JP2011149099A - Steel forging and assembling type crankshaft - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the hydrogen crack resistance of a steel forging made of carbon steel by structural designing without depending on a means of adding alloy elements. <P>SOLUTION: The steel forging has a composition comprising 0.15 to 0.5% C, ≤0.6% (not including 0%) Si, 0.5 to 1.5% Mn, 0.1 to 2.5% Ni, 0.1 to 2.5% Cr, 0.01 to 0.7% Mo, 0.0002 to 0.01% S and ≤0.002% (not including 0%) O, and the balance iron with inevitable impurities, and in which the steel cross-section in the position of the depth D/4(D: the circle equivalent diameter of the cross-section of the steel forging) is composed of a sound part composed of a ferrite structure or a ferrite-pearlite mixed structure, and the balance (hereinafter, referred to as a macro-segregated part), the ratio of the sound part to the steel cross-section is ≥90 area%, and (the average grain size of the pearlite)/(the average grain size of the ferrite) in the macro-segregated part is ≥3.0. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to forged steel products widely used in industrial fields such as machinery, ships, and generators, and more particularly to crank journals and crank throws, and assembled crankshafts obtained therefrom.

  There are two types of crankshafts, which are transmission members for driving sources of diesel engines used in ships, generators, and the like. Among them, assembly type crankshafts are used for large diesel engines, and forged steel products are mainly used for crank journals and crank throws. In order to obtain the tensile strength and forgeability of 500 MPa or more necessary for the product at a low cost, conventionally, carbon steel with a low amount of Mn, Cr, etc. added at low C is used for quenching or normalizing treatment. Forged steel products mainly composed of a ferrite-pearlite mixed structure are used after tempering. Originally, it is said that hydrogen cracking is unlikely to occur in carbon steel having a tensile strength of less than 800 MPa, but hydrogen cracking may occur at a temperature near room temperature when the temperature is lowered during heat treatment or the like. In general, hydrogen cracking is likely to occur with increasing strength and fatigue. For this reason, various technologies have been proposed for low alloy steels with high fatigue strength in terms of steel refining technology, thermal history, and component composition. In terms of refining technology, the upper limit of the amount of hydrogen at the time of refining molten steel is regulated, and dehydrogenation treatment is carried out in actual operation when it exceeds the upper limit.

  For example, as in Patent Document 1, a method is proposed in which inclusions are reduced by secondary refining, and at the time of RH vacuum degassing, the inclusions are removed by refluxing molten steel between a ladle and a degassing tank. Yes. From the viewpoint of thermal history, for example, by holding at a high temperature for a long time, hydrogen is diffused and escaped, and the hydrogen content is actually reduced. However, in the present technology, particularly in the case of large forged steel products, the effect of preventing hydrogen cracking is small as compared with the fact that the rate of content decrease due to long-time holding at high temperatures is slow, and time and cost are required. In addition, hydrogen concentration cannot be controlled in the macro-segregation part of alloy elements in which hydrogen cracking occurs in large forged steel products.

  From the aspect of component composition, as disclosed in Patent Document 2, by increasing the S content in the steel, MnS inclusions are introduced into the steel to prevent hydrogen concentration, thereby preventing hydrogen cracking. A method for improving the above has been proposed. However, in this technique, there is a concern that coarse MnS-based inclusions do not become hydrogen trap sites but instead become hydrogen cracking origins. In carbon steel, an increase in the S content does not lead to an improvement in hydrogen cracking resistance.

  Patent Document 3 proposes to increase the content of Ti, Zr, Hf, and Nb in the steel to regulate the number of inclusions of 20 μm or more, the degree of circularity, and the number of inclusions of 1 to 10 μm. . As a result, Ti, Zr, Hf, and Nb compounds are introduced into the steel to prevent hydrogen concentration, thereby improving hydrogen cracking resistance. In addition, as a method of suppressing hydrogen embrittlement in the field of steel wires, suppression of hydrogen intrusion due to external factors such as corrosion or suppression of hydrogen diffusion using precipitated carbonitride by tempering is known. However, these differ in hydrogen behavior from hydrogen cracking that occurs in a shorter time than when corrosion occurs, such as during cooling or standing at room temperature.

JP 2003-183722 A JP 2003-268438 A JP 2006-336092 A

  The dehydrogenation treatment as described above is limited in reducing the amount of hydrogen in terms of treatment time and cost. In general, production is controlled with a hydrogen amount of 1 to several ppm level. However, since hydrogen cracking occurs with a small amount of hydrogen, hydrogen cracking cannot be completely prevented with this level of control. In particular, in the case of large products, the rate of decrease in hydrogen content due to long-time holding at high temperatures is slow, and the effect of preventing hydrogen cracking is small compared to the time and cost. Further, the means by increasing the S concentration does not lead to much improvement in hydrogen cracking resistance in carbon steel. In addition, the means for increasing the Ti, Zr, Hf, and Nb concentrations certainly improve the hydrogen cracking resistance, but the increase in these concentrations increases the cost of the product, but the hydrogen cracking resistance commensurate with the cost increase. Is not effective.

  The present invention has been made paying attention to the above-mentioned circumstances, and it is intended to improve the hydrogen cracking resistance of a forged steel product made of carbon steel by controlling the form of the structure without using the means of adding alloy components. Objective.

