JP4830318B2 - Method for producing non-tempered high-tensile steel with excellent surface properties - Google Patents

Description

  The present invention relates to a method for producing off-heat treated high-strength steel used as offshore structures, bridges, shipbuilding, line pipes, construction machinery, and construction steel materials mainly requiring earthquake resistance, and in particular, tensile strength of 550 MPa. As described above, the present invention relates to a method suitable for manufacturing a material having a yield ratio of 80% or less and excellent surface properties.

  In recent years, steel sheets having excellent earthquake resistance are required for building structures and the like from the viewpoint of ensuring safety during an earthquake. In addition, it is clear that steel plates with lower yield ratios are more excellent in earthquake resistance, and steel materials for construction and the like are obliged to use steel materials with a yield ratio of 80% or less.

  Patent documents 1-7 etc. are proposed about the low yield ratio steel material for ensuring earthquake resistance. In Patent Document 1, the slab heating temperature is lowered and the toughness is improved by defining the reduction ratio in the non-recrystallization temperature range to be 30% or more, and the high strength is achieved by controlling the cooling rate and the cooling stop temperature. It has both low yield ratio and high toughness.

  However, in this method, since the deformation resistance is high due to low temperature heating of the slab and a load is applied to the rolling mill and the finishing temperature is low, strict temperature control is required, and stable production is difficult.

  In Patent Document 2, the cooling rate at the time of accelerated cooling is controlled by changing the water density, which enables cooling at almost the same cooling rate even at different plate thicknesses, the same strength regardless of the plate thickness, Techniques have been proposed that make it possible to obtain a yield ratio.

  However, the stop temperature of accelerated cooling is in the range of 400 to 550 ° C. In this case, residual stress is generated due to cooling, and the generation of distortion due to gas cutting or the like in the processing process becomes a problem.

  In Patent Document 3, a high yield strength high strength steel is manufactured by controlling the accelerated cooling rate after hot rolling to 1 ° C./s or more and cooling to 750 to 600 ° C. However, the hot rolling end temperature is set to 950 ° C. or more, and in this case, there is a problem that scale flaws occur frequently.

  In Patent Document 4, both high strength and low YR are achieved by controlling the cooling rate during accelerated cooling after the end of hot rolling to 1 to 5 ° C./s. However, at such a slow cooling rate of 1 to 5 ° C / s, it is necessary to add a large amount of expensive elements such as Cu, Ni and Mo in order to ensure high strength. To increase.

  In Patent Document 5, the low yield ratio is reduced by controlling the accelerated cooling rate after the hot rolling or the cooling rate at the time of reheating and quenching to 5 ° C / s or more, and further controlling the heating rate at the time of tempering. Have achieved. However, temperature rise rate control during tempering requires strict temperature management and time management in actual production, and stable production is difficult.

  In Patent Document 6, two-phase region heat treatment is performed in order to ensure a low yield ratio. However, although the two-phase region heat treatment is a technique that can stably secure a low yield ratio, there are problems such as an increase in manufacturing cost due to an increase in the number of heat treatments performed offline and an increase in the manufacturing period.

In Patent Document 7, an attempt is made to ensure a high strength and a low yield ratio by controlling the cooling rate at the accelerated cooling rate after completion of hot rolling to 0.3 to 3 ° C./s. At the cooling rate, it is essential to add a large amount of alloying elements in order to ensure high strength, and as a result, the manufacturing cost increases significantly.
JP-A-5-320752 JP-A-5-339631 JP-A-6-340924 Japanese Unexamined Patent Publication No. 9-3596 Japanese Patent Laid-Open No. 9-3595 Japanese Patent Laid-Open No. 10-306316 JP 2001-226737 A

  However, the techniques described in Patent Documents 1 to 7 require expensive alloying element addition and strict production condition management, and further, offline two-phase region heat treatment and tempering heat treatment, and a method for producing thick steel plates used in large quantities. It was not always a reasonable method.

  Then, this invention aims at providing the method of manufacturing a low yield ratio high tensile steel cheaply and simply.

The object of the present invention is achieved by the following means.
1. % By mass
C: 0.05 to 0.18%
Si: 0.05 to 0.5%,
Mn: 0.6 to 2.0%,
P: 0.02% or less S: 0.005% or less Al: 0.1% or less Nb: 0.005 to 0.03%
Furthermore, in mass%
Cu: 0.1 to 1.0%
Ni: 0.1 to 2.0%
Cr: 0.05-1.0%
Mo: 0.05-1.0%
B: 0.0005 to 0.002%
Ti: 0.005 to 0.05%
V: 0.005 to 0.1%
The steel containing one or more of the following, the balance being Fe and inevitable impurities, and the carbon equivalent Ceq represented by the formula (1) being 0.38 to 0.42 % is in the range of 1000 to 1250 ° C. , And then hot-rolled so that the rolling end temperature is 800 to 950 ° C. on the surface, and then cooled to the cooling stop temperature indicated by equation (3) at the center of the plate thickness within the cooling rate range indicated by equation (2). A method for producing non-tempered high-tensile steel having a tensile strength excellent in surface properties of 550 MPa or more and a yield ratio of 80% or less.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, C, Si, Mn, Ni, Cr, Mo, V: Content of each element (mass%)
−50 × Ceq + 25 ≦ Vc ≦ −50 × Ceq + 39 (2)
Where Vc is the cooling rate (° C./s)
T ≦ 2000 × Nb + 580 (when 0.005 ≦ Nb ≦ 0.02)
T ≦ 620 (when 0.020 <Nb ≦ 0.03) (3)
Here, Nb: Amount of Nb added (mass%), T: Cooling stop temperature (° C)

  According to the present invention, it is possible to produce a low-yield ratio high-tensile steel sheet inexpensively and easily, which is extremely useful industrially.

  The present invention defines the component composition and manufacturing conditions.

  [Component composition]% is mass%.

C: 0.05-0.18%
C is effective in securing the strength of the steel, and 0.05% or more is necessary to exert its effect. However, excessive addition increases the sensitivity to low-temperature weld cracking. Therefore, in this invention, it limits to 0.05 to 0.18% of range.

Si: 0.05-0.5%
Si acts as a deoxidizing element and needs to be contained in an amount of 0.05% or more in terms of steelmaking, but if it exceeds 0.5%, the base material toughness decreases. For this reason, Si is limited to the range of 0.05 to 0.5%.

Mn: 0.6-2%,
Mn is an element that increases the hardenability of steel and improves its strength. To ensure this effect, it must contain 0.6% or more. On the other hand, if the content exceeds 2%, the weldability deteriorates remarkably. For this reason, in the present invention, Mn is limited to a range of 0.6 to 2%.

P: 0.02% or less P is an element inevitably contained in steel as an impurity, and it is desirable to reduce it as much as possible in order to deteriorate the toughness of steel. In particular, the content exceeding 0.02% significantly deteriorates toughness. Therefore, P is limited to 0.02% or less.

S: 0.005% or less
S is an element inevitably contained in steel as an impurity, and is desirably reduced as much as possible in order to deteriorate the toughness of steel and the drawing in the thickness direction tensile test. In particular, the content exceeding 0.005% significantly deteriorates the above characteristics. Therefore, S is limited to 0.005% or less.

Al: 0.1% or less
Al acts as a deoxidizer and is most commonly used in the deoxidation process of molten steel. If the content exceeds 0.1%, a coarse oxide is formed, and the ductility of the base material is remarkably deteriorated. Therefore, Al is limited to 0.1% or less.

Nb: 0.005-0.03%
Nb is an element effective for increasing the strength by precipitation strengthening, and 0.005% or more is necessary to exert the effect. On the other hand, when 0.03% or more is added, the toughness of the welded portion deteriorates. Therefore, the range of Nb addition amount is set to 0.005 to 0.03%.

  In the present invention, Nb is added as an essential additive to increase the strength and to increase the accelerated cooling stop temperature. Increasing the accelerated cooling stop temperature has the effect of reducing the residual stress in the steel sheet due to cooling. The higher the accelerated cooling stop temperature, the smaller the residual stress in the steel sheet. Distortion can be reduced.

