As I saw, the Winter Soldier's metal hand is very powerful. And in many combats with the Avengers it's been shown. But in Captain America: Civil War, when both groups are combating, Spider-Man, here just a kid, was able to lift and bend the Winter Soldier's metal hand very easily. I don't understand how he can do this?
From this answer on SciFi.SE;
Bucky Barnes does not have superhuman abilities.
Neither does Steve Rogers. Instead, Steve is simply at peak physical fitness for a human being (to the point where he may appear to be superhuman). It would seem that after Captain America: The Winter Soldier, Bucky also shares these traits.
From Captain America: Civil War, I believe we can assume that Bucky and Steve are about equal when it comes to physical strength, endurance and speed.
However, despite being "leveled up" to peak human, they are both significantly weaker than Peter Parker.
Peter Parker was bitten by a radioactive spider, which gave him the proportionate strength of a spider. This means that if a spider were scaled up to the size of a human (along with their strength), it would be as physically strong as Peter Parker.
In fact, we get an indication of Peter's raw strength in Captain America: Civil War. Tony is showing Peter vidoes of Spider-Man's good deeds all across New York, and in one of them, Peter stops a van from driving into a bus at the last second by simply picking it up.
Being as spiders can lift many times their own body weight, so can Peter Parker, which makes him one of the strongest characters in the Marvel Cinematic Universe to date; and that is why he can block Bucky's punches and move his arm around so easily.
Both Cap and Bucky are on about equal strength, with Cap being stronger but handicapped by his friendship to Bucky. In Age of Ultron, we see Cap flip a moving motorcycle over his head. It's 600 pounds dry weight. Cap isn't moved at all. That's super human, as the human weightlifting record is a 400 lbs snatch.
In Civil War, Bucky does much of the same, flipping a motorcycle around in the air without skipping a beat. This bike is clearly smaller than the one Capt flipped.
But in Civil War, we see a video of Spider-Man dead stopping a moving van. Tony Stark explicitly states it is 3000 pounds.
That's 6 times what Capt was able to do. And that's EXPLICITLY why Tony seeks out Parker. Because he knew he needed a heavy hitter in order to bring down two Enhanced Humans like Capt and Bucky.
Spider-Man is in a completely different strength class. His strength dwarfs Capt and Bucky's strength. Even an enhanced human with an Vibranium Arm like Bucky can't produce the same amount of force of a 3000 pound car moving at 40 MPH. He lacks the pure mass/density to do so.
In the Marvel Cinematic Universe, both Captain America and the Winter Soldier possess powers even surpassing what is said to be peak human. The MCU takes a great deal of it's inspiration and characterization from the Ultimate Marvel series.
Captain America has displayed strength beyond even peak human since his first canonical showing in the MCU by lifting a motorcycle with three showgirls on it. That's a lifting feat in the 900-1,250 pound range. In Age of Ultron, he threw a motorcycle at an attacker. In Civil War, he held back a helicopter from taking off with only one hand.
Cap has also been able to sprint at speeds reaching that of 30-45 miles an hour for long periods of time as shown in Civil War when he, Winter Soldier, and the Black Panther outran cars. Not a car, CARS.
Considering that Winter Soldier was enhanced with the same serum he was, albeit a different version and in smaller amounts, yet still able to fight on the same terms as Cap, display similar feats and still hold his own, I'd consider the two of them superhuman.
Spider-Man is a completely different story. Even at their highest showing, Cap and the Winter Soldier aren't any stronger than the 2-3 ton range. Spider-Man, even at his weakest and as a child, is blatantly superhuman. Considering that the MCU has shades of the Ultimate Universe, what with nearly all the non-tech Earth-based powers coming from Erskine's Super-Soldier serum, I'd say that this Spider-Man is definitely not going to be rocking Normal Spider-Man strength.
Normal Comic (616 Universe for those that know) Spider-Man is in the 35-45 ton range as of right now. That's strong enough to lift two armored trucks overhead.
Ultimate Comics has Peter Parker Spider-Man (there's a distinction) at around a 3-5 ton range. Near the end of his run, even while near death, burnt, bruised, bleeding and with a bullet in him, he lifted a truck (approximately 11 tons=22,000 lbs).
This MCU Spider-man is faster, stronger, more agile and more...well...super than Cap and the Soldier, even with the Soldier's metal arm.