JPS5821011B2 - Manufacturing method for steel sheets with excellent low-temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a steel plate with excellent low-temperature toughness, especially a steel plate for low-temperature use of class 1 with a tensile strength of 40 to 60 Kp/1, which has high impact fracture absorption energy and a significantly low ductility-brittle transition temperature. This relates to a manufacturing method.

Recently, in the shipbuilding industry, construction of carrier vessels and storage tanks for liquefied petroleum gases such as LNG and LPG has become active, and the low-temperature steel plates used for these tanks, secondary barriers, and internal structural members have excellent toughness at low temperatures. requested.

Furthermore, for oil and natural gas line pipe materials used in cold regions, there is a demand for steel sheets that have high impact fracture absorption energy and a significantly low ductility-brittle transition temperature.

To meet these demands, materials such as high-strength steel for line pipes have been subjected to so-called controlled rolling to improve their toughness by making the grains extremely fine. There is a tendency for small laminations to appear and so-called separations to occur frequently, which is a problem in terms of material reliability.

In particular, it has been recognized that materials that are prone to separation tend to be prone to hydrogen-induced cracking, so improvements are needed.

On the other hand, such separation does not occur with heat-treated steels that have been put into practical use, such as steels that have been quenched, tempered, or normalized, but the ductility-brittle transition temperature during impact fracture can be controlled. If it were to be made as low as that of a rolled material, it would be necessary to add a large amount of an expensive alloying element such as Ni, which was a practical limit.

In view of these points, the inventors have been developing steel sheets that have high impact fracture absorption energy and at the same time a significantly low ductility-brittle transition temperature. A new method for manufacturing low-temperature steel sheets has been established that advantageously avoids the above-mentioned problems by applying appropriate heat treatment.

The gist of this invention is as follows.

Weight: 0: 0.04-0.16%, Si: 0.10-0
．． 50%, Mn; 1.0-2.0%, Altota
A; 0.010-0.060% and Nb; 0.0
Contains 1 to 0.06%, and optionally 0.15%
% or less of V, O of 0.50 or less, Or of 0.30 or less, and Ni of 0.50 or less, with the balance consisting of Fe and inevitable impurities. is heated to a temperature above which 0.01% or more of Nb is dissolved in solid solution, and then a cumulative reduction rate of 30 to 85 in a temperature range of 900°C or less, and a rolling end temperature of 700°C to 850°C.
After hot rolling under the following conditions and once air cooling, 850
A method for producing a steel sheet with excellent low-temperature toughness, characterized by carrying out a normalizing treatment in a temperature range of ~950°C.

In carrying out the present invention, P and S as impurities in the steel can be allowed at 0.03% or less, o, and o2% or less, respectively.

In other words, this invention is characterized by the combination of the steps of subjecting the steel with the above-mentioned specific components to controlled rolling under specific conditions, thereby once making the grains extremely fine, and then normalizing the steel again under specific conditions. , by forming fine grains in the steel that could not be obtained with conventional simply normalized materials, thereby significantly improving both strength and toughness.

Now, in order to obtain fine ferrite grains by normalizing treatment, it is first necessary that the grains be fine in the state before the heat treatment, that is, in the as-rolled state.

Therefore, this invention is not only capable of hot rolling in a temperature range of 900°C or higher after heating the steel to a temperature at which 0.011 or more of Nb, an essential component, forms a solid solution, but also in the non-recrystallized region of 900°C or lower. Furthermore, it is based on the ultra-fine graining effect of the controlled rolling method by transforming and generating ferrite grains from austenite grain boundaries elongated by cumulative reduction and deformation bands within the grains, and the above-mentioned solid solution Nb is removed during or after rolling. The method utilizes the fact that Nb (0, N) is finely precipitated at ferrite grain boundaries and subgrain grain boundaries during cooling, and is further normalized afterwards.

Therefore, it is significant to avoid the drawbacks associated with the above-mentioned controlled rolling method.

FIGS. 1 and 2 show the relationship between various mechanical properties after heat treatment with respect to cumulative rolling reduction of 900° C. or lower and finishing rolling temperature.

In Figure 1, the sample steel is 0.06, 0-0.25, se
-1.7%, Mn -0.03%, AAtota/
-0,04 section, Nb-0,02%V-0,2%N1 (
7) Using converter molten steel, this steel slab was reheated to 1150°C to dissolve Nb at 0.04% and
Apply a cumulative reduction rate of 60 at °C, followed by 900 to 850
The cumulative reduction rate was varied by 20 to 90 points in the temperature range of °C.

Thereafter, the material was cooled in air, reheated to 900.degree. C. and normalized, and then subjected to a 27ERV notch Charpy impact test and a tensile test.

In addition, FIG. 1 shows the relationship between the cumulative reduction rate of 900 to 850° C. and vTrs after normalizing treatment.

