JPH0217608B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a soft wire rod for cold heating which can omit the spheroidizing process. (Prior art) In JP-A-54-143716, softening is achieved by low-temperature rolling in which the finishing rolling entrance temperature is specified in the range of 600 to 900°C, but since there is no specification of the exit temperature, Due to the temperature increase, the desired softening is not achieved. Furthermore, Japanese Patent Application Laid-Open No. 58-107416,
Although softening is attempted by specifying the finish rolling temperature and rolling reduction rate, there are problems with the durability of the rolls due to low temperature rolling, the increased motor load during rolling, and the tendency for surface flaws to occur on the wire. As mentioned above, various methods for softening rolled wire rods have been proposed, but
At present, it has not yet reached the point where it can be stably supplied industrially. (Problems to be Solved by the Invention) The present invention provides a method for producing a cold-melting wire rod having a granular cementite structure by a process of omitting spheroidizing annealing. (Means for Solving the Problem) The present inventors have proposed that, as a means of obtaining a granulated cementite structure in a rolled wire rod state, the addition of B accelerates the completion of high-temperature pearlite transformation in a short period of time, and We have developed a component system in which part of the Mn contained in the steel is replaced with Cr and B, and the N and O levels are kept low in order to fully bring out the effects of B. By rolling steel of this composition system at a final finishing temperature of 950°C or lower and cooling the temperature below 0.3°C/sec in the range of 750°C to 650°C, granulation of cementite is achieved, and ferrite is The supersaturated carbon inside is brought to an equilibrium state, and softening is achieved. That is, the present invention has been made based on the above knowledge, and the gist thereof is that C: 0.20 to
0.60%, Si: 0.10% or less, Mn: 0.20-0.40%,
P: 0.020% or less, S: 0.020% or less, Cr: 0.20~
0.60%, B: 0.0005~0.0050%, Ti: 0.005~0.030
%, Al: 0.010 to 0.060%, N: 0.0060% or less,
O: 0.0040% or less, the balance Fe and unavoidable impurities are hot-rolled in a finishing mill, and the final finishing temperature is 950°C or less, so that the recrystallized austenite grain size of the rolled wire rod is 11 or more, and then cooling conditions at 0.3℃/
sec or less. The present invention will be explained in detail below. First, C is an essential element for imparting the required strength to products such as bolts and nuts during the quenching and tempering treatment after cold heating. If it is less than 0.20%, the required strength cannot be obtained. On the other hand, if it exceeds 0.60%, the strength of the rolled wire becomes too high and it can no longer be used as a cold-melting material, so it is limited to a range of 0.20 to 0.60%. Si increases the strength of rolled wire through its solid solution hardening effect. The content was limited to less than 0.1%, which makes the effect of solid solution hardening negligible. Even if Si is lowered in this way, the combination of C, Mn, Cr, and B does not reduce the hardenability required during hardening treatment. Next, although Mn is an essential element for improving hardenability and fixing S, the amount added is reduced to mainly for fixing S, and Cr and B are added in a well-balanced combination instead. In particular, B not only compensates for hardenability, but also
One of the most important points of the present invention is that it is effective in promoting softening. In other words, S45C steel, a conventional mechanical structural steel,
The JIS standard stipulates that Mn should contain 0.60 to 0.90%, but by reducing the amount of Mn and adding Cr and B instead, the ferrite transformation start temperature and pearlite The start and end temperatures of transformation become higher, promoting softening. Here, the reason why the amounts of Mn, Cr, and B added are limited as described above will be explained below. In order to complete the pearlite transformation in the high temperature range in a short time, it is better to replace Mn with Cr as much as possible, but if Mn is less than 0.2%, the S in the steel cannot be sufficiently fixed, and hot Unable to suppress fragility. On the other hand, if Mn exceeds 0.4%, pearlite transformation at high temperatures cannot be completed in a short time even if Cr is added, so the Mn content should be reduced from 0.2% to
Limited to 0.4%. Cr is an essential element for promoting pearlite transformation at high temperatures, but if the amount added is less than 0.2%, sufficient effect will not be exhibited. On the other hand, if it exceeds 0.6%, the tensile strength of the rolled wire rod will be too high and it will be difficult to ensure the same cold-melting properties as the spheroidized annealed material.
%. Furthermore, B is an essential element for promoting pearlite transformation and improving hardenability at high temperatures. If the amount added is less than 0.0005%, these effects will not be exhibited. If it exceeds 0.0050%, coarse B compounds will precipitate and the toughness will deteriorate, so 0.0005% to 0.0050%
limited to. In addition, the total amount of Mn and Cr is 0.4 to 0.8
% is desirable. In the component system of the present invention that requires the addition of B, N and O
It is particularly important to specify the upper limit of B from the viewpoint of exhibiting the effect of B and cooling properties. That is, N is
If it exceeds 0.0060%, it will be included at the same time as B addition.
Along with forming TiN, a hard non-deformable inclusion, BN precipitates, which not only inhibits the promotion of high-temperature pearlite transformation and the effect of improving hardenability.
Since it deteriorates cold cooling properties, it is specified at 0.0060% or less. In addition, if O is high, oxide inclusions such as Al ₂ O ₃ and TiO ₂ are formed, which deteriorates cold water properties, so O:
