JP5385661B2 - Steel with improved impact deformation resistance - Google Patents

Description

The present invention relates to a technique for improving resistance when a steel is deformed at a strain rate of 10 ² / sec or more (hereinafter referred to as resistance during impact deformation (impact deformation resistance)), and more specifically, a strain rate of 10 ^0. The present invention relates to a technique for improving impact deformation resistance when compared to resistance when steel is deformed at a speed of less than / sec (hereinafter referred to as resistance during static deformation (static deformation resistance)).

  Steel materials are used in a wide range of fields such as machine parts and electrical parts, and steel materials with various strengths are used depending on the application. Whether or not the steel material satisfies the required strength is usually determined based on deformation resistance (tensile and compressive strength) during static deformation. By the way, even if the static deformation resistance is constant, the impact deformation resistance of steel varies. When an unexpected stress such as a collision is applied to a steel material, it is difficult to predict whether the steel material is broken or not based on the static deformation resistance. Therefore, in consideration of a predetermined safety factor, a steel material having a strength sufficiently higher than the required strength is used.

  However, in order to increase the strength of the steel material, for example, an expensive element is added or a special manufacturing method is adopted. Moreover, the usage-amount of steel materials itself may be increased and destruction may be prevented. For this reason, problems such as an increase in the price of steel materials, an increase in the size of parts, and an increase in weight are incurred.

  By the way, although it is unrelated to impact deformation resistance and static deformation resistance, the technique which improves the cold workability of steel materials is known (patent documents 1-4 etc.). In Patent Document 1, strain aging occurs due to solid solution C and solid solution N at the time of cold working, and deformation resistance is increased. On the other hand, by adding B or Al, solid solution C and solid solution N are added. It is disclosed that if it is fixed, the deformation resistance can be reduced and the cold workability of the steel material is improved. Each of these patent documents does not include B as an essential element as an element for fixing N, and does not disclose increasing the heating temperature of the steel material before rolling while using B as an essential element. The maximum heating temperature of the B-added steel is, for example, 910 ° C. (Patent Document 1), 882 ° C. (Patent Document 2), 960 ° C. (Patent Document 3), and 930 ° C. (Patent Document 4). )

JP 2000-8139 A JP 2000-204433 A JP 2001-303189 A JP 2001-342544 A

  The present invention has been made by paying attention to the above-described circumstances, and its purpose is not to increase the static deformation resistance of the steel material too much (even if the safety factor is not increased too much), but at the time of impact deformation. It is to suppress the destruction of the steel material.

  As a result of intensive studies to solve the above problems, the present inventors fixed the solid solution N in the steel with B while making the structure in the steel a predetermined ferrite-pearlite structure, and sufficiently coarsened the BN. Thus, the ratio of the impact deformation resistance to the static deformation resistance (that is, the impact deformation resistance / static deformation resistance, hereinafter sometimes referred to as the static / dynamic ratio) can be increased, that is, the strength at the time of impact deformation with respect to the static deformation resistance. The present invention has been completed by finding that it is possible to prevent the deterioration, and therefore, even if the static deformation resistance of the steel material is not increased too much (even if the safety factor is not increased too much), the destruction of the steel material during impact deformation can be suppressed. .

That is, the steel with improved impact deformation resistance according to the present invention is
About component composition
C: 0.05 to 0.5% (meaning by mass, the same applies to chemical components), Si: 0.005 to 0.50%, Mn: 0.2 to 0.80%, P: 0.050 % Or less (excluding 0%), S: 0.005 to 0.05%, Cr: 0.05 to 0.30%, Al: 0.005 to 0.06%, B: 0.0005 to 0 .0055%, N: 0.0005 to 0.008%, solid solution N: 0.0010% or less, balance: iron and inevitable impurities,
The structure is a ferrite-pearlite structure having a ferrite fraction of 55 to 97 area%,
As for the inclusions, the diameter of the largest nitride-based inclusion containing B is 100 nm or more, and the number of nitride-containing inclusions containing B whose diameter is 100 nm or more is 0.010 to 0 μm ² per 1 μm ² . It has a gist in that it is .05.