The forged steel product of the present invention that has achieved the above object is:
C: 0.15 to 0.5% (meaning mass%; the same applies to the component composition),
Si: 0.6% or less (excluding 0%),
Mn: 0.5 to 1.5%
Ni: 0.1 to 2.5%,
Cr: 0.1 to 2.5%,
Mo: 0.01 to 0.7%,
S: 0.0002 to 0.01%
O: 0.002% or less (excluding 0%),
And the balance consists of iron and inevitable impurities,
The steel cross section at the position of depth D / 4 (D: equivalent circle diameter of the cross section of the forged steel product) is described as a healthy part and a remaining part (hereinafter referred to as “macro-segregation part”) composed of a ferrite structure or a ferrite-pearlite mixed structure. ), And the ratio of the healthy part to the steel cross section is 90 area% or more, and (pearlite average particle diameter) / (ferrite average particle diameter) in the macrosegregation part is 3.0 or more. It is a product.

  It is preferable that (average particle diameter of pearlite) / (average particle diameter of ferrite) in the macrosegregation part is 3.5 or more, and further, the cementite interval in the pearlite is 0.30 μm or more.

  The forged steel product of the present invention may further contain Cu: 0.5% or less (excluding 0%).

  The forged steel product of the present invention may further contain V: 0.3% or less (excluding 0%).

  The forged steel product of the present invention may further contain Ca: 0.1% or less (excluding 0%).

  The forged steel product of the present invention may further contain at least one selected from the group consisting of Ti, Zr, and Hf: a total of 0.1% or less (excluding 0%).

In the forged steel product, the inclusions observed at the macrosegregated portion at the position of depth D / 4 (D: equivalent circle diameter of the cross section of the forged steel product) all have a major axis of 20 μm or less and a major axis of 1 to It is recommended that the density of inclusions of 10 μm is 5 to 500 / cm ² .

  The forged steel product of the present invention is used, for example, as a crank journal or a crank throw. The present invention also provides an assembled crankshaft having the crank journal or the crank throw.

  In the forged steel product of the present invention, (average particle size of pearlite) / (average particle size of ferrite) in the macro-segregation part is 3.0 or more, that is, the ferrite particles causing hydrogen cracking are relatively small. By doing so, the cause of hydrogen cracking propagating between a plurality of ferrite grains and pearlite grains can be suppressed, so that a forged steel product having improved hydrogen cracking resistance can be provided.

It is a photograph of the macrosegregation part which appears in the section of a forged steel product. 4 is a schematic graph in which the horizontal axis represents the pearlite particle size ratio, and the vertical axis represents the time (relative value) taken until cracking occurred in the forged steel product. (A) is the SEM image of the healthy part of a forged steel product, (b) is the SEM image of the macrosegregation part of a forged steel product. (A) is the SEM image of the macrosegregation part of the conventional forged steel goods, (b) is the one part schematic diagram. (A) is the SEM image of the macrosegregation part of the forged steel product of this invention, (b) is the one part schematic diagram. It is an example of the heat processing for manufacturing the forged steel product of this invention.

  With the goal of elucidating the cause of hydrogen cracking, the inventors have investigated the effect of the steel structure on hydrogen cracking of forged steel products (especially large forged products: meaning forged steel products weighing 20 tons or more). The investigation proceeded from the correlation between the crack initiation point and the hydrogen behavior during cooling. As a result, forged steel products (especially large forged steel products) have a macro-segregation part of alloying elements unlike continuous cast materials, and hydrogen cracking tends to occur from the segregation part due to the concentration of hydrogen in the macro-segregation part. I found. This phenomenon is estimated to occur as follows.

  That is, after the ferrite transformation during cooling, the steel structure becomes a mixed state of ferrite and austenite. Since ferrite and austenite are different in hydrogen solid solubility and hydrogen diffusion rate, hydrogen is concentrated in the austenite part. When austenite is transformed into ferrite, pearlite, or bainite, hydrogen is concentrated in the strained portion accompanying the transformation, and as a result, it is estimated that hydrogen cracking occurs.

  For reference, FIG. 1 shows a photograph of a macrosegregation portion appearing in a cross section of a forged steel product. The left part of FIG. 1 shows a streak-like macro segregation part that can be observed without using a microscope, and the right part of FIG. 1 is a micrograph focusing on one macro segregation part. The part that appears whitish other than the streak-like macro-segregation part is a healthy part.

  The structure of the macro-segregation part is mainly composed of pearlite structure in general carbon steel, and cracks are generated starting from proeutectoid ferrite existing at the grain boundary triple point between pearlite structures. Propagates the grain boundary between ferrite grains and pearlite grains.

  Therefore, in order to improve hydrogen cracking resistance, it is considered that one of the effective means is to prevent the formation of these macro-segregation parts, but especially in the case of large forged steel products, the cooling rate is reduced in all parts of the steel material. Since it is difficult to make it uniform, there is a limit to the suppression of the macro segregation part.

  In general forged steel products (particularly large forged steel products), the cooling rate is slow (about 1 to 2 ° C./min), and thus the structure is mainly composed of pearlite. Therefore, the present inventor controls the formation of macro segregation part (particularly segregation part mainly composed of pearlite structure) to suppress the hydrogen concentration in the macro segregation part, and to generate and propagate (propagation) cracks. It has been found that a forged steel product having excellent hydrogen cracking resistance can be obtained by suppressing it.

