JPH0625379B2 - Manufacturing method of high carbon cold rolled steel sheet with excellent toughness after heat treatment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention has good workability and provides excellent toughness only by tempering at an extremely low temperature after quenching after cold working. As shown, the present invention relates to a method for manufacturing a high-carbon cold-rolled steel sheet, which is suitable as a chain material or a gear part that requires high altitude and high toughness, and has a low cost.

<Prior art and its problems> Currently, high-carbon steel sheets are very demanding as materials for chain materials, automobile gears, safety belt buckles, washers, agricultural machinery parts, etc. The high carbon steel sheet provided is generally 650 to 750 after hot rolling.
Sphericalized at ℃, shipped from the factory,
After being formed or punched into the desired shape by the (user), quenching into the oil bath from the temperature range of 750 to 950 ° C and tempering in the relatively high temperature range of 400 to 650 ° C It is used after being applied.

The performance required for these high carbon steel sheets is that the hardness after spheroidizing is low and good elongation is obtained, the prescribed hardness is obtained by quenching and tempering, and the toughness during use is good. , And so on.

Therefore, when the high carbon steel material steel sheet is used for the above-mentioned applications, it has been conventionally performed to add a large amount of Cr, Mo, etc. in order to improve the above properties. That is, SCM435 steel specified by JIS, 0.40 to 0.80% C (hereinafter,% representing the component ratio is% by weight) -0.30% or less Si-0.40 to 1.20% Mn
-0.06% sol.Al-0.90-1.10% Cr-0.90-1.10% Mo-0.
45 to 0.55% Ni steel was applied for this kind of application. However, steel with a relatively large amount of Cr-Mo added is expensive, and it is difficult to apply it to parts that require high dimensional accuracy due to poor workability and parts with complex shapes. Is
The current situation is that improvement is required.

Therefore, in order to improve the workability of the Cr-Mo-added steel, the Si content is controlled to 0.15% or less and the C content is suppressed to a low level. In addition to this, in order to secure strength and suppress embrittlement. It has also been proposed to add Ni, Ti, Nb or B (JP-A-57-43959). However,
It can be concluded that it is not realistic to expect sufficient workability from the viewpoint that the proposed steel is still difficult to spheroidize since it still contains a large amount of Cr and Mo. Met.

Moreover, as mentioned earlier, this kind of high carbon steel sheet is processed into the desired shape at the customer site, and then subjected to quenching / tempering treatment to ensure hardness and toughness. It was also pointed out that the high tempering temperature is a considerable obstacle in terms of workability and work cost.

<Means for Solving Problems> The inventors of the present invention, in view of the above problems found in conventional high carbon steel sheets, refrain from using expensive additive elements that lead to cost increase, and In order to provide a relatively inexpensive high carbon steel sheet that has sufficient workability that can sufficiently respond to processing and that can secure sufficient strength and toughness only by performing a simple heat treatment after the processing, As a result of research from various viewpoints, the following findings (a) to (c) were obtained. That is, (a) Carbon steel (SC material) is used as a base, and an appropriate amount of M
After quenching a steel sheet with n, Ti and B added, and then tempering it at a lower temperature, it is possible to obtain high strength equal to or better than that of conventional Cr-Mo-added steel. (b) Strength after heat treatment Comparing the workability before heat treatment between the same conventional Cr-Mo-added steel and the above-mentioned carbon steel added with appropriate amounts of Mn, Ti and B, the above-mentioned carbon steel added with appropriate amounts of Mn, Ti and B is conventional. Is superior to Cr-Mo-added steel of
While conventional carbon steels did not become practical steels because their toughness was remarkably reduced, steel sheets containing appropriate amounts of Ti and B were sufficiently satisfactory even with the above-mentioned low-temperature tempering, which has a low operating cost. Excellent toughness is ensured, and extremely high toughness is exhibited even after quenching. (D) In the case of high carbon steel sheet, spheroidizing annealing is necessary to give workability according to customer's request, but If cold rolling with a rolling reduction of 20% or more is performed before spheroidizing annealing, spheroidizing during spheroidizing annealing is promoted, softening is further promoted, and workability is further improved.

