JPH10296396A - Production of bar steel for hot-forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a bar steel without causing hot-forging crack by heating a slab or a billet containing each specific mass % of C, Si, Mn, P, S, T.Al, T.O, Ni, Cr, Ca and rolling it into bar-steel so that rolling ratio is specific value or lower. SOLUTION: Molten steel having the components containing, by mass, 0.08-0.61% C, 0.15-0.35% Si, 0.30-1.65% Mn, <=0.030% P, 0.005-0.030% S, 0.005-0.040% T.Al, 5-30 ppm T.O, <=0.25% Ni, 0-0.70% Cr as base component and further, containing 5-30 ppm Ca is cast into bloom for hot-forging by using a mold 3. Then, the bloom 7 after cutting is heated, and thereafter, the bloom is formed into a slab 10 by using a blooming mill 9 at 2-4 passes, or the molten steel is continuously cast to the billet and cut into a prescribed length. The bloomed and rolled slab 10 or the billet cut into the prescribed length is heated and the bar steel 13 is rolled so that the rolling ratio of the cast billet is <=9.5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a steel bar for hot forging by using a medium-section bloom, and provides a method for manufacturing a steel bar in which surface cracking due to MnS does not occur in a hot forging, quenching and grinding process. Is what you do.

[0002]

2. Description of the Related Art S10C-S58C, SMn420-S
Steel materials for machine structures such as Mn443 and SMnC are used in large quantities in automobiles and industrial machine parts, and are mainly processed by hot forging. In hot forging, the hot deformation resistance and deformability of steel are important and are affected by the content of C and alloying elements, the crystal grain size during forging, the size and shape of parts, etc. For the steel, the heating temperature and forging conditions are optimized according to the steel type and parts.

[0003] Hot forged parts are usually subjected to quenching and tempering after forging and then ground (or cut) into various parts, so that a required amount of S is added to steel. However, since S causes cracking as a cause of red heat embrittlement, a method of adding a predetermined amount of Mn and changing S to MnS to prevent hot forging cracking is generally adopted. However, Mn
S, unlike oxide-based inclusions, has plasticity even at a relatively low temperature, and is stretched during lumping or bar rolling, which adversely affects the workability of the steel material. Further, due to quenching after forging or thermal strain during grinding, cracks may be generated and propagated with MnS as a starting point. Therefore, it is said that a reduction in the amount of MnS and a reduction in the MnS elongation L / W (length / thickness ratio) are effective in preventing the occurrence of surface cracks through the forging-grinding integrated process.

[0004] As a countermeasure, MnS has conventionally been reduced in size by optimizing S in steel and improving center segregation in continuous casting. Also, iron and steel, 64 (1978)
According to 1.145, morphological control of MnS is effective as a method for suppressing the stretching of MnS during hot rolling, and elements (Me) such as REM and Ca are converted so that MnS becomes (Mn, Me) S. It describes a method of suppressing the stretching of MnS by adding and making it form a solid solution. As a method of making MnS difficult to be stretched, although it does not form a solid solution in MnS, iron and steel, 52 (1966) 4.741, and electric steel making, 53
(1982) 3.195 describes the addition of Te to steel to add M
Methods are described to reduce the nS L / W and improve mechanical properties.

[0005]

SUMMARY OF THE INVENTION MnS is an element of Mn in molten steel.
n and S are concentrated between dendrite trees during the solidification process,
It is formed in the temperature range of 00 to 1400 ° C., and is distributed at the grain boundaries as granular precipitates of several μm to several tens μm. Mn to crystallize
The grain size of S increases when the O concentration in steel is high, such as low-carbon free-cutting steel, or when the cooling rate of the slab in the above temperature range is lower. Since the steel for machine structural use has a low O concentration in the steel due to deoxidation, it is most important to optimize the cooling rate. On the other hand, the cooling rate of the slab is affected by the slab cross-sectional size and secondary cooling. Further, when the slab size is large, MnS is elongated in a spindle shape or a linear shape by slab rolling or bar rolling, and MnS in the bar is increased.
L / W of S increases.

