JPH0579729B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention has a tensile strength of 70 kg by direct quenching.
f/mm ² or more and a yield ratio of 90% or less, the method of manufacturing high-toughness, low-yield-ratio high-strength steel plates, with the aim of increasing the deformation of steel plates and increasing the safety of steel structures. The aim is to achieve advantageous application to bridges, architecture, penstocks, pressure vessels, etc. In general, mild steel with a tensile strength of 40Kgf/ ^mm2 class has a low yield ratio of about 60 to 70%, but as the tensile strength of steel increases, the yield ratio tends to increase. High-strength steel with a tensile strength of 70 to 110 Kgf/mm ² , which has been increasingly used in recent years, usually has a yield ratio as high as 90% or more, so it is an important consideration when designing buildings. The yield ratio is considered to indicate the margin from when a steel plate yields until it breaks, and the lower the value, the greater the deformation, and the greater the uniform elongation and total elongation, which is advantageous in terms of the safety of steel structures. It is clear that Furthermore, from the perspective of improving the fatigue properties of steel structures, there is a need for the development of high-strength steel plates with low yield ratios. (Prior technology) In order to manufacture high-strength steel with a tensile strength of 70 to 110 Kgf/ ^mm2 , it is necessary to make the structure mainly martensite in order to ensure its strength, but if as-quenched, the toughness is low and the plate The strength in the thickness direction is uneven.
Therefore, conventionally, tempering treatment at a temperature of about 600°C after quenching has been used to improve the toughness of steel plates and make the strength uniform in the thickness direction. It is an unavoidable problem that the ratio is high, exceeding 90%. As an attempt to solve this problem, it has been considered to omit the tempering process and utilize the low yield ratio of as-quenched steel sheets, but as mentioned above, simply as-quenched steel sheets have low toughness, especially in the thickness direction. Because of the non-uniform strength, steel plates that can be put to practical use have not yet been manufactured. In addition, an attempt was made to lower the yield ratio by creating a two-phase mixed structure in which ferrite was mixed with martensite using a two-phase region quenching method.
It has been reported about mm ² class high tensile strength steel ("Basic characteristics of low yield ratio 80 kg class high strength steel and its welded parts", Journal of the Welding Society of Japan, 3-3, 1985 (reference)). However, in this case as well, since ferrite is soft, in order to obtain the same strength as conventional hardened and tempered steel sheets, it is necessary to increase the carbon equivalent compared to conventional steel, and this requires welding of steel sheets, which is most important when constructing steel structures. The disadvantage of increased cracking susceptibility is inevitable. (Problems to be solved by the invention) Quenching and tempering of conventional 70 to 110 Kgf/mm grade ² steel, which has a carbon equivalent equal to or less than that of the tempered steel, has the same strength, and has a yield ratio of less than 90%. In addition to making it possible to manufacture high-strength steels with low yield ratios,
It is an object of the present invention to reduce the strength distribution difference in the thickness direction observed in conventional as-quenched steel. (Means for Solving the Problems) This invention includes: C: 0.04 to 0.14 wt% (hereinafter simply indicated in %), Si: 0.03 to 0.20%, Mn: 0.60 to 1.40%, Cr: 0.30 to 1.20%, Mo: 0.30 to 1.20%, V: 0.03 to 0.10%, Al: 0.02 to 0.09%, B: 0.0003 to 0.003% and N: 0.0045% or less, and the carbon equivalent Ceq. expressed by the following formula is 0.38 to 0.38
A steel slab containing 1000 to 1250
After heating to ℃, the rolling finishing temperature on the steel plate surface is 780 ~
Tensile strength of 70 Kgf/mm ² or more, yield ratio by direct quenching method, characterized by hot rolling at 850℃ and quenching starting within 150 seconds after the end of the rolling.
A method for manufacturing a high tensile strength steel plate with a low yield ratio of 90% or less (first invention). Ceq.=(C)+1/24(Si)+1/6(Mn)+1/5
(Cr) + 1/4 (Mo) + 1/40 (Ni) + 1/14 (V) (The element symbol in the formula is the alloy component content (%)) Further, in the first invention, this invention further includes Ni. This is a method for manufacturing an extra-thick high-strength steel plate (second invention) using a steel slab containing 3.20% or less as a starting material. The basis of the idea of this invention is roughly as follows. 1 After rolling and finishing at a low temperature in which unrecrystallized austenite is the main component, the martensite is strengthened by direct quenching, and at this time, the hardenability improving effect of B is particularly effectively utilized to achieve a low carbon equivalent. Quantify. 2. Reduce strength changes in the plate thickness direction by selecting an appropriate rolling finishing temperature where unrecrystallized austenite is the main component. 3. Find out the chemical composition necessary to achieve high toughness with the above direct quenching process, and use the composition design based on it. The inventors melted a large amount of steel and created a slab of it.
After heating to various temperatures of 950-1250℃, 950-750℃
By finishing rolling at various temperatures, holding at that temperature for a predetermined period of time, and then quenching,
The effects of finishing rolling temperature and the time from completion of rolling to quenching on the strength and toughness of steel sheets were investigated in detail. As a result, when direct quenching is performed after rolling in the recrystallized austenite temperature range, sufficient toughness cannot be obtained with quenching, regardless of whether the time from completion of rolling to quenching is short or long. It has been found that there is only a very narrow range of suitable times to obtain toughness. On the other hand, when quenching is performed after finishing rolling in a temperature range in which unrecrystallized austenite is the main component, even if the holding time after finishing rolling and quenching changes or even if the holding time is considerably long, quenching is still sufficient. A steel plate with low yield ratio and high toughness was obtained. In the case of rolling mainly composed of unrecrystallized austenite, the behavior of B greatly contributes to the fact that high strength and toughness can be stably obtained even if the time from completion of rolling to quenching changes. . In other words, when B exists at austenite grain boundaries, it lowers grain boundary energy and improves hardenability.
When the rolling temperature is low, the movement speed of the austenite grain boundaries is low and the diffusion speed of B is also low, so that B can stably exist at the austenite grain boundaries for a long time.
Furthermore, in the case of low-temperature rolling, strain is introduced into the austenite, and this strain is carried over into the martensite, which has the effect of increasing strength. Another reason why the toughness is good even after quenching is that the unrecrystallized austenite is deformed as shown in the metallographic micrograph in Figure 1, and its diameter is small, so it is difficult for the martensite to become fine. Can be mentioned. As an example, the following chemical components (%), C/0.10,
Si/0.14, Mn/0.94, Ni/0.85, Cr/0.51,
Mo/0.46, V/0.051, Al/0.040, B/0.001,
Figure 2 shows the results of investigating the effects of these changes on the strength and toughness of the steel plates produced by manufacturing steel plates with a thickness of 50 mm by varying the rolling finishing temperature and the time until direct quenching for steel slabs with a diameter of N/0.0035. Shown below. As is clear from the figure, in the case of a recrystallized austenite structure, the strength changes greatly depending on the time from completion of rolling to direct quenching as described above, and it is recognized that the toughness is poor in the case of low strength. On the other hand, in the case where the unrecrystallized austenite was the main component, no changes in strength and toughness over time were observed until the test was held for 150 seconds, and both showed sufficient values. The yield ratio is also low at less than 85%. Figure 3 shows the hardness distribution in the thickness direction of a steel plate obtained by rolling the above-mentioned steel slab mainly in the unrecrystallized austenite region and directly quenching it. The hardness distribution of the as-quenched material when the reheating and quenching method was applied is also shown. In the former case, the hardenability is high at the center of the sheet thickness and the surface layer of the steel sheet is hardened because the temperature at the center of the sheet is high and the temperature is lower as it approaches the surface layer of the steel sheet. Hardenability deteriorates as the area increases. On the other hand, the cooling rate during quenching is slower at the center of the plate thickness. Therefore, these elements are in harmony and have uniform hardness (in other words, strength) as shown in the figure. (Function) Next, the reason for limiting each component will be described. C is the most effective component for strengthening martensite, but if it is less than 0.04%, the strengthening effect is small, and in order to obtain strength, it is necessary to add large amounts of other alloy components, which is not preferable. On the other hand, if it exceeds 0.14%, martensite becomes brittle and its toughness deteriorates. Si has a role as an alloying element in addition to acting as a deoxidizing agent, and is an extremely important component in this invention because it affects the precipitation of carbides.
If the Si content is less than 0.03%, no effect as a deoxidizing agent can be obtained, and on the other hand, if the amount exceeds 0.20%, the effect of improving toughness by reducing the Si content cannot be expected. Another major feature of the results shown in FIG. 3 is that it is significantly affected by Si, which needs to be in the range of 0.03 to 0.2%. Mn is required to be at least 0.60% to ensure strength, but if it exceeds 1.40%, weldability and workability deteriorate, so the content should be in the range of 0.60 to 1.40%. If Cr is less than 0.30%, the effect of increasing strength is poor, while if it exceeds 1.20%, carbides will precipitate during direct quenching, contributing to deterioration of toughness. Mo is necessary from the viewpoint of improving hardenability and grain size regulating effect, and 0.3% or more is required to obtain this effect. However, if it exceeds 1.2%, the effect decreases, so it is limited to 0.3 to 1.2% from the economic point of view. V is necessary to reduce the softening degree of the weld heat affected zone, but if it is less than 0.03%, this effect will hardly be obtained, while if it exceeds 0.10%, it will deteriorate the toughness of the heat affected zone, which is undesirable. Limited to ~0.10%. Al is necessary as a deoxidizing agent, but if it is less than 0.02%, the deoxidizing effect will be small, while if it exceeds 0.09%, the toughness will be significantly reduced. Therefore, the addition range is limited to 0.02 to 0.09%. B is an extremely important component because it improves the hardenability of steel sheets even in extremely small amounts. In particular, it can be said to be the most important component in the development of the steel of this invention. However, if the amount added is less than 0.0003%, no effect of improving hardenability due to B can be expected;
If it exceeds 0.003%, B precipitates are formed, which not only reduces the amount of B effective for improving hardenability, but also B precipitates themselves deteriorate hardenability, which is not preferable. N forms nitrides such as BN, which not only reduces the hardenability improvement effect of B but also causes toughness deterioration, so it is preferable to reduce it as much as possible.
When the content is 0.0045% or less, B can work effectively without impairing the toughness of the steel plate. Next, if the carbon equivalent Ceq. is less than 0.38%, 70Kg
It becomes difficult to simultaneously obtain a tensile strength of f/mm ² or more and good toughness, and the weld heat affected zone becomes softened. On the other hand, when Ceq. The essential components have been explained above, but in this invention, for the purpose of ensuring toughness, in the range of 3.20% or less
Ni can be contained. Here is the amount of Ni
The reason why it is limited to 3.20% or less is because even if it is added in a large amount exceeding 3.20%, no further improvement in toughness can be expected, but rather it impairs economic efficiency. Furthermore, in order to avoid variations in material quality due to changes in slab heating temperature and to manufacture steel plates of stable material, it is necessary to limit the slab heating temperature. In this invention, when the heating temperature exceeds 1250°C, strength is obtained by improving hardenability due to coarsening of austenite grains, but toughness deteriorates; on the other hand, when the slab heating temperature is less than 1000°C, In this case, the austenite grains become finer, and the resulting deterioration of hardenability causes a decrease in strength, and along with this, the toughness also deteriorates. Therefore, in order to produce steel plates with stable strength and toughness, the temperature of the slab before rolling must be 1000-1250℃.
need to be heated to a temperature range of The finishing temperature of rolling also has a large effect on the material quality of the steel sheet. The rolling finishing temperature is 780 as the steel plate surface temperature.
A range of ~850°C is suitable. This is because, in this invention, it is important to carry out rolling at a low temperature where unrecrystallized austenite is the main component, and to ensure fine grains and hardenability. As the temperature increases, the hardenability of steel decreases, and as shown in Figure 4, the strength and
On the other hand, if the surface temperature exceeds 850℃, the temperature in the center of a thick steel plate will decrease.
This is because the temperature reaches 900°C or higher, resulting in large recrystallized austenite grains, which also leads to deterioration of toughness. Figure 4 shows C/0.10, Si/0.14, Mn/0.94, Ni/
0.85, Cr//0.51, Mo/0.46, V/0.051, Al/
A steel slab of 0.040, B/0.001, N/0.0035,
After heating to 1150℃, plate thickness is adjusted at various finishing temperatures.