  Such an effect of increasing the cooling stop temperature due to the addition of Nb cannot be further improved even if 0.02% or more is added. Therefore, the range of Nb addition is preferably 0.005 to 0.02%.

Ceq: 0.3-0.42%
Ceq is required to be 0.3% or more to ensure the strength of the welded joint, which is indispensable as a welded structure. However, if it is 0.42% or more, the weldability deteriorates. Therefore, Ceq is set to 0.3 to 0.42%.
However, Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 where C, Si, Mn, Ni, Cr, Mo, V: Content of each element Amount (mass%).

The above is the basic component composition of the present invention, but when further improving the characteristics, one or more of Cu, Ni, Cr, Mo, B, Ti, V can be added.
Cu: 0.1 to 1.0%
Cu is an element effective for increasing the strength without deteriorating the toughness, and in order to exert its effect, addition of 0.1% or more is necessary. However, the addition of 1.0% or more frequently causes surface defects during hot rolling. Therefore, when added, Cu is made 0.1 to 1.0%.

Ni: 0.1-2.0%
Ni is an element effective for increasing the strength without deteriorating the toughness, and 0.1% or more of addition is necessary to exert the effect. However, the addition of 2.0% or more increases the alloy cost. Therefore, when Ni is added, the content of Ni is set to 0.1 to 2.0%.

Cr: 0.05-1.0%
Cr is an element effective for increasing the strength without significantly increasing the alloy cost, and 0.05% or more of addition is necessary to exert the effect. However, weldability deteriorates when 1.0% or more is added. Therefore, when added, Cr is made 0.05 to 1.0%.

Mo: 0.05-1.0%
Mo is an element effective for increasing the hardenability and increasing the strength, and 0.05% or more of addition is necessary to exert the effect. However, weldability deteriorates when 1.0% or more is added. Therefore, when added, Mo is made 0.05 to 1.0%.

B: 0.0005-0.002%
B is an element effective for increasing the hardenability and increasing the strength by adding a very small amount, and 0.005% or more is necessary to exert the effect. However, addition of 0.002% or more deteriorates weldability. Therefore, when adding, B is 0.0005 to 0.002%.

Ti: 0.005-0.05%
Ti is an element effective for improving the toughness of the base metal and the welded joint, and 0.005% or more is necessary to exert its effect. However, when 0.05% or more is added, the weldability deteriorates. Therefore, when Ti is added, Ti is made 0.005 to 0.05%.

V: 0.005-0.1%
V is an element effective for increasing the strength by precipitation strengthening, and 0.005% or more is necessary to exert the effect. However, the weldability is deteriorated by addition of 0.1% or more. Therefore, when added, V is 0.005 to 0.1%.
[Production conditions]
Heating temperature: 1000-1250 ° C
When the heating temperature is 1000 ° C. or less, the deformation resistance increases, and a load is applied to the rolling apparatus. Also, at 1250 ° C or higher, surface flaws occur frequently during hot working. Therefore, heating temperature shall be 1000-1250 degreeC.

Hot rolling finish temperature: 800 ~ 950 ℃
When the hot rolling finish temperature is 800 ° C. or lower, the strength cannot be ensured, and when it is 950 ° C. or higher, the toughness deteriorates and the surface properties of the steel sheet deteriorate due to the scale wrinkles. Therefore, the hot rolling end temperature is set to 800 to 950 ° C.

Cooling rate
In order to achieve both a tensile strength of 550 MPa and a yield ratio of 80% or less, it is necessary to cool each component at an optimal cooling rate.

  After heating steel with a Ceq amount of 0.30 to 0.42% to 1150 ° C, hot rolling was finished at 880 ° C on the surface of the steel sheet, and after rolling to a plate thickness of 20 mm, the cooling rate was 580 in the range of 3 to 45 ° C / s. Cooled to ° C. A JIS Z2201 No. 5 test piece was collected from the obtained steel plate, a tensile test was performed, and a cooling rate range in which both strength and yield ratio were compatible was investigated.

