JPH0663026B2 - Manufacturing method of high strength and high toughness boron-added thick steel plate by direct quenching process. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) Related to obtaining a thick steel sheet excellent in strength and toughness by a direct quenching / tempering step of quenching in a cooling process after hot rolling and then tempering. The technical contents of this specification based on the results of the developed research are located in the technical field related to the production of so-called tempered thick steel plate.

As a method of manufacturing a thick steel sheet having excellent strength and toughness, generally, tempering heat treatment of reheating, quenching and tempering after rolling has been carried out. Alternatively, for the purpose of continuity, it is useful to use the thermal energy applied to slab heating as it is for quenching after rolling.

In this direct quenching and tempering process (DQ-T), compared to the conventional reheating quenching and tempering process (RHQ-T), in addition to the above process advantages, the heating temperature is high and quenching is performed in the cooling process. Therefore, it is generally said that at the time of quenching, the alloy components of the steel are sufficiently dissolved and the austenite is homogeneous, so that the quenchability is improved.

However, the advantage of this increase in the amount of solid solution elements is that it was brought about by quenching while the alloy elements were in a supersaturated solid solution state at the end of rolling, and the non-equilibrium state was used. It was done. Therefore, in the DQ-T process, the time from the end of rolling to the quenching greatly affects the material of the steel.

Therefore, in the actual manufacturing process, even in the same steel sheet, there is a time lag in entering the quenching device, which causes material variations, which is an important point in using the DQ-T process.

(Prior Art) As a solution to this problem, in Japanese Patent Publication No. 58-3011, the time from the end of rolling to quenching is limited, but in tempered high-strength steel with a strength level of 60 kgf / mm ² or more, boron is used. Although it is an indispensable condition to effectively utilize and secure the hardenability, this study has not been added.

Further, in the specification of Japanese Patent Application No. 59-133085 (published in Japanese Patent Publication No. 5-33286), which the inventors previously proposed, this is a countermeasure focusing on the behavior of boron.
Strict conditions are added to the l, B, and N amounts.

A conventional technique has not been proposed yet for solving the variation in the material due to the DQ condition that relaxes the compositional constraint, especially in the case of boron-added steel.

(Problems to be Solved by the Invention) Regarding the behavior of boron in the DQ process, which is closely related to the hardenability of steel, the inventors have found that the amounts of Al, B and N in the boron-added steel have a significant effect. It was clarified based on various experimental results. However, the limitation of the component amount obtained by this is very strict because the products of Al amount and N amount and B amount and N amount are within a very narrow range.

The inventors have further investigated the behavior of boron from the relationship with the DQ condition, and as a result, if the slab heating temperature and the finishing temperature are appropriately selected, the boron is effective in a very wide range of Al × N and B × N. It was found to be available.

Based on this finding, by optimizing the DQ conditions when manufacturing boron-added high-strength steel in the DQ-T process and stably using B, it is possible to solve the variation in the material derived from the DQ process. It is the purpose of the invention.

(Means for Solving Problems) The above-mentioned purpose is achieved by a procedure having the following points as the main points.

1. C: 0.04 to 0.15 wt%, Si: 0.01 to 0.30 wt%, Mn: 0.10.
-2.0 wt% and Al: 0.02-0.15 wt%, B: 0.0003-0.0030 wt%, and N:
0.0010-0.0100wt%, Al (wt%) × N (wt%)> 10 ^-4 and / or B (wt%) ×
Boron-added low alloy steel containing N (wt%)> 2.5 × 10 ^-6 is used as a raw material, heated to a temperature of 1180 ° to 1300 ° C, hot-rolled, and finished rolling at 850 ° C or higher. After completion of rolling, in the process of holding at a temperature close to the rolling finishing temperature or cooling following the completion of rolling, the time of 1 minute or more and 6 minutes or less in the temperature range of 850 ° C. or higher is waited for 300 minutes.
A method for producing high strength and high toughness boron-added thick steel sheet by direct quenching process, which is characterized by the combination of rapid cooling to a temperature of ℃ or below and subsequent tempering.

The basis of the completion of the present invention is that the relationship between the behavior of boron from the end of rolling to the quenching and the quenching effect of steel was clarified as follows.

(Operation) Now, regarding the time setting from the end of rolling of the boron-added steel to the quenching, set appropriate conditions in consideration of changes in boron, which is an essential component for the hardenability of the steel, during that period. There is a need.

That is, during this holding, boron is considered to undergo changes such as diffusion of solid solution B into austenite grain boundaries and precipitation of segregated boron nitride.

Since such a change in boron occurs depending on the elapsed time after rolling, it is considered that the hardenability of steel also differs depending on the degree of this change. Particularly, the precipitation of boron nitride and the like is an important change because it directly leads to deterioration of the hardenability of steel.

The inventors changed the slab heating temperature and the holding temperature after rolling for the boron-added low alloy steel, and examined the change in tensile strength with the time from the end of rolling to the quenching as a parameter. Table 1 shows the steel composition for this experiment.

As DQ conditions, the slab heating temperature is 1220 ℃, and the holding temperature after hot rolling is 900.
When the temperature is set to ℃ (a), when the holding temperature is lowered to 850 ℃ and 800 ℃ (b),
(C) When the slab heating temperature was lowered to 1150 ° C (d) DQ was performed for each, and then tempering treatment was performed under the condition of 630 ° C for 40 minutes. Fig. 1 collectively shows changes in the tensile strength depending on the holding time from the end of rolling to the quenching.

Under any of the DQ conditions, the austenite grains are recrystallized within 1 minute after hot rolling, resulting in improvement in hardenability. However, if the holding time after that becomes longer, it behaves differently depending on the DQ condition. That is, when the slab heating temperature is as high as 1220 ° C, the strength does not decrease even if the holding time is 3 minutes or more and stable high strength is maintained, while the slab heating temperature is lowered as 1150 ° C. A sharp drop in strength is observed when the holding time is about 2 minutes. Similarly, when the holding temperature drops from 850 ° C to 800 ° C, the strength decreases remarkably with the holding time.

Based on the above results, in the present invention, the slab heating temperature is set to 1180 ° C. or higher to obtain the solid solution effect of B, and 1300 ° C. or lower to suppress coarsening of austenite grains,
The rolling finishing temperature is 850 ° C or higher to suppress BN precipitation during rolling, and 950 ° C or lower to suppress coarsening of recrystallized γ grains to improve toughness, and the holding temperature after rolling is BN after rolling.
The temperature was limited to 850 ° C or higher in order to suppress the precipitation of γ and cause recrystallization of γ grains.

Fig. 2 (a) shows the results of the boron distribution when the slab heating temperature was 1220 ℃ and 1150 ℃ both when the holding temperature was 900 ℃ and the holding time was 2 minutes by the fission track etching method. Shown in (b).

As is clear from this, by setting the slab heating temperature within the range of the present invention, it is found that the precipitation of boron during holding after rolling is significantly suppressed. This also leads to securing the solid solution B in order to effectively use boron. However, even if the slab heating temperature is set to 1180 ° C or higher, unless the holding temperature after rolling is set to 850 ° C or higher, precipitation of boron occurs and hardenability deteriorates. That is, by setting the slab heating temperature and the holding temperature within the range of the present invention, it becomes possible to secure the solid solution B for a long time after rolling, and the quenching effect of B can be effectively utilized.

In the present invention, the above DQ condition is further limited to 1 minute or more and 6 minutes or less as the time from the end of rolling to the quenching. However, if it is less than 1 minute, boron is sufficient in the recrystallization of austenite grains immediately after rolling. Since hardenability deteriorates because segregation at the austenite grain boundaries is not possible, recrystallization is completed, and time is provided for B to segregate at the grain boundaries. However, the time of more than 6 minutes is not necessary because the production efficiency decreases, the effect does not increase. Quenching is necessary to completely complete the martensitic transformation and obtain a good quenching structure.
It is necessary to rapidly cool to below 300 ° C.

In the present invention, the use of boron is indispensable, and therefore B is required to be 0.0003 wt% or more. Further, Al is required to be 0.02% or more in order to obtain deoxidizing and fine grain effects.

However, addition of B exceeding 0.0030wt%, A exceeding 0.15%
No addition of l was found to enhance the effect, and on the contrary, the weldability was deteriorated, so the upper limit was set to the above value.

Further, it is not preferable that the amount of N exceeds 0.01 wt% because it impairs the toughness of the base metal, but it is difficult to make it less than 0.001 wt% from the steelmaking technology.

This invention eliminates the need to limit the values of (Al, N) or (BN) in the specification of Japanese Patent Application No. 59-133085 by setting the heating temperature to a value within the specified range, and is described in the prior application. The above ranges of component products are excluded from the scope of the present invention. That is, (Alwt%) × (Nwt%)> 10 ⁻⁴ and / or (Bwt
%) × (Nwt%)> 2.5 × 10 ⁻⁶ , B can be effectively used in DQ by applying the present invention.

In general, the strength level of steel sheet that contains a large amount of B is 70 kg.
f / mm ² or more, and the strength level is 70 kgf in this invention as well.
The following C, Si, and Mn are indispensable in order to achieve a value of / mm ² or more.

C is an essential component for ensuring hardenability and strength,
0.04wt% or more is required. But 0.15w for weldability
It must be below t%.

Si is added for deoxidation and securing strength, but if it is less than 0.01 wt%, its effect is not sufficient, and if it exceeds 0.3 wt%, toughness deteriorates.

Mn is added in an amount of 0.1 wt% or more to ensure hardenability of steel, but 2.0 wt% is added so as not to adversely affect toughness and weldability.
Below.

Furthermore, commercially available steel sheets of 70 Kgf / mm ² or more have Cu, Ni, Cr, Mo,
Each element such as V, Nb, and Ti is added. In the present invention as well, since there is no adverse effect on the effective use of B, one or two or more of each element can be added within the desirable range described below depending on the required performance of the steel sheet.

Cu is added in an amount of 0.1 wt% or more when it is expected that the strength will be improved. However, addition of Cu in excess of 0.6 wt% is not suitable in terms of toughness and weldability.

Ni is also 0.1 w to secure hardenability and improve low temperature toughness.
Although t% or more may be added, addition of more than 3.0 wt% is not appropriate from the economical aspect.

Cr is an element that improves hardenability, oxidation resistance and high temperature strength, and may be added in an amount of 0.1 wt% or more, but is set to 0.7 wt% or less so as not to deteriorate weldability and base material toughness.

Mo may be added in an amount of 0.01 wt% or more in order to secure hardenability, but it is 0.70 wt% or less in terms of weldability and cost.

V may be added in an amount of 0.01 wt% or more to secure the strength,
It should be 0.10 wt% or less for the base metal toughness and weldability.

Nb can be added in an amount of 0.01 to 2.0 wt% to secure the strength.

Ti has the effect of fixing N as TiN, and can be added in an amount of 0.001 wt% or more in order to increase the strength.
Addition of more than 0.1 wt% is disadvantageous because of base metal toughness.

In the present invention, after direct quenching, a tempering process is performed in order to adjust a sufficient toughness level and strength within a required range.

(Example) Using a steel having the composition shown in Table 2 as a test material, a block having a plate thickness of 100 mm was heated at 1250 ° C or 1150 ° C for 1 hour and rolled to a plate thickness of 30 mm. By changing the rolling finish temperature from 900 ℃ to 800 ℃,
After the rolling was completed, it was placed in a furnace prepared separately and set to a temperature equal to the rolling finishing temperature and held for 60 seconds or 360 seconds.

Then, it was rapidly cooled to 150 ° C at a cooling rate of 10 ° C / s between 800 and 500 ° C.

Next, the tempering was performed at 630 ° C. for 40 minutes.

After the heat treatment, a Charpy test and a tensile test were performed. The results are shown in Table 3.

Only in the case according to the present invention (slab heating temperature, finishing temperature), strength even after holding for 6 minutes after rolling,
The toughness was excellent, but under the remaining conditions, the strength was sharply reduced and the toughness was also deteriorated after holding for 6 minutes.

As is clear from the above examples, according to the method of the present invention, DQ
It can be seen that the material of the high-strength and high-toughness steel by the −T process can be manufactured at a high level and the “variation” can be minimized.

This shows that a high-quality thick steel plate can be manufactured without "variation" in material due to "deviation" in time when entering the quenching device at the top and bottom of the rolled plate.

Although the above-mentioned embodiment describes the case of holding for 1 minute or more at a temperature equal to the rolling finishing temperature, in addition to this, the holding is performed at 850 ° C. in the cooling process after the rolling and before the quenching for quenching. The same effect can be obtained by setting the elapsed time up to 1 minute or more.

(Effect of the invention) When B-added steel with a wide range of components is manufactured by the DQ-T process,
There is no variation in strength and toughness that often occurs in the past, and thick steel plates with high levels can be stably obtained.

[Brief description of drawings]

Fig. 1 is a comparative graph showing the change in tensile strength depending on the time from the end of rolling to the quenching, and Fig. 2 (a) and (b) show the boron distribution change after the slab heating temperature 2 minutes after rolling. It is a metallographic micrograph which shows the influence.

Claims (1)

[Claims] 1. C: 0.04 to 0.15 wt%, Si: 0.01 to 0.30 wt
%, Mn: 0.10 to 2.0 wt%, Al: 0.02 to 0.15 wt%, B: 0.0003 to 0.0030 wt%, and N: 0.0010 to 0.0100 wt%, Al (wt%) × N (wt%) > 10 ⁻⁴ and / or B (wt
%) × N (wt%)> 2.5 × 10 ⁻⁶ , using a boron-containing low alloy steel contained as a material, heating it to a temperature of 1180 ° to 1300 ° and hot rolling it, and ending the rolling at 850 ° C. or higher. After completion of this rolling, in the process of holding at a temperature close to the rolling finish temperature or cooling subsequent to the completion of rolling, a time of 1 minute or more and 6 minutes or less in a temperature range of 850 ° C. or higher, 300
A method for producing high-strength, high-toughness boron-added thick steel sheet by a direct quenching process, which is characterized by rapid cooling to a temperature of ℃ or below, followed by tempering.