It was specified as 0.0040% or less. By using compositional conditions characterized by reduced Si, Mn, and addition of Cr and B while keeping N and O within the specified range, the rolled wire rod can be made much softer than conventional cold-melting wire rods. , it is possible to provide quenching and tempering strength comparable to that of mechanical structural steel. Furthermore, Al is added to prevent coarsening of the austenite grain size during the quenching process after cold-melting, and if it is less than 0.010%, the effect will be small;
If it exceeds 0.060%, the effect of suppressing austenite grain coarsening will be saturated, and the cold-hardening properties will deteriorate, so it is limited to 0.010 to 0.060%. Both P and S are elements harmful to cooling properties.
In either case, if it exceeds 0.020%, the negative effects become noticeable, so the content was limited to less than this. In the present invention, the softening effect is large due to the regulation of the above component ranges. Next, in the present invention, when hot rolling steel comprising the above components, the final finishing temperature is set to 950°C or less, thereby increasing the recrystallized austenite grain size.
Must be 11 or more. The most preferred temperature is 700-950°C. 700 or less is possible, but it becomes impractical due to low temperature rolling and increased motor load. Figure 1 shows the relationship between final finishing temperature and particle size. By finishing into such fine grains, a large number of transformation nuclei are introduced, and granular cementite transformation becomes possible. Regarding the cooling conditions after hot rolling, it was specified that cooling should be performed at a rate of 0.3° C./sec or less within a range of 750 to 650° C., which includes the ferrite-pearlite transformation region. When the temperature is outside the range of 750 to 650°C, the transformation range is outside.
A granular cementite structure is obtained by cooling at 0.3°C/sec or less. If it is more than this, it will become hard and the original purpose cannot be achieved. Furthermore, the supersaturated carbon in the ferrite is brought to an equilibrium state. (Example) Table 1 shows the chemical composition of the test materials, the hot rolling conditions, and the mechanical properties and cold hardening properties of the wire rods in the hot rolled state and in the quenched and tempered state. Here, the hot rolled state is 10mmφ, and the quenched and tempered material is 10mm.
After drawing the φ wire to 8.5mmφ, 850℃ oil quenching → 600℃
It is water-cooled after tempering. The evaluation of cold-melting property was carried out after drawing a 10mmφ wire rod to 9.5mmφ.
It was determined by the presence or absence of cracks when a compression test was performed at an upsetting rate of 40% using a 9.5mmφ x 1425mm test piece with a 0.5mm deep V-notch. ○ indicates that no cracks occurred, △ indicates that fine cracks were observed with a magnifying glass, and × indicates that cracks occurred. In the same table, test numbers 7, 8, and 10 to 15 are examples of the present invention, and the others are comparative examples. The softening standards for the hot rolled wire rod were S45C equivalent, tensile strength equivalent to spheroidized annealed material: 58 Kg/mm ² or less, and reduction of area: 55% or more. In addition, the guaranteed strength standard after quenching and tempering was a tensile strength of 86 kg/mm ² or more. As seen in the table, all of the rolled wire rods of the present invention have a tensile strength of 58 Kg/mm ² or less and a granular cementite structure. Furthermore, the cold-melting properties are comparable to those of spheroidized annealed materials, and the tensile strength after quenching and tempering is 86 Kg/mm ² or more. On the other hand, Comparative Examples Nos. 1 to 6 and 17 do not have B added, so the softening is insufficient and the structure is ferrite-pearlite, or the strength after quenching and tempering is insufficient. This is an example where it is not possible. No.9
This is an example in which the steel becomes soft and becomes a granular cementite structure, but the required strength after quenching and tempering cannot be obtained because the Cr content is too low. No. 16 is an example in which S is not sufficiently fixed due to too little Mn, resulting in poor cold hardening properties, and No. 18 is an example in which the required softening cannot be achieved because Si is too high.
No. 19 is an example in which N is too high, No. 20 is too high in O, and No. 21 is in an example in which B is too high, resulting in poor cooling properties. No.
22 to 25 are materials equivalent to JIS S45C. All of them had poor cooling properties. However, when No. 22 is annealed to spheroidize, the tensile strength is
52Kg/mm ² and a reduction of 59%, resulting in good cold-filling properties.
In this way, materials equivalent to S45C require spheroidizing annealing before cold-melting.

【table】

[Table] Circle mark: Inventive material (effects of the invention) As is clear from the above examples, the present invention is a method for producing a cold-rolling wire rod having a granular cementite structure in a hot-rolled state. The industrial effect is extremely large in that chemical annealing treatment can be omitted.

[Brief explanation of drawings]

FIG. 1 is a graph of finish rolling temperature and rolling recrystallization grain size.

Claims (1)

[Claims] 1. C: 0.20 to 0.60%, Si: 0.10% or less, Mn: 0.20 to 0.40%, P: 0.020% or less, S: 0.020% or less, Cr: 0.20 to 0.60%, B: 0.0005 to 0.0050%, Ti: 0.005 to 0.030% Al: 0.010 to 0.060%, N: 0.0060% or less O: 0.0040% or less A steel consisting of the balance Fe and unavoidable impurities is hot rolled in a finishing mill, and the final finishing temperature is 950°C. By setting the recrystallized austenite grain size of the rolled wire rod to 11 or more, and the subsequent cooling conditions,
A method for producing a wire rod for cold heating having a granular cementite structure, characterized in that the temperature is 0.3°C/sec or less in the range of 750°C to 650°C.