel sheet is carburized and quenched, the hardness may decrease. In normal carburization (carbon potential is 0.8% or more, effective hardening depth is 1 mm or more, surface hardness is 700 HV or more), the surface hardness is hardly lowered, but the carbon potential (CP) is 0. When a surface hardness of about 500 to 650 HV is targeted by slight carburization of less than 8%, the surface hardness is likely to decrease.

The causes of the surface hardness reduction in the boron steel sheet are considered as follows (1) to (3).
(1) Solid solution B in the steel surface layer portion is reduced by the high-temperature heating applied during the steel plate manufacturing stage or during quenching after forming the part (de-B phenomenon).
(2) The hardenability of the surface layer portion is reduced by the removal of B, and an incompletely hardened structure is formed in the surface layer portion during quenching to reduce the surface hardness.
(3) Especially when the plate thickness is large, the cooling rate at the time of quenching is lowered, so the surface hardness is significantly reduced.

Techniques for preventing the formation of incompletely quenched structures have been proposed so far.
For example, Patent Document 1 uses boron steel having a C content of 0.1 to 0.3% by mass, and prevents the generation of coarse grains during carburizing by finely depositing Ti and Nb carbonitrides. Further, Cr and Mo are added to make up for a decrease in hardenability due to de-B, and generation of an incompletely hardened structure formed to a depth of 0.2 to 0.7 mm from the surface is prevented. However, in Patent Document 1, it is necessary to gradually cool a temperature range of 800 to 500 ° C. at a cooling rate of 1 ° C./second or less in order to finely precipitate Ti and Nb carbonitrides, resulting in poor productivity.

  Patent Document 2 uses boron steel having a C content of 0.10 to 0.30 mass%, and the upper limit of the Mn and Cr contents is 1.5 mass%. However, in Patent Document 2, the hardenability of the de-B portion can be complemented by adding Mn and Cr to the upper limit, but these elements that are solid solution strengthening elements greatly reduce the workability. On the other hand, in Patent Document 2, when the contents of Mn and Cr are small, it is difficult to supplement the decrease in hardenability due to de-B. Addition of Ni leads to an increase in material cost.

  Patent Document 3 uses boron steel having a C content of 0.10 to 0.30 mass%, and the structure of the non-carburized part is a two-phase structure composed of martensite containing 10 to 70 area% of ferrite. The steel material for gears with a small distortion by heat treatment is proposed. However, in Patent Document 3, since the ferrite exists in the metal structure after the heat treatment, a decrease in fatigue characteristics, impact toughness, and the like can be considered.

  Patent Document 4 uses boron steel having a C content of 0.10 to 0.40 mass%, and the ratio of the area ratio of carbide in the surface layer portion to the area ratio of carbide in the center portion of the plate thickness is 0.90 or more. By setting the X value calculated by the alloy component to 24 or more, the poorly quenched layer in the surface layer portion is improved. However, in patent document 4, in order to improve the hardenability of a surface layer part, it is necessary to add Mn and Cr excessively. On the other hand, excessive addition of Mn and Cr deteriorates various cold workability such as stretch flange processing, punching processing, bending processing and the like.

JP 2001-303172 A JP 04-297521 A (Patent No. 3024245) JP 2001-032036 A (Patent No. 3989138) JP 2010-215961 A

As described above, various techniques for preventing the formation of an incompletely quenched structure (ferrite or bainite) have been proposed by the prior art, but the carbon potential (C.P.) is low, that is, when a slight carburizing and quenching is performed. There has been a problem that the countermeasures against the decrease in surface hardness are still insufficient.
Accordingly, the present invention has been made in view of the above-described problems, and is excellent in workability, and boron steel that can achieve the desired quenching hardness in the surface layer portion even in carburizing and quenching at a low carbon potential. It is to provide steel sheets.