On the other hand, in Fig. 2, the same test steel as in Fig. 1 was heated through the same slab heating, and the same reduction in the range of 1050 to 950°C was applied, followed by a cumulative reduction of 50 times below 900°C. The relationship between vTrs and Y, S was obtained by fixing the rolling finish temperature to , changing the rolling finishing temperature, and then performing normalizing treatment under the same conditions as in the case of FIG. 1.

In Figure 1, an improvement in toughness is seen at a cumulative reduction rate of 30 below 900°C, and an improvement in toughness is seen at a cumulative reduction rate of 50 below 900°C.
As shown in FIG. 1, the lower the rolling finishing temperature, the better the strength and toughness after normalizing treatment as shown in FIG. 2.

However, if the cumulative rolling reduction below 900°C is less than 30 degrees or the rolling finishing temperature exceeds 850°C, no improvement in toughness will be seen, and if the cumulative rolling reduction below 900°C is 85%.
Even if the finishing temperature exceeds 700° C. or lowers the finishing temperature to less than 700° C., the above effects are saturated and the rolling efficiency is unnecessarily deteriorated.

For this reason, the upper limit of the cumulative reduction rate below 900°C is 8
The lower limit was set to 5 and the lower limit was set to 30. The upper limit of the finishing temperature was 850°C and the lower limit was 700°C.

We have already emphasized that in order to obtain fine ferrite grains through normalizing treatment, it is desirable to have fine grains as rolled, but at the same time, these grains are difficult to grow during heating to the heat treatment temperature. is necessary.

In other words, when the α → γ transformation occurs on the way to the normalizing temperature, the nucleus of γ grain generation is mainly at the grain boundaries, and therefore the grain boundary area is large, that is, the grains must be fine until immediately before the transformation. This is the reason for the formation of fine grains after metamorphosis.

By the way, in general, even if the ferrite grains are fine before heat treatment, grain growth occurs from around 400℃ during heating to the normalizing temperature, so steel with a normal composition cannot be sintered even if the grains are fine as rolled. Normally, the effect will be weakened by the conditioning process.

However, as in this invention, solid solution Nb is added to o or o1% before rolling.
In the case where Nb(ON) is present in the ferrite grain boundaries and subgrain boundaries during rolling or during subsequent cooling, the precipitates are
Since they do not form a solid solution below the transformation temperature, even if they are heated again to the heat treatment temperature after rolling, they maintain their effect of suppressing the growth of ferrite grains and the disappearance of subgrains until just before the α→γ transformation. As a result, the fine grain effect of normalizing treatment can be further improved by this invention.

When we investigated the relationship between the normalizing temperature and various mechanical properties, we found that it was around 800°C and 1050°C, as shown in Figure 3.
In this invention, the lower limit of the normalizing temperature range is set to 850°C, and the upper limit is set to 950°C, since brittle formation occurs at the above temperatures.

By the way, Fig. 3 uses the same test steel as in Fig. 1, and after applying the same slab heating and the same reduction in the 1050 to 950°C range, the 50% cumulative temperature was measured at 900 to 800°C. Relationship between vTrs and Charpy impact absorption energy (vE-50°G) at -50°C when the reduction rate is applied, air-cooled, and then the normalizing temperature (holding time 1 hr) is varied in the range of 750 to 1050° It is.

The reasons for limiting the ingredients in this invention will be explained below.

If C is less than 0.04 CI), the strength of the steel plate will decrease and the weld heat affected zone (hereinafter abbreviated as HAZ) will be greatly softened, so the lower limit of the C content was set to 0.04%.

Also, if C exceeds 0.16%, the base material becomes dusty. The upper limit was set at 0.16% because the hardening of the welded area and the deterioration of cracking resistance were significant.

Si is an element that is inevitably included for deoxidation during steel refining, but if it becomes less than 0.1%, the toughness of the base material deteriorates, so the lower limit was set to 0.1%.

On the other hand, there is too much Si. If this happens, the cleanliness of the steel deteriorates and the toughness decreases, so the upper limit was set to 0.50.

If Mn is less than 1.0%, the strength and toughness of the steel sheet will decrease, and the HAZ will become more softened, so the lower limit was set at 1.0%.

On the other hand, if there is too much Mn, the toughness of the HAZ deteriorates, so the upper limit was set to 2.0.

Ad is required to be added so that at least 0.01% of All is dissolved in solid solution for deoxidation of steel.AA! tota
The lower limit of l was set to 0.01.

On the other hand, if the solid solution Al content exceeds 0.06%, not only the toughness of the HAZ but also the toughness of the weld metal will deteriorate significantly.

For this reason, the upper limit of Alltotal was set to o, o6o%.

As mentioned above, Nb is at least o at the rolling temperature.
If o1% or more is not dissolved in solid solution, the fine grain effect that improves the transition temperature cannot be obtained, and for this reason, the lower limit of the total Nb amount is set to o, o1
%.

