JP4589242B2 - Hot-rolled non-tempered steel bar excellent in toughness and method for producing the same - Google Patents

Description

In the present invention, even if it is a non-heat treated material as it is hot-rolled, the tensile strength TS is 700 MPa or more, the minimum value of the impact value _U E20 at 20 ° C. is 100 J / cm ² or more, and the average value is 150 J / cm ² or more. The present invention relates to a hot-rolled non-tempered steel bar excellent in toughness and a method for producing the same.
The hot-rolled non-tempered steel bar in the present invention is a product in which fine precipitates are deposited during cooling after rolling the steel bar to improve the characteristics, and the product does not require post-processing tempering treatment. It means steel bar.

Structural parts used in automobiles and other transportation equipment and construction machinery include carbon steel for machine structural use and tempered steel tempered and tempered alloy steel for machine structural use. Non-tempered steel whose strength is secured by adjustment is used.
Non-tempered steel used for such applications generally has a ferrite-pearlite dual-phase structure to which V or Nb is added. Compared to tempered steel, the yield strength can be increased when the tensile strength is the same. When the drawing value and impact value are low and the yield strength is the same, there is a problem that the tensile strength, that is, the hardness is excessively increased and the machinability is lowered.

As a technique for solving the above problems, Patent Document 1 and Patent Document 2 have ferrite, bainitic ferrite, and pseudo-martensite in order to obtain a high-strength, high yield ratio, and high-toughness non-heat treated steel. A technique is disclosed in which a steel material having a structure is subjected to aging treatment at 600 ° C. or lower after cold working to precipitate Cu and Ti—Nb-based carbides.
However, it is not practical to strictly control the ratio of multiple structures in actual production, and when using precipitation strengthening by adding a large amount of Cu, a large amount of expensive Ni is added to prevent hot cracking. As a structural component that is consumed in large quantities, there is a problem in terms of cost.

Therefore, the present inventors have not reduced productivity even in actual production, have a tensile strength of 700 MPa or more and a yield ratio of 0.85 or more with an inexpensive component composition, and are hot rolling molds that are excellent in toughness. In order to obtain a non-tempered steel bar, a technique for dispersing fine precipitates having a particle size of less than 10 nm in a ferrite single-phase structure and a component composition and manufacturing method therefor have been invented (Patent Document 3).
However, the technique shown in Patent Document 3 has a problem that variation in toughness is large. That is, the impact value _U E20 at 20 ° C., which is an index of toughness, is as high as 100 J / cm ² or more, but this value is an average value, and the impact value is 50 J / cm ² or less for each impact value. There was a case.
JP 2001-123224 A JP 2001-131680 A Japanese Patent Application No. 2003-120402

Therefore, the present invention is a hot rolling type non-adjustment which can stably obtain a tensile strength of 700 MPa or more and an impact value _U E20 of 100 J / cm ² or more which can be easily produced in actual production with an inexpensive component composition. The object is to provide a quality steel bar together with its advantageous production method.

When the present inventors have dispersed and precipitated fine precipitates having a particle size of less than 10 nm in a ferrite single-phase structure to enhance the precipitation of ferrite, the strength can be increased without impairing the machinability even with non-tempered steel. In addition to reducing the segregation of Mn and reducing the crystal grain size of ferrite to 35 μm or less, it is clear that toughness is improved, impact value _U E20 is improved, and variation in impact value is also reduced. became.
The present invention has been completed based on the above findings, and the gist thereof is as follows.

In invention 1, the component composition of steel is C: 0.040 to 0.150% in mass%, Si: 0.5% or less, Mn: 0.5 to 3.0%, Al: 0.1% or less. Ti: 0.03-0.35%, Mo: 0.05-0.8%, balance Fe and inevitable impurities, Mn segregation ratio R satisfies the following formula (1), and the structure is a crystal grain A hot-rolled non-tempered steel bar having a ferrite single phase having a diameter of 35 μm or less and fine precipitates having a particle diameter of less than 10 nm dispersed in the ferrite.
R = Mn (max) / Mn (min) ≦ 1.5 (1)
Mn (max): Mn content of the highest part of Mn
Mn (min): Mn content in the lowest part of Mn

Invention 2 is a hot-rolled non-tempered steel bar according to Invention 1, wherein the component composition of the steel satisfies the following formula (2).
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (2)

  Invention 3 is the hot-rolled non-heat treated steel bar according to Invention 1 or 2, wherein the fine precipitate is a carbide of Ti and Mo.

  Invention 4 is characterized by further including one or more of Nb: 0.08% or less, V: 0.15% or less, and W: 1.5% or less as a component composition of steel. It is a hot-rolling type non-tempered steel bar according to invention 1.

The invention 5 is the hot-rolled non-tempered steel bar according to the invention 4, wherein the component composition of the steel further satisfies the following formula (3).
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50 (3)

  Invention 6 is the hot-rolled non-tempered steel bar according to Invention 4 or 5, wherein the fine precipitate is a carbide containing Ti, Mo, and at least one of Nb, V, and W.

