JPS5940890B2 - Method for manufacturing steel materials with excellent low-temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to steel materials such as steel pipes used for transporting and storing low-temperature fluids such as liquefied low-temperature gases, or in atmospheres such as extremely cold regions.

As is well known, the toughness of steel materials rapidly decreases at low temperatures.
It exhibits so-called low-temperature embrittlement.

Conventional methods for improving such low-temperature brittleness, that is, imparting low-temperature toughness, include methods such as adding alloying elements, especially large amounts of Ni, etc., and methods such as double quenching. Both have the problem of significantly increasing costs.

Therefore, there has been a strong demand for the development of a method for simply and easily manufacturing steel pipes and other steel materials with excellent low-temperature toughness using inexpensive steel with regular components.

By the way, the present inventors have developed a method for manufacturing seamless steel pipes,
We are proceeding with research and development of a method in which steel billets are pierced and rolled through primary hot working, then finish rolled to adjust the outer diameter through secondary hot working, and then quenched immediately without cooling (direct quenching). ing.

The method for producing seamless steel pipes using direct quenching will be explained in more detail with reference to Fig. 1. First, a steel billet 1 as a raw material is heated to about 1200°C or higher in a heating furnace 2 such as a rotary furnace. This is then hot worked in a series of piercing and rolling processes 3 consisting of a piercer, elongator, plug mill, reeler mill, etc. to obtain a cross-sectional size and shape close to that of the final product, and then in a reheating furnace such as a walking beam furnace. 4 and reheated to about 900°C, the reheated steel material (base pipe) is finished to a predetermined outer diameter by a finish rolling process 5 such as a sizer mill or a stretch reducer, and then immediately heated by a quenching device 6. This is a hardening method.

In this method, the process from heating the steel billet 1 to quenching is carried out in one continuous process, but if the quenching device 6, which is the final stage of the continuous process, stops for some reason, the billet 1 is processed and processed from the upstream side. It is necessary to discharge the steel material sent from the system to the outside of the system at a stage close to the quenching device 6 and temporarily store it in stock.

In this case, generally, the steel material is discharged from the system and stored at the stage where the finish rolling process 5 of the sizer mill, etc. is completed, that is, at the stage where it has been processed to the shape and dimensions of the final product. It is considered normal that the stocked steel materials are heated and quenched.

However, in this case, it is not so-called direct quenching, which is quenching immediately after hot working, but normal quenching.
The quality of the product obtained is significantly inferior to that obtained by direct rolling, and for example, when the same tempering conditions are adopted, the tensile strength is significantly inferior.

Under these circumstances, the inventors of the present invention decided to send the steel discharged from the reheating furnace 4 as it is to the cooling bed side immediately before finish rolling in the sizer mill etc. when the quenching equipment is stopped as described above, and to store the steel material as it is in the cooling bed. I thought about doing it.

When the quenching equipment is restored, the stocked steel material is charged into the reheating furnace 4 and heated to around 900°C again, and then slightly hot rolled in a finish rolling process 5 such as a sizer mill. When we conducted an experiment in which we immediately quenched (directly quenched), we found that it not only has superior strength, but also
It has been discovered that it is possible to obtain steel pipes with excellent low-temperature toughness.

Therefore, we repeated the experiment and found that by adjusting the working ratio in hot rolling (finish rolling) immediately before quenching, it was possible to obtain a steel material with excellent low-temperature toughness from a low-carbon steel with a normal composition. They discovered that the above conditions can be satisfied and came up with this invention.

That is, in the method for manufacturing a low-temperature steel material of the present invention, for example, in the process of manufacturing a series of seamless steel pipes as described above, Ar1 is discharged from the system and stored immediately before secondary hot working (finish rolling) in a sizer mill or the like. Like steel cables and pipes that have been cooled below their transformation point, we use low-carbon steel materials of normal composition that have already been processed to a cross-sectional size and shape close to the cross-sectional size and shape of the final product. It is hot-rolled at a processing ratio of , and then immediately quenched (directly quenched).

