JPH0762212B2 - High carbon thin steel sheet with high toughness and heat crack resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is excellent in impact resistance and wear resistance after heat treatment, and is less likely to be damaged by hydrogen during use, and further, it has high manufacturability and workability. It is suitable for chain parts, gear parts, clutch parts, seat belt buckles, and washers, and has high toughness and high resistance to heat cracking (referred to as "heat cracking resistance"). is there.

Here, "heat cracking" refers to cracking that occurs due to repeated heating by applying stress such as friction and rapid cooling by water or the like.

(Prior art) Generally, chain parts, gear parts, clutch parts, seat belt buckles, washers parts, etc. are high carbon alloy steel cold rolled steel plates such as SCM435, SCM445 or SCr435, SCr440 etc. specified in JIS G3311, and S45CM- It is usually manufactured by using S50CM high carbon cold-rolled steel sheet as a raw material, forming it, and then hardening it by heat treatment such as quenching and tempering. Here, the material steel sheet for each product is soft and easy to process before forming, desired strength is obtained only by heat treatment performed after forming, and sufficient impact resistance when used as a product and Since it is required to exhibit wear resistance, a material with a high carbon content as described above is selected as a material, and in general, spheroidizing to make it soft when shipping thin steel sheets from steel manufacturers. Annealed. Then, the thin steel sheet after shipping is formed into a desired shape by the user and subjected to heat treatment such as quenching and tempering to give necessary characteristics. In this case, the impact resistance and wear resistance of the product are In particular, since the tempering temperature affects the tempering temperature, the tempering temperature of "as-quenched" or "up to 650 ° C" is carefully selected depending on the form and situation of use. Usually, the tempering temperature at this time is 180 to 450 degrees.

(Problems to be solved by the invention) However, in the above-mentioned quenching and tempering type high carbon thin steel sheet specified in JIS, impact resistance and further prevention of cracking due to hydrogen are inadequate despite careful selection of heat treatment conditions. It is sufficient, and when applied to a chain if cooled, it is difficult to completely prevent brittle fracture due to contact impact of a chain cover or the like.

In addition, when brakes are continuously applied to a brake disc or the like, cracks may occur when the heating portion due to friction is repeatedly rapidly cooled by water or the like. As described above, this crack is called "heat crack".

Therefore, as a preventive measure for this, users perform austempering for SCM435 etc. to improve toughness, but with this method the equipment that makes the process complicated compared to quenching and tempering is becoming larger. Therefore, there was an inconvenience that the cost would rise.

Based on such circumstances and the future cost reduction requirements expected for members requiring high impact resistance and wear resistance such as the above-mentioned chain member and disk member, the inventors of the present invention,
In order to meet these requirements, the toughness of wear-resistant materials is improved without resorting to special heat treatment, and impact resistance comparable to austempering can be obtained by simple quenching and tempering, and cracks due to hydrogen absorption. We have come to realize that it is essential to develop materials that are highly effective in preventing the occurrence of

(Means for Solving the Problems) From the above viewpoints, the inventors of the present invention have sufficient wear resistance and impact resistance as materials for disk plates, chains, and the like, and have good workability. Research was conducted in order to provide a thin steel sheet that does not cause inconvenience such as cracking even when subjected to impact, friction, etc. on the surface during rolling process, forming process to final product, and running in the product state.

Here, the following findings were obtained regarding the improvement of impact resistance and the prevention of thermal cracking due to hydrogen penetration.

(1) Improving impact resistance (a) 3ppm or more in high carbon alloy mesh
(ppm and% are based on weight) B is segregated at the austenite grain boundaries to exert an action of strengthening the bond strength of the grain boundaries, resulting in a remarkable suppression of the occurrence of grain boundary cracks. thing.

(B) However, in this case, if only B is simply added, the added B combines with N in the steel to form BN, so that the intergranular cracking suppressing effect of B disappears.
However, when a specific amount of Ti (0.005 to 0.050%) is added together with B, Ti is preferentially bonded to N over B and Ti is added.
Since N is generated, the solid solution B increases, and the segregation amount of B to the austenite grain boundaries also increases, so that the grain boundary crack suppressing effect of B can be sufficiently secured.

(C) Further, in addition to this, when the content of P in the steel is reduced to a specific value or less, the amount of P segregated in the austenite grain boundaries is reduced, and grain boundary embrittlement, which causes brittle fracture, is suppressed. Further toughness improvement.

(D) Further, generally, when steel is tempered at a temperature of about 300 ° C. after quenching, so-called “low temperature temper embrittlement” occurs and becomes extremely brittle, but addition of B is also very effective against this embrittlement. To be.