First, the present inventors considered that it is extremely difficult to prevent the de-B phenomenon inherent to boron steel in the current manufacturing process, and it would be advisable to allow the de-B to occur to some extent. . In view of this point, the present inventors have studied to solve the above problems, and as a result, have obtained the following knowledge regarding the stretch flangeability and surface layer hardenability of boron steel.
(1) Ensuring stretch flangeability (local ductility) and preventing hardenability deterioration caused by de-B can be achieved by strictly controlling the chemical composition.
(2) In the metal structure, stretch flangeability can be further improved by controlling the amount of spheroidized carbide and the distance between carbides.
The inventors of the present invention ensure sufficient surface layer hardenability while securing stretch flangeability of boron steel by taking measures with the chemical composition of (1) and the metal structure of (2). As a result, the present invention has been completed.
That is, the present invention includes C: 0.10 to 0.50 mass%, Si: 0.50 mass% or less, Mn: 0.20 to 1.8 mass%, Cr: 0.20 to 2.0 mass%. , P: 0.02 mass% or less, S: 0.02 mass% or less, Ti: 0.01-0.20 mass%, Al: 0.002-0.10 mass%, B: 0.0005-0 .0050% by mass, the balance being Fe and inevitable impurities, carbides are dispersed in ferrite, the average distance between the carbides is 0.8 μm or more, and the X value defined by the following formula (1) Is a carburizing steel plate having excellent workability, having a Y value defined by the following formula (2) of 30 or more and a hardness of 150 HV or less.
X = 30C + 30Si + 20Mn + 5Cr + 150S + 80Ti-1.5 Average distance between carbides (1)
Y = 5C + 22Mn + 32Cr (2)

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate of the boron steel which is excellent in workability and can obtain high quenching hardness in a surface layer part in the carburizing quenching with a low carbon potential can be provided.

It is a perspective view of the stretch flange processed goods obtained in the Example. It is a graph which shows the relationship between stretch flange workability and X value. It is a graph which shows the relationship between stretch flange workability and notch tensile elongation (Elv value). It is a graph which shows the relationship between the quenching hardness and Y value in the position of a depth of 100 micrometers from the surface. It is a graph which shows the relationship between the depth from the surface, and quenching hardness.

Hereinafter, the carburized steel sheet of the present invention will be described in detail.
[Chemical composition]
C: 0.10 to 0.50 mass%
C is an important alloying component for ensuring the hardness necessary for machine structural members including automobile parts, and a desired hardness can be obtained with a C content of 0.10% by mass or more. However, when a large amount of C exceeding 0.50% by mass is contained, the hardenability and the quenching hardness are sufficiently ensured, but the stretch flangeability of the steel sheet is remarkably deteriorated.

Si: 0.5% by mass or less Si is one of elements having a great influence on ductility. If Si is added excessively, the ferrite is hardened by the solid solution strengthening action, which causes cracks during molding. Further, when the Si content increases, scale flaws tend to be generated on the surface of the steel sheet during the production process, leading to a reduction in surface quality. Furthermore, grain boundary oxidation occurs during carburizing heating, leading to a reduction in fatigue life. Therefore, in this invention, the upper limit of Si content was set to 0.5 mass%.

Mn: 0.20 to 1.8% by mass
Mn suppresses ferrite transformation in the cooling process after quenching heating, and enhances the hardenability of the steel material by making the structure mainly martensite even at a relatively slow cooling rate. Mn is an alloy component effective for toughening. However, if the Mn content is less than 0.20% by mass, the hardenability is significantly lowered, and high-temperature products such as pearlite and upper bainite are formed during cooling, and quenching is necessary as a mechanical structural member including automobile parts. Hardness cannot be obtained. On the other hand, when a large amount of Mn exceeding 1.8% by mass is contained, the ferrite is cured and the stretch flangeability is deteriorated.

Cr: 0.20 to 2.0% by mass
Cr is an alloy component effective for improving hardenability, and the effect of adding Cr becomes remarkable when the content is 0.20% by mass or more. On the other hand, when the Cr content is less than 0.20% by mass, not only the hardness is insufficient, but also the dependency on the cooling rate at the time of quenching is increased, so that the quenching hardness tends to be unstable. On the other hand, if a large amount of Cr exceeding 2.0% by mass is contained, the workability is remarkably deteriorated.

