JPH10277705A - Production of non-heat-treated bar steel for high toughness hot-forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably obtain high toughness even if the heat treatment after hot-forging is omitted in the production process of machine parts by heating and pressing under condition of specific temp. and reduction of area, forming into a steel billet and further, rolling this steel billet into a bar steel. SOLUTION: The steel containing 0.70-1.30 in the range of Ceq shown in the equation I, and 0.20-0.60% C, 0.10-1.5% Si, 0.60-2.00% Mn, 0.10-1.0% Cr, 0.03-0.35% V, 0.01-0.07% Ti, 0.0030-0.0200% N and 0.005-0.050% Al, is produced and continuously cast. At this time, the casting and the cooling is executed so that the temp. range of the solidus temp.-1000 deg.C becomes >=15 deg.C/min cooling speed. Thereafter, the surface temp. of a cast bloom is cooled to <=900 deg.C and charged into a heating furnace as hot slab state without cooling at <=500 deg.C for >=20 min. At this time, after heating the steel slab so that the heating time T1 and the staying time in a heating furnace satisfy the inequality II, blooming is applied at 10-35% reduction of area per one pass and the rolling reduction is executed at 2-6 passes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-heat treated steel bar for hot forging having high toughness.

[0002] Machine parts for automobiles and industrial machines are formed by hot forging carbon steel for machine structure into a predetermined shape, then quenching.
Often manufactured by tempering (tempering heat treatment) and cutting. In this manufacturing process, a large amount of energy and cost are required for the refining heat treatment performed after the hot forging, and omitting them has great industrial significance. The present invention provides a method for producing a non-heat treated steel for hot forging having high toughness even if the heat treatment after the hot forging is omitted.

[0003]

2. Description of the Related Art In order to omit the above-mentioned heat treatment after hot forging, a so-called microalloy type non-heat treated steel in which a small amount of elements such as V and Nb are added to steel for machine structural use is well known. . However, the metallographic structure of this type of non-heat treated steel after hot forging shows a ferrite and pearlite structure that has been significantly coarsened, and even if it is machined into mechanical parts, its toughness is extremely low. Its application is limited to fields that do not require high toughness, and in fact it cannot be applied to important parts such as underbody parts that require high toughness. In order to solve this drawback, it has been proposed to add a small amount of Ti to prevent coarsening of crystal grains and improve toughness (for example, JP-A-56-38448). It is not stable, and it is not a perfect measure.

[0004] As one of methods for stably securing the toughness of the non-heat treated material, the casting size is reduced to increase the cooling speed of the slab, and the slab is rolled into a steel bar by omitting slab rolling. (For example, Japanese Patent Application Laid-Open No. 62-2537).
25) In this case, the application to large parts is limited due to the reduction ratio due to the small slab size.

[0005] In producing this type of non-heat treated steel, by taking some measures to ensure toughness, it has become possible to achieve stable toughness even after the sizing process. In this case, the application of the non-heat treated steel to large parts can be expanded, and the merit is extremely large. On the other hand, when non-heat-treated steel is manufactured through such a lumping process, it depends on the component system, but is not limited to carbon such as AlN, TiN, V (C, N), nitride or ( Precipitation of sulfides such as Mn, Fe) S, etc. is large, and surface defects are likely to occur in a slab heating step or a lumping step due to a decrease in hot ductility caused by the sulfides. Depending on the degree, the yield in slab rolling is greatly reduced, and costs for repairing the flaws are required, leading to a significant increase in manufacturing costs.

[0006]

As described above, as described above, Ti
Is an additive element suitable for refining the final grains of hot-forged non-refined steel. By optimizing the steel production method used for forging, the effect of refining Ti crystal grains is maximized. Can be used for

[0007] Therefore, components and production methods that maximize the effect of Ti are still being sought, and in particular, a production method that enables a significant cost reduction while satisfying predetermined mechanical properties industrially. Are increasingly demanding.

According to the present invention, high toughness can always be obtained stably even if the heat treatment after hot forging is omitted in the above-mentioned process of manufacturing mechanical parts. It is an object of the present invention to provide a method for producing a non-heat-treated steel for hot forging, which can be applied to a large-sized component whose application has been postponed. In addition, another object of the present invention is to provide a method for suppressing the occurrence of surface flaws, which are likely to occur during slab rolling in the present non-heat-treated steel, and preventing an increase in production cost due to a decrease in yield or flaw care.