The present inventors have investigated the surface cracks of a crankshaft in the case of manufacturing a crankshaft by hot-forging a steel bar and then grinding the burrs on the surface. According to the results, surface cracks are likely to occur during grinding, and the occurrence locations are many where the inside of the material steel bar has become the surface of the crankshaft due to metal flow. When the degree of stretching of MnS is large,
It has been found that cracks are likely to occur because they serve as crack initiation points and propagation paths.

As described above, in order to prevent surface cracking of a hot forged part, L / W of MnS in a steel bar as a material is used.
Has to be adjusted to an appropriate value or less, and the establishment of a manufacturing method therefor has been an issue.

[0008]

Means for Solving the Problems The present invention has been made to solve the above problems, and the means thereof are as follows. (1) By mass, C: 0.08 to 0.61%, Si: 0.
15 to 0.35%, Mn: 0.30 to 1.65%, P ≦
0.030%, S: 0.005 to 0.030%, T.P. A
l: 0.005 to 0.040%; O: 5 to 30 pp
A method for producing a steel bar for hot forging comprising m, Ni ≦ 0.25%, Cr: 0 to 0.70% as basic components and further containing Ca: 5 to 30 ppm, wherein the molten steel of the above components is bloom cast. After continuous casting into slabs and cutting to a predetermined length, the slabs are heated and formed into steel slabs by slab-rolling in 2 to 4 passes, or molten steel of the above-described components is continuously cast into billet slabs, and a predetermined length is formed. The slab that has been cut into pieces and the slab or the billet slab that has been cut to a predetermined length are heated, and the bar is rolled so that the rolling ratio from the slab is 9.5 or less. .

(2) C: 0.08 to 0.61 by mass
%, Si: 0.15 to 0.35%, Mn: 0.30
1.65%, P ≦ 0.030%, S: 0.005 to 0.5%
030%, T.P. Al: 0.005 to 0.040%;
O: 5 to 30 ppm, Ni ≦ 0.25%, Cr: 0 to 0
0.70% as a basic component, and Ca: 5 to 30 pp
A method for producing a steel bar for hot forging containing m and Te: 5 to 30 ppm, wherein molten steel of the above-described component is continuously cast into a bloom slab, cut into a predetermined length, and then heated to 2 to
It is formed into a slab by 4 pass slab rolling, or the molten steel of the above component is continuously cast into a billet slab and cut to a predetermined length. The billet slab is heated, and the bar is rolled so that the rolling ratio from the slab is 63 or less.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical composition of a steel material for machine structural use is specified by Japanese Industrial Standards (JIS). That is, the carbon steel for machine structure is JIS G4051, and the manganese steel for machine structure and the manganese chrome steel for machine structure are JIS G4051.
SG 4160 respectively. The basic chemical composition of the steel material targeted in the present invention has a range defined by JIS as the upper and lower limits.

However, S is set to 0.1 in order to impart machinability.
005% is the lower limit. Al is used for adjusting the crystal grain size.
The lower limit is 005%, and the addition of more than 0.040% saturates the crystal grain size adjusting action, so the upper limit is 0.040%. T. O is empirically 5 to 30 based on the values of the various components.
ppm and is defined in this range.

Further, when Ca to be added is added to molten steel, calcium aluminate (12CaO · 7Al ₂ O) is added.
₃ ), and the remaining Ca is effectively used for controlling the form of MnS. T. Since O is 5 to 30 ppm, C
a is set to 5 to 30 ppm, which is more than the stoichiometric ratio. As for Te, a Te / S ratio of 0.05 to 0.1 is required, and since S is 0.005 to 0.030%,
It is specified as 30 ppm.