The results of an investigation of the relationship between the steel sheet surface temperature and strength and toughness during finish rolling of steel materials obtained by rolling to 50 mm and then directly quenching after 60 seconds are shown. (Example) Table 1 shows a total of 3 steel ingots, including a steel ingot with a chemical composition according to the present invention and a steel ingot with a chemical composition outside the preferred composition range.
After melting a steel ingot and manufacturing steel plates with a thickness of 50 mm by varying the slab heating temperature, rolling finishing temperature, and time from the end of rolling to quenching, each steel plate was measured at the 1/4 and 1/2 thickness positions. YS, TS and −
This is the result of examining the absorbed energy of 2 mmV notch at 60°C.

【table】

[Table] Chemical composition, slab heating temperature, rolling finishing temperature,
If the time from the end of rolling to quenching is both within the scope of this invention, the plate thickness is 1/4 and 1/2.
A high tensile strength steel plate with a low yield ratio and good strength and toughness was obtained at any position. On the other hand, if any one of the above manufacturing conditions is lacking, the strength and toughness at the 1/2t position will be low even if the chemical composition is the same, and even if strength is obtained, the toughness at the 1/2t position will be extremely low. Furthermore, even if the manufacturing conditions were within the range of this invention, if the chemical composition was outside the range, the toughness was low at both the 1/4t and 1/2t positions. (Effect of the invention) The safety of steel structures such as bridges, buildings, marine structures, penstocks, and pressure vessels that use class ² high tensile strength steel with a tensile strength of 70 to 110 Kgf/mm can be improved. It can be widely applied to the following fields.

[Brief explanation of the drawing]

Figure 1 is a metallographic micrograph showing the deformation of unrecrystallized austenite, and Figure 2 shows the effects of finishing rolling temperature and time from completion of rolling to direct quenching on the strength and toughness of steel sheets. The graph, FIG. 3, is a comparison graph showing the distribution of hardness in the sheet thickness direction, and FIG. 4 is a graph showing the influence of rolling finishing temperature on the strength and toughness of the steel sheet.

Claims (1)

[Claims] 1 C: 0.04-0.14wt%, Si: 0.03-0.20wt%, Mn: 0.60-1.40wt%, Cr: 0.30-1.20wt%, Mo: 0.30-1.20wt%, V: 0.03 ~0.10wt%, Al: 0.02~0.09wt%, B: 0.0003~0.003wt% and N: 0.0045wt% or less, and the carbon equivalent Ceq. expressed by the following formula is 0.38 ~
A steel slab containing Fe within a range satisfying 0.65wt%, with the remainder being substantially Fe, from 1000 to
After heating to 1250℃, the rolling finishing temperature is reached at the steel plate surface.
After hot rolling to a temperature of 780 to 850℃,
Characterized by starting quenching within 150 seconds,
A method for producing a high tensile strength steel plate with a tensile strength of 70 Kgf/mm ² or more and a yield ratio of 90% or less using a direct quenching method. Ceq.=(C)+1/24(Si)+1/6(Mn)+1/5
(Cr) + 1/4 (Mo) + 1/40 (Ni) + 1/14 (V) (Element symbols in the formula are alloy component content (wt%)) 2 C: 0.04 to 0.14 wt%, Si: 0.03 to 0.20wt%, Mn: 0.60-1.40wt%, Cr: 0.30-1.20wt%, Mo: 0.30-1.20wt%, V: 0.03-0.10wt%, Al: 0.02-0.09wt%, B: 0.0003-0.003wt %, N: 0.0045wt% or less and Ni: 3.20wt% or less, carbon equivalent Ceq. expressed by the following formula is 0.38 ~
A steel slab containing Fe within a range satisfying 0.65wt%, with the remainder being substantially Fe, from 1000 to
After heating to 1250℃, the rolling finishing temperature is reached at the steel plate surface.
After hot rolling to a temperature of 780 to 850℃,
Characterized by starting quenching within 150 seconds,
A method for producing a high tensile strength steel plate with a tensile strength of 70 Kgf/mm ² or more and a yield ratio of 90% or less using a direct quenching method. Ceq.=(C)+1/24(Si)+1/6(Mn)+1/5
(Cr) + 1/4 (Mo) + 1/40 (Ni) + 1/14 (V) (Element symbols in the formula are alloy component content (wt%))