  Invention 7 is steel component composition by mass% S: 0.01-0.1%, Pb: 0.2% or less, Ca: 0.005% or less, Bi: 0.1% or less B: The hot-rolled non-tempered steel bar according to inventions 1 to 6, characterized by containing one or more of 0.02% or less.

In invention 8, the component composition of steel is C: 0.040 to 0.150% in mass%, Si: 0.5% or less, Mn: 0.5 to 3.0%, Al: 0.1% or less. , Ti: 0.03-0.35%, Mo: 0.05-0.8%, the balance Fe and unavoidable impurities, and Mn segregation ratio R satisfying the following formula (1) 1100 Heating to 1270 ° C, hot rolling at a finishing temperature of 850 to 1100 ° C, and then cooling at a cooling rate of 1.0 ° C / s or less. .
R = Mn (max) / Mn (min) ≦ 1.5 (1)
Mn (max): Mn content of the highest part of Mn
Mn (min): Mn content in the lowest part of Mn

A ninth aspect of the present invention is the method for producing a hot-rolled non-tempered steel bar according to the eighth aspect, wherein the total area reduction in the final two passes of the hot rolling is 30% or more.
here,
Total area reduction ratio in final 2 passes (%) = ((cross-sectional area before rolling in final 2 passes) − (cross-sectional area after rolling in final 2 passes)) / (cross-sectional area before rolling in final 2 passes) X100

Invention 10 is the method for producing a hot-rolled non-tempered steel bar according to Invention 8 or 9, wherein the component composition of the steel satisfies the following formula (2).
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (2)

  Invention 11 is characterized in that the composition of steel further includes one or more of Nb: 0.08% or less, V: 0.15% or less, and W: 1.5% or less in mass%. It is a manufacturing method of the hot rolling type non-tempered steel bar according to any one of inventions 8 to 10.

Invention 12 is the method for producing a hot-rolled non-tempered steel bar according to Invention 11, wherein the component composition of the steel further satisfies the following expression (3).
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50 (3)

  Invention 13 is steel component composition by mass% S: 0.01-0.1%, Pb: 0.2% or less, Ca: 0.005% or less, Bi: 0.1% or less B: A method for producing a hot-rolled non-tempered steel bar according to any one of inventions 8 to 12, characterized by containing one or more of 0.02% or less.

According to the present invention, a steel bar having a tensile component of 700 MPa or more and an impact value _U E20 of 100 J / cm ² or more that can be easily produced by actual production with an inexpensive component composition is tempered. The present invention is extremely useful in industry.

The component composition, microstructure and production conditions of the present invention will be specifically described below.
1. The reason why the component composition is limited will be described. In addition, all content (%) of each element in a component composition means the mass%.

C: 0.040 to 0.150%
When C is less than 0.040%, the precipitation amount of fine precipitates is insufficient, and a tensile strength of 700 MPa or more cannot be obtained. On the other hand, if added over 0.150%, the precipitate becomes coarse and a tensile strength of 700 MPa or more cannot be obtained, so the upper limit is made 0.150%.

Si: 0.5% or less Si is added to improve cold workability, but even if added over 0.5%, the effect is impaired, so the upper limit is made 0.5%. More preferably, it is 0.15% or less.

Mn: 0.5 to 3.0%
In the present invention, the precipitation behavior of precipitates is closely related to the progress of transformation from austenite to ferrite (hereinafter referred to as ferrite transformation), and the transformation start temperature of ferrite transformation and precipitation of precipitates that occur during cooling after rolling. When the difference from the starting temperature is small and the ferrite transformation and precipitation compete, the precipitate is finely dispersed in the ferrite. Mn contributes to competing ferrite transformation and precipitation by lowering the ferrite transformation temperature and reducing the difference between the transformation initiation temperature of the ferrite transformation and the precipitation initiation temperature of the precipitate. , 0.5% or more needs to be added. On the other hand, if added over 3.0%, a low-temperature transformation phase such as bainite is generated in addition to ferrite, so that precipitation strengthening due to fine precipitates is insufficient and the strength is lowered, so the upper limit is made 3.0%. .

Mn segregation ratio R: 1.5 or less Mn is an element that causes microsegregation. When the amount of Mn varies due to microsegregation, the transformation start temperature of ferrite transformation also varies accordingly, and the degree of competition between ferrite transformation and precipitation As a result, the size of the precipitates is different for each ferrite grain, the microstructure becomes non-uniform, and the strength varies between crystal grains. Not only is the toughness deteriorated due to the unevenness of the microstructure and the strength fluctuation between the crystal grains, but also the variation thereof increases, so it is necessary to reduce the segregation ratio R of Mn. Here, the segregation ratio R of Mn is expressed by R = Mn (max) / Mn (min)
Mn (max) represents the amount of Mn in the highest portion of Mn, and Mn (min) represents the amount of Mn in the lowest portion of Mn.

