JP6787238B2 - Manufacturing method of steel for machine structure - Google Patents

Description

The present invention relates to a method for producing a steel material for machine structure.

Most of the mechanical parts used in automobiles and industrial machines are formed by hot forging or cold forging. Therefore, high forging properties are required for steel materials for machine structural use, which are materials for machine parts. Further, when manufacturing a machine part, cutting is performed on the machine structural steel material which is the material of the machine part, if necessary. Therefore, steel materials for machine structural use are required to have not only high forgeability but also high machinability.

If MnS is formed in the steel material, the machinability of the steel is enhanced. However, if coarse MnS is formed, the coarse MnS becomes a stress concentration source, and the forgeability of the steel material may decrease. If fine MnS can be dispersed in the steel material, the forgeability of the steel material can be improved while improving the machinability of the steel material. Therefore, there is a demand for a technique for finely dispersing MnS in a steel material.

Techniques for finely dispersing MnS in steel materials are JP-A-2003-2903081 (Patent Document 1), JP-A 1-228643 (Patent Document 2), and International Publication No. 01/066814 (Patent Document 3). , Japanese Patent Application Laid-Open No. 8-92687 (Patent Document 4), and Japanese Patent Application Laid-Open No. 10-1746 (Patent Document 5).

The mechanical structural steel disclosed in Patent Document 1 is characterized in that fine sulfide-based inclusions containing MnS as a main component are present in the steel in an amount of 5000 pieces / mm ² or more per unit area. Mn and S in the steel are contained in an atomic% ratio of around 1: 1 of Mn / S = 0.6 to 1.4, and the S content is set to 0.05 to 0.40% in mass%. As a result, Patent Document 1 describes that the above-mentioned fine sulfide-based inclusions can be obtained.

In the method disclosed in Patent Document 2, at least one of Zr, Ti, Ce, Y, and Hf is dissolved in molten steel having a pre-deoxidation dissolved oxygen concentration of 20 to 60 ppm by weight. Casting into a continuous casting machine or mold, the average cooling rate at the 1/2 thickness position of the slab or steel ingot is 50 ° C./min or more at the liquidus temperature to 1400 ° C. and 1 to 50 ° C./ Minutes. It is described in Patent Document 2 that MnS can be finely dispersed and precipitated in steel by this method.

The steel disclosed in Patent Document 3 has a mass% of C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: It contains 0.003 to 0.2%, S: 0.003 to 0.5%, Zr: 0.003 to 0.01%, Al: 0.01% or less, patent-O: 0.02. % Or less, patent-N: 0.02% or less, the average aspect ratio of MnS is 10 or less, the maximum aspect ratio is 30 or less, and the balance is composed of Fe and unavoidable impurities. It is described in Patent Document 3 that MnS can be finely dispersed and the shape of MnS can be maintained in a spherical shape by the above chemical composition.

The non-tamed steel for hot forging disclosed in Patent Document 4 includes Si: 2% or less, S: 0.10% or less, N: 0.02% or less, O: 0.010% or less, and is inevitable. In steel containing impurity elements, C: 0.10 to 0.6%, Mn: 0.3 to 2.5%, Cr: 0.05 to 2.5%, V: 0.03 to 0.5. %, Al: 0.060% or less, Ti: 0.005 to 0.03%, and 1 × 10 ² to 1 × 10 ⁶ inclusions having an average particle size of 0.1 to 5 μm / Contains mm ² . This inclusion is a Ti acid / nitride, MnS, and a composite compound mainly composed of the Ti acid / nitride and MnS. The non-tamed steel for hot forging containing the inclusions is hot in a steel material in which the temperature range of 1500 to 900 ° C is cooled at a cooling rate of 1 ° C. It is described in Patent Document 4 that it can be produced by forging and then air-cooling.

The spring steel described in Patent Document 5 has C: 0.25 to 0.70%, Si: 1.0 to 2.1%, Mn: 0.05 to 0.49%, Cr in mass%. : 0.05 to 2.0%, S: 0.02% or less, N: 10 to 200 ppm, and Ti: 0.001 to 0.2%, Cu: 0.05 to 0.5% , Ca: 0.0002 to 0.01%, Zr: 0.001 to 0.10%, contains at least one element selected from the group consisting of residual iron and unavoidable impurities. Further, in the above spring steel, a sulfide or sulfide containing Mn as a main component existing in a cross section having a depth of 0.3 mm or more from the surface and a width of 20 mm ² set from a region not including the central portion. There are 20 or less composite compounds having a major axis of 8 μm or more. Patent Document 5 states that this spring steel is produced by casting a molten steel satisfying each of the above component compositions at an average solidification rate of 20 ° C./min or more, and rolling or drawing the obtained casting. Is described in.

Japanese Unexamined Patent Publication No. 2003-293081 Japanese Unexamined Patent Publication No. 1-228643 International Publication No. 01/0668114 Japanese Unexamined Patent Publication No. 8-92687 Japanese Unexamined Patent Publication No. 10-1746

However, the techniques disclosed in Patent Documents 1 to 5 do not mention the relationship between the C content and MnS. Therefore, when the C content in the steel material is high, MnS is unlikely to become fine, and the forgeability and machinability of the steel material may decrease.

An object of the present invention is to provide a method for producing a steel material for machine structure, which has high forgeability and high machinability regardless of the C content.

The method for producing a steel material for machine structure according to the present invention is, in mass%, C: 0.1 to 1.0%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P. : 0.003 to 0.2%, S: 0.01 to 0.05%, N: 0.01% or less, T.I. O: 0.0018% or less, Cr: 0 to 2.0%, Ni: 0 to 2.0%, Mo: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 0. 2%, Ti: 0 to 0.1%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, Zr: 0 to 0.01%, Te: 0 to 0.005%, Bi: A molten steel containing 0 to 0.15%, Pb: 0 to 0.15%, and B: 0 to 0.004% and having a chemical composition in which the balance is Fe and impurities, is ingot-cast or continuously cast. It includes a casting process of solidifying the cast material by a method to produce a cast material, and a process of reheating the cast material and then performing hot working to produce a steel material for machine structure. In the casting process, the molten steel is solidified at a cooling rate according to the C content of the molten steel.

