JPH01319629A - Production of cr-mo steel sheet having excellent toughness - Google Patents

Abstract

PURPOSE:To develop the Cr-Mo steel sheet having excellent high-temp. strength and low-temp. toughness by heating the hot rolled sheet of a Cr-Mo steel having a specific compsn. at the specific heating-up rate obtd. from a rolling finish temp., then subjecting the sheet to tempering after hardening or normalizing. CONSTITUTION:The slab of the Cr-Mo steel having the compsn. which contains, by weight %, 0.03-0.17% C, 0.02-0.5% Si, 0.1-3.0% Mn, 0.5-13% Cr, 0.3-3.0% Mo+W, 0.03-0.5% V, 0.01-0.2% Nb, 0.003-0.05% Al, <0.01% N, <0.02% P, <0.02% S or is further incorporated therein with 0.0002-0.005% B alone or in combination with 0.005-0.05% Ti is heated and is hot rolled to a sheet material. The rolled sheet is heated up by the heating-up rate range HR determined from the rolling end temp. Tf by the equation I and is then subjected to the normalizing-tempering or hardening-tempering treatment, by which the steel sheet having the high-temp. strength and low-temp. toughness suitable as the material for the high-temp. apparatus of petrochemical and petroleum refining systems is obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing Cr-Mo steel used in high-temperature equipment such as chemical plants such as petroleum refining and petrochemical plants, or power plants such as thermal and nuclear power plants. It is something.

[Prior Art] In plants such as chemical reaction vessels, Cr-Mo steel containing O to 2.596 Cr in addition to 0.5 to 1% Mo is used depending on required properties. In particular, 2.25Cr
-IMo steel is preferred for demanding applications due to its excellent high temperature properties.

In recent years, Cr- containing V or Nb has been used to improve the operational efficiency of petroleum refining blunts, etc., or for coal liquefaction plants.
Mo steel has been developed. Steels with increased Cr content up to 3% or more are also being considered for use in coal ltlization blunts.

Furthermore, 7-13% Cr steel with a high Cr content has been used for a relatively long time in equipment such as power plants that have severe operating temperature conditions. In addition, in recent years, high Cr steel containing Nb or Nb has been used as a structural material for steam generators in fast breeder reactors.
” has been.

These Cr-Mo steels are used at high temperatures and are required to have excellent high-temperature strength, and also require excellent low-temperature toughness from the viewpoint of ensuring safety in pressure tests.

Conventionally, Cr-Mo steel used for chemical reaction vessels, etc. is rolled, heated to a predetermined temperature (usually 930'C), normalized or quenched, and then tempered and used. There are many. Due to the high temperature and high pressure operating conditions of plants, there is a demand for higher strength steel materials, and as shown in Japanese Patent Application Laid-open No. 80-238452, V,
Efforts are being made to improve high-temperature strength by adding elements such as Nb.

However, normalizing or quenching at high temperatures, which is essential for effective utilization of alloying elements, causes coarsening of austenite crystal grains and reduces low-temperature toughness. In this way, high-temperature strength and low-temperature toughness are antinomy, and it is difficult to achieve both.

[Problems to be Solved by the Invention] Chemical reaction vessels are used at high temperatures and require tensile strength and creep strength at high temperatures. In addition, chemical reaction vessels are required to undergo periodic pressure tests, and the materials used must also have excellent low-temperature toughness. In addition, generally Ct・
- Since the toughness of Mo steel gradually decreases during use at high temperatures, it is possible to extend the period during which Mo steel has excellent toughness at the initial stage of use.

The present invention provides a method for producing Cr-Mo steel that maintains fine-grained austenite even during normalization or quenching at high temperatures to improve high-temperature properties and has excellent low-temperature toughness after tempering. The purpose is to

[Means for Solving the Problems] The present inventors have determined that the austenite grain size, which has a decisive influence on the low-temperature toughness of quenched and tempered Cr-Mo steel,
Various methods of maintaining fine grains even during high-temperature normalizing or quenching were investigated.

As a result, it was difficult to achieve the objective by simply controlling the conditions during the quenching process, and it was discovered that fine austenite grains could be maintained during normalizing or quenching at high temperatures only by adjusting the temperature at the end of rolling. .

The present invention was made based on the above-mentioned knowledge, and the gist thereof is: C: 0.03 to 0.17% in weight%;
Si: 0.02-0.5%, Mn: 0.1
~3.0%, Cr: 0.5~13%, Mo+W:
0J-3.0%, VO103~0.5%, Nb:O,
O1~0.2%, AΩ: r, -0.003~0405%, N・0. Below old %, P:
0.02% or less, S: 0.02% or less as a basic component, and if necessary, B.0.0002-0005% may be contained alone or in combination with Ti: 0.005-(]05%. 1100 steel consisting of residual Fe and unavoidable impurities
After heating to ~1280°C, it is rolled, and the rolling temperature (T
f) The range of heating rate (HR) determined from This is a method for manufacturing a Cr-Mo steel sheet with excellent toughness, which is characterized by performing binding.