FIG. 1 shows that 0.095C-0.1Si-0.22Ti-0.41Mo-0.012P-0.012S-0.038Al-0.0031N steel is used as the base and Mn content is varied in the range of 1.0-2.0%. A steel with a segregation ratio R changed by changing the casting temperature and casting speed at the time of casting was prepared. After heating this to 1220 ° C, the total area reduction rate in the final two passes was 40%, the finishing temperature was 880 ° C, and the diameter was 100 mmφ. 3 shows the relationship between the tensile strength TS and impact value _U E20 and the segregation ratio R when hot-rolled into a steel bar. In addition, it was set as air cooling after hot rolling, and the cooling rate in that case (average cooling rate from after rolling to 500 degreeC) was 0.18 degreeC / s.

Here, when measuring the segregation ratio R of Mn, EDX analysis is performed on 20 arbitrary ferrite grains, and the Mn output count is regarded as the Mn amount, and the Mn amount of the ferrite grain having the highest Mn is determined. By setting Mn (max) and Mn amount of ferrite grains having the lowest Mn as Mn (min),
The segregation ratio R = Mn (max) / Mn (min) was calculated.

The tensile strength was measured using a small test piece having a parallel part diameter of 6 mmφ and a parallel part length of 40 mm. The impact value _U E20, were taking into account the variations were subjected to arbitrary U notched impact test piece No. JIS3 from position to twenty sampled test bars.

  The obtained steel bar was also subjected to structural observation. Specifically, specimens for tissue observation were collected from a total of 20 locations in the vicinity where the impact specimens were collected, the tissues were identified, and the average cross-sectional area of the crystal grains was determined for each specimen using the cutting method of JIS G 0552. From this, the crystal grain size of each test piece was calculated as the diameter of the equivalent circle, and the average value of the whole steel bar was obtained by taking the average value of 20 places in total. When the structure was identified and the crystal grain size was measured by this method, the structure was a ferrite single phase regardless of the position of the steel bar, and the crystal grain size was 20 to 30 μm.

As can be seen from FIG. 1, the impact value _U E20 is classified by the segregation ratio R of Mn. When the segregation ratio R is 1.5 or less, the minimum value of the impact value _U E20 is 100 J / cm ² or more and the average value is 150 J / cm ^2. That's it. Moreover, since the average value of impact values is close to the maximum value, it can be said that there are few things that are below the average value of impact values. On the other hand, when the segregation ratio R exceeds 1.5, the lowest impact value is 60 J / cm ² or less and the average value is 130 J / cm ² or less, which is inferior in toughness. In addition, the average value of the impact value is an intermediate value between the maximum value and the minimum value, and the variation of the impact value is large. Regarding the tensile strength TS, the influence of the segregation ratio is not particularly observed, and no correlation with the impact value is observed.

From the above, in order to obtain excellent toughness with a minimum impact value _U E20 of 100 J / cm ² or more and an average value of 150 J / cm ² or more, the segregation ratio R of Mn needs to be 1.5 or less. The method for measuring the segregation ratio R of Mn is not particularly limited. As described above, the segregation ratio R is obtained by measuring the output count of Mn for a plurality of ferrite grains by EDX analysis and taking the ratio between the maximum value and the minimum value. Can be evaluated.

Al: 0.1% or less Al acts as a deoxidizer, forms N and AlN, and improves the quenching effect of B.
However, even if added over 0.1%, the effect is saturated, so the upper limit is made 0.1%. More preferably, it is 0.05% or less.

Ti: 0.03-0.35%
Ti is added to finely precipitate precipitates including Ti-based carbides and Ti-Mo-based carbides, and improve strength. In order to ensure a tensile strength of 700 MPa or more, addition of 0.03% or more is necessary. On the other hand, if added over 0.35%, precipitates are coarsened, and the strength and toughness are lowered. 0.03 to 0.35%. A more preferable range is 0.03 to 0.20%.

Mo: 0.05-0.8%
Mo is added to finely precipitate precipitates including Mo-based carbides and Ti-Mo-based carbides, and to improve strength. Further, Mo has a slow diffusion rate, and when it precipitates together with Ti, it has an advantage that the growth rate of the precipitate is reduced and a fine precipitate is easily obtained. Here, in order to ensure a tensile strength of 700 MPa or more, addition of 0.05% or more is necessary. On the other hand, addition over 0.8% seems to generate a low-temperature transformation phase such as bainite in addition to ferrite. Therefore, precipitation strengthening due to fine precipitates is insufficient and the strength is lowered, so the range of Mo is set to 0.05 to 0.8%. A more preferable range is 0.15 to 0.50%.

In the above component composition, when the following formula (2) is satisfied particularly with respect to the atomic ratio of the amounts of C, Ti and Mo, it is advantageous for the refinement of precipitates.
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (2)
This parameter affects the size of the precipitate, and when the range is 0.50 to 1.50, it is preferable because the formation of fine precipitates having a particle size of less than 10 nm is facilitated.

  It should be noted that Ti and Mo in the carbides of fine Ti—Mo carbides are observed to have an atomic ratio of Ti / Mo of 0.2 to 2.0, and finer carbides of 0.7 to 1.5. It was done.

  Although the essential components have been described above, in the present invention, one or more of Nb, V, and W can be added in order to further improve the strength and toughness.

Nb: 0.08% or less Nb forms fine precipitates together with Ti and Mo and contributes to an increase in strength. Moreover, ductility and toughness are improved by refining the structure and adjusting the crystal grains. However, if added over 0.08%, it becomes excessively fine, and on the contrary, the toughness is lowered. Therefore, the added amount is made 0.08% or less. More preferably, it is 0.04% or less.

V: 0.15% or less V also forms fine precipitates together with Ti and Mo and contributes to an increase in strength. However, if added over 0.15%, the precipitates become coarse, so the addition amount is 0.15 % Or less. More preferably, it is 0.10% or less.

W: 1.5% or less W also forms fine precipitates together with Ti and Mo and contributes to an increase in strength. However, if added over 1.5%, the precipitates become coarse, so the addition amount is 1.5%. The following. More preferably, it is 1.0% or less.

When the above-described Nb, V, and W are added, if the following formula (3) is satisfied with respect to the atomic ratio of these elements and the amounts of C, Ti, and Mo, it is advantageous for refinement of precipitates.
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50 (3)
This parameter affects the size of the precipitate, and when the range is set to 0.50 or more and 1.50 or less, formation of fine precipitates having a particle diameter of less than 10 nm is facilitated, which is preferable.
In addition, in the fine carbide containing one or more of Nb, V, and W, the atomic ratio (Ti + Nb + V) / (Mo + W) of Ti, Mo, Nb, V, and W in the carbide is 0. .2 to 2.0, and 0.7 to 1.5 for finer carbides were observed.

Furthermore, in the present invention, in order to improve the machinability at the time of processing the part, S is set to 0.01% or more and 0.1% or less, Pb: 0.2% or less, Ca: 0.005% or less, Bi : 0.1% or less, B: 0.02% or less can be added alone or in combination.
Here, the reason why S is set to 0.01 or more and 0.1% or less is that when S is less than 0.01%, machinability cannot be improved. It is because toughness falls.
In addition, since the ductility and toughness of Pb, Ca, Bi, and B exceed the respective upper limit values, ductility and toughness are reduced. Therefore, the addition amount is Pb: 0.2% or less, Ca: 0.005% or less, Bi : 0.1% or less, B: 0.02% or less.

In addition, for the purpose of improving strength, ductility and toughness, one or more of Cr, Ni and Cu are added in a range of Cr: 0.5% or less, Ni 0.5% or less, Cu 0.5% or less. It doesn't matter.
P and N, which are unavoidable impurities, are undesirable elements for toughness, so it is desirable to reduce P and N. Specifically, it is preferable to restrict P to 0.03% or less. N is preferably regulated to 0.01% or less, and more preferably 0.005% or less. In addition, the effect of this invention is not impaired by the presence or absence and content of these elements.

2. Regarding Microstructure In the present invention, the microstructure is a structure in which fine precipitates having a grain size of less than 10 nm are dispersed and precipitated in a ferrite single-phase structure having a crystal grain size of 35 μm or less. The reason will be described.

Ferrite: Single-phase structure The structure in the present invention has a toughness comparable to that of the tempered material by forming a ferrite single-phase structure. Here, the ferrite single-phase structure means that the area ratio of ferrite is 95% or more, preferably 98% or more by observing the cross-sectional structure with a 200-fold optical microscope.

The crystal grain size of ferrite: 35 μm or less In the present invention, the toughness is improved by setting the crystal grain size of ferrite to 35 μm or less,
The minimum value of the impact value _U E20 is 100 J / cm ² or more, and the average value is 150 J / cm ² or more.

The details will be described below based on the test results.
0.062C-0.15Si-1.35Mn-0.12Ti-0.25Mo-0.023P-0.017S-0.033Al-0.0045N steel with the Mn segregation ratio R of 1.32 by adjusting the casting temperature and casting speed during casting Prepared. This was heated to 1130 ° C., hot-rolled into a steel bar having a diameter of 150 mmφ, and air-cooled to room temperature after rolling (the average cooling measure up to 500 ° C. was 0.11 ° C./s). During the hot rolling, various rolling pass schedules (temperature and area reduction ratio of each rolling pass) were changed in order to change the crystal grain size of ferrite.

The obtained steel bar was subjected to a structure observation and a tensile test and an impact test. Here, with respect to the impact test, 20 U-notch impact test pieces of JIS3 were collected from any position of the steel bar in consideration of variation, and used for the test. In the structure observation, the structure was identified and the crystal grain size was measured in the same manner as when the influence of the segregation ratio R of Mn was investigated.
As a result of the structure observation, the structure was a ferrite single phase under any condition. The ferrite grain size was changed from about 15 μm to about 55 μm because the pass schedule in hot rolling was changed.