The method for producing a steel material for machine structure according to the present invention can produce a steel material for machine structure having high forgeability and high machinability regardless of the C content.

FIG. 1 is a diagram showing the relationship between the carbon content and the solidification cooling rate.

Hereinafter, embodiments of the present invention will be described in detail.

The present inventors have investigated and studied a technique for finely dispersing MnS in a steel material. As a result, the present inventors obtained the following findings.

(A) Factors for formation of coarse MnS In steel materials, coarse MnS is considered to be formed by the following process. In the casting process, the growth of dendrites (dendritic crystals) also progresses along with the progress of the formation of solidified parts in the molten steel. At that time, Mn and S are concentrated in the liquid phase of the region (hereinafter referred to as the final solidified portion) between the secondary arms of the dendrite (branches generated from the main axis (trunk) of the dendrite). As the solidification further progresses, solidification proceeds in the final solidification portion between the secondary arms and MnS is precipitated. If a large number of dendrites are formed, a large number of secondary arms are also formed, and as a result, a large number of fine (narrow) final solidified portions are formed. In this case, fine MnS is generated in the narrow final solidified portion. As a result, fine MnS is dispersed and formed.

Based on the above process, the present inventors thought that if the secondary arm spacing was narrowed, the formation of coarse MnS could be suppressed and fine MnS could be dispersed.

Therefore, the present inventors focused on the above-mentioned secondary arm spacing of dendrites and investigated the solidification structure morphology of various steel types. As a result, the present inventors have found that there is a correlation between the carbon (C) content and the secondary arm spacing. As a result of further studies by the present inventors, the following findings were obtained regarding the relationship between the cooling rate and C content in the casting process and the dendrite secondary arm spacing.

(A) As the degree of supercooling increases, finer dendrites are generated and the secondary arm spacing becomes narrower. That is, the faster the cooling rate and the faster the solidification progresses, the narrower the secondary arm spacing becomes.
(B) The higher the C content, the larger the interface energy between the solid phase and the liquid phase, and the more difficult it is for the solid-liquid interface to take a complicated shape. Therefore, the higher the C content, the wider the secondary arm spacing of the dendrite.
(C) When the cooling rate is the same, the higher the C content, the wider the solid-liquid coexistence temperature range and the longer it takes to solidify. Therefore, the distance between the secondary arms of the dendrite becomes wide.

Based on the above findings (a) to (c), the present inventors disperse fine MnS by cooling and solidifying the molten steel at a cooling rate corresponding to the C content of the molten steel in the casting process. I found that it can be generated.

(B) Method for making MnS finer As described above, when the cooling rate is the same, the higher the C content, the wider the dendrite secondary arm spacing. On the other hand, when the C content is the same, the slower the cooling rate, the wider the dendrite secondary arm spacing. Further, it is considered that the narrower the interval between the secondary arms, the more the coarsening of MnS is suppressed, and the finer MnS is dispersed and formed. Therefore, the present inventors further investigated a method for narrowing the secondary arm spacing.

The present inventors define the average cooling rate from the liquidus line temperature to the solidus line temperature in the temperature range of steel in the casting process as "solidification cooling rate" (° C./min), and use this solidification cooling rate. noticed.

Here, the "solidification cooling rate" in the present specification is more specifically defined as follows. When a steel ingot is manufactured by the ingot method, the average cooling rate from the liquidus line temperature to the solidus line temperature at the center of gravity of the ingot is defined as the solidification cooling rate (° C./min). When a slab is manufactured by the continuous casting method, the solidification cooling rate (° C / min) is the average cooling rate from the liquidus line temperature to the solidus line temperature at the center of the surface cut perpendicular to the longitudinal direction of the slab. ).

If the solidification cooling rate is increased, the degree of supercooling is increased, and the dendrite secondary arm spacing can be narrowed. In this case, it is considered that the formation of coarse MnS can be suppressed and MnS can be finely dispersed in the steel material. On the other hand, the dendrite secondary arm spacing is also affected by the C content. Therefore, the present inventors investigated the solidification cooling rate, the C content, and the size of MnS.

FIG. 1 is a diagram showing the relationship between the C content _CC (mass%) and solidification cooling rate V (° C./s) in molten steel and the size of MnS in the cast material after the casting process. FIG. 1 was obtained by the following method.

By mass%, C: 0.1 to 1.0%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0.2%, S: 0.01-0.05%, N: 0.01% or less, T.I. A plurality of molten steels containing O: 0.0018% or less and having a chemical composition in which the balance was composed of Fe and impurities were prepared. The C content of each molten steel was set to a different value in a wide range of 0.1 to 1.0. The content of elements other than the C content of each molten steel was within the above range.

Continuous casting was carried out using a plurality of prepared molten steels to produce a plurality of cast materials (shards). The cross section (cross section) perpendicular to the longitudinal direction of each casting was the same. The solidification cooling rate V in each casting was specified by the method described later. In each casting, all the conditions other than the solidification cooling rate V were the same. The solidification cooling rate V was adjusted by the method described later.

Further, bulk rolling and finish rolling by a continuous rolling mill were carried out on each cast material under the following conditions to produce a steel bar having a diameter of 60 mm. Specifically, the cast material was heated to 1200 ° C. Billets were produced by performing bulk rolling on the heated cast material. The cross section of the billet (the cross section perpendicular to the longitudinal direction) was the same. The billet was heated to 1200 ° C. The billets after heating were subjected to finish rolling by a continuous rolling mill to produce steel bars having a diameter of 60 mm. The cumulative surface reduction rate in hot working (bulk rolling and finish rolling) from cast material to steel bar was 94%.

