JP6593111B2 - Zr-containing forging steel - Google Patents

Description

  The present invention relates to a Zr-containing forging steel material that can prevent nozzle clogging during continuous casting.

  In recent forged parts for automobile engines and forged parts for undercarriage, non-heat treated steel for hot forging that can omit the tempering treatment (quenching-tempering treatment) is applied. The non-tempered steel material is a steel material whose component composition is designed so that excellent mechanical properties can be realized with air cooling or air cooling after hot forging.

  One of the parts to which non-tempered steel is widely applied is an engine connecting rod (hereinafter sometimes referred to as “connecting rod”). The connecting rod is a part that transmits the movement of the piston of the engine to the crankshaft, and is composed of two parts, a cap and a rod.

  The connecting rod is attached to the crankshaft by fitting the cap and the rod into a large end portion with the crankshaft in between, and fastening them with bolts.

  Conventionally, a connecting rod is manufactured by forging a cap and a rod separately, or mechanically cutting a forged piece and then machining the mating surface of the cap and the rod with high accuracy. Have been made. Also, the mating surfaces are often subjected to pin processing so that the mating surfaces of the cap and the rod do not shift, and there is a problem that the manufacturing process becomes more complicated and the manufacturing cost increases in the production of the connecting rod.

  Therefore, in recent years, after the cap and rod are forged into a single shape, the inner end of the large end is notched, and impact tensile stress is applied in the cold, and the cap and rod are split into fractures. Is used as it is as a mating surface, and a method of attaching to the crankshaft is adopted.

  In this construction method, machining of the mating surfaces can be omitted, and by using the irregularities on the fracture surface, pin processing for preventing misalignment is not necessary, so that the processing cost of the parts can be reduced. Furthermore, since the area of the mating surface can be reduced by eliminating the pins, the connecting rod itself can be reduced in size and weight.

  A steel material used for the production of a fracture split type connecting rod requires a high degree of fracture separation and machinability. In recent years, from the viewpoint of fuel efficiency, there is a strong movement to increase the strength and weight of the steel itself. For this reason, development of the steel material which has high intensity | strength, fracture-separation property, and machinability is calculated | required.

  On the other hand, Patent Document 1 discloses a non-tempered steel for hot forging manufactured without impairing manufacturability and mechanical properties and without adding Pb or the like and having excellent break separation and machinability. Steel has been proposed.

  Specifically, the amount of MnS inclusions is dispersed in a large amount and finely by optimizing the amounts of C and V to improve the break separation and further controlling the amounts of Zr, Ca, and Al simultaneously. As a result, the fracture separation is improved and the machinability is improved without impairing the mechanical properties and manufacturability. According to Patent Document 1, it is possible to obtain a steel material having both high strength, fracture separation, and machinability.

  However, in the steel material of Patent Document 1, since Zr is added, a high melting point Zr-based oxide is generated in the molten steel. The high melting point Zr-based oxide is easily agglomerated and coalesced in the molten steel, and is deposited and deposited as a coarse Zr-based oxide on the inner wall of the continuous casting nozzle. For this reason, Zr-added steel is a steel type that easily causes clogging of the continuous casting nozzle.

  During continuous casting, when the continuous casting nozzle is blocked and the flow of molten steel in the continuous casting nozzle is obstructed, the molten steel surface in the mold fluctuates and the slab quality deteriorates. In the worst case, when the continuous casting nozzle is completely blocked, the continuous casting operation must be stopped, and the production efficiency is lowered.

  Several countermeasures have been proposed so far for nozzle blockage during continuous casting of Zr-containing steel.

For example, Patent Document 2 proposes a method in which Ca is added in a ladle after Al deoxidation, and Zr is added immediately after the addition of Ca to prevent clogging of the immersion nozzle. However, in this method, CaO produced by the addition of Ca and ZrO ₂ produced by the addition of Zr may form a high melting point composite oxide of CaO · ZrO ₂ , and worsen the nozzle clogging.

Patent Document 3 proposes a method of preventing nozzle clogging by disposing a refractory containing ZrO _{2 in} a part of a sliding nozzle, an immersion nozzle or the like in continuous casting of Zr-containing steel. However, when ZrO ₂ is used as the material of the nozzle, there is a problem that the manufacturing of the nozzle becomes complicated and the cost increases.