(E) However, in some cases, hydrogen embrittlement and fatigue embrittlement, which have a large effect on thermal cracking due to coarsening of austenite grains, may not be completely prevented by simply adding B or reducing the P content. A specific amount of strictly adjusted ingredients
When Nb (0.005 to 0.100%) is further added, the austenite grains are effectively refined, the absorbed energy is improved, and hydrogen embrittlement and cracking due to fatigue are significantly suppressed.

(2) Prevention of occurrence of thermal cracking due to hydrogen absorption (f) Impurity elements in steel, mainly Mn, Si, and P, have a great influence on the occurrence of cracking due to hydrogen absorption, and especially the amount of Mn and P added Cleaning the steel by reducing the amount of hydrogen has a great effect on preventing the occurrence of cracks due to hydrogen absorption.

(G) The frequency of cracking due to hydrogen absorption increases as the size of austenite grains increases, but when Nb and Ti are added in combination, a very stable carbide or nitride is formed, which has the effect of suppressing the growth of these austenite grains. However, the refining of austenite grains after quenching and tempering by this addition has the effect of significantly suppressing the intrusion of hydrogen from the steel surface layer into the steel interior, and delays the occurrence of cracks.

(H) Generally, quenching is required to increase the toughness of high carbon steel sheets.
Although deterioration of formability and punchability before tempering was unavoidable, addition of Mo as a steel component did not lead to deterioration of toughness after quenching and tempering without deterioration of the above-mentioned formability and punchability, especially "low temperature temper brittleness". The deterioration of toughness, which is called “,” can be effectively prevented.

(I) In addition, reduction of Mn content also greatly contributes to improvement of toughness by suppressing MnS generation, and the expected decrease in strength due to Mn reduction can be sufficiently compensated by the addition effect of a small amount of Cr and Mo. The toughness is improved more than expected by the synergistic effect of the addition of Cr and Mo.

The present invention has been completed based on the above findings, "a thin steel sheet produced by rolling is C: 0.30 to 0.60%, Si: 0.70% or less, Mn: 0.05 to 1.00%, P: 0.030% Below, Cr: 0.5 to 1.5%, Mo: 0.6% or less, Ti: 0.005 to 0.050%, B: 3 to 20ppm, Nb: 0.005 to 0.100%, sol.Al: 0.08% or less, N: more than 0.002% 0.010% Below, by constituting the composition with the balance being essentially Fe, excellent wear resistance, toughness (impact resistance), heat crack resistance,
In addition, "the point that good workability is imparted" is also featured.

(Operation) Here, the reason why the component composition of the thin steel sheet according to the present invention is numerically limited as described above will be explained.

(A) C In order to obtain the desired hardness and wear resistance in the C steel sheet, the C content needs to be 0.30% or more, while if it exceeds 0.60%, the hardness increases excessively and sufficient hardness is obtained. Since the absorbed energy is not obtained and the occurrence of cracks due to hydrogen absorption is greatly increased, the C content is set to 0.30 to 0.60%.

(B) Si is not particularly required to be positively added, but if the content of Si exceeds 0.70%, the steel sheet tends to become hard and brittle, so the Si content is set to 0.70% or less.

(C) The applications of the high carbon steel sheet to which the present invention is added with Mn Cr and Mo are gears, chains and the like, and unlike general wear resistant steel sheets, it is necessary to reduce Mn in order to improve toughness. In particular, in the steel sheet of the present invention, if it is contained in an amount of more than 1.0%, it tends to be hardened by heat treatment and becomes too hard, resulting in a decrease in toughness. On the other hand, if the Mn content is less than 0.05%, the amount of solid solution S increases and embrittlement occurs during hot working, impairing the manufacturability of the steel sheet. Therefore, the Mn content is 0.05 to 1.00%. Although specified, the addition of 0.80% or less is particularly desirable to suppress the occurrence of cracks due to hydrogen absorption.

(D) The lower the P P content, the better the toughness, and the P content is 0.030.
% Or less, but it is desirable to limit it to 0.02% or less.

(E) Cr Cr is a component added mainly for the purpose of improving hardenability, and therefore 0.5% or more is added. On the other hand, if its content exceeds 1.5%, it causes the steel to harden and becomes brittle, so the Cr content was set to 0.5 to 1.5%.

(F) Mo Addition of Mo produces the effect of maintaining high toughness after heat treatment without deteriorating the workability of the steel sheet before heat treatment (before quenching / tempering).