  A forged steel product excellent in hydrogen cracking resistance according to the present invention has a predetermined chemical component and is observed in a steel cross section at a position of depth D / 4 (D: equivalent circle diameter of cross section of forged steel product). The structure is 90 area% or more of ferrite structure or ferrite-pearlite mixed structure, and the ratio of pearlite average particle diameter / ferrite average particle diameter is 3.0 or more in the remaining macrosegregation part (perlite is the main structure). As a result, hydrogen concentration at the strained portion in the macro segregation portion is prevented, and good hydrogen cracking property can be secured.

  Furthermore, it is preferable that the cementite interval in pearlite is 0.30 μm or more as a structure in which cracks are more difficult to propagate.

  Hereinafter, the basic configuration of the present invention will be described in order. First, the content of chemical components suitable for the forged steel product of the present invention will be described.

1. Chemical composition of forged steel C: 0.15 to 0.5%
C is an element that contributes to improving the strength of forged steel products. In order to ensure sufficient strength for the forged steel product, it is desirable to contain C in an amount of 0.15% or more, preferably 0.20% or more, more preferably 0.30% or more. However, if the amount of C is too large, the toughness of the forged steel product is deteriorated. Therefore, it is suppressed to 0.5% or less, preferably 0.45% or less, more preferably 0.42% or less.

Si: 0.6% or less (excluding 0%)
Since Si is a deoxidizing element and acts as an element for improving the strength of forged steel products, it is allowed to be contained. However, if the amount of Si is too large, the reverse V segregation of the forged steel product becomes remarkable and coarse inclusions are generated. Therefore, it is 0.6% or less, preferably 0.45% or less, more preferably 0.35% or less ( 0% not included).

Mn: 0.5 to 1.5%
Mn is an element that enhances the hardenability of the forged steel product and contributes to improving the strength. In order to ensure sufficient strength and hardenability, 0.5% or more, preferably 0.7% or more, more preferably 0.8%. Add 9% or more. Note that Mn is an element that promotes bainite formation, and if the amount of Mn is too large, it promotes reverse V segregation, so 1.5% or less, preferably 1.45% or less, more preferably 1.4% or less. And

Ni: 0.1 to 2.5%
Ni is an element useful as an element for improving the toughness of forged steel products, and is contained in an amount of 0.1% or more, preferably 0.15% or more. On the other hand, if the amount of Ni becomes excessive, the cost increases, so it is 2.5% or less, preferably 2.0% or less, more preferably 1.5% or less.

Cr: 0.1 to 2.5%
Cr is an element that enhances the hardenability of the forged steel product and improves the toughness. Their action is effectively exhibited by containing 0.1% or more, preferably 0.2% or more. Note that Cr is an element that promotes the formation of bainite, and if the amount of Cr is too large, reverse V segregation is promoted and coarse inclusions are formed, so 2.5% or less, preferably 2.3% or less. Is desirable.

Mo: 0.01 to 0.7%
Mo is an element that effectively acts to improve the hardenability, strength, and toughness of the forged steel product, and in order to exert these effects effectively, it is desirable to contain 0.01% or more, preferably 0.05% or more. . However, since Mo has a small equilibrium distribution coefficient, when the amount of Mo becomes excessive, microsegregation (normal segregation) is likely to occur. Moreover, when the amount of Mo becomes excessive, it leads to a cost increase. Therefore, the Mo amount is 0.7% or less, preferably 0.5% or less.

S: 0.0002 to 0.01%
S combines with Mn, Mg, Ca, etc. in the steel and promotes reverse V segregation to form S-based inclusions. S-type inclusions having an elongated shape are likely to be coarse inclusions having a large major axis and can be the starting point of hydrogen cracking. Therefore, in order to reduce coarse S-based inclusions, the S content is set to 0.01% or less, preferably 0.0015% or less. On the other hand, the fine S-based inclusions in the macro-segregation part structure form a large number of stress fields, easily capture surplus hydrogen, and have an effect of improving the hydrogen cracking resistance of the macro-segregation part structure. In order to ensure such fine S-based inclusions, the S content is set to 0.0002% or more, preferably 0.0004% or more, more preferably 0.0006% or more.

O: 0.002% or less (excluding 0%)
O (oxygen) is an element that forms oxide inclusions such as SiO ₂ , Al ₂ O ₃ , MgO, and CaO. By reducing O as much as possible, coarse inclusions can be suppressed and fine inclusions can be precipitated. Therefore, the amount of O is made 0.002% or less, preferably 0.001% or less. However, it is difficult to make O (oxygen) 0% in industrial production.

  The basic components of the forged steel product (steel) used in the present invention are as described above, and the remaining component is substantially iron, but it is of course acceptable to mix inevitable impurities. Further, the forged steel product of the present invention may further contain other elements in a range that does not adversely affect the effects of the present invention.

Cu: 0.5% or less (excluding 0%)
Cu has the effect of improving the toughness of the forged steel product and the refinement of the structure, and may be contained in the steel in order to exert these effects. In order to effectively exhibit such an action, for example, it is recommended to contain Cu by 0.01% or more. However, if the amount of Cu becomes excessive, the cost increases, so it is 0.5% or less, preferably 0.4% or less (excluding 0%).

V: 0.3% or less (excluding 0%)
V has the effect of strengthening precipitation and refining the structure of forged steel products, and is an element useful for increasing the strength. In order to effectively exhibit such an action, it is recommended to contain V by 0.01% or more, preferably 0.02% or more. However, even if V is contained excessively, the above action is saturated and economically wasteful, so the amount is desirably 0.3% or less, preferably 0.15% or less (excluding 0%). .

Ca: 0.1% or less (excluding 0%)
Ca has an action capable of suppressing the stretchability of sulfide in a forged steel product, and in order to exert this action effectively, for example, 0.0001% or more may be contained in the steel. However, even if the amount of Ca is excessive, this action is saturated, so the content is made 0.1% or less (excluding 0%).

Ti, Zr, Hf: 0.1% or less in total (excluding 0%)
Ti, Zr, and Hf have the effect | action of the toughness improvement and structure refinement | miniaturization of a forged steel product, In order to exhibit these effect | actions effectively, you may contain any 1 or more types in steel. In order to exert these effects, for example, 0.001% or more may be contained in the steel. However, when the Ti, Zr, and Hf contents become excessive, coarse carbides are precipitated, so the total amount is 0.1% or less, preferably 0.01% or less (excluding 0%).

  Examples of other elements that are allowed to be positively added are Al (for example, 0.001% or more and 0.01% or less), which is effective for deoxidation and cracking resistance of steel in the steelmaking process, and hardenability improvement. Examples include B having an effect, W, Nb, Ta, Ce, Zr, and Te, which are solid solution strengthening elements or precipitation strengthening elements, and these can be contained alone or in combination of two or more. These additive elements are preferably about 1% or less in total. As will be described later, it is desirable to control N which is an inevitable impurity in order to introduce fine inclusions.

2. Forged steel structure fraction (1) Basic structure The steel structure observed in the steel cross section at a depth of D / 4 has a ferrite structure or ferrite in order to obtain the necessary tensile strength and required forgeability of the forged steel product. And the pearlite mixed structure is 90 area% or more (preferably 95 area% or more, more preferably 97 area% or more). In the portion corresponding to the ferrite structure or the ferrite and pearlite mixed structure, hydrogen cracking is rarely generated. Therefore, this portion is hereinafter referred to as “sound part”. The remaining portion is described as a “segregation portion” and is a structure mainly composed of a pearlite structure, which will be described in detail later. As another structure, a retained austenite structure or the like may exist. In the present invention, the depth D / 4 refers to the distance from the side surface of the forged steel product. The “D” means a circle equivalent diameter (a diameter of a circle having the same area as the cross-sectional area) of a cross section of the forged steel product (a cross section perpendicular to the longitudinal direction of the forged steel product). Therefore, if the forged steel product is cylindrical, D is the diameter of the circle.

(2) Discrimination between healthy part and macro-segregation part A method for discriminating between a healthy part and a macro-segregation part will be described. First, a 5 cm × 5 cm region of the steel cross section is photographed and imported to electronic equipment as grayscale data. Since the macro-segregation part is mainly composed of a pearlite structure (the pearlite structure is approximately over 70% by area), the macro segregation part appears darker than the healthy part (the pearlite structure is approximately 50 to 70% by area) (see FIG. 1 described above). Using this, {(average lightness of healthy part) + (average lightness of macro-segregation part)} / 2 is set as a threshold value, and (white) part having a lightness higher than this threshold value is determined as a healthy part by image processing. On the contrary, the part (black) whose brightness is lower than the threshold value is defined as a macro-segregation part. By such a method, the healthy part and the macro-segregated part are distinguished from each other, and the ratio (area%) of the healthy part to the steel cross section is obtained.

  Using the above method, the above-described image processing is performed in another observation region (5 cm × 5 cm), and the average value of these two fields of view is used as the final healthy part ratio (area%). In the present invention, in order to obtain the necessary tensile strength and necessary forgeability for forged steel products, the proportion of the healthy part (ferrite structure or ferrite and pearlite mixed structure) is set to 90 area% or more as described above. A value obtained by subtracting the percentage of healthy part (area%) from 100 area% is defined as the percentage of macro segregated part (area%).

Average brightness of healthy and macrosegregated areas:
The average brightness of the healthy part is determined as follows. First, in order to confirm that a region having a relatively high brightness (white) in a 5 cm × 5 cm region is a healthy part, tissue observation is performed using an FE-SEM described later. After confirming that the structure is a healthy part (ferrite structure or ferrite and pearlite mixed structure), a value obtained by averaging the lightness of any 10 points of the healthy part is defined as the average brightness. Similarly, for the macro segregation part, the average brightness of the macro segregation part is a value obtained by averaging the lightness values of any 10 points of the macro segregation part.

3. Structure and grain size of macro segregation zone Hydrogen cracking of forged steel products mainly occurs at the macro segregation zone. In many cases, the crack does not pass through the interface of the macro-segregation part but passes through the macro-segregation part. Since the occurrence and propagation of hydrogen cracking is presumed to depend on the structure of the segregation part, it is important to control the structure of the macrosegregation part in order to suppress hydrogen cracking. When the occurrence and propagation of hydrogen cracking is observed in detail, the structure of the macrosegregation part is mainly composed of pearlite structure, and ferrite (so-called pro-eutectoid) is formed at the grain boundary triple point of the pearlite structure (the boundary between the three pearlite grains). It is inferred that hydrogen cracks occur due to transformation at the interface between pro-eutectoid ferrite and pearlite grains, and hydrogen cracks propagate along the respective crystal grain boundaries. From such inference, in order to suppress the propagation of hydrogen cracking in the structure mainly composed of pearlite, the value of the ratio of (average particle size of pearlite) / (average particle size of ferrite) in the macro segregation part (hereinafter referred to as “ Need to be controlled). The meaning of the average particle diameter will be described later.