In the manufactured steel bar having a diameter of 60 mm, the maximum particle size of MnS was determined by the method described later. FIG. 1 was created using the C content of each molten steel, the solidification cooling rate V, and the obtained maximum particle size of MnS.

The “Δ” mark in FIG. 1 indicates that the maximum particle size of MnS is 20 μm or more, and coarse MnS is formed. The “◯” mark indicates that the maximum particle size of MnS is less than 20 μm, and fine MnS is formed.

With reference to FIG. 1, when the maximum particle size of MnS is less than 20 μm (marked with “◯” in FIG. 1), it can be seen that there is a correlation between the C content _CC and the solidification cooling rate V. That is, by changing the solidification cooling rate according to the C content, in other words, by cooling and solidifying the molten steel at the solidification cooling rate according to the C content, fine MnS is dispersed and formed. can do.

Further, the higher the C content, the faster the solidification cooling rate V required to reduce the maximum particle size of MnS to less than 20 μm. That is, the higher the C content _{C C,} if faster solidification cooling rate V, can be reduced MnS.

Further, referring to FIG. 1, the maximum particle size of MnS changed discontinuously in the vicinity of C content _CC = 0.45%. As a result of further examination of this point, if the casting step is carried out at a solidification cooling rate satisfying the formulas (1) to (3), the formation of coarse MnS is suppressed, and fine MnS is dispersed and generated. As a result, it was found that a steel material for machine structure having high forgeability and high machinability can be produced.
Fn1 ≤ V <30 (1)
If a _C C <0.45:
Fn1 = 0.06 × (0.8 + _CC ) (2)
If a _C C ≧ 0.45:
Fn1 = 0.09 × (0.1 + _CC ) (3)
Here, the C content (mass%) in the molten steel is substituted for "Cc".
When the dendrite secondary arm spacing of the cast material is defined as λ2 (μm), the solidification cooling rate V is defined by the equation (4).
V = (1/60) × (λ2 / 770) ^{− (1 / 0.41)} (4)

The reason why the maximum particle size of MnS changes discontinuously when the C content Cc = 0.45% is not clear, but the following matters can be considered. When the C content is around Cc = 0.45%, the interface energy between the solid phase and the liquid phase changes, and the shape of the solid-liquid interface changes. Specifically, if lower than the C content _{C C} is 0.45%, [delta] phase is generated by the bcc structure, if the C content _{C C} is higher than 0.45%, gamma phase of fcc structure generated To do. Since the phases produced are different, the interfacial energy between the solid phase and the liquid layer changes discontinuously at the boundary of C content _CC = 0.45%. As a result, as shown in FIG. 1, discontinuous results occur at the maximum particle size of MnS with the C content _CC = 0.45 as a boundary.

If the solidification cooling rate V satisfies the formula (1), the maximum particle size of MnS in the machine structural steel material becomes less than 20 μm. On the other hand, if the solidification cooling rate V is too fast, excessive strain is generated during solidification, which causes cracks in the solid phase. Therefore, the upper limit of the solidification cooling rate V is 30 ° C./s.

The method for producing a steel material for machine structural use according to the present invention completed based on the above findings is, in terms of mass%, C: 0.1 to 1.0%, Si: 0.01 to 1.5%, Mn: 0. 05 to 2.0%, P: 0.003 to 0.2%, S: 0.01 to 0.05%, N: 0.01% or less, T.I. O: 0.0018% or less, Cr: 0 to 2.0%, Ni: 0 to 2.0%, Mo: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 0. 2%, Ti: 0 to 0.1%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, Zr: 0 to 0.01%, Te: 0 to 0.005%, Bi: A molten steel containing 0 to 0.15%, Pb: 0 to 0.15%, and B: 0 to 0.004% and having a chemical composition in which the balance is Fe and impurities, is ingot-cast or continuously cast. It includes a casting process of solidifying the cast material by a method to produce a cast material, and a process of reheating the cast material and then performing hot working to produce a steel material for machine structure. In the casting process, the molten steel is solidified at a cooling rate according to the C content of the molten steel.

Preferably, in the casting step, the solidification cooling rate V (° C./sec), which is the average cooling rate from the liquidus line temperature to the solidus line temperature, satisfies the formula (1).
Fn1 ≤ V <30 (1)
Here, Fn1 is defined by the equations (2) and (3).
If a _C C <0.45:
Fn1 = 0.06 × (0.8 + _CC ) (2)
If a _C C ≧ 0.45:
Fn1 = 0.09 × (0.1 + _CC ) (3)
Here, _{C C} of formula (2) and (3) indicates the C content of the molten steel (mass%).
When the dendrite secondary arm spacing of the cast material is defined as λ2 (μm), the solidification cooling rate V is defined by the equation (4).
V = (1/60) × (λ2 / 770) ^{− (1 / 0.41)} (4)

The chemical composition is one or 2 selected from the group consisting of Cr: 0.01 to 2.0%, Ni: 0.05 to 2.0%, and Mo: 0.05 to 1.0%. It may contain more than a seed.

The chemical composition is one or 2 selected from the group consisting of V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, and Ti: 0.005 to 0.1%. It may contain more than a seed.

The chemical composition is one or 2 selected from the group consisting of Ca: 0.0002 to 0.005%, Mg: 0.0003 to 0.005%, and Zr: 0.0003 to 0.01%. It may contain more than a seed.

The chemical composition is one or 2 selected from the group consisting of Te: 0.0003 to 0.005%, Bi: 0.001 to 0.15%, and Pb: 0.01 to 0.15%. It may contain more than a seed.

The chemical composition may contain B: 0.0005 to 0.004%.

Hereinafter, embodiments of the present invention will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[Manufacturing method of steel for machine structure]
The method for producing a steel material for machine structural use according to the present invention includes a process of casting molten steel to produce a cast material (casting process) and a process of performing hot working on the cast material to produce a steel material for machine structural structure ( Hot working process) and included. Hereinafter, each step will be described.

[Casting process]
At the beginning of the casting process, molten steel is prepared. The chemical composition of molten steel contains the following elements.

C: 0.1 to 1.0%
Carbon (C) increases the strength of steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, hard carbides are precipitated and the machinability of the steel is lowered. Therefore, the C content is 0.1 to 1.0%. The lower limit of the C content is preferably 0.12%, more preferably 0.16%. The preferred upper limit of the C content is 0.85%, more preferably 0.70%.

Si: 0.01-1.5%
Silicon (Si) reinforces ferrite in steel. Si also increases the temper softening resistance of steel. If the Si content is too low, these effects cannot be obtained. On the other hand, if the Si content is too high, the steel becomes embrittlement. If the Si content is too high, the deformation resistance of the steel at high temperatures will further increase. Therefore, the Si content is 0.01-1.5%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%. The preferred upper limit of the Si content is 1.0%, more preferably 0.6%.

Mn: 0.05 to 2.0%
Manganese (Mn) combines with S in the steel to form MnS and enhances the machinability of the steel. Mn further enhances the hardenability of steel and enhances the strength after quenching. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, these effects will be saturated. If the Mn content is too high, the strength of the steel becomes too high, and the cold workability is lowered. Therefore, the Mn content is 0.05 to 2.0%. The preferred lower limit of the Mn content is 0.1%, more preferably 0.3%. The preferred upper limit of the Mn content is 1.7%, more preferably 1.2%.

P: 0.003 to 0.2%
Phosphorus (P) enhances the machinability of steel. If the P content is too low, this effect cannot be obtained. On the other hand, if the P content is too high, the strength of the steel is increased, and the cold workability, hot workability, and castability are lowered. Therefore, the P content is 0.003 to 0.2%. The preferred lower limit of the P content is 0.005%, more preferably 0.01%. The preferred upper limit of the P content is 0.1%, more preferably 0.05%.

S: 0.01-0.05%
Sulfur (S) combines with Mn in the steel to form MnS and enhances the machinability of the steel. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, the forging property is lowered, and cracks are likely to occur during hot forging and cold forging. Therefore, the S content is 0.01 to 0.05%. The lower limit of the S content is preferably 0.012%, more preferably 0.015%. The preferred upper limit of the S content is 0.045%, more preferably 0.04%.

N: 0.01% or less Nitrogen (N) is inevitably contained. N dissolves in steel and increases the strength of steel. However, if the N content is too high, the tool life will be shortened because it will be hardened in the vicinity of the cutting edge due to dynamic strain aging in cutting. Therefore, the N content is 0.01% or less. The preferred lower limit of the N content is 0.001%, more preferably 0.002%. The preferred upper limit of the N content is 0.008%, more preferably 0.006%.

T. O: 0.0018% or less Oxygen (O) is an impurity. In this specification, T.I. O means the total amount of oxides in steel. T. If O is too high, coarse oxides may be formed. As a result, the coarse oxide becomes the starting point of cracking, and the forgeability of the steel is lowered. Therefore, T.I. O is 0.0018% or less. T. The preferred lower limit of O is 0.0003%, more preferably 0.0005%. T. The preferred upper limit of O is 0.0016%, more preferably 0.0014%.

The balance of the chemical composition of molten steel for machine structural steel according to the embodiment of the present invention consists of Fe and impurities. Here, the impurities are those mixed from ore, scrap, or the manufacturing environment as raw materials when the molten steel of the steel material for machine structure is industrially manufactured, and adversely affect the steel material for machine structure. Means something that is acceptable to the extent that it does not exist.

[Arbitrary element]
The chemical composition of the molten steel for machine structural steel according to the present invention may further contain one or more selected from the group consisting of Cr, Ni, and Mo instead of a part of Fe. All of these elements are optional elements and enhance the hardenability of steel.

Cr: 0-2.0%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr enhances the hardenability of the steel and enhances the strength of the steel. Cr further increases temper softening resistance. If even a small amount of Cr is contained, these effects can be obtained to some extent. However, if the Cr content is too high, Cr carbides will be formed and the steel will be embrittled. Therefore, the Cr content is 0 to 2.0%. The lower limit of the Cr content is preferably 0.01%, more preferably 0.02%. The preferred upper limit of the Cr content is 1.8%, more preferably 1.5%.

Ni: 0-2.0%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni enhances the hardenability of steel and enhances the strength of steel. Ni further enhances the ductility and corrosion resistance of steel. If even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, these effects will be saturated. Therefore, the Ni content is 0 to 2.0%. The lower limit of the Ni content is preferably 0.05%, more preferably 0.1%. The preferred upper limit of the Ni content is 1.5%, more preferably 1.0%.

Mo: 0-1.0%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo enhances the hardenability of the steel and enhances the strength of the steel. Mo further increases temper softening resistance. If even a small amount of Mo is contained, these effects can be obtained to some extent. However, if the Mo content is too high, these effects will saturate. Therefore, the Mo content is 0 to 1.0%. The preferred lower limit of the Mo content is 0.05%, more preferably 0.08%. The preferred upper limit of the Mo content is 0.8%, more preferably 0.5%.

The chemical composition of the molten steel may further contain one or more selected from the group consisting of V, Nb, and Ti instead of a part of Fe. All of these elements are optional elements and form carbides, nitrides and carbonitrides (collectively referred to as "carbonitrides and the like") to increase the strength of steel.

V: 0-1.0%
Vanadium (V) is an optional element and may not be contained. When contained, V forms carbonitrides and the like. The carbonitride of V or the like refines the crystal grains by the pinning effect and enhances the strength of the steel. If even a small amount of V is contained, this effect can be obtained to some extent. However, if the V content is too high, the mechanical properties of the steel will rather deteriorate. Therefore, the V content is 0 to 1.0%. The preferred lower limit of the V content is 0.05%, more preferably 0.08%. The preferred upper limit of the V content is 0.8%, more preferably 0.5%.