  The steel of the present invention further includes (a) Ti: 0.005% or less (not including 0%), Nb: 0.005% or less (not including 0%), and V: 0.005% or less ( At least one selected from the group consisting of: (b) Mo: 0.2% or less (not including 0%), (c) Cu: 0.05% or less (not including 0%) ) And / or Ni: 0.05% or less (not including 0%) may be contained, and the properties of the steel are further improved depending on the type of components to be contained.

  In the steel with improved impact deformation resistance according to the present invention, a steel material having the above component composition is hot-worked at a heating temperature of 1150 to 1250 ° C. and a finishing temperature of 850 to 1000 ° C., and then at a rate of 0.1 to 5 ° C./second. It can manufacture by cooling to 400 degrees C or less.

  According to the present invention, the solute N in the steel is fixed with B and the BN is sufficiently coarsened while the structure in the steel is a predetermined ferrite-pearlite structure. Even if the ratio is increased and the static deformation resistance of the steel material is not increased too much (even if the safety factor is not increased too much), the destruction of the steel material during impact deformation can be suppressed.

  In the steel of the present invention, the solute N is 0.0010% or less (preferably 0.0008% or less, more preferably 0.0006% or less). By reducing the solid solution N and coarsening the nitride inclusions containing B, the static ratio of deformation resistance of the steel material can be increased.

  If it demonstrates in detail, the influence which the solid solution N in steel will differ in static deformation resistance (tensile and compressive strength) and impact deformation resistance. When steel is statically deformed, a work hardening phenomenon occurs due to the occurrence of dynamic strain aging due to diffusion of solid solution N and fixing dislocations during movement. On the other hand, when the steel undergoes impact deformation, that is, when the moving speed of dislocations becomes faster than the diffusion of solute N, dynamic strain aging does not occur. Furthermore, the deformation resistance when the steel undergoes impact deformation has a strain rate dependency and a temperature dependency. Strain rate dependence refers to a phenomenon in which deformation resistance increases due to the thermal activation process of dislocations. The temperature dependency is a phenomenon that occurs when the heat generated by the processing heat is not diffused to the surroundings and is deformed in a pseudo-adiabatic manner, and is a phenomenon in which the deformation resistance is lowered as the strain rate increases. As described above, the solute N does not contribute to the improvement of the impact deformation resistance, and has the effect of lowering the static ratio of the deformation resistance of the steel material in order to improve only the static deformation resistance. In the present invention, the static ratio of the deformation resistance of the steel material can be increased by reducing the solid solution N.

  Furthermore, in the steel of the present invention, B is used to fix solute N, and B-containing nitride inclusions (nitrides, carbonitrides, etc., for example, BN, hereinafter simply referred to as BN) formed thereby. Is coarsening. When fine BN is precipitated, the static resistance ratio of the deformation resistance of the steel material is reduced. On the other hand, when coarse BN is precipitated, the static resistance ratio of the deformation resistance of the steel material can be increased.

  The reason why coarse BN can increase the static-dynamic ratio is presumed as follows. That is, the dislocation organization state differs between static deformation and impact deformation. During static deformation, dislocations are organized and organized. Fine BN exhibits an effect of increasing resistance when dislocations are organized. On the other hand, at the time of impact deformation, dislocations are generated all at once and randomized. If there is fine BN, the resistance at the initial stage of deformation is increased by stopping the movement of dislocations that are randomly generated. However, after that, the site where the dislocation has not occurred is deformed, so that the deformation is localized. That is, because of the non-uniform deformation in which the upper and lower yield points occur, the impact deformation resistance in the high strain region does not increase. On the other hand, coarse BN has no effect of fixing dislocations. Therefore, the steel can be uniformly deformed and the upper and lower yield points do not appear. Therefore, reducing the fine BN and increasing the coarseness can increase the static ratio of the deformation resistance of the steel material.

