JPS6410564B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention provides a wear-resistant steel that has excellent soundness, good steel plate shape, and small difference in hardness distribution in the thickness direction by quenching treatment or quenching-tempering treatment. Regarding the manufacturing method, in detail, when a steel billet of a specific composition is rolled under specific conditions and then subjected to direct quenching or normal quenching, the cooling rate and cooling stop temperature are controlled to ensure soundness. The present invention relates to a method for producing wear-resistant steel having excellent properties, good steel sheet shape, and little difference in hardness distribution in the thickness direction. [Prior art and its problems] Wear-resistant steel with excellent soil and sand abrasion resistance has conventionally been manufactured by quenching or quenching-tempering steel, but improvements in weldability and low cost have been produced. It is desired to reduce the amount of added components from the viewpoint of chemical processing, and for this reason, it is increasingly being manufactured as is after the quenching process. Furthermore, there is a trend toward manufacturing by a direct quenching process that can further reduce the amount of added components. In addition, the hardness required for such wear-resistant steel was conventionally determined by the surface Brinell hardness H _B
The hardness used to be about 360, but recently the hardness H _B
High hardness of over 400 is increasingly required. The hardness of steel is approximately determined by the martensite fraction of the structure and the amount of carbon contained, and in order to manufacture steel with high hardness, it is necessary to increase the amount of carbon in proportion to the hardness. For steels with surface Brinell hardness H _B > 400,
Approximately 0.2% carbon must be added. but,
When the added components of steel increase in this way, quench cracking, hydrogen cracking, etc. are more likely to occur during quenching. For this reason, extreme care is required when hardening. Particularly in the direct quenching method, it is necessary to carefully perform dehydrogenation, etc. in advance. However, like traditional manufacturing processes,
When it is cooled to around room temperature during quenching,
For example, by degassing at the molten steel stage or slow cooling at the slab stage, the amount of hydrogen in the steel can be reduced to 2ppm in advance.
Even with the following conditions, it is difficult to completely prevent quench cracking and hydrogen cracking. Furthermore, in the conventional manufacturing process, when the product is provided after being quenched, it is difficult to correct the quenching distortion. [Object of the Invention] In view of the above-mentioned current situation, the present invention provides a steel sheet that is free from defects such as hydrogen cracking, has excellent soundness, has a small amount of distortion, and has a good shape through quenching treatment or quenching-tempering treatment, and It is an object of the present invention to provide a method for producing wear-resistant steel that can obtain wear-resistant steel that has sufficiently high surface hardness and a small difference in hardness distribution in the thickness direction. [Summary of the Invention] The method for producing wear-resistant steel of the present invention includes: C: 0.05 to 0.40wt%, Si: 0.1 to 0.8wt%, Mn: 0.5 to 2.0wt%, Ti: 0.005 to 0.10wt%, B: Contains 0.0005 to 0.005wt%, Sol.Al: 0.005 to 0.10wt%, N: 0.005wt% or less, H: 0.0002wt% or less, and Ti/N≧3.0, and the remainder: Fe and inevitable impurities. When quenching the steel material obtained by heating the steel billet to a temperature of 1000℃ or higher and then finishing rolling at a temperature _{of 3 points} or higher of Cool at a cooling rate of /sec or more,
It is characterized by stopping cooling at a temperature of 150-300℃ to obtain wear-resistant steel. [Structure of the Invention] This invention uses a steel billet of a specific composition as a raw material, rolling it under specific conditions, and then subjecting it to direct quenching or normal quenching with a controlled cooling rate and cooling stop temperature. Thus, a wear-resistant steel is obtained which is free from defects such as hydrogen cracking, has excellent soundness, has a small amount of distortion, has a good steel plate shape, and has a small difference in hardness distribution in the thickness direction. In this invention, as a material, C: 0.05~
0.40wt%, Si: 0.1~0.8wt%, Mn: 0.5~2.0wt
%, Ti: 0.005-0.10wt%, B: 0.0005-0.005wt
%, Sol.Al: 0.005 to 0.10wt%, N: 0.005wt% or less, H: 0.0002wt% or less, and Ti/
A steel slab with N≧3.0 and the balance consisting of Fe and unavoidable impurities is used. The reason why the composition of the steel slab is limited as described above is as follows. C: Since high hardness is required for wear-resistant steel, C
is required to be 0.05wt% or more, but from the viewpoint of weldability and toughness, the upper limit is set to 0.40wt%, and 0.05 to 0.40wt%.