More specifically, the method of the present invention comprises CO less than 15%, Si 0.1-1.0%, Mn 0.4-2.0%,
Ac
3. After heating to a temperature above the transformation point and below the austenite grain enlargement starting temperature and hot rolling so that the cross-sectional shrinkage ratio R becomes 0.015 or above, immediately quenching, and then heating to a temperature below the Ac1 transformation point. It is characterized by being tempered.

However, here, the cross-sectional shrinkage ratio R is the area shrinkage ratio of the cross section perpendicular to the main rolling direction before and after hot rolling, or more precisely, the cross-sectional area before hot rolling is 81, and the cross-sectional area after hot rolling is The area 82 and the area are calculated by the following formula.

2 R=1-- 1 The manufacturing method of the present invention will be explained in more detail below.

As mentioned above, the composition range of the steel material targeted by this invention is CO less than 15%, Si 0.1 to 1.0%, Mn 0.
4 to 2.0%, AI 0.01 to 0.10%, and the remainder substantially Fe and unavoidable impurities. The reason for limiting the components is as follows.

If C exceeds 0.15%, toughness decreases.

Further, the lower limit of C is not particularly limited, but it is usually desirable for practical use to be around 0.08% from the viewpoint of weldability and strength.

Si is added for the purpose of deoxidizing and increasing strength, and for this purpose 0.1% or more is required.

On the other hand, if Si exceeds 1.0%, toughness decreases rapidly.

It is necessary to add Mn in an amount of 0.4% or more for the purpose of improving strength and toughness, but if it exceeds 2.0%, segregation and welding defects will occur.

AI is added to deoxidize and combine with N in steel to refine the grains, but for this purpose, it is necessary to have an amount of 0.10% or more, and if AI exceeds 0.10%, the above-mentioned The effect of is saturated.

In addition, in order to further improve low-temperature toughness, N
One or both of iO, 05-9.5% and CuO, 05-0.5% may be added depending on the purpose.

In addition, to ensure strength and hydrogen-induced cracking resistance, CrO
, 05-2.5%, Mo 0.05-1.5%, NbO
, 01-0.1%, Vo, 01-0.2%, TiO, 0
05-0.1%, Bo, 0005-0.005%, Ca
O1002-0.0050%, REMo, 005-0.
One or more selected from 0.050% may be added.

The manufacturing method of this invention uses steel having the above-mentioned composition as a material, but the material used in the manufacturing method of this invention is a raw pipe that has been processed in advance to have a cross-sectional size and shape close to the cross-section of the product. etc.

For example, in a series of manufacturing processes for seamless steel pipes as shown in FIG.・Main pipes that have already been subjected to fire hot processing,
Alternatively, the raw tube is discharged from the system just before the quenching device 6 in FIG.
A base tube that has already been subjected to secondary hot rolling, such as (Slightly different from the cross-sectional dimensions and shape of the final product in the method)
etc. are eligible.

In the method of the present invention, such materials such as raw pipes are first heated to a temperature above the Ac3 transformation point and below the austenite crystal grain coarsening starting temperature.

This heating needs to be above the Aca transformation point in order to uniformly transform the steel into austenite and to sufficiently dissolve the alloying elements in the steel. It is desirable to heat the steel to a temperature as low as possible, and it is necessary to heat it to at least the austenite crystal grain coarsening starting temperature.

The thus heated material is immediately rolled using a rolling mill such as a sizer mill or a stretch reducer depending on the shape and dimensions of the product to be obtained, with a cross-sectional shrinkage ratio R1, that is, an area S1 before rolling in a cross section perpendicular to the main rolling direction. Hot rolling is performed so that the value of (1-82/S1) determined by the subsequent area S2 is 0.015 or more.

The present inventors found through experiments that this cross-sectional shrinkage ratio is appropriate, as shown in the examples described later, and the steel material after quenching and tempering can only be achieved by setting the cross-sectional shrinkage ratio to 0.015 or more. If the cross-sectional shrinkage ratio is less than 0.015, good low-temperature toughness cannot be obtained.