Nb: 0-0.2%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb forms carbonitrides and the like. Nb carbonitrides and the like refine the crystal grains by the pinning effect and increase the strength of the steel. If even a small amount of Nb is contained, this effect can be obtained to some extent. However, if the Nb content is too high, carbonitrides and the like are excessively formed, and the mechanical properties of the steel are rather deteriorated. Therefore, the Nb content is 0 to 0.2%. The preferred lower limit of the Nb content is 0.005%, more preferably 0.01%. The preferred upper limit of the Nb content is 0.15%, more preferably 0.1%.

Ti: 0-0.1%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti forms carbonitrides and the like. Ti carbonitrides and the like refine the crystal grains by the pinning effect and increase the strength of the steel. Ti further combines with O in the steel to form a soft oxide, enhancing the machinability of the steel. If even a small amount of Ti is contained, these effects can be obtained to some extent. However, if the Ti content is too high, these effects will saturate. Therefore, the Ti content is 0 to 0.1%. The preferred lower limit of the Ti content is 0.005%, more preferably 0.01%. The preferred upper limit of the Ti content is 0.08%, more preferably 0.06%.

The chemical composition of the steel material for mechanical structure according to the present invention may further contain one or more selected from the group consisting of Ca, Mg, and Zr instead of a part of Fe. All of these elements are arbitrary elements, and all of them control the morphology of MnS.

Ca: 0 to 0.005%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca combines with O in the steel to form a soft oxide, increasing the machinability of the steel. Ca further dissolves in MnS and suppresses stretching of MnS due to rolling or forging. As a result, the anisotropy of the steel is reduced. If even a small amount of Ca is contained, these effects can be obtained to some extent. However, if the Ca content is too high, hard oxides are excessively formed, and the machinability of the steel is rather lowered. If the Ca content is too high, the yield during production is further reduced. Therefore, the Ca content is 0 to 0.005%. The preferred lower limit of the Ca content is 0.0002%, more preferably 0.0004%. The preferred upper limit of the Ca content is 0.004%, more preferably 0.003%.

Mg: 0 to 0.005%
Magnesium (Mg) is an optional element and may not be contained. When contained, Mg combines with O in steel to form an oxide. The formed oxide becomes a precipitation nucleus of MnS and finely disperses MnS. As a result, the forgeability of the steel is increased, and the anisotropy of the steel is further reduced. If even a small amount of Mg is contained, this effect can be obtained to some extent. However, if the Mg content is too high, this effect will saturate. If the Mg content is too high, the yield during production is further reduced. Therefore, the Mg content is 0 to 0.005%. The preferable lower limit of the Mg content is 0.0003%, more preferably 0.0005%. The preferred upper limit of the Mg content is 0.004%, more preferably 0.003%.

Zr: 0-0.01%
Zirconium (Zr) is an optional element and may not be contained. When contained, Zr combines with O in steel to form oxides. The formed oxide becomes a precipitation nucleus of MnS and finely disperses MnS. As a result, the forgeability of the steel is increased, and the anisotropy of the steel is further reduced. If even a small amount of Zr is contained, this effect can be obtained to some extent. However, if the Zr content is too high, this effect will saturate. If the Zr content is too high, the yield during production is further reduced. Therefore, the Zr content is 0-0.01%. The preferred lower limit of the Zr content is 0.0003%, more preferably 0.0005%. The preferred upper limit of the Zr content is 0.005%, more preferably 0.003%.

The chemical composition of the molten steel for mechanical structural steel according to the present invention may further contain one or more selected from the group consisting of Te, Bi, and Pb instead of a part of Fe. All of these elements are optional elements and enhance the machinability of steel.

Te: 0-0.005%
Tellurium (Te) is an optional element and may not be contained. When contained, Te enhances the machinability of steel. Te further suppresses the elongation of MnS. As a result, the anisotropy of the steel is reduced. If even a small amount of Te is contained, these effects can be obtained to some extent. However, if the Te content is too high, cracks may occur during casting. Therefore, the Te content is 0 to 0.005%. The preferred lower limit of the Te content is 0.0003%, more preferably 0.0005%. The preferred upper limit of the Te content is 0.004%, more preferably 0.003%.

Bi: 0 to 0.15%
Bismuth (Bi) is an optional element and may not be contained. When contained, Bi enhances the machinability of steel. If even a small amount of Bi is contained, this effect can be obtained to some extent. However, if the Bi content is too high, this effect will be saturated. If the Bi content is too high, further defects may occur in the steel during casting. Therefore, the Bi content is 0 to 0.15%. The preferred lower limit of the Bi content is 0.001%, more preferably 0.002%. The preferred upper limit of the Bi content is 0.1%, more preferably 0.05%.

Pb: 0 to 0.15%
Lead (Pb) is an optional element and may not be contained. When contained, Pb enhances the machinability of steel. If even a small amount of Pb is contained, this effect can be obtained to some extent. However, if the Pb content is too high, this effect will saturate. If the Pb content is too high, further defects may occur in the steel during casting. Therefore, the Pb content is 0 to 0.15%. The preferred lower limit of the Pb content is 0.01%, more preferably 0.03%. The preferred upper limit of the Pb content is 0.12%, more preferably 0.1%.

The chemical composition of molten steel for machine structural steel according to the present invention may further contain B instead of a part of Fe.

B: 0 to 0.004%
Boron (B) is an optional element and may not be contained. When contained, B dissolves in the steel to increase hardenability and increase the strength of the steel. B is further precipitated as a nitride to enhance the machinability of the steel. If even a small amount of B is contained, these effects can be obtained to some extent. However, if the B content is too high, these effects will be saturated. If the B content is too high, the mechanical properties of the steel will rather deteriorate. Therefore, the B content is 0 to 0.004%. The preferable lower limit of the B content is 0.0005%, more preferably 0.0008%. The preferred upper limit of the B content is 0.0035%, more preferably 0.003%.

The molten steel having the above chemical composition is melted by a well-known method. For example, molten steel is produced by performing primary refining in a converter or electric furnace. Further, the molten steel after the primary refining may be further subjected to the secondary refining by AOD (Argon Oxygen Decarburization), VOD (Vacuum Oxygen Decarburization), RH (RUHRSTAHL-HERAEUS) or the like.

Through the above steps, molten steel having the above chemical composition is melted. As a matter of course, the chemical composition of the mechanical structural steel material of the present invention is also the same as the chemical composition of the molten steel. Therefore, the mechanical structural steel material has the above-mentioned chemical composition.

The above-mentioned molten steel is cast to produce a cast material. Casting as used herein includes casting by the ingot forming method and casting by the continuous casting method. Therefore, the casting material referred to in the present specification is a steel ingot or a slab. The slabs are, for example, slabs, blooms and billets. In the casting process, the molten steel described above is poured into a mold and cooled. The molten steel solidifies by cooling to produce a cast material.

[Cooling in the casting process]
In the casting process, the molten steel is cooled and solidified at a cooling rate according to the C content of the molten steel. As described above, by cooling the molten steel at a cooling rate (° C./s) according to the C content, the dendrite secondary arm spacing can be narrowed, and fine MnS can be dispersed and produced. As a result, a steel material for machine structure having high forgeability and high machinability can be manufactured.

The cooling rate can be adjusted by, for example, the following method. In the case of the ingot method, the cooling rate can be adjusted, for example, by changing the size of the mold. For example, if the cooling rate is to be increased, the cross-sectional area of the cross section perpendicular to the longitudinal direction of the mold (hereinafter referred to as the cross section) may be reduced. If the cooling rate is to be slowed down, the cross-sectional area of the cross section of the mold may be increased.

Similarly, in the case of the continuous casting method, the cooling rate can be adjusted by changing the size of the mold, for example. Specifically, the cooling rate can be adjusted by adjusting the area of the cross section of the mold. In the case of the continuous casting method, in the roll group arranged downstream of the mold of the continuous casting machine, a plurality of fluid nozzles for cooling the slab are arranged between the rolls. Therefore, the cooling rate can be adjusted by adjusting the flow rate of the fluid (cooling liquid typified by water, air, or a mixed fluid of the cooling liquid and air) injected from these plurality of fluid nozzles.

[About equations (1) to (3)]
Preferably, the solidification cooling rate V (° C./s) defined above satisfies equation (1).
Fn1 ≤ V <30 (1)
The solidification cooling rate (° C./s) of the slab is substituted for V in the formula (1). Fn1 in the formula (1) is defined by the formulas (2) and (3).
If a _C C <0.45:
Fn1 = 0.06 × (0.8 + _CC ) (2)
If a _C C ≧ 0.45:
Fn1 = 0.09 × (0.1 + _CC ) (3)
The C content (mass%) of the molten steel is substituted for " _CC " in the formulas (2) and (3).

The solidification cooling rate is the average of the cooling rates from the liquidus line to the solidus line and is determined based on the dendrite secondary arm spacing. The range from the liquid phase line to the solid phase line is, for example, 1500 ° C to 1300 ° C. The method for determining the solidification cooling rate based on the secondary arm spacing of the dendrite of the cast material is as follows.

When the cast material is a steel ingot, the dendrite secondary arm spacing λ2 (μm) is measured in an observation region of 5 mm × 5 mm including the position of the center of gravity of the ingot. When the casting material is a slab, the dendrite secondary arm spacing λ2 (μm) is measured in an observation region of 5 mm × 5 mm including the center of the cross section of the slab in the center of the observation region. Using the measured dendride secondary arm spacing λ2 (μm), the solidification cooling rate V (° C./s) is defined by the following equation (4).
V = (1/60) × (λ2 / 770) ^{− (1 / 0.41)} (4)

From equation (4), the dendrite secondary arm spacing λ2 depends on the solidification cooling rate V. Therefore, the solidification cooling rate V can be obtained by measuring the dendrite secondary arm spacing λ2.

When the solidification cooling rate V defined in the formula (4) satisfies the formula (1), as shown in FIG. 1, the formation of coarse MnS is suppressed and fine MnS is dispersed and generated. Therefore, the forgeability and machinability of the machine structural steel material are improved.

[Hot working process]
The cast material produced by the casting process is hot-worked to produce a steel material for machine structure.

In the hot working process, the cast material is usually hot-worked one or more times. Before each hot working, the material (cast material or intermediate material produced by hot working the cast material one or more times) is heated. After that, hot working is performed on the material. Hot working is, for example, hot forging or hot rolling. When performing hot working a plurality of times, the first hot working is, for example, lump rolling or hot forging, and the next hot working is, for example, finish rolling using a continuous rolling mill. In a continuous rolling mill, for example, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a row. The material after hot working is cooled by a well-known cooling method such as air cooling. In the hot working step, the heating temperature of the material before hot working is a well-known temperature, for example, 950 to 1300 ° C.

Through the above steps, the steel material for machine structure according to the present embodiment is manufactured. The mechanical structural steel material is, for example, steel bar or wire rod. The mechanical structural steel material manufactured by the above manufacturing method can obtain high forging property and high machinability. Hereinafter, the present invention will be described in more detail with reference to Examples.

Molten steels (test numbers 1 to 74) having the chemical compositions shown in Tables 1 to 3 were melted in a 150 kg high frequency furnace.

Using the molten steel, a steel ingot was cast by an ingot forming method. For each ingot, the solidification cooling rate was adjusted by changing the size of the mold. Regarding the molds used for the molten steel of each test number, Tables 4 to 6 show a mold of 200 mm × 300 mm × 700 mm as mold A, a mold of 220 mm × 300 mm × 700 mm as mold B, and a mold of 180 mm × 300 mm × 700 mm as mold C. .. The degree of superheat of molten steel at the time of casting (difference between the molten steel temperature at the time of casting and the liquidus temperature) of each test number was set to 30 ° C.

Subsequently, the solidification cooling rate was determined by the following method. For test numbers 1, 2, and 22, the dendrite secondary arm spacing λ2 at the center of gravity of the steel ingot was measured by the above method, and the solidification cooling rate V was determined based on the formula (4). As for the other test numbers, since the mold size was the same as that of any of test numbers 1, 2, and 22, the same solidification cooling rate V as the test number having the same mold size among the test numbers 1, 2 and 22 I considered it.

The solidification cooling rate V (° C./s) obtained by the above method is shown in Tables 4 to 6. When the solidification cooling rate V satisfies the equation (1), it is determined that the cooling condition is satisfied (“◯” in Tables 4 to 6). On the other hand, when the solidification cooling rate V does not satisfy the formula (1), it is determined that the cooling conditions are not satisfied (“x” in Tables 4 to 6).

Subsequently, hot working was performed on each of the manufactured steel ingots to manufacture a steel material for machine structure. Specifically, the ingot was heated to 1250 ° C. Hot forging (forging) was performed on the heated steel ingot to produce a round bar having a diameter of 60 mm. The temperature of the steel material (round bar) at the end of forging was 1150 ° C.

[Evaluation test]
[MnS maximum particle size measurement test]
The maximum particle size of MnS on the round bar of each test number was determined by the following method. A test piece containing the R / 2 part (the center position of the straight line (radius) connecting the center of the round bar and the outer surface in the cross section) of the cross section (cross section perpendicular to the longitudinal direction) of the round bar of each test number. Created. Of the test pieces, the test pieces were resin-filled and mirror-polished so that the cross section including the R / 2 portion had a microscopic surface. After polishing the observation surface, the observation surface was etched with 3% alcohol nitrate (Nital corrosive liquid). The etched observation surface was observed with a 500x optical microscope, and an arbitrary 50 visual fields (area per visual field was 9000 μm ² ) were specified. A photographic image of the specified 50 fields of view was generated. In each field of view, MnS was identified based on contrast. The area was determined for each of the plurality of identified MnS. The diameter (diameter equivalent to a circle) when the obtained area was converted into a circle having the same area was defined as the particle size (μm) of the MnS. The maximum particle size of MnS determined in 50 fields of view was defined as the maximum particle size (μm) of MnS on the round bar of the test number. The maximum particle size (μm) of MnS is shown in Tables 4 to 6.

[Forgeability evaluation test]
The forgeability evaluation test was carried out by the following method. A part of a round bar having a diameter of 60 mm for each test number was cut. A cutting process was performed on the cut round bar to prepare an inset test piece having a diameter of 20 mm and a length of 30 mm. The central axis of the stationary test piece coincided with the central axis of the round bar having a diameter of 60 mm before cutting.

A thermocouple was attached to the stationary test piece. The stationary test piece to which the thermocouple was attached was heated to 1000 ° C. by high frequency heating. Within 3 seconds after heating, stationary forging was performed on the stationary test piece. At the time of stationary forging, strain was gradually applied to the stationary test piece, and it was confirmed whether or not cracks were generated after the strain was applied. If no cracks occurred, further distortion was applied. When a fine crack (length 0.5-1.0 mm) was first observed with the naked eye or with a magnifying glass, it was determined that the crack had occurred. The strain when cracks occur in the stationary test piece is defined as the critical strain ε (%) and is defined by Eq. (5).
ε = (H ₀ −H) / H ₀ (5)
Here, H ₀ is the length (mm) of the stationary test piece before forging, and H is the length (mm) of the stationary test piece after forging.

Tables 4 to 6 show the critical strain ε (%). For mechanical structural steel materials, it is desirable that the critical strain ε is 85% or more. Therefore, when the critical strain ε was 85% or more, it was judged that the forgeability was high (“◯” in Tables 4 to 6). On the other hand, when the critical strain ε was less than 85%, it was judged that the forgeability was low (“x” in Tables 4 to 6).

[Machinability evaluation test]
The machinability evaluation test was carried out by the following method. A steel bar having a diameter of 60 mm for each test number was cut to a length of 20 mm to obtain a test piece. The test piece was perforated.

Specifically, the cumulative hole depth up to the breakage of the drill was measured by changing the outer peripheral speed of the drill. A high-speed steel straight drill was used as the drill. The nose radius of the drill was 3 mm and the tip angle was 118 °. Drilling was performed at a cutting speed of 10 to 70 m / min, a feed rate of 0.25 mm / rev, and a hole depth of 9 mm. When the cumulative hole depth (hole depth x number of holes) reached 1000 mm, drilling with one drill was completed. In this case, the drill was replaced, the outer peripheral speed of the drill was increased, and the test was repeated until it was damaged. The maximum drill outer peripheral speed capable of drilling a cumulative hole depth of 1000 mm was defined as the outer peripheral cutting speed _VL1000 (m / min) and used as an index of machinability.

Tables 4 to 6 show the maximum cutting speed _VL1000 (m / min). For machine structural steel materials, it is desirable that the maximum cutting speed _VL1000 is 20 m / min. Therefore, when the maximum cutting speed _VL1000 is 20 m / min or more, it is judged that the machinability is high (“◯” in Tables 4 to 6). On the other hand, when the cutting speed _VL1000 was less than 20 m / min, it was judged that the machinability was low (“x” in Tables 4 to 6).

[Test results]
With reference to Tables 1-6, test numbers 1-60 were suitable for chemical composition and cooling conditions. Therefore, the maximum particle size of MnS is less than 20 μm. Therefore, the critical strain ε was 85% or more, showing high forging property. Further, the maximum cutting speed _VL1000 was 20 m / min or more, showing high machinability.

On the other hand, in the steel materials of test numbers 61 to 72, although the chemical composition was appropriate, the solidification cooling rate was lower than that of Fn1. Therefore, the maximum particle size of MnS is 20 μm or more. As a result, the critical strain ε was lower than 85%, and the forgeability was low.