[0009]

Means for Solving the Problems The present inventors have conducted various studies in order to solve the above problems and completed the present invention. That is, the first aspect of the present invention is as follows: (I) Ceq. Is in the range of 0.70 to 1.30, C: 0.20 to 0.60%, Si: 0.10 to 1.5% Mn: 0.60 to 2.00% Cr: 0.10 to 1.30%. 0% V: 0.03 to 0.35% Ti: 0.01 to 0.07% N: 0.0030 to 0.0200% Al: 0.005 to 0.050%, with the balance substantially being When a steel consisting of unavoidable impurities and Fe is melted using a converter or an electric furnace and further cast by a continuous casting method, the temperature range from the solidus temperature to 1000 ° C. is set at a cooling rate of 15 ° C./min or more. Casting,
After cooling, the slab surface temperature is cooled to 900 ° C. or less, and without being cooled to a temperature of 500 ° C. or less for 20 minutes or more, the slab is charged into a heating furnace as a hot slab. After heating so that the heating temperature (T1) and the heating furnace residence time (t1) satisfy the expression (2), two to six passes are performed by slab rolling at a reduction rate of 10 to 35% per pass. This is a method for producing a non-heat treated steel bar for high toughness hot forging, characterized by forming a steel slab by applying a reduction, and further rolling the steel slab.

[0010] Ceq. (%) = C% + ／ × Si% + ／ × Mn% + 1/9 × Cr% + 1.54 × V% (1) Furnace heating time: t1 (min) ≦ (1181-T1) / 0.77 (2) where T1: heating temperature (° C.) where 900 ≦ T1 ≦
1181 (° C.) The second aspect of the present invention is as follows: (II) In the process of cooling the continuous cast slab, at least a range of 10 mm or more from the surface layer of the cast slab is once cooled to the Ar 1 point temperature or lower, and then loaded into the heating furnace And heating the cast slab. The method for producing a non-heat treated steel bar for high toughness hot forging as described in (I) above, wherein the slab is heated.

Further, the third aspect of the present invention is that (III) the heating temperature (T2) and the heating furnace time in the heating furnace: t2 (min) ≦ ( 1240-T2) /1.33 (3) where T2: heating temperature (° C.), provided that 900 ≦ T2 ≦
1240 (° C.) After heating so that (t2) satisfies the expression (3), the steel slab is rolled into a steel bar, and the high toughness hot work according to the above (I) or (II), This is a method for producing a non-heat treated steel bar for forging.

The first aspect of the invention is that the chemical composition is restricted to a specific range, and when casting molten steel having that composition, the cooling rate after solidification is increased and the slab is transferred to a heating furnace. To control the temperature condition of the slab at the time of charging and the heating condition at the time of heating the slab to a specific range, and then apply predetermined processing by slab rolling to form a slab for bar rolling. Thus, the present inventors have found that it is possible to prevent coarsening of crystal grains generated during heating of a hot structure and to stably obtain high toughness in a non-steel-forged steel part used as hot forged.

A second aspect of the present invention is the same as the first aspect, except that the surface layer of the slab is once cooled to a temperature not higher than the Ar1 point before being charged into the heating furnace. In order to achieve high toughness while hot forging in non-heat treated steel forged parts, a method for improving surface flaws, which is a problem especially when heating and slab rolling high nitrogen steel slabs It was found.

A third invention point is that hot forging is performed by combining the first and second inventions with controlling a heating condition for heating a steel slab to a specific range. The present invention has found a method for more effectively preventing the coarsening of the crystal grains generated during the heating of a non-heat-treated steel, and more stably achieving high toughness in hot-forged non-heat treated steel parts.

[0015]

Embodiments of the present invention will be described below.

The reasons for limiting the chemical components in the present invention are as follows.

C is an element necessary for controlling the ferrite and pearlite structures of the non-heat-treated forged product and for increasing the strength of the part by precipitation hardening in combination with V. If the amount is less than 0.20%, it is necessary. The amount of alloying elements for obtaining strength increases, which is uneconomical. If it exceeds 0.6%, the strength becomes too high and the toughness is impaired.

Since Si is 0.10% or more as a deoxidizing agent, and if it exceeds 1.5%, it becomes harder than necessary.
The upper limit was 1.5%.

Mn, together with C, Si, and Cr, is an important element that controls the strength of a non-heat treated steel product.
On the other hand, if it exceeds 2.0%, the difficulty in production increases and, on the contrary, the machinability decreases, so it must be avoided.