P: 0.02% by mass or less P is an element that deteriorates ductility and toughness. When the P content exceeds 0.02% by mass, the toughness of the prior austenite grain boundaries deteriorates after quenching, so that Fatigue properties are reduced. Therefore, in this invention, the upper limit of P content was set to 0.02 mass%.

S: 0.02% by mass or less S is a very important factor governing stretch flangeability. That is, S produces MnS-based inclusions and deteriorates the local ductility. In stretch flange processing, the generated MnS becomes the starting point of breakage, and cracks are likely to occur. Therefore, in this invention, S content is controlled to 0.02 mass% or less, preferably 0.005 mass% or less.

Ti: 0.01-0.20 mass%
Ti is a component added to adjust the deoxidation of molten steel, but also exhibits a denitrification action.
Moreover, since N dissolved in the steel sheet is fixed as a nitride, the effective B amount for improving the hardenability is increased. Further, carbonitride is formed, and the effect of preventing the coarsening of crystal grains during quenching heating is exhibited. In order to stably obtain these actions, a Ti content of 0.01% by mass or more is necessary. However, when a large amount of Ti exceeding 0.20% by mass is contained, not only is it economically disadvantageous, but it also causes deterioration of stretch flangeability.

Al: 0.002 to 0.10% by mass
Al is a component used as a deoxidizer for molten steel, and also exhibits an effect of fixing N. Such an effect becomes remarkable when the Al content is 0.002% by mass or more. However, when a large amount of Al exceeding 0.10% by mass is contained, the cleanliness of the steel is impaired, surface flaws are easily generated, and the surface quality of the steel sheet is deteriorated.

B: 0.0005-0.0050 mass%
B significantly improves the hardenability of the steel material by adding a very small amount. Moreover, the effect | action which strengthens a grain boundary is exhibited by reducing the distortion energy of a grain boundary. It is a necessary alloy component in order to stably obtain the necessary hardness for machine structural members such as automobile parts. Such an effect of addition of B becomes significant when the content is 0.0005% by mass or more. However, even if B exceeding 0.0050 mass% is added, the effect is saturated, and conversely, the toughness is deteriorated.

X value: 40 or less Even when each alloy element is within the above-mentioned range, MnS inclusions, ferrite hardness, grain size of spheroidized carbides and average distance between carbides are complicatedly related to stretch flangeability of the steel sheet. It has a great influence on it. In the present invention, it is important that the content of each alloy component is adjusted so that the X value defined by the following formula (1) is 40 or less in the above range.
X = 30C + 30Si + 20Mn + 5Cr + 150S + 80Ti-1.5 Average distance between carbides (1)
When an alloy design having an X value of 40 or less as an index of stretch flangeability is employed, the local ductility is improved, and the steel material satisfies the stretch flangeability required in the production of parts.
Here, in Formula (1), the value (mass%) of the content rate of each component is substituted for the term of each element. For the average distance between carbides, a value in μm is substituted.

Y value: 30 or more Y value is an index showing the hardenability of the de-B portion on the steel sheet surface, in order to optimize the contents of C, Mn and Cr for improving hardenability and to obtain the required quenching hardness. Used to determine the C, Mn, and Cr content. In the present invention, it is important that the content of each alloy component is adjusted so that the Y value defined by the following formula (2) is 30 or more in the above range.
Y = 5C + 22Mn + 32Cr (2)
When an alloy design with a Y value of 30 or more is adopted, the quenching hardness of the steel sheet surface can be stably obtained.
Here, in Formula (2), the value (mass%) of the content rate of each component is substituted for the term of each element.