Up to 0.06%, the strength increases as the amount added increases without deteriorating the base material toughness; however, when added in excess of 0.06%, the toughness of the HAZ deteriorates significantly.

Therefore, the upper limit was set at 0.06%.

Next, as selected components, Ni 0.50% or less, Or 0.3
Adding one or more of 0% or less, OuO, 50% or less, and Vo, 15% or less improves the strength and toughness of the base material even more than the above, and increases the thickness that can be manufactured. The reason for limiting the amount added is as described below.

- Ni improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of HAZ, but for the purpose of this invention, it is not necessary to add more than 0.5%; Anything more than that will increase manufacturing costs, so set the upper limit to 0.
．． It was set at 5%.

Or has the function of suppressing the band-like pearlite structure that tends to form in the structure after normalization, thereby improving toughness, but on the other hand, it increases the hardenability of HAZ, resulting in a decrease in toughness and cracking resistance. Taking this into consideration, the upper limit was set at 0.3%.

Ou not only has almost the same effect as Ni, but also improves corrosion resistance, but if it exceeds 0.50%, cranks are likely to occur during hot rolling and the surface quality of the steel sheet deteriorates, so the upper limit should be set to 0. .50%.

(2) is an attempt to improve the strength and toughness of the base material of the steel plate according to the present invention, increase the thickness of the steel plate that can be manufactured, and ensure the strength of the joint at the welded part.

However, if the amount added is too large, the toughness of the base material and HAZ will deteriorate significantly, so the upper limit was set at 0.15%.

Next, the present invention will be explained in detail.

Table 1 shows the chemical composition and manufacturing conditions of the test steel plates obtained by controlled rolling in a thick plate mill and normalization, and Table 2 shows the mechanical properties of the steel plates.

Comparative steels #1 and #2 both contain no Nb, and #1
The rolling finishing temperature was set at 90°C according to the commonly used rolling method.
0℃, #2 has a cumulative reduction of 50 inches at 900℃ or less, and the rolling finishing temperature is 850℃,
Comparative steels #3 and #4 both contain 0.02% Nb, but #3 is not subjected to a reduction of 900°C or less, and #4 is #4.
These steels were subjected to the same heat treatment as No. 2, but the Charpy shock transition temperature vTrs after normalization was -600C to -75°C, and even #4, which is the highest, was only 18
While it only shows a value of about 2°C, 0.02% N
Steel plates #5 and #6 according to the present invention, which were normalized by adding B and giving a cumulative reduction rate of 50% or 80, respectively, at 900°C or less had vTrs of -120°C and -140°C.
The steel sheet according to the present invention is characterized in that the impact transition temperature is significantly improved, and the tensile strength and impact absorption energy are also improved.

Test steel plates #10, #11, and #12 are examples in which Ni, Or, Ou, and V are added in small amounts with the aim of improving the strength to some extent, increasing the strength to 50 Ky1m1M, and increasing the thickness to 279. Comparative steels #7, #8, and #9 that do not contain Nb have a vTr of about 182°C or less.
s, the steel plate according to the present invention exhibits a value of vTrs of -118 to -150°C, and the impact transition temperature is significantly improved, despite the increase in strength.

The steel sheet according to the present invention not only has excellent Charpy impact properties but also extremely good DWTT properties as shown in Table 2, and there is no fear of separation.

[Brief explanation of drawings]

Figure 1 shows the rolling finishing temperature fixed at 850°C and 90°C.
If the cumulative reduction rate is changed below 0°C, then 900°
Charpy impact transition temperature (v
A graph showing the relationship between Trs) and yield stress (y, s,),
Figure 2 also shows vTrs, Y, and S of the steel shown in Figure 1, which was normalized under the same conditions as the steel shown in Figure 1, when the cumulative reduction rate below 900°C was fixed at 50 degrees and the rolling finishing temperature was changed.
, and Figure 3 shows the relationship between 900°G and 8
vTrs when the normalizing temperature (holding time 1 hr) of a steel plate given a cumulative reduction rate of 50% in a temperature range of 50°C is changed
and Charpy impact absorption energy (v
It is a graph showing the relationship between E and 50°C.

Claims (1)

[Claims] 1 Weight: 0; 0.04-0.16, Si: 0.10
~o, so%, Mn; 1.0-2.0%, A#to
tad; 0.010-0.060 and Nb; 0
．． 01 to 0.06, and further 0.0 as necessary.
Contains one or more selected from 15% or less of V, 0.50% or less of O, 0.30% or less of Or, and 0.50% or less of Ni, and the remainder is substantially Fe and Steel containing unavoidable impurities is heated above the temperature at which 1% or more of Nb is dissolved in solid solution, and then the cumulative reduction rate is 30 to 85% in a temperature range of 900°C or less, and the rolling end temperature is 700°C.
A method for producing a steel sheet with excellent low-temperature toughness, characterized by hot rolling at a temperature of 850°C or less, once air cooling, and then normalizing in a temperature range of 850 to 950°C.