JP6443324B2 - Steel material and manufacturing method thereof - Google Patents

Description

The present invention relates to a steel material, particularly a steel material having an excellent strength-toughness balance and excellent surface hardenability. Specifically, after manufacturing by hot working such as hot rolling, heat treatment such as normalizing, quenching, and tempering is not performed, but cutting is performed directly from the state of hot working and then induction hardening is performed. The present invention relates to a non-heat treated steel material excellent in strength-toughness balance, which is suitable as a material to be manufactured, specifically, a material for a shaft of a construction machine or industrial machine, a pin of an automobile, or the like. As a strength-toughness balance suitable for this type of component, etc., it is necessary to ensure a strength of 900 MPa or more and a toughness of 40 J / cm ² or more.

  Originally, steel is an excellent material that can achieve refinement of the structure and improvement in strength by introducing a large amount of strain by heat treatment (quenching) without introducing strain into the metal structure by processing. On the other hand, there is a strong demand to reduce energy consumption in the above heat treatment and to omit the heat treatment step itself.

  As a method for responding to such a request, for example, a precipitate forming element such as V is added to steel, and fine precipitates of V and carbon are dispersed in the steel at the time of cooling after hot working. There is a technique to increase the strength of the steel material after processing. Thus, the steel which makes it possible to omit the heat treatment by adding a precipitate forming element is called non-tempered steel.

  However, additive elements such as V are expensive, and there are merits in energy consumption reduction and process saving by omitting heat treatment, but a rise in the price of steel used as a material for parts is inevitable.

  On the other hand, Patent Document 1 proposes a method of obtaining high strength (high yield strength and yield ratio) using Nb carbonitride instead of V. If the steel described here is used, it is possible to maintain good mechanical properties and to reduce expensive additive elements for converting the steel into non-tempered steel.

Patent Publication 2010-280978

  By the way, parts of construction machines and industrial machines are often subjected to induction hardening to obtain final products. In the above-mentioned non-heat treated steel, when the surface layer is hardened by induction hardening, it is important to improve the quality of the surface layer part, specifically, to suppress the variation in hardness of the induction hardened layer. .

  The present invention has been made in view of the current situation, the purpose of which is a steel material with less variation in hardness in the induction-hardened layer when induction-hardened, and excellent balance between strength and toughness, It is to be realized in non-heat treated steel to which V or the like is not added.

  In order to solve the above problems, the inventors diligently investigated the relationship between the hardness variation of the induction hardening layer when the induction hardening layer is formed on the surface layer of the steel material and the structure before induction hardening. It was found that the structure before induction hardening with the least variation was a martensite structure, and the present invention was conceived. The gist configuration of the present invention is as follows.

1. % By mass
C: 0.30 to 0.55%,
Si: 0.05 to 1.50%,
Mn: less than 1.55%,
Cr: 0.85 to 2.00% or less,
Al: 0.010 to 0.050%,
N: 0.0020 to 0.0200%,
Nb: 0.005 to 0.100%,
P: 0.030% or less (including 0%),
S: 0.030% or less (including 0%),
Cu: 0.20% or less (including 0%) and Ni: 0.20% or less (including 0%)
The balance is Fe and the inevitable impurities component composition, and the structure of the surface layer part from the surface to a depth of 7 mm is a martensite-based structure with a martensite area ratio of 75% or more, the martensite-based structure The lower layer of the structure has a bainite main structure with an area ratio of bainite of 75% or more, the thickness of the bainite main structure is 5 mm or more, and the hardness difference between the surface layer part and the center part is 60 or more in terms of Vickers hardness. Steel material.

2. The component composition further includes:
Ti: 0.001 to 0.100 mass%
The steel material according to 1 above, comprising:

3. The component composition further includes:
B: 0.0002 to 0.0050 mass%
The steel material as described in 1 or 2 above.

4). The steel material having the component composition described in any one of 1 to 3 above is subjected to hot working and formed into a circular cross-section material, and then cooled from a temperature range of 800 to 1000 ° C. at a depth of 7 mm from the surface. Is cooled to 10 ° C./s or more, and after the elapse of time P [s] determined by the following equation (1) from the start of the cooling, cooling is performed by switching to air cooling.

P [s] = 0.01 × r ² ± r / 10 (1)
Where r: radius of the circular cross-section material at the cooling site (mm)

According to the present invention, the hardness of the induction-hardened layer is less variable than the case where the induction-hardened layer is formed in the structure composed of conventional ferrite and pearlite, and the strength and toughness of the intermediate portion between the steel material center and the surface layer are reduced. Steel materials of 900 MPa or more and 40 J / cm ² or more can be provided.