Note that the upper limit of the cross-sectional shrinkage ratio in hot rolling is not particularly limited (・However, as shown in the examples, the effect of improving low-temperature toughness is saturated at a cross-sectional shrinkage ratio of about 0.025 to 0.030. Even if the cross-sectional shrinkage ratio is increased above, the effect does not increase,
Furthermore, since the material targeted by this invention has been processed in advance to have a cross-sectional size and shape close to that of the product, the cross-sectional shrinkage ratio is usually about 0.10 or less.

In addition, in this hot rolling, it is desirable to apply compressive load as much as possible at once to obtain a cross-sectional shrinkage ratio of 0.0] or 5 or more, and from that point of view, it is desirable to obtain a cross-sectional shrinkage ratio of 0.0] or 5 or more, and from that point of view, it is desirable to apply a compressive load as much as possible at once to obtain a cross-sectional shrinkage ratio of 5 or more.
However, the process of passing through a set of rolls to which pressure is applied is 1.
It is desirable to roll the sheet to a cross-sectional shrinkage ratio of 0.015 or more.

In the case of rolling with 4 passes or more, even if the cross-sectional shrinkage ratio for the entire hot rolling process is 0.015 or more, the cross-sectional shrinkage ratio per pass becomes significantly small, and therefore good low-temperature toughness may not be obtained. be.

After hot rolling as described above, water quenching is immediately performed.

That is, it is hardened before being cooled to a critical temperature.

For example, in the case of a steel pipe, it is preferable to use a method of cooling the pipe by flowing a water flow along its longitudinal direction, that is, an axial flow, on both the inner and outer surfaces of the pipe, but the method is not necessarily limited to this.

After being hardened in this manner, it is tempered to a temperature below the Ac1 transformation point.

This tempering is usually preferably carried out at a temperature of 600° C. or higher, and after tempering, it is rapidly cooled in a conventional manner.

``In this way, after hot rolling, a steel material such as a steel pipe, which is the final product in the manufacturing method of the present invention, can be obtained by quenching and tempering.

In addition, when using raw pipes discharged and cooled from the middle of the seamless steel pipe manufacturing line shown in FIG. 1 as the raw material in the manufacturing method of the present invention, the walking beam furnace, etc. shown in FIG. 1 is used. It is desirable to use the reheating furnace 4 for heating before hot rolling in the manufacturing method of the present invention.

That is, for example, as shown by the broken line in FIG.
A is discharged and stocked from the cooling bed 7 side.
r1 When using raw pipe cooled below the transformation point as the material,
As shown by the solid line in Fig. 2, the cooled raw tube is reinserted into the reheating furnace 4 and heated to a temperature above the Ac3 transformation point and below the austenite crystal grain coarsening starting temperature. It is sufficient to hot-roll the trench reducer or the like to a cross-sectional shrinkage ratio of 0.015 or more in the finish rolling step 5, and immediately quench and temper it.

For example, as shown by the broken line in FIG.
Even when using a steel pipe that has been cooled by being discharged to a cooling bed γ' outside the line, the steel pipe is reinserted into the reheating furnace 4 and heated to the above temperature, as shown by the solid line in Fig. 3. Heating is performed again in finish rolling step 5 until the cross-sectional shrinkage ratio is 0.
．． 015 or higher, immediately quenched, and then tempered.

The steel material obtained as described above has extremely excellent low-temperature toughness.

The reason may be as follows.

That is, in order to set the heating temperature for hot rolling to be below the austenite grain coarsening starting temperature (but above the Ac3 point) and to perform hot rolling with a relatively large cross-sectional shrinkage ratio of 0.015 or more, The first reason is that the austenite crystal grains (before quenching) are significantly refined, and the crystal grains after direct quenching and quenching are also refined, which prevents crack propagation. Can be mentioned.

In addition, strain or lattice defects introduced into the steel material by hot rolling function as precipitation sites for carbide formation during tempering, and high-temperature tempering causes the carbide precipitation to become spheroidal and dispersed. The second reason is that sulfide-based nonmetallic inclusions are expanded and separated by rolling, which improves the occurrence of brittle cracks caused by carbides and sulfides and propagation characteristics.

