JPS62253724A - Production of wire rod for cold forging having granular cementite structure - Google Patents

Abstract

PURPOSE:To obtain the title soft wire rod for cold forging capable of omitting spheroidizing annealing by hot-rolling steal having a specified composition with a finishing mill, specifying the final finishing temp. to obtain more than a desired recrystallization grain size, and controlling the cooling conditions. CONSTITUTION:The steel contg., by weight, 0.20-0.60% C, <=0.10% Si, 0.20-0.40% Mn, <=0.020% P, <=0.020% S, 0.20-0.60% Cr, 0.0005-0.0050% B, 0.005-0.030% Ti, 0.010-0.060% Al, <=0.0060% N, and <=0.040% O is treated as follows. Namely, the final finishing temp. is controlled to <=950 deg.C at the time of hot-rolling the steel, and the recrystallized austenite grain size of the rolled wire rod is adjusted to >=11. The hot-rolled steel is cooled at 750-650 deg.C including the ferrite-pearlite transformation range at the cooling rate of <=0.3 deg.C/sec. Consequently, the desired wire rod for cold forging having a granular cementite structure is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing a soft cold-forging wire rod in which the spheroidization process can be omitted.

(Prior art) In JP-A-54-143716, the finish rolling inlet temperature f is specified in the range of 600 to 900 C, and softening is attempted by low-temperature rolling, but the outlet temperature is not specified.
Due to the temperature increase during rolling, the desired softening cannot be achieved. Furthermore, in JP-A-58-107416, softening is achieved by specifying the finish rolling temperature and rolling reduction ratio, but due to low-temperature rolling, the durability of the rolls, the motor load during rolling increases, and the surface of the wire rod is reduced. This is a problem because it tends to cause scratches.

As mentioned above, various methods for softening rolled wire rods have been proposed, but at present, it has not been possible to stably supply industrially the conventional spheroidized annealed material discs. (Problems to be Solved) The present invention provides a method for producing a cold-melting gland material having a granular cementite structure by a process that eliminates 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 fCr and B, and the effect of B is fully brought out to suppress the N and O levels to low values at T7t.

This compositional steel is rolled at a final finishing temperature of 950C or less,
Moreover, by setting the subsequent cooling conditions to 0.3C/See or less in the range of 750C to 650C, cementite becomes granular, and the supersaturated carbon in the ferrite shifts to an equilibrium state, resulting in softening. achieved.

That is, the present invention has been made based on the above findings, and its gist 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: 0.0
005~o, ooso%, Ti:0.005~0.0
30%, A9. =0.010~0.060chi, N:
0.0060% or less, 0:0.0040% or less, remainder F
The recrystallized austenite grain size of the rolled wire rod is made to be 11 or more by hot rolling the steel consisting of E and unavoidable impurities in a finishing rolling mill and the final finishing temperature is 950C or less, and the subsequent cooling conditions are changed from 750C to 650C. It is characterized by being 0.3 C/sec or less in the range of .

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 forging. If it is less than 0.20%, the required strength cannot be obtained. On the other hand, if it exceeds 0.601 t, the strength of the rolled wire becomes too high and it can no longer be used as a cold forged material, so it is limited to a range of 0.20 to 0.60%.

Si increases the strength of the rolled wire due to its solid solution hardening effect. 0, which makes it possible to ignore the effects of solid solution hardening.
The content was limited to less than 1%. Even if the SI is lowered in this way, the combination of C, Mn, Cr, and B does not reduce the hardenability required during hardening treatment.

Next, 1cMn is an essential element for improving hardenability and fixing S, but the amount added is reduced to a level that mainly fixes S.
Instead, C and Bt are added in a well-balanced combination. One of the most important points of the present invention is that B is particularly effective not only in compensating for hardenability but also in promoting softening.

In other words, 5450 steel, which is a conventional mechanical structural steel, is JIS
The standard stipulates that Mn should be contained in the range of 0.60 to 0.90%, but the amount of Mn can be reduced and K instead be used.
By adding Cr and Bt, the specific bearerite transformation start temperature as well as the pearlite transformation start and end temperatures become high (and softening is promoted).

Here, the reason why the amounts of Mn, C'' 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 f Cr as much as possible.
If n is less than 0.21, S in the steel cannot be sufficiently fixed and hot embrittlement cannot be suppressed. on the other hand,
When Mn exceeds 0.4%, even if Cr is added,
Since pearlite transformation at high temperatures cannot be completed in a short time, the amount of Mn is limited to 0.2 to 0.4%.

C" is an essential element for promoting pearlite transformation at high temperatures, but if its addition amount is less than 0.2%, it will not exhibit sufficient effect. On the other hand, if it exceeds 0.6%, it will cause damage to the rolled wire rod. Since the tensile strength is too high and it becomes difficult to secure cold forgeability comparable to that of spheroidized annealed materials, it is limited to 0.2 to 0.6 inches.

Furthermore, B is an essential element for promoting pearlite transformation and improving hardenability at high temperatures. The amount added is o, ooo
If it is less than s%, these effects will not be exhibited. 0.0050
If it exceeds 0.0005% to 0.00%, the content is limited to 0.0005% to 0.00%, since coarse compounds will precipitate and deteriorate the toughness. Note that the total amount of sMn and Cr is preferably 0.4 to 0.8%.

In the component system of the present invention that requires the addition of B, it is particularly important to specify the upper limit values of N and O from the viewpoint of exerting the effect of B and cold forgeability. That is, when N exceeds 0.0060%, Ti, which is contained at the same time as B addition, forms hard non-deformable inclusions of TiN, and BN precipitates, promoting high-temperature pearlite transformation and reducing hardenability. 0.0060% because it not only inhibits the improvement effect but also worsens cold forging property.
It is specified below.

Furthermore, if O is too high, oxide-based inclusions such as Al2O and Tio2 are formed, which deteriorates cold forgeability.
0040% or less is specified. Within the specified range of N and O, S
Due to the compositional conditions characterized by reduced i, Mn, and addition of Cr and B, it is possible to achieve a much softer rolled wire rod compared to conventional cold-forging wire rods, and it is also possible to harden pan plates for machine structures. - Full tempering strength can be imparted.

Furthermore, M is added for the purpose of preventing coarsening of austenite grain size during quenching treatment after cold forging.
If it is less than 0.060%, the effect is small (on the other hand, if it exceeds 0.060%, the effect of suppressing austenite grain coarsening is saturated, and the cold forgeability is rather deteriorated,
It was limited to 0.060%.

Both P and S are elements harmful to cold forging properties.

In any case, if it exceeds 0.020%, the adverse effects become significant, 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 t:' or less, so that the recrystallized austenite grain size is 11 or more. Most preferred is 700-950C. Although it is possible to achieve a rolling temperature of 700 or less, 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.

The cooling conditions after hot rolling are in the range of 750 to 650C, which includes the ferrite-pearlite transformation region, and 0.
075, which specifies cooling at 3 C/sec or less.
When the temperature is outside the range of 0 to 650C, the transformation region is outside. 0.3C
By cooling at less than /sec, a granular cementite structure is obtained. If it exceeds this value, it will become hard (and the original purpose cannot be achieved. However, the supersaturated carbon in the ferrite will shift 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 forgeability of the wire rods in the hot rolled state and in the quenched and tempered state. Here, the hot rolled state is IQ
IIIφ, quenched and tempered material is 1Qililφ wire rod 8.
After wire drawing to 5mm diameter, 850C oil quenching → 600C tempering followed by water cooling treatment.

Cold forgeability was evaluated by drawing a 101Ilφ wire rod to 9.5I111φ and attaching a V notch with a depth of Q, 51111*9.
5mmφX1425I111B test piece, upsetting rate 40
It was determined by the presence or absence of cracking at a certain time during a compression test. ○
The mark indicates that no cracking occurred, the Δ mark indicates that fine cracks were observed with a magnifying glass, and the X mark indicates that cracking occurred.

In the same table, test numbers &7, 8.10 to 15 are examples of the present invention,
The others are comparative examples. The softening standards for hot rolled wire rods are:
Tensile strength equivalent to 545C and equivalent to spheroidized annealed material: 58ky
/a: 2 or less, aperture: 55% or more.

In addition, the guaranteed strength standard after quenching and tempering is tensile strength 85
kg/tx1 or more.

As shown in the table, all of the rolled wire rods of the present invention have a tensile strength of 513 ky/11 m" or less and a granular cementite structure. In addition, the cold forgeability is comparable to that of spheroidized annealed wire rods. The tensile strength after quenching and tempering is 85 kg/11 ra" or more.

On the other hand, Comparative Examples A1 to A6 and A17 do not have B added, so they are insufficiently softened and have a ferrite-pearlite structure, or have insufficient strength after quenching and tempering. This is an example where it is not possible. Although iFx 9 becomes soft and becomes a granular cementite structure, it is an example in which the required strength after quenching and tempering cannot be obtained because the Cr content is too small.

A16 is an example in which S is not sufficiently fixed due to too little Mn and cold forgeability is poor, and A18 is an example in which the required softening cannot be achieved because Si is too high.

A19 has too high N, &m has too high 0, and black 21 has too high B, so the cold forgeability is poor. boiled 22~2
5 is a material equivalent to JIS fl') S45C. All have poor cold forging properties.

However, when A22'fc spheroidizing annealing is performed, the tensile strength is 52 kg/11112, the reduction of area is 59%, and the cold forgeability is good. In this way, 84' 5 C-equivalent materials require spheroidizing annealing before cold forging.

(Effects of the Invention) As is clear from the above examples, the present invention provides cold rolled steel having a granular cementite structure in a hot rolled state.

This is a method for manufacturing wire rods for windshields, and has an extremely large industrial effect in that it can omit the spheroidizing annealing treatment.

[Brief explanation of drawings]

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

Claims (1)

[Claims] C: 0.20 to 0.60%, Si: 0.10% or less Mn in weight%
: 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: 0.005-0.0
30% Al: 0.010-0.060%, N: 0.0060%
The following O: 0.0040% or less A steel consisting of the balance Fe and unavoidable impurities is hot rolled in a finishing rolling mill, and the final finishing temperature is 950°C or less, so that the recrystallized austenite grain size of the rolled wire is 11 or more, A method for producing a cold forging wire having a granular cementite structure, characterized in that the subsequent cooling conditions are 0.3° C./sec or less in the range of 750° C. to 650° C.