FIG. 2 shows the relationship between the ferrite grain size, the tensile strength TS, and the impact value _U E20.
The impact value _U E20 varies depending on the ferrite particle size. When the ferrite particle size is 35 μm or less, the minimum value of the impact value _U E20 is 100 J / cm ² or more and the average value is 150 J / cm ² or more. Yes. In contrast minimum value of those ferrite grain diameter exceeds 35μm impact value _U E20 is 60 J / cm ² or less, and the average value of a 120 J / cm ² or less, a large deterioration of the toughness.

Here, it is considered that the impact value _U E20 greatly changes at the boundary of the ferrite particle size of 35 μm because the transition temperature is about 20 ° C. when the ferrite particle size is 35 μm. That is, when the ferrite particle size is finer than 35 μm, the transition temperature is lower than 20 ° C., so a high impact value _U E20 is shown. Conversely, when the ferrite particle size is coarser than 35 μm, the transition temperature is Since the temperature is higher than 20 ° C., it is assumed that only a low impact value _U E20 is shown. Although the tensile strength also varies depending on the ferrite particle size, the degree is about 30 MPa, and the ferrite particle size dependency is small. In addition, if the tensile test value changes to such a degree, the impact on the impact value can be ignored.
From the above, in order to obtain excellent toughness with a minimum impact value _U E20 of 100 J / cm ² or more and an average value of 150 J / cm ² or more, the crystal grain size of ferrite needs to be 35 μm or less.

Particle size of fine precipitates in ferrite: less than 10 nm When the particle size of precipitates is 10 nm or more, the tensile strength of 700 MPa or more necessary for machine structural parts of transport machines and construction machines including automobiles cannot be obtained. On the other hand, when a fine precipitate having a particle size of less than 10 nm is precipitated in the ferrite single phase structure, the yield ratio is increased, and a high yield ratio comparable to the tempered material is obtained. When the yield ratio is high, an increase in tensile strength is suppressed with respect to an increase in yield strength and the hardening of the steel can be reduced, so that machinability comparable to tempered steel is obtained. Fine precipitates are deposited during cooling after hot rolling.

  The smaller the particle size of the fine precipitates, the more effective the strength increase. Desirably, the fine precipitate is 5 nm, more preferably 3 nm or less, and the fine precipitate is a carbide containing a composite of Ti and Mo, or further Nb, V , Carbides containing one or more of W are preferred.

With respect to the number of fine precipitates, 1000 / μm ³ or more, more desirably 5000 / μm ³ or more, it is easy to obtain a tensile strength of 700 MPa or more.

  Although the distribution form of these fine precipitates is not particularly defined, it is desirable that the fine precipitates are uniformly dispersed and precipitated in the matrix. In the present invention, if the size of the precipitate satisfies 90% or more of the total precipitate, the target tensile strength of 700 MPa or more can be obtained. However, a precipitate having a size of 10 nm or more consumes the precipitate-forming element and adversely affects the strength.

  In addition, the effect of the present invention is not impaired even if a small amount of Fe carbide is contained in addition to the above-described precipitate, but if a large amount of Fe carbide having an average particle size of 1 μm or more is contained, the toughness is inhibited. In the invention, it is desirable that the upper limit of the size of Fe carbide contained is 1 μm and the content is 1% or less of the entire precipitate.

Below, the calculation method of the ratio which occupies for all the precipitates of a fine precipitate is demonstrated.
An electron microscope sample is prepared by an electropolishing method using a twin jet method and observed at an acceleration voltage of 200 kV. At that time, the defocus is defocused from the normal focus in order to control the crystal orientation of the parent phase so that the fine precipitate has a measurable contrast with respect to the parent phase and to minimize the number of precipitates. Observe by method. Moreover, the sample thickness of the area | region which measured the deposit particle | grains is evaluated by measuring an elastic scattering peak and an inelastic scattering peak intensity using an electron energy loss spectroscopy.

By this method, the measurement of the number of particles and the measurement of the sample thickness can be executed for the same region. The number of particles and the particle size are measured at four locations of a 0.5 μm × 0.5 μm region of the sample, and the precipitates distributed per 1 μm ² are calculated as the number for each particle size. From this value and the thickness of the sample, the number of precipitates distributed per 1 μm ^{3 of} particle diameter is calculated, and the ratio of the precipitate having a particle diameter of less than 10 nm to the total precipitate is calculated.

3. The manufacturing conditions desirable below are described.

Casting conditions In the present invention, in order to suppress the segregation of Mn at the time of casting the steel so that the segregation ratio R of Mn is within the above range, the casting temperature is set to 1530 ° C. or less, and the casting speed (drawing speed) is 1.5 m. Must be less than / min.