  In Patent Document 4, when manufacturing a molten steel containing 0.1 to 1.5% by mass of Zr and containing 8% by mass or more of Cr, the molten steel after the reduction treatment in the reduction process of chromium oxide in a refining furnace. A method has been proposed in which the composition is [Al] ≧ 0.15 mass%, or [Si] ≧ 0.8 mass%.

  This method relates to a Cr-containing steel having a very large amount of Zr, and also has a large amount of [Al] and [Si] as the molten steel composition. Therefore, this method is applied to the production of general carbon steel. It is difficult to do.

Patent Document 5, and deoxidation with Al and Ti, to control the molten steel oxide Al ₂ O ₃ -Ti ₂ O ₃ based oxide, and a total oxygen concentration in the molten steel 0.008 wt% A method of adding Zr in an amount specified by a predetermined relational expression to the molten steel described below has been proposed. Since this method utilizes deoxidation by Ti, it is difficult to apply to other than steel having a high Ti content.

  Patent Document 6 proposes a method of adjusting the amount of dissolved oxygen in molten steel before addition of Zr to 0.0002 to 0.015% and then adding an amount of Zr corresponding to the amount of dissolved oxygen.

In this method, the amount of Zr added is defined using only the amount of dissolved oxygen. However, since Zr is a strong deoxidizing element, ZrO ₂ is produced by reducing the oxide present in the molten steel. Therefore, even a dissolved oxygen amount is the same, also different amounts of ZrO ₂ generating Different oxide amount in the molten steel, accordingly, also be different embodiment of nozzle clogging.

  In Patent Document 7, in the method of continuously casting Zr-containing steel containing 0.001 to 0.1% by mass of Zr, the inner wall of the immersion nozzle is composed of a refractory having an MgO content of 60 to 85% by mass. A method has been proposed. However, when MgO is used as the nozzle material, the manufacture of the nozzle becomes complicated and the cost increases. Further, when MgO is used as the nozzle material, there is a problem that the thermal shock resistance is lowered.

Japanese Patent No. 5251873 Japanese Patent Laid-Open No. 01-143740 JP 07-299549 A JP 2008-101259 A JP 2010-280953 A JP 2012-046815 A JP 2012-143767 A

  Conventional prevention measures against nozzle clogging that occurs when continuously casting a Zr-containing steel material having strength, fracture separability, and machinability used in the production of fracture split type connecting rods are effective as described above. It may work but may not work well. That is, it is difficult to reliably prevent nozzle clogging during continuous casting and to obtain a Zr-containing steel material used for manufacturing a fractured split type connecting rod.

  Therefore, in the present invention, in continuous casting of a Zr-containing steel material, in view of the conventional nozzle clogging prevention measures, the nozzle clogging is surely prevented, and the strength, rupture separability, and An object is to obtain a Zr-containing forging steel material having machinability, and an object is to provide a Zr-containing forging steel material that can solve the problem.

  The inventors diligently studied the component composition of the Zr-containing forging steel material that solves the above problems. As a result, using a standardized amount of low-melting-point oxide inclusions as the main element and further defining the component composition of the steel, the nozzle clogging can be achieved while maintaining good castability of the molten steel during continuous casting. It has been found that a Zr-containing forging steel material having strength, break separation property, and machinability suitable for producing a fracture split type connecting rod can be obtained. This finding will be described later.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) Component composition is mass%, C: 0.20-0.60%, Si: 0.15-2.50%, Mn: 0.20-2.00%, P: 0.010 0.150%, S: 0.040 to 0.150%, V: 0.10 to 0.50%, Al: 0.001 to 0.010%, Zr: 0.0005 to 0.0050%, Ca : 0.0004 to 0.0050%, N: 0.002 to 0.020% included, O: limited to 0.0035% or less, the balance is made of Fe and unavoidable impurities, contained in the steel material, An oxide inclusion having a width perpendicular to the rolling direction of 5 μm or more, and normalized as (% Al ₂ O ₃ ) + (% CaO) + (% ZrO ₂ ) = 100% (% is mass%). The number of oxide inclusions satisfying the following formula (1) is 20% or more in total in terms of the number ratio. Forging steel.
4.5 × (% CaO) + (% ZrO ₂ ) ≧ 135 (1)
Here, (% Al ₂ O ₃ ): mass% of Al ₂ O ₃
(% CaO): mass% of CaO
(% ZrO ₂ ): mass% of ZrO ₂

  (2) The above-mentioned component composition is further in mass%, Cr: 0.05 to 2.00%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%, The Zr-containing forging steel according to (1) above, comprising two or more types.