Whether fine BN is reduced and BN is appropriately coarse is determined by the diameter of the maximum BN and the number of BN having a diameter of 100 nm or more. In the present invention, the maximum BN diameter is 100 nm or more (preferably 120 nm or more, more preferably 150 nm or more). The number of BN having a diameter of 100 nm or more is 0.05 or less (preferably 0.045 or less, more preferably 0.040 or less). By controlling the maximum BN diameter and the number of BN having a diameter of 100 nm or more, it is possible to prevent sticking of movable dislocations at the time of impact deformation and increase the static / dynamic ratio. When BN is coarsened, the number of BN having a diameter of 100 nm or more is usually 0.010 or more, preferably 0.015 or more, more preferably 0.020 or more. The number of BN is the number per 1 μm ² of the steel material.

  The steel structure of the present invention is a ferrite-pearlite structure in which pearlite is present in the form of islands in ferrite. However, a part of perlite may be replaced with cementite, bainite, martensite, or the like. In the ferrite single phase structure, the upper and lower yield points tend to appear, and the impact deformation resistance in the high strain region does not increase. On the other hand, when pearlite is present in the ferrite matrix in the form of islands, the impact deformation resistance and the static / dynamic ratio can be increased. Therefore, the ferrite fraction is usually 97 area% or less (preferably 95 area% or less, more preferably 93 area% or less, and further preferably 90 area% or less). However, even if the pearlite increases too much, the pearlite islands start to bond with each other, the deformation does not become uniform at the interface between the ferrite and the pearlite, and the impact deformation resistance does not increase. Therefore, when the ferrite fraction is 55 area% or more (preferably 58 area% or more, more preferably 60 area% or more), the impact deformation resistance and the static / dynamic ratio can be increased.

  As described above, in the steel of the present invention, the amount of solute N, the size of BN, and the steel structure are controlled, and the static-dynamic ratio can be increased. In addition, the component composition of this steel is as follows normally.

C: 0.05-0.5%
C is an essential element for imparting strength to steel. C forms a certain amount of pearlite and has an effect of preventing upper and lower yield points from occurring during impact deformation. In order to ensure such an effect, the C content is 0.05% or more (preferably 0.08% or more, more preferably 0.10% or more). However, when the amount of C becomes excessive, the pearlite fraction increases, and on the contrary, the static ratio of deformation resistance decreases. Therefore, the C content is 0.5% or less (preferably 0.45% or less, more preferably 0.40% or less).

Si: 0.005-0.50%
Si is an element useful as a deoxidizing agent. If this amount is too small, deoxidation becomes insufficient, and gas defects are generated and cracks are likely to occur. Therefore, the Si amount is 0.005% or more (preferably 0.010% or more, more preferably 0.015% or more). However, when the amount of Si is excessive, not only the deformation resistance increases due to solid solution strengthening, but also the deformation ability is reduced and cracks are generated. Therefore, the Si amount is 0.50% or less (preferably 0.4% or less, more preferably 0.35% or less).

Mn: 0.2 to 0.80%
Mn is useful for deoxidation and desulfurization. If this amount is too small, deoxidation and desulfurization are insufficient, and cracks are likely to occur due to generation of gas defects and grain boundary segregation of FeS. Mn is an element useful for improving quenching and tempering softening resistance during heat treatment after cold working. In order to effectively exhibit such an action, the amount of Mn is 0.2% or more (preferably 0.23% or more, more preferably 0.25% or more). However, when the amount of Mn becomes excessive, the ferrite-pearlite growth rate after hot working decreases, and bainite harmful to cold workability and cracking is likely to occur. Therefore, the amount of Mn is 0.80% or less (preferably 0.7% or less, more preferably 0.65% or less).

P: 0.050% or less (excluding 0%)
P segregates at the ferrite grain boundaries and degrades the cold workability. Further, P causes an increase in deformation resistance and occurrence of cracks by strengthening the solid solution of ferrite. Therefore, P is desirably reduced as much as possible from the viewpoint of cold workability and the like, but an extreme reduction leads to an increase in steelmaking cost. Therefore, the P content is set to 0.050% or less (preferably 0.04% or less, more preferably 0.03% or less). In addition, P is an element inevitably contained in steel, and it is difficult to reduce the amount to 0 in industrial production.

S: 0.005-0.05%
When S is combined with Fe, it precipitates in the form of a film on the grain boundary as FeS and deteriorates the cold workability. Therefore, the entire amount must be combined with Mn and precipitated as MnS. However, when the precipitation amount of MnS increases too much, it causes deterioration of cold workability and generation of cracks. Therefore, the S amount is 0.05% or less (preferably 0.04% or less, more preferably 0.03% or less). However, extreme reduction of S degrades machinability. Therefore, the S amount is 0.005% or more (preferably 0.007% or more, more preferably 0.010% or more).

Cr: 0.05-0.30%
Cr is an element useful for suppressing dynamic strain aging during static deformation by fixing solute C. In order to effectively exhibit such action, the Cr content is 0.05% or more (preferably 0.08% or more, more preferably 0.010% or more). However, even if the amount of Cr is excessive, the action is saturated and workability is deteriorated and cracks are generated. Therefore, the Cr content is 0.30% or less (preferably 0.2% or less, more preferably 0.15% or less).

Al: 0.005-0.06%
Al is an element useful as a deoxidizing element during melting, and if this amount is too small, deoxidation becomes insufficient, and gas defects are generated and cracks are likely to occur. Therefore, in order to effectively exhibit the deoxidizing action, the Al amount is 0.005% or more (preferably 0.007% or more, more preferably 0.010% or more). However, when the amount of Al becomes excessive, it binds with solute N during hot working, so that coarse BN having a diameter of 100 nm or more is reduced. That is, the amount of N that can be combined with B is reduced, and as a result of BN becoming finer, upper and lower yield points are likely to occur, and the static-dynamic ratio is lowered. Therefore, the Al content is 0.06% or less (preferably 0.055% or less, more preferably 0.050% or less).

B: 0.0005 to 0.0055%
B suppresses dynamic strain aging during static deformation by fixing solute N, and forms a coarse BN to prevent non-uniform deformation during impact deformation, thereby reducing the static resistance ratio of deformation resistance. It is an important element to enhance. B also has an effect of suppressing a decrease in grain boundary strength due to P ferrite grain boundary segregation. In order to fully exhibit these actions, the B content is 0.0005% or more (preferably 0.0008% or more, more preferably 0.0010% or more). However, even if the amount of B becomes excessive, the action is saturated and cracking is likely to occur. Further, when the amount of B becomes excessive, the number of BN having a diameter of 100 nm or more increases and the static / dynamic ratio decreases. Therefore, the B content is 0.0055% or less (preferably 0.0052% or less, more preferably 0.0050% or less).

N (total N amount in steel): 0.0005 to 0.008%
N dissolved in steel generates dynamic strain aging during static deformation, but does not generate dynamic strain aging during impact deformation, which causes a reduction in the static ratio of deformation resistance. In order to reduce this solid solution N as much as possible, the N amount is 0.008% or less (preferably 0.0075% or less, more preferably 0.0070% or less). However, since it takes cost to reduce the amount of N, the amount of N is usually 0.0005% or more (preferably 0.0010% or more, more preferably 0.0015% or more).

  The basic component composition of the steel of the present invention is as described above, and the balance is usually substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Furthermore, the steel of the present invention may contain the following selective components as necessary.

Selected from the group consisting of Ti: 0.005% or less (not including 0%), Nb: 0.005% or less (not including 0%), and V: 0.005% or less (not including 0%) At least one of Ti, Nb and V is an element effective for reducing solid solution N by bonding with N and suppressing dynamic strain aging at the time of static deformation. You may make it contain. In order to sufficiently exhibit this action, the Ti, Nb and V amounts are each preferably 0.0015% or more, more preferably 0.0020% or more. However, these nitrides are formed in preference to BN, but are less likely to be coarser than BN. Therefore, when they are excessive, non-uniform deformation is likely to occur during impact deformation, and the static-dynamic ratio is lowered. Therefore, the amounts of Ti, Nb, and V are set to 0.005% or less (preferably 0.0045% or less, more preferably 0.0040% or less), respectively.

Mo: 0.2% or less (excluding 0%)
Mo is an element that has the effect of regulating the size of crystal grains and suppressing non-uniform deformation during impact deformation, and may be contained in steel as necessary. In order to sufficiently exhibit this action, the Mo amount is preferably 0.02% or more, more preferably 0.03% or more. However, even if an excessive amount of Mo is added, the effect is saturated, so the Mo amount is set to 0.2% or less (preferably 0.18% or less, more preferably 0.15% or less).

Cu: 0.05% or less (not including 0%) and / or Ni: 0.05% or less (not including 0%)
Both Cu and Ni are elements that contribute to improvement of work hardening by solid solution in steel, and may be contained in steel as necessary. In order to sufficiently exhibit this action, the amounts of Cu and Ni are each preferably 0.008% or more, more preferably 0.01% or more. However, excessive amounts of Cu and Ni may promote non-uniform deformation during impact deformation. Therefore, the amount of Cu and Ni is determined to be 0.05% or less (preferably 0.04% or less, more preferably 0.03% or less).

  As described above, the steel of the present invention not only contains predetermined components, but also includes and controls the inclusions. This steel can be manufactured by sufficiently heating the cast steel slab to completely dissolve BN, then hot working at a relatively low temperature, and cooling at a predetermined cooling rate. By completely dissolving BN during heating, fine BN can be eliminated. Further, by lowering the hot working temperature, it is possible to reliably precipitate BN that has been dissolved, and to prevent generation of solute N. In addition, by adjusting the cooling rate, the growth and organization state of BN can be controlled, and inclusions of the desired size and the desired organization can be achieved.

  More specifically, the steel slab after casting is heated after being subjected to partial rolling as necessary and before hot working (hot rolling, etc.). In the present invention, this steel slab (billet, etc.) is heated. The temperature is set to 1150 ° C. or higher (preferably 1180 ° C. or higher). If the heating temperature is low, the progress of solid solution and coarsening of BN becomes inappropriate, and the maximum BN diameter or the number of BN having a diameter of 100 nm or more is out of the predetermined range. However, even if the heating temperature is too high, the effect on BN solid solution is saturated and the end of the steel slab (such as billet) is deformed, making hot working difficult. Therefore, the heating temperature is usually 1250 ° C. or lower (preferably 1230 ° C. or lower, more preferably 1210 ° C. or lower).

  In hot working (hot rolling, hot forging, etc.), the end temperature is controlled. The hot working finish temperature is 1000 ° C. or lower (preferably 980 ° C. or lower). If the end temperature of hot working is too high, solid solution N remains. On the other hand, if the processing end temperature is too low, hot processing cannot be performed efficiently. Therefore, the processing end temperature is usually 850 ° C. or higher (preferably 900 ° C. or higher).

  The cooling rate after the hot working is finished is controlled for the purpose of controlling the growth of BN and the tissue state. The cooling rate is 0.1 ° C./second or more (preferably 0.15 ° C./second or more, more preferably 0.20 ° C./second or more). If the cooling rate is too slow, BN changes to Al nitride during cooling, and therefore the maximum BN diameter or the number of BN having a diameter of 100 nm or more is out of the predetermined range. On the other hand, if the cooling rate is too high, transformations other than the ferrite phase (bainite transformation, martensitic transformation) occur, and the desired ferrite fraction cannot be obtained. Further, precipitation of nitride becomes insufficient, and solid solution N remains. Therefore, the cooling rate is 5 ° C./second or less (preferably 4.5 ° C./second or less, more preferably 4.0 ° C./second or less). The cooling rate means a rate during cooling from the start of cooling to a temperature of 400 ° C. If properly cooled to 400 ° C., BN does not change its size or composition below that temperature. Therefore, at 400 ° C. or lower, the cooling rate is not particularly limited, and a cooling method suitable for the manufacturing process, such as cooling, air cooling, oil cooling, or water cooling, may be selected as appropriate.

By appropriately controlling the inclusions and structure of the steel as described above, the steel of the present invention having improved impact deformation resistance, that is, a high static resistance ratio of deformation resistance can be produced. In the steel of the present invention, the ratio of the impact deformation resistance (MPa) when the strain rate is 10 ² / sec and the static deformation resistance (MPa) when the strain rate is 10 ⁰ / sec (impact deformation resistance / static The deformation resistance (ie, static ratio) is preferably 0.75 or more, more preferably 0.80 or more, and still more preferably 0.85 or more.

  Further, the steel of the present invention may be subjected to a skin pass. By imparting a small strain to the steel of the present invention, movable dislocations can be introduced into the steel to further suppress the occurrence of upper and lower yield points during impact deformation. However, when the amount of strain is too large, dislocations form cell walls, and conversely, upper and lower yield points are likely to occur. Therefore, it is recommended to apply a true strain of 0.2 or less (more preferably 0.1 or less) to the steel by a skin pass. As the skin pass, for example, drawing (drawing) processing can be performed.

  The steel obtained by hot working as described above is usually a wire rod or steel bar, and is good by cold working (for example, cold forging, cold forging, cold rolling, etc.). Steel parts can be produced while maintaining the static ratio. Steel parts obtained by cold working include, for example, bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, washers, tappets, saddles, bulgs , Inner case, Clutch, Sleeve, Outer race, Sprocket, Core, Stator, Anvil, Spider, Rocker arm, Body, Flange, Drum, Fitting, Connector, Pulley, Metal fitting, York, Base, Valve lifter, Spark plug, Pinion gear, In addition to steering shafts, common rails, etc., there are mechanical parts and electrical parts.

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

(1) Manufacture of steel The test steel was manufactured by two methods of performing hot rolling (manufacturing method 1) or hot forging (manufacturing method 2) as hot working.

(A) Manufacturing method 1
150 kg of steel having the composition shown in Tables 1 and 2 below was melted in a vacuum induction furnace and cast into an ingot having an upper surface: φ245 mm × lower surface: φ210 mm × length 480 mm. The ingot was heated to 1200 ° C. and hot forged into a 155 mm square billet. The billet end portion was cut and welded to a dummy billet of 155 mm square × 9 to 10 m long. This dummy billet was hot-rolled into a round bar of φ80 mm at the heating temperature and processing end temperature shown in Tables 3 to 5 below. The round bar was cooled to 400 ° C. at the cooling rates shown in Tables 3 to 5 below, and then cooled to room temperature. After cooling, some steels were given a true strain (area reduction strain) of 0.1 by drawing.

(B) Manufacturing method 2
150 kg of steel having the composition shown in Table 1 below was melted in a vacuum induction furnace and cast into an ingot having an upper surface: φ245 mm × lower surface: φ210 mm × length 480 mm. The ingot was heated to 1200 ° C. and hot forged into a 155 mm square billet. This billet was hot forged into a round bar of φ80 mm at a heating temperature (1150 to 1200 ° C.) and a processing end temperature (950 ° C.) shown in Table 6 below. The round bar was cooled to 400 ° C. at a cooling rate shown in Table 6 (1.0 ° C./second), and then cooled to room temperature. After cooling, some steels were given a true strain (amount of area reduction) of 0.1 by drawing.

(2) Measurement and calculation of solute N amount The solute N amount of the test steel produced as described above was determined in the following manner based on JIS G 1228.

(A) Measurement of total N amount The total N amount in steel was measured using an inert gas melting method-thermal conductivity method. Specifically, a sample cut from the test steel is placed in a crucible, melted in an inert gas stream, extracted N, transported to a thermal conductivity cell, and the change in thermal conductivity is measured. Asked.

(B) Measurement of N content in nitrogen compounds (ammonia distillation separation indophenol blue spectrophotometry)
About 0.5 g of a sample cut out from the test steel was dissolved in a 10% AA-based electrolytic solution and subjected to constant current electrolysis. The resulting insoluble residue (N compound) was filtered through a polycarbonate filter having a hole size of 0.1 μm. The obtained insoluble residue was decomposed by heating in a chip made of sulfuric acid, potassium sulfate and pure copper, and the decomposition product was combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation was performed, and the distilled ammonia was absorbed into dilute sulfuric acid. Furthermore, phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) were added to form a blue complex, and the absorbance was measured using a photometer to determine the amount of N in the compound. The 10% AA electrolyte solution is a non-aqueous solvent electrolyte solution consisting of 10% acetone, 10% tetramethylammonium chloride, and the remainder methanol, and does not generate a passive film on the steel surface.

(C) Calculation of the amount of solute N The amount of solute N in the steel was calculated by subtracting the amount of N in the compound from the total amount of N in the steel determined by the above method. The results are shown in Tables 3-6 below.

(3) Measurement of ferrite fraction The ferrite fraction of the test steel produced as described above was measured as follows. First, a sample obtained by hot rolling or hot forging was cut at the center in the longitudinal direction of the hot rolled material or hot forged material, and a sample was taken from the cut surface. The sample was embedded in resin, the sample surface was mirror-polished using emery paper and a diamond buff, and then subjected to nital corrosion. Using an optical microscope (observation magnification: 100 times, observation area: 5850 μm ² ), photographs were taken at five locations at the D / 4 depth (D: diameter of the test steel) of the test steel. The photographic image was binarized using Image Pro Plus, the ferrite phase was white and the other phases were black, and the respective fractions were determined, and the average value of the five locations was calculated as the ferrite fraction (area%). The results are shown in Tables 3-6 below.

(4) Measurement of the maximum BN diameter and the number of BN having a diameter of 100 nm or more The maximum BN diameter and the number of BN having a diameter of 100 nm or more of the test steel produced as described above were measured as follows. First, an extraction replica sample was prepared from the depth D / 4 position of the test steel (D: diameter of the test steel), and a transmission electron microscope (TEM) (observation magnification: 7,500 to 60,000 times, observation area: 21 μm ² ), 5 TEM photographs were arbitrarily taken. Using Image Pro Plus, the photographic image was binarized, the parent phase was white and BN was black, and the maximum BN diameter and the number of BN having a diameter of 100 nm or more were calculated. Here, the diameter of the BN means the equivalent circle diameter of the BN (that is, the average value of the diameter of the circumscribed circle and the diameter of the inscribed circle) obtained from Image Pro Plus. The average values of these values in five fields are shown in Tables 3 to 6 below.
The presence or absence of B in the N compound was determined by composition analysis by energy dispersive X-ray spectroscopy (EDS) attached to TEM.

(5) Evaluation of Static Ratio and Cracks The static ratio and the presence or absence of cracks of the test steel produced as described above were evaluated as follows.

First, a sample of φ10 mm × length 15 mm was cut out from the depth D / 4 position of the test steel (D: diameter of the test steel). By using a processing for master, the samples are cold forged under the conditions shown in Tables 3 to 6 below at the cold working temperature, the compression rate of 80%, and the strain rate of 10 ⁰ / sec or strain rate of 10 ² / sec. The static deformation resistance (MPa) at a strain rate of 10 ⁰ / sec and the impact deformation resistance (MPa) at a strain rate of 10 ² / sec were measured. The static ratio was calculated from these values. The results are shown in Tables 3-6 below.

Furthermore, the surface of the sample cold forged at a strain rate of 10 ² / sec was observed with a stereomicroscope (observation magnification 20 times) to confirm the presence or absence of cracks. The results are shown in Tables 3-6 below.

  The following can be seen from Tables 1-6. Experiment No. 1 satisfying the requirements of the component composition, solute N amount, steel structure and BN of the present invention. 1-4, 6-14, 16-32, 35-40, 42 and 60-69 show an excellent static ratio, no cracks, and improved impact characteristics. On the other hand, those not satisfying the requirements of the present invention were confirmed to have a small static ratio or cracks, as described later.

  Experiment No. 2 with a small number of BN having a diameter of 100 nm or more (hereinafter simply referred to as “number”). No. 5 with a large amount of solute N and a small maximum BN diameter (hereinafter simply referred to as “maximum diameter”). 15. Experiment No. with small maximum diameter 33, Experiment No. with many pieces 34 and Experiment No. with a large amount of solute N. No. 41 has a low static motion ratio, and the impact characteristics are not improved.

  Experiment No. with a large amount of C and a low ferrite fraction. 43 (steel No. 34) was confirmed to be cracked. On the other hand, Experiment No. 2 with a small amount of C and a high ferrite fraction No. 44 (steel No. 35) has a low static motion ratio and cracks.

  Experiment No. which satisfies the requirements of the steel structure and BN of the present invention but does not satisfy the requirements of the component composition. Although 45-52 (steel No. 36-43) has a favorable static motion ratio, the crack was confirmed.

  Experiment No. with a small amount of Al and a large amount of dissolved N In 53 (steel No. 44), the static-dynamic ratio decreased. On the other hand, Experiment No. with a large amount of Al. 54 (steel No. 45) has a small number and a low static motion ratio.

  Experiment No. containing no B 55 (steel No. 46) has a large amount of solute N because coarse BN is not formed. Since 56 (steel No. 47) has a small maximum diameter and a large amount of solute N, all of them have a low static ratio. On the contrary, the experiment No. Since 57 (steel No. 48) has many numbers, its static / dynamic ratio is low.

  Experiment No. with a large amount of total N and a large amount of dissolved N 58 (steel No. 49) has a low static ratio. In addition, in Experiment No. 2 where Ti amount is large and BN is not formed. 59 (steel No. 50) has a low static ratio.

Claims (5)

About component composition
C: 0.05 to 0.5% (meaning mass%, the same applies to chemical components below),
Si: 0.005 to 0.50%,
Mn: 0.2 to 0.80%,
P: 0.050% or less (excluding 0%),
S: 0.005 to 0.05%,
Cr: 0.05-0.30%,
Al: 0.005 to 0.06%,
B: 0.0005 to 0.0055%,
N: 0.0005 to 0.008%,
Solid solution N: 0.0010% or less,
The rest: iron and inevitable impurities
The structure is a ferrite-pearlite structure having a ferrite fraction of 55 to 97 area%,
As for the inclusions, the diameter of the largest nitride-based inclusion containing B is 100 nm or more, and the number of nitride-containing inclusions containing B whose diameter is 100 nm or more is 0.010 to 0 μm ² per 1 μm ² . .05 Steel with improved impact deformation resistance, characterized by the number of steels.   Further, Ti: 0.005% or less (excluding 0%), Nb: 0.005% or less (not including 0%), and V: 0.005% or less (not including 0%) The steel according to claim 1 containing at least one selected.   The steel according to claim 1 or 2, further containing Mo: 0.2% or less (not including 0%).   The steel according to any one of claims 1 to 3, further containing Cu: 0.05% or less (not including 0%) and / or Ni: 0.05% or less (not including 0%). A method for producing the steel according to any one of claims 1 to 4,
The steel material having the composition according to any one of claims 1 to 4 is hot-worked at a heating temperature of 1150 to 1250 ° C and a processing end temperature of 850 to 1000 ° C, and then 400 ° C at a rate of 0.1 to 5 ° C / second. A method for producing steel with improved impact deformation resistance, characterized by cooling to: