JPH01132740A - Steel sheet for heat treatment - Google Patents

Abstract

PURPOSE:To provide a steel sheet for heat treatment which is as soft as a low-carbon steel, has good workability and has the heat treatment performance equiv. to the heat treatment performance of an ordinary high-carbon steel at the time of a hardening-tempering treatment by incorporating respectively prescribed ratios of C, Si, Mn, Al, N, P, S, and Ca into said steel sheet and consisting the balance substantially Fe. CONSTITUTION:The above-mentioned steel sheet for heat treatment is constituted by incorporating, by weight %, 0.30-l.20% C, 0.30-2.00% Si, 0.05-1.50% Mn, 0.001-0.100% Al, <=0.006% N, <=0.020% P, <=0.015% S, and 0.0010-0.0200% Ca (where 1-10 Ca/S) and the balance Fe and unavoidable impurities into the steel sheet. This steel sheet has the structure mainly composed of a ferrite phase and fine graphite grain having <=10mum diameter and has <=50kg/mm<2> tensile strength. The formability before the heat treatment is greatly improved if said steel sheet is used for various mechanical parts such as cutters, springs and wear-resistant parts and, therefore, the simplification of working stages and the complication of forming shapes are enabled and a high effect is obtainable in terms of saving of the stages, labor and cost.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention is based on the good workability of bending a low carbon steel plate.
The present invention aims to propose a steel plate for heat treatment that has high carbon steel plate bending hardenability and also has excellent toughness after quenching and tempering treatment.

In general, carbon steel materials used after undergoing a heat treatment process such as quenching and tempering have at least about Q, 3wt of carbon.
% (hereinafter referred to simply as %), and is called high carbon steel.As such high carbon steel has high hardness, excellent strength and wear resistance, it is used for knives, springs, and other various mechanical parts. Widely used in the field.

In these application fields, steel sheets for heat treatment are subjected to various processes such as cutting, punching, drilling, and bending prior to heat treatment, but steel with high hardenability has high strength and cannot be processed as described above. Have difficulty.

In order to compensate for this, it is common to perform a softening treatment such as spheroidizing annealing in advance, but there is a limit to the degree of softening in the spheroidized cementite structure obtained by such treatment. It is difficult to obtain workability comparable to that of low carbon steel.

In other words, the composition of high carbon steel for heat treatment is usually determined from the viewpoint of hardenability, so it has to be difficult to process. Expecting good workability on par with steel was an impossible order.

When using high carbon steel in this way, there are constraints such as the inability to process complex shapes, as well as limitations on forming methods,
Problems such as molding machines and the like, as well as the increase in processing man-hours and time mentioned above, caused problems in manufacturing costs.

As another means to solve the above-mentioned difficulties in high carbon steel,
For parts that require complex forming, low-carbon steel with good workability is used and processed to the desired shape, and then carburized or further nitrided to ensure hardenability. Although methods have been adopted to
Needless to say, such carburizing and nitriding methods involve an increase in the number of man-hours, which is economically disadvantageous.

(Prior Art) In JP-A No. 60-52551, in order to further improve the workability of carbon steel materials without requiring particularly troublesome processes and equipment, and while ensuring the necessary strength, The content of P and S is P (%) XS (%) ≦10
It has been proposed that it is useful to induce the formation of a graphite phase by reducing X 10-' as much as possible, resulting in a structure mainly composed of a ferrite phase and a graphite phase.

In this case, C013 should be suitable for use as heat treatment steel.
% or more, the tensile strength ranges from approximately 50 kgf/mm' to 35 kgf/mm'', so the effect of improving workability is still not sufficient.

In addition, JP-A-60-128245 also discloses that workability is improved when the structure is mainly composed of ferrite and graphite. However, this example is not intended for materials that are subjected to hardening treatment, but is limited to structural materials that exclusively utilize the vibration damping properties of the graphite phase in the structure. From the perspective of this, the larger the grain size of the graphite phase in the structure, the better. Structures with graphite grains are not compatible for the following reasons.