It is the schematic which shows the hardness investigation position of an induction hardening layer. It is a figure which compares and shows the dispersion | variation in the hardness of the induction hardening layer of the steel materials from which the structure | tissue before induction hardening differs. It is a figure which compares and shows the structure after induction hardening of the steel materials from which the structure | tissue before induction hardening differs. It is a figure which shows the cooling rate and continuous cooling transformation diagram of each position of steel materials. It is a figure which shows the cooling rate and continuous cooling transformation diagram of each position of steel materials. It is a figure which compares and shows the static bending test result of the steel materials from which the hardness difference between the center of steel materials and a surface layer differs.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be specifically described.
The inventors first began studying from grasping the variation in hardness in the induction-quenched portion when induction hardening was performed on the steel material. That is, SCM435 defined in JIS G4053 was used as a raw material, and this was hot-worked into a cylindrical shape of 70 mmφ. From this hot-worked material, a round bar steel in which the entire structure is ferrite and pearlite as it is rolled, and a round bar steel whose martensite main structure is martensite with a surface area of at least 10 mm from the surface by quenching with an area ratio of 75% or more. Each of these round bar steels was induction hardened under exactly the same conditions, and the hardness and effective hardened layer depth (ECD) of the obtained induction hardened layer were investigated.
Since the induction hardened layer is uniformly obtained on the surface layer of the steel material, as shown in FIG. 1, the thickness direction (radial direction) of the induction hardened layer 2 at 12 positions T equally divided in the circumferential direction of the round steel bar 1. ) Vickers hardness was measured at intervals of 0.2 mm, and the variation in hardness at each depth position was investigated based on the hardness measurement value. The effective hardened layer depth (ECD) was measured according to JIS G0557. The results of this investigation are shown in FIG.

  As shown in FIG. 2, the variation in hardness of the round bar steel whose structure before induction hardening is ferrite and pearlite is clearly larger than that of the round bar steel whose main structure is martensite. In addition, with respect to the effective hardened layer depth (ECD), the round steel bar having a martensite main structure is deeper. From these results, it is understood that the martensite-based structure is superior in the induction hardenability when the effects of the structure are compared in steels having the same chemical composition.

  Next, a micrograph of the microstructure of the induction hardened layer is shown in FIG. From this figure, it can be seen that the black and white contrast of the round bar steel of the ferrite and pearlite structure before induction hardening is clearly larger than that of the round bar steel mainly composed of the martensite. From this, in the ferrite and pearlite structures, there is a difference in carbon concentration between the ferrite portion with a low carbon concentration and the pearlite portion with a high carbon concentration, so that spots that follow this difference remain after induction hardening, leading to variation in hardness. On the other hand, since the martensite main structure is a carbon supersaturated and homogeneous structure, it can be inferred that a homogeneous structure can be obtained even after induction hardening.

  By the way, when the structure before induction hardening is martensite-based, the hardness variation and ECD after induction hardening are better than those of ferrite and pearlite, indicating that non-tempered steel is induction hardened. It can be understood that this technique realizes the omission of the heat treatment process in exchange for the deterioration of. From such a point of view, if the focus is on induction hardenability, it is usual to use tempered steel while avoiding non-tempered steel. However, since there is a demand for reduction or omission of the heat treatment process, there is a strong demand for non-tempered steel having high-frequency hardenability equivalent to that of tempered steel even in a hot-rolled structure.

Now, as shown in FIG. 3 above, if the superiority or inferiority of induction hardenability is caused by unevenness in the carbon concentration in the tissue before induction hardening, it is only necessary to make the tissue with less unevenness in carbon concentration. Become. Here, as a steel structure that can be built by hot rolling, it is martensite that carbon is homogeneously dissolved in the matrix.
And in order to obtain martensite after hot rolling, it is necessary to quench the steel material immediately after hot rolling. One method is to obtain martensite by dipping red hot steel products still in the austenite temperature range after hot rolling in water, hot water or oil and quenching.