%. Si: Si is useful as a deoxidizing element and needs to be present in an amount of 0.1 wt% or more, but if it exceeds 0.8 wt%, weldability and toughness deteriorate, so it is set at 0.1 to 0.8 wt%. Mn: Mn is required at 0.50wt% or more to ensure hardenability and improve toughness, but if it exceeds 2.0wt%, toughness may deteriorate, so 0.5 to 2.0wt%
%. Ti, N: Ti 0.005-0.10wt%, N 0.005wt
% or less and Ti/N≧3.0 is as follows:
B, which is effective in improving hardenability during hardening,
This is to ensure sufficient capacity. In other words, in high N steel without Ti addition, B in the steel combines with N and has no effect on improving hardenability, but when Ti is added, Ti combines with N and fixes N. B no longer combines with N and exhibits the effect of improving hardenability. This Ti
If it is less than 0.005wt%, N fixation will be insufficient,
Hardenability due to B cannot be ensured. Also, even at high N levels, a sufficient amount of
By including Ti, hardenability due to B can be ensured, but if Ti exceeds 0.1wt%, it not only increases cost but also forms large TiN, which is preferable from the viewpoint of steel quality. do not have. FIG. 1 is a graph showing the relationship between the thickness of the steel material after quenching and the difference in hardness distribution in the thickness direction ΔH _v (ΔH _v =H _v,Max −H _v,nio ). When Ti/N is 2.7 or less, ΔH _v is quite large even if the plate thickness is thin, and in order to maintain ΔH _v ≦150 up to a thickness of about 100 mm, it is necessary to set Ti/N ≧ 3.0. . N is 0.005wt% or less, and Ti is
If Ti/N≧3.0 in the range of 0.005 to 0.1wt%,
Ti is N without causing the above problems.
When combined with B, B enters a solid solution state, making it possible to improve hardenability. Moreover, even in the case of relatively thick steel plates, it is possible to manufacture wear-resistant steel with little difference in hardness distribution. B: 0.0005wt% or more of B is required to improve hardenability, but since adding a large amount is harmful to weldability, the upper limit is set at 0.005wt%, and 0.0005 to 0.005wt
%. Sol.Al: Al is essential as a deoxidizing element.
0.005wt% or more is required, but if it exceeds 0.10wt%, the toughness will deteriorate due to AlN etc., so the upper limit is set at 0.10wt%, and 0.005~
It was set to 0.10wt%. H: H generally has the effect of increasing cracking susceptibility, and this tendency is particularly noticeable in high-strength steel materials, so it is desirable to have a small amount. but,
By setting the quenching stop temperature to 150 to 300℃, cracking susceptibility can be reduced even with the same amount of hydrogen, and H is
There is no problem in application up to 0.0002wt% (2ppm).
Therefore, H was set to 0.0002wt% or less. In this invention, as a steel billet as a material, Cu: 1.0wt% or less, Cu: 1.0wt% or less,
Ni: 1.0wt% or less, Cr: 2.0wt% or less, Mo:
At least one of the following, or A component composition containing two or more types can be used. The reason for limiting these Cu etc. is as follows. Cu: Cu improves hardenability, but if added in large amounts it causes hot embrittlement, so 1.0wt%
The following was made. Ni: Ni improves toughness and hardenability, but is expensive, so it was set at 1.0 wt% or less. Cr, Mo: Cr and Mo improve hardenability, but
If added in large amounts, it is harmful to weldability and toughness, so Cr and Mn were set at 2.0wt% or less and 1.0wt% or less, respectively. V, Nb: V and Nb increase strength through precipitation hardening, but if added in large amounts they impair weldability, so V and Nb were both set to 0.1 wt% or less and Nb 0.1 wt% or less. Ca, Mg, REM: Ca, Mg, and REM combine with S to form shape-controlled sulfides and improve low-temperature toughness, but when added in large amounts, they reduce the cleanliness of steel, so both Ca0 .01wt% or less,
Mg was 0.01wt% or less and REM was 0.01wt% or less. In this invention, the above-mentioned steel billet is heated to a temperature of 1000°C or higher and then rolled, the rolling is finished at an Ar point of ₃ or more, and the steel material obtained by rolling is then heated to a temperature of _A3 or higher. Cool and quench, that is, for direct quenching, cool from a temperature of Ar ₃ or higher, for normal quenching, reheat to a temperature of Ac ₃ or higher, and then cool from a temperature of Ac ₃ or higher. It is then quenched. Here, the heating temperature of the steel billet is set to 1000°C or higher in order to increase rolling efficiency and to ensure the quenching start temperature (temperature at Ar ₃ points or higher) when using the direct quenching method. . The reason why the finishing temperature of rolling is set to ₃ or more Ar points is because of the relationship between rolling efficiency and quenching operation. In other words, in direct quenching, the rolling is immediately followed by quenching, but since quenching requires the steel to be rapidly cooled from a single-phase austenite state, rolling is performed at least as early as before direct quenching. The finishing temperature must be above the starting point of austenite → ferrite transformation (Ar ₃ point). On the other hand, in normal quenching, after hot rolling, the steel material is cooled to a temperature of at least Ar ₁ point or higher, and then reheated to a temperature of Ac ₃ or higher before quenching. Although finishing temperature does not have as much direct meaning as it does in quenching, at temperatures below the Ar ₃ point, the aforementioned ferrite precipitation begins and the rolling load increases, so rolling should be finished at a temperature above the Ar ₃ point. is desirable. Furthermore, if rolling is performed at a temperature below the Ar ₃ point, distortion will be introduced into the ferrite, and the austenite obtained when heated to a temperature above the Ac ₃ point may become mixed grains, resulting in non-uniform hardenability. In order to bring the surface hardness of steel to a predetermined high value through quenching, it is necessary to obtain a complete martensitic structure near the surface layer of the steel, and for this purpose the cooling rate during quenching must be at least 3°C/sec or higher. It is necessary that If the cooling rate is less than 3° C./sec, not only the surface hardness of the wear-resistant steel obtained will be insufficient, but also the hardness distribution in the thickness direction will become concave, which is not preferable. The cooling stop temperature for quenching is for quench cracking, hydrogen cracking,
From the viewpoint of preventing distortion etc., the temperature is set at 150 to 300°C.
Wear-resistant steel is obtained by quenching rolled steel and is sometimes used without tempering, so it is important to maintain the quality of the rolled steel after quenching. The cooling stop temperature becomes important. Figure 2 is a graph showing the relationship between the water-cooling stop temperature of quenching and the UST defect density determined by ultrasonic testing (UST) of quenched steel. As can be seen from Figure 2, as the water cooling stop temperature increases,
UST defect density shows a decreasing trend. Also, even if the amount of hydrogen in the steel is the same, the one with higher hardness has UST
High susceptibility to defect occurrence. Therefore, the harder the steel, the higher the cooling stop temperature for quenching and the lower the hydrogen concentration in the steel. When this water cooling stop temperature is less than 150℃, the UST defect density is quite high even if the hydrogen concentration is kept low.
For steel materials with hardness H _B of 450 to 500, the hydrogen concentration is
The US defect density cannot be reduced unless it is 1.0 ppm or less. On the other hand, when the stopping temperature is set to 150 to 300℃, even steel with hardness H _B of 450 to 500,
When the hydrogen concentration in steel is 2.0 ppm or less, the UST defect density can be reduced to a level that poses no practical problem. From the above, the lower limit of the cooling stop temperature for quenching is determined.
The temperature shall be 150℃. If the stopping temperature is less than 150°C, delayed fracture may occur unless the hydrogen content in the steel is kept sufficiently low. When the stopping temperature is set to 150°C or higher, hydrogen can be diffused during the slow cooling process after quenching. Stopping cooling at a temperature of 150°C or higher is also effective in preventing distortion due to quenching. The upper limit of the stopping temperature is 300℃. This is 300
This is because if quenching is stopped at a high temperature exceeding 0.degree. C., sufficient hardness cannot be obtained. As mentioned above, the main purpose of this invention is to produce wear-resistant steel as it is after quenching, but in order to adjust the hardness and other mechanical properties,
It goes without saying that this also includes the possibility of tempering at a temperature below Ac ₁ point after quenching, if necessary. In that case, when the tempering temperature is below 400℃, the normal tempering time is adopted, and when the tempering temperature is 400℃
When the Ac value exceeds ₁ point and the hardness decreases significantly, it is recommended to perform short-term tempering for less than 1 minute. According to the present invention, it is possible to obtain a wear-resistant steel with excellent low-temperature toughness, which has few internal flaws, low distortion, and has a hardness distribution difference ΔH _v in the thickness direction of 150 or less. [Example] Next, an example of the present invention will be described. Producing wear-resistant steel from steel slabs a to f containing the chemical components shown in Table 1 under the manufacturing conditions shown in Table 2,
Invention steel (Steel No. 1, 5, 8, 9, 11, 13) and comparative steel (Steel No. 2-4, 6-7, 10, 12, 14-19)
I got it. Table 2 also shows the test results of the surface hardness H _B of the steel of the present invention and the comparative steel. In addition, the first
Among the steel slabs a to f in the table, a to d have chemical compositions within the range of steel slabs targeted for the steel of the present invention, and e to
f is the chemical composition of the steel slab targeted for the steel of the present invention.

【table】

【table】

〔Effect of the invention〕

As explained above, according to the present invention, there is no internal defect such as hydrogen cracking, the steel plate has excellent soundness, the amount of distortion is small and the shape of the steel plate is good, the surface hardness is sufficiently high, and the hardness distribution difference in the plate thickness direction. It is possible to produce wear-resistant steel with small wear resistance.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the thickness of the steel material after quenching and the hardness distribution difference in the thickness direction, and Figure 2 is a graph showing the relationship between the water cooling stop temperature of quenching and the UST defect density of the steel material after quenching. It is.

Claims (1)

[Claims] 1 C: 0.05-0.40wt%, Si: 0.1-0.8wt%, Mn: 0.5-2.0wt%, Ti: 0.005-0.10wt%, B: 0.0005-0.005wt%, Sol.Al : 0.005 to 0.10wt%, N: 0.005wt% or less, H: 0.0002wt% or less, and Ti/N≧3.0, the balance being Fe and unavoidable impurities. After heating to a temperature of _Ar3
When quenching the steel material obtained by finishing rolling at a temperature of 3 or higher, cool the steel material at a cooling rate of 3℃/sec or higher from the temperature of point ₃ or higher, and stop cooling at a temperature of 150 to 300℃. A method for producing wear-resistant steel, characterized by: 2 C: 0.05-0.40wt%, Si: 0.1-0.8wt%, Mn: 0.5-2.0wt%, Ti: 0.005-0.10wt%, B: 0.0005-0.005wt%, Sol.Al: 0.005-0.10wt% , contains N: 0.005wt% or less, H: 0.0002wt% or less, and Ti/N≧3.0, furthermore, Cu: 1.0wt% or less, Ni: 1.0wt% or less, Cr: 2.0wt%. At least one of the following: Mo: 1.0wt% or less, Nb: 0.1wt% or less, V: 0.1wt% or less, Ca: 0.01wt% or less, Mg: 0.01wt% or less, rare earth elements: 0.01wt% or less After heating a steel piece containing Fe and unavoidable impurities to a temperature of 1000℃ or higher, Ar ₃
When quenching the steel material obtained by finishing rolling at a temperature of 3 or higher, cool the steel material at a cooling rate of 3℃/sec or higher from the temperature of point ₃ or higher, and stop cooling at a temperature of 150 to 300℃. A method for producing wear-resistant steel, characterized by: