JP2020002455A - Steel bar or steel product, and manufacturing method therefor - Google Patents

Abstract

To provide a steel bar or a steel product, excellent in delayed fracture resistance without need for expensive alloy elements, and a manufacturing method therefor.SOLUTION: There is provided a steel bar or a steel product having a component composition containing, by mass%, C:0.12 to 0.35%, Si:1.5 to 3.0%, Mn:2.0 to 5.0%, N:0.02% or less, P:0.03% or less, S:0.02% or less, and the balance:Fe with impurities, and a structure having, by area%, martensite:90% or more, the balance:one or more kind or bainite, retained austenite, pearlite, and ferrite, an old austenite particle diameter measured at any position on the cross section of 5 μm or less, and tensile strength of the bar steel or the steel product in a longer direction of over 1300 MPa.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a steel rod excellent in delayed fracture resistance or a steel product using the steel rod as a raw material, and a method for producing the same.

  For example, a steel product made of a steel rod such as a torsion bar in a vehicle suspension device that is subjected to high tension during use, or a part of the reinforcing bar of a structure including a reinforcing bar such as a PC pile, Since a high tensile stress of about 70% of the tensile strength is applied, a steel rod having excellent delayed fracture resistance is used. In order to obtain such a steel rod, an expensive alloy element such as Mo, V, Ti, or Nb is added in addition to Cr.

  In particular, in a PC pile or the like, the above-described steel rod having excellent delayed fracture resistance is adopted as a steel material in a portion to which a tensile stress is applied, and a reinforcing bar or a steel rod (a non-tensile material) in other portions to which a tensile stress is not applied. ) Used ordinary steel.

  However, in recent years, it has been pointed out that when a deformation stress such as bending or bending is applied to a PC pile, there is a risk that delayed fracture may occur due to the stress, thereby ensuring the safety of the PC pile and the like. For this reason, a certain degree of delayed fracture resistance has also been required for non-tensile steel bars and steel bars.

  It is considered that the delayed fracture is caused by atomic hydrogen contained in the steel material mainly gathering at the crystal grain boundaries due to stress applied to the steel material and causing grain boundary fracture. For this reason, low-cost steel materials having excellent delayed fracture resistance have been required for non-tensile materials such as reinforcing bars and steel bars and torsion bars for vehicle suspensions.

Patent Literature 1 discloses that the content of C is 0.15 to 0.50% by weight, the content of Si and Mn is 2.0% or less at maximum, the tempered martensite structure is included, and the aspect ratio of the prior austenite grains is 2 or more. A high-strength steel rod excellent in delayed fracture resistance, characterized in that the size of the boundary carbide is 0.2 μm or less and the strength is 145 kgf / mm ² or more, and a method for producing the same are described.

  The invention of Patent Literature 1 performs a rolling process at a rolling reduction of 30% or more in a low-temperature region to perform martensitic transformation while maintaining austenite elongated grains, thereby maintaining a high critical diffusible hydrogen amount and a high strength. However, in order to achieve a large rolling reduction in a low temperature region, a large-scale rolling equipment is required, and there is a disadvantage that the equipment cost rises.

  Further, in Patent Document 2, in mass%, C: 0.05 to 0.20%, Si: 1.0 to 3.5%, Mn: 4.5 to 5.5%, Al: 0.001 -0.080%, P: 0.030% or less, S: 0.020% or less, N: 0.010% or less, a high-hardness steel having a microstructure having an average crystal grain size of 2.6 μm or less. A high-hardness steel having an ultrafine martensite structure and having a Vickers hardness of 490 or more in the martensite structure is described.

  The invention of Patent Literature 2 is to uniformly heat to 1200 ° C., then cool to a temperature range of 600 to 400 ° C., perform a process of at least plastic equivalent strain 1.1 by forging in the temperature range, and then to room temperature. It is obvious that air cooling requires forging and cannot be applied to the production of steel bars by rolling.

Japanese Patent No. 3153072 Japanese Patent No. 5896458

  In view of such circumstances, the present invention provides high-strength steel rods and steel products that have high strength, have workability and toughness, and do not rely on expensive additional elements, and have excellent cost performance. It is an object of the present invention to provide a method for manufacturing the same.

