JPH04202629A - Production of cold rolled high carbon steel strip excellent in workability - Google Patents

Abstract

PURPOSE:To obtain a homogeneous and sufficiently softened cold rolled steel strip in a short time by specifying conditions in primary annealing at the time of improving the material of a hot rolled strip of high carbon steel with specific composition by means of primary annealing and secondary annealing. CONSTITUTION:A hot rolled strip of a high carbon steel having a composition consisting of, by weight, 0.6-1.3% C, <=0.5% Si, <=1% Mn, <=1.6% Cr, and the balance Fe is subjected to first stage soaking and slow cooling consisting of soaking and holding at a temp. between the Ac1 point and 780 deg.C for >=1hr in a furnace with an atmosphere consisting of >=50vol.% of H2 gas and the balance N2 gas and cooling down to a temp. right under the Ar1 point at <=60 deg.C/hr cooling rate. Subsequently, this steel strip is subjected to second stage soaking and slow cooling consisting of soaking and holding at a temp. right under the Ac1 point for 3-20hr and cooling down to a temp. not higher than the Ar1 point at >=60 deg.C/hr cooling rate. The steel strip subjected to primary annealing consisting of these first stage and second stage soakings and slow coolings mentioned above is cold-rolled and then subjected to secondary annealing at a temp. in the region between 600 deg.C and a temp. right under the Ac1 point.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a high carbon cold rolled steel strip with excellent workability and heat treatability, which is used as a material for cutlery, springs, washers, springs, and other mechanical parts. Relating to a manufacturing method.

[Conventional technology]

Blades, mainsprings, washers, springs, and other mechanical parts are made of high-carbon cold-rolled steel strip (JIS G 3311), and undergo various processing processes such as punching, bending, press working, and cutting, as well as quenching and tempering. It is manufactured through other heat treatment steps. In order to improve and stabilize the quality of these products and reduce manufacturing costs, it is necessary that the raw material, high-carbon cold-rolled steel strip, is soft, has good workability, and has an excellent homogeneity of structure. It is.

As methods for improving the material quality of high-carbon cold-rolled steel strips for this purpose, annealing treatment (secondary annealing treatment) for hot-rolled steel strips and annealing treatment after cold rolling (secondary annealing treatment) are performed. High carbon steel strip is hot-rolled with [pro-eutectoid ferrite pearlite]
It exhibits a brittle and hard structure consisting of , or "pearlite", or "pearlite + pro-eutectoid cementite", and moreover, the structure is non-uniform. - The subsequent annealing treatment is a heat treatment for increasing workability by changing the hot rolled structure into a spheroidized carbide structure, and also improving the homogeneity of the structure. In addition, the secondary annealing treatment after cold rolling is a heat treatment that softens the copper strip by causing recovery and recrystallization in the texture of the copper strip, which has been strained by cold rolling. 60
This is achieved by heating and maintaining the temperature just below the 0°c-Act point for an appropriate period of time (approximately 5 to 25 hours).

[Problem to be solved by the invention]

The following three methods are typical annealing treatments for changing the hard and brittle structure of the high carbon hot rolled steel strip into a uniform spheroidized carbide structure.

(a) After soaking to just above Ac1 point, slowly cool.

(b) After repeating up and down of Ac1 point, it is slowly cooled.

(c) Soak for a long time under AC and stippling.

The above (a) and (b) are heat treatment methods that require strict temperature control, and are useful for heat treatment of small items such as tools, but they are not suitable for heat treatment of bulky and heavy items such as hot-rolled steel strips. It is difficult to carry out the heat treatment in a Hell type furnace (temperature distribution inside the furnace tends to be uneven), and it is difficult to obtain the desired heat treatment effect.

On the other hand, the annealing method (c) has a relatively wide tolerance for temperature control and is a versatile method. Conventionally, this method has been used exclusively as an annealing treatment method for belts that require a corbel annealing furnace. is being carried out. However, since the heat treatment requires a long time of about 40 hours, energy consumption is enormous and there are also problems in processing efficiency. Moreover, the softening effect of the annealing treatment is not necessarily sufficient, and therefore there are restrictions on the reduction rate in the next process of cold rolling, and depending on the thickness of the copper strip, it may be necessary to perform intermediate annealing during cold rolling. Therefore, there are problems in that disadvantages such as making the process more complicated and increasing the number of cold rolling baths are inevitable.