Heating temperature: 1100-1270 ° C
In the present invention, in order to precipitate precipitates finely during cooling after hot rolling, it is necessary to dissolve the precipitates precipitated on the slab before hot rolling in a heating furnace. At that time, when the heating temperature is less than 1100 ° C., the Ti—Mo-based carbide is not sufficiently dissolved, so the temperature is set to 1100 ° C. or higher. On the other hand, if the heating temperature is higher than 1270 ° C., the austenite grain size becomes coarse, and it becomes difficult to make the ferrite grain size 35 μm or less, so the upper limit of the heating temperature is 1270 ° C.

Finishing temperature: 850-1100 ° C
In the present invention, the precipitation behavior of the precipitate is closely related to the progress of the ferrite transformation, and the difference between the transformation start temperature of the ferrite transformation that occurs during cooling after rolling and the precipitation start temperature of the precipitate is small. When precipitation competes, the precipitate is finely dispersed and precipitated in the ferrite. In order to compete with ferrite transformation and precipitation, it is necessary to lower the ferrite transformation start temperature, but when the finishing temperature in hot rolling is low, the strain introduced in the rolling increases the ferrite transformation start temperature and causes precipitation. Inhibits refinement of objects.

  Therefore, it is necessary to set the finishing temperature to 850 ° C. or higher where the influence of rolling distortion does not appear. On the other hand, if the finishing temperature is too high, the ferrite grains become coarse and it becomes difficult to make the ferrite grain size 35 μm or less. Therefore, the upper limit of the finishing temperature is set to 1100 ° C.

Cooling rate: 1.0 ° C./s or less In the present invention, fine precipitates are deposited during cooling after hot rolling. In that case, if the cooling rate after hot rolling exceeds 1.0 ° C./s, precipitation does not proceed sufficiently, and a tensile strength of 700 MPa or more cannot be obtained. Therefore, the cooling rate after hot rolling needs to be 1.0 ° C./s or less. Further, if the cooling rate is 1.0 ° C./s or less, the steel of the present invention is a low C steel, so that a ferrite single phase is obtained. In addition, since precipitation is substantially complete | finished by 500 degreeC, what is necessary is just to cool to 500 degreeC after a hot rolling by the cooling rate of 1.0 degrees C / s or less.

Total area reduction: 30% or more In the present invention, in order to obtain excellent toughness, it is necessary to refine the crystal grain size of ferrite. For this purpose, it is desirable to increase the area reduction in hot rolling. In that case, it is effective to increase the total area reduction in the final two passes of hot rolling. Specifically, if the total area reduction ratio is 30% or more, the ferrite grain size can be easily refined, which is preferable.

  Steels having the compositions shown in Table 1 were melted and hot-rolled into steel bars having predetermined dimensions under the conditions shown in Tables 2 and 3. During melting, each component element was changed, and the segregation ratio R of Mn was also changed by changing the casting temperature and casting speed during casting. Here, steel Nos. 1 to 24 and 27 have a casting temperature of 1530 ° C. or less and a casting speed of 1.5 m / min or less, and steel Nos. 25 and 26 are set to satisfy these conditions. Regarding the segregation ratio R of Mn, as described above, an EDX analysis is performed on 20 arbitrary ferrite grains, the output count of Mn is regarded as the Mn amount, and the Mn amount of the ferrite grain having the highest Mn is defined as Mn ( max), and the amount of Mn of the ferrite grain having the lowest Mn was calculated as Mn (min) from the ratio of the two. In addition to the steel of the present invention that satisfies the scope of the present invention prepared in this way and the comparative steel that deviates from the scope of the present invention, non-tempered steel of S45C tempered material (steel No. 27) was also melted as conventional steel.

  In the hot rolling, the heating temperature, pass schedule (temperature and area reduction ratio of each reduction pass), finishing temperature, and cooling rate to 500 ° C. after rolling were changed. Here, the cooling rate was changed by changing the finished dimensions of the rolling and air cooling after rolling.

The steel bar thus obtained was subjected to structure observation, tensile test, and impact test.
The tensile test was performed using a small test piece having a parallel part diameter of 6 mmφ and a parallel part length of 40 mm.

As for the impact test, 20 JIS3 U-notch impact test specimens were collected from arbitrary positions of the steel bar in consideration of variation, and the impact value _U E20 at 20 ° C. was obtained by using the specimens for the test.

  As described above, the tissue observation specimens were collected from a total of 20 locations in the vicinity of the impact specimen. Using this, after corroding the steel bar cross section with nital, the structure was identified with an optical microscope, and the average cross-sectional area of the crystal grains was determined for each test piece by the cutting method of JIS G 0552. The crystal grain size of each test piece was calculated, and the average crystal grain size of the whole steel bar was calculated by taking an average value of 20 places in total. In addition, a thin film sample is prepared by electrolytic polishing, and the particle size of the precipitate is measured by observation with a transmission electron microscope (TEM) according to the method described above, and an energy dispersive X-ray spectrometer (EDX) is used in combination. The precipitate was identified.

Tables 2 and 3 show the results of the above-described structure observation, tensile test, and impact test.
The numbers in the table are used to classify the individual results, and the steel numbers and hot rolling conditions are combined (for example, steel numbers) so that the combination of the test steel and hot rolling conditions is clearly indicated. If 1 is hot-rolled under condition A, start with 1-A).

  Regarding the structure, the case where a low-temperature transformation phase such as F for ferrite, P for pearlite, or bainite is generated and its volume fraction exceeds 5% or more is abbreviated as T. The average particle size is described for the precipitate. The variation in particle diameter was ± 1 nm at the maximum for precipitates of less than 10 nm, and ± 3 nm to ± 5 nm for precipitates larger than that. In the case where pearlite or a low temperature transformation phase was generated in the structure, the measurement of the crystal grain size and the particle size of the precipitate was omitted.

Table 2 shows the hot rolling conditions within the scope of the present invention, and shows the influence of the steel composition. In the present invention examples where both the steel composition and the hot rolling conditions satisfy the scope of the present invention, the tensile strength of 700 MPa or more is shown. As for the impact value _U E20, the minimum value is 100 J / cm ² or more and the average value is 150 J / cm ² or more, which is significantly higher than the value of conventional steel (No. 27-A). It can be seen that the toughness can be obtained stably.

On the other hand, in the comparative example in which the steel composition is out of the scope of the present invention, either the tensile strength or the impact value _U E20 does not satisfy the scope of the present invention.
No. 17-A has low C, the amount of fine precipitates is insufficient, and the tensile strength is low.
No. 18-A has high C, precipitates are coarsened, and tensile strength is low.
No. 19-A has a low Mn, so the ferrite transformation and precipitation do not compete sufficiently, and the precipitate precipitates coarsely, resulting in low tensile strength.

Since No. 20-A has a high Mn, a low-temperature transformation phase is generated, and the precipitation strength due to fine precipitates is insufficient, so that the tensile strength is low. Although seems to be due to formation of the low-temperature transformation phase, for the impact value _U E20, minimum value 102J / cm ^2, the mean value has become a 161J / cm ^2, the value for the aim of the present invention only Only show values above.

No. 21-A has low Ti, so the amount of fine precipitates is insufficient and the tensile strength is low. on the other hand,
No. 22-A is high in Ti, precipitates are coarsened, the tensile strength is low, and the impact value _U E20 is also below the target value.

No. 23-A has low Mo, so the amount of fine precipitates is insufficient and the tensile strength is low. on the other hand,
No. 24-A has a high Mo, so a low-temperature transformation phase is formed, and the tensile strength is low due to insufficient precipitation strengthening by fine precipitates. Further, as with No. 20-A having a high Mn, the impact value _U E20 has a minimum value of 105 J / cm ² and an average value of 158 J / cm ² , which is slightly higher than the target value of the present invention. Only show.

Since No. 25-A has a high segregation ratio R of Mn, although the tensile strength is slightly higher than the target lower limit, both the minimum value and the average value of the impact value _U E20 are below the target and inferior in toughness.
Since No. 26-A has a higher segregation ratio R of Mn, the minimum value and average value of the impact value _U E20 are far below the target and are inferior in toughness. In addition, the size of the precipitate is as coarse as 18 nm and the tensile strength is low.

Table 3 shows the results of hot rolling under various rolling conditions using steel No. 2 which is the steel of the present invention.
In the present invention example in which steel No. 2 which is the present invention steel is hot-rolled under the conditions of the present invention, a tensile strength of 700 MPa or more is obtained, and the impact value _U E20 also has a minimum value of 100 J / cm ² or more, The average value is 150 J / cm ² or more, significantly exceeding the value of conventional steel (No. 27-A), and excellent toughness can be obtained stably.

Here, paying attention to the total area reduction rate in the final two passes of hot rolling, as can be seen from the comparison of No.2-D to No.2-H, even when the total area reduction rate is less than 30% (No .2-G, H), the ferrite grain size is approximately 30 μm, which is 35 μm or less, which is the range of the present invention, and the minimum value of the impact value _U E20 is about 110 _J / cm ² and the average value is about 160 _J / cm ^2. Excellent toughness is obtained, but when the total area reduction is 30% or more (No.2-D, E and F), the ferrite grain size is reduced to 20 μm or less, and the minimum value of the impact value _U E20 is Around 130 J / cm ² , the average value is 170 J / cm ² or more. From this, when the total area reduction in the final two passes of hot rolling is 30% or more, finer ferrite is promoted and the toughness is further improved.

On the other hand, in the comparative example in which the hot rolling conditions are out of the scope of the present invention, either the tensile strength or the impact value _U E20 does not satisfy the present invention.
No.2-I has a low heating temperature, and the precipitate deposited on the slab before hot rolling does not sufficiently dissolve in the heating furnace. Low. In addition, regarding the precipitate, there are a mixture of what seems to have finely precipitated during cooling after rolling, and what seems to be the undissolved residue of the precipitate deposited on the slab, and the average particle size of the precipitate is It was 100 nm or more.
On the other hand, in No. 2-J, since the heating temperature is high and the austenite at the time of heating is coarse, the ferrite is also coarsened to 41 μm, and the impact value _U E20 is inferior.

In No. 2-K, the finishing temperature is low, and the strain introduced by rolling raises the start temperature of the ferrite transformation and prevents competition between the ferrite transformation and precipitation, resulting in coarse precipitates and low tensile strength. On the other hand, in No.2-L, since the finishing temperature is high, the ferrite is coarse and the impact value _U E20 is inferior.
Since No.2-M has a high cooling rate after hot rolling, fine precipitates are not sufficiently precipitated during cooling, and the tensile strength is low.

The present invention is a hot-rolled non-heat treated steel bar with a tensile strength of 700 MPa or more and an impact value _U E20 of 100 J / cm ² or more which can be stably obtained, and is a structural component used in transportation equipment and construction machinery including automobiles. It can be used in place of tempered steel that has been quenched and tempered.

It is a diagram showing the relationship between impact value _U E20 and tensile strength at segregation ratio R and 20 ° C. for Mn. It is a diagram showing the relationship between impact value _U E20 and the tensile strength in the crystal grain size and 20 ° C. of ferrite.

Claims (13)

The component composition of the steel is C: 0.040 to 0.150% in mass%, Si: 0.5% or less, Mn: 0.5 to 3.0%, Al: 0.1% or less, Ti: 0 0.03 to 0.35%, Mo: 0.05 to 0.8%, balance Fe and inevitable impurities, Mn segregation ratio R satisfies the following formula (1), and the structure has a crystal grain size of 35 μm or less. A hot-rolled non-tempered steel bar having a ferrite single phase and fine precipitates having a particle size of less than 10 nm dispersed in ferrite.
R = Mn (max) / Mn (min) ≦ 1.5 (1)
Mn (max): Mn content of the highest part of Mn
Mn (min): Mn content in the lowest part of Mn The hot rolling type non-tempered steel bar according to claim 1, wherein the component composition of the steel satisfies the following formula (2).
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (2)   The hot-rolled non-tempered steel bar according to claim 1 or 2, wherein the fine precipitate is a carbide of Ti or Mo.   The steel composition further comprises one or more of Nb: 0.08% or less, V: 0.15% or less, and W: 1.5% or less in terms of mass%. Hot rolled non-tempered steel bar. Furthermore, the component composition of steel satisfy | fills following (3) Formula, The hot-rolling type non-heat-treated steel bar of Claim 4 characterized by the above-mentioned.
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50 (3)   The hot-rolled non-heat treated steel bar according to claim 4 or 5, wherein the fine precipitate is a carbide containing Ti, Mo, and at least one of Nb, V, and W.   As a component composition of steel, S: 0.01 to 0.1% by mass%, Pb: 0.2% or less, Ca: 0.005% or less, Bi: 0.1% or less, B: 0 The hot-rolled non-tempered steel bar according to claim 1, comprising 0.02% or less of one kind or two or more kinds. The component composition of the steel is C: 0.040 to 0.150% in mass%, Si: 0.5% or less, Mn: 0.5 to 3.0%, Al: 0.1% or less, Ti: 0 0.03 to 0.35%, Mo: 0.05 to 0.8%, balance Fe and unavoidable impurities, and Mn segregation ratio R is heated to 1100 to 1270 ° C satisfying the following formula (1) And hot-rolling at a finishing temperature of 850 to 1100 ° C., and then cooling at a cooling rate of 1.0 ° C./s or less.
R = Mn (max) / Mn (min) ≦ 1.5 (1)
Mn (max): Mn content of the highest part of Mn
Mn (min): Mn content in the lowest part of Mn The method for producing a hot-rolled non-tempered steel bar according to claim 8, wherein the total area reduction in the final two passes of hot rolling is 30% or more.
here,
Total area reduction ratio in final 2 passes (%) = ((cross-sectional area before rolling in final 2 passes) − (cross-sectional area after rolling in final 2 passes)) / (cross-sectional area before rolling in final 2 passes) X100 The method for producing a hot-rolled non-heat treated steel bar according to claim 8 or 9, wherein the component composition of the steel satisfies the following formula (2).
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96)] ≦ 1.50 (2)   The component composition of the steel further includes one or more of Nb: 0.08% or less, V: 0.15% or less, and W: 1.5% or less in terms of mass%. The method for producing a hot-rolled non-heat treated steel bar according to any one of 10. Furthermore, the component composition of steel satisfy | fills following (3) Formula, The manufacturing method of the hot-rolling type non-tempered steel bar of Claim 11 characterized by the above-mentioned.
0.50 ≦ (C / 12) / [(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 184)] ≦ 1.50 (3)   As a component composition of steel, S: 0.01 to 0.1% by mass%, Pb: 0.2% or less, Ca: 0.005% or less, Bi: 0.1% or less, B: 0 The method for producing a hot-rolled non-tempered steel bar according to any one of claims 8 to 12, comprising 0.02% or less of one kind or two or more kinds.

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