[Metal structure]
Average distance between carbides: 0.8 μm or more The carburized steel sheet of the present invention has an annealed structure in which carbides are dispersed in a ferrite matrix. According to the study by the present inventors, the average distance between carbides is particularly important as a factor governing local ductility such as stretch flangeability, and when the average distance between carbides is 0.8 μm or more, It was found that it was possible to impart high stretch flangeability, which was difficult to achieve with steel plates. When the average distance between carbides in the steel sheet is less than 0.8 μm, it facilitates the connection and growth of microvoids generated from carbides during the forming process, and the local ductility is sensitive to the presence of such microscopic defects. descend. Therefore, in this invention, the average distance between carbide | carbonized_materials of a steel plate was prescribed | regulated to 0.8 micrometer or more.

The average distance between carbides refers to the value (μm) of L obtained by substituting the carbide area ratio f and the average carbide particle size D (μm) in the steel sheet cross section into the following equation.
L = ((π / (4 × f)) ^(1/2) −1) × D (3)
Here, the area ratio f of the carbide is a value obtained by f = C / 6.67 when the C content of the steel sheet is C (mass%). This represents the area ratio of cementite as a value proportional to the actual C content, assuming 100% cementite (f = 1) when the C content is 6.67% by mass. For example, when the C content of the steel sheet is 0.36% by mass, f = 0.36 / 6.67 = 0.054.
The average carbide particle size D is a value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field in the observation of the metal structure of the cross section of the steel sheet for all the measured carbides. Specifically, the area of each carbide is measured, and the equivalent circle diameter is calculated from the area. The area can be measured using an image processing apparatus. And the sum total of the circle equivalent diameter of all the measured carbide | carbonized_materials is calculated | required, and the value which remove | divided the sum total with the total number of measurement carbide | carbonized_material is set to average carbide particle diameter D (micrometer). In order to increase the reliability of the numerical value, the observation visual field is an area where the total number of measured carbides is 300 or more.

  As the distance between the carbides is set higher in the annealing temperature, the hot rolled pearlite is divided and spheroidized, and fine carbides dissolve in the ferrite matrix and disappear. The distance between them increases. Conversely, the lower the holding temperature, the smaller the average distance between carbides. Although the distance between the carbides after annealing changes depending on the length of the heating and holding time, the influence of temperature is larger than the time. When the holding time is less than 0.5 hour, the hot-rolled pearlite is not sufficiently divided and spheroidized, and the structure after annealing becomes inappropriate.

〔mechanical nature〕
Hardness: 150 HV or less Hardness (Vickers hardness) is the simplest indicator of workability. A softer one has higher ductility and better workability. In order to obtain a steel plate with excellent stretch flangeability, it is necessary to set it to 150 HV or less.

Hereinafter, the manufacturing method of the steel plate for carburizing of this invention is demonstrated.
[Hot-roll pickling process]
The slab heating temperature before hot rolling may be 1150 to 1350 ° C. as in the case of general carbon steel. The hot rolling finishing temperature is 800 to 900 ° C. When the temperature is lower than 800 ° C., the deformation resistance is increased and the sheet passing property is lowered. When it exceeds 900 degreeC, an austenite particle size will coarsen and the toughness of a hot rolled material will fall. The winding temperature is 500 to 700 ° C. When the temperature is below 500 ° C., the hot-rolled material is cured and the productivity is lowered. Above 700 ° C, the amount of pro-eutectoid ferrite increases, the distribution of cementite becomes non-uniform, and the lamellar spacing of the pearlite increases, making it difficult to spheroidize the cementite, and workability after annealing. Decreases. After winding, it is subjected to pickling by a normal method. Then, if necessary, cold rolling can be performed to adjust the plate thickness.

[Annealing process]
The steel plate that has been pickled in the previous step and cold-rolled as necessary is subjected to annealing. As a simple annealing method, a method in which the steel sheet is soaked for 0.5 h or more in a temperature range of 650 to 740 ° C. can be adopted. Thereby, the layered carbide in the pearlite is divided and spheroidized to give processability. The holding time may be approximately 50 hours or less. The soaking is to be held in a predetermined temperature range up to the center of the plate thickness.