Next, the reason for defining the rolling ratio (cross-sectional area ratio) from the slab to the steel bar in the present invention will be described. First,
The present inventors investigated the relationship between the rate of occurrence of surface cracks when grinding a crankshaft and L / W of MnS in a steel bar. The results are shown in FIG. 1, and the incidence of surface cracking increases with L / W when L / W exceeds 5. The reason for this is that, at the site where the inside of the raw steel bar is exposed to the surface of the crankshaft due to the metal flow of hot forging as described above, MnS that has been stretched against thermal strain during quenching or grinding acts as a notch, This is because it is a starting point of crack generation and promotes propagation. As described above, in order to prevent cracking, it is necessary to make L / W of MnS in a steel bar 5 or less.

MnS is soft and plastic in a temperature range of about 1200 to 900 ° C. where slab rolling and bar rolling are performed, so that MnS is stretched together with plastic deformation of steel. For example, the present inventors investigated SMn443 and obtained the results shown in FIG. 2 regarding the relationship between the rolling ratio λ from the slab to the bar and the L / W of MnS in the bar. As shown in the figure, L / W increases with increasing λ.
Is clearly increasing. However, Ca and Te
When L is added, the rate of increase in L / W with respect to the increase in λ is smaller than when L is not added.

Now, when MnS in the slab is approximated to a sphere having a diameter D and MnS in a steel bar is approximated to a cylinder having a diameter W and a length L, and the rolling ratio from the slab to the steel bar is λ, the following equations are obtained. Holds. L = Dλ (1) W = D (1 / λ) ^1/2 (2) From formulas (1) and (2), L / W = λ ^3/2 (3)

From equation (3), {2/3}｝ {(log (L / W) / logλ)} = 1 (4) The left side (hereinafter abbreviated as ν) of equation (4) is MnS MnS for plastic deformation of steel, corresponding to the ratio of the deformation strain of steel to the deformation strain of steel.
Shows the relative degree of plastic deformation.

Then, the logarithm of the horizontal axis and the vertical axis of FIG. 2 is obtained and plotted again to obtain FIG. The gradient in FIG. 3 shows the degree of plastic deformation ν of MnS. When Ca is not added, the deformation is almost ν = 1 in the region where the rolling ratio is small, but ν increases as the rolling ratio increases. Tends to be smaller. The reason is presumed that in a region where the rolling ratio is large, MnS is stretched and divided, and L / W is measured to be apparently small. On the other hand, Ca or Ca and Te
When is added, ν becomes small, and the plastic deformation of MnS is clearly suppressed.

When plotting the relationship between L / W and logλ, a linear relationship is established as shown in FIG.
Table 1 is obtained by obtaining λ at which / W is 5 or less from the figure.

[0019]

[Table 1]

As described above, in the present invention, the upper limit of the rolling ratio is set to C
It is stipulated that 9.5 is added when a is added, and 63 when Ca and Te are added.

[0021] Regarding the lower limit of the rolling ratio, generally, when the rolling ratio is 4.0 or more, the austenitic grain size number of the steel bar becomes 7 or more, and the mechanical properties (TS, RA, It is desirable to set it to 4.0 because _{u ERT} satisfies the JIS target value.

As described above, according to the present invention, the rolling ratio from the slab to the steel bar is 9.5 when Ca is added in an amount of 5 to 30 ppm.
When 5 or less, 5 to 30 ppm of Ca and 5 to 30 ppm of Te are added, the content is specified as 63 or less. In addition,
In the present invention, when the rolling ratio is 9.5 or less, only Ca may be added. Only when the rolling ratio is 9.5 to 63, C
a and Te may be added.

Next, the reason why the slab is formed into a steel slab by two- to four-pass slab rolling will be described. Bulking and rolling 1
In the temperature range of 200 to 900 ° C., MnS is soft and easily stretched. Therefore, when multi-pass rolling with a small reduction per pass is performed, the permeability of the reduction is reduced and the plastic deformation of the surface layer to the intermediate portion of the slab is reduced. And MnS in that portion is excessively stretched.