The present invention has been made on the basis of the above findings. "C: 0.40 to 0.80%, Si: 0.30% or less, Mn: 1.0 to 2.0%, sol.Al: 0.06% or less, Ti: 0.005 to 0.025% , B: 0.0003 to 0.0030%, or Cr: 0.25% or less, with the balance being Fe and unavoidable impurities, the steel is hot-rolled and then cold rolled with a reduction of 20% or more. A high-carbon cold-rolled steel sheet that exhibits good workability by performing hot rolling and then spheroidizing annealing and that can secure sufficient hardness and excellent toughness by performing a relatively simple heat treatment. The main feature is that it can be manufactured at low cost.

As described above, the present invention exerts excellent punching workability and forming workability, and further, after performing these workings,
With oil quenching at about 950 ° C, or even 4
The present invention relates to a method for producing a high carbon cold-rolled steel sheet that exhibits high strength and good toughness by performing a tempering treatment at a temperature lower than 00 ° C (preferably 300 ° C or less). The reasons for limiting the composition and the steel plate manufacturing conditions as described above are as follows.

A) Steel component composition (a) C C is a component necessary to secure the desired strength in the steel sheet, but if its content is less than 0.40%, the tensile strength after quenching and tempering will be lower than the desired value. On the other hand, 0.80
If it is contained in excess of 0.1%, the tensile strength increases but the toughness deteriorates. Therefore, the C content was set to 0.40 to 0.80%.

(b) Si The Si component is solid-dissolved in the ferrite phase and strengthens it to prevent cracking during cold working, so it is added up to 0.30%, but it should be contained in excess of 0.30%. Since the formability of the steel sheet deteriorates, the Si content was set to 0.30% or less.

(c) Mn The Mn component has the effect of improving the hardenability of steel sheets,
If the content is less than 1.0%, the desired effect due to the above action cannot be obtained, while if it exceeds 2.0%, the ductility decreases and the cold workability deteriorates, so the Mn content is It was set at 1.0 to 2.0%.

(d) sol.Al The sol.Al component is a deoxidizing agent added to prevent the formation of oxide inclusions, and if deoxidizing is sufficient, it does not always remain in the steel sheet. It doesn't matter. However,
If the sol.Al component exceeds 0.06% and remains in the steel sheet, the ductility during cold working decreases, so the sol.Al content is 0.06%.
The following was set.

(e) Ti The Ti component has the effect of refining ferrite grains to improve ductility and fixing solid solution N to improve cold workability, but if the content is less than 0.005%, it is due to the above effect. The desired effect cannot be obtained, and on the other hand, if the content exceeds 0.025%, deterioration of hardenability and precipitation of TiC become problems, so the Ti content was set to 0.005 to 0.025%.

(f) BB component has the effect of improving the hardenability of the steel sheet and strengthening the crystal grain boundaries to improve the toughness after heat treatment, but if the content is less than 0.0003%, the desired effect due to the above effect is obtained. On the other hand, if more than 0.0030% is contained, no further improvement effect can be obtained, and rather the toughness may be deteriorated. Therefore, the B content is 0.0003 to 0.0030.
Defined as%.

(g) Cr Cr has the function of preventing the formation of graphite, improving the hardenability of the steel sheet, and improving the strength, so it is a component added when it is necessary to improve the properties by these functions. However, if the content exceeds 0.25%, the workability after spheroidizing annealing deteriorates, so the Cr content is 0.25%.
Limited to:

B) Steel plate manufacturing conditions (a) Hot rolling Hot rolling conditions are not specified, but in hot rolling of steel, the deformation resistance during rolling is large, so the finishing temperature should be 800 ° C or higher. In addition, it is desirable to carry out the winding at a temperature of 500 to 700 ° C. in order to suppress the refinement of the lamellar spacing of pearlite.

(b) Cold rolling As described above, the spheroidization can be further improved by performing the cold rolling before the spheroidizing annealing.

In other words, when the cold rolling is carried out, the lamellar of the "perlite structure having a relatively large lamellar spacing" obtained by hot rolling is divided, and spheroidization is promoted, so products with high dimensional accuracy and thin wall are required. It is very advantageous when applied to. However, if the reduction ratio of this cold rolling is less than 20%, the lamella is not sufficiently divided, and the spheroidization due to the subsequent annealing does not improve as desired. Therefore, the reduction ratio of the cold rolling is 20% or more. It was decided to carry out.

If there is a possibility that the cold rolling may cause edge cracks, it is preferable to subject the hot-rolled sheet to softening annealing in the usual manner. Of course, it is not always necessary to spheroidize cementite in the annealing in this case, and it is sufficient to carry out the annealing at about 600 ° C. or higher as usual.

(c) Spheroidal Annealing As described above, this type of steel sheet is manufactured on the premise of cold working by the customer. However, in general, as-rolled, the workability is insufficient, so in order to ensure sufficient elongation during processing at the customer, cementite in the steel sheet structure, particularly lamellar lamella of cementite separated by cold rolling, is required. Spheroidizing annealing for spheroidizing the structure is essential. The spheroidizing annealing conditions are not particularly limited, and it is sufficient to carry out at 650 to 750 ° C as usual.

The hardness is Rockwell B by means of the conditions described above.
Scale (H _{R B)} at 85 or less, good cold-rolled steel sheet punching processability and moldability can be obtained. Moreover this steel sheet, quenching after between "cold is usually carried out at customer processing (750 ~
Even after carrying out "tempering" at a low temperature of 100 to less than 400 ° C after "heating and holding at 950 ° C for 1 minute or more and then rapidly cooling in an oil bath", sufficiently excellent strength and toughness are exhibited, Further, it has the property of exhibiting good toughness without performing tempering treatment, which is carried out by heating and holding at the predetermined temperature for 1 minute or more. If the temperature is set to 300 ° C. or lower, the advantage becomes more remarkable, which is desirable.

In this connection, cold-rolled steel sheet produced in the condition as defined in the present invention, the indicates the Rockwell C scale after quenching and tempering treatment (H _{R C)} over 40 hardness (strength), and sufficiently excellent It also has toughness.

Next, the present invention will be described more specifically by way of examples.

<Example> Example 1 Each slab (2
00 mm thickness) was created.

Next, after heating each of these slabs to 1200 ° C. for 30 minutes, hot rolling was performed at a finishing temperature of 880 ° C. to obtain a hot rolled steel plate with a plate thickness of 4 mm, and It was cooled to 600 ° C. on a roll and wound up.

Then, multiple samples were taken from each of the obtained hot-rolled steel sheets,
Annealing under various conditions as shown in Table 2 was carried out to achieve spheroidization of cementite.

Hardness at room temperature was measured using a Rockwell B scale for each of the steel sheets after the spheroidizing annealing, and the results are also shown in Table 2.

Incidentally, FIG. 1 is a graph of the results shown in Table 2.

Separately from this, among the hot rolled steel sheet coils wound after the hot rolling, those having steel types A and B which are the subject steels of the present invention are subjected to pickling to a sheet thickness of 4.0 to 2.0 mm. The room temperature hardness of the cold-rolled steel sheet obtained by cold rolling and then annealing at 680 ° C. for 16 hours to make the cementite spheroidized was measured. The results are shown in Table 3.

From the results shown in Table 3, spheroidizing of cementite progresses in a more desirable state when cold rolling with a predetermined reduction rate is performed during spheroidizing annealing, and a high carbon steel sheet with better workability is obtained. You can see that you can get it.

Example 2 Each slab (200 mm thick) having the composition as shown in Table 1 was prepared by a conventional method.

Next, after heating each of these slabs to 1200 ° C. for 30 minutes, hot rolling at a finishing temperature of 880 ° C. is performed to obtain a hot rolled steel sheet with a thickness of 4 mm, which is cooled to 600 ° C. on a hot run table and then wound. It was Then, the wound coil was pickled and then subjected to softening annealing at 680 ° C. for 16 hours.

