JPS63183123A - Production of high tensile steel having excellent low-temperature toughness after linear and spotty reheating - Google Patents

Abstract

PURPOSE:To produce a high tensile steel having excellent low-temp. toughness after linear and spotty reheating by subjecting a low-C steel slab having a specific compsn. consisting of C, Si, Mn, S, Al, Ti, N, and Fe to hot rolling and cooling under specific conditions. CONSTITUTION:The steel slab which contains 0.01-0.08wt.% C, 0.05-0.50% Si, 0.80-2.0% Mn, <=0.030% S, 0.010-0.060% Al, 0.005-0.020% Ti, 0.0020-0.0080% N, contains further, >=1 kinds among <=0.70% Cu, <=2.0% Ni, <=0.50% Cr, <=0.50% Mo, 0.01-0.10% V, 0.005-0.030% Nb, and 0.005-0.030% Ca at need, consists of the balance Fe and unavoildable impurities and has <=0.36% carbon equiv. is heated to 950-1,150 deg.C. The steel is then subjected to hot rolling at <=50% cumulative draft at <=850 deg.C and Ar3 point + or - 40 deg.C rolling finish temp. The hot rolled sheet is subjected to accelerated cooling down to a 600-300 deg.C range at >=2 deg.C/sec cooling rate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-strength steel that retains excellent low-temperature toughness even after linear and spot reheating over a wide range of conditions.

Applying linear or dotted reheating processing (hereinafter simply referred to as linear heating) to the surface of Jubei Nithorn Banner steel plate is, for example, as described in the Transactions of the Society of Naval Architects of Japan No. 133. , In ship construction, bending of hull members, correction after bending,
It is often used to remove distortion after welding or molding.

Conventionally, the maximum temperature of linear heating allowed for high-strength steel plates that handle 50 kgf/mm" is 900°C. However, during the actual work, the maximum temperature is 900°C. In many cases, construction is being carried out, and therefore, the temperature may actually exceed 900°C during construction.In this way, in linear heating, construction is carried out at a temperature higher than the allowable heating temperature. In this case, the problem arises that the mechanical properties, especially the toughness, of the parts heated to high temperatures deteriorate significantly.On the other hand, in order to improve construction efficiency, it is necessary to raise the linear heating temperature as high as possible. be.

As a result of intensive research to solve the above-mentioned conventional problems in linear heating, the inventors have determined the hot rolling conditions for steel slabs with C reduced to a predetermined amount and the subsequent We have discovered that by optimally controlling the accelerated cooling conditions, it is possible to produce high-strength steel sheets that ensure the toughness of the linearly heated parts, even when linearly heated at temperatures as high as 900°C or higher, for example. , which led to the present invention.

The method for manufacturing high-strength steel with excellent low-temperature toughness after linear and spot reheating processing according to the present invention is to solve the problem of co, o in weight percent.
i~0.08% Si0.05~0.50% Mn 0.80~2.0% S 0.030% or less Afo, 010~0.060% Ti 0.005~0.020% N 0.0020 Heating a steel slab consisting of ~0.0080% balance iron and unavoidable impurities and having a carbon equivalent of 0.36% or less to a temperature in the range of 950 to 1150 °C,
The cumulative reduction rate at 850°C or lower is 50% or more, and
After hot rolling with the finishing temperature of Ar=±40°C, the cooling rate is 2°C/2°C to a temperature in the range of 600 to 300°C.
It is characterized by accelerated cooling in seconds or more.

First, the chemical composition of the steel used in the method according to the present invention will be explained.

C is the most important alloying element in the method of the present invention and is essential for ensuring strength, and the amount of C influences the toughness of the linear heating section. Therefore, in order to ensure strength, it is necessary to add at least 0.01%. However, if too much is added, a large amount of bainite will be generated during cooling after linear heating, hardening the steel, and will not only reduce the toughness, but also deteriorate the toughness of the weld heat affected zone. shall be 0.08% or less.

St is an effective element for deoxidizing steel and ensuring strength, but if it is less than 0.05%, its effect is poor and 0.
．． When it exceeds 50%, the toughness of the base metal and weldability are deteriorated, so the upper limit is set to 0.50%.

Mn is also an element necessary to ensure the strength of the base metal, and therefore it is necessary to add a small amount (0.80% in both cases).However, when adding too much, coarse bainite is generated. It hardens by quenching, and the toughness of the base metal deteriorates, and the weldability and toughness of the linear heated part also deteriorate significantly.
The upper limit of the amount added was set to 2.0%.