[For production] The present invention will be explained in more detail below.

C is an effective element for increasing the strength at room temperature and high temperature, and requires at least 0.003% of the strength level required for steel for chemical reaction vessels.

As the amount of C increases, the toughness of the steel material decreases and the weldability also deteriorates, so the upper limit is set at 0.17%.

Sl is added in an amount of 0.02% or more to deoxidize and increase strength, but if the amount added is too large, the toughness decreases, so the 11th limit is added to 0.05%.
shall be.

Mn is an effective element for fixing S and increasing strength, but if the amount added is too large, the transformation point will drop too much, and the amount of retained austenite will increase during normalizing and quenching, so it is 0.1 to 3
,0%.

P is an element that not only micro-segregates in steel and significantly increases the directional difference in toughness (7), but also segregates at grain boundaries during tempering and post-weld heat treatment and reduces toughness, so it must be reduced. Therefore, the upper limit is set to 0.02%.

S forms non-metallic inclusions MnS in steel, increases the directional difference in toughness, and lowers the "second part side energy" in the Nyalpy test, so the first limit is set to 0.02%.

Cr not only increases hardenability, but also precipitates carbonitrides during tempering and post-weld heat treatment, thereby improving high-temperature strength. Further, Cr is added in an amount of 0.5% or more in order to stabilize carbonitrides and improve the hydrogen corrosion resistance of the steel. However, 13%
The addition of super is unnecessary for steel for chemical reaction vessels.
” 1st period is 13%.

Mo and W are added to increase high temperature strength, particularly creep rupture strength. [7] However, if less than 0.3% is added alone or jointly, the effect is not significant, and if it exceeds 3.0%, the effect is saturated, so the amount added is set to 0.3 to 3.0%.

(2) itself forms a carbonitride, which increases the strength, and also forms a solid solution in the Cr carbonitride, which has the effect of further stabilizing the Cr carbonitride. However, if the addition amount is less than 0.03%, the effect is not sufficient, and if it exceeds 0.5%, the effect is saturated and the effect corresponding to the addition amount cannot be obtained.
．． 5%.

Nb is an element that forms stable carbonitrides during the thermal history of heating and cooling7, and has the effect of improving the high temperature strength of steel. In addition, deformation-induced precipitation occurs during rolling, which prevents movement of grain boundaries and prevents [If crystal grains from becoming coarser]. For this reason, 0
Although it is added in an amount of 0.0% or more, if it exceeds 0.2%, the effect commensurate with the amount added cannot be obtained, so it is economically suppressed to 0.2% or less.

8ρ is an essential element for deoxidizing steel, and based on this I], the lower limit is set at 0.003%. However, as the amount of addition of ρ increases, the creep rupture strength is impaired, so the limit of addition is reduced to 0.
．． 05%.

Like C, N increases the strength of steel, or in normal melting methods, it forms pores in the steel ingot when added in an amount of 0.08% or more. If the pores are not bonded even after rolling, rolling and toughness will deteriorate, so the upper limit is set at 0.08%.

B is an element that increases hardenability when added in a small amount, and is added when hardenability is required. Hardening improvement effect is 0
．． It is accepted from 0.0002%, or there is no point in increasing the amount beyond 0.0050%. For this reason, the amount added should be 0.0002~
It shall be 0.005%.

Ti combines with N and has the effect of preventing B from becoming ineffective in improving hardenability and becoming N. Therefore, 0.005% or more of T1 can be added together with B. However, if the amount of nitrogen in the steel is too high, a large amount of TiN will be formed and the cleanliness will be impaired.
In order to regulate the amount of O44% or less, the amount of Ti added is 0.0
5% or less is sufficient.

Next, the rolling conditions will be described.

Steel having the above chemical composition is obtained by melting in a converter or electric furnace, then subjecting it to ladle refining or vacuum descaling treatment as necessary, and is usually made into an ingot using a mold or one-way solidification mold. After that, it is divided into slabs.

Slabs may be manufactured directly from molten steel by continuous casting. Any method of soaking and pressing may be used in the blooming process. That is, the slab may be soaked after cooling, or C may be carried out by charging the slab as a hot slab into a soaking furnace. 1100-12
After soaking at 80°C, it is rolled or forged into a slab. The slab thickness is preferably about 1.3 to 2.5 times the product plate thickness.

It is essential that the slab be heated at a temperature at which some or all of the N 1.1 and (2) contained in the steel dissolve into solid solution. Therefore, the slab heating temperature is set to 1100° C. or higher. However, if the slab heating temperature is too high, the structure will become coarse and it will be difficult to refine it by rolling.
The upper limit is 1280°C.

The heated slab is rolled by multiple passes of rolling.