  The present inventors have already developed a delayed fracture test method capable of simulating the delayed fracture phenomenon of a high-strength steel material and evaluating the delayed fracture characteristic with the amount of hydrogen required for fracture. From the test results using this method, by controlling the heating temperature and the holding temperature during the tempering in the heat treatment of the steel rod, the holding time, the amount of hydrogen required for delayed fracture can be increased, that is, the delayed fracture resistance can be improved. With the knowledge described above, the present invention was completed.

  In this delayed fracture test method, a test material consisting of a PC steel rod having an annular notch having a shape shown in FIG. 1 is charged with hydrogen by a constant current, and then, in air, a balance weight 2 and a fulcrum shown in FIG. In this test, a constant load tensile stress of 70% of the tensile strength is applied to the test material 1 by a cantilever type testing machine having No. 3 and the time required for fracture is measured.

  On the other hand, a test material having the same shape is charged with a cathode under the same conditions, and the amount of hydrogen charged in the test material is measured by gas chromatography. At this time, the measurement is performed by heating at a heating rate of 100 ° C./h, and two peaks as shown in FIG. 3 are recognized in the hydrogen release profile. Among these, the peak on the low temperature side indicates the amount of hydrogen that can diffuse at room temperature, and thus this was defined as the diffusible hydrogen amount.

  When the rupture time thus obtained and the amount of diffusible hydrogen at that time are graphed, a curve as shown in FIG. 4 is drawn. From this figure, the maximum amount of hydrogen Hc that does not break even after 100 hours from the start of the test is determined, this is defined as the critical diffusible hydrogen amount, and the delayed fracture resistance of the steel material is evaluated by the magnitude of this value. .

  As a result of investigating the occurrence and propagation of delayed fracture cracks in PC steel rods by this delayed fracture test method, the following was found.

  The relationship between the prior austenite grain size before quenching and the critical diffusible hydrogen content of PC steel rods of various components was measured. As shown in FIG. 5, the prior austenite grain size in a general quenched and tempered martensite structure was obtained. Is 12 to 15 μm on average, the critical diffusible hydrogen content is about 0.20 ppm, whereas when the austenite grain size is refined to 5 μm or less on average, the critical diffusible hydrogen content becomes 0 μm. .45 ppm or more, and an improvement in delayed fracture resistance was observed.

  Next, similarly, when the critical diffusible hydrogen amount was measured for various PC steel rods having different tensile strengths, the critical diffusible hydrogen amount tended to decrease as the tensile strength increased, as shown in FIG. It was also found that even with steel rods having the same tensile strength, differences in the critical diffusible hydrogen content were caused by differences in plasticity and structure.

  In the present invention, in consideration of application of a PC pile to a non-tensile material, a torsion bar, and the like and cost performance, a tensile strength of more than 1300 MPa is considered from the viewpoint of preventing grain boundary fracture which is dominant to delayed fracture. The composition and structure capable of achieving the average value of the prior austenite particle size of 5 μm or less and the production process are limited, and the gist thereof is as follows.

(1) The component composition is represented by mass%
C: 0.12-0.35%,
Si: 1.5 to 3.0%,
Mn: 2.0-5.0%,
N: 0.02% or less,
P: 0.03% or less,
S: 0.02% or less,
The balance: Fe and impurities,
On a cross section perpendicular to the longitudinal direction of the steel bar or the steel product, the structure is, by area percentage, martensite: 90% or more, and the balance: bainite, retained austenite, pearlite, one or more of ferrite,
The prior austenite grain size measured at an arbitrary position on the cross section is 5 μm or less;
A steel bar or a steel product, wherein a tensile strength in a longitudinal direction of the steel bar or the steel product is more than 1300 MPa.

  (2) The composition as described in (1) above, wherein the component composition further includes one or two types of Cr: 3.0% or less and B: 0.005% or less by mass%. Steel bars or steel products.

  (3) The component composition further includes one or more of Cu: 2.0% or less, Ni: 2.0% or less, and Mo: 1.0% or less by mass%. The steel bar or steel product according to the above (1) or (2), which is characterized by the following.

  (4) The composition of the components is further, in mass%, Al: 0.2% or less, V: 0.5% or less, Ti: 0.1% or less, Nb: 0.2% or less, REM: 0. The steel rod or the steel product according to any one of the above (1) to (3), comprising one or more kinds of not more than 02%.