For the steel materials of test numbers 73 and 74, the cooling rate was appropriate, the maximum particle size of MnS was less than 20 μm, and the critical strain ε was 85% or more. On the other hand, the S content was too low. Therefore, the maximum cutting speed _VL1000 was less than 20 m / min, and the machinability was low.

Molten steel (test numbers 75 to 78) having the chemical composition shown in Table 7 was melted using a converter, a ladle refining facility (LF: Ladle Furnace), and an RH (Ruhrstahl-Hausen) vacuum degassing device. ..

Subsequently, continuous casting was carried out at a casting speed of 1.3 to 1.5 m / min using a rectangular mold of 220 mm × 220 mm to produce a slab (220 mm × 220 mm) from the molten steel. The degree of superheat of molten steel at the time of casting was 30 ° C., and the amount of secondary cooling specific water was 0.20 l / kg. Billets were produced by performing bulk rolling on the produced slabs. Finish rolling was carried out on the billets using a continuous rolling mill to produce steel bars (steel materials for machine structure) having a diameter of 60 mm. The heating temperature of the slab during slab rolling was 1200 ° C, and the heating temperature of the billet during finish rolling was 1100 ° C.

In the same manner as in Example 1, the solidification cooling rate V (° C./s), the maximum particle size of MnS (μm), the critical strain ε (%), and the maximum cutting speed _VL1000 (m / min) were determined. Further, the cooling conditions, forgeability, and machinability were also determined in the same manner as in Example 1. For test number 75, the dendrite secondary arm spacing λ2 at the center of gravity of the slab was measured by the above method, and the solidification cooling rate V was determined from the measurement result. For test numbers 76 to 78, the solidification cooling rate V was considered to be equal to test number 75 because the mold size, casting speed, molten steel superheat degree, and secondary cooling specific water amount were the same as those of test number 75. These results are shown in Table 8.

[Test results]
With reference to Tables 7 and 8, test numbers 75 and 76 had appropriate chemical composition and cooling conditions. Therefore, the maximum particle size of MnS is less than 20 μm. As a result, the critical strain ε was 85% or more, showing high forgeability. Further, the maximum cutting speed _VL1000 was 20 m / min or more, showing high machinability.

In the steel materials of test numbers 77 and 78, although the chemical composition was appropriate, the solidification cooling rate V was lower than Fn1. Therefore, the maximum particle size of MnS is 20 μm or more. As a result, the critical strain ε was lower than 85%, and the forgeability was low.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

Claims (7)

By mass%
C: 0.1 to 1.0%,
Si: 0.01-1.5%,
Mn: 0.05-2.0%,
P: 0.003 to 0.2%,
S: 0.01-0.05%,
N: 0.01% or less,
T. O: 0.0018% or less,
Cr: 0-2.0%,
Ni: 0-2.0%,
Mo: 0-1.0%,
V: 0-1.0%,
Nb: 0-0.2%,
Ti: 0-0.1%,
Ca: 0-0.005%,
Mg: 0-0.005%,
Zr: 0-0.01%,
Te: 0-0.005%,
Bi: 0-0.15%,
A molten steel containing Pb: 0 to 0.15% and B: 0 to 0.004% and having a chemical composition in which the balance is Fe and impurities is solidified by an ingot method or a continuous casting method to obtain a cast material. The casting process to manufacture and
After reheating the cast material, it is provided with a step of performing hot working to manufacture a steel material for machine structure.
In the casting step, a method for producing a steel material for machine structure, in which the molten steel is solidified at a cooling rate corresponding to the C content of the molten steel. The method for manufacturing a steel material for machine structure according to claim 1.
In the casting step, a method for producing a steel material for machine structure, wherein the solidification cooling rate V (° C./sec), which is the average cooling rate from the liquidus temperature to the solidus temperature, satisfies the formula (1).
Fn1 ≤ V <30 (1)
Here, Fn1 in the equation (1) is defined by the equations (2) and (3).
If a _C C <0.45:
Fn1 = 0.06 × (0.8 + _CC ) (2)
If a _C C ≧ 0.45:
Fn1 = 0.09 × (0.1 + _CC ) (3)
Here, _{C C} in the formula (2) and (3) indicates the C content of the molten steel (mass%).
When the dendrite secondary arm spacing of the cast material is defined as λ2 (μm), the solidification cooling rate V is defined by the equation (4).
V = (1/60) × (λ2 / 770) ^{− (1 / 0.41)} (4) The method for manufacturing a steel material for machine structure according to claim 1 or 2.
The chemical composition is
Cr: 0.01-2.0%,
A method for producing a steel material for machine structure, which contains one or more selected from the group consisting of Ni: 0.05 to 2.0% and Mo: 0.05 to 1.0%. The method for manufacturing a steel material for machine structure according to any one of claims 1 to 3.
The chemical composition is
V: 0.05 to 1.0%,
A method for producing a steel material for machine structure, which contains one or more selected from the group consisting of Nb: 0.005 to 0.2% and Ti: 0.005 to 0.1%. The method for manufacturing a steel material for machine structure according to any one of claims 1 to 4.
The chemical composition is
Ca: 0.0002 to 0.005%,
Mg: 0.0003 to 0.005%, and
Zr: A method for producing a steel material for machine structure, which contains one or more selected from the group consisting of 0.0003 to 0.01%. The method for manufacturing a steel material for machine structure according to any one of claims 1 to 5.
The chemical composition is
Te: 0.0003 to 0.005%,
Bi: 0.001 to 0.15%, and
Pb: A method for producing a steel material for machine structure, which contains one or more selected from the group consisting of 0.01 to 0.15%. The method for manufacturing a steel material for machine structure according to any one of claims 1 to 6.
The chemical composition is
B: A method for producing a steel material for machine structure, which contains 0.0005 to 0.004%.

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