The larger the reduction amount and the lower the rolling end temperature, the finer the austenite grains or the smaller the short axis dimension of elongated grains. The reduction amount, ie, the ratio of slab thickness to product thickness, is preferably 2 or more. The lower the rolling end temperature, the more finely divided the structure is.
Industrially, a temperature of 700° C. or higher is preferable because the lower the temperature, the higher the shape resistance and the greater the load on the rolling mill.

The rolled steel plate is cooled, surface treated, and then heated for normalizing or hardening. The heating temperature, that is, the quenching temperature is high for Cr, Mo.

Additive elements such as Nb and V are dissolved in solid solution and work effectively to improve high-temperature strength. However, even at 980°C, which is 50°C higher than normal normalizing or quenching temperature,
The grains become coarse and good toughness may not be obtained after tempering.

However, as a result of a systematic test regarding the rolling end temperature and heating rate, it was found that if heating is performed within the heating rate range corresponding to the rolling end temperature, no coarse grains will be generated even at 980°C, and uniform fine grains will be obtained. I knew it was possible.

014%C-0,25%5t-0,45%Mn-0
,008%P-0,005%S-2.58%Cr-1
．． 07%Mo-0,23%V-0,039%Nb
The results for -0,007%Al -0008%N steel are shown in Figures 1 and 2.

As shown in FIG. 1, there is an optimum heating rate range in order to obtain fine grains by high temperature quenching (980'C).

As shown in Fig. 2, the range of the optimum heating rate (HR) in item 1 changes depending on the rolling end temperature (T, ), )≧(1400−Tf)/300.In other words, it is clear that the lower the rolling end temperature, the higher the temperature increase rate.

Figure 3 shows the relationship between the Charpy transition temperature after tempering (690'CX 30 hours) with respect to the temperature increase rate during normalizing heating, for the case of the rolling end temperature (900'C) in Figure 2. ing. The excellent fracture surface transition temperature of -40° C. or lower completely coincides with the fine grain region shown in FIG. 1, and the toughness improving effect of the present invention is clear.

After hardening or normalizing, it is tempered and used for manufacturing various plants. The tempering conditions are not particularly limited, and the tempering conditions are within the conventional temperature range (650 - b). After tempering, the austenite grain size was measured, and a tensile test and a Charpy test were conducted to determine the tensile strength and Charpy fracture surface transition temperature.

The results are shown in Table 2.

According to the present invention, indicated by Q as shown in Table 2,
Uniform and fine austenite grains are obtained even after high-temperature quenching heating at 980° C. or higher, and both the tensile strength and toughness (fracture surface transition temperature) after tempering are good.

On the other hand, in the comparative examples indicated by x, the temperature increase rate is outside the range required from the finishing rolling temperature, so the γ grain size does not become fine and the toughness level is low.

[Effects of the Invention] Although the Cr-Mo steel sheet manufactured by this method can obtain fine austenite grains without going through complicated steps such as repeated quenching, and has high strength by high-temperature quenching, Excellent toughness equivalent to that obtained by quenching at normal quenching temperatures can be obtained.

It is extremely useful as copper for chemical plants or power generation branes, which require excellent high-temperature strength and good low-temperature toughness, and has high industrial value.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between temperature increase rate and γ grains after normalizing, Figure 2 is a diagram showing the combination of rolling end temperature and temperature increase rate to obtain fine grains by normalizing, Figure 3 is a diagram showing the relationship between the temperature increase rate during normalizing and the Charpy fracture surface transition temperature. Agent Patent Attorney Tatsuo Chanoki Noborugaki C
'C/h) Ascent car) I ('Cy force-209=

Claims (1)

[Claims] 1. In weight%: C: 0.03 to 0.17%, Si: 0.02 to 0.5%
, Mn: 0.1-3.0%, Cr: 0.5-13%, M
o+W: 0.3-3.0%, V: 0.03-0.5%, Nb: 0.01-0.2%,
Al: 0.003 to 0.05%, N: 0.01% or less,
P: 0.02% or less, S: 0.02% or less Steel consisting of residual Fe and unavoidable impurities is 1100-128
After heating to 0°C and rolling, the range of heating rate (H_R) determined from the rolling finishing temperature (T_f), (1700-
Production of a Cr-Mo steel sheet with excellent toughness, characterized by carrying out normalizing and tempering or quenching and tempering after heating at T_f)/300≧ log(H_R)≧(1400-T_f)/300. Method. 2. In weight%, C: 0.03-0.17%, Si: 0.02-0.5%
, Mn: 0.1-3.0%, Cr: 0.5-13%,
Mo+W: 0.3-3.0%, V: 0.03-0.5%, Nb: 0.01-0.2%,
Al: 0.003 to 0.05%, N: 0.01% or less,
The basic components are P: 0.02% or less, S: 0.02% or less, and B: 0.0002 to 0.00.
Cr having excellent toughness according to claim 1, characterized in that the steel contains 5% of Ti alone or in combination with 0.005 to 0.05% of Ti, and is composed of residual Fe and unavoidable impurities. - A method for producing a Mo steel plate.