  However, this method has various difficulties. For example, it is well known that as-quenched steel is very brittle. That is, it is easy to cause a burning crack in conveyance after rolling, and in that case, it is often in a state where it cannot be shipped as a product. Even if the entire steel is immersed in the cooling medium in this way, there is no guarantee that the entire steel will be cooled uniformly. If the cooling medium is water, a vapor film is formed on the surface of the steel material due to the boiling of the water, and the water vapor that has boiled and vaporized rises in the direction opposite to the gravity in the water, so that the steel material is easily burnt. When the spots are formed, not only the quality cannot be guaranteed, but also the expansion rate at the time of transformation from austenite and the temperature at which transformation occurs vary depending on the structure, which causes the steel material to be distorted or cause a large residual stress. In order to avoid this, it is easily imagined that even if the oil is rapidly cooled with an oil that is difficult to boil and suitable for uniform cooling, the cost of managing and maintaining the equipment is enormous.

  Under the above technical background, the present inventors have studied various methods for establishing good induction hardenability in non-heat treated steel. As a result, it was conceived that only the surface layer portion subjected to induction hardening after hot rolling has a martensite main structure, and portions other than the surface layer having the martensite main structure have substantially a ferrite and pearlite structure.

  That is, after the hot rolling, only the surface layer of the steel material is rapidly cooled to form a martensite main structure, and then the rapid cooling is stopped to make the surface layer suitable for induction hardening, while the inner structure is not brittle. The structure is other than martensite (high toughness). By adopting such a structure, it is possible to improve the induction hardenability while ensuring the toughness of the steel material itself without causing the above-described problems.

  It is also important to increase the strength of the steel material itself. Here, when examining the strength and toughness of steel materials, the test piece used for the investigation of strength and toughness is the No. 4 test piece (tensile test piece) specified in JIS Z2241, and the U-notch test piece (Charpy) specified in JIS Z2242. It is often an impact test piece). However, these test pieces are smaller than the 70 mmφ round bar used for the investigation of the induction hardenability this time. In such a case, it is important where the test piece is taken from 70 mmφ, that is, where the strength and toughness of the 70 mmφ round bar represent the 70 mmφ round bar. Therefore, investigating the strength and toughness of specimens taken in parallel with the longitudinal direction of the round bar from the middle position between the center and the surface of the round bar can accurately determine the strength and toughness of round bars of various diameters. After confirming that it was most suitable for comparison, the following experiments were conducted. In addition, the reason why the intermediate position between the center and the surface is suitable for the sampling position of the test piece is generally that the element such as P that has an adverse effect on the strength during casting is segregated at the center, and the diameter of the specimen is This is because it is not suitable as the strength and toughness that represents a thick steel material, and if the test piece is collected from the surface side of the middle part, the influence of residual stress cannot be excluded.

  In addition, the parallel part diameter of No. 4 test piece (tensile test piece) prescribed | regulated to JISZ2241 is 14 mm, and a grip part is thicker than this and diameter 20mm in many cases. In other words, a round bar having a diameter of less than about 40 mmφ results in a test piece including the center when the No. 4 test piece is sampled from the middle. In order to avoid this, in such a case, it was decided to use a 14A test piece. Even when this 14A test piece faced the above problem, the test piece was taken from the center position. The same was done for Charpy specimens.

When the strength and toughness were investigated using the specimens collected as described above, when the martensite main structure was formed on the surface layer and the balance was ferrite and pearlite, the toughness was 40 J / cm ² or more. It was revealed that sufficient strength as a quality steel, specifically, a strength of 900 MPa or more cannot be obtained. In order to secure a strength of 900 MPa or more and a toughness of 40 J / cm ² or more, it has been newly found that it is effective to form a bainite structure immediately below a martensite structure formed by rapid cooling.

  By the way, as is clear from the continuous cooling chart of steel, the formation of the bainite phase immediately below the martensite phase formed on the surface layer of the steel material is understood as a matter of course metallurgically. However, a unique heat treatment that forms the martensite phase only on the surface layer of the steel material described above, that is, after quenching the hot-rolled material to form the martensite phase on the hot-rolled material surface layer, the rapid cooling is stopped, and the remainder is ferrite. It is very difficult to form bainite directly under the martensite layer because heat treatment is performed to cool the air to form pearlite. This situation will be described below with reference to schematic diagrams.