(5) The steel material having the component composition described in any one of (1) to (4) is hot-rolled, tempered after final finish rolling, and tempered to obtain a steel material according to any one of (1) to (4). In a manufacturing method for manufacturing a steel rod or a steel product,
The final finish rolling temperature is 600 ° C. or more and 850 ° C. or less, and the rolling reduction is 20% or more and 30% or less;
After the final finish rolling, within 10 seconds, cool to 300 ° C. at a cooling rate of 30 ° C./sec or more, then cool to 100 ° C. or less,
A method for producing a steel rod or a steel product, wherein the tempering is performed at a temperature of 150 ° C. or more and 250 ° C. or less for 10 seconds or more.

  According to the present invention, a steel rod or a steel product having a tensile strength of 1300 MPa or more and excellent in workability and toughness and a method for producing the same can be provided by rolling without depending on expensive additional elements.

It is a top view of the test piece used for a delayed fracture test of a steel bar. It is explanatory drawing of a delayed fracture test apparatus. It is a figure showing the profile of the amount of hydrogen released in hydrogen content analysis. It is a figure which shows the relationship between delayed fracture and critical diffusible hydrogen amount. It is a figure which shows the relationship between a prior-austenite particle size and a critical diffusible hydrogen amount. It is a figure which shows the relationship between tensile strength and critical diffusible hydrogen amount.

The present invention will be described.
<Component composition>
First, the component composition will be described. Hereinafter, “%” related to the component composition means “% by mass”.
C: 0.12% or more, 0.35% or less C is an element necessary for obtaining a strength of 1300 MPa or more by quenching / tempering, but if it exceeds 0.35%, toughness and delayed fracture resistance Therefore, the upper limit was set to 0.35% because of the possibility of adversely affecting the properties. 0.12% or more is added to secure the strength, but 0.15% or more is preferable, and 0.20% or more is more preferable to increase the strength.

Si: 1.5% or more and 3.0% or less Si is an element that contributes to deoxidation of steel and improvement of strength. In order to obtain the effect of addition, 1.5% or more is added, but if it exceeds 3.0%, the toughness is significantly reduced, so the upper limit was made 3.0%.

Mn: 2.0% or more and 5.0% or less Mn is an element that contributes to the improvement of hardenability by suppressing the deoxidation of steel and the transformation of ferrite during cooling. Although 0.0% or more is added, if it exceeds 5.0%, the delayed fracture resistance decreases, so the upper limit was made 5.0%.

N: 0.02% or less N is an element inevitably mixed in the steel rod manufacturing stage. When Al is used as a deoxidizing agent, N is combined with Al to form AlN finely, and austenite grain boundaries are formed. It is an element responsible for the pinning effect of movement suppression. If N exceeds 0.02%, the steel becomes brittle, so the upper limit was made 0.02%.

P: 0.03% or less P is an element which is an impurity, and segregates at the grain boundary to embrittle the steel. If P exceeds 0.03%, the delayed fracture resistance decreases, so the upper limit is made 0.03%. Preferably it is 0.015% or less. The lower limit includes 0%, but it is technically difficult to set the lower limit to 0%, and practically 0.001% is a practical lower limit.

S: 0.02% or less S is an impurity and has an adverse effect on delayed fracture resistance, so the upper limit is made 0.02%. Preferably it is 0.015% or less. The lower limit includes 0%, but it is technically difficult to set the lower limit to 0%, and practically 0.001% is a practical lower limit.

The balance: Fe and impurities The balance of the component composition is Fe and impurities. The impurities are elements that are mixed in from the steel raw material and / or in the steelmaking process, and are allowed as long as the characteristics of the present invention are not impaired. The impurities are, for example, 0.01% or less of Pb, Bi, Te, Sn, W, Co, As, Mg, Zr, In, and REM.

  The steel rod or the steel product of the present invention may further contain 3.0% or less of Cr and / or 0.005% or less of B for the purpose of improving hardenability. Further, one or more of Cu: 2.0% or less, Ni 2.0% or less, and Mo: 1.0% or less can be contained mainly for the purpose of improving corrosion resistance.

  Further, in order to reduce the prior austenite grain size, Al: 0.2% or less, V: 0.5% or less, Ti: 0.1% or less, Nb: 0.2% or less, REM: 0.02% or less Or one or more of these.