Cr is an important element for improving the strength of the tempered steel product like Mn described above. If it is less than 0.1%, its effect is not sufficient. Since the material improvement effect was small and not economical, it was excluded from the claims.

V is an element that enhances the strength by being combined with C and N in the steel during cooling after hot forging, and its effect cannot be obtained if it is less than 0.03%, while it exceeds 0.35%. Even if it is added, it is only too hard, so 0.35% was made the upper limit.

C, Si, Mn, Cr and V are all elements that increase the strength of the as-forged as-heat-treated non-heat treated steel part, and the amount of Ceq. Is 0.70 or more,
Must be 1.30 or less. It is specified that the hardness of machine parts used in automobiles or industrial occasions is such an altitude since the hardness is in the range of 200 to 360 in Vickers hardness, and therefore Ceq. Is less than 0.70, the required hardness cannot be obtained.
If it exceeds 1.30, it becomes too hard and must be avoided.

Ti and N are elements necessary to form nitrides in the steel and to prevent the crystal grains from becoming coarse during heating in hot forging. If the Ti content is less than 0.1%, no effect can be obtained, and if it exceeds 0.07%, the toughness is rather deteriorated. Therefore, the upper limit is set to 0.07%.

On the other hand, if N is less than 0.0030%, the effect of preventing the crystal grains from being coarsened cannot be obtained, and if N exceeds 0.0200%, the toughness is reduced, so that such addition must be avoided.

A1 is an element necessary for deoxidizing steel.
If less than 005%, no effect is obtained, and 0.050%
Even if it is added in excess, no further effect can be obtained, so that it is excluded from the claims.

Incidentally, S, which is an element for improving machinability,
Even if Pb, Bi, Te, Ca, etc. are added, this effect is not impaired at all.

Next, the necessity of controlling the speed after completion of solidification when casting molten steel will be described. After a certain part in the slab is solidified, that is, 100% after the solidus temperature is reduced.
The average cooling rate in the temperature range up to 0 ° C is 15 ° C / m
If it is not cooled at a cooling rate of more than in, the heat until rolling to the subsequent product, how to devise the processing history, but it is not possible to finely disperse Ti's charcoal, nitride during hot forging, As a result, the crystal grains become coarse and the toughness is lost.

As described above, in order to obtain high toughness as it is during hot forging, it is necessary to disperse Ti charcoal and nitride finely during hot forging to prevent coarsening of crystal grains.
For that purpose, it is a necessary condition to increase the cooling rate in the continuous casting process.

For example, as proposed in Japanese Patent Application Laid-Open No. 62-253725, when the casting size is reduced, the cooling rate of the slab is increased, and when the slab rolling is omitted, the steel is rolled into a steel bar. Not subjected to reheating in the lumping process,
i Since the growth of charcoal and nitride can be prevented, it is easy to achieve finer charcoal and nitride.

However, when the cast slab cooled under the conditions of the present invention is subjected to slab rolling and once formed into a steel slab and then rolled at a steel bar factory or the like, coarsening of Ti coal and nitride is prevented. Therefore, it is necessary to further restrict the temperature history and processing conditions from the continuous casting process to the billet through the slab rolling to the slab to a specific range. It has been found that non-heat treated steel for hot forging in which high toughness appears stably can be manufactured even after passing through.

That is, in the continuous casting step and subsequent steps, the surface temperature of the slab is cooled to 900 ° C. or less, and
The heating piece was charged into the heating furnace without cooling to a temperature of 20 ° C. or less for 20 minutes or more, and the heating was performed so that the heating temperature (T1) and the heating furnace time (t1) satisfied the formula (2). Then, by performing rolling from outer cast slab to steel slab by applying reduction of 2 to 6 passes at a reduction rate of 10 to 35% per pass by slab rolling, Ti is further reduced. Charcoal and nitride can be finely and sufficiently precipitated, and a non-heat treated steel for hot forging having excellent toughness can be manufactured.

The heating furnace charging temperature is set to 900 ° C. or less, and 50
The reason why a restriction such as charging a heating piece into a heating furnace without cooling it to a temperature of 0 ° C. or less for 20 minutes or more is given by taking a temperature history before charging the heating furnace. Ti dissolved in supersaturation by cooling in the process
Is promoted, and the Ostwald growth of the deposited charcoal and nitride is suppressed.
Fine distribution of i-charcoal and nitride can be realized.