When the rapid cooling to the steel material is stopped halfway and switched to air cooling, the steel material tends to reach a uniform temperature throughout. Taking this as a temperature change from the surface, recuperation occurs where the surface layer once cooled to room temperature rises again due to heat from the center. That is, after the rapid cooling stop, the temperature rises again in the surface layer part that has been cooled rapidly and the temperature has dropped, and in the vicinity of the surface layer immediately below. This state is shown in FIG. 4 superimposed on a continuous cooling diagram (CCT diagram) of steel. In FIG. 4, a line indicating recuperation returns from the Ms point to the γ region, and finally enters the ferrite + pearlite (F + P) region. The structure after transformation of iron that dissolves carbon is determined by the temperature at which transformation occurs. This is because the form of carbon after transformation is determined by the ease of carbon diffusion, and the ease of carbon diffusion is determined by temperature. That is, in steel cooling, no matter what heat treatment history goes through in the γ region, the final transformation structure is determined by the temperature at the time of transformation, so the steel having a CCT diagram as shown in FIG. In this case, all the portions other than the portion that has undergone martensitic transformation during cooling are transformed at the time of recuperation, and inevitably become a structure of ferrite and pearlite. That is, all but the outermost layer that has undergone martensitic transformation becomes ferrite and pearlite.
On the other hand, as shown in FIG. 5, in the steel in which only the ferrite and pearlite regions are on the right side (the slower cooling rate side) of the CCT diagram, the transformation at the time of recuperation is bainite. It was found that such steels, that is, steels that undergo bainite transformation during recuperation, have higher strength inside, specifically inside the round bar as described above, than steels that undergo ferrite and pearlite transformations. .

  Therefore, component design for causing bainite transformation at the time of recuperation, more specifically, in the CCT diagram, only the F + P region is shifted to the low cooling rate side without significantly affecting the bainite region. As a result of searching for the component design of the steel having the CCT diagram as shown in FIG. 5, it was found that it is effective to add a certain amount or more of Cr and an appropriate amount of Nb. Both Cr and Nb are elements that concentrate in cementite at the time of transformation. However, in a relatively high temperature region where pearlite is formed, the diffusion rate is slower than that of carbon, so that the effect of delaying the formation of cementite is exhibited. On the other hand, the cementite formed during the bainite transformation is not formed by the diffusion of carbon, but the carbon discharged from the ferrite phase that is finely precipitated due to a decrease in the critical radius of the ferrite precipitation nuclei with a large supercooling. Therefore, it is not affected by Cr and Nb. Therefore, the addition of Cr and Nb is a preferable element for promoting the formation of bainite during recuperation as described above. In the technique described in Patent Document 1 described above, Nb is added for precipitation strengthening, but in the present invention, Nb is added as an element that affects the transformation behavior during cooling and recuperation.

Up to this point, the martensite structure of the steel material layer for improving the induction hardenability and the bainite structure for increasing the intermediate strength have been described, but there is no particular limitation on the structure on the center side other than the above.
However, considering the strength and tenacity of the entire steel material apart from the intermediate strength, the hardness distribution of the cross section close to the so-called U-shape, in which only the surface layer is as hard as possible, has a hardness distribution of quenched and tempered steel. Thus, it became clear that the room for bending as a round bar is larger than that of the flat hardness distribution. That is, L1 shown in FIG. 6 shows a three-point bending test result of a round bar obtained by quenching and tempering 70 mmφ SCM435 and setting the difference in Vickers hardness between the surface layer and the center to 32. Similarly, L2 shows the results of a three-point bending test of a round bar which was left in the air after quenching so as to form martensite only on the surface layer, and then tempered as appropriate and the difference in hardness between the surface layer and the center was 64. ing.

  Here, the difference in Vickers hardness between the surface layer and the center was calculated from the hardness measured as follows. That is, the hardness of the center of the steel material is an average value of Vickers hardness measured at five equally divided points on a circumference having a radius of 5 mm from the center. Similarly, the hardness of the surface layer is an average value of Vickers hardness measured at five arbitrary points in the circumferential direction from a depth position of 2 mm from the surface.

  As shown in FIG. 6, the greater the difference in hardness between the surface layer and the center, the greater the stroke to break. The stroke of the three-point bending indicates the bending as a round bar, and indicates that there is room for bending more when the hardness difference between the surface layer and the center is larger. In this way, in order to increase the room for bending with respect to external stress in the final round bar product and to differentiate from the quenching / tempering material, the difference in hardness between the surface layer and the center is 60 or more in terms of Vickers hardness. As a result, the present invention has been completed.