  Of these elements, Al binds preferentially to N contained in steel, as described above, to form AlN that exhibits a pinning effect at the prior austenite grain boundaries. V, Ti, and Nb form nitrides or carbonitrides that exhibit a pinning effect at the former austenite grain boundaries, like Al. REM functions to reduce the size of the oxide and enhance the pinning effect.

  The upper limits of these optional elements are defined by the balance between the effect and cost of each element, and the upper limit is a value at which even if added beyond the upper limit, an effect commensurate with the increased cost is hardly obtained. .

<Organization>
Next, the structure on a cross section perpendicular to the longitudinal direction of the bar or the steel product (hereinafter, may be simply referred to as “structure”) will be described. % Relating to the organization means “area ratio”.

Martensite: 90% or more In the steel bar and the steel product of the present invention, in order to secure a tensile strength of more than 1300 MPa in the longitudinal direction of the steel bar or the steel product, martensite is made 90% or more. Preferably it is 93% or more.

Remainder: one or more of bainite, retained austenite, pearlite, ferrite One or two types of bainite, retained austenite, pearlite, ferrite, which are structures other than martensite and inevitably formed. That is all.

  The structure on the cross section perpendicular to the longitudinal direction of the steel bar or the steel product was cut out of the steel bar or the steel product so that the cross section perpendicular to the longitudinal direction of the steel bar or the steel product could be observed, and the cross section was mirror-polished. Thereafter, the tissue fraction (area ratio) was corroded with a nital solution (a solution obtained by dissolving 3 g of nitric acid in 100 ml of ethanol and, if necessary, adding a surfactant) for 5 to 30 seconds, and washed with water. Observe with an optical microscope.

  The tissue fraction is calculated by marking bainite, retained austenite, pearlite, and ferrite in a tissue photograph taken with an optical microscope, and measuring the tissue area using an ordinary image analyzer.

Old austenite grain size: 5 μm or less The old austenite grain size on a cross section perpendicular to the longitudinal direction of a bar or steel product is 5 μm or less in order to maintain required workability and toughness and ensure excellent delayed fracture resistance. And Preferably it is 3 μm or less.

  For the old austenite grain size on the cross section perpendicular to the long direction of the bar or steel product, cut the bar or steel product so that the cross section perpendicular to the long direction of the bar or steel product can be observed, and mirror the After polishing, the structure is revealed by the corrosion method described in JIS G 0551 (2013), observed and photographed with an optical microscope, and can be obtained by the cutting method described in the JIS standard.

  In addition, regarding the former austenite grain size on the cross section perpendicular to the long direction of the bar or steel product, cut out the bar or steel product so that the cross section perpendicular to the long direction of the bar or steel product can be observed, and Is mirror-polished, and then immersed in a mixed solution of picric acid and ethanol (a solution obtained by adding 4 g of picric acid to 100 ml of alcohol) for 5 minutes to allow austenite grain boundaries to emerge, and then to include the outermost surface A 1000-fold photograph is taken with an optical microscope, and the average particle equivalent diameter can be determined using an image analyzer.

<Mechanical properties>
Next, mechanical characteristics will be described.

Tensile strength: more than 1300 MPa In the steel bar and the steel product of the present invention, in order to expand the uses of the steel bar and the steel product, the tensile strength in the longitudinal direction of the steel bar or the steel product was set to more than 1300 MPa. The tensile strength can be measured by the method described in JIS Z 2241 (2011).

  In the case of a steel rod having a substantially circular cross section, the steel rod is cut out to a length capable of being subjected to a tensile test. In the case of a processed steel product, the cut out steel product is processed into a proportional test piece described in the above JIS standard, and a tensile stress is applied in a longitudinal direction of the steel product to measure.

<Production method>
Next, a manufacturing method will be described. Basically, a material such as a billet having the above-described composition is subjected to rough rolling, hot rolling, and heat treatment to produce a steel rod or the like having desired dimensions and performance.

  In the hot rolling, the final finish rolling is performed at a temperature of 600 ° C. or more and 850 ° C. or less, and a rolling reduction of 20% or more and 30% or less. Within 10 seconds after the final finish rolling, the sample is cooled to 300 ° C at a cooling rate of 30 ° C / sec or more, and then cooled to 100 ° C or less at a required cooling rate. After that, tempering is performed at a temperature of 150 ° C. or more and 250 ° C. or less for 10 seconds or more to obtain a steel bar having a prior austenite particle size of 5 μm or less and a tensile strength of more than 1300 MPa.