In the present invention, the amount of S is limited to 0.030% or less in order to increase the impact absorption energy of the base material and the linear heating section.

Af is an element added to killed steel for deoxidation,
If it is less than 0.010%, deoxidation is insufficient,
Since the toughness of the base material deteriorates, it is necessary to add 0.010% or more. However, if too much is added, Al oxide-based nonmetallic inclusions are likely to be generated, which deteriorates the base material toughness, the toughness of the linear heated part, and the lamellar tear resistance properties, so the upper limit of the amount added is set to o, o. Let it be so%.

Ti generates TiN during reheating of a steel slab, suppresses coarsening of austenite grains, and refines ferrite grains through intragranular nucleation during austenite-ferrite transformation during accelerated cooling after hot rolling. Addition amount is 0.
When OO is less than 5%, it is difficult to effectively obtain the above-mentioned effects, and on the other hand, when added in excess, the toughness of the linear heating section is deteriorated, so the upper limit is set to 0.020%.

N is necessary to combine with Ti to produce the above-mentioned TiN, and is at least 0. Contains 20% OO. On the other hand, when the content exceeds 0.0080%, it not only deteriorates the linear heating section but also the toughness of the weld metal, so the upper limit was set at 0°0080%.

In addition to the above elements, the steel used in the present invention may contain Cu, Ni, Cr%Mo, depending on the required characteristics of the product to be obtained.
At least one or two elements selected from 5V and Nb can be added.

Cu is useful for adjusting strength and improving weldability, and is also effective in improving base material toughness. However, if too much Cu is added, Cu cracking occurs during rolling of the steel, so the upper limit of the amount added is set to 0.70%.

Ni has the effect of improving the strength and toughness of the base metal and the toughness of the linear heated part without having a detrimental effect on weldability. However, when adding too much, hardenability increases and bainite is generated during the cooling process after reheating.
Since it deteriorates the toughness of the linear heated part, the upper limit of the amount added is 2.
．． Set to 0%.

Cr is useful as an inexpensive element that improves strength and toughness, but adding too much deteriorates weld cracking resistance, so it is desirable to keep the amount to the minimum that can ensure the required material quality. In the invention, the upper limit of the amount added is 0.50%.

Mo improves strength and toughness, but when added in excess, it causes quench hardening of the base metal and linear heating part, deteriorating base metal toughness, linear heating characteristics, and weldability, so the amount added is The range shall be 0.50% or less.

(2) and Nb produce carbides, which are effective in increasing the strength of the steel due to their precipitation-strengthening action and improving toughness due to the refinement of crystal grains. In order to effectively obtain such effects, it is necessary to use 0.010% or more for ■, and o for Nb.
, oos% or more is required. but,
When either element is added in excess, it produces bainite, which causes deterioration of toughness and reduces weld cracking resistance, so the upper limit of the amount added is 0.1 for ■.
0%, and 0.030% for Nb.

Ca makes MnS spheroidal, improves shock absorption energy, and also reduces defects in steel materials caused by hydrogen. In order to effectively obtain such effects, the amount added should be 0.0005~
0. The range is OO30%.

In the method of the present invention, the steel used is
(Ceq) is required to be 0.36% or less.

As an index for evaluating the hardenability of steel, Ceq is closely related to strength and toughness, and in order to improve linear heating characteristics, the lower the value, the more advantageous it is. Even if the amount of alloying elements in steel is defined as described above, if Ceq is large, deterioration of toughness cannot be avoided, so in the present invention, Ceq of steel is set to 0.36% or less.

The method of the present invention involves hot rolling a steel slab having the above-mentioned chemical composition and Ceq.
Predetermined strength and toughness can be ensured by heating to a temperature in the range of 50°C, making the cumulative reduction rate at 850°C or less 50% or more, and hot rolling at the finishing temperature of Ar 3 points ±40°C. It is necessary to do so.

Then, according to the method of the present invention, after such hot rolling,
Average cooling rate 2℃/ to temperatures ranging from 600 to 300℃
Perform accelerated cooling in seconds or more. Cooling stop temperature is 600℃
When the temperature is higher than 300°C, it is not possible to obtain the desired strength, and on the other hand, when the temperature is lower than 300°C, the toughness deteriorates due to the formation of bainite.

As described above, according to the method of the present invention, a steel slab having a predetermined chemical composition is hot rolled under predetermined conditions and then accelerated cooling under predetermined conditions. High-strength steel that maintains low-temperature toughness can also be obtained by linear heating at a wide range of temperatures, particularly at temperatures as high as 900° C. or higher.

