JPH0344422A - Manufacture of high carbon thin steel sheet - Google Patents

Abstract

PURPOSE:To stably manufacture the high carbon steel sheet for a high hardness member having effectively suppressed decarburizing reaction on a surface layer, at the time of manufacturing the high carbon thin steel sheet, by subjecting a hot rolled sheet of a high carbon steel to softening in a specified temp. area without executing descaling and, if required, thereafter subjecting it to cold rolling and annealing. CONSTITUTION:A medium-high carbon slab having the compsn. contg., by weight, 0.30 to 1.20% C, <1.0% Si, <1.50% Mn, <0.050% P, <0.050% S and 0.01 to 0.10% Sb or furthermore contg. one or more kinds among <1.50% Cr, <0.50% Mo and <2.0% Ni is hot rolled into a hot rolled sheet, which is thereafter coiled at >=450 deg.C and is subjected to air cooling. Since the decarburization on the surface of the hot rolled sheet can effectively be prevented by the incorporation of Sb, descaling can be unnecessitated. Then, the sheet is subjected to softening in the temp. area of 600 to (Ac1+40) deg.C or 600 to (Ac +40) deg.C and, if required, is furthermore subjected to one or plural times of cold rolling and annealing. Its decarburization at the time of heat treatment such as spheroidizing needed for the softening and quenching, tempering and austempering for improving the strength can effectively be suppressed, by which the high hardness steel sheet can stably be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a high carbon thin steel sheet with excellent decarburization resistance. More specifically, the present invention focuses on the surface layer formed during heat treatment processes such as spheroidizing annealing necessary for softening, quenching/tempering to impart desired strength, or austempering in the manufacturing process of high carbon thin steel sheets. The present invention relates to a method for producing a high carbon thin steel sheet that can effectively suppress decarburization of the steel and is very effective for streamlining the production of high hardness members such as gears, washers, cutlery, etc.

(Prior art) Generally, high hardness parts such as gears, washers, cutlery, saws, and washers are manufactured using SK7M to SK7M specified in JIS G3311.
Low Mn-based steels with a very high C content, such as SKIM, and high carbon cold-rolled steel sheets, such as S45CM to SS70C, are used as materials.

The manufacturing method is to pickle the hot-rolled steel sheet and then annealing it as necessary to produce a product as a hot-rolled sheet, or to further cold-roll it, followed by spheroidizing annealing, and to form a suitable product. Adjust the intensity. It is common to use the products obtained by punching and bottom-shaping them after hardening them by subsequent heat treatment such as quenching and tempering.

Here, the raw material steel plates for each of the above products are soft and easy to process before forming, and the desired strength can only be obtained through heat treatment after forming, and they have sufficient hardness and durability to be used as products. Since it is required to exhibit wear resistance, a material with a high CI is selected in terms of -a.

(Problem to be solved by the invention) However, as the CI increases, the hardness of conventional materials increases in the form of hot-rolled sheets, which poses significant restrictions on formability during processing into products or rolling reduction during cold rolling. I was receiving it. For this reason, the manufacturing process has had many problems, such as a prolonged annealing time and an increased number of cold rolling or annealing times. Therefore, the present inventors aimed to refine the structure of hot-rolled sheets for the purpose of improving the formability of hot-rolled sheets and improving the rolling reduction during cold rolling, and found that the necessity of softening hot-rolled sheets was solved. I came to realize that.

In order to satisfy both of these requirements, rapid cooling after finish rolling in the hot rolling process and annealing of the hot rolled sheet are required. However, the problem is that the hardness of the hot rolled sheet increases due to rapid cooling after rolling, making pickling before annealing difficult. Therefore, the present inventors investigated a means of performing softening annealing without descaling the hot rolled sheet by pickling.

For these annealing, generally (Ac, 50)~^0
A box annealing process is used in which the product is soaked for a long period of 6 to 24 hours at a temperature range of 1℃ or Ac+ to (Ac, +30)'C, and the atmosphere at this time is an inert atmosphere such as Nls Ar. Carburizing is carried out in a carburizing atmosphere such as coke gas or methane, and this atmospheric gas is carefully selected to prevent decarburization. However, even in such an atmosphere, a decarburized layer is formed on the temporary surface layer due to the attached scale, which poses a problem.

The following two types of causes can be considered for decarburization from the temporary surface layer in these heat treatment steps.

First, if oxide scale remains on the temporary surface layer during the hot rolling process, F, which is the main component of this scale during soaking,
ed, is decomposed and 0. occurs and separates from the temporary surface layer. The remaining scale turns into pure iron, forming a decarburized layer on the surface of the plate.

Second, 0 in the atmospheric gas! When the concentration increases, 02 separates into ○ atoms in the plate surface layer and invades the inside from the temporary surface layer. By combining this ○ atom with C in the temporary surface layer,
Co and cot are formed and released from the temporary surface layer to the outside. After this, a decarburized layer is formed.

The decarburized layer formed in this way not only reduces the hardness of the plate surface and significantly deteriorates its wear resistance, but also causes a decrease in strength, so extreme care must be taken to prevent its occurrence. It is something that must be done.

Considering these circumstances and the above-mentioned demand for cost reduction in the manufacturing process of tool steel, the present inventors have found that in order to meet these demands, it is possible to effectively suppress these decarburizations in the material itself. We came to realize that it was necessary to develop a pure variety.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a high carbon thin steel sheet with excellent decarburization resistance.

(Means for Solving the Problems) Therefore, in order to achieve the above object, the present inventors have conducted various studies on means for improving formability or rolling reduction in cold rolling. The following new findings were obtained regarding the structure of the plate, the suppression of hardness increase during the structure formation process, and the prevention of decarburization during the countermeasure process.

(a) In order to obtain a fine structure in a hot-rolled sheet, it is necessary to
Requires winding at a relatively low temperature range of 50°C or higher.

(b) Furthermore, hot-rolled sheets with such fine &II textures generally have high hardness, and after completion of hot rolling, they may be reheated to an appropriate temperature range of 600°C or higher and subjected to softening annealing as necessary. What is necessary to do.

(c) In conventional hot-rolled sheets, such reheating produces a decarburized layer on the surface layer of the sheet, so in the present invention, a zero carbon layer is formed on the steel sheet.
．． If 0.1% or more of sb is added and this steel plate is rolled into a thin steel sheet, the contained sb will be converted into 0□ gas O in the surface layer during heating such as annealing or quenching.
In order to suppress the decomposition into atoms, the intrusion of O atoms into the temporary surface layer is prevented, and the reaction between C and O in the temporary surface layer is suppressed, so decarburization is effectively prevented.

(d) The decarburization prevention effect of sb is generally
It should be effective not only for general high carbon steel sheets called SC materials and SK materials in the standards, but also for Cr-Mo or NJ low alloy high carbon steel sheets called SCM materials and SKS materials.

(e) Similar to P and Sn, when sb is heated to the austenite region, it segregates at the austenite grain boundaries, reduces the prior austenite grain boundary strength after cooling, and has the property of causing grain boundary fracture there. There is.

Therefore, from the viewpoint of grain boundary strengthening, it is necessary to set an upper limit on the amount of sb added.

Based on the findings shown in (a) to e), the present inventors conducted further studies, and as a result, completed the present invention.

Here, the gist of the present invention is, in weight percent, C: 0.30 to 1.20%, Si: 1.00% or less, Mn: 1.50% or less, P: 0.050%.
Below, S: 0.050% or less, Sb: 0.01~
0.10%, and if necessary, one or more alloy components of Cr: 1.50% or less, Mo: 0.50% or less, and Ni: 2.00% or less, the remainder Fe and unavoidable After hot-rolling a steel billet with a steel composition consisting of impurities, it is coiled at a temperature of 450°C or higher, and after cooling, it becomes 600~(Ac++40) 'C1 or 600~(^cca+40)'' without descaling. This method of manufacturing a high carbon thin steel sheet is characterized by performing softening annealing in a temperature range of c, and further performing cold rolling and annealing one or more times as necessary.

(Function) Hereinafter, the present invention will be explained in detail along with the function and effect. In addition, in this specification, "1%" means "wt%" unless otherwise specified.
” shall mean.

First, the reason for limiting the composition of the steel slab used in the present invention will be explained.

(a) C In order to impart desired hardness and wear resistance to the steel plate,
The C content must be 0.30% or more, while 1
．． If the content exceeds 20%, the amount of cementite will increase and the toughness will significantly deteriorate, making it extremely difficult to ensure the durability of the product. Therefore, the C content is 0
．． It was set at 30-1.20%.

(b) St It is necessary to add Si to give the product appropriate hardness, but if it is added in excess of 1.00%, the steel plate tends to become hard and brittle, so the Si content is 1.0
It was set as 0% or less.

(C) An In wear-resistant steel plates, Mn is generally added in large amounts to improve wear resistance, but in the steel plates of Zero Invention, the upper limit is 1.50% for this purpose. . However, if added in excess of 1.50%, it will deteriorate the toughness of the steel and lead to damage to the product during use.
It is not preferable to add more than this amount. Therefore, the Mn content was determined to be 1.50% or less.

Note that since a decrease in Mn leads to a decrease in hardenability, it is desirable to add at least 0.30% or more of Mn.

(d) PP P segregates in the austenite grain boundaries of steel and has a great effect on the toughness of the product after quenching and tempering. It goes without saying that the lower the P content, the better the toughness. And P content is 0.050%
The P content is set at 0.050% or less, since if it exceeds P, it is likely to segregate at grain boundaries and cause grain boundary embrittlement, but it is preferably limited to 0.020% or less.

(e) S The lower the S content in ordinary steel sheets, the better; however, especially in high-strength steel sheets such as those involved in the present invention, the presence of MnS has a significant effect on toughness deterioration, so the S content is reduced to 0.05.
Although it is set to 0% or less, it is desirable to suppress it to 0.020% or less.

(f) Sb When 0.01% or more of Sb is added to high carbon steel and rolled into a thin steel plate, the contained Sb converts into O atoms of O gas in the surface layer during heating such as annealing or quenching. In order to suppress the decomposition of , O atoms are prevented from penetrating into the surface layer. Therefore, since the reaction between C and O in the temporary surface layer is suppressed, decarburization is effectively prevented.

However, like P and Sn, when sb is heated to the austenite region, it segregates at the austenite grain boundaries, reduces the prior austenite grain boundary strength after cooling, and causes grain boundary fracture there.

Therefore, from the viewpoint of grain boundary strengthening, the upper limit of the amount of sb added was set to 0.
．． It is necessary to limit it to 10%.

For this reason, the amount of sb added was limited to 0.01 to 0.10%, but it is more desirable to add about 0.02 to 0.08% from the viewpoint of ensuring effective decarburization suppressing action or ensuring toughness. .

Furthermore, in addition to the above &IIfi, at least one of Cr, Mo, and Ni may be added to the steel billet according to the present invention, if necessary, which improves the hardenability of the steel according to the present invention. and are added to further improve mechanical properties such as processability, and the amounts and reasons for these additions will be explained below.

(Cr) Cr is a component that is added as needed mainly for the purpose of improving hardenability, but if it is contained in excess of 1.50%, it will cause the steel to become hard and brittle. In the steel billet used in the invention, Cr is added as necessary to improve hardenability, and the upper limit is 1.50%.Also, when the purpose is to improve hardenability, the target is 0.15% or more. It is desirable to add Cr.

(b)M.

Mo is a component added as necessary, and the addition of MO has the effect of maintaining high toughness after heat treatment without deteriorating the workability of the steel sheet before heat treatment (quenching, tempering).

Generally, when steel is tempered at a temperature of around 300° C. after quenching, it causes so-called "low temperature tempering embrittlement J" and becomes extremely brittle. Mo addition is effective against this embrittlement.

For this reason, in the steel sheet according to the present invention, Mo shall be added with an upper limit of 0.50% as necessary6.However, even if this upper limit is exceeded, the effect of improving toughness will be saturated; This upper limit should not be exceeded as it will also lead to an increase in costs.

In addition, in order to obtain this toughness improvement effect, M of 0.15% or more is required.
It is desirable to add o.

Q) Ni Ni has the effect of improving the workability of steel, and is a component that is actively added as necessary in the present invention, but it is desirable that its content be 2.00% or less. .

This addition of Ni effectively suppresses the occurrence of cracks during rolling, and also facilitates processing by the user.

This is considered to be to reduce the deformation resistance of the ferrite matrix.

On the other hand, adding Ni in an amount exceeding 2.0% only increases the manufacturing cost of the steel sheet and the effect of improving workability is saturated, so the upper limit of the amount added was set at 2.0%.

In the present invention, after hot rolling and winding a steel billet having the above composition, softening annealing is performed in a specific temperature range without descaling or scaling.The manufacturing conditions will be described below. .

0) It is desirable to carry out the hot rolling in the desired temperature range of Arc or Ac from the viewpoint of completely dissolving cementite into austenite.

Next, the steel plate that has been hot rolled in this way is rolled at 450 mm.
Winding in a temperature range of ℃ or higher, preferably 500℃ or higher,
Cooling. The reason why the winding temperature is limited in this way is to take advantage of the fact that the structure of the obtained hot rolled sheet becomes finer, so that the cementite is efficiently spheroidized during annealing. Therefore, the lower the winding temperature condition is, the more effective it is, but since problems such as cracking occur due to hardening during winding, the winding temperature was limited to 450° C. or higher. More preferably, the temperature is 500°C or higher. In addition, the upper limit of this winding temperature is set at 650°C from the viewpoint of refining the structure of the hot rolled sheet.
According to the findings of the present inventors, increasing the cooling rate during cooling after winding promotes spheroidization and refinement of cementite during annealing. The higher the speed, the more desirable it is.

(G) Softening annealing temperature conditions after coiling As mentioned above, hot-rolled sheets coiled at a relatively low temperature of 450°C or higher have a large hardness, especially when the cooling rate to the coiling temperature is high. , there is a risk that problems such as breakage may occur during the pickling process before cold rolling or during the cold rolling process.

As a preventive measure, it is necessary to heat the hot-rolled sheet to a temperature range of 600 to (Ac+ + 40) °C- or 600 to (Acc-+ 40)'C and then cool it without descaling by pickling. be. Annealing temperature is 600℃
If it is less than (Ac + + 40) 'C or (Ac
This is because if the temperature exceeds +40°C, cementite will precipitate in a lamellar form during cooling, which will have an adverse effect on cold rolling.

The atmospheric conditions at this time are an inert atmosphere such as N2 or Ar, a carburizing atmosphere such as coke gas or methane, or the atmosphere, and this atmospheric gas is adjusted as appropriate depending on the scale formation situation to prevent decarburization. However, the conventional S
Compared to steel without the addition of b, the selection conditions are very wide.

Furthermore, as the cooling conditions after annealing, it is desirable to cool at a relatively slow rate of 100'C/hr or less.

In this way, hot-rolled sheets that have been softened and annealed without descaling are usually descaled by pickling, followed by one or more cold rolling and spheroidizing annealing as necessary. conduct. In this way, it is possible to obtain a high carbon thin steel sheet that is extremely soft and has a large cold rolling ratio compared to the conventional case where a hot rolled sheet is pickled and then cold rolled.

The present invention will be further explained in detail with reference to Examples, but these are merely illustrative of the present invention and are not intended to limit the present invention.

Example 1 A steel billet N (LA or billet NαG) having a chemical lJI value shown in Table 1 was hot rolled under the following conditions: finish rolling temperature: 850°C, cooling rate = 50″C/Sec, coiling temperature = 550°C. After rolling, the scale is still attached 6
Annealing was performed by soaking at 50°C, 1680'C, or 740'C for 24 hours. In addition, the heating and cooling rate in hot-rolled sheet annealing was 40° C./hr.

Table 2 (Note) Vehicles outside the scope of the present invention Figure 1 graphically shows the influence of the amount of sb added on the generation of a decarburized layer from the surface layer (depth of decarburized from the surface layer).

From this result, it was confirmed that steel billet NnA-NαE, which is a steel billet with sb added within the range according to the present invention, is suitable for efficiently suppressing decarburization under various temperature conditions. .

Example 2 Steel slabs shown in Table 1 were hot rolled under the following conditions and then subjected to softening annealing without descaling, and after completing the softening annealing, they were pickled. At this time, the change in hardness of the hot rolled sheet depending on the annealing temperature and the change in the critical cold reduction after annealing were investigated, and the results are shown in FIG.

The hot rolling conditions of this example are Finishing temperature: 850'C Cooling rate: 100'C/Sec Coiling temperature:
The temperature was 500℃. Note that the heating and cooling rate during annealing of the hot rolled sheet was 40° C./hr.

From these results, it is recognized that annealing at a temperature lower than 600° C. results in too high hardness and a very low critical cold reduction rate. From this, it was confirmed that softening and improvement of the critical cold reduction ratio can be achieved by annealing in the temperature range of 600° C. or higher, which is the range of the present invention.

Example 3 In this example, the influence of the coiling temperature conditions limited in the present invention was investigated using the same steel shown in Table 1.

The results are summarized in Figure 3.

The hot rolling and annealing conditions at this time were as follows.

Finishing temperature = 850°C Cooling rate: 100°C/Sec Hot rolled plate annealing conditions: 680°C in nitrogen for 24 hours From these results, it was recognized that the lower the winding temperature, the lower the hardness after annealing.

Note that the cooling rate after finishing hot rolling is not particularly limited in the present invention, but as shown in Fig. 4, it has been observed that an increase in the cooling rate causes softening after annealing. There is. The hot rolling and annealing conditions at this time were: Finishing temperature: 850°C Coiling conditions: Coiling at 500°C after cooling at each cooling rate Hot rolled sheet Annealing conditions: Heating and cooling rate of hot rolled sheet in nitrogen at 680°C x 2 Jhr :40°C/hr.

As mentioned above, by refining the cementite structure, spheroidization during annealing can be made more efficiently, and for such refining of cementite, the higher the cooling rate, the more desirable it is. A rate of about 10° C./sec is effective.

Example 4 Steel slabs kJ to N [LYO steel grades shown in Table 2 were annealed after hot rolling without descaling according to the conditions of the present invention, followed by cold rolling, and then finish annealing. A thin steel plate with a thickness of 2.5 mm was prepared. We investigated the hardness of these steel plates after quenching and tempering, the absorbed energy, and the depth of decarburization from the surface layer.
The results are shown in Table 2.

(Left below) As shown in these results, it is recognized that the steel sheet obtained by the method of the present invention is superior to the comparative example in both absorbed energy and decarburization suppression. Ta.

Furthermore, Cr: 1, 50% or less, Mo: 0.
50% or less and Ni: 1 of the alloy components of 2.00% or less
Tests were also conducted for cases containing one or more species, and the same absorption energy and decarburization suppressing effects as in this example were confirmed.

(Effects of the Invention) The present invention, configured as described above, allows for spheroidizing annealing necessary for softening the material and imparting the desired strength while effectively suppressing surface decarburization. It has the effect of being able to be quenched, tempered, austempered, etc., and is extremely useful industrially.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the amount of sb added and the depth of decarburization; Figure 2 is a graph showing the relationship between the amount of sb added and the depth of decarburization; ! ! A graph showing the relationship between temperature and the limit cold reduction or hardness after annealing the hot rolled sheet; FIG. 3 is a graph showing the relationship between the coiling temperature and the limit cold reduction or the hardness after annealing the hot rolled sheet; and FIG. 4 is a graph showing the relationship between cooling rate and critical cold reduction or hardness after annealing a hot rolled sheet.

Claims (3)

[Claims] (1) Weight percentage: C: 0.30 to 1.20%, Si: 1.00% or less, M
n: 1.50% or less, P: 0.050% or less, S: 0.
050% or less, Sb: 0.01 to 0.10%, balance Fe
After hot rolling, a steel billet having a steel composition consisting of unavoidable impurities is rolled up in a temperature range of 450°C or higher, and after cooling, it is heated to 600~(Ac_1+40°C) without descaling.
)C or a temperature range of 600 to (Ac_c_m+40)C. (2) The method for manufacturing a high carbon thin steel sheet according to claim 1, characterized in that after performing the softening annealing, cold rolling and annealing are further performed one or more times. (3) In terms of weight percentage, the steel piece further contains Cr: 1.50%
Claim 1 containing one or more of the following alloy components: Mo: 0.50% or less and Ni: 2.00% or less
Or the method for manufacturing a high carbon thin steel sheet according to claim 2.