The present invention has been made to solve the above-mentioned problems regarding the production of high carbon cold rolled steel strip.

[Means and effects for solving the problems] The method for producing a high carbon cold rolled steel strip according to the present invention includes C: 0.6 to 1.3% by weight, Si: 0.5% by weight or less, Mn: 1 Weight% or less, C
r: 1.6% by weight or less, the remainder substantially consists of Fe, and if desired, a part of Fe may be Cu: 0.30% by weight or less, Ni: 2.20% by weight or less, W: 2.50% by weight A high-carbon hot-rolled steel strip having a chemical composition substituted with one or more elements selected from the group of Lid% or less, Mo+2.50% by weight, V:Q, and 3Q% by weight was heated with 50 volumes of H2 gas. % or more and the remainder is N2 gas in a mixed gas atmosphere furnace, and after soaking for more than 1 hour in the temperature range of Ac+ point to 780℃, 60''C
1st stage soaking/slow cooling to just below A F 1 point at a cooling rate of /Hr or less, and cooling rate of 60"C/Hr or less after soaking and holding for 3 to 20 hours just below A e 1 point So Ar1
After being subjected to a primary annealing process consisting of a second stage of soaking and slow cooling to a temperature below 600°
It is characterized by performing secondary annealing treatment in the temperature range below c-Ar+stipple.

The present invention will be explained in detail below.

Hot rolling of the high carbon steel having the above chemical composition is carried out according to a conventional method, and there are no particular restrictions on hot rolling conditions, but it is preferable to wind up the hot rolled steel strip after completion of phase transformation. The structure of a hot-rolled steel strip coiled at a high temperature before the completion of phase transformation exhibits a structure in which coarse layered pearlite has developed, whereas the structure of a hot-rolled steel strip coiled after completion of phase transformation is It has a homogeneous structure consisting of "ferrite + fine pearlite", "fine pearlite", or "fine pearlite + pro-eutectoid cementite", so there is less segregation of austenite phase during annealing treatment, and the distribution of spheroidized carbides is reduced. This is effective in suppressing deviation and variation in particle size.

The annealing treatment (-second annealing) of the hot rolled copper strip in the present invention is performed using N
The mixed gas C (hereinafter referred to as "hydrogen-rich gas") in which 2 gases account for 50% or more of the volume and the remainder is N2 gas is used as the atmospheric gas. The normal atmospheric gas in an annealing furnace is NX (H, ≦2%, CO≦3%) or HNX
(H, 510%, remaining N2) etc., but instead of this, H
The reason why we decided to use a high hydrogen gas containing z gas as its main component is that it has a higher thermal conductivity than the above-mentioned NX gas and HNX gas that contains N2 gas as its main component. This is because it enables the strict control of temperature distribution required when performing annealing, and is suitable for controlling a narrow temperature range.In addition, in annealing using NX or HNX as an atmospheric gas, A noticeable oxide film (temper color) occurs, but when high hydrogen gas is used as the atmospheric gas,
Temper color is significantly reduced, and the effect is that the heating efficiency of the copper strip is increased.

The primary annealing treatment using high hydrogen gas as the atmospheric gas is AC1
The first stage of soaking/slow cooling process involves soaking to a point of 780°C and then slow cooling.
It consists of soaking and slow cooling steps.

Soaking in the first stage is a process in which pearlite in the hot-rolled structure is dissolved in austenite, and the subsequent slow cooling is a process in which solid solution C is precipitated with undissolved residual carbides as nuclei, resulting in spheroidized carbide rice grains. This is the process of generating. The reason why the upper limit of the soaking temperature (above the A cI point) was set at 780°C is that if the heating temperature exceeds this temperature, undissolved carbide, which becomes the core of forming spherical carbide powder, cannot remain. be.

Further, the reason why the soaking time is set to 1 hour or longer is because if the soaking time is shorter than that, the amount of undissolved pearlite will increase and the formation of spheroidized carbonized rice grains will become insufficient. Note that a soaking time of up to about 3 hours is sufficient, and soaking for a longer period of time than that is not particularly necessary.

The reason why the slow cooling rate following the first stage soaking was set to 60"C/Hr or less is because at a cooling rate exceeding that, the precipitation of solid solution C is suppressed, so that spheroidization with undissolved carbides as the nucleus occurs. Carbide formation is insufficient, and a large amount of C is added to austenite.
This is because if %Ar remains in solid solution, transformation occurs and forms a layered pearlite structure.

Moreover, the reason why the temperature reached by slow cooling is set to just below the Ar1 point is because there is no need to lower the temperature to a lower temperature than that, and because of the thermal economy when performing the subsequent second stage soaking.

The soaking in the second stage soaking and slow cooling process is a process for growing the spheroidized carbonized rice grains produced in the first stage soaking and slow cooling process. The reason why the soaking temperature is set to just below the AC1 point is to promote the growth of spheroidized carbide rice grains due to the precipitation of solid solution C. The reason why the soaking time was set to 3 to 20 hours is because if the soaking time is less than 3 hours, the precipitation of solid solution C and the growth of spheroidized carbide rice grains will be insufficient, and the spheroidized carbide rice grains will become fine. 1
structure, and as a result, sufficient softening cannot be achieved (
This is because even after 20 hours, the spheroidized carbonized rice grains do not grow and the resulting softening effect does not increase, which is disadvantageous in terms of thermoeconomic efficiency and processing efficiency.

The cooling subsequent to the second stage soaking is a process for stabilizing the spheroidized carbonized rice grains produced in the previous processes and further precipitating solid solution C in the ferrite. This cooling is Ac1
Even though the temperature is lower than that of the austenite, the transformation has not yet been completed. Therefore, if the rapid cooling exceeds 60°C/Hr, the transformation will occur while the C in the austenite remains in solid solution. As a result, layered pearlite is produced, resulting in a mixed structure of layered pearlite and spheroidized carbonized rice grains. For this reason, it was decided to cool down to the A11 point or lower at a cooling rate that did not exceed 60°C 10r.

After the primary annealing treatment consisting of the first and second soaking and slow cooling steps, cold rolling (reduction ratio of about 20% or more) is performed, -
The high-carbon hot-rolled copper strip subjected to the secondary annealing treatment has -
Unlike the hard and brittle structure of hot-rolled A, it has good workability and is softer than those subjected to conventional annealing. Therefore, the workability in cold rolling is excellent, and cold rolling can be successfully achieved without problems such as edge breakage. In addition, the cold reduction rate of high carbon hot rolled steel strip subjected to conventional annealing treatment is approximately 7.
5% is the limit, and intermediate annealing is required to perform further cold rolling, but in the high carbon hot rolled steel strip that has been annealed according to the present invention, it exceeds 75%, which is - '9
The cold rolling reduction ratio can be expanded to 0%. This means that the number of cold rolling and annealing treatments can be reduced.
This means simplifying the process.

After cold rolling, annealing treatment is performed again. This annealing treatment (secondary annealing) is a heat treatment for causing recovery and recrystallization of the texture formed by cold rolling and changing it to a soft recrystallized texture in which work hardening has been eliminated. The heat treatment is achieved by heating and holding at 600° C. to just below A e 1 point for an appropriate time. The lower limit is set at 600°C because at lower temperatures, recrystallization and grain growth of ferrite will be insufficient, resulting in a lack of softening effect.
The reason why the upper limit is set at just below 1 point is that if it exceeds that point, an austenite phase will be formed and layered pearlite will appear during the cooling process, resulting in hardening. The secondary annealing treatment, unlike the above-mentioned secondary annealing treatment, does not necessarily need to be performed in a high hydrogen gas atmosphere, and may be performed using NX gas, HNX gas, or the like. In this heat treatment, soaking is not so important, and approximately 5 to 25
Since it is sufficient to heat and hold the annealing for about an hour, it is not necessarily necessary to perform batch annealing, and it is also possible to perform the annealing in a continuous annealing furnace. Further, there is no particular need to control the cooling rate; for example, natural cooling may be used.

In addition, when performing secondary annealing after cold rolling in a batch annealing furnace, the cold rolling process and secondary annealing process may be combined as necessary to prevent seizure of the overlapping surface of the rope coil. An electrolytic cleaning treatment process is carried out on the surface of the steel strip. Further, after the secondary annealing treatment, skin pass rolling is performed, and cutting to a predetermined size according to the groove specifications is also performed, which is no different from usual.

Figure 1 (1) (n) shows the primary annealing treatment (first stage soaking/slow cooling and second stage soaking/slow cooling) of a hot rolled steel strip in a high hydrogen gas atmosphere according to the present invention. The heat cycle satisfies the specifications of the present invention, but the atmosphere was replaced with a high hydrogen gas and the conventional general annealing atmosphere was N.
In the case of X gas (all batch annealing), the hardness of the top part (T), middle part (M) and end part (E) of the hot rolled steel strip after annealing was measured and the softening effect was compared. FIG. The chemical composition of the test hot rolled steel strip (winding after hot rolling is normal winding after completion of phase transformation) is C:
0.86%, Si: 0.22-0.23%, Mn: 0.
46~0.47%, Cr:0.015~O', 016%
, the remainder is essentially Fe, and the high hydrogen gas is 75% Hz
Nz, NX gas 2% H22, 6% CON z
wo'F-h Sore is used.

(As shown in Figures [] and [H], the annealing effect using NX gas as the atmosphere gas is extremely incomplete, and the lack of softening in the middle and end parts is particularly noticeable (Figure [■])
On the other hand, in the case of a high hydrogen gas atmosphere, sufficient softening was achieved with little variation over the entire band from the top to the end (Figure 19). From a comparison of the two, it was found that the primary annealing treatment of the hot-rolled steel strip in the present invention must be performed in a high hydrogen gas atmosphere, and the use of high hydrogen gas (improving thermal conductivity) ensures that the prescribed heat cycle is executed. I know it will happen.

FIG. 2 shows the primary annealing treatment (first
The hardness of the cold-rolled steel strip obtained by performing the secondary annealing treatment after cold rolling and the hardness of the hot-rolled steel strip obtained by the conventional method of lengthening the hot-rolled steel strip just below the AC1 point. The hardness of cold-rolled steel strips obtained by performing a second annealing treatment of time heating and holding (processing time: 40 Hr) and a secondary annealing treatment after cold rolling are shown in comparison. The atmosphere of the primary annealing treatment of the present invention is high hydrogen gas, and the secondary annealing treatment is high hydrogen gas or NX gas,
NX gas was used for the primary and secondary annealing treatments in the conventional method. In addition, the secondary annealing treatment was performed at 690°C x 20
Hr → air cooling. Note that the chemical composition of the test material, the winding after hot rolling, the composition of high hydrogen gas, NX gas, etc. are the same as those in the test shown in FIG. 1 above.

In the figure, al and a2 are the hardnesses of the cold-rolled steel strip produced by the method of the present invention (al: high hydrogen gas used in secondary annealing treatment, a2
: NX gas was used in the same treatment), and b is the hardness of the cold rolled steel strip by the conventional method. All are shown as the average value of the measured values of the top, middle and end parts of the copper strip (N=9).As shown in the figure, the method of the present invention (al, a2
) has a superior softening effect compared to the conventional method (b). In addition, the reduction rate of cold rolling by the conventional method is about 75%, which is the upper limit, but by the method of the present invention, it is possible to perform cold rolling that exceeds this, and it is possible to achieve a high reduction rate of 85%. Even cold-rolled steel strip has sufficient softness. In addition, al
Since there is no substantial difference in the degree of softening between a2 (high hydrogen gas used in the secondary annealing treatment) and a2 (NX gas used in the same treatment), the secondary annealing treatment in the present invention is different from the primary annealing treatment. It can be seen that the annealing treatment does not necessarily have to be carried out in a high hydrogen gas atmosphere, and that a conventional annealing treatment atmosphere may be applied.

Next, the reasons for limiting the composition of the high carbon steel used in the present invention will be explained. % indicating content is weight %.

C: 0.6 to 1.3% C is an element that has the effect of imparting strength, hardenability, wear resistance, etc. to the steel plate. The lower the C content, the softer it becomes and the better the workability becomes, but when it is less than 0.6%, A c 1
When heated above a point, pearlite rapidly dissolves into austenite, making it difficult to leave undissolved carbide necessary for forming spheroidized carbide grains. On the other hand, if it exceeds 1.3%, graphite will form and become brittle, making it difficult to perform cold rolling. For this reason, 0.6~
It is set at 1.3%.

Si: 0.5% or less Si is added as a deoxidizing agent, and also strengthens ferrite.
It is an element that has effects such as improving hardenability. However, 0
．． If it exceeds 5%, the toughness deteriorates, so the upper limit is set at 0.5%.

Mn: 1% or less Mn is an element that provides strength necessary for toning and also has a cementite stabilizing effect. However, if it exceeds 1%, there is a strong tendency for tempering embrittlement at high temperatures, so the upper limit is set at 1%.

Cr: 1.6% or less Cr gives the copper strip the necessary strength and wear resistance, and also has the effect of preventing graphitization, but if it exceeds 1.6%, hardenability decreases and tempering embrittlement occurs. 1.
6% or less.

Cu: 0.3% or less Cu is an element that has the effect of making it easier to remove the surface scale of hot rolled steel strips by pickling, etc. However, if it exceeds 0.3%, it causes brittleness, so .3% is the upper limit.

Ni: 2.2% or less Ni has the effect of reinforcing the ferrite base and increasing the tensile strength. However, if it exceeds 2.2%, graphitization tends to occur, and increasing the amount is disadvantageous in terms of cost. Therefore, the upper limit is set at 2.2%.

W: 2.5% or less W has the effect of increasing tempering softening resistance, but 2.
When it exceeds 5%, stable carbides are generated during annealing, which causes uneven hardness during quenching.

Therefore, it is set to 2.5% or less.

Mo: 0.3% or less Mo, like W, has the effect of increasing resistance to temper softening, and is also an effective element for improving toughness.

However, if more than 0.3% is added, the hardenability decreases, so the upper limit is set at 0.3%.

V:Q, 3% or less ■ has the effect of refining austenite grains and increasing hardenability, and is also effective in improving temper softening resistance and improving toughness. However, if it exceeds 0.3% When added,
This causes embrittlement (therefore, the upper limit is set at 0.3%).

Note that impurities such as P and S may be mixed within the range normally allowed for this type of steel. For example, the gist of the present invention is not impaired by the presence of 0.04% or less of P, 0.03% or less of S, or the like.

〔Example〕

A hot-rolled strip of high carbon steel (coiled after phase transformation) is subjected to secondary annealing treatment, then cold rolled, and then subjected to secondary annealing treatment to obtain a product cold-rolled steel strip. Test pieces were taken from the top, middle, and end portions of the test piece, and the hardness was measured.

Table 1 (1) shows the chemical composition of the test steel, Table 1 [■] shows the primary annealing treatment conditions, cold rolling reduction ratio, secondary annealing treatment conditions, and hardness measurement results of the product steel strip (average value). are also shown. In the table, "high hydrogen gas" is 75%Hz -Nz, r
"NX gas" is 2% H2-2°6% Co-N2.

Nos. 1 to 5 are invention examples, Nos. 11ll to 15 are comparative examples,
Among the comparative examples, Nα11 to 13 are examples that deviate from the provisions of the present invention in that NX gas was used in the atmosphere for the secondary annealing treatment, and No. 14 was an example in which the primary annealing treatment was performed at 800°C in an NX gas atmosphere. In No. 15, the primary and secondary annealing treatments satisfy the provisions of the present invention, but the chemical composition of the tempering zone is inappropriate. This is an example of (insufficient C content).

All of the invention side stones 1 to 5 have obtained a good softening effect that surpasses the comparative examples Nα11 to 15. Comparative examples Nα11 to 14
(-NX gas used for secondary annealing treatment) is compared with the invention example, even if the primary annealing treatment is three times longer than that of the invention example, as with Na 11-13, the hardness of the product steel strip is high;
Further, even if the scorching temperature in the first stage of primary annealing is too high as in Nα14, softening and hardening are poor. When the softening effects of these comparative examples and inventive examples are compared taking into account the cold rolling reduction ratio, the difference is obvious. Further, as is clear from Comparative Example Nα15, in order to ensure the softening effect according to the present invention, the chemical composition of the copper strip as well as the annealing treatment conditions must satisfy the provisions of the present invention.

[Effects of the Invention] According to the method of the present invention, a homogeneous and sufficiently softened cold-rolled steel strip having a fine spheroidized carbide structure can be obtained. In addition, the primary annealing treatment of the hot rolled steel strip in the present invention can be accomplished in an extremely short time, unlike conventional methods.
It has a large energy saving effect and significantly improves productivity.

Moreover, since the hot-rolled steel strip subjected to the secondary annealing treatment is sufficiently softened, unlike the conventional method, cold rolling with a high rolling reduction can be achieved without performing an intermediate annealing treatment.

Therefore, the present invention is extremely useful as a method for manufacturing high-carbon cold-rolled steel strips that are supplied as raw materials for machine parts such as cutlery, springs, washers, springs, etc., and reduces the manufacturing cost and improves productivity of the materials. In addition, great effects can be obtained in improving and stabilizing the quality of mechanical parts.

[Brief explanation of the drawing]

FIG. 1 CI) [I[) is a graph showing the hardness of the hot rolled steel strip after primary annealing treatment, and FIG. 2 is a graph showing the hardness of the cold rolled steel strip.

Claims (1)

[Claims] 1. C: 0.6 to 1.3% by weight, Si: 0.5% by weight or less, Mn: 1% by weight or less, Cr: 1.6% by weight or less, the balance is substantially Fe. A high-carbon hot rolled steel strip having a chemical composition of Soaking and slow cooling in the first stage of cooling to just below the A_r_1 point at a cooling rate of ℃/Hr or less,
After soaking for 3 to 20 hours just below point A_c_1, 60℃/
After being subjected to a primary annealing process consisting of a second stage of soaking and slow cooling in which the material is cooled to a temperature of A_r_1 or less at a cooling rate of Hr or less,
A method for producing a high carbon cold rolled steel sheet with excellent formability and heat treatability, which comprises performing cold rolling and then performing secondary annealing treatment in a temperature range of 600° C. to just below the A_c_1 point. 2, C: 0.6 to 1.3% by weight, Si: 0.5% by weight or less, Mn: 1% by weight or less, Cr: 1.6% by weight or less, and Cu: 0.3% by weight or less, Ni : 2.2% by weight or less, W: 2.50% by weight or less, Mo: 0.30% by weight or less, V: 0.30% by weight or less, the remainder substantially A high carbon hot-rolled steel strip having a chemical composition consisting of Fe is heated in an atmosphere furnace containing 50% by volume or more of H_2 gas and the balance being N_2 gas from point A_c_1 to 78
After soaking in a temperature range of 0℃ for more than 1 hour, the first stage soaking/slow cooling is performed at a cooling rate of 60℃/Hr or less to just below the A_r_1 point, and soaking for 3 to 20 hours to just below the A_c_1 point. After holding, it is subjected to a primary annealing process consisting of a second stage of soaking and slow cooling in which it is cooled to a temperature of A_r_1 or less at a cooling rate of 60°C/Hr or less, followed by cold rolling and then 600°C to A_
A method for producing a high carbon cold-rolled steel sheet with excellent formability and heat treatability, characterized by performing a secondary annealing treatment in a temperature range just below c_1 point.