  (3) The Zr-containing forging steel as described in (1) or (2) above, wherein the component composition further includes Mg: 0.0005 to 0.0050% by mass.

  According to the present invention, there is provided a Zr-containing forging steel material having strength, fracture separability, and machinability, which reliably prevents nozzle clogging during continuous casting, and is suitable for production of a fracture split type connecting rod. Can be provided.

3 is a ZrO ₂ —CaO—Al ₂ O ₃ ternary phase diagram at 1873K. FIG. It is a figure which shows the relationship between the number ratio (%) of the inclusion which satisfies (1) Formula, and a nozzle obstruction | occlusion parameter | index.

The Zr-containing forging steel material of the present invention (hereinafter sometimes referred to as “the present invention steel material”)
Component composition is mass%, C: 0.20-0.60%, Si: 0.15-2.50%, Mn: 0.20-2.00%, P: 0.010-0.150 %, S: 0.040 to 0.150%, V: 0.10 to 0.50%, Al: 0.001 to 0.010%, Zr: 0.0005 to 0.0050%, Ca: 0.00. 0004 to 0.0050%, N: 0.002 to 0.020% included, O: limited to 0.0035% or less, the balance is made of Fe and unavoidable impurities, contained in the steel, in the rolling direction When the vertical inclusions are oxide inclusions having a width of 5 μm or more and normalized as (% Al ₂ O ₃ ) + (% CaO) + (% ZrO ₂ ) = 100% (% is mass%), The number of oxide inclusions satisfying the formula (1) is 20% or more in total in terms of the number ratio.
4.5 × (% CaO) + (% ZrO ₂ ) ≧ 135 (1)
Here, (% Al ₂ O ₃ ): mass% of Al ₂ O ₃
(% CaO): mass% of CaO
(% ZrO _2): ZrO ₂ mass%

  In addition, the steel composition of the present invention further has a component composition of mass%, Cr: 0.05 to 2.00%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%. 1 type or 2 types or more and / or Mg: 0.0005 to 0.0050% is contained, It is characterized by the above-mentioned.

  Hereinafter, the steel material of the present invention will be described. First, the background from the idea to the steel material of the present invention will be described.

  In order to find a method for preventing clogging of a continuous casting nozzle in the production of a non-heat treated steel for hot forging excellent in fracture separation and machinability described in Patent Document 1, the present inventors Focusing on the behavior of inclusions in molten steel, which is the main cause, various production conditions were changed, continuous casting tests were conducted, clogging status of continuous casting nozzles, nozzle deposits, inclusions in molten steel, and their relationships Scoured. As a result, the following became clear.

(a) The deposit when the continuous casting nozzle is clogged is a deposit mainly composed of a high melting point ZrO ₂ —CaO-based inclusion, and partly a low melting point ZrO ₂ —CaO—Al ₂ O ₃ inclusion. Contains deposits.
(b) The inclusions in the molten steel are almost the same as the deposits attached to the continuous casting nozzle. That is, the cause of the clogging of the continuous casting nozzle is an inclusion present in the molten steel.

  Next, the operating conditions were changed, and the relationship between the blockage of the continuous casting nozzle and the inclusions in the molten steel was investigated. As a result, the following became clear.

(c) inclusion in the molten steel is, when only high-melting inclusions ZrO ₂ -CaO-based, although clogging of the continuous casting nozzle vigorously occurs, inclusions in the molten steel _{is, ZrO 2 -CaO-Al 2 O} 3 system In the case of containing a lot of low melting point inclusions, clogging of the continuous casting nozzle does not occur.

  Based on the above knowledge, the present inventors said that it is necessary to increase the amount of low melting point inclusions present in the molten steel to a predetermined amount or more in order to reliably prevent clogging of the continuous casting nozzle. I came up with the idea.

FIG. 1 shows a ZrO ₂ —CaO—Al ₂ O ₃ ternary phase diagram at 1873K (T. Murakami, H. Fukuyama, T. Kishida, M. Susa and K. Nagata, Metallurgical Materials Transactions B, 31B, 25.) In FIG. 1, the colored portion is a region of a liquid phase or a solid-liquid coexisting phase at 1873K. FIG. 1 is a diagram showing the molar concentration (mol%), but the above region can be expressed by the following equation (1) when converted to mass%.
4.5 × (% CaO) + (% ZrO ₂ ) ≧ 135 (1)
(% CaO): mass% of CaO
(% ZrO ₂ ): mass% of ZrO ₂

Inclusions that satisfy the above formula (1) form low-melting-point ZrO ₂ —CaO—Al ₂ O ₃ inclusions that do not cause nozzle clogging during continuous casting. Therefore, the present inventors investigated the relationship between the number ratio of inclusions satisfying the above equation (1) and nozzle clogging. The results are shown in FIG.

The number ratio (%) of inclusions satisfying the expression (1) on the horizontal axis is oxide inclusions contained in the steel bar and having a width perpendicular to the rolling direction of 5 μm or more, (% Al ₂ O ₃ ) + (% CaO) + (% ZrO ₂ ) = number ratio of the number of oxide inclusions satisfying the above equation (1) when normalized to 100% (% is mass%). .

For example, when the composition of certain oxide inclusions is (% Al ₂ O ₃ ) = 40%, (% CaO) = 50%, (% ZrO ₂ ) = 10%, the above formula (1) is satisfied. . On the other hand, the composition of some oxide _{inclusions, (% Al 2 O 3)} = 40%, (% CaO) = 10%, if the _{(% ZrO 2) = 50%} , does not satisfy the above equation (1) .

  When ten oxide inclusions are detected and five of the compositions satisfy the above formula (1) according to the above criteria, the number of oxide inclusions satisfying the above formula (1) The number ratio is 50%.

  The nozzle blockage index on the vertical axis is the ratio (actual opening / original opening) between the actual opening of the continuous casting nozzle and the actual opening calculated from the molten steel throughput and the molten steel head. The larger the index value, the more frequently nozzle clogging occurs.

  The target of the nozzle blockage index is 1 or less. It can be seen from FIG. 2 that the blockage of the continuous casting nozzle can be remarkably suppressed when the number ratio (%) of inclusions satisfying the above expression (1) is 20% or more.

Based on the above findings, the present inventors are oxide inclusions having a width perpendicular to the rolling direction of 5 μm or more, which are contained in steel bars, and are (% Al ₂ O ₃ ) + (% CaO). When normalized as + (% ZrO ₂ ) = 100% (% is% by mass), the number of oxide inclusions satisfying the above formula (1) is 20% or more in terms of the number ratio. Stipulated.

  Inclusions to be counted are oxide inclusions. There are sulfide inclusions in the inclusions, but most of the sulfide inclusions are inclusions that are generated during or after solidification of the molten steel and have little effect on nozzle clogging during continuous casting. Therefore, sulfide inclusions are not counted.

Since the oxide inclusions produced are Al ₂ O ₃ , CaO, and ZrO ₂ , in the calculation of the number ratio of oxide inclusions, (% Al ₂ O ₃ ) + (% CaO) + It was normalized as (% ZrO ₂ ) = 100% (% is mass%). By this standardization, the relationship between the number ratio of oxide inclusions and the nozzle clogging index can be objectively evaluated.

  Small inclusions whose width perpendicular to the rolling direction is less than 5 μm have little adverse effect on nozzle clogging. In many cases, they are compounded with sulfide inclusions in steel bars and steels for accurate composition analysis. Therefore, the oxide inclusions to be counted are oxide inclusions whose width perpendicular to the rolling direction is 5 μm or more.

The number ratio of oxide inclusions that are 5 μm or more in width perpendicular to the rolling direction and satisfy the above formula (1) is (% Al ₂ O ₃ ) + (% CaO) + (% ZrO ₂ ) = 100% (% is mass%), and the total is 20% or more. When the number ratio is less than 20%, as shown in FIG. 2, the nozzle blockage index rapidly increases. The number ratio is preferably 40% or more, more preferably 60% or more (see FIG. 2).

  Although the aspect of the inclusion characterizing the steel of the present invention is as described above, a specific method for obtaining the steel of the present invention is as follows.

  The component composition of the molten steel primarily refined in a converter or electric furnace is adjusted, and required amounts of Al, Zr, and Ca are added. If it is added in order of decreasing deoxidizing power, it is easy to reduce O in the molten steel. Therefore, the addition is preferably performed in the order of Al, Zr, and Ca. In particular, Al is preferably added as early as possible. For example, Al is preferably added when steel is output from a converter or an electric furnace.