  When coarse pro-eutectoid ferrite is generated at the grain boundary triple point of the pearlite structure, the strain at the grain boundary triple point at which the hardness difference between the pearlite structure and the ferrite structure is generated tends to be concentrated. In addition, since the hydrogen diffusion coefficient differs between the ferrite structure and the pearlite structure, hydrogen tends to concentrate at the grain boundary triple points. By overlapping these, hydrogen cracks are likely to occur at the grain boundary triple points, and it is assumed that the generated cracks propagate along the grain boundaries of these structures where there is a difference in hardness between the ferrite structure and the pearlite structure.

  When the pearlite particle size ratio in the macro-segregation part is about 2 or more and less than 3.0 as in the past, the present inventors have a structure form in which coarse pro-eutectoid ferrite is generated in the pearlite structure, and the grain boundaries of the pearlite grains Because the size of proeutectoid ferrite existing at the triple point increases, and cracks that occur when the proeutectoid ferrites are connected to each other easily propagate through the grain boundaries of pearlite grains, hydrogen concentration and cracking are promoted. I found a tendency. Therefore, the pearlite particle size ratio in the macro segregation part is controlled to 3.0 or more to suppress the formation of coarse pro-eutectoid ferrite.

  The pearlite particle size ratio is preferably 3.2 or more, more preferably 3.5 or more. The upper limit of the pearlite particle size ratio is not particularly defined. However, when it is about 20, the structure form is substantially pearlite mainly, and the influence of proeutectoid ferrite precipitated at the grain boundary triple points is eliminated. 10 may be the upper limit.

  It should be noted that if a coarse pearlite structure is present in the macro-segregation part, crack propagation is likely to occur, and if coarse pearlite grains are present, the linearity of the structure is considered to be higher and cracks are likely to propagate. It is desirable to suppress the formation of pearlite grains. Therefore, it is desirable to control the maximum particle size of the pearlite structure to 100 μm or less. The thickness is preferably 70 μm or less, more preferably 50 μm or less. Furthermore, since it is considered that cracks are likely to propagate particularly when the pearlite structure particle size is high, the ratio of (maximum pearlite particle size) to (average particle size of pearlite) is preferably 3.0 or less. .

  As described above, the generated crack propagates at the interface between the ferrite structure and the pearlite structure or the pearlite grain boundary. In the case of pearlite with a narrow cementite interval, since the hardness is generally high, the strain at the grain boundary triple point is also large. Furthermore, since the hydrogen diffusion coefficient of pearlite with a narrow cementite interval decreases, hydrogen tends to concentrate in the pearlite and tends to promote crack propagation at the interface between the ferrite structure and the pearlite structure or at the pearlite grain boundary. . In order to suppress crack propagation, the cementite spacing in the pearlite is preferably 0.30 μm or more. More preferably, it is 0.4 μm or more.

  FIG. 2 is a schematic graph in which the horizontal axis represents the pearlite particle size ratio and the vertical axis represents the time (relative value) taken until cracking occurred in the forged steel product. As shown in FIG. 2, the portion having a pearlite particle size ratio of about 1 is a portion representing a healthy portion of a conventional or forged steel product of the present invention, and the portion having a pearlite particle size ratio of about 2.5 is a conventional forged steel product. The part of 3.0 or more shows the macro-segregation part of the forged steel product of the present invention.

  From FIG. 2, the macrosegregation part of the conventional forged steel product is inferior in hydrogen cracking resistance, but by making the average particle size of ferrite relatively small with respect to the average particle size of pearlite (the pearlite particle size ratio is increased). Thus, it can be seen that even the macro-segregation part exhibits the same hydrogen cracking resistance as the healthy part.

  For reference, FIG. 3A is an SEM image of a healthy portion of a forged steel product, and FIG. 3B is an SEM image of a macro-segregated portion of the forged steel product. 4A is an SEM image of a macro segregation part of a conventional forged steel product, FIG. 4B is a schematic diagram of a part thereof, FIG. 5A is an SEM image of a macro segregation part of the forged steel product of the present invention, b) is a partial schematic view thereof.

4). Inclusions Coarse inclusions in the structure of the macro-segregation part serve as starting points and propagation paths for hydrogen cracking and adversely affect hydrogen cracking resistance. When coarse inclusions having a major axis exceeding 20 μm are present, the influence of hydrogen cracking at the interface between the inclusions and the steel structure becomes larger than the influence due to the size of the crystal grain size described above. Accordingly, it is recommended that the longest inclusion observed in the 2 mm × 2 mm field of view of the macro steel cross section be controlled to preferably 20 μm or less, more preferably 15 μm or less.

Conversely, when fine inclusions with a major axis of 1 to 10 μm are present in the macrosegregation structure, a large number of stress fields are formed, and it is easy to capture surplus hydrogen exceeding the solid solubility limit, and hydrogen concentration in the strained part Therefore, hydrogen cracking resistance is improved. Therefore, the density of inclusions having a major axis of 1 to 10 μm in the structure of the macrosegregation part is 5 pieces / cm ² or more, preferably 10 pieces / cm ² or more, more preferably 20 pieces / cm ² or more. On the other hand, when the density of the fine inclusions is excessive, it adversely affects mechanical properties such as toughness of the forged steel product, and therefore is 500 pieces / cm ² or less, preferably 200 pieces / cm ² or less, more preferably 100 pieces / cm ² or less. Fine inclusions with a major axis of less than 1 μm also have the effect of improving hydrogen cracking resistance. However, in order to improve measurement efficiency, fine inclusions with a major axis of less than 1 μm were excluded from consideration.

  The kind of the inclusion is not limited, and examples thereof include S-based inclusions; Ti-based inclusions; oxide-based inclusions such as Al, S, Ca, Mg, and Mn; In the present invention, the longest inclusion and the density of inclusions having a longest diameter of 1 to 10 μm are values observed at a position of depth D / 4 (D: equivalent circle diameter of forged steel) from the surface. These values can be measured by the methods shown in the following examples.

5. Manufacturing Method (1) Structure Control The forged steel product of the present invention can be manufactured by controlling the steel structure and inclusions as follows, for example. As a method of controlling the steel structure, (A) in order to make the ferrite-pearlite mixed structure 90 area% or more, and (B) to make the pearlite particle size ratio 3.0 or more, quenching from a temperature of Ac ₃ points or more. Alternatively, the austenite grain size can be reduced by repeating a basic process including a heat treatment process such as normalization and a molding process into a product shape (for example, hot forging or cold forging) once or 2 to 5 times. As a result, the crystal grain size of the pearlite structure can be reduced, the variation of the pearlite structure grain size can be reduced, and the cementite interval in the pearlite can be increased. Further, the pearlite particle size ratio increases as the number of basic steps is increased. If only a ferrite-pearlite mixed structure is obtained, heating to Ac ₁ to Ac ₃ points (austenite-ferrite two-phase temperature range) is sufficient, but the requirements of the present invention are not satisfied.

  In order to refine the austenite grains, it is desirable to perform the molding process at a high processing rate. Further, the crystal grain size of the pearlite structure can be refined even if the molding step is performed during heat treatment such as quenching or normalization. FIG. 6 is an example of heat treatment for producing the forged steel product of the present invention, and shows an example in which the above basic process is performed twice.

In the cooling step of the heat treatment step, (A) hydrogen is diffused from the ferrite structure or ferrite-pearlite mixed structure to the austenite structure (that is, the amount of hydrogen in the ferrite structure or ferrite-pearlite mixed structure is reduced), and (B ) In order to make the ratio of the macrosegregation part less than 10% by area, natural cooling is performed so that it passes from Ac ₃ point or more to Fs point (ferrite transformation start temperature) to Ps point (pearlite transformation start temperature) of the continuous cooling transformation curve. (5 ° C./min or less). Slow cooling like natural cooling can increase the cementite spacing in the pearlite, but the effect is higher when there are fewer alloy components of Mn, Cr, Mo, V. In order to make the cementite interval 0.30 μm or more, it is necessary that Mn ≦ 1.4%, Cr ≦ 2.3%, Mo ≦ 0.5%, and V ≦ 0.15%.

  After the heat treatment as described above, the forged steel product is finally homogenized at, for example, 600 ° C. to 900 ° C. for 1 hour to 20 hours, and then tempered.

  In the heat treatment as described above, the temperature may be controlled in the vicinity of a depth D / 4 from the surface, but it is difficult to measure the temperature at this position in a large steel ingot. Therefore, for example, a forged steel product may be manufactured by controlling the temperature based on the surface temperature, and the temperature control may be appropriately performed by feeding back the result of the structure observation.

(2) Control of Inclusions In the present invention, there is no limitation on the method of controlling inclusions, but it is important to control the amounts of Si, Cr, S, and O in the component range of the present invention. In order to prevent hydrogen cracking, it is desirable to actively add elements that form fine inclusions and increase the inclusions that crystallize during solidification, but the amount added is such that coarse inclusions are not formed. It is desirable to suppress to the amount.

  Furthermore, it is good to control S type inclusions and Ti type inclusions by the following method. For example, by adding Ca that has the effect of spheroidizing S-based inclusions, the abundance ratio of CaS inclusions with a small major axis is increased, the average size of S-based inclusions is decreased, and the number of fine inclusions is increased. be able to. In addition, when Ti is added to form a precipitate, most of Ti is combined with N in the steel to become a nitride. Therefore, the size of Ti inclusions can be controlled by controlling the amount of Ti added according to the amount of N in the steel. These inclusions may be present not only in the segregated portion mainly composed of the pearlite structure, but also in the ferrite structure and the pearlite structure (that is, the entire forged steel product). However, when considering the mechanical properties of the forged steel product, it is better to have less inclusions in the ferrite or pearlite structure.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

(Evaluation with preliminary specimen)
First, 150 kg of an ingot having chemical components A to N shown in Table 1 was melted by vacuum induction melting. Next, the solidified steel ingot was demolded, and then heated to about 1200 ° C. to perform hot forging to finish a forged material having a cross-sectional diameter of about 100 mm. Hot forging was performed by extending the steel ingot body with a press and then forming it into a round cross section using a dedicated tool.

Then, this steel ingot is heated to 870 ° C. at 50 ° C./hour (the temperature of all the listed steels is not less than ₃ points of Ac) and held for 2 hours, and then cooled to room temperature (1-2 ° C./min) Was repeated once or twice to control the crystal grain size.

  Subsequently, it heated to 640 degreeC with the temperature increase rate of 50 degreeC / hour, and it hold | maintained at this temperature for 10 hours, Then, the preliminary test piece (A-N) was obtained.

Further, the steel type A was formed into a round cross section after hot forging in the same manner as described above, and thereafter the steel ingot was formed at a temperature of 720 ° C. at 50 ° C./hour (at a temperature not lower than Ac ₁ point of steel A and less than Ac ₃ points). The temperature was kept for 2 hours and then air-cooled to room temperature (1-2 ° C./min) once. Subsequently, it heated to 640 degreeC with the temperature increase rate of 50 degreeC / hour, and it hold | maintained at this temperature for 10 hours, Then, the preliminary test piece Aa was obtained by standing to cool (1 degrees C / min or less). The production conditions for each preliminary test piece are listed in Table 2.

The preliminary test piece has an EBSP (Electron Back Scattering Pattern) detector after taking a sample from a position at a depth D / 4 from the side surface and performing electropolishing to prevent transformation of retained austenite. The tissue type and area ratio were measured with a FE-SEM (Field Emission Type Scanning Electron Microscope). EBSP determines the crystal orientation of the electron beam incident position by making an electron beam incident on the sample surface and analyzing the Kikuchi pattern obtained from the reflected electrons generated at this time. If the crystal orientation is measured at a predetermined pitch, the orientation distribution on the sample surface can be measured. The sample set in the lens barrel of the SEM apparatus is irradiated with an electron beam at intervals of 0.1 μm in a measurement range of 150 μm × 150 μm, and an EBSP image projected on the screen is taken with a high-sensitivity camera, and is sent to a computer Each hue (each structure) was color-mapped by taking it as an image, analyzing the image with a computer, and comparing it with a pattern obtained by simulation using a known crystal system. The area ratio of each tissue (region) mapped in this way was determined. Moreover, the crystal grain distribution was obtained from all the crystal grains in the field of view of each structure, and the average grain size and the maximum grain size were calculated. The average particle diameter means an arithmetic average (arithmetic mean) value of the particle diameter (equivalent circle diameter) in the observation visual field, and the maximum particle diameter means the maximum particle diameter in the observation visual field. In addition, as hardware and software related to the above analysis, TexSEM Laboratories Inc. OIM (Orientation Imaging Microscopy ^™ ) system can be used.

  In addition, using the SEM device, the steel cross section is observed at a magnification of about 100 times in a measurement range of 2 mm × 2 mm, the maximum major axis of inclusions and the number of inclusions having a major axis of 1 to 10 μm are measured, and the density is calculated. did.

The hydrogen cracking resistance of each preliminary test product was evaluated by the time until hydrogen cracking breakage (hereinafter abbreviated as “hydrogen cracking time”) as follows. In the large product, cracking occurs due to the hydrogen present in the original, but in the small product such as the preliminary test product, the hydrogen escapes, so this hydrogen cracking resistance was evaluated by charging with hydrogen. Each preliminary test product is processed into a dumbbell shape with a length of 150 mm and a distance between marked lines of 10 mm, a central part is 4 mm in diameter, a gripping part at both ends is 8 mm in diameter, and a screw is provided over a length of 15 mm. A test piece for evaluation of crackability was prepared. The test piece was completely immersed in the aqueous solution so as to be surrounded by ₂ mol / L H ₂ SO ₄ +0.01 mol / L KSCN aqueous solution. Each test piece is immersed in the aqueous solution, catholyzed at a current density of 1.0 A / dm ^2, and a tensile load of 350 MPa, which is below the yield point, is applied in the major axis direction while adding hydrogen, until breakage occurs. The time (hydrogen cracking time) was measured. This test was performed three times for each preliminary test product, and the average value of the hydrogen cracking time was determined. The results are shown in Table 2. The test was conducted for up to 100 hours, and those that did not break still were described as “> 100” in Table 2. Those whose hydrogen cracking time is 50 hours or less are evaluated as being poor in hydrogen cracking resistance, those whose time exceeds 50 hours are evaluated as being excellent in hydrogen cracking resistance, and those exceeding 85 hours are hydrogen cracking resistance It was evaluated as being particularly excellent.

  Preliminary test piece No. 1 that does not satisfy any of the requirements of the present invention (1) chemical component, (2) healthy part area ratio (90 area% or more), and (3) pearlite particle size ratio (3.0 or more). Nos. 3, 16 to 19 have a hydrogen cracking time of 50 hours or less and are inferior in hydrogen cracking resistance. On the other hand, preliminary test piece No. The hydrogen cracking time of 1,2,4-15,20,21 greatly exceeds 50 hours. From these results, it is understood that the hydrogen cracking resistance is improved if the pearlite particle size ratio of the present invention and the requirements of the predetermined structure form are satisfied.