In addition, looking only at the grain size, in order to improve notch toughness, the austenite grain size after hot rolling (J
IS) is preferably 6.0 or higher.

However, if the chemical composition of the steel contains alloying elements that improve notch toughness, such as Ni or Nb, the cross-sectional shrinkage ratio may be small; on the other hand, if these alloying elements are not contained, the cross-sectional shrinkage ratio may be small. However, what is important in the method of the present invention is that if the cross-sectional shrinkage ratio is equal to 100%, an effect of improving notch toughness can always be obtained, and excellent low-temperature toughness can be obtained.

That is, according to the method of the present invention, even when not specifically containing an alloying element that improves notch toughness, the first reason and The second reason works together to provide good low-temperature toughness.

Examples of this invention are described below.

Example Two types of steel materials A and B shown in Table 1 were pre-hot worked to a diameter of 80 to 105 m, IL1 wall thickness of 8 to 16 m.
After processing this into a raw material tube of 500 m in size, it is thoroughly heated to 910 to 930°C in a walking beam furnace, and then it is rolled into various cross-sectional shrinkage ratios in 1 to 3 passes in a slotted hot rolling mill. It was hot rolled.

Next, it is immediately water-cooled and quenched using an axial flow quenching device on both the inside and outside surfaces, and then tempered to about 605°C to 640°C.
The yield strength of each steel material was set to 40 and 9 f/-.

A sub-size 2mm V notch Charpy impact test was conducted on each of the steel materials obtained in the above-mentioned examples.
The transition temperature (vTrs) exhibiting the percent brittle fracture surface ratio was investigated.

The transition temperature vTrs of steels A and B of each component is shown in FIG. 4 in correspondence with each cross-sectional shrinkage ratio R.

Practically speaking, it can be determined that low-temperature toughness is excellent if vTrs is below -50°C, but as is clear from Figure 4, if the cross-sectional shrinkage ratio R increases from about 0.010 to 0.020, Not only in the case of steel B containing Ni, which has a sharp decrease in vTrs and has the effect of improving toughness, but also in the case of steel A, which does not substantially contain Ni, the cross-sectional shrinkage ratio is 0.01.
5 or more, it was possible to ensure vTrs≦−50°C.

As is clear from the above explanation, according to the method of the present invention, steel materials such as steel pipes with excellent low-temperature toughness can be easily and easily produced using inexpensive steel with ordinary components. It is possible to provide steel materials such as steel pipes at low cost.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of the manufacturing process of seamless steel pipes as the background of this invention, and FIGS. 2 and 3 are block diagrams showing examples of applying the invention to the manufacturing process of seamless steel pipes, respectively. Figure 4 shows the cross-sectional shrinkage ratio R and Charpy impact test fracture surface transition temperature (vTrs
) is a graph showing the relationship between

Claims (1)

[Claims] I C0.15% or less, Si 0.1-1.0%, M
Steel material with n0.4 to 2.0%, AIo, 01 to 0.10%, the balance consisting essentially of Fe and unavoidable impurities, and which has been processed in advance to have a cross-sectional shape and dimensions close to those of the product. is used as a material and heated to a temperature above the material's A c 3 transformation point and below the austenite crystal grain coarsening start temperature, so that the cross-sectional shrinkage ratio R determined by the following equation (1) is 0.
．． 1. A method for producing a steel material having excellent low-temperature toughness, which comprises hot rolling the steel material to a temperature of 0.015 or higher, immediately quenching it, and then tempering it at a temperature below the Ac1 transformation point. 2 R = 1-1 ... (1) 1 However, Sl is the area of the cross section perpendicular to the main rolling direction before hot rolling, and S2 is the area perpendicular to the main rolling direction after hot rolling. Calculate the area of the cross section of the eggplant. 2 The cross-sectional shrinkage ratio R in the hot rolling is 5 or more and 0
．． 10. The manufacturing method according to claim 1, wherein rolling is performed so that the thickness is 10 or less.