JPH03183739A - Manufacture of high toughness non-heat treated steel for hot forging, its bar steel - Google Patents

Abstract

PURPOSE:To obtain the non-heat treated steel and bar steel for hot forging having excellent strength and toughness, at the time of subjecting a molten steel contg. specified amounts of Mn, Cr, V and Ti to continuous casting, by regulating its cooling in a specified temp. range, executing casting and thereafter directly rolling the steel into a bar steel. CONSTITUTION:A steel constituted of, by weight, 0.20 to 0.60% C, 0.10 to 2.00% Si, 0.50 to 2.00% Mn, 0.10 to 1.20% Cr, 0.03 to 0.20% V, 0.005 to 0.070% Ti, <0.005% Al, 0.0010 to 0.0100% O, 0.005 to 0.020% N and the balance Fe with inevitable impurities is subjected to continuous casting. The casting is executed under the condition that >=20 deg.C/min cooling rate is regulated in the temp. range from its solidifying temp. to 1000 deg.C. After that, the steel is directly rolled into a bar steel without executing blooming. The bar steel is heated to about <=1270 deg.C and is forged into machine parts. After that, the machine parts are cooled in the temp. range of about 800 to 400 deg.C at 0.1 to 5.0 deg.C/sec average. The bar steel has excellent impact properties as hot-forged state and is useful as a non-heat treated steel for hot forging.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a steel that exhibits a fine metal structure as hot forged and has high strength and high UJ properties, a method for manufacturing this beak, and a method for manufacturing this steel. This invention relates to a method of manufacturing parts using the method, and is suitable for use in automobile parts and industrial machine parts. [Prior Art] Many steel parts for automobiles and industrial TNI SATs are formed by cutting raw steel bars into predetermined lengths, heating them, and hot forging them. is usually about 1
Since hot forging is performed by heating to a temperature of 200° C. or higher, the metal structure of the hot-forged parts is extremely coarse, and the strength and toughness of the parts are usually poor. Therefore, in order to change the structure of parts to a fine structure with excellent mechanical properties, they are usually subjected to thermal refining treatment, that is, quenching and tempering. However, heat treatment requires a large amount of cost, so in recent years, steels that have sufficient strength and toughness even if heat treatment is omitted, so-called non-temperature steel for hot forging (non-heat forged steel) have been developed. (tempered steel) is now in demand. PJ4T1B non-tempered steel has already been put into practical use, and when classified from the viewpoint of required toughness, it can be divided into normal toughness type and high toughness type. If only strength is required in view of the usage conditions of the target part, then non-tempered steel that utilizes precipitation hardening of V, Nb, etc. may be used, for example. This type is, for example, JP-A-58-52458.
It is disclosed in the publication No. In addition, in the case of high-toughness molds that require both strength and toughness, it is almost essential that the structure as hot-forged be fine, and the structure will not coarsen even when heated to high temperatures. You must do so. Although the aforementioned V and Nb precipitates also have the effect of refining crystal grains, they are not effective in the extremely high hot forging temperature range. The present inventors have already completed the invention for non-thermal steel that has a fine structure as hot forged and has high toughness, and T
Prevention of tissue coarsening has been achieved using
-253725, Japanese Patent Application No. 63-318279)
, These inventions have made it possible to obtain parts that have a fine structure, high strength, and high toughness even after hot forging, and are now being applied to automobile suspension parts. . [Problem to be solved by the invention] As a result of the above-mentioned prior invention, hot forged non-tempered steel has been improved in toughness, and conventional heat-treated products are being replaced with non-tempered steel. The demands for higher performance and lighter weight are becoming more and more intense.1 Further improvements in mechanical properties, especially toughness, are required. In order to satisfy this requirement, the present invention provides a heat-forged non-thermal treated steel that has even better strength and toughness. [Means for solving the problem] (1) In weight%, C: 0.20-0.60. Si: 0.10
-2.00°Mn: 0.50-2.00.
Cr: 0.10-1.20°V:
[), [+3-0.2[1,T f : [1,1)(
15-0,071). Aβ: less than 0.005, O: o, ooto-o
, oiooo. N: 0.005-0.020. (2) A high-toughness non-thermal steel for hot forging that has excellent toughness even after hot forging, with the remainder consisting of Fe and unavoidable impurities.
High-grade steel having the composition described in item +1) above, which is cast into a slab under conditions such that the cooling rate is 20°C/n+fn or higher in the temperature range of 0°C, and then immediately rolled into a steel bar without blooming. Method for producing a non-tempered steel bar for toughness hot forging (3) The steel bar produced by the production method described in item (2) above is heated to a temperature of 1270°C or less, and after being forged into machine parts, the steel bar is heated from 800°C to 400°C. Average temperature range of 0.1-5.0
A method for producing high-toughness hot-forged non-thermal steel parts, characterized by cooling at a speed range of 1270° C./sec and not performing thermal refining treatment (4) The steel described in item +1) above is heated to 1270° C. After heating to a temperature of Method for manufacturing high-toughness hot-forged non-tempered steel parts (5) Series M! 100% from the freezing point when casting
It is cast into a slab under conditions such that the temperature range of 0°C becomes a cooling rate of 20°C/lll1n or more, and then heated to a temperature of 1250°C or less and formed into a mechanical part at a processing ratio of 2 or more,
This is a method for manufacturing a high-toughness hot-forged non-thermal steel part having the components described in item (1) above, which is characterized in that no heat-refining treatment is performed. As previously mentioned, it is essential to refine the structure in order to achieve high toughness, but in order to refine the structure as hot-forged, it is necessary to prevent the austenite structure from coarsening during high-temperature heating. is one effective means. In order to prevent coarsening of the austenite structure, the present inventors utilized the effect of fine precipitates to prevent movement of grain boundaries, the so-called blocking effect. The effect of bottling is greater as the precipitates are finer and larger, but Ti carbonitrides are very finely dispersed and have low solubility at high temperatures, so they are not particularly effective in preventing coarsening of the austenite structure. This is a well-known fact. The key point of the steel of the present invention is to utilize Ti oxides in order to further finely disperse Ti carbonitrides (see 1 above).
Item 2), and the key point of the method for manufacturing the steel of the present invention is to control the cooling rate after solidification to produce finer precipitation (Section 2 above). Furthermore, in the method for manufacturing parts using the steel of the present invention manufactured in this manner, the forging heating temperature is regulated to suppress the growth of Ti carbonitrides and the morphology of the structure is controlled in order to obtain high toughness. This is the main point (Section 3 above), or it is effective to reduce the number of times of heating (Section 5 above), and in order to control the morphology of the structure and produce parts with uniform quality,
Cooling in foam after forging is also effective (item 4 above)
. [Function] The reasons for the limitations of the present invention will be explained below. C: C is an element that forms various carbides and is dissolved in steel to determine the strength and toughness of parts, and 0.20% is required to obtain the necessary strength. If it is less than 0.02%, a large amount of alloying elements are required to obtain strength, which is uneconomical. Moreover, when C is added in excess of 0.60%, toughness decreases. Si: Si is an element that acts as a deoxidizer and strengthens the steel by forming a solid solution in the shell, and needs to be present in an amount of 0.10% or more. However, when added in an amount exceeding 2.00%, toughness decreases. Mn: Mn is an element that increases hardenability, refines the structure, and improves strength and toughness, and requires 0.50% or more, but if added in large amounts, a structure that is unfavorable for toughness will develop. Therefore, the upper limit was set at 2.00%. Cr: Like Mn, Cr is an element that improves strength and toughness by refining the structure. The effect cannot be expected if it is less than 0.10%, but if added in a large amount, the effect will be saturated and a structure that is unfavorable for toughness will likely develop, so the upper limit is set at 1.20%. V: v is a nitride that refines the structure during heating for forging, increases the toughness of as-hot-forged steel, and also reduces V during cooling after forging.
Significantly strengthens steel by precipitating as carbides1
0.03% or more is necessary for strengthening, but even if added in a large amount, the hardenability increases and becomes too hard, so the upper limit is set at 0.20%. Ti: Ti is an element that plays an important role in the present invention. That is, when Ti is used as a deoxidizing agent, it combines with zero when the steel is in a high temperature state after casting, becomes an oxide, and is finely dispersed in the steel. Furthermore, solid solution Ti remaining in the steel combines with N and C during cooling to become carbonitrides, and Ti carbonitrides tend to be more finely dispersed as the number of Ti oxides increases. Ti carbonitride has the effect of preventing coarsening of the austenite structure during heating for forging and refining the structure after forging, thereby significantly improving the toughness of as-forged parts. In order to exhibit the effect of preventing coarsening of the austenite structure, T
It is necessary for i to be 0.005% or more, but if it is added in excess of 0.070%, toughness and fatigue strength will decrease. Since Aβ:A2 is a strong deoxidizer, adding a large amount will prevent the formation of Ti oxide, which is important for the present invention. Therefore, AJ2 is limited to less than 0.005%. 0:0 is necessary to form Ti oxide and finely disperse it in the steel. In order to form a sufficient Ti oxide, 0.0010% or more is required, but O exceeding 0.0100% forms coarse inclusions and deteriorates toughness, machinability, etc. Most of the NUN becomes Ti nitride, which is finely dispersed in the steel and refines the austenite structure during heating for forging.Minimum of 0.005% of N is required to refine the austenite structure during forging heating. Since it is difficult to manufacture, the upper limit is set to 0.
020%. In claim (2), the cooling rate in the temperature range from the freezing point to 1000° C. during continuous casting determines the size of the Ti carbonitride. For this reason, J! This is important because it greatly affects the effect of preventing coarsening of the austenite structure when ultimately forging parts. Cooling rate from freezing point to 1000℃ 2
If it is less than 0°C/min, the Ti carbonitride becomes coarse and the effect of preventing coarsening of the austenite structure decreases.
In addition, coarse Ti carbonitrides are generated which are harmful to fatigue properties, machinability, etc. Therefore, when casting by continuous casting, the cooling rate in the temperature range from the freezing point to 1000°C is 20"C.
/win, and above. Since blooming rolling is usually performed after heating at high temperature for a long time, Ti carbonitrides grow, reducing the effect of preventing coarsening of the structure. Therefore, it is necessary to directly roll the slab into a steel bar without performing blooming. In claim (3), the heating temperature during forming by forging is 12 to suppress the growth of Ti carbonitrides and maintain the effect of preventing structure coarsening.
The temperature must be 70℃ or less. If forged by heating to a temperature exceeding 1270℃, the toughness will decrease, and the impact value required for, for example, automobile suspension parts (when using JIS B test pieces, 8 kgf-m/c+++ at room temperature
' ). Also, during cooling following forging, the transformation temperature range is 800°C.
It is necessary to cool the steel at a relatively high cooling rate in the temperature range from .degree. C. to 400.degree. C. to improve toughness. Therefore, 8
The average cooling rate in the temperature range from 00℃ to 40[1”C
, i-s, o°C/see. If the cooling rate is less than 0.1°C/sec, the hardness required for mechanical structural parts (Vickers hardness of 210 or more) cannot be obtained, and if it exceeds 540°C/sec, the hardness will become too high. . In claim (4), the advantage of cooling in the foam obtained by adding a foaming agent to water after forming by hot forging is that compared to other cooling methods, uniform cooling over the entire part is achieved. The structure can be controlled to be optimal for hot-forged, non-thermal treated steel. The foam can be made from an aqueous solution of a surfactant or a water-soluble polymer, and the cooling rate can be adjusted by changing the water content of the foam. The water content of the foam for pearlite and bainai is 90 g/10
It is necessary to be less than 0 m1, but l g / 1
If it is less than 00+sβ, the cooling rate is slow and the strength may decrease, so the lower limit is limited to l g / 100+mt2 or more. In claim (5), in order to finely precipitate Ti carbonitride, when casting by continuous casting, the temperature range from the freezing point to 1000°C is 20°C.
As mentioned earlier, the slab should be cast under conditions that result in a cooling rate of ℃/min. In order to produce a product, it is necessary to subject the slab to forging without going through a rolling process. 1 The additional heat temperature is T
i The temperature is limited to 1270° C. or lower to suppress the growth of carbonitrides. In addition, in order to compress defects in the slab, eliminate the influence of the casting structure on the material quality of the forged part, and make it highly tough, a working ratio of 2 or more is required. [Example] Example 1 An example regarding claim (1) is shown below. 150 kg of molten steel having the components shown in Table 1 was melted in a vacuum melting furnace, cast into an ingot, cooled, heated to 1230° C., and formed into a steel bar with a diameter of 30 mm. Using this steel bar as raw material, we heated it to 1200℃ for 2 hours to simulate the thermal history of hot forging when manufacturing Ill machine parts.
The mixture was heated for 0 minutes and allowed to cool. JIS No. U notch Charpy impact test pieces were cut out from these steel bars, and the impact values were measured. Also, the Vickers hardness in the cross section perpendicular to the longitudinal direction of the steel bar (measured load 10 kg)
was also measured. It is clear from Table 1 that the steel of the invention exhibits excellent impact values. Example 2 An example regarding claim (2) is shown below. Molten steel adjusted to the components shown in Table 2 (shell side of the present invention shown in claim +1) has a cross-sectional size of 162 x 162 mm. 247 x 300 mm, 350 x 56 Gm+a after casting with a continuous casting machine and cooling, 247 x 30
A slab measuring 0 mm, 350 x 560 mm was heated to a temperature of 1250°C and bloomed into a billet with a cross-sectional size of 162 x 162 mm. In addition, a part of the 162×162+nm slab was similarly heated to 1250℃ and heated to 120X.
It was bloomed into a billet with a cross section of 120 mm. The average cooling rate from the freezing point during casting to 1000℃ is 162
45℃/sec with 162mm slab, 247x3
25℃/see, 350 with a 00mm long piece
The temperature was 9°C/sec for one piece of 560mm. These billets were 1) heated to 50° C. and rolled into a steel bar with a diameter of 40 mm to obtain a steel bar for testing. This steel bar was used as a raw material, and was heated to 1200° C. for 20 minutes and allowed to cool in order to simulate the thermal history of hot forging when manufacturing mechanical parts. The manufacturing process is shown in Figure 1. JIS No. a U notch Charpy impact test pieces were cut out from these steel bars.
(The Ii impact value was measured. The Vickers hardness in the cross section perpendicular to the longitudinal direction of the steel bar (measured load 10 kg) was also measured. These results are shown in Table 3. In this case, it can be seen that the hardness and impact value are improved by cooling within the cooling rate range after casting shown in claim (2).Furthermore, a slab with a cross-sectional size of 162 x 162 + nn was directly rolled into a steel bar. Comparing the case where the steel bar is bloomed to -12 (lx 120ma+) and then rolled into a steel bar, higher hardness and toughness are obtained when the former step is taken, and claim (2)
It is clear that the properties of the final forged part are improved by manufacturing it with the method described in . Table 1 Example 3 An example related to claim (3) is shown below. The steels of the invention examples shown in Example 2 and Table 2 were cast using a continuous casting machine with a cross-sectional size of 162 x 162 mm and cooled. The average cooling rate to 1000℃ after casting is 44℃/
It was min. After cooling the #s piece to room temperature, 1)
A steel bar that was reheated to 50°C and rolled into a steel bar with a diameter of 70III1) was used as a forging material. These raw steel bars are heated to a temperature of 1210°C to 1300°C and forged into a knuckle spindle for automobiles, and then cooled to a temperature of 0.05-6 between 800°C and 400°C using blast cooling, slow cooling with a heat insulating material, etc. Cooling was performed at a cooling rate of .7°C/llln. After cooling, hardness test pieces and impact test pieces (JIS No. 3) were cut out from an automotive knuckle spindle.
An impact test was conducted. The influence of cooling rate on hardness and impact value is shown in Figure 2. In FIG. 2, the heating temperature was kept constant at 1200°C. From Figure 2, the cooling rate between 800 and 400°C is 0.
It can be seen that excellent hardness and impact value can be obtained by setting the temperature to 1-5.0°C/sec. In addition, in order to investigate the effect of forging heating temperature on hardness and impact value, after forging a knuckle spindle by changing the forging heating temperature, it was cooled at 1.0'C/min between 800 and 400°C, and the hardness and impact value were was measured. The results are shown in FIG. From the same figure, the forging heating temperature is 1
By setting the temperature to 270℃ or less, 8kgf-m/
It can be seen that the impact value is good (Ii) of c- or more. Example 4 An example related to claim (4) is shown below. The steel of the present invention shown in Table 2 above was X
It was cast using a 162++UA continuous KC casting machine, and the cooling rate from the freezing point to 1000°C was 44°C/min on average. After the slab was cooled to room temperature, 1) it was reheated to 50° C. and rolled into a steel bar with a diameter of 70 mm, which was used as a forging material. Table 4 shows the knuckle spindle, which was heated to 1230°C and then cooled, and the water-containing fltl-90g.
The mean hardness and standard deviation of the knuckle spindles cooled in bubbles of /100+nn are shown as 0. Bubble detection reduces the variation in hardness, resulting in a uniform material. However, 0.5 g / 100m12
When cooling in foam, the cooling rate is slow, resulting in a decrease in hardness. Table 1 Example 5 An example related to claim (5) is shown below. The steel shown in Table 2 has a cross section of 60 x 60 mm and 120 mm.
The poem was printed using a continuous casting machine of x 120mm. The cooling rate from the freezing point after casting to 1000℃ is 60 x 601)
125℃/5h with 1I slab, 120x 1
For a 20 mm slab, it was 82°C/sin. 60x60m+n 1 sushi piece is left as it is, and 120x
A 120mm slab is heated to 1250℃ and has a diameter of 70+n.
After rolling to +n and cooling, each was reheated to 1200°C and forged into a steel bar with a diameter of 50 mm and 40 n + m + 30 mm. Diameter 50mm, 40mm from 60x60mm slab,
30IllI1) The processing ratio (ratio of cross-sectional area) to the steel bar is 1.4, 2.3, and 4.0, respectively, and 120X
The total machining ratio from the 120mm tJ piece is 5.
8, 9.0.16.0. JIS No. 3 U notch Charpy impact test pieces were cut from these steel bars, and the impact value and Vickers hardness (measured load: 10 kg) in a cross section perpendicular to the longitudinal direction of the steel bars were measured. (Table 5) 60Xb generally has high toughness, but especially when the processing ratio is 2 or more, the toughness is good, and the impact value of conventional tempered steel is 8kgf-III/
It can be seen that more than c- is obtained. 120X 120t
In the case of a steel bar made by once blooming rolling an atan piece and forging it, the working ratio is large, but the impact value is less than 8 kgf1/c-. Table 5 [Effects of the Invention] As shown above, the high-toughness hot-forged non-tempered steel of the present invention and the steel bar produced by the manufacturing method of the present invention have excellent impact properties as hot-forged, and It is very useful as a non-thermal steel for forging. Furthermore, the method for manufacturing high-toughness non-tempered parts of the present invention is an effective method for manufacturing parts that have a high impact value and uniform hardness in an as-forged state.

[Brief explanation of drawings]

Figure 1 is a manufacturing process diagram of the test material steel bar, and Figure 2 is the Vickers hardness (Hv) and +20 when the knuckle spindle was heated to 1200°C in Example 2, forged, and then cooled at different cooling rates. Figure 3 shows the change in impact value (JIS No. 3 impact test piece) at °C. Figure 3 shows the Vickers hardness (Hv) and It is a figure showing the change in impact value (JIS No. 3 impact test piece) at +20°C.

Claims (5)

[Claims] (1) In weight%, C: 0.20-0.60, Si: 0.10-2.00,
Mn: 0.50-2.00, Cr: 0.10-1.20
, V: 0.03-0.20, Ti: 0.005-0.0
70, Al: less than 0.005, O: 0.0010-0.
0100, N: 0.005-0.020, and the remainder is Fe and unavoidable impurities. (2) When casting by continuous casting, 1000
℃ temperature range to 20℃/min. A high-toughness non-heat-treated steel bar for hot forging having the composition as set forth in claim (1), which is cast into a slab under the above cooling rate conditions and then immediately rolled into a steel bar without blooming. Production method. (3) The steel bar manufactured by the manufacturing method according to claim (2) is heated to a temperature of 1270°C or less, and after being forged into mechanical parts, the temperature range from 800°C to 400°C is 0.1-5.0°C on average.
°C/sec. A method for manufacturing high toughness hot forged non-thermal steel parts characterized by cooling at a speed range of . (4) After heating the steel according to claim (1) to a temperature of 1270°C or less and forming it by hot forging, a foam having a water content of 1 to 90 g/100 ml is obtained by adding a foaming agent to water. A method for producing a high-toughness hot-forged non-tempered steel part, which is cooled in a medium and has a ferrite-pearlite structure. (5) When casting by continuous casting, 1000% from the freezing point
℃ temperature range to 20℃/min. The component according to claim (1), characterized in that it is cast into a slab under conditions that provide a cooling rate of at least 1,250° C. or less, and then heated to a temperature of 1,250° C. or less and formed into a machine part at a processing ratio of 2 or more. A method for producing high toughness hot forged non-thermal steel parts.