After the addition of Al, the molten steel is stirred by an RH degassing refining device, a ladle heating refining device, a simple molten steel processing facility, etc., and the Al ₂ O ₃ inclusions are floated and separated. Thereafter, Zr and Ca are preferably added in this order to form an Al ₂ O ₃ —ZrO ₂ —CaO-based composite oxide (inclusion).

  The form of Zr and Ca added to the molten steel is not particularly limited. Zr may be added in the form of, for example, pure Zr, Zr—Si alloy, Fe—Zr alloy or the like. What is necessary is just to add Ca with forms, such as Ca-Si alloy. Even after the addition of Zr and Ca, the molten steel is sufficiently agitated to float and separate the inclusions.

Various methods can be considered for controlling the inclusions to inclusions having a predetermined composition depending on the manufacturing process and the like. For example, a method of controlling the slag composition, maintaining sufficient equilibrium by carrying out sufficient stirring, and controlling the composition of inclusions in the molten steel to be almost the same as the slag composition, increasing the liquid phase ratio of the slag, And the like, and the ZrO ₂ oxide can be easily absorbed and controlled to have a predetermined inclusion composition.

  The molten steel thus produced is continuously cast according to a conventional method to obtain a slab. Continuous casting should be performed with care to avoid mixing oxygen sources as much as possible. This is because when the oxygen source is mixed into the molten steel, the inclusion composition changes and the O concentration increases, which is prevented. The continuously cast slab is subjected to hot rolling in accordance with a conventional method to produce a steel material for forging (the steel material of the present invention).

  The steel material of the present invention is a steel material that can be subjected to hot forging or cold forging without being subjected to heat treatment without being subjected to heat treatment, but after being subjected to heat treatment for property improvement, it is subjected to hot forging or cold forging. Also good.

  Next, the reason for limiting the component composition of the steel of the present invention will be described. The limitation of the component composition is mainly necessary for ensuring excellent fracture separation and machinability in addition to controlling the composition of inclusions. Hereinafter,% related to the component composition means mass%.

C: 0.20% to 0.60%
C is an element that ensures the tensile strength of the part and increases the fraction of the pearlite structure having low ductility and toughness and contributes to the improvement of break separation. If it is less than 0.20%, the effect of addition cannot be sufficiently obtained, so C is made 0.20% or more. Preferably it is 0.25% or more.

  On the other hand, if it exceeds 0.60%, the structure becomes coarse and the yield ratio decreases, so C is made 0.60% or less. Preferably it is 0.55% or less.

Si: 0.15 to 2.50%
Si is an element that strengthens ferrite by solid solution strengthening, lowers ductility and toughness, and contributes to improvement in fracture separation. If it is less than 0.15%, the effect of addition cannot be sufficiently obtained, so Si is made 0.15% or more. Preferably it is 0.30% or more.

  On the other hand, if it exceeds 2.50%, the fraction of the ferrite structure becomes excessive and, on the contrary, the fracture separability decreases, so Si is made 2.50% or less. Preferably it is 2.30% or less.

Mn: 0.20 to 2.00%
Mn is an element that strengthens ferrite by solid solution strengthening, lowers ductility and toughness, and contributes to improvement in fracture separation. If it is less than 0.20%, the effect of addition cannot be sufficiently obtained, so Mn is made 0.20% or more. Preferably it is 0.30% or more.

  On the other hand, if it exceeds 2.00%, the lamellar spacing of the pearlite becomes small, the ductility and toughness of the pearlite increase, the fracture separability decreases, and the bainite structure is easily generated, and the fracture separability is greatly increased. Since it falls, Mn shall be 2.00% or less. Preferably it is 1.75% or less.

P: 0.010 to 0.150%
P is an element that reduces the ductility and toughness of ferrite and pearlite and contributes to the improvement of fracture separation. If it is less than 0.010%, the effect of addition cannot be sufficiently obtained, so P is made 0.010% or more. Preferably it is 0.015% or more.

  On the other hand, if it exceeds 0.150%, the hot ductility is lowered, and cracks and wrinkles are likely to occur during hot working, so P is made 0.150% or less. Preferably it is 0.100% or less.

S: 0.040 to 0.150%
S is an element that combines with Mn to form MnS (MnS inclusions) and contributes to improvement of machinability. In addition, when S is added in a small amount of Zr and Ca and the amount of Al added is limited, MnS-based inclusions having a small aspect ratio that becomes a propagation path of cracks during fracture division are finely dispersed in a large amount in steel. Thus, it is an element that contributes to the improvement of break separation.

  If it is less than 0.040%, the effect of addition cannot be sufficiently obtained, so S is made 0.040% or more. Preferably it is 0.080% or more. On the other hand, if it exceeds 0.150%, the hot ductility is lowered, and cracks and wrinkles are likely to occur during hot working, so S is made 0.150% or less. Preferably it is 0.100% or less.

V: 0.10 to 0.50%
V mainly forms carbides and carbonitrides during cooling after hot forging, strengthens ferrite by precipitation strengthening, reduces ductility and toughness, improves fracture separability, yield ratio It is an element that contributes to improvement. If it is less than 0.10%, the effect of addition cannot be sufficiently obtained, so V is made 0.10% or more. Preferably it is 0.30% or more.

  On the other hand, if it exceeds 0.50%, the effect of addition is saturated, so V is 0.50% or less. Preferably it is 0.35% or less.

Al: 0.001 to 0.010%
Zr: 0.0005 to 0.0050%
Ca: 0.0004 to 0.0050%
The addition amount of Al, Zr, and Ca is simultaneously controlled to control the inclusion composition within a predetermined range. In addition, a large amount of MnS inclusions having a small aspect ratio can be finely dispersed in the steel. Since the MnS inclusions serve as a propagation path of cracks at the time of fracture splitting, the MnS inclusions have an effect of improving fracture separation.

  Al is a deoxidizing element and is an element that acts to adjust the inclusion composition to a predetermined range. If it is less than 0.001%, the deoxidation effect and the effect of adjusting the inclusion composition cannot be obtained sufficiently, so Al is made 0.001% or more. Preferably it is 0.003% or more, More preferably, it is 0.005% or more.

On the other hand, if it exceeds 0.010%, Al ₂ O ₃ is preferentially generated, the inclusion composition not only deviates from the predetermined range, but also the Zr system that acts to uniformly disperse MnS-based inclusions and Al prevents the generation of Ca-based oxides and promotes the generation of coarse MnS-based inclusions that impair mechanical properties such as break separation and fatigue, so Al is made 0.010% or less. Preferably it is 0.008% or less, More preferably, it is 0.007% or less.

Zr is a deoxidizing element and is an element that forms ZrO ₂ and acts to control the inclusion composition within a predetermined range. Since ZrO ₂ serves as a crystallization and precipitation nucleus for MnS inclusions, Zr is an element that increases the crystallization and precipitation sites of MnS inclusions and contributes to uniform and fine dispersion of MnS inclusions.

  If it is less than 0.0005%, the effect of addition cannot be sufficiently obtained, so Zr is made 0.0005% or more. Preferably it is 0.0010% or more, More preferably, it is 0.0015. On the other hand, if it exceeds 0.0050%, the effect of addition is saturated, so the content is made 0.0050% or less. Preferably it is 0.0035% or less, More preferably, it is 0.0030%.

  Ca is a deoxidizing element, and is an element that forms CaO and acts to control the inclusion composition within a predetermined range. Ca is dissolved in MnS inclusions to form a composite sulfide. It is also an element that forms and lowers its deformability and suppresses the stretching of MnS inclusions during rolling or hot forging.

  If it is less than 0.0004%, the effect of addition cannot be sufficiently obtained, so Ca is made 0.0004% or more. Preferably it is 0.0008% or more, More preferably, it is 0.00120% or more. On the other hand, if it exceeds 0.0050%, the effect of addition is saturated, so Ca is made 0.0050% or less. Preferably it is 0.0040% or less, More preferably, it is 0.0030% or less.

N: 0.002 to 0.020%
N forms V nitride and V carbonitride that function as ferrite transformation nuclei during cooling after hot forging, promotes ferrite transformation, and suppresses the formation of bainite structure that significantly impairs fracture separation. It is an element that makes the action. Preferably it is 0.005% or more. More preferably, it is 0.008% or more.

  On the other hand, if it exceeds 0.020%, the hot ductility is lowered and cracks and wrinkles are likely to occur during hot working, so N is made 0.020% or less. Preferably it is 0.017% or less, More preferably, it is 0.015% or less.

O: 0.0035% or less O is an element that makes it difficult to control the composition of inclusions when it is excessively present in the steel of the present invention. If it exceeds 0.0035%, the composition control of inclusions becomes extremely difficult, so O is limited to 0.0035% or less. Preferably it is 0.0030% or less, More preferably, it is 0.0025% or less.

  The component composition of the steel of the present invention further strengthens ferrite, lowers ductility and toughness, and obtains good fracture separability, so that Cr: 0.05 to 2.00%, Nb: 0.005 to 0 One or two or more of 0.050% and Ti: 0.005 to 0.050% may be included.

Cr: 0.05-2.00%
Cr, like Mn, is an element that strengthens ferrite by solid solution strengthening, reduces ductility and toughness, and contributes to the improvement of fracture separation. If it is less than 0.05%, the effect of addition cannot be sufficiently obtained, so Cr is made 0.05% or more. Preferably it is 0.10% or more.

  On the other hand, if it exceeds 2.00%, the lamellar spacing of the pearlite is reduced, the ductility and toughness of the pearlite is increased, the fracture separability is lowered, and the bainite structure is easily formed, and the fracture separability is increased. Therefore, Cr is made 2.00% or less. Preferably it is 1.50% or less.

Nb: 0.005 to 0.050%
Nb is an element that contributes to the improvement of fracture separation by forming carbides and carbonitrides, strengthening ferrite by precipitation strengthening, reducing ductility and toughness, and cooling during hot forging. is there. If it is less than 0.005%, the effect of addition cannot be sufficiently obtained, so Nb is made 0.005% or more. Preferably it is 0.010% or more.

  On the other hand, if it exceeds 0.050%, the effect of addition is saturated, so Nb is made 0.050% or less. Preferably it is 0.035% or less.

Ti: 0.005 to 0.050%
Ti is an element that contributes to the improvement of fracture separability by forming carbides and carbonitrides, strengthening ferrite by precipitation strengthening, lowering ductility and toughness during cooling after hot forging. is there. If less than 0.005%, the effect of addition cannot be sufficiently obtained, so Ti is made 0.005% or more. Preferably it is 0.010% or more.

  On the other hand, if it exceeds 0.050%, the effect of addition is saturated and the machinability may be lowered, so Ti is made 0.050% or less. Preferably it is 0.035% or less.

  Further, the component composition of the steel of the present invention may contain 0.0005 to 0.0050% of Mg in order to control the dispersion mode of inclusions and further improve the break separation property.

Mg: 0.0005 to 0.0050%
Mg is a deoxidizing element, and becomes an crystallization and precipitation nucleus of MnS-based inclusions, and is an element that forms Mg oxide that functions to uniformly disperse MnS-based inclusions and improve anisotropy. . Mg is an element that acts as a solid solution in MnS inclusions to form a composite sulfide, lowers its deformability, and suppresses stretching of MnS inclusions during rolling and hot forging. It is.

  If it is less than 0.0005%, the effect of addition cannot be sufficiently obtained, so Mg is made 0.0005% or more. Preferably it is 0.0010% or more. On the other hand, if it exceeds 0.0050%, a large amount of inclusions and clusters thereof are produced in large quantities, and mechanical properties such as fatigue properties are deteriorated, so Mg is made 0.0050% or less. Preferably it is 0.0040% or less.

  The component composition of the steel of the present invention may include Te, Zn, Sn, etc. as long as the characteristics of the steel of the present invention are not impaired in addition to the above elements.

  Although trace amounts of Cu, Ni, and Mo do not particularly affect the properties of the steel of the present invention, both Cu and Ni as unavoidable impurities are 0. It is preferable to limit it to 15% or less, and similarly to limit Mo to 0.01% or less.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
When the molten steel primarily refined in a converter having a capacity of 300 tons was put into a ladle, Al metal was added and Al deoxidation was performed. Next, degassing treatment and component adjustment were performed using a ladle heating type refining apparatus and an RH type degassing refining apparatus. Thereafter, Zr was added to the molten steel using an Fe—Zr alloy. After the addition of Zr, stirring was performed over a period of time equal to or longer than the uniform mixing time, and the inclusions were removed.

  The uniform mixing time is the time required for the added alloy element to asymptotically fall within a predetermined range near the final value, and is a time correlated with the stirring force. The uniform mixing time can be obtained by a tracer experiment, and using a relational expression with a known stirring power density (for example, Shigeo Asai, Tetsuo Okamoto, Kosei, whip, iron and steel 68, 426). Can be estimated.

