JPS62139815A - Manufacture of high strength and toughness thick steel plate with added boron by direct quenching process - Google Patents

Abstract

PURPOSE:To manufacture thick steel plate superior in strength and toughness, by hot rolling low alloy steel material with B added having a specified compsn. under a specified condition, thereafter, heat treating the plate. CONSTITUTION:Slab of low alloy steel with B added contg. by weight, 0.04-0.15% C, 0.01-0.30% Si, 0.10-2.0% Mn and 0.02-0.15% Al, 0.0003-0.0030% B, 0.0010-0.100% N under Al(%)XN(%)>10<-4> and B(%)XN(%)>2.5X10<-6> of further one or >=2 kinds among <0.6% Cu, <3.0% Ni, <0.7% Cr, <0.7% Mo, <0.1% V, <2.0% Nb, <0.1% Ti is heated to 1,180-1,300 deg.C. This is hot rolled to thick steel plate so that rolling is finished at >=850 deg.C, the plate is held at temp near rolling finish temp., or held at >=850 deg.C temp. range during cooling after hot rolling for the lapse of 1-6min, then rapidly cooled to <=300 deg.C, then temper treated.

Description

[Detailed Description of the Invention] (Industrial Application Field) Related to obtaining thick steel plates with excellent strength and toughness through a direct quenching/tempering process in which quenching is performed during the cooling process after hot rolling, and then tempering is performed. The technical content of this specification, which is based on the results of the development research conducted by the present inventors, is located in the technical field related to the production of so-called tempered thick steel plates.

The general method for manufacturing thick steel plates with excellent strength and toughness is to heat and quench the steel again after rolling.

Tempering heat treatment has been used for tempering, but it has not been possible to directly use the thermal energy applied to slab heating for post-rolling quenching in order to omit or make the manufacturing process more continuous. Useful.

In this direct quenching and tempering process (DQ-T),
In addition to the process advantages mentioned above, compared to the conventional reheating quenching and tempering process (RHQ-T"), the heating temperature is high and quenching occurs during the cooling process, so the alloy components of the steel are sufficiently hardened during quenching. It is generally said that the advantage of improved hardenability is that the austenite is homogeneous.

However, this advantage of increasing the amount of solid solution elements is brought about by quenching while the alloying elements remain in a supersaturated solid solution state at the end of rolling, making use of the non-equilibrium state. This is what I did. Therefore, in the DQ-T process, the time from the end of rolling to quenching greatly affects the material quality of the steel.

Therefore, in the actual manufacturing process, even within the same steel plate, the time lag in entering the quenching device causes variations in material quality, and this is an important point when 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 boron is Despite the fact that effective utilization and ensuring hardenability are essential conditions, this consideration has not been taken into account.

In addition, the patent application No. 59-13308 previously proposed by the inventors
The specification of No. 5 focuses on the behavior of boron, but for this reason, strict conditions are added to the amounts of Al, B, and N.

No prior art has yet been proposed for solving material variations, particularly in boron-added steel, by using DQ conditions that relax this component restriction.

(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, kl of boron-added steel.

The inventors have clarified based on detailed experimental results that B and Nfi have a significant effect.

However, the limit on the amount of components determined by this is the amount of Al and N
It was very difficult to keep the amount and the product of the amount of B and the amount of N within a very narrow range.

The inventors further investigated the behavior of boron in relation to the DQ conditions, and found that if the slab heating temperature and finishing temperature are appropriately selected, one shift of A'A×N and B×N can be achieved in a very wide range. It was discovered that boron can be used effectively.

Based on this knowledge, it is possible to solve the variation in material quality caused by the DQ process by optimizing the OQ conditions when manufacturing boron-added high-strength steel using the DQ-T process and stably utilizing B. This is the object of the invention.

(Means for resolving the problem) The above purpose will be achieved through the steps outlined below.

1. C: 0.04-0.15wt%, Si:
0.01-0.30wt%.

Mn: 0. II) ~2. Contains 0wt%,
Al: 0.02-0.15wt%, B: 0.0
003-0.0030wt%.

and N: 0.0010 to 0.0100 wt%, 1
(wt%)×N(wt%) >10-' and/or B(
wt%) × N (wt%) > 2.5
Finishing rolling at 850°C or higher; After this rolling, holding at a temperature close to the finishing rolling temperature or during the cooling process following the completion of rolling, rolling in a temperature range of 850°C or higher for 1 minute or more and 6 minutes. Wait for up to 3 minutes
A method for producing a high-strength, high-toughness, boron-added thick steel plate by a direct quenching process, characterized by combining rapid cooling to a temperature of 00°C or lower and then tempering (first invention).

2. C: 0.04-0.15wt%, Si: 0
．． 01-0.30wt%.

Mn: 0.10-2.0 wt%, [:u:0.6
wt% or less, Ni: 3.0wt% or less, [:r:0
．． 7 wt% or less, Mo: 0.7 wt% or less.

Contains one or two of V: 0.1 wt% or less, Nt]: 2.0 wt% or less, Ti: 0.1 wt% or less, and further Aj2: 0.02 to 0.15 wt%, B
: 0.0003 to 0.0030wt%, and N :
0.0010 ~ O, oioowt%, i (wt%)
×N (wt%) >10-' and/or B (wt%) ×
Boron-added low alloy steel containing N (wt%) >2.5 x 10-6 is used as a material, heated to a temperature of 1180° to 1300°C and hot rolled,
Finishing rolling at 850°C or higher; After this rolling, holding at a temperature close to the finishing rolling temperature or during the cooling process following the completion of rolling, rolling in a temperature range of 850°C or higher for 1 minute or more and 6 minutes. Wait for up to 3 minutes
A method for producing a high-strength, high-toughness, boron-added thick steel plate by a direct quenching process (second invention), characterized by combining rapid cooling to a temperature of 00° C. or lower, and then tempering.

In this way, a boron-added high-strength, high-toughness tempered steel sheet having a tensile strength of 60 kgf/mm2 or more can be advantageously produced.

The clarification of the relationship between the behavior of boron and the quenching effect of steel during the period from the end of rolling to quenching as described below is the basis for the completion of this invention.

(Function) When setting the time from the end of rolling to quenching of boron-added steel, appropriate conditions should be set taking into consideration the changes in the steel plate during that time, especially the change in boron, which is an essential component for the hardenability of steel. There is a need to.

That is, during this holding, boron is thought to undergo changes such as (1) diffusion of solid solution B to austenite grain boundaries, (2) segregation, and (2) precipitation of boron nitride.

Since such a change in boron occurs depending on the elapsed time after rolling, the hardenability of the steel is considered to vary depending on the degree of this change. Particularly, the precipitation of boron nitrides (2) is an important change because it is directly linked to deterioration of the hardenability of steel.

The inventors investigated changes in tensile strength of boron-added low alloy steel by varying the slab heating temperature and post-rolling holding temperature and using the time from the end of rolling to quenching as a parameter.

Table 1 shows the steel composition for this experiment.

As DO conditions, ■ When the slab heating temperature is 1220℃ and the holding temperature after hot rolling is 900℃ (A), ■ When the holding temperature is lowered to 850℃ and 800℃ (B), (C) ■ Slab When the heating temperature is lowered to 1150℃ (d)
Each was subjected to OQ, and then tempered at 630° C. for 40 minutes. Figure 1 summarizes the changes in tensile strength depending on the holding time from the end of rolling to quenching.

Under any DO conditions, austenite grains recrystallize within 1 minute after hot rolling, resulting in improved hardenability. However, when the subsequent retention time becomes longer, the behavior changes depending on the OQ conditions.

In other words, 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 high strength is stably maintained.
When the temperature is lowered to 150°C, a rapid decrease in strength is observed after a holding time of about 2 minutes. Similarly, the holding temperature is 850℃
When the temperature decreases from 800°C to 800°C, the strength decreases significantly depending on the holding time.

Based on the above results, in this 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, and the rolling finishing temperature is set during rolling. The holding temperature after rolling is 850°C or higher to suppress the ON precipitation of BN, and 950°C or lower to suppress the coarsening of recrystallized γ grains and improve toughness. The temperature was limited to 850°C or higher in order to cause recrystallization.

Figure 2 (a) shows the results of examining the boron distribution using the fission track etching method when the slab heating temperature was 1220°C and 1150°C, and when the holding temperature was 900°C and the holding time was 2 minutes. Shown in (b).

As is clear from this, by setting the slab heating temperature within the range of the present invention, boron precipitation during holding after rolling is significantly suppressed. This also leads to securing solid solution B in order to effectively utilize boron.

However, even if the slab heating temperature is 1180° C. or higher, if the holding temperature after rolling is not 850° C. or higher, boron will still precipitate and hardenability will deteriorate. That is, by setting the slab heating temperature and holding temperature within the range of the present invention, it becomes possible to ensure solid solution B for a long time after rolling, and the hardening effect of B can be effectively utilized.

In this invention, in addition to the above DQ conditions, the time from completion of rolling to quenching is limited to 1 minute or more and 6 minutes or less, but if it is less than 1 minute, boron is not sufficiently absorbed during austenite grain recrystallization immediately after rolling. Since hardenability deteriorates because B cannot segregate at austenite grain boundaries, time is given for recrystallization to complete and for B to segregate at grain boundaries. However, if the time exceeds 6 minutes, the production efficiency will decrease and the effect will not increase, so it is not necessary. In order to completely complete the martensitic transformation and obtain a good hardened structure,
It is necessary to rapidly cool down to below 00°C.

In this invention, the use of boron is essential, and therefore B is required in an amount of 0.0003 wt% or more. Also, Aβ
is required to be 0.02% or more in order to obtain deoxidizing and fine grain effects.

However, the addition of B exceeding 0.0030 wt%, 0.1
Addition of Aβ exceeding 596% did not increase the effect, and on the contrary worsened the weldability, so the upper limit was set at the above value.

Furthermore, it is not preferable for the amount of N to exceed 0.01 wt% because it impairs the toughness of the base metal, but it is difficult to reduce the amount to less than 0.001 wt% due to steel manufacturing technology.

This invention is achieved by setting the heating temperature to a value within a specific range (
(1.
There is no need to limit the value of N) or (BN), and the range of the component products described in the previous application is excluded from the scope of the present invention. That is, (Aj'wt%)X(Nwt%>>io
-' and/or (8wt%) x (Nwt%) >2
．． In the case of a range of 5 x 10-6, B can be effectively used at 00 o'clock by applying the present invention.

H! , B, and N is to limit the range of alloy components required to satisfy the strength level TS of the steel sheet of the present invention of 70 kgf/mm 2 or more.

The layer is an essential component to ensure hardenability and strength.
O,Q 4wt% or more is required. However, for the sake of weldability, it is necessary to keep it at 0.15 wt% or less.

Si is added to deoxidize and ensure strength, but it is 0.01w
If it is less than t%, the effect will not be sufficient, and if it exceeds 0.3wt%, the toughness will deteriorate.

) Jn is added in an amount of 0.1 wt% or more to ensure the hardenability of the steel, but 2. Qwt% or less.

C'u is 0.1w when expected to improve strength by this.
It is added in an amount of t% or more, but adding more than 0.5II+t% is not appropriate from the viewpoint of toughness, weldability, and properties.

Ni is also added to ensure hardenability and improve low temperature toughness.
．． 1 wt% or more may be added, but 3. It is not appropriate to add more than Qwt% from an economic point of view.

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 it improves weldability,
The content should be 0.7% or less so as not to deteriorate the toughness of the base material.

MO is set to 0.0 to ensure hardenability. Q1wt% or more may be added, but from the viewpoint of weldability and cost, 0.70w
t% or less.

0.01wt% or more may be added to ensure strength, but 0.01wt% or more may be added to ensure base material toughness and weldability. It is necessary to keep it below 1Qwt%.

Others are set to 0.0 to ensure strength. O1-2. Qwt% can be added.

L has the effect of fixing N as TiN and can be added in an amount of 0.01 wt% or more to increase strength, but it can be added in an amount of 0.01 wt% or more to improve base material toughness. Addition exceeding lvt% is disadvantageous.

Among the above alloy components, C, Si, ! , ln are essential, Cu, Ni, Cr, Mo, v, Nb
Although one or more types of Ti and Ti are added depending on the required strength level and performance, the addition of these alloy components does not affect the effective use of B, which is the basis of this invention. In this invention, after direct quenching, a tempering treatment is performed to adjust the sufficient toughness level and strength within the required range.

(Example) A 100 mm thick block of steel having the composition shown in Table 2 was heated at 1250° C. or 1150° C. for 1 hour and rolled to a plate thickness of 30 mm+. The finishing temperature of rolling was varied between 900°C and 800°C, and after the rolling was completed, it was placed in a separately prepared furnace set at the same temperature as the finishing rolling temperature.
It was held for 0 seconds or 360 seconds.

After that, the cooling rate is 10℃/s between 800 and 500℃.
It was rapidly cooled to 50°C.

Next, tempering was performed at 630°C and 4Q+nin.

After heat treatment, Charpy test and tensile test were conducted.

The results are shown in Table 3.

Even when holding after rolling for 6 minutes only in the case according to this invention (slab heating temperature, finishing temperature), the strength
Although the toughness was excellent, under the remaining conditions, the strength suddenly decreased and the toughness also deteriorated after being held for 6 minutes.

As is clear from the above examples, by this invention method, D
It can be seen that high-strength, high-toughness steel can be manufactured using the Q-T process to a high standard and with minimal "variation."

This shows that it is possible to produce high-quality thick steel plates without "variation" in material quality due to the time difference J between the top and bottom of the rolled plate entering the quenching device.

In the above embodiment, the case was described in which the temperature was maintained at the same temperature as the finishing rolling temperature for 1 minute or more. The same effect can be obtained by setting the elapsed time to 1 minute or more.

(Effects 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 occurred in the past, and thick steel plates with high standards can be stably produced.

[Brief explanation of drawings]

Figure 1 is a comparison graph showing the change in tensile strength depending on the time from completion of rolling to quenching. Figures 2 (a) and (b) are graphs showing changes in boron distribution when the slab heating temperature changes 2 minutes after rolling. This is a metallographic micrograph showing the influence of Fig. 1/l end ridge fig, a, Yode Riki Gin Rulo Wei l〃2nd
Figure (2L) (b) z(7θP

Claims (1)

[Claims] 1. C: 0.04~0.15wt%, Si: 0, 01~
0.30 wt%, Mn: 0.10 to 2.0 wt%, Al: 0.02 to 0.15 wt%, B: 0.00
03~0.0030wt%, and N:0.0010~0
．． 0100wt%, Al (wt%) x N (wt%)>
10^-^4 and/or B (wt%) x N (wt%)>
The material is boron-added low alloy steel containing 2.5 x 10^-^6, heated to a temperature of 1800° to 1300°C and hot rolled, and the rolling is completed at 850°C or higher. After that, in the process of holding at a temperature close to the finishing rolling temperature or the cooling process following the completion of rolling, 850
High-strength, high-strength steel produced by a direct quenching process, characterized by a combination of cooling to a temperature of 300°C or less after waiting for at least 1 minute to 6 minutes at a temperature above 300°C, followed by tempering. A method for manufacturing tough boron-added thick steel plates. 2, C: 0.04~0.15wt%, Si: 0.01~
0.30wt%, Mn: 0.10-2.0wt%, Cu
: 0.6wt% or less, Ni: 3.0wt% or less, Cr:
0.7wt% or less, Mo: 0.7wt% or less, V: 0.
1wt% or less, Nb: 2.0wt% or less, Ti: 0.1
Contains one or two of the following wt%, further Al: 0.02 to 0.15 wt%, B: 0.0003 to
0.0030wt%, and N: 0.0010-0.01
00wt%, Al(wt%)×N(wt%)>10^
-^4 and/or B (wt%) x N (wt%) > 2.5
The material is boron-added low alloy steel containing in 850 in the process of holding at a temperature close to the finishing temperature or cooling following the completion of rolling.
High-strength, high-strength steel produced by a direct quenching process, characterized by a combination of cooling to a temperature of 300°C or less after a period of 1 minute or more and less than 6 minutes at a temperature above 300°C, followed by tempering. A method for manufacturing tough boron-added thick steel plates.