When the slab is charged into a heating furnace at a temperature exceeding 900 ° C., the sites where Ti coals and nitrides precipitate are sparse, and the coals and nitrides become uneven in the heating furnace. In addition, it is coarsely precipitated, and the coarsening of crystal grains during hot forging cannot be sufficiently suppressed.

On the other hand, when the slab cooled to a temperature of 500 ° C. or less for 20 minutes or more is heated, it is already precipitated (M
The tendency of Ti coals and nitrides to precipitate on precipitates such as (n, Fe) S increases, and depending on the distribution of (Mn, Fe) S etc., the distribution of Ti coals and nitrides becomes non-uniform, It is preferable to avoid such conditions because coarse grains are easily generated during hot forging and the toughness varies accordingly.

Further, the heating conditions of the slab before the slab rolling are restricted so as to satisfy the expression (2), because the precipitation of fine coal and nitride of Ti is promoted and the coarsening of char and nitride is promoted. It is for suppressing.

Further, the slab to which the above-mentioned heat history has been applied in the continuous casting step and the heating step before slab rolling is subjected to two- to six-pass passes by slab rolling at a reduction in area of 10 to 35% per stage. When a reduction is applied to form the slab into a billet, Ti charcoal,
Precipitation of nitride is promoted by the strain introduced by slab rolling, and the amount of fine carbon and nitride precipitated can be effectively increased.
The reason for limiting the area reduction rate per stage to 10 to 35% is as follows:
If it is less than 10%, the deformation does not penetrate sufficiently to the center of the material,
Sufficient distortion cannot be introduced at the center. The area reduction rate is 35%
This is because if it exceeds 300 mm, rolling becomes difficult because the surface quality, the shape and dimensional accuracy of the slab are secured, and it becomes difficult to bite into the reduction roll.

The reason for limiting the number of passes is that no strain is introduced in the 0 pass, and it is extremely difficult to form a slab having the dimensional accuracy required for product rolling in the 1 pass, while the number of passes is 6 passes. In the above, the effect of promoting the precipitation of charcoal and nitride of Ti saturates. On the other hand, in the case of continuous rolling, an increase in capital investment due to an increase in the number of rolling mills and the like, and in the case of a reversing mill, the productivity is increased. This is not a good idea because it causes a decrease in the manufacturing cost and an increase in the manufacturing cost.

In the present invention, 10% to 35% per pass
After applying the reduction of 2 to 6 passes at the reduction of area, even if the reduction is performed at a reduction of 10% or less per pass in order to adjust the shape of the billet, the effect is not impaired at all.

As described above, in the present invention, the heat history from the continuous casting to the sizing step, the heating time and the heating temperature in the sizing step, and the rolling conditions are limited to specific ranges, and the fine Ti By combining controlled heating and rolling technology to promote nitride precipitation, it is possible to increase the cooling rate in the slab, and to achieve a finer dispersion of Ti char and nitride. Also, when casting into a slab having a cross-sectional size such that the cooling rate in the slab is slower and a slab for product rolling can not be obtained without passing through the lumping step, the high toughness type non-heat treated steel Manufacturing becomes possible.

On the other hand, in steel bar products used for automobile parts and the like, it is necessary to secure a certain reduction ratio, for example, 4 to 8 or more, in order to secure the soundness of the center of the product.
When the cross-sectional size of the slab is determined, the size of a product that can be manufactured to secure a reduction ratio is defined. Therefore, if the slab cross-sectional size capable of realizing the characteristics as a high-toughness non-heat treated steel can be expanded by applying the present invention, the characteristics as a high toughness non-heat-treated steel are satisfied, and further, the reduction ratio is secured. As a result, the size of a product that satisfies the soundness of the product center can be greatly expanded.

Next, an embodiment of the present invention will be described.

When a non-heat treated steel slab is charged into a heating furnace as a hot slab and subjected to slab rolling, carbon, nitride such as Al, Ti, V, etc. added for refining crystal grains. And (Mn, Fe)
The precipitation of sulfides such as S is large, and their precipitation at the austenite (γ) grain boundaries lowers the hot ductility.
Surface flaws are likely to occur in the slab heating step and in the ingot slab step, and particularly in high nitrogen steel with an N concentration of 80 ppm or more, the surface cracking sensitivity is extremely poor due to the precipitation of nitrides. In such a steel type, the yield in slab rolling is greatly reduced due to surface flaws, and costs for inspection and maintenance of flaws are increased, leading to a significant increase in manufacturing costs. It is necessary to improve hot ductility.