Usually, the size of a billet used for a hot forging bar is 115 mm square to 180 mm square, and the bar size is 40 m.
Since the diameter is from mφ to 130 mmφ, the rolling ratio from the slab to the bar is about 4.0 (115 mm square → 65 mmφ) to 2
5.8 (180 mm square → 40 mm φ). Therefore,
Assuming that the rolling ratio from the slab to the steel bar is 4.0 to 63, the rolling ratio from the slab to the steel slab is in the range of about 1 to 2.4. If the rolling ratio from the slab to the slab is 1, the slab rolling is omitted, while if the rolling ratio is more than 1 and less than 2.4, the slab can be formed into a slab by 2-4 pass rolling. is there.

[0025] When forming into a slab by slab rolling of 5 passes or more, as described above, the Mn in the surface layer to the middle portion of the slab is increased.
Since S is excessively stretched and the slab size becomes large and the cooling rate becomes low, the size of MnS crystallized in the solidification process becomes large. As a result, MnS is easily stretched and L /
There is a problem that W increases. Therefore, in the present invention, for the purpose of minimizing the elongation of MnS, the slab rolling is defined as 2 to 4 passes. On the other hand, when the bar size is small and the steel bar can be rolled as it is, the slab rolling is omitted.

FIG. 5 shows an example of the embodiment of the present invention. 1
Is a ladle, 2 is a tundish, 3 is a mold, 4 is a secondary cooling zone, 5 is a guide roll, 6 is a cutting machine, 7 is a slab, 8 is a slab heating furnace, 9 is a slab rolling mill, 10 Is a billet, 11 is a billet heating furnace, 12 is a bar rolling mill, and 13 is a bar.

In the first embodiment of the present invention, 5 to 30 ppm of Ca is contained in the molten steel in the ladle 1, and in the method of the second embodiment, 5 to 30 ppm of Ca and 5 to 30 ppm of Te.
And, using the mold 3, for example, a slab size 220
After casting a bloom of hot-forging steel of mm square and heating the cut slab 7, it is formed into a 162 mm square steel slab 10 with 2 to 4 passes using a slab rolling machine 9. The steel slab was rolled into a bar 13 having a diameter of 120 mm to 40 mm. In this case, the rolling ratio from the slab to the bar is 4.28 to 38.6.
It is. As a result of investigating the obtained steel bars, the L / W of MnS was maintained at 5 or less in any of the methods, and a good level of mechanical properties was ensured. Then, the steel bar was hot forged, quenched and tempered, ground and processed into a crankshaft, but no surface crack was generated.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail below. In a 270 ton converter and RH degassing apparatus, hot forging steel SMn443 was melted and the chemical composition was 0.42% C
-0.24% Si-1.50% Mn-0.014% P-
0.014% S-0.025% T. Al-12 ppm
T. After adjusting to O-0.02% Ni-0.22% Cr, Ca is adjusted to 5 to 30 depending on the rolling ratio from the slab to the bar.
ppm, or 5-30 ppm of Ca and 5-30 pp of Te
m added.

FIG. 5 shows an outline of a blooming continuous caster, in which a 162 mm square and 220 mm square medium section bloom is cast in a tundish at a superheat degree of molten steel of 20 to 40 ° C., a secondary cooling specific water amount of 0.4 l / kg, and casting. Speed 2.5-1.8m / m
After casting as in, the slab is cut to a predetermined length,
The slab of 162 mm square was cooled as it was without slab rolling, and the slab of 220 mm square was charged into a heating furnace and heated for about 1 hour so that the average cross-sectional temperature was 1100 ° C. It was formed into a 162 mm square steel slab by 2 to 4 pass rolling using a 900 mmφ HV type bulking mill.

It should be noted that Ca in the molten steel in the ladle before casting is 5 to 5%.
When added in the range of 30 ppm, the rolling ratio from the slab to the steel bar was set in the range of 6.6 to 9.5. In addition, Ca5
When Te was added in the range of 5 to 30 ppm in addition to -30 ppm, the rolling ratio was in the range of 9.6 to 63.