Final finishing rolling temperature: 600 ° C. or more and 850 ° C. or less When the final finishing rolling temperature is less than 600 ° C., the deformation resistance of the material to be rolled increases, a large load is applied to the rolling mill, and recrystallization hardly proceeds. Since the prior austenite grain size is hard to be reduced, the final finish rolling temperature was set to 600 ° C. or higher. On the other hand, if the final finish rolling temperature exceeds 850 ° C., recrystallization proceeds too much and the prior austenite grain size does not become fine, so the final finish rolling temperature was set to 850 ° C. or lower.

Start of cooling after rolling: Within 10 seconds After the final finish rolling, if it is maintained at the final finishing temperature or slowly cooled, recrystallization progresses during that time, and it becomes difficult to refine the austenite grain size. Thereafter, cooling was started within 10 seconds.

Cooling rate after rolling: 30 ° C / sec or more up to 300 ° C After final finishing rolling, cooling is performed within 10 seconds from the final finishing temperature to 300 ° C at a cooling rate of 30 ° C / sec or more to suppress the progress of recrystallization. , Miniaturization of the prior austenite grain size. As a means for cooling from the final finishing rolling temperature to 300 ° C. at a cooling rate of 30 ° C./sec or more, EDC (Easy Drawing Conveyer) cooling in which a wound coil is immersed in a water tank can be applied.

Cooling temperature range from 300 ° C .: to 100 ° C. or less After cooling to 300 ° C. at a cooling rate of 30 ° C./sec or more, cool to 100 ° C. at a required cooling rate.

Tempering temperature: 150 ° C or more and 250 ° C or less Tempering time: 10 seconds or more In order to secure required strength and toughness, tempering was performed at 150 ° C or more and 250 ° C or less, avoiding a tempering embrittlement temperature of more than 250 to 350 ° C. . If the tempering time exceeds 10 seconds, the required strength cannot be secured, so the tempering time was set to 10 seconds or less.

  Next, an example of the present invention will be described. The conditions in the example are one condition example adopted for confirming the operability and effects of the present invention. It is not limited. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example 1)
Steel rods having a diameter of 10 mm having various component compositions were produced under various rolling and cooling conditions, and the tensile strength TS and the critical diffusible hydrogen amount were measured. The results are shown in Tables 1 and 2. Table 1 shows the component composition, and Table 2 shows the rolling / cooling conditions, tensile strength TS, and critical diffusible hydrogen amount.

  The method for observing and measuring the microstructure and the method for measuring the prior austenite grain size are the methods described above. In addition, the tensile test was performed by cutting a steel bar into a length of 300 mm and working as a test piece without processing.

  In Table 1, Inventive Examples 1 to 7 are examples satisfying the basic component composition of the present invention, and Inventive Examples 8 to 17 are applied examples containing a prescribed amount of an optional additive element below Cr. is there.

  Each of the component composition of the present invention and the invention examples manufactured under rolling and cooling conditions satisfied a tensile strength of 1300 MPa or more and a critical diffusible hydrogen amount: at least 0.10 ppm or more.

  In Comparative Example 1, the C amount was lower than the range of the present invention, and the tensile strength did not reach 1300 MPa. In Comparative Example 2, although the C content exceeds the range of the present invention and the tensile strength exceeds 1300 MPa, the critical diffusible hydrogen content is as low as 0.02 ppm and cannot be put to practical use.

  In Comparative Example 3, the component composition satisfies the range of the present invention, but the tempering temperature was too high, and the critical diffusible hydrogen amount according to the tensile strength was not obtained. In Comparative Example 4, the component composition satisfies the range of the present invention, but the tempering temperature is too low, and the critical diffusible hydrogen amount according to the tensile strength has not been obtained. In Comparative Example 5, the cooling rate after the final finish rolling was slow, the martensitic transformation did not sufficiently proceed, and the tensile strength did not reach the specified value.

  In Comparative Example 6, the amount of Si was below the range of the present invention, and the amount of critical diffusible hydrogen was as low as 0.03 ppm. In Comparative Example 8, the Si content exceeded the range of the present invention, the N content was 0.025%, the upper limit of the present invention exceeded 0.02%, ferrite transformation progressed, and the tensile strength reached the specified value. Did not.

