JP4687153B2 - Production method of low yield ratio high strength steel - Google Patents

Description

  The present invention relates to a method for producing off-tempered high-tensile steel used as offshore structures, bridges, shipbuilding, line pipes, construction machinery, and steel materials for construction that mainly require earthquake resistance, and in particular, plate thickness of 40 mm or less. And a suitable manufacturing method for those having a yield ratio of 80% or less.

  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. Furthermore, it is desirable that a member used for a beam material that is most required to have plastic deformability when an earthquake occurs, has a lower yield ratio.

Patent Documents 1 to 9 and the like have been proposed for low yield ratio steel materials for ensuring earthquake resistance.
In Patent Document 1, the slab heating temperature is lowered, and the reduction rate in the non-recrystallization temperature range is defined as 30% or more to improve toughness, and by controlling the cooling rate and cooling stop temperature, It describes that the strength, the low yield ratio, and the high toughness are compatible.

  However, in the method described in Patent Document 1, 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 during accelerated cooling is controlled by changing the water density, and cooling can be performed at almost the same cooling rate even in different plate thicknesses, with the same strength regardless of the plate thickness, A technique is described that makes it possible to obtain a yield ratio.

  However, in the method described in Patent Document 2, when a thin material having a thickness of 40 mm or less is cooled at a low cooling rate by reducing the water density, it is difficult to perform uniform cooling over the entire surface of the steel plate, and cooling strain is not generated. growing.

  Patent Document 3 describes that high-strength steel with a low yield ratio 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, in order to obtain high strength at a cooling stop temperature of 600 ° C. or higher, it is necessary to add a large amount of expensive elements such as Cu, Ni, and Mo, and as a result, the manufacturing cost is greatly increased.

  Patent Document 4 describes that 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 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, and as a result, the manufacturing cost increases significantly.

  Patent Document 5 includes a low yield ratio by controlling the accelerated cooling rate after completion of hot rolling or the cooling rate during reheating and quenching to 5 ° C / s or more, and further controlling the temperature rising rate during tempering. Is described. However, temperature rise rate control during tempering requires strict temperature management and time management in actual production, and stable production is difficult.

  Patent Document 6 describes that a 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, the increase in the number of heat treatments performed offline increases the manufacturing cost and prolongs the manufacturing period.

  Patent Document 7 describes securing a high strength and a low yield ratio by controlling the cooling rate during accelerated cooling after the end of hot rolling to 0.3 to 3 ° C./s. However, at such a slow cooling rate, a large amount of alloying element is indispensable for securing high strength, and as a result, the manufacturing cost is greatly increased.

  Patent Documents 8 and 9 describe that after rolling, preliminary cooling is performed, air cooling is performed as soon as a predetermined temperature is reached, and then cooling is performed again to achieve a low yield ratio. Although the air cooling time after the pre-cooling is managed by a formula using the pre-cooling stop temperature and the transformation point as parameters, the air cooling time is substantially increased, resulting in a decrease in productivity.

In particular, when the technology described in Patent Document 8 and Patent Document 9 is applied to a thin material having a thickness of 40 mm or less, if the air cooling time is very long, the steel sheet is distorted even if there is a slight temperature difference between the upper and lower surfaces of the steel sheet. Occurs, and a flat steel plate cannot be manufactured.
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 JP 2000-256736 JP 2000-87138 A

  As described above, the techniques described in Patent Documents 1 to 9 require expensive alloy element addition, strict production condition management, and further, off-line two-phase region heat treatment and tempering heat treatment, and are used in large quantities. However, it was not always a reasonable method for the production.

  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: 0.05-0.5%
Mn: 0.6-2.0%
P: 0.02% or less
S: 0.005% or less
Al: Contains 0.1% or less, the balance is Fe and inevitable impurities, and the steel with a carbon equivalent Ceq of 0.3 to 0.45% represented by the formula (1) is heated in the range of 1000 to 1250 ° C, and then the rolling finish temperature After hot rolling to a temperature of 800 to 950 ° C on the steel sheet surface, cool to 550 to 650 ° C at the center of the plate thickness, and then cool to 500 ° C or less again after holding for 4 to 20s air cooling. A method for producing high-tensile steel with a sheet thickness of 40 mm or less and a yield ratio of 80% or less.

Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Where C, Si, Mn, Ni, Cr, Mo, V: Content of each element (% by mass)
2. As a steel component,
Cu: 0.1 to 1.0%
Ni: 0.1-2.0%
Cr: 0.05-1.0%
Mo: 0.05-1.0%
B: 0.0005-0.002%
Nb: 0.005-0.05%
Ti: 0.005-0.05%
V: 0.005-0.1%
The method for producing high-tensile steel according to 1, wherein the sheet thickness is 40 mm or less and the yield ratio is 80% or less.
3. The method for producing a high-strength steel having a thickness of 40 mm or less and a yield ratio of 80% or less according to 1 or 2, characterized by tempering in a temperature range of 200 to Ac1 (° C.) after cooling to 500 ° C. or less.

  According to the present invention, it is possible to produce a high strength steel having a low yield ratio with a thin material at low cost and easily, which is extremely useful industrially.

  In this invention, a component composition and manufacturing conditions are prescribed | regulated.

  [Ingredient composition] All% are mass%.

C: 0.05-0.18%
C is an element that increases the strength of steel, and 0.05% or more is necessary to secure 490 MPa or more, which is the tensile strength of a generally used steel sheet. However, excessive addition increases the low-temperature weld crack sensitivity. Therefore, it is limited to the range of 0.05 to 0.18%.

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, it is limited to a range of 0.05 to 0.5%.

Mn: 0.6-2.0%
Mn is an element that increases the hardenability of the steel and improves the strength. In order to secure this effect, Mn is required to be contained in an amount of 0.6% or more. On the other hand, if the content exceeds 2.0%, the weldability deteriorates remarkably. For this reason, it limits to the range of 0.6 to 2.0%.

P: 0.02% or less P 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. 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.

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. When further improving the characteristics, one or more of Cu, Ni, Cr, Mo, B, Ti, Nb, and 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 made into 0.0005 to 0.0020%.

Nb: 0.005-0.05%
Nb is an element effective for increasing the strength by precipitation strengthening, and 0.005% or more is necessary to exert the effect. However, when 0.05% or more is added, the weldability deteriorates. Therefore, when adding, Nb shall be 0.005-0.05%.
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 the 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 of the slab is 1000 ° C. or less, the deformation resistance increases, and a load is applied to the rolling mill. Also, at 1250 ° C or higher, surface flaws occur frequently during hot working. Therefore, the heating temperature of a slab shall be 1000-1250 degreeC.

Hot rolling finish temperature: 800 ~ 950 ℃
When the hot rolling end temperature is 800 ° C. or lower, the strength cannot be secured, and when it is 950 ° C. or higher, the toughness deteriorates. Therefore, the hot rolling end temperature is set to 800 to 950 ° C. The temperature is the temperature at the steel sheet surface.

Cooling conditions Cooling is performed until the temperature reaches 550 to 650 ° C (after primary cooling) at the center of the plate thickness of the steel sheet (after primary cooling), then kept in the temperature range for 4 to 20 seconds, and then cooled again to 500 ° C or less (secondary cooling). , Achieve low YR.

  By cooling the central portion of the plate thickness to 550 to 650 ° C., a state of supercooled austenite is obtained, and it is possible to promote the formation of ferrite by maintaining air cooling from this state.

  By keeping the air cooling for 4 s or more in this temperature range, the ferrite fraction becomes 20% or more and a low yield ratio is obtained. On the other hand, if air cooling is maintained for more than 20 s, the ferrite fraction becomes 60% or more, the tensile strength decreases, and the yield ratio increases. Therefore, it is cooled to 550 to 650 ° C. and kept air cooled for 4 to 20 seconds.

  Figure 1 shows the relationship between the primary cooling stop temperature and tensile properties. The test steel is 0.08C-0.25Si-1.35Mn-0.018P-0.002S-0.20Ni-0.20Cu (number is mass%) as a chemical component. After heating to 1150 ℃, the steel plate surface temperature is 850 ℃ Then, the rolling was finished and the plate thickness was reduced to 25 mm, and then accelerated cooling to a temperature range of 500 to 700 ° C. at the center of the plate thickness. After holding the air cooling for 10 seconds, accelerated cooling to 400 ° C. was performed again.

  JIS Z2201 No. 5 test piece was collected from the obtained steel sheet, and the relationship between the primary cooling stop temperature and the tensile properties was investigated. (The primary cooling stop temperature refers to the cooling stop temperature immediately after the completion of rolling. Carried out at 500-700 ° C.). As a result, it was confirmed that low YR was achieved by setting the primary cooling stop temperature to 550 to 650 ° C.

  The reason why low YR is achieved when the primary cooling stop temperature is 550 to 650 ° C. is that when the temperature is stopped in this temperature range, a microstructure with an appropriate ferrite fraction can be obtained. When cooling to 550 ° C. or lower and when cooling is stopped at 650 ° C. or higher, the ferrite fraction becomes less than 20%, so low YR cannot be achieved.

  FIG. 2 shows the results of investigating the relationship between the air cooling holding time (air cooling time) after the primary cooling stop and the tensile properties. After heating the steel having the same composition as the test steel in Fig. 1 to 1150 ° C, finishing rolling at 850 ° C at the surface temperature of the steel sheet to a plate thickness of 20mm, accelerated cooling to 600 ° C at the center of the plate thickness ( (Primary cooling), and after maintaining air cooling for 0 to 30 seconds in the temperature range, accelerated cooling (secondary cooling) to 400 ° C. was performed again.

  JIS Z2201 No. 5 test specimen was collected from the obtained steel sheet, and the relationship between the air cooling holding time (air cooling time) and the tensile properties was investigated. As a result, it was confirmed that low YR was achieved by setting the air cooling holding time (air cooling time) to 4 to 20 s.

  This is because, as described above, a microstructure with an appropriate ferrite fraction can be obtained by setting the air cooling holding time (air cooling time) to 4 to 20 s.

  Figure 3 investigated the relationship between the secondary cooling stop temperature and tensile properties. After heating the steel with the same composition as the test steels in Figs. 1 and 2 to 1150 ° C, rolling at a surface temperature of 850 ° C and finishing to a plate thickness of 32 mm, accelerated cooling to 600 ° C at the center of the plate thickness ( (Primary cooling), and after cooling for 15 seconds, the mixture was cooled again to 300 to 550 ° C. (secondary cooling).

  A JIS Z2201 No. 5 test piece was collected from the obtained steel sheet, and the relationship between the secondary cooling stop temperature and the tensile properties was investigated. The secondary cooling stop temperature was the second cooling stop temperature after holding the air cooling, and this test was performed at 300 to 550 ° C. As a result, it was confirmed that a low YR was achieved by setting the secondary cooling stop temperature to 500 ° C. or lower.

  Accordingly, in the present invention, the cooling condition is that after cooling to 550 to 650 ° C. (after primary cooling), the air cooling is maintained for 4 to 20 seconds, and cooling is again performed to 500 ° C. or less (secondary cooling). In the above description, the primary cooling and the secondary cooling are accelerated cooling. However, in the present invention, the cooling rate can be appropriately selected so that a microstructure with an appropriate ferrite fraction can be obtained.

Tempering temperature: 200 ℃ ~ Ac1
The tempering heat treatment is performed to reduce residual stress in the steel sheet due to water cooling. At temperatures below 200 ° C, the residual stress reduction effect cannot be obtained, and the strength and other materials change significantly above the Ac1 point. Therefore, it is set to 200 ° C. to Ac1.

  Steels having the chemical components shown in Table 1 were produced into steel plates having various plate thicknesses (40 mm or less) under the conditions shown in Table 2. The component compositions of the test steel were all within the scope of the present invention, and the steel sheet was manufactured by changing the manufacturing conditions over a wide range. Table 2 also shows the tensile properties. The tensile test was carried out with JIS Z2201 No. 5 test piece.

  All of the developed steels satisfy YR of 80% or less, while the comparative steel yields because at least one of the primary cooling stop temperature, the air cooling holding time, or the secondary cooling stop temperature is outside the scope of the present invention. The ratio is over 80%.

FIG. 5 is a graph showing the influence of the primary cooling stop temperature on the tensile properties, where (a) shows the influence of the primary cooling stop temperature on YR (%), and (b) shows the effects of the primary cooling stop temperature on YS and TS (MPa). FIG. It is a figure which shows the influence of the air-cooling holding time (air-cooling time) on a tensile characteristic, (a) is the influence of the air-cooling holding time (air-cooling time) on YR (%), (b) is on YS and TS (MPa). The figure which shows the influence of air cooling holding time (air cooling time). The figure which shows the influence of the secondary cooling stop temperature on the tensile properties, (a) is the influence of the secondary cooling stop temperature on YR (%), (b) is the secondary cooling stop on YS, TS (MPa) The figure which shows the influence of temperature.

Claims (3)

% By mass, C: 0.05 to 0.18%
Si: 0.05-0.5%
Mn: 0.6-2.0%
P: 0.02% or less
S: 0.005% or less
Al: Contains 0.1% or less, the balance is Fe and inevitable impurities, and the steel with a carbon equivalent Ceq of 0.3 to 0.45% represented by the formula (1) is heated in the range of 1000 to 1250 ° C, and then the rolling finish temperature After hot rolling to a temperature of 800 to 950 ° C on the steel sheet surface, cool to 550 to 650 ° C at the center of the plate thickness, and then cool to 500 ° C or less again after holding for 4 to 20s air cooling. A method for producing high-tensile steel with a sheet thickness of 40 mm or less and a yield ratio of 80% or less.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Where C, Si, Mn, Ni, Cr, Mo, V: Content of each element (% by mass) As a steel component,
Cu: 0.1 to 1.0%
Ni: 0.1-2.0%
Cr: 0.05-1.0%
Mo: 0.05-1.0%
B: 0.0005-0.002%
Nb: 0.005-0.05%
Ti: 0.005-0.05%
V: 0.005-0.1%
The method for producing a high-strength steel having a thickness of 40 mm or less and a yield ratio of 80% or less according to claim 1, wherein one or more of these are contained. The method for producing a high-strength steel having a thickness of 40 mm or less and a yield ratio of 80% or less according to claim 1 or 2, wherein the steel is cooled to 500 ° C or less and then tempered in a temperature range of 200 to Ac1 (° C).

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