In order to prevent the decrease in hot ductility due to the precipitates and the deterioration of surface flaws due to the precipitates, the surface layer portion of the slab is once cooled to a temperature not higher than the Ar1 point temperature, and the temperature is reduced during the subsequent temperature rise process. It is most effective to make the γ grains fine by transformation. If the microstructure range of the surface layer structure is insufficient, it may not be possible to sufficiently prevent the occurrence of surface flaws, and it is necessary to miniaturize at least a range of 10 mm or more from the surface layer of the cast slab. It must be cooled below the Ar1 point temperature.

On the other hand, there was a concern that the properties of the non-heat treated steel would change because the behavior of precipitation of carbon and nitride was greatly affected by such thermal history. As a result, as long as the temperature conditions described in claim 1 are not deviated, even if a heat history for refining γ grains and improving hot ductility is applied to the surface layer of the slab, the above-described carbon and nitrides can be used. No remarkable difference was observed in the distribution state and the tendency of the crystal grains to become coarse during hot forging.

As a method of temporarily cooling the surface layer of the slab to a temperature not higher than the Ar 1 point temperature in a short time before charging the heating furnace, a method of cooling with water or mist spray, a method of cooling by dipping in a water tank or the like, or a method of simply allowing to cool down And any other method that can achieve predetermined cooling conditions.

Finally, an embodiment of the present invention will be described.

If the heating conditions before rolling the slab into bar steel are inappropriate, precipitates effective for preventing coarsening of γ grains during hot forging form a solid solution in the matrix or grow coarsely. The effect of preventing coarsening of γ grains may be reduced, and the toughness of the hot forged material may be significantly reduced.

According to the third aspect of the present invention, a non-heat treated steel for hot forging, which constantly and stably exhibits high toughness as it is in hot forging, by regulating the heating conditions before rolling to such a steel bar. It is an invention that enables the production of

The present inventors have investigated the effect of the heating conditions on the non-tempering properties when the slab is heated before rolling the steel bars. As a result, the heating temperature is low as long as the hot rolling of the steel bars is possible. , The heating furnace staying time is short, the crystal grain coarsening during hot forging can be prevented stably, and the heating temperature is too high, or if the heating temperature is too long, the crystal grain coarsening prevention effect and toughness improvement The effect varied.

Regarding the cause, when the heating temperature is high, the charcoal and nitride of Ti already precipitated in the steel slab are dissolved or only a part of the charcoal and nitride are coarsened by Ostwald ripening. However, since only large coals and nitrides remain, if the residence time in the heating furnace is long even if the heating temperature is not so high, the size of the charcoal and nitrides of Ti increases due to Ostwald growth. Therefore, it is considered that the effect of suppressing the coarsening of crystal grains occurs.

As a result of the investigations by the present inventors, it has been found that the production of a non-heat treated steel which can stably prevent crystal grain coarsening on an industrial scale and obtain high toughness while hot forging is required before rolling a steel bar. The heating temperature (T2) and the heating furnace time (T2) of (3)
It was necessary to control to satisfy the equation. The reason why the heating temperature (T2) is set to 900 ° C. or higher and 1240 ° C. or lower is that
If the heating temperature is lower than 900 ° C., the rolling load is extremely large, so that it is difficult to roll the steel bars and cannot be industrially manufactured.
If the temperature exceeds 40 ° C., the toughness of the hot forged material is significantly reduced for the above-mentioned reason.

[0052]

The present invention will be described in more detail with reference to the following examples.

Example 1 In each example, steel having the chemical components shown in Table 1 was melted in a converter, and the cross-sectional size of the bloom slab was 350 mm × 560 mm, 300 mm × 3.
00mm, 270mm × 270mm, 220mm × 22
Casting was performed while changing the size to 0 mm and 162 mm x 162 mm, and the cooling rate in the slab was changed. After casting, the slab is charged into a heating furnace as a hot slab and heated, and the cross-sectional size is 162 mm x
Excluding the slab of 162 mm, the sectional size of 16
15% reduction of area per pass on 2mm x 162mm slab
Rolled at ２５25%, then allowed to cool to room temperature.
For comparison, a 220 mm × 220 mm slab is rolled at a reduction ratio of 10% or less per step, or the cross-sectional size is 162 m.
In order to examine the influence of the number of passes, the slab of mx 162 mm was heated only, and then allowed to cool without slab rolling.