(Evaluation by this test)
With reference to the results of the preliminary test steel as described above, the test product was manufactured from a large steel ingot and evaluated. Steel with chemical composition shown in Table 1 above, steel type B, steel type E, steel type F or K, respectively, is melted by a molten steel processing facility equipped with an electrode arc heating function, and a 40 ton class (total height 3 m, diameter 1.5 m) Cast into a mold. The amount of hydrogen in the molten metal stage was 3 ppm as measured by Hydris. The solidified steel ingot was demolded at around 1000 ° C., then heated to about 1200 ° C., hot forged at the same temperature, and finished into a forged product having a cross-sectional diameter of 150 mm. In hot forging, the steel ingot body was stretched with a press machine and then formed into a round cross section using a dedicated tool.

  Each forged product is slowly cooled naturally from the surface temperature of about 900 ° C. to room temperature (about 0.5 ° C./min), then heated again to about 600 ° C., and then slowly cooled to room temperature (about 0.5 ° C./min).

  The test product was manufactured as described above. The microstructure at the position of depth D / 4 from the surface is a ferrite-pearlite mixed structure for steel types B, E and F, and the area ratio is 90% or more, and the pearlite in the macrosegregation part. The particle size ratio was 3.0 or more. On the other hand, the steel type K also had a ferrite-pearlite mixed structure, but its area ratio was less than 90%, and the pearlite particle size ratio at the macrosegregation part was less than 3.0.

  The evaluation of hydrogen cracking resistance of this test product was examined by using the above-mentioned SEM apparatus for the presence of fine cracks in the ferrite-pearlite structure at a measurement range of 150 μm × 150 μm and a magnification of 1000 times. The results are shown in Table 3. In addition, it was determined that a crack occurred when a crack having a length of 1 μm or more was present.

  As can be seen from Table 3, the steel type K which does not satisfy any of the requirements of the present invention (chemical composition, healthy part area ratio, pearlite particle size ratio) caused fine cracks, but the steel type B satisfies all the requirements of the present invention. , E, and F, it was found that fine cracks did not occur and hydrogen cracking resistance was improved.

(Measurement of cementite spacing)
Next, among the above samples, the test No. 1 in which the pearlite particle size ratio is in a suitable range (3.5 or more). About 1,2,4,5,7,9-13,20,21, the steel cross-section was observed by the magnification of about 1000 times, and the cementite space | interval in pearlite was measured. As a result, as shown in Table 4 below, the test No. 1 in which the cementite interval in the pearlite is 0.30 μm or more is obtained. For Nos. 1, 2, 4, 5, 7, and 9-13, the hydrogen cracking time exceeded 85 hours as shown in Table 2, while the cementite spacing in pearlite was less than 0.30 μm. As for Tables 20 and 21, as shown in Table 2, the hydrogen cracking time was less than 85 hours. From the above results, it can be seen that hydrogen cracking resistance is further improved when the pearlite particle size ratio is 3.5 or more and the cementite interval in the pearlite is 0.30 μm or more. In addition, about the measurement of a cementite space | interval, ten pearlite grains were arbitrarily selected in the said observation visual field of a steel cross section, and the average value of the cementite space | interval in each pearlite grain was taken. The cementite spacing in one pearlite grain was determined by dividing the length of the pearlite grain in the direction perpendicular to the cementite layer by the number of cementite layers.

Claims (9)

C: 0.15 to 0.5% (meaning mass%; the same applies to the component composition),
Si: 0.6% or less (excluding 0%),
Mn: 0.5 to 1.5%
Ni: 0.1 to 2.5%,
Cr: 0.1 to 2.5%,
Mo: 0.01 to 0.7%,
S: 0.0002 to 0.01%
O: 0.002% or less (excluding 0%),
And the balance consists of iron and inevitable impurities,
The steel cross section at the position of depth D / 4 (D: equivalent circle diameter of the cross section of the forged steel product) is described as a healthy part and a remaining part (hereinafter referred to as “macro-segregation part”) composed of a ferrite structure or a ferrite-pearlite mixed structure. ), The ratio of the healthy part to the steel cross section is 90 area% or more, and (average particle diameter of pearlite) / (average particle diameter of ferrite) in the macrosegregation part is 3.0 or more. Forged steel products characterized by   2. The forged steel product according to claim 1, wherein (average particle diameter of pearlite) / (average particle diameter of ferrite) in the macrosegregation portion is 3.5 or more, and a cementite interval in the pearlite is 0.30 μm or more.   The forged steel product according to claim 1 or 2, further comprising Cu: 0.5% or less (not including 0%).   The forged steel product according to any one of claims 1 to 3, further comprising V: 0.3% or less (not including 0%).   Further, the forged steel product according to any one of claims 1 to 4, further comprising Ca: 0.1% or less (not including 0%).   Furthermore, the forged steel products in any one of Claims 1-5 which contain any 1 or more types selected from the group which consists of Ti, Zr, and Hf in total 0.1% or less (0% is not included). The inclusions observed in the macrosegregation part at the position of depth D / 4 (D: equivalent circle diameter of the cross section of the forged steel product) are all inclusions whose major axis is 20 μm or less and whose major axis is 1 to 10 μm. density forgings according to claim 1 which is 5 to 500 pieces / cm ^2.   The forged steel product according to claim 1, which is a crank journal or a crank throw.   9. An assembled crankshaft comprising the crank journal or crank throw according to claim 8.