Table 2 shows examples of the present invention together with comparative examples. The mechanical properties were evaluated by taking a test piece from the center of the bar and performing a tensile test and an impact test. The L / W of MnS was evaluated by measuring a thickness and a length by randomly extracting a few μm to several tens μm of MnS with a microscope after collecting and polishing a sample from a 1 / 2r portion of the steel bar.

[0032]

[Table 2]

In the examples of the present invention, both the mechanical properties and the L / W of MnS were good, and no cracking occurred when the crankshaft was processed. On the other hand, in the comparative example, the L / W of MnS exceeded 5, and cracks were observed on the surface after working on the crankshaft.

[0034]

According to the present invention, the slab is formed from the middle section bloom by two to four passes of slab rolling, or the slab rolling is omitted.
Since the rolling ratio from the slab to the bar is limited by the content of the steel, it is possible to produce a bar without hot forging cracks, and its industrial application effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between L / W of MnS and the incidence of crankshaft cracking.

FIG. 2 is a view showing a relationship between a rolling ratio from a slab to a steel bar and L / W of MnS.

FIG. 3 is a diagram showing a relationship between logλ and 2/3 log (L / W).

FIG. 4 is a diagram showing a relationship between logλ and L / W.

FIG. 5 shows an example of the embodiment.

[Explanation of symbols]

 Reference Signs List 1 ladle 2 tundish 3 mold 4 secondary cooling zone 5 guide roll 6 cutting machine 7 slab 8 slab heating furnace 9 slab rolling mill 10 steel slab 11 steel slab heating furnace 12 bar rolling mill 13 steel bar

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI B22D 11/12 B22D 11/12 A C22C 38/00 301 C22C 38/00 301Y 38/58 38/58 (72) Inventor Takashi Yoshioka No. 12, Nakamachi, Muroran City, Hokkaido Nippon Steel Corporation Muroran Steel Corporation (72) Inventor Koichi Isobe 12, Nakamachi, Muroran City, Hokkaido Nippon Steel Corporation Muroran Steel Corporation

Claims (2)

[Claims] 1. A mass, C: 0.08 to 0.61%, S
i: 0.15 to 0.35%, Mn: 0.30 to 1.65
%, P ≦ 0.030%, S: 0.005 to 0.030
%, T. Al: 0.005 to 0.040%; O: 5
３０30 ppm, Ni ≦ 0.25%, Cr: 0０〜0.70
% Is a basic component and a method for producing a steel bar for hot forging further containing 5 to 30 ppm of Ca, wherein the molten steel of the component is continuously cast into a bloom cast piece, cut into a predetermined length, and then cut into a predetermined length. Is heated and formed into a steel slab by slab rolling of 2 to 4 passes, or the molten steel of the above-described component is continuously cast into a billet slab and cut into a predetermined length, and the slab or the specified slab or A method for producing a steel bar for hot forging, comprising heating a billet slab cut to a length, and rolling the steel bar so that a rolling ratio from the slab is 9.5 or less. 2. C: 0.08 to 0.61% by mass, S
i: 0.15 to 0.35%, Mn: 0.30 to 1.65
%, P ≦ 0.030%, S: 0.005 to 0.030
%, T. Al: 0.005 to 0.040%; O: 5
３０30 ppm, Ni ≦ 0.25%, Cr: 0０〜0.70
% Is a basic component, and further comprises a steel bar for hot forging further containing 5 to 30 ppm of Ca and 5 to 30 ppm of Te. After cutting, the slab is heated and formed into a steel slab by slab rolling of 2 to 4 passes, or the molten steel of the component is continuously cast into a billet slab and cut to a predetermined length, and the slab is slab-rolled. A method for producing a steel bar for hot forging, comprising: heating a steel slab or a billet slab cut to a predetermined length and rolling the steel bar so that a rolling ratio from the slab is 63 or less.