[0054]

[Table 1]

The steel slab thus produced was heated and rolled in a steel bar factory and processed into a steel bar having a diameter of 70 mm. This steel bar was forged into a front axle for a truck by hot forging, and allowed to cool after forging. In this as-forged state, impact test specimens and the like were sampled from the center of the axle in parallel with the axial direction, and materials such as toughness were investigated.

Table 2 shows the results of the investigation. FIG. 1 shows the relationship between the slab cooling rate and the impact value with hot forging, and FIG. 2 shows the relationship between the slab heating conditions and the impact value in a heating furnace before slabging.
From these figures and tables, high toughness was obtained in the example to which the method of the present invention was applied, but the cooling rate during forging and the heating conditions before slab rolling did not satisfy the formula (2). The toughness is lower than the examples of the present invention in the examples and the examples in which rolling was performed at a reduction in area of 10% or less per pass and the comparative example in which a slab having a cross-sectional size of 162 mm x 162 mm was heated but not subjected to slab rolling. You can see that.

Also, by applying the present invention, the cross-sectional size that can be manufactured even after the sizing process is 270 mm × 2
From a 70 mm slab, it can be manufactured with a reduction ratio of 6 to a product with a diameter of 124 mm, but the cross-sectional size is the same as the size of the bar for rolling steel bars and 162 mm x
From 162 mm slabs, only products up to 75 mm in diameter can be manufactured.

[0058]

[Table 2]

[0059]

[Table 3]

(Example 2) In Example 2, a bloom slab having the same component as in Example 1 and having a cross-sectional size of 220 mm x 220 mm was immersed in a water tank before being charged into the heating furnace, and then inserted into the heating furnace. Even if the range of cooling to the Ar1 point temperature or less was changed by variously changing the immersion time, slab rolling was performed, and the state of occurrence of surface flaws in the obtained steel slab was observed.

Table 3 shows the results of evaluation by scoring the state of flaw occurrence in the steel slab. FIG. 3 shows the relationship between the range of the surface area of the slab that was cooled to the Ar 1 point temperature or lower and the flaw score. Further, the steel slab having a rating of 2 or less, which can be industrially redirected to steel bar rolling, was forged into a front axle for a truck by the same method as in the example, and the toughness was adjusted as it was.

[0062]

[Table 4]

As shown in Table 3 and FIG. 3, by adopting the method described in claim 2, it is possible to stably achieve a flaw score of 1 or less. No remarkable difference was observed between the cases where the method was performed and the case where the method was not performed.

Therefore, by applying the invention described in claim 2, the characteristics of the non-heat-treated steel are not impaired, and the steel sheet can be produced at the billet stage as compared with the case where the method described in claim 2 is not performed. It is possible to manufacture more economically by suppressing the reduction of the yield due to the scraping and the like, and the cost increase due to the care and inspection of flaws.

(Embodiment 3) In Embodiment 3, Embodiments 1 and 2
The cross section size is 270mm in the steel type of the same composition as
Steel slabs manufactured from bloom slabs of × 270 mm and 220 mm × 220 mm were heated and rolled under various conditions to form steel bars, and the toughness of the hot forged material was investigated in the same manner as in Example 1.

The results of the investigation are shown in Table 4 and FIG. As shown in these figures, the cross-sectional size is 220 mm x 220 m
m, the cross-sectional size is 270 mm x 270 mm, even though the heating conditions before the steel bar rolling deviate from the conditions described in claim 3, while the reduction in toughness is relatively small. In the case of manufacturing from bloom cast slab, the toughness under hot forging was significantly reduced because the heating conditions were out of the conditions described in claim 3.

From these results, it can be seen that the application of the invention according to claim 3 is more stable than the case where it is not applied,
Higher toughness can be obtained more reliably.

[0068]

[Table 5]

[0069]

[Table 6]

Example 4 Steel having the chemical components shown in Table 5 was melted in a converter, and the sectional size of the bloom slab was 350 mm × 560 mm, 300 mm × 3 in the same manner as in Example 1.
00mm, 270mm × 270mm, 220mm × 22
The casting was changed to 0 mm, and the cooling rate in the slab was changed. After casting, the slab was charged into a heating furnace as a hot slab, heated, and then rolled into a slab having a cross-sectional size of 162 mm x 162 mm by slab rolling, and then allowed to cool to room temperature. The steel slab thus produced was heated and rolled in a steel bar factory and processed into a steel bar having a diameter of 85 mm. This steel bar was forged into a front axle for a truck by hot forging, and allowed to cool after forging. In this as-forged state, an impact test piece was sampled from the center of the axle in parallel with the axial direction, and the material such as toughness was examined.