The carburized steel sheet of the present invention obtained as described above is processed into a predetermined machine part by a general method, and then the carbon potential (CP) is 0.2% or more and 0.8%. It is subjected to carburizing and quenching treatment with less than carburizing atmosphere.
The carbon potential is C% of the Fe surface in the material in equilibrium with the carburizing gas, and the desired C.I. P. It is generally performed to adjust so that.

  Steels having the compositions shown in Table 1 were melted, and each steel was hot-rolled under conditions of a slab heating temperature of 1230 ° C, a hot rolling finishing temperature of 850 ° C, and a coiling temperature of 600 ° C, pickling, Annealing was performed at a heat pattern of 710 ° C. and soaking 40 h to obtain a steel sheet (test material) having a thickness of 5.0 mm. However, no. 8 and no. 16 only, annealing conditions were 580 ° C. and soaking was maintained for 25 h.

In order to evaluate local ductility, the notch tensile elongation Elv was evaluated. First, a JIS No. 5 test piece was cut out from the test material so that the rolling direction of the test material coincided with the length direction of the test piece, and an opening angle was formed at both edge portions of the central portion in the longitudinal direction of the parallel part of the test piece A V-notch having a depth of 45 degrees and a depth of 2 mm was processed, and this was used as a notched tensile test piece. The distance between the gauge points was 5 mm.
A tensile test was performed with this test piece, and the distance between the gauge points after the fracture was measured to obtain the elongation percentage (%) with respect to the distance between the gauge points before the test, which was defined as a notch tensile elongation Elv. The Elv value is an index indicating local ductility, and the local ductility can be more appropriately quantitatively evaluated as compared with the local elongation calculated as total elongation-uniform elongation in a normal tensile test. The results are shown in Table 1. The higher the notch tensile elongation Elv (%), the higher the local ductility.

The stretch flangeability was evaluated as follows.
First, a disk having a diameter of 150 mm was punched from the test material, and this was used as a base plate (blank material) for stretch flange processing. Next, prior to stretch flange processing, a punching hole having a diameter of 25 mm was provided in the center of the base plate. Using this hole as an initial hole, a punch with a diameter of 40 mm was pushed in, and stretch flange processing for processing a vertical wall with a height of 40 mm was performed. At this time, the burr generated when the initial hole was punched was set so as to be on the punch side at the time of stretch flange processing. A schematic view of the obtained stretched flange processed product is shown in FIG.
The presence or absence of processing cracks on the flange upper end face was evaluated. The evaluation is as follows: grade 5 with no cracks, grade 4 with a crack length of less than 10% of the plate thickness, grade 3 with a crack length of 10% to less than 25% of the plate thickness, A rating of 2 or more and less than 50% of the plate thickness was rated 2, and a rating of 1 or more was 50% or more. The results are shown in Table 1.

  Even if the alloy component is within the scope of the invention, the X value exceeds 40. 31, no. 40 and no. In No. 51, the notch tensile elongation decreased, and cracks occurred on the flange upper end surface in the stretch flange processing. In addition, No. in which the alloy element exceeds the upper limit. 9, no. 13, no. 14, no. 26, no. 29, no. 30, no. 35, no. 48 and no. No. 50 and No. 50 having an average distance between carbides of less than 0.8 μm. 8, no. 16 and no. No. 24, and No. 24 whose material hardness exceeds 150 HV. 2, no. 5, no. 6, no. 23, no. 32, no. 33, no. 36, no. 41, no. 42, no. 43 and no. For No. 46, the notch tensile elongation decreased and cracking occurred in the stretch flange processing.

  When the stretch flange workability is organized by X value, as shown in FIG. 2, when the X value defined by X = 30C + 30Si + 20Mn + 5Cr + 150S + 80Ti-1.5 average distance between carbides is 40 or less, there is no work cracking and the stretch flangeability is excellent. It is clear that

  Stretch flange workability also has a close correlation with notch tensile elongation Elv. When the stretch flange workability is arranged by notch tensile elongation Elv (%), as shown in FIG. 3, it was found that the stretch flange workability is good when the notch tensile elongation Elv is about 45% or more. It was also found that when the notch tensile elongation Elv exceeds 55%, cracks do not occur and it becomes favorable.