General Practice■ Table 1 shows the chemical composition of the test steel, Ceq, Acs and A.
Steels A to E are inventive steels, and Steels EH are comparative steels.

After heating these steel slabs to a temperature of 950 to 1150°C, they are rolled at a cumulative reduction rate of 50 to 60% at 850°C or less, and finished into a steel plate with a thickness of 25 to 3B++n.
Immediately thereafter, accelerated cooling was carried out to a temperature of 500°C at a cooling rate of 10°C/sec. Table 2 shows the tensile properties and impact properties of the steel plate thus obtained.

Next, using the above sample steel, linear heating was performed at the temperatures shown in Table 2, and an impact test was conducted on the reheated portion. FIG. 1 shows the linear heating temperature and the impact value CvE-a at 40°C of the linear heating part. ).

In the case of comparative tIIF, if the linear heating temperature becomes 950°C or higher, νE-,. However, according to the invention steel B, when heated to 1050°C and air cooled, vE-a
o indicates 18 kgf-m, and the steel of the present invention is 100 kgf-m.
Even when heated to 0°C and immediately cooled with water, vE, o
shows 12 kgf-m, and thus it is clear that the steel of the present invention has excellent toughness after linear heating.

Figure 2 shows the relationship between the cooling time from (AC++50)'C to 300°C and vE-4° when the test steel was linearly heated to 950°C and then air cooled.
According to the invention steel E, vE-4° is 22 kgf-a+ when the cooling time is 7 seconds, and it has excellent low-temperature toughness, whereas according to comparative fiG, the cooling time is 22 kgf-a+.
Even if it is for 5 seconds, vE-4° is 5 kgf-m, and the deterioration of low-temperature toughness is significant.

Table 3 shows the tensile strength and impact value of comparative steel H when linear heating was repeated two and three times under the same conditions.
By this repeated linear heating, vE-ao becomes 3
．． 2 kgf-, and the deterioration of low-temperature toughness is significant.

On the other hand, in the steels A and C of the present invention, vE, even when repeated linear heating was performed.

is 13.4 kgf-ta or more, and it is clear that excellent low-temperature toughness is maintained even after repeated linear heating.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the relationship between the linear heating temperature and the impact value (VB-4°) at 40°C of the linear heating part, and Fig. 2 is a graph showing the relationship between the linear heating temperature and the impact value (VB-4°) at 40°C. (AC
3+ 50) Immediate heating time from 'C to 300°C and vE- of the obtained linear heating section. It is a graph showing the relationship between Patent applicant: Kobe Steelbo Co., Ltd. Agent: Patent attorney: Itsuki Makino

Claims (2)

[Claims] (1) In weight%: C0.01-0.08% Si0.05-0.50% Mn0.80-2.0% S0.030% or less Al0.010-0.060% Ti0.005-0.020 %N0.0020~0.0080% The balance consists of iron and unavoidable impurities, carbon equivalent is 0.3
A steel slab of 6% or less was heated to a temperature in the range of 950 to 1150°C, and hot-rolled with a cumulative reduction rate of 50% or more at 850°C or less, and a finishing temperature of Ar_3 ± 40°C. A method for producing high-strength steel having excellent low-temperature toughness after linear and spot reheating processing, characterized in that the process is followed by accelerated cooling to a temperature in the range of 600 to 300°C at a cooling rate of 2°C/sec or more. (2) In weight% (a) C0.01~0.08% Si0.05~0.50% Mn0.80~2.0% S0.030% or less Al0.010~0.060% Ti0.005~ 0.020%, and N0.0020 to 0.0080%, furthermore, (b) Cu0.70% or less, Ni2.0% or less, Cr0.50% or less, Mo0.50% or less, V0.01 ~0.10%, Nb0.005~0.030%, and Ca0.0005~0.0030%, with the balance consisting of iron and inevitable impurities, and carbon equivalent is 0.3
A steel slab of 6% or less was heated to a temperature in the range of 950 to 1150°C, and hot-rolled with a cumulative reduction rate of 50% or more at 850°C or less, and a finishing temperature of Ar_3 ± 40°C. A method for producing high-strength steel having excellent low-temperature toughness after linear and spot reheating processing, characterized in that the process is followed by accelerated cooling to a temperature in the range of 600 to 300°C at a cooling rate of 2°C/sec or more.