[0071]

[Table 7]

Table 6 shows the inspection results. It can be seen that the examples of the present invention have extremely high toughness as hot forged as compared with the comparative examples, and that stable high toughness is obtained.

[0073]

[Table 8]

[0074]

[Table 9]

[0075]

As described above, by adopting the method for producing a non-heat treated steel for hot forging according to the present invention, the method is always stable even if the heat treatment after the hot forging is omitted in the production process of the machine parts. Not only can high toughness be obtained and non-heat treated steel bars can be applied to important safety parts such as underbody parts of automobiles,
By making it possible to stably achieve high toughness even after the lumping process, it becomes possible to apply to large parts, which have been postponed in the past. At the same time, it is possible to suppress the occurrence of surface flaws, which are likely to occur when slab-rolling non-heat-treated steel, and to prevent an increase in production cost due to a decrease in yield or maintenance of flaws. More economical production of high quality steel is possible.

[Brief description of the drawings]

FIG. 1 shows the relationship between the slab cooling rate from the solidus temperature to 1000 ° C. in continuous cast slab and the impact value after cooling with hot forging.

FIG. 2 shows a relationship between a heating condition of a continuous cast slab in a slab heating furnace and an impact value after cooling by hot forging.

FIG. 3 is a graph showing a relationship between a range of a surface portion of a slab before being charged to a slab heating furnace, which is cooled to a temperature of Ar1 point, and a flaw performance of a steel slab after slab rolling.

FIG. 4 is a view showing a relationship between heating conditions of a steel slab in a steel bar heating furnace and an impact value after cooling by hot forging.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/00 301 C22C 38/00 301Y 38/28 38/28 (72) Inventor Tsutomu Tanaka 12 Nakamachi, Muroran Nippon Steel Corporation Muroran Works, Ltd.

Claims (3)

[Claims] 1. The method according to claim 1, wherein Ceq. Is 0.70
C: 0.20 to 0.60%, Si: 0.10 to 1.5% Mn: 0.60 to 2.00% Cr: 0.10 to 1.0% V: 0.03 to 0.35% Ti: 0.01 to 0.07% N: 0.0030 to 0.0200% Al: 0.005 to 0.050%, with the balance being substantially unavoidable impurities When a steel made of Fe is melted using a converter or an electric furnace, and further cast by a continuous casting method, a temperature range from the solidus temperature to 1000 ° C. is cooled at a cooling rate of 15 ° C./min or more.
After cooling, the slab surface temperature is cooled to 900 ° C. or less, and without being cooled to a temperature of 500 ° C. or less for 20 minutes or more, the slab is charged into a heating furnace as a hot slab. After heating so that the heating temperature (T1) and the heating furnace residence time (t1) satisfy the expression (2), two to six passes are performed by slab rolling at a reduction rate of 10 to 35% per pass. A method for producing a non-heat-treated non-heat-treated steel bar for hot forging, comprising: applying a reduction to form a steel slab, and further rolling the steel slab. Ceq. (%) = C% + ／ × Si% + ／ × Mn% + 1/9 × Cr% + 1.54 × V% (1) Furnace heating time: t1 (min) ≦ (1181-T1) / 0.77 (2) where T1: heating temperature (° C.) where 900 ≦ T1 ≦
1181 (° C) 2. A method of cooling a continuously cast slab, wherein at least a range of at least 10 mm or more from the surface layer of the cast slab is cooled to a temperature not higher than the Ar1 point temperature and then charged into a heating furnace to heat the cast slab. The method for producing a non-heat treated steel bar for high toughness hot forging according to claim 1. 3. When the slab is heated before the bar is rolled, the slab is heated to a heating temperature (T2) and a heating furnace sintering time (t2) satisfying the formula (3). The method for producing a non-heat treated steel bar for high toughness hot forging according to claim 1, wherein the steel bar is rolled. Heating furnace time: t2 (min) ≦ (1240−T2) /1.33 (3) where T2: heating temperature (° C.), but 900 ≦ T2 ≦
1240 (° C)