Evaluation of hardenability performed the carburizing process using the stretch flange processed goods shown in FIG.
The carburizing treatment was carried out in a carburizing atmosphere having a carbon potential (C.P.) of 0.6% at 880 ° C. for 3 hours and subsequently quenched in oil at 60 ° C. As the carburizing gas at this time, a mixed gas adjusted to a composition of H ₂ : 40%, CO: 20%, N ₂ : 40% was used. About the processed product after carburizing and quenching, a test piece for observation of hardness and metal structure was cut out from a portion (flange bottom) having a plate thickness of 5 mm.
For the hardness measurement, 10 points of hardness at a position of a depth of 100 μm from the surface layer of the plate thickness section were randomly measured and averaged. In addition, the metal structure was mirror-polished and then corroded with 2% nital and observed with an optical microscope to determine the presence or absence of a poorly quenched layer.

  No. having a Y value of less than 30 even when the alloy component is within the scope of the invention. 1, no. 4, no. 6, no. 12, no. 17, no. 18, no. 19, no. 38 and no. No. 46, or Mn or Cr content below the lower limit. 34 and no. In 52, hardness at a position of 100 μm depth from the surface layer was low, and an incompletely quenched structure was observed.

  When the hardness at the position of 100 μm depth from the surface layer is arranged by Y value, as shown in FIG. 4, when the Y value defined by Y value = 5C + 22Mn + 32Cr is 30 or more, the hardness at the position of 100 μm depth from the surface layer Is stable and shows a high value, and it is clear that ferrite and bainite are not observed even in the metal structure, and that good hardenability is exhibited.

  FIG. 5 shows an example in which the hardness distribution in the thickness direction after carburizing and quenching was measured for the inventive example (No. 21) and the comparative example (No. 1). As shown in FIG. 5, in the steel plate of the comparative example, a region where the hardness is reduced to about 350 Hv in the vicinity of the depth from the surface of 50 to 300 μm is recognized, which is a poor surface hardness due to poor quenching. is there. On the other hand, in the steel sheet of the present invention, since the hardness defect portion is not recognized, it is clear that high quenching hardness is obtained in the surface layer portion even by carburizing and quenching at a low carbon potential.

  As shown in the above examples, the present invention appropriately adjusts the alloy components and the content thereof, and X = 30C + 30Si + 20Mn + 5Cr + 150S + 80Ti-1.5 The X value defined by the average distance between carbides is 40 or less. When the Y value specified by 5C + 22Mn + 32Cr is set to 30 or more and the hardness of the annealed material is set to 150 HV or less, excellent stretch flangeability can be obtained, and quenching by de-B of the plate thickness surface layer portion generated during carburizing heating. It is possible to improve defects.

Claims (1)

C: 0.10 to 0.50 mass%, Si: 0.50 mass% or less, Mn: 0.20 to 1.8 mass%, Cr: 0.20 to 2.0 mass%, P: 0.02 % By mass, S: 0.02% by mass or less, Ti: 0.01-0.20% by mass, Al: 0.002-0.10% by mass, B: 0.0005-0.0050% by mass, the balance Is Fe and inevitable impurities, carbides are dispersed in ferrite, the average distance between the carbides is 0.8 μm or more, the X value defined by the following formula (1) is 40 or less, The carburizing steel plate having excellent workability having a Y value defined by the following formula (2) of 30 or more and a hardness of 150 HV or less has a carbon potential of 0.2% or more and less than 0.8%. Mechanical structural member obtained by carburizing and quenching .
X = 30C + 30Si + 20Mn + 5Cr + 150S + 80Ti-1.5 Average distance between carbides (1)
Y = 5C + 22Mn + 32Cr (2)

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