Furthermore, Ca was added to the molten steel using a Ca—Si alloy wire. The composition control of inclusions, controlling the slag basicity (ratio of the CaO concentration and SiO ₂ concentration) increases the liquid phase ratio of the slag, to promote the reaction between the molten steel, it is easy to absorb the ZrO ₂ oxide It was carried out by the method.

There are _two methods for increasing the liquid phase ratio of slag: controlling the basicity of slag (ratio of CaO concentration to SiO ₂ concentration) and adding CaF ₂ or Na ₂ O to the slag. May be.

  The molten steel of the non-heat treated steel material for hot forging thus melted was continuously cast. The continuous casting was carried out with a bloom 4-strand continuous caster having a mold cross-sectional size of 560 mm × 350 mm. The superheat degree (the value obtained by subtracting the liquidus temperature of the steel having the composition of the molten steel from the temperature of the molten steel) of the molten steel in the tundish during continuous casting was 10 to 60 ° C. The throughput of molten steel (amount of cast molten steel per unit time) was 0.5 to 1.5 t / min.

  In Table 1, the component composition (mass%) of the molten steel of an invention example and a comparative example is shown.

Table 2 shows the slag liquid phase ratio (%) of the ladle slag at 1500 ° C., the number ratio (%) of inclusions satisfying the above formula (1), Al ₂ O ₃ + CaO + ZrO ₂ in the invention examples and comparative examples. = The average composition of inclusions when normalized to 100% and the nozzle clogging index are shown.

  The liquid phase ratio of ladle slag was estimated from the slag composition by thermodynamic equilibrium calculation (Wataru Yamada, Toru Matsumiya, Nippon Steel Technical Report, 342 (1991), 38, reference). The nozzle clogging index is a ratio between the actual opening of the continuous casting nozzle and the original opening calculated from the molten steel throughput and the molten steel head. The larger the nozzle blockage index, the more frequently nozzle blockage occurs, and the target is 1 or less (see FIG. 2).

  In Invention Examples 1 to 35, the liquid phase ratio of the ladle slag is as high as 85% or more, and as a result, the number ratio of inclusions satisfying the formula (1) is 20% or more, and the nozzle The clogging index was 1 or less, nozzle clogging was prevented, and continuous casting was possible without problems.

  On the other hand, in Comparative Examples 36 to 48, the liquid phase ratio of the ladle slag is as low as less than 85%, the inclusion number ratio that satisfies the formula (1) is less than 20%, and nozzle clogging frequently occurs. In some comparative examples, casting was interrupted.

  As described above, according to the present invention, it is possible to reliably prevent nozzle clogging during continuous casting, and to provide Zr having strength, fracture separation, and machinability suitable for the production of a fracture split type connecting rod. A steel material for containing forging can be provided. Therefore, the present invention has high applicability in the steel industry.

Claims (3)

Component composition is mass%, C: 0.20-0.60%, Si: 0.15-2.50%, Mn: 0.20-2.00%, P: 0.010-0.150 %, S: 0.040 to 0.150%, V: 0.10 to 0.50%, Al: 0.001 to 0.010%, Zr: 0.0005 to 0.0050%, Ca: 0.00. 0004 to 0.0050%, N: 0.002 to 0.020% included, O: limited to 0.0035% or less, the balance consists of Fe and unavoidable impurities, contained in the steel material, in the rolling direction a vertical width oxide inclusions of more than _{5μm, (% Al 2 O 3} ) + (% CaO) + (% ZrO 2) = 100% (% is wt%) and when normalized, ( % Al ₂ O ₃₎ is more than 9%, the (% CaO) is 14% or more, and (% ZrO ₂₎ is not less than 5%, the following formula (1) The number of oxide inclusions for legs, Zr-containing forging steel, characterized in that it is in the number ratio of a total of 20% or more.
4.5 × (% CaO) + (% ZrO ₂ ) ≧ 135 (1)
Here, (% Al ₂ O ₃ ): mass% of Al ₂ O ₃
(% CaO): mass% of CaO
(% ZrO ₂ ): mass% of ZrO ₂   Further, the component composition is one or more of mass%, Cr: 0.05 to 2.00%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%. The Zr-containing forging steel material according to claim 1, comprising:   The Zr-containing forging steel material according to claim 1 or 2, wherein the component composition further includes Mg: 0.0005 to 0.0050% in mass%.

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