  Figure 1 shows the relationship between the amount of Ceq and the cooling rate. When the cooling rate is slower than (-50 × Ceq + 25) ° C / s, the tensile strength is less than 550 MPa, and when the cooling rate is higher than (-50 × Ceq + 39) ° C / s, the yield ratio Becomes 80% or more. Therefore, the cooling rate range was set to −50 × Ceq + 25 ≦ Vc ≦ −50 × Ceq + 39.

Cooling stop temperature In order to secure the tensile strength of 550 MPa or more, which is the newly developed steel, it is necessary to cool to the optimal cooling stop temperature for each Nb addition amount. After heating steel containing Ceq amount of 0.40% and Nb addition amount of 0.005-0.03% to 1150 ° C, hot rolling was finished at 880 ° C on the steel sheet surface, and after rolling to a plate thickness of 20mm, the cooling rate was 10 ° C Cooled to 450-700 ° C at / s. JIS Z2201 No. 5 test piece was collected from the obtained steel sheet, and a tensile test was performed to investigate the cooling stop temperature range in which the strength can be secured at each Nb addition amount.

  Fig. 2 shows the relationship between the Nb addition amount and the cooling stop temperature. When the amount of Nb added is 0.02% or less, the tensile strength is 550 MPa or more when the cooling stop temperature is (2000 × Nb-580) ° C. or less. In the case of addition exceeding 0.02%, the effect of further increasing the cooling stop temperature cannot be obtained. Therefore, when the Nb addition amount is 0.005 to 0.02%, the cooling stop temperature range is T ≦ 2000 × Nb-580 (° C.), and when the Nb addition amount exceeds 0.02%, T ≦ 620 (° C.). .

  Steels having the chemical components shown in Table 1 were produced into steel plates using the conditions shown in Table 2. Table 2 shows the tensile properties and surface properties. Tensile tests were carried out with JIS Z2201 No. 5 test pieces for plate thicknesses of 40 mm or less, and evaluations were carried out with JIS Z 2201 No. 4 test pieces taken from 1/4 plate thickness positions for plate thicknesses exceeding 40 mm.

  The developed steel was confirmed to have excellent tensile properties of 550 MPa and a yield ratio of 80% or less, and excellent surface properties. On the other hand, the comparative steel has a cooling rate or cooling stop temperature outside the range of the present invention, so that the strength is 550 MPa or less or the yield ratio is 80% or more. Moreover, when the hot rolling finish temperature was 950 ° C. or higher, it was observed that scale wrinkles were generated.

The figure which shows the influence which the amount of Ceq and a cooling rate have on tensile properties. The figure which shows the influence which Nb addition amount and cooling stop temperature have on tensile characteristics.

Claims (1)

% By mass
C: 0.05 to 0.18%
Si: 0.05 to 0.5%,
Mn: 0.6 to 2.0%,
P: 0.02% or less S: 0.005% or less Al: 0.1% or less Nb: 0.005 to 0.03%
Furthermore, in mass%
Cu: 0.1 to 1.0%
Ni: 0.1 to 2.0%
Cr: 0.05-1.0%
Mo: 0.05-1.0%
B: 0.0005 to 0.002%
Ti: 0.005 to 0.05%
V: 0.005 to 0.1%
The steel containing one or more of the following, the balance being Fe and inevitable impurities, and the carbon equivalent Ceq represented by the formula (1) being 0.38 to 0.42 % is in the range of 1000 to 1250 ° C. , And then hot-rolled so that the rolling end temperature is 800 to 950 ° C. on the surface, and then cooled to the cooling stop temperature indicated by equation (3) at the center of the plate thickness within the cooling rate range indicated by equation (2). A method for producing non-tempered high-tensile steel having a tensile strength excellent in surface properties of 550 MPa or more and a yield ratio of 80% or less.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, C, Si, Mn, Ni, Cr, Mo, V: Content of each element (mass%)
−50 × Ceq + 25 ≦ Vc ≦ −50 × Ceq + 39 (2)
Where Vc is the cooling rate (° C./s)
T ≦ 2000 × Nb + 580 (when 0.005 ≦ Nb ≦ 0.02)
T ≦ 620 (when 0.020 <Nb ≦ 0.03) (3)
Here, Nb: Amount of Nb added (mass%), T: Cooling stop temperature (° C)

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