It is the thickness of the martensite main structure introduced into the surface layer, but it is impossible to form a hardened layer of 6 mm or more by induction hardening from the viewpoint of component manufacture. If there is enough. The thickness of the bainite main structure is 5 mm or more, more preferably 6 mm or more, and further preferably 7 mm or more from the viewpoint of securing sufficient tensile strength.
The martensite main structure formed on the surface layer may be martensite with an area ratio of 75% or more in order to suppress hardness variation after induction hardening. Moreover, the bainite main structure | tissue formed in a minimum should just be 75% or more of bainite by area ratio from a viewpoint of intensity | strength and toughness ensuring.

Hereinafter, each requirement of the present invention will be described in detail.
[Ingredient composition]
Hereinafter, unless otherwise indicated, the% display regarding the chemical composition of steel means the mass%.
C: 0.30 to 0.55%
C is an element that precipitates as a solid solution or fine carbide in Fe and strengthens the steel material. Further, it is a very useful element that can be adjusted in hardenability by solid solution in austenite during heat treatment and strengthening austenite by solid solution strengthening. However, if C is contained in an amount exceeding 0.55%, the hardenability of the steel material becomes too high, and even the transformation at the time of recuperation becomes martensite, and sufficient toughness cannot be obtained at an intermediate position. On the other hand, if the C content is less than 0.30%, the hardenability becomes too low. As a result, even if the F + P region in the CCT diagram is shifted to the low cooling rate side by adding Cr, Nb, etc. It becomes difficult to obtain bainite due to transformation during heating. Therefore, the content of C is set to 0.30 to 0.55%. Preferably it is 0.33 to 0.50% of range.

Si: 0.05 to 1.50%
Si is an element that dissolves in Fe and strengthens the steel. Further, it is a component that exhibits temper softening resistance during tempering and is effective in maintaining strength after tempering. However, Si is an element that promotes decarburization, and if it exceeds 1.50%, decarburization during heating in hot rolling becomes unavoidable. On the other hand, if the Si content is less than 0.05%, sufficient solid solution strengthening and temper softening resistance cannot be obtained. Therefore, the Si content is set to 0.05 to 1.50%. Preferably it is 0.08 to 1.30% of range.

Mn: Less than 1.55% Mn is effective for solid solution strengthening in Fe and strengthening the steel material, and at the same time dramatically increasing the hardenability of the steel material. Specifically, it is an element that shifts the B region as well as the F + P region to the low cooling rate side. For this reason, when Mn is contained excessively, the martensitic transformation will be overtaken even at the time of recuperation. Therefore, the upper limit is 1.55%, preferably 1.20%. More preferably, it is 1.00%. On the other hand, steel products inevitably contain S derived from a raw material that degrades the performance of the steel material, but Mn forms a compound with this S and has the effect of removing the adverse effects of S on the steel material. Therefore, it may be contained so long as it does not affect the hardenability, for example, about 0.50%, more preferably about 0.30%.

Cr: 0.85-2.00%
As described above, Cr is an additive element that has the effect of forming a solid solution in Fe to solidify and strengthen the steel material and shifting only the F + P region to the low cooling rate side. However, when Cr is added in excess of 2.00%, the austenite being heated is solid-solution strengthened by Cr. As a result, the entire hardenability is increased and martensitic transformation occurs even during recuperation. End up. On the other hand, if it is less than 0.85%, the above effects cannot be obtained sufficiently. Therefore, the Cr content is set to 0.85 to 2.00%. Preferably it is 0.90 to 1.80% of range.

Al: 0.010 to 0.050%
Al is an element that fixes oxygen inevitably mixed from the atmosphere as an oxide in the manufacturing process of steel products and renders the adverse effects of oxygen on steel materials harmless. Furthermore, it is an element that forms an N and nitride, which will be described later, and has an effect of suppressing coarsening of the austenite grain size of the steel being heated. For that purpose, it is contained at 0.010% or more. However, if Al is contained in excess of 0.050%, a coarse oxide is formed, which may be a starting point of cracks in the steel material. Therefore, the content of Al is set to 0.010 to 0.050%. Preferably it is 0.025 to 0.045% of range.

N: 0.0020 to 0.0200%
N is an element capable of obtaining the effect of suppressing the coarsening of the austenite grain size of the steel material being heated by forming the above-described Al and nitride. For that purpose, 0.0020% or more is contained. However, if the content exceeds 0.0200%, it dissolves in Fe and causes so-called strain aging at room temperature, which may significantly reduce the elongation of the steel material. Therefore, the content of N is set to 0.0020 to 0.0200%. Preferably it is 0.0040 to 0.0180% of range.

Nb: 0.005 to 0.100%
As described above, Nb is an element useful for shifting only the F + P region to the low cooling rate side. If it is 0.005% or less, this effect cannot be sufficiently obtained. On the other hand, when it contains 0.100% or more, it precipitates as a carbonitride at the austenite grain boundary at the time of casting, and causes a grain boundary crack. Therefore, the Nb content is set to 0.005 to 0.100%. Preferably it is 0.007 to 0.080% of range.

P: 0.030% or less (including 0%)
P is a harmful element that segregates at the austenite grain boundaries and embrittles the prior austenite grain boundaries and lowers the toughness of the steel material even when cooled to room temperature. Therefore, the lower the value, the better, but the upper limit allowable as an impurity is 0.030%. Preferably it is 0.025% or less, and may be 0%.

S: 0.030% or less (including 0%)
S is a harmful element that dissolves in the steel and lowers the strength of the steel. The adverse effects of S can be removed by adding Mn to form a compound. However, if the amount exceeds the limit, not only the effect of Mn cannot be completely removed, but also Mn forms a large amount of S and a compound, resulting in the strengthening and hardenability of the steel, which is the original purpose of adding Mn. Improvement cannot be obtained. From these viewpoints, the lower the S content, the better, but the allowable upper limit is 0.030%. Preferably it is 0.025% or less, and may be 0%.

Cu: 0.20% or less (including 0%)
In the present invention, Cu is an element that adversely affects steel materials. That is, Cu forms a liquid phase at the grain boundaries on the steel surface during hot rolling of the steel material, and causes the occurrence of surface cracks by reducing the bonding strength of the grain boundaries. In order to prevent this, the amount of Cu may be controlled below a certain level, and this upper limit is 0.20%. This upper limit is preferably 0.17% and may be 0%.

Ni: 0.20% or less (including 0%)
Since Ni increases the hardenability of the steel material, if it is added excessively, it becomes difficult to adjust the hardenability only by suppressing the addition of elements that adjust the hardenability such as C, Mn, and Cr. Further, since Ni is a strong austenite stabilizing element, residual austenite remains in martensite formed on the steel surface at the time of cooling, and the aging deterioration during use after becoming a part is accelerated. In order to suppress this, it is necessary to manage the amount of Ni below a certain level, and this upper limit is 0.20%. This upper limit is preferably 0.15%, more preferably 0.12%, and may be 0%.

  The balance of the above basic components is Fe and inevitable impurities. Inevitable impurities include elements mixed from slabs such as Ca and Mg, and rare earth elements (REM). Furthermore, if necessary, Ti and / or B can be added in the following range.

Ti: 0.001 to 0.100%
Ti is an element that forms carbides and nitrides at a high temperature and suppresses the coarsening of the austenite grain size of the steel material being heated and improves the toughness of the steel material, so it is added at 0.001% or more. May be. However, if the effect of addition of Ti exceeds 0.100%, the Ti compound may be coarsened and the effect may not be obtained. Therefore, when adding Ti, it is 0.001 to 0.100% of range, preferably 0.0040 to 0.080%.

B: 0.0002 to 0.0050%
B is an element that dramatically improves the hardenability of steel even in a small amount, and may be added. If the addition amount is less than 0.0002%, the effect of improving hardenability cannot be obtained. On the other hand, if it exceeds 0.0050%, it is combined with N to form BN during casting, which causes grain boundary cracking. Therefore, when adding B, it was made into the range of 0.0002 to 0.0050%, preferably 0.0005 to 0.0030%.

[Organization]
As described above, it is important that the steel material has a martensite main structure whose surface layer has a martensite area ratio of 75% or more and a bainite main structure below the martensite main structure. That is, by setting it as such a structure, the intensity | strength and toughness which were excellent as steel materials can be ensured, suppressing the hardness dispersion | variation in this quenching layer at the time of performing induction hardening.
Here, the surface layer is a region from the surface of the steel material to a depth of 7 mm, and the lower layer, which is a bainite main structure, is a region having a thickness of 5 mm or more. The ratio of the martensite structure in the surface layer is 75% or more. Of course, it may be 100%. Similarly, the ratio of the bainite structure in the lower layer is 75% or more. Of course, it may be 100%.

  The structure other than the surface layer region and the lower layer region is not particularly limited. For example, any of ferrite, pearlite, and bainite may be used, but ferrite and pearlite are used from the viewpoint of obtaining good bending by making the central portion high toughness.

[Hardness regulation]
It is important that the difference in hardness between the center of the steel material and the surface layer is 60 or more in terms of Vickers hardness. This is because, in order to give the strength and tenacity of the entire steel material, the hardness distribution in the depth direction from the surface of the steel material has more room for bending as a steel material than the uniform hardness in the depth direction. .

[Production method]
After performing hot working on the steel material having the above-described composition, cooling is performed at a cooling rate of 10 ° C./s or more at a depth of 7 mm from the surface from a temperature range of 800 to 1000 ° C. After the elapse of time P [s] determined by the following equation (1) from the start of the above, the cooling is switched to air cooling.
P [s] = 0.01 × r ² ± r / 10 (1)
Where r: radius of the circular cross-section material at the cooling site (mm)
First, as described above, it is important that the steel material has a martensite main structure. For this purpose, after hot working on the circular cross-section material, rapid cooling is performed at a cooling rate of 10 ° C./s or more from a temperature range of 800 to 1000 ° C. Preferably, cooling is performed at a rate of 11 ° C./s or more, more preferably 12 ° C./s or more. In addition, in order to make the surface layer from the surface to a depth of 7 mm as a martensite main structure, cooling at a rate of 10 ° C./s or more from a temperature range of 800 to 1000 ° C. is performed at a position of a depth of 7 mm from the surface of the steel material. Control at the temperature.

  Moreover, if the finishing temperature of hot working is in the range of 800 to 1000 ° C., the steel material after hot working may be immediately subjected to cooling treatment. Of course, cooling may be performed after reheating in the range of 800 to 1000 ° C.

  The reason why this rapid cooling is set to P [s] according to the above equation (1) is as follows. That is, as described above, it is effective to form a martensite main body structure on the surface layer by rapid cooling, and to form a bainite main body structure under this surface layer, but to form bainite under the surface layer (martensite main body structure layer). In the CCT diagram shown in FIG. 5, after forming martensite on the surface layer, it is necessary to undergo a temperature history in which the portion under the surface layer that has not undergone martensite transformation undergoes bainite transformation during recuperation and subsequent cooling. In order to pass through this temperature history, it is necessary to stop the rapid cooling at a certain stage. At this time, if the rapid cooling time P is too long, the martensitic transformation proceeds to a region near the center, and the bainite main structure becomes larger than necessary, and the hardness of the center increases. The hardness difference between the surface layer portion and the center portion cannot be ensured, and the toughness is deteriorated. On the other hand, if the rapid cooling time P is too short, a portion of the surface layer that has not undergone martensite transformation undergoes pearlite transformation after recuperation during air cooling after the rapid cooling stop, and a bainite main structure is formed in this region. Can not be. That is, the rapid cooling time P has an appropriate value in order to secure the area of the bainite main structure and to give the necessary altitude difference between the surface layer portion and the central portion. And it is thought that the appropriate value of this rapid cooling time becomes long, so that the diameter of steel materials is large. Therefore, the inventors investigated the appropriate range of the rapid cooling time for each radius of the circular cross-section material in the cooling region using the steel material as a circular cross-section material, and if the above formula (1) is satisfied, the martensite main structure It was confirmed that the above-mentioned bainite main structure was formed under the surface layer, and the above-described hardness difference could be imparted to the surface layer portion and the center portion. Therefore, after the elapse of the rapid cooling time P [s] according to the above equation (1), it is necessary to switch to air cooling for cooling.

  The steel material having the above-described component composition is subjected to air cooling that is allowed to stand in the atmosphere after P [s] to room temperature, so that a surface layer having a depth of 7 mm or more from the surface that is a martensite-based structure and a surface layer below this surface layer It becomes a structure | tissue which has the layer of the bainite main structure of thickness 5mm or more.

EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.
Steel type No. having the component composition shown in Table 1. 1 to 79 were melted using a 150 kg vacuum melting furnace to produce a steel ingot, and then once hot-forged into a 120 mm diameter cylinder, this cylinder was held at 1150 ° C. for 1 hour and then air-cooled. After normalization, a round bar having a diameter shown in Table 2 was cut out.
In Table 1, the steel type No. 1-56 is a compatible steel whose chemical composition falls within the range defined in the present invention. On the other hand, steel type No. 57-77 is a comparative steel in which the underlined component deviates from the content range defined in the present invention. Furthermore, steel type No. Reference numerals 78 and 79 are examples of addition of selected components of the present invention, and are comparative steels outside the range of addition of these selected components.

The above round bar is further cut into a length of 180 mm, heated to 800-900 ° C. to the axial center thereof to austenite the whole steel, and then in a water bath under the conditions shown in Table 2, time: P (s ) Was cooled with water, and then allowed to cool in the atmosphere.
The cooling rate at a depth of 7 mm from the surface of the round bar was measured by drilling a hole to a predetermined position of the round bar and installing a contact-type thermocouple in the hole. The range for calculating the cooling rate was from the start of water cooling until the time P (s) passed, and the water cooling time: P was measured with a stopwatch.

Various test specimens were collected from the intermediate position of the round bar produced as described above (D / 4 depth position from the surface: D is the diameter of the round bar), and TS (tensile strength [MPa]) and toughness were obtained. (Charpy impact test value [J / cm ² ]) was investigated. The tensile test piece was a No. 4 test piece defined in JIS Z2201, and the tensile speed was measured at 10 mm / min at the stroke speed of the tensile tester. The Charpy impact test piece was a U-notch test piece (height under notch 8 mm) defined by JIS Z2242, and the measurement temperature was 25 ° C. The specimen for microstructural observation cut out a specimen having a thickness of 10 mm in the direction perpendicular to the C section, and the C section was used for observation. The observation surface was mirror-polished and corroded with a nital solution, and then observed with an optical microscope with a magnification of 100 times to determine the microstructure.

  The test results are summarized in Table 3. Further, the difference in Vickers hardness between the surface layer portion and the center portion was calculated from the hardness measured as follows. That is, the hardness of the central part of the steel material is an average value of Vickers hardness measured at five equally divided points on the circumference having a radius of 5 mm from the center. Similarly, the hardness of the surface layer portion is an average value of Vickers hardness measured at five arbitrary points in the circumferential direction from a depth position of 2 mm from the surface. The measurement of Vickers hardness was based on JIS Z2244.

  Finally, the hardness variation of the round bar surface layer after induction hardening was compared between the case where the previous structure was a martensite structure and the case where the structure was ferrite and pearlite. A 4 mm induction hardened layer is formed on each of the above test pieces, the Vickers hardness at a position 3 mm from the surface is measured at 12 locations, and the average is obtained by applying a martensite layer to the surface layer before induction hardening with the same steel material. A comparison was made between the case of formation and the case of ferrite and pearlite. The ratio (%) is also shown in Table 3 using the case where the previous structure is ferrite and pearlite as the denominator and the case where the martensite layer is formed on the surface layer as the numerator.

Claims (4)

% By mass
C: 0.30 to 0.55%,
Si: 0.05 to 1.50%,
Mn: less than 1.55%,
Cr: 0.85 to 2.00% or less,
Al: 0.010 to 0.050%,
N: 0.0020 to 0.0200%,
Nb: 0.005 to 0.100%,
P: 0.030% or less (including 0%),
S: 0.030% or less (including 0%),
Cu: 0.20% or less (including 0%) and Ni: 0.20% or less (including 0%)
The balance is Fe and the inevitable impurities component composition, and the structure of the surface layer part from the surface to a depth of 7 mm is a martensite-based structure with a martensite area ratio of 75% or more, the martensite-based structure The lower layer of the structure has a bainite main structure with an area ratio of bainite of 75% or more, the thickness of the bainite main structure is 5 mm or more, and the hardness difference between the surface layer part and the center part is 60 or more in terms of Vickers hardness. Steel material. The component composition further includes:
Ti: 0.001 to 0.100 mass%
The steel material of Claim 1 containing. The component composition further includes:
B: 0.0002 to 0.0050 mass%
The steel material according to claim 1 or 2 containing. A steel material having the composition according to any one of claims 1 to 3 is subjected to hot working and formed into a circular cross-section material, and then cooled at a depth of 7 mm from the surface from a temperature range of 800 to 1000 ° C. Cooling is performed at a speed of 10 ° C./s or more, and after the time P [s] determined by the following equation (1) from the start of the cooling, cooling is performed by switching to air cooling . The structure is a martensite main structure with a martensite area ratio of 75% or more, and has a bainite main structure with a bainite area ratio of 75% or more under the martensite main structure, and the thickness of the bainite main structure. Is 5 mm or more, The manufacturing method of the steel materials whose hardness difference of the said surface layer part and center part is 60 or more by Vickers hardness .
P [s] = 0.01 × r ² ± r / 10 (1)
Where r: radius of the circular cross-section material at the cooling site (mm)