Next, the softened and annealed steel sheet was cold-rolled from a sheet thickness of 4.0 mm to 2.0 mm, and then annealed again at 680 ° C. for 16 hours to make cementite spheroidized.

Subsequently, each of the spheroidized and annealed steel sheets thus treated is heated and held at 845 ° C. for 30 minutes and then rapidly quenched into an oil bath, and a) as-quenched, b) quenched After that, five samples were prepared: one that was tempered at 200 ° C, one that was tempered at 300 ° C after quenching, one that was tempered at 400 ° C after quenching, and one that was tempered at 500 ° C after quenching.

Then, "hardness", "tensile strength" and "impact absorption energy" were measured for these samples, and the results are shown in Table 4, and some of them are shown in Figs.
It is displayed as a graph in the figure.

The tensile test was carried out by collecting JIS No. 5 test pieces, and the Charpy impact test was carried out by electron beam welding by stacking five 2 mm-thick plate materials in the same manner as in Example 1.
This was carried out at 0 ° C. for a 0 mm square bar, which was processed and used as a JIS No. 3 V notch test piece.

As is clear from the results shown in Table 4, FIG. 2 and FIG. 3, the cold-rolled steel sheets manufactured according to the conditions specified in the present invention are all tempered at a very low temperature. The results show that it has "high tensile strength of 180 kgf / mm ² or more" and "excellent impact absorption energy of 6.0 kg-m / mm ² or more".

In addition, what is shown in Table 5 is the JI The characteristics required for the high carbon steel sheet are compared by adding S45C specified in S, and it is clear from Table 5 that the steel sheet according to the present invention satisfies all the required characteristics. Moreover, it can be understood that it is very advantageous in terms of cost when the constituent components and the treatment steps are examined.

<Summary of Effects> As described above, according to the present invention, the chain material,
It is possible to stably provide low-cost high-carbon cold-rolled steel sheets that sufficiently satisfy various properties required as materials for automobile gears, safety belt buckles, various washers, and civil engineering machinery and agricultural machinery parts. It is possible to further improve the performance of the devices using the, and bring about an extremely useful effect in industry.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the spheroidizing annealing temperature and the steel plate hardness (workability) summarized from the results of Example 1. FIG. 2 is a graph showing the relationship between the tempering temperature and the tensile strength of the steel sheet, which is summarized from the results of Example 2. FIG. 3 is a graph showing the relationship between the tempering temperature and the impact absorption energy of the steel sheet, which is summarized from the results of Example 2.

Claims (4)

[Claims] 1. By weight ratio, C: 0.40 to 0.80%, Si: 0.30% or less, Mn: 1.0 to 2.0%, sol.Al: 0.06% or less, Ti: 0.005 to 0.025%, B: 0.0003 to 0.0030% Characterized in that the steel having the composition of the composition containing Fe and the unavoidable impurities is hot-rolled, then cold-rolled at a rolling reduction of 20% or more, and then spheroidized.
A method for producing a high carbon cold rolled steel sheet having excellent toughness after heat treatment. 2. A method for producing a high carbon cold-rolled steel sheet having excellent toughness after heat treatment according to claim 1, wherein annealing is carried out before cold rolling. 3. By weight ratio, C: 0.40 to 0.80%, Si: 0.30% or less, Mn: 1.0 to 2.0%, sol.Al: 0.06% or less, Cr: 0.25% or less, Ti: 0.005 to 0.025%, B: 0.0003 to 0.0030% of steel, the balance of which is Fe and inevitable impurities, is steel hot-rolled, then cold-rolled at a reduction of 20% or more, and then spheroidized. Characterized by,
A method for producing a high carbon cold rolled steel sheet having excellent toughness after heat treatment. 4. A method for producing a high carbon cold-rolled steel sheet excellent in toughness after heat treatment according to claim 3, wherein annealing is carried out before cold rolling.