  In Comparative Example 7, the Mn content exceeded the range of the present invention, and the tensile strength did not reach the specified value. The cause is presumed to be that the Mn content is too high, the martensitic transformation start temperature (Ms point) decreases, and the retained austenite after quenching increases. In Comparative Example 9, the Mn content was below the range of the present invention, and the tensile strength did not reach the specified value. This is presumed to be due to a decrease in hardenability due to an insufficient amount of Mn.

  In Comparative Example 10, although the component composition satisfies the range of the present invention, the rolling finish temperature was too high, the prior austenite particle size was large, and the critical diffusible hydrogen amount in accordance with the tensile strength was not obtained. There is a high possibility that the application range will be restricted.

  In Comparative Example 11, although the component composition satisfies the range of the present invention, the rolling reduction in the final finish rolling was too low, the prior austenite grain size became large, and the critical diffusible hydrogen amount according to the tensile strength was obtained. Therefore, there is a high possibility that the applicable range as a material is restricted.

  In Comparative Example 12, the Ni content of the optional additive element was 2.3%, which exceeded the upper limit of the range of the present invention, and the tensile strength did not reach the specified value. In Comparative Example 13, the amount of Cr was too large, ferrite was formed, and the tensile strength was lowered. In Comparative Example 14, the amount of Cu was too large, cracks occurred during rolling, and measurement was not possible. In Comparative Example 15, the rolling finish temperature was too low, and work-induced transformation cracking occurred during rolling, and measurement was not possible.

  In Comparative Example 16, the amount of Al and the amount of N exceeded the upper limit of the range of the present invention, and a large amount of AlN was generated at the grain boundary, and the critical diffusible hydrogen amount corresponding to the tensile strength could not be obtained. In Comparative Example 17, the Ni content of the optional additive element was 2.3%, which exceeded the upper limit of the range of the present invention, and the rolling reduction at the time of finish rolling was too large to cause cracking at the time of rolling. could not.

  As described above, according to the present invention, a steel rod or a steel product excellent in workability and toughness having a tensile strength of 1300 MPa or more and a method for producing the same are provided by rolling without depending on expensive additive elements. be able to. Since the steel bar or the steel product of the present invention is particularly suitable as a material such as a non-tensile member of a PC pile or a torsion bar of a suspension device, the present invention has high industrial applicability.

1 test material 2 balance weight 3 fulcrum

Claims (5)

Ingredient composition in mass%
C: 0.12-0.35%,
Si: 1.5 to 3.0%,
Mn: 2.0-5.0%,
N: 0.02% or less,
P: 0.03% or less,
S: 0.02% or less,
The balance: Fe and impurities,
On a cross section perpendicular to the longitudinal direction of the steel bar or the steel product, the structure is, by area percentage, martensite: 90% or more, and the balance: bainite, retained austenite, pearlite, one or more of ferrite,
The prior austenite grain size measured at an arbitrary position on the cross section is 5 μm or less;
A steel bar or a steel product, wherein a tensile strength in a longitudinal direction of the steel bar or the steel product is more than 1300 MPa.   2. The steel rod according to claim 1, wherein the composition further includes one or two types of Cr: 3.0% or less and B: 0.005% or less in mass%. 3. Steel products.   The component composition further comprises, by mass%, one or more of Cu: 2.0% or less, Ni: 2.0% or less, and Mo: 1.0% or less. The steel rod or the steel product according to claim 1.   Further, the composition of the components is, by mass%, Al: 0.2% or less, V: 0.5% or less, Ti: 0.1% or less, Nb: 0.2% or less, REM: 0.02% or less. The steel rod or the steel product according to any one of claims 1 to 3, comprising one or more of the following. The steel rod or the steel product according to any one of claims 1 to 4, wherein the steel material having the component composition according to any one of claims 1 to 4 is hot-rolled, subjected to final finish rolling, and then tempered. In a manufacturing method for manufacturing
The final finish rolling temperature is 600 ° C. or more and 850 ° C. or less, and the rolling reduction is 20% or more and 30% or less;
After the final finish rolling, within 10 seconds, cool to 300 ° C. at a cooling rate of 30 ° C./sec or more, then cool to 100 ° C. or less,
A method for producing a steel rod or a steel product, wherein the tempering is performed at a temperature of 150 ° C. or more and 250 ° C. or less for 10 seconds or more.

Images