Generally, when steel is tempered at a temperature of around 300 ° C. after quenching, so-called “low temperature temper embrittlement” occurs and becomes extremely brittle. However, tempering at the above temperature may be necessary in some cases when it is desired to obtain a desired hardness. In fact, the above-mentioned "low temperature temper embrittlement" is remarkable particularly in the case of a thick sample and tends to be reduced in a thin plate, so that "tempering at around 300 ° C" is sometimes adopted. However, in that case, depending on the usage conditions, the problem of reduced toughness still remains. The addition of Mo is also very effective against such embrittlement.

Further, in the steel sheet according to the present invention, the effect is improved as the Mo content is increased, but the effect becomes remarkable particularly from 0.1% or more. On the other hand, even if added over 0.6%, the effect is saturated, so the Mo content was set to 0.6 or less, preferably 0.1 to 0.6%.

(G) Ti Ti has the effects of improving the hardenability of steel and increasing the hardness and tensile strength of steel by forming and finely dispersing TiN. In addition, the formation of TiN promotes solid solution of B, and also exerts an action of promoting grain boundary strengthening of austenite. However, when the Ti content is less than 0.005%, the desired effect due to the above action cannot be obtained, while on the other hand, 0.05
The Ti content is set to 0.005 to 0.050%, because if the content exceeds 0% in excess, not only will the cost increase, but it will lead to hardening of the steel and lose its advantages. The upper limit of Ti is 0.
It is preferably set to 03%.

(H) BB is an extremely important element, which not only improves the hardenability of steel, but also exerts the action of strengthening the grain boundary by segregating as solid solution B in the grain boundary, and brittleness at a content of 3 ppm or more. The effect of significantly suppressing the occurrence of destruction is secured.

However, even if added in excess of 20 ppm, the above effect will be saturated, resulting in cost increase.
It was set at 20 ppm.

(I) Nb Nb has a function of refining austenite grains to improve both hardness, tensile strength and toughness of the steel, particularly when added in combination with Ti. In particular, in B-added steel, austenite is likely to coarsen during reheating, so addition of Nb is extremely effective for refining austenite, and is also very effective for preventing fracture due to hydrogen embrittlement. However, if the content is less than 0.005%, the desired effects due to the above action cannot be secured, while even if the content exceeds 0.100%, these effects reach a saturated state, so the Nb content is 0.00
It was set at 5 to 0.100%.

(J) Sol.Al Al is a component added as necessary as a deoxidizing agent for steel, but if the content of Sol.Al exceeds 0.08%, not only the cost increases but also hardening of the steel sheet occurs. So there is no advantage. As described above, the content was determined to be 0.08% or less because the content up to 0.08% of Sol.Al is allowed.

(K) Although the inclusion of N N is effective in improving the hardness and tensile strength of steel,
It has the property of forming BN and inhibiting the grain boundary strengthening effect of B. In order to maintain the grain boundary strengthening effect of B, the content is
The limit was 0.006% or less. When Mo is added, the effect of preventing embrittlement is exhibited, and even when N is 0.01% or less, it is effective. Also
If it is less than 0.002%, the austenite grains become coarse and the crack suppressing ability deteriorates, so the lower limit is made more than 0.002%.

(L) Others It is preferable that S is low also in ordinary steel, and particularly in the high strength steel plate according to the present invention, the S content is preferably suppressed to 0.0040% or less. It is advisable to employ any of the Ca-Si injection treatment methods. However, if the Ca content exceeds 0.02%, it becomes a large inclusion to deteriorate the toughness, so it is necessary to keep in mind the Ca addition treatment within the range where the Ca content does not exceed 0.02%.

The thin steel sheet according to the present invention is manufactured by melting steel containing the above components and the balance being substantially Fe, and rolling the steel to a desired thickness by hot rolling and cold rolling.
During this, annealing for softening may be performed. In any case, finally, annealing is performed to improve workability by performing workability, and this final annealing is suitably at 650 to 880 ° C.

The thin steel sheet produced as described above is usually processed by a user and then heat-treated to obtain desired hardness and performance.

Examples The effects of the present invention will be described by way of examples in comparison with comparative examples.

Example 1 First, a steel having a composition containing Mo as shown in Table 1 was smelted, and then hot-rolled into a hot-rolled sheet having a thickness of 3 mm, followed by softening by annealing and then 2.0 mm. Cold rolled to thickness. In the rolling process, steel plate No. 1
Although rolling was completed without any trouble, it was confirmed that steel sheet No. 2 having a high P content tends to have an increased hardness after rolling.

Next, this cold rolled steel sheet was subjected to batch annealing of “holding at 680 ° C. for 16 hours”.

The hardness of the steel sheet after annealing is also shown in Table 1.

Then, samples were taken from these annealed plates, heated to 850 ° C., quenched with oil cooling, and further tempered at various temperatures. The tempering time was 15 minutes.

The hardness of each sample after the above heat treatment was measured, a Charpy V-notch test piece was prepared, and an impact test at 0 ° C. was performed to determine the impact absorption energy. The results were summarized by the tempering temperature and shown in FIG. Shown in.

The following can be seen from the results shown in FIG.

At the C content level of the test steel sheet, the hardness depends on the application
H _R C45 or higher is required, also generally hardness so it decreases the tempering temperature is increased to obtain a high hardness (high wear resistance) is required tempering cold (300 ° C. or less in the steel plate). Further, the comparative steel sheet No. 2 in which the Ti content, the Nb content and the B content are 0 has a small absorbed energy under any tempering condition.

On the other hand, it is clear that the steel sheet No. 1 of the present invention shows energy of 6.0 kg · m or more at any tempering temperature and is excellent in impact resistance.

Example 2 Similar to Example 1, the hot rolled steel plate having a thickness of 4.0 mm was changed to a cold rolled steel plate having a thickness of 3.0 mm (the composition of components is as shown in Table 1), and the cold rolled steel plate was used as a disc plate (brake disc plate). Punch out
After holding at 850 ° C for 15 minutes, oil-cooled quenching and 250 ° C
A tempering process was performed for 30 minutes, and this was assembled to prepare a braking device as shown in FIG. 2A shows a disc plate 1 (thickness: 3.00 mm, diameter: 250 mm), and FIG. 2B shows a braking pad 2. Braking is performed while rotating the disc plate 1 of FIG. 2 (a) at 900 rpm.
At this time, a water spray is intermittently sprayed onto the disk to prevent the disk from being heated. At this time, the endurance time until minute cracks (heat cracks) occurred in the voids (arrows in FIG. 2 (c)) in the disk was measured.

(At this time, braking is performed by sandwiching the mild steel plate Hv = 100 from both sides of the disk.) The results are shown in Table 2 together with the surface hardness of the disk.

Also from the results shown in Table 2, it is clear that the disk made of the steel sheet of the present invention in which the P content is reduced and Ti, B and Nb are added has very few cracks and is excellent in durability. Is.

Example 3 Steels having the respective component compositions shown in Table 3 were melted, hot-rolled,
Cold rolling and annealing were sequentially performed to manufacture a 2.5 mm thick thin steel plate.

These steel sheets, hardness after annealing (H _R B), and which quenching and tempering treatment (treatment conditions third is as shown in Table) was followed hardness (H _R C) and Charpy impact test absorbed energy (At 0 ° C.) was measured, and the results are also shown in Table 3.

As is apparent from the results shown in Table 3, satisfying thin steel sheet No.3~17 of the present invention, in both hardness after annealing (H _R B) is relatively low and soft though the heat treatment (quenching and tempering) after hardness is high (H _R C ≧ 45), yet with respect to the impact absorption energy is high (5 kg · m or higher), smaller even impact absorption energy either in the comparative steel sheet, It can be confirmed that the impact resistance is poor.

(Effects of the Invention) As described above, according to the present invention, it is possible to realize a high toughness high carbon thin steel sheet that is easy to manufacture and process and has excellent wear resistance and impact resistance. Chain parts,
It can be applied to materials such as gear parts and clutch parts to further improve the performance of these products, resulting in extremely useful effects in industry.

[Brief description of drawings]

FIG. 1 shows “changes in hardness and impact absorbed energy depending on tempering temperature” of the thin steel sheets manufactured in Examples.
FIG. 2 (a), FIG. 2 (b), and FIG. 2 (c) are explanatory views of the procedure of a crack test of the brake disc manufactured in the example. 1: Brake disc (disc plate), 2: Braking pad (soft steel plate)

Claims (1)

[Claims] 1. A weight ratio of C: 0.30 to 0.60%, Si: 0.70% or less, Mn: 0.05 to 1.00%, P: 0.030% or less, Cr: 0.5 to 1.5%, Mo: 0.6% or less, Ti: 0.005 to 0.050%, Nb: 0.005 to 0.100%, B: 3 to 20ppm, sol.Al: 0.08% or less, N: more than 0.002% and 0.010% or less, the balance being essentially Fe, wear resistance, toughness, heat cracking High carbon thin steel sheet with excellent properties.