Generally, when steel is heated to the austenitizing temperature, it is easier for C in the steel to dissolve into the austenite phase when it is in the graphite state than when it is in the cementite state. In particular, this tendency becomes stronger as the graphite grains become coarser. As described above, if the solubility of C in austenite is poor, it will not be possible to obtain a predetermined quench hardness during quenching after austenitization, so the steel cannot be used as a heat treatment steel.

(Problems to be Solved by the Invention) In view of the above-mentioned problems with steel for heat treatment, a steel that is as soft as low carbon steel and has good workability in cutting, punching, drilling, bending, etc. In addition to this, it also has heat treatment performance comparable to that of ordinary high carbon steel when quenching and tempering is performed without the need for time-consuming processes such as carburizing and nitriding. It is an object of this invention to provide a steel plate.

(Means for Solving the Problems) The above object can be advantageously achieved by a configuration having the following points as its main points.

C: 0.30-1.20% Si: 0.30-2.00% Mn: 0.05-1.50% AIl: 0.001-0.100% N: 0.0
060% or less P: 0.020% or less S: 0.015% or less and Ca: 0.0010 to 0.0200% but Ca/s
1 to 10, with the balance being Fe and unavoidable impurities, and has a structure mainly composed of a ferrite phase and fine graphite grains with a diameter of 10 μm or less, and has a tensile strength of 50
kgf/mm2 or less, with excellent workability and hardenability,
A steel plate for heat treatment (first invention), characterized in that it also has excellent toughness after quenching and tempering treatment.

C: 0.30-1.20%, Si: 0.30-2.00%, Mn: 0.05-1.50%, Eight β: 0.001-0.100% N: 0.0060% Hereinafter, P: 0.020% or less, S: 0.015% or less, and Ca: 0.0010 to 0.0200wt%, however, Ca
/S1~1O1B: 0.0005~0.0500%
, with the remainder being Fe and unavoidable impurities,
It has a structure consisting mainly of ferrite phase and fine graphite grains with a diameter of 10 μm or less, and has a tensile strength of 50 kgf/m.
m2 or less, has excellent workability and hardenability, and is hardenable.
A steel plate for heat treatment (second invention) characterized by having excellent toughness after tempering treatment.

C: 0.30-1.20%, Si: 0.30-2.00%, Mn: 0.05-1.50%, AN: 0.001-0.100% N: 0.0060% Hereinafter, P: 0.020% or less, S: 0.015% or less, Ca: 0.0010 to 0.0200wt%, but Ca
/Sl-10, and one or more selected from the group consisting of Cr, Mo, and Ni: 0.20 to 1, 9 wt%, with the balance being Fe and unavoidable impurities. , has a structure mainly consisting of a ferrite phase and fine graphite grains with a diameter of 10 μm or less, has a tensile strength of 50 kgf/mm” or less, has excellent workability and hardenability, and has a hardening-tempering treatment. Steel plate for heat treatment (No. 3), which is characterized by excellent toughness
invention).

C: 0.30-1.20% S+: 0.30-2.00% Mn: 0.05-1.50% Aβ: 0.001-0.100% N: 0.0060% or less P: 0 .020% or less S: 0.015% or less Ca: 0.0010-0.0200wt% However, Ca
/S1~1O1B: 0.0005~0.0500%
and one or more selected from the group consisting of Cr, Mo, and Ni: 0.20 to 1, 9 wt%, the balance being Fe and inevitable impurities, and forming a ferrite phase. It has a structure mainly composed of fine graphite grains with a diameter of 10 μm or less, has a tensile strength of 50 kgf/mm” or less, has excellent workability and hardenability, and has excellent toughness after quenching and tempering. A steel plate for heat treatment (fourth invention).

Each of the inventions 1 to 4 has the material characteristics of a material that is soft and has good workability on par with low carbon steel when processed, and has good hardenability on the same level as ordinary high carbon steel when heat treated. This is a steel sheet with excellent toughness, wear resistance, and strength properties.In order to achieve these material properties, the microstructure of the steel is changed to a structure in which a fine graphite phase is uniformly dispersed in a phalrite phase (hereinafter referred to as In order to obtain such a microstructure, the chemical composition of the steel must be adjusted, and if necessary, the rolling conditions during hot rolling must be adjusted, as well as the subsequent annealing conditions. Adjustments are made.

Any of the steel sheets for heat treatment listed above can be obtained in the form of a hot-rolled sheet that has been subjected to a prescribed hot rolling process and then annealed to change the microstructure to a ferrite-graphite structure. It can also be obtained in the form of a cold-rolled plate by applying the above-mentioned annealing treatment after hot or cold rolling using a hot-rolled plate as a raw material at a rolling temperature range of 500°C or less. In particular, the excellent workability during cold rolling reduces the load on cold rolling operations, which is advantageous. It is also possible to provide a cold-rolled sheet by subjecting it to annealing treatment.

In hot rolling, it is more desirable to advance recrystallization of each grain in order to uniformly disperse the fine graphite, rather than setting conditions that allow each grain to be made as fine as possible during the hot rolling process.

In warm or cold rolling, the sheet is finished to a predetermined thickness at a rolling reduction of 20% or more.

The annealing treatment conditions are 500°C to 750°C, preferably 6
Suitable for holding between 50°C and A1 transformation point for 1 to 200 hours.

(Function) The reasons for numerical limitations in each of the above inventions will be explained in detail below.

C is an essential element to ensure hardenability, and 0.30% or more is required due to requirements such as wear resistance, hardness, and strength characteristics of various products manufactured using the heat-treated steel sheets listed above. It is. The reason why the upper limit is set at 1.20% is that a C content exceeding this not only saturates the hardenability, but also increases the amount of undissolved cementite or graphite phase during austenitization before quenching. This is because it causes deterioration of impact resistance properties after treatment.

Si is still an essential element for the following two reasons.

First of all, the steel base is strengthened by solid solution hardening, and after quenching, it is easier to obtain high strength that cannot be achieved by quench hardening with carbon alone, thereby improving wear resistance and increasing hardness. This is because it is possible to achieve this goal.

The second reason is to obtain a good fine graphite structure. That is, as described above, the microstructure of high carbon steel as hot-rolled is a structure containing ferrite and pearlite, or bainite therein, and usually has very high strength, so that its formability is extremely poor. In order to improve this, the microstructure is changed to the desired ferrite/graphite structure by annealing, but Si acts to make it easier to change cementite into graphite grains during annealing, and after annealing, It makes it easier to obtain a ferrite/graphite structure, which contributes to softening and improving workability. Furthermore, Sl also serves to improve the hardenability by improving the solubility of graphite grains in austenite during heating before hardening.

The mechanism by which 81 promotes the conversion of cementite into graphite is as follows.

Since Sl is a non-carbide forming element, it is difficult to dissolve in cementite in an equilibrium state, but when it is dissolved in a non-equilibrium state, it greatly destabilizes cementite. A after hot rolling
r, the production rate of cementite due to transformation is very fast, so the composition of the produced cementite heavily inherits the composition ratio of the parent phase before transformation, and as a result, excess S above the equilibrium solubility
It will contain i. As the Si content of the base material increases, the amount of excess Sl in cementite also increases, which increases the degree of destabilization of cementite and facilitates its conversion into graphite.

In order to advantageously obtain the above two effects, Si should be 0.
It is necessary to set the content to 3 to 2.0%, and the reason for limiting it to 2.0% or less is from the viewpoint of manufacturing costs, that is, even if it is added in excess of 2.0%, the wear resistance related to solid solution hardening will decrease. This is because the improvement in graphitization during annealing is saturated, and the manufacturing cost only increases.

Mn is an element that improves hardenability, and has a particularly great effect of lowering the critical cooling rate in the hardening process.
If the amount of Mn is increased, it becomes possible to slow down the cooling rate during quenching from the perspective of preventing quenching distortion, and from this perspective it can be said that it is an effective element. When the amount is large, it significantly stabilizes cementite and inhibits graphitization.
．． If it exceeds 5%, such adverse effects will become significant, and graphitization during annealing will be delayed, making it difficult to obtain the desired ferrite-graphite structure, so the upper limit should be set to 1.5%.
%.

Furthermore, the lower limit of Mn was set as 0.05% because if the Mn content is lower than this, fixation of S as an impurity element becomes insufficient and hot embrittlement is likely to occur.

Af improves the cleanliness of steel as a deoxidizing element and AI! In order to expect the effect of reducing solid solution N that inhibits graphitization, N is required to be 0.001% or more, but this effect is saturated when it exceeds 0.100%, so 0.
．． The range is 0.001% to 0.100%.

N dissolves in the form of replacing C in cementite and has the effect of significantly stabilizing it, making it difficult to obtain a ferrite-graphite structure during annealing.
Must be below 0.060%.

The first reason is that P has an adverse effect on both hardenability and workability due to its influence on the transformation characteristics and segregation of steel, and P is dissolved in small amounts in cementite. Because it has the effect of stabilizing this, it is undesirable for the second reason to show an effect of inhibiting the formation of ferrite/graphite structures during annealing.In order to avoid such adverse effects of P, the content should be 0.020% or less. There must be. However, it is not necessary to unnecessarily lower it beyond economical efficiency, and it is not necessary to lower it to less than about 0.002%.

S tends to form non-metallic inclusions, worsening workability, and has the effect of inhibiting graphitization during annealing, so it must be kept at 0.015% or less. However, S
For the same reason as for P, it is not necessary to lower it below about 0.0005%.

Ca is an element that plays a particularly important role and has the following effects. The first effect is to control the shape of the sulfide as CaS into a spherical shape that does not adversely affect workability. The second effect is to fix S as CaS to eliminate the harmful effects of a trace amount of 7IJ-3, which has a large negative effect on graphitization.

Thirdly, CaS acts as a graphite nucleus during graphitization, increasing the speed of graphitization and improving the refinement and uniformity of graphite grains.

In order to obtain such effects, it is important to control the amount of Ca added and the Ca/S ratio within an appropriate range.

The above effects cannot be obtained unless the amount of Ca is at least 0.0010% or more. However, if it exceeds 0.0200%, the effect will be saturated and the amount of Ca-based nonmetallic inclusions will increase, which will conversely deteriorate workability, which is not preferable.

In addition, when the Ca/S ratio is less than 1, S as CaS
The fixation of CaS becomes insufficient, and free S remains, which dissolves in cementite and increases its stability, inhibiting graphitization.At the same time, since the amount of CaS decreases, the graphitization nucleation effect decreases and the desired result is not achieved. No effect is obtained. However, when the Ca/S ratio exceeds 10, CaS aggregates, becomes coarser, and becomes unevenly distributed, reducing the number of nuclear sites and worsening the distribution state of the final graphite grains. As a result, the desired uniform and fine graphite structure cannot be obtained.

B is also a useful element. That is, the first effect is
As is conventionally known, it is a useful element for improving hardenability, and the second is that it combines with N in steel to form BN, reducing IJ-N that inhibits graphitization. The third is promoting the destabilization of cementite, which is necessary to promote graphitization, and the third is BN and Fe23 (CB).
Precipitates such as No. 6 exhibit effects such as acting as a nucleus during graphitization.

To expect the above effect, B is at least 0.0005
% or more is required. However, if the amount is added in excess of 0.0500, the effect will be saturated and this will only cause economic disadvantage.

Both Cr and Mo increase the hardenability and resistance to temper softening after quenching, so if used in an appropriate amount, they increase the heat treatment performance of the steel. However, on the other hand, it has the effect of substituting with Fe in the hot-rolled cementite and forming a solid solution therein, thereby stabilizing it, and therefore also having the effect of inhibiting graphitization. In order to improve heat treatment performance, it is desirable to add 0.20% or more of any element, but 1.0% or more is desirable.
If it exceeds 1.0%, graphitization will deteriorate significantly and the desired ferrite+fine graphite structure will not be obtained, so 1.0% is set as the upper limit.

Ni has the effect of reducing the critical cooling rate during hardening, so it contributes to improving hardenability, does not form carbides, does not have a negative effect on graphitization like the above-mentioned Cr and Mo, and is rather On the contrary, it is a useful element in that it has a slightly supportive effect. In order to obtain this effect, it is necessary to add at least 0.20%, but it is an expensive element and if it exceeds 1.0% it will be economically disadvantageous, so the upper limit is set at 1.0%. did.

Here, Cr, Mo, and Ni are the same effective components in that they contribute to improving hardenability.

Next, as mentioned above, in order to simultaneously satisfy workability and hardenability, each invention is limited to having a ferrite/graphite structure, and the reason for this is as explained below based on the inventors' research results. be.

Figure 1 shows C: 0.62%, Si: 1.62%, Mn
:0.78%.

AR: 0.015%, N: 0.0023%,
P: 0.007%.

Using a small specimen taken from a gmm thick hot rolled steel strip with a composition of S: 0.001% and Ca: 0.0015%, the graphitization ratio in the structure was changed by various methods. , the tensile properties and Charpy impact properties were investigated by adjusting the structure to consist mainly of ferrite and fine ferrite, with the remaining C being spheroidized cementite.

Moreover, FIG. 2 shows the difference in hardenability when the graphite particle diameter is different. This evaluation of hardenability was carried out for cases where the graphitization rate was 80% or more and the average graphite particle diameter was variously different, and the heating and holding time at 860°C was varied variously.
The average hardness of the cross section is shown when quenching is performed at a cooling rate of .

According to the results shown in Figures 1 and 2, (1) Tensile properties and impact properties depend on the graphitization ratio, and if this graphitization ratio exceeds 80%, the tensile strength is low and the elongation and tit properties are also good. (2) On the other hand, hardenability depends on the average particle size of graphite, and in the case of large graphite grains exceeding 10 μm, the heating time required for austenitization is significantly longer (3) In this way, graphite formation It was found that by increasing the ratio and creating a structure in which fine gelaphyte, whose average grain size was adjusted to 10 μm or less, is mixed with ferrite, a special feature that satisfies workability and hardenability at the same time was obtained. .

Here, it is practical to select the range of graphitization annealing conditions from two viewpoints: obtaining an annealed structure most advantageous for workability through sufficient softening, and low annealing cost. For example, in low-temperature annealing where the annealing temperature range is less than 500°C, the progress of softening is extremely slow;
If the temperature exceeds 0°C, the proportion of the austenite phase during annealing increases, and this portion remains as a pearlite phase after annealing, which is undesirable as it becomes a cause of inhibiting the uniformity of the softened structure. Therefore, the annealing temperature range is 500 to 750℃.
is recommended, and the appropriate annealing time is about 1 to 200 hours, but the annealing temperature is low and the culm requires a long time.

In addition, from the viewpoint of the material, the optimal range of annealing temperature is 650
C to the A1 transformation point, and in particular, if a temperature just below the A1 transformation point is selected, a good material can be obtained with short annealing.

In addition, even if an annealing cycle is adopted, for example, once heated to a temperature in the α + γ two-phase temperature region, and then held at a temperature below the A1 transformation point at a very slow cooling rate, the annealing time can be shortened and the material quality can be improved.

Next, when performing cold or warm rolling prior to this graphitization annealing, from the viewpoint of hardenability and cold workability, it is possible to improve the microstructure consisting of ferrite and fine uniform graphite grains more efficiently. It becomes a means to obtain. In other words, a hot-rolled sheet with an appropriate amount of Ca added to the chemical composition according to the amount of S in the steel so as to act as graphitization nuclei is subjected to cold or warm rolling prior to graphitization annealing. , the graphitization nucleation effect becomes more pronounced and the graphitization rate increases, so that a final microstructure in which graphite grains are finely and uniformly distributed can be easily obtained.

By the way, this graphitization process first involves nucleation of graphite, then decomposition of cementite,
Grain growth progresses in the order of solid solution of C into the matrix and diffusion into graphite grains. The nucleation process is a very important factor in this, and the more nucleation sites there are and the more uniformly they are distributed, the finer and more uniform the final graphite grains will be.

Furthermore, if the number of nuclei increases, the diffusion distance of C after cementite decomposition becomes shorter, so that the graphite growth rate increases.

In addition, in the case of B addition, precipitates such as BN or Fe23(CB)6 act as graphitization nuclei, and when cold or warm rolling is added to this, microscopic formation occurs around these precipitates. As a result, the dislocation density increases significantly, increasing the nucleation effect.

In addition, since a large number of point defects introduced by cold or warm rolling serve as nucleation sites, nucleation becomes even easier.

In addition, cementite is destabilized by the above-mentioned rolling and becomes easily decomposed, and furthermore, the dislocations introduced by rolling act as a diffusion path for C dissolved in the matrix, thereby increasing the growth rate of graphite. It is.

In addition, the increase in dislocation density due to cold or warm rolling as described above increases the number of ferrite grain recrystallization nuclei during the next step of annealing, so that grain refinement of the ferrite phase matrix after annealing is achieved. be done. As a result, it also contributes to improving toughness and strength-elongation balance. By accumulating such multiple effects, the effects become even more apparent and extremely effective and good results can be obtained.

In order to exhibit such an effect, a reduction ratio of 20% or more is required in the warm or cold rolling temperature range of 500° C. or less. If this rolling temperature exceeds 500°C, the number of effectively acting dislocations and point defects will decrease due to strain recovery and recrystallization of the ferrite matrix after rolling, and the desired effect will not be fully exerted. be. Also, rolling reduction rate 2
If it is less than 0%, the proportion of point defects and dislocations introduced by rolling is too small, so it is difficult to obtain any effect.

Example 1 A steel whose chemical composition is shown in Table 1 was hot-rolled in a conventional manner to obtain a hot-rolled steel strip with a thickness of 8 mm, and then this hot-rolled steel strip was annealed in a prescribed manner. Table 2 shows the annealing conditions, tensile properties after annealing, Charpy properties, and hardness and toughness results after quenching and tempering.

The tensile properties were measured using a 3mm thick JIS No. 5 tensile test piece, and the hardness was determined by heating at 850°C for 30ml, oil quenching at a cooling rate of 70°C/sec, and then tempering at 250°C for 60ml. This is the later result.

Ferrite, which is a microstructural feature according to this invention.
In order to clarify the material characteristics due to the graphite structure, the relationship between the tensile strength shown in Table 2 and the hardness after quenching and tempering is shown in Figure 3, and the relationship between tensile strength and elongation is shown in Figure 4 for comparison. .

From Figures 3 and 4, the tensile strength of this invention is 50 kg.
f/mm2 or less, and although it is a high carbon steel, its tensile properties show low strength and high ductility comparable to low carbon steel, and the hardness after quenching and tempering is comparable to the ferrite and spheroidized cementite of the comparative steels. It can be seen that it has the same hardenability as structural steel. Furthermore, the hardenability is further improved by containing Cr, tJo, and Ni.

Further, from Table 2, it can be seen that the invention steel has significantly superior impact properties compared to the comparison steel, which is a steel with a spheroidized cementite structure, and this effect is particularly significant when B is added.

(Effects of the Invention) According to the present invention, high carbon steel for heat treatment, which conventionally had poor workability, has mechanical properties of softness and good workability comparable to low carbon steel, and excellent mechanical properties equivalent to conventional high carbon steel. A steel with good hardenability can be obtained. If the steel of the present invention is used for various mechanical parts such as knives, springs, and wear-resistant parts, the formability before heat treatment will be significantly improved, making it possible to simplify the processing process and make the formed shape more complex. Significant effects can be obtained in terms of process, labor and cost savings.

[Brief explanation of the drawing]

Figure 1 is a graph showing the effect of graphite ratio on mechanical properties and hardenability. Figure 2 is a graph showing the effect of graphite grain size on hardenability. Figure 3 is a graph showing tensile strength and hardenability. - A graph showing a comparison of hardness after tempering; FIG. 4 is a graph showing the relationship between tensile strength and elongation of the invention steel and comparative steel.

Claims (1)

[Claims] 1. C: 0.30 to 1.20 wt%, Si: 0.30 to 2.00 wt%, Mn: 0.05 to 1.50 wt%, Al: 0.001 to 0.100 wt% %, N: 0.0060 wt% or less, P: 0.020 wt% or less, S: 0.015 wt% or less, and Ca: 0.0010 to 0.0200 wt%, but Ca/
It has a composition of s1 to s10 with the balance being Fe and unavoidable impurities, has a structure mainly composed of a ferrite phase and fine graphite grains with a diameter of 10 μm or less, and has a tensile strength of 50K.
gf/mm^2 or less, with excellent workability and hardenability,
A steel plate for heat treatment, which is characterized by having excellent toughness after quenching and tempering. 2, C: 0.30-1.20 wt%, Si: 0.30-2,00 wt%, Mn: 0.05-1.50 wt%, Al: 0.001-0.100 wt%, N: 0. 0060wt% or less, P: 0.020wt% or less, S: 0.015wt% or less, Ca: 0.0010 to 0.0200wt%, but Ca/
s1~10, B: 0.0005~0.0500wt%, the balance is Fe and unavoidable impurities, it has a structure mainly composed of ferrite phase and fine graphite grains with a diameter of 10 μm or less, and has a tensile strength. 50kgf/mm
^2 or less, has excellent workability and hardenability, and is hardenable.
A steel plate for heat treatment that is characterized by excellent toughness after tempering. 3. C: 0.30-1.20wt% Si: 0.30-2.00wt% Mn: 0.05-1.50wt% Al: 0.001-0.100wt%, N: 0.0060wt% or less P: 0.020wt% or less S: 0.015wt% or less Ca: 0.0010 to 0.0200wt% However, Ca/
s1 to 10, and one or more selected from the group consisting of Cr, Mo, and Ni: 0.20 to 1.0 wt%, with the balance being Fe and unavoidable impurities. , has a structure consisting mainly of a ferrite phase and fine graphite grains with a diameter of 10 μm or less, and has a tensile strength of 50 kgf/
A steel plate for heat treatment, which is characterized by having a hardness of less than mm^2, excellent workability and hardenability, and excellent toughness after quenching and tempering. 4, C: 0.30-1.20wt% Si: 0.30-2.00wt% Mn: 0.05-1.50wt% Al: 0.001-0.100wt%, N: 0.0060wt% or less P: 0.020wt% or less S: 0.015wt% or less and Ca: 0.001 to 0.0200wt% However, Ca/S
1 to 10, B: 0.0005 to 0.0500 wt%, and one or more selected from the group consisting of Cr, Moe, and Ni: 0.2 to 1.0 wt%, the remainder It has a composition of Fe and unavoidable impurities, has a structure mainly composed of a ferrite phase and fine graphite grains with a diameter of 10 μm or less, and has a tensile strength of 50 kgf/
A steel plate for heat treatment, which is characterized by having a hardness of less than mm^2, excellent workability